Electrolyte and secondary battery

Abstract

The invention provides an electrolyte and a secondary battery, and belongs to the technical field of secondary batteries, the electrolyte comprises a lithium salt, a solvent, a first additive and a second additive, the first additive comprises fluoroethylene carbonate; and the second additive comprises a compound shown as a formula (1): # imgabs0 #, wherein R1 and R2 in the formula (1) are independently selected from substituent groups with the carbon atom number of 1-6, the unsaturation degree of 0-4 and the heteroatom number of 0-3. According to the electrolyte provided by the invention, an orderly-arranged, compact and stable SEI film is further formed by utilizing a copolymerization reaction between FEC and the compound as shown in the formula (1), so that side reaction between the electrolyte and a negative electrode can be further prevented on the premise of not increasing impedance, the cycling stability of the electrolyte is improved, and the cycling performance of the battery is improved; meanwhile, in the high-temperature storage process, the acidity increase caused by FEC decomposition is effectively relieved through the excellent acidity inhibition capability of the compound shown in the formula (1), so that the high-temperature storage performance of the battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to an electrolyte and a secondary battery. Background Art

[0002] Lithium-ion batteries are widely used in consumer electronics, power vehicles and energy storage due to their long cycle life, high operating voltage, low self-discharge rate, no memory effect and environmental friendliness.

[0003] Currently, fluoroethylene carbonate (FEC) is a widely used electrolyte additive in lithium-ion batteries. FEC can react with lithium salts to form a stable and dense solid electrolyte interface film (Solid Electrolyte Interphase, SEI), and form a complex with lithium salts in the SEI film, thereby inhibiting the dissolution of lithium salts in the electrolyte and avoiding side reactions between the electrode and the electrolyte, effectively improving the cycle life of the battery and significantly improving the low-temperature performance of the battery.

[0004] However, during high-temperature storage of electrolytes containing FEC, FEC will undergo a dehydrofluorination reaction, causing the acidity of the electrolyte to continue to increase, destroying the negative electrode interface and causing side reactions, ultimately leading to battery capacity decay.

[0005] Therefore, it is necessary to design an electrolyte and a secondary battery to solve the above technical problems. Summary of the invention

[0006] In view of the above shortcomings of the prior art, the present invention provides an electrolyte and a secondary battery to improve the technical problem that the existing FEC-containing electrolyte is prone to dehydrofluorination reaction under high temperature environment, resulting in increased acidity of the electrolyte and affecting the storage and cycle performance of the battery.

[0007] To achieve the above-mentioned object and other related objects, the present invention provides an electrolyte, which includes a lithium salt, a solvent, a first additive and a second additive; wherein the first additive includes fluoroethylene carbonate; and the second additive includes a compound represented by formula (1):

[0008]

[0009] R1 and R2 in formula (1) are each independently selected from a substituent having 1 to 6 carbon atoms, 0 to 4 unsaturations and 0 to 3 heteroatoms, wherein the heteroatom is selected from any one of a nitrogen atom, a sulfur atom, an oxygen atom, a boron atom and a phosphorus atom.

[0010] In an example of the present invention, the mass proportion of the first additive in the electrolyte is 0.5% to 1%; and / or the mass proportion of the second additive in the electrolyte is 0.05% to 3%.

[0011] In an example of the present invention, the mass proportion of the second additive in the electrolyte is 0.1% to 0.5%.

[0012] In one example of the present invention, R1 and R2 in the formula (1) are independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino or heterocyclic ring, and the heterocyclic ring is selected from any one of pyrridine, pyrrole, thiophene, thiazole and furan.

[0013] In one example of the present invention, the compound represented by formula (1) includes at least one of the following compounds:

[0014]

[0015]

[0016] In an example of the present invention, the electrolyte further includes a third additive, and the third additive is selected from at least one of propenyl-1,3-sultone and tetravinylsilane.

[0017] In one example of the present invention, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methanesulfonate and lithium trifluoromethylsulfonate; the mass proportion of the lithium salt in the electrolyte is 12% to 16%.

[0018] In one example of the present invention, the solvent includes carbonate and carboxylate; the carbonate is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate and fluoroethylene carbonate, and the carboxylate is selected from at least one of ethyl formate, ethyl acetate, propyl acetate and ethyl propionate; the mass proportion of the solvent in the electrolyte is 70% to 80%.

[0019] The present invention also provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte described in any one of the above examples.

[0020] In the electrolyte provided by the present invention, the first additive FEC and the second additive compound represented by formula (1) are used in combination, and the copolymerization reaction between FEC and the compound represented by formula (1) is utilized to further form an orderly arranged, dense and stable SEI film, which can further prevent the electrolyte from having side reactions with the negative electrode without increasing the impedance, thereby improving the cycle stability of the electrolyte and the cycle performance of the battery; at the same time, during the high-temperature storage process, the excellent acidity inhibition ability of the compound represented by formula (1) can effectively alleviate the increase in acidity caused by the decomposition of FEC, thereby improving the high-temperature storage performance of the battery. DETAILED DESCRIPTION

[0021] The following is an explanation of the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the features in the following embodiments and the embodiments can be combined with each other without conflict. It should also be understood that the terms used in the embodiments of the present invention are intended to describe specific embodiments, rather than to limit the scope of protection of the present invention. The test methods for which specific conditions are not specified in the following examples are usually carried out under conventional conditions or according to the conditions recommended by the manufacturers.

[0022] For simplicity, only some numerical ranges are explicitly disclosed herein, and each point or single value between the range endpoints is included in the range. Thus, each point or single value can be combined with any other point or single value as its own lower limit or upper limit or with other lower limits or upper limits to form an unspecified range.

[0023] In this document, unless otherwise specified, “%” refers to mass percentage.

[0024] In this article, substituents refer to atoms or groups of atoms that replace the main chain or ring in organic compounds. They can replace a hydrogen atom or other atoms in the molecule and affect the chemical and physical properties of the molecule. The type and position of the substituent have an important influence on the reactivity, polarity, solubility and other properties of the molecule.

[0025] Unsaturation, also known as hydrogen deficiency index or ring plus double bond index, is a quantitative indicator of the degree of unsaturation of organic compound molecules. For hydrocarbons with molecular formula CnHm and derivatives of hydrocarbons with molecular formula CnHmOx, if m<2n+2, the hydrocarbon and its hydrocarbon radical have a certain degree of unsaturation Ω, that is, compared with open-chain alkanes with the same number of carbon atoms in the organic compound molecule, the unsaturation of the organic compound increases by 1 for every 2 hydrogen atoms reduced.

[0026] Heteroatom is a general term for non-carbon atoms in organic chemistry. The most common heteroatoms are nitrogen, sulfur and oxygen.

[0027] The first aspect of the present invention provides an electrolyte, which uses a first additive FEC and a second additive compound represented by formula (1) in combination, and further forms an orderly arranged, dense and stable SEI film by utilizing the copolymerization reaction between FEC and the compound represented by formula (1), which can further prevent the electrolyte from having side reactions with the negative electrode without increasing the impedance, thereby improving the cycle stability of the electrolyte and the cycle performance of the battery; and utilizing the alkaline properties of the compound represented by formula (1) to inhibit the increase in electrolyte acidity caused by the high-temperature decomposition of FEC, thereby improving the chemical stability of the electrolyte and improving the high-temperature storage performance of the battery.

[0028] The electrolyte includes a lithium salt, a solvent, a first additive, and a second additive. The solvent is the main part of the electrolyte and is used to dissolve the lithium salt and the additive. After the lithium salt is dissolved in the organic solvent, a large amount of active lithium ions can be released, so that the electrolyte has good conductivity. The additive is an auxiliary substance of the electrolyte and can be selected according to actual needs, for example, it can be a film-forming additive, a flame retardant additive, a high voltage additive, etc.

[0029] In the present invention, the first additive includes fluoroethylene carbonate (FEC); and the second additive includes the compound represented by formula (1).

[0030] Wherein, the chemical formula of formula (1) is as follows:

[0031]

[0032] R1 and R2 in formula (1) are each independently selected from a substituent having 1 to 6 carbon atoms, 0 to 4 unsaturations and 0 to 3 heteroatoms. The types of the substituents R1 and R2 in formula (1) do not affect each other. The number of carbon atoms in the above substituents can be any value from 1 to 6, such as 1, 3, 5 or 6, and the degree of unsaturation can be any value from 0 to 4, such as 0, 1, 3 or 4. The heteroatom is a non-carbon atom, and the heteroatom is selected from any one of a nitrogen (N) atom, a sulfur (S) atom, an oxygen (O) atom, a boron (B) atom, and a phosphorus (P) atom. For example, the heteroatom can be a nitrogen atom, a sulfur atom, or an oxygen atom, but is not limited thereto. The number of heteroatoms can be any value from 0 to 3. For example, 0, 1, 2 or 3.

[0033] The first additive FEC has excellent film-forming properties and can react with lithium salts to form a SEI interface film during the charge and discharge process. The formed SEI film can prevent the electrolyte from directly contacting the negative electrode and causing side reactions, thereby improving the cycle stability of the electrolyte and the cycle performance of the battery. Among them, in the SEI film, the complex formed by the reaction of FEC and lithium salts can inhibit the further dissolution of lithium salts in the electrolyte into the SEI film at the negative electrode interface, slowing down the growth rate of the SEI film, thereby ensuring the formation of a tightly structured SEI film while avoiding excessive thickness of the SEI film and increasing the battery impedance. However, FEC will undergo a hydrogen fluoride removal reaction in a high temperature environment, resulting in an increase in the acidity of the electrolyte, destroying the SEI film on the surface of the negative electrode, and causing high-temperature storage capacity attenuation.

[0034] The compound shown in the second additive formula (1) is an imide compound. On the one hand, it has alkaline characteristics. After being added to the electrolyte in combination with FEC, it can effectively inhibit the increase in acidity caused by the decomposition of FEC, thereby improving the chemical stability of the electrolyte and improving the high-temperature storage performance of the battery. On the other hand, the carbonyl and imino groups of the cyclic imide molecules in the compound shown in formula (1) can form intramolecular or intermolecular hydrogen bonds, which makes the cyclic imide molecules have good molecular stability and can promote the orderly arrangement of molecules. In the process of participating in the formation of the SEI film, the compound of formula (1) can preferentially form a uniform prepolymer and then form a SEI film, thereby bringing the high structural strength and orderly arrangement characteristics between the cyclic imide molecules into the SEI film, forming a chemically stable and densely arranged SEI film, so as to improve the cohesion and mechanical strength of the SEI film, and help the SEI film to maintain integrity at high temperatures.

[0035] In some embodiments, the mass proportion of the first additive FEC in the electrolyte is any value between 0.5% and 1%, for example, it can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%. The inventors have found that when the mass proportion of the first additive in the electrolyte is limited to the above range, it can ensure the effect of FEC on the improvement of the battery cycle performance while not causing excessive attenuation of the battery's high temperature storage performance. If the mass proportion of the first additive FEC in the electrolyte is less than 0.5%, it is impossible to form a SEI film of sufficient thickness at the negative electrode interface, and the protection effect on the negative electrode is poor; if the mass proportion of the first additive FEC in the electrolyte is higher than 1%, then too much FEC is likely to cause excessive increase in the acidity of the electrolyte at high temperature, so that the second additive cannot effectively adjust the pH of the electrolyte at an appropriate content, causing continuous damage to the negative electrode SEI film, causing the battery's high temperature storage capacity to decay.

[0036] In some embodiments, the mass proportion of the second additive formula (1) compound in the electrolyte is any value in the range of 0.05% to 3%, for example, it can be 0.05%, 0.1%, 0.2%, 0.3%, 0.5%, 0.7%, 1%, 1.5%, 2%, 2.5% or 3%. Optionally, the mass proportion of the second additive in the electrolyte is any value in the range of 0.1% to 0.5%, for example, it can be 0.1%, 0.2%, 0.3%, 0.4% or 0.5%. The inventors have found that when the mass proportion of the second additive in the electrolyte is limited to the above range, the acidity increase of the electrolyte caused by the decomposition of FEC at high temperature can be effectively slowed down, and the formation of a stable and appropriately thick SEI film at the negative electrode interface can be promoted, thereby improving the electrochemical stability of the battery without excessively increasing the battery impedance. When the mass proportion of the second additive in the electrolyte is too small, a SEI film of sufficient thickness cannot be formed at the negative electrode interface, and it is not enough to suppress the increase in electrolyte acidity caused by the high-temperature decomposition of FEC; when the mass proportion of the second additive in the electrolyte is too large, it will cause the SEI film formed at the negative electrode interface to be too thick, thereby affecting the ionic conductivity of the negative electrode and having a negative impact on the battery's cycle performance.

[0037] In some embodiments, R1 and R2 in the compound of formula (1) are independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino or heterocycle. Wherein, heterocycle means that the atoms constituting the ring include at least one non-carbon atom in addition to carbon atoms, and these non-carbon atoms are heteroatoms. As an example, heterocycle includes but is not limited to any one of pyridine, pyrrole, thiophene, thiazole, and furan. That is, R1 and R2 can be selected from any one of alkyl, alkenyl, alkynyl, carbonyl, ester, amino and heterocycle, and can be all the same, partially the same, or different.

[0038] In some embodiments, the compound represented by formula (1) is selected from at least one of compound (1-1), compound (1-2), compound (1-3) and compound (1-4). Optionally, the compound represented by formula (1) is selected from compound (1-1). The chemical formulas of compound (1-1), compound (1-2), compound (1-3) and compound (1-4) are as follows:

[0039]

[0040]

[0041] In some embodiments, the electrolyte further includes a third additive, which is selected from at least one of propenyl-1,3-sultone (PST) and tetravinylsilane (TVSi). The third additive helps to promote the formation of a stable SEI film at the negative electrode interface, which can further enhance the film-forming effect of the electrolyte at the negative electrode interface, enhance the protective effect on the negative electrode interface, and improve the charge and discharge cycle performance of the battery.

[0042] In some embodiments, the third additive is selected from one or a combination of PST and TVSi, and the mass proportion of the third additive in the electrolyte is any value in the range of 0.05% to 3%, for example, 0.05%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2%, 2.3%, 2.5%, 2.7% or 3%. When the mass proportion of the third additive in the electrolyte is within the above range, it can promote the formation of an interface film at the interface of the negative electrode and even the positive electrode, while improving the high temperature stability of the electrode interface film, and does not excessively increase the impedance of the battery. When the mass proportion of the third additive in the electrolyte is too small, it will lead to insufficient film formation of the negative electrode interface SEI film and even the positive electrode interface solid electrolyte interface film (Cathode Electrolyte Interphase, CEI), making it difficult to effectively protect the electrode interface, thereby having a negative impact on the battery's cycle capacity; when the mass proportion of the third additive in the electrolyte is too large, it will cause the negative electrode interface SEI film impedance to be too high, which will have a negative impact on the battery's cycle performance.

[0043] In addition, it should be noted that the lithium salt in the electrolyte can be selected from conventional lithium salts in the art. As an example, the lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethyl)sulfonyl imide (LiFSI), lithium bis(trifluoromethyl)sulfonyl imide (LiTFSI), lithium acetate (CH3COOLi), lithium methane sulfonate (CH3SO3Li) and lithium trifluoromethyl sulfonate (CF3SO3Li). Preferably, the lithium salt is selected from lithium hexafluorophosphate (LiPF6) with good comprehensive performance, or lithium hexafluorophosphate (LiPF6) is mainly used, and other lithium salts are added in appropriate amounts to take advantage of the advantages and improve the performance of the electrolyte. In some embodiments, the mass percentage of the lithium salt in the electrolyte is any value between 12% and 16%, for example, it can be 12%, 13%, 14%, 15% or 16%. For example, in one example, the lithium salt includes lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI), and the mass ratio of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) in the electrolyte is (13-14):(1-2).

[0044] The solvent in the electrolyte can be selected from conventional solvents suitable for lithium-ion batteries in the art. The solvent includes at least one of carbonate and carboxylate, wherein the carbonate is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (EMC) and fluoroethylene carbonate (FEC), and the carboxylate is selected from one or more of ethyl formate (EF), ethyl acetate (EA), propyl acetate (PA) and ethyl propionate (EP). That is, the solvent can be selected from any one of the above-mentioned solvents, or any two or a combination of two. As an example, the solvent is a combination of EF and EC, a combination of EC, DMC and EMC, or a combination of EC, DMC, PC and EA, and the like. In some embodiments, the solvent includes at least carbonate and carboxylate, and the mass proportion of the solvent in the electrolyte is generally any value between 70% and 88%, for example, 70%, 73%, 75%, 78%, 80%, 83%, 85%, 86%, 87% or 88%; optionally, the mass proportion of the solvent in the electrolyte is any value between 70% and 80%, for example, it can be 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77% or 78%.

[0045] In addition, in some implementations, the solvent may further include at least one of ethers and nitriles, wherein the ethers are selected from at least one of ethylene glycol dimethyl ether and diethanol diethyl ether, and the nitriles include one or more of acetonitrile, propionitrile, butyronitrile, and valeronitrile.

[0046] The electrolyte of the present invention can be prepared according to conventional preparation methods. For example, the electrolyte is prepared in a glove box, the argon or nitrogen content in the glove box is 99.999%, the actual oxygen content in the glove box is 0.1ppm, and the moisture content is less than 10ppm. In the glove box, after the solvent is mixed evenly according to the set ratio, the fully dried lithium salt and additives are added to the above solvent, mixed evenly, and prepared into an electrolyte. Among them, the content of each component in the electrolyte is the weight percentage calculated based on the total weight of the electrolyte.

[0047] The second aspect of the present invention also provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte described above. During the battery charging and discharging process, lithium ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet to play an isolation role; the electrolyte plays a role in conducting lithium ions between the positive electrode sheet and the negative electrode sheet. It should be noted that the structural type of the above-mentioned secondary battery is not limited, and the secondary battery can be any conventional structure of the secondary battery on the market, such as a soft-pack battery, a square shell battery or a cylindrical battery.

[0048] In addition, the secondary battery of the present invention can be used in the form of a single cell, a battery module or a battery pack for electronic devices to power them. Electronic devices include but are not limited to mobile phones, tablets, laptops, electric toys, battery cars, new energy vehicles, ships, spacecraft, etc. Among them, electric toys can include fixed or mobile electric toys, for example, game consoles, electric car toys, electric ship toys and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles and spacecrafts, etc. New energy vehicles can be pure electric vehicles, hybrid vehicles or extended-range vehicles, etc.

[0049] The composition and preparation method of the secondary battery are described in detail below.

[0050] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. Among them, the positive current collector is, for example, a foil formed after surface treatment of nickel, titanium, aluminum, nickel, silver, stainless steel or carbon. In addition to the foil, the positive current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam or non-woven fabric. Among them, the thickness of the positive current collector is, for example, 8μm-15μm. In one embodiment, the positive current collector is, for example, aluminum foil, and the thickness of the aluminum foil is, for example, 13μm. The positive current collector has two surfaces opposite to each other in its own thickness direction, and the positive active material layer is disposed on any one or both of the two opposite surfaces of the positive current collector. The positive active material layer includes a positive active material, a positive conductive agent and a positive binder. There is no specific restriction on the positive active material, the positive conductive agent and the positive binder here, and those skilled in the art can choose according to actual needs.

[0051] The positive electrode active material can be selected from any material suitable for lithium-ion batteries, that is, any compound that can reversibly embed and deintercalate lithium ions can be used. As an example, the positive electrode active material can be selected from ternary materials, lithium-containing phosphates, and spinel materials, wherein the ternary materials include lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide doped with metal ions, lithium nickel cobalt aluminum oxide doped with metal ions, etc.; lithium-containing phosphates include lithium iron manganese phosphate, lithium iron phosphate, lithium iron manganese phosphate, etc. The binder of the positive electrode is, for example, selected from any one or more of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate (Polyacrylate), polyvinyl ether (Polyvinylether), polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene (Polyhexafluoropropylene) or styrene butadiene rubber (Polymerized Styrene Butadiene Rubber, SBR), etc. The positive electrode conductive agent is, for example, one selected from carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc., or a combination of a plurality of them mixed in any ratio.

[0052] The negative electrode plate includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be made of a material with good electrical conductivity and mechanical strength, such as copper foil. The negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active material layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder and a thickener. The specific types of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder are not specifically limited here. Materials known in the art that can be used in lithium-ion batteries can be used, and those skilled in the art can select according to actual needs.

[0053] The negative electrode active material is selected from compounds capable of intercalating and deintercalating lithium ions. As an example, the negative electrode active material is selected from one or more combinations of carbon-based materials and silicon-based materials. Carbon-based materials include, for example, soft carbon, hard carbon, artificial graphite, natural graphite, etc., and silicon-based materials include, for example, elemental silicon, silicon oxide compounds, silicon carbon compounds, etc. The negative electrode conductive agent is selected from one of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc., or a combination of two or more of them mixed in any proportion. The negative electrode binder is selected from any one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), or a combination of several mixed in any proportion; the thickener is selected from sodium carboxymethyl cellulose CMC-Na or lithium carboxymethyl cellulose CMC-Li.

[0054] The diaphragm is selected from conventional types in the art, such as polyethylene film (PE), polypropylene film (PP), glass fiber film, polyethylene film or composite film, etc. The thickness of the diaphragm is 9-18 μm, the air permeability is 180-380 s / 100 mL, and the porosity is 30-50%.

[0055] The battery assembly is carried out according to conventional methods, for example: after all the above-mentioned pole pieces, separators and electrolytes are prepared, the negative pole piece, separator and positive pole piece are stacked and wound in sequence, so that the separator is located between the positive and negative poles to play a role of isolation. Then the electrode assembly formed by winding or stacking is packaged into a shell, transferred to a vacuum oven at 120°C for drying, injected with 3.0g / Ah of the above-prepared electrolyte, and then sealed, and a formation process is carried out to finally prepare a lithium-ion battery with a capacity of 1Ah.

[0056] The technical scheme of the present invention is described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available products or can be prepared by conventional methods in the art.

[0057] Example 1

[0058] This embodiment provides an electrolyte, which includes a lithium salt, a solvent, a first additive, and a second additive. The solvent is a composition of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), and the volume ratio of the three is 3:4:4; the lithium salt is lithium hexafluorophosphate (LiPF6), and the mass proportion of the lithium salt in the electrolyte is 12%; the first additive is fluoroethylene carbonate (FEC), and the mass proportion of the first additive in the electrolyte is 0.5%; the second additive is a compound shown in formula (1-1), and the mass proportion of the second additive in the electrolyte is 0.3%.

[0059] The preparation process of the electrolyte is as follows: In an argon atmosphere glove box with a water content of <10ppm, EC, DMC, and EMC are first mixed in a volume ratio of 3:4:4 to form a solvent; then lithium salt LiPF6 and additives are added to the solvent, and the electrolyte is obtained after mixing evenly. Among them, the amount of LiPF6 added is 12% of the total weight of the electrolyte, the amount of the first additive FEC compound added is 0.5% of the total weight of the electrolyte, and the amount of the second additive formula (1-1) compound added is 0.3% of the total weight of the electrolyte.

[0060] Example 2

[0061] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass proportion of the first additive FEC in the electrolyte is adjusted to 0.2%.

[0062] Example 3

[0063] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass proportion of the first additive FEC in the electrolyte is adjusted to 1%.

[0064] Example 4

[0065] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass proportion of the first additive FEC in the electrolyte is adjusted to 1.2%.

[0066] Example 5

[0067] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass proportion of the second additive compound of formula (1-1) in the electrolyte is adjusted to 0.01%.

[0068] Example 6

[0069] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass proportion of the second additive compound of formula (1-1) in the electrolyte is adjusted to 0.05%.

[0070] Example 7

[0071] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass proportion of the second additive compound of formula (1-1) in the electrolyte is adjusted to 3%.

[0072] Example 8

[0073] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the mass proportion of the second additive compound of formula (1-1) in the electrolyte is adjusted to 3.2%.

[0074] Example 9

[0075] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the compound represented by formula (1-2) is used as the second additive.

[0076] Example 10

[0077] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the compound represented by formula (1-3) is used as the second additive.

[0078] Embodiment 11

[0079] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that the compound represented by formula (1-4) is used as the second additive.

[0080] Comparative Example 1

[0081] This comparative example provides an electrolyte of the same system as that of Example 1. The difference between this comparative example and Example 1 is that the first additive FEC and the second additive compound of formula (1-1) are not added to the electrolyte.

[0082] Comparative Example 2

[0083] This comparative example provides an electrolyte of the same system as that of Example 1. The difference between this comparative example and Example 1 is that the first additive FEC is not added to the electrolyte.

[0084] Comparative Example 3

[0085] This comparative example provides an electrolyte of the same system as that of Example 1. The difference between this comparative example and Example 1 is that the second additive compound of formula (1-1) is not added to the electrolyte.

[0086] The electrolytes prepared in Examples 1-11 and Comparative Examples 1-3 were respectively assembled in lithium ion secondary batteries. The preparation process of the lithium ion secondary batteries is as follows:

[0087] Preparation of positive electrode sheet: Mix the positive electrode active material LiFePO4, the positive electrode binder polyvinylidene fluoride and the positive electrode conductive agent Super P in a mass ratio of 97:1:2, add the solvent N-methylpyrrolidone, and stir until it is uniform and transparent under the action of a vacuum mixer to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on an aluminum foil, and the aluminum foil is dried at room temperature and then transferred to an oven for drying, and then cold pressed and cut to obtain a positive electrode sheet.

[0088] Preparation of negative electrode sheets: Mix the negative electrode active material artificial graphite, the negative electrode conductive agent Super P, the negative electrode thickener sodium carboxymethyl cellulose (CMC-Na), and the negative electrode binder styrene-butadiene rubber (SBR) in a mass ratio of 96:1:1:2, add deionized water and fully stir until uniform under the action of a vacuum mixer to obtain a negative electrode slurry; evenly coat the negative electrode slurry on the negative electrode collector copper foil, dry it at room temperature, and then transfer it to an oven for drying, and obtain a negative electrode sheet through cold pressing, slitting and other processes.

[0089] Selection of diaphragm: A polypropylene film with a thickness of 12 μm was used as the diaphragm.

[0090] Preparation of battery: Place the positive electrode sheet, separator, and negative electrode sheet in order, with the separator between the positive electrode sheet and the negative electrode sheet to play an isolating role, and wind to form a bare battery cell. Then wrap it with aluminum plastic film, transfer it to a vacuum oven and dry it at 120°C, inject the above-prepared electrolyte at 3.0g / Ah, seal it, and perform electrolyte formation to obtain a soft-pack lithium-ion battery with a capacity of 1Ah.

[0091] The performance of lithium-ion batteries assembled with the electrolytes provided in Examples 1-11 and Comparative Examples 1-3 was tested, wherein the electrolyte parameters and test results of Examples 1 to 11 and Comparative Examples 1 to 3 are shown in Table 1, and the test method is described as follows.

[0092] Battery cycle performance test: At room temperature of 25℃, charge the lithium-ion battery to 3.65V at a constant current rate of 1C, and then charge at a constant voltage of 3.65V to a current of 0.05C; after the battery is left to stand for 10 minutes, discharge it to 2.5V at a constant current rate of 1C, and record the first discharge capacity C0. After repeating the above charge and discharge steps 2000 times, record the discharge capacity C1 of the battery at the 2000th cycle, and then record the capacity retention rate of the battery after 2000 cycles at room temperature = C1 / C0×100%.

[0093] Battery high temperature storage performance test: At room temperature of 60℃, the lithium-ion battery is charged to 3.65V at a constant current rate of 0.33C, then charged to a current of 0.05C at a constant voltage at a voltage of 3.65V, and then discharged to 2.5 at a constant current rate of 0.33C, left to stand for 30min, and repeated 2-3 times, and the last 0.33C discharge capacity C0 of the lithium-ion battery is recorded. The fully charged lithium-ion battery is placed in a high temperature oven at 60℃ for 60 days, and then the lithium-ion battery is taken out and charged and discharged at room temperature at a rate of 0.33C / 0.33C in the voltage range of 2.5V-3.65V. After 2 cycles, the last discharge capacity C1 is recorded. The capacity recovery rate of the lithium-ion battery after 60 days of high temperature storage relative to that before storage is calculated according to the following formula: Capacity recovery rate (%) = (C1 / C0) × 100%.

[0094] Table 1: Performance test results of electrolyte assembled batteries in Examples 1 to 11 and Comparative Examples 1 to 3

[0095]

[0096]

[0097] The contents in Table 1 are all the mass percentages of the components relative to the electrolyte.

[0098] By comparing the test results of Examples 1-11 and Comparative Example 1, it can be seen that the use of FEC and the compound of formula (1) as additives in the electrolyte can not only inhibit the acidification of the electrolyte at high temperature, improve the chemical stability of the electrolyte at high temperature, and improve the high-temperature storage performance of the battery; but also form a SEI film with tightly and orderly arranged molecules at the negative electrode interface, thereby reducing the dissolution of lithium salt in the electrolyte in the SEI film, isolating the contact side reactions between the negative electrode and the electrolyte, and effectively improving the cycle performance of the battery.

[0099] By comparing the test results of Example 1, Examples 5-11 and Comparative Examples 2-3, it can be seen that compared with Comparative Example 2, only using the compound of formula (1) as an additive results in poor film formation of the electrolyte at the negative electrode, and cannot effectively improve the battery cycle performance; or only using FEC as an additive in Comparative Example 3 results in severe acidification of the electrolyte under high temperature conditions, causing the battery high temperature storage capacity to decay; the embodiment of this scheme introduces the compound of formula (1) and an appropriate amount of FEC into the electrolyte as an additive, which can not only maintain the improvement effect of FEC on the battery cycle performance, but also effectively inhibit the acidification of the electrolyte at high temperature through the compound of formula (1), thereby improving the high temperature storage performance of the battery.

[0100] By comparing the test results of Examples 1 to 4, it can be known that when the mass proportion of the first additive FEC in the electrolyte is limited to an appropriate range of 0.5% to 1%, the impedance of the battery can be increased while promoting the formation of a stable SEI film at the negative electrode interface, and the acid substances released by decomposition at high temperature can also be neutralized by the second additive. When the mass proportion of the first additive FEC in the electrolyte is less than 0.5%, the SEI film at the negative electrode interface is insufficiently formed, and it is difficult to effectively protect the negative electrode, thereby negatively affecting the cycle capacity of the battery; when the mass proportion of the first additive FEC in the electrolyte is higher than 1%, too much FEC will cause the impedance of the SEI film at the negative electrode interface to be too high, and it will also cause the electrolyte to have an acidity that exceeds the suitable adjustment range of the second additive at high temperature, causing continuous damage to the negative electrode SEI film, thereby negatively affecting the cycle capacity and high temperature storage capacity of the battery.

[0101] By comparing the test results of Example 1 and Examples 5 to 8, it can be seen that when the mass proportion of the second additive compound of formula (1) in the electrolyte is limited to an appropriate range of 0.05% to 3%, it is possible to effectively suppress the high-temperature acidification of the electrolyte without excessively increasing the battery impedance.

[0102] From the test results of Examples 1, 9, 10, and 11, it can be seen that compound (1-1), compound (1-2), compound (1-3), and compound (1-4) have the same structure and functional groups as the compound of formula (1), so when used with FEC as an additive, they can achieve the effect of improving the battery cycle performance and high temperature storage performance. Therefore, the use of compounds of formula (1) with different types of R1 and R2 groups has little effect on the battery cycle performance and high temperature storage performance.

[0103] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. An electrolyte, characterized in that: The invention comprises a lithium salt, a solvent, a first additive and a second additive; the first additive comprises fluoroethylene carbonate; and the second additive comprises a compound represented by formula (1): R1 and R2 in formula (1) are each independently selected from a substituent having 1 to 6 carbon atoms, 0 to 4 unsaturations and 0 to 3 heteroatoms, wherein the heteroatom is selected from any one of a nitrogen atom, a sulfur atom, an oxygen atom, a boron atom and a phosphorus atom.

2. The electrolyte according to claim 1, characterized in that The mass percentage of the first additive in the electrolyte is 0.5% to 1%.

3. The electrolyte according to claim 1 or 2, characterized in that The mass percentage of the second additive in the electrolyte is 0.05% to 3%.

4. The electrolyte according to claim 3, characterized in that The mass proportion of the second additive in the electrolyte is 0.1% to 0.5%.

5. The electrolyte according to claim 1, characterized in that R1 and R2 in the formula (1) are each independently selected from an alkyl group, an alkenyl group, an alkynyl group, a carbonyl group, an ester group, an amino group or a heterocycle, and the heterocycle is selected from any one of pyridine, pyrrole, thiophene, thiazole and furan.

6. The electrolyte according to claim 1 or 5, characterized in that The compound represented by formula (1) includes at least one of the following compounds:

7. The electrolyte according to claim 1, characterized in that The electrolyte further includes a third additive, and the third additive is selected from at least one of propenyl-1,3-sultone and tetravinylsilane.

8. The electrolyte according to claim 1, characterized in that The lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methanesulfonate and lithium trifluoromethylsulfonate; the lithium salt accounts for 12% to 16% by mass in the electrolyte.

9. The electrolyte according to claim 1, characterized in that The solvent includes carbonate and carboxylate; the carbonate is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate and fluoroethylene carbonate, and the carboxylate is selected from at least one of ethyl formate, ethyl acetate, propyl acetate and ethyl propionate; the mass proportion of the solvent in the electrolyte is 70% to 80%.

10. A secondary battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte according to any one of claims 1 to 9.

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