Non-aqueous electrolyte and lithium ion battery

Abstract

The invention relates to the field of lithium batteries, in particular to a non-aqueous electrolyte and a lithium ion battery. The non-aqueous electrolyte comprises a lithium salt, an organic solvent and an additive; the organic solvent comprises cyclic carbonate and fluorinated chain ester, the additive comprises an additive A and an additive B, the additive A comprises a compound as shown in a structural formula 1, and the additive B comprises at least one of lithium difluoro (oxalato) borate, lithium difluorophosphate, lithium bis (trifluoromethane sulfonimide) or lithium tetrafluoroborate; the lithium ion battery satisfies a + b / 7 < = (c + d) / 10, 1 < = a < = 3, 3 < = b < = 10, 20 < = c < = 40, and 10 < = d < = 30. According to the non-aqueous electrolyte disclosed by the invention, the high-content additive A can better ensure the thermal stability of the battery and improve the high-temperature storage performance of the battery; the high-content additive B can improve the inorganic component content of the negative electrode SEI film and enhance the lithium conductivity; the solubility of the additive A and the additive B is improved by using a large amount of cyclic carbonate and fluorinated chain ester, and the cycle performance and the high-temperature performance of the battery are remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a non-aqueous electrolyte and a lithium ion battery with excellent high-temperature storage and high-temperature cycle performance. BACKGROUND

[0002] Lithium ion batteries have been widely used in electric vehicles, various electric tools, energy storage devices and various electronic products due to their high specific energy density and long cycle life. With the expansion of the application range of lithium ion batteries, the battery industry has put forward higher requirements for the energy density of batteries.

[0003] In order to design a lithium ion battery with high energy density, in addition to the continuous optimization of its space utilization (high compaction density, high active material ratio), increasing the operating voltage of the lithium ion battery is also one of the important ways to increase the energy density of the battery. Increasing the charge cut-off voltage can make the lithium ion battery obtain new capacity on the original basis, which has an immediate effect on the improvement of energy density.

[0004] However, as the voltage of the lithium ion battery increases, the structural stability of the material becomes worse, and the oxidizability of the material surface to the electrolyte becomes stronger. These two reasons will lead to the intensification of the side reaction between the positive electrode material and the electrolyte interface, and the high-temperature performance of the battery will deteriorate seriously, resulting in the reduction of the cycle performance of the battery. Therefore, how to effectively cope with the deterioration of the cycle performance and storage performance of the high-voltage lithium ion battery at high temperature is an industry problem at present. SUMMARY

[0005] In order to solve the above technical problems, the present application provides a non-aqueous electrolyte, so that the lithium ion battery using the non-aqueous electrolyte can have excellent high-temperature storage and high-temperature cycle performance.

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

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

[0008] The organic solvent comprises a cyclic carbonate and a fluorinated chain ester;

[0009] The cyclic carbonate comprises a fluorinated cyclic carbonate and a non-fluorinated cyclic carbonate, the fluorinated cyclic carbonate comprises fluorinated ethylene carbonate, and the non-fluorinated cyclic carbonate comprises ethylene carbonate and / or propylene carbonate;

[0010] The fluorinated chain ester comprises 2,2-difluoroethyl acetate and / or methyl trifluoroethyl carbonate;

[0011] The additive comprises an additive A and an additive B;

[0012] The additive A comprises a compound shown in structural formula 1:

[0013]

[0014] Structural formula 1

[0015] X is selected from or R1 and R2 are each independently selected from H, or R1 and R2 are not simultaneously selected from H, and at least one of X, R1 and R2 contains a sulfur atom;

[0016] The additive B comprises at least one of lithium difluoro(oxalato)borate, lithium difluorophosphate, lithium bis(trifluoromethanesulfonyl)imide or lithium tetrafluoroborate;

[0017] The mass percentage content of the additive A is a%, the mass percentage content of the additive B is b%, the mass percentage content of the cyclic carbonate is c%, and the mass percentage content of the fluorinated chain ester is d%, based on the mass of the non-aqueous electrolyte, and the non-aqueous electrolyte satisfies the following conditions:

[0018] a+b / 7≤(c+d) / 10, 1≤a≤3, 3≤b≤10, 20≤c≤40, and 10≤d≤30.

[0019] The non-aqueous electrolyte of the present application adds a high content of the compound shown in structural formula 1 as the additive A and a high content of a lithium salt type additive as the additive B, and the organic solvent comprises a cyclic carbonate and a fluorinated chain ester. The inventor has found, through a large amount of research, that when the relationship between the mass percentage content a of the additive A, the mass percentage content b of the additive B, the mass percentage content c of the cyclic carbonate and the mass percentage content d of the fluorinated chain ester in the non-aqueous electrolyte satisfies a+b / 7≤(c+d) / 10, 1≤a≤3, 3≤b≤10, 20≤c≤40, and 10≤d≤30, the combination of the high content of the additive A and the additive B makes the lithium ion battery have good positive and negative electrode protection and low impedance performance; the high dielectric constant and the suitable fluidity of the cyclic carbonate and the fluorinated chain ester improve the solubility of the additive A and the additive B in the non-aqueous electrolyte, thereby improving the high-temperature storage performance and high-temperature cycle performance of the battery, and also taking into account the safety performance.

[0020] It is speculated that due to the dissolution of high content of additive A and high content of additive B in the non-aqueous electrolyte, additive A can form a good interface film with high thermal stability at the positive and negative electrodes respectively, and reduce the thickness of the CEI film and the SEI film, and compared with the conventional additive amount, high content of additive A can further reduce the side reaction of the electrolyte at the interface of the electrode sheet, reduce the consumption of active lithium, improve the interface stability, and the interface film can also inhibit the dissolution of metal ions in the positive electrode and improve the film formation of the negative electrode, enhance the stability of the negative electrode, thereby effectively improve the high temperature cycle performance of the battery; since the addition of additive A may increase the battery impedance and reduce the conductivity, the present application synchronously adds high content of additive B, which can improve the content of inorganic components in the negative electrode SEI film, reduce the desolvation energy of lithium ions, and improve the conduction of lithium ions; the large anion group makes the negative charge fully separate and domain, easy to dissociate, and has high conductivity; at the same time, high content of lithium salt type additive B can also reduce the amount of free solvent and further reduce the probability of its reduction in the negative electrode or oxidation in the positive electrode, improve the high temperature performance, inhibit the generation of gas, and further improve the kinetic and thermodynamic performance of the lithium ion battery. However, high content of additive A and additive B will cause the solubility of the electrolyte to decrease sharply, the turbidity to rise obviously, and finally the problems such as particles on the surface of the battery cell to occur, thereby causing a series of safety hazards. Therefore, the present application introduces a large amount of cyclic carbonate and fluorinated chain ester as solvents, the cyclic carbonate and fluorinated chain ester have high dielectric constant and suitable flowability, can promote the dissociation of lithium salt and additive and form solvation structure, improve the solubility of additive A and additive B, make it possible to add high content of additive in the electrolyte, and maximize the guarantee of the stability of the positive electrode of the battery; also can prevent the electrolyte from entering the active material and corroding the active material, significantly improve the thermal stability of the battery, the change of the thickness of the battery during high temperature storage and applying heat shock can be significantly reduced, the heat shock durability can be improved, the direct contact between the positive electrode and the electrolyte can be effectively prevented, even if the battery is stored at high temperature and continuously charged and discharged, the decomposition of the electrolyte can be inhibited, and the high temperature storage and high temperature cycle life of the battery are improved.

[0021] The present application limits the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate and the mass percentage content d of fluorinated chain ester in the non-aqueous electrolyte, improves the solubility of additive A and additive B, significantly improves the thermal stability and kinetic performance of the battery electrolyte, inhibits the dissolution of metal ions in the positive electrode, so that the electrolyte has good positive electrode protection effect and low impedance performance at the same time, thereby making the lithium ion battery have high temperature cycle performance and high temperature storage performance.

[0022] When the relationship a+b / 7>(c+d) / 10 between the mass percentage content a of the additive A, the mass percentage content b of the additive B, the mass percentage content c of the cyclic carbonate, and the mass percentage content d of the fluorinated chain ester in the non-aqueous electrolyte is a+b / 7>(c+d) / 10, it means that the amount of the additive A and the additive B does not match the amount of the cyclic carbonate and the fluorinated chain ester of the organic solvent, i.e., the content of the cyclic carbonate and / or the fluorinated chain ester in the non-aqueous electrolyte is too low, or the content of the additive A and / or the additive B is too high. At this time, the additive A and the additive B cannot be completely dissolved, resulting in a large turbidity of the electrolyte and obvious particles on the surface of the lithium ion battery after the electrolyte is injected, and the lithium ion battery prepared has serious safety hazards. Especially when the fluorinated chain ester is too low, the conventional carbonate solvent cannot well dissolve the lithium salt type additive, so that the additive A and the additive B are precipitated or the content decreases in the non-aqueous electrolyte, thereby affecting the film forming effect and kinetic performance under high voltage. At the same time, due to the decrease of the content of the fluorinated chain ester, the oxidation resistance brought by the fluorine substituent in the non-aqueous electrolyte is greatly reduced, and the performance of the lithium ion battery is deteriorated.

[0023] The additive A can participate in the formation of stable protective films on both the positive and negative electrodes, and can reduce the thickness of the SEI film and the CEI film. Compared with the conventional additive amount, the high solubility of the additive A in the non-aqueous electrolyte can further reduce the side reaction of the electrolyte at the interface of the electrode sheet, inhibit the dissolution of metal ions in the positive electrode, and enhance the protection effect of the positive electrode of the lithium ion battery. If the content a of the additive A in the non-aqueous electrolyte is too high, the battery impedance is too large, the charge and discharge polarization and heat production of the battery significantly increase, causing a series of negative effects such as thermal decomposition of lithium salt, and finally deteriorating the electrochemical performance of the battery; at the same time, too high additive A cannot be dissolved in the electrolyte, resulting in turbidity of the electrolyte. If the a value is too low, the protective film formed is unstable and cannot effectively improve the high temperature performance of the battery. Specifically, in some embodiments of the present application, the mass percentage content a% of the additive A in the non-aqueous electrolyte is 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.1%, 2.2%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, or a range consisting of any two of these values. Preferably, the mass percentage content a% of the additive A in the non-aqueous electrolyte is 2% to 3%.

[0024] The lithium salt type additive B is hydrolyzed in the non-aqueous electrolyte to generate difluorophosphate, difluoroboric oxalate and other fluorine acid ions, which can participate in the formation of the SEI film, increase the content of LiF in the protective film component, and further improve the stability and reduce the impedance of the SEI film. At the same time, compared with the conventional additive amount, the high content of the lithium salt type additive B makes the effective components of the additive in the non-aqueous electrolyte more sufficient, more effectively improves the content of inorganic components in the SEI film, reduces the desolvation energy of lithium ions and improves the conduction ability of lithium ions; and can reduce the amount of free solvent, reduce the probability of reduction at the negative electrode or oxidation at the positive electrode, inhibit the electrolyte gas production, and improve the high temperature performance. If the content b of the additive B in the non-aqueous electrolyte is too low, it is difficult to form a good protective film on the negative electrode surface, so as to effectively improve the impedance and high temperature performance of the lithium ion battery. Specifically, in some embodiments of the present application, the mass percentage content b% of the additive B in the non-aqueous electrolyte is 3%, 3.5%, 4%, 4.2%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10% or a range composed of any two of these values. Preferably, the mass percentage content b% of the additive B in the non-aqueous electrolyte is 4.2% to 9%. More preferably, the mass percentage content b% of the additive B in the non-aqueous electrolyte is 5% to 9%.

[0025] When the additive B includes lithium difluoroboric oxalate, a boron-containing oxalate interface film can be generated on the negative electrode surface during decomposition, which has better density and ion conductivity than LiPF6, can inhibit solvent co-intercalation and side reactions, reduce impedance, and improve the cycle and high temperature storage capacity of the battery.

[0026] When the additive B includes lithium difluorophosphate, the inorganic component content of the negative electrode SEI film can be improved during film formation, the lithium ion conductivity is enhanced, and the kinetic performance of the battery is improved.

[0027] When the additive B includes lithium bis-trifluoromethanesulfonimide, the thermal decomposition temperature of lithium bis-trifluoromethanesulfonimide is above 200℃, and the thermal stability is high, which can improve the high temperature storage performance of the battery.

[0028] When the additive B includes lithium tetrafluoroborate, the desolvation energy of lithium ions at low temperature can be reduced, the migration rate of lithium ions is improved, and the low temperature discharge performance of the battery is improved.

[0029] The organic solvent of the present application contains a large amount of cyclic carbonate, which has a high dielectric constant and a certain polarity, and can increase the solubility of additives in the non-aqueous electrolyte. Specifically, the cyclic carbonate solvent with a high dielectric constant can better shield the electrostatic interaction between additive ions, making the lithium salt type additive B more stable in the solution and less likely to agglomerate or precipitate, thereby increasing its solubility and maintaining good conductivity of the electrolyte in the lithium ion battery. If the content c of cyclic carbonate in the non-aqueous electrolyte is too high, the viscosity of the electrolyte at low temperature will increase significantly, the migration speed of lithium ions in the electrolyte will slow down, and the charge and discharge performance of the battery will be affected. At the same time, due to the large molecular structure of cyclic carbonate, a high content will narrow the lithium ion migration channel in the electrolyte, increasing the internal resistance of the battery. If the content c is too low, the lithium salt will not be fully dissolved, the concentration of lithium ions in the electrolyte will decrease, and the ionic conductivity will decrease, resulting in a slow migration speed of lithium ions during the charge and discharge process of the battery, and causing problems such as increased turbidity of the electrolyte, obvious particles on the surface of the battery after liquid injection, etc. At the same time, cyclic carbonate can participate in the SEI film formation process, and a low content may cause the uniformity of the SEI film to decrease, resulting in increased film impedance and battery performance degradation. Specifically, in some embodiments of the present application, the mass percentage content c of cyclic carbonate in the non-aqueous electrolyte is 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, or a range formed by any two of these values. Preferably, the mass percentage content c of cyclic carbonate in the non-aqueous electrolyte is 30% to 40%.

[0030] The organic solvent of the present application also contains fluorinated chain ester, which includes 2,2-difluoroethyl acetate and / or methyl trifluoroethyl carbonate. The fluorinated chain ester has a high dielectric constant, moderate viscosity and good kinetic performance, can promote the dissociation of lithium salt and additives and form a solvation structure, and increase the solubility of lithium salt and additives in the non-aqueous electrolyte. In the present application, the solubility of additives A and B in the electrolyte is increased by adding more than the conventional amount of cyclic carbonate and fluorinated chain ester, and controlling the ratio of the two. When the content d of fluorinated chain ester in the non-aqueous electrolyte is too high, the viscosity of the electrolyte increases and the flowability of the electrolyte at low temperature deteriorates, the migration of lithium ions in the electrolyte is hindered, and the internal resistance of the battery increases. Too high d value can also cause the SEI film to be too thick, thereby increasing the migration path length of lithium ions on the electrode surface, intensifying the polarization of the battery, and shortening the cycle life. At the same time, it can also cause problems such as corrosion of electrode materials, reduction of thermal stability and safety performance of the battery, etc. If d is too low, a large amount of lithium salt will precipitate, the electrolyte will become turbid, and the content of effective components in the electrolyte will decrease, and the SEI film will not form well, affecting the cycle and safety performance of the lithium ion battery. Specifically, in some embodiments of the present application, the mass percentage content d% of fluorinated chain ester in the non-aqueous electrolyte is 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30% or a range formed by any two of these values. Preferably, the mass percentage content d% of fluorinated chain ester in the non-aqueous electrolyte is 20% to 30%.

[0031] Specifically, in some embodiments of the present application, in the compound represented by structural formula 1, X is selected from or , R1 and R2 are each independently selected from H, , or R1 and R2 are not simultaneously selected from H. At least one of X, R1 and R2 contains a sulfur atom, and X, R1 and R2 do not simultaneously contain a sulfur atom.

[0032] As an example, the additive A is selected from one or more of the following compounds:

[0033]

[0034] The additive A includes both a sulfur-containing ring structure and a carbonate-containing ring structure, and the interface film component formed by the cooperation of the carbonate-containing ring structure and the sulfur-containing ring structure is more stable and dense.

[0035] In some embodiments of the present application, in the compound represented by structural formula 1, X is selected from , R1 and R2 are each independently selected from H or R1 and R2 are not simultaneously selected from H.

[0036] As an example, the additive A is selected from one or more of the following compounds:

[0037]

[0038] The additive A has a polycyclic structure, compared with a monocyclic structure of vinyl sulfate, each ring of the polycyclic structure is involved in the formation of the electrode surface interfacial film, the formed interfacial film components are more stable and dense, which has the effect of improving the interfacial film structure strength, and thus is conducive to improving the high temperature stability.

[0039] Specifically, in some embodiments of the present application, the fluorinated chain ester includes 2,2-difluoroethyl acetate (DFEA, CAS: 1550-44-3) and / or methyl trifluoroethyl carbonate (FEMC, CAS: 156783-95-8). Since the fluorinated solvent can well improve the oxidation resistance and high temperature stability of the solvent, but will inevitably increase the viscosity and reduce the flowability of the solvent, so that the impedance of the solvent increases. Therefore, the inventors, through analysis of a large amount of experimental data, prefer two fluorinated solvents, DFEA and FEMC, which have good oxidation resistance and good flowability, to balance the thermodynamic performance and kinetic performance.

[0040] Specifically, in some embodiments of the present application, the cyclic carbonate includes fluorinated cyclic carbonate and non-fluorinated cyclic carbonate, the fluorinated cyclic carbonate includes fluorinated ethylene carbonate, and the non-fluorinated cyclic carbonate includes ethylene carbonate and / or propylene carbonate, the mass percentage content of the fluorinated ethylene carbonate is c1%, the mass percentage content of the ethylene carbonate is c2%, and the mass percentage content of the propylene carbonate is c3%, based on the mass of the non-aqueous electrolyte, the cyclic carbonate satisfies: 1≤(c2+c3) / c1≤2.

[0041] The inventors have found that the performance of a lithium ion battery can be further improved by regulating the mass ratio of ethylene carbonate (EC) and propylene carbonate (PC) in the non-fluorinated cyclic carbonate and the ratio of the fluorinated cyclic carbonate (FEC) in the cyclic carbonate. First, the stability of the SEI film must be ensured. Due to the unique effect of the fluorine substituent, the negative electrode reduction potential of the fluorinated cyclic carbonate (FEC) appears earlier than that of ethylene carbonate (EC), especially in the high-voltage lithium cobaltate system. The negative electrode SEI film is mainly derived from the reduction and decomposition of the fluorinated cyclic carbonate (FEC). It has been found that when (c2+c3) / c1≤2, the content of FEC in the non-aqueous electrolyte can be sufficient. Since the dielectric constant of ethylene carbonate (EC) and propylene carbonate (PC) is higher than that of fluorinated cyclic carbonate (FEC), it means that when the content of ethylene carbonate and propylene carbonate is too low, the overall solubility of the non-aqueous electrolyte will be weak, which cannot meet the solubility requirements of additive B and additive A, thereby affecting the overall performance of the battery. After a large number of studies, the inventors have found that when the ratio of the mass of the fluorinated ethylene carbonate (FEC) to the total mass of the ethylene carbonate (EC) and the propylene carbonate (PC) satisfies 1≤(c2+c3) / c1≤2, the performance of the battery can achieve the best balance, which provides support for the efficient and stable operation of the battery system.

[0042] In some preferred embodiments of the present application, the mass percentage content c1% of the fluorinated ethylene carbonate is 10% to 20% based on the mass of the non-aqueous electrolyte. Controlling the mass percentage content of FEC within the above range can avoid the problem of increased battery impedance caused by the excessive growth of the solid electrolyte interface film (SEI) caused by the introduction of excessive FEC.

[0043] In some preferred embodiments of the present application, the mass percentage content c2% of the ethylene carbonate (EC) is 5% to 15% based on the mass of the non-aqueous electrolyte. Controlling the mass percentage content of EC within the above range can make the electrolyte have a suitable viscosity, a high ionic conductivity, and good electrochemical stability, which can further improve the high-temperature cycle performance and high-temperature storage performance of the lithium ion battery, and also take into account the safety performance.

[0044] In some preferred embodiments of the present application, the mass percentage content c3% of the propylene carbonate (PC) is 5% to 15% based on the mass of the non-aqueous electrolyte. Controlling the mass percentage content of PC within the above range can make the electrolyte have a suitable viscosity, a high ionic conductivity, and good electrochemical stability, which can further improve the high-temperature cycle performance and high-temperature storage performance of the lithium ion battery, and also take into account the safety performance.

[0045] Specifically, in some embodiments of the present application, the organic solvent further includes a linear carbonate and / or a carboxylic acid ester.

[0046] In some embodiments of the present application, the linear carbonate includes one or more of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0047] In some embodiments of the present application, the carboxylic acid ester includes one or more of propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isopentyl acetate, propyl propionate, ethyl propionate, methyl butyrate, and ethyl n-butyrate.

[0048] Specifically, in some embodiments of the present application, the electrolyte salt in the non-aqueous electrolyte includes lithium hexafluorophosphate (LiPF6), and the mass percentage of the lithium hexafluorophosphate (LiPF6) is greater than or equal to the mass percentage of the additive B. When the above range is satisfied, the amount of free solvent can be reduced, and the probability of reduction at the negative electrode or oxidation at the positive electrode can be reduced, thereby improving the high-temperature performance and inhibiting the generation of gas. When the content of LiPF6 is within a certain range, increasing the content of LiPF6 can improve the ion migration rate of the electrolyte of the lithium ion battery to a certain extent, thereby improving the kinetic performance of the lithium ion battery. Specifically, the mass percentage of the lithium hexafluorophosphate is 10-18%.

[0049] Specifically, in some embodiments of the present application, the non-aqueous electrolyte further includes a basic additive, and the basic additive includes one or more of a sulfonic acid lactone compound, a cyclic sulfate compound, a phosphate compound, and a borate compound.

[0050] In some preferred embodiments, the mass percentage of the basic additive is 0.01%-10% based on the mass of the non-aqueous electrolyte. Preferably, the content of the basic additive is 0.1%-5%.

[0051] In some preferred embodiments, the sulfonic acid lactone compound includes at least one of 1,3-propane sulfonic acid lactone, 1,4-butane sulfonic acid lactone, or 1,3-propylene sulfonic acid lactone.

[0052] In some preferred embodiments, the cyclic sulfate compound includes at least one of ethylene sulfate, propylene sulfate, or methyl ethylene sulfate.

[0053] In some preferred embodiments, the phosphate compound includes at least one of the compounds shown in structural formula 2:

[0054]

[0055] Structural formula 2

[0056] In the structural formula 2, R 31 , R 32 , and R 33each independently selected from a C1-C5 saturated hydrocarbon group, a C1-C5 unsaturated hydrocarbon group, a C1-C5 halogenated hydrocarbon group, -Si(CmH2m+1)3, m is a natural number from 1 to 3; more preferably, the compound of structural formula 1 includes at least one of tris(trimethylsilyl) phosphate, tris(triethylsilyl) phosphate, tripropargyl phosphate, dipropargylmethyl phosphate, dipropargylethyl phosphate, dipropargylpropyl phosphate, dipropargyltrifluoromethyl phosphate, dipropargyl-2,2,2-trifluoroethyl phosphate, dipropargyl-3,3,3-trifluoropropyl phosphate, dipropargylhexafluoroisopropyl phosphate, triallyl phosphate, diallylmethyl phosphate, diallylethyl phosphate, diallylpropyl phosphate, diallyltrifluoromethyl phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, diallylhexafluoroisopropyl phosphate.

[0057] In some preferred embodiments, the borate compound includes at least one of tris(trimethylsilyl) borate and tris(triethylsilyl) borate.

[0058] In a second aspect, the present application also provides a lithium ion battery, which includes a positive electrode, a negative electrode, and the non-aqueous electrolyte as described above.

[0059] Preferably, the positive electrode includes a positive electrode material layer including a positive electrode active material, and the positive electrode active material includes lithium cobaltate.

[0060] Specifically, in some embodiments of the present application, the positive electrode further includes a positive electrode binder and a positive electrode conductive agent. The positive electrode binder includes one or more 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-vinyl fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, a thermoplastic polyimide, a thermoplastic resin of polyethylene and polypropylene, an acrylic resin, and a styrene butadiene rubber. The positive electrode conductive agent includes one or more of conductive carbon black, conductive carbon spheres, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene, and reduced graphene oxide.

[0061] Specifically, in some embodiments of the present application, the positive electrode sheet further includes a positive electrode current collector, and the positive electrode material layer is disposed on a surface of the positive electrode current collector. The positive electrode current collector includes an electron-conducting metal material, and preferably, the positive electrode current collector includes one or more of aluminum, nickel, tin, copper, and stainless steel.

[0062] Preferably, the negative electrode comprises a negative electrode material layer comprising a negative electrode active material, the negative electrode active material comprising artificial graphite and / or natural graphite. The negative electrode further comprises a negative electrode binder and a negative electrode conductive agent, which can be the same as the positive electrode binder and the positive electrode conductive agent respectively, and will not be described here again.

[0063] Specifically, in some embodiments of the present application, the lithium ion battery further comprises a separator, the separator being located between the positive electrode and the negative electrode.

[0064] The separator is a conventional separator selected from one or more of ceramic separators, polymer separators, non-woven fabrics, inorganic-organic composite separators. For example, single-layer polypropylene (PP) separators, single-layer polyethylene (PE) separators, double-layer PP / PE separators, double-layer PP / PP separators, and triple-layer PP / PE / PP separators.

[0065] The non-aqueous electrolyte of the present application uses additive A and additive B as additives. Additive A can effectively improve the high-temperature cycle performance of the battery, and a high content of additive A can better ensure the thermal stability of the battery and improve the high-temperature storage performance of the battery. A high content of additive B can improve the inorganic content of the negative electrode SEI film and enhance the lithium conductivity. At the same time, a large amount of cyclic carbonates and fluorinated chain esters are used to improve the solubility of additive A and additive B. By limiting the relationship between the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonates, and the mass percentage content d of fluorinated chain esters in the non-aqueous electrolyte, that is, a+b / 7≤(c+d) / 10, 1≤a≤3, 3≤b≤10, 20≤c≤40, 10≤d≤30, the solubility of additive A and additive B in the non-aqueous electrolyte can be greatly improved, thereby improving the high-temperature storage performance and high-temperature cycle performance of the lithium ion battery using the above non-aqueous electrolyte, and also considering the safety performance. DETAILED DESCRIPTION

[0066] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the embodiments in the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0067] Embodiment 1-1

[0068] The preparation method of the lithium ion battery in this embodiment comprises the following steps:

[0069] 1) Preparation of the positive electrode sheet:

[0070] LiCoO2, conductive carbon black Super-P and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1.5:1.5, and then they were dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. The slurry was uniformly coated on both sides of an aluminum foil, and then the positive electrode material layer was obtained after drying, calendering and vacuum drying, and the aluminum lead was welded on the positive electrode sheet by using an ultrasonic welding machine.

[0071] 2) Preparation of negative electrode sheet:

[0072] The negative active material graphite, conductive carbon black Super-P, binder styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 96.3:1.0:1.2:1.5, and then they were dispersed in deionized water to obtain a negative electrode slurry. The slurry was coated on both sides of a copper foil, and then the negative electrode sheet was obtained after drying, calendering and vacuum drying, and the nickel lead was welded on the negative electrode sheet by using an ultrasonic welding machine.

[0073] 3) Preparation of electrolyte:

[0074] In an argon-filled glove box (moisture <10 ppm, oxygen <10 ppm), cyclic carbonates, fluorinated chain esters and propyl propionate were mixed uniformly, wherein the cyclic carbonates included ethylene carbonate (EC), propylene carbonate (PC) and fluorinated ethylene carbonate (FEC), and the fluorinated chain esters included 2,2-difluoroethyl acetate (DFEA) and methyl trifluoroethyl carbonate (FEMC) in a mass ratio of 1:1; then LiPF6, additive A (compound 4) and additive B (lithium bistrifluoromethanesulfonylimide) were added. Based on the mass of the non-aqueous electrolyte, the mass content of LiPF6 was 15 wt%, and the mass contents of the cyclic carbonates, the fluorinated chain esters, additive A and additive B in the electrolyte were as shown in Table 1.

[0075] 4) Battery assembly:

[0076] The above-prepared positive electrode sheet, separator and negative electrode sheet were assembled into a laminated soft-pack battery cell.

[0077] 5) Liquid injection and formation of the battery cell

[0078] The above-prepared electrolyte was injected into the battery cell in a glove box with a dew point controlled below -40°C, and then vacuum packaged and left for 72 h. Then the conventional formation of the first charge was carried out in the following steps: 0.05C constant current charging for 180 min, 0.1C constant current charging for 120 min, 0.2C constant current charging for 120 min, secondary vacuum sealing, and then further full charging (100% SOC) at a current of 0.2C, and after standing at room temperature for 72 h, full discharging (0% SOC) at a current of 0.2C.

[0079] Examples 1-2 to 1-44 and Comparative Examples 1-1 to 1-15

[0080] The present examples and comparative examples are used to illustrate the lithium ion battery disclosed in the present application, including most of the operation steps in Example 1-1 described above, the difference is that the composition and content of the organic solvent, electrolyte salt and additive in the non-aqueous electrolyte, as shown in the respective tables.

[0081] The lithium ion battery prepared in each example and comparative example is tested for performance according to the following method:

[0082] I. High temperature storage performance test

[0083] The lithium ion battery after formation is charged to 4.5 V at room temperature with 1 C constant current and constant voltage, the initial discharge capacity and initial battery thickness of the battery are measured, then after 30 days of storage in 60℃ environment, discharged to 3 V at 1 C, the retention capacity and recovery capacity of the battery and the thickness of the battery after storage are measured. The calculation formula is as follows:

[0084] Battery capacity retention rate (%) = retention capacity / initial capacity x 100%;

[0085] Battery capacity recovery rate (%) = recovery capacity / initial capacity x 100%;

[0086] Thickness expansion rate (%) = (thickness of battery after storage-initial thickness of battery) / initial thickness of battery x 100%.

[0087] II. High temperature cycle performance test

[0088] The lithium ion battery prepared above is placed in an oven at a constant temperature of 45℃, charged to 4.5 V at a current of 1 C, then charged at a constant voltage until the current drops to 0.02 C, then discharged to 3.0 V at a current of 1 C, and so on, the cycle capacity retention rate and impedance growth rate of the battery after 500 cycles at 45℃ are calculated. The calculation is as follows:

[0089] Cycle capacity retention rate (%) = [(capacity after corresponding cycle number-initial capacity after first cycle) / initial capacity after first cycle] x 100%;

[0090] Impedance growth rate (%) = [(discharge impedance after corresponding cycle number-discharge impedance after first cycle) / discharge impedance after first cycle] x 100%.

[0091] III. 136℃ hot box test:

[0092] The lithium ion battery after formation is charged at room temperature at 0.5C constant current and constant voltage to 4.5V (LiCoO2 / artificial graphite battery), then charged at constant voltage until the current drops to 0.02C, and after 5min rest, discharged at 0.5C constant current to 3.0V. Then the battery is placed in a blast oven to rise from room temperature to 136℃ at 5℃ / min and keep for 30min. The state of the battery surface is monitored during the temperature rising process and the holding process. If the battery valve opens, catches fire or explodes, it is a fail test; if the battery does not open the valve, does not catch fire and does not explode, it is a pass test. 5 batteries are tested in each group, and the pass rate = the number of passes / 5.

[0093] Table 1 shows the parameters required for preparing lithium ion batteries in Examples 1-1 to 1-25 and Comparative Examples 1-1 to 1-12, and the differences between the components in the non-aqueous electrolyte of each example and comparative example and Example 1-1 are shown in Table 1.

[0094] Table 1

[0095]

[0096]

[0097] Table 2 shows the test results of the lithium ion batteries prepared in Examples 1-1 to 1-25 and Comparative Examples 1-1 to 1-12.

[0098] Table 2

[0099]

[0100]

[0101] As can be seen from the test results of Examples 1-1 to 1-25 and Comparative Examples 1-1 to 1-12, the lithium ion battery provided by the present application, which uses a high content of a compound with a specific structure as additive A and a high content of a lithium salt type additive B, and limits the organic solvent to contain a cyclic carbonate and a fluorinated chain ester; and also limits the relationship between the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, and the mass percentage content d of fluorinated chain ester in the non-aqueous electrolyte to satisfy a+b / 7≤(c+d) / 10, 1≤a≤3, 3≤b≤10, 20≤c≤40, and 10≤d≤30, can obtain a lithium ion battery that takes into account excellent high-temperature storage performance, high-temperature cycle performance, and safety performance.

[0102] As can be seen from the test results of Example 1-1 and Comparative Examples 1-1 to 1-12, when any one of the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, and the mass percentage content d of fluorinated chain ester in the non-aqueous electrolyte does not meet the range or the relationship a+b / 7≤(c+d) / 10, the synergistic effect between the parameters cannot be achieved, and the thermal stability and kinetic performance of the battery electrolyte cannot be improved well, which may lead to impedance growth during battery cycling, positive metal ion dissolution, and battery performance degradation, and thus a lithium ion battery with high-temperature storage, cycling performance, and safety performance cannot be obtained.

[0103] When the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, and the mass percentage content d of fluorinated chain ester in the non-aqueous electrolyte further satisfy 2≤a≤3, 4.2≤b≤9, 30≤c≤40, and 20≤d≤30, the high-temperature cycling performance and high-temperature storage performance of the prepared lithium ion battery can be further improved.

[0104] As can be seen from the test results of Example 1-1 to 1-18 and Example 1-19 to 1-25, when the relationship between the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, and the mass percentage content d of fluorinated chain ester in the non-aqueous electrolyte is within the above range, and the ratio of the mass of ethylene carbonate (EC) and propylene carbonate (PC) c2+c3 to the mass of fluorinated ethylene carbonate (FEC) c1 satisfies 1≤(c2+c3) / c1≤2, the battery performance can achieve the best balance, and the battery system can be ensured to operate more efficiently and stably.

[0105] As shown in Table 3, the parameters required for preparing lithium ion batteries in Example 1-1 and Example 1-26 to 1-30 and Comparative Examples 1-13 to 1-15 are shown, and the difference between Example 1-26 to 1-30 and Comparative Examples 1-13 to 1-15 and Example 1-1 is the type of fluorinated chain ester and the type of cyclic carbonate shown in Table 3, and the remaining parameters and preparation steps are the same as described in Example 1-1.

[0106] Table 3

[0107]

[0108] Note: " / " in the table means that the item is not present, DFEA is 2,2-difluoroethyl acetate, FEMC is methyl trifluoroethyl carbonate, and when the fluorinated chain ester is two types, the mass ratio is 1:1.

[0109] The test results of the lithium ion batteries prepared in Example 1-1 and Examples 1-26 to 1-30 and Comparative Examples 1-13 to 1-15 are shown in Table 4.

[0110] Table 4

[0111]

[0112] From the test results in Table 4, it can be seen that when the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, the mass percentage content d of fluorinated chain ester, the relationship a+b / 7≤(c+d) / 10 meet the relevant requirements, and the ratio of the total mass c2+c3 of ethylene carbonate (EC) and propylene carbonate (PC) to the mass c1 of fluorinated ethylene carbonate (FEC) in the cyclic carbonate is the same, the lithium ion battery with excellent high-temperature cycle performance and excellent high-temperature storage performance can be obtained by using 2,2-difluoroethyl acetate (DFEA) and / or methyl trifluoroethyl carbonate as the fluorinated chain ester organic solvent and using fluorinated ethylene carbonate, ethylene carbonate and / or propylene carbonate as the cyclic carbonate organic solvent, but when other cyclic carbonates or fluorinated chain esters outside the scope of the present application are used as solvents, the high-temperature cycle performance, high-temperature storage performance and safety performance of the lithium ion battery will be deteriorated.

[0113] The test results of the lithium ion batteries prepared in Example 1-1 and Examples 1-31 to 1-34 are shown in Table 5; the difference between Examples 1-31 to 1-34 and Example 1-1 is the type of additive A shown in Table 5, and the remaining parameters and preparation steps are the same as described in Example 1-1.

[0114] Table 5

[0115]

[0116] From the test results in Table 5, it can be seen that when the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, the mass percentage content d of fluorinated chain ester, the relationship a+b / 7≤(c+d) / 10 meet the relevant requirements, and any one compound containing both sulfate group and carbonate group is used as additive A, the lithium ion battery with excellent high-temperature cycle performance, excellent high-temperature storage performance and good safety performance can be obtained, which indicates that the battery system of the present application has universality for different compounds containing both sulfate group and carbonate group as additive A.

[0117] The test results of the lithium ion batteries prepared in Example 1-1, Example 1-35 to Example 1-37 are shown in Table 6; the differences between Example 1-35 to Example 1-37 and Example 1-1 are the additive B types shown in Table 6, and the rest of the parameters and preparation steps are the same as those described in Example 1-1.

[0118] Table 6

[0119]

[0120] It can be seen from the test results in Table 6 that when the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, the mass percentage content d of fluorinated chain ester, and the relationship a+b / 7≤(c+d) / 10 in the non-aqueous electrolyte meet the relevant requirements, and lithium bis-trifluoromethanesulfonimide, lithium difluoroboric oxalate, lithium difluorophosphate or lithium tetrafluoroborate are added as additive B, lithium ion batteries with excellent high-temperature cycle performance, excellent high-temperature storage performance and good safety performance can be obtained, which shows that the battery system of the present application has universality for different types of additive B.

[0121] The test results of Example 1-1, Example 1-38 to Example 1-42 are shown in Table 7; the differences between Example 1-38 to Example 1-42 and Example 1-1 are the content of electrolyte salt lithium hexafluorophosphate and additive B in the non-aqueous electrolyte and the ratio of their contents shown in Table 7, and the rest of the parameters and preparation steps are the same as those described in Example 1-1.

[0122] Table 7

[0123]

[0124] It can be seen from the test results in Table 7 that when the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, the mass percentage content d of fluorinated chain ester, and the relationship a+b / 7≤(c+d) / 10 in the non-aqueous electrolyte meet the relevant requirements, and the content of electrolyte salt lithium hexafluorophosphate is greater than or equal to the content of additive B, or the content of electrolyte salt lithium hexafluorophosphate in the non-aqueous electrolyte is between 10% and 18%, the high-temperature cycle performance, high-temperature storage performance and safety performance of the lithium ion batteries prepared are better. The possible reason for analysis is that when the content of electrolyte salt lithium hexafluorophosphate is greater than or equal to the content of additive B, the amount of free solvent can be reduced, thereby reducing the probability of its reduction at the negative electrode or oxidation at the positive electrode, improving the high-temperature performance and inhibiting the generation of gas; and a high content of LiPF6 can improve the ion migration rate of the electrolyte, thereby further improving the kinetic performance of the lithium ion battery.

[0125] The test results of the lithium ion batteries prepared in Examples 1-1, 1-43 to 1-44 are shown in Table 8. Examples 1-43 to 1-44 differ from Example 1-1 in the type and content of the base additive shown in Table 8, and the rest of the parameters and preparation steps are the same as those described in Example 1-1.

[0126] Table 8

[0127]

[0128] As can be seen from the test results in Table 8, the lithium ion battery of the present application, when used in combination with the base additive, still has a good improvement effect on the impedance performance and high temperature performance of the lithium ion battery.

[0129] The parameters required for the lithium ion batteries prepared in Examples 2-1 to 2-25 and Comparative Examples 2-1 to 2-12 are shown in Table 9. Examples 2-1 to 2-25 and Comparative Examples 2-1 to 2-12 differ from Example 1-1 in the parameters and the type of additive A shown in Table 9, and the rest of the parameters and preparation steps are the same as those described in Example 1-1. The specific differences are that Examples 2-1 to 2-25 and Comparative Examples 2-1 to 2-12 use only compound 6 containing a sulfate group as additive A, and the differences between the components in the non-aqueous electrolyte of each example and comparative example and Example 1-1 are shown in Table 9.

[0130] Table 9

[0131]

[0132] Table 10 shows the test results of the lithium ion batteries prepared in Examples 2-1 to 2-25 and Comparative Examples 2-1 to 2-12.

[0133] Table 10

[0134]

[0135] As can be seen from the test results of Examples 2-1 to 2-25 and Comparative Examples 2-1 to 2-12, the lithium ion battery provided by the present application uses a high content of a compound with a specific structure as additive A and a high content of a lithium salt type additive B, and the organic solvent is limited to contain a cyclic carbonate and a fluorinated chain ester; meanwhile, the relationship between the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, and the mass percentage content d of fluorinated chain ester in the non-aqueous electrolyte satisfies a+b / 7≤(c+d) / 10, 1≤a≤3, 3≤b≤10, 20≤c≤40, and 10≤d≤30, so that a lithium ion battery with excellent high-temperature storage performance, high-temperature cycle performance, and safety performance can be obtained.

[0136] As can be seen from the test results of Examples 2-1 to 2-25 and Comparative Examples 2-1 to 2-12, the lithium ion battery provided by the present application uses a high content of a compound with a specific structure as additive A and a high content of a lithium salt type additive B, and the organic solvent is limited to contain a cyclic carbonate and a fluorinated chain ester; meanwhile, the relationship between the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, and the mass percentage content d of fluorinated chain ester in the non-aqueous electrolyte satisfies a+b / 7≤(c+d) / 10, 1≤a≤3, 3≤b≤10, 20≤c≤40, and 10≤d≤30, so that a lithium ion battery with excellent high-temperature storage performance, high-temperature cycle performance, and safety performance can be obtained.

[0137] When the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, and the mass percentage content d of fluorinated chain ester in the non-aqueous electrolyte further satisfy 2≤a≤3, 4.2≤b≤9, 30≤c≤40, and 20≤d≤30, the high-temperature cycle performance and high-temperature storage performance of the prepared lithium ion battery can be further improved.

[0138] As can be seen from the test results of Examples 2-1 to 2-18 and Examples 2-19 to 2-25, when the relationship between the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, and the mass percentage content d of fluorinated chain ester in the non-aqueous electrolyte is within the above range, and the ratio of the mass of ethylene carbonate (EC) and propylene carbonate (PC) c2+c3 to the mass of fluorinated ethylene carbonate (FEC) c1 satisfies 1≤(c2+c3) / c1≤2, the performance of the battery can achieve the best balance, and the battery system can be ensured to operate more efficiently and stably.

[0139] The parameters required for preparing the lithium ion battery in Example 2-1 and Examples 2-26 to 2-30 and Comparative Examples 2-13 to 2-15 are shown in Table 11. Examples 2-26 to 2-30 and Comparative Examples 2-13 to 2-15 differ from Example 2-1 in the type and content of the fluorinated chain ester and the cyclic carbonate shown in Table 2, and the rest of the parameters and the preparation steps are the same as described in Example 2-1.

[0140] Table 11

[0141]

[0142] Note: " / " in the table means that the item is not present. DFEA is 2,2-difluoroethyl acetate, FEMC is methyl trifluoroethyl carbonate, and when two types of fluorinated chain ester are present, the mass ratio is 1:1.

[0143] The test results of the lithium ion batteries prepared in Example 2-1 and Examples 2-26 to 2-30 and Comparative Examples 2-13 to 2-15 are shown in Table 12.

[0144] Table 12

[0145] As can be seen from the test results in Table 12, when the mass percentage content a of additive A, the mass percentage content b of additive B, the mass percentage content c of cyclic carbonate, the mass percentage content d of fluorinated chain ester, the relationship a+b / 7≤(c+d) / 10 meets the relevant requirements, and the mass ratio of c2+c3 to the mass c1 of fluorinated ethylene carbonate (FEC) in the cyclic carbonate is the same, using 2,2-difluoroethyl acetate (DFEA) and / or methyl trifluoroethyl carbonate as the fluorinated chain ester organic solvent and using fluorinated ethylene carbonate, ethylene carbonate and / or propylene carbonate as the cyclic carbonate organic solvent, lithium ion batteries with excellent high-temperature cycle performance and excellent high-temperature storage performance can be obtained, but when other cyclic carbonates or fluorinated chain esters outside the scope of the present application are used as solvents, the high-temperature cycle performance, high-temperature storage performance and safety performance of the lithium ion battery will be degraded.

[0146] The test results of the lithium ion batteries prepared in Example 2-1, Example 2-31 and Comparative Example 2-16 are shown in Table 13. Example 2-31 differs from Example 2-1 and Comparative Example 2-16 in the type of additive A shown in Table 13, and the rest of the parameters and the preparation steps are the same as described in Example 2-1.

[0147] Table 13

[0148]

[0149] From the test results of Table 13, it can be seen that when the mass percentage content a of the additive A, the mass percentage content b of the additive B, the mass percentage content c of the cyclic carbonate, the mass percentage content d of the fluorinated chain ester, and the relationship a+b / 7≤(c+d) / 10 meet the relevant requirements, and only a compound containing a sulfate group is added as the additive A, a lithium ion battery with excellent high-temperature cycle performance, excellent high-temperature storage performance, and good safety performance can be obtained, which indicates that the battery system of the present application is universal for different compounds containing only a sulfate group as the additive A.

[0150] From the test results of Example 2-1, Example 2-31, and Comparative Example 2-16, it can be seen that when DTD is used to replace the additive A, the high-temperature cycle performance of Comparative Example 2-16 is significantly deteriorated, and slight lithium precipitation occurs, indicating that the additive A of the present application has a more improved effect on the battery performance than the single-ring structure of vinyl sulfate, because the multi-ring structure of the first additive of the present application is individually ring-opened to participate in the formation of the electrode surface interfacial film, the formed interfacial film components are more stable and dense, and have the effect of improving the interfacial film structure strength, thereby being beneficial to improving the high-temperature stability, while the single-ring structure of DTD cannot form a good synergistic effect with other components, and has poor improvement effect on the safety performance and cycle performance of the lithium ion battery.

[0151] As shown in Table 14, the test results of the lithium ion batteries prepared in Example 2-1, Example 2-32 to Example 2-34 are shown; the difference between Example 2-32 to Example 2-34 and Example 2-1 is the type of additive B shown in Table 14, and the rest of the parameters and preparation steps are the same as described in Example 2-1.

[0152] Table 14

[0153]

[0154] From the test results of Table 14, it can be seen that when the mass percentage content a of the additive A, the mass percentage content b of the additive B, the mass percentage content c of the cyclic carbonate, the mass percentage content d of the fluorinated chain ester, and the relationship a+b / 7≤(c+d) / 10 meet the relevant requirements, and lithium bis(trifluoromethanesulfonyl)imide, lithium difluorophosphate, or lithium difluoroboric oxalate is added as the additive B, a lithium ion battery with excellent high-temperature cycle performance, excellent high-temperature storage performance, and good safety performance can be obtained, which indicates that the battery system of the present application is universal for different additives B.

[0155] The test results of the lithium ion batteries prepared in Example 2-1, Example 2-35 to Example 2-39 are shown in Table 15; the differences between Example 2-35 to Example 2-39 and Example 2-1 are the content and content ratio of the electrolyte salt lithium hexafluorophosphate and the additive B in the non-aqueous electrolyte shown in Table 15, and the rest of the parameters and preparation steps are the same as those described in Example 2-1.

[0156] Table 15

[0157]

[0158] From the test results in Table 15, it can be seen that when the mass percentage content a of the additive A, the mass percentage content b of the additive B, the mass percentage content c of the cyclic carbonate, the mass percentage content d of the fluorinated chain ester in the non-aqueous electrolyte, the relationship a+b / 7≤(c+d) / 10 meets the relevant requirements, and at the same time when the content of the electrolyte salt lithium hexafluorophosphate is greater than or equal to the content of the additive B, or the content of the electrolyte salt lithium hexafluorophosphate in the non-aqueous electrolyte is between 10% and 18%, the high-temperature cycle performance and high-temperature storage performance of the prepared lithium ion battery are better. The analysis reason may be that when the content of the electrolyte salt lithium hexafluorophosphate is greater than or equal to the content of the additive B, the amount of free solvent can be reduced, thereby reducing the probability of its reduction at the negative electrode or oxidation at the positive electrode, improving the high-temperature performance, and inhibiting the generation of gas; and a high content of LiPF6 can improve the ion migration rate of the electrolyte, thereby further improving the kinetic performance of the lithium ion battery.

[0159] The test results of the lithium ion batteries prepared in Example 2-1, Example 2-40 to Example 2-41 are shown in Table 16; the differences between Example 2-40 to Example 2-41 and Example 2-1 are the types and contents of the basic additives in the non-aqueous electrolyte shown in Table 16, and the rest of the parameters and preparation steps are the same as those described in Example 2-1.

[0160] Table 16

[0161]

[0162] From the test results in Table 16, it can be seen that the lithium ion battery of the present application still has a good improvement effect on the impedance performance and high-temperature performance of the lithium ion battery when used in combination with the basic additive.

[0163] The above describes the present application with the help of specific examples, but it should be understood that the specific description here should not be understood as limiting the essence and scope of the present application, and various modifications made by those skilled in the art after reading the above examples are within the scope of the present application.

Claims

1. A nonaqueous electrolyte, characterized by comprising: The electrolyte salt, the organic solvent and the additive are included. The organic solvent includes a cyclic carbonate and a fluorinated chain ester; The cyclic carbonate includes a fluorinated cyclic carbonate and a non-fluorinated cyclic carbonate, the fluorinated cyclic carbonate includes fluorinated ethylene carbonate, and the non-fluorinated cyclic carbonate includes ethylene carbonate and / or propylene carbonate; The fluorinated chain ester includes 2,2-difluoroethyl acetate and / or methyl trifluoroethyl carbonate; The additive includes an additive A and an additive B; The additive A includes a compound shown in structural formula 1: Structural formula 1 wherein X is selected from or R1, R2are each independently selected from H, , or , R1and R2are not simultaneously selected from H, and at least one of X, R1and R2contains a sulfur atom; The additive B includes at least one of lithium difluoro(oxalato)borate, lithium difluorophosphate, lithium bis(trifluoromethanesulfonyl)imide or lithium tetrafluoroborate; The mass percentage of the additive A is a%, the mass percentage of the additive B is b%, the mass percentage of the cyclic carbonate is c%, and the mass percentage of the fluorinated chain ester is d% based on the mass of the non-aqueous electrolyte, and the non-aqueous electrolyte satisfies the following conditions: a+b / 7≤(c+d) / 10, 1≤a≤3, 3≤b≤10, 20≤c≤40, and 10≤d≤30.

2. The nonaqueous electrolyte according to claim 1, wherein The non-aqueous electrolyte satisfies at least one of the following conditions: （1）2≤a≤3； （2）4.2≤b≤9； （3）30≤c≤40； （4）20≤d≤30。 3. The nonaqueous electrolyte according to claim 1, wherein The compound shown in structural formula 1 includes at least one of the following compounds:

4. The nonaqueous electrolyte according to claim 1, wherein The mass percentage of the fluorinated ethylene carbonate is c1%, the mass percentage of the ethylene carbonate is c2%, and the mass percentage of the propylene carbonate is c3% based on the mass of the non-aqueous electrolyte, and the cyclic carbonate satisfies 1≤(c2+c3) / c1≤2.

5. The nonaqueous electrolyte according to claim 4, wherein The non-aqueous electrolyte satisfies at least one of the following conditions: （1）10≤c1≤20； （2）5≤c2≤15； （3）5≤c3≤15。 6. The nonaqueous electrolyte according to claim 1, wherein The electrolyte salt includes lithium hexafluorophosphate, and the mass percentage of the lithium hexafluorophosphate is greater than or equal to the mass percentage of the additive B.

7. The nonaqueous electrolyte according to claim 1, wherein The non-aqueous electrolyte further includes a basic additive, the basic additive includes one or more of a sulfolane compound, a cyclic sulfate compound, a phosphate compound and a borate compound; and / or, The mass percentage of the basic additive is 0.01% to 10% based on the mass of the non-aqueous electrolyte.

8. The nonaqueous electrolyte according to claim 1, wherein The sulfolane compound includes at least one of 1,3-propane sultone, 1,4-butane sultone or 1,3-propene sultone; and / or, The cyclic sulfate compound includes at least one of ethylene sulfate, propylene sulfate or methyl ethylene sulfate; and / or, The phosphate compound includes at least one of the compounds shown in structural formula 2: Structural formula 2 In the structural formula 2, R 31 , R 32 , R 33 are each independently selected from a C1-C5 saturated hydrocarbon group, a C1-C5 unsaturated hydrocarbon group, a C1-C5 halogenated hydrocarbon group, -Si(C m H 2m+1 )3, and m is a natural number of 1 to 3; and / or, The borate compound includes at least one of tris(trimethylsilyl)borate and tris(triethylsilyl)borate.

9. A lithium-ion battery, characterized by The non-aqueous electrolyte includes a positive electrode, a negative electrode and the non-aqueous electrolyte according to any one of claims 1 to 8.

10. The lithium-ion battery of claim 9, wherein the lithium-ion battery is a lithium-ion battery. The positive electrode includes a positive electrode material layer including a positive electrode active material, and the positive electrode active material includes lithium cobaltate; The negative electrode includes a negative electrode material layer including a negative electrode active material, and the negative electrode active material includes artificial graphite and / or natural graphite.

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