Electrolyte and lithium ion battery thereof

Abstract

The application discloses an electrolyte and a lithium ion battery thereof. The electrolyte comprises a lithium salt, an organic solvent and an additive. The additive comprises an additive A, an additive B, an additive C and a film-forming additive D. The additive A, the additive B and the additive C respectively account for 0.1-3 wt% of the total mass of the electrolyte. The additive A is a silicon-based pyrophosphoric ester compound with a general structure of formula I, the additive B is a lithium salt of bis-trifluoromethyl pyroboric acid with a general structure of formula II, and the additive C is a cyclic pyrocarbonate compound with a general structure of formula III. In formula I, R1-R 12 alkane groups with 1-5 carbon atoms, a trifluoromethyl group and a nitrile group. In formula II, X1, X2, X3 and X4 are selected from alkane groups with 1-5 carbon atoms, alkene groups, alkyne groups and trifluoromethyl groups.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries and electrolyte thereof, in particular to an electrolyte and a lithium ion battery thereof. BACKGROUND

[0002] Lithium ion batteries are widely used in 3C digital, power tools, aerospace, energy storage, electric vehicles and other fields due to their high specific energy, no memory effect, long cycle life and other advantages. In lithium ion batteries, high-voltage cathode materials are widely used in portable electronic devices such as mobile phones and notebook computers, as well as electric vehicles and large-scale energy storage devices due to their high energy density, environmental friendliness, long cycle life and other advantages.

[0003] Current lithium ion batteries have problems of insufficient high-temperature cycle performance, high-temperature storage performance, heat shock resistance and low-temperature discharge capacity. SUMMARY

[0004] The purpose of the present application is to provide an electrolyte and a lithium ion battery thereof, to improve the high-temperature cycle performance, and / or the high-temperature storage performance, and / or the heat shock resistance, and / or the low-temperature discharge capacity of the lithium ion battery.

[0005] The present application discloses an electrolyte, comprising a lithium salt, an organic solvent and an additive; the additive comprises an additive A, and / or an additive B, and / or an additive C, and / or a film-forming additive D;

[0006] The additive A, the additive B and the additive C each account for 0.1-3wt% of the total mass of the electrolyte; the additive A is a silicon-based pyrophosphoric acid ester compound of formula I, the additive B is a lithium salt of bis-trifluoromethyl pyroboric acid of formula II, and the additive C is a cyclic pyrocarbonate compound of formula III:

[0007]

[0008] In formula I, R1-R 12 are selected from alkyl groups of 1-5 carbon atoms, trifluoromethyl groups, nitrile groups; in formula II, X1, X2, X3, X4 are selected from alkyl groups of 1-5 carbon atoms, alkenyl groups, alkynyl groups, trifluoromethyl groups; in formula II, Y is selected from alkyl groups of 1-5 carbon atoms, alkenyl groups, alkynyl groups, fluorine atoms, hydrogen atoms and alkyl groups of 1-5 carbon atoms, alkenyl groups, alkynyl groups substituted by fluorine atoms;

[0009] The film-forming additive D is selected from two or more of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, methylene methane disulfonate, propene sultone, butanedinitrile, hexanedinitrile, 1,3,6-hexanetricarbonitrile, lithium difluorophosphate, lithium difluoro(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, wherein at least one is fluoroethylene carbonate;

[0010] The content of the fluoroethylene carbonate in the film-forming additive D is 0.5-20 wt% of the total mass of the electrolyte, and the content of the other components is 0.1-8 wt% of the total mass of the electrolyte.

[0011] Optionally, the organic solvent includes one or more of a chain carbonate organic solvent, a cyclic carbonate organic solvent, a carboxylic acid ester organic solvent, a silicon-based organic solvent, and a difluoro-based organic solvent.

[0012] Optionally, the organic solvent includes a silicon-based organic solvent and a difluoro-based organic solvent.

[0013] Optionally, the silicon-based organic solvent has a general structure of Formula IV and / or V, and the difluoro-based organic solvent has a general structure of Formula VI and / or VII:

[0014]

[0015] wherein R1, R2, R3, R4 are selected from a siloxane with 1-5 carbon atoms, and X1, X2 are selected from an alkyl with 1-5 carbon atoms substituted with two fluorine atoms on the same methyl group.

[0016] Optionally, the additive A has at least one of the following specific structural formulas:

[0017]

[0018] Optionally, the additive B has at least one of the following specific structural formulas:

[0019]

[0020] Optionally, the additive C has at least one of the following specific structural formulas:

[0021]

[0022] Optionally, the organic solvent is selected from three or more of vinyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, tetrahydrofuran, a silicon-based organic solvent, and a difluoro-based organic solvent, wherein at least two are a silicon-based organic solvent and a difluoro-based solvent.

[0023] Optionally, the specific structural formula of the silicon-containing organic solvent is at least one of the following structural formulae:

[0024]

[0025] The specific structural formula of the difluoro-containing organic solvent is at least one of the following structural formulae:

[0026]

[0027] The application further discloses a lithium ion battery comprising the electrolyte.

[0028] The functional groups such as silanone and siloxane in the electrolyte, the silane in the additive A and the silane in the additive B can improve the stability of the electrode interface film, inhibit solvent decomposition, and improve the thermal shock and high-temperature storage performance. The pyrocarbonate structure in the additive C can promote the formation of a stable SEI film between the non-aqueous solvent and lithium ions, effectively improve the cycle life of the battery, and improve the rate performance of the battery. The FEC in the film-forming additive D can continuously modify the SEI film damaged during the cycle process, thereby improving the cycle performance of the battery. The additive B and the film-forming additive D can improve the low-temperature discharge capacity of the battery. DETAILED DESCRIPTION

[0029] It should be understood that the terms used herein, the disclosed specific structures and functional details are only for the purpose of describing specific embodiments and are representative, but the application can be embodied in many alternative forms, and should not be interpreted as being limited to the embodiments described herein.

[0030] The application will be described in detail below with reference to optional embodiments.

[0031] As an embodiment of the application, an electrolyte is disclosed, comprising a lithium salt, an organic solvent and an additive; the additive comprises an additive A, and / or an additive B, and / or an additive C, and / or a film-forming additive D; the additive A, the additive B and the additive C each account for 0.1-3wt% of the total mass of the electrolyte; the additive A is a silane-based pyrophosphoric acid ester compound with the general structural formula of formula I, the additive B is a lithium salt of a bistrifluoromethyl pyroborate compound with the general structural formula of formula II, and the additive C is a cyclic pyrocarbonate compound with the general structural formula of formula III:

[0032]

[0033] In formula I, R1-R 12alkyl group of 1-5 carbon atoms, a trifluoromethyl group, a nitrile group; in formula II, X1, X2, X3, X4 are selected from an alkyl group of 1-5 carbon atoms, an alkenyl group, an alkynyl group, a trifluoromethyl group; in formula II, Y is selected from an alkyl group of 1-5 carbon atoms, an alkenyl group, an alkynyl group, a fluorine atom, a hydrogen atom, and an alkyl group of 1-5 carbon atoms, an alkenyl group, an alkynyl group substituted by a fluorine atom.

[0034] The film-forming additive D is selected from two or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propanesultone (PS), vinyl sulfate (DTD), methane disulfonate methylene (MMDS), propene sulfonic acid lactone (PST), succinonitrile (SN), adiponitrile (ADN), 1,3,6-hexanetricarbonitrile (HTCN), lithium difluorophosphate (LiPO2F2), lithium difluoro(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), wherein at least one is fluoroethylene carbonate (FEC). In the film-forming additive D, the content of fluoroethylene carbonate (FEC) is 0.5-20wt% of the total mass of the electrolyte, and the content of other components is 0.1-8wt% of the total mass of the electrolyte.

[0035] The electrolyte of the application, the additive A has a siloxane structure or a silanitrile group which can effectively remove a small amount of water and the decomposition product HF of the electrolyte at high temperature in the electrolyte, avoiding the erosion of the electrode interface film by HF. In addition, the siloxane group and the borate can regulate the LiF component in the SEI film, and the SEI film formed has many holes and good permeability, effectively reducing the internal resistance of the battery. The additive B can be preferentially reduced on the negative electrode surface to participate in the formation of the negative electrode SEI film, and the SEI film formed is rich in B-O bonds with stronger mechanical properties and better toughness, improving the continuous repairability of the SEI film during the cycle process, and effectively improving the cycle performance of the battery. The fluorinated group is conducive to the formation of SEI and CEI components represented by LiF.

[0036] The silanitrile, siloxane and other functional groups in the additive B and the additive A have a synergistic effect which can improve the stability of the electrode interface film, inhibit the decomposition of the solvent, and improve the thermal shock and high-temperature storage performance.

[0037] The bicarbonate structure in the additive C can promote the formation of a stable SEI film between the non-aqueous solvent and lithium ions, alleviate the side reaction between the negative electrode and the organic solvent, and improve the charge and discharge performance of the battery. The SEI film formed by the participation of bicarbonate contains more organic components, and the film has better toughness and mechanical properties, and low interface impedance. Therefore, the cycle life of the battery can be effectively improved, and the rate performance of the battery can be improved.

[0038] The FEC in the film-forming additive D can continuously modify the SEI film damaged in the circulation process, avoid the side reaction of the electrode interface directly contacting with the electrolyte, and effectively improve the battery cycle performance. The FEC is easy to decompose to generate HF at high temperature, and the additive A can effectively remove HF, so the additive A and the additive D have a synergistic effect.

[0039] Further, if the above four additives are used together in the electrolyte, they can interact with each other, and compared with using only one or two or three of them, the performance of the electrolyte can be effectively improved, especially in the electrolyte system using a silicon-containing organic solvent and a double-fluorinated solvent, the battery performance improvement effect is more significant.

[0040] Specifically, the organic solvent includes one or more of a chain carbonate organic solvent, a cyclic carbonate organic solvent, a carboxylic acid ester organic solvent, a silicon-containing organic solvent, and a double-fluorinated organic solvent.

[0041] Specifically, the organic solvent includes a silicon-containing organic solvent and a double-fluorinated organic solvent. The Si-O bond in the silicon-containing organic solvent can withstand higher temperature and has better thermal stability, effectively improving the heat shock resistance and high-temperature storage performance of the battery. The silicon-containing organic solvent reduces the viscosity of the original solvent, improves the shuttling ability of lithium ions in the solvent, and effectively improves the low-temperature rate performance of the battery. The F atom in the double-fluorinated organic solvent has strong electronegativity and weak polarity, so the fluorinated solvent has high oxidation resistance. The inventors have found through a large number of experiments that the double-fluorinated solvent has stronger oxidation resistance and better performance. If the fluorine atoms are replaced too much or the hydrocarbon chains are too long, the viscosity of the solvent will increase, which is not conducive to improving the low-temperature rate performance of the battery.

[0042] The silicon-containing organic solvent and the fluorinated solvent used in the present application can have a synergistic effect, on the one hand, reducing the viscosity of the electrolyte, and on the other hand, improving the thermal stability and oxidation resistance of the electrolyte solvent. Further, through the synergistic effect of the additive A, the additive B, the additive C, the additive D, and the simultaneous use of the silicon-containing organic solvent and the double-fluorinated solvent, the electrolyte has excellent film-forming performance on the electrode surface, the additives can effectively remove the acidic by-products of the electrolyte, stabilize the structure of the electrode material, inhibit the thermal decomposition of the solvent, so that the high-temperature cycle performance, high-temperature storage performance, heat shock resistance, and low-temperature discharge performance of the lithium ion battery are effectively improved.

[0043] Specifically, the proportion of the additive A, the additive B and the additive C in the total mass of the electrolyte can be 0.1wt%, 0.3wt%, 0.5wt%, 0.7wt%, 1wt%, 1.3wt%, 1.5wt%, 1.8wt%, 2wt%, 2.3wt%, 2.5wt%, 3wt%. The proportion of the content of the fluoroethylene carbonate (FEC) in the total mass of the electrolyte can be 0.5wt%, 1wt%, 2wt%, 3wt%, 5wt%, 8wt%, 10wt%, 13wt%, 15wt%, 17wt%, 20wt%. The proportion of the content of the other components in the film-forming additive D in the total mass of the electrolyte can be 0.1wt%, 0.5wt%, 1wt%, 2wt%, 3wt%, 5wt%, 8wt%.

[0044] Specifically, the structure general formula of the silicon-containing organic solvent is the following formula IV and / or V, and the structure general formula of the difluoro-containing organic solvent is the following formula VI and / or VII:

[0045]

[0046] wherein R1, R2, R3, R4 are selected from siloxane with 1-5 carbon atoms, and X1, X2 are selected from alkyl with 1-5 carbon atoms substituted by two fluorine atoms on the same methyl group.

[0047] Specifically, the specific structure formula of the additive A is at least one of the following structure formula:

[0048]

[0049] Specifically, the specific structure formula of the additive B is at least one of the following structure formula:

[0050]

[0051] Specifically, the specific structure formula of the additive C is at least one of the following structure formula:

[0052]

[0053] Specifically, the organic solvent is selected from three or more of the following: vinyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, tetrahydrofuran, silicon-containing organic solvent, difluoro-containing organic solvent, wherein at least two are silicon-containing organic solvent and difluoro-containing solvent.

[0054] Specifically, the specific structure formula of the silicon-containing organic solvent is at least one of the following structure formula:

[0055] The difluoro-containing organic solvent has the following structure formula:

[0056] The specific structure of the solvent is at least one of the following structure:

[0057]

[0058] Specifically, the lithium salt is selected from one or more of hexafluorophosphate, hexafluoroarsenate, perchlorate, lithium trifluorosulfonate, lithium difluoro(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methide. The concentration of the lithium salt in the electrolyte is 0.5M-2M.

[0059] The application also discloses a lithium ion battery comprising the electrolyte as described above. Specifically, the lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte as described above. The positive electrode sheet comprises a positive electrode current collector and a positive electrode film, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film, the positive electrode film comprises a positive electrode active material, a conductive agent and a binder, and the negative electrode film comprises a negative electrode active material, a conductive agent and a binder; the positive electrode active material is LiNi 1-x-y-z Co x Mn y Al z O2 wherein: 0≤x≤1, 0≤y≤1, 0≤z≤1 and 0≤x+y+z≤1; the negative electrode active material is selected from graphite and / or silicon, such as natural graphite, artificial graphite, mesophase carbon microbeads (MCMB for short), hard carbon, soft carbon, silicon, silicon or SiO w x, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithiated TiO2-Li4Ti5O 12 Li-Al alloy.

[0060] Specifically, non-limiting examples of the binder include polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, butadiene-styrene rubber, acrylated butadiene-styrene rubber, epoxy resin, nylon and the like.

[0061] Specifically, non-limiting examples of the conductive agent include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, etc.), metal-based materials (e.g., metal powder, metal fibers, etc., including, for example, copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives) and mixtures thereof.

[0062] The application is further illustrated by the following examples.

[0063] Example 1

[0064] The present embodiment is used to illustrate the lithium ion battery and the preparation method thereof disclosed by the present application, and includes the following operation steps:

[0065] Preparation of electrolyte: the electrolyte was prepared according to the formulation of Example 1 shown in Table 1.

[0066] Preparation of positive electrode sheet: the positive electrode active material (LiCoO2), conductive agent (CNT (Carbon Nanotube)), and binder (PVDF (polyvinylidene fluoride)) were mixed in N-methylpyrrolidone solvent in a mass ratio of 97:1.5:1.5, and stirred sufficiently to form a uniform positive electrode slurry. The slurry was coated on the positive electrode current collector Al foil, dried, cold-pressed, and a positive electrode sheet was obtained.

[0067] Preparation of negative electrode sheet: the negative electrode active material (silicon-oxygen-carbon composite material), conductive agent (acetylene black), binder (styrene-butadiene rubber), and thickening agent (sodium carboxymethyl cellulose) were mixed in a proper amount of deionized water solvent in a mass ratio of 95:2:2:1, and stirred sufficiently to form a uniform negative electrode slurry. The slurry was coated on the negative electrode current collector Cu foil, dried, cold-pressed, and a negative electrode sheet was obtained.

[0068] Preparation of lithium ion battery: the PE porous polymer film was used as the separator.

[0069] The positive electrode sheet, the separator, and the negative electrode sheet were stacked in order, with the separator between the positive electrode and the negative electrode to play a separating role, and then the stacked electrode sheet and the separator were wound to obtain a winding core. The winding core was placed in an aluminum plastic film bag shaped by punching, the electrolyte prepared above was injected into the dried electrode core, and the battery was prepared after vacuum packaging, standing, and formation.

[0070] Examples 2-18

[0071] Examples 2-18 are used to illustrate the lithium ion battery electrolyte, lithium ion battery, and preparation method thereof disclosed by the present application, and include most of the operation steps in Example 1, with the difference being that:

[0072] In the preparation operation of the electrolyte: the additive was added to the organic solvent in a mass percentage content shown in Table 1 in Examples 2-18.

[0073] Comparative Example 1

[0074] Comparative Example 1 is used to compare the lithium ion battery electrolyte, lithium ion battery, and preparation method thereof disclosed by the present application, and includes most of the operation steps in Example 1, with the difference being that:

[0075] In the preparation operation of the electrolyte: the additive was added to the organic solvent in a mass percentage content shown in Table 1 in Comparative Example 1.

[0076] Table 1

[0077]

[0078] The lithium ion batteries prepared in the above Examples 1-18 and Comparative Example 1 were subjected to the following performance tests:

[0079] High-temperature storage test of the battery:

[0080] Test method: the full-state battery (0.7C constant current and constant voltage charging to 4.45V, with a cutoff current of 0.05C) was placed in an environment at 85°C for 18h, and after 12h, the hot thickness, voltage, and internal resistance were tested; 0.2C constant current discharge to 3.0V, and then the recovery capacity was recorded after 3 cycles (the third week capacity was taken).

[0081] High-temperature cycle performance test:

[0082] At 45°C, the battery after being divided was charged to 4.45V at 1C constant current and constant voltage, with a cutoff current of 0.05C, and then discharged to 3.0V at 0.5C constant current, and the cycle was repeated, and after 500 cycles of charging and discharging, the capacity retention rate at the 500th cycle was calculated, and the calculation formula was as follows:

[0083] Capacity retention rate at the 500th cycle (%) = (discharge capacity at the 500th cycle / discharge capacity at the first cycle) x 100%.

[0084] 60°C 30d high-temperature storage test:

[0085] The battery was charged and discharged at 0.2C (4.45V-3.0V) once at room temperature, and the discharge capacity C0 before storage was recorded, then the battery was charged to 4.45V at constant current and constant voltage to full state, and the thickness d1 of the battery before high-temperature storage was tested using a vernier caliper (by connecting the two diagonal lines of the above battery with a straight line, the intersection of the two diagonal lines is the test point of the thickness of the battery), the battery was placed in a constant temperature oven at 60°C for 30 days, after the storage was completed, the battery was taken out and the hot thickness d2 of the battery after storage was tested, and the thickness expansion rate of the battery after 30 days of storage at 60°C was calculated; after the battery was cooled at room temperature for 24h, the battery was discharged again at 0.2C constant current to 3.0V, and then charged to 4.45V at 0.2C constant current and constant voltage, and the discharge capacity C1 and the charge capacity C2 of the battery after storage were recorded, and the capacity remaining rate and the recovery rate of the battery after 30 days of storage at 60°C were calculated, and the calculation formula was as follows:

[0086] Thickness expansion rate after 30 days of storage at 60°C = (d2-d1) / d1*100%;

[0087] Capacity remaining rate after 30 days of storage at 60°C = C1 / C0*100%;

[0088] Capacity recovery rate after 30 days storage at 60°C = C2 / C0*100%.

[0089] Low temperature discharge performance test:

[0090] At 25°C ambient condition, discharge the battery after separation at 0.2C to 3.0V, and stand for 5min; then charge at 0.2C to 4.45V, when the voltage of the battery reaches 4.45V, change to 4.45V constant voltage charging, until the charging current is less than or equal to the given cutoff current 0.05C, and stand for 5min; transfer the full charged battery to the high and low temperature box, set to -20°C, and stand for 120min after the temperature of the box reaches; then discharge to the terminal voltage 3.0V at 0.2C, and stand for 5min; then adjust the temperature of the high and low temperature box to 25°C±3°C, and stand for 60min after the temperature of the box reaches; charge to 4.45V at 0.2C, when the voltage of the battery reaches 4.45V, change to 4.45V constant voltage charging, until the charging current is less than or equal to the given cutoff current 0.05C; stand for 5min; calculate the capacity retention rate of -20°C low temperature discharge to 3.0V. The calculation formula is as follows:

[0091] Capacity retention rate of -20°C discharge to 3.0V (%) = (-20°C discharge to 3.0V discharge capacity / 25°C discharge to 3.0V discharge capacity) x 100%.

[0092] Thermal shock performance:

[0093] At 25°C ambient condition, discharge to 3.0V at the given current 0.2C; stand for 5min; charge to 4.45V at the charging current 0.2C, when the voltage of the battery reaches 4.45V, change to 4.45V constant voltage charging, until the charging current is less than or equal to the given cutoff current 0.05C; after standing for 1h, put the battery into the oven, and increase the temperature of the oven to 130±2°C at the speed of 5±2°C / min, and stop after keeping for 60min, the judgment standard is that the battery does not catch fire or explode.

[0094] Table 2

[0095]

[0096] (1) Thermal shock: according to the comparative examples and examples, when adding additive A or additive B alone, the thermal shock pass rate can be improved. Because the siloxane and silanitrile groups in additive A can remove HF decomposed at high temperature in the electrolyte, and avoid more electrode side reactions caused by HF under thermal shock, leading to thermal runaway of the battery; the SEI film structure formed by the B-O bond in additive B is more stable, so the thermal shock capacity of the battery can be improved. When additive A and additive B are used together, the thermal shock pass rate is further improved.

[0097] (2) High temperature cycle: according to the comparative examples and examples, the addition of the additives ABCD alone can improve the high temperature cycle performance of the battery, the combination of two kinds of additives can further improve the high temperature cycle performance, and the combination of four kinds of additives can most significantly improve the performance of the battery. The results of Example 10 show that when the content of additive A or additive B exceeds 3%, the cycle performance of the battery decreases, because too much additive A and additive B increase the electrode interface impedance and deteriorate the battery performance. The results of Example 11 show that when the content of the silicon-containing organic solvent and the fluorinated solvent exceeds 8%, the battery performance is also deteriorated to some extent, and too much silicon-containing organic solvent and fluorinated solvent can affect the performance of other solvents in the electrolyte.

[0098] (3) High temperature storage performance: according to the comparative examples and examples, the addition of the additives ABD alone can improve the high temperature storage performance of the battery, and the addition of FEC in the film-forming additive D can increase the thickness expansion during high temperature storage, because FEC is prone to decomposition and gas production at high temperature. When the four kinds of additives are combined, the thickness change rate and the remaining recovery capacity during high temperature storage can be significantly improved.

[0099] (4) Low temperature discharge: according to the comparative examples and examples, the addition of additive B and film-forming additive D alone can improve the low temperature discharge capacity of the battery, mainly because additive B lithium borate can form a SEI film with higher conductivity, and LiPO2F2 in the film-forming additive D can form a lithium oxyfluorophosphate SEI film with higher conductivity. The results of Examples 8 and 9 show that the silicon-containing organic solvent can improve the low temperature discharge capacity of the battery, because the silicon-containing organic solvent can reduce the viscosity of the electrolyte. The results of Examples 10 and 11 show that when the content of additive A or additive B exceeds 3%, too much additive A and additive B increase the electrode interface impedance and deteriorate the low temperature performance, and when the content of the silicon-containing organic solvent and the fluorinated solvent exceeds 8%, the viscosity of the electrolyte is also increased, which affects the performance of other solvents and deteriorates the low temperature performance.

[0100] (5) According to the comparative examples and examples, the combination of the four kinds of additives, the silicon-containing organic solvent and the fluorinated solvent system can most significantly improve the performance of the battery.

[0101] The above is a further detailed description of the present application in combination with specific optional embodiments, which cannot be considered as limiting the specific implementation of the present application to these descriptions. For ordinary skilled persons in the technical field to which the present application belongs, without departing from the concept of the present application, a number of simple deductions or substitutions can be made, which should be considered as falling within the protection scope of the present application.

Claims

1. An electrolyte, characterized by, The electrolyte comprises lithium salt, organic solvent and additive; the additive comprises additive B or a combination of additive B and at least one of additive A, additive C and film forming additive D; The additive A, additive B and additive C each accounts for 0.1-3wt% of the total mass of the electrolyte; the additive A is a silicon-based pyrophosphoric acid ester compound of general structure of formula I, the additive B is a lithium salt of bis-trifluoromethyl pyroboric acid of general structure of formula II, and the additive C is a cyclic pyrocarbonate compound of general structure of formula III: In Equation I, R1-R 12 The group is selected from alkane groups, trifluoromethyl groups, and nitrile groups with 1-5 carbon atoms; in Formula II, X1, X2, X3, and X4 are selected from alkane groups, alkenyl groups, alkynyl groups, and trifluoromethyl groups with 1-5 carbon atoms; in Formula II, Y is selected from alkane groups, alkenyl groups, alkynyl groups, fluorine atoms, hydrogen atoms, and alkane groups, alkenyl groups, and alkynyl groups with 1-5 carbon atoms that are substituted with fluorine atoms. The film forming additive D is selected from two or more of fluorinated ethylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, methane dithioformate, propene sultone, butanedinitrile, hexanedinitrile, 1,3,6-hexanetricarbonitrile, lithium difluorophosphate, lithium difluoro(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, lithium bisoxalate borate and lithium difluoro oxalate borate, at least one of which is fluorinated ethylene carbonate; The content of the fluorinated ethylene carbonate in the film forming additive D accounts for 0.5-20wt% of the total mass of the electrolyte, and the content of other components accounts for 0.1-8wt% of the total mass of the electrolyte.

2. The electrolyte of claim 1, wherein The organic solvent comprises one or more of chain carbonate organic solvent, cyclic carbonate organic solvent, carboxylic acid ester organic solvent, silicon-based organic solvent and double-fluorinated organic solvent.

3. The electrolyte of claim 2, wherein The organic solvent comprises silicon-based organic solvent and double-fluorinated organic solvent.

4. The electrolyte of claim 3, wherein The silicon-based organic solvent has general structure of formula IV and / or V, and the double-fluorinated organic solvent has general structure of formula VI and / or VII: wherein R1, R2, R3 and R4 are selected from siloxane with 1-5 carbon atoms, and X1 and X2 are selected from alkyl with 1-5 carbon atoms, which is substituted by two fluorine atoms on the same methyl group.

5. The electrolyte of claim 1, wherein The additive A has at least one of the following specific structural formulae:

6. The electrolyte of claim 1, wherein The additive B has at least one of the following specific structural formulae:

7. The electrolyte of claim 1, wherein The additive C has at least one of the following specific structural formulae:

8. The electrolyte of claim 2, wherein The organic solvent is selected from three or more of vinyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, tetrahydrofuran, silicon-based organic solvent and double-fluorinated organic solvent, at least two of which are silicon-based organic solvent and double-fluorinated solvent.

9. The electrolyte of claim 4, wherein The silicon-based organic solvent has at least one of the following specific structural formulae: The double-fluorinated organic solvent has at least one of the following specific structural formulae:

10. A lithium-ion battery, characterized by, The electrolyte comprises the electrolyte according to any one of claims 1-9.

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