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 a compound represented by Formula I, and the second additive includes vinylene ester. The electrolyte provided by this invention, on the one hand, utilizes the tertiary amino group in the compound of Formula I to adjust the pH of the electrolyte, making the chemical environment of the electrolyte more stable; on the other hand, it leverages the property of vinylene ester to polymerize on the positive electrode surface under high voltage initiation, guiding the unsaturated double bond of the compound of Formula I to participate in the polymerization process on the positive electrode surface. During the polymerization process, amine substances and salts generated by the reaction of amine groups with acidic substances are introduced to form a polymer film with high mechanical strength and chemical stability on the positive electrode surface.

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, the upper limit of the operating voltage of cathode materials for high-capacity lithium-ion batteries (such as ternary materials or lithium iron phosphate materials) has been increasing and gradually approaching the material limit after development. At high voltages (>3.80V), the electronic impedance and polarization of cathode materials increase, making them prone to lattice collapse caused by uneven delithiation during charge and discharge. This leads to an increase in side reactions between the cathode and the electrolyte, resulting in a decrease in battery cycle life. At the same time, the acidity of the carboxylic acid ester system electrolyte is prone to increase at high temperatures, making it more likely to react with the cathode material, which will further deteriorate the battery capacity and cycle life.

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

[0005] 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 the electrolyte is prone to side reactions with the positive electrode material under high temperature and high voltage environment, which deteriorates the battery cycle performance.

[0006] 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 a compound represented by Formula I, and the second additive comprises a vinylene ester;

[0007]

[0008] In Formula I, R1, R2, R3, and R4 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 heteroatom is selected from any one of nitrogen, sulfur, oxygen, boron, and phosphorus atoms.

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

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

[0011] In one example of the present invention, the electrolyte further includes a third additive, which 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 each independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino, or heterocyclic groups, wherein the heterocyclic group 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 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, which is selected from at least one of fluoroethylene carbonate, propylene-1,3-sulfonyl lactone, ethylene 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 selected from lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methanesulfonate, and lithium trifluoromethylsulfonate; the lithium salt has a mass content of 12% to 17% in the electrolyte.

[0017] In one example of the present invention, the solvent comprises carbonates and carboxylic esters; 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 ester 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, a negative electrode, a separator, and an electrolyte as described in any of the above examples.

[0019] The electrolyte provided by this invention introduces the compound shown in Formula I and vinylene as additives. On the one hand, the tertiary amino group in the compound of Formula I is used to adjust the pH of the electrolyte, making the chemical environment of the electrolyte more stable. On the other hand, the property of vinylene to polymerize on the surface of the positive electrode under high voltage is used to guide the unsaturated double bond of the compound of Formula I to participate in the polymerization process on the surface of the positive electrode. During the polymerization process, amine substances and salts generated by the reaction of amine groups with acidic substances are introduced to form a polymer film with high mechanical strength and chemical stability on the surface of the positive electrode. This effectively protects the integrity of the positive electrode structure, prevents direct contact between the electrolyte and the negative electrode, reduces irreversible consumption of lithium ions, and improves the cycle performance and life of the battery. Detailed Implementation

[0020] 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.

[0021] 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.

[0022] In this document, terms such as "multiple," "various," and "repeatedly" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" indicates one or more types. Terms such as "further," "even further," and "particularly" are used to describe purposes and indicate differences in content, but should not be construed as limiting the scope of protection of this invention.

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

[0027] The first aspect of the present invention provides an electrolyte in which the pH of the electrolyte is adjusted by introducing a compound of Formula I and vinylene as additives to avoid the increase of electrolyte acidity at high temperature, thereby improving the chemical stability of the electrolyte; and it 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 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 a compound represented by Formula I, and the second additive comprises vinylene (VC).

[0030] The chemical formula of Formula I is shown below:

[0031]

[0032] In Formula I, R1, R2, R3, and R4 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. That is, the types of substituents R1, R2, R3, and R4 in Formula I do not affect each other; they can be completely identical substituents, or partially identical substituents, such as R1 and R2 being the same, or R1, R2, and R3 being the same, etc., or they can be 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, selected from any one of nitrogen (N), sulfur (S), oxygen (O), boron (B), and phosphorus (P) atoms. For example, a heteroatom can be a nitrogen atom, a sulfur atom, or an oxygen atom, but is not limited to these. The number of heteroatoms can be any value from 0 to 3. For example, 0, 1, 2, or 3.

[0033] The compound shown in Formula I, the first additive, contains a tertiary amino group (R3N, where R represents an alkyl group, and the tertiary amino group is formed by the combination of a trimethyl group and an amino group (NH2)). Due to the lone pair of electrons on the nitrogen atom, the tertiary amino group exhibits basicity and can chemically react 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. Furthermore, the molecule of the compound shown in Formula I also contains unsaturated double bonds. The unsaturated double bonds in the molecule of the compound shown in Formula I can open under the initiation of the electric field and oxidative environment on the electrode surface and undergo copolymerization reactions with other components in the electrolyte (such as solvent molecules, other additives, etc.) to form a polymer film.

[0034] The unsaturated double bonds of the second additive, VC, act as initiators for cationic polymerization of the positive electrode. They can initiate a copolymerization reaction between VC and the compound shown in Formula I, along with solvent molecules, on the high-potential positive electrode surface, forming a polymer film on the positive electrode surface. This copolymerization reaction introduces tertiary amines and their salts from reactions with acidic substances into the polymer film, increasing the active sites and improving its ionic conductivity, mechanical strength, and chemical stability. Simultaneously, this copolymerization reaction can mitigate the increase in impedance caused by the reduction of tertiary amines in the electrolyte to excess lithium nitride on the negative electrode surface by introducing tertiary amines into the polymer film on the positive electrode surface. This polymer film can cover the positive electrode surface, preventing direct contact between the electrolyte and the positive electrode, thereby reducing side reactions between the positive electrode and the electrolyte, minimizing irreversible consumption of active lithium ions in the battery, and improving battery capacity and cycle life. 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 battery charging and discharging, protect the integrity of the electrode structure, and thus further improve the rate performance and cycle life of the battery.

[0035] In some embodiments, the mass content of the first additive in the electrolyte is any value between 0.05% and 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 between 0.1% and 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 contents of the first and second additives in the electrolyte are within the aforementioned range, they can promote the formation of a stable solid electrolyte interphase (CEI) film at the positive electrode interface without excessively increasing the battery impedance. However, if the contents of the first and second additives in the electrolyte are excessive, it will lead to an overly thick CEI film at the positive electrode interface, thereby affecting the ionic conductivity of the positive electrode and negatively impacting the battery's cycle performance.

[0038] In some embodiments, the electrolyte further includes a third additive, which includes thiophene (SF). VC, under high voltage initiation, can copolymerize with SF at the positive electrode surface, thereby introducing SF into the CEI film. Since the polymer of VC has insulating properties and the polymer of SF has good electronic conductivity, introducing SF into the CEI film formed by VC polymerization allows the insulating and conductive portions of the CEI film to be uniformly intercalated, reducing the local electronic impedance of the CEI film surface. This ensures the overall insulation of the CEI film while allowing uneven lithium delithiation in the positive electrode area to rapidly self-discharge through the CEI film surface, achieving charge balance at the positive electrode interface, ensuring uniform lithium delithiation at the positive electrode interface, reducing redox side reactions caused by electron exchange at the positive electrode interface, and reducing electrolyte decomposition losses.

[0039] In some embodiments, the mass content of the second additive VC in the electrolyte 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 1.5%, or 3% and 1%, respectively. Limiting the second additive VC and the third additive SF to the above ranges can promote the formation of a CEI film with sufficient mechanical strength and avoid the formation of an excessively thick CEI film that would excessively increase the battery impedance. Simultaneously, keeping the mass content of the third additive SF lower than that of the second additive VC can prevent the insulation of the CEI film from being compromised due to excessive content of highly conductive SF polymer, ensuring the chemical stability of the CEI film and preventing the CEI film from reacting with the electrolyte and causing electrolyte decomposition.

[0040] In some embodiments, R1, R2, R3, and R4 in the compound of Formula I are each independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino, or heterocyclic. A heterocyclic ring is defined as a ring comprising at least one non-carbon atom in addition to a carbon atom; these non-carbon atoms are called heteroatoms. As an example, heterocyclic rings include, but are not limited to, any one of pyridine, pyrrole, thiophene, thiazole, and furan. For example, a heterocyclic ring 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 rings; they can be all the same, partially the same, or all different. As an example, R1, R2, R3, and R4 can all be alkyl; or R1 and R2 can be alkyl, R3 can be alkenyl, and R4 can be alkynyl; or R1 can be alkyl, R2 can be ester, R3 can be amino, and R4 can be 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 shown below:

[0042]

[0043] In some embodiments, the electrolyte further includes a fourth additive selected from at least one of fluoroethylene carbonate (FEC), propylene-1,3-sulfonyl lactone (PST), ethylene sulfate (DTD), and tetravinylsilane (TVSi). The fourth additive helps form a stable solid electrolyte interphase (SEI) film at the negative electrode interface, further improving the film-forming effect of the electrolyte at the negative electrode interface, enhancing the protection of the negative electrode interface, and improving the charge-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, 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 an SEI film at the negative electrode interface without excessively increasing the battery impedance. When the content of the fourth additive in the electrolyte is too low, it will lead to insufficient SEI film formation at the negative electrode interface, making it difficult to effectively protect the negative electrode, thereby negatively affecting the cycle capacity of the battery; when the content of the fourth additive in the electrolyte is too high, it will lead to excessively high SEI film impedance at the negative electrode interface, negatively affecting the battery cycle performance.

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

[0046] 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 organic solvent can 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 includes at least carbonates and carboxylic acid esters, and the mass content of the solvent in the electrolyte is typically 70% to 85%, for example, 70%, 75%, 80%, 81%, 83%, or 85%, etc. When carboxylic acid esters are present in the solvent, the first additive compound (Formula I) and the second additive VC can polymerize to form a CEI film, which can introduce the carbonyl group of the carboxylic acid ester into the formed CEI film to form sites for lithium ion binding, thereby reducing the ion conduction impedance of the CEI film and achieving a balance between stability and kinetics.

[0047] In addition, in some implementations, the solvent may 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 valerate.

[0048] The electrolyte of this invention can be prepared using conventional methods. For example, the electrolyte can be prepared in a glove box with an argon or nitrogen content of 99.999%, an actual oxygen content of 0.1 ppm, and a moisture content of less than 10 ppm. In the glove box, the solvent is mixed evenly according to a predetermined ratio. 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.

[0049] This invention also provides a secondary battery, comprising a positive electrode, a negative electrode, a separator, and the electrolyte described in any of the above embodiments. During battery charging and discharging, lithium ions repeatedly insert and extract between the positive and negative electrode. The separator is disposed between the positive and negative electrode, serving as an isolation layer; the electrolyte conducts lithium ions between the positive and negative electrode. It should be noted that the structural type of the above-described 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.

[0050] 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.

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

[0052] 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.

[0053] 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 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 of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc., or a combination of two or more in any proportion.

[0054] 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.

[0055] 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).

[0056] The diaphragm is selected from conventional types in the art, such as polyethylene (PE), polypropylene (PP), glass fiber membrane, polyethylene membrane, or composite membrane. The diaphragm thickness 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] Battery assembly is performed using conventional methods. For example, after preparation, 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, and then injected with 3.0 g / Ah of the prepared electrolyte before sealing. Electrolysis is then performed to ultimately produce a 1Ah lithium-ion battery.

[0058] In some embodiments, the secondary battery uses a positive electrode active material with a high upper limit of operating voltage (e.g., >3.80V), such as a high-nickel ternary material or lithium iron phosphate material. While using the above-mentioned positive electrode active materials can improve battery capacity and fast-charging performance, they are also prone to lattice collapse due to uneven lithium delithiation during charging and discharging, and increase side reactions with the electrolyte, resulting in loss of active material. However, using the electrolyte in the above embodiments in the secondary battery can effectively protect the positive electrode interface, further improving the battery's cycle life compared to existing technologies. In one example, the positive electrode active material in the secondary battery is lithium iron phosphate. Lithium iron phosphate not only suffers from the problem of unstable kinetic performance of high-voltage positive electrode materials, but also reacts with carboxylic acid esters in the electrolyte at high temperatures (>45°C), increasing the acidity of the electrolyte. Using the electrolyte in the above embodiments in the secondary battery can effectively adjust the pH of the electrolyte using a first additive, improving the chemical stability of the electrolyte, and can form a CEI film on the positive electrode surface with the help of a second additive, thus achieving effective protection of the positive electrode interface without excessively affecting its ion exchange capacity.

[0059] 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.

[0060] Example 1

[0061] 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), propylene carbonate (PC), and ethyl acetate (EA), with a volume ratio of 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), with a lithium salt content of 14% by mass and a LiPF6 to LiFSI mass ratio of 1:1. The first additive is a compound of formula I-1, with a first additive content of 0.3% by mass in the electrolyte. The second additive is vinylene (VC), with a second additive content of 3% by mass in the electrolyte.

[0062] The electrolyte preparation process 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, along with additives, are added to the solvent and mixed thoroughly to obtain a non-aqueous electrolyte. The amount of LiPF6 and LiFSI added is 7% of the total weight of the electrolyte, and the types and amounts of each additive are shown in Table 1.

[0063] Example 2

[0064] This embodiment provides an electrolyte with the same system as in Example 1. The difference between this embodiment and Example 1 is that a fourth additive is added to the electrolyte. The fourth additive includes fluoroethylene carbonate (FEC) and ethylene 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 with the same system as in Example 2. The difference between this embodiment and Example 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 with the same system as in Example 2. The difference between this embodiment and Example 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 with the same system as in Example 2. The difference between this embodiment and Example 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 with the same system as in Example 2. The difference between this embodiment and Example 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 with the same system as in Example 2. The difference between this embodiment and Example 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 with the same system as in Example 2. The difference between this embodiment and Example 2 is that the compound shown in Formula I-2 is used as the first additive.

[0077] Example 9

[0078] This embodiment provides an electrolyte with the same system as in Example 2. The difference between this embodiment and Example 2 is that the compound shown in Formula I-3 is used as the first additive.

[0079] Example 10

[0080] This embodiment provides an electrolyte with the same system as in Example 2. The difference between this embodiment and Example 2 is that the compound shown in Formula I-4 is used as the first additive.

[0081] Example 11

[0082] This embodiment provides an electrolyte with the same system as in Example 2. The difference between this embodiment and Example 2 is that a third additive, thiophene (SF), is added to the electrolyte instead of a fourth additive; and the mass content of the second additive, VC, 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 with the same system as that in Example 11. The difference between this embodiment and Example 11 is that a fourth additive, FEC and DTD, are added to the electrolyte. The mass content of FEC added to the electrolyte is 1%, and the mass content of DTD added is 1%.

[0085] Example 13

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

[0087] Example 14

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

[0089] Example 15

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

[0091] Example 16

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

[0093] Example 17

[0094] This embodiment provides an electrolyte with the same system as that in Example 12. The difference between this embodiment and Example 12 is that the compound shown in Formula I-2 is used as the first additive.

[0095] Example 18

[0096] This embodiment provides an electrolyte with the same system as in Example 12. The difference between this embodiment and Example 12 is that the compound shown in Formula I-3 is used as the first additive.

[0097] Example 19

[0098] This embodiment provides an electrolyte with the same system as in Example 12. The difference between this embodiment and Example 12 is that the compound shown in Formula I-4 is used as the first additive.

[0099] Comparative Example 1

[0100] 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 first additive is added to the electrolyte.

[0101] Comparative Example 2

[0102] This comparative example provides an electrolyte with the same system as that in Example 2. The difference between this comparative example and Example 2 is that no first additive is added to the electrolyte.

[0103] Comparative Example 3

[0104] This comparative example provides an electrolyte with the same system as Example 2. The difference between this comparative example and Example 2 is that a third additive is added to the electrolyte instead of a 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 with the same system as that in Example 12. The difference between this comparative example and Example 12 is that the mass content of the second additive VC in the electrolyte 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 with the same system as that in Example 12. The difference between this comparative example and Example 12 is that the mass content of the second additive VC and the mass content of the third additive SF in the electrolyte are adjusted to 1%.

[0109] Comparative Example 6

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

[0113] 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.

[0114] Selection of diaphragm: A 12μm thick polypropylene membrane was used as the diaphragm.

[0115] 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.

[0116] The performance of lithium-ion batteries assembled with the electrolytes provided in Examples 1 to 19 and Comparative Examples 1 to 6 was tested. The electrolyte parameters and test results of Examples 1 to 19 and Comparative Examples 1 to 6 are shown in Table 1. The test methods are as follows:

[0117] (1) Beginning of Life Direct Current Resistance (BOL DCR) Test: Adjust the temperature of the constant temperature chamber to 25℃, place the battery in the constant temperature chamber and let it stand for 10 minutes, then charge it at a constant current rate of 0.33C to 3.85V, and then charge it at a constant voltage rate of 3.85V to a current rate of 0.05C; after the battery stands for 30 minutes, discharge it at a constant current rate of 0.33C to 2.0V, and let it stand for 10 minutes. Repeat this cycle twice, and take the capacity of the last discharge as C0. Charge the battery at a constant current rate of 0.33C to 3.85V, and then charge it at a constant voltage rate of 0.05C; after the battery stands for 30 minutes, discharge it at a constant current rate of 0.33C to 50% of the C0 capacity, and record the voltage V0 at the end of the stand for 1 hour; 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) Cyclic performance test: Adjust the temperature of the constant temperature chamber to 45℃, place the battery in the constant temperature chamber and let it stand for 30 minutes, and charge it at a constant current of 2C to 3.85V, and then charge it at a constant voltage of 3.85V to a current of 0.05C; after letting the battery stand for 10 minutes, discharge it at a constant current of 1C to 2.0V; after cycling the above charging and discharging steps 800 times, record the discharge capacity C1 of the cell at the 800th cycle, then the capacity retention rate after 800 cycles at 45℃ = 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 mass percentages of the component relative to the electrolyte.

[0123] Comparing the test results of Example 1 and Comparative Example 1, and Examples 2 to 10 and Comparative Example 2, it can be seen that, compared with the existing electrolytes that only use VC as a positive electrode film-forming additive, the present application introduces compound I and VC together as additives in the electrolyte to introduce amine functional groups into the positive electrode interface polymer film to improve the mechanical strength and chemical stability of the CEI film, and introduces carbonyl groups of carboxylic acid esters into the positive electrode interface polymer film to improve the ionic conductivity of the CEI film, thus achieving a balance between positive electrode interface stability and kinetics, and effectively improving the battery impedance and cycle performance compared with the comparative examples.

[0124] Comparing the test results of Examples 2 and 12 to 14, it can be seen that introducing 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 uneven delithiation region at the positive electrode interface to self-discharge quickly, thereby ensuring uniform delithiation at the positive electrode interface, improving the cycle performance of the battery, and thus further improving the cycle life of the battery.

[0125] Comparing the test results of Examples 12 to 14 and Comparative Example 3, it can be seen that in Comparative Example 3, only SF was added to the electrolyte along with Compound I as an additive. This introduced excessive conductive material into the electrode interface film formed by the additive, damaging the chemical stability of the interface film and causing side reactions between the electrolyte and the interface film, thus deteriorating the high-temperature cycle capacity of the battery. Compared to Comparative Example 3, this embodiment uses appropriate amounts of VC and SF as additives in the electrolyte to avoid introducing excessive conductive material into the electrode interface film, which would damage the overall insulating properties of the CEI film, as in Comparative Example 3. The electrolyte in this embodiment 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] Comparing the test results of Example 12 and Comparative Example 4, and Example 13 and Comparative Example 5, it can be seen that this solution, by limiting the SF content in the electrolyte to be lower than the VC content, aims to improve both the local impedance and chemical stability of the positive electrode interface. However, if the comparative examples add SF to the electrolyte in a quantity greater than or equal to VC, excessive SF additives will introduce too much conductive material into the CEI film formed at the positive electrode interface, damaging the insulation stability of the CEI film and causing a reaction between the CEI film and the electrolyte. This impairs the protective effect of the CEI film on the positive electrode, promotes electrolyte decomposition, and consequently worsens the high-temperature cycle capacity of the battery compared to the embodiments of this solution.

[0127] Comparing the test results of Examples 2 to 7, Example 12, and Examples 15 to 16, it can be seen that when the mass content of the first additive in the electrolyte is limited to an appropriate range of 0.05% to 3%, a stable CEI film can be formed at the positive electrode interface without excessively increasing the battery impedance. When the mass content of the first additive in the electrolyte is less than 0.05%, the CEI film formation at the positive electrode interface will be insufficient, making it difficult to effectively protect the positive electrode, thus negatively affecting the cycle capacity of the battery. When the mass percentage of the additive in the electrolyte is greater than 3%, the CEI film impedance at the positive electrode interface will be too high, negatively affecting the cycle performance of the battery.

[0128] The test results from Examples 2, 8 to 10, 12, and 17 to 19 show that compounds I-1, I-2, I-3, and I-4, having the same structure and functional groups as compounds of Formula I, exhibit the same improvement effects on battery impedance, high-temperature storage, and cycle performance when used as additives in combination with VC or VC and SF. Therefore, using Formula I compounds with different types of R1, R2, R3, and R4 groups has little impact on battery conductivity and high-temperature cycle performance.

[0129] 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 comprises the compound shown in Formula I: Formula I; In Formula I, R1, R2, R3, and R4 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 heteroatom is selected from any one of nitrogen, sulfur, oxygen, boron, and phosphorus atoms. The second additive includes vinylene esters; The first additive has a mass content of 0.05% to 3%, and the second additive has a mass content of less than or equal to 3%.

2. The electrolyte according to claim 1, characterized in that, The first additive has a mass content of 0.1% to 0.5% in the electrolyte.

3. The electrolyte according to claim 1, characterized in that, The electrolyte further includes a third additive, which 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%.

4. The electrolyte according to claim 1, characterized in that, In Formula I, R1, R2, R3, and R4 are each independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino, or heterocyclic groups, wherein the heterocyclic group is selected from any one of pyridine, pyrrole, thiophene, thiazole, and furan.

5. The electrolyte according to claim 1 or 4, characterized in that, The compound represented by Formula I includes at least one of the following compounds: I-1； I-2； I-3； I-4。 6. 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-sulfonyl lactone, ethylene sulfate and tetravinylsilane, and the mass content of the fourth additive in the electrolyte is 0.1% to 2%.

7. 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 has a mass content of 12% to 17% in the electrolyte.

8. The electrolyte according to claim 1, characterized in that, The solvent includes carbonates and carboxylic acid esters; 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.

9. 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 8.

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