Battery electrolyte film-forming additive, battery electrolyte and application thereof

Abstract

The invention provides a battery electrolyte film-forming additive, a battery electrolyte and application of the battery electrolyte film-forming additive, the battery electrolyte film-forming additive is one or more of alkyl phosphonate alkoxy silane compounds, the compound has a chemical structure shown in a formula (I), R1, R2, R3, R4 and R5 are independently selected from one of C1-C10 alkyl groups, n is larger than or equal to 1 and smaller than or equal to 10, and n is an integer. P with electrophilic property in the formula shows relatively high reduction potential; oxygen atoms react with active points of a silicon negative electrode, so that the activity of a passive film layer can be reduced; the high-bond-energy Si-O bond and chain alkyl structure can endow the passivation film with flexibility and stability. Under the synergistic cooperation of the structures, the battery electrolyte film-forming additive can generate a compact passive film layer with flexible and stable characteristics on a silicon negative electrode, the continuous consumption of the battery electrolyte caused by the expansion of silicon particles in the circulation process is inhibited, the interface impedance is reduced, and the battery performance is improved. # imgabs0 #

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a battery electrolyte film-forming additive, a battery electrolyte and applications thereof. Background Art

[0002] Lithium-ion batteries have become one of the important and indispensable chemical energy sources in the fields of digital electronics, electric vehicles, energy storage applications, aerospace, etc. due to their high operating voltage, high specific energy density, long cycle life, and environmental friendliness. The electrolyte in lithium-ion batteries is an important component of lithium-ion batteries and is called the "blood" of lithium-ion batteries. It is usually composed of lithium salts, solvents, and additives, and has an important impact on the cycle performance, rate performance, and safety performance of lithium-ion batteries.

[0003] The most promising direction for the application of high-energy-density lithium-ion batteries is the silicon-carbon material system. However, there are also many problems with silicon-carbon negative electrodes: the volume expansion and contraction of silicon particles during deintercalation leads to particle pulverization, shedding, and electrochemical performance failure; the continuous growth of the solid electrolyte layer (SEI) on the surface of silicon particles comes from the continuous side reaction with the electrolyte, which leads to poor cycle performance of high-energy-density batteries using silicon-carbon negative electrode materials, etc.

[0004] In the existing methods, the cycle stability performance of lithium-ion batteries can be further improved by adding battery electrolyte film-forming additives to the battery electrolyte. The existing commonly used battery electrolyte film-forming additives include vinylene carbonate (VC), fluoroethylene carbonate (FEC), propylene sulfate (PCS), etc. Adding them to the electrolyte has a certain effect on improving battery performance. However, the film layer formed by traditional battery electrolyte film-forming additives is not flexible and stable enough, and may also bring safety, cost and environmental problems. These shortcomings limit its application in the new generation of high energy density lithium-ion batteries, especially it cannot meet the ideal application effect in the silicon-carbon negative electrode battery system.

[0005] When traditional battery electrolyte film-forming additives are applied to silicon-carbon negative electrode lithium-ion batteries formed by silicon-carbon material systems, the formed film layer is prone to rupture during the cycle, resulting in continuous decomposition of the electrolyte and consumption of active lithium in the electrolyte, increasing impedance, and reducing the battery's cycle stability and energy density. The characteristics of silicon-carbon negative electrode materials require that the formed film layer not only have good stability, but also have high flexibility to adapt to the volume expansion and contraction of silicon particles during charging and discharging. The film formed by traditional additives is often relatively rigid and cannot meet this basic requirement.

[0006] Therefore, developing new battery electrolyte additives that can form more stable and flexible battery electrolytes and applying them to silicon-carbon negative electrode lithium-ion batteries is an important research direction in the field of lithium-ion batteries. Summary of the invention

[0007] The main purpose of the present invention is to provide a battery electrolyte film-forming additive, a battery electrolyte and its application, so as to solve the problems in the prior art due to the poor stability and flexibility of the battery electrolyte film-forming additive and the problem that it cannot be applied to silicon-carbon negative electrode lithium-ion batteries with higher requirements on stability and flexibility.

[0008] In order to solve the above problems, the present invention provides a battery electrolyte film-forming additive, wherein the battery electrolyte film-forming additive is one or more alkylphosphonate alkoxysilanes, and the alkylphosphonate alkoxysilane has a chemical structure shown in the following formula (I): Wherein, R1, R2, R3, R4 and R5 are each independently selected from one of C1 to C10 alkyl groups, 1≤n≤10, and n is an integer. The use of the above-mentioned battery electrolyte film-forming additive can effectively generate a passivation film layer with a dense structure and both flexible and stable characteristics. The presence of the passivation film layer can inhibit the continuous consumption of battery electrolyte caused by the expansion of silicon particles during the cycle, and can also reduce the interface impedance, thereby improving the cycle stability of lithium-ion batteries with silicon-carbon negative electrode battery systems.

[0009] Further, R1, R2 and R3 are each independently selected from one of C1-C5 alkyl groups, R4 and R5 are each independently selected from one of C1-C10 alkyl groups, 1≤n≤3, and n is an integer; preferably, R1, R2 and R3 are each independently selected from one of C1-C10 alkyl groups, R4 and R5 are each independently selected from one of C1-C2 alkyl groups, 1≤n≤3; and n is an integer. The above definition can further enhance the use effect of the battery electrolyte film-forming additive.

[0010] Furthermore, R1, R2, R3, R4 and R5 are independently selected from methyl, ethyl, propyl and butyl, 3≤n≤5; and n is an integer. Selecting the above specific types of structures can further make the battery electrolyte film-forming additive have a better use effect, which is beneficial to improving the cycle stability of the battery.

[0011] Furthermore, the battery electrolyte film-forming additive is selected from one or more of the following structures:

[0012] Preferably, the battery electrolyte film-forming additive is a compound of compound S1 and compound S4, and the weight ratio of the two is (1-3):2; or, the battery electrolyte film-forming additive is a compound of compound S3 and compound S4, and the weight ratio of the two is 2:(3-5); or, the battery electrolyte film-forming additive is a compound of compound S2 and compound S3, and the weight ratio of the two is 1:(1-3). Compounding the battery electrolyte film-forming additive in the above manner can make the obtained mixed battery electrolyte film-forming additive have better system miscibility characteristics, which is beneficial to make the passivation film layer formed in the battery have better density, stability and flexibility.

[0013] According to another aspect of the present invention, a battery electrolyte is provided, which includes a lithium salt, an organic solvent and the above-mentioned battery electrolyte film-forming additive. Applying the above-mentioned battery electrolyte additive to the battery electrolyte is beneficial to reducing the internal resistance of the battery and improving the cycle stability of the lithium battery.

[0014] Furthermore, by weight percentage, the lithium salt in the battery electrolyte is 7-16%, the battery electrolyte film-forming additive is 0.5-9%, and the balance is an organic solvent; preferably, by weight percentage, the lithium salt in the battery electrolyte is 10-15%, the battery electrolyte film-forming additive is 0.5-3%, and the balance is an organic solvent. Controlling the ratio of lithium salt, battery electrolyte film-forming additive and organic solvent in the battery electrolyte within the above range allows the battery electrolyte to have good conductivity and electrochemical stability while also allowing the above components to have good compatibility, which can further improve the performance of the battery electrolyte.

[0015] Further, the organic solvent is one or more of organic carbonate, ionic liquid, polyether, aromatic ether, C1-C40 alkyl ether, cyclic ether, carboxylate, sulfone, nitrile, dinitrile and polynitrile; preferably, the organic solvent is one or more of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate; preferably, the lithium salt is one or more of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium trifluoromethylsulfonate, lithium bis(pentafluoroethylsulfonylimide), lithium bis(trifluoromethylsulfonylimide), lithium bis(fluorosulfonylimide), lithium difluoro(bis(oxalate)phosphate and lithium tetrafluoro(oxalate)phosphate; further preferably, the lithium salt is lithium hexafluorophosphate. The above-mentioned types of lithium salts and the battery electrolyte film-forming additive of the present invention can better play a synergistic role.

[0016] Further, the battery electrolyte film-forming additive is a compound of compound S1 and compound S4, the organic solvent is dimethyl sulfoxide, and the lithium salt is lithium difluorooxalate borate; or, the battery electrolyte film-forming additive is a compound of compound S3 and compound S4, the organic solvent is aromatic polyoxyethylene ether, and the lithium salt is lithium trifluoromethyl sulfonate; or, the battery electrolyte film-forming additive is a compound of compound S2 and compound S3, the organic solvent is hexanetrinitrile, and the lithium salt is bis(fluorosulfonyl imide) lithium; wherein the structural formulas of compound S1, compound S2, compound S3, and compound S4 are as follows:

[0017] Using them in the above manner is beneficial to further improve the stability and flexibility of the passivation film layer formed during the battery cycle.

[0018] According to the third aspect of the present invention, there is also provided a silicon-carbon negative electrode lithium ion battery, the silicon-carbon negative electrode lithium ion battery comprising the above-mentioned battery electrolyte. Introducing the above-mentioned battery electrolyte film-forming additive into the silicon-carbon negative electrode lithium ion battery is conducive to improving the cycle stability of the battery.

[0019] Furthermore, the silicon-carbon negative electrode lithium-ion battery also includes a positive electrode containing a cathode active material, a negative electrode containing an anode active material, and a battery separator; preferably, the cathode active material is a material capable of releasing lithium ions; further preferably, the cathode active material is one or more of transition metal phosphates, transition metal oxide lithium salts, and metal sulfides; preferably, the anode active material is a material capable of receiving lithium ions; further preferably, the anode active material is one or more of silicon material, silicon-carbon material, a composite material of silicon monoxide and graphite, and a composite material of silicon-carbon and silicon monoxide; preferably, the battery separator is at least one of a polyimide separator, a polyethylene separator, a polypropylene separator, and a polyethylene terephthalate separator; further preferably, the battery separator is a polyethylene separator or a polypropylene separator. Selecting the above-mentioned types of cathode, anode, and separator materials can further improve the cycle stability performance of the silicon-carbon negative electrode lithium-ion battery.

[0020] The present invention provides a battery electrolyte film-forming additive, a battery electrolyte and its application. The battery electrolyte film-forming additive is composed of alkylphosphonate alkoxysilane, and the component structure contains not only Si-O bonds and chain alkyl ((CH2)n) structures with high bond energy, but also P atoms with good electrophilic properties. The P with electrophilic properties in the structure can show a higher reduction potential, so that the battery preferentially generates a dense and stable passivation film at the silicon negative electrode during the formation process; the oxygen atoms in the structure react with the active points of the silicon negative electrode to reduce the activity of the passivation film layer, thereby further inhibiting the decomposition of the battery electrolyte at the active points of the silicon negative electrode during the circulation process; the high bond energy Si-O bonds and chain alkyl ((CH2)n) structures in the structure can make the passivation film more flexible and stable. Under the synergistic effect of the above-mentioned structural features, alkyl phosphonate alkoxysilane can be used as a film-forming additive for battery electrolyte to effectively generate a passivation film layer with a dense structure and both flexible and stable properties on the silicon negative electrode. The presence of this passivation film layer can inhibit the continuous consumption of battery electrolyte caused by the expansion of silicon particles during the cycle, and can also reduce the interfacial impedance, thereby improving the cycle stability of lithium-ion batteries with silicon-carbon negative electrode cell systems. DETAILED DESCRIPTION

[0021] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0022] As described in the background technology section, when traditional battery electrolyte film-forming additives are applied to silicon-carbon negative electrode lithium-ion batteries formed by silicon-carbon material systems, the formed film layer is prone to rupture during the cycle, resulting in continuous decomposition of the battery electrolyte and consumption of active lithium in the battery electrolyte, increasing impedance, and thus reducing the battery's cycle stability and energy density. Therefore, finding a battery electrolyte film-forming additive that allows the film layer it forms to have both stability and flexibility is the key to solving the above problems.

[0023] In order to solve the above problems, the present invention provides a battery electrolyte film-forming additive, wherein the battery electrolyte film-forming additive is one or more alkylphosphonate alkoxysilanes, and the alkylphosphonate alkoxysilane has a chemical structure shown in the following formula (I):

[0024]

[0025] Wherein, R1, R2, R3, R4 and R5 are each independently selected from one of C1 to C10 alkyl groups, 1≤n≤10, and n is an integer.

[0026] The component structure of the alkylphosphonate alkoxysilane in the above-mentioned battery electrolyte film-forming additive contains not only P atoms with good electrophilic properties, but also Si-O bonds with high bond energy and chain alkyl ((CH2)n) structures. The electrophilic P in the structure can show a higher reduction potential, so that the battery preferentially generates a dense and stable passivation film on the silicon negative electrode during the formation process; the oxygen atoms in the structure react with the active points of the silicon negative electrode to reduce the activity of the passivation film layer, thereby further inhibiting the decomposition of the battery electrolyte at the active points of the silicon negative electrode during the circulation process; the high bond energy Si-O bonds and chain alkyl ((CH2)n) structure can make the passivation film more flexible and stable. Under the synergistic effect of the above structural features, alkylphosphonate alkoxysilane as a battery electrolyte film-forming additive can effectively generate a passivation film layer with a dense structure and both flexible and stable characteristics on the silicon negative electrode. The presence of the passivation film layer can inhibit the continuous consumption of battery electrolyte caused by the expansion of silicon particles during the cycle, and can also reduce the interface impedance, thereby improving the cycle stability of lithium-ion batteries in silicon-carbon negative electrode battery systems. In addition, the alkyl substituents R1, R2, R3, R4 and R5 in the alkylphosphonate alkoxysilane are each independently selected from one of the C1-C10 alkyl groups, and 1≤n≤10 is controlled, and n is an integer. The above structural features can further improve the flexibility of the formed passivation film layer. Controlling the component structure of the battery electrolyte film-forming additive within the above range can not only make it have good system compatibility, but also make the passivation film layer formed by the battery electrolyte film-forming additive during use have better flexibility and stability, effectively improving the use effect of the battery electrolyte film-forming additive.

[0027] In a preferred embodiment, R1, R2 and R3 are each independently selected from one of C1-C5 alkyl groups, R4 and R5 are each independently selected from one of C1-C10 alkyl groups, 1≤n≤3, and n is an integer. By making the component structure of the battery electrolyte film-forming additive within the above range, the battery electrolyte film-forming additive can be better involved in the battery system, and the passivation film layer formed has better stability and flexibility. Preferably, R1, R2 and R3 are each independently selected from one of C1-C10 alkyl groups, R4 and R5 are each independently selected from one of C1-C2 alkyl groups, 1≤n≤3; and n is an integer. The above characteristics of the alkyl chain can further enhance the use effect of the battery electrolyte film-forming additive.

[0028] In a preferred embodiment, R1, R2, R3, R4 and R5 are independently selected from methyl, ethyl, propyl and butyl, 3≤n≤5; and n is an integer. The substituents R1, R2, R3, R4 and R5 in the alkylphosphonate alkoxysilane are selected from the above specific types, and the number of chain alkyl groups is controlled within the above range, which can further make the battery electrolyte film-forming additive have better stability and flexibility, and can effectively improve the cycle stability of the battery.

[0029] In a preferred embodiment, the battery electrolyte film-forming additive is selected from one or more of the following structures:

[0030] Preferably, the battery electrolyte film-forming additive is a compound of compound S1 and compound S4, and the weight ratio of the two is (1-3):2; or, the battery electrolyte film-forming additive is a compound of compound S3 and compound S4, and the weight ratio of the two is 2:(3-5); or, the battery electrolyte film-forming additive is a compound of compound S2 and compound S3, and the weight ratio of the two is 1:(1-3). In particular, after a large number of experiments, the present invention creatively discovered that the use of the above-mentioned battery electrolyte film-forming additives in the above-mentioned proportions can achieve better results. This may be because after mixing the above-mentioned battery electrolyte film-forming additives with chain alkyl groups of different lengths and substituent side chains of different lengths, the obtained mixed battery electrolyte film-forming additive has better system miscibility characteristics, and the passivation film layer formed in the battery has better density, stability and flexibility, which can further improve the battery's cycle stability performance.

[0031] It should be noted that the present invention provides a simpler and more convenient method for preparing a film-forming additive for a battery electrolyte. The method uses Bu3N as a catalyst, selects a corresponding substituted alkenylphosphonic acid diester and a corresponding alkyl and methoxy-substituted silane in a molar ratio of 1: (8-16), adds the above two raw materials and the catalyst to anhydrous ethanol, reacts the mixed system at 60-80°C for 8-14h, and removes the anhydrous ethanol solvent and the unreacted silane raw material by vacuum distillation to obtain a battery electrolyte film-forming additive. The method is simple to operate, especially convenient for post-processing, and is a method that can be used for large-scale production.

[0032] According to another aspect of the present invention, a battery electrolyte is also provided, the battery electrolyte comprising a lithium salt, an organic solvent and the above-mentioned battery electrolyte film-forming additive. The battery electrolyte additive is applied to the battery electrolyte, and a passivation film layer having a dense structure and both flexible and stable characteristics can be formed during the cycle process. The presence of the passivation film layer can inhibit the continuous consumption of the battery electrolyte during the cycle process, and can also reduce the internal resistance of the battery and improve the cycle stability of the lithium battery.

[0033] In a preferred embodiment, by weight percentage, the lithium salt in the battery electrolyte is 7-16%, the battery electrolyte film-forming additive is 0.5-9%, and the balance is an organic solvent. By controlling the ratio of lithium salt, battery electrolyte film-forming additive and organic solvent in the battery electrolyte within the above range, the battery electrolyte can have good conductivity and electrochemical stability while also having good compatibility of the above components, further reducing the internal resistance of the battery and improving the cycle stability of the lithium battery. Preferably, by weight percentage, the lithium salt in the battery electrolyte is 10-15%, the battery electrolyte film-forming additive is 0.5-3%, and the balance is an organic solvent. The above effect is better when the components in the battery electrolyte are controlled within the above preferred range.

[0034] By way of example but not limitation, the organic solvent is one or more of organic carbonate, ionic liquid, polyether, aromatic ether, C1-C40 alkyl ether, cyclic ether, carboxylate, sulfone, nitrile, dinitrile and polynitrile. The organic solvent in the battery electrolyte can play the role of a mixed system, and the above-mentioned types of organic solvents and lithium salts and battery electrolyte film-forming additives all have good compatibility. Preferably, the organic solvent is one or more of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate. The above-mentioned types of organic solvents are more common in actual use and have better effects. Also by way of example but not limitation, the lithium salt is one or more of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium perchlorate, lithium bis oxalate borate, lithium difluoro oxalate borate, lithium trifluoromethyl sulfonate, lithium bis (pentafluoroethyl sulfonyl imide), lithium bis (trifluoromethyl sulfonyl imide), lithium bis (fluorosulfonyl imide), lithium difluoro (bis oxalate) phosphate and lithium tetrafluoro (oxalate) phosphate. The above-mentioned types of lithium salts and the battery electrolyte film-forming additives of the present invention can play a good synergistic role. Further preferably, the lithium salt is lithium hexafluorophosphate, which is more common in practice and has better effects. When the organic solvents ethylene carbonate, diethyl carbonate and ethyl methyl carbonate are used in combination, any proportion is acceptable. In the embodiment of the present invention, ethylene carbonate, diethyl carbonate and ethyl methyl carbonate are selected and mixed in a mass ratio of 3:3:4.

[0035] In a preferred embodiment, the battery electrolyte film-forming additive is a compound of compound S1 and compound S4, the organic solvent is dimethyl sulfoxide, and the lithium salt is lithium difluorooxalate borate; or, the battery electrolyte film-forming additive is a compound of compound S3 and compound S4, the organic solvent is aromatic polyoxyethylene ether, and the lithium salt is lithium trifluoromethyl sulfonate; or, the battery electrolyte film-forming additive is a compound of compound S2 and compound S3, the organic solvent is hexanetrinitrile, and the lithium salt is bis(fluorosulfonyl imide) lithium; wherein the structural formulas of compound S1, compound S2, compound S3, and compound S4 are as follows:

[0036] The inventors discovered during the experiment that, according to the above-mentioned matching method, the mixed battery electrolyte film-forming additive, lithium salt and organic solvent have a better matching effect. This may be because, under the above-mentioned matching method, there is a certain synergistic effect between the battery electrolyte film-forming additive and the lithium salt and organic solvent, which will affect the compatibility between the three and the stability and flexibility of the passivation film layer formed during the battery cycle, making it have a better effect.

[0037] According to the third aspect of the present invention, a silicon-carbon negative electrode lithium-ion battery is also provided, and the silicon-carbon negative electrode lithium-ion battery includes the above-mentioned battery electrolyte. During the charging and discharging process of the silicon-carbon negative electrode, the volume expansion and contraction of silicon particles during deintercalation lead to particle pulverization, shedding and electrochemical performance failure, and the continuous side reaction with the battery electrolyte will cause the continuous growth of the solid electrolyte layer (SEI) on the surface of the silicon particles. After the battery electrolyte film-forming additive proposed by the present invention is introduced into the silicon-carbon negative electrode lithium-ion battery, the above-mentioned problems can be effectively suppressed, thereby reducing the irreversible consumption of the negative electrode surface, reducing the internal resistance of the silicon-carbon negative electrode lithium ion battery, and improving the coulombic efficiency and cycle stability characteristics of the silicon-carbon negative electrode lithium ion battery. In addition, the battery electrolyte additive also has good electrochemical stability, can maintain the stability of the battery electrolyte in a wider voltage range, further enhances the safety performance of the battery, and improves the operating temperature range of the battery.

[0038] In a preferred embodiment, the silicon-carbon negative electrode lithium ion battery also includes a positive electrode containing a cathode active material, a negative electrode containing an anode active material and a battery separator; preferably, the cathode active material is a material capable of releasing lithium ions; further preferably, the cathode active material is one or more of transition metal phosphates, transition metal oxide lithium salts and metal sulfides; preferably, the anode active material is a material capable of receiving lithium ions; further preferably, the anode active material is one or more of silicon material, silicon-carbon material, a composite material of silicon monoxide and graphite, and a composite material of silicon-carbon and silicon monoxide; preferably, the battery separator is at least one of a polyimide separator, a polyethylene separator, a polypropylene separator and a polyethylene terephthalate separator; further preferably, the battery separator is a polyethylene separator or a polypropylene separator. In the process of preparing silicon-carbon negative electrode lithium-ion batteries, by selecting the above-mentioned types of cathode, anode and diaphragm materials, they can better interact with the battery electrolyte containing the battery electrolyte film-forming additive proposed in the present invention, thereby significantly improving the energy density of the silicon-carbon negative electrode lithium-ion battery, further reducing the internal stress problem of the battery caused by the volume change of the silicon-carbon negative electrode material during the charging and discharging process, and improving the cycle stability performance of the silicon-carbon negative electrode lithium-ion battery.

[0039] The present application is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in the present application.

[0040] Example 1

[0041] (1) Preparation of film-forming additives for battery electrolytes

[0042] Diethyl propylene phosphonate and methyltrimethoxysilane were added to anhydrous ethanol in a molar ratio of 1:10 and mixed evenly, and Bu3N was added as a catalyst, and the mixed system was reacted at 70°C for 12 hours. After the reaction was completed, the reaction mixture was distilled under reduced pressure to remove anhydrous ethanol and unreacted methyltrimethoxysilane to obtain the battery electrolyte film-forming additive S1. The chemical reaction formula is as follows:

[0043]

[0044] (2) Preparation of battery electrolyte

[0045] In a glove box filled with argon (the water content in the glove box is less than 0.1ppm, and the oxygen content is less than 0.1ppm), ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) are mixed evenly in a mass ratio of 3:3:4, and then lithium hexafluorophosphate (LiPF6) is slowly added thereto, and after the lithium salt is completely dissolved, the battery electrolyte film-forming additive S1 is added, and the battery electrolyte is obtained after being stirred evenly. In the above-mentioned battery electrolyte, by weight percentage, the organic solvent is 87.5%, the lithium salt is 12%, and the battery electrolyte film-forming additive S1 is 0.5%.

[0046] (3) Preparation of battery positive electrode

[0047] According to weight percentage, 92% NCM811, 4% acetylene black and 4% PVDF binder were added to N-methylpyrrolidone to prepare slurry, and the obtained slurry was coated on 12 micron thick aluminum foil, dried and rolled to obtain the positive electrode sheet of the battery.

[0048] (4) Preparation of battery negative electrode

[0049] According to weight percentage, 87% silicon carbon, 9% acetylene black and 4% SBR binder are mixed, deionized water is added thereto, and then the slurry is coated on an 8-micron copper foil, dried and rolled to obtain a battery negative electrode sheet.

[0050] (5) Preparation of batteries

[0051] The leakage point is controlled in a dry environment below -50°C, and the battery positive electrode plate, battery separator (model 92233), and battery negative electrode plate are stacked in order to make a battery cell. The stacking process must ensure that the separator completely separates the positive electrode plates, and the negative electrode completely covers the positive electrode. Then, use the glue-attached pole ear to encapsulate it in an aluminum-plastic film of a fixed size to form a soft-pack battery to be filled with liquid. Subsequently, the battery electrolyte is injected into the soft-pack battery, and then it is sealed, formed, aged, and divided into two parts to obtain an experimental battery for testing.

[0052] Example 2

[0053] The difference between Example 2 and Example 1 is that the prepared battery electrolyte film-forming additives are different, and when preparing the battery electrolyte, the addition ratios of the organic solvent, the lithium salt and the battery electrolyte film-forming additive are different.

[0054] (1) Preparation of film-forming additives for battery electrolytes

[0055] Vinylphosphonic acid dimethyl ester and methyltriethoxysilane were added to anhydrous ethanol in a molar ratio of 1:8 and mixed evenly, and Bu3N was added as a catalyst, and the mixed system was reacted at 60°C for 10 hours. After the reaction was completed, the reaction mixture was distilled under reduced pressure to remove anhydrous ethanol and unreacted methyltriethoxysilane to obtain the battery electrolyte film-forming additive S2. The chemical reaction formula is as follows:

[0056]

[0057] (2) Preparation of battery electrolyte

[0058] In a glove box filled with argon (the water content in the glove box is less than 0.1ppm, and the oxygen content is less than 0.1ppm), ethylene carbonate, diethyl carbonate and ethyl methyl carbonate are mixed evenly in a mass ratio of 3:3:4, and then lithium hexafluorophosphate is slowly added thereto, and after the lithium salt is completely dissolved, the battery electrolyte film-forming additive S1 is added, and the mixture is stirred evenly to obtain a battery electrolyte. In the above battery electrolyte, by weight percentage, the organic solvent is 87%, the lithium salt is 12%, and the battery electrolyte film-forming additive S2 is 1%.

[0059] Example 3

[0060] The difference between Example 3 and Example 1 is that the prepared battery electrolyte film-forming additives are different, and when preparing the battery electrolyte, the addition ratios of the organic solvent, the lithium salt and the battery electrolyte film-forming additive are different.

[0061] (1) Preparation of film-forming additives for battery electrolytes

[0062] Vinylphosphonic acid diethyl ester and ethyltripropoxysilane were added to anhydrous ethanol in a molar ratio of 1:8 and mixed evenly, and Bu3N was added as a catalyst, and the mixed system was reacted at 80°C for 8h. After the reaction was completed, the reaction mixture was distilled under reduced pressure to remove anhydrous ethanol and unreacted ethyltripropoxysilane to obtain the battery electrolyte film-forming additive S3. The chemical reaction formula is as follows:

[0063]

[0064] (2) Preparation of battery electrolyte

[0065] In a glove box filled with argon (the water content in the glove box is less than 0.1ppm, and the oxygen content is less than 0.1ppm), ethylene carbonate, diethyl carbonate and ethyl methyl carbonate are mixed evenly in a mass ratio of 3:3:4, and then lithium hexafluorophosphate is slowly added thereto, and after the lithium salt is completely dissolved, the battery electrolyte film-forming additive S1 is added, and the mixture is stirred evenly to obtain a battery electrolyte. In the above battery electrolyte, by weight percentage, the organic solvent is 86.5%, the lithium salt is 12%, and the battery electrolyte film-forming additive S2 is 1.5%.

[0066] Example 4

[0067] The difference between Example 4 and Example 1 is that the prepared battery electrolyte film-forming additives are different, and when preparing the battery electrolyte, the addition ratios of the organic solvent, the lithium salt and the battery electrolyte film-forming additive are different.

[0068] (1) Preparation of film-forming additives for battery electrolytes

[0069] Dimethyl propylene phosphonate and ethyl tributyloxysilane were added to anhydrous ethanol in a molar ratio of 1:16 and mixed evenly, and Bu3N was added as a catalyst, and the mixed system was reacted at 75°C for 14 hours. After the reaction was completed, the reaction mixture was distilled under reduced pressure to remove anhydrous ethanol and unreacted ethyl tributyloxysilane to obtain the battery electrolyte film-forming additive S4. The chemical reaction formula is as follows:

[0070]

[0071] (2) Preparation of battery electrolyte

[0072] In a glove box filled with argon (the water content in the glove box is less than 0.1ppm, and the oxygen content is less than 0.1ppm), ethylene carbonate, diethyl carbonate and ethyl methyl carbonate are mixed evenly in a mass ratio of 3:3:4, and then lithium hexafluorophosphate is slowly added thereto, and after the lithium salt is completely dissolved, the battery electrolyte film-forming additive S1 is added, and the mixture is stirred evenly to obtain a battery electrolyte. In the above battery electrolyte, by weight percentage, the organic solvent is 86%, the lithium salt is 12%, and the battery electrolyte film-forming additive S2 is 2%.

[0073] Example 5

[0074] The difference between Example 5 and Example 1 is that in the prepared battery electrolyte, by weight percentage, the organic solvent accounts for 92.5%, the lithium salt accounts for 7%, and the battery electrolyte film-forming additive S1 accounts for 0.5%.

[0075] Example 6

[0076] The difference between Example 6 and Example 1 is that in the prepared battery electrolyte, the organic solvent accounts for 75%, the lithium salt accounts for 16%, and the battery electrolyte film-forming additive S1 accounts for 9% by weight.

[0077] Example 7

[0078] The main difference between Example 7 and Example 1 is that in the prepared battery electrolyte, the organic solvent accounts for 87%, the lithium salt accounts for 10%, and the battery electrolyte film-forming additive S1 accounts for 3% by weight.

[0079] Example 8

[0080] The main difference between Example 8 and Example 1 is that in the prepared battery electrolyte, the organic solvent accounts for 84.5%, the lithium salt accounts for 15%, and the battery electrolyte film-forming additive S1 accounts for 0.5% by weight.

[0081] Example 9

[0082] The difference between Example 9 and Example 1 is that in the battery electrolyte, the battery electrolyte film-forming additive is compound S1 and compound S4 mixed in a mass ratio of 1:2, the organic solvent is dimethyl sulfoxide (DMSO), and the lithium salt is lithium difluorooxalatoborate (LiBF2(C2O4)).

[0083] Example 10

[0084] The difference between Example 10 and Example 1 is that in the battery electrolyte, the battery electrolyte film-forming additive is compound S1 and compound S4 mixed in a mass ratio of 3:2, the organic solvent is dimethyl sulfoxide, and the lithium salt is lithium difluorooxalate borate.

[0085] Embodiment 11

[0086] The difference between Example 11 and Example 1 is that in the battery electrolyte, the battery electrolyte film-forming additive is compound S3 and compound S4 mixed in a mass ratio of 2:3, the organic solvent is aromatic polyoxyethylene ether, and the lithium salt is lithium trifluoromethanesulfonate (LiCF3SO3).

[0087] Example 12

[0088] The difference between Example 12 and Example 1 is that in the battery electrolyte, the battery electrolyte film-forming additive is compound S3 and compound S4 mixed in a mass ratio of 2:5, the organic solvent is aromatic polyoxyethylene ether, and the lithium salt is lithium trifluoromethanesulfonate.

[0089] Example 13

[0090] The difference between Example 13 and Example 1 is that in the battery electrolyte, the battery electrolyte film-forming additive is compound S2 and compound S3 mixed in a mass ratio of 1:1, the organic solvent is hexanetrinitrile, and the lithium salt is lithium bis(fluorosulfonyl imide) (Li[(CF3SO2)2N]).

[0091] Embodiment 14

[0092] The difference between Example 14 and Example 1 is that in the battery electrolyte, the battery electrolyte film-forming additive is compound S2 and compound S3 mixed in a mass ratio of 1:3, the organic solvent is hexanetrinitrile, and the lithium salt is bis(fluorosulfonyl imide) lithium.

[0093] Embodiment 15

[0094] The difference between Example 15 and Example 4 is that the amount of the battery electrolyte film-forming additive added to the battery electrolyte is 0.3%.

[0095] Example 16

[0096] The difference between Example 16 and Example 4 is that the amount of the battery electrolyte film-forming additive added to the battery electrolyte is 10%.

[0097] Comparative Example 1

[0098] The difference from Example 1 is that the film-forming additive in the battery electrolyte is 2% fluoroethylene carbonate (FEC).

[0099] Comparative Example 2

[0100] The difference from Example 1 is that the film-forming additive of the battery electrolyte is 1% vinylene carbonate (VC).

[0101] Comparative Example 3

[0102] The difference from Example 1 is that the film-forming additive of the battery electrolyte is 1% diethyl sulfate (DTD).

[0103] Comparative Example 4

[0104] The difference from Example 1 is that no battery electrolyte film-forming additive is added during the process of preparing the battery electrolyte.

[0105] In order to intuitively compare the compositions of the battery electrolytes prepared in the examples and comparative examples, the components of the battery electrolytes are summarized here. The results are shown in Table 1. It should be noted that the data in the table involve ratios that refer to their mass ratios.

[0106] Table 1

[0107]

[0108]

[0109] The batteries prepared in the above examples and comparative examples were tested for relevant performances, and the results are shown in Table 2. The specific test methods are as follows:

[0110] The above-mentioned soft-pack battery was formed and divided into different capacities using a battery test cabinet. The formation process was 0.03C constant current charging to 3.0V, 0.05C constant current charging to 3.4V, and 0.1C constant current charging to 3.7V; the aging process was 45°C for 12 hours; the division process was 0.33C constant current and constant voltage charging to 4.25V, the cut-off current was 0.05C, and 0.33C constant current discharge to 2.75V was repeated 3 times. Then the battery was charged and discharged at 1C / 1C constant current and constant voltage for 1000 cycles at room temperature, and the voltage range was set to 2.75-4.25V to test its first coulomb efficiency and capacity retention rate. An internal resistance meter was used to test the impedance of the battery before and after 1000 cycles, and the internal resistance growth rate was equal to the difference before and after the cycle divided by the internal resistance of the battery before the cycle. A vernier caliper was used to test the thickness of the battery before and after 1000 cycles, and the thickness growth rate was equal to the difference in battery thickness before and after the cycle divided by the battery thickness before the cycle.

[0111] Table 2

[0112]

[0113]

[0114] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0115] In Examples 1 to 16, when the battery electrolyte film-forming additive of the present invention is used in a battery, in a battery cycle test, the first coulombic efficiency and the capacity retention rate after 1000 cycles are significantly improved, and the internal resistance and battery thickness are also significantly reduced during the cycle. In particular, the above effect will be better if the amount of the battery electrolyte film-forming additive added to the battery electrolyte is controlled within a preferred range. The prepared battery electrolyte additive is compounded and used in accordance with corresponding proportions, and is combined with corresponding lithium salts and organic solvents, which is more conducive to improving the comprehensive performance of the battery.

[0116] On the other hand, in Comparative Examples 1 to 3, conventional battery electrolyte film-forming additives are used. Although the battery performance is improved compared to Comparative Example 4 in which no battery electrolyte film-forming additives are used, the effect is not ideal, and is far from the effect of using the battery electrolyte film-forming additive prepared by the present invention.

[0117] In summary, the battery electrolyte additive of alkylphosphonate alkoxysilane provided by the present invention can significantly improve the initial charge and discharge coulombic efficiency and capacity retention rate in the battery cycle test, and can also effectively reduce the internal resistance and battery thickness, inhibit the impedance growth caused by the reaction of the battery electrolyte with the lithiated silicon negative electrode during the cycle process, and at the same time can also protect the silicon negative electrode from repeated lithium insertion and extraction caused by particle breakage and differentiation, which is beneficial to improving the cycle stability of the battery.

[0118] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A battery electrolyte film-forming additive, characterized in that: The battery electrolyte film-forming additive is one or more alkylphosphonate alkoxysilanes, and the alkylphosphonate alkoxysilane has a chemical structure shown in the following formula (I): Wherein, R1, R2, R3, R4 and R5 are each independently selected from one of C1 to C10 alkyl groups, 1≤n≤10, and n is an integer.

2. The battery electrolyte film-forming additive according to claim 1, characterized in that: The R1, the R2 and the R3 are each independently selected from one of C1 to C5 alkyl groups, the R4 and the R5 are each independently selected from one of C1 to C10 alkyl groups, 1≤n≤3, and n is an integer; Preferably, the R1, the R2 and the R3 are each independently selected from one of C1-C10 alkyl groups, the R4 and the R5 are each independently selected from one of C1-C2 alkyl groups, 1≤n≤3; and n is an integer.

3. The battery electrolyte film-forming additive according to claim 1, characterized in that: The R1, the R2, the R3, the R4 and the R5 are each independently selected from a methyl group, an ethyl group, a propyl group and a butyl group, 3≤n≤5; and n is an integer.

4. The battery electrolyte film-forming additive according to any one of claims 1 to 3, characterized in that: The battery electrolyte film-forming additive is selected from one or more of the following structures: Preferably, the battery electrolyte film-forming additive is a compound of compound S1 and compound S4, and the weight ratio of the two is (1-3):2; or, the battery electrolyte film-forming additive is a compound of compound S3 and compound S4, and the weight ratio of the two is 2:(3-5); or, the battery electrolyte film-forming additive is a compound of compound S2 and compound S3, and the weight ratio of the two is 1:(1-3).

5. A battery electrolyte, characterized in that: The battery electrolyte comprises a lithium salt, an organic solvent and the battery electrolyte film-forming additive according to any one of claims 1 to 4.

6. The battery electrolyte according to claim 5, characterized in that In terms of weight percentage, the lithium salt in the battery electrolyte is 7-16%, the battery electrolyte film-forming additive is 0.5-9%, and the balance is the organic solvent; Preferably, by weight percentage, the lithium salt in the battery electrolyte is 10-15%, the battery electrolyte film-forming additive is 0.5-3%, and the balance is the organic solvent.

7. The battery electrolyte according to any one of claims 5 or 6, characterized in that The organic solvent is one or more of organic carbonate, ionic liquid, polyether, aromatic ether, C1-C40 alkyl ether, cyclic ether, carboxylate, sulfone, nitrile, dinitrile and polynitrile; preferably, the organic solvent is one or more of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate; Preferably, the lithium salt is one or more of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium trifluoromethylsulfonate, lithium bis(pentafluoroethylsulfonylimide), lithium bis(trifluoromethylsulfonylimide), lithium bis(fluorosulfonylimide), lithium difluoro(bis(oxalate)phosphate and lithium tetrafluoro(oxalate)phosphate; further preferably, the lithium salt is lithium hexafluorophosphate.

8. The battery electrolyte according to claim 7, characterized in that: The battery electrolyte film-forming additive is a compound of compound S1 and compound S4, the organic solvent is dimethyl sulfoxide, and the lithium salt is lithium difluorooxalate borate; or The battery electrolyte film-forming additive is a compound of compound S3 and compound S4, the organic solvent is an aromatic polyoxyethylene ether, and the lithium salt is lithium trifluoromethanesulfonate; or The battery electrolyte film-forming additive is a compound of compound S2 and compound S3, the organic solvent is hexanetrinitrile, and the lithium salt is bis(fluorosulfonyl imide) lithium; The structural formulas of the compound S1, the compound S2, the compound S3 and the compound S4 are as follows:

9. A silicon-carbon negative electrode lithium ion battery, characterized in that: The silicon-carbon negative electrode lithium-ion battery comprises the battery electrolyte described in any one of claims 5 to 8.

10. The silicon-carbon negative electrode lithium ion battery according to claim 9, characterized in that: The silicon-carbon negative electrode lithium ion battery also includes a positive electrode containing a cathode active material, a negative electrode containing an anode active material and a battery separator; Preferably, the cathode active material is a material capable of releasing lithium ions; further preferably, the cathode active material is one or more of transition metal phosphates, transition metal oxides, lithium salts, and metal sulfides; Preferably, the anode active material is a material capable of receiving lithium ions; further preferably, the anode active material is one or more of a silicon material, a silicon-carbon material, a composite material of silicon monoxide and graphite, and a composite material of silicon carbon and silicon monoxide; Preferably, the battery separator is at least one of a polyimide separator, a polyethylene separator, a polypropylene separator and a polyethylene terephthalate separator; further preferably, the battery separator is a polyethylene separator or a polypropylene separator.

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