CN115513513A

CN115513513A - Secondary battery and electric equipment

Info

Publication number: CN115513513A
Application number: CN202211320303.2A
Authority: CN
Inventors: 陈庆; 曾城
Original assignee: Sunwoda Electric Vehicle Battery Co Ltd
Current assignee: Sunwoda Electric Vehicle Battery Co Ltd
Priority date: 2022-10-26
Filing date: 2022-10-26
Publication date: 2022-12-23

Abstract

The embodiment of the application discloses secondary battery and consumer, secondary battery is through adding first lithium salt and the cooperation of manganese iron lithium oxide positive pole active material in electrolyte, and in battery charge-discharge cycle in-process, first lithium salt in the electrolyte takes place reduction reaction in preference to organic solvent and cosolvent in the electrolyte on negative pole active material layer, constructs the SEI layer that one deck ionic conductivity is higher for the negative pole piece, restraines Mn ²⁺ And deposition on the SEI film layer is carried out, so that the dissolution of Mn in the positive active material is avoided, and the high-temperature cycle and high-temperature storage capacity of the secondary battery are improved.

Description

Secondary battery and electric equipment

Technical Field

The application relates to the technical field of batteries, in particular to a secondary battery and electric equipment.

Background

The LMFP (lithium manganese iron phosphate) is a new-generation positive electrode material developed on the basis of the LFP (lithium iron phosphate), and although the LMFP and the LFP have the same specific capacity density, the LMFP has higher working voltage, so the energy density of the LMFP is improved to a certain extent.

LMFP has lower electron conductivity and ion conductivity, which causes greater polarization, and due to Mn ³⁺ /Mn ² ⁺ The two-phase reaction has larger lattice mismatch and is greatly influenced by polarization. The Zingiber (John-Teller) effect of manganese ions causes manganese to dissolve out, reducing the cycle life of the batteryIts life is long. Mn due to the Jiangtai (John-Teller) effect ³⁺ Enriched on the surface of positive electrode particles and distorted MnO ₆ Octahedra, leading to lattice distortion and reduced structural stability, affecting stability and cyclability. Meanwhile, the dissolved manganese ions are subjected to reduction reaction at the negative electrode and are separated out, so that the SEI film is damaged, more active lithium is consumed in the repair process of the SEI film, and the cycle life of the battery is influenced. While the commonly used lithium salt LiPF in the electrolyte ₆ Lithium hexafluorophosphate is prone to the formation of HF (hydrofluoric acid) at high temperatures, and fluorosolvent FEC (fluoroethylene carbonate) also forms HF, which promotes the dissolution process of Mn. Meanwhile, the manganese-deficient phase is generated in the anode material due to the dissolution of manganese, the lithium ion diffusion kinetics is slowed down in the subsequent charging and discharging process, the polarization voltage of the secondary battery is increased, the irreversible capacity loss is caused, and the rapid capacity attenuation and the poor rate performance in the charging and discharging cycle process are finally caused.

Disclosure of Invention

The embodiment of the application provides a secondary battery and electric equipment, which can solve the problems of capacity attenuation and rate performance reduction caused by dissolution of manganese ions in the existing lithium iron manganese phosphate secondary battery.

A first aspect of the present application provides a secondary battery, including a positive electrode sheet and an electrolyte, wherein the positive electrode sheet includes a positive active material, and the positive active material includes a manganese iron lithium oxide; the total molar weight of manganese and iron in the lithium manganese iron oxide is 100%, the molar percentage of the manganese is n1%, the molar percentage of the iron is n2%, and n1 is more than n2; the electrolyte comprises a first lithium salt, and the first lithium salt comprises at least one of lithium nitrate, lithium nitrite, lithium borate, lithium phosphate, lithium fluoroborate, lithium sulfate and lithium chloride.

Optionally, the content of the first lithium salt is less than or equal to 1.5wt% based on the total mass of the electrolyte.

Optionally, the content of the first lithium salt is 0.1wt% to 1wt%.

Optionally, n1 is more than or equal to 60.

Optionally, n1-n2 is less than or equal to 40.

Optionally, the electrolyte further comprises a second lithium salt, the second lithium salt comprising lithium hexafluorophosphate; the mass ratio of the first lithium salt to the second lithium salt is (0.01-0.13): 1.

optionally, the mass ratio of the first lithium salt to the second lithium salt is (0.02-0.09): 1.

optionally, the mass ratio of the first lithium salt to the second lithium salt is (0.02-0.04): 1.

optionally, the electrolyte further comprises an organic solvent, and the organic solvent comprises cyclic carbonate and chain carbonate.

Optionally, the electrolyte further comprises a film forming additive, wherein the film forming additive comprises at least one of vinylene carbonate, 1,3-propane sultone and fluoroethylene carbonate.

Optionally, the electrolyte further includes a cosolvent, and the cosolvent includes at least one of an ether solvent, dimethyl sulfoxide, sulfolane, tetramethylurea, and an amide compound.

A second aspect of the present application provides a power consumer including the secondary battery provided in the first aspect of the present application, including but not limited to an electric vehicle, an energy storage device, and the like.

The beneficial effects of this application lie in, provide a secondary battery and consumer, secondary battery is through adding first lithium salt and the cooperation of lithium iron manganese phosphate positive active material in electrolyte, and at battery charge-discharge cycle in-process, the reduction reaction takes place on negative pole active material layer in preference to organic solvent and cosolvent in the electrolyte in the first lithium salt in the electrolyte, constructs the higher SEI layer of one deck ionic conductivity for negative pole piece, and the suppression Mn suppresses the more SEI layer of Mn ²⁺ And deposition on the SEI film layer is carried out, so that the dissolution of Mn in the positive active material is avoided, and the high-temperature cycle and high-temperature storage capacity of the secondary battery are improved.

Detailed Description

The technical solutions in the embodiments of the present application will be described clearly and completely below, and it should be understood 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. Furthermore, it should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the invention, are given by way of illustration and explanation only, and are not intended to limit the scope of the invention.

In the detailed description and claims, a list of items connected by the term "at least one of can mean any combination of the listed items. For example, if items a and B are listed, the phrase "at least one of a and B" means a only; only B; or A and B. In another example, if items A, B and C are listed, the phrase "at least one of A, B and C" means a only; or only B; only C; a and B (excluding C); a and C (excluding B); b and C (excluding A); or A, B and all of C. Item a may comprise a single element or multiple elements. Item B may comprise a single element or multiple elements. Item C may comprise a single element or multiple elements. At least one of the terms has the same meaning as at least one of the terms.

In the present specification, the numerical range indicated by the term "to" means a range including numerical values described before and after the term "to" as the minimum value and the maximum value, respectively.

The embodiment of the application provides a secondary battery and power consumption equipment, secondary battery is through adding first lithium salt and the cooperation of lithium iron manganese phosphate positive active material in electrolyte, and at battery charge-discharge cycle in-process, the reduction reaction takes place on negative pole active material layer in organic solvent and cosolvent in the electrolyte in preference to first lithium salt in the electrolyte, constructs the higher SEI layer of one deck ionic conductivity for the negative pole piece, restraines Mn ²⁺ And deposition on the SEI film layer is carried out, so that the dissolution of Mn in the positive active material is avoided, and the high-temperature cycle and high-temperature storage capacity of the secondary battery are improved.

In an embodiment of the present application, a secondary battery is provided, where the secondary battery includes a positive electrode plate, a negative electrode plate, a separator, an electrolyte, and a housing.

I. Positive pole piece

The positive pole piece comprises a positive current collector and a positive active material layer arranged on the positive current collector.

Positive electrode active material layer

The positive electrode active material layer may be one or more layers. Each of the multiple layers of positive electrode active material may contain the same or different positive electrode active material. The positive electrode active material is any substance capable of reversibly intercalating and deintercalating metal ions such as lithium ions.

In some embodiments, the positive electrode active material includes lithium iron manganese oxide, wherein the molar percentage of the manganese element is n1% and the molar percentage of the iron element is n2% based on 100% of the total molar amount of the manganese element and the iron element in the lithium iron manganese oxide, and n1> n2 is satisfied.

In some embodiments, n1 ≧ 60, then n2 ≦ 40.

In some embodiments, n1-n2 ≦ 40.

In some embodiments, the lithium iron manganese oxide comprises doped and/or clad lithium iron manganese phosphate.

In some embodiments, the general structural formula of the lithium iron manganese oxide includes Li _a M _y Mn _x Fe _1-x-y PO ₄ Wherein a is more than or equal to 0.95 and less than or equal to 1.1,0 and less than or equal to 0.05,0.55 and less than or equal to 0.8, and M comprises one or more of Mg, al, ti and Co.

In some embodiments, a is 1,y and 0,x is 0.6, the lithium iron manganese oxide in the positive electrode active material has the structural formula LiMn _0.6 Fe _0.4 PO ₄ (ii) a a is 1,y and 0,x is 0.7, the structural formula of the lithium iron manganese oxide in the positive electrode active material is LiMn _0.7 Fe _0.3 PO ₄ 。

In some embodiments, the lithium iron manganese oxide preferably has the formula LiMn _0.6 Fe _0.4 PO ₄ . The electron conductivity and the ion conductivity of the positive electrode active material with too high manganese content are low, and the Zingiber (John-Teller) effect of manganese is serious, namely, the structure of the positive electrode material can be damaged due to the high manganese content, and the positive electrode materialThe dissolution of manganese is more serious, an SEI layer of a negative electrode is damaged, the capacity retention rate and the rate capability of the secondary battery at 45 ℃ are influenced, and the improvement of the energy density of the battery is limited due to the fact that the content of manganese is too low.

In addition, the positive electrode active material layer also comprises a positive electrode conductive agent and a positive electrode binder.

Positive electrode conductive agent

The kind of the positive electrode conductive agent is not limited, and any known conductive agent can be used. Examples of the positive electrode conductive agent may include, but are not limited to, natural graphite, artificial graphite, and the like; carbon black such as acetylene black; carbon materials such as amorphous carbon such as needle coke; a carbon nanotube; graphene, and the like. The positive electrode conductive agents may be used alone or in any combination.

Positive electrode binder

The type of the positive electrode binder used for producing the positive electrode active material layer is not particularly limited, and in the case of the coating method, any material may be used as long as it is soluble or dispersible in the liquid medium used for producing the electrode. Examples of positive electrode binders may include, but are not limited to, one or more of the following: resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and cellulose nitrate; rubbery polymers such as Styrene Butadiene Rubber (SBR), nitrile Butadiene Rubber (NBR), fluororubber, isoprene rubber, and ethylene-propylene rubber; thermoplastic elastomer polymers such as styrene-butadiene-styrene block copolymers or hydrogenated products thereof, ethylene-propylene-diene terpolymers (EPDM), styrene-ethylene-butadiene-ethylene copolymers, styrene-isoprene-styrene block copolymers or hydrogenated products thereof; syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene- α -olefin copolymer, and other soft resinous polymers; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymer; and a polymer composition having ion conductivity of alkali metal ions (particularly lithium ions). The positive electrode binder may be used alone or in any combination thereof.

Positive current collector

The kind of the positive electrode current collector is not particularly limited, and it may be any material known to be suitable for use as a positive electrode current collector. Examples of the positive electrode current collector may include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc.; carbon materials such as carbon cloth and carbon paper; a composite of a polymer and a metal layer. In some embodiments, the positive current collector is a metallic material. In some embodiments, the positive current collector is aluminum.

The form of the positive electrode current collector is not particularly limited. When the positive electrode collector is a metal material, the form of the positive electrode collector may include, but is not limited to, a metal foil, a metal cylinder, a metal coil, a metal plate, a metal foil, a metal expanded metal, a stamped metal, a foamed metal, and the like. When the positive electrode collector is a carbon material, the form of the positive electrode collector may include, but is not limited to, a carbon plate, a carbon thin film, a carbon cylinder, and the like. In some embodiments, the positive current collector is a metal foil. In some embodiments, the metal foil is mesh-shaped. The thickness of the metal foil is not particularly limited. In some embodiments, the metal foil has a thickness greater than 1 μm, greater than 3 μm, or greater than 5 μm. In some embodiments, the metal foil has a thickness of less than 1mm, less than 50 μm, or less than 20 μm. In some embodiments, the thickness of the metal foil is within a range consisting of any two of the above values.

II. Electrolyte solution

The electrolyte includes a lithium salt, an organic solvent, a film-forming additive, and a cosolvent.

Lithium salt

In some embodiments, the lithium salt comprises a first lithium salt comprising at least one of lithium nitrate, lithium nitrite, lithium borate, lithium phosphate, lithium fluoroborate, lithium sulfate, lithium chloride, lithium fluoride.

In some embodiments, the first lithium salt is present in an amount less than or equal to 1.5wt%, based on the total mass of the electrolyte. Specifically, the content of the first lithium salt may be 0.1wt%, 0.2wt%, 0.3wt%, 0.5wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.2wt%, 1.3wt%, 1.5wt%, or a range consisting of any two thereof.

In some embodiments, the first lithium salt is present in an amount of 0.1wt% to 1.0wt%, and specifically, the first lithium salt may be present in an amount of 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, or any two thereof.

In some embodiments, the lithium salt further comprises a second lithium salt comprising lithium hexafluorophosphate; the mass ratio of the first lithium salt to the second lithium salt is (0.01-0.13): 1.

in some embodiments, the mass ratio of the first lithium salt to the second lithium salt is (0.02 to 0.09): 1.

in some embodiments, the mass ratio of the first lithium salt to the second lithium salt is (0.02 to 0.04): 1.

organic solvent

In some embodiments, the organic solvent includes cyclic carbonates and chain carbonates.

Specifically, the organic solvent is one or more mixed solvents of EC (ethylene carbonate), DEC (diethyl carbonate), DMC (dimethyl carbonate), PC (polycarbonate), and EMC (ethyl methyl carbonate). Among them, EC (ethylene carbonate) and PC (polycarbonate) are cyclic carbonates. DEC (diethyl carbonate), DMC (dimethyl carbonate) and EMC (ethyl methyl carbonate) are chain carbonates.

Film forming additive

In some embodiments, the film-forming additive comprises at least one of vinylene carbonate, 1,3-propane sultone, fluoroethylene carbonate (FEC). In some embodiments, fluoroethylene carbonate (FEC) is preferred.

Cosolvent

In some embodiments, the co-solvent comprises at least one of an ether solvent, dimethyl sulfoxide, sulfolane, tetramethylurea, an amide compound. The ether solvent includes, but is not limited to, at least one of tetraethylene glycol dimethyl ether, ethylene glycol dimethyl ether, propylene glycol methyl ether, and ethylene glycol monomethyl ether.

It will be appreciated that the choice of co-solvent includes, but is not limited to, the ranges described above, so long as it is capable of dissolving the first lithium salt. Compared with other options, the cosolvent has higher solubility for the first lithium salt, so that the first lithium salt can be better introduced into the electrolyte.

In the charge-discharge cycle process of the secondary battery, the first lithium salt in the electrolyte is subjected to a reduction reaction in preference to the organic solvent on the negative electrode plate, so that an SEI (solid electrolyte interphase) layer with higher ionic conductivity is constructed for the negative electrode plate, and the SEI layer can prevent Mn (manganese) in the electrolyte ²⁺ The deposition of the positive electrode material LMFP can cause the structural damage caused by the dissolution of Mn in the positive electrode material LMFP, and the lithium loss and the gas generation caused by the fracture and the repair of an SEI layer can be avoided.

In a secondary battery with a negative active material comprising graphite and a positive active material comprising lithium manganese iron phosphate (LMFP), a first lithium salt is used for electronic insulation of an SEI layer formed by a negative pole piece, so that manganese can be prevented from depositing on the surface of the negative active material layer and damaging the SEI layer on the surface of the negative active material layer. The accessible is carried out EDS scanning and is seen having the existence of manganese metal to negative pole piece surface to observe the secondary cell that this application embodiment provided to Mn ²⁺ The inhibiting effect of the deposition of (2).

By adding a first lithium salt (such as lithium nitrate), a film-forming additive and a cosolvent, a nitrogen-rich SEI layer is generated at the position of a negative pole piece, and Mn in the electrolyte can be reduced ²⁺ The deposition of the anode plate and the contact of the cathode plate and the electrolyte are isolated, so that a large number of side reactions are reduced, and the generation of gas is inhibited. Wherein LiNO is ₃ The organic solvent introduced into the electrolyte through the co-solvent is more, and the lithium salt (e.g., lithium nitrate) is contained at a higher concentration. The higher the concentration of lithium nitrate is, the more compact the SEI layer is generated, and the better the protection effect on the negative electrode is. But the SEI layer is too dense to facilitate lithium ion diffusion, and the rate performance of the battery is reduced. After the lithium nitrate is added, a better SEI layer is generated, and Mn is reduced ²⁺ So as to avoid the dissolution of Mn, SEI protects the negative electrode and avoids the deposition of Mn ions, and the concentration of Mn ions in the electrolyte reaches saturation, so that the electrolyte is difficult to continuously dissolve the Mn ionsThereby avoiding the dissolution of Mn in the anode, stabilizing the structural stability of the LMFP anode material, and being beneficial to the performance exertion of LMFP and the improvement of the high-temperature cycle and high-temperature storage performance of the battery.

III, negative pole piece

The negative pole piece comprises a negative pole current collector and a negative pole active material layer arranged on at least one surface of the negative pole current collector, wherein the negative pole active material layer contains a negative pole active material, and the negative pole active material contains graphite.

The negative pole piece is a single-sided pole piece or a double-sided pole piece, when the negative pole piece is the single-sided pole piece, the negative active material layer is arranged on one surface of the negative current collector, and when the negative pole piece is the double-sided pole piece, the negative active material layer is arranged on two surfaces of the negative current collector. The negative pole piece can also have a single-sided negative pole piece area and a double-sided negative pole piece area.

Negative current collector

In some embodiments, the negative current collector includes, but is not limited to, metal foil, metal cylinder, metal coil, metal plate, metal film, metal expanded metal, stamped metal, foamed metal, and the like. In some embodiments, the negative current collector is a metal foil. In some embodiments, the negative current collector is a copper foil. As used herein, the term "copper foil" includes copper alloy foils.

In some embodiments, the negative electrode current collector is a conductive resin. In some embodiments, the conductive resin includes a film obtained by evaporating copper on a polypropylene film.

Negative electrode active material layer

The anode active material layer may be one or more layers, and each of the plurality of anode active material layers may include the same or different anode active materials. The negative electrode active material is any substance capable of reversibly intercalating and deintercalating metal ions such as lithium ions. In some embodiments, the chargeable capacity of the negative active material is greater than the discharge capacity of the positive active material to prevent lithium metal from being precipitated on the negative electrode sheet during charging.

In some embodiments, the thickness of the negative electrode active material layer refers to the thickness of the negative electrode active material layer coated on a single side of the negative electrode current collector. In some embodiments, the thickness of the single-sided negative active material layer is 15 μm or more. In some embodiments, the thickness of the single-sided negative electrode active material layer is 20 μm or more. In some embodiments, the thickness of the single-sided negative active material layer is 30 μm or more. In some embodiments, the thickness of the single-sided negative active material layer is 150 μm or less. In some embodiments, the thickness of the single-sided negative electrode active material layer is 120 μm or less. In some embodiments, the thickness of the single-sided negative active material layer is 100 μm or less. In some embodiments, the thickness of the negative active material layer is within a range consisting of any two of the above values. When the thickness of the negative electrode active material layer is within the above range, the electrolyte may permeate into the vicinity of the negative electrode current collector interface, improving the charge-discharge characteristics of the secondary battery at high current density; while the volume ratio of the negative electrode current collector to the negative electrode active material is within an appropriate range, the capacity of the secondary battery can be secured.

In some embodiments, the negative active material layer includes a negative active material, a conductive agent, a binder, and a dispersant.

Negative electrode active material

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

Conductive agent

In some embodiments, the conductive agent comprises one or more of carbon black, graphite, carbon fibers, carbon nanotubes, or graphene, preferably carbon black.

Binder

The binder may improve adhesion between the anode active materials. The type of the binder is not particularly limited, and may be any material that is stable to the electrolyte solution or the solvent used in the production of the electrode. In some embodiments, the binder comprises sodium carboxymethylcellulose and styrene butadiene rubber.

Dispersing agent

In some embodiments, the dispersing agent comprises diethyl hexanol, which is an environment-friendly organic compound, low in price, wide in source, low in surface tension, easy to adsorb and spread on the liquid surface, and capable of shearing and rubbing materials under mechanical force, and having internal friction among particles, and under the action of various forces, raw material particles tend to be highly dispersed, so that the slurry is more uniform, the dispersion effect is good, the prepared dry pole piece is uniform in thickness, the problem of wrinkling and the like is avoided, the stability of the pole piece is enhanced, the transmission efficiency of lithium ions between a positive pole and a negative pole is improved, the electrochemical polarization is reduced, the non-faradaic reaction process is accelerated, and the requirements of power battery rate multiplying performance and cycle life are met.

IV, isolating film

In order to prevent short circuit, a separator is generally provided between the positive electrode tab and the negative electrode tab. In this case, the electrolyte of the present application is generally used by penetrating the separator.

V, applications

The embodiment of the application also provides an electric device which comprises the secondary battery. As a typical application, the secondary battery may be used for electric toys, electric tools, battery cars, electric automobiles, energy storage devices, ships, spacecraft, and the like.

The following description will be made of a method for manufacturing a secondary battery provided in the present application with reference to specific examples:

example 1

Preparing an electrolyte:

LiNO as a first lithium salt ₃ Dissolving in cosolvent DMSO (dimethyl sulfoxide) to obtain uniform mixed solution. 1mol of LiPF ₆ Dissolved in 1L of a mixed solvent of EC, DEC and FEC, the volume ratio of EC, DEC and FEC in the mixed solvent being 4.5 ₆ Electrolyte with the concentration of 1 mol/L. Adding a solvent containing a first lithium salt to a solution containing LiPF ₆ To obtain a product containing LiNO ₃ The electrolyte of (1). Wherein LiN is contained in the obtained electrolyteO ₃ Is 0.5wt%, liNO ₃ And LiPF ₆ The mass ratio of (A) to (B) is 0.04.

Preparing a positive pole piece:

mixing the positive active material LiMn _0.6 Fe _0.4 PO ₄ The conductive agent carbon black and the binder polyvinylidene fluoride (PVDF) are mixed according to the mass ratio of 96.5:2.3:1.2, mixing, adding solvent N-methyl pyrrolidone (NMP), stirring in vacuum until the system is uniform to obtain anode slurry, then uniformly coating the anode slurry on the upper and lower surfaces of an anode current collector aluminum foil, airing at room temperature, transferring to an oven, and then compacting at a density of 2.3g/cm ³ And (5) obtaining the positive pole piece after cold pressing and slitting under the condition.

Preparing a negative pole piece:

mixing a negative electrode active material graphite, a conductive agent CNT (carbon nano tube), a thickening agent CMC (carboxymethyl cellulose), and a binder SBR (styrene butadiene latex) 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 electrode current collector copper foil, airing at room temperature, transferring the negative electrode current collector copper foil to an oven, and continuously drying the negative electrode current collector copper foil until the compaction density is 1.7g/cm ³ And carrying out cold pressing and slitting under the condition of (1) to obtain the negative pole piece.

Preparing an isolating membrane:

polyethylene film was chosen as the release film.

Preparing a lithium ion battery:

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

Example 2

A secondary battery was fabricated in accordance with the method of example 1, except that the following differences were changed to example 1:

the first lithium salt in the electrolyte is LiNO ₃ 0.4wt% LiNO ₃ And LiPF ₆ The mass ratio of (A) to (B) is 0.03.

Example 3

the first lithium salt in the electrolyte is LiNO ₃ 0.2wt% LiNO, content ₃ And LiPF ₆ The mass ratio of (A) to (B) is 0.02.

Example 4

the positive active material comprises LiMn _0.7 Fe _0.3 PO ₄ The lithium iron manganese phosphate material.

Example 5

the first lithium salt in the electrolyte is LiNO ₃ 1.0wt% LiNO, the content ₃ And LiPF ₆ The mass ratio of (A) to (B) is 0.09.

Example 6

the positive active material comprises LiMn _0.8 Fe _0.2 PO ₄ The lithium iron manganese phosphate material.

Example 7

the first lithium salt in the electrolyte is LiNO ₃ 0.1wt% LiNO ₃ And LiPF ₆ The mass ratio of (A) to (B) is 0.01.

Example 8

the first lithium salt in the electrolyte is LiNO ₃ 1.5wt% LiNO, the content ₃ And LiPF ₆ The mass ratio of (2) is 0.13.

Example 9

A secondary battery was fabricated in accordance with the method of example 1, except for the following differences from example 1:

the positive active material comprises LiMn _0.9 Fe _0.1 PO ₄ The lithium iron manganese phosphate material.

Example 10

the first lithium salt in the electrolyte is lithium nitrite.

Example 11

the first lithium salt in the electrolyte is lithium borate.

Example 12

the first lithium salt in the electrolyte is lithium chloride.

Comparative example 1

the electrolyte does not contain the first lithium salt as described above.

The method for testing the secondary battery according to the present invention will be described below.

High temperature cycle performance testing of batteries

Standing for 10 minutes, charging to 4.25V at 1C multiplying power with constant current, then charging at constant voltage until the current is 0.05C, and then discharging to 2.8V with 1C constant current, wherein the charging and discharging cycle is one. The cycle number when the discharge capacity is reduced to 80% is the cycle life, taking the first discharge capacity as 100%.

And (4) finishing conditions: 3000cycles or a discharge capacity of less than 80%. Lithium ion batteries prepared in examples and comparative examples were fully charged at a rate of 1C and fully discharged at a rate of 1C at 45℃, actual capacities at that time were recorded, and then tested at 45℃ by the above-described cycle test method, and a high-temperature cycle curve was drawn when the test was completed. And judging the high-temperature cycle performance of the battery by taking a 500-cycle post-simulation curve as a high-temperature cycle curve due to longer cycle time.

Wherein, table 1 is a table showing the test results of the secondary batteries manufactured in the respective examples and the secondary battery manufactured in the comparative example after the high temperature cycle performance test. Table 2 is a table showing the results of testing the room temperature discharge rate performance of the secondary batteries manufactured in the respective examples and the secondary battery manufactured in the comparative example. Table 3 is a table showing the results of the room temperature charge rate performance test of the secondary batteries manufactured in the respective examples and the secondary battery manufactured in the comparative example.

The room-temperature discharge rate test method of the secondary battery is as follows:

the discharge capacity of the secondary batteries of comparative example 1 and examples 1 to 12, each of which was 5 batteries, was measured at 25 ℃ by sequentially constant-current charging at a rate of 1C to 4.25V, then constant-voltage charging at 4.25V to a current of 0.05C, and discharging at rates of 0.33C, 0.5C, 1C, 1.5C, 2C, and 3C to 2.8V, respectively.

Discharge capacity ratio (%) at different rates = discharge capacity at different rates/0.33C discharge capacity × 100%.

Charge capacity ratio (%) at different magnification = charge capacity at different magnification/0.33C charge capacity × 100%.

TABLE 1

TABLE 2

TABLE 3

As can be seen from Table 1, referring to examples 1 and 4, when the first lithium salt was added to the electrolyte, liNO was used as the first lithium salt ₃ (lithium nitrate), and LiNO ₃ When the content of the electrolyte is 0.5wt%, the capacity retention rate of the secondary battery is the highest and reaches 99% after 500 cycles at 45 ℃. Further, liNO was used as the first lithium salt ₃ (lithium nitrate) LiNO ₃ The content of the lithium manganese iron phosphate in the electrolyte is 0.5wt%, and the structural formula of the lithium manganese iron phosphate in the positive active material is LiMn _0.6 Fe _0.4 PO ₄ Or LiMn _0.7 Fe _0.3 PO ₄ In this case, the capacity retention rate of the secondary battery is as high as 99% at 45 ℃ after 500 cycles.

Referring to examples 2, 5 and 7, when LiNO is contained in the secondary battery ₃ When the content of the electrolyte is less than 0.5wt% or more than 0.5wt%, the capacity retention rate of the secondary battery after 500 cycles at 45 ℃ cannot reach 99%, such as LiNO ₃ When the content of the electrolyte exceeds 0.5wt% to 1.0wt% or even 1.5wt%, an additional solvent is required to dissolve excessive LiNO ₃ Addition of excessive solvent affects the cycle capacity retention rate of the secondary battery, liNO ₃ Addition of lithium nitrate too much easily causes re-precipitation of lithium nitrate in the electrolyte, resulting in a decrease in the cycle capacity retention rate to 97%, whereas LiNO ₃ When the content of the electrolyte is too low, for example, only 0.1wt% as shown in example 7, a dense SEI film layer cannot be formed on the negative electrode plate, so that Mn deposition occurs on the surface of the negative electrode plate, and the cycle capacity retention rate of the secondary battery is reduced to 93%.

In addition, referring to examples 6 and 9, when the structural formula of lithium iron manganese phosphate in the positive electrode active material was LiMn _0.8 Fe _0.2 PO ₄ Or LiMn _0.9 Fe _0.1 PO ₄ Even if LiNO is present ₃ The content of 0.5wt% in the electrolyte also affects the capacity retention rate of the secondary battery after 500 cycles at 45 ℃, resulting in the capacity retention rate of the secondary battery after 500 cycles at 45 ℃ being reduced to 95% and 96%.

In addition, referring to comparative example 1, comparative example 1 is different from example 1 only in that the first lithium salt (LiNO) is not added in comparative example 1 ₃ ) The capacity retention rate of the secondary battery obtained in comparative example 1 after 500 cycles at 45 ℃ is reduced to 86%, and the high-temperature cycle performance is poor.

In addition, referring to examples 10 to 12, the first lithium salt in the corresponding secondary batteries did not use nitrate, and the capacity retention rates of the prepared secondary batteries after 500 cycles at 45 ℃ were 90%, 89%, and 89%, respectively.

The higher the concentration of the lithium nitrate is, the more compact the SEI layer is generated, and the better the protection effect on the negative electrode is. But the SEI layer is too dense to facilitate lithium ion diffusion, and the rate performance of the battery is reduced.

As can be seen from Table 2, referring to examples 1 and 4, when the first lithium salt was added to the electrolyte and LiNO was used as the first lithium salt ₃ (lithium nitrate), and LiNO ₃ When the content of the lithium manganese phosphate in the electrolyte is 0.5wt%, the structural formula of the lithium manganese phosphate in the positive active material is LiMn _0.6 Fe _0.4 PO ₄ Or LiMn _0.7 Fe _0.3 PO ₄ When the rate of room temperature discharge capacity of the secondary battery was maintained at 100% at different rate discharge capacities, indicating that the rate performance of the secondary battery was not degraded, referring to example 5, when LiNO was used ₃ When the content in the electrolyte was increased to 1wt%, the rate performance of the secondary battery at room temperature was decreased to 99% at a discharge capacity ratio of 1.5C and 98% at a discharge capacity ratio of 3C, indicating that the discharge rate performance of the secondary battery was decreased because of excessive LiNO ₃ The SEI layer is too dense to facilitate lithium ion diffusion, and the discharge rate performance of the secondary battery is deteriorated, when LiNO is used in reference to example 7 ₃ When the content of the electrolyte is only 0.1wt%, secondary electricity is generatedThe rate capability of the cell at room temperature is reduced to 99% at a discharge capacity ratio of 0.5C and 98% at a discharge capacity ratio of 1C, because of too little LiNO ₃ Resulting in insufficient compactness of the SEI layer and reduced discharge rate performance of the secondary battery, but due to LiNO ₃ The decrease is not so large.

In addition, referring to examples 6 and 9, when the structural formula of lithium manganese iron phosphate in the positive electrode active material was LiMn _0.8 Fe _0.2 PO ₄ Or LiMn _0.9 Fe _0.1 PO ₄ Even if LiNO is present ₃ The content in the electrolyte is 0.5wt%, and the discharge capacity ratio of the corresponding secondary battery at 0.5C is reduced to 99%. Referring to examples 10 to 12, the first lithium salt in the corresponding secondary batteries was not nitrate, and the discharge capacity ratio of the resulting secondary batteries was significantly decreased.

Referring to comparative example 1, since comparative example 1 did not add LiNO to the electrolyte ₃ The discharge rate performance of the secondary battery at room temperature is gradually reduced along with the increase of the discharge rate, the reduction is serious, the discharge capacity ratio of 3C is reduced to 92%, and the reduction of the discharge rate performance of the secondary battery is obvious.

As can be seen from Table 3, referring to example 1, when the first lithium salt was added to the electrolyte, and the first lithium salt used was LiNO ₃ (lithium nitrate), and LiNO ₃ When the content of the lithium manganese phosphate in the electrolyte is 0.5wt%, the structural formula of the lithium manganese phosphate in the positive active material is LiMn _0.6 Fe _0.4 PO ₄ Or LiMn _0.7 Fe _0.3 PO ₄ When the charge rate performance of the secondary battery did not decrease much, the charge capacity ratio of 3C decreased to 84%, when LiNO, as in example 5 ₃ When the content in the electrolyte solution was increased by 1.0wt%, the charge capacity ratio of 3C was decreased to 80%, referring to example 7, when LiNO was used ₃ When the content in the electrolyte was decreased by 0.1wt%, the charge capacity ratio of 3C was decreased to 75%.

Referring to comparative example 1, since comparative example 1 did not add LiNO to the electrolyte ₃ Resulting in that the charge rate performance of the secondary battery at room temperature is gradually increased as the charge rate is increasedThe charge rate performance of the secondary battery is remarkably reduced, and the charge capacity ratio of 3C is reduced to 68%.

In addition, referring to examples 6 and 8, when the structural formula of lithium manganese iron phosphate in the positive electrode active material was LiMn _0.8 Fe _0.2 PO ₄ Or LiMn _0.9 Fe _0.1 PO ₄ Even if LiNO is present ₃ The content in the electrolyte was 0.5wt%, and the charge capacity ratio of the corresponding secondary battery was remarkably decreased. Referring to examples 10 to 12, the first lithium salt in the corresponding secondary battery did not use nitrate, and the discharge capacity ratio of the obtained secondary battery was remarkably decreased.

The foregoing detailed description is directed to a secondary battery and an electric device provided in the embodiments of the present application, and specific examples are applied herein to illustrate the principles and implementations of the present application, and the above description of the embodiments is only provided to help understand the method and the core concept 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. A secondary battery comprises a positive pole piece and electrolyte, and is characterized in that the positive pole piece comprises a positive active material, and the positive active material comprises manganese iron lithium oxide;

the total molar weight of manganese and iron in the lithium manganese iron oxide is 100%, the molar percentage of the manganese is n1%, the molar percentage of the iron is n2%, and n1 is more than n2;

the electrolyte comprises a first lithium salt, and the first lithium salt comprises at least one of lithium nitrate, lithium nitrite, lithium borate, lithium phosphate, lithium fluoroborate, lithium sulfate and lithium chloride.

2. The secondary battery according to claim 1, wherein the content of the first lithium salt is less than or equal to 1.5wt% based on the total mass of the electrolyte.

3. The secondary battery according to claim 1, wherein n 1. Gtoreq.60.

4. The secondary battery according to claim 1, wherein n1-n2 ≦ 40.

5. The secondary battery of claim 1, wherein the general structural formula of the lithium iron manganese oxide comprises Li _a M _y Mn _x Fe _1-x-y PO ₄ Wherein a is more than or equal to 0.95 and less than or equal to 1.1,0 and less than or equal to 0.05,0.55 and less than or equal to 0.8, and M comprises one or more of Mg, al, ti and Co.

6. The secondary battery of claim 1, wherein the electrolyte further comprises a second lithium salt comprising lithium hexafluorophosphate;

the mass ratio of the first lithium salt to the second lithium salt is (0.01-0.13): 1.

7. the secondary battery according to claim 1, wherein the electrolyte further comprises an organic solvent, and the organic solvent comprises a cyclic carbonate and a chain carbonate.

8. The secondary battery of claim 1, wherein the electrolyte solution further comprises a film forming additive comprising at least one of vinylene carbonate, 1,3-propane sultone, and fluoroethylene carbonate.

9. The secondary battery of claim 1, wherein the electrolyte further comprises a co-solvent comprising at least one of an ether solvent, dimethyl sulfoxide, sulfolane, tetramethylurea, and an amide compound.

10. An electric device comprising the secondary battery according to any one of claims 1 to 9.