Electrolyte and secondary battery

Abstract

This invention provides an electrolyte and a secondary battery, belonging to the field of secondary battery technology. The electrolyte includes a lithium salt, a solvent, a first additive, and a second additive. The first additive includes fluoroethylene carbonate; the second additive includes a compound represented by formula (1): R1 and R2 in formula (1) are each independently selected from substituents with 1 to 6 carbon atoms, an unsaturation degree of 0 to 4, and a heteroatom number of 0 to 3. The electrolyte provided by this invention, through the copolymerization reaction between FEC and the compound represented by formula (1), further forms an ordered and densely stable SEI film, which can further prevent the electrolyte from undergoing side reactions with the negative electrode without increasing impedance, thereby improving the cycle stability of the electrolyte and enhancing the cycle performance of the battery. At the same time, during high-temperature storage, the excellent acidity suppression ability of the compound represented by formula (1) effectively alleviates the acidity increase caused by FEC decomposition, thereby improving the high-temperature storage performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, specifically to an electrolyte and a secondary battery. Background Technology

[0002] Lithium-ion batteries are widely used in consumer electronics, electric vehicles, and energy storage due to their advantages such as 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 interphase (SEI) film. In the SEI film, it forms a complex with lithium salts, thereby inhibiting the dissolution of lithium salts in the electrolyte and avoiding side reactions between the electrodes and the electrolyte. This effectively improves the cycle life of the battery and also significantly enhances the low-temperature performance of the battery.

[0004] However, when electrolytes containing FEC are stored at high temperatures, the FEC undergoes a defluorination reaction, which causes the acidity of the electrolyte to rise continuously, damaging 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-mentioned technical problems. Summary of the Invention

[0006] In view of the above-mentioned shortcomings of the prior art, the present invention provides an electrolyte and a secondary battery to improve the technical problem that existing FEC-containing electrolytes are prone to defluorination reaction at high temperatures, which leads to an increase in electrolyte acidity and affects battery storage and cycle performance.

[0007] To achieve the above and other related objectives, the present invention provides an electrolyte comprising a lithium salt, a solvent, a first additive, and a second additive; wherein the first additive comprises fluoroethylene carbonate; and the second additive comprises a compound represented by formula (1):

[0008]

[0009] In formula (1), R1 and R2 are each independently selected from substituents with 1 to 6 carbon atoms, 0 to 4 degrees of unsaturation, and 0 to 3 heteroatoms. The heteroatoms are selected from any one of nitrogen, sulfur, oxygen, boron, and phosphorus atoms.

[0010] In one example of the present invention, the first additive accounts for 0.5% to 1% of the mass of the electrolyte; and / or, the second additive accounts for 0.05% to 3% of the mass of the electrolyte.

[0011] In one example of the present invention, the second additive accounts for 0.1% to 0.5% of the mass of the electrolyte.

[0012] In one example of the present invention, R1 and R2 in formula (1) are each independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino or heterocyclic, wherein the heterocyclic is selected from any one of pyridine, 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 one example of the present invention, the electrolyte further includes a third additive selected from at least one of propylene-1,3-sulfonyl lactone 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 lithium salt accounts for 12% to 16% of the mass of the electrolyte.

[0018] In one example of the present invention, the solvent comprises carbonate and carboxylic acid ester; 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 carboxylic acid ester is selected from at least one of ethyl formate, ethyl acetate, propyl acetate and ethyl propionate; the solvent accounts for 70% to 80% of the electrolyte by mass.

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

[0020] In the electrolyte provided by this invention, the first additive FEC and the second additive compound shown in formula (1) are used together. By utilizing the copolymerization reaction between FEC and the compound shown in formula (1), an ordered and dense and stable SEI film is further formed. This can further prevent the side reaction between the electrolyte and the negative electrode without increasing the impedance, thereby improving the cycle stability of the electrolyte and enhancing the cycle performance of the battery. At the same time, during high-temperature storage, the excellent acidity suppression ability of the compound shown in formula (1) can effectively alleviate the acidity increase caused by FEC decomposition, thereby improving the high-temperature storage performance of the battery. Detailed Implementation

[0021] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other. It should also be understood that the terminology used in the embodiments of the present invention is for describing specific implementation schemes and not for limiting the scope of protection of the present invention. Test methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or according to the conditions recommended by the respective manufacturers.

[0022] For simplicity, this article only explicitly discloses some numerical ranges, and every point or single value between the endpoints of the range is included within the range. Therefore, each point or single value can be used as its own lower or upper limit and combined with any other point or single value, or combined with other lower or upper limits to form a range that is not explicitly stated.

[0023] Unless otherwise specified, "%" in this article refers to the percentage content by mass.

[0024] In this article, substituents refer to atoms or groups of atoms that replace those on the main chain or rings of organic compounds. They can replace a hydrogen atom or other atoms in a molecule, affecting its chemical and physical properties. The type and position of substituents have a significant impact on the reactivity, polarity, solubility, and other properties of a molecule.

[0025] Unsaturation degree, also known as the hydrogen deficiency index or cycloaddition double bond index, is a quantitative indicator of the degree of unsaturation in organic compound molecules. For hydrocarbons with the molecular formula CnHm and their derivatives with the molecular formula CnHmOx, if m < 2n+2, then the hydrocarbon and its hydrocarbon group possess a certain degree of unsaturation Ω. That is, compared to open-chain alkanes with the same number of carbon atoms, for every two fewer hydrogen atoms, the degree of unsaturation of the organic compound increases by 1.

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

[0027] The first aspect of the present invention provides an electrolyte in which a first additive FEC and a second additive compound of formula (1) are used together. By utilizing the copolymerization reaction between FEC and the compound of formula (1), an ordered and densely stable SEI film is further formed. This can further prevent the side reaction between the electrolyte and the negative electrode without increasing the impedance, thereby improving the cycle stability of the electrolyte and enhancing the cycle performance of the battery. Furthermore, by utilizing the alkaline properties of the compound of formula (1), the acidity of the electrolyte caused by the high-temperature decomposition of FEC is suppressed, thereby improving the chemical stability of the electrolyte and improving the high-temperature storage performance of the battery.

[0028] The electrolyte comprises lithium salt, solvent, first additive, and second additive. The solvent, as the main component of the electrolyte, is used to dissolve the lithium salt and additives. After the lithium salt dissolves in the organic solvent, it can release a large number of active lithium ions, giving the electrolyte good conductivity. The additives, as auxiliary substances of the electrolyte, can be selected according to actual needs, such as film-forming additives, flame-retardant additives, high-voltage additives, etc.

[0029] In this invention, the first additive comprises fluoroethylene carbonate (FEC); the second additive comprises the compound shown in formula (1).

[0030] The chemical formula of formula (1) is shown below:

[0031]

[0032] In formula (1), R1 and R2 are each independently selected from substituents having 1 to 6 carbon atoms, an unsaturation degree of 0 to 4, and a heteroatom number of 0 to 3. The types of 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. 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 to these. The number of heteroatoms can be any value from 0 to 3, such as 0, 1, 2, or 3.

[0033] The first additive, FEC, exhibits excellent film-forming properties. During charge and discharge, it reacts with lithium salts to form an SEI (Sediment Inlet) interfacial film. This SEI film prevents direct contact between the electrolyte and the negative electrode, thus avoiding side reactions and improving the electrolyte's cycle stability and overall battery cycle performance. Specifically, within the SEI film, the complex formed by the reaction of FEC and lithium salt inhibits further dissolution of lithium salt from the electrolyte into the SEI film at the negative electrode interface, slowing down the SEI film's growth rate. This ensures a tightly structured SEI film while preventing excessive thickness and thus avoiding increased battery impedance. However, at high temperatures, FEC undergoes a defluorination reaction, leading to increased electrolyte acidity, which damages the SEI film on the negative electrode surface and causes high-temperature storage capacity decay.

[0034] The second additive, compound (1), is an imide compound. On the one hand, it has basic properties. When added to the electrolyte in combination with FEC, it can effectively suppress the increase in acidity caused by FEC decomposition, thereby improving the chemical stability of the electrolyte and enhancing 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 (1) can form intramolecular or intermolecular hydrogen bonds, which gives the cyclic imide molecules good molecular stability and promotes the orderly arrangement between molecules. In the process of participating in the formation of the SEI film, compound (1) can preferentially form a uniform prepolymer and then form the SEI film, thereby bringing the high structural strength and orderly arrangement characteristics between cyclic imide molecules into the SEI film, forming a chemically stable and densely arranged SEI film, which improves the cohesion and mechanical strength of the SEI film and helps the SEI film maintain its integrity at high temperatures.

[0035] In some embodiments, the mass percentage of the first additive FEC in the electrolyte is any value between 0.5% and 1%, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%. The inventors have found that limiting the mass percentage of the first additive in the electrolyte within the above range ensures that FEC improves battery cycle performance without causing excessive degradation of the battery's high-temperature storage performance. If the mass percentage of the first additive FEC in the electrolyte is less than 0.5%, a sufficiently thick SEI film cannot be formed at the negative electrode interface, resulting in poor protection of the negative electrode. If the mass percentage of the first additive FEC in the electrolyte is greater than 1%, excessive FEC at high temperatures can easily cause an excessive increase in electrolyte acidity, preventing the second additive from effectively regulating the electrolyte pH at an appropriate content, leading to continuous damage to the negative electrode SEI film and causing a decrease in the battery's high-temperature storage capacity.

[0036] In some embodiments, the mass percentage of the second additive compound of formula (1) in the electrolyte is any value from 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 percentage of the second additive in the electrolyte is any value from 0.1% to 0.5%, for example, 0.1%, 0.2%, 0.3%, 0.4%, or 0.5%. The inventors have found that limiting the mass percentage of the second additive in the electrolyte within the above range can effectively slow down the increase in acidity of the electrolyte caused by FEC decomposition at high temperatures and promote the formation of a stable and appropriately thick SEI film at the negative electrode interface, thereby improving the electrochemical stability of the battery without excessively increasing the battery impedance. When the mass percentage of the second additive in the electrolyte is too low, a sufficiently thick SEI film cannot be formed at the negative electrode interface, nor can it suppress the increase in electrolyte acidity caused by the high-temperature decomposition of FEC. When the mass percentage of the second additive in the electrolyte is too high, it will lead to an excessively thick SEI film at the negative electrode interface, thereby affecting the ionic conductivity of the negative electrode and negatively impacting the cycle performance of the battery.

[0037] In some embodiments, R1 and R2 in the compound of formula (1) are each independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino, or heterocyclic. A heterocyclic ring is defined as a ring consisting of at least one non-carbon atom in addition to a carbon atom; these non-carbon atoms are heteroatoms. As an example, heterocyclic rings include, but are not limited to, any one of pyridine, pyrrole, thiophene, thiazole, and furan. That is, R1 and R2 can both be selected from any one of alkyl, alkenyl, alkynyl, carbonyl, ester, amino, and heterocyclic rings; they can be all the same, partially the same, or both 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 compounds (1-1), (1-2), (1-3), and (1-4) are shown below:

[0039]

[0040]

[0041] In some embodiments, the electrolyte further includes a third additive selected from at least one of propylene-1,3-sulfonyl lactone (PST) and tetravinylsilane (TVSi). The third additive helps promote the formation of a stable SEI film at the negative electrode interface, further enhancing the film-forming effect of the electrolyte at the negative electrode interface, strengthening the protection of the negative electrode interface, and improving the charge-discharge cycle performance of the battery.

[0042] In some embodiments, the third additive is selected from one or a combination of two of PST and TVSi, and the mass percentage of the third additive in the electrolyte is any value between 0.05% and 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 percentage of the third additive in the electrolyte is within the above range, it can promote the formation of an interface film at the interface between the negative electrode and even the positive electrode, improve the high-temperature stability of the electrode interface film, and not excessively increase the impedance of the battery. When the mass percentage of the third additive in the electrolyte is too low, the formation of the SEI film at the negative electrode interface and even the solid electrolyte interphase (CEI) film at the positive electrode interface will be insufficient, making it difficult to effectively protect the electrode interface and thus negatively affecting the cycle capacity of the battery. When the mass percentage of the third additive in the electrolyte is too high, the impedance of the SEI film at the negative electrode interface will be too high, negatively affecting the cycle performance of the battery.

[0043] Furthermore, it should be noted that the lithium salt in the electrolyte can be any type of lithium salt conventional in the art. For example, the lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethyl)sulfonylimide (LiTFSI), lithium acetate (CH3COOLi), lithium methanesulfonate (CH3SO3Li), and lithium trifluoromethylsulfonate (CF3SO3Li). Preferably, the lithium salt is selected from lithium hexafluorophosphate (LiPF6) which has better overall performance, or lithium hexafluorophosphate (LiPF6) is used as the main component, with appropriate amounts of other lithium salts added to leverage their advantages and comprehensively improve electrolyte performance. In some embodiments, the mass percentage of the lithium salt in the electrolyte is any value between 12% and 16%, for example, 12%, 13%, 14%, 15%, or 16%. For example, in one example, the lithium salt includes lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonyl imide (LiFSI), with the mass ratio of lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonyl imide (LiFSI) in the electrolyte being (13-14):(1-2).

[0044] The solvent in the electrolyte can be any solvent conventionally used in lithium-ion batteries. The solvent includes at least one of carbonates and carboxylic acid esters, 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 carboxylic acid ester 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 any one of the solvents listed above, or any two or a combination of two. For example, the solvent could be a combination of EF and EC, a combination of EC, DMC, and EMC, or a combination of EC, DMC, PC, and EA, etc. In some embodiments, the solvent comprises at least carbonates and carboxylic acid esters, and the mass percentage of the solvent in the electrolyte is typically any value between 70% and 88%, such as 70%, 73%, 75%, 78%, 80%, 83%, 85%, 86%, 87%, or 88%. Optionally, the mass percentage of the solvent in the electrolyte is any value between 70% and 80%, such as 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, or 78%.

[0045] In addition, in some implementations, the solvent may 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 valerate.

[0046] The electrolyte of this invention can be prepared using conventional methods. For example, the electrolyte can be prepared in a glove box, where the argon or nitrogen content is 99.999%, the actual oxygen content is 0.1 ppm, and the moisture content is less than 10 ppm. In the glove box, the solvent is mixed evenly according to a predetermined ratio, and then the fully dried lithium salt and additives are added to the solvent and mixed evenly to prepare the electrolyte. The content of each component in the electrolyte is a weight percentage calculated based on the total weight of the electrolyte.

[0047] A second aspect of this invention also provides a secondary battery, comprising a positive electrode, a negative electrode, a separator, and the electrolyte described above. During the charging and discharging process, lithium ions repeatedly insert and extract between the positive and negative electrodes. The separator is disposed between the positive and negative electrodes, serving as an isolation layer. The electrolyte conducts lithium ions between the positive and negative electrodes. It should be noted that the structural type of the aforementioned secondary battery is not limited; the secondary battery can be any commercially available conventional structure, such as a pouch battery, a prismatic battery, or a cylindrical battery.

[0048] Furthermore, 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 powering electronic devices. Electronic devices include, but are not limited to, mobile phones, tablets, laptops, electric toys, electric vehicles, new energy vehicles, ships, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc.

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

[0050] The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive current collector is, for example, a foil formed by surface treatment of materials such as nickel, titanium, aluminum, silver, stainless steel, or carbon. Besides foil, the positive current collector can also be used in any combination of one or more forms such as film, mesh, porous, foam, or non-woven fabric. The thickness of the positive current collector is, for example, 8 μm-15 μm. In one embodiment, the positive current collector is, for example, an aluminum foil, and the thickness of the aluminum foil is, for example, 13 μm. The positive current collector has two surfaces opposite each other in its thickness direction, and the positive active material layer is disposed on either 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. No specific limitations are placed on the positive active material, positive conductive agent, and positive binder here; those skilled in the art can select them according to actual needs.

[0051] The positive electrode active material can be any material suitable for lithium-ion batteries, i.e., compounds that can reversibly insert and deintercalate lithium ions. For example, the positive electrode active material can be selected from ternary materials, lithium-containing phosphates, and spinel materials. Ternary materials include lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide doped with metal ions, and lithium nickel cobalt aluminum oxide doped with metal ions, etc.; lithium-containing phosphates include lithium manganese iron phosphate, lithium iron phosphate, and lithium manganese iron phosphate, etc. The binder for the positive electrode is selected from, for example, any one or more of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexanefluoropropylene, or polymerized styrene-butadiene rubber (SBR). The positive electrode conductive agent is selected from one or a combination of multiple materials such as carbon black, acetylene black, graphene, carbon nanotubes, and carbon nanofibers in any proportion.

[0052] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative current collector can be made of a material with good conductivity and mechanical strength, such as copper foil. The negative current collector has two surfaces opposite each other in its own thickness direction, and the negative active material layer is disposed on either or both of the two opposite surfaces of the negative current collector. The negative active material layer includes a negative active material, a negative conductive agent, a negative binder, and a thickener. No specific limitations are placed on the specific types of negative active material, negative conductive agent, and negative binder; materials known in the art for use in lithium-ion batteries can be used, and those skilled in the art can select them according to actual needs.

[0053] The negative electrode active material is selected from compounds capable of intercalating and deintercalating lithium ions. For example, the negative electrode active material is selected from one or more combinations of carbon-based and silicon-based materials. Carbon-based materials include, for example, soft carbon, hard carbon, artificial graphite, and natural graphite. Silicon-based materials include, for example, elemental silicon, silicon oxide compounds, and silicon-carbon compounds. The negative electrode conductive agent is selected from one or a combination of two or more of the following: carbon black, acetylene black, graphene, carbon nanotubes, and carbon nanofibers, in any proportion. The negative electrode binder is selected from any one or a combination of several of the following: polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR), 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 field, such as polyethylene (PE), polypropylene (PP), glass fiber membrane, polyethylene membrane, or composite membrane. The diaphragm thickness is 9-18 μm, the air permeability is 180-380 s / 100 mL, and the porosity is 30-50%.

[0055] Battery assembly is performed using conventional methods. For example, after all the electrodes, separators, and electrolytes are prepared, the negative electrode, separator, and positive electrode are stacked and wound sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. The wound or stacked electrode assembly is then encapsulated in a housing, transferred to a vacuum oven at 120°C for drying, injected with 3.0 g / Ah of the prepared electrolyte, sealed, and subjected to a formation process to finally produce a 1 Ah lithium-ion battery.

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

[0057] Example 1

[0058] This embodiment provides an electrolyte comprising 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) in a volume ratio of 3:4:4; the lithium salt is lithium hexafluorophosphate (LiPF6), and the lithium salt accounts for 12% of the electrolyte by mass; the first additive is fluoroethylene carbonate (FEC), and the first additive accounts for 0.5% of the electrolyte by mass; the second additive is the compound shown in formula (1-1), and the second additive accounts for 0.3% of the electrolyte by mass.

[0059] The electrolyte preparation process 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 mixture is stirred evenly to obtain the electrolyte. Specifically, 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 compound (1-1) added is 0.3% of the total weight of the electrolyte.

[0060] Example 2

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

[0062] Example 3

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

[0064] Example 4

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

[0066] Example 5

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

[0068] Example 6

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

[0070] Example 7

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

[0072] Example 8

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

[0074] Example 9

[0075] This embodiment provides an electrolyte with the same system as in Example 1. The difference between this embodiment and Example 1 is that the compound shown in formula (1-2) is used as a second additive.

[0076] Example 10

[0077] This embodiment provides an electrolyte with the same system as in Example 1. The difference between this embodiment and Example 1 is that the compound shown in formula (1-3) is used as a second additive.

[0078] Example 11

[0079] This embodiment provides an electrolyte with the same system as in Example 1. The difference between this embodiment and Example 1 is that the compound shown in formula (1-4) is used as a second additive.

[0080] Comparative Example 1

[0081] This comparative example provides an electrolyte with the same system as in Example 1. The difference between this comparative example and Example 1 is that no first additive FEC and second additive compound of formula (1-1) are added to the electrolyte.

[0082] Comparative Example 2

[0083] This comparative example provides an electrolyte with the same system as that in 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 with the same system as that in Example 1. The difference between this comparative example and Example 1 is that no second additive compound of formula (1-1) is added to the electrolyte.

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

[0087] Preparation of the positive electrode sheet: The positive electrode active material LiFePO4, the positive electrode binder polyvinylidene fluoride, and the positive electrode conductive agent Super P were mixed at a mass ratio of 97:1:2. N-methylpyrrolidone solvent was added, and the mixture was stirred under vacuum until it became homogeneous and transparent, obtaining the positive electrode slurry. The positive electrode slurry was uniformly coated onto aluminum foil, which was then air-dried at room temperature and transferred to an oven for further drying. After cold pressing and slitting, the positive electrode sheet was obtained.

[0088] Preparation of negative electrode sheet: The negative electrode active material artificial graphite, negative electrode conductive agent Super P, negative electrode thickener sodium carboxymethyl cellulose (CMC-Na), and negative electrode binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96:1:1:2. After adding deionized water, the mixture is stirred thoroughly under the action of a vacuum mixer until uniform to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, dried at room temperature, and then transferred to an oven for drying. The negative electrode sheet is obtained through cold pressing, slitting and other processes.

[0089] Selection of diaphragm: A 12μm thick polypropylene membrane was used as the diaphragm.

[0090] Battery fabrication: The positive electrode, separator, and negative electrode are placed sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. These are then wound to form a bare cell. An aluminum-plastic film is then wrapped around the cell, and the cell is dried in a vacuum oven at 120°C. The prepared electrolyte is injected at a rate of 3.0 g / Ah, and the cell is sealed. Electrolysis is then performed to obtain a 1 Ah soft-pack lithium-ion battery.

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

[0092] Battery cycle performance test: At room temperature (25℃), the lithium-ion battery was charged to 3.65V at a constant current rate of 1C, and then charged at a constant voltage of 3.65V until the current reached 0.05C. After the battery was allowed to rest for 10 minutes, it was discharged to 2.5V at a constant current rate of 1C, and the initial discharge capacity C0 was recorded. After repeating the above charge-discharge steps 2000 times, the discharge capacity C1 of the battery on the 2000th cycle was recorded. The capacity retention rate of the battery after 2000 cycles at room temperature was then calculated as C1 / C0 × 100%.

[0093] High-temperature storage performance test of the battery: At room temperature of 60℃, the lithium-ion battery was charged to 3.65V at a constant current rate of 0.33C, and then charged at 3.65V at a constant voltage rate until the current reached 0.05C. The battery was then discharged to 2.5V at a constant current rate of 0.33C, allowed to stand for 30 minutes, and this process was repeated 2-3 times. The discharge capacity C0 of the lithium-ion battery at the last 0.33C discharge was recorded. The fully charged lithium-ion battery was stored in a 60℃ oven for 60 days. Afterward, the battery was removed and subjected to charge-discharge cycles at room temperature at a rate of 0.33C / 0.33C within a voltage range of 2.5V-3.65V for two cycles. The discharge capacity C1 of the battery at the last 0.33C discharge was recorded. The capacity recovery rate of the lithium-ion battery after 60 days of high-temperature storage relative to its initial value was calculated using 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 mass percentages of the component relative to the electrolyte.

[0098] Comparing the test results of Examples 1-11 and Comparative Example 1, it can be seen that using FEC and the compound of formula (1) as additives in the electrolyte can 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; it can also form a tightly ordered SEI film at the negative electrode interface, thereby reducing the dissolution of lithium salt in the electrolyte in the SEI film, isolating the contact side reaction between the negative electrode and the electrolyte, and effectively improving the cycle performance of the battery.

[0099] Comparing the test results of Examples 1, 5-11 and Comparative Examples 2-3, it can be seen that, compared with Comparative Example 2, which only used the compound of Formula (1) as an additive, the electrolyte had poor film formation effect at the negative electrode and could not effectively improve the battery cycle performance; or Comparative Example 3, which only used FEC as an additive, the electrolyte was severely acidified at high temperature, resulting in the battery's high-temperature storage capacity decay; In this embodiment, the compound of Formula (1) and an appropriate amount of FEC are introduced into the electrolyte to be used as additives, which can maintain the improvement effect of FEC on battery cycle performance, and can effectively inhibit the acidification of the electrolyte at high temperature through the compound of Formula (1), thereby improving the battery's high-temperature storage performance.

[0100] Comparing the test results of Examples 1 to 4, it can be seen that when the mass percentage of the first additive FEC in the electrolyte is limited to an appropriate range of 0.5% to 1%, a stable SEI film can be formed at the negative electrode interface without excessively increasing the battery impedance. The acidic substances released from its decomposition at high temperatures can also be neutralized by the second additive. When the mass percentage of the first additive FEC in the electrolyte is less than 0.5%, the SEI film formation at the negative electrode interface will be insufficient, making it difficult to effectively protect the negative electrode, thus negatively impacting the battery's cycle capacity. When the mass percentage of the first additive FEC in the electrolyte is greater than 1%, excessive FEC will lead to excessively high impedance of the SEI film at the negative electrode interface, and at high temperatures, the acidity of the electrolyte will increase beyond the suitable adjustment range of the second additive, causing continuous damage to the negative electrode SEI film, thus negatively impacting the battery's cycle capacity and high-temperature storage capacity.

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

[0102] The test results of Examples 1, 9, 10, and 11 show that compounds (1-1), (1-2), (1-3), and (1-4) have the same structure and functional groups as the compound of formula (1). Therefore, when used in combination with FEC as an additive, they can all improve the battery cycle performance and high-temperature storage performance. Thus, the use of compounds of formula (1) with different types of R1 and R2 groups has little impact 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 invention. Any person skilled in the art can 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 those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. An electrolyte, characterized in that, Includes lithium salt, solvent, first additive, and second additive; the first additive includes fluoroethylene carbonate; the second additive includes a compound represented by formula (1): In formula (1), R1 and R2 are each independently selected from substituents with 1 to 6 carbon atoms, 0 to 4 degrees of unsaturation, and 0 to 3 heteroatoms. The heteroatoms are selected from any one of nitrogen, sulfur, oxygen, boron, and phosphorus atoms. The first additive has a mass percentage of 0.5% to 1% in the electrolyte, and the second additive has a mass percentage of 0.05% to 3% in the electrolyte.

2. The electrolyte according to claim 1, characterized in that, The second additive accounts for 0.1% to 0.5% of the mass of the electrolyte.

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

4. The electrolyte according to claim 1 or 3, characterized in that, The compound represented by formula (1) includes at least one of the following compounds:

5. The electrolyte according to claim 1, characterized in that, The electrolyte also includes a third additive selected from at least one of propylene-1,3-sulfonyl lactone and tetravinylsilane.

6. 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% of the mass of the electrolyte.

7. The electrolyte according to claim 1, characterized in that, The solvent comprises carbonates and carboxylic acid esters; the carbonate is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, and the carboxylic acid ester is selected from at least one of ethyl formate, ethyl acetate, propyl acetate, and ethyl propionate; the solvent accounts for 70% to 80% of the electrolyte by mass.

8. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte according to any one of claims 1 to 7.

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