Lithium ion battery electrolyte and application thereof

Abstract

This invention proposes a lithium-ion battery electrolyte and its application. The electrolyte comprises at least the following components: a fluorinated solvent, including 2,2-difluoroethyl acetate, fluoroethylene carbonate, and 2,2-difluoroethyl ethyl carbonate; a lithium salt, including a first lithium salt comprising lithium hexafluorophosphate; and an additive, including a first additive comprising lithium difluorobis(oxalato)phosphate. The lithium-ion battery electrolyte and its application proposed in this invention can improve the electrolyte's high-voltage resistance and kinetic performance, reduce battery impedance, and enhance the battery's fast-charging performance and high-temperature cycle stability.

Description

Technical Field

[0001] This invention relates to the field of power battery technology, specifically to a lithium-ion battery electrolyte and its application. Background Technology

[0002] With the rapid development of the new energy vehicle market, the demand for high-energy-density lithium-ion batteries is becoming increasingly urgent. Increasing battery voltage is a straightforward and feasible method to improve energy density. However, as battery voltage increases, higher demands are placed on the high-voltage resistance of the electrolyte. While using fluorinated solvents can improve the high-voltage resistance of the electrolyte, it also reduces conductivity, increases viscosity and impedance, leading to a deterioration in battery kinetics and cycle stability. Summary of the Invention

[0003] This invention proposes a lithium-ion battery electrolyte and its application. The lithium-ion battery electrolyte and its application provided by this invention can improve the high voltage resistance and kinetic performance of the electrolyte, reduce the battery impedance, and improve the battery's fast charging performance and high temperature cycle stability.

[0004] To address the aforementioned technical problems, the present invention provides a lithium-ion battery electrolyte, comprising at least the following components:

[0005] Fluorinated solvents, including 2,2-difluoroethyl acetate, fluoroethylene carbonate and 2,2-difluoroethyl ethyl carbonate;

[0006] Lithium salts, including a first lithium salt comprising lithium hexafluorophosphate; and

[0007] Additives, including a first additive, wherein the first additive comprises lithium difluorobis(oxalato)phosphate.

[0008] In one embodiment of the present invention, the mass ratio of 2,2-difluoroethyl acetate, 2,2-difluoroethyl ethyl carbonate, and fluoroethylene carbonate is (3-5):(6-4):1.

[0009] In one embodiment of the present invention, the content of 2,2-difluoroethyl acetate in the electrolyte is 10wt%-40wt%.

[0010] In one embodiment of the present invention, the content of 2,2-difluoroethyl ethyl carbonate in the electrolyte is 20wt%-40wt%.

[0011] In one embodiment of the present invention, the content of the fluoroethylene carbonate in the electrolyte is 5wt%-30wt%.

[0012] In one embodiment of the present invention, the content of the first additive in the electrolyte is 0.1wt%-1wt%.

[0013] In one embodiment of the present invention, the additive further includes a second additive, the second additive including triallyl isocyanurate, and the content of the second additive in the electrolyte is 0.01wt%-1wt%.

[0014] In one embodiment of the present invention, the mass ratio of the first additive to the second additive is (0.63-1.67):1.

[0015] In one embodiment of the present invention, the content of the first lithium salt in the electrolyte is 11wt%-20wt%, and the lithium salt further includes a second lithium salt, the second lithium salt being selected from at least one of lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide, and the content of the second lithium salt in the electrolyte is less than 1wt%.

[0016] The present invention also provides a lithium-ion battery, comprising at least:

[0017] The positive electrode sheet includes a positive electrode active material, wherein the structural formula of the positive electrode active material is Li. a Ni x Mn y O 4-z M z Wherein, 0.90≤a≤1.1, 0.4≤x≤0.6, 1.4≤y≤1.6, 0≤z≤0.1, and M is selected from at least one of Cl, Br, I, S, Se, Te or F;

[0018] Negative electrode plate;

[0019] A diaphragm is disposed between the positive electrode and the negative electrode; and

[0020] The electrolyte is selected from the lithium-ion battery electrolytes mentioned above.

[0021] In summary, this invention proposes a lithium-ion battery electrolyte and its application. The fluorinated solvent improves the electrolyte's high-voltage resistance, enhances its kinetic properties, and reduces its viscosity, thus preventing a decrease in conductivity. The fluorinated solvent and the first additive work synergistically to form a stable solid electrolyte interphase (SEI) film at the negative electrode-electrochemical interface and a stable chemical-electrochemical interface (CEI) film at the positive electrode-electrochemical interface, ensuring the stability of the electrode-electrolyte interface at high temperatures and significantly improving the battery's fast-charging performance and high-temperature cycle stability. The second additive further improves the battery's high-temperature cycle stability. The second lithium salt increases the electrolyte's conductivity, thereby further improving the battery's impedance and fast-charging performance. Detailed Implementation

[0022] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0023] It should be understood that the invention can be embodied in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0024] The technical solution of the present invention will be further described in detail below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] This invention proposes a lithium-ion battery electrolyte, comprising at least a fluorinated solvent, a lithium salt, and additives. The fluorinated solvent includes 2,2-difluoroethyl acetate (DFEA), fluoroethylene carbonate (FEC), and 2,2-difluoroethylethyl carbonate (DFDEC). The lithium salt includes a first lithium salt, specifically lithium hexafluorophosphate (LiPF6). The additive includes a first additive, specifically lithium difluorobis(oxalato) phosphate (LiODFP). In the lithium-ion battery electrolyte provided by this invention, the fluorinated solvent improves the electrolyte's high-voltage resistance and kinetic performance. Furthermore, the fluorinated solvent and the first additive work synergistically to form a stable SEI film at the interface between the negative electrode and the electrolyte, and a stable CEI film at the interface between the positive electrode and the electrolyte, thereby significantly improving the battery's fast-charging performance and high-temperature cycle stability.

[0026] In one embodiment of the present invention, the fluorinated solvent, for example, is a difluorinated solvent, which possesses sufficient oxidative stability and suitable dissociation ability, significantly improving the high-voltage resistance and kinetic performance of the electrolyte. Specifically, in fluorinated solvents, fluorine atoms have strong electron-withdrawing properties, which enhances their high-voltage resistance and oxidation resistance. However, with increasing fluorination degree, the dissociation ability of the fluorinated solvent for lithium salts decreases, and the reduction resistance of the fluorinated solvent on the negative electrode side also decreases. Therefore, compared to monofluorinated solvents (e.g., ethyl 2-fluorocarbonate) and trifluorinated solvents (e.g., ethyl 2,2,2-trifluorocarbonate), difluorinated solvents possess sufficient oxidative stability and suitable dissociation ability, significantly improving the high-voltage resistance and kinetic performance of the electrolyte.

[0027] In one embodiment of the present invention, in the difluorinated solvent, the fluorine atoms in DFEA have strong electron-withdrawing properties, which improves its high-voltage resistance, thereby ensuring the stability of the electrolyte interface on the positive electrode side. At the same time, the ethyl acetate solvent of DFEA type has a low viscosity, thereby avoiding a decrease in the conductivity of the electrolyte.

[0028] In one embodiment of the present invention, for the fluorinated solvent, the content of DFEA in the electrolyte is, for example, 10wt%-40wt%, the content of DDFEC in the electrolyte is, for example, 20wt%-40wt%, and the content of FEC in the electrolyte is, for example, 5wt%-30wt%, and the mass ratio of DFEA, DDFEC and FEC is, for example, (3-5):(6-4):1. By controlling the mass ratio of DFEA, DDFEC and FEC, the viscosity of the electrolyte can be reduced, the dissociation ability and high-temperature stability of the electrolyte can be improved, thereby balancing the battery's impedance, fast-charging performance and high-temperature cycle stability.

[0029] In one embodiment of the present invention, the first additive is, for example, LiODFP, which can improve the fast-charging performance and high-temperature cycle stability of the battery. Specifically, in conventional carbonate solvent systems, LiODFP causes rapid capacity decay and battery failure due to insufficient intrinsic oxidation stability of the electrolyte and its inability to withstand high voltage. However, by introducing LiODFP into the fluorinated solvent provided by the present invention, the battery does not fail rapidly because the fluorinated solvent has high oxidation stability. Moreover, LiODFP can be reduced at the negative electrode to generate a stable SEI film protecting the interface between the negative electrode and the electrolyte. Simultaneously, it can undergo oxidative decomposition at the positive electrode before the solvent, thereby participating in the formation of a stable CEI film. The products of LiODFP decomposition are rich in inorganic elements F and P with high ionic conductivity, which can effectively inhibit the decomposition of the electrolyte on the surfaces of the positive and negative electrodes and reduce the leakage of transition metal ions from the positive electrode side. Therefore, the synergistic effect of LiODFP and the fluorinated solvent can improve the fast-charging performance and high-temperature cycle stability of the battery.

[0030] In one embodiment of the present invention, the content of the first additive in the electrolyte is, for example, 0.1wt%-1wt%, and further, for example, 0.3wt%-0.8wt%. By controlling the content of the first additive, the ion conductivity and high-temperature stability of the interfacial phase can be improved, the reaction at the solid-liquid interface can be reduced, and the solubility of the first additive in the fluorinated solvent can be increased, thereby taking into account the battery impedance, fast charging performance and high-temperature cycle stability.

[0031] In one embodiment of the present invention, the additive further includes a second additive, such as triallyl isocyanurate (TAIC), the structural formula of which is, for example, [insert structural formula here]. By introducing a second additive, the second additive will participate in the film-forming reaction, which will help to further generate a stable interface layer and reduce the decomposition of electrolyte components at the electrode interface. At the same time, the second additive will also promote the dissociation of the first additive, slow down the decomposition rate of the first additive at the solid-liquid interface, and generate a denser interface phase, thereby improving the high-temperature cycle stability of the battery.

[0032] In one embodiment of the present invention, the content of the second additive in the electrolyte is, for example, 0.01wt%-1wt%. By controlling the content of the second additive, the high-temperature cycle stability of the battery can be significantly improved without affecting the battery's impedance and fast charging performance.

[0033] In one embodiment of the present invention, the mass ratio of the first additive to the second additive is, for example, (0.63-1.67):1. By controlling the mass ratio of the first additive to the second additive, the battery's impedance, fast-charging performance, and high-temperature cycle stability can be improved. Specifically, when the mass ratio of the first additive to the second additive is too large or too small, the battery's high-temperature cycle stability, impedance, and fast-charging performance will all be affected.

[0034] In one embodiment of the present invention, the additive further includes a third additive, such as a negative electrode film-forming additive, which is selected from at least one of vinylene carbonate (VC), 1,3-propanesultone (PS), or ethylene sulfate (DTD), and the content of the third additive in the electrolyte is, for example, 0.1wt%-5wt%.

[0035] In one embodiment of the present invention, the content of lithium salt in the electrolyte is, for example, 12wt%-20wt%. The content of the first lithium salt in the electrolyte is, for example, 11wt%-20wt%.

[0036] In one embodiment of the present invention, the lithium salt further includes a second lithium salt, which is selected from at least one of lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). By introducing the second lithium salt, the conductivity of the electrolyte can be improved due to its stronger dissociation ability, thereby improving the battery impedance and fast-charging performance. The content of the second lithium salt in the electrolyte is, for example, less than 1 wt%. By controlling the content of the second lithium salt, it is possible to avoid the easy reaction between a high content of the second lithium salt and the aluminum foil under high voltage, thereby preventing the continuous decomposition of the electrolyte and improving the battery impedance, fast-charging performance, and high-temperature cycle stability.

[0037] In one embodiment of the present invention, when preparing the electrolyte, battery-grade DFEA, DFDEC, and FEC are mixed uniformly in a glove box under a stable gas atmosphere such as argon according to a mass ratio to obtain a fluorinated solvent. Lithium salt and additives are then added to the fluorinated solvent and mixed uniformly to prepare a lithium-ion battery electrolyte. The moisture content in the glove box is, for example, less than 10 ppm.

[0038] This invention also proposes a lithium-ion battery, comprising a positive electrode, a separator, a negative electrode, and an electrolyte. The separator is located between the positive and negative electrodes to prevent short circuits between them, allowing lithium ions to pass through. The electrolyte fills the space between the positive electrode, separator, and negative electrode, and is the aforementioned lithium-ion battery electrolyte, serving to conduct ions. The lithium-ion battery can be, for example, a primary or secondary battery. A secondary battery can be, for example, a pouch battery, a hard-case battery, or a cylindrical battery. This invention does not specifically limit the type or category of lithium-ion batteries.

[0039] In one embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive active layer coated on at least one surface of the positive current collector. The positive current collector is, for example, a foil formed by surface treatment of materials such as nickel, titanium, aluminum, silver, stainless steel, or carbon. Besides foil, the positive current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam, or nonwoven fabric.

[0040] In one embodiment of the present invention, the positive electrode active layer includes a positive electrode active material, a conductive agent, and a binder. The ratio of the positive electrode active material, the conductive agent, and the binder can be selected according to actual needs. In this embodiment, the positive electrode active material includes, for example, lithium nickel manganese oxide, with the chemical formula, for example, Li. a Ni x Mn y O 4-z M z Wherein, 0.90≤a≤1.1, 0.4≤x≤0.6, 1.4≤y≤1.6, 0≤z≤0.1, and M is selected from at least one of Cl, Br, I, S, Se, Te, or F. Further, in this embodiment, the surface of the positive electrode active material is provided with a coating layer, the material of which includes, for example, carbon.

[0041] In one embodiment of the present invention, the conductive agent is selected from at least one of conductive carbon black (SuperP), acetylene black, carbon nanotubes, or graphene, and the binder is selected from at least one of polyvinylidene fluoride (PVDF), poly(ethylene oxide) (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate (polyacrylate), polyvinyl ether (polyvinyl ether), polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexanefluoropropylene (polyhexafluoropropylene), or polymerized styrene-butadiene rubber (SBR).

[0042] In one embodiment of the present invention, the positive electrode current collector is, for example, aluminum foil, the conductive agent is, for example, SuperP, the binder is, for example, PVDF, and the chemical formula of the positive electrode active material is, for example, Li. 0.95 Ni 0.45 Mn 1.54 O4, and the surface of the positive electrode active material is coated with a carbon coating layer. Specifically, when preparing Li containing a carbon coating layer... 0.95 Ni 0.45 Mn 1.54 When using O4 as the positive electrode active material, the carbon source is pre-sintered in a stable gas atmosphere such as nitrogen to obtain carbon material. Then, the carbon material and Li... 0.95 Ni 0.45 Mn 1.54 The O4 positive electrode active material is mixed evenly and then subjected to a secondary sintering process to obtain Li containing a carbon coating layer. 0.95 Ni 0.45 Mn 1.54 O4 positive electrode active material. The carbon source is selected from at least one of fructose, polyethylene glycol, galactose, polyvinylpyrrolidone, or tannic acid. The pre-sintering temperature is, for example, 550°C, and the pre-sintering time is, for example, 5 hours. The secondary sintering temperature is, for example, 350°C, and the secondary sintering time is, for example, 12 hours.

[0043] In one embodiment of the present invention, when preparing the positive electrode, Li containing a carbon coating layer is used... a Ni x Mn y O 4-z M zPositive electrode active material, PVDF, and Super P are mixed in a mass ratio of 98:1:1, dissolved in an organic solvent, and stirred under vacuum until the system is homogeneous to obtain a positive electrode slurry. The positive electrode slurry is then uniformly coated onto aluminum foil, air-dried at room temperature, and then transferred to an oven for drying. After cold pressing and slitting, the positive electrode sheet is obtained. The organic solvent is, for example, N-methylpyrrolidone (NMP).

[0044] In one embodiment of the present invention, the negative electrode sheet includes, for example, a negative electrode current collector and a negative electrode active layer coated on at least one surface of the negative electrode current collector. The negative electrode current collector is selected from, for example, a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foamed copper current collector, or a stainless steel current collector.

[0045] In one embodiment of the present invention, the negative electrode active layer includes a negative electrode active material, a conductive agent, a thickener, and a binder. The proportions of the negative electrode active material, conductive agent, thickener, and binder can be selected according to actual needs. In this embodiment, the negative electrode active material is selected from at least one of graphite, silicon, or carbon dioxide. Further, the negative electrode active material is, for example, selected from at least one of artificial graphite, natural graphite, soft carbon, hard carbon, pure silicon, silicon oxides, and silicon carbide compounds.

[0046] In one embodiment of the present invention, the conductive agent is selected from at least one of Super P, acetylene black, Ketjen black, carbon nanotubes, or graphene. The thickener includes, for example, sodium carboxymethyl cellulose (CMC-Na). The binder is selected from at least one of PVDF, PEO, PA, polypropylene, polyacrylate, polyethylene ether, PMMA, polyhexamethylene propylene, or SBR.

[0047] In one embodiment of the present invention, the negative electrode current collector is, for example, copper foil, the negative electrode active material is, for example, artificial graphite, the conductive agent is, for example, SuperP, the thickener is, for example, CMC-Na, and the binder is, for example, SBR. Specifically, the negative electrode active material, conductive agent, thickener, and binder are mixed in a mass ratio of 96:1:1:2, and deionized water solvent is added. The mixture is then thoroughly stirred and mixed under the action of a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry is coated onto copper foil, and after being dried at room temperature, it is transferred to an oven for drying. After cold pressing and slitting, the negative electrode sheet is obtained.

[0048] In one embodiment of the present invention, the separator is, for example, a conventional separator, a ceramic separator, a polymer separator, a non-woven fabric separator, or an inorganic-organic composite separator. Specifically, the separator is, for example, a single-layer polypropylene (PP) membrane, a single-layer polyethylene (PE) membrane, a double-layer PP / PE membrane, a double-layer PP / PP membrane, or a triple-layer PP / PE / PP membrane, wherein the thickness of the separator is, for example, 9μm-15μm. In this embodiment, for example, a single-layer PP membrane with a thickness of 12μm is selected as the separator.

[0049] In one embodiment of the present invention, the above-mentioned positive electrode, separator and negative electrode are placed in sequence, so that the separator is placed between the positive electrode and the negative electrode to play a role in isolation, and is put into an aluminum-plastic film, transferred to a vacuum oven at 120°C for drying, and then injected with the lithium-ion battery electrolyte prepared above at 3.0 g / Ah and sealed. After electrolyte formation, a soft-pack lithium-ion battery with a capacity of 1Ah is obtained.

[0050] The present invention will be explained in more detail below by referring to embodiments, which should not be construed as limiting. Appropriate modifications can be made within the scope of the present invention, and all such modifications fall within the technical scope of the present invention.

[0051] Example 1

[0052] Preparation of the positive electrode: Fructose is pre-sintered in a stable gas atmosphere such as nitrogen to obtain carbon material, and then the carbon material and Li... 0.95 Ni 0.45 Mn 1.54 The O4 highly active material is mixed evenly and then subjected to a secondary sintering process to obtain Li containing a carbon coating. 0.95 Ni 0.45 Mn 1.54 O4 positive electrode active material. Li containing a carbon coating layer... 0.95 Ni 0.45 Mn 1.54 O4 positive electrode active material, PVDF and Super P are mixed in a mass ratio of 98:1:1, dissolved in NMP solvent, and stirred under vacuum until the system is homogeneous to obtain positive electrode slurry. The positive electrode slurry is then uniformly coated on aluminum foil, dried at room temperature and then transferred to an oven for drying. After cold pressing and slitting, positive electrode sheet is obtained.

[0053] Preparation of negative electrode sheet: Artificial graphite, SuperP, CMC-Na and SBR are mixed in a mass ratio of 96:1:1:2, and deionized water solvent is added. The mixture is then stirred and mixed thoroughly under vacuum to obtain a negative electrode slurry. The negative electrode slurry is coated on copper foil, and then dried at room temperature before being transferred to an oven for drying. After rolling and cutting, the negative electrode sheet is obtained.

[0054] Electrolyte preparation: In an argon atmosphere glove box with a moisture content of less than 10 ppm, DFEA, DFDEC, and FEC were mixed uniformly at a mass ratio of 3:6:1 according to the electrolyte composition described in Table 1 to obtain a fluorinated solvent. LiPF6 and LiODFP were then added to the fluorinated solvent and mixed uniformly to prepare the lithium-ion battery electrolyte. The contents of LiPF6 and LiODFP in the electrolyte were 13 wt% and 0.5 wt%, respectively.

[0055] Membrane selection: A single-layer PP membrane with a thickness of 12μm was selected as the membrane.

[0056] Battery preparation: The positive electrode, separator and negative electrode are placed in sequence and placed in an aluminum-plastic film. After drying in a vacuum oven at 120°C, the lithium-ion battery electrolyte prepared above is injected at 3.0 g / Ah and then sealed. After electrolyte formation, a soft-pack lithium-ion battery with a capacity of 1Ah is obtained.

[0057] Example 2

[0058] The mass ratio of DFEA, DFDEC and FEC is 4:5:1, and the other steps are the same as in Example 1.

[0059] Example 3

[0060] The mass ratio of DFEA, DFDEC and FEC is 5:4:1, and the other steps are the same as in Example 1.

[0061] Example 4

[0062] The content of LiODFP in the electrolyte is 0.01 wt%, and the other steps are the same as in Example 2.

[0063] Example 5

[0064] The content of LiODFP in the electrolyte is 1 wt%, and the other steps are the same as in Example 2.

[0065] Example 6

[0066] The electrolyte contains TAIC, and the content of TAIC in the electrolyte is 0.01 wt%. Other steps are the same as in Example 2.

[0067] Example 7

[0068] The TAIC content in the electrolyte is 0.5 wt%, and the other steps are the same as in Example 6.

[0069] Example 8

[0070] The electrolyte contains LiFSI, and the content of LiFSI in the electrolyte is 1 wt%. The other steps are the same as in Example 2.

[0071] Example 9

[0072] The LiFSI content in the electrolyte is 1.2 wt%, and the other steps are the same as in Example 8.

[0073] Example 10

[0074] The mass ratio of LiODFP to TAIC in the electrolyte was 0.32:0.5, and the other steps were the same as in Example 7.

[0075] Example 11

[0076] The mass ratio of LiODFP to TAIC in the electrolyte was 0.8:0.5, and the other steps were the same as in Example 7.

[0077] Example 12

[0078] The mass ratio of LiODFP to TAIC in the electrolyte is 1:0.5, and the other steps are the same as in Example 7.

[0079] Comparative Example 1

[0080] The electrolyte contains 2,2,2-trifluoroethyl ccetate (TFEA), and the mass ratio of DFEA, TFEA, DFDEC and FEC is 0:4:5:1. The other steps are the same as in Example 2.

[0081] Comparative Example 2

[0082] The electrolyte contains 2,2,2-trifluoroethyl carbonate (TFDEC), and the mass ratio of DFEA, DFDEC, TFDEC and FEC is 4:0:5:1. The other steps are the same as in Example 2.

[0083] Comparative Example 3

[0084] The content of LiODFP in the electrolyte is 0, and the other steps are the same as in Example 2.

[0085] Comparative Example 4

[0086] The electrolyte contains VC, and the contents of VC and LiODFP in the electrolyte are 0.5 wt% and 0 wt%, respectively. Other steps are the same as in Example 2.

[0087] Comparative Example 5

[0088] The electrolyte contains DTD, and the content of DTD in the electrolyte is 0.5 wt%. Other steps are the same as those in Comparative Example 4.

[0089] Comparative Example 6

[0090] The positive electrode active material is lithium cobalt oxide, and the other steps are the same as those in Comparative Example 3.

[0091] Comparative Example 7

[0092] The electrolyte contains LiODFP, and the content of LiODFP in the electrolyte is 0.5 wt%. Other steps are the same as those in Comparative Example 6.

[0093] The content of raw material components and the mass ratio of raw materials required for the electrolytes prepared in each embodiment and comparative example are shown in Table 1. The contents of the first lithium salt, second lithium salt, first additive, second additive, and third additive are mass percentages calculated based on the total mass of the electrolyte.

[0094] Table 1. Composition of electrolytes in Examples 1-12 and Comparative Examples 1-7

[0095]

[0096]

[0097] In this invention, the lithium-ion batteries prepared with different electrolyte ratios and positive electrode active materials in Examples 1-12 and Comparative Examples 1-7 were subjected to performance tests, and the test results are shown in Table 2.

[0098] In one embodiment of the present invention, for example, a direct current resistance (DCR) test is performed on a lithium-ion battery. Specifically, at 25°C, the battery is discharged to 50% state of charge (SOC) at a 1C current, and the battery voltage is detected and recorded as the initial voltage V1. Then, the current is increased to 4C and maintained for 30 seconds. After the battery voltage stabilizes, the stable voltage is recorded as the final voltage, and the difference between the final voltage and the initial voltage is calculated. The ratio of this difference to the 4C current value is the battery's DCR.

[0099] In one embodiment of the present invention, for example, a fast-charging performance test is performed on a lithium-ion battery. Specifically, at 25°C, the battery is sequentially charged at different rates (0.33C, 1C, 2C, and 3C) to the upper limit cutoff voltage, then charged at a constant voltage at the upper limit cutoff voltage until the current is less than 0.05C. After resting for 10 minutes, it is discharged at a constant current of 0.33C to the lower limit cutoff voltage. Each charging rate is cycled twice, and then the cycle is adjusted to the next rate. The charging capacity of the second cycle at 0.33C and the second cycle at 3C are recorded as the charging capacity of the lithium-ion battery at 0.33C and 3C, respectively. The capacity retention rate at the high-rate 3C charging is calculated using the following formula:

[0100] 3C capacity retention rate (%) = (3C charging capacity / 0.33C charging capacity) × 100%.

[0101] In the examples 1-12 and 1-5, the upper limit cutoff voltage of the lithium nickel manganese oxide / graphite battery is 4.85V and the lower limit cutoff voltage is 3.4V. In the examples 6-7, the upper limit cutoff voltage of the lithium cobalt oxide / graphite battery is 4.45V and the lower limit cutoff voltage is 2.8V.

[0102] In one embodiment of the present invention, a high-temperature cycle stability test is performed on a lithium-ion battery, for example. Specifically, the battery is charged and discharged at 45°C with a charge / discharge rate of 1C / 1C within the charge / discharge cutoff voltage, and the battery capacity is measured. The test ends when the battery capacity reaches 80% of the first cycle capacity (State of Health (SOH), and the number of cycles at 45°C is recorded.

[0103] In the examples 1-12 and 1-5, the lithium nickel manganese oxide / graphite batteries had a charge / discharge cutoff voltage of 3.4V-4.85V, while in the examples 6-7, the lithium cobalt oxide / graphite batteries had a charge / discharge cutoff voltage of 2.8V-4.45V.

[0104] Table 2 shows the performance test results of lithium-ion batteries in Examples 1-12 and Comparative Examples 1-7.

[0105] Group DCR(mOhm) 3C Capacity Retention Rate (%) Number of cycles at 45℃ (80% SOH) Example 1 118 75 524 Example 2 112 79 505 Example 3 108 81 479 Example 4 113 80 486 Example 5 121 77 516 Example 6 113 79 511 Example 7 118 80 526 Example 8 102 81 501 Example 9 119 76 464 Example 10 120 78 518 Example 11 115 81 537 Example 12 121 76 497 Comparative Example 1 130 73 453 Comparative Example 2 146 71 469 Comparative Example 3 102 72 475 Comparative Example 4 99 71 440 Comparative Example 5 116 68 459 Comparative Example 6 131 63 396 Comparative Example 7 139 67 412

[0106] Please refer to Tables 1 and 2. Comparing Example 2 and Comparative Examples 1-2, it can be seen that when the fluorinated solvent contains TFEA or TFDEC, the battery impedance increases significantly, and the 3C capacity retention and 45°C cycle count decrease. This indicates that compared to difluorinated solvents, trifluorinated solvents have higher viscosity and lower dissociation capacity, leading to reduced electrolyte wettability and conductivity. Consequently, the battery impedance and fast-charging performance are affected. Furthermore, trifluorinated solvents are more prone to decomposition and defluorination, disrupting the stability of the interfacial phase and reducing the battery's high-temperature cycling stability. Therefore, choosing difluorinated solvents can improve battery impedance, fast-charging performance, and high-temperature cycling stability.

[0107] Please refer to Tables 1 and 2. Comparing Examples 1-3, it can be seen that in the fluorinated solvent composed of DFEA, DDFEC, and FEC, as the DFEA content increases, the battery impedance and the number of cycles at 45°C decrease, while the 3C capacity retention increases. This indicates that, compared to DDFEC, DFEA has lower viscosity and higher dissociation ability. Therefore, a higher DFEA content can improve the wettability and conductivity of the electrolyte, thereby improving the battery impedance and fast-charging performance. However, DFEA has low stability at high temperatures and easily decomposes to generate a large number of interfacial byproducts, leading to SEI film instability and affecting the high-temperature cycling stability of the battery. Therefore, by controlling the mass ratio of DFEA, DDFEC, and FEC in the fluorinated solvent, it is possible to balance the battery impedance, fast-charging performance, and high-temperature cycling stability.

[0108] Please refer to Tables 1 and 2. Comparing Example 2 and Comparative Examples 3-5, it can be seen that when the fluorinated solvent contains the first additive LiODFP, the 3C capacity retention rate and the number of cycles at 45°C of the battery are significantly increased. When the fluorinated solvent contains the third additive VC and / or DTD, the 3C capacity retention rate and the number of cycles at 45°C of the battery are both decreased. This indicates that in fluorinated solvents, conventional film-forming additives such as the third additive can affect the fast-charging performance and high-temperature cycling stability of the battery. However, the combined use of the fluorinated solvent and the first additive LiODFP can significantly improve the fast-charging performance and high-temperature cycling stability of the battery.

[0109] Please refer to Tables 1 and 2. Comparing Examples 2 and 4-5, it can be seen that as the content of the first additive, LiODFP, increases from 0.01 wt% to 0.5 wt%, the battery impedance and 3C capacity retention do not change significantly, but the number of cycles at 45°C increases significantly. This indicates that the preferential decomposition of LiODFP can significantly improve the ion conductivity and high-temperature stability of the interfacial phase, reduce the reaction at the solid-liquid interface, and thus improve the high-temperature cycle stability of the battery. Meanwhile, the battery impedance and fast-charging performance are not affected. However, when the content of the first additive, LiODFP, continues to increase from 0.5 wt% to 1 wt%, although the number of cycles at 45°C continues to increase, the battery impedance increases significantly, and the 3C capacity retention decreases significantly. This indicates that as the LiODFP content continues to increase, the solubility of LiODFP in fluorinated solvents decreases significantly, the electrolyte becomes turbid, and the battery impedance and fast-charging performance are significantly affected. Therefore, by controlling the content of the first additive, LiODFP, it is possible to balance the battery impedance, fast-charging performance, and high-temperature cycle stability.

[0110] Please refer to Tables 1 and 2. Comparing Examples 2 and 6, it can be seen that when the electrolyte contains the second additive TAIC, although the battery impedance increases slightly and the 3C capacity retention remains unchanged, the number of cycles at 45°C increases significantly. This indicates that the combined use of the first additive LiODFP and the second additive TAIC can further improve the high-temperature cycling stability of the battery. Specifically, the second additive TAIC participates in the film-forming reaction, which helps to further assist in the formation of a stable interface layer and reduces the decomposition of electrolyte components at the electrode interface. At the same time, the second additive TAIC also promotes the dissociation of the first additive LiODFP, slowing down the decomposition rate of the first additive LiODFP at the solid-liquid interface, thereby generating a denser interface phase, which helps to improve the high-temperature cycling stability of the battery.

[0111] Please refer to Tables 1 and 2. Comparing Examples 6-7, it can be seen that as the content of the second additive TAIC increases from 0.01 wt% to 0.5 wt%, although the battery impedance increases slightly, the 3C capacity retention and the number of cycles at 45°C also increase significantly. This indicates that while the battery impedance is slightly affected by the increase in the content of the second additive TAIC, the high-temperature cycle stability and fast-charging performance of the battery are significantly improved. Therefore, by controlling the content of the second additive TAIC, it is possible to balance the battery impedance, fast-charging performance, and high-temperature cycle stability.

[0112] Please refer to Tables 1 and 2. Comparing Examples 10-12 and Example 7, it can be seen that when the mass ratio of the first additive LiODFP and the second additive TAIC increases from 0.32:0.5 to 0.8:0.5, the battery impedance decreases, while the 3C capacity retention and the number of cycles at 45°C significantly increase. This indicates that when the mass ratio of the first additive LiODFP and the second additive TAIC increases from 0.32:0.5 to 0.8:0.5, the battery's impedance performance, fast charging capability, and high-temperature cycle stability are significantly improved. However, when the mass ratio of the first additive LiODFP and the second additive TAIC continues to increase from 0.8:0.5 to 1:0.5, the battery impedance increases significantly, and the 3C capacity retention and the number of cycles at 45°C significantly decrease. This indicates that when the mass ratio of the first additive LiODFP and the second additive TAIC is too large, the battery's impedance performance, fast charging capability, and high-temperature cycle stability are affected. Therefore, by controlling the mass ratio of the first additive LiODFP and the second additive TAIC, the battery's impedance, fast charging performance, and high-temperature cycle stability can be improved.

[0113] Please refer to Tables 1 and 2. Comparing Examples 2 and 8, it can be seen that when the electrolyte contains a second lithium salt, LiFSI, the number of cycles at 45°C decreases slightly, but the impedance decreases significantly, and the 3C capacity retention rate increases significantly. This indicates that because the second lithium salt, LiFSI, has a stronger dissociation capability, it can improve the conductivity of the electrolyte, thereby improving the battery's impedance and fast-charging performance. Therefore, the second lithium salt, LiFSI, can improve impedance and fast-charging performance.

[0114] Please refer to Tables 1 and 2. Comparing Examples 8-9, it can be seen that as the content of the second lithium salt, LiFSI, increases, the battery impedance gradually increases, while the number of cycles at 45°C and the 3C capacity retention gradually decrease. This indicates that under high voltage, the second lithium salt, LiFSI, readily reacts with the aluminum foil, causing continuous electrolyte decomposition, which in turn affects the battery's impedance, fast-charging performance, and high-temperature cycle stability. Therefore, by controlling the content of the second lithium salt, LiFSI, the battery's impedance, fast-charging performance, and high-temperature cycle stability can be improved.

[0115] Please refer to Tables 1 and 2. Comparing Example 2, Comparative Example 3, and Comparative Examples 6-7, it can be seen that in lithium cobalt oxide cathode active materials, when the electrolyte also contains a fluorinated solvent and the first additive LiODFP, although the number of cycles at 45°C and the 3C capacity retention rate of the battery increase, the increase is very small, and the impedance also increases significantly. This indicates that for the lithium cobalt oxide cathode active material system, the fluorinated solvent and the first additive LiODFP have little effect on improving the battery's fast-charging performance and high-temperature cycle stability, and affect the battery's impedance performance. Therefore, the fluorinated solvent and the first additive LiODFP have a poor overall improvement effect on the lithium cobalt oxide cathode active material system, thus limiting their application in this system. Therefore, compared to lithium cobalt oxide cathode active materials, the fluorinated solvent and the first additive LiODFP have a better overall improvement effect on the lithium nickel manganese oxide cathode active material system.

[0116] This invention also provides an electronic device comprising at least one of the aforementioned lithium-ion batteries, which provides electrical energy. The electronic device can be a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, or power tool, etc. In one embodiment of this invention, the vehicle is, for example, a new energy vehicle, which can be a pure electric vehicle, a hybrid electric vehicle, or a range-extended electric vehicle, etc. Spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc. The electronic device includes the aforementioned lithium-ion battery, and therefore the advantages of including the aforementioned lithium-ion battery are not elaborated here.

[0117] In summary, this invention proposes a lithium-ion battery electrolyte and its application. By introducing a fluorinated solvent into the electrolyte, the high-voltage resistance of the electrolyte can be improved, its kinetic properties can be enhanced, and its viscosity can be reduced, thus preventing a decrease in conductivity. By introducing a first additive into the electrolyte, the fluorinated solvent and the first additive work synergistically to form a stable SEI film at the interface between the negative electrode and the electrolyte, and a stable CEI film at the interface between the positive electrode and the electrolyte, ensuring the stability of the electrode-electrolyte interface at high temperatures and significantly improving the battery's fast-charging performance and high-temperature cycle stability. Introducing a second additive into the electrolyte further improves the battery's high-temperature cycle stability. Introducing a second lithium salt into the electrolyte increases its conductivity, thereby further improving the battery's impedance and fast-charging performance.

[0118] The above description is merely a preferred embodiment of this application and an explanation of the technical principles used. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to the technical solutions formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the inventive concept. For example, technical solutions formed by replacing the above-mentioned features with technical features with similar functions disclosed in this application (but not limited to) each other.

[0119] Apart from the technical features described in the specification, the other technical features are known to those skilled in the art. To highlight the innovative features of this invention, the other technical features will not be described in detail here.

Claims

1. A lithium-ion battery electrolyte, characterized in that, It includes at least the following components: Fluorinated solvents, including 2,2-difluoroethyl acetate, fluoroethylene carbonate and 2,2-difluoroethyl ethyl carbonate; Lithium salts, including a first lithium salt comprising lithium hexafluorophosphate; and Additives, including a first additive, wherein the first additive comprises lithium difluorobis(oxalato)phosphate.

2. The electrolyte for lithium ion batteries according to claim 1, characterized in that, The mass ratio of the 2,2-difluoroethyl acetate, the 2,2-difluoroethyl ethyl carbonate, and the fluoroethylene carbonate is (3-5):(6-4):

1.

3. The electrolyte for lithium ion batteries according to claim 1, characterized in that, The content of 2,2-difluoroethyl acetate in the electrolyte is 10wt%-40wt%.

4. The electrolyte for lithium ion batteries according to claim 1, characterized in that, The content of 2,2-difluoroethyl ethyl carbonate in the electrolyte is 20wt%-40wt%.

5. The electrolyte for lithium ion batteries according to claim 1, characterized in that, The content of the fluoroethylene carbonate in the electrolyte is 5wt%-30wt%.

6. The electrolyte for lithium-ion batteries according to claim 1, characterized in that, The content of the first additive in the electrolyte is 0.1wt%-1wt%.

7. The electrolyte for lithium-ion batteries according to claim 1, characterized in that, The additive also includes a second additive, which comprises triallyl isocyanurate, and the content of the second additive in the electrolyte is 0.01wt%-1wt%.

8. The lithium-ion battery electrolyte of claim 7, wherein, The mass ratio of the first additive to the second additive is (0.63-1.67):

1.

9. The lithium-ion battery electrolyte according to claim 1, characterized in that, The first lithium salt has a content of 11wt%-20wt% in the electrolyte, and the lithium salt further includes a second lithium salt, which is selected from at least one of lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide, and the content of the second lithium salt in the electrolyte is less than 1wt%.

10. A lithium-ion battery, characterized by, include: The positive electrode sheet includes a positive electrode active material, wherein the structural formula of the positive electrode active material is Li. a Ni x Mn y O 4-z M z Wherein, 0.9≤a≤1.1, 0.4≤x≤0.6, 1.4≤y≤1.6, 0≤z≤0.1, and M is selected from at least one of Cl, Br, I, S, Se, Te or F; Negative electrode plate; A diaphragm is disposed between the positive electrode and the negative electrode; and The electrolyte is selected from the lithium-ion battery electrolyte according to any one of claims 1-9.

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