WO2024065150A1

WO2024065150A1 - Positive electrode active material and preparation method therefor, and positive electrode sheet, secondary battery and electric device comprising same

Info

Publication number: WO2024065150A1
Application number: PCT/CN2022/121562
Authority: WO
Inventors: 袁天赐; 蒋耀; 张欣欣; 欧阳楚英; 康伟斌; 吴凌靖; 陈尚栋
Original assignee: 宁德时代新能源科技股份有限公司
Priority date: 2022-09-27
Filing date: 2022-09-27
Publication date: 2024-04-04

Abstract

The present application provides a positive electrode active material and a preparation method therefor, and a positive electrode sheet, a secondary battery and an electric device comprising same. The positive electrode active material comprises a core and a shell covering the core; the core comprises LimAxMn1-yByP1- zCzO4-nDn; the shell comprises crystalline pyrophosphate LiaMP2O7 and/or Mb(P2O7)c, crystalline phosphate XPO4, crystalline borate YpBqOr and carbon, and the shell comprises one or more coatings; and each coating independently comprises one or more of crystalline pyrophosphate LiaMP2O7 and/or Mb(P2O7)c, crystalline phosphate XPO4, crystalline borate YpBqOr and carbon. The positive electrode active material of the present application can enable the secondary battery to have a relatively high energy density, and also have improved cycle performance, safety performance and/or rate performance.

Description

Positive electrode active material, preparation method thereof, positive electrode sheet, secondary battery and electric device containing the same

Technical Field

The present application belongs to the field of battery technology, and specifically relates to a positive electrode active material, a preparation method thereof, a positive electrode plate containing the same, a secondary battery, and an electrical device.

Background technique

In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields. With the application and promotion of secondary batteries, their safety performance has received more and more attention. Lithium manganese phosphate has become one of the most popular positive electrode active materials due to its advantages such as high capacity, good safety performance and abundant raw material sources. However, lithium manganese phosphate is prone to manganese ion dissolution during charging, resulting in rapid capacity decay.

Summary of the invention

The purpose of the present application is to provide a positive electrode active material, a preparation method thereof, a positive electrode plate, a secondary battery and an electrical device containing the same, which can enable a secondary battery using the positive electrode active material to have a higher energy density and improved cycle performance, safety performance, and/or rate performance.

In a first aspect, the present application provides a positive electrode active material having a core-shell structure, which comprises a core and a shell covering the core.

The core comprises _LimAxMn1 _- _yByP1 _- _zCzO4 _- _nDn , the A comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IIB, Group VB and Group VIB, _and optionally comprises one or more elements selected from Group Zn, Al, Na, K, Mg, Nb, Mo and W, the B comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB and Group VIIIB, and optionally comprises one or more elements selected from Group Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, the C comprises one or more elements selected from Group IIIA, Group IVA, Group VA and Group VIA, and optionally comprises The core is electrically neutral.

The shell comprises crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , crystalline phosphate XPO ₄ , crystalline borate Y _p B _q O _r and carbon, and the shell comprises one or more coating layers, each coating layer independently comprises one or more of crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , crystalline phosphate XPO ₄ , crystalline borate Y _p B _q O _r and carbon, crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) c The M in _c each independently includes one or more metal elements selected from transition metals, group IA, group IIA, group IIIA, group IVA, group VA and lanthanides, and optionally includes one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values of a, b and c satisfy the following conditions: the crystalline pyrophosphate Li _a MP ₂ O ₇ or M _b (P ₂ O ₇ ) _c maintains electrical neutrality, the X comprises one or more metal elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA and lanthanides, optionally comprising one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, the Y comprises one or more metal elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA and lanthanides, optionally comprising one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, _1≤p≤4 , 1≤q≤7, 2≤r≤12, and the values of p, q and r satisfy the following conditions: the crystalline borate _YpBqOr _maintains electrical neutrality.

After extensive research, the inventors discovered that by modifying and coating lithium manganese phosphate, a new type of positive electrode active material with a core-shell structure can be obtained. The positive electrode active material can achieve significantly reduced manganese ion dissolution and reduced lattice change rate. When used in secondary batteries, it can improve the cycle performance, rate performance, safety performance of the secondary batteries and increase the capacity of the secondary batteries.

In any embodiment of the present application, the interplanar spacing of the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c ranges from 0.293 nm to 0.470 nm, and the angle of the crystal direction (111) ranges from 18.00° to 32.00°.

In any embodiment of the present application, the interplanar spacing of the crystalline phosphate XPO ₄ ranges from 0.244 nm to 0.425 nm, and the angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

Crystalline pyrophosphate and crystalline phosphate within the above-mentioned range of interplanar spacing and angle can more effectively inhibit the lattice change rate and manganese ion dissolution of lithium manganese phosphate during lithium insertion and extraction, thereby improving the high-temperature cycle performance and high-temperature storage performance of secondary batteries.

In any embodiment of the present application, the carbon is a mixture of SP2 carbon and SP3 carbon. Optionally, the molar ratio of the SP2 carbon to the SP3 carbon is any value in the range of 0.1 to 10, and more preferably any value in the range of 2.0 to 3.0. The present application improves the comprehensive performance of the secondary battery by limiting the molar ratio of the SP2 carbon to the SP3 carbon to the above range.

In any embodiment of the present application, in the core, the ratio of y to 1-y is 1:10 to 1:1, and can be 1:4 to 1:1. Thus, the cycle performance and rate performance of the secondary battery are further improved.

In any embodiment of the present application, in the core, the ratio of z to 1-z is 1:9 to 1:999, and can be 1:499 to 1:249. Thus, the cycle performance and rate performance of the secondary battery are further improved.

In any embodiment of the present application, in the core, the B includes one or more elements selected from Fe, Ti, V, Ni, Co and Mg, and optionally includes at least two elements selected from Fe, Ti, V, Ni, Co and Mg. Simultaneous doping of two or more of the above elements at the Mn position is beneficial to enhancing the doping effect, further reducing the lattice change rate on the one hand, and further reducing the surface oxygen activity on the other hand.

In any embodiment of the present application, in the core, the C includes an element selected from B (boron), S, Si and N. This can further improve the rate performance of the secondary battery.

In any embodiment of the present application, q:r is 1:3.

In any embodiment of the present application, the shell includes a first coating layer coating the inner core, a second coating layer coating the first coating layer, a third coating layer coating the second coating layer, and a fourth coating layer coating the third coating layer, the fourth coating layer includes carbon, and the first coating layer, the second coating layer, and the third coating layer independently include any one of crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , crystalline phosphate XPO ₄ , and crystalline borate Y _p B _q O _r .

In any embodiment of the present application, the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the fourth coating layer includes carbon, and the second coating layer and the third coating layer each independently include crystalline phosphate XPO ₄ or crystalline borate Y _p B _q O _r , optionally, the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the second coating layer includes crystalline phosphate XPO ₄ , the third coating layer includes crystalline borate Y _p B _q O _r , and the fourth coating layer includes carbon. This can further improve the cycle performance, safety performance, and/or rate performance of the secondary battery.

In any embodiment of the present application, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, and can be optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

In any embodiment of the present application, the coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, and can be optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

In any embodiment of the present application, the coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, and can be optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

In any embodiment of the present application, the coating amount of the fourth coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, and more optionally greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

The coating amount of the four coating layers is preferably within the above range, so that the core can be fully coated and the cycle performance, safety performance, and/or rate performance of the secondary battery can be further improved without sacrificing the gram capacity of the positive electrode active material.

In any embodiment of the present application, the thickness of the first coating layer is 1 nm to 15 nm.

In any embodiment of the present application, the thickness of the second coating layer is 1 nm to 15 nm.

In any embodiment of the present application, the thickness of the third coating layer is 1 nm to 15 nm.

In any embodiment of the present application, the thickness of the fourth cladding layer is 2 nm to 25 nm.

The thickness of the four coating layers is preferably within the above range, so that the core can be fully coated and the cycle performance, safety performance, and/or rate performance of the secondary battery can be further improved without sacrificing the gram capacity of the positive electrode active material.

In any embodiment of the present application, based on the weight of the positive electrode active material, the manganese content is in the range of 10% to 35% by weight, optionally in the range of 15% to 30% by weight, and more optionally in the range of 17% to 20% by weight. At this time, it is possible to effectively avoid the problems of poor structural stability and density reduction of the positive electrode active material that may be caused by excessive manganese content, thereby improving the performance of the secondary battery such as circulation, storage and compaction density; and it is possible to avoid the problems such as low voltage platform that may be caused by too low manganese content, thereby improving the energy density of the secondary battery.

In any embodiment of the present application, based on the weight of the positive electrode active material, the content of phosphorus is in the range of 12 wt% to 25 wt%, and can be optionally in the range of 15 wt% to 20 wt%. At this time, the following situations can be effectively avoided: if the content of phosphorus is too high, the covalency of P-O may be too strong, which may affect the conductivity of small polarons, thereby affecting the conductivity of the positive electrode active material; if the content of phosphorus is too low, the stability of the pyrophosphate and/or phosphate lattice structure in the core and the shell may be reduced, thereby affecting the overall stability of the positive electrode active material.

In any embodiment of the present application, based on the weight of the positive electrode active material, the weight ratio of manganese element to phosphorus element ranges from 0.90 to 1.25, and can be optionally 0.95 to 1.20. At this time, the following situations can be effectively avoided: if the weight ratio is too large, it may cause an increase in the dissolution of manganese ions, affecting the stability of the positive electrode active material and the cycle performance and storage performance of the secondary battery; if the weight ratio is too small, the discharge voltage platform of the positive electrode active material may decrease, thereby reducing the energy density of the secondary battery.

In any embodiment of the present application, the lattice change rate of the positive electrode active material before and after complete lithium deintercalation is 4% or less, optionally 3.8% or less, and more optionally 2.0% to 3.8%. In this case, the positive electrode active material can improve the capacity and rate performance of the secondary battery.

In any embodiment of the present application, the Li/Mn antisite defect concentration of the positive electrode active material is less than 4%, optionally less than 2.2%, and more optionally 1.5% to 2.2%. By keeping the Li/Mn antisite defect concentration within the above range, it is possible to avoid Mn ²⁺ hindering the transmission of Li ⁺ , while improving the capacity and rate performance of the positive electrode active material.

In any embodiment of the present application, the compaction density of the positive electrode active material at 3T is 2.2 g/cm ³ or more, and can be 2.2 g/cm ³ or more and 2.8 g/cm ³ or less. This is beneficial to improve the volume energy density of the secondary battery.

In any embodiment of the present application, the surface oxygen valence state of the positive electrode active material is below -1.90, and can be optionally -1.90 to -1.98. Thus, by limiting the surface oxygen valence state of the positive electrode active material to the above range as described above, the interfacial side reaction between the positive electrode active material and the electrolyte can be reduced, thereby improving the cycle performance and storage performance of the secondary battery.

A second aspect of the present application provides a method for preparing a positive electrode active material, comprising the following steps of providing a core material and a coating step.

The step _{of providing a core material: the core comprises LimAxMn1-yByP1-zCzO4} _- _nDn _, _the _A _comprises _one or more elements selected from Group IA, Group IIA, Group IIIA, Group IIB, Group VB and Group VIB, and optionally comprises one or more elements selected from Group Zn, Al, Na, K, Mg, Nb, Mo and W, the B comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB and Group VIIIB, and optionally comprises one or more elements selected from Group Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, the C comprises one or more elements selected from Group IIIA, Group IVA, Group VA and Group VIA, and optionally comprises The core is electrically neutral.

Coating step: providing coating solutions including pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , phosphate XPO ₄ , borate Y _p B _q O _r and a carbon source respectively, adding the core material into the coating solution and mixing, and obtaining a positive electrode active material through sintering, wherein the positive electrode active material has a core-shell structure, comprising the core and a shell coating the core, the shell comprising crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , crystalline phosphate XPO ₄ , crystalline borate Y _p B _q O _r and carbon, and the shell comprising one or more coating layers, each coating layer independently comprising one or more of crystalline pyrophosphate Li a MP ₂ O ₇ and/or M b (P 2 O 7 ) c , crystalline phosphate XPO 4 , crystalline borate Y _p B q O r and carbon, the crystalline pyrophosphate Li _a MP 2 O 7 and/or M _b (P ₂ O ₇ ) _c , crystalline phosphate XPO ₄ , crystalline borate Y _p B _q O _r and carbon, M in M b ( _P 2 O ₇ ) c and/or M _b (P ₂ O ₇ ) _c each independently includes one or more metal elements selected from transition metals, group IA, group IIA, group IIIA, group IVA, group VA and lanthanides, and optionally includes one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values of a, b and c satisfy the following conditions: the crystalline pyrophosphate Li _a MP ₂ O ₇ or M _b (P ₂ O ₇ ) c _c maintains electrical neutrality, the X comprises one or more metal elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA and lanthanides, optionally comprising one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, the Y comprises one or more metal elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA and lanthanides, optionally comprising one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, _1≤p≤4 , 1≤q≤7, 2≤r≤12, and the values of p, q and r satisfy the following conditions: the crystalline borate _YpBqOr _maintains electrical neutrality.

In any embodiment of the present application, the step of providing a core material comprises the following steps: step (1): mixing and stirring a manganese source, a dopant of element B and an acid in a container to obtain manganese salt particles doped with element B; step (2): mixing the manganese salt particles doped with element B with a lithium source, a phosphorus source, a dopant of element C, optionally a dopant of element A and optionally a dopant of element D in a solvent to obtain a slurry, and sintering under the protection of an inert gas atmosphere to obtain a core material.

In any embodiment of the present application, the mixing in step (1) is performed at a temperature of 20°C to 120°C, optionally 40°C to 120°C.

In any embodiment of the present application, the stirring in step (1) is performed at 400 rpm to 700 rpm for 1 hour to 9 hours, optionally 3 hours to 7 hours.

When the heating temperature and stirring time during the preparation of the core particles are within the above range, the prepared core and the positive electrode active material prepared therefrom have fewer lattice defects, which is beneficial to inhibiting the dissolution of manganese ions and reducing the interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the cycle performance and safety performance of the secondary battery.

In any embodiment of the present application, the step (2) is mixed at a temperature of 20° C. to 120° C., optionally 40° C. to 120° C., for 1 hour to 12 hours.

In any embodiment of the present application, the sintering in step (2) is sintering at 600° C. to 950° C. for 4 to 10 hours in an inert gas or a mixed atmosphere of inert gas and hydrogen.

In any embodiment of the present application, the coating step includes coating crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , coating crystalline phosphate XPO ₄ , coating crystalline borate Y _p B _q O _r and coating carbon.

In any embodiment of the present application, optionally, the first coating step is a step of coating crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the second coating step is a step of coating crystalline phosphate XPO ₄ , the third coating step is a step of coating crystalline borate Y _p B _q O _r , and the fourth coating step is a step of coating carbon, wherein the positive electrode active material obtained has a core-shell structure, comprising the core and a shell coating the core, the shell comprising a first coating layer coating the core, a second coating layer coating the first coating layer, a third coating layer coating the second coating layer, and a fourth coating layer coating the third coating layer, the first coating layer comprises crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the second coating layer comprises crystalline phosphate XPO ₄ , the third coating layer comprises crystalline borate Y _p B _q O _r , and the fourth coating layer comprises carbon.

In any embodiment of the present application, the step of coating the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c comprises the following steps: adding a source of element M, a phosphorus source and an acid, and optionally a lithium source, to a solvent to obtain a coating solution, thoroughly mixing the material to be coated with the above-mentioned coating solution, drying, and then sintering to obtain a material coated with crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c .

In any embodiment of the present application, the step of coating the crystalline phosphate XPO ₄ includes the following steps: adding a source of element X, a phosphorus source and an acid to a solvent to obtain a coating liquid, fully mixing the material to be coated with the above-mentioned coating liquid, drying, and then sintering to obtain a material coated with the crystalline phosphate XPO ₄ .

In any embodiment of the present application, the step _of coating the crystalline _borate _YpBqOr comprises the following steps: adding a source of element Y and a boron source into a solvent to obtain a coating solution, fully mixing the material to be coated with the coating solution, drying, and then sintering to obtain a material coated _with the crystalline borate _YpBqOr _.

In any embodiment of the present application, the step of coating carbon includes the following steps: adding a carbon source to a solvent to obtain a coating solution, adding the material to be coated to the above coating solution, mixing evenly, drying, and then sintering to obtain a carbon-coated material.

In any embodiment of the present application, in the step of coating the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the pH of the solution in which the source of element M, the phosphorus source and the acid and optionally the lithium source are dissolved is controlled to be 3.5 to 6.5, and then stirred and reacted for 1 to 5 hours, and then the solution is heated to 50° C. to 120° C. and maintained at this temperature for 2 to 10 hours.

In any embodiment of the present application, the sintering in the step of coating the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c is performed at 650° C. to 800° C. for 2 to 6 hours.

In any embodiment of the present application, in the step of coating the crystalline phosphate XPO ₄ , the source of element X, the phosphorus source and the acid are dissolved in a solvent, stirred and reacted for 1 to 10 hours, and then the solution is heated to 60° C. to 150° C. and maintained at this temperature for 2 to 10 hours.

In any embodiment of the present application, the sintering in the step of coating the crystalline phosphate XPO ₄ is performed at 500° C. to 700° C. for 6 to 10 hours.

In any embodiment of the present application, the sintering in the step of coating the crystalline _borate _YpBqOr is performed at 300°C to 500°C for 2 hours to ₁₀ hours.

In any embodiment of the present application, the sintering in the carbon coating step is performed at 700° C. to 800° C. for 6 hours to 10 hours.

By controlling the conditions of the coating step within the above range, the capacity, cycle performance, high temperature storage performance, rate performance, etc. of the secondary battery prepared using the positive electrode active material can be ensured or even improved.

The method for preparing the positive electrode active material described in the present application has a wide range of raw material sources, low cost, simple process, and is conducive to industrialization.

The third aspect of the present application provides a positive electrode plate, which includes a positive electrode collector and a positive electrode film layer arranged on at least one surface of the positive electrode collector, the positive electrode film layer includes the positive electrode active material of the first aspect of the present application or the positive electrode active material prepared by the method of the second aspect of the present application, and the content of the positive electrode active material in the positive electrode film layer is more than 10 weight%, and can be optionally 90 weight% to 99.5 weight%, based on the total weight of the positive electrode film layer.

The positive electrode plate of the present application is used in a secondary battery, and can improve the energy density, cycle performance, safety performance, and/or rate performance of the secondary battery.

A fourth aspect of the present application provides a secondary battery, comprising the positive electrode active material of the first aspect of the present application or the positive electrode active material prepared by the method of the second aspect of the present application or the positive electrode plate of the third aspect of the present application.

A fifth aspect of the present application provides an electrical device, comprising the secondary battery of the fourth aspect of the present application.

The positive electrode sheet, secondary battery, and electrical device of the present application include the positive electrode active material of the present application, and thus have at least the same advantages as the positive electrode active material.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on the drawings without creative work.

FIG. 1 is a schematic diagram of a battery cell according to an embodiment of the present application.

FIG. 2 is an exploded schematic diagram of an embodiment of a battery cell of the present application.

FIG. 3 is a schematic diagram of an embodiment of a battery module of the present application.

FIG. 4 is a schematic diagram of an embodiment of a battery pack of the present application.

FIG. 5 is an exploded schematic diagram of the embodiment of the battery pack shown in FIG. 4 .

FIG. 6 is a schematic diagram of an embodiment of an electric device including the secondary battery of the present application as a power source.

In the drawings, the drawings may not be drawn according to the actual scale. The reference numerals are as follows: 1 battery pack, 2 upper box, 3 lower box, 4 battery module, 5 battery cell, 51 housing, 52 electrode assembly, 53 cover plate.

Detailed ways

Below, the embodiments of the positive electrode active material, the preparation method thereof, the positive electrode sheet, the secondary battery and the electric device containing the positive electrode active material of the present application are specifically disclosed with appropriate reference to the drawings. However, there are cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The range defined in this way can be inclusive or exclusive of end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60-120 and 80-110 is listed for a specific parameter, it is understood that the range of 60-110 and 80-120 is also expected. In addition, if the

minimum range values

1 and 2 are listed, and if the

maximum range values

3, 4 and 5 are listed, the following ranges can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "a-b" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" represents that all real numbers between "0-5" have been fully listed herein, and "0-5" is just the abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

If not otherwise specified, all embodiments and optional embodiments of the present application may be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of the present application.

Unless otherwise specified, all technical features and optional technical features of the present application may be combined with each other to form a new technical solution, and such a technical solution should be deemed to be included in the disclosure of the present application.

If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

If there is no special explanation, the "include" and "comprising" mentioned in this application represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

As used herein, the terms "plurality", "multiple", and "multi-layers" refer to two, two or more layers.

As used herein, "about" a numerical value indicates a range, which means a range of ±10% of the numerical value.

In this article, the term "coating layer" refers to a material layer coated on the lithium manganese phosphate core, and the material layer may completely or partially coat the lithium manganese phosphate core. The use of "coating layer" is only for the convenience of description and is not intended to limit the present invention. In addition, each coating layer may be completely coated or partially coated. Similarly, the term "thickness of the coating layer" refers to the thickness of the material layer coated on the lithium manganese phosphate core in the radial direction of the lithium manganese phosphate core.

In this article, the median particle size Dv50 refers to the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 50%. In this application, the median particle size Dv50 of the material can be measured by laser diffraction particle size analysis. For example, referring to the standard GB/T19077-2016, a laser particle size analyzer (such as Malvern Master Size 3000) is used for measurement.

The inventors of the present application have found in actual operation that the existing lithium manganese phosphate (LiMnPO ₄ ) positive electrode active material has a serious dissolution of manganese ions during deep charge and discharge. Although the prior art has attempted to coat lithium manganese phosphate with lithium iron phosphate to reduce interface side reactions, this coating cannot prevent the dissolved manganese ions from continuing to migrate into the electrolyte. After the dissolved manganese ions migrate to the negative electrode, they are reduced to metallic manganese. The metallic manganese produced in this way is equivalent to a "catalyst" that can catalyze the decomposition of the SEI film (solid electrolyte interphase) on the surface of the negative electrode to produce by-products; part of the by-products is gas, which causes the secondary battery to swell and affects the safety performance of the secondary battery; in addition, another part of the by-products is deposited on the surface of the negative electrode, which will hinder the passage of lithium ions in and out of the negative electrode, causing the impedance of the secondary battery to increase, thereby affecting the kinetic performance of the secondary battery. In addition, in order to replenish the lost SEI film, the active lithium inside the electrolyte and the battery is also continuously consumed, which will also have an irreversible effect on the capacity retention rate of the secondary battery.

After extensive research, the inventors discovered that by doping and modifying lithium manganese phosphate and coating the lithium manganese phosphate, a new type of positive electrode active material with a core-shell structure can be obtained. The positive electrode active material can achieve significantly reduced manganese ion dissolution and reduced lattice change rate. When used in secondary batteries, it can improve the cycle performance, rate performance, safety performance of the secondary batteries and increase the capacity of the secondary batteries.

Positive electrode active material

Specifically, the first aspect of the present application proposes a positive electrode active material with a core-shell structure, which includes a core and a shell covering the core.

The inner core includes _LimAxMn1 _- _yByP1 _- _zCzO4 _- _nDn , wherein A includes one or more _elements selected from Group IA, Group IIA, Group IIIA, Group IIB, Group VB and Group VIB, B includes one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB and Group VIIIB, C includes one or more elements selected from Group IIIA, Group IVA, Group VA and Group VIA, D includes one or more elements selected from Group VIA and Group VIIA, m is selected from the range of 0.900 to 1.100, x is selected from the range of 0 to 0.100, y is selected from the range of 0.001 to 0.500, z is selected from the range of 0.001 to 0.100, n is selected from the range of 0 to 0.100, and the inner core is electrically neutral.

The shell includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , crystalline phosphate XPO ₄ , crystalline borate Y _p B _q O _r and carbon, and the shell includes one or more coating layers, and each coating layer independently includes one or more of crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , crystalline phosphate XPO ₄ , crystalline borate Y _p B _q O _r and carbon. The M in the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c each independently includes one or more metal elements selected from transition metals, group IA, group IIA, group IIIA, group IVA, group VA and lanthanides, 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values of a, b and c satisfy the following conditions: the crystalline pyrophosphate Li _a MP ₂ O ₇ or M _b (P ₂ O ₇ ) _c is kept electrically neutral. The X includes one or more metal elements selected from transition metals, group IA, group IIA, group IIIA, group IVA, group VA and lanthanides. The Y includes one or more metal elements selected from transition metals, group IA, group IIA, group IIIA, group IVA, group VA and lanthanides, 1≤p≤4, 1≤q≤7, 2≤r≤12, and the values of p, q and r satisfy the following conditions: the crystalline borate Y _p B _q O _r is kept electrically neutral.

Unless otherwise specified, in the above chemical formula, when A is more than two elements, the above-mentioned limitation on the numerical range of x is not only a limitation on the stoichiometric number of each element as A, but also a limitation on the sum of the stoichiometric numbers of each element as A. For example, when A is more than two elements A1, A2...An, the stoichiometric numbers x1, x2...xn of A1, A2...An each need to fall within the numerical range of x defined in this application, and the sum of x1, x2...xn also needs to fall within the numerical range. Similarly, for the case where B, C and D are more than two elements, the limitation on the numerical range of the stoichiometric numbers of B, C and D in this application also has the above meaning.

In an optional embodiment, when B is one, two, three or four elements selected from the above range, _By _is _Qn1Dn2En3Kn4 , n1+ _n2 + _n3 +n4=y, and n1, n2, n3, n4 are all positive numbers and not zero at the same time, Q, D, E, K are each independently an element selected from the above range, and optionally, at least one of Q, D, E, K is Fe. Optionally, one of n1, n2, n3, n4 is zero, and the others are not zero; more optionally, two of n1, n2, n3, n4 are zero, and the others are not zero; further optionally, three of n1, n2, n3, n4 are zero, and the others are not zero. In the core _LimAxMn1 _- _yByP1 _- _zCzO4 _-nDn _, it is advantageous to dope one, two, three or four of the above B elements at the Mn position, _and optionally, one, two or three of the above B elements; in addition, it is advantageous to dope one or two C elements at the P position, which is conducive to uniform distribution of the doping elements.

In the kernel _LimAxMn1 _- _yByP1 _- _zCzO4 _- _nDn , the size of m is affected by _the valence state of the doping elements A, B, C, and D and the size of the doping amount to ensure that the entire kernel system is electrically neutral. If the value of m is too small, the lithium content of the entire kernel system will be reduced, affecting the capacity of the positive electrode active material, so the m value is limited to 0.900 to 1.100. The y value will limit the total amount of all doping elements in the Mn position. If y is too small, that is, the doping amount is too small, the doping element will not work. If y exceeds 0.5, the Mn content in the system will be less, affecting the voltage platform of the positive electrode active material, so the y value is limited to 0.001 to 0.500. The C element is doped in the P position, because the PO tetrahedron is relatively stable, and the z value is too large to affect the stability of the positive electrode active material, so the z value is limited to 0.001 to 0.100.

In the kernel, m is selected from the range of 0.900 to 1.100, for example, m can be 0.900, 0.991, 0.992, 0.993, 0.994, 0.995, 0.996, 0.997, 0.998, 1.000, 1.001, 1.002, 1.003, 1.004, 1.005, 1.006. Optionally, m is selected from the range of 0.995 to 1.002.

In the kernel, x is selected from the range of 0 to 0.100, for example, x can be 0, 0.001, 0.005, 0.08, 0.1. Optionally, x is selected from the range of 0.001 to 0.100, 0.001 to 0.005.

In the kernel, y is selected from the range of 0.001 to 0.500, for example, y can be 0.001, 0.100, 0.200, 0.250, 0.300, 0.350, 0.400, 0.450, 0.500.

In the kernel, z is selected from the range of 0.001 to 0.100, for example, z may be 0.001, 0.002, 0.003, 0.004, 0.005, or 0.100.

In the kernel, n is selected from the range of 0 to 0.100, for example, n can be 0, 0.001, 0.005, 0.08, 0.1. Optionally, n is selected from the range of 0.001 to 0.100, 0.001 to 0.005. Optionally, n is selected from the range of 0.001 to 0.100, 0.001 to 0.005.

The x is selected from the range of 0 to 0.100, and the n is selected from the range of 0 to 0.100, that is, the Li position and the O position of the lithium manganese phosphate may be undoped or doped.

In some embodiments, optionally, the x is selected from the range of 0.001 to 0.100, that is, the element A is doped at the Li position of the lithium manganese phosphate.

In some embodiments, optionally, the n is selected from the range of 0.001 to 0.100, that is, the element D is doped at the O position of the lithium manganese phosphate.

In some embodiments, optionally, the x is selected from the range of 0.001 to 0.100, and the n is selected from the range of 0.001 to 0.1, that is, the Li position and the O position of the lithium manganese phosphate are doped at the same time.

The positive electrode active material of the present application can improve the capacity, cycle performance and safety performance of the secondary battery. Although the mechanism is not clear, it is speculated that the positive electrode active material of the present application may have a special core-shell structure. By doping the lithium manganese phosphate core, it can not only effectively reduce the dissolution of manganese ions, thereby reducing the manganese ions that migrate to the negative electrode, reducing the electrolyte consumed by the decomposition of the SEI film, and improving the cycle performance and safety performance of the secondary battery, but also promote the adjustment of the Mn-O bond, reduce the lithium ion migration barrier, promote lithium ion migration, and improve the rate performance of the secondary battery. The shell comprises _crystalline pyrophosphate _LiaMP2O7 and/or _Mb ( _P2O7 ) _c , crystalline phosphate _XPO4 , crystalline _borate _YpBqOr and carbon, and the shell comprises one or more _coating layers, the crystalline pyrophosphate can further _increase the migration resistance of manganese ions, reduce their dissolution, reduce the surface impurity lithium content, reduce the contact between the inner core and the electrolyte, thereby reducing the interface _side reaction, reducing the gas production, and improving the high temperature storage performance, cycle performance and safety performance of the secondary battery; the crystalline phosphate can effectively reduce the interface side reaction between the positive electrode active material and the electrolyte, thereby improving the high temperature cycle and storage performance of the secondary battery; the crystalline borate has excellent lithium ion and electron conductivity, can further reduce the dissolution of manganese ions, and reduce the surface impurity lithium content, thereby further reducing the interface side reaction between the positive electrode active material and the electrolyte, and at the same time the high temperature cycle and storage performance of the secondary battery are further improved; the carbon can effectively improve the conductivity and desolvation ability of the positive electrode active material, thereby further improving the safety performance and kinetic performance of the secondary battery.

In addition, in the core, the element B doped at the Mn position of the lithium manganese phosphate also helps to reduce the lattice change rate of the lithium manganese phosphate during the lithium insertion and extraction process, improves the structural stability of the lithium manganese phosphate positive electrode active material, greatly reduces the dissolution of manganese ions and reduces the oxygen activity on the particle surface; the element C doped at the P position also helps to change the difficulty of the Mn-O bond length change, thereby improving electronic conductivity and reducing the lithium ion migration barrier, promoting lithium ion migration, and improving the rate performance of the secondary battery; the element A doped at the Li position also helps to reduce the lattice change rate of the lithium manganese phosphate during the lithium insertion and extraction process; the element D doped at the O position helps to reduce interface side reactions.

In addition, the entire core system maintains electrical neutrality, which can ensure that there are as few defects and impurities in the positive electrode active material as possible. If there is an excess of transition metal (such as manganese) in the positive electrode active material, since the material system itself has a relatively stable structure, the excess transition metal is likely to precipitate in the form of a single substance or form an impurity phase inside the lattice. Maintaining electrical neutrality can minimize such impurities. In addition, ensuring the electrical neutrality of the system can also generate lithium vacancies in the positive electrode active material in some cases, thereby making the kinetic performance of the positive electrode active material better.

Therefore, the positive electrode sheet and electrical devices such as secondary batteries using the positive electrode active material of the present application can have a higher energy density and improved cycle performance, safety performance, and/or rate performance.

In some embodiments, optionally, the A includes one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo and W. By selecting the Li-site doping element within the above range, the lattice change rate can be further reduced, thereby further improving the rate performance of the secondary battery.

In some embodiments, optionally, the B includes one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, more optionally, the B includes one or more elements selected from Fe, Ti, V, Ni, Co and Mg, and further optionally, the B includes at least two elements selected from Fe, Ti, V, Ni, Co and Mg. Simultaneous doping of two or more of the above elements at the Mn position in the lithium manganese phosphate positive electrode active material is conducive to enhancing the doping effect. On the one hand, it further reduces the lattice change rate, thereby inhibiting the dissolution of manganese ions and reducing the consumption of electrolyte and active lithium ions. On the other hand, it is also conducive to further reducing the surface oxygen activity and reducing the interface side reactions between the positive electrode active material and the electrolyte, thereby improving the cycle performance and high temperature storage performance of the secondary battery.

In some embodiments, optionally, the C includes one or more elements selected from B (boron), S, Si and N. More optionally, the C includes one element selected from B (boron), S, Si and N. By selecting the P-site doping element within the above range, the rate performance of the secondary battery can be further improved.

In some embodiments, optionally, the D includes one or more elements selected from S, F, Cl and Br. By selecting the O-position doping element within the above range, the side reactions of the interface can be further reduced and the high temperature stability of the secondary battery can be improved.

In some embodiments, optionally, the M includes one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al.

In some embodiments, optionally, X includes one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al.

In some embodiments, optionally, Y includes one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al.

In some embodiments, in the core, the ratio of y to 1-y is 1:10 to 1:1, and can be 1:4 to 1:1. Here, y represents the sum of the stoichiometric numbers of the Mn-position doping elements B. When the above conditions are met, the energy density and cycle performance of the secondary battery using the positive electrode active material can be further improved.

In some embodiments, in the core, the ratio of z to 1-z is 1:9 to 1:999, optionally 1:499 to 1:249. Here, z represents the sum of the stoichiometric numbers of the P-site doping elements C. When the above conditions are met, the energy density and cycle performance of the secondary battery using the positive electrode active material can be further improved.

In some embodiments, q:r is 1:3.

The average particle size of the kernel prepared in this application ranges from 50nm to 500nm, and Dv50 is 200nm to 300nm. The primary particle size of the kernel is in the range of 50nm to 500nm, and Dv50 is 200nm to 300nm. If the average particle size of the kernel is too large (more than 500nm), the capacity of the secondary battery using the material will be affected; if the average particle size of the kernel is too small, its specific surface area is large, it is easy to agglomerate, and it is difficult to achieve uniform coating.

Through process control (for example, fully mixing and grinding the materials of various sources), it is possible to ensure that each element is evenly distributed in the lattice without aggregation. The main characteristic peak positions in the X-ray diffraction (XRD) spectrum of the doped lithium manganese phosphate of the present application are consistent with those of the undoped LiMnPO ₄ , indicating that the doping process does not introduce an impurity phase. Therefore, the improvement in the core performance is mainly due to element doping, rather than impurity phases. After preparing the positive electrode active material described in the present application, the inventors of the present application cut the middle region (core region) of the prepared positive electrode active material particles by a focused ion beam (FIB for short), and tested them by transmission electron microscopy (TEM for short) and X-ray energy spectrum analysis (EDS for short) and found that the elements were evenly distributed without aggregation.

In the present application, crystalline means that the crystallinity is above 50%, i.e., 50% to 100%. Crystallinity less than 50% is called glassy state (or amorphous state). Crystallinity of crystalline pyrophosphate, crystalline phosphate and crystalline borate described in the present application is 50% to 100%. Pyrophosphate, phosphate and borate with a certain degree of crystallinity are not only conducive to giving full play to the function of pyrophosphate coating layer to hinder the dissolution of manganese ions, excellent lithium ion conduction ability of phosphate coating layer, reduce interface side reactions and excellent lithium ion and electron conduction ability of borate coating layer, but also enable the coating layer to be better lattice matched, so as to achieve a tighter combination of coating layer.

In the present application, the crystallinity of crystalline pyrophosphate, crystalline phosphate and crystalline borate can be tested by conventional technical means in the art, such as density method, infrared spectroscopy, differential scanning calorimetry and nuclear magnetic resonance absorption method, and can also be tested by, for example, X-ray diffraction. The specific X-ray diffraction test method may include the following steps: take a certain amount of positive electrode active material powder, measure the total scattering intensity by X-ray, which is the sum of the scattering intensity of the entire space material, and is only related to the intensity of the primary ray, the chemical structure of the positive electrode active material powder, and the total number of electrons participating in the diffraction, that is, the mass, and has nothing to do with the order of the sample; then separate the crystalline scattering and the non-crystalline scattering from the diffraction pattern, and the crystallinity is the ratio of the scattering of the crystalline part to the total scattering intensity. It should be noted that in the present application, the crystallinity of the pyrophosphate, phosphate and borate in the coating layer can be adjusted, for example, by adjusting the process conditions of the sintering process, such as sintering temperature, sintering time, etc.

In some embodiments, the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c in the shell has a lattice spacing ranging from 0.293 nm to 0.470 nm, and a crystal direction (111) angle ranging from 18.00° to 32.00°.

In some embodiments, the interplanar spacing of the crystalline phosphate XPO ₄ in the shell is in the range of 0.244 nm to 0.425 nm, and the angle of the crystal direction (111) is in the range of 20.00° to 37.00°.

The crystalline pyrophosphate and crystalline phosphate can be characterized by conventional techniques in the art, or by transmission electron microscopy (TEM). Under TEM, the core and the coating can be distinguished by measuring the interplanar spacing.

The specific test method for the interplanar spacing and angle of the crystalline pyrophosphate and crystalline phosphate in the coating layer may include the following steps: take a certain amount of coated positive electrode active material sample powder in a test tube, inject a solvent such as alcohol into the test tube, and then fully stir and disperse it, and then use a clean disposable plastic pipette to take an appropriate amount of the above solution and drop it on a 300-mesh copper mesh. At this time, part of the powder will remain on the copper mesh, and the copper mesh and the sample will be transferred to the TEM sample cavity for testing to obtain the original TEM test image. The original image obtained by the above TEM test is opened in the diffractometer software, and Fourier transform is performed to obtain the diffraction pattern. The distance from the diffraction spot to the center position in the diffraction pattern is measured to obtain the interplanar spacing, and the angle can be calculated according to the Bragg equation.

The difference between the interplanar spacing range of crystalline pyrophosphate and the existence of crystalline phosphate can be directly judged by the value of the interplanar spacing.

In some embodiments, the carbon in the shell is a mixture of SP2 carbon and SP3 carbon. Optionally, the molar ratio of the SP2 carbon to the SP3 carbon is any value in the range of 0.1 to 10, and more preferably any value in the range of 2.0 to 3.0.

In some embodiments, the molar ratio of the SP2 carbon to the SP3 carbon may be about 0.1, about 0.2, about 0.3, about 0.4, about 0.5, about 0.6, about 0.7, about 0.8, about 0.9, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9 or about 10, or any range of any of the above values.

By selecting the form of carbon in the shell, the comprehensive electrochemical performance of the secondary battery can be improved. Specifically, by using a mixed form of SP2 carbon and SP3 carbon and limiting the ratio of SP2 carbon to SP3 carbon within a certain range, the following situations can be effectively avoided: if the carbon in the shell is all amorphous SP3, the conductivity is poor; if it is all graphitized SP2, although the conductivity is good, there are few lithium ion pathways, which is not conducive to the deintercalation of lithium ions. In addition, limiting the molar ratio of SP2 carbon to SP3 carbon within the above range can not only achieve good conductivity, but also ensure the passage of lithium ions, so it is beneficial to improve the kinetic performance and cycle performance of the secondary battery.

The mixing ratio of the SP2 form and the SP3 form of the carbon can be controlled by sintering conditions such as sintering temperature and sintering time. For example, when sucrose is used as a carbon source, after sucrose is cracked at high temperature, it is deposited and at the same time, carbon with both SP3 form and SP2 form is produced under the action of high temperature. The ratio of SP2 form carbon and SP3 form carbon can be regulated by selecting high temperature cracking conditions and sintering conditions.

The structure and characteristics of carbon can be measured by Raman spectroscopy. The specific test method is as follows: By separating the peaks of the energy spectrum of the Raman test, I _d /I _g (I _d is the peak intensity of SP3 form carbon, and I _g is the peak intensity of SP2 form carbon) is obtained to confirm the molar ratio of the two.

In some embodiments, the shell includes multiple coating layers, and each coating layer independently includes one or more of crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , crystalline phosphate XPO ₄ , crystalline borate Y _p B _q O _r and carbon.

In some embodiments, the shell includes a first coating layer coating the inner core, a second coating layer coating the first coating layer, a third coating layer coating the second coating layer, and a fourth coating layer coating the third coating layer, the fourth coating layer includes carbon, and the first coating layer, the second coating layer, and the third coating layer independently include any one of crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , crystalline phosphate XPO ₄ , and crystalline borate Y _p B _q O _r .

For example, the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the second coating layer includes crystalline phosphate XPO ₄ , and the third coating layer includes crystalline borate Y _p B _q O _r . Alternatively, the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the second coating layer includes crystalline borate Y _p B _q O _r , and the third coating layer includes crystalline phosphate XPO ₄ . Alternatively, the first coating layer includes crystalline phosphate XPO ₄ , the second coating layer includes crystalline borate Y _p B _q O _r , and the third coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c . Alternatively, the first coating layer comprises crystalline phosphate XPO ₄ , the second coating layer comprises crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , and the third coating layer comprises crystalline borate Y _p B _q O _r . Alternatively, the first coating layer comprises crystalline borate Y _p B _q O _r , the second coating layer comprises crystalline phosphate XPO ₄ , and the third coating layer comprises crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c . Alternatively, the first coating layer comprises crystalline borate Y _p B _q O _r , the second coating layer comprises crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , and the third coating layer comprises crystalline phosphate XPO ₄ .

In some embodiments, optionally, the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the fourth coating layer includes carbon, and the second coating layer and the third coating layer each independently include crystalline phosphate XPO ₄ or crystalline borate Y _p B _q O _r .

In some embodiments, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, and can be optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

In some embodiments, the coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, and can be optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

In some embodiments, the coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, and can be optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

In some embodiments, the coating amount of the fourth coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, and more optionally greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

In the present application, the coating amount of each of the above four coating layers is not zero.

In the positive electrode active material with a core-shell structure described in the present application, the coating amount of the four coating layers is preferably within the above range, so that the inner core can be fully coated and at the same time, the cycle performance, safety performance, and/or rate performance of the secondary battery can be further improved without sacrificing the gram capacity of the positive electrode active material.

In some embodiments, the first cladding layer has a thickness of 1 nm to 15 nm.

In some embodiments, the second cladding layer has a thickness of 1 nm to 15 nm.

In some embodiments, the third coating layer has a thickness of 1 nm to 15 nm.

In some embodiments, the fourth cladding layer has a thickness of 2 nm to 25 nm.

In the positive electrode active material with a core-shell structure described in the present application, the thickness of the four coating layers is preferably within the above range, thereby being able to fully coat the inner core and at the same time further improve the cycle performance, safety performance, and/or rate performance of the secondary battery without sacrificing the gram capacity of the positive electrode active material.

The thickness test of the coating layer is mainly carried out through FIB. The specific method may include the following steps: randomly select a single particle from the positive electrode active material powder to be tested, cut a thin slice with a thickness of about 100nm from the middle position or near the middle position of the selected particle, and then perform TEM test on the thin slice to measure the thickness of the coating layer, measure 3-5 positions, and take the average value.

In some embodiments, optionally, the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the second coating layer includes crystalline phosphate XPO ₄ , the third coating layer includes crystalline borate Y _p B _q O _r , and the fourth coating layer includes carbon.

The inventor unexpectedly discovered in further research that by coating the inner core with a first coating layer including crystalline pyrophosphate, the migration resistance of manganese ions can be further increased, their dissolution can be reduced, and the surface impure lithium content can be reduced, and the contact between the inner core and the electrolyte can be reduced, thereby reducing interfacial side reactions, reducing gas production, and improving the high-temperature storage performance, cycle performance and safety performance of the secondary battery; by further coating with a crystalline phosphate coating layer having excellent lithium ion conductivity, the interfacial side reactions between the positive electrode active material and the electrolyte can be effectively reduced, thereby improving the high-temperature cycle and storage performance of the secondary battery; by further coating with a crystalline borate coating layer having excellent lithium ion and electron conductivity, the dissolution of manganese ions can be further reduced, and the surface impure lithium content can be reduced, thereby further reducing the interfacial side reactions between the positive electrode active material and the electrolyte, and at the same time, the high-temperature cycle and storage performance of the secondary battery are further improved; by further coating with a carbon layer as the fourth coating layer, the safety performance and kinetic performance of the secondary battery can be further improved.

In the present application, since metal ions are difficult to migrate in pyrophosphate, pyrophosphate as the first coating layer can effectively isolate the doped metal ions from the electrolyte. The structure of crystalline pyrophosphate is stable, so crystalline pyrophosphate coating can effectively inhibit the dissolution of transition metals and improve cycle performance.

The bonding between the first coating layer and the core is similar to a heterojunction, and the strength of the bonding is limited by the degree of lattice matching. When the lattice mismatch is below 5%, the lattice matching is better, and the two are easy to bond tightly. Tight bonding can ensure that the coating will not fall off during subsequent cycles, which is beneficial to ensure the long-term stability of the positive electrode active material. The degree of bonding between the first coating layer and the core is mainly measured by calculating the mismatch between the core and each lattice constant of the coating. In the present application, after B and C elements are doped into the core, the matching degree between the core and the first coating layer is improved compared to undoped elements, and the core and the pyrophosphate coating layer can be more tightly bonded together.

The reason why crystalline phosphate is chosen as the second coating layer is, firstly, because it has a high lattice matching degree with the first coating material, crystalline pyrophosphate (the mismatch degree is only 3%); secondly, the stability of phosphate itself is better than that of pyrophosphate, and using it to coat pyrophosphate is beneficial to improve the stability of the positive electrode active material. The structure of crystalline phosphate is very stable, and it has excellent lithium ion conductivity. Therefore, the use of crystalline phosphate for coating can effectively reduce the interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the high-temperature cycle performance and high-temperature storage performance of the secondary battery. The lattice matching mode between the second coating layer and the first coating layer is similar to the combination between the first coating layer and the core mentioned above. When the lattice mismatch is less than 5%, the lattice matching is better, and the two are easy to combine tightly.

The main reason for choosing borate as the third coating layer is that it has lower surface activity compared to phosphate, which can further reduce the surface impurity lithium content, reduce the decomposition of the electrolyte, and further inhibit the dissolution of manganese ions. Therefore, the use of crystalline borate for coating can further reduce the interfacial side reactions between the positive electrode active material and the electrolyte, thereby further improving the high-temperature cycle performance and high-temperature storage performance of the secondary battery.

The main reason for using carbon as the fourth coating layer is that the carbon layer has good electronic conductivity. Since electrochemical reactions occur when used in secondary batteries, the participation of electrons is required. Therefore, in order to increase the electron transfer between particles and the electron transfer at different positions on the particles, carbon with excellent conductive properties can be used to coat the positive electrode active material. Carbon coating can effectively improve the conductivity and desolvation ability of the positive electrode active material.

Therefore, the positive electrode sheet of the positive electrode active material having four coating layers in the above coating sequence and electrical devices such as secondary batteries can have further improved cycle performance, safety performance, and/or rate performance.

In some embodiments, the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , and a coating amount of the first coating layer is greater than 0 and less than or equal to 6 wt %, optionally greater than 0 and less than or equal to 5.5 wt %, and more optionally greater than 0 and less than or equal to 2 wt %, based on the weight of the core.

In some embodiments, the second coating layer includes crystalline phosphate XPO ₄ , and a coating amount of the second coating layer is greater than 0 and less than or equal to 6 wt %, optionally greater than 0 and less than or equal to 5.5 wt %, and more optionally 2 wt % to 4 wt %, based on the weight of the core.

In some embodiments, the third coating layer includes crystalline _borate _YpBqOr , and a coating amount of the third coating layer is greater than 0 and less than or equal to 6 _wt %, optionally greater than 0 and less than or equal to 5.5 wt%, and more optionally 2 wt% to 5 wt%, based on the weight of the core.

In some embodiments, the fourth coating layer includes carbon, and the coating amount of the fourth coating layer is greater than 0 and less than or equal to 6 weight%, optionally greater than 0 and less than or equal to 5.5 weight%, and more optionally greater than 0 and less than or equal to 2 weight%, based on the weight of the core.

In the positive electrode active material with a core-shell structure described in the present application, the coating amount of the four coating layers with the above-mentioned coating order is preferably within the above-mentioned range, thereby being able to fully coat the inner core and at the same time further improve the cycle performance, safety performance, and/or rate performance of the secondary battery without sacrificing the gram capacity of the positive electrode active material.

For the first coating layer, by keeping the coating amount within the above range, the following situations can be effectively avoided: too little coating amount means that the coating layer is thin and may not be able to effectively hinder the migration of transition metals; too much coating amount means that the coating layer is too thick, which will affect the migration of Li ⁺ and further affect the rate performance of the positive electrode active material.

For the second coating layer, by keeping the coating amount within the above range, the following situations can be effectively avoided: too much coating amount may affect the overall platform voltage of the positive electrode active material; too little coating amount may not achieve sufficient coating effect.

For the third coating layer, by keeping the coating amount within the above range, the following situations can be effectively avoided: too much coating amount may cause the coating layer to be too thick, increasing the battery impedance; too little coating amount may lead to insufficient inhibition of manganese ion dissolution, and at the same time, the improvement of lithium ion and electron transport performance is not significant.

For the fourth coating layer, the carbon coating mainly plays the role of enhancing the electron transfer between particles. However, since the structure also contains a large amount of amorphous carbon, the carbon density is relatively low. Therefore, if the coating amount is too large, it will affect the compaction density of the electrode.

In some embodiments, the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , and the thickness of the first coating layer is 1 nm to 15 nm. In some embodiments, the thickness of the first coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, or in any range of any of the above values. Optionally, the thickness of the first coating layer is 1 nm to 10 nm. This can avoid the adverse effects on the kinetic properties of the positive electrode active material that may be generated when it is too thick, and can avoid the problem that the migration of transition metal ions may not be effectively hindered when it is too thin.

In some embodiments, the second coating layer includes crystalline phosphate XPO ₄ , and the thickness of the second coating layer is 1nm to 15nm. In some embodiments, the thickness of the second coating layer may be about 2nm, about 3nm, about 4nm, about 5nm, about 6nm, about 7nm, about 8nm, about 9nm, about 10nm, about 11nm, about 12nm, about 13nm, about 14nm, about 15nm, or in any range of any of the above values. Optionally, the thickness of the second coating layer is 2nm to 15nm. At this time, the surface structure of the second coating layer is stable, and the side reaction with the electrolyte is small, so it can effectively reduce the interface side reaction, thereby improving the high temperature cycle performance and high temperature storage performance of the secondary battery.

In some embodiments, the third coating layer includes crystalline _borate _YpBqOr , _and the thickness of the third coating layer is 1nm to 15nm. In some embodiments, the thickness of the third coating layer may be about 2nm, about 3nm, about 4nm, about 5nm, about 6nm, about 7nm, about 8nm, about 9nm, about 10nm, about 11nm, about 12nm, about 13nm, about 14nm, about 15nm, or in any range of the above arbitrary numerical values. Optionally, the thickness of the third coating layer is 1nm to 10nm. Thus, the dissolution of manganese ions and the decomposition of the electrolyte can be further suppressed, while further promoting the transmission of lithium ions and electrons.

In some embodiments, the fourth coating layer includes carbon, and the thickness of the fourth coating layer is 2nm to 25nm. In some embodiments, the thickness of the fourth coating layer may be about 2nm, about 3nm, about 4nm, about 5nm, about 6nm, about 7nm, about 8nm, about 9nm, about 10nm, about 11nm, about 12nm, about 13nm, about 14nm, about 15nm, about 16nm, about 17nm, about 18nm, about 19nm, about 20nm, about 21nm, about 22nm, about 23nm, about 24nm or about 25nm, or in any range of any of the above values. Thus, the conductivity of the positive active material can be improved and the compaction density of the positive electrode sheet prepared using the positive active material can be improved.

In some embodiments, based on the weight of the positive electrode active material, the manganese content is in the range of 10 wt % to 35 wt %, optionally in the range of 15 wt % to 30 wt %, and more optionally in the range of 17 wt % to 20 wt %.

In some embodiments, based on the weight of the positive electrode active material, the content of phosphorus element is in the range of 12 wt % to 25 wt %, optionally in the range of 15 wt % to 20 wt %.

In some embodiments, based on the weight of the positive electrode active material, the weight ratio of the manganese element to the phosphorus element ranges from 0.90 to 1.25, and optionally from 0.95 to 1.20.

In the present application, in the case where manganese is contained only in the core of the positive electrode active material, the content of manganese may correspond to the content of the core.

In the present application, limiting the content of the manganese element within the above-mentioned range can effectively avoid problems such as poor structural stability of the positive electrode active material and decreased density that may be caused by an excessively high manganese content, thereby improving the cycle, storage and compaction density performance of the secondary battery; and can avoid problems such as low voltage platform that may be caused by an excessively low manganese content, thereby improving the energy density of the secondary battery.

In the present application, limiting the content of the phosphorus element within the above range can effectively avoid the following situations: if the content of the phosphorus element is too large, the covalency of P-O may be too strong and affect the conductivity of small polarons, thereby affecting the conductivity of the positive electrode active material; if the content of the phosphorus element is too small, the stability of the pyrophosphate and/or phosphate lattice structure in the core and the shell may be reduced, thereby affecting the overall stability of the positive electrode active material.

The weight ratio of manganese to phosphorus has the following effects on the performance of secondary batteries: if the weight ratio is too large, it means that there is too much manganese, and the dissolution of manganese ions increases, affecting the stability and capacity of the positive electrode active material, and thus affecting the cycle performance and storage performance of the secondary battery; if the weight ratio is too small, it means that there is too much phosphorus, which is easy to form an impurity phase, which will cause the discharge voltage platform of the positive electrode active material to drop, thereby reducing the energy density of the secondary battery.

The measurement of manganese and phosphorus can be carried out by conventional technical means in the art. In particular, the following method is used to determine the content of manganese and phosphorus: the material is dissolved in dilute hydrochloric acid (concentration 10-30%), the content of each element in the solution is tested by ICP, and then the content of manganese is measured and converted to obtain its weight percentage.

In some embodiments, the lattice change rate of the positive electrode active material before and after complete lithium deintercalation is 4% or less, optionally 3.8% or less, and more optionally 2.0% to 3.8%.

The lithium insertion and extraction process of lithium manganese phosphate (LiMnPO ₄ ) is a two-phase reaction. The interfacial stress of the two phases is determined by the lattice change rate before and after lithium insertion and extraction. The smaller the lattice change rate, the smaller the interfacial stress and the easier the Li ⁺ transmission. Therefore, reducing the lattice change rate of the inner core will help enhance the transmission capacity of Li ⁺ , thereby improving the rate performance of the secondary battery. The positive electrode active material with a core-shell structure described in the present application can achieve a lattice change rate before and after lithium insertion and extraction of less than 4%, so the use of the positive electrode active material can improve the rate performance of the secondary battery. The lattice change rate can be measured by methods known in the art, such as X-ray diffraction (XRD) spectra.

In some embodiments, the Li/Mn antisite defect concentration of the positive electrode active material is less than 4%, optionally less than 2.2%, and more optionally 1.5% to 2.2%. The Li/Mn antisite defect described in the present application refers to the position of Li ⁺ and ^Mn2+ interchanged in the _LiMnPO4 lattice. Accordingly, the Li/Mn antisite defect concentration refers to the percentage of Li ⁺ interchanged with ^Mn2+ to the total amount of Li ⁺ . In the present application, the Li/Mn antisite defect concentration can be tested, for example, according to JIS K 0131-1996.

The positive electrode active material described in the present application can achieve the above-mentioned lower Li/Mn antisite defect concentration. Although the mechanism is not very clear, the inventors of the present application speculate that since Li ⁺ and Mn2 ⁺ will exchange positions in the _LiMnPO4 lattice, and the Li ⁺ transmission channel is a one-dimensional channel, it will be difficult for Mn2 ⁺ to migrate in the Li ⁺ channel, thereby hindering the transmission of Li ⁺ . Therefore, the positive electrode active material with a core-shell structure described in the present application has a low Li/Mn antisite defect concentration within the above range, so it can avoid Mn2 ⁺ from hindering the transmission of Li ⁺ , while improving the capacity and rate performance of the positive electrode active material.

In some embodiments, the compaction density of the positive electrode active material at 3T is 2.2 g/cm ³ or more, and can be 2.2 g/cm ³ or more and 2.8 g/cm ³ or less. The higher the compaction density, the greater the weight of the positive electrode active material per unit volume, so increasing the compaction density is beneficial to increasing the volume energy density of the secondary battery. The compaction density can be measured according to GB/T 24533-2009.

In some embodiments, the surface oxygen valence state of the positive electrode active material is below -1.90, and can be optionally -1.90 to -1.98. The stable valence state of oxygen is -2. The closer the valence state is to -2, the stronger its electron-acquiring ability is, that is, the stronger its oxidizing property is. Normally, its surface valence state is below -1.7. The present application limits the surface oxygen valence state of the positive electrode active material to the above range as described above, thereby reducing the interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the cycle performance and storage performance of the secondary battery. The surface oxygen valence state can be measured by methods known in the art, such as by electron energy loss spectroscopy (EELS).

In some embodiments, the average particle size of the primary particles of the positive electrode active material ranges from 50nm to 500nm, and the volume median particle size Dv50 ranges from 200nm to 300nm. Since the particles will agglomerate, the actual measured secondary particle size after agglomeration may be 500nm to 40000nm. The size of the positive electrode active material particles will affect the processing of the material and the compaction density performance of the pole piece. By selecting the average particle size of the primary particles within the above range, the following situations can be effectively avoided: the average particle size of the primary particles of the positive electrode active material is too small, which may cause particle agglomeration, difficulty in dispersion, and require more binder, resulting in poor brittleness of the pole piece; the average particle size of the primary particles of the positive electrode active material is too large, which may cause larger gaps between particles and reduce the compaction density.

Through the above scheme, the lattice change rate of lithium manganese phosphate and the dissolution of manganese ions during the lithium insertion and extraction process can be effectively suppressed, thereby improving the high-temperature cycle performance and high-temperature storage performance of the secondary battery.

Preparation

The second aspect of the present application provides a method for preparing a positive electrode active material, which can prepare the positive electrode active material of the first aspect of the present application.

Specifically, the preparation method includes the following steps of providing a core material and a coating step.

In some embodiments, the step of providing a core material includes the following steps (1) and (2).

Step (1): Mix and stir a manganese source, a dopant of element B and an acid in a container to obtain manganese salt particles doped with element B.

Step (2): mixing the manganese salt particles doped with element B with a lithium source, a phosphorus source, a dopant of element C, optionally a dopant of element A and optionally a dopant of element D in a solvent to obtain a slurry, and sintering under an inert gas atmosphere to obtain a core material.

The preparation method of the present application has no particular restrictions on the source of materials. The source of a certain element may include one or more of the element's simple substance, sulfate, halide, nitrate, organic acid salt, oxide or hydroxide. The precursor is the source that can achieve the purpose of the preparation method of the present application.

In some embodiments, the dopant of element A is selected from one or more of element A's simple substance, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

In some embodiments, the dopant of element B is one or more selected from the group consisting of a simple substance of element B, a carbonate, a sulfate, a chloride, a nitrate, an organic acid salt, an oxide, and a hydroxide.

In some embodiments, the dopant of element C is one or more selected from inorganic acids, organic acids, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of element C.

In some embodiments, the dopant of element D is selected from one or more of a simple substance of element D and an ammonium salt.

In the present application, the manganese source may be a manganese-containing substance known in the art that can be used to prepare lithium manganese phosphate. As an example, the manganese source may be one or more selected from elemental manganese, manganese dioxide, manganese phosphate, manganese oxalate, and manganese carbonate.

In the present application, the acid may be one or more selected from hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, silicic acid, silicic acid and organic acids such as oxalic acid. In some embodiments, the acid is a dilute organic acid with a concentration of 60 wt % or less.

In the present application, the lithium source may be a lithium-containing substance known in the art that can be used to prepare lithium manganese phosphate. As an example, the lithium source is one or more selected from lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.

In the present application, the phosphorus source may be a phosphorus-containing substance known in the art that can be used to prepare lithium manganese phosphate. As an example, the phosphorus source is one or more selected from diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate and phosphoric acid.

In the present application, the amount of each dopant added for the elements A, B, C, and D depends on the target doping amount.

In some embodiments, after the manganese source, the dopant of element B and the acid react in a solvent to obtain a manganese salt suspension doped with element B, the suspension is filtered, dried, and sand-milled to obtain manganese salt particles doped with element B having a particle size of 50-200 nm.

In some embodiments, the slurry in step (2) is dried to obtain a powder, and then the powder is sintered to obtain the core material.

In some embodiments, the mixing in step (1) is performed at a temperature of 20°C to 120°C, optionally 40°C to 120°C.

In some embodiments, the stirring in step (1) is performed at 400 rpm to 700 rpm for 1 hour to 9 hours, optionally 3 hours to 7 hours.

Optionally, the reaction temperature in step (1) can be carried out at about 30°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C or about 120°C; the stirring in step (1) is carried out for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours or about 9 hours; optionally, the reaction temperature and stirring time in step (1) can be in any range of the above-mentioned arbitrary values.

In some embodiments, the step (2) is mixed at a temperature of 20°C to 120°C, optionally 40°C to 120°C for 1 to 12 hours. Optionally, the reaction temperature in the step (2) can be about 30°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C or about 120°C; the mixing in the step (2) is carried out for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, about 10 hours, about 11 hours or about 12 hours; optionally, the reaction temperature and mixing time in the step (2) can be in any range of the above-mentioned arbitrary values.

When the temperature and time during the preparation of the core particles are within the above range, the core obtained and the positive electrode active material prepared therefrom have fewer lattice defects, which is beneficial to inhibiting the dissolution of manganese ions and reducing the interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the cycle performance and safety performance of the secondary battery.

In some embodiments, during the preparation of the core material, the solution pH is controlled to be 3.5 to 6, optionally, the solution pH is controlled to be 4 to 6, and more optionally, the solution pH is controlled to be 4 to 5. It should be noted that in the present application, the pH of the obtained mixture can be adjusted by methods commonly used in the art, for example, by adding an acid or a base.

In some embodiments, optionally, in step (2), the molar ratio of the manganese salt particles doped with element B to the lithium source and the phosphorus source is 1:(0.5-2.1):(0.5-2.1), more optionally, the molar ratio of the manganese salt particles doped with element B to the lithium source and the phosphorus source is about 1:1:1.

In some embodiments, the sintering conditions in the process of preparing the inner core material are: sintering at 600°C to 950°C for 4 to 10 hours in an inert gas or a mixed atmosphere of inert gas and hydrogen; optionally, the sintering may be performed at about 650°C, about 700°C, about 750°C, about 800°C, about 850°C or about 900°C for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours; optionally, the sintering temperature and sintering time may be within any range of the above-mentioned any numerical values.

In the process of preparing the core material, if the sintering temperature is too low or the sintering time is too short, the crystallinity of the core will be low, which will affect the overall performance. If the sintering temperature is too high, impurities will easily appear in the core, thus affecting the overall performance. If the sintering time is too long, the core particles will grow larger, thus affecting the capacity, compaction density and rate performance.

In some embodiments, optionally, the protective atmosphere is a mixed gas of 70-90 volume % nitrogen and 10-30 volume % hydrogen.

In some embodiments, the coating step includes coating crystalline _{pyrophosphate} _LiaMP2O7 and/or _Mb ( _P2O7 ₎ _c , coating crystalline phosphate _XPO4 , coating crystalline _borate _YpBqOr _, and coating carbon. The specific coating order of the above coating _steps is not specifically limited and can be adaptively adjusted according to the specific structure of the shell of the required positive electrode active material.

Optionally, the step of coating the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c comprises the following steps: adding a source of element M, a phosphorus source and an acid and optionally a lithium source to a solvent to obtain a coating solution, fully mixing the material to be coated with the coating solution, drying, and then sintering to obtain a material coated with the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c . The material to be coated can be a core material, a material coated with one coating layer, a material coated with two coating layers, or a material coated with three coating layers according to actual conditions.

Optionally, the step of coating the crystalline phosphate XPO ₄ comprises the following steps: adding a source of element X, a phosphorus source and an acid into a solvent to obtain a coating solution, fully mixing the material to be coated with the coating solution, drying, and then sintering to obtain a material coated with the crystalline phosphate XPO _4. The material to be coated can be a core material, a material coated with one coating layer, a material coated with two coating layers, or a material coated with three coating layers according to actual conditions.

Optionally, the step of coating the crystalline borate Y _p B _q O _r comprises the following steps: adding a source of element Y and a boron source to a solvent to obtain a coating solution, fully mixing the material to be coated with the coating solution, drying, and then sintering to obtain a material coated with the crystalline borate Y _p B _q O _r . The material to be coated can be a core material, a material coated with one coating layer, a material coated with two coating layers, or a material coated with three coating layers according to actual conditions.

Optionally, the step of coating carbon includes the following steps: adding a carbon source to a solvent to obtain a coating solution, adding a material to be coated to the coating solution, mixing evenly, drying, and then sintering to obtain a carbon-coated material. The material to be coated can be a core material, a material coated with one coating layer, a material coated with two coating layers, or a material coated with three coating layers according to actual conditions.

In some embodiments, optionally, the first coating step is a step of coating crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the second coating step is a step of coating crystalline phosphate XPO ₄ , the third coating step is a step of coating crystalline borate Y _p B _q O _r , and the fourth coating step is a step of coating carbon, wherein the positive electrode active material obtained has a core-shell structure, which includes the core and a shell coating the core, the shell includes a first coating layer coating the core, a second coating layer coating the first coating layer, a third coating layer coating the second coating layer, and a fourth coating layer coating the third coating layer, the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the second coating layer includes crystalline phosphate XPO ₄ , the third coating layer includes crystalline borate Y _p B _q O _r , and the fourth coating layer includes carbon.

In some embodiments, the source of the element M is one or more selected from the group consisting of a simple substance, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide of the element M.

In some embodiments, the source of the element X is one or more selected from the group consisting of a simple substance of the element X, a carbonate, a sulfate, a chloride, a nitrate, an organic acid salt, an oxide, and a hydroxide.

In some embodiments, the source of the element Y is one or more selected from the group consisting of a simple substance of element Y, a carbonate, a sulfate, a chloride, a nitrate, an organic acid salt, an oxide, and a hydroxide.

In the present application, the added amount of each source of the elements M, X, and Y depends on the target coating amount.

As an example, the carbon source is one or more selected from starch, sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and citric acid.

In the present application, the amount of the carbon source added can be determined based on the residual carbon value and the target coating amount.

In some embodiments, in the step of coating the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the pH of the solution in which the source of element M, the phosphorus source and the acid, and optionally the lithium source, are dissolved is controlled to be 3.5 to 6.5, and then stirred and reacted for 1 to 5 hours, and then the solution is heated to 50° C. to 120° C. and maintained at this temperature for 2 to 10 hours.

Optionally, in the step of coating crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the reaction is fully carried out. Optionally, the reaction is carried out for about 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours or about 5 hours. Optionally, the reaction time of the reaction can be within any range of the above values.

Optionally, in the step of coating the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the pH of the solution is controlled to be 4 to 6.

Optionally, in the step of coating crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the solution is heated to about 55°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C or about 120°C, and maintained at this temperature for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours; optionally, the heated temperature and the holding time may be in any range of any of the above values.

In some embodiments, the sintering in the step of coating the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c is performed at 650° C. to 800° C. for 2 to 6 hours. Alternatively, the sintering may be performed at about 650° C., about 700° C., about 750° C., or about 800° C. for about 2 hours, about 3 hours, about 4 hours, about 5 hours, or about 6 hours; alternatively, the sintering temperature and sintering time may be within any range of the above values.

In the step of coating the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , by controlling the sintering temperature and time within the above range, the following situations can be effectively avoided: when the sintering temperature is too low and the sintering time is too short, the crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c will have low crystallinity and more amorphous substances, which will lead to a decrease in the effect of inhibiting metal dissolution, thereby affecting the cycle performance and storage performance of the secondary battery; when the sintering temperature is too high, impurities will appear in the formed coating layer, which will also affect its effect of inhibiting metal dissolution, thereby affecting the cycle performance and storage performance of the secondary battery; when the sintering time is too long, the thickness of the formed coating layer will increase, affecting the migration of Li ⁺ , thereby affecting the capacity and rate performance of the positive electrode active material.

In some embodiments, in the step of coating the crystalline phosphate XPO ₄ , the source of element X, the phosphorus source and the acid are dissolved in a solvent, stirred and reacted for 1 to 10 hours, and then the solution is heated to 60° C. to 150° C. and maintained at this temperature for 2 to 10 hours.

Optionally, in the step of coating the crystalline phosphate XPO ₄ , the reaction is fully carried out. Optionally, the reaction is carried out for about 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours. Optionally, the reaction time of the reaction can be within any range of the above values.

Optionally, in the step of coating the crystalline phosphate XPO ₄ , the solution is heated to about 65°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C, about 120°C, about 130°C, about 140°C or about 150°C, and maintained at this temperature for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours; optionally, the temperature of the heating and the holding time may be in any range of any of the above values.

In the step of providing the core material, the step _of coating the crystalline _{pyrophosphate} _LiaMP2O7 and/or _Mb ( _P2O7 ) _c and the step of coating the crystalline phosphate _XPO4 , before sintering, that is, in the preparation of the core material for chemical reaction (that is _, step (1) and step (2)) and in the preparation of the coating liquid of each coating layer, by selecting appropriate reaction temperature and reaction time as described above, the following situations can be effectively avoided: when the reaction temperature is too low, the reaction cannot occur or the reaction rate is slow; when the temperature is too high, the product decomposes or forms impurities; when the reaction time is too long, the product particle size is large, which may increase the time and difficulty of subsequent processes; when the reaction time is too short, the reaction is incomplete and the obtained product is small.

In some embodiments, the sintering in the step of coating the crystalline phosphate XPO ₄ is performed at 500° C. to 700° C. for 6 to 10 hours. Alternatively, the sintering may be performed at about 550° C., about 600° C. or about 700° C. for about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours; alternatively, the sintering temperature and sintering time may be within any range of the above values.

In the step of coating the crystalline phosphate XPO ₄ , by controlling the sintering temperature and time within the above range, the following situations can be effectively avoided: when the sintering temperature is too low and the sintering time is too short, the crystalline phosphate XPO ₄ will have low crystallinity and more amorphous state, which will reduce the performance of the surface reaction activity of the positive electrode active material, thereby affecting the cycle performance and storage performance of the secondary battery; when the sintering temperature is too high, it will cause the appearance of impurities in the formed coating layer, which will also affect its effect of reducing the surface reaction activity of the positive electrode active material, thereby affecting the cycle performance and storage performance of the secondary battery; when the sintering time is too long, the thickness of the formed coating layer will increase, affecting the voltage platform of the positive electrode active material, thereby reducing the energy density of the secondary battery, etc.

In some embodiments, the sintering in the step of coating the crystalline _borate _YpBqOr is performed at 300°C to 500°C for 2 hours to 10 hours. Alternatively, the sintering may be performed at about 300°C, _about 350°C, about 400°C, about 450°C or about 500°C for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours; alternatively, the sintering temperature and sintering time may be within any range of the above values.

In the step of coating the crystalline _borate _YpBqOr _, by controlling the sintering temperature and time within the above ranges, the following situations can be avoided: when the sintering temperature is too low and the sintering time is too short, the crystallinity of the crystalline _borate _YpBqOr is low, the _amorphous state is more, and the coating effect is poor, the inhibition of manganese ion dissolution is insufficient, and the improvement of lithium ion and electron transport performance is not significant; when the sintering temperature is too high and the sintering time is too long, the thickness of the formed coating layer will increase, the battery impedance will increase, and the kinetic performance and energy density of the secondary battery will be affected.

In some embodiments, the sintering in the carbon coating step is performed at 700° C. to 800° C. for 6 to 10 hours. Alternatively, the sintering may be performed at about 700° C., about 750° C., or about 800° C. for about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours; alternatively, the sintering temperature and sintering time may be within any range of the above values.

In the step of coating carbon, by controlling the sintering temperature and time within the above range, the following situations can be effectively avoided: when the sintering temperature is too low, the degree of graphitization of the carbon material will decrease, affecting its conductivity, thereby affecting the capacity of the positive electrode active material; when the sintering temperature is too high, the degree of graphitization of the carbon material will be too high, affecting the transmission of Li ⁺ , thereby affecting the capacity of the positive electrode active material, etc.; when the sintering time is too short, the coating layer of the formed layer will be too thin, affecting its conductivity, thereby affecting the capacity of the positive electrode active material; when the sintering time is too long, the formed coating layer will be too thick, affecting the compaction density of the positive electrode active material, etc.

In the above-mentioned step of coating crystalline _{pyrophosphate} _LiaMP2O7 and/ _or _Mb ( _P2O7 ) _c , the step of coating crystalline phosphate _XPO4 , the step of coating crystalline borate _YpBqOr _, _and the step of coating _carbon , the drying can be carried out at a drying temperature of 100°C to 200°C, optionally 110°C to 190°C, more optionally 120°C to 180°C, even more optionally 120°C to 170°C, and most optionally 120°C to 160°C, and the drying time can be 3 hours to 9 hours, optionally 4 hours to 8 hours, more optionally 5 hours to 7 hours, and most optionally about 6 hours.

The positive electrode active material prepared by the preparation method of the positive electrode active material described in the present application has a reduced amount of manganese ion dissolution in the secondary battery after cycling, and has improved high temperature storage performance, cycle performance and rate performance. In addition, the raw material source is wide, the cost is low, the process is simple, and it is conducive to industrialization.

Positive electrode

The third aspect of the present application provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes the positive electrode active material of the first aspect of the present application or the positive electrode active material prepared by the method of the second aspect of the present application, and the content of the positive electrode active material in the positive electrode film layer is 10% by weight or more, and can be 90% by weight to 99.5% by weight, based on the total weight of the positive electrode film layer. The positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is arranged on any one or both of the two opposite surfaces of the positive electrode current collector.

The positive electrode film layer does not exclude other positive electrode active materials other than the positive electrode active material of the first aspect of the present application or the positive electrode active material prepared by the method of the second aspect of the present application. For example, the positive electrode film layer may also include other positive electrode active materials other than the above-mentioned four-layer coated positive electrode active materials. Optionally, the other positive electrode active materials may include at least one of lithium transition metal oxides and modified compounds thereof. As an example, the other positive electrode active materials may include at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and their respective modified compounds.

In some embodiments, the positive electrode film layer may further include a positive electrode conductive agent. The present application has no particular restrictions on the type of the positive electrode conductive agent. As an example, the positive electrode conductive agent includes at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

In some embodiments, the positive electrode film layer may further include a positive electrode binder. The present application has no particular restrictions on the type of the positive electrode binder. As an example, the positive electrode binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylic resin.

In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may be selected from at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

The positive electrode film layer is usually formed by coating the positive electrode slurry on the positive electrode current collector, drying and cold pressing. The positive electrode slurry is usually formed by dispersing the positive electrode active material, optional conductive agent, optional binder and any other components in a solvent and stirring them uniformly. The solvent can be N-methylpyrrolidone (NMP), but is not limited thereto.

Secondary battery

The fourth aspect of the present application provides a secondary battery, which includes the positive electrode plate of the third aspect of the present application.

Secondary batteries, also known as rechargeable batteries or storage batteries, refer to batteries that can be used continuously by recharging the active materials after the battery is discharged. Generally, secondary batteries include electrode assemblies and electrolytes, and the electrode assemblies include positive electrode sheets, negative electrode sheets, and separators. The separator is arranged between the positive electrode sheet and the negative electrode sheet, and mainly plays the role of preventing the positive and negative electrodes from short-circuiting, while allowing active ions to pass through. The electrolyte plays the role of conducting active ions between the positive electrode sheet and the negative electrode sheet.

The secondary battery mentioned in the embodiments or implementations of the present application refers to a single physical module including one or more battery cells to provide higher voltage and capacity. For example, the secondary battery mentioned in the present application may include a battery cell, a battery module or a battery pack, etc. A battery cell is the smallest unit that makes up a secondary battery, which can realize the function of charging and discharging alone. The present application has no particular restrictions on the shape of the battery cell, which can be cylindrical, square or any other shape. Figure 1 is a battery cell 5 of a square structure as an example.

In some embodiments, the battery cell includes an electrode assembly, and the single cell may further include an outer package. The electrode assembly may be made of a positive electrode sheet, a negative electrode sheet, and a separator, etc., by a winding process and/or a lamination process, and the outer package may be used to encapsulate the above-mentioned electrode assembly. The outer package may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT) and polybutylene succinate (PBS).

In some embodiments, as shown in FIG. 2 , the outer package may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The electrode assembly 52 is encapsulated in the receiving cavity. The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, which can be adjusted according to demand.

In some embodiments of the present application, battery cells can be assembled into a battery module, and the number of battery cells contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. FIG. 3 is a schematic diagram of a battery module 4 as an example. As shown in FIG. 3, in the battery module 4, multiple battery cells 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other manner. The multiple battery cells 5 can be further fixed by fasteners.

Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of battery cells 5 are received in the receiving space.

In some embodiments, the battery modules can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack. Figures 4 and 5 are schematic diagrams of a battery pack 1 as an example. As shown in Figures 4 and 5, the battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 is used to cover the lower box body 3 and form a closed space for accommodating the battery module 4. Multiple battery modules 4 can be arranged in the battery box in any manner.

[Positive electrode]

The positive electrode plate used in the secondary battery of the present application is the positive electrode plate described in any embodiment of the third aspect of the present application.

[Negative electrode]

In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector and including a negative electrode active material. For example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

The negative electrode active material may be a negative electrode active material for a secondary battery known in the art. As an example, the negative electrode active material includes, but is not limited to, at least one of natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may include at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material. The tin-based material may include at least one of elemental tin, tin oxide, and tin alloy material. The present application is not limited to these materials, and other conventionally known materials that can be used as negative electrode active materials for secondary batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

In some embodiments, the negative electrode film layer may further include a negative electrode conductive agent. The present application has no particular restrictions on the type of the negative electrode conductive agent. As an example, the negative electrode conductive agent may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

In some embodiments, the negative electrode film layer may further include a negative electrode binder. The present application has no particular restrictions on the type of the negative electrode binder. As an example, the negative electrode binder may include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

In some embodiments, the negative electrode film layer may further include other additives. As an example, the other additives may include a thickener, such as sodium carboxymethyl cellulose (CMC), a PTC thermistor material, and the like.

In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, a copper foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may be selected from at least one of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

The negative electrode film layer is usually formed by coating the negative electrode slurry on the negative electrode current collector, drying and cold pressing. The negative electrode slurry is usually formed by dispersing the negative electrode active material, optional conductive agent, optional binder, and other optional auxiliary agents in a solvent and stirring them uniformly. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited thereto.

The negative electrode plate does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode plate described in the present application also includes a conductive primer layer (for example, composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In some other embodiments, the negative electrode plate described in the present application also includes a protective layer covering the surface of the negative electrode film layer.

[Electrolytes]

The present application has no specific limitation on the type of the electrolyte, which can be selected according to the needs. For example, the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (ie, an electrolyte solution).

In some embodiments, the electrolyte is an electrolyte solution including an electrolyte salt and a solvent.

The type of the electrolyte salt is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the electrolyte salt may include at least one of lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO ₂ F ₂ ), lithium difluorobisoxalatophosphate (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP).

The type of the solvent is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the solvent may include ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and at least one of diethyl sulfone (ESE).

In some embodiments, the electrolyte may further include additives, for example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high temperature performance, additives that improve battery low temperature power performance, etc.

[Isolation film]

Secondary batteries using electrolytes and some secondary batteries using solid electrolytes also include a separator. The separator is arranged between the positive electrode plate and the negative electrode plate, and mainly plays the role of preventing the positive and negative electrodes from short-circuiting, while allowing active ions to pass through. The present application has no particular restrictions on the type of the separator, and any known porous structure separator with good chemical stability and mechanical stability can be selected.

In some embodiments, the material of the isolation membrane may include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane may be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer are the same or different.

The preparation method of the secondary battery of the present application is well known. In some embodiments, the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, the separator, and the negative electrode sheet can be formed into an electrode assembly through a winding process or a lamination process, and the electrode assembly is placed in an outer package, and the electrolyte is injected after drying. After vacuum packaging, standing, forming, shaping and other processes, a battery cell is obtained. Multiple battery cells can also be further connected in series, in parallel or in mixed connection to form a battery module. Multiple battery modules can also be connected in series, in parallel or in mixed connection to form a battery pack. In some embodiments, multiple battery cells can also directly form a battery pack.

Electrical devices

The fifth aspect of the present application provides an electrical device, which includes the secondary battery of the present application. The secondary battery can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device can be, but is not limited to, a mobile device (such as a mobile phone, a tablet computer, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc.

The electrical device may select a specific type of secondary battery, such as a battery cell, a battery module or a battery pack, according to its usage requirements.

Fig. 6 is a schematic diagram of an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirements of the electric device for high power and high energy density, a battery pack or a battery module can be used as a power source.

As another example, the electric device may be a mobile phone, a tablet computer, a notebook computer, etc. The electric device is usually required to be light and thin, and a battery cell may be used as a power source.

Example

The following examples describe the disclosure of the present application in more detail, and these examples are only for illustrative purposes, as it is obvious to those skilled in the art that various modifications and variations are made within the scope of the disclosure of the present application. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing, and the instruments used in the examples are commercially available.

The sources of raw materials involved in the embodiments of this application are as follows:

名称name	化学式Chemical formula	厂家factory	规格Specification
碳酸锰Manganese carbonate	MnCO ₃ MnCO ₃	山东西亚化学工业有限公司Shandong West Asia Chemical Industry Co., Ltd.	1Kg1Kg
碳酸锂Lithium carbonate	Li ₂CO ₃ Li ₂ CO ₃	山东西亚化学工业有限公司Shandong West Asia Chemical Industry Co., Ltd.	1Kg1Kg
碳酸镁Magnesium carbonate	MgCO ₃ MgCO ₃	山东西亚化学工业有限公司Shandong West Asia Chemical Industry Co., Ltd.	1Kg1Kg
碳酸锌Zinc carbonate	ZnCO ₃ ZnCO ₃	武汉鑫儒化工有限公司Wuhan Xinru Chemical Co., Ltd.	25Kg25Kg
碳酸亚铁Ferrous carbonate	FeCO ₃ FeCO ₃	西安兰之光精细材料有限公司Xi'an Lanzhiguang Fine Materials Co., Ltd.	1Kg1Kg
硫酸镍Nickel Sulfate	NiCO ₃ NiCO ₃	山东西亚化学工业有限公司Shandong West Asia Chemical Industry Co., Ltd.	1Kg1Kg
硫酸钛Titanium sulfate	Ti(SO ₄) ₂ Ti(SO ₄ ) ₂	山东西亚化学工业有限公司Shandong West Asia Chemical Industry Co., Ltd.	1Kg1Kg
硫酸钴Cobalt sulfate	CoSO ₄ CoSO ₄	厦门志信化学有限公司Xiamen Zhixin Chemical Co., Ltd.	500g500g
二氯化钒Vanadium dichloride	VCl ₂ VCl ₂	上海金锦乐实业有限公司Shanghai Jinjinle Industrial Co., Ltd.	1Kg1Kg
二水合草酸Oxalic acid dihydrate	C ₂H ₂O ₄.2(H ₂O) C ₂ H ₂ O ₄ .2(H ₂ O)	上海金锦乐实业有限公司Shanghai Jinjinle Industrial Co., Ltd.	1Kg1Kg
磷酸二氢铵Ammonium dihydrogen phosphate	NH ₄H ₂PO ₄ NH ₄ H ₂ PO ₄	上海澄绍生物科技有限公司Shanghai Chengshao Biotechnology Co., Ltd.	500g500g
蔗糖sucrose	C ₁₂H ₂₂O ₁₁ C ₁₂ H ₂₂ O ₁₁	上海源叶生物科技有限公司Shanghai Yuanye Biotechnology Co., Ltd.	100g100g
稀硫酸Dilute sulfuric acid	H ₂SO ₄ _H2SO4	深圳海思安生物技术有限公司Shenzhen Hisian Biotechnology Co., Ltd.	质量分数60％Mass fraction 60%
稀硝酸Dilute nitric acid	HNO ₃ HNO ₃	安徽凌天精细化工有限公司Anhui Lingtian Fine Chemical Co., Ltd.	质量分数60％Mass fraction 60%
亚硅酸Silicic acid	H ₂SiO ₃ H ₂ SiO ₃	上海源叶生物科技有限公司Shanghai Yuanye Biotechnology Co., Ltd.	100g，质量分数99.8％100g, mass fraction 99.8%

I. Battery Preparation

Example 1

Step 1: Preparation of positive electrode active materials

Step S1: Preparation of co-doped manganese oxalate

689.6g of manganese carbonate, 455.27g of ferrous carbonate, 4.65g of cobalt sulfate, and 4.87g of vanadium dichloride were added to a mixer and fully mixed for 6 hours. The obtained mixture was then transferred to a reactor, and 5L of deionized water and 1260.6g of oxalic acid dihydrate were added, heated to 80°C, and stirred at a speed of 500rpm for 6 hours, mixed evenly, until the reaction was terminated without bubble generation, and a co-doped manganese oxalate suspension was obtained. The suspension was then filtered, dried at 120°C, and sand-milled to obtain manganese oxalate particles with a particle size of 100nm.

Step S2: Preparation of core Li _0.997 Mn _0.60 Fe _0.393 V _0.004 Co _0.003 P _0.997 S _0.003 O ₄

Take 1793.1g of manganese oxalate prepared in (1), 368.3g of lithium carbonate, 1146.6g of ammonium dihydrogen phosphate and 4.9g of dilute sulfuric acid, add them into 20L of deionized water, stir them thoroughly, and evenly mix and react at 80°C for 10 hours to obtain a slurry. The slurry is transferred to a spray drying device for spray drying and granulation, and dried at a temperature of 250°C to obtain a powder. In a protective atmosphere (90% nitrogen and 10% hydrogen), the powder is sintered in a roller kiln at 700°C for 4 hours to obtain the above-mentioned core material.

Step S3: Preparation of the first coating layer coating solution

_Li2FeP2O7 solution was prepared by dissolving _7.4g _lithium carbonate, 11.6g ferrous carbonate, 23.0g ammonium dihydrogen phosphate and 12.6g oxalic acid dihydrate in 500mL deionized water, controlling the pH to 5, and then stirring and reacting at room temperature for 2 hours to obtain a solution, and then heating the solution to 80°C and maintaining this temperature for 4 hours to obtain a first coating layer coating solution.

Step S4: Coating of the first coating layer

The doped 1571.9 g lithium manganese phosphate core material obtained in step S2 is added to the first coating layer coating solution (coating material content is 15.7 g) obtained in step S3, and the mixture is stirred and mixed for 6 hours. After being evenly mixed, the mixture is transferred to a 120°C oven and dried for 6 hours, and then sintered at 650°C for 6 hours to obtain a pyrophosphate-coated material.

Step S5: Preparation of the second coating layer coating solution

3.7 g of lithium carbonate, 11.6 g of ferrous carbonate, 11.5 g of ammonium dihydrogen phosphate and 12.6 g of oxalic acid dihydrate were dissolved in 1500 mL of deionized water, and then stirred and reacted for 6 hours to obtain a solution, and then the solution was heated to 120°C and maintained at this temperature for 6 hours to obtain a second coating layer coating solution.

Step S6: Coating of the second coating layer

Add 1586.8 g of the pyrophosphate-coated material obtained in step S4 to the second coating layer coating liquid (coating material content is 47.1 g) obtained in step S5, stir and mix thoroughly for 6 hours, and after mixing evenly, transfer to a 120°C oven and dry for 6 hours, and then sinter at 700°C for 8 hours to obtain a two-layer coated material.

Step S7: Preparation of the third coating layer coating solution

28.3 g of lithium hydroxide and 13.7 g of boron oxide were added into 500 mL of deionized water to obtain a third coating layer coating solution.

Step S8: Coating of the third coating layer

1633.9 g of the two-layer coated material obtained in step S6 is added to the third coating layer coating liquid (coating material content is 31.4 g) obtained in step S7, and after mixing evenly, it is transferred to a 120°C oven for drying for 6 hours, and then sintered at 400°C for 10 hours to obtain a three-layer coated material.

Step S9: Preparation of the fourth coating layer aqueous solution

37.3 g of sucrose was dissolved in 500 g of deionized water, and then stirred and fully dissolved to obtain a sucrose aqueous solution.

Step S10: Coating of the fourth coating layer

1665.3 g of the three-layer coated material obtained in step S8 was added to the sucrose solution obtained in step S9, and the mixture was stirred and mixed for 6 hours. After being evenly mixed, the mixture was transferred to a 150°C oven and dried for 6 hours, and then sintered at 700°C for 10 hours to obtain a four-layer coated material, i.e., the positive electrode active material.

Step 2: Preparation of positive electrode

The positive electrode active material, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were added to N-methylpyrrolidone (NMP) at a weight ratio of 97:1.2:1.8, and stirred and mixed to obtain a positive electrode slurry. The positive electrode slurry was then evenly coated on aluminum foil at a coating surface density of 0.018g/ ^cm2 , and dried, cold pressed, and cut to obtain a positive electrode sheet.

Step 3: Preparation of negative electrode

The negative electrode active material artificial graphite, hard carbon, conductive agent acetylene black, binder styrene butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) were dissolved in solvent deionized water at a weight ratio of 90:5:2:2:1, and stirred and mixed to prepare a negative electrode slurry. The negative electrode slurry was evenly coated on the negative electrode current collector copper foil at a coating surface density of 0.0075g/ ^cm2 , and the negative electrode sheet was obtained after drying, cold pressing, and slitting.

Step 4: Preparation of electrolyte

In an argon atmosphere glove box ( _H2O <0.1ppm, _O2 <0.1ppm), organic solvents ethylene carbonate (EC)/ethyl methyl carbonate (EMC) were mixed uniformly in a volume ratio of 3/7, 12.5 wt% (based on the weight of the ethylene carbonate/ethyl methyl carbonate solvent) of _LiPF6 was added and dissolved in the organic solvent, and stirred uniformly to obtain an electrolyte.

Step 5: Preparation of isolation membrane

A commercially available PP-PE copolymer microporous film (from Zhuogao Electronic Technology Co., Ltd., Model 20) with a thickness of 20 μm and an average pore size of 80 nm was used.

Step 6: Preparation of the full battery

The positive electrode sheet, separator, and negative electrode sheet obtained above are stacked in order, with the separator placed between the positive and negative electrodes to play a role of isolation, and then wound to obtain an electrode assembly. The electrode assembly is placed in an outer package, injected with the above electrolyte and packaged to obtain a full battery (hereinafter also referred to as "full battery").

【Preparation of button cell】

The positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were added to N-methylpyrrolidone (NMP) at a weight ratio of 90:5:5, and stirred in a drying room to form a slurry. The slurry was coated on aluminum foil, dried, and cold pressed to form a positive electrode sheet. The coating surface density was 0.015g/ ^cm2 , and the compaction density was 2.0g/ ^cm3 .

A lithium sheet was used as the negative electrode, and a solution of 1 mol/L _LiPF6 in ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) in a volume ratio of 1:1:1 was used as the electrolyte. Together with the positive electrode sheet prepared above, they were assembled into a button battery (hereinafter also referred to as "button battery") in a button box.

Example 2-33

The positive electrode active materials of Examples 2-33 were prepared in a manner similar to that of Example 1. The differences in the preparation of the positive electrode active materials are shown in Tables 1 to 7. The other processes are the same as those of Example 1.

Examples 34-51

The positive electrode active materials of Examples 34 to 51 were prepared in a manner similar to that of Example 1. The differences in the preparation of the positive electrode active materials are shown in Tables 8 to 10. The other processes are the same as those of Example 1.

Comparative Examples 1-18

The positive electrode active materials of Comparative Examples 1-18 were prepared in a manner similar to Example 1, and the differences in the preparation of the positive electrode active materials are shown in Tables 1 to 7. Comparative Examples 1-2, 4-10 and 12 were not coated with the first coating layer, so there were no steps S3 and S4; Comparative Example 1-11 was not coated with the second coating layer, so there were no steps S5 and S6, and Comparative Examples 1-18 were not coated with the third coating layer, so there were no steps S7 and S8.

In addition, in all the embodiments and comparative examples of the present application, unless otherwise indicated, the first coating layer material, the second coating layer material, and the third coating layer material used are assumed to be crystalline.

Table 2: Preparation of the first coating layer coating solution (step S3)

Table 3: Coating of the first coating layer (step S4)

Table 4: Preparation of the second coating layer coating solution (step S5)

Table 5: Coating of the second coating layer (step S6)

Table 6: Coating of the third coating layer (step S8)

Table 7: Coating of the fourth coating layer (step S10)

Table 8: Investigation of the first coating layer material

Table 9: Investigation of the second coating material

Table 10: Investigation of the third coating layer material

II. Performance Evaluation

1. Lattice change rate measurement method

Under a constant temperature of 25°C, the positive electrode active material sample was placed in an X-ray powder diffractometer (model: Bruker D8 Discover), and the sample was tested at 1°/minute. The test data was sorted and analyzed, and the lattice constants a0, b0, c0 and v0 were calculated with reference to the standard PDF card (a0, b0 and c0 represent the lengths of the unit cell in each direction, v0 represents the unit cell volume, which can be directly obtained through XRD refinement results).

The positive electrode active material sample is prepared into a buckle battery by using the buckle battery preparation method in the above embodiment, and the buckle battery is charged at a small rate of 0.05C until the current is reduced to 0.01C. Then the positive electrode plate in the buckle battery is taken out and immersed in dimethyl carbonate (DMC) for 8 hours. Then it is dried, powdered, and particles with a particle size of less than 500nm are screened out. Take a sample and calculate its unit cell volume v1 in the same way as the above test of the fresh sample, and (v0-v1)/v0×100% is shown in the table as the lattice change rate (unit cell volume change rate) before and after complete lithium deintercalation.

2. Li/Mn antisite defect concentration

The XRD results tested in the "lattice change rate measurement method" are compared with the PDF (Powder Diffraction File) card of the standard crystal to obtain the Li/Mn antisite defect concentration. Specifically, the XRD results tested in the "lattice change rate measurement method" are imported into the General Structural Analysis System (GSAS) software to automatically obtain the refinement results, which include the occupancy of different atoms. The Li/Mn antisite defect concentration is obtained by reading the refinement results.

3. Surface oxygen valence state

Take 5g of the above-prepared positive electrode active material sample and prepare it into a buckle battery according to the buckle battery preparation method described in the above embodiment. The buckle battery is charged at a small rate of 0.05C until the current is reduced to 0.01C. Then take out the positive electrode plate in the buckle battery and soak it in DMC for 8 hours. Then dry it, scrape the powder, and screen out the particles with a particle size of less than 500nm. The obtained particles are measured by electron energy loss spectroscopy (EELS, the instrument model used is Talos F200S) to obtain the energy loss near-edge structure (ELNES), which reflects the state density and energy level distribution of the element. According to the state density and energy level distribution, the number of occupied electrons is calculated by integrating the valence band state density data, thereby inferring the valence state of the surface oxygen after charging.

4. Compacted density

5 g of the positive electrode active material powder prepared above was placed in a special compaction mold (American CARVER mold, model 13 mm), and then the mold was placed on a compaction density instrument. A pressure of 3T (tons) was applied, and the thickness of the powder under pressure (the thickness after pressure relief, the area of the container used for testing was 1540.25 mm ² ) was read on the instrument, and the compaction density was calculated by ρ=m/v.

5. Initial gram capacity measurement method of button cell

The button cells prepared in the above embodiments and comparative examples were charged to 4.3 V at 0.1 C, then charged at a constant voltage at 4.3 V until the current was less than or equal to 0.05 mA, left to stand for 5 minutes, and then discharged to 2.0 V at 0.1 C. The discharge capacity at this time was the initial gram capacity, recorded as D0.

6. 3C charging constant current ratio

Under a constant temperature environment of 25°C, let the fresh full batteries prepared in the above embodiments and comparative examples stand for 5 minutes and discharge to 2.5V at 1/3C. Let stand for 5 minutes, charge to 4.3V at 1/3C, and then charge at constant voltage at 4.3V until the current is less than or equal to 0.05mA. Let stand for 5 minutes, and record the charging capacity at this time as C0. Discharge to 2.5V at 1/3C, let stand for 5 minutes, and then charge to 4.3V at 3C, let stand for 5 minutes, and record the charging capacity at this time as C1. The 3C charging constant current ratio is C1/C0×100%. The higher the 3C charging constant current ratio, the better the rate performance of the secondary battery.

7. Battery expansion test after full battery storage at 60℃ for 30 days

The full batteries prepared in the above-mentioned embodiments and comparative examples at 100% state of charge (SOC) were stored at 60°C. The open circuit voltage (OCV) and AC internal resistance (IMP) of the battery were measured before, during and after storage to monitor the SOC, and the volume of the battery was measured. The full battery was taken out after each 48 hours of storage, and the open circuit voltage (OCV) and internal resistance (IMP) were tested after standing for ¹ hour, and the battery volume was measured by the water displacement method after cooling to room temperature. The water displacement method is to first measure the gravity _F1 of the battery separately using a balance that automatically converts the dial data, and then completely place the battery in deionized water (density is known to be 1g/cm3), and measure the gravity _F2 of the battery at this time. The buoyancy _Fbuoyancy of the battery is _F1 - _F2 , and then according to the Archimedean principle _Fbuoyancy =ρ×g× _{Vdisplacement} , the battery volume V=( _F1 - _F2 )/(ρ×g) is calculated.

From the OCV and IMP test results, the batteries of all embodiments maintained an SOC of more than 99% throughout the test until the end of storage.

After storage for 30 days, the battery volume was measured, and the percentage increase in the battery volume after storage relative to the battery volume before storage was calculated.

8. Full battery cycle performance test at 45°C

At a constant temperature of 45°C, charge the full battery to 4.3V at 1C, then charge at a constant voltage at 4.3V to a current of ≤0.05mA, let stand for 5 minutes, and then discharge to 2.5V at 1C. The capacity is recorded as D0. Repeat the above process until the capacity decays to 80% of D0. Record the number of repetitions at this time, which is the number of cycles corresponding to 80% capacity retention at 45°C.

9. Transition metal Mn (and Mn-doped Fe) dissolution test

After cycling at 45°C until the capacity decays to 80%, the full batteries prepared in the above embodiments and comparative examples are discharged at a rate of 0.1C to a cut-off voltage of 2.0V. Then the battery is disassembled, the negative electrode plate is taken out, 30 discs of unit area (1540.25 ^mm2 ) are randomly taken from the negative electrode plate, and the inductively coupled plasma emission spectrum (ICP) is tested using Agilent ICP-OES730. According to the ICP results, the amount of Fe (if the Mn position of the positive electrode active material is doped with Fe) and Mn is calculated, thereby calculating the dissolution amount of Mn (and Fe doped at the Mn position) after cycling. The test standard is based on EPA-6010D-2014.

10. Measurement of manganese and phosphorus in positive electrode active materials

5 g of the positive electrode active material prepared above was dissolved in 100 mL of reverse aqua regia (concentrated hydrochloric acid: concentrated nitric acid = 1:3), the content of each element in the solution was tested by ICP, and then the content of manganese or phosphorus was measured and converted (the amount of manganese or phosphorus/the amount of positive electrode active material × 100%) to obtain its weight ratio.

11. Coating thickness test

The thickness test of the coating layer is mainly carried out by cutting a thin slice of about 100 nm thick from the middle of a single particle of the positive electrode active material prepared above by FIB, and then performing TEM test on the thin slice to obtain the original TEM test image.

The original image obtained from the above TEM test was opened in the DigitalMicrograph software, and the coating layer was identified through the lattice spacing and angle information, and the thickness of the coating layer was measured.

The thickness of the selected particles was measured at three locations and the average value was taken.

12. Crystal plane spacing and angle test

Take 1g of each positive electrode active material powder prepared above and put it in a 50mL test tube, and inject 10mL of 75% alcohol by mass into the test tube, stir and disperse for 30 minutes, then use a clean disposable plastic pipette to take an appropriate amount of the above solution and drop it on a 300-mesh copper grid. At this time, some powder will remain on the copper grid. Transfer the copper grid and the sample to the TEM (Talos F200s G2) sample chamber for testing to obtain the original TEM test image.

The original image obtained by the above TEM test was opened in the DigitalMicrograph software, and Fourier transform was performed (automatically completed by the software after clicking the operation) to obtain the diffraction pattern. The distance from the diffraction spot to the center position in the diffraction pattern was measured to obtain the crystal plane spacing, and the angle was calculated according to the Bragg equation.

By comparing the obtained crystal plane spacing and corresponding angle data with their standard values, different materials in the coating layer can be identified.

13. Determination of the molar ratio of SP2 and SP3 in the third coating carbon

This test is performed by Raman spectroscopy. By separating the peaks of the energy spectrum of the Raman test, I _d /I _g is obtained, where I _d is the peak intensity of the carbon in the SP3 form and I _g is the peak intensity of the carbon in the SP2 form, thereby confirming the molar ratio of the two.

14. Determination of the chemical formula of the core and the composition of different coating layers

The internal microstructure and surface structure of the positive electrode active material were characterized with high spatial resolution using spherical aberration electron microscopy (ACSTEM), and the core chemical formula of the positive electrode active material and the composition of different coating layers were obtained using three-dimensional reconstruction technology.

Table 11 shows the performance data of the positive electrode active materials, butt-charged or fully-charged in Examples 1-51 and Comparative Examples 1-18 measured according to the above performance test method.

Table 12 shows the thickness of each coating layer in the positive electrode active materials prepared in Examples 1-17 and Comparative Examples 3-4 and 12, and the weight ratio of manganese element to phosphorus element.

Table 13 shows the interplanar spacing and angles of the first coating layer material and the second coating layer material in the positive electrode active materials prepared in Examples 1 and 34-46.

Table 11

Table 12

Table 13

It can be seen from Table 11 that, compared with the comparative example, the embodiment achieves a smaller lattice change rate, a smaller Li/Mn antisite defect concentration, a larger compaction density, a surface oxygen valence state closer to -2, less Mn and Fe dissolution after cycling, and better battery performance, such as better high-temperature storage performance and high-temperature cycling performance.

It can be seen from Table 12 that by doping and coating the Mn and P sites of lithium manganese iron phosphate (containing 35% manganese and about 20% phosphorus), the manganese content and the weight content ratio of manganese to phosphorus in the positive electrode active material are significantly reduced; in addition, by comparing Examples 1-17 with Comparative Examples 3, 4 and 12, combined with Table 11, it can be seen that the reduction of manganese and phosphorus in the positive electrode active material will reduce the dissolution amount of manganese ions and iron ions and improve the performance of the battery prepared therefrom.

As can be seen from Table 13, the interplanar spacing and angle of the first coating layer and the second coating layer in the positive electrode active material of the present application are within the range described in the present application. As can be seen from Table 11, the use of the first coating layer and the second coating layer containing other elements within the scope of the present application also obtains a positive electrode active material with good performance and achieves good battery performance results.

The inventors then investigated the effect of the coating sequence of the coating layers on the battery performance. The positive electrode active materials of Examples 52-56 were prepared in a manner similar to Example 1. The differences in the preparation of the positive electrode active materials are shown in Table 14. The other processes were the same as those of Example 1.

Table 15 shows the performance data of the positive electrode active materials, butt-charged or fully-charged in Examples 52-56 measured according to the above performance test method.

Table 14

序号Serial number	第一包覆层First coating layer	第二包覆层Second coating layer	第三包覆层The third coating layer	第四包覆层Fourth coating layer
实施例1Example 1	1％Li ₂FeP ₂O ₇ ₁ _% _Li2FeP2O7	3％LiFePO ₄ 3% _LiFePO4	2％Li ₃BO ₃ 2 _% _Li3BO3	1％碳1% Carbon

实施例52Embodiment 52	1％Li ₂FeP ₂O ₇ ₁ _% _Li2FeP2O7	2％Li ₃BO ₃ 2 _% _Li3BO3	3％LiFePO ₄ 3% _LiFePO4	1％碳1% Carbon

实施例53Embodiment 53	3％LiFePO ₄ 3% _LiFePO4	1％Li ₂FeP ₂O ₇ ₁ _% _Li2FeP2O7	2％Li ₃BO ₃ 2 _% _Li3BO3	1％碳1% Carbon
实施例54Embodiment 54	3％LiFePO ₄ 3% _LiFePO4	2％Li ₃BO ₃ 2 _% _Li3BO3	1％Li ₂FeP ₂O ₇ ₁ _% _Li2FeP2O7	1％碳1% Carbon
实施例55Embodiment 55	2％Li ₃BO ₃ 2 _% _Li3BO3	1％Li ₂FeP ₂O ₇ ₁ _% _Li2FeP2O7	3％LiFePO ₄ 3% _LiFePO4	1％碳1% Carbon
实施例56Embodiment 56	2％Li ₃BO ₃ 2 _% _Li3BO3	3％LiFePO ₄ 3% _LiFePO4	1％Li ₂FeP ₂O ₇ ₁ _% _Li2FeP2O7	1％碳1% Carbon

Table 15

It can be seen from Tables 14 and 15 that when the first coating layer includes crystalline pyrophosphate Li _a MP ₂ O ₇ and/or M _b (P ₂ O ₇ ) _c , the second coating layer includes crystalline phosphate XPO ₄ , the third coating layer includes crystalline borate Y _p B _q O _r , and the fourth coating layer includes carbon, the battery has better overall performance.

It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

A positive electrode active material with a core-shell structure, comprising a core and a shell covering the core,

The core comprises LimAxMn1 - yByP1 - zCzO4 - nDn , the A comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IIB, Group VB and Group VIB, and optionally comprises one or more elements selected from Group Zn, Al, Na, K, Mg, Nb, Mo and W, the B comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB and Group VIIIB, and optionally comprises one or more elements selected from Group Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, the C comprises one or more elements selected from Group IIIA, Group IVA, Group VA and Group VIA, and optionally comprises The D comprises one or more elements selected from Group VIA and Group VIIA, optionally one or more elements selected from S, F, Cl and Br, the m is selected from the range of 0.900 to 1.100, optionally from the range of 0.995 to 1.002, the x is selected from the range of 0 to 0.100, optionally from the range of 0.001 to 0.005, the y is selected from the range of 0.001 to 0.500, the z is selected from the range of 0.001 to 0.100, the n is selected from the range of 0 to 0.100, optionally from the range of 0.001 to 0.005, and the core is electrically neutral;

The shell comprises crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , crystalline phosphate XPO 4 , crystalline borate Y p B q O r and carbon, and the shell comprises one or more coating layers, each coating layer independently comprises one or more of crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , crystalline phosphate XPO 4 , crystalline borate Y p B q O r and carbon, crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c Each of the M in c independently comprises one or more metal elements selected from transition metals, group IA, group IIA, group IIIA, group IVA, group VA and lanthanides, and optionally comprises one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values of a, b and c satisfy the following conditions: the crystalline pyrophosphate Li a MP 2 O 7 or M b (P 2 O 7 ) c maintains electrical neutrality, the X comprises one or more metal elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA and lanthanides, optionally comprising one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, the Y comprises one or more metal elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA and lanthanides, optionally comprising one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, 1≤p≤4, 1≤q≤7 , 2≤r≤12, and the values of p, q and r satisfy the following conditions: the crystalline borate YpBqOr maintains electrical neutrality.
The positive electrode active material according to claim 1, wherein

The crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c has a lattice spacing ranging from 0.293 nm to 0.470 nm, and a crystal orientation (111) angle ranging from 18.00° to 32.00°; and/or,

The interplanar spacing of the crystalline phosphate XPO 4 ranges from 0.244 nm to 0.425 nm, and the angle of the crystal orientation (111) ranges from 20.00° to 37.00°.
The positive electrode active material according to claim 1 or 2, wherein the carbon is a mixture of SP2 carbon and SP3 carbon, and optionally, the molar ratio of the SP2 carbon to the SP3 carbon is any value in the range of 0.1 to 10, and more optionally any value in the range of 2.0 to 3.0.
The positive electrode active material according to any one of claims 1 to 3, wherein

In the kernel, the ratio of y to 1-y is from 1:10 to 1:1, optionally from 1:4 to 1:1; and/or,

In the kernel, the ratio of z to 1-z is from 1:9 to 1:999, optionally from 1:499 to 1:249; and/or,

In the core, the B includes one or more elements selected from Fe, Ti, V, Ni, Co and Mg, and optionally includes at least two elements selected from Fe, Ti, V, Ni, Co and Mg; and/or,

In the core, the C includes one element selected from the group consisting of B (boron), S, Si and N.
The positive electrode active material according to any one of claims 1 to 4, wherein q:r is 1:3.
The positive electrode active material according to any one of claims 1 to 5, wherein the shell includes a first coating layer coating the inner core, a second coating layer coating the first coating layer, a third coating layer coating the second coating layer, and a fourth coating layer coating the third coating layer, the fourth coating layer includes carbon, and the first coating layer, the second coating layer, and the third coating layer each independently include any one of crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , crystalline phosphate XPO 4 , and crystalline borate Y p B q O r .
The positive electrode active material according to claim 6, wherein the first coating layer comprises crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , the fourth coating layer comprises carbon, and the second coating layer and the third coating layer independently comprise crystalline phosphate XPO 4 or crystalline borate Y p B q O r , optionally, the first coating layer comprises crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , the second coating layer comprises crystalline phosphate XPO 4 , the third coating layer comprises crystalline borate Y p B q O r , and the fourth coating layer comprises carbon.
The positive electrode active material according to claim 6 or 7, wherein

The coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, and can be greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core; and/or,

The coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, and can be greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core; and/or,

The coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, and can be greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core; and/or,

The coating amount of the fourth coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, and more optionally greater than 0 and less than or equal to 2% by weight, based on the weight of the core.
The positive electrode active material according to any one of claims 6 to 8, wherein

The thickness of the first coating layer is 1 nm to 15 nm; and/or,

The thickness of the second coating layer is 1 nm to 15 nm; and/or,

The thickness of the third coating layer is 1 nm to 15 nm; and/or,

The thickness of the fourth cladding layer is 2 nm to 25 nm.
The positive electrode active material according to any one of claims 1 to 9, wherein

Based on the weight of the positive electrode active material, the manganese content is in the range of 10 wt % to 35 wt %, optionally in the range of 15 wt % to 30 wt %, and more optionally in the range of 17 wt % to 20 wt %; and/or,

Based on the weight of the positive electrode active material, the content of phosphorus is in the range of 12 wt % to 25 wt %, and optionally in the range of 15 wt % to 20 wt %; and/or,

The weight ratio of the manganese element to the phosphorus element is in the range of 0.90 to 1.25, and optionally 0.95 to 1.20, based on the weight of the positive electrode active material.
The positive electrode active material according to any one of claims 1 to 10, wherein the positive electrode active material satisfies at least one of the following conditions (1) to (4):

(1) The lattice change rate of the positive electrode active material before and after complete lithium deintercalation is 4% or less, optionally 3.8% or less, and more preferably 2.0% to 3.8%;

(2) The Li/Mn antisite defect concentration of the positive electrode active material is 4% or less, optionally 2.2% or less, and more preferably 1.5% to 2.2%;

(3) The compaction density of the positive electrode active material at 3T is 2.2 g/cm 3 or more, and can be 2.2 g/cm 3 or more and 2.8 g/cm 3 or less;

(4) The surface oxygen valence state of the positive electrode active material is -1.90 or less, and can be selected from -1.90 to -1.98.
A method for preparing a positive electrode active material comprises the following steps:

The step of providing a core material: the core comprises LimAxMn1-yByP1-zCzO4 - nDn , the A comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IIB, Group VB and Group VIB, and optionally comprises one or more elements selected from Group Zn, Al, Na, K, Mg, Nb, Mo and W, the B comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB and Group VIIIB, and optionally comprises one or more elements selected from Group Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, the C comprises one or more elements selected from Group IIIA, Group IVA, Group VA and Group VIA, and optionally comprises The D comprises one or more elements selected from Group VIA and Group VIIA, optionally one or more elements selected from S, F, Cl and Br, the m is selected from the range of 0.900 to 1.100, optionally from the range of 0.995 to 1.002, the x is selected from the range of 0 to 0.100, optionally from the range of 0.001 to 0.005, the y is selected from the range of 0.001 to 0.500, the z is selected from the range of 0.001 to 0.100, the n is selected from the range of 0 to 0.100, optionally from the range of 0.001 to 0.005, and the core is electrically neutral;

Coating step: providing coating solutions including pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , phosphate XPO 4 , borate Y p B q O r and a carbon source respectively, adding the core material into the coating solution and mixing, and obtaining a positive electrode active material through sintering, wherein the positive electrode active material has a core-shell structure, comprising the core and a shell coating the core, the shell comprising crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , crystalline phosphate XPO 4 , crystalline borate Y p B q O r and carbon, and the shell comprising one or more coating layers, each coating layer independently comprising one or more of crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , crystalline phosphate XPO 4 , crystalline borate Y p B q O r and carbon, the crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , crystalline phosphate XPO 4 , crystalline borate Y p B q O r and carbon, M in M b ( P 2 O 7 ) c and/or M b (P 2 O 7 ) c each independently includes one or more metal elements selected from transition metals, group IA, group IIA, group IIIA, group IVA, group VA and lanthanides, and optionally includes one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values of a, b and c satisfy the following conditions: the crystalline pyrophosphate Li a MP 2 O 7 or M b (P 2 O 7 ) c c maintains electrical neutrality, the X comprises one or more metal elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA and lanthanides, optionally comprising one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, the Y comprises one or more metal elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA and lanthanides, optionally comprising one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb and Al, 1≤p≤4 , 1≤q≤7, 2≤r≤12, and the values of p, q and r satisfy the following conditions: the crystalline borate YpBqOr maintains electrical neutrality.
The preparation method according to claim 12, wherein the step of providing the core material comprises the following steps: step (1): mixing and stirring a manganese source, a dopant of element B and an acid in a container to obtain manganese salt particles doped with element B; step (2): mixing the manganese salt particles doped with element B with a lithium source, a phosphorus source, a dopant of element C, optionally a dopant of element A and optionally a dopant of element D in a solvent to obtain a slurry, and sintering under the protection of an inert gas atmosphere to obtain the core material.
The preparation method according to claim 13, wherein

The step (1) is performed mixing at a temperature of 20°C to 120°C, optionally 40°C to 120°C; and/or,

The stirring in step (1) is carried out at 400 rpm to 700 rpm for 1 hour to 9 hours, optionally 3 hours to 7 hours; and/or,

The step (2) is performed at a temperature of 20° C. to 120° C., optionally 40° C. to 120° C., for mixing for 1 to 12 hours; and/or,

The sintering in the step (2) is carried out at 600° C. to 950° C. for 4 to 10 hours in an inert gas or a mixed atmosphere of inert gas and hydrogen.
The preparation method according to any one of claims 12 to 14, wherein the coating step comprises a step of coating crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , a step of coating crystalline phosphate XPO 4 , a step of coating crystalline borate Y p B q O r , and a step of coating carbon,

Optionally, the first coating step is a step of coating crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , the second coating step is a step of coating crystalline phosphate XPO 4 , the third coating step is a step of coating crystalline borate Y p B q O r , and the fourth coating step is a step of coating carbon, wherein the positive electrode active material obtained has a core-shell structure, comprising the core and a shell coating the core, the shell comprising a first coating layer coating the core, a second coating layer coating the first coating layer, a third coating layer coating the second coating layer, and a fourth coating layer coating the third coating layer, the first coating layer comprising crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , the second coating layer comprising crystalline phosphate XPO 4 , the third coating layer comprising crystalline borate Y p B q O r , and the fourth coating layer comprising carbon;

Optionally, the step of coating the crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c comprises the following steps: adding a source of element M, a phosphorus source and an acid and optionally a lithium source to a solvent to obtain a coating solution, fully mixing the material to be coated with the coating solution, drying, and then sintering to obtain a material coated with the crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c ;

Optionally, the step of coating the crystalline phosphate XPO 4 comprises the following steps: adding a source of element X, a phosphorus source and an acid into a solvent to obtain a coating solution, fully mixing the material to be coated with the coating solution, drying, and then sintering to obtain a material coated with the crystalline phosphate XPO 4 ;

Optionally, the step of coating the crystalline borate Y p B q O r comprises the following steps: adding a source of element Y and a boron source into a solvent to obtain a coating solution, fully mixing the material to be coated with the coating solution, drying, and then sintering to obtain a material coated with the crystalline borate Y p B q O r ;

Optionally, the step of coating carbon includes the following steps: adding a carbon source into a solvent to obtain a coating solution, adding a material to be coated into the coating solution, mixing evenly, drying, and then sintering to obtain a carbon-coated material.
The preparation method according to claim 15, wherein

In the step of coating crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c , the pH of the solution containing the source of element M, the phosphorus source, the acid and optionally the lithium source is controlled to be 3.5 to 6.5, and then stirred and reacted for 1 to 5 hours, and then the solution is heated to 50° C. to 120° C. and maintained at this temperature for 2 to 10 hours; and/or,

The sintering in the step of coating the crystalline pyrophosphate Li a MP 2 O 7 and/or M b (P 2 O 7 ) c is performed at 650° C. to 800° C. for 2 to 6 hours; and/or,

In the step of coating the crystalline phosphate XPO 4 , the source of element X, the phosphorus source and the acid are dissolved in a solvent, stirred and reacted for 1 to 10 hours, and then the solution is heated to 60° C. to 150° C. and maintained at the temperature for 2 to 10 hours; and/or,

The sintering in the step of coating the crystalline phosphate XPO 4 is carried out at 500° C. to 700° C. for 6 to 10 hours; and/or,

The sintering in the step of coating the crystalline borate YpBqOr is performed at 300°C to 500° C for 2 hours to 10 hours; and/or,

The sintering in the carbon coating step is performed at 700° C. to 800° C. for 6 hours to 10 hours.
A positive electrode sheet, comprising a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, the positive electrode film layer comprising the positive electrode active material described in any one of claims 1 to 11 or the positive electrode active material prepared by the preparation method described in any one of claims 12 to 16, and the content of the positive electrode active material in the positive electrode film layer is more than 10 weight%, and can be optionally 90 weight% to 99.5 weight%, based on the total weight of the positive electrode film layer.
A secondary battery comprises the positive electrode active material according to any one of claims 1 to 11, or the positive electrode active material prepared by the preparation method according to any one of claims 12 to 16, or the positive electrode sheet according to claim 17.
An electrical device comprising the secondary battery according to claim 18.