Electrolyte and battery comprising same

Abstract

The present invention provides an electrolyte and a battery comprising same. The electrolyte of the present invention comprises a solvent, a lithium salt, and additives. The additives comprise additive A, additive B, and additive C. Additive A is selected from fluoroethylene carbonate, additive B is selected from a silane borate compound represented by formula I, and additive C is selected from a multi-membered cyclic ester compound represented by formula II. By means of synergy among additive A, additive B, and additive C, the electrolyte of the present invention forms on the surface of a negative electrode an interfacial protective film having excellent thermal stability, high density, good flexibility, good mechanical strength, and low impedance. The protective film can enable free deintercalation of lithium ions while effectively protecting the electrode from side reactions with the electrolyte, thereby enabling the battery to have excellent room-temperature and high-temperature cycle performance, high-temperature storage performance, and low-temperature discharge performance.

Description

Electrolyte and battery comprising the same

[0001] This application claims priority to the Chinese patent application No. 202411110414.X, filed on August 14, 2024, and entitled "Electrolyte and battery comprising the same", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application belongs to the technical field of lithium ion batteries, and relates to an electrolyte and a battery comprising the same. BACKGROUND

[0003] With the progress of science and technology and the diversification of energy demand, traditional energy is being consumed at an accelerated rate, and countries are accelerating their strategic deployment of new energy technologies. Power batteries are rising with the development of new energy vehicles, and energy storage batteries are also emerging with the development of clean energy. Lithium ion batteries have the advantages of green environmental protection, good cycle performance, no memory effect, etc., and have been widely used in the fields of electronic digital, energy storage, electric vehicles, communication equipment, etc.

[0004] Carbon-based materials and silicon-based materials are the two most commonly used negative electrode materials at present, and they play a crucial role in improving the application of lithium ion batteries. However, the performance of these two materials is highly dependent on the properties of the interface film on their surface. In particular, the volume expansion problem of silicon-based materials is more prominent, and they are more dependent on the interface film.

[0005] To solve the above problems, researchers have constructed a stable interface film on the electrode surface by adding a new type of functional additive to the electrolyte to protect the negative electrode material and reduce the side reactions on the electrode surface. However, the existing functional additives have limited improvement on the electrode surface side reactions, and the electrochemical performance of the battery is still not ideal when working in high and low temperature environments. SUMMARY

[0006] The present application provides an electrolyte and a battery comprising the same. The electrolyte of the present application can form an interface protection film with excellent thermal stability, high density, good flexibility, good mechanical strength and low impedance on the negative electrode surface by using fluoroethylene carbonate (FEC), silane borate compounds represented by formula I and multi-ring ester compounds represented by formula II as additives. The protection film can effectively protect the electrode from side reactions with the electrolyte, while allowing lithium ions to be freely deintercalated, thereby making the battery have excellent room temperature and high temperature cycle performance, high temperature storage performance and low temperature discharge performance.

[0007] The first aspect of the present application provides an electrolyte, comprising a solvent, a lithium salt and an additive, the additive comprising an additive A, an additive B and an additive C; the additive A is selected from fluoroethylene carbonate, the additive B is selected from a compound shown in formula I, and the additive C is selected from a compound shown in formula II:

[0008] A-L1-B-L2-C formula II;

[0009] In formula I, R1 to R9 are each independently selected from halogen-substituted or unsubstituted C1-C8 alkyl, halogen-substituted or unsubstituted C2-C8 alkenyl, halogen-substituted or unsubstituted C2-C8 alkynyl, halogen or cyano;

[0010] In formula II, A, B, C are each independently selected from H, halogen-substituted or unsubstituted five-membered or six-membered cyclic carbonate, halogen-substituted or unsubstituted five-membered or six-membered cyclic sulfate, halogen-substituted or unsubstituted five-membered or six-membered cyclic sulfonate, halogen-substituted or unsubstituted five-membered or six-membered cyclic carboxylate, and at most one of A, B, C is selected from H, and at least one of A, B, C is selected from halogen-substituted or unsubstituted five-membered or six-membered cyclic carbonate; L1 and L2 are each independently selected from a single bond, halogen-substituted or unsubstituted C1-C8 alkylene, halogen-substituted or unsubstituted C1-C8 alkenylene or an ether bond.

[0011] In the present application, C1-C8 alkyl refers to a saturated aliphatic hydrocarbon group with a carbon atom number of 1-8, which can be a straight-chain alkyl or a branched-chain alkyl, including but not limited to methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, n-pentyl, n-butyl, etc.

[0012] C2-C8 alkenyl refers to an aliphatic hydrocarbon group containing an unsaturated carbon-carbon double bond with a carbon atom number of 2-8, which can be a straight-chain alkenyl or a branched-chain alkenyl, including but not limited to vinyl, propenyl, n-butenyl, isobutenyl, etc.

[0013] C2-C8 alkynyl refers to an aliphatic hydrocarbon group containing an unsaturated carbon-carbon triple bond with a carbon atom number of 2-8, which can be a straight-chain alkynyl or a branched-chain alkynyl, including but not limited to ethynyl, propynyl, butynyl, isobutynyl, etc.

[0014] C1-C8 alkylene refers to a divalent group obtained by losing two H from an alkane molecule with a carbon atom number of 1-8, including but not limited to methylene, ethylene, propylene, etc.

[0015] C2-C8 alkenylene refers to a divalent group obtained by losing two H from an alkene molecule with a carbon atom number of 1-8, including but not limited to vinylene, propenylene, butenylene, etc.

[0016] For the case that the above substituent is substituted by halogen, taking halogen-substituted C1-C8 alkyl as an example, the halogen-substituted C1-C8 alkyl refers to that the H atom on the alkyl group is further substituted by halogen, including but not limited to fluoromethyl, difluoromethyl, trifluoromethyl, chloromethyl, bromomethyl, etc.

[0017] Halogen includes F, Cl, Br and I.

[0018] The electrolyte of the application at least includes three types of additives, namely, additive A, additive B and additive C. The FEC as the additive A can form an interface protection film including polyvinylidene carbonate and LiF components on the surface of the negative electrode. The polyvinylidene carbonate has good elasticity and flexibility and can inhibit the expansion of the negative electrode material during the cycle process. The LiF can enhance the mechanical strength of the interface protection film. However, the interface protection film is unstable and easy to be broken down at high temperature, which leads to the oxidation-reduction side reaction of the negative electrode material and the electrolyte, serious gas production, and further poor cycle performance and serious capacity attenuation of the battery.

[0019] The Si-O covalent bond in the silane borate additive B shown in formula I has very strong stability and is not easy to be broken down at high temperature. The additive B is adsorbed on the surface of the interface protection film formed by the FEC, which improves the high-temperature stability of the interface protection film, overcomes the defect of easy decomposition at high temperature caused by the FEC alone, avoids the problem of poor cycle performance and serious capacity attenuation of the battery at high temperature, and further has the following beneficial effects after the addition of the additive B:

[0020] On the one hand, the electron-deficient B atom in the additive B can complex the transition metal ions dissolved out of the positive electrode during the charging and discharging process, and inhibit the oxidation and decomposition of the electrolyte by the transition metal ions.

[0021] On the other hand, the electron-deficient B atom in the additive B can also adsorb the HF with extremely strong corrosion in the electrolyte, and avoid the damage of the HF to the electrode material.

[0022] On the other hand, the electron-deficient B atom in the additive B can also adsorb the HF with extremely strong corrosion in the electrolyte, and avoid the damage of the HF to the electrode material.

[0023] However, the silane structure in the additive B can eliminate the LiF in the interface protection film, resulting in a decrease in the mechanical strength of the interface protection film. The cyclic ester compound shown in Formula II includes at least two rings, and at least one ring is a cyclic carbonate. Compared with a monocyclic component, the cyclic ester compound shown in Formula II has stronger film-forming ability, can form a more comprehensive interface protection film with higher coverage density on the electrode surface, makes up for the defects brought by the additive B, and because the composition of the compound shown in Formula II includes a cyclic carbonate component, the film formed thereby has good compatibility with the interface protection film formed by FEC, further inhibits the decomposition of the electrolyte on the electrode surface, and enables the battery to have good cycle performance.

[0024] In summary, the electrolyte of the present application can form an interface protection film with excellent thermal stability, good mechanical strength and low impedance on the negative electrode surface through the synergistic effect of the additive A, the additive B and the additive C. The protection film can effectively protect the electrode from side reactions with the electrolyte, and also enables the lithium ions to be freely deintercalated, thereby reducing the migration resistance of the lithium ions, and further enabling the battery to have excellent normal temperature and high temperature cycle performance, high temperature storage performance and low temperature discharge performance.

[0025] In a preferred embodiment, the additive B is selected from one or more of the following compounds:

[0026] The silane borate compound with the above structure not only enables the battery to have good high and low temperature cycle performance, but also has the advantage of being easy to prepare. Among them, the compounds shown in Formula Ib, Formula Ic, Formula Id, Formula Ie and Formula If have unsaturated carbon-carbon double bonds and carbon-carbon triple bonds, which can be polymerized to form a polymer to form a more comprehensive wrapping on the surface of the interface protection film formed by FEC, further improving the stability of the interface protection film.

[0027] In a preferred embodiment, the additive C is selected from one or more of the following compounds:

[0028] When the content of the additive A, the additive B and the additive C in the electrolyte is too low, it is difficult to form a dense and stable interface protection film, and when the content is too high, it can cause the thickness of the interface protection film to grow excessively, resulting in an increase in the internal impedance of the battery. Based on the synergistic effect of the three components and in order to enable the battery to have lower impedance and form a more dense and stable interface protection film, the content of the additives A, B and C needs to be controlled within a suitable range.

[0029] In a preferred embodiment, the mass content of the additive B is 0.1wt% to 1.5wt% based on the total mass of the electrolyte. For example, the mass content of the additive B can be 0.1wt%, 0.3wt%, 0.5wt%, 0.7wt%, 0.9wt%, 1.1wt%, 1.3wt%, 1.5wt% or a range between any two of the above values based on the total mass of the electrolyte.

[0030] In a preferred embodiment, the mass content of the additive C is 0.1wt% to 1.5wt% based on the total mass of the electrolyte. For example, the mass content of the additive C can be 0.1wt%, 0.3wt%, 0.5wt%, 0.7wt%, 0.9wt%, 1.1wt%, 1.3wt%, 1.5wt% or a range between any two of the above values based on the total mass of the electrolyte.

[0031] In a preferred embodiment, the mass content of the additive A is 1wt% to 15wt% based on the total mass of the electrolyte. For example, the mass content of the additive A can be 1wt%, 3wt%, 5wt%, 7wt%, 9wt%, 11wt%, 13wt%, 15wt% or a range between any two of the above values based on the total mass of the electrolyte.

[0032] Further, the mass content of the additive A is not less than 20 times of the mass content of the additive B.

[0033] The silane structure in the additive B can eliminate LiF in the interface protection film. By limiting the mass content of the additive A and the additive B within the above ratio range, the problem of mechanical strength reduction of the interface protection film caused by the elimination of LiF component in the interface protection film formed by FEC can be avoided.

[0034] In a specific embodiment, the electrolyte of the present application further comprises an additive D selected from one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), diethyl sulfate (DTD), 1,3-propane sultone (PS), 1,3-propene sultone (PST), ethylene sulfite (ES), tris(trimethylsilyl) phosphate. The additive D is a functional additive commonly used in electrolyte, and the addition of the additive D to the electrolyte can further improve the cycle performance of the battery.

[0035] In a preferred embodiment, the mass content of the additive D is 0.5wt% to 5wt% based on the total mass of the electrolyte. Controlling the mass content of the additive D within the above range can avoid the problem of increased battery impedance caused by the excessive growth of the solid electrolyte interface film (SEI) caused by the excessive addition of the additive D. Exemplarily, the mass content of the additive D can be 0.5wt%, 1.0wt%, 1.5wt%, 2.0wt%, 2.5wt%, 3.0wt%, 3.5wt%, 4.0wt%, 4.5wt%, 5.0wt% or a range between any two of the above values.

[0036] The present application does not make specific limitation to the type of lithium salt in the electrolyte, which can be selected from the lithium salts commonly used in electrolytes, including but not limited to one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium bisoxalate borate, lithium difluoro oxalate borate, lithium difluoro di-oxalate phosphate, preferably lithium hexafluorophosphate.

[0037] The present application also does not make specific limitation to the type of solvent in the electrolyte, which can be selected from the solvents commonly used in electrolytes, including but not limited to one or more of propylene carbonate, ethyl methyl carbonate (EMC), ethylene carbonate (EC), dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone, sulfolane, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate.

[0038] The present application does not make specific limitation to the content of lithium salt in the electrolyte, as long as it can be completely dissolved in the electrolyte without easy precipitation and make the electrolyte have a suitable lithium ion concentration, which can refer to the conventional content of lithium salt in the electrolyte. In a specific embodiment, the mass content of the lithium salt is 12wt% to 15wt% based on the total mass of the electrolyte. Exemplarily, the mass content of the lithium salt can be 12wt%, 13wt%, 14wt%, 15wt% or a range between any two of the above values.

[0039] The present application does not make specific limitation to the content of solvent in the electrolyte, as long as it can completely dissolve the lithium salt and make the electrolyte have a high ionic conductivity, which can refer to the conventional content of solvent in the electrolyte. In a specific embodiment, the mass content of the solvent is 10wt% to 80wt% based on the total mass of the electrolyte. Exemplarily, the mass content of the solvent can be 10wt%, 20wt%, 30wt%, 40wt%, 50wt%, 60wt%, 70wt%, 80wt% or a range between any two of the above values.

[0040] The second aspect of the present application provides a battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the electrolyte is the electrolyte provided by the first aspect of the present application.

[0041] The battery of the present application has low internal impedance and good high-low temperature cycle performance due to the electrolyte capable of forming an interface protection film with excellent stability and compactness on the surface of the electrode, and the electrolyte itself having good oxidation resistance.

[0042] In a preferred embodiment, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer on one side or both sides of the negative electrode current collector, and the negative electrode active material layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material and / or a carbon-based material.

[0043] The present application does not make specific limitations on the type of silicon-based material, which can be silicon-carbon material, silicon-oxygen material, nano-silicon, silicon alloy, etc.

[0044] The present application also does not make specific limitations on the type of carbon-based material, which can be one or more of graphite, hard carbon, soft carbon, and mesocarbon microbeads.

[0045] In a preferred embodiment, the negative electrode active material comprises a silicon-based material, and the content of the silicon-based material in the negative electrode active material is 5wt%-50wt%.

[0046] The silicon-based material can make the negative electrode sheet have higher gram capacity, but it is prone to swelling. Controlling the content of the silicon-based material in the negative electrode active material within the above range can have both smaller swelling degree and higher gram capacity.

[0047] Further, the mass content of fluoroethylene carbonate in the electrolyte is 5wt%-15wt%. FEC in the above higher content range can effectively form a film to inhibit the swelling of the silicon-based material, and after the additive B and the additive C are compounded with the FEC in the above content range, the gas production phenomenon caused by excessive FEC content in the battery can be significantly inhibited, so that the battery has excellent high-low temperature cycle performance and storage performance.

[0048] The present application does not make specific limitations on the type of negative electrode current collector, which can be selected from conventional negative electrode current collectors used in the art, such as copper foil.

[0049] The negative electrode active material layer mainly comprises a negative electrode active material, and also comprises conventional components such as binder and conductive agent.

[0050] The present application does not make specific limitations on the conductive agent in the negative electrode sheet, which can be selected from conventional conductive agents used in the art, including but not limited to one or more of acetylene black, conductive carbon black, ketjen black, conductive graphite, carbon nanotube, conductive carbon fiber, and graphene.

[0051] The binder in the negative electrode sheet is not particularly limited, and can be selected from the binders conventionally used in the art, including but not limited to one or more of polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, butadiene rubber, and polyethylene oxide.

[0052] The negative electrode sheet of the present application can be prepared by conventional methods in the art, for example, by dispersing the negative electrode active material, the conductive agent, and the binder in a solvent to form a negative electrode active slurry, and then coating the negative electrode active slurry on a negative electrode current collector, and drying to obtain the negative electrode sheet.

[0053] To improve the energy density of the battery, the negative electrode sheet can have a higher compaction density so that the negative electrode sheet has a higher gram capacity. In a preferred embodiment, the compaction density of the negative electrode sheet is 1 g / cm 3 ~ 4 g / cm 3 . For example, the compaction density of the negative electrode sheet can be 1 g / cm 3 , 2 g / cm 3 , 3 g / cm 3 , 4 g / cm 3 , or a range defined by any two of the above values.

[0054] The positive electrode sheet of the present application comprises a current collector and a positive electrode active material layer disposed on the surface of the current collector, wherein the positive electrode active material layer is mainly composed of a positive electrode active material, and further comprises conventional components such as a binder and a conductive agent.

[0055] The type of the positive electrode active material is not particularly limited, and can be selected from the positive electrode active materials conventionally used in the art, including but not limited to one or more of lithium cobaltate, lithium manganate, lithium nickel cobalt manganate ternary material, lithium nickel manganate, and lithium-rich manganese-based material. Preferably, the lithium nickel cobalt manganate ternary material.

[0056] The type of the conductive agent and the binder in the positive electrode sheet is not particularly limited, and the selection range can refer to the type of the conductive agent and the binder in the negative electrode sheet, which is not described herein again.

[0057] The current collector in the positive electrode sheet is not particularly limited, and can be selected from the positive electrode current collectors conventionally used in the art, such as aluminum foil.

[0058] The positive electrode sheet of the present application can be prepared by conventional methods in the art, for example, by dispersing the positive electrode active material, the conductive agent, and the binder in a solvent to obtain a positive electrode active material layer slurry, and then coating the slurry on a positive electrode current collector, and drying to obtain the positive electrode sheet.

[0059] To improve the energy density of the battery, the positive electrode sheet can have a higher compaction density so that the positive electrode sheet has a higher gram capacity. In a preferred embodiment, the compaction density of the positive electrode sheet is 2 g / cm 3 ~ 5 g / cm 3 For example, the compaction density of the positive electrode sheet can be 2 g / cm 3 , 2.5 g / cm 3 , 3 g / cm 3 , 3.5 g / cm 3 , 4 g / cm 3 , 4.5 g / cm 3 , 5 g / cm 3 or a range defined by any two of the above values.

[0060] The function of the separator is to separate the positive and negative electrode sheets and provide a channel for the migration of lithium ions. The type of separator is not specifically limited in the present application and can be selected from commonly used separators in the art, such as polypropylene separators, polyethylene separators, etc.

[0061] The preparation method of the lithium ion battery is not specifically limited in the present application and the lithium ion battery can be prepared by using a conventional preparation method in the art. For example, the positive electrode sheet, the separator and the negative electrode sheet are sequentially stacked and placed with the separator between the positive electrode sheet and the negative electrode sheet, and an electric core is obtained by a stacking or winding process, and then the lithium ion battery of the present application can be obtained by going through processes such as baking, liquid injection, formation, packaging, etc.

[0062] The implementation of the present application has at least the following beneficial effects:

[0063] The electrolyte provided by the present application can form an interface protection film on the negative electrode surface by compounding FEC, a silane borate additive represented by Formula I and a multi-ring cyclic ester additive represented by Formula II, which has excellent thermal stability, high density, good flexibility, good mechanical strength and low impedance. The protection film can effectively inhibit the expansion of the negative electrode material during the cycle process, can also avoid the capacity attenuation and gas production problems of the battery caused by the side reaction of the negative electrode material and the electrolyte, and can also make the lithium ions free to deintercalate, reduce the migration resistance of the lithium ions, and thus make the battery have excellent room temperature and high temperature cycle performance, high temperature storage performance and low temperature discharge performance. DETAILED DESCRIPTION

[0064] To make the purpose, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0065] The electrolyte and the battery provided by the application will be further described in detail through specific examples.

[0066] Unless otherwise specified, the reagents, materials and instruments used in the following examples are conventional reagents, conventional materials and conventional instruments in the art, which can be obtained by commercial purchase, and the reagents involved can also be obtained by conventional methods in the art.

[0067] The electrolyte and the lithium ion battery of Examples 1-37 and Comparative Examples 1-8 of the application are prepared according to the following methods:

[0068] 1. Preparation of electrolyte

[0069] In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), EC, DEC and EMC were mixed uniformly in a mass ratio of 3:5:2, and then additives and LiPF6 were added. The mass content of LiPF6 was 14.5wt% based on the total mass of the electrolyte, and the types of additives and their mass contents in the electrolyte are listed in Table 1.

[0070] 2. Preparation of positive electrode sheet

[0071] The positive electrode active material lithium nickel cobalt manganese oxide (NCM811), the binder polyvinylidene fluoride (PVDF) and the conductive agent acetylene black were mixed in a weight ratio of 96.5:2:1.5, N-methyl pyrrolidone (NMP) was added, and the mixture was stirred under the action of a vacuum stirrer until the mixture became a uniform positive electrode slurry; the positive electrode slurry was uniformly coated on an aluminum foil with a thickness of 7μm; the coated aluminum foil was baked in an oven with three different temperature gradients (90℃, 100℃, 85℃), and then dried in an oven at 120℃ for 8h, followed by rolling, to control the positive electrode sheet compaction density at 3.5g / cm 3 , and the positive electrode sheet was obtained by slitting.

[0072] 3. Preparation of negative electrode sheet

[0073] The mixture of the negative electrode active material graphite and silicon monoxide (mass ratio of graphite to silicon monoxide was 4:1), the thickening agent sodium carboxymethyl cellulose (CMC-Na), the binder styrene-butadiene rubber, the conductive agent acetylene black and the conductive agent single-walled carbon nanotube (SWCNT) were mixed in a weight ratio of 95.9:1:2:1:0.1, deionized water was added, and the negative electrode slurry was obtained under the action of a vacuum stirrer; the negative electrode slurry was uniformly coated on a copper foil with a thickness of 6μm; after drying (temperature: 85℃, time: 5h) and rolling, the negative electrode sheet compaction density was controlled at 1.65g / cm 3 , and the negative electrode sheet was obtained by die cutting.

[0074] In the negative active material of Examples 1 to 36 and Comparative Examples 1 to 7, a mixture of graphite and silicon monoxide (mass ratio of graphite to silicon monoxide: 4:1) was used. In the negative active material of Example 37 and Comparative Example 8, graphite was used.

[0075] 4. Preparation of the separator

[0076] A polyethylene separator having a thickness of 8 μm was selected.

[0077] 5. Preparation of the lithium ion battery

[0078] The above positive electrode sheet, the separator, and the negative electrode sheet were sequentially stacked and wound to obtain a bare cell without electrolyte injection. The bare cell was placed in an outer packaging foil, and the above prepared electrolyte was injected into the dried bare cell. The lithium ion battery was obtained by performing processes such as vacuum packaging, standing, formation, shaping, and sorting.

[0079] Table 1

[0080] In Table 1, the compound having a CAS number of 1431298-10-0 has the following structural formula:

[0081] Test Example

[0082] The lithium ion battery prepared above was subjected to the following performance tests:

[0083] 1. Capacity retention rate at 25°C for 500 cycles

[0084] Test method: At 25°C, the battery was charged at 1C to a cutoff voltage of 4.2V, and then charged at 4.2V to a cutoff current of 0.05C. Then, the battery was discharged at 1C to 2.75V. The above processes were repeated for 500 cycles. The discharge capacity Q1 of the first cycle and the discharge capacity Q500 of the 500th cycle were recorded. The capacity retention rate at 25°C for 500 cycles was calculated by Q500 / Q1 x 100%.

[0085] 2. Capacity retention rate at 45°C for 200 cycles

[0086] Test method: At 45°C, the battery was charged at 1C to a cutoff voltage of 4.2V, and then charged at 4.2V to a cutoff current of 0.05C. Then, the battery was discharged at 1C to 2.75V. The above processes were repeated for 200 cycles. The discharge capacity Q1 of the first cycle and the discharge capacity Q200 of the 200th cycle were recorded. The capacity retention rate at 45°C for 200 cycles was calculated by Q200 / Q1 x 100%.

[0087] 3. High-temperature storage performance

[0088] Test method: at 25℃, the battery is charged at 1C constant current to 4.2V, then charged at 4.2V constant voltage to 0.05C, then discharged at 0.5C constant current to 2.75V, the discharge capacity is recorded as C2, then charged at 1C constant current to 4.2V, charged at 4.2V constant voltage to 0.05C, then the battery is transferred to high temperature 60℃ for 7 days, then discharged at 0.5C constant current, the discharge capacity is recorded as C3, the capacity retention rate of the battery after 7 days storage at 60℃ is calculated by C3 / C2x100%.

[0089] The initial thickness of the battery after sorting is recorded as h1, and the thickness after 7 days storage at high temperature 60℃ is h2, the thickness expansion rate of the battery after 7 days storage at 60℃ is calculated by (h2-h1) / h1x100%.

[0090] 4. DC internal resistance (DCIR) growth rate

[0091] Test method: at 25℃, the battery is charged at 1C constant current to 4.2V, then discharged at 1C for 30min, after 1h, then discharged at 2C (I 2C ) for 10s, record the voltage values V1 and V2 before and after 2C discharge, calculate the DCIR1 of the battery at 50% SOC (the calculation formula of DCIR1 is: DCIR1=(V1-V2) / I 2C ). After 7 days storage at 60℃, after 3 weeks of charge and discharge at 1C constant current (charged at 1C constant current to 4.2V, then discharged at 1C for 30min, after 1h, this is 1 week), the battery is charged at 1C constant current to 4.2V, then discharged at 1C for 30min, after 1h, then discharged at 2C (I 2C ) for 10s, record the voltage values V3 and V4 before and after 2C discharge, calculate the DCIR2 of the battery at 50% SOC (the calculation formula of DCIR2 is: DCIR1=(V3-V4) / I 2C ). DC internal resistance (DCIR) growth rate=(DCIR2-DCIR1) / DCIR1x100%.

[0092] 5. Low temperature discharge performance test

[0093] Test method: at 25℃, the battery is charged at 1C constant current to 4.2V, to 0.05C constant current, discharged at 1C constant current to 2.75V, record the discharge capacity as Q1. At 25℃, the battery is charged at 1C constant current to 4.2V, to 0.05C constant current, then discharged at 1C to 2.75V at-20℃, record the discharge capacity Q2. The discharge capacity retention rate at-20℃=Q2 / Q1x100%.

[0094] The test results of the above properties are shown in Table 2.

[0095] Table 2

[0096] From the data in Table 2, the following conclusions can be drawn:

[0097] 1) According to the comparison of Examples 1-6 and Comparative Example 4, it can be seen that when FEC is not added, the cycle performance of the battery at 25°C and 45°C and the discharge capacity retention at -20°C are significantly poorer. After the addition of FEC, an SEI film can be formed on the surface of the negative electrode, thereby helping to improve the cycle performance of the battery at high and low temperatures. However, when the content of FEC is too low, the film formation quality is poor, and the room temperature and high temperature cycle performance is relatively poor. When the content of FEC is too high, it is difficult to effectively inhibit the gas generation of the battery, resulting in a significant increase in the thickness expansion rate and DCIR of the battery during storage at 60°C, and the cycle performance of the battery at 45°C is significantly deteriorated.

[0098] 2) From the comparison of Examples 4, 7-11 and Comparative Examples 1 and 3, it can be seen that the silane borate additives B with different substituents can make the battery have excellent 25°C and 45°C cycle performance, low temperature discharge performance, and overall have a low 60°C storage thickness expansion rate, DCIR growth rate, and low temperature discharge capacity retention. When tri-tert-butyl borate is used as additive B without a silane structure, the stability of the SEI film at high temperature cannot be improved, leading to gas generation and a serious increase in DCIR during decomposition at high temperature, and the cycle performance at 45°C and the storage performance at 60°C are significantly poorer. When no additive B is contained, the battery not only has serious gas generation leading to significant deterioration of the high temperature performance and storage performance, but also has a significant decrease in low temperature discharge performance.

[0099] 3) From the comparison of Examples 4, 12-15, it can be seen that when the amount of silane borate additive B is too small, it is difficult to effectively inhibit the expansion of the negative electrode material, and the battery impedance is large, resulting in poor high temperature performance and low temperature discharge performance of the battery. When the amount of silane borate additive B is too large, the battery impedance is significantly increased, and the 25°C and 45°C cycle performance of the battery is significantly decreased. In addition, during high temperature storage, the interface film constructed by the additive B can well inhibit the side reactions, and thus still has a good storage capacity retention rate, a low gas generation amount, and a low DCIR growth rate.

[0100] 4) From the comparison of Example 4, 16-23 and Comparative Examples 2, 5, 6, it can be seen that when no additive C is added, the cycle performance of the battery at 25℃ and 45℃ is relatively poor, and the use of additive C with different types of substituents can make the battery have excellent high and low temperature cycle performance, high temperature storage performance and low temperature discharge performance. However, when using a single ring of vinyl sulfate or a double ring of vinyl sulfate, it is difficult to effectively inhibit the expansion of the negative electrode material, and the 25℃ and 45℃ cycle performance of the battery is relatively poor.

[0101] 5) From the comparison of Example 4, 24-27, it can be seen that the content of additive C should not be too high, and when the content of additive C increases, the impedance of the battery increases significantly, resulting in certain deterioration of the room temperature and high temperature cycle performance and low temperature discharge performance.

[0102] 6) From the comparison of Example 4, 28-36, it can be seen that when a suitable amount of additive D with film-forming function such as VC, VEC, DTD, PS, PST, ES, etc. is further added on the basis of additives A, B and C, the cycle performance of the battery at 25℃ and 45℃, the storage performance at 60℃ and the low temperature discharge performance can be improved, and from the comparison of Example 28 and Example 34-36, it can be seen that when the content of additive D is too high, the internal resistance of the battery increases significantly, resulting in poor cycle performance at room temperature and high temperature. The reason may be that when the content of additive D increases, the SEI film grows excessively, resulting in increased impedance.

[0103] 7) From the comparison of Example 4 and Comparative Example 7, it can be seen that when the electrolyte only includes 10% FEC as an additive, the battery has a serious gas generation phenomenon, the thickness expansion rate at 60℃ and the DCIR increase significantly, and the cycle performance of the battery at 25℃ and 45℃ and the low temperature discharge performance at -20℃ are significantly deteriorated.

[0104] 8) From the comparison of Example 37 and Comparative Example 8, it can be seen that when the negative electrode active material is graphite, the use of FEC in combination with additive B and additive C (Example 37) can make the battery have excellent cycle performance at 25℃ and 45℃, storage performance at 60℃ and low temperature discharge performance at -20℃.

[0105] The above examples are only used to illustrate the technical solutions of the present application, and are not limiting; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing examples, or make equivalent substitutions for part or all of the technical features; and these modifications or substitutions do not make the essence of the corresponding technical solution deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. An electrolyte comprising a solvent, a lithium salt and an additive, characterized in that, The additives include an additive A, an additive B, and an additive C; the additive A is selected from fluoroethylene carbonate, the additive B is selected from a compound shown in formula I, and the additive C is selected from a compound shown in formula II: In formula I, R1 to R9 are each independently selected from halogen-substituted or unsubstituted C1-C8 alkyl, halogen-substituted or unsubstituted C2-C8 alkenyl, halogen-substituted or unsubstituted C2-C8 alkynyl, halogen or cyano; In formula II, A, B, C are each independently selected from H, halogen-substituted or unsubstituted five- or six-membered cyclic carbonate, halogen-substituted or unsubstituted five- or six-membered cyclic sulfate, halogen-substituted or unsubstituted five- or six-membered cyclic sulfonate, halogen-substituted or unsubstituted five- or six-membered cyclic carboxylate, and at most one of A, B, C is selected from H, and at least one of A, B, C is selected from halogen-substituted or unsubstituted five- or six-membered cyclic carbonate; L1 and L2 are each independently selected from a single bond, halogen-substituted or unsubstituted C1-C8 alkylene, halogen-substituted or unsubstituted C2-C8 alkenylene, or an ether bond.

2. The electrolyte according to claim 1, characterized in that, The additive B is selected from one or more of the following compounds:

3. The electrolyte of claim 1, wherein The additive C is selected from one or more of the following compounds:

4. The electrolyte according to any one of claims 1 to 3, characterized in that, The mass content of the additive B is 0.1wt%-1.5wt% based on the total mass of the electrolyte; And / or, the mass content of the additive C is 0.1wt%-1.5wt% based on the total mass of the electrolyte; And / or, the mass content of the additive A is 1wt%-15wt% based on the total mass of the electrolyte.

5. The electrolyte according to claim 4, characterized in that, The mass content of the additive A is not less than 20 times of the mass content of the additive B.

6. The electrolyte according to any one of claims 1 to 3, characterized in that, The electrolyte further comprises an additive D, the additive D is selected from one or more of vinylene carbonate, vinyl ethylene carbonate, ethylene sulfate, 1,3-propane sulfonate lactone, 1,3-propylene sulfonate lactone, vinyl sulfite, tris(trimethylsilyl)phosphate.

7. The electrolyte according to claim 6, characterized in that The mass content of the additive D is 0.5wt%-5wt% based on the total mass of the electrolyte.

8. The electrolyte of claim 1, wherein, The lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bisfluorosulfonimide, lithium bis-trifluoromethanesulfonimide, lithium bisoxalate borate, lithium difluorooxalate borate, lithium difluorodioxalate phosphate; And / or, the solvent is selected from one or more of propylene carbonate, methyl ethyl carbonate, vinyl carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate.

9. The electrolyte of claim 8, wherein, The mass content of the lithium salt is 12wt%-15wt% based on the total mass of the electrolyte; And / or, the mass content of the solvent is 10wt%-80wt% based on the total mass of the electrolyte.

10. A battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, characterized by The electrolyte is the electrolyte according to any one of claims 1-9.

11. The battery of claim 10, wherein, The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer located on one side or both sides of the negative electrode current collector, the negative electrode active material layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material and / or a carbon-based material.

12. The battery of claim 11, wherein, The negative electrode active material comprises a silicon-based material, and the content of the silicon-based material is 5wt%-50wt%. The negative electrode active material comprises a silicon-based material, and the content of the silicon-based material is 5wt%-50wt%.

13. The battery according to claim 11 or 12, characterized in that The mass content of fluoroethylene carbonate in the electrolyte is 5wt%-15wt%. The mass content of fluoroethylene carbonate in the electrolyte is 5wt%-15wt%.

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