A special additive for electrolyte, electrolyte and lithium-ion battery

Abstract

This invention discloses a special electrolyte additive, an electrolyte, and a lithium-ion battery. The trimethylsiloxysilane compound and fluorosulfonic anhydride compound of this invention have low LUMO and high HOMO values, respectively, and can preferentially undergo redox reactions in the solvent. This allows them to form a more stable CEI film and SEI film on the positive and negative electrode surfaces, respectively, which can effectively slow down the dissolution of metal ions from the positive electrode and inhibit their reduction deposition on the negative electrode. At the same time, the fluorosulfonic anhydride compound, together with the trimethylsiloxysilane compound, participates in the formation of the SEI film, further improving the mechanical strength of the SEI film, thereby alleviating the side reactions between the electrode and the electrolyte and improving the cycle stability of the battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a special additive for electrolyte, an electrolyte, and a lithium-ion battery. Background Technology

[0002] With the continuous expansion of the markets for new energy vehicles and portable electronic devices, the demand for high-performance and high-safety batteries is becoming increasingly urgent, providing a real impetus for the research and development of lithium batteries. Electrolyte, as one of the four main materials of lithium batteries, is the carrier of ion transport in the battery, playing a role in conducting lithium ions between the positive and negative electrodes. Mainstream lithium battery electrolytes are typically formulated from electrolyte lithium salts, organic solvents, and additives in a certain proportion. Among these, electrolyte additives are characterized by small dosage and rapid effect, but they play a crucial role in improving specific electrolyte properties. Through the screening and proportioning of small amounts of additives, the performance of the electrolyte can be significantly improved, making it a hot research area in the field of battery electrolytes in recent years.

[0003] Under high voltage, the cathode material structure is unstable and has high oxidation activity, easily catalyzing the oxidative decomposition of the electrolyte and generating a large amount of gas. During cycling, transition metal ions from the cathode easily dissolve into the electrolyte, hindering Li reduction deposition at the anode. + Furthermore, the migration of electrolytes and the volume expansion of the silicon anode lead to SEI film rupture, exacerbating side reactions at the electrode / electrolyte interface, resulting in accelerated cell capacity decay and degraded cell performance. Therefore, developing a new electrolyte additive to construct a stable electrode / electrolyte interface to meet the application requirements of lithium batteries under room temperature cycling is particularly important. Summary of the Invention

[0004] This invention addresses the problems in the prior art by disclosing a special electrolyte additive, an electrolyte, and a lithium-ion battery. The special electrolyte additive in this invention can form a stable interface film on the positive and negative electrode surfaces of the battery, effectively slowing down the dissolution of metal ions from the positive electrode and inhibiting their reduction and deposition on the negative electrode; it also alleviates side reactions at the electrode and electrolyte interface and improves the cycle stability of the battery.

[0005] This invention is achieved through the following technical solution:

[0006] In a first aspect, the present invention discloses a special additive for electrolytes, comprising a trimethylsiloxysilane compound represented by structural formula I and a fluorosulfonic anhydride compound represented by structural formula II:

[0007] Ⅰ

[0008] II

[0009] R1-R7 are each independently selected from hydrogen, halogen, nitrile, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, substituted or unsubstituted alkyl of C1-C10, substituted or unsubstituted alkoxy of C1-C10, substituted or unsubstituted alkenyl of C2-C10, substituted or unsubstituted alkynyl of C2-C10, and substituted or unsubstituted aryl of C6-C15. When R1-R7 are substituted, the substituent is selected from alkyl of C1-C4, alkoxy of C1-C4, thiohydroxy, thioether, ketone, aldehyde, ester, ether, amino, imino, amide, nitro, carboxylic acid, carbonate, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, carbamate, halogen, and nitrile. The halogen includes F, Cl, and Br, and at least one of R2-R7 is selected from those containing an F group.

[0010] As a further embodiment, in the structural formula I, R1 is selected from hydrogen, halogen, nitrile, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, substituted or unsubstituted alkyl of C1-C6, substituted or unsubstituted alkoxy of C1-C6, substituted or unsubstituted alkenyl of C2-C8, substituted or unsubstituted alkynyl of C2-C8, and substituted or unsubstituted aryl of C6-C12. When R1 is substituted, the substituent is selected from alkyl of C1-C2, alkoxy of C1-C2, thiohydroxy, thioether, ketone, aldehyde, ester, ether, amino, imino, amide, nitro, carboxylic acid, carbonate, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, urethane, halogen, and nitrile.

[0011] Furthermore, R1 is selected from one of the following: substituted or unsubstituted alkyl groups of C1-C4, substituted or unsubstituted alkoxy groups of C1-C4, substituted or unsubstituted alkenyl groups of C2-C4, and substituted or unsubstituted alkynyl groups of C2-C4. When R1 is substituted, the substituent is selected from one of the following: alkyl groups of C1-C2, alkoxy groups of C1-C2, isocyanate groups, isothiocyanate groups, carbamate groups, halogen groups, and nitrile groups.

[0012] Furthermore, R1 is selected from one of the C1-C4 alkyl groups or C1-C4 alkoxy groups with substituent-terminated ends, wherein the substituent is selected from one of the isocyanate group, isothiocyanate group, urethane group, halogen group, and nitrile group. The substituent at the end has higher reactivity, which is more conducive to the formation of a stable CEI film on the positive electrode surface and inhibits the dissolution of transition metals in the positive electrode.

[0013] Furthermore, R1 is selected from cyano-terminated C1-C4 alkyl groups or cyano-terminated C1-C4 alkoxy groups. The cyano group can be anchored on the surface of the cathode material to form a stable complex with the cathode metal ions and inhibit the dissolution of the cathode metal ions.

[0014] As examples of specific trimethylsiloxysilane compounds, the following compounds are given:

[0015] Compound 1-1

[0016] Compounds 1-2

[0017] As a further embodiment, the fluorosulfonic anhydride compound has the structure shown in Formula III:

[0018] III

[0019] R4 and R5 are each independently selected from one of hydrogen, halogen, nitrile, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, substituted or unsubstituted alkyl of C1-C6, substituted or unsubstituted alkoxy of C1-C6, substituted or unsubstituted alkenyl of C2-C8, substituted or unsubstituted alkynyl of C2-C8, and substituted or unsubstituted aryl of C6-C12. When R4 and R5 are substituted, the substituent is selected from one of alkyl of C1-C2, alkoxy of C1-C2, thiohydroxy, thioether, ketone, aldehyde, ester, ether, amino, imino, amide, nitro, carboxylic acid, carbonate, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, urethane, halogen, and nitrile. At least one of R4 and R5 is selected from one containing an F group. When fluorosulfonic anhydride compounds have the structure shown in Formula III, it is beneficial to reduce the interaction force between the substituents and the sulfonic anhydride groups and improve the stability of the structure.

[0020] Furthermore, one of R4 and R5 is selected from hydrogen, and the other is selected from F, a C1-C4 fluoroalkyl group, a C1-C4 fluoroalkoxy group, a C2-C4 fluoroalkenyl group, and a C2-C4 fluoroalkynyl group.

[0021] Furthermore, one of R4 and R5 is selected from hydrogen, and the other is selected from F and a C1-C4 fluoroalkyl group having at least 3 F atoms.

[0022] Furthermore, one of R4 and R5 is selected from hydrogen, and the other is selected from a C1-C4 fluoroalkyl group with at least 3 F atoms. F has strong electronegativity and weak polarity. Introducing relatively more F atoms into the additive is beneficial to improving the wettability of the electrode, thereby improving the film-forming ability of the electrolyte. At the same time, the introduction of F atoms is also beneficial to improving the ion transport capacity of the electrolyte.

[0023] As examples of specific fluorosulfonic anhydride compounds, the following compounds are given:

[0024] Compound 2-1

[0025] Compound 2-2

[0026] Compounds 2-3

[0027] The trimethylsiloxysilane compounds of this application have a lower LUMO value than the solvent, making it easier for them to gain electrons and undergo reduction reactions on the negative electrode surface. The fluorosulfonic anhydride compounds have a higher HOMO value, making it easier for them to lose electrons and undergo oxidation reactions on the positive electrode surface. This is beneficial for both compounds to form more stable CEI and SEI films on the positive and negative electrode surfaces, respectively. At the same time, the fluorosulfonic anhydride compounds also participate in the formation of the SEI film, and the F element in their structure can further improve the wettability of the electrode and enhance the film-forming ability of the electrolyte additives. The trimethylsiloxysilane compounds form an SEI film containing a Si-O-Si network on the surface of the silicon negative electrode material, which can further stabilize the electrode / electrolyte interface. The SEI film is also rich in Si, S, and F elements, giving it good lithium-ion conductivity, stability, and mechanical strength. In addition, trimethylsiloxy can react with HF in the electrolyte, inhibiting the damage of HF to the cell material and current collector. Therefore, the combination of trimethylsiloxysilane compounds and fluorosulfonic anhydride compounds in this application can synergistically improve the mechanical strength and ion transport efficiency of the SEI and CEI films, effectively slow down the dissolution of positive electrode metal ions, inhibit their reduction and deposition on the negative electrode, reduce side reactions between the electrode and the electrolyte, and improve the cycle stability of lithium-ion batteries.

[0028] In a second aspect, the present invention discloses an electrolyte comprising an organic solvent, a lithium salt, and additives, wherein the additives include conventional additives and special additives disclosed in the first aspect of the present invention.

[0029] In some embodiments, the additive accounts for 3 to 20% of the mass of the electrolyte.

[0030] In some embodiments, the trimethylsiloxysilane compound accounts for 0.1-3.5% of the mass of the electrolyte, preferably 0.5-3%. The fluorosulfonic anhydride compound accounts for 0.1-5.5% of the mass of the electrolyte, preferably 0.5-5%. When the mass percentages of the trimethylsiloxysilane compound and the fluorosulfonic anhydride compound are too low, the film stability and mechanical strength on the positive and negative electrode surfaces are poor, failing to adequately mitigate side reactions between the electrodes and the electrolyte. When the mass percentages are too high, the film impedance is too large, which is detrimental to lithium ion insertion and extraction, and instead degrades the cell performance.

[0031] In some embodiments, the organic solvent is selected from one or more of carbonate organic solvents, carboxylic acid ester organic solvents, ether organic solvents, sulfone organic solvents, and phosphate ester organic solvents.

[0032] In some embodiments, the carbonate organic solvent is selected from cyclic carbonates or linear carbonates, wherein the cyclic carbonate is selected from at least one of ethylene carbonate, vinylene carbonate, fluoroethylene carbonate, and propylene carbonate, and the linear carbonate is selected from at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dioctyl carbonate, and methyl trifluoroethyl carbonate.

[0033] In some embodiments, the carboxylic acid ester organic solvent is selected from alkyl carboxylic acid esters or fluorocarboxylic acid esters, wherein the alkyl carboxylic acid ester is selected from at least one of ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, ethyl butyrate, γ-butyrolactone, γ-valerolactone, and δ-valerolactone, and the fluorocarboxylic acid ester is selected from at least one of ethyl fluoroacetate, methyl trifluoroacetate, ethyl trifluoroacetate, propyl trifluoroacetate, 2,2,2-trifluoroethyl difluoroacetate, methyl pentafluoropropionate, and 2,2-difluoroethyl acetate.

[0034] In some embodiments, the ether organic solvent is selected from at least one of ethylene glycol dimethyl ether, diethylene glycol monomethyl ether, diethylene glycol dimethyl ether, propylene glycol methyl ether, dipropylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, propylene glycol ethyl ether, propylene glycol butyl ether, and tetrahydrofuran.

[0035] In some embodiments, the sulfone organic solvent is selected from at least one of sulfolane, methyl ethyl sulfone, dimethyl sulfone, and vinyl sulfone.

[0036] In some embodiments, the phosphate ester organic solvent is selected from at least one of trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, triphenyl phosphate, diphenyl octyl phosphate, dimethyl methyl phosphate, and diethyl ethyl phosphate.

[0037] Preferably, the organic solvent is selected from carbonate organic solvents. Carbonate organic solvents have low viscosity, which is beneficial to the migration of lithium ions in the electrolyte, thereby improving the rate performance of the battery. At the same time, they have good stability and are not easily decomposed during battery cycling. They also have good compatibility with lithium salts and other additive components, which is beneficial to the cycle stability of the battery.

[0038] More preferably, the organic solvent is selected from a mixture of cyclic carbonates and linear carbonates. Using the mixture as an organic solvent can balance the viscosity and dielectric constant of the electrolyte, thereby optimizing the ionic conductivity and fluidity of the electrolyte.

[0039] More preferably, the organic solvent is selected from at least three of ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0040] In some embodiments, the lithium salt is selected from at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalate borate), lithium hexafluorophosphate, lithium hexafluoroantimonyate, lithium hexafluoroarsenate, lithium difluorooxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0041] Preferably, the lithium salt is selected from at least two of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(oxalate)borate, and lithium difluorooxalateborate.

[0042] In some embodiments, the conventional additive is selected from at least one of film-forming additives, overcharge protection additives, flame retardant additives, high-temperature additives, low-temperature additives, dehydrating additives, and deacidifying additives.

[0043] In some embodiments, the film-forming additive is selected from at least one of vinylene carbonate, adiponitrile, succinic anhydride, maleic anhydride, and citrate anhydride.

[0044] In some embodiments, the overcharge protection additive is selected from at least one of biphenyl, cyclohexylbenzene, and tert-butylbenzene.

[0045] In some embodiments, the flame retardant additive is selected from at least one of trimethyl phosphate, triphenyl phosphate, tris(trimethylsilane) phosphate, and pentafluoroethoxyphosphazene.

[0046] In some embodiments, the high-temperature additive is selected from at least one of vinyl sulfite, methylene disulfonate, 1,3-sulfonyl lactone, 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, thiodipropionitrile, ethylene glycol dipropionitrile ether, and 1,3,6-hexanetrionitrile.

[0047] In some embodiments, the low-temperature additive is selected from at least one of ethylene carbonate, fluoroethylene carbonate, ethylene sulfate, cyclohexanehexanone, and tris(trimethylsilane)borate.

[0048] Thirdly, the present invention discloses a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte disclosed in the second aspect of the present invention.

[0049] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising a positive active material. The positive current collector is selected from metal foil or composite positive current collectors, and the positive active material may be a known positive active material for lithium-ion batteries. For example, the positive active material may include at least one of the following materials: lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium manganese phosphate.

[0050] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material. The negative electrode current collector is selected from metal foil or composite negative electrode current collector, and the negative electrode active material may be a negative electrode active material known in the art for lithium-ion batteries. For example, the negative electrode active material may include at least one of the following materials: graphite, soft carbon, hard carbon, elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, silicon alloy, elemental tin, tin oxide, and tin alloy.

[0051] In some embodiments, the diaphragm is selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride.

[0052] The features and beneficial effects of this invention are as follows:

[0053] (1) The trimethylsiloxysilane compounds and fluorosulfonic anhydride compounds of the present invention have low LUMO values ​​and high HOMO values, respectively, and can preferentially undergo reduction and oxidation reactions in the solvent, so that the two can form SEI films and CEI films with more stable composition and structure on the negative electrode and positive electrode surfaces, respectively. This can effectively slow down the dissolution of metal ions from the positive electrode and inhibit their reduction and deposition on the negative electrode. At the same time, the fluorosulfonic anhydride compounds, together with the trimethylsiloxysilane compounds, participate in the formation of the SEI film, further improving the mechanical strength of the SEI film, thereby alleviating the side reactions between the electrode and the electrolyte and improving the cycle stability of the battery.

[0054] (2) The F element in the fluorosulfonic anhydride compounds of the present invention can further enhance the wettability of the electrode and improve the film-forming ability of the electrolyte additive.

[0055] (3) The trimethylsiloxysilane compound of the present invention forms an SEI film containing a Si-O-Si network on the surface of silicon anode material, which can further stabilize the electrode / electrolyte interface.

[0056] (4) The SEI film formed on the negative electrode surface by the present invention is rich in Si, S and F elements, which can improve its ionic conductivity, stability and mechanical strength.

[0057] (5) The trimethylsiloxy group of the present invention can react with HF in the electrolyte to inhibit the damage of HF to the cell material and current collector, and further improve the stability of the electrode / electrolyte interface.

[0058] Therefore, the present invention uses trimethylsiloxysilane compounds and fluorosulfonic anhydride compounds in combination as special additives for electrolytes, which can effectively improve the cycle stability of batteries. Detailed Implementation

[0059] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in this application is for the purpose of describing particular embodiments only and is not intended to limit this application; the terms "comprising" and "having" and any variations thereof in the specification, claims and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0060] In this application, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0061] In the description of this application, the term "multiple" means two or more (including two), and "at least one" means one or more (including one, two, three, etc.).

[0062] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0063] Example 1

[0064] (1) Preparation of electrolyte: Ethyl carbonate, propylene carbonate, diethyl carbonate and methyl ethyl carbonate are mixed in a mass ratio of 1:1:2:6. After mixing evenly, 13.5wt% LiPF6 is added in sequence. After being fully dissolved, conventional additives and special additives are added. The conventional additive is 1wt% vinylene carbonate, and the special additives are 2wt% trimethylsiloxysilane compound 1-1 and 3wt% fluorosulfonic anhydride compound 2-1. After being fully mixed and dissolved, it is ready for use.

[0065] (2) Preparation of positive electrode sheet: Lithium-rich manganese-based positive electrode active material, conductive carbon black, carbon nanotubes and binder polyvinylidene fluoride were mixed in a mass ratio of 96:1.5:0.5:2, and then dispersed in N-methyl-2-pyrrolidone. After thorough stirring, a positive electrode slurry was obtained. The positive electrode slurry was uniformly coated on the positive electrode current collector aluminum foil, dried, rolled and die-cut to obtain a positive electrode sheet that meets the requirements.

[0066] (3) Preparation of negative electrode sheet: The negative electrode active materials graphite, silicon carbide, conductive carbon black, binder styrene-butadiene rubber, polyacrylic acid and thickener carboxymethyl cellulose are mixed in a mass ratio of 63:31.5:1.2:2:1.8:0.5 and then dispersed in deionized water to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, dried and then rolled and die-cut to obtain a negative electrode sheet that meets the requirements.

[0067] (4) Preparation of lithium-ion battery: The positive and negative electrode sheets and separator prepared by the above method are stacked in sequence, with the separator located in the middle of the positive and negative electrode sheets. The stacked cells are fixed with tape, and then cold-pressed and packaged to form a lithium-ion battery with a thickness of 4.6 mm, a length of 85 mm and a width of 50 mm. The cells are vacuum baked at 80°C for 24 h to obtain cells to be injected with electrolyte. The prepared electrolyte is injected into the cells in a glove box with the dew point controlled below -40°C. After standing at high temperature for 24 h and standing at room temperature for 12 h, the cells are formed, sealed twice and tested for capacity to complete the battery manufacturing.

[0068] Example 2

[0069] The difference from Example 1 is that the trimethylsiloxysilane compound added to the electrolyte is 0.1 wt% of compound 1-1.

[0070] Example 3

[0071] The difference from Example 1 is that the trimethylsiloxysilane compound added to the electrolyte is 0.5 wt% of compound 1-1.

[0072] Example 4

[0073] The difference from Example 1 is that the trimethylsiloxysiloxane compound added to the electrolyte is 3 wt% of compound 1-1.

[0074] Example 5

[0075] The difference from Example 1 is that the fluorosulfonic anhydride compound added to the electrolyte is 0.1 wt% of compound 2-1.

[0076] Example 6

[0077] The difference from Example 1 is that the fluorosulfonic anhydride compound added to the electrolyte is 5 wt% of compound 2-1.

[0078] Example 7

[0079] The difference from Example 1 is that the fluorosulfonic anhydride compound added to the electrolyte is 0.5 wt% of compound 2-1.

[0080] Example 8

[0081] The difference from Example 1 is that the trimethylsiloxysilane compound added to the electrolyte is 2 wt% of compound 1-2.

[0082] Example 9

[0083] The difference from Example 1 is that the fluorosulfonic anhydride compound added to the electrolyte is 3 wt% of compound 2-2.

[0084] Example 10

[0085] The difference from Example 1 is that the fluorosulfonic anhydride compound added to the electrolyte is 3 wt% of compound 2-3.

[0086] Comparative Example 1

[0087] The difference from Example 1 is that no trimethylsiloxysilane compounds are added to the electrolyte.

[0088] Comparative Example 2

[0089] The difference from Example 1 is that no fluorosulfonic anhydride compounds are added to the electrolyte.

[0090] Comparative Example 3

[0091] The difference from Example 1 is that no trimethylsiloxysilane compounds and fluorosulfonic anhydride compounds are added to the electrolyte.

[0092] Comparative Example 4

[0093] The difference from Example 1 is that the electrolyte contains 0.02 wt% of compound 1-1 of trimethylsiloxysilane and 0.02 wt% of compound 2-1 of fluorosulfonic anhydride.

[0094] Comparative Example 5

[0095] The difference from Example 1 is that the electrolyte contains 10 wt% of compound 1-1 of trimethylsiloxysilane and 10 wt% of compound 2-1 of fluorosulfonic anhydride.

[0096] Comparative Example 6

[0097] The difference from Example 1 is that the trimethylsiloxysilane compound added to the electrolyte is replaced with trivinylsilane.

[0098] Comparative Example 7

[0099] The difference from Example 1 is that the trimethylsiloxysilane compound added to the electrolyte is replaced with tris(trimethylsilyl)silane.

[0100] The lithium-ion batteries prepared in Examples 1-8 and Comparative Examples 1-7 were subjected to room temperature cycle performance tests, and the test procedures are as follows:

[0101] At 25℃, the capacitor was charged to 4.50V at a constant current and constant voltage of 0.33C, with a cutoff current of 0.05C. Then, it was discharged to 2.5V at a constant current of 0.33C. The initial discharge capacity was recorded as C0. This charge-discharge cycle was repeated 300 times, and the discharge capacity on the 300th cycle was recorded as C1. The capacity retention rate during room temperature cycling was calculated using the following formula: Capacity retention rate = C1 / C0 × 100%. The performance test results are shown in Table 1.

[0102] Validation Result Analysis

[0103] Table 1

[0104]

[0105] First, we can see from the data results in Table 1 that, compared with Comparative Example 3 without special additives, the addition of trimethylsiloxysilane compounds and fluorosulfonic anhydride compounds to Comparative Examples 1-2 alone improved the room temperature cycle performance of the battery to a certain extent, but showed a significant difference compared with Examples 1-10. This indicates that the two special additives have a synergistic effect when used together, forming a more stable interface film on the positive and negative electrode surfaces, and improving the cycle stability of the battery to a greater extent.

[0106] As can be seen from Examples 1, 1, 6, and 7, replacing the trimethylsiloxysilane compounds of this application with trivinylsilane and tris(trimethylsilyl)silane also improves the cycle performance of the battery to some extent, but the improvement effect of Example 1 is more obvious. Although trivinylsilane and tris(trimethylsilyl)silane can also form films on the negative electrode surface, the trimethylsiloxysilane compounds of this application can form a Si-O-Si network SEI film on the silicon negative electrode surface. The introduction of Si element is also beneficial to further improve the ionic conductivity of the SEI film. At the same time, it can also have a synergistic effect with fluorosulfonic anhydride compounds. Therefore, the formed SEI film is more stable, has higher mechanical strength, and is more conducive to improving the cycle stability of the battery.

[0107] Building upon this, we further discovered that trimethylsiloxysilane compounds and fluorosulfonic anhydride compounds with different structures exhibited certain differences in their effects on battery performance. As shown in Examples 1 and 8, compared to isocyanate groups, nitrile groups had a more significant effect on improving battery cycle performance. We speculate that this is because nitrile groups can more stably anchor to the surface of the cathode material, forming stable complexes with cathode metal ions and inhibiting the dissolution of metal ions. Therefore, we preferred nitrile groups as the end-capping substituents. From Examples 1, 9, and 10, we first found that when the substituents in the fluorosulfonic anhydride compounds were located at the oxygen para-position (Examples 1 and 9), the performance improvement effect on the battery was more significant. We believe that this structure can reduce the interaction force between the substituents and the sulfonic anhydride groups to a certain extent, thereby improving the stability of the compound structure. Furthermore, when the structure of Example 9 contains more F atoms, the cycle performance of the battery is further improved. F has strong electronegativity and weak polarity. Introducing relatively more F atoms into the additive is beneficial to improving the wettability of the electrode and thus improving the film-forming ability of the electrolyte. At the same time, the introduction of F atoms is also beneficial to improving the ion transport ability of the electrolyte and thus improving the cycle performance of the battery. Therefore, we prefer the fluorosulfonic anhydride compound shown in Formula III, and the structure containing at least 3 F atoms in the substituents.

[0108] Meanwhile, we investigated the effect of the mass ratio of trimethylsiloxysilane compounds and fluorosulfonic anhydride compounds in the electrolyte on the battery cycle performance. As shown in Examples 1-7, the battery cycle performance gradually improved with increasing mass ratio of the special additives in the electrolyte. However, when the mass ratio of trimethylsiloxysilane compounds and fluorosulfonic anhydride compounds was too low (Comparative Example 4), the film stability and mechanical strength on the positive and negative electrode surfaces were poor, failing to adequately mitigate side reactions between the electrodes and the electrolyte. When the mass ratio was too high (Comparative Example 5), the film impedance was too large, hindering lithium ion insertion and extraction, and consequently degrading the cell performance. Therefore, we preferably used 0.5-3% of the trimethylsiloxysilane compounds and 0.5-5% of the fluorosulfonic anhydride compounds in the electrolyte.

[0109] In summary, the special electrolyte additives in this invention can form a stable interfacial film on the positive and negative electrode surfaces of the battery, effectively slowing down the dissolution of positive electrode metal ions and inhibiting their reduction and deposition on the negative electrode; alleviating side reactions at the electrode and electrolyte interface and improving the cycle stability of the battery.

[0110] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A lithium-ion battery electrolyte, comprising special electrolyte additives, characterized in that, This includes trimethylsiloxysilane compounds represented by structural formula I and fluorosulfonic anhydride compounds represented by structural formula II: Ⅰ Ⅱ R1-R7 are each independently selected from hydrogen, halogen, nitrile, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, substituted or unsubstituted alkyl of C1-C10, substituted or unsubstituted alkoxy of C1-C10, substituted or unsubstituted alkenyl of C2-C10, substituted or unsubstituted alkynyl of C2-C10, and substituted or unsubstituted aryl of C6-C15. When R1-R7 are substituted, the substituent is selected from alkyl of C1-C4, alkoxy of C1-C4, thiohydroxy, thioether, ketone, aldehyde, ester, ether, amino, imino, amide, nitro, carboxylic acid, carbonate, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, urethane, halogen, and nitrile. The halogen includes F, Cl, and Br, and at least one of R2-R7 is selected from those containing an F group. The trimethylsiloxysilane compound has a mass percentage of 0.1-3.5% in the electrolyte, and the fluorosulfonic anhydride compound has a mass percentage of 0.1-5.5% in the electrolyte.

2. The lithium-ion battery electrolyte according to claim 1, characterized in that, In the structural formula I, R1 is selected from hydrogen, halogen, nitrile, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, substituted or unsubstituted alkyl of C1-C6, substituted or unsubstituted alkoxy of C1-C6, substituted or unsubstituted alkenyl of C2-C8, substituted or unsubstituted alkynyl of C2-C8, and substituted or unsubstituted aryl of C6-C12. When R1 is substituted, the substituent is selected from alkyl of C1-C2, alkoxy of C1-C2, thiohydroxy, thioether, ketone, aldehyde, ester, ether, amino, imino, amide, nitro, carboxylic acid, carbonate, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, urethane, halogen, and nitrile.

3. The lithium-ion battery electrolyte according to claim 1, characterized in that, In the structural formula I, R1 is selected from one of the following: substituted or unsubstituted alkyl groups of C1-C4, substituted or unsubstituted alkoxy groups of C1-C4, substituted or unsubstituted alkenyl groups of C2-C4, and substituted or unsubstituted alkynyl groups of C2-C4. When R1 is substituted, the substituent is selected from one of the following: alkyl groups of C1-C2, alkoxy groups of C1-C2, isocyanate groups, isothiocyanate groups, carbamate groups, halogen groups, and nitrile groups.

4. The lithium-ion battery electrolyte according to claim 1, characterized in that, In the structural formula I, R1 is selected from one of the substituent-terminated C1-C4 alkyl groups and the substituent-terminated C1-C4 alkoxy groups, wherein the substituent is selected from one of the isocyanate group, isothiocyanate group, urethane group, halogen group, and nitrile group.

5. The lithium-ion battery electrolyte according to claim 1, characterized in that, In the structural formula I, R1 is selected from nitrile-terminated C1-C4 alkyl groups or nitrile-terminated C1-C4 alkoxy groups.

6. The lithium-ion battery electrolyte according to claim 1, characterized in that, The fluorosulfonic anhydride compounds have the structure shown in Formula III: III R4 and R5 are each independently selected from one of hydrogen, halogen, nitrile, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, substituted or unsubstituted alkyl of C1-C6, substituted or unsubstituted alkoxy of C1-C6, substituted or unsubstituted alkenyl of C2-C8, substituted or unsubstituted alkynyl of C2-C8, and substituted or unsubstituted aryl of C6-C12. When R4 and R5 are substituted, the substituent is selected from one of alkyl of C1-C2, alkoxy of C1-C2, thiohydroxy, thioether, ketone, aldehyde, ester, ether, amino, imino, amide, nitro, carboxylic acid, carbonate, trimethylsilyl, trimethylsiloxy, isocyanate, isothiocyanate, urethane, halogen, and nitrile. At least one of R4 and R5 is selected from one containing an F group.

7. The lithium-ion battery electrolyte according to claim 1, characterized in that, In the aforementioned structural formula III, one of R4 and R5 is selected from hydrogen, and the other is selected from one of F, C1-C4 fluoroalkyl, C1-C4 fluoroalkoxy, C2-C4 fluoroalkenyl, and C2-C4 fluoroalkynyl.

8. The lithium-ion battery electrolyte according to claim 1, characterized in that, In the aforementioned structural formula III, one of R4 and R5 is selected from hydrogen, and the other is selected from F and a C1-C4 fluoroalkyl group having at least 3 F atoms.

9. The lithium-ion battery electrolyte according to claim 1, characterized in that, In the aforementioned structural formula III, one of R4 and R5 is selected from hydrogen, and the other is selected from a C1-C4 fluoroalkyl group with at least 3 F atoms.

10. The lithium-ion battery electrolyte according to claim 1, characterized in that, It also includes organic solvents, lithium salts, and conventional additives.

11. The lithium-ion battery electrolyte according to claim 1, characterized in that, The additives account for 3-20% of the mass of the electrolyte.

12. The lithium-ion battery electrolyte according to claim 1, characterized in that, The trimethylsiloxysilane compound has a mass percentage of 0.5-3% in the electrolyte; the fluorosulfonic anhydride compound has a mass percentage of 0.5-5% in the electrolyte.

13. The lithium-ion battery electrolyte according to claim 10, characterized in that, The organic solvent is selected from one or more of the following: carbonate organic solvents, carboxylic acid ester organic solvents, ether organic solvents, sulfone organic solvents, and phosphate ester organic solvents; The carbonate organic solvent is selected from cyclic carbonates or linear carbonates, wherein the cyclic carbonate is selected from at least one of ethylene carbonate, vinylene carbonate, fluoroethylene carbonate, and propylene carbonate, and the linear carbonate is selected from at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dioctyl carbonate, and methyl trifluoroethyl carbonate. The carboxylic acid ester organic solvent is selected from alkyl carboxylic acid esters or fluorocarboxylic acid esters, wherein the alkyl carboxylic acid ester is selected from at least one of ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, ethyl butyrate, γ-butyrolactone, γ-valerolactone, and δ-valerolactone, and the fluorocarboxylic acid ester is selected from at least one of ethyl fluoroacetate, methyl trifluoroacetate, ethyl trifluoroacetate, propyl trifluoroacetate, 2,2,2-trifluoroethyl difluoroacetate, methyl pentafluoropropionate, and 2,2-difluoroethyl acetate; The ether organic solvent is selected from at least one of ethylene glycol dimethyl ether, diethylene glycol monomethyl ether, diethylene glycol dimethyl ether, propylene glycol methyl ether, dipropylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, propylene glycol ethyl ether, propylene glycol butyl ether, and tetrahydrofuran; The sulfone organic solvent is selected from at least one of sulfolane, methyl ethyl sulfone, dimethyl sulfone, and vinyl sulfone; The phosphate ester organic solvent is selected from at least one of trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, triphenyl phosphate, diphenyl octyl phosphate, dimethyl methyl phosphate, and diethyl ethyl phosphate.

14. The lithium-ion battery electrolyte according to claim 10, characterized in that, The organic solvent is selected from carbonate organic solvents.

15. The lithium-ion battery electrolyte according to claim 10, characterized in that, The organic solvent is selected from a mixture of cyclic carbonates and linear carbonates.

16. The lithium-ion battery electrolyte according to claim 10, characterized in that, The organic solvent is selected from at least three of ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate.

17. The lithium-ion battery electrolyte according to claim 10, characterized in that, The lithium salt is selected from at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(oxalate borate), lithium hexafluorophosphate, lithium hexafluoroantimonyate, lithium hexafluoroarsenate, lithium difluorooxalate phosphate, and lithium tetrafluorooxalate phosphate.

18. The lithium-ion battery electrolyte according to claim 10, characterized in that, The lithium salt is selected from at least two of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(oxalate)borate, and lithium difluorooxalateborate.

19. The lithium-ion battery electrolyte according to claim 10, characterized in that, The conventional additives are selected from at least one of film-forming additives, overcharge protection additives, flame retardant additives, high-temperature additives, low-temperature additives, dehydrating additives, and deacidifying additives; The film-forming additive is selected from at least one of vinylene carbonate, adiponitrile, succinic anhydride, maleic anhydride, and citrate anhydride; The overcharge protection additive is selected from at least one of biphenyl, cyclohexylbenzene, and tert-butylbenzene; The flame retardant additive is selected from at least one of trimethyl phosphate, triphenyl phosphate, tris(trimethylsilane) phosphate, and pentafluoroethoxyphosphazene; The high-temperature additive is selected from at least one of vinyl sulfite, methylene disulfonate, 1,3-sulfonyl lactone, 1,3-propane sulfonyl lactone, 1,4-butane sulfonyl lactone, thiodipropionitrile, ethylene glycol dipropionitrile ether, and 1,3,6-hexanetrionitrile. The low-temperature additive is selected from at least one of ethylene ethylene carbonate, fluoroethylene carbonate, ethylene sulfate, cyclohexanehexanone, and tris(trimethylsilane)borate.

20. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and a lithium-ion battery electrolyte as described in any one of claims 1-19.

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