A non-aqueous electrolyte and a lithium-ion battery

Abstract

The present invention relates to the technical field of lithium-ion batteries, and particularly relates to a non-aqueous electrolyte and a lithium-ion battery. The non-aqueous electrolyte includes a lithium salt and an organic solvent; in the non-aqueous electrolyte of the present invention, at least one carboxylic acid ester such as methyl propionate and ethyl butyrate is used as the organic solvent, two different lithium salts are adopted and one of them is lithium hexafluorophosphate, and the relationship among the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of the lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte satisfies 1.8 ≤ (10×d + n×b) / a ≤ 18, 10 ≤ a ≤ 60, 30 ≤ b ≤ 98, 0.8 ≤ n ≤ 1.5, and 3.2 ≤ d ≤ 10. When these conditions are met, the high-rate and high-safety characteristics of the battery can be balanced, the safety hazards of high-rate lithium battery products are reduced, and at the same time, it is beneficial to further improve the charging rate of existing batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium - ion batteries, and particularly to a non - aqueous electrolyte and a lithium - ion battery that take into account the rate performance and safety performance of the battery. Background Art

[0002] Due to its advantages such as high working voltage, wide working temperature range, large energy density and power density, no memory effect, and long cycle life, lithium - ion batteries have been widely used in the fields of 3C digital products such as mobile phones and laptop computers, as well as in the field of new energy vehicles. In recent years, the development of lithium - ion batteries has tended towards high energy density, high rate, and high safety.

[0003] Under this market trend, battery materials tend to use high - compaction, low - porosity positive and negative electrode materials and thin - layer separators to increase the proportion of positive and negative active substances in the battery and improve the energy density of the battery. However, in such a battery system, the porosity is very low, resulting in insufficient diffusion and infiltration of the electrolyte on the electrode plate, seriously affecting the battery cycle rate, leading to lithium deposition during battery cycling at high rates, and further causing the battery cycle to fail due to voltage drop. In order to develop a battery that takes into account both high energy density and high rate, how to design an electrolyte with high ionic conductivity and high lithium - ion transference number has become the key; in order to meet the high - safety characteristics of the battery, the electrolyte needs to further satisfy good compatibility with the battery at high temperatures and high pressures.

[0004] Currently, the market mainly uses two solvents, ethyl acetate and ethyl propionate, to reduce the viscosity of the electrolyte and improve the lithium - conducting ability of the electrolyte, but the safety performance of the battery has been seriously deteriorated, restricting the popularization and use of fast - charging electrolytes. Summary of the Invention

[0005] In order to solve the above - mentioned technical problems, the present invention provides a non - aqueous electrolyte and a lithium - ion battery that take into account the rate performance and safety performance of the battery.

[0006] The present invention adopts the following technical solutions:

[0007] A non - aqueous electrolyte, the non - aqueous electrolyte comprising a lithium salt and an organic solvent;

[0008] The organic solvent comprises at least one carboxylic acid ester such as methyl propionate and ethyl butyrate;

[0009] The lithium salt comprises at least two kinds, and one of them is lithium hexafluorophosphate;

[0010] The non - aqueous electrolyte satisfies the following conditions:

[0011] 1.8 ≤ (10×d + n×b) / a ≤ 18, 10 ≤ a ≤ 60, 30 ≤ b ≤ 98, 0.8 ≤ n ≤ 1.5, 3.2 ≤ d ≤ 10;

[0012] Among them, a is the mass percentage of carboxylic acid ester in the non-aqueous electrolyte, with the unit of wt%;

[0013] b is the mass percentage of lithium hexafluorophosphate in the lithium salt, with the unit of wt%;

[0014] n is the total content of lithium salt in the non-aqueous electrolyte, with the unit of mol / L;

[0015] d is the thermal expansion rate of the non-aqueous electrolyte, with the unit of %;

[0016] The test method for the thermal expansion rate of the non-aqueous electrolyte is as follows: Place the non-aqueous electrolyte in a container with a volume scale, and then place the non-aqueous electrolyte in a high and low temperature test chamber and keep it at 25°C for 2 hours. Read and record the volume of the non-aqueous electrolyte as V1; Raise the temperature of the high and low temperature test chamber to 85°C and keep it at 85°C for 2 hours. Read and record the volume of the non-aqueous electrolyte as V2; The thermal expansion rate d of the non-aqueous electrolyte = (V2 - V1) / V1 × 100%.

[0017] The non-aqueous electrolyte of the present invention uses at least one carboxylic acid ester such as methyl propionate and ethyl butyrate as an organic solvent, and two different lithium salts, one of which is lithium hexafluorophosphate. Through a large number of studies, the inventors found that when the relationship between the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of the lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte satisfies 1.8 ≤ (10×d + n×b) / a ≤ 18, 10 ≤ a ≤ 60, 30 ≤ b ≤ 98, 0.8 ≤ n ≤ 1.5, and 3.2 ≤ d ≤ 10, they can jointly affect the rate performance and safety performance of the battery, forming a unified whole with an interaction. It is speculated that methyl propionate and ethyl butyrate have excellent dielectric constant and low viscosity characteristics, which helps to balance the rate performance and safety performance of the battery; by controlling the total amount of the lithium salt to maintain the high conductivity characteristics of the electrolyte, and using the interfacial film formed by lithium hexafluorophosphate on the negative electrode to improve the high-temperature stability of the battery, while the introduction of other lithium salts further contributes to the stability of the interfacial film; the limitation of the thermal expansion rate of the electrolyte can control the high-temperature performance of the battery and prevent safety problems such as battery fire. When the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of the lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte are in a synergistic state, the rate performance and safety performance of the battery can be balanced. Preferably, the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of the lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte satisfy 2.5 ≤ (10×d + n×b) / a ≤ 12.5; more preferably, the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of the lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte satisfy 3.2 ≤ (10×d + n×b) / a ≤ 12.5.

[0018] In the present invention, the organic solvent is at least one carboxylic acid ester selected from methyl propionate and ethyl butyrate. Carboxylic acid esters are superior to conventional carbonate solutions in enhancing the rate performance of batteries. However, the compatibility of carboxylic acid esters with other components of the battery is poor, and most carboxylic acid esters cause significant corrosion to lithium metal and graphite negative electrode sheets at high temperatures, leading to battery fires. Among carboxylic acid ester solvents, compared with propyl formate, ethyl acetate, propyl acetate, and propyl propionate, methyl propionate has more excellent high-temperature negative electrode compatibility; compared with methyl acetate, ethyl formate, and methyl formate, it has higher melting and boiling points, better thermodynamic stability, excellent dielectric constant, and low viscosity characteristics. It is an optimized solvent that takes into account both the rate performance and thermal safety performance of the battery. Ethyl butyrate has one more methyl group extended on the basis of methyl propionate, has a similar dipole moment and polarity, and also has good high-temperature compatibility at the battery negative electrode. Specifically, in some embodiments of the present invention, the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte is 10wt%, 12wt%, 15wt%, 18wt%, 20wt%, 24wt%, 25wt%, 27wt%, 30wt%, 32wt%, 35wt%, 38wt%, 40wt%, 42wt%, 45wt%, 48wt%, 50wt%, 52wt%, 55wt%, 58wt%, 60wt% or an intermediate value composed of any two of these values. If the value of a is too low, its improvement of the conductive characteristics of the electrolyte cannot be effectively reflected; if the value of a is too high, the thermodynamic stability of the electrolyte will decrease, and the volume expansion of the electrolyte at high temperatures will cause uneven system distribution. Preferably, the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte is 15wt% - 50wt%; more preferably, the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte is 15wt% - 30wt%.

[0019] The content of the lithium salt is one of the key factors affecting the conductive characteristics of the non-aqueous electrolyte. In order to maintain the high conductivity characteristics of the non-aqueous electrolyte, in some embodiments of the present invention, the total content n of the lithium salt in the non-aqueous electrolyte is 0.8mol / L, 0.85mol / L, 0.9mol / L, 0.95mol / L, 1.0mol / L, 1.05mol / L, 1.1mol / L, 1.15mol / L, 1.2mol / L, 1.25mol / L, 1.3mol / L, 1.35mol / L, 1.4mol / L, 1.45mol / L, 1.5mol / L or an intermediate value composed of any two of these values. If the value of n is too low, the conductivity of the electrolyte is low; if the value of n is too high, the dissociation of anions and cations is incomplete, resulting in a decrease in the lithium ion transference number and a decrease in conductivity. Preferably, the total content n of the lithium salt in the non-aqueous electrolyte is 1.0mol / L - 1.4mol / L.

[0020] The lithium salt of the present invention comprises lithium hexafluorophosphate. As a traditional lithium salt, lithium hexafluorophosphate can form a SEI interfacial film rich in LiF at the negative electrode of the battery, improving the high-temperature stability of the battery interface. At the same time, the content of lithium hexafluorophosphate in the lithium salt needs to be above 30 wt% to ensure the strength of the battery SEI and the interfacial repair during the cycle storage process. However, the interfacial film formed by a single lithium hexafluorophosphate cannot support the efficient insertion and extraction of lithium ions at the negative electrode of the battery. Therefore, other lithium salts need to be introduced to improve the interfacial lithium conduction efficiency. Specifically, in some embodiments of the present invention, the mass percentage content b of lithium hexafluorophosphate in the lithium salt is 30 wt%, 35 wt%, 38 wt%, 40 wt%, 45 wt%, 48 wt%, 50 wt%, 52 wt%, 55 wt%, 58 wt%, 60 wt%, 62 wt%, 65 wt%, 68 wt%, 70 wt%, 72 wt%, 75 wt%, 78 wt%, 80 wt%, 82 wt%, 85 wt%, 88 wt%, 90 wt%, 92 wt%, 95 wt%, 98 wt% or an intermediate value composed of any two of these values. Preferably, the mass percentage content b of lithium hexafluorophosphate in the lithium salt is 60 wt% - 97 wt%.

[0021] Specifically, in some embodiments of the present invention, the lithium salt further comprises one or more of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluoro(oxalato)borate (LiODFB), lithium difluoro(oxalato)phosphate (LiODFP), lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2). By jointly using lithium hexafluorophosphate with any one or more of the above lithium salts, compared with the single addition of lithium hexafluorophosphate, the present invention introduces higher contents of inorganic components such as Li2O and Li3N to improve the interfacial lithium conduction efficiency. At the same time, the introduction of elements such as B, S, and P can improve the stability of the interfacial film to a certain extent.

[0022] The thermal expansion rate of the non-aqueous electrolyte is a thermodynamic property of the electrolyte at high temperatures and is determined by the overall composition of the non-aqueous electrolyte. Since the maximum temperature rise of the battery during the rate cycling process can reach 30 - 40 °C, the non-uniformity of the temperature inside the battery will lead to the generation of non-uniformity in the system. The greater the thermal expansion rate of the electrolyte, the more obvious the dilution phenomenon of the electrolyte in the high-temperature region, and the non-uniformity inside the battery is amplified; at the same time, the side reactions intensify at high temperatures, further causing an increase in the consumption of the electrolyte, resulting in local electrolyte dryness, lithium deposition in the battery, a sharp drop in the cycle capacity, and battery fire. Specifically, in some embodiments of the present invention, the thermal expansion rate d of the non-aqueous electrolyte is 3.2%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10% or the intermediate value formed by any two of these values. Preferably, the thermal expansion rate d of the non-aqueous electrolyte is 3.2% - 8%.

[0023] For the test method of the thermal expansion rate of the non-aqueous electrolyte, the present invention sets 85 °C as the heating height. Since the 85 °C test is the highest temperature in the current conventional battery tests, the thermal expansion rate at the extreme temperature is more representative. More specifically, the test method for the thermal expansion rate of the non-aqueous electrolyte of the present invention is as follows: Place the non-aqueous electrolyte in a glass bottle with a volume scale and fill it to 2 / 3 of the volume of the container. Then place the non-aqueous electrolyte in a high and low temperature test chamber with a glass observation window, set the temperature of the high and low temperature test chamber to 25 °C and keep it warm for 2 h. Read the volume of the non-aqueous electrolyte parallel to the concave liquid surface of the non-aqueous electrolyte from the glass observation window and record it as V1; Heat the high and low temperature test chamber to 85 °C at a rate of 5 °C / min and keep it warm at 85 °C for 2 h. Read the volume of the non-aqueous electrolyte parallel to the concave liquid surface of the non-aqueous electrolyte from the glass observation window and record it as V2; The thermal expansion rate d of the non-aqueous electrolyte = (V2 - V1) / V1 × 100%.

[0024] The thermal expansion rate of the non-aqueous electrolyte is closely related to the composition of the lithium salt, organic solvent, and additive in the non-aqueous electrolyte, and can be regulated by adding free low-boiling solvents, lithium salts, and additives with stronger polarity. Specifically, the thermal expansion rate of the non-aqueous electrolyte is most affected by free solvent molecules. The more free low-boiling solvents, the higher the thermal expansion rate; the larger the anion of the lithium salt, the lower the ability to combine with solvent molecules, and the thermal expansion rate of the non-aqueous electrolyte will relatively increase; the addition of additives with stronger polarity such as FEC, VC, and nitriles will preferentially combine with lithium ions and anions to form a solvent shell layer, pushing aside solvent molecules, resulting in an increase in free solvent molecules and an increase in the thermal expansion rate; however, due to the high-boiling characteristics of the additive molecules themselves, the overall thermal expansion rate will be reduced to a certain extent, and the thermal expansion rate of the electrolyte can be fine-tuned by adding additives.

[0025] Specifically, in some embodiments of the present invention, the conductivity σ of the non-aqueous electrolyte satisfies: 6.5 mS / cm to 10 mS / cm.

[0026] Specifically, in some embodiments of the present invention, the organic solvent further includes at least one of cyclic carbonate solvents, linear carbonate solvents, carboxylic acid ester solvents, and ether solvents.

[0027] In some preferred embodiments, the cyclic carbonate solvents include at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinylene carbonate, and butylene carbonate.

[0028] In some preferred embodiments, the linear carbonate solvents include at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate.

[0029] In some preferred embodiments, the carboxylic acid ester solvents include at least one of ethyl acetate, ethyl propionate, propyl propionate, methyl acrylate, ethyl difluoroacetate, methyl acetate, methyl butyrate, isobutyl methyl ester, trimethylacetic acid methyl ester, and trimethylacetic acid ethyl ester, excluding methyl propionate and ethyl butyrate.

[0030] In some preferred embodiments, the ether solvents include at least one of ethylene glycol dimethyl ether, 1,3-dioxolane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0031] Specifically, in some embodiments of the present invention, the non-aqueous electrolyte further includes an additive, and the additive includes at least one of cyclic sulfate compounds, sulfonic acid lactone compounds, cyclic carbonate compounds, phosphate compounds, and nitrile compounds.

[0032] In some preferred embodiments, the cyclic sulfate compounds include at least one of 4-methyl vinyl sulfate, vinyl sulfate, and propylene sulfate.

[0033] In some preferred embodiments, the sulfonic acid lactone compounds include at least one of 1,3-propane sultone, 1,4-butane sultone, and allyl-1,3-sultone.

[0034] In some preferred embodiments, the cyclic carbonate compounds include at least one of vinylene carbonate, ethylene vinylene carbonate, methylene ethylene carbonate, fluoroethylene carbonate, trifluoromethyl ethylene carbonate, difluoroethylene carbonate, and the compound shown in the following structural formula 1:

[0035] ;

[0036] In the shown structural formula 1, R21 , R 22 , R 23 , R 24 , R 25 , R 26 are each independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group;

[0037] In some preferred embodiments, the compound represented by Structural Formula 1 includes at least one of the compounds represented by Compounds 1-1 to 1-6 below:

[0038] Compound 1-1; Compound 1-2; Compound 1-3; Compound 1-4; Compound 1-5; Compound 1-6.

[0039] In some preferred embodiments, the phosphate compound includes at least one of tris(trimethylsilyl) phosphate, tris(triethylsilyl) phosphate, and the compound represented by the following Structural Formula 2:

[0040] Structural Formula 2;

[0041] In the Structural Formula 2, R 31 , R 32 , R 33 are each independently selected from a C1-C5 saturated hydrocarbon group, an unsaturated hydrocarbon group, and a halogenated hydrocarbon group, and at least one of R 31 , R 32 , R 33 is an unsaturated hydrocarbon group; more preferably, the compound represented by Structural Formula 2 includes at least one of triallyl phosphate, diallylmethyl phosphate, diallylethyl phosphate, diallylpropyl phosphate, diallyltrifluoromethyl phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, diallylhexafluoroisopropyl phosphate, tripropargyl phosphate, dipropargylmethyl phosphate, dipropargylethyl phosphate, dipropargylpropyl phosphate, dipropargyltrifluoromethyl phosphate, dipropargyl-2,2,2-trifluoroethyl phosphate, dipropargyl-3,3,3-trifluoropropyl phosphate, and dipropargylhexafluoroisopropyl phosphate.

[0042] In some preferred embodiments, the nitrile compound includes at least one of succinonitrile, glutarodinitrile, ethylene glycol bis(propionitrile) ether, hexane trinitrile, adiponitrile, pimelonitrile, suberonitrile, azelaonitrile, and sebaconitrile.

[0043] In some embodiments, based on the total mass of the non-aqueous electrolyte being 100%, the content of the additive is 0.01 wt% to 10 wt%. Preferably, the content is 0.1 wt% to 5 wt%; more preferably, the content is 0.1 wt% to 2 wt%. Specifically, the content of any optional substance in the additive can be 0.01 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2 wt%, 2.2 wt%, 2.5 wt%, 2.8 wt%, 3 wt%, 3.2 wt%, 3.5 wt%, 3.8 wt%, 4 wt%, 4.5 wt%, 5 wt%.

[0044] In a second aspect, the present invention also provides a lithium-ion battery, comprising a positive electrode, a negative electrode, and the above-mentioned non-aqueous electrolyte.

[0045] The negative electrode includes a negative electrode material layer containing a negative electrode active material. Specifically, in some embodiments of the present invention, the negative electrode material is any one or several of a silicon-based material and a carbon material. The silicon-based material is selected from one or more of a silicon material, a silicon oxide material, a silicon-carbon material, and a silicon alloy material; preferably, the silicon material is a nano-silicon material; preferably, the silicon oxide material is a SiOx material, where 0 ≤ x < 2; preferably, the silicon-carbon material is: a silicon-based material containing silicon and a carbon material, and / or a silicon-based material containing SiOy and a carbon material, where 0 ≤ y < 2; preferably, the silicon alloy material is a Mg2Si alloy material and / or an Fe2Si alloy material. The carbon material is selected from one or more of artificial graphite, natural graphite, composite graphite, graphene, and hard carbon; preferably, the carbon material is an artificial graphite material.

[0046] Specifically, in some embodiments of the present invention, the negative electrode material layer further includes a negative electrode binder and a negative electrode conductive agent.

[0047] The negative electrode binder includes one or more of thermoplastic resins such as polyvinylidene fluoride, copolymers of vinylidene fluoride, polytetrafluoroethylene, copolymers of vinylidene fluoride - hexafluoropropylene, copolymers of tetrafluoroethylene - hexafluoropropylene, copolymers of tetrafluoroethylene - perfluoroalkyl vinyl ether, copolymers of ethylene - tetrafluoroethylene, copolymers of vinylidene fluoride - tetrafluoroethylene, copolymers of vinylidene fluoride - trifluoroethylene, copolymers of vinylidene fluoride - trichloroethylene, copolymers of vinylidene fluoride - fluoroethylene, copolymers of vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene, thermoplastic polyimide, polyethylene, and polypropylene; acrylic resins; and styrene-butadiene rubber.

[0048] The negative electrode conductive agent includes one or more of conductive carbon black, conductive carbon spheres, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene, or reduced graphene oxide.

[0049] Specifically, in some embodiments of the present invention, the negative electrode further includes a negative electrode current collector, and the negative electrode material layer is disposed on the surface of the negative electrode current collector. The negative electrode current collector includes a metal material capable of conducting electrons. Preferably, the negative electrode current collector includes one or more of aluminum, nickel, tin, copper, and stainless steel.

[0050] Specifically, in some embodiments of the present invention, the positive electrode includes a positive electrode material layer, and the positive electrode material layer includes a positive electrode active material. The positive electrode active material may include LiFe 1-x’ M’ x’ PO4, LiMn 2-y’ M y’ O4, and LiNi x Co y Mn z M 1-x-y-z O2, where M’ is selected from one or more of Mn, Mg, Co, Ni, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, Zr, W, V, or Ti, M is selected from one or more of Fe, Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, Zr, W, V, or Ti, and 0 ≤ x’ < 1, 0 ≤ y’ ≤ 1, 0 ≤ y ≤ 1, 0 ≤ x ≤ 1, 0 ≤ z ≤ 1, x + y + z ≤ 1. The positive electrode active material may further include one or several of sulfides, selenides, and halides. More preferably, the positive electrode active material may include LiCoO2, LiFePO4, LiFe 0.4 Mn 0.6 PO4, LiMn2O4, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.5 Co 0.2Mn 0.2 Al 0.1 O2, LiNi 0.5 Co 0.2 Al 0.3 One or more of O2.

[0051] Specifically, in some embodiments of the present invention, the positive electrode further includes a positive electrode current collector, and the positive electrode material layer is disposed on the surface of the positive electrode current collector. The material of the positive electrode current collector may be the same as that of the positive electrode current collector, which will not be elaborated herein.

[0052] Specifically, in some embodiments of the present invention, the positive electrode material layer further includes a positive electrode binder and a positive electrode conductive agent, and the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent are blended to obtain the positive electrode material layer. The positive electrode binder and the positive electrode conductive agent may be the same as the negative electrode binder and the negative electrode conductive agent respectively, which will not be elaborated herein.

[0053] Specifically, in some embodiments of the present invention, the lithium ion battery further includes a separator, and the separator is located between the positive electrode and the negative electrode.

[0054] The separator can be an existing conventional separator, which can be a ceramic separator, a polymer separator, a non-woven fabric, an inorganic-organic composite separator, etc., including but not limited to single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP, and triple-layer PP / PE / PP separators.

[0055] For the non-aqueous electrolyte of the present invention, at least one carboxylic acid ester such as methyl propionate and ethyl butyrate is used as an organic solvent, two different lithium salts are used and one of them is lithium hexafluorophosphate, and the relationship between the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of the lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte satisfies 1.8 ≤ (10×d + n×b) / a ≤ 18, 10 ≤ a ≤ 60, 30 ≤ b ≤ 98, 0.8 ≤ n ≤ 1.5, and 3.2 ≤ d ≤ 10. It can take into account the high rate and high safety characteristics of the battery, reduce the safety hazards of high rate lithium battery products, and is beneficial to further improving the charging rate of existing batteries. Detailed Embodiments

[0056] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work belong to the protection scope of the present invention.

[0057] Example 1

[0058] The preparation method of the lithium-ion battery in this example includes the following steps:

[0059] 1) Preparation of the positive electrode sheet:

[0060] Mix the positive electrode active material lithium cobalt oxide LiCoO2, conductive carbon black Super-P, and binder polyvinylidene fluoride (PVDF) evenly according to a mass ratio of 94:3:3, and then disperse them in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. Coat the slurry evenly on both sides of the aluminum foil, and after drying, rolling, and vacuum drying, weld the aluminum lead wire with an ultrasonic welder to obtain the positive electrode sheet.

[0061] 2) Preparation of the negative electrode sheet:

[0062] Mix the negative electrode active material artificial graphite, conductive carbon black Super-P, binder styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) according to a mass ratio of 94:1:2.5:2.5, and then disperse them in deionized water to obtain a negative electrode slurry. Coat the slurry on both sides of the copper foil, and after drying, rolling, and vacuum drying, weld the nickel lead wire with an ultrasonic welder to obtain the negative electrode sheet.

[0063] 3) Preparation of the electrolyte:

[0064] Mix ethylene carbonate (EC), methyl propionate (MP), and ethyl methyl carbonate (EMC) according to a mass ratio of EC:MP:EMC = 1:1:1, then add lithium hexafluorophosphate (LiPF6) to a molar concentration of 1 mol / L, and add lithium difluoro(oxalato)borate at 0.5 wt%. The thermal expansion rate of the obtained electrolyte is tested to be 6.22%.

[0065] 4) Preparation of the battery cell:

[0066] Place a separator between the positive electrode plate and the negative electrode plate, then wind the sandwich structure composed of the positive electrode plate, the negative electrode plate, and the separator, and then flatten the wound body and put it into an aluminum foil packaging bag, and bake it in vacuum at 85 °C for 48 h to obtain the battery cell to be filled with liquid.

[0067] 5) Filling and formation of the battery cell

[0068] In a glove box with the dew point controlled below -40°C, the above-prepared electrolyte was injected into the battery cells, vacuum-packaged, and left standing for 6 hours. Then, the following steps were carried out for the first charging and formation: constant current charging at 0.05C for 180 minutes, constant current charging at 0.1C for 120 minutes, constant current charging at 0.1C for 120 minutes, secondary vacuum sealing, and then further constant current charging at a current of 0.5C until 3.65V, followed by constant voltage charging until the current dropped to 0.02C. After standing for 5 minutes, it was discharged at a constant current of 0.5C to 3.0V, obtaining a LiCoO₂ / artificial graphite lithium-ion battery.

[0069] Examples 2 - 26 and Comparative Examples 1 - 9

[0070] This example and the comparative examples are used to contrastively illustrate the lithium-ion battery disclosed in the present invention, including most of the operation steps in Example 1 above. The differences are as follows: the composition and content of the organic solvent, lithium salt, and additive in the non-aqueous electrolyte, and the thermal expansion rate of the non-aqueous electrolyte are shown in Table 1.

[0071] Table 1

[0072] Group MP content a (wt%) Types and contents of other carboxylic acid esters Total lithium salt content n (mol / L) Types and contents of the second lithium salt <![CDATA[Percentage b (wt%) of LiPF6 in the total amount of lithium salts]]> Thermal expansion rate d (%) of the electrolyte (10 × d + n × b) / a Additive Example 1 29 / 1.05 0.5% LiODFB 96.2 6.22 5.63 / Example 2 10 / 1.05 0.5% LiODFB 96.2 6.01 16.11 / Example 3 15 / 1.05 0.5% LiODFB 96.2 6.43 11.02 / Example 4 50 / 1.05 0.5% LiODFB 96.2 6.95 3.41 / Example 5 60 / 1.05 0.5% LiODFB 96.2 7.16 2.88 / Example 6 29 / 0.80 0.5% LiODFB 95.0 6.27 4.78 / Example 7 29 / 1.00 0.5% LiODFB 96.0 6.20 5.45 / Example 8 10 EB 10% 1.50 <![CDATA[6.0% LiBF4]]> 57.3 3.20 11.8 / Example 9 / EB 60% 1.05 0.5% LiODFB 96.2 5.78 2.65 / Example 10 / EB 10% 1.05 0.5% LiODFB 96.2 5.36 15.46 / Example 11 / EB 30% 1.05 0.5% LiODFB 96.2 5.67 5.26 / Example 12 29 / 1.40 1.0% LiODFB 94.3 6.04 6.63 / Example 13 29 / 1.50 0.5% LiODFB 97.3 5.83 7.04 / Example 14 29 / 1.05 0.8% LiODFB 0.39% LiBOB 8% LiFSI 30.0 7.78 3.77 / Example 15 29 / 1.05 0.75% LiODFB 4.5% LiFSI 60.0 7.20 4.66 / Example 16 29 / 1.05 0.4% LiODFB 97.0 6.22 5.66 / Example 17 29 / 1.05 0.26% LiBOB 98.0 6.23 5.70 / Example 18 18 30% MA 1.05 <![CDATA[0.5% LiODFB0.5%LiPO2F2]]> 92.4 8.00 9.83 / Example 19 15 40% MA 1.05 <![CDATA[0.5% LiODFB 1.0% LiBF4]]> 88.6 10.00 12.87 / Example 20 60 / 0.80 0.5% LiODFB 0.5% LiBOB 4.5% LiFSI 45.0 7.93 1.92 / Example 21 50 / 0.95 0.75% LiODFB 4.5% LiFSI 55.8 7.20 2.50 / Example 22 15 40% EB 1.25 2% LiTFSI 95.5 6.81 12.50 / Example 23 10 20% EP 1.30 <![CDATA[0.5% LiODFB1.0%LiBF4]]> 90.8 6.20 18.00 / Example 24 29 / 1.05 0.5% LiODFB 96.2 5.86 5.50 FEC 1.5% Example 25 29 / 1.05 0.5% LiODFB 96.2 6.14 5.60 VC 1.5% Example 26 29 / 1.05 0.5% LiODFB 96.2 6.08 5.58 HTCN 1.5% Comparative Example 1 8 20% EP 1.05 3% LiFSI 77.1 5.91 17.51 / Comparative Example 2 63 / 1.10 0.5% LiODFB 96.4 7.17 2.82 / Comparative Example 3 29 / 0.70 0.5% LiODFB 94.3 6.31 4.45 / Comparative Example 4 29 / 1.60 0.6% LiODFB 97.0 5.77 7.34 / Comparative Example 5 15 / 1.00 9% LiFSI 28.0 8.32 7.41 / Comparative Example 6 29 / 1.05 0.1% LiBOB 99.2 6.22 5.74 / Comparative Example 7 17 55% MA 1.05 0.5% LiODFB 96.2 10.50 12.12 / Comparative Example 8 60 / 0.80 8% LiFSI 20.0 9.03 1.77 / Comparative Example 9 10 / 1.50 0.5% LiODFB 97.3 4.30 18.90 /

[0073] Where: MP - methyl propionate, EB - ethyl butyrate, MA - methyl acrylate, EP - ethyl propionate, LiPF₆ - lithium hexafluorophosphate, LiODFB - lithium difluorooxalate borate, LiBOB - lithium bis(oxalato)borate, LiFSI - lithium bis(fluorosulfonyl)imide, LiTFSI - lithium bis(trifluoromethanesulfonyl)imide, LiBF₄ - lithium tetrafluoroborate, LiPO₂F₂ - lithium difluorophosphate, FEC - fluoroethylene carbonate, VC - vinylene carbonate, HTCN - ethanetricarbonitrile.

[0074] The lithium-ion batteries prepared in each example and comparative example were subjected to performance tests as follows:

[0075] 4C / 4C cycle performance test of the battery

[0076] The lithium-ion battery was placed in a constant temperature environment at 25°C, charged at a constant current of 4C until 4.45V, then charged at a constant voltage until the current dropped to 0.02C, and then discharged at a constant current of 4C to 3V. This cycle was repeated 500 times, and the discharge capacity of the first time and the last time were recorded.

[0077] The capacity retention rate of the cycle was calculated according to the following formula:

[0078] Battery capacity retention rate (%) = Discharge capacity of the last time / Discharge capacity of the first time × 100%;

[0079] The battery thermal abuse test passed the critical stability:

[0080] a. Conducted in accordance with the thermal abuse test specification of the national standard "GB 31241-2022 Safety Technical Specification for Lithium-Ion Batteries and Battery Packs for Portable Electronic Products", repeat the test on 5 batteries. If all 5 batteries pass the test, record that the battery passes the 130 °C test;

[0081] b. If the battery passes the 130 °C test, raise the temperature in the test chamber by 1 °C and repeat the test steps in a. until the battery catches fire or explodes. Record the highest test temperature that the battery can pass as the critical temperature for the battery to pass thermal abuse;

[0082] c. If the battery fails to pass the 130 °C test, lower the temperature in the test chamber by 1 °C and repeat the test steps in a. until all 5 batteries pass the test at the corresponding temperature. Record this test temperature of the battery as the critical temperature for the battery to pass thermal abuse.

[0083] The performance test results of the lithium-ion batteries prepared in each example and comparative example are shown in Table 2.

[0084] Table 2

[0085] Group Capacity retention rate (%) of the battery after 500 cycles Critical stability (°C) passed by the battery thermal abuse test Example 1 85.3 135 Example 2 81.3 135 Example 3 85.1 134 Example 4 85.7 133 Example 5 84.2 132 Example 6 82.3 133 Example 7 85.2 135 Example 8 78.3 140 Example 9 83.9 137 Example 10 80.2 138 Example 11 83.4 137 Example 12 85.0 137 Example 13 81.4 134 Example 14 80.0 136 Example 15 84.3 136 Example 16 85.6 134 Example 17 80.3 135 Example 18 86.5 133 Example 19 82.2 131 Example 20 80.1 133 Example 21 80.1 135 Example 22 84.8 136 Example 23 79.1 133 Example 24 84.3 133 Example 25 80.5 135 Example 26 83.2 136 Comparative Example 1 8.9 131 Comparative Example 2 75.2 120 Comparative Example 3 6.3 126 Comparative Example 4 43.2 127 Comparative Example 5 66.2 133 Comparative Example 6 11.4 133 Comparative Example 7 75.1 123 Comparative Example 8 43.3 125 Comparative Example 9 10.2 134

[0086] From the test results of Examples 1-26 and Comparative Examples 1-9 in Table 2, it can be seen that for the non-aqueous electrolyte of the present invention, by using at least one carboxylic acid ester such as methyl propionate and ethyl butyrate as the organic solvent, and at least two lithium salts including lithium hexafluorophosphate, and when the relationship between the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of the lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte satisfies 1.8 ≤ (10×d + n×b) / a ≤ 18, 10 ≤ a ≤ 60, 30 ≤ b ≤ 98, 0.8 ≤ n ≤ 1.5, 3.2 ≤ d ≤ 10, the prepared lithium-ion battery has good rate performance and high-temperature safety performance.

[0087] From the test results of Examples 1-23, it can be seen that when the relationship between the mass percentage content a of the carboxylic acid ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of the lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte satisfies the corresponding relationship, when using any one or more of methyl propionate or ethyl butyrate as the organic solvent and combining with other organic solvents, the prepared lithium-ion batteries all have better rate performance and safety performance.

[0088] As can be seen from the test results of Examples 12-23, when the relationship among the mass percentage content a of carboxylic ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte satisfies the corresponding relationship, when lithium hexafluorophosphate is combined with one or more other different types of lithium salts, the prepared lithium-ion batteries all have better rate performance and safety performance.

[0089] As can be seen from the test results of Example 1 and Examples 24-26, when the relationship among the mass percentage content a of carboxylic ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte satisfies the corresponding relationship, and further combined with conventional additives, the rate performance and safety performance of the prepared lithium-ion batteries are better, indicating that the battery system of the present invention has universality for different additives.

[0090] As can be seen from the test results of Example 1 and Comparative Examples 1-9, when any one of the content of the mass percentage content a of carboxylic ester in the non-aqueous electrolyte, the mass percentage content b of lithium hexafluorophosphate in the lithium salt, the total content n of lithium salt in the non-aqueous electrolyte, and the thermal expansion rate d of the non-aqueous electrolyte is too large or too small, or does not satisfy the relevant relational formula, it will have an impact on the rate performance and / or safety performance of the lithium-ion battery, further proving the synergistic effect among the above contents.

[0091] The present invention has been further described above with the aid of specific examples. However, it should be understood that the specific description here should not be construed as a limitation on the essence and scope of the present invention. Various modifications made by those of ordinary skill in the art to the above examples after reading this specification all fall within the scope protected by the present invention.

Claims

1. A non-aqueous electrolyte, characterized in that, The non-aqueous electrolyte includes a lithium salt and an organic solvent; The organic solvent includes at least one carboxylate ester such as methyl propionate and ethyl butyrate; The lithium salt includes at least two kinds, and one of them is lithium hexafluorophosphate; The non-aqueous electrolyte satisfies the following conditions: 1.8 ≤ (10×d + n×b) / a ≤ 18, 10 ≤ a ≤ 50, 30 ≤ b ≤ 98, 0.8 ≤ n ≤ 1.5, 3.2 ≤ d ≤ 10; Wherein, a is the mass percentage content of the carboxylate ester in the non-aqueous electrolyte, with the unit of wt%; b is the mass percentage content of lithium hexafluorophosphate in the lithium salt, with the unit of wt%; n is the total content of the lithium salt in the non-aqueous electrolyte, with the unit of mol / L; d is the thermal expansion rate of the non-aqueous electrolyte, with the unit of %; The test method for the thermal expansion rate of the non-aqueous electrolyte is as follows: Place the non-aqueous electrolyte in a container with a volume scale, and then place the non-aqueous electrolyte in a high and low temperature test chamber and keep it at 25°C for 2 hours. Read and record the volume of the non-aqueous electrolyte as V1; Raise the temperature of the high and low temperature test chamber to 85°C and keep it at 85°C for 2 hours. Read and record the volume of the non-aqueous electrolyte as V2; The thermal expansion rate d of the non-aqueous electrolyte = (V2 - V1) / V1 × 100%.

2. The non-aqueous electrolyte according to claim 1, wherein The non-aqueous electrolyte satisfies the following conditions: 2.5 ≤ (10×d + n×b) / a ≤ 12.

5.

3. The non-aqueous electrolyte according to claim 1, characterized in that, The mass percentage content a of the carboxylate ester in the non-aqueous electrolyte is 15wt% - 50wt%.

4. The non-aqueous electrolyte according to claim 1, wherein The mass percentage content b of lithium hexafluorophosphate in the lithium salt is 60wt% - 97wt%.

5. The non-aqueous electrolyte according to claim 1, characterized in that, The total content n of the lithium salt in the non-aqueous electrolyte is 1.0mol / L - 1.4mol / L.

6. The non-aqueous electrolyte according to claim 1, characterized in that, The thermal expansion rate d of the non-aqueous electrolyte is 3.2% - 8%.

7. The non-aqueous electrolyte according to claim 1, wherein, The lithium salt also includes one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluoro(oxalato)borate, lithium difluoro(oxalato)phosphate, lithium bis(oxalato)borate, lithium tetrafluoroborate, and lithium difluorophosphate.

8. The non-aqueous electrolyte according to claim 1, characterized in that, The organic solvent also includes at least one of cyclic carbonate solvents, linear carbonate solvents, carboxylate ester solvents, and ether solvents; and / or, The cyclic carbonate solvents include at least one of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinylene carbonate, and butylene carbonate; and / or, The linear carbonate solvents include at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and methyl propyl carbonate; and / or, The carboxylate ester solvents include at least one of ethyl acetate, ethyl propionate, propyl propionate, methyl acrylate, ethyl difluoroacetate, methyl acetate, methyl butyrate, isobutyl methyl acetate, trimethylacetic acid methyl ester, and trimethylacetic acid ethyl ester other than methyl propionate and ethyl butyrate; and / or, The ether solvents include at least one of ethylene glycol dimethyl ether, 1,3-dioxolane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

9. The non-aqueous electrolyte according to claim 1, characterized in that, The non-aqueous electrolyte also includes an additive, and the additive includes at least one of cyclic sulfate compounds, sultone compounds, cyclic carbonate compounds, phosphate ester compounds, and nitrile compounds; and / or, Based on the total mass of the non-aqueous electrolyte being 100%, the content of the additive is 0.01 wt% to 10 wt%; and / or, The cyclic sulfate compound includes at least one of vinyl methyl sulfate, ethylene sulfate, and propylene sulfate; and / or, The sultone compound includes at least one of 1,3-propane sultone, 1,4-butane sultone, and allyl-1,3-sultone; and / or, The cyclic carbonate compound includes at least one of vinylene carbonate, ethylene vinylene carbonate, ethylene methylene carbonate, fluoroethylene carbonate, trifluoromethyl ethylene carbonate, difluoroethylene carbonate, and the compound represented by the following structural formula 1: Structural formula 1 In the shown structural formula 1, R 21 , R 22 , R 23 , R 24 , R 25 , R 26 are each independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group; and / or, The phosphate compound includes at least one of tris(trimethylsilyl) phosphate, tris(triethylsilyl) phosphate, and the compound represented by the following structural formula 2: Structural formula 2 In the said structural formula 2, R 31 , R 32 , R 33 are each independently selected from saturated hydrocarbon groups, unsaturated hydrocarbon groups, and halogenated hydrocarbon groups having 1 to 5 carbon atoms, and at least one of R 31 , R 32 , R 33 is an unsaturated hydrocarbon group; and / or, The nitrile compound includes at least one of succinonitrile, glutarodinitrile, ethylene glycol bis(propionitrile) ether, hexane trinitrile, adiponitrile, pimelonitrile, suberonitrile, azelaonitrile, and sebaconitrile.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, and the non-aqueous electrolyte according to any one of claims 1-9.

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