Low-temperature safe lithium ion battery

Abstract

The invention discloses a low-temperature safe lithium ion battery which comprises a positive electrode active material, a negative electrode active material and an electrolyte, the positive electrode active material comprises lithium vanadium phosphate; the negative electrode active material comprises a graphite / hard carbon composite material; in the electrolyte, the mass ratio of the electrolyte solvent to the electrolyte additive to the lithium salt is (60-100): (1-10): (10-20); in the electrolyte solvent, the mass ratio of the chain carbonate to the non-halogenated cyclic carbonate to the halogenated cyclic carbonate to the acrylate is (50-70): (5-13); (1-5): (4-12); the lithium salt comprises fluorine-containing lithium phosphate salt and fluorine-containing lithium sulfimide salt; the electrolyte additive is dinitrile; the problems that the lithium ion battery is poor in cycle performance and rate capability and the internal resistance of the battery is increased at low temperature are solved.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion batteries, and in particular to a low-temperature safe lithium-ion battery. Background Art

[0002] In recent years, lithium-ion battery technology has become increasingly mature. Due to its high energy density and low cost, it has been widely used as an electrochemical energy storage device in portable electronic products, electric vehicles, and energy storage grids. However, poor safety and poor low-temperature performance have always been the shortcomings of lithium-ion batteries. The ambient temperature will seriously affect the cycle stability, service life and safety of lithium-ion batteries. It is difficult for them to work in the low-temperature environment of low-latitude areas in winter, and they are even unable to meet the requirements of energy storage and release under ultra-low temperature extreme conditions such as the polar regions, deep sea, and space. The electrochemical reaction kinetics often decrease sharply, and lithium precipitation occurs.

[0003] At present, the commercialized cathode materials for lithium-ion batteries mainly include lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4) and ternary materials (Li(Ni x Co y Mn z )O2), etc.; Different types of lithium-ion batteries have their own advantages and disadvantages. For example, lithium cobalt oxide has high specific capacity and good cycle performance, but high cost and poor safety; lithium manganese oxide has low cost, simple preparation, and good rate performance, but the material structure is unstable and manganese is easily dissolved under high temperature conditions; lithium iron phosphate has good cycle performance and safety, but low electronic conductivity and small lithium ion diffusion coefficient. The development of high-performance positive electrode materials is the key to improving the overall performance of lithium-ion batteries. Lithium vanadium phosphate (Li3V2(PO4)3, LVP) is a lithium-ion battery positive electrode material with great market potential that has been developed in recent years. Lithium vanadium phosphate has a monoclinic structure Li3V2(PO4)3 compound, which not only has good safety, but also has higher Li + Li3V2(PO4)3 is considered a superior cathode material to LiCoO2 due to its high ion diffusion coefficient, higher discharge voltage, and energy density. It combines the advantages of lithium cobalt oxide and lithium iron phosphate, overcoming their shortcomings. The synthesis process for Li3V2(PO4)3 is simple, making it easy to industrialize. This cathode material also boasts excellent electrochemical performance, safety, and a high discharge voltage. However, its relatively low ionic and electronic conductivity limits its further application. Therefore, it is necessary to address the low-temperature performance of lithium-ion batteries and improve cell safety. Summary of the Invention

[0004] The present invention addresses the problems existing in the prior art, such as poor cycle performance, poor rate performance, and increased battery internal resistance of ternary materials or lithium iron phosphate at low temperatures. The present invention discloses a low-temperature safe lithium-ion battery using lithium vanadium phosphate as the positive electrode material. The purpose is to provide a lithium vanadium phosphate battery suitable for high rate, high cycle performance, and low battery internal resistance at low temperatures. The battery is used to solve the problems in the prior art, such as the tendency of side reactions between the electrolyte and the electrode material in low-temperature, high-rate environments, resulting in excessive growth of the positive electrode solid electrolyte interface, damage to the structure of the positive electrode material, and poor compatibility between the positive electrode and the electrolyte. The present invention uses chain carbonates, non-halogenated cyclic carbonates, halogenated cyclic carbonates, and acrylates as electrolyte solvents, and at the same time compounds dinitriles that can reduce electrolyte viscosity and improve rate performance at low temperatures. Based on the physical and chemical properties of the electrolyte solvents, their impact on battery performance, and the synergistic effects between the electrolyte solvents, the specific selection of various electrolyte solvents and the amount and proportion of each component are finely optimized; the present invention also takes into account the composition and ratio of lithium salts in the electrolyte, as well as the combined use of negative electrode active materials; the stability of the battery is ensured, polarization is reduced, charge transfer resistance is reduced, conductivity is improved, the diffusion coefficient of lithium ions is increased, the reversible extraction and embedding of lithium ions is promoted, phase change during charging and discharging is reduced, and the electrochemical properties of the lithium vanadium phosphate positive electrode material are significantly improved, especially the cycle stability and rate performance of the lithium vanadium phosphate battery at low temperatures are improved, and the internal resistance is reduced.

[0005] The present invention is achieved through the following technical solutions:

[0006] The present invention first provides a low-temperature safe lithium-ion battery, comprising a positive electrode active material, a negative electrode active material and an electrolyte;

[0007] The positive electrode active material includes lithium vanadium phosphate;

[0008] The negative electrode active material includes a graphite / hard carbon composite material;

[0009] The electrolyte comprises an electrolyte solvent, an electrolyte additive and a lithium salt;

[0010] In the electrolyte, the mass ratio of the electrolyte solvent, the electrolyte additive and the lithium salt is (60-100): (1-10): (10-20);

[0011] The electrolyte solvent includes cyclic carbonate, chain carbonate and acrylate;

[0012] The cyclic carbonates include non-halogenated cyclic carbonates and halogenated cyclic carbonates;

[0013] In the electrolyte solvent, the mass ratio of the chain carbonate, non-halogenated cyclic carbonate, halogenated cyclic carbonate, and acrylate is (50-70): (5-13); (1-5): (4-12);

[0014] The electrolyte additive is dinitrile;

[0015] The lithium salt includes at least a fluorine-containing lithium phosphate salt and a fluorine-containing lithium sulfonyl imide salt.

[0016] As a preferred solution, the electrolyte additive is one or more of adiponitrile, succinonitrile, glutaronitrile, ethylene glycol dinitrile, and perfluoro dinitrile.

[0017] As a further preferred embodiment, the electrolyte additive is adiponitrile.

[0018] As a preferred solution, the fluorine-containing lithium phosphate salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, and lithium tetrafluorophosphate.

[0019] As a further preferred embodiment, the fluorine-containing lithium phosphate salts are lithium hexafluorophosphate and lithium difluorophosphate.

[0020] As a preferred solution, the fluorine-containing lithium sulfonyl imide salt is selected from one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium fluorosulfonyltrifluoromethylsulfonyl imide, and lithium perfluoroalkylsulfonyl imide.

[0021] As a further preferred embodiment, the fluorine-containing lithium sulfonyl imide salt is lithium bis(fluorosulfonyl imide).

[0022] As a preferred solution, in the electrolyte, the mass ratio of the electrolyte solvent, the electrolyte additive and the lithium salt is 79:5:16.

[0023] As a preferred solution, in the electrolyte solvent, the mass ratio of the chain carbonate, non-halogenated cyclic carbonate, halogenated cyclic carbonate, and acrylate is 59:9:3:8.

[0024] As a preferred solution, the chain carbonate includes one or more of ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, and ethylpropyl carbonate.

[0025] As a further preferred embodiment, the chain carbonate includes ethyl methyl carbonate and diethyl carbonate.

[0026] As a preferred solution, the non-halogenated cyclic carbonate includes one or more of ethylene carbonate, vinylene carbonate, vinylethylene carbonate, styrene carbonate, butadiene carbonate, and allyl carbonate.

[0027] As a further preferred embodiment, the non-halogenated cyclic carbonate includes ethylene carbonate and vinylene carbonate.

[0028] As a preferred solution, the halogenated cyclic carbonate includes one or more of fluoroethylene carbonate, difluoroethylene carbonate, chloroethylene carbonate, and chlorovinylene carbonate.

[0029] As a further preferred embodiment, the halogenated cyclic carbonate includes fluoroethylene carbonate.

[0030] As a preferred solution, the acrylic ester includes one or more of methyl acrylate, ethyl acrylate, and butyl acrylate.

[0031] As a further preferred embodiment, the acrylate includes methyl acrylate.

[0032] As an embodiment, in the electrolyte solvent, the mass ratio of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acrylate, fluoroethylene carbonate and vinylene carbonate is (4.5-11): (30-40): (20-30): (4-12): (1-5): (0.5-2).

[0033] As a further preferred embodiment, in the electrolyte solvent, the mass ratio of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acrylate, fluoroethylene carbonate and vinylene carbonate is 8:35:24:8:3:1.

[0034] As a solution, the mass ratio of the fluorine-containing lithium phosphate salt to the fluorine-containing lithium sulfonyl imide salt in the lithium salt is (9.5-18.5): (0.5-1.5).

[0035] As a further preferred embodiment, the mass ratio of the fluorine-containing lithium phosphate salt to the fluorine-containing lithium sulfonyl imide salt in the lithium salt is 15:1.

[0036] As a further preferred embodiment, the mass ratio of lithium hexafluorophosphate to lithium difluorophosphate is (8.5-15.5):(1-3);

[0037] As a further preferred solution, the mass ratio of lithium hexafluorophosphate to lithium difluorophosphate is 13:2.

[0038] As a further solution, the low-temperature safe lithium-ion battery further includes a positive electrode sheet and a negative electrode sheet.

[0039] The positive electrode sheet is an aluminum foil coated with the positive electrode active material lithium vanadium phosphate, and the negative electrode sheet is a copper foil coated with the negative electrode active material graphite / hard carbon composite material;

[0040] When the positive electrode active material lithium vanadium phosphate is coated on the aluminum foil and the negative electrode active material graphite / hard carbon composite material is coated on the copper foil, an adhesive and a conductive agent are also used.

[0041] As an embodiment, the binder uses an oil-based binder, and the binder includes at least one of a thermoplastic resin, an acrylic resin, sodium carboxymethyl cellulose, and styrene butadiene rubber.

[0042] As a preferred embodiment, the thermoplastic resin includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride, polytetrafluoroethylene, a copolymer of vinylidene fluoride-hexafluoropropylene, a copolymer of tetrafluoroethylene-hexafluoropropylene, a copolymer of tetrafluoroethylene-perfluoroalkyl vinyl ether, a copolymer of ethylene-tetrafluoroethylene, a copolymer of vinylidene fluoride-tetrafluoroethylene, a copolymer of vinylidene fluoride-trifluoroethylene, a copolymer of vinylidene fluoride-trichloroethylene, a copolymer of vinylidene fluoride-fluoroethylene, a copolymer of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, polyethylene and polypropylene.

[0043] As a preferred solution, the acrylic resin includes at least one of vinyl acrylate resin, methyl acrylate resin, butyl acrylate resin, acrylic styrene resin, acrylate resin, acrylate copolymer resin, acrylic resin, and acrylic emulsion resin.

[0044] As a further preferred solution, the binder is polyvinylidene fluoride (PVDF).

[0045] As an approach, the conductive agent includes at least one of a carbon material, a conductive polymer, and a metal conductor.

[0046] As a preferred solution, the carbon material includes one or more of conductive carbon black (Super P), conductive graphite, carbon fiber, carbon nanotube (CNT), Ketjen black, and graphene.

[0047] As a further preferred solution, the conductive agent is carbon black (Super P).

[0048] As a solution, the mass ratio of the lithium vanadium phosphate, polyvinylidene fluoride, and conductive carbon black is (90-99): (1-5): (1-5).

[0049] As a preferred solution, the mass ratio of the lithium vanadium phosphate, polyvinylidene fluoride, and conductive carbon black is 92:5:3.

[0050] As a solution, the mass ratio of the graphite / hard carbon composite material, polyvinylidene fluoride, and conductive carbon black is (90-99): (1-5): (1-5).

[0051] As a preferred solution, the mass ratio of the graphite / hard carbon composite material, polyvinylidene fluoride, and conductive carbon black is 94:3:3.

[0052] As a further solution, a low-temperature safe lithium-ion battery further includes a diaphragm, wherein the diaphragm is selected from one of a polyethylene film, a polypropylene film, a glass fiber film or a composite film;

[0053] As a further preferred solution, the diaphragm is a polyethylene film coated with a nano-aluminum oxide coating.

[0054] As a second aspect of the present invention, a method for preparing a low-temperature safe lithium-ion battery is also provided, comprising the following steps:

[0055] S1: Prepare the positive electrode sheet according to the target stoichiometric ratio and control the surface density of the positive electrode sheet to 8-25 mg / cm 2 ; Roller compaction density is 2.0-2.5g / cm 3 ;

[0056] S2: Prepare the negative electrode sheet according to the target stoichiometric ratio and control the surface density of the negative electrode sheet to 5-9 mg / cm 2 ; Roller compaction density is 1.2-1.7g / cm 3 ;

[0057] S3: Preparation of electrolyte: mixing electrolyte solvent, electrolyte additive and lithium salt according to the target stoichiometric ratio;

[0058] S4: Assemble the positive electrode sheet and separator prepared in S1, the negative electrode sheet prepared in S2, and the electrolyte prepared in S3 into a lithium-ion battery.

[0059] As a further solution, the S1 is to mix the positive electrode active material, the binder, and the conductive agent according to the target stoichiometric ratio, apply the solvent on the aluminum foil, and obtain the positive electrode sheet after drying, rolling, and cutting.

[0060] As a further solution, the S2 is to mix the negative electrode active material, the binder, and the conductive agent according to the target stoichiometric ratio, apply the mixture on the copper foil, and obtain the negative electrode sheet after drying, rolling, and cutting.

[0061] As a further preferred solution, the surface density of the positive electrode sheet coating is 15 mg / cm 2 ; Roller compaction density is 2.2g / cm 3 ;

[0062] As a further preferred solution, the surface density of the negative electrode sheet coating is 7 mg / cm 2 ; Roller compaction density is 1.3g / cm 3

[0063] As a further solution, the housing of the S4 lithium-ion battery can be a soft pack / square / cylindrical.

[0064] As a further solution, the baking parameters for the drying in S1 and S2 are baking at 85° C. for 24 hours, with nitrogen circulation every 2 hours.

[0065] As a further solution, the injection amount of the lithium-ion battery is 3-5 g / Ah.

[0066] As a further solution, the formation parameters of the lithium-ion battery are: 0.05C charging to 3.5V, 0.1C charging to 4.2V.

[0067] The characteristics and beneficial effects of the present invention are:

[0068] (1) The low-temperature safe lithium-ion battery of the present invention uses lithium vanadium phosphate as its positive electrode active material and achieves better low-temperature performance. This is mainly due to the reasonable selection and precise compounding of electrolyte solvents, the selection of negative electrode materials, the selection and dosage of electrolyte additives, and the selection and ratio of lithium salts. Through multi-dimensional collaborative innovation, excellent low-temperature electrochemical performance is achieved.

[0069] The present invention adopts a lithium vanadium phosphate positive electrode, which has the advantages of high energy density. Through multi-dimensional collaborative design, a lithium vanadium phosphate-based low-temperature and high-performance battery system is constructed. On the one hand, a fluorine-containing lithium phosphate salt / fluorine-containing sulfonyl imide lithium salt double salt system is adopted, and the PO bond positive electrode interface anchoring improves the ion transfer efficiency, and the fluorine-containing sulfonyl imide lithium salt forms a CEI film rich in LiF and sulfur-containing compounds. Combined with the multi-electrolyte solvent complex, the dielectric, viscosity, polarity and other aspects are precisely regulated to improve the ionic conductivity at low temperature, inhibit the dissolution of vanadium in the positive electrode active material, inhibit the crystallization of the electrolyte at low temperature, and reduce the interface side reaction at low temperature; on the other hand, a dinitrile is introduced to form a dicyano synergistic mechanism, and its flexible long chain reduces the solvation energy and the viscosity of the electrolyte. The dicyano group forms a surface metal transition state coordination with the surface of the lithium vanadium phosphate, and simultaneously optimizes the positive electrode interface dynamics; on the third hand, a graphite / hard carbon composite material is adopted, and the porous buffering effect of hard carbon and the high capacity characteristics of graphite are coordinated to further improve the low-temperature cycle capacity retention rate and break through the bottleneck of traditional graphite low-temperature pulverization. The synergistic effect of the above angles, at a certain compounding ratio, solves the problems of poor cycle performance, rate performance, and severe impedance increase of lithium vanadium phosphate batteries at low temperatures, and improves the safety performance of lithium vanadium phosphate batteries at low temperatures.

[0070] (2) The present invention also provides a method for preparing a low-temperature safe lithium-ion battery. Since the viscosity and fluidity of the positive and negative electrode slurries decrease at low temperatures, the present invention further optimizes the surface density, rolling parameters and formation parameters of the coating of the positive and negative electrode sheets, thereby optimizing the compaction density and uniformity of the slurry on the sheets, and further improving the performance of the battery at low temperatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 The capacity retention rate and coulombic efficiency diagram of Example 1 of the present invention after 300 cycles at -20°C and 0.2C;

[0072] Figure 2 This is a graph showing the capacity retention of Example 1 and Comparative Example 2 after 100 cycles at -10°C and 3C;

[0073] Figure 3 This is a graph showing the capacity retention rate of Example 1 of the present invention and Comparative Example 3 after 100 cycles at -10°C 3C. DETAILED DESCRIPTION

[0074] The present invention will be further described below through the accompanying drawings and specific embodiments. However, it should be understood that these embodiments are only used for more detailed description and should not be understood as limiting the present invention in any form, that is, they are not intended to limit the scope of protection of the present invention.

[0075] The present invention addresses the problems existing in the prior art, such as poor cycle performance, poor rate performance, and increased battery internal resistance of ternary materials or lithium iron phosphate at low temperatures. The present invention discloses a low-temperature safe lithium-ion battery using lithium vanadium phosphate as the positive electrode material. The purpose is to provide a lithium vanadium phosphate battery suitable for high rate, high cycle performance, and low battery internal resistance at low temperatures. The battery is used to solve the problems in the prior art, such as the tendency of side reactions between the electrolyte and the electrode material in low-temperature, high-rate environments, resulting in excessive growth of the positive electrode solid electrolyte interface, damage to the structure of the positive electrode material, and poor compatibility between the positive electrode and the electrolyte. The present invention uses chain carbonates, non-halogenated cyclic carbonates, halogenated cyclic carbonates, and acrylates as electrolyte solvents, and at the same time compounds dinitriles that can reduce electrolyte viscosity and improve rate performance at low temperatures. Based on the physical and chemical properties of the electrolyte solvents, their impact on battery performance, and the synergistic effects between the electrolyte solvents, the specific selection of various electrolyte solvents and the amount and proportion of each component are finely optimized; the present invention also takes into account the composition and ratio of lithium salts in the electrolyte, as well as the combined use of negative electrode active materials; the stability of the battery is ensured, polarization is reduced, charge transfer resistance is reduced, conductivity is improved, the diffusion coefficient of lithium ions is increased, the reversible extraction and embedding of lithium ions is promoted, phase change during charging and discharging is reduced, and the electrochemical properties of the lithium vanadium phosphate positive electrode material are significantly improved, especially the cycle stability and rate performance of the lithium vanadium phosphate battery at low temperatures are improved, and the internal resistance is reduced.

[0076] The present invention is achieved through the following technical solutions:

[0077] The present invention first provides a low-temperature safe lithium-ion battery, comprising a positive electrode active material, a negative electrode active material and an electrolyte;

[0078] The positive electrode active material includes lithium vanadium phosphate;

[0079] The negative electrode active material includes a graphite / hard carbon composite material;

[0080] The electrolyte comprises an electrolyte solvent, an electrolyte additive and a lithium salt;

[0081] In the electrolyte, the mass ratio of the electrolyte solvent, the electrolyte additive and the lithium salt is (60-100): (1-10): (10-20);

[0082] The electrolyte solvent includes cyclic carbonate, chain carbonate and acrylate;

[0083] The cyclic carbonates include non-halogenated cyclic carbonates and halogenated cyclic carbonates;

[0084] In the electrolyte solvent, the mass ratio of the chain carbonate, non-halogenated cyclic carbonate, halogenated cyclic carbonate, and acrylate is (50-70): (5-13); (1-5): (4-12);

[0085] The lithium salt includes at least a fluorine-containing lithium phosphate salt and a fluorine-containing lithium sulfonyl imide salt.

[0086] The present invention uses lithium vanadium phosphate as the positive electrode active material. Through the selection and ratio of lithium salts and electrolyte solvents, the compounding of electrolyte additives, and the selection of negative electrode active materials, the present invention solves the problems of rapid battery capacity decay, low conductivity, poor cycle performance, poor rate performance, and increased impedance of lithium-ion batteries at low temperatures from multiple angles. The present invention uses lithium vanadium phosphate as the positive electrode active material to overcome the above problems. Based on the multiple oxidation states of vanadium, it can provide multiple voltage platforms and high energy density. Its polyanionic framework provides excellent low and high temperature stability and cycle life. + Occupies octahedral sites and diffuses along one-dimensional chain channels, so at low temperatures, Li + Diffusion is easily hindered by lattice contraction, Li +The increase in migration activation energy leads to a decrease in the diffusion coefficient, slowing the electron and ion conduction rates. At low temperatures, there is still room for improvement in the capacity, conductivity, cycle performance, and rate performance of lithium vanadium phosphate batteries. The composition and ratio of conventional lithium salts and conventional electrolyte solvents make it difficult to further improve the performance of lithium vanadium phosphate batteries at low temperatures. Therefore, it is necessary to develop a low-temperature safe lithium vanadium phosphate system that takes into account ion transport, interface stability, cycle performance, and rate performance.

[0087] Therefore, the present invention first selects fluorine-containing lithium phosphate and fluorine-containing lithium sulfonyl imide from the perspective of lithium salt compounding in the electrolyte. The present invention is based on the interaction between the phosphoric acid group in the fluorine-containing phosphate and the PO4 on the surface of the positive electrode active material. 3- The skeleton can form a closer interface contact through the weak interaction of PO bonds, reducing the physical gap between the electrolyte and the positive electrode, and improving the Li + The interface transmission efficiency of the positive electrode active material lithium vanadium phosphate is improved, which overcomes the problem of decreased interface transmission capacity at low temperature. The phosphoric acid groups in the fluorinated phosphate interact with the PO4 3- The chemical similarity of the skeleton can reduce the corrosion of the electrolyte on the positive electrode skeleton, reduce the dissolution of transition metal vanadium, and reduce the interface side reaction, thereby further improving the cycle performance at low temperature; In addition, the fluorinated sulfonyl imide lithium salt used in the present invention has the characteristics of high dissociation degree and low viscosity, which can significantly reduce the viscosity of the electrolyte at low temperature, enhance the lithium ion migration rate in the electrolyte and the Li in the lithium vanadium phosphate positive electrode active material. + The two sulfonyl groups are connected by nitrogen atoms to form a highly symmetrical delocalized charge structure, which weakens the dissociation and shuttling ability of Li + electrostatic attraction, providing sufficient free Li + , directly alleviate the low temperature, lithium vanadium phosphate positive electrode due to Li + Polarization problem caused by slow diffusion; and fluorinated sulfonyl imide lithium salts preferentially decompose under high pressure to generate LiF and sulfur-containing compounds. These products participate in the construction of the positive electrode electrolyte interface (CEI) film: LiF with high lithium conductivity is embedded and deposited in the CEI film, providing fast Li + The sulfide can fill the pores of the CEI membrane and inhibit the continuous penetration of the electrolyte into the positive electrode material and the side reaction. - It can neutralize the trace HF produced by the decomposition of fluorinated lithium phosphate salts, further reducing the corrosion effect of byproducts on the electrode. Therefore, fluorinated lithium phosphate salts and fluorinated lithium sulfonimide salts improve the cycle performance and rate performance at low temperatures from different angles. The two play a synergistic role in mutual cooperation and are indispensable.

[0088] The present invention further selects chain carbonate, non-halogenated cyclic carbonate, halogenated cyclic carbonate and acrylate as compound electrolyte solvents from the perspective of electrolyte solvent types and ratios, combines the physical and chemical properties of different electrolyte solvents, and the interaction between lithium salt and positive electrode active materials; balances the freezing point, viscosity, dielectric constant and oxidation potential of the electrolyte solvent, thereby improving the solubility, dispersibility and Li + Dissociation ability, electrolyte wetting ability and film-forming ability on positive and negative electrodes, reduce the decomposition of lithium vanadium phosphate and the generation of side reactions at the positive electrode interface, reduce impedance at low temperatures, and further solve the problems of rapid capacity attenuation, low conductivity, poor cycle performance and rate performance of lithium vanadium phosphate batteries at low temperatures;

[0089] Specifically, when a chain carbonate with a higher mass ratio is used, due to its linear structure, it has a lower freezing point, viscosity and dielectric constant, which reduces the Li + The migration resistance at low temperature gives the electrolyte excellent low-temperature fluidity, alleviates the concentration polarization phenomenon at low temperature, avoids the local concentration imbalance caused by the crystallization of fluorinated lithium phosphate salts and fluorinated lithium sulfonyl imide salts in the electrolyte, and enables it to maintain a high and stable ionic conductivity in an extremely low temperature environment. The high fluidity helps the electrolyte to fully infiltrate the pores of the positive electrode active material, thereby improving the interface ion transmission efficiency. The present invention also compounds a certain amount of non-halogenated cyclic carbonate, which forms a "high-low dielectric" synergistic solvation shell with the chain carbonate due to its high dielectric constant, optimizing Li + The desolvation process improves the dissociation degree of lithium salt and reduces the desolvation energy barrier at the positive electrode interface; the present invention also increases the oxidation potential of the electrolyte by adding a small amount of halogenated cyclic carbonate and utilizing the strong electron-withdrawing effect of halogen atoms, thereby inhibiting solvent decomposition and gas production under high voltage and high rate. Its strongly electronegative halogen group can be adsorbed on the surface of lithium vanadium phosphate, reducing the dissolution of vanadium ions, while improving the mechanical strength and ion conductivity of the SEI film, forming a high-conductivity ion interface film rich in lithium halide, improving the mechanical strength of the film layer and the lithium ion migration number, and at the same time, the low interfacial energy of the halogenated cyclic carbonate can relieve the volume deformation stress of the electrode and enhance the cycle performance at high voltage and high rate at low temperature; the present invention also regulates the polarity of the solvent by adding a small amount of acrylate, enhances the dispersion uniformity of fluorine-containing lithium phosphate salt and fluorine-containing sulfonyl imide lithium salt in the electrolyte, and polymerizes on the electrode surface during the charge and discharge process to form a polyacrylate-based flexible interface layer. This interfacial layer, combined with the halogenated cyclic carbonate to form a membrane with strong mechanical strength, remains flexible at low temperatures, giving the SEI membrane excellent low-temperature ductility and further reducing the impedance at low temperatures, thereby supporting the high-rate cycling performance of the battery in low-temperature environments. By limiting the mass proportion of acrylate, the side reaction that may occur with vanadium ions on the surface of the positive electrode active material due to its high reduction potential is controlled.

[0090] The present invention also compounds dinitrile to reduce electrolyte viscosity and improve performance at low temperatures. Dinitrile can optimize ion transport at low temperatures and improve performance in extreme environments, especially at low temperatures. Because dinitrile has a high dielectric constant, it can enhance the electrolyte's ability to dissolve lithium salts, maintain a high lithium ion concentration at low temperatures, and reduce the risk of salting out. At the same time, it has a weak coordination ability, and its coordination strength with lithium ions is lower than that of carbonate solvents, which can reduce the solvation energy of lithium ions and promote the desolvation process of lithium ions at low temperatures. It has a flexible long-chain methylene group, which determines that it has lower viscosity, high ionic conductivity and low freezing point compared to single nitrile. The dicyano group of dinitrile can simultaneously react with Li + It coordinates with the transition metal on the positive electrode surface to stabilize the positive electrode interface layer. When compounded with electrolyte solvents containing carbonates, it improves the wettability of the electrolyte to the graphite negative electrode, reduces the uneven lithium deposition at low temperatures, and solves the core problems of salting out, high impedance, and uneven lithium deposition in low-temperature electrolytes.

[0091] The present invention also optimizes the negative electrode active material. Since the layered structure of the graphite negative electrode is very stable, lithium ions are embedded in the "interlayers". The stable reversible structure makes the cycle life better. However, at low temperatures, the cycle life and rate performance are significantly reduced. This is because the graphite will undergo a large volume expansion during the cycle. The graphite is prone to particle rupture due to local stress concentration, which affects the cycle performance of the graphite. The low lithium insertion potential of graphite and the slow lithium ion diffusion at low temperatures lead to rapid capacity decay at low temperatures and the risk of dendrite growth. The present invention uses a graphite / hard carbon composite material (GrHC) as the negative electrode active material. The porous properties of hard carbon can alleviate the volume expansion caused by lithium ion insertion / extraction at low temperatures, disperse stress, and reduce electrode pulverization. Hard carbon has a disordered nanoporous structure and a high specific surface area, which can buffer the volume change when lithium ions are embedded in the graphite interlayer, and at the same time can improve the lithium ion insertion ability at low temperatures. It promotes the reversible extraction and insertion of lithium ions. Through the synergistic effect of the composite material: graphite provides high capacity, and hard carbon improves structural stability, a significant extension of the low-temperature cycle life is achieved. Graphite and hard carbon are evenly distributed, providing more active sites for lithium insertion on the graphite surface. The multi-component structure of the graphite / hard carbon composite material is more conducive to the infiltration of the electrolyte in the negative electrode;

[0092] Based on the above optimization, the coordinated design and optimization of electrolyte solvent-electrolyte additive-negative electrode active material can achieve ion transmission, interface stability and safety assurance of lithium vanadium phosphate batteries; at the same time, the raw materials required by the present invention are low in cost, simple to prepare, and have more superior low-temperature performance, making it suitable for use in power supply devices of various mobile electronic products and electric vehicles.

[0093] As an example, the electrolyte additive is one or more of adiponitrile, succinonitrile, glutaronitrile, ethylene glycol dinitrile, and perfluoro dinitrile.

[0094] As a further preferred example, the electrolyte additive is adiponitrile.

[0095] Compared with other dinitriles, the flexible long chain composed of four methylene groups (-CH2-) of adiponitrile gives the molecule conformational freedom and can maintain low viscosity at low temperatures. Adiponitrile forms a VOC≡N coordination bond with the surface of lithium vanadium phosphate through the cyanide group to inhibit vanadium dissolution, resulting in better performance.

[0096] As a further preferred example, the fluorine-containing lithium phosphate salt is one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluorophosphate, and lithium monofluorophosphate.

[0097] As a further preferred example, the fluorine-containing lithium phosphate salts are lithium hexafluorophosphate and lithium difluorophosphate.

[0098] As a further preferred example, the fluorine-containing lithium sulfonyl imide salt is one or more of lithium bisfluorosulfonyl imide, lithium bistrifluoromethylsulfonyl imide, lithium fluorosulfonyl trifluoromethylsulfonyl imide, and lithium perfluoroalkylsulfonyl imide.

[0099] As a further preferred example, the fluorine-containing lithium sulfonyl imide salt is lithium bis(fluorosulfonyl)imide.

[0100] Lithium hexafluorophosphate can still maintain a certain ionic conductivity at low temperatures. When combined with electrolyte solvents, it can meet the basic needs of ordinary lithium-ion batteries in mild low-temperature environments. However, there is still room for improvement. Using lithium hexafluorophosphate alone may cause crystal precipitation due to solvent solidification or a decrease in its own solubility, resulting in electrolyte failure. At low temperatures, the impedance of the SEI film at the electrode interface increases significantly, and lithium hexafluorophosphate itself has limited improvement on the low-temperature stability of the SEI film, which can easily lead to increased battery polarization and a sudden drop in capacity.

[0101] The compounding of lithium difluorophosphate can alleviate the low solubility of lithium hexafluorophosphate when used alone and provide more PO4 3- , forming a closer positive electrode interface contact, reducing the physical gap between the electrolyte and the positive electrode, further ensuring the interface transmission capacity of the positive electrode active material at low temperatures, and lithium difluorophosphate can form a thinner and more uniform SEI film on the negative electrode surface, significantly reducing the resistance of lithium ions crossing the interface at low temperatures, improving the efficiency of lithium ion insertion / extraction, and reducing the repeated rupture and regeneration of the SEI film during low-temperature cycling, which can reduce the growth rate of the battery internal resistance and extend the low-temperature cycle life;

[0102] The present invention also introduces a certain amount of lithium bis(fluorosulfonyl)imide, which is conducive to meeting the requirements of high voltage and long cycle. The lithium bis(fluorosulfonyl)imide forms a solid electrolyte layer rich in LiF on the positive electrode side of the battery, which can effectively prevent the organic solvent of the present invention from contacting the positive electrode material, reducing the occurrence of side reactions under low temperature operation, thereby protecting the positive electrode material and improving the cycle stability of the battery. - The strong electron-withdrawing property weakens the coordination effect between anions and cations in lithium salts and increases Li + The activity of LiFSI can form a thinner CEI film with better stability, more suitable density and better stability under high pressure, and can make the flow rate of lithium ions in a suitable range, so that the secondary battery has better low-temperature floating charge performance and safety performance, and has better anti-overcharge performance under low temperature and high pressure; LiFSI anion (FSI - ) has strong dissociation ability and can maintain a high Li + The migration number is high, and the electrolyte system has a low viscosity, which significantly improves low-temperature ionic conductivity. At low temperatures, the ionic conductivity of the electrolyte containing lithium bis(fluorosulfonyl)imide decreases slightly, supporting battery operation at extremely low temperatures and further improving low-temperature cycling performance. Therefore, we have compounded lithium hexafluorophosphate, lithium difluorophosphate, and lithium bis(fluorosulfonyl)imide.

[0103] As a preferred example, in the electrolyte, the mass ratio of the electrolyte solvent, the electrolyte additive and the lithium salt is 79:5:16;

[0104] As a preferred example, in the electrolyte solvent, the mass ratio of the chain carbonate, non-halogenated cyclic carbonate, halogenated cyclic carbonate, and acrylate is 59:9:3:8;

[0105] As a preferred example, the chain carbonate includes one or more of ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, and ethylpropyl carbonate.

[0106] As a further preferred example, the chain carbonate includes ethyl methyl carbonate and diethyl carbonate.

[0107] In low temperature environments, the performance differences of different chain carbonates are mainly reflected in viscosity, melting point, dielectric constant and compatibility with lithium salts. Especially when used with fluorine-containing lithium phosphate salts and fluorine-containing lithium sulfonyl imide salts, the present invention prefers ethyl methyl carbonate and diethyl carbonate, which have low viscosity at low temperatures, can remain liquid, and can better dissolve fluorine-containing lithium phosphate salts and fluorine-containing lithium sulfonyl imide salts. Ethyl methyl carbonate and diethyl carbonate with moderate polarity can better dissolve lithium salts and reduce Li +The migration resistance in the electrolyte can be reduced, and the excessive solvation of lithium salts due to excessive polarity can be avoided, thereby further improving the Li+ intercalation and deintercalation kinetics of the positive electrode active material lithium vanadium phosphate.

[0108] As a preferred example, the non-halogenated cyclic carbonate includes one or more of ethylene carbonate, vinylene carbonate, vinylethylene carbonate, styrene carbonate, butadiene carbonate, and allyl carbonate.

[0109] As a further preferred example, the non-halogenated cyclic carbonate includes ethylene carbonate and vinylene carbonate.

[0110] As a non-halogenated cyclic carbonate, ethylene carbonate contains a saturated five-membered ring and a strong polar ester group, has a high dielectric constant, and effectively shields Li in lithium salts. + The electrostatic attraction between the electrolyte and the fluorine-containing anions increases the dissociation degree of the lithium salt and overcomes the decrease in the ion migration rate at low temperatures. Vinylene carbonate improves the film-forming property of the electrolyte on the positive electrode, inhibits solvent decomposition, and can significantly reduce the positive electrode interface impedance, which is beneficial to the migration of lithium ions. The formed interface film can effectively inhibit the side reactions between the electrolyte and the positive electrode, inhibit the dissolution of transition metal ions in the positive electrode material, and further improve the positive electrode interface stability, thereby improving the cycle performance of the lithium vanadium phosphate battery at low temperatures.

[0111] As a preferred example, the halogenated cyclic carbonate includes one or more of fluoroethylene carbonate, difluoroethylene carbonate, chloroethylene carbonate, and chlorovinylene carbonate.

[0112] As a further preferred example, the halogenated cyclic carbonate includes fluoroethylene carbonate.

[0113] Compared with difluoroethylene carbonate, chloroethylene carbonate and chlorovinyl carbonate, fluoroethylene carbonate has a higher oxidation potential, and the SEI film it forms has stronger lithium conductivity. Due to its ring structure and single fluorine substitution, the LiF content in the SEI film generated by its decomposition is higher and the interface impedance is lower, further reducing the internal resistance of the battery.

[0114] As a preferred example, the acrylate includes one or more of methyl acrylate, ethyl acrylate, and butyl acrylate;

[0115] As a further preferred example, the acrylate includes methyl acrylate;

[0116] As an example, in the electrolyte solvent, the mass ratio of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acrylate, fluoroethylene carbonate and vinylene carbonate is (4.5-11): (30-40): (20-30): (4-12): (1-5): (0.5-2);

[0117] As a further preferred example, in the electrolyte solvent, the mass ratio of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acrylate, fluoroethylene carbonate and vinylene carbonate is 8:35:24:8:3:1;

[0118] Reasonable distribution of the mass ratio of different electrolyte solvents makes the electrolyte solvent have a lower freezing point, lower viscosity and suitable dielectric constant, thereby optimizing the fluidity of the electrolyte solvent at low temperature, making the lithium salt more evenly dispersed in the electrolyte, and the electrolyte fully infiltrates the pores of the positive electrode active material, inhibiting solvent decomposition and gas production under high voltage and high rate, giving the SEI film excellent low-temperature ductility, further reducing the impedance at low temperature, reducing interfacial side reactions, and improving the cycle and rate performance at low temperature.

[0119] As an example, the mass ratio of the fluorine-containing lithium phosphate salt to the fluorine-containing lithium sulfonyl imide salt in the lithium salt is (9.5-18.5): (0.5-1.5).

[0120] As a further preferred example, the mass ratio of lithium hexafluorophosphate, lithium difluorophosphate, and lithium bis(fluorosulfonyl)imide in the lithium salt is 13:2:1;

[0121] As a further preferred example, the mass ratio of the fluorine-containing lithium phosphate salt to the fluorine-containing lithium sulfonyl imide salt in the lithium salt is 15:1.

[0122] As a further preferred example, the mass ratio of lithium hexafluorophosphate to lithium difluorophosphate is (8.5-15.5):(1-3);

[0123] As a further preferred example, the mass ratio of lithium hexafluorophosphate to lithium difluorophosphate is 13:2.

[0124] At this mass ratio, a higher mass ratio of lithium hexafluorophosphate is used, which is rich in PO4 3- The interface transmission capacity of the positive electrode active material is guaranteed at low temperature, and the interface side reaction is reduced. On this basis, the appropriate amount of lithium difluorophosphate can alleviate the low solubility of lithium hexafluorophosphate at low temperature when used alone, improve the solubility of lithium salt, and provide more PO4 3- The interfacial transport capacity of the positive electrode active material at low temperatures is further guaranteed; the addition of a relatively small proportion of lithium bis(fluorosulfonyl)imide to form a LiF-rich solid electrolyte layer on the positive electrode side can effectively prevent the organic solvent of the present invention from contacting the positive electrode material, reducing the occurrence of side reactions under low-temperature operation, but does not affect the interfacial transport capacity due to excessive thickness of the interface layer.

[0125] As a further example, the low-temperature safe lithium-ion battery further includes a positive electrode sheet and a negative electrode sheet.

[0126] The positive electrode sheet is an aluminum foil coated with the positive electrode active material lithium vanadium phosphate, and the negative electrode sheet is a copper foil coated with the negative electrode active material graphite / hard carbon composite material;

[0127] When the positive electrode active material lithium vanadium phosphate is coated on the aluminum foil and the negative electrode active material graphite / hard carbon composite material is coated on the copper foil, an adhesive and a conductive agent are also used.

[0128] As an example, the binder uses an oil-based binder, and the binder includes at least one of thermoplastic resin, acrylic resin, sodium carboxymethyl cellulose, and styrene butadiene rubber.

[0129] As a preferred example, the thermoplastic resin includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride, polytetrafluoroethylene, a copolymer of vinylidene fluoride-hexafluoropropylene, a copolymer of tetrafluoroethylene-hexafluoropropylene, a copolymer of tetrafluoroethylene-perfluoroalkyl vinyl ether, a copolymer of ethylene-tetrafluoroethylene, a copolymer of vinylidene fluoride-tetrafluoroethylene, a copolymer of vinylidene fluoride-trifluoroethylene, a copolymer of vinylidene fluoride-trichloroethylene, a copolymer of vinylidene fluoride-fluoroethylene, a copolymer of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, polyethylene and polypropylene.

[0130] As a preferred example, the acrylic resin includes at least one of vinyl acrylate resin, methyl acrylate resin, butyl acrylate resin, acrylic styrene resin, acrylate resin, acrylate copolymer resin, acrylic resin, and acrylic emulsion resin.

[0131] As a further preferred example, the binder is polyvinylidene fluoride (PVDF).

[0132] As an example, the conductive agent includes at least one of a carbon material, a conductive polymer, and a metal conductor.

[0133] As a preferred example, the carbon material includes one or more of conductive carbon black (Super P), conductive graphite, carbon fiber, carbon nanotube (CNT), Ketjen black, and graphene.

[0134] As a further preferred example, the conductive agent is carbon black (Super P).

[0135] As an example, the mass ratio of lithium vanadium phosphate, polyvinylidene fluoride, and conductive carbon black is (90-99): (1-5): (1-5).

[0136] As a preferred example, the mass ratio of the lithium vanadium phosphate, polyvinylidene fluoride, and conductive carbon black is 92:5:3.

[0137] As an example, the mass ratio of the graphite / hard carbon composite material, polyvinylidene fluoride, and conductive carbon black is (90-99): (1-5): (1-5).

[0138] As a preferred example, the mass ratio of the graphite / hard carbon composite material, polyvinylidene fluoride, and conductive carbon black is 94:3:3.

[0139] As a further example, the diaphragm is selected from one of a polyethylene film, a polypropylene film, a glass fiber film or a composite film;

[0140] As a further example, a low-temperature safe lithium-ion battery further includes a separator, wherein the separator is selected from one of a polyethylene film, a polypropylene film, a glass fiber film, or a composite film;

[0141] As a further preferred example, the diaphragm is a polyethylene film coated with a nano-aluminum oxide coating.

[0142] As a second aspect of the present invention, a method for preparing a low-temperature safe lithium-ion battery is also provided, comprising the following steps:

[0143] S1: Prepare the positive electrode sheet according to the target stoichiometric ratio and control the surface density of the positive electrode sheet to 8-25 mg / cm 2 ; Roller compaction density is 2.0-2.5g / cm 3 ;

[0144] S2: Prepare the negative electrode sheet according to the target stoichiometric ratio and control the surface density of the negative electrode sheet to 5-9 mg / cm 2 ; Roller compaction density is 1.2-1.7g / cm 3 ;

[0145] S3: Preparation of electrolyte: mixing electrolyte solvent, electrolyte additive and lithium salt according to the target stoichiometric ratio;

[0146] S4: Assemble the positive electrode sheet and separator prepared in S1, the negative electrode sheet prepared in S2, and the electrolyte prepared in S3 into a lithium-ion battery.

[0147] As a further solution, the S1 is to mix the positive electrode active material, the binder, and the conductive agent according to the target stoichiometric ratio, and apply the solvent on the aluminum foil. After drying, rolling, and cutting, the positive electrode sheet is obtained.

[0148] As a further solution, the S2 is to mix the negative electrode active material, the adhesive and the conductive agent according to the target stoichiometric ratio, apply the mixture on the copper foil, and obtain the negative electrode sheet after drying, rolling and cutting.

[0149] As a further preferred solution, the surface density of the positive electrode sheet coating is 15 mg / cm2 ; Roller compaction density is 2.2g / cm 3 .

[0150] As a further preferred solution, the surface density of the negative electrode sheet coating is 7 mg / cm 2 ; Roller compaction density is 1.3g / cm 3 .

[0151] As a further solution, the housing of the S4 lithium-ion battery can be a soft pack / square / cylindrical.

[0152] As a further solution, the baking parameters for the drying in S1 and S2 are baking at 85° C. for 24 hours, with nitrogen circulation every 2 hours.

[0153] As a further solution, the injection amount of the lithium-ion battery is 3-5 g / Ah.

[0154] As a further solution, the formation parameters of the lithium-ion battery are: 0.05C charging to 3.5V, 0.1C charging to 4.2V.

[0155] In order to facilitate understanding of the present invention, the present invention will be described in more detail below, and embodiments of the present invention are given, but the scope of the present invention is not limited thereby.

[0156] Example 1:

[0157] Positive electrode sheet: The positive electrode active material lithium vanadium phosphate, conductive agent conductive carbon black Super P, and binder polyvinylidene fluoride (PVDF) are thoroughly stirred and mixed in N-methylpyrrolidone (NMP) solvent system, coated on aluminum foil, dried, and rolled to obtain the positive electrode sheet. The coating surface density is 15mg / cm 2 ; Roller press to 2.2g / cm 3 .

[0158] Negative electrode sheet: The negative electrode active material graphite / hard carbon composite material, conductive agent conductive carbon black Super P, and binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in N-methylpyrrolidone (NMP) solvent system, coated on copper foil, dried, and rolled to obtain the negative electrode sheet. The coating surface density is 7mg / cm 2 ; Roller pressed to 1.3g / cm 3 .

[0159] Diaphragm: Polyethylene (PE) is used as the base film and a nano-aluminum oxide coating is coated on the base film as the diaphragm.

[0160] Electrolyte: In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), EC (ethylene carbonate): EMC (ethyl methyl carbonate): DEC (diethyl carbonate): MA (methyl acrylate): FEC (fluoroethylene carbonate): AND (adiponitrile): VC (vinylene carbonate) were mixed in a mass ratio of 8:35:24:8:3:5:1. Lithium hexafluorophosphate, lithium difluorophosphate, and lithium bis(fluorosulfonyl)imide in a mass ratio of 13:2:1 were added to the mixed solution and then slowly added, and stirred until they were completely dissolved to obtain the electrolyte in this embodiment.

[0161] The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence and wound in the same direction to obtain a bare battery cell, which is then packaged with aluminum-plastic film. The battery after liquid injection undergoes conventional processes such as packaging, storage, formation, aging, secondary packaging, and capacity division to obtain a lithium-ion battery.

[0162] Comparative Example 1:

[0163] Positive electrode sheet: The positive electrode active material, lithium vanadium phosphate, conductive agent, conductive carbon black Super P, and binder, polyvinylidene fluoride (PVDF), were thoroughly stirred and mixed in an N-methylpyrrolidone (NMP) solvent system. The mixture was then coated onto aluminum foil, dried, and roll-pressed to produce the positive electrode sheet. The coating had an area density of 14 mg / cm², which was then rolled to 2.3 g / cm³.

[0164] Negative electrode sheet: The negative electrode active material graphite / hard carbon composite material, the conductive agent conductive carbon black Super P, and the binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in an N-methylpyrrolidone (NMP) solvent system, coated on copper foil, dried, and roll-pressed to obtain a negative electrode sheet with a coated surface density of 6.5 mg / cm2; rolled to 1.4 g / cm3.

[0165] Diaphragm: Polyethylene (PE) is used as the base film and a nano-aluminum oxide coating is coated on the base film as the diaphragm.

[0166] Electrolyte: In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), EC (ethylene carbonate): EMC (ethyl methyl carbonate): DEC (diethyl carbonate): MA (methyl acrylate): FEC (fluoroethylene carbonate): DTD (vinyl sulfate): VC (vinylene carbonate) were mixed in a mass ratio of 8:37:24:8:3:1:1, and lithium hexafluorophosphate, lithium difluorophosphate and lithium bis(fluorosulfonyl)imide) were added slowly in a mass ratio of 13:2:1 to the mixed solution, and stirred until they were completely dissolved to obtain the electrolyte in this embodiment.

[0167] The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence and wound in the same direction to obtain a bare battery cell, which is then packaged with aluminum-plastic film. The battery after liquid injection undergoes conventional processes such as packaging, storage, formation, aging, secondary packaging, and capacity division to obtain a lithium-ion battery.

[0168] Comparative Example 2:

[0169] The difference from Example 1 is that the positive electrode active material is lithium iron phosphate (LFP), and the rest is the same as Example 1;

[0170] Comparative Example 3:

[0171] The difference from Example 1 is that the positive electrode active material is sodium ferric pyrophosphate (NFPP), and the other parts are the same as Example 1.

[0172] The above examples and comparative examples were subjected to electrochemical performance tests:

[0173] In order to illustrate the effectiveness of the present invention in detail, Example 1 and Comparative Examples 1-3 were subjected to performance tests:

[0174] (1) Battery Low-Temperature Cycling Performance Test: Place the lithium-ion battery in a -20°C thermostat for 2 hours to allow the lithium-ion battery to reach a constant temperature. Cycle the constant-temperature lithium-ion battery at 0.2C for 300 cycles, and record the discharge capacity and the average coulombic efficiency over 300 cycles. The capacity retention rate at the 300th cycle is calculated as follows: discharge capacity at the 300th cycle / discharge capacity at the first cycle * 100%.

[0175] (2) Battery Low-Temperature Rate Performance Test: Place the lithium-ion battery in a -10°C thermostat for 2 hours to allow the lithium-ion battery to reach a constant temperature. Cycle the constant-temperature lithium-ion battery at 3C charge and discharge, and record the discharge capacity. Cycle Capacity Retention at 100th Cycle = Discharge Capacity at 100th Cycle / Discharge Capacity at First Cycle * 100%.

[0176] like Figure 1 As shown, in Example 1, under the lithium vanadium phosphate system, the electrolyte system plays an important role in the low-temperature performance of the battery, and both the capacity retention rate and the coulombic efficiency are high.

[0177] like Figure 2-3 As shown, when Example 1 is compared with Comparative Examples 2-3, when the positive electrode active material is replaced with other materials, the capacity retention rate is lower after 100 cycles of 3C charge and discharge at low temperature, which shows that the lithium vanadium phosphate positive electrode active material plays a key role in the low-temperature rate performance.

[0178] In summary, the present invention provides a lithium vanadium phosphate battery suitable for high rate, high cycle performance, and low battery internal resistance growth at low temperature, which solves the problem that the positive electrode material in the prior art is prone to side reactions between the electrolyte and the electrode material in a low temperature, high rate environment, causing excessive growth of the positive electrode solid electrolyte interface, destroying the structure of the positive electrode material, and causing the positive electrode to have poor compatibility with the electrolyte. The present invention uses chain carbonates, non-halogenated cyclic carbonates, halogenated cyclic carbonates, and acrylates as electrolyte solvents, and simultaneously compounds dinitrile with reduced electrolyte viscosity and improved performance at low temperatures, further considering the composition and ratio of lithium salts in the electrolyte, and the combined use of positive electrode materials, ensuring the stability of the positive electrode material, reducing polarization, reducing charge transfer resistance, improving electrical conductivity, improving the diffusion coefficient of lithium ions, promoting reversible extraction and embedding of lithium ions, reducing phase change during charge and discharge, and significantly improving the electrochemical properties of lithium vanadium phosphate positive electrode materials, especially, improving the cycle stability and rate performance of lithium vanadium phosphate batteries at low temperatures, and reducing the increase in internal resistance.

[0179] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A low-temperature safe lithium-ion battery, characterized in that: including a positive electrode active material, a negative electrode active material and an electrolyte; The positive electrode active material includes lithium vanadium phosphate; The negative electrode active material includes a graphite / hard carbon composite material; The electrolyte comprises an electrolyte solvent, an electrolyte additive and a lithium salt; The mass ratio of the electrolyte solvent, the electrolyte additive and the lithium salt is (60-100): (1-10): (10-20); The electrolyte solvent includes cyclic carbonate, chain carbonate and acrylate; The cyclic carbonates include non-halogenated cyclic carbonates and halogenated cyclic carbonates; In the electrolyte solvent, the mass ratio of the chain carbonate, non-halogenated cyclic carbonate, halogenated cyclic carbonate, and acrylate is (50-70): (5-13); (1-5): (4-12); The electrolyte additive is dinitrile; The lithium salt includes at least a fluorine-containing lithium phosphate salt and a fluorine-containing lithium sulfonyl imide salt.

2. A low-temperature safe lithium-ion battery according to claim 1, characterized in that: The electrolyte additive is one or more of adiponitrile, succinonitrile, glutaronitrile, ethylene glycol dinitrile, and perfluoro dinitrile; Preferably, the electrolyte additive is adiponitrile.

3. A low-temperature safe lithium-ion battery according to claim 1, characterized in that: The fluorine-containing lithium phosphate salt is one or more of lithium hexafluorophosphate, lithium difluorophosphate, and lithium tetrafluorophosphate; Preferably, the fluorine-containing lithium phosphate salt is lithium hexafluorophosphate and lithium difluorophosphate; Preferably, the fluorine-containing lithium sulfonyl imide salt is selected from one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium fluorosulfonyltrifluoromethylsulfonyl imide, and lithium perfluoroalkylsulfonyl imide; More preferably, the fluorine-containing lithium sulfonyl imide salt is lithium bis(fluorosulfonyl imide).

4. A low-temperature safe lithium-ion battery according to claim 1, characterized in that: In the electrolyte, the mass ratio of the electrolyte solvent, the electrolyte additive and the lithium salt is 79:5:

16.

5. A low-temperature safe lithium-ion battery according to claim 1, characterized in that: In the electrolyte solvent, the mass ratio of the chain carbonate, the non-halogenated cyclic carbonate, the halogenated cyclic carbonate, and the acrylate is 59:9:3:

8.

6. A low-temperature safe lithium-ion battery according to claim 1, characterized in that: The chain carbonate includes one or more of ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate; Preferably, the chain carbonate includes ethyl methyl carbonate and diethyl carbonate; Preferably, the non-halogenated cyclic carbonate includes one or more of ethylene carbonate, vinylene carbonate, vinylethylene carbonate, styrene carbonate, butadiene carbonate, and allyl carbonate; Preferably, the non-halogenated cyclic carbonate includes ethylene carbonate and vinylene carbonate; Preferably, the halogenated cyclic carbonate includes one or more of fluoroethylene carbonate, difluoroethylene carbonate, chloroethylene carbonate, and chlorovinylene carbonate; Preferably, the halogenated cyclic carbonate comprises fluoroethylene carbonate; Preferably, the acrylate includes one or more of methyl acrylate, ethyl acrylate, and butyl acrylate; Preferably, the acrylate comprises methyl acrylate; Further preferably, the mass ratio of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acrylate, fluoroethylene carbonate and vinylene carbonate is (4.5-11): (30-40): (20-30): (4-12): (1-5): (0.5-2); Further preferably, in the electrolyte solvent, the mass ratio of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acrylate, fluoroethylene carbonate and vinylene carbonate is 8:35:24:8:3:

1.

7. A low-temperature safe lithium-ion battery according to claim 1, characterized in that: The mass ratio of the fluorine-containing lithium phosphate salt to the fluorine-containing lithium sulfonyl imide salt in the lithium salt is (9.5-18.5): (0.5-1.5); Preferably, the mass ratio of the fluorine-containing lithium phosphate salt to the fluorine-containing lithium sulfonyl imide salt in the lithium salt is 15:1; Further preferably, the mass ratio of lithium hexafluorophosphate to lithium difluorophosphate is (8.5-15.5):(1-3); Further preferably, the mass ratio of lithium hexafluorophosphate to lithium difluorophosphate is 13:

2.

8. The low-temperature safe lithium-ion battery according to claim 1, characterized in that: The low-temperature safety lithium-ion battery also includes a positive electrode sheet and a negative electrode sheet. The positive electrode sheet is an aluminum foil coated with the positive electrode active material lithium vanadium phosphate, and the negative electrode sheet is a copper foil coated with the negative electrode active material graphite / hard carbon composite material; When the positive electrode active material lithium vanadium phosphate is coated on the aluminum foil and the negative electrode active material graphite / hard carbon composite material is coated on the copper foil, an adhesive and a conductive agent are also used; Preferably, the binder uses an oil-based binder, and the binder includes at least one of a thermoplastic resin, an acrylic resin, sodium carboxymethyl cellulose, and styrene butadiene rubber; Preferably, the thermoplastic resin includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride, polytetrafluoroethylene, a copolymer of vinylidene fluoride-hexafluoropropylene, a copolymer of tetrafluoroethylene-hexafluoropropylene, a copolymer of tetrafluoroethylene-perfluoroalkyl vinyl ether, a copolymer of ethylene-tetrafluoroethylene, a copolymer of vinylidene fluoride-tetrafluoroethylene, a copolymer of vinylidene fluoride-trifluoroethylene, a copolymer of vinylidene fluoride-trichloroethylene, a copolymer of vinylidene fluoride-fluoroethylene, a copolymer of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, polyethylene and polypropylene; Preferably, the acrylic resin includes at least one of vinyl acrylate resin, methyl acrylate resin, butyl acrylate resin, acrylic styrene resin, acrylate resin, acrylate copolymer resin, acrylic resin, and acrylic emulsion resin; Preferably, the binder is polyvinylidene fluoride (PVDF); Preferably, the conductive agent includes at least one of a carbon material, a conductive polymer, and a metal conductor; Preferably, the carbon material includes one or more of conductive carbon black (Super P), conductive graphite, carbon fiber, carbon nanotube (CNT), Ketjen black, and graphene; Preferably, the conductive agent is carbon black (Super P); Preferably, the mass ratio of lithium vanadium phosphate, polyvinylidene fluoride, and conductive carbon black is (90-99): (1-5): (1-5); Preferably, the mass ratio of lithium vanadium phosphate, polyvinylidene fluoride, and conductive carbon black is 92:5:3; Preferably, the mass ratio of the graphite / hard carbon composite material, polyvinylidene fluoride, and conductive carbon black is (90-99): (1-5): (1-5); Preferably, the mass ratio of the graphite / hard carbon composite material, polyvinylidene fluoride, and conductive carbon black is 94:3:

3.

9. A low-temperature safe lithium-ion battery according to claim 1, characterized in that: Also includes the diaphragm; The diaphragm is selected from one of polyethylene film, polypropylene film, glass fiber film or composite film; Preferably, the diaphragm is a polyethylene film coated with a nano-aluminum oxide coating.

10. The method for preparing a low-temperature safe lithium-ion battery according to any one of claims 1 to 9, characterized in that: The steps include: S1: Prepare the positive electrode sheet according to the target stoichiometric ratio and control the surface density of the positive electrode sheet to 8-25 mg / cm 2 ; Roller compaction density is 2.0-2.5g / cm 3 ; S2: Prepare the negative electrode sheet according to the target stoichiometric ratio and control the surface density of the negative electrode sheet to 5-9 mg / cm 2 ; Roller compaction density is 1.2-1.7g / cm 3 ; S3: Preparation of electrolyte: mixing electrolyte solvent, electrolyte additive and lithium salt according to the target stoichiometric ratio; S4: Assemble the positive electrode sheet and separator prepared in S1, the negative electrode sheet prepared in S2, and the electrolyte prepared in S3 into a lithium-ion battery; Preferably, the S1 is to mix the positive electrode active material, the binder and the conductive agent according to the target stoichiometric ratio, apply the solvent on the aluminum foil, and obtain the positive electrode sheet after drying, rolling and cutting. Preferably, the S2 is to mix the negative electrode active material, the binder and the conductive agent according to the target stoichiometric ratio, apply the mixture on the copper foil, and obtain the negative electrode sheet after drying, rolling and cutting. Preferably, the surface density of the positive electrode sheet coating is 15 mg / cm 2 ; Roller compaction density is 2.2g / cm 3 ; Preferably, the surface density of the negative electrode sheet coating is 7 mg / cm 2 ; Roller compaction density is 1.3g / cm 3 .

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