Electrolyte and secondary battery

Abstract

The present 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 a compound represented by a formula I, and the second additive comprises vinylene ester. According to the electrolyte provided by the invention, on one hand, the pH value in the electrolyte is adjusted by utilizing tertiary amino groups in the compound in the formula I, so that the chemical environment of the electrolyte is more stable; on the other hand, through the characteristic that vinylene is polymerized on the surface of the positive electrode to form a film under the initiation of high voltage, the unsaturated double-bond compound shown in the formula I is guided to participate in the process of polymerizing and forming the film on the surface of the positive electrode, and amido substances and salt substances generated by the reaction of amido and acidic substances are introduced in the process of polymerizing and forming the film; therefore, a polymer film with relatively high mechanical strength and chemical stability is formed on the surface of the positive electrode.

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] At present, the upper limit of the working voltage of the positive electrode materials (such as ternary materials or lithium iron phosphate materials) of high-capacity lithium-ion batteries has been increasing after development and is gradually approaching the material limit. The electronic impedance and polarization of the positive electrode materials at high voltage (>3.80V) increase, and the lattice collapse caused by uneven lithium removal is very likely to occur during the charge and discharge process, resulting in an increase in side reactions between the positive electrode and the electrolyte, which in turn causes the battery cycle life to decrease. At the same time, the acidity of the electrolyte of the carboxylate system is very easy to increase in a high temperature environment, and it is more likely to have side reactions with the positive electrode material, which will further deteriorate the capacity and cycle life of the battery.

[0004] Therefore, it is necessary to design an electrolyte and a secondary battery to solve the above technical problems. Summary of the invention

[0005] 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 in the prior art that the electrolyte is prone to side reactions with the positive electrode material under high temperature and high voltage environment, thereby deteriorating the battery cycle performance.

[0006] 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 a compound shown in formula I, and the second additive includes vinyl ester;

[0007]

[0008] R1, R2, R3, and R4 in formula I 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.

[0009] In an example of the present invention, in the electrolyte, the mass content of the first additive is 0.05% to 3%, and the mass content of the second additive is less than or equal to 3%.

[0010] In an example of the present invention, the mass content of the first additive in the electrolyte is 0.1% to 0.5%.

[0011] In an example of the present invention, the electrolyte further includes a third additive, and the third additive includes thiophene; in the electrolyte, the mass content of the second additive is X, and the mass content of the third additive is Y; wherein X>Y and 2%≤X+Y≤4%.

[0012] In one example of the present invention, R1, R2, R3, and R4 in Formula I are independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino or heterocycle, and the heterocycle 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 I includes at least one of the following compounds:

[0014]

[0015] In one example of the present invention, the electrolyte further includes a fourth additive, the fourth additive is selected from at least one of fluoroethylene carbonate, propylene-1,3-sultone, vinyl sulfate and tetravinylsilane, and the mass content of the fourth additive in the electrolyte is 0.1% to 2%.

[0016] 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 content of the lithium salt in the electrolyte is 12% to 17%.

[0017] 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.

[0018] 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.

[0019] The electrolyte provided by the present invention introduces the compound represented by formula I and vinyl ester as additives. On the one hand, the tertiary amino group in the compound represented by formula I is used to adjust the pH value in the electrolyte, so that the chemical environment of the electrolyte is more stable. On the other hand, the characteristic of vinyl ester to polymerize and form a film on the surface of the positive electrode under high voltage is used to guide the compound represented by formula I with unsaturated double bonds to participate in the polymerization and film forming process on the surface of the positive electrode, and in the polymerization and film forming process, the amine substance and the salt substance generated by the reaction of the amine group with the acidic substance are introduced to form a polymer film with high mechanical strength and chemical stability on the surface of the positive electrode, thereby effectively protecting the integrity of the positive electrode plate structure, preventing direct contact between the electrolyte and the negative electrode, reducing the irreversible consumption of lithium ions, and improving the cycle performance and life of the battery. DETAILED DESCRIPTION

[0020] 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.

[0021] 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.

[0022] In this document, "multiple", "multiple", "multiple times", etc., unless otherwise specified, refer to a number greater than 2 or equal to 2. For example, "one or more" means one or greater than or equal to two. "Further", "further", "particularly", etc. are used for descriptive purposes to indicate differences in content, but should not be understood as limiting the scope of protection of the present invention.

[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] Number of heteroatoms: In organic chemistry, non-carbon atoms are collectively referred to as heteroatoms. The most common heteroatoms are nitrogen atoms, sulfur atoms, and oxygen atoms.

[0027] The first aspect of the present invention provides an electrolyte, which adjusts the pH of the electrolyte by introducing a compound represented by formula I and vinyl ester as additives to avoid an increase in the acidity of the electrolyte at high temperature, thereby improving the chemical stability of the electrolyte; and can also polymerize on the surface of a high-potential positive electrode to form a polymer film with high mechanical strength and high stability, thereby reducing side reactions between the electrode and the electrolyte and improving the cycle life 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 the compound represented by formula I, and the second additive includes vinylene ester (VC).

[0030] Wherein, the chemical formula of formula I is as follows:

[0031]

[0032] R1, R2, R3, and R4 in Formula I are independently selected from substituents having 1 to 6 carbon atoms, 0 to 4 unsaturations, and 0 to 3 heteroatoms. That is, the types of substituents R1, R2, R3, and R4 in Formula I do not affect each other, and can be completely identical substituents, or partially identical substituents, such as R1 and R2 are the same or R1, R2, and R3 are the same, etc., or completely different substituents. 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. Heteroatoms are non-carbon atoms, and heteroatoms are selected from any one of nitrogen (N) atoms, sulfur (S) atoms, oxygen (O) atoms, boron (B) atoms, and phosphorus (P) atoms. For example, heteroatoms can be nitrogen atoms, sulfur atoms, or oxygen atoms, but are not limited thereto. The number of heteroatoms can be any value from 0 to 3. For example, 0, 1, 2 or 3, etc.

[0033] The compound shown in the first additive formula I contains a tertiary amino group (R3N, R represents an alkyl group, and the tertiary amino group is formed by combining a trimethyl group with an amino group (NH2)). The tertiary amino group exhibits alkalinity due to the lone pair of electrons on the nitrogen atom, and can react chemically with acidic substances in the electrolyte (such as acidic substances produced by the decomposition of lithium salts), thereby adjusting the pH of the electrolyte and making the chemical environment of the electrolyte more stable. In addition, the compound molecule shown in formula I also contains an unsaturated double bond. The unsaturated double bond in the compound molecule shown in formula I can be opened under the electric field and oxidative environment of the electrode surface, and copolymerize with other components in the electrolyte (such as solvent molecules, other additives, etc.) to form a polymer film.

[0034] The unsaturated double bond of the second additive VC acts as an initiator of cationic polymerization of the positive electrode, and can initiate a copolymerization reaction between VC and the compound shown in Formula I and solvent molecules on the surface of the positive electrode with high potential, so that VC and the compound shown in Formula I form a polymer film on the surface of the positive electrode. The copolymerization reaction of VC and the compound shown in Formula I introduces tertiary amino groups and salt substances generated by the reaction of tertiary amino groups with acidic substances into the polymer film to increase the active sites of the polymer film, improve the ionic conductivity, mechanical strength and chemical stability of the polymer film, and at the same time, the copolymerization reaction can also introduce tertiary amino groups into the polymer film on the surface of the positive electrode to slow down the increase in impedance caused by the reduction of tertiary amino groups in the electrolyte on the surface of the negative electrode to generate excessive lithium nitride. The polymer film can cover the surface of the positive electrode, which can prevent the direct contact between the electrolyte and the positive electrode, thereby reducing the side reactions between the positive electrode and the electrolyte, reducing the irreversible consumption of active lithium ions in the battery, and improving the capacity and cycle life of the battery. In addition, based on its good mechanical strength and chemical stability, the polymer film can better resist the volume changes and mechanical stress generated during the battery charging and discharging process, protect the integrity of the electrode structure, and thus further improve the battery's rate performance and cycle life.

[0035] In some embodiments, the mass content of the first additive in the electrolyte is any value in the range of 0.05% to 3%, for example, 0.05%, 0.1%, 0.2%, 0.3%, 0.5%, 0.7%, 1%, 1.5%, 2%, 2.5% or 3%. Optionally, the mass content of the first additive in the electrolyte is any value in the range of 0.1% to 0.5%, for example, 0.1%, 0.2%, 0.3%, 0.4% or 0.5%.

[0036] In some embodiments, the mass content of the second additive is any value less than or equal to 3%, for example, it can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.7%, 1%, 1.3%, 1.5%, 1.7%, 2%, 2.3%, 2.5%, 2.7% or 3%.

[0037] It should be noted that when the content of the first additive and the second additive in the electrolyte is within the above range, it is possible to promote the formation of a stable solid electrolyte interface film (Cathode Electrolyte Interphase, CEI) at the positive electrode interface without excessively increasing the impedance of the battery. When the content of the first additive and the second additive in the electrolyte is too much, the CEI film formed at the positive electrode interface will be too thick, thereby affecting the ionic conductivity of the positive electrode and having a negative impact on the cycle performance of the battery.

[0038] In some embodiments, the electrolyte further includes a third additive, and the third additive includes thiophene (SF). Among them, VC can copolymerize with SF on the positive electrode surface under high voltage initiation, thereby introducing SF into the CEI film. Since the polymer of VC is insulating, and the polymer of SF has good electronic conductivity, introducing SF into the CEI film formed by VC polymerization can make the insulating part and the conductive part in the CEI film evenly embedded together, reducing the local electronic impedance on the surface of the CEI film, thereby ensuring the overall insulation of the CEI film, so that the area where the positive electrode plate is unevenly de-lithiated can be quickly self-discharged through the surface of the CEI film, realizing the charge balance of the positive electrode interface, ensuring the uniform de-lithiation of the positive electrode interface, reducing the redox side reactions caused by electron exchange at the positive electrode plate interface, and reducing the decomposition loss of the electrolyte.

[0039] In some embodiments, in the electrolyte, the mass content of the second additive VC is X, and the mass content of the third additive SF is Y; the mass contents of the second additive VC and the third additive SF satisfy the following relationship: X>Y and 2%≤X+Y≤4%, for example, the mass contents of the second additive and the third additive can be 1.5% and 0.5%, 2% and 1%, 2.5% and 1%, 2.5% and 1%, or 3% and 1%, respectively. Among them, limiting the second additive VC and the third additive SF within the above range can promote the formation of a CEI film with sufficient mechanical strength, and avoid the formation of an overly thick CEI film and excessively increase the battery impedance. At the same time, making the mass content of the third additive SF lower than the mass content of the second additive VC can avoid the destruction of the insulation of the CEI film caused by the excessive content of the highly conductive SF polymer in the CEI film, ensure the chemical stability of the CEI film, and avoid the CEI film from reacting with the electrolyte to cause the decomposition of the electrolyte.

[0040] In some embodiments, R1, R2, R3, and R4 in the compound of formula I are independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino, or heterocyclic. Wherein, heterocyclic 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, heterocyclic includes but is not limited to any one of pyridine, pyrrole, thiophene, thiazole, and furan. For example, the heterocyclic can be pyridine, thiophene, or furan, etc. That is, R1, R2, R3, and R4 can all be selected from any one of alkyl, alkenyl, alkynyl, carbonyl, ester, amino, and heterocyclic, and can be all the same, partially the same, or not the same. As an example, R1, R2, R3, and R4 can all be alkyl; or R1 and R2 are alkyl, R3 is alkenyl, and R4 is alkynyl; or R1 is alkyl, R2 is ester, R3 is amino, and R4 is alkenyl; and so on.

[0041] In some embodiments, the compound represented by Formula I is selected from at least one of Compound I-1, Compound I-2, Compound I-3 and Compound I-4. Optionally, the compound represented by Formula I is selected from Compound I-1. The chemical formulas of Compound I-1, Compound I-2, Compound I-3 and Compound I-4 are as follows:

[0042]

[0043] In some embodiments, the electrolyte further includes a fourth additive, which is selected from at least one of fluoroethylene carbonate (FEC), propenyl-1,3-sultone (PST), diethylene sulfate (DTD) and tetravinylsilane (TVSi). The fourth additive helps to form a stable solid electrolyte interface film (Solid Electrolyte Interphase, SEI) 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.

[0044] In some embodiments, the mass content of the fourth additive in the electrolyte is any value between 0.1% and 2%, for example, it can be 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8% or 2%. When the total content of the fourth additive in the electrolyte is within the above range, it can promote the formation of the SEI film at the negative electrode interface without excessively increasing the impedance of the battery. When the content of the fourth additive in the electrolyte is too little, it will lead to insufficient film formation of the SEI film at the negative electrode interface, making it difficult to effectively protect the negative electrode, thereby having a negative impact on the battery's cycle capacity; when the content of the fourth additive in the electrolyte is too much, it will cause the impedance of the SEI film at the negative electrode interface to be too high, which will have a negative impact on the battery's cycle performance.

[0045] 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 17%, for example, it can be 12%, 13%, 14%, 15%, 16% or 17%. 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 (1-14):(1-16).

[0046] 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 organic 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 so on. In some embodiments, the solvent includes at least carbonate and carboxylate, and the mass content of the solvent in the electrolyte is generally 70% to 85%, for example, 70%, 75%, 80%, 81%, 83% or 85%, etc. When carboxylate substances exist in the solvent, when the first additive compound of formula I and the second additive VC are polymerized into a film, the carbonyl group of the carboxylate can be introduced into the formed CEI membrane to form a site in the CEI membrane for lithium ion binding, thereby reducing the ion conduction impedance of the CEI membrane and achieving a balance between stability and kinetics.

[0047] In addition, in some implementations, the solvent may further include at least one of ethers and nitriles, wherein the ethers are selected from ethylene glycol dimethyl ether and / or diethanol diethyl ether, and the nitriles include one or more of acetonitrile, propionitrile, butyronitrile, and valeronitrile.

[0048] 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.

[0049] The present invention also provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte described in any one of the above embodiments. 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 isolating 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 may not be limited, and the secondary battery may be any conventional structure secondary battery on the market, such as a soft-pack battery, a square shell battery or a cylindrical battery.

[0050] 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.

[0051] The composition and preparation method of the secondary battery are described in detail below:

[0052] 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.

[0053] 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 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 two or more selected from the group consisting of ... in any proportion.

[0054] 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.

[0055] 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.

[0056] 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 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL, and the porosity is 30% to 50%.

[0057] The battery assembly is carried out according to conventional methods, for example: after preparation, the negative electrode sheet, the separator, and the positive electrode sheet are stacked and wound in sequence, so that the separator is located between the positive and negative electrodes 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, and the above-prepared electrolyte 3.0g / Ah is injected and then sealed, and the electrolyte is formed, and finally a lithium-ion battery with a capacity of 1Ah is prepared.

[0058] In some embodiments, the secondary battery uses a positive electrode active material with a high upper limit of working voltage (such as> 3.80V), such as a high nickel ternary material or a lithium iron phosphate material. Although the use of the above-mentioned positive electrode active material can improve the battery capacity and fast charging performance, it is also easy to cause lattice collapse caused by uneven lithium removal during the charging and discharging process, and the side reactions between the electrolyte and the electrolyte increase, resulting in active material loss. The electrolyte in the above-mentioned embodiment used in the secondary battery can effectively protect the positive electrode interface, which can further improve the cycle life of the battery compared to the prior art. In one example, the positive electrode active material in the secondary battery is selected from lithium iron phosphate material. The lithium iron phosphate material not only has the problem of unstable kinetic performance of high-voltage positive electrode materials, but also reacts with carboxylic acid esters in the electrolyte in a high temperature environment (> 45°C) to increase the acidity of the electrolyte. The electrolyte in the above-mentioned embodiment of the secondary battery can effectively adjust the pH of the electrolyte using the first additive, improve the chemical stability of the electrolyte, and can cooperate with the second additive to form a CEI film on the positive electrode surface, so as to achieve effective protection of the positive electrode interface without excessively affecting the ion exchange capacity of the positive electrode interface.

[0059] 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.

[0060] Example 1

[0061] This embodiment provides an electrolyte, which includes a lithium salt, a solvent, a first additive, and a second additive. Wherein, the solvent is a composition of ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC) and ethyl acetate (EA), and the volume ratio of the four is EC: DMC: PC: EA = 2.5: 3: 0.5: 4; the lithium salt is a composition of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI), and the mass content of the lithium salt in the electrolyte is 14%, and the mass ratio of LiPF6 to LiFSI is 1: 1; the first additive is a compound shown in formula I-1, and the mass content of the first additive in the electrolyte is 0.3%, and the second additive is vinylene carbonate (VC), and the mass content of the second additive in the electrolyte is 3%.

[0062] The preparation process of the electrolyte is as follows: In an argon atmosphere glove box with a water content of <10ppm, EC, DMC, PC, and EA are first mixed in a volume ratio of 2.5:3:0.5:4 to form a solvent; then lithium salts LiPF6 and LiFSI and additives are added to the solvent and mixed evenly to obtain a non-aqueous electrolyte. Among them, the addition amount of LiPF6 and LiFSI is 7% of the total weight of the electrolyte, and the type and addition amount of each additive are shown in Table 1.

[0063] Example 2

[0064] This embodiment provides an electrolyte of the same system as that of Embodiment 1. The difference between this embodiment and Embodiment 1 is that a fourth additive is further added to the electrolyte. The fourth additive includes fluoroethylene carbonate (FEC) and diethylene sulfate (DTD). The mass content of the fourth additive in the electrolyte is 2%, and the mass ratio of FEC to DTD is 1:1.

[0065] Example 3

[0066] This embodiment provides an electrolyte of the same system as that of Embodiment 2. The difference between this embodiment and Embodiment 2 is that the mass content of the first additive in the electrolyte is adjusted to 0.05%.

[0067] Example 4

[0068] This embodiment provides an electrolyte of the same system as that of Embodiment 2. The difference between this embodiment and Embodiment 2 is that the mass content of the first additive in the electrolyte is adjusted to 0.1%.

[0069] Example 5

[0070] This embodiment provides an electrolyte of the same system as that of Embodiment 2. The difference between this embodiment and Embodiment 2 is that the mass content of the first additive in the electrolyte is adjusted to 0.5%.

[0071] Example 6

[0072] This embodiment provides an electrolyte of the same system as that of Embodiment 2. The difference between this embodiment and Embodiment 2 is that the mass content of the first additive in the electrolyte is adjusted to 1%.

[0073] Example 7

[0074] This embodiment provides an electrolyte of the same system as that of Embodiment 2. The difference between this embodiment and Embodiment 2 is that the mass content of the first additive in the electrolyte is adjusted to 3%.

[0075] Example 8

[0076] This embodiment provides an electrolyte of the same system as that of Embodiment 2. The difference between this embodiment and Embodiment 2 is that the compound represented by Formula I-2 is used as the first additive.

[0077] Example 9

[0078] This embodiment provides an electrolyte of the same system as that of Embodiment 2. The difference between this embodiment and Embodiment 2 is that the compound represented by Formula I-3 is used as the first additive.

[0079] Example 10

[0080] This embodiment provides an electrolyte of the same system as that of Embodiment 2. The difference between this embodiment and Embodiment 2 is that the compound represented by Formula I-4 is used as the first additive.

[0081] Embodiment 11

[0082] This embodiment provides an electrolyte of the same system as that of Embodiment 2. The difference between this embodiment and Embodiment 2 is that the third additive thiophene (SF) is added to the electrolyte instead of the fourth additive; and the mass content of the second additive VC in the electrolyte is adjusted to 2%, and the mass content of the third additive SF is adjusted to 1%.

[0083] Example 12

[0084] This embodiment provides an electrolyte of the same system as that of Embodiment 11. The difference between this embodiment and Embodiment 11 is that a fourth additive FEC and DTD are added to the electrolyte, and the mass content of FEC added to the electrolyte is 1%, and the mass content of DTD added is 1%.

[0085] Embodiment 13

[0086] This embodiment provides an electrolyte of the same system as that of Embodiment 12. The difference between this embodiment and Embodiment 12 is that, in the electrolyte, the mass content of the second additive VC is adjusted to 1.5%, and the mass content of the third additive SF is adjusted to 0.5%.

[0087] Embodiment 14

[0088] This embodiment provides an electrolyte of the same system as that of Embodiment 12. The difference between this embodiment and Embodiment 12 is that in the electrolyte, the mass content of the second additive VC is adjusted to 3%, and the mass content of the third additive SF is adjusted to 1%.

[0089] Embodiment 15

[0090] This embodiment provides an electrolyte of the same system as that of Embodiment 12. The difference between this embodiment and Embodiment 12 is that, in the electrolyte, the mass content of the first additive in the electrolyte is adjusted to 0.05%.

[0091] Example 16

[0092] This embodiment provides an electrolyte of the same system as that of Embodiment 12. The difference between this embodiment and Embodiment 12 is that, in the electrolyte, the mass content of the first additive in the electrolyte is adjusted to 3%.

[0093] Embodiment 17

[0094] This embodiment provides an electrolyte of the same system as that of Embodiment 12. The difference between this embodiment and Embodiment 12 is that the compound represented by Formula I-2 is used as the first additive.

[0095] Embodiment 18

[0096] This embodiment provides an electrolyte of the same system as that of Embodiment 12. The difference between this embodiment and Embodiment 12 is that the compound represented by Formula I-3 is used as the first additive.

[0097] Embodiment 19

[0098] This embodiment provides an electrolyte of the same system as that of Embodiment 12. The difference between this embodiment and Embodiment 12 is that the compound represented by Formula I-4 is used as the first additive.

[0099] Comparative Example 1

[0100] 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 is not added to the electrolyte.

[0101] Comparative Example 2

[0102] This comparative example provides an electrolyte of the same system as that of Example 2. The difference between this comparative example and Example 2 is that the first additive is not added to the electrolyte.

[0103] Comparative Example 3

[0104] This comparative example provides an electrolyte of the same system as that of Example 2. The difference between this comparative example and Example 2 is that the third additive is added to the electrolyte instead of the second additive, and the mass content of the third additive in the electrolyte is 3%.

[0105] Comparative Example 4

[0106] This comparative example provides an electrolyte of the same system as Example 12. The difference between this comparative example and Example 12 is that in the electrolyte, the mass content of the second additive VC is adjusted to 1%, and the mass content of the third additive SF is adjusted to 2%.

[0107] Comparative Example 5

[0108] This comparative example provides an electrolyte of the same system as Example 12. The difference between this comparative example and Example 12 is that in the electrolyte, the mass content of the second additive VC is adjusted to 1%, and the mass content of the third additive SF is adjusted to 1%.

[0109] Comparative Example 6

[0110] This comparative example provides an electrolyte of the same system as Example 12. The difference between this comparative example and Example 12 is that in the electrolyte, the mass content of the second additive VC is adjusted to 0.5%, and the mass content of the third additive SF is adjusted to 0.5%.

[0111] The electrolytes prepared in Examples 1 to 19 and Comparative Examples 1 to 6 were used in lithium ion secondary batteries to verify the efficacy of the present invention. The preparation process of the lithium ion secondary battery is as follows:

[0112] 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.

[0113] 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.

[0114] Selection of diaphragm: A polypropylene film with a thickness of 12 μm was used as the diaphragm.

[0115] 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.

[0116] The performance test was performed on the lithium ion batteries assembled with the electrolytes provided in Examples 1 to 19 and Comparative Examples 1 to 6, wherein the electrolyte parameters and test results of Examples 1 to 19 and Comparative Examples 1 to 6 are shown in Table 1, and the test method is as follows:

[0117] (1) Beginning of Life Direct Current Resistance (BOL DCR) test: Adjust the temperature of the thermostat to 25℃, place the battery in the thermostat and let it stand for 10 minutes, and charge it to 3.85V at a constant current rate of 0.33C, and then charge it to a current rate of 0.05C at a voltage of 3.85V; after the battery is left to stand for 30 minutes, discharge it to 2.0V at a constant current rate of 0.33C, let it stand for 10 minutes, repeat twice, and take the last discharge capacity as C0. Charge the battery to 3.85V at a constant current rate of 0.33C, and then charge it to a current rate of 0.05C at a constant voltage; after the battery is left to stand for 30 minutes, discharge it to 50% C0 capacity at a current rate of 0.33C, let it stand for 1 hour, and record the static terminal voltage V0; finally, discharge the battery at a constant current rate of 4C0 for 30 seconds, and record the voltage V1. Then BOL DCR = (V0-V1) / 4C0.

[0118] (2) Cycle performance test: adjust the temperature of the thermostat to 45°C, place the battery in the thermostat and let it stand for 30 minutes, and charge it to 3.85V at a constant current of 2C, and then charge it to a current rate of 0.05C at a voltage of 3.85V; after the battery is allowed to stand for 10 minutes, discharge it to 2.0V at a constant current of 1C; after cycling the above charge and discharge steps for 800 times, record the discharge capacity C1 of the battery cell at the 800th cycle, then the capacity retention rate after 800 cycles at 45°C = C1 / C0 × 100%.

[0119] Table 1: Performance test results of electrolyte assembled batteries in Examples 1 to 19 and Comparative Examples 1 to 6

[0120]

[0121]

[0122] The contents in Table 1 are all the mass percentages of the components relative to the electrolyte.

[0123] By comparing the test results of Example 1 and Comparative Example 1 as well as Examples 2 to 10 and Comparative Example 2, it can be known that compared with the existing electrolyte that only uses VC as a positive electrode film-forming additive, the present application scheme introduces a combination of the compound of formula I and VC in the electrolyte as additives, so as to introduce amine functional groups into the positive electrode interface polymer film to improve the mechanical strength and chemical stability of the CEI membrane, and introduces the carbonyl group of the carboxylate into the positive electrode interface polymer film to improve the ionic conductivity of the CEI membrane, thereby achieving a balance between positive electrode interface stability and kinetics, and effectively improving the impedance and cycle performance of the battery compared to the comparative example.

[0124] By comparing the test results of Example 2 and Examples 12 to 14, it can be known that continuing to introduce SF with a mass content lower than VC into the electrolyte can further improve the electronic conductivity of the CEI film formed at the positive electrode interface, reduce the local impedance of the positive electrode interface, and enable the non-uniform lithium desorption area of ​​the positive electrode interface to quickly self-discharge, so as to ensure uniform lithium desorption at the positive electrode interface, improve the cycle performance of the battery, and further increase the cycle life of the battery.

[0125] By comparing the test results of Examples 12 to 14 and Comparative Example 3, it can be seen that in Comparative Example 3, only SF is added to match the compound of Formula I as an additive, which will introduce too much conductive material into the electrode interface film formed by the additive and destroy the chemical stability of the interface film, causing the electrolyte and the interface film to react with each other, thereby deteriorating the high-temperature cycle capacity of the battery. Compared with Comparative Example 3, the embodiment of this scheme uses appropriate amounts of VC and SF as additives in the electrolyte to avoid introducing too much conductive material into the electrode interface film as in Comparative Example 3 and destroying the overall insulation properties of the CEI film. The electrolyte of this scheme can reduce the local impedance of the CEI film while ensuring the chemical stability of the CEI film and improving the high-temperature cycle life of the battery.

[0126] By comparing the test results of Example 12 and Comparative Example 4, and Example 13 and Comparative Example 5, it can be known that the present solution limits the SF content added to the electrolyte to be lower than the VC content, so as to take into account the improvement of the local impedance and chemical stability of the positive electrode interface. However, if the SF added to the electrolyte in the comparative example is greater than or equal to VC, too much SF additive will introduce too much conductive material into the CEI film formed at the positive electrode interface, and destroy the insulation stability of the CEI film, causing the CEI film to react with the electrolyte, thereby damaging the protective effect of the CEI film on the positive electrode, promoting the loss and decomposition of the electrolyte, and thus making the high temperature cycle capacity of the battery worse than that of the present solution.

[0127] By comparing the test results of Examples 2 to 7, Example 12, and Examples 15 to 16, it can be known that when the mass content of the first additive in the electrolyte is limited to an appropriate range of 0.05% to 3%, the impedance of the battery can be increased while promoting the formation of a stable CEI film at the positive electrode interface. When the mass content of the first additive in the electrolyte is less than 0.05%, the CEI film at the positive electrode interface is insufficiently formed, making it difficult to effectively protect the positive electrode, thereby negatively affecting the cycle capacity of the battery; when the mass percentage of the additive in the electrolyte is higher than 3%, the impedance of the CEI film at the positive electrode interface is too high, which has a negative impact on the cycle performance of the battery.

[0128] From the test results of Examples 2, 8 to 10, 12, and 17 to 19, it can be seen that the compounds of Formula I-1, Formula I-2, Formula I-3, and Formula I-4 have the same structure and functional groups as the compound of Formula I, so when used as additives with VC or with VC and SF, they have the same improvement effect on the impedance, high temperature storage, and cycle performance of the battery. Therefore, the use of compounds of Formula I with different types of R1, R2, R3, and R4 groups has little effect on the conductivity and high temperature cycle performance of the battery.

[0129] 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: including a lithium salt, a solvent, a first additive, and a second additive; The first additive comprises a compound represented by formula I: R1, R2, R3, and R4 in Formula I are each independently selected from a substituent having 1 to 6 carbon atoms, 0 to 4 unsaturations, and 0 to 3 heteroatoms, wherein the heteroatoms are selected from any one of nitrogen atoms, sulfur atoms, oxygen atoms, boron atoms, and phosphorus atoms; The second additive includes a vinyl ester.

2. The electrolyte according to claim 1, characterized in that In the electrolyte, the mass content of the first additive is 0.05% to 3%, and the mass content of the second additive is less than or equal to 3%.

3. The electrolyte according to claim 2, characterized in that The mass content of the first additive in the electrolyte is 0.1% to 0.5%.

4. The electrolyte according to claim 1 or 2, characterized in that The electrolyte further includes a third additive, and the third additive includes thiophene; in the electrolyte, the mass content of the second additive is X, and the mass content of the third additive is Y; wherein X>Y and 2%≤X+Y≤4%.

5. The electrolyte according to claim 1, characterized in that R1, R2, R3, and R4 in Formula I 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 I comprises at least one of the following compounds:

7. The electrolyte according to claim 1, characterized in that The electrolyte further includes a fourth additive, which is selected from at least one of fluoroethylene carbonate, propylene-1,3-sultone, vinyl sulfate and tetravinylsilane, and the mass content of the fourth additive in the electrolyte is 0.1% to 2%.

8. The electrolyte according to claim 1, characterized in that The lithium salt comprises at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methanesulfonate and lithium trifluoromethylsulfonate; the mass content of the lithium salt in the electrolyte is 12% to 17%.

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.

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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