CN115663118A

CN115663118A - Positive electrode plate, secondary battery and power utilization device

Info

Publication number: CN115663118A
Application number: CN202211428686.5A
Authority: CN
Inventors: 季姣燕
Original assignee: Sunwoda Electric Vehicle Battery Co Ltd
Current assignee: Sunwoda Electric Vehicle Battery Co Ltd
Priority date: 2022-11-15
Filing date: 2022-11-15
Publication date: 2023-01-31

Abstract

The application discloses a positive pole piece,A secondary battery and an electric device. The positive pole piece comprises a positive active material, wherein the positive active material comprises manganese-iron-lithium oxide and a ternary material; the positive pole piece satisfies the following conditions: 46.7<Z/X<189, wherein Z is the active specific surface area of the positive pole piece and is expressed in cm ² And X is the ratio of the mass of the ternary material to the mass of the positive electrode active material. The composition of the positive active material and the active specific surface area are controlled, and the high-temperature cycle performance of the battery is improved.

Description

Positive electrode plate, secondary battery and power utilization device

Technical Field

The application relates to the technical field of electrochemistry, in particular to a positive pole piece, a secondary battery and an electric device.

Background

Lithium manganese iron phosphate (LMFP) is a promising positive electrode material in secondary batteries because of its ability to function as a cathode materialHas the characteristics of high voltage window, excellent energy density and good cycle stability, but when LMFP is taken as a positive electrode material, mn is used ³⁺ /Mn ²⁺ Intrinsic low electron/ion conductivity and due to Mn ³⁺ /Mn ²⁺ The two-phase reaction has larger lattice mismatch and is more influenced by polarization. On the other hand, it may be due to Mn ³⁺ John-Teller Effect of (1) to produce Mn ²⁺ And Mn ⁴⁺ Mn is easy to dissolve out, mn in the LMFP electrode material is accelerated to dissolve in the electrolyte at high temperature, and decomposed Mn ²⁺ The electrolyte is conducted through the electrolyte and deposited on the surface of graphite of the negative electrode of the battery, an SEI film is damaged in the subsequent circulation process, the SEI film is continuously regenerated and repaired, and a large amount of active lithium is consumed. Meanwhile, the positive electrode material generates a manganese-deficient phase due to manganese dissolution, and lithium ion diffusion is hindered in the subsequent charging and discharging processes, so that the polarization of the battery is increased, the capacity loss is increased, and the results of fast high-temperature capacity attenuation and poor capacity retention rate in the charging and discharging cycle process are caused.

Disclosure of Invention

The purpose of the present application is to provide a positive electrode sheet, a secondary battery and an electric device, which can improve the high-temperature cycle performance of the battery by controlling the composition of a positive electrode active material and the active specific surface area.

Based on the above, the present application provides a positive electrode plate, which includes a positive active material, wherein the positive active material includes a manganese iron lithium oxide and a ternary material; the positive pole piece satisfies the following conditions: 46.7<Z/X<189, wherein Z is the active specific surface area of the positive pole piece and is expressed in cm ² And X is the ratio of the mass of the ternary material to the mass of the positive electrode active material.

Further, X is more than 0.3 and less than 0.9.

Further, 44.8 < Z < 56.7.

Further, the chemical formula of the ternary material includes Li _a Ni _x Co _y M _z A _e O ₂ M comprises Mn and Al, wherein a is more than or equal to 0.8 and less than 1.2, x is more than or equal to 0.3 and less than or equal to 1, Y is more than or equal to 0.1 and less than or equal to 0.3, z is more than or equal to 0.1 and less than or equal to 0.4, e is more than or equal to 0 and less than or equal to 0.1, A comprises Zr, sr, ti, B, mg, sn, W, Y, ba, nb,At least one of Mo, ta, si, la, er, nd, gd and Ce.

Furthermore, the ternary material has a layered structure, and x is more than or equal to 0.6 and less than or equal to 0.7.

Further, the chemical formula of the lithium manganese iron oxide includes Li _c Mn _b Fe _1-b PO ₄ Wherein b is more than 0 and less than 1, c is more than 0.8 and less than 1.2.

Further, 104<Z*PD<177, wherein PD is the compacted density of the positive pole piece and has the unit of g/cm ³ 。

Further, PD is more than or equal to 2.2 and less than or equal to 3.5.

The application also provides a secondary battery, which comprises the positive pole piece.

The application also provides an electric device, and the electric device comprises the secondary battery, and the secondary battery is used as a power supply of the electric device.

The beneficial effect of this application lies in: compared with the prior art, the secondary battery comprises a positive active material, wherein the positive active material comprises manganese-iron-lithium oxide and a ternary material; the positive pole piece satisfies the following conditions: 46.7<Z/X<189, Z is the active specific surface area of the anode pole piece, and the unit is cm ² And X is the ratio of the mass of the ternary material to the mass of the positive electrode active material. The application improves the high-temperature cycle performance of the battery while reducing the side reaction of the anode active material and the electrolyte, reduces the polarization of the battery, and stabilizes the high-capacity release of the battery.

Detailed Description

The technical solutions in the embodiments of the present application are clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present application, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application. In addition, in the description of the present application, the term "including" means "including but not limited to". Various embodiments of the present application may exist in a range of forms; it is to be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the application; accordingly, the described range descriptions should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, it is contemplated that the description of a range from 1 to 6 has specifically disclosed sub-ranges such as, for example, from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as individual numbers within a range such as, for example, 1, 2, 3, 4, 5, and 6, as applicable regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any number (fractional or integer) recited within the range so indicated.

In order to solve the problems of fast capacity attenuation and poor rate capability of a secondary battery in the prior art, the embodiment of the application provides a positive pole piece. It should be noted that the following description of the embodiments is not intended to limit the preferred order of the embodiments.

The positive pole piece comprises a positive active material, wherein the positive active material comprises manganese-iron-lithium oxide and a ternary material; the positive pole piece satisfies the following conditions: 46.7<Z/X<189 wherein Z is the active specific surface area of the positive pole piece and the unit is cm ² And/g, X is the ratio of the mass of the ternary material to the mass of the positive electrode active material.

In the process of charging the battery, active substances of the positive pole piece, such as lithium ions, sodium ions and the like, are oxidized under the action of an electric field to release electrons, complete charge exchange on the surface of the positive active material, enter the electrolyte, pass through the diaphragm, migrate to the negative pole and are embedded into the active substances of the negative pole. In the embodiment, the manganese-iron-lithium oxide and the ternary material are mixed to be used as the positive active material of the positive pole piece, compared with the conventional manganese-iron-lithium oxide, the appropriate mixing ratio of the manganese-iron-lithium oxide and the ternary material can improve the active specific surface area of the positive pole piece, reduce the side reaction of the manganese-iron-lithium oxide and the electrolyte, and meanwhile, the ternary material can also increase the conductivity of the positive pole piece, reduce the polarization phenomenon of the battery, stabilize the high-capacity performance of the battery and improve the high-temperature cycle performance of the battery.

The active specific surface area of the positive pole piece is different from that of the positive pole piece in the traditional technology, the active specific surface area of the positive pole piece is the active specific surface area of the positive pole piece when the positive pole piece is actually reacted in the charging and discharging process, the number of active sites of the positive pole piece during charging and discharging can be accurately represented, the size of the active specific surface area of the positive pole piece is related to the type, the particle size and the specific surface area of a positive active material, the porosity and the roughness of the positive pole piece are related to the porosity and the roughness of the positive pole piece, and the porosity and the roughness of the positive pole piece can be adjusted by changing the proportion of each component (such as the positive active material, a binder and a conductive agent) in the pulping process or the size of cold pressing pressure. In the positive pole piece of the embodiment, when the Z/X ratio is 46.7-189, the high-temperature cycle performance of the battery can be improved, the polarization phenomenon of the battery is reduced, and the high-capacity release of the battery is stabilized. Wherein Z represents the active specific surface area of the positive pole piece, and X represents the ratio of the mass of the ternary material to the mass of the positive pole active material. When the ratio of Z to X is more than 189, the electrochemical activity of the positive pole piece is too high, the active particles are contacted with the electrolyte too much, side reactions are increased, a large amount of active lithium is consumed, and the periphery of the ternary material is completely coated by the manganese-iron-lithium oxide with poor conductivity when the mixing proportion of the ternary material is small, so that the diffusion of lithium ions is indirectly hindered. When the ratio of Z to X is less than 46.7, the cost in preparing the battery is high and the high-temperature cycle performance of the battery is affected.

To further enhance the electrochemical performance of the cell, in some embodiments, 49.8 plus Z/X <133, further Z/X is at a value in the range of any one or any two of 49.8, 65.4, 90.5, 129.6, 133.

The larger the active specific surface area of the positive pole piece is, the more active sites in the positive pole piece are, the faster the exchange speed of active substances and electrons is, and the better the dynamic performance of the battery is, but too many active sites can increase the side reaction of active materials and electrolyte, and deteriorate the cycle performance of the battery, while too few active sites can cause the reduction of chemical reaction in a battery system, and cause the deterioration of the capacity and the cycle performance of the battery. Based on this, in some embodiments of the present application, 44.8 < Z < 56.7.

In this example, the activity of the positive electrode sheetWhen the specific surface area is larger than 56.7, the electrochemical activity of the positive pole piece is too high, and the contact of active particles and electrolyte is too much, so that the side reaction is increased, a large amount of active lithium is consumed, and the performance of the battery is deteriorated. When the active specific surface area of the positive pole piece is less than 44.8cm ² At/g, the chemical reaction in the battery system is reduced, the efficiency of lithium ion deintercalation is reduced, and the battery capacity and cycle performance are reduced.

In order to maintain the chemical reaction rate in the cell in a more reasonable range so that the cycling performance of the cell is maintained in a more optimal state, in some embodiments, 47.8 < Z < 50.8.

In some embodiments, 0.3 < X < 0.9. The ratio X of the mass of the lithium iron manganese oxide to the mass of the positive active material is controlled within the range, so that the compatibility of the positive material and the electrolyte can be improved, the side reaction of the lithium iron manganese oxide and the electrolyte is reduced, the conductivity of a positive pole piece can be improved, and the polarization of the battery is reduced. In order to further enable the positive pole piece to have more reasonable conductivity and active sites and improve the performance of the battery, in some embodiments, X is more than 0.4 and less than 0.8.

In some embodiments, the ternary material has a chemical formula comprising Li _a Ni _x Co _y M _z A _e O ₂ M comprises Mn and Al, wherein a is more than or equal to 0.8 and less than 1.2, x is more than or equal to 0.3 and less than or equal to 1, Y is more than or equal to 0.1 and less than or equal to 0.3, z is more than or equal to 0.1 and less than or equal to 0.4, e is more than or equal to 0 and less than or equal to 0.1, A comprises at least one of Zr, sr, ti, B, mg, sn, W, Y, ba, nb, mo, ta, si, la, er, nd, gd and Ce.

In some embodiments, the ternary material has a layered structure. The ternary material has the advantages of high voltage platform, high energy density, high tap density, excellent conductivity, mature production process and the like, and can effectively improve the electrochemical performance of the battery and reduce the production cost. In addition, the ternary material with the layered structure has better stability, and the cycle performance and the safety performance of the battery are improved. In some embodiments, x is greater than or equal to 0.6 and less than or equal to 0.7, and the nickel content is kept in the range, so that the cycling stability of the battery can be further improved, the comprehensive performance of the battery is improved, if the nickel content is too low, the capacity of the battery is reduced, and if the nickel content is too high, lithium and nickel in the ternary material are seriously mixed and arranged, so that the cycling stability of the battery is influenced.

In some embodiments, the formula of the lithium iron manganese oxide includes Li _c Mn _b Fe _1-b PO ₄ Wherein b is more than 0 and less than 1, c is more than 0.8 and less than 1.2. The lithium manganese iron oxide comprises lithium manganese iron phosphate, the lithium manganese iron phosphate has the energy density advantage and the low-temperature performance advantage compared with lithium iron phosphate, and compared with ternary materials, the lithium manganese iron phosphate has an olivine structure, is more stable in structure during charging and discharging, and has better safety and cycle stability compared with ternary materials. The blending of the lithium iron manganese phosphate and the ternary material can give consideration to the energy density and stability of the material, and is beneficial to improving the performance of the battery.

In some embodiments, 104<Z*PD<177, wherein PD is the compacted density of the positive pole piece and has the unit of g/cm ³ . In the embodiment, the value of Z PD is controlled to be 104-177, so that the long-term cycle performance of the battery can be effectively improved. In some embodiments, Z × PD takes on the value: 104. 130, 145, 160, 177, or any range of two values.

In some embodiments, 2.2 ≦ PD ≦ 3.5. The compaction density of the positive pole piece is controlled within the range, and the energy density of the positive pole piece and the infiltration performance of the positive pole piece to electrolyte can be considered, so that the energy density of the battery can be effectively improved, and the polarization of the battery can be reduced.

In some embodiments, the positive electrode sheet includes a positive electrode current collector, and the positive electrode active material is disposed on at least one surface of the positive electrode current collector. The positive electrode current collector may be selected according to actual requirements, and is preferably an aluminum foil.

In some embodiments, the positive electrode sheet further includes a conductive agent and a binder, and the types and contents of the conductive agent and the binder are not particularly limited and can be selected according to actual needs. In some embodiments, the conductive agent may include conductive carbon black, carbon nanotubes, graphene, and the like, and the binder may include polyvinylidene fluoride.

The application also provides a secondary battery, which comprises a positive pole piece, a negative pole piece, a diaphragm and electrolysisAnd (4) liquid. The positive pole piece comprises a positive active material, and the positive active material comprises manganese-iron-lithium oxide and a ternary material; the positive pole piece satisfies the following conditions: 46.7<Z/X<189 wherein Z is the active specific surface area of the positive pole piece and the unit is cm ² And X is the ratio of the mass of the ternary material to the mass of the positive electrode active material.

In some embodiments, the negative electrode tab includes a negative electrode current collector and a negative active material, a binder, and a conductive agent overlying the negative electrode current collector. The kind and content of the negative electrode active material, the binder, and the conductive agent are not particularly limited and may be selected according to actual needs.

In some embodiments, the negative electrode material may be selected from one or more of graphite, soft carbon, hard carbon, carbon fiber, silicon-based material, tin-based material.

In some embodiments, the negative electrode material is graphite.

In some embodiments, the main components of the electrolyte include a lithium salt, an organic solvent, and an additive. The kind and composition of the lithium salt and the organic solvent are not particularly limited, and may be selected according to actual requirements. The lithium salt may include lithium hexafluorophosphate, lithium bis-fluorosulfonylimide, and the like, the solvent may include ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, propyl propionate, and the like, and the additive may include lithium difluorophosphate, lithium bis-oxalato-borate, succinonitrile, and the like.

In some embodiments, the separator material may be any common separator material, such as a separator made of polyethylene, polypropylene, polyvinylidene fluoride, and various composite films.

In some embodiments, the separator material is a polypropylene film.

In some embodiments, the method of manufacturing the secondary battery described above includes the steps of:

the positive pole piece, the isolation film and the negative pole piece are sequentially stacked, the isolation film is positioned between the positive pole piece and the negative pole piece to play a role of isolation, the positive pole piece and the negative pole piece are wound into a bare cell and then are filled into a shell, the bare cell is baked at 65-95 ℃ to remove water, then electrolyte is injected and the shell is sealed, and the secondary battery is obtained after the working procedures of standing, hot cold pressing, formation, clamping, capacity grading and the like.

The present application further provides an electric device including the secondary battery described above.

In some embodiments, the powered device of the present application is, but not limited to, a backup power source, an electric motor, an electric car, an electric motorcycle, a power assisted bicycle, a bicycle, an electric tool, a household large battery, and the like.

In the application, the active specific surface area of the positive electrode plate can be obtained by the following test method, and the test method comprises the following steps:

assembling a positive pole piece to be tested, a reference electrode, a negative pole piece and electrolyte into a button half cell, wherein the electrolyte contains electrochemical redox probe molecules with the concentration of C, the redox potential of the probe molecules is 2-4V, and the probe molecules are dissolved in the electrolyte;

testing a series of linear sweep voltammetry curves of different button type half cells at different sweep speeds V by using an electrochemical workstation to obtain oxidation peak current i _p (Oxidation) and reduction Peak Current i _p (reduction) and then in the resulting series of peak currents i of the button half-cells _p Plotting the square root of the sweep speed V to obtain a slope K; according to Randles-Sevick equation ip =2.69 × 10 ⁵ *n ^2/3 cD ^1/ ² AV ^1/2 Wherein ip is peak current, n is electron transfer number, D is diffusion coefficient, D (oxidation) =1.41 x 10 ^-6 cm ² (s, D (reduction) =1.26 x 10) ^-6 cm ² And v is the sweep rate, A is the active surface area of the positive pole piece, and c is the concentration of the probe molecules. The active specific surface area of the positive pole piece is obtained by dividing the active surface area A of the positive pole piece by the weight m of the positive pole piece.

When a certain potential is applied, electrons are transferred to active sites on the surface of an active material in the positive pole piece through the current collector, and then the probe molecules undergo an oxidation-reduction reaction.

In the above test method, the three electrodes are glassy carbon electrodes. The electrolyte is common electrolyte, and comprises electrolyte salt and organic solvent; including one or more of cyclic carbonate, carboxylate and the like.

In the above test method, the redox potential of the probe molecule is 3V to 4V. The concentration of the probe molecules is 0.03 mol/L-0.08 mol/L; further preferably 0.05mol/L.

In the test method, the probe molecule is one of ruthenium hexammoniate, ferric chloride and ferrocene, and is preferably ferrocene, because ferrocene has the advantages of excellent reversibility in a battery system due to low oxidation-reduction potential (3V-3.4V), no influence of a positive electrode system and a negative electrode system on the peak position and the like, the method obtains peak currents of an oxidation peak and a reduction peak by testing a cyclic voltammetry curve of a ferrocene solution on the surface of an active substance at different sweeping speeds, and obtains a slope by fitting the current i and the sweeping speed V, thereby calculating the reaction activity specific surface area of the pole piece. The method takes iron ions as pointer elements, oxidation/reduction sites of the iron ions on the surface of a pole piece are taken as pole piece active sites, and a linear relation is established according to Randles-Sevick equation and cyclic voltammetry curve to obtain the active reaction area.

In the test method, the sweep speed is 0.01mv/s to 3.50mv/s, the sweep voltage range is 2.9 to 3.5v, D (oxidation) =1.41 x 10 ^-6 cm ² (s, D (reduction) =1.26 x 10) ^-6 cm ² /s。

The present application has been repeated several times, and the present invention will now be described in further detail with reference to some test results, which will be described in detail below with reference to specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present application.

Example 1

Preparing a positive pole piece: dv50 are LMFP (LiMn) of 5 μm and 8 μm, respectively _0.5 Fe _0.5 PO ₄ ) Ternary materials (LiNi) _0.5 Co _0.2 Mn _0.3 O ₂ ) Mixing the materials according to the mass ratio of 7: 2.3:1.2, mixing, adding NMP (N-methyl pyrrolidone) serving as a solvent, stirring in vacuum until the system is uniform to obtain anode slurry, uniformly coating the anode slurry on the upper surface and the lower surface of an aluminum foil of an anode current collector, airing at room temperature, transferring the anode slurry to an oven, and carrying out cold pressing (cold pressing)The pressure is 1.70 MPa), and the positive pole piece is obtained after the slitting process, wherein the compaction density of the obtained positive pole piece is 3.0g/cm ³ The active specific surface area of the positive pole piece is 56.7cm ² /g。

Preparing a negative pole piece: the method comprises the following steps of mixing graphite serving as a negative electrode active material, a conductive agent CNT, a thickening agent CMC and a binder SBR according to a mass ratio of 96.5:0.8:0.9:1.8, mixing, adding solvent deionized water, stirring in vacuum until the system is uniform to obtain negative electrode slurry, then uniformly coating the negative electrode slurry on the upper surface and the lower surface of a negative current collector copper foil, airing at room temperature, transferring the negative current collector copper foil to an oven for continuous drying, and obtaining a negative electrode plate after cold pressing and slitting, wherein the compaction density is 1.6g/cm ³ 。

Preparing an electrolyte: lithium hexafluorophosphate using ethylene carbonate as solvent was prepared as an electrolyte

Preparation of lithium ion secondary battery: and (3) stacking the positive pole piece, the negative pole piece and the polyethylene film isolating film according to the sequence of the positive pole piece, the diaphragm and the negative pole piece, winding, encasing, packaging, drying, adding an electrolyte, and performing vacuum packaging, standing, formation, capacity grading and other processes to obtain the lithium ion secondary battery.

Example 2

The preparation method is the same as that in example 1, except that in the process of preparing the positive pole piece, the mass ratio of the LMFP to the ternary material is 6: 2.35:1.15, the cold pressing pressure is 1.74MPa.

Example 3

The preparation method is the same as that in example 1, except that in the process of preparing the positive pole piece, the mass ratio of the LMFP to the ternary material is 5:2.4:1.1, and the cold pressing pressure is 1.78MPa.

Example 4

The preparation method is the same as that in example 1, except that in the process of preparing the positive pole piece, the mass ratio of the LMFP to the ternary material is 3: 2.45:1.05 and the cold pressing pressure is 1.84MPa.

Example 5

The preparation method is the same as that in example 1, except that in the process of preparing the positive pole piece, the mass ratio of the LMFP to the ternary material is 2: 2.5:1.0, and the cold pressing pressure is 1.88MPa.

Example 6

The preparation method is the same as that in example 1, except that in the process of preparing the positive pole piece, the mass ratio of the LMFP to the ternary material is 4: 2.55:0.95 and the cold pressing pressure is 1.91MPa.

Example 7

The preparation method is the same as that in example 1, except that in the process of preparing the positive pole piece, the mass ratio of the LMFP to the ternary material is 1: 2.6:0.9 and the cold pressing pressure is 1.92MPa.

Example 8 to example 14:

the preparation method is the same as example 3, except that: and (3) obtaining the positive pole pieces with different compaction densities by adjusting the coating weight of the positive active material.

Example 15:

the preparation method is the same as that of the embodiment 1, except that in the preparation process of the positive pole piece, the ternary material is selected from LiNi _1/3 Co _1/3 Mn _1/3 O ₂ 。

Example 16:

the preparation method is the same as that of the embodiment 1, except that in the preparation process of the positive pole piece, the ternary material is selected from LiNi _0.8 Co _0.1 Mn _0.1 O ₂ 。

Example 17:

the preparation method is the same as that of the embodiment 1, except that in the preparation process of the positive pole piece, the ternary material is selected from LiNi _0.6 Co _0.2 Mn _0.2 O ₂ 。

Example 18:

the preparation method is the same as that of the embodiment 1, except that in the preparation process of the positive pole piece, the ternary material is selected from LiNi _0.7 Co _0.2 Mn _0.1 O ₂ 。

Example 19: the preparation method is the same as that of the embodiment 1, except that in the preparation process of the positive pole piece, the ternary material is selected from LiNi _0.4 Co _0.2 Mn _0.4 O ₂ 。

Comparative example 1:

the preparation method is the same as that of example 1, except that the positive pole piece does not contain a ternary material.

Comparative example 2:

the preparation method is the same as example 1, except that the mass ratio of the LMFP to the ternary material is 5.

Comparative example 3:

the preparation method is the same as example 1, except that the mass ratio of the LMFP to the ternary material is 4.

High-temperature cycle performance test of lithium ion secondary batteries:

and (2) standing the lithium ion secondary battery at 45 ℃ for 10 minutes, carrying out constant current charging to 4.25V at a rate of 1C, then carrying out constant voltage charging until the current is less than or equal to 0.05C, standing for 10 minutes, and then carrying out constant current discharging to 2.8V at a rate of 1C, wherein the discharge capacity is a charge-discharge cycle, and the discharge capacity at the time is recorded as the discharge capacity of the 1 st cycle of the lithium ion secondary battery. The lithium ion secondary battery was subjected to 500-cycle charge/discharge tests in accordance with the above-described method, and the discharge capacity at the 500 th cycle was recorded. The capacity retention (%) of the lithium ion secondary battery after 500 cycles of 1C/1C at 45 ℃ was = the discharge capacity at 500 cycles/the discharge capacity at 1 cycle × 100%.

The testing method of the internal resistance of the lithium ion secondary battery comprises the following steps:

the lithium ion secondary battery is placed at 25 ℃ for 5 minutes, is charged to 4.2V at a constant current of 1C rate, is charged at a constant voltage until the current is less than or equal to 0.05C, and the state of charge (SOC) of the battery is 100 percent, is placed for 5 minutes, is discharged at a constant current of 1C rate, and is adjusted to 50 percent. The lithium ion secondary battery, 50% SOC, was left to stand for 10 minutes and was subjected to constant current discharge at a rate of 4C for 30 seconds. The voltage U1 at the last 1 second, the voltage U2 at 4C-rate constant current discharge for the last 1 second, and the current I at 4C-rate constant current discharge were recorded. Internal resistance of the lithium ion secondary battery = (U2-U1)/I.

TABLE 1 parameters and test results for examples 1 to 19 of the present application, and comparative examples 1 to 3

And (4) analyzing results: the lithium iron manganese phosphate and the ternary material are mixed to serve as active materials of the positive pole piece, and the high-temperature cycle performance of the battery can be obviously improved. And when Z/X is in the range of 46.7 to 189, the cycle performance of the battery is more excellent, and if Z/X is out of the above range, such as comparative examples 2 and 3, the high temperature cycle performance of the battery is significantly deteriorated.

In the foregoing embodiments, the descriptions of the respective embodiments have respective emphasis, and for parts that are not described in detail in a certain embodiment, reference may be made to related descriptions of other embodiments.

The positive electrode plate, the secondary battery and the electric device provided by the embodiment of the present application are introduced in detail, and a specific example is applied to explain the principle and the implementation manner of the present application, and the description of the above embodiment is only used to help understand the method and the core idea of the present application; meanwhile, for those skilled in the art, according to the idea of the present application, there may be variations in the specific embodiments and the application scope, and in summary, the content of the present specification should not be construed as a limitation to the present application.

Claims

1. The positive pole piece is characterized by comprising a positive active material, wherein the positive active material comprises ferromanganeseLithium oxide and ternary materials; the positive pole piece meets the following requirements: 46.7<Z/X<189 wherein Z is the active specific surface area of the positive pole piece and the unit is cm ² And X is the ratio of the mass of the ternary material to the mass of the positive electrode active material.

2. The positive electrode sheet according to claim 1, wherein 0.3 < X < 0.9.

3. The positive electrode sheet according to claim 1, wherein 44.8 < Z < 56.7.

4. The positive electrode sheet according to claim 1, wherein the chemical formula of the ternary material comprises Li _a Ni _x Co _y M _z A _e O ₂ M comprises Mn and Al, wherein a is more than or equal to 0.8 and less than 1.2, x is more than or equal to 0.3 and less than or equal to 1, Y is more than or equal to 0.1 and less than or equal to 0.3, z is more than or equal to 0.1 and less than or equal to 0.4, e is more than or equal to 0 and less than or equal to 0.1, A contains at least one of Zr, sr, ti, B, mg, sn, W, Y, ba, nb, mo, ta, si, la, er, nd, gd and Ce.

5. The positive electrode sheet according to claim 4, wherein the ternary material has a layered structure, x is 0.6-0.7.

6. The positive electrode sheet of claim 1, wherein the chemical formula of the lithium iron manganese oxide comprises Li _c Mn _b Fe _1-b PO ₄ Wherein b is more than 0 and less than 1, c is more than 0.8 and less than 1.2.

7. The positive electrode tab according to any one of claims 1 to 6, wherein 104 is<Z*PD<177, wherein PD is the compacted density of the positive pole piece and has the unit of g/cm ³ 。

8. The positive electrode plate as claimed in claim 6, wherein PD is 2.2. Ltoreq. PD.ltoreq.3.5.

9. A secondary battery comprising the positive electrode sheet according to any one of claims 1 to 8.

10. An electric device, characterized by comprising the secondary battery according to claim 9 as a power supply source for the electric device.