Anhydrous electrolyte and lithium-ion battery

Abstract

This invention provides an anhydrous electrolyte and a lithium-ion battery, specifically relating to the field of electrolyte technology. The anhydrous electrolyte comprises an organic solvent, a lithium salt, and additives. The additives include lithium difluorodioxazophosphate and substituents R1, R2, R3, and R4 in the compound formula, each independently selected from substituents having 1-6 carbon atoms, an unsaturation degree of 0-4, and 0-3 heteroatoms. The heteroatoms are selected from any one of nitrogen, sulfur, oxygen, boron, and phosphorus atoms. This anhydrous electrolyte can improve the low-temperature cycle performance of lithium-ion batteries.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in power, energy storage, and digital fields due to their advantages such as high energy density, light weight, no memory effect, environmental friendliness, and long service life, and have broad application prospects.

[0003] Electrolyte is a crucial component of lithium-ion batteries, playing a vital role in conducting lithium ions between the positive and negative electrodes. However, at low temperatures, electrolyte viscosity increases, ionic conductivity decreases, and the impedance of the solid-state electrolyte membrane increases, leading to increased diffusion resistance of lithium ions and severe performance degradation of lithium-ion batteries at low temperatures. Therefore, providing a low-temperature electrolyte is essential for improving the low-temperature performance of lithium-ion batteries. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the present invention provides an anhydrous electrolyte and a lithium-ion battery to improve the low-temperature performance of lithium-ion batteries.

[0005] To achieve the above and other related objectives, the present invention provides an anhydrous electrolyte comprising: an organic solvent, a lithium salt, and an additive; wherein the additive comprises lithium difluorodioxanol phosphate and a compound represented by Formula I:

[0006]

[0007] In Formula I, R1, R2, R3, and R4 are each independently selected from substituents having 1-6 carbon atoms, an unsaturation degree of 0-4, and a heteroatom number of 0-3. The heteroatom is selected from any one of nitrogen, sulfur, oxygen, boron, and phosphorus atoms.

[0008] In one example of the present invention, the content of lithium difluorodioxazophosphate in the anhydrous electrolyte is 0.1% to 0.8%, and the content of the compound represented by Formula I is 0.05% to 3%.

[0009] In one example of the present invention, the content of the compound represented by Formula I in the anhydrous electrolyte is 0.1% to 0.5%.

[0010] 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.

[0011] In one example of the present invention, the compound represented by Formula I includes any one or more of the following compounds:

[0012]

[0013] In one example of the present invention, the additive further includes one or more of vinylene carbonate, vinyl sulfate, fluorovinyl carbonate, propylene-1,3-sulfonyl lactone, and tetravinylsilane.

[0014] In one example of the present invention, the organic solvent includes carbonates and carboxylic acid esters, wherein the carbonates include one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and fluoroethylene carbonate; and the carboxylic acid esters include one or more of ethyl formate, ethyl acetate, propyl acetate, and ethyl propionate.

[0015] In one example of the present invention, the organic solvent further includes: ether solvents and / or nitrile solvents, wherein the ether solvents include one or both of ethylene glycol dimethyl ether and diethanol diethyl ether, and the nitrile solvents include one or more of acetonitrile, propionitrile, butyronitrile, and valerate.

[0016] In one example of the present invention, the lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methanesulfonate, and lithium trifluoromethylsulfonate, and the total content of lithium salt in the electrolyte is 12% to 17%.

[0017] In another aspect, the present invention provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and any of the above-described anhydrous electrolyte; the positive electrode comprises lithium iron phosphate material.

[0018] This invention introduces lithium difluorodioxanol phosphate and the compound shown in Formula I as additives into anhydrous electrolyte. Lithium difluorodioxanol phosphate can reduce impedance and improve the low-temperature performance of the battery, but it has the opposite effect in acidic environments. The tertiary amino group in the compound shown in Formula I can interact with acidic substances in the electrolyte (such as acidic substances produced by the decomposition of lithium salts), adjusting the electrolyte's pH and making the chemical environment of the electrolyte more stable. This suppresses the negative effects of lithium difluorodioxanol phosphate in acidic environments, thus achieving the effect of improving low-temperature impedance and enhancing low-temperature cycling performance. Furthermore, the salt formed by the compound shown in Formula I with acidic substances can participate in the membrane formation process, increasing the ionic conductivity and stability of the membrane.

[0019] Furthermore, the unsaturated double bonds of the compound shown in Formula I can undergo polymerization in the electrolyte. During battery charging, the electric field and oxidative environment on the electrode surface trigger the opening of these double bonds, which then copolymerize with other components in the electrolyte (such as solvent molecules and other additives) to form a polymer film. This polymer film can cover the electrode surface, protecting the electrode, reducing side reactions between the electrode and the electrolyte, preventing direct contact between the electrolyte and the negative electrode, reducing irreversible lithium-ion consumption, and thus improving the battery's cycle performance and coulombic efficiency.

[0020] In addition, the compound shown in Formula I can be adsorbed onto the electrode surface through physical or chemical adsorption, and can undergo cross-linking reactions between molecules or with other components in the electrolyte to form a cross-linked network structure. This cross-linked structure can enhance the mechanical strength and stability of the membrane, enabling it to better resist volume changes and mechanical stresses generated during battery charging and discharging, thereby protecting the integrity of the electrode structure. 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 described therein can be combined with each other.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0023] In this document, when referring to numerical ranges, unless otherwise specified, the distribution of selectable values ​​within a numerical range is considered continuous, including the two endpoints of the range (i.e., the minimum and maximum values), and every value between these two endpoints. When multiple numerical ranges are provided to describe a feature or property, these numerical ranges can be combined.

[0024] 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.

[0025] Unless otherwise specified, "%" in this article refers to the percentage content by mass.

[0026] 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.

[0027] Unsaturation, 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. Its molecular formula is C2. n H m The hydrocarbon and its molecular formula is C n H m O x For hydrocarbon derivatives, if m < 2n + 2, then the hydrocarbon and its hydrocarbon group have a certain degree of unsaturation Ω. That is, compared with open-chain alkanes with the same number of carbon atoms, the degree of unsaturation of the organic compound increases by 1 for every 2 hydrogen atoms removed.

[0028] 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.

[0029] Lithium iron phosphate (LFP) batteries have become one of the best choices for electric vehicles due to their low cost and stable structure. However, with the arrival of winter (low temperatures), the significant reduction in the driving range of electric vehicles has become one of the obstacles to their development. Therefore, developing an electrolyte that can improve the low-temperature performance of lithium-ion batteries is crucial for the development of the lithium-ion battery industry and even the electric vehicle industry as a whole.

[0030] The inventors of this application discovered that lithium difluorodioxalate phosphate (LiODFP) is a good cathode film-forming additive that can reduce battery impedance and improve low-temperature performance. However, it is rarely used in lithium-ion batteries, especially in lithium iron phosphate (LiFePO4, or LFP) systems. The reason is that organic solvents and lithium salts in the electrolyte react to produce acid, leading to an increase in electrolyte acidity. In an acidic environment, LiODFP, in turn, increases battery impedance, thus negating its positive effects. Therefore, the inventors propose introducing a substance that can inhibit the increase in electrolyte acidity to work in conjunction with LiODFP, thereby achieving the effect of LiODFP in improving low-temperature impedance and enhancing low-temperature cycling performance.

[0031] A first aspect of this invention provides an anhydrous electrolyte comprising an organic solvent, a lithium salt, and an additive. The additive comprises lithium difluorodioxazophosphate and a compound represented by Formula I.

[0032]

[0033] In Formula I, R1, R2, R3, and R4 are each independently selected from substituents with 1-6 carbon atoms, an unsaturation degree of 0-4, and 0-3 heteroatoms. 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 1-6, such as 1, 3, 5, or 6, and the degree of unsaturation can be any value from 0-4, 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 0-3. For example, 0, 1, 2, or 3.

[0034] The compound shown in Formula I contains a tertiary amino group (R3N, where R represents an alkyl group). Due to the lone pair of electrons on the nitrogen atom, the tertiary amino group exhibits basicity. It can react chemically with acidic substances in the electrolyte (such as acidic substances produced by the decomposition of lithium salts, carboxylic acid ester solvents, etc.), thereby adjusting the pH of the electrolyte and making the chemical environment of the electrolyte more stable. This allows lithium difluorodioxalatoborate to improve low-temperature impedance and enhance low-temperature cycle performance, thus improving the low-temperature performance of lithium-ion batteries.

[0035] Furthermore, the salts formed by amine groups and acidic substances can participate in the membrane formation process, increasing the membrane's ionic conductivity and stability. Moreover, the compound molecule shown in Formula I contains unsaturated double bonds. During battery charging, the electric field and oxidative environment on the electrode surface trigger the opening of these double bonds, which then copolymerize with other components in the electrolyte (such as solvent molecules and other additives) to form a polymer film. This polymer film can cover the electrode surface, protecting the electrode, reducing side reactions between the electrode and the electrolyte, preventing direct contact between the electrolyte and the negative electrode, reducing irreversible lithium-ion consumption, and thus improving the battery's cycle performance and coulombic efficiency.

[0036] In addition, the compound molecules shown in Formula I can also be adsorbed onto the electrode surface through physical adsorption or chemical adsorption, and cross-linking reactions can occur between molecules or between molecules and other components in the electrolyte to form a cross-linked network structure. This cross-linked structure can enhance the mechanical strength and stability of the membrane, enabling it to better resist volume changes and mechanical stresses generated during battery charging and discharging, thereby protecting the integrity of the electrode structure.

[0037] In one embodiment, the content of lithium difluorodioxazone phosphate in the anhydrous electrolyte is 0.1% to 0.8%, for example, 0.1%, 0.3%, 0.5%, or 0.8%. The content of the compound represented by Formula I is 0.05% to 3%, for example, 0.05%, 0.1%, 1%, 2%, or 3%. Further, the content of the compound represented by Formula I is 0.1% to 0.5%, for example, 0.1%, 0.3%, or 0.5%.

[0038] In some embodiments, R1, R2, R3, and R4 in Formula I are each independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino, or heterocyclic groups. A heterocyclic group refers to a ring whose constituent atoms include at least one non-carbon atom in addition to a carbon atom; these non-carbon atoms are called heteroatoms. As an example, heterocyclic groups include, but are not limited to, any one of pyridine, pyrrole, thiophene, thiazole, and furan. For example, a heterocyclic group 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 groups; 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.

[0039] Furthermore, the compound represented by Formula I is selected from one or a combination of two or more of Compound I-1, Compound I-2, Compound I-3 and Compound I-4.

[0040]

[0041]

[0042] In some embodiments, the additive may also include one or more of vinylene carbonate (VC), vinyl sulfate (DTD), fluoroethylene carbonate (FEC), propylene-1,3-sulfonyl lactone (PST), and tetravinylsilane (TVSI). VC can undergo an electrochemical reaction on the negative electrode surface during the initial charge-discharge of the lithium battery to form a solid electrolyte interphase (SEI) film, improving the battery's initial efficiency and cycle performance. DTD can improve the battery's cycle performance and reduce capacity decay and internal resistance. FEC decomposes to form fluoride ions, which can react with lithium salts in the solvent to produce LiF, which is difficult to decompose and has good insulating properties, thus forming the SEI film. PST can form a solid electrolyte interphase film on the battery electrode surface, which can inhibit the co-intercalation and reductive decomposition of solvent molecules at the negative electrode, thereby improving the cycle performance and high-temperature performance of the lithium-ion battery. TVSI can significantly improve the performance of the lithium-ion battery under high temperature and high pressure conditions, and so on. Specific selections can be made according to actual production needs. For example, in addition to lithium difluorodioxazophosphate and the compound shown in Formula I, the additive may also include VC, or VC and FEC, or VC, DTD and FEC, etc., to make the battery performance more fully realized. It should be noted that the amount of the above additives can be set according to the conventional proportions in the art, and no specific limitations are made here.

[0043] The organic solvent, as the main component of the electrolyte, can be any solvent conventionally used in lithium-ion batteries. In one embodiment, the organic solvent includes carbonates and carboxylic esters, wherein the carbonates include cyclic carbonates and linear carbonates. Cyclic carbonates have a higher dielectric constant, which is beneficial for the dissociation of lithium salts and improves the ionic conductivity of the electrolyte. Linear carbonates have lower melting points and viscosity, which is beneficial for improving the performance of the electrolyte at low temperatures. Carboxylic esters have even lower melting points and viscosity than linear carbonates. Adding low-viscosity carboxylic esters significantly increases the conductivity of the electrolyte, and the bond length between carboxylic esters and lithium is greater than that of carbonates, indicating that they are easier to desolvate, which helps improve the cell's dynamic performance. Using carbonates and carboxylic esters together can achieve a synergistic effect and improve the overall performance of the electrolyte.

[0044] The carbonate is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and fluoroethylene carbonate (FEC), and the carboxylic acid ester is selected from one or more of ethyl formate, ethyl acetate, propyl acetate, and ethyl propionate. That is, the organic solvent can be selected from any one of the carbonates and carboxylic acid esters listed above, or any two or a combination of two. For example, the organic solvent could be a combination of ethylene carbonate, dimethyl carbonate, and methyl formate, or a combination of ethyl formate and ethylene carbonate, etc.

[0045] In other embodiments, the organic solvent may also include ether solvents and / or nitrile solvents. Ether solvents can reduce battery impedance and suppress low-temperature lithium dendrite growth. Nitrile solvents enable the electrolyte to maintain good ionic conductivity and high ion intercalation and deintercalation capabilities at lower temperatures, effectively improving battery efficiency. The ethers are selected from one or both of ethylene glycol dimethyl ether and diethanol diethyl ether, but are not limited to these. Nitrile solvents include, but are not limited to, acetonitrile, propionitrile, butyronitrile, or valerate. A single nitrile solvent can be used, such as butyronitrile; or a combination of two or more, such as a combination of acetonitrile and propionitrile. The content of organic solvent in the anhydrous electrolyte is typically 70% to 80%, for example, 70%, 80%, or 75%.

[0046] The lithium salt can be selected from conventional lithium salts suitable for lithium-ion batteries. To obtain a better electrolyte, the lithium salt typically needs to have the following characteristics: low dissociation energy and high solubility. Low dissociation energy ensures that the electrolyte formed after the lithium salt dissolves has high conductivity, thereby achieving high battery rate; high solubility ensures that there are enough lithium ions in the electrolyte for transport; good stability and good film-forming properties ensure that the electrolyte is not continuously consumed during subsequent cycles.

[0047] In some embodiments, the lithium salt includes 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). The lithium salt can be a single lithium salt or a mixture of lithium salts. For example, the lithium salt can be lithium hexafluorophosphate, or a mixture of lithium hexafluorophosphate and lithium bis(trifluoromethyl)sulfonylimide, or a mixture of lithium hexafluorophosphate, lithium bis(trifluoromethyl)sulfonylimide, and lithium acetate, but is not limited thereto. Preferably, the lithium salt is a mixture of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide. Lithium hexafluorophosphate has high safety and strong overall performance: it has a moderate ion transport number, a moderate dissociation constant, good antioxidant properties, and good aluminum foil passivation ability. Lithium bis(fluorosulfonyl)imide has high conductivity, good thermal stability, and good ion migration rate and interface stability. Using lithium hexafluorophosphate in combination with lithium difluorosulfonylimide can improve the overall performance of the electrolyte. The mass ratio of lithium hexafluorophosphate to lithium difluorosulfonylimide is 14:1 to 1:16, for example, 14:1, 1:1, 1:16, etc.

[0048] In some embodiments, the total lithium salt content in the electrolyte is 12% to 17%, specifically 12%, 14%, 16%, or 17%, etc.

[0049] The anhydrous electrolyte of this invention can be prepared using conventional methods. As an example, in a glove box under an argon atmosphere with a water content of <10 ppm, the organic solvent is mixed uniformly according to a predetermined ratio. Then, the thoroughly dried lithium salt and additives are added to the organic solvent and mixed uniformly to prepare the electrolyte. The content of each component in the electrolyte, except for the organic solvent, is a weight percentage calculated based on the total weight of the electrolyte.

[0050] A second aspect of the present invention provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and the anhydrous electrolyte described above. The separator is disposed between the positive and negative electrodes, serving as an isolation layer. During battery charging and discharging, lithium ions repeatedly insert and extract between the positive and negative electrodes, while the anhydrous electrolyte conducts lithium ions between them.

[0051] Specifically, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. 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 to 15 μm. In this embodiment, the positive current collector is, for example, aluminum foil, and the thickness of the aluminum foil is, for example, 13 μm. The positive active material layer can be disposed on one surface of the positive current collector or on both surfaces. The positive active material layer includes a positive active material, a conductive agent, and a binder. In this invention, the positive active material is lithium iron phosphate (LiFePO4) material, but it is not limited to this; it can also be a ternary positive electrode material, etc. The binder for the positive electrode is selected from one or more of the following: 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 conductive agent for the positive electrode is selected from one or at least two of the following: conductive carbon black (Super P), acetylene black, graphene, carbon nanotubes, and carbon nanofibers.

[0052] The negative electrode plate includes a negative current collector and a negative active material layer provided on at least one surface of the negative current collector. The negative current collector is, for example, selected from one of a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foam copper current collector, a stainless steel current collector, etc., and the thickness of the negative current collector is, for example, 8 μm to 15 μm. In this embodiment, the negative current collector is a copper foil, and the thickness of the copper foil is, for example, 13 μm. The negative active material layer is provided on one surface of the negative current collector, or can also be provided on both surfaces. The negative active material layer includes a negative active material, a conductive agent, a binder, and a thickening agent. The specific types of the negative active material, the conductive agent, and the binder are not specifically limited here, and materials known in the art that can be used in lithium-ion batteries can be used. Those skilled in the art can select according to actual needs.

[0053] The negative active material is selected from compounds capable of intercalating and deintercalating lithium ions. For example, graphite-based negative materials and / or silicon-based negative materials can be selected. Among them, the graphite-based negative materials include, but are not limited to, natural graphite, artificial graphite, soft carbon, hard carbon, etc.; the silicon-based negative materials include silicon oxides SiO x (0 < x < 2), silicon-carbon composites, silicon单质, etc. The conductive agent of the negative electrode is selected from one of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc. or a composition of two or more of them mixed in any proportion. The binder of the negative electrode is selected from any one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), or a composition of several of them mixed in any proportion; the thickening agent is selected from sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0054] The separator is selected from conventional types in the art. For example, a polyethylene film (Polyethylene, PE), a polypropylene film (Polypropylene, PP), a glass fiber film, a polyethylene film, or a composite film, etc. The thickness of the separator is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL; the porosity is 30% to 50%.

[0055] The lithium-ion battery can be prepared according to the conventional methods in the art, as shown below:

[0056] (1) Preparation of the positive electrode plate

[0057] Disperse the above-mentioned positive active material, conductive agent, and binder in a solvent (such as N-methylpyrrolidone, abbreviated as NMP) to form a uniform positive electrode slurry; coat the positive electrode slurry on the positive current collector, and after processes such as drying and cold pressing, obtain the positive electrode plate. Among them, the proportion between the components in the positive electrode slurry can be set according to the conventional proportion and is not limited here.

[0058] (2) Preparation of negative electrode sheet

[0059] The aforementioned negative electrode active material, binder, thickener, and conductive agent are dispersed in deionized water to form a uniform negative electrode slurry. The negative electrode slurry is then coated onto a negative electrode current collector, and after drying and cold pressing, a negative electrode sheet is obtained. The proportions of the components in the negative electrode slurry can be set according to conventional proportions and are not limited here.

[0060] (3) Battery assembly

[0061] The prepared positive electrode, separator, and negative electrode are placed sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. The bare cell is obtained by winding or stacking the electrodes. The bare cell is then installed into a battery casing, and after assembly, electrolyte injection, formation, and capacity testing, a lithium-ion battery is obtained.

[0062] The lithium-ion battery of this invention can be used in the form of a single cell, a battery module, or a battery pack to power 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.

[0063] 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 all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the embodiments are all commercially available.

[0064] Example 1

[0065] This embodiment provides an anhydrous electrolyte comprising an organic solvent, a lithium salt, and additives. The organic solvent is a composition of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl propionate (EA), and propylene carbonate (PC) in a mass ratio of EC:DMC:EA:PC = 2.5:3:4:0.5. The lithium salt is 11% lithium hexafluorophosphate (LiPF6) and 4% lithium difluorosulfonylimide (LiFSI). The additives include 2.5% VC, 0.5% FEC, 1% DTD, 0.05% compound I-1, and 0.5% LiODFP. Except for the solvent, the contents of the other components are weight percentages calculated based on the total weight of the electrolyte.

[0066] The electrolyte preparation process is as follows: In an argon atmosphere glove box with a water content of <10ppm, the solvent is first mixed evenly in a mass ratio of EC:DMC:EA:PC = 2.5:3:4:0.5 to form an organic solvent; then, lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonyl imide (LiFSI), and additives are added to the organic solvent in the above ratio and mixed evenly to obtain the electrolyte.

[0067] This embodiment also provides a lithium-ion battery, which includes a positive electrode, a separator, a negative electrode, and the electrolyte of this embodiment. The battery preparation process is as follows:

[0068] Preparation of positive electrode sheet: The positive electrode active material LiFePO4, the binder polyvinylidene fluoride, and the conductive agent carbon black (Super P) are mixed in a mass ratio of 97:1:2. N-methylpyrrolidone (NMP) is added, and the mixture is stirred under vacuum until the system is homogeneous and transparent to obtain the positive electrode slurry. The positive electrode slurry is uniformly coated on aluminum foil, dried at room temperature, and then transferred to an oven for drying. The positive electrode sheet is then obtained through cold pressing, slitting, and other processes.

[0069] Preparation of negative electrode sheet: The negative electrode active material artificial graphite, conductive agent carbon black (Super P), thickener sodium carboxymethyl cellulose (CMC-Na), and binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96:1:1:2. Deionized water is added, and the mixture is thoroughly stirred and mixed evenly under the action of a vacuum mixer to obtain a negative electrode slurry. The negative electrode slurry is evenly coated on copper foil, dried at room temperature, and then transferred to an oven for drying. The negative electrode sheet is then obtained through cold pressing, slitting, and other processes.

[0070] Selection of diaphragm: 12μm polypropylene film (PP) was used as the diaphragm.

[0071] Battery assembly: The positive electrode, separator, and negative electrode are placed in sequence, with the separator positioned between the positive and negative electrodes to act as a separator. The cells are then wound to form a bare cell, which is then assembled in a steel shell, liquefied metallization, and capacity testing to obtain a lithium-ion battery.

[0072] Referring to Table 1, the present invention also provides Examples 2-11 and Comparative Examples 1-2, wherein the electrolyte components of Examples 2-11 and Comparative Examples 1-2 are the same as those of Example 1, except that the additives shown in Table 1 are different.

[0073] Table 1: Types, contents, and battery performance of additives in Examples 1 to 11 and Comparative Examples 1 to 2

[0074]

[0075] The lithium-ion batteries of Examples 1-11 and Comparative Examples 1-2 were subjected to performance tests, and the test results are shown in Table 1. The test methods are as follows:

[0076] (1) Low-temperature initial DC resistance (BOL DCR) test:

[0077] Set the temperature of the constant temperature chamber to -20℃ and let it stand for 10 minutes. Charge the battery with a constant current of 0.33C to 3.8V, then charge it with a constant current of 0.05C at 3.8V, and let it stand for 30 minutes. Discharge the battery with a constant current of 0.33C to 2.0V, and let it stand for 10 minutes. Repeat this cycle twice, and record the discharge capacity of the last discharge as C0. Charge the battery with a constant current of 0.33C to 3.8V, then charge it with a constant voltage of 0.05C, and let it stand for 30 minutes. Discharge the battery with a constant current of 0.33C to 50% C0, and let it stand for 1 hour. Record the voltage V0 at the end of the standing period. Discharge the battery with a constant current of 4C0 for 30 seconds and record the voltage V1. Then, BOL DCR = (V0 - V1) / 4C0.

[0078] (2) Low-temperature cycling test:

[0079] Adjust the temperature of the constant temperature chamber to 10℃ and stabilize it for 30 minutes; charge at a constant current of 0.5C to 3.8V, then charge at a constant voltage of 0.05C, let it stand for 10 minutes, and then discharge at a constant current of 1C to 2.0V. After cycling this charge and discharge cycle 800 times, record the discharge capacity of the cell at the first cycle as C0 and the discharge capacity of the cell at the 800th cycle as C1. Then the capacity retention rate after 800 cycles at 10℃ = C1 / C0 × 100%.

[0080] Referring to Table 1, comparing Examples 1-6 and Comparative Examples 1 and 2, it can be concluded that, under the same conditions, while adding LiODFP alone (Comparative Example 2) can improve the low-temperature cycle performance of lithium-ion batteries to some extent compared to not adding LiODFP (Comparative Example 1), the DCR value also increases. This is because the acidic substances produced by lithium salt decomposition lead to an increase in electrolyte acidity, and LiODFP, in an acidic environment, has a reverse effect, increasing battery impedance. In Examples 1 to 6, with the addition of compound I-1, battery impedance gradually decreases and low-temperature cycle performance gradually increases. When the addition amount of compound I-1 exceeds 0.5%, battery impedance rebounds, and low-temperature cycle performance decreases significantly. This is because the tertiary amino group in the compound I-1 molecule interacts with the acidic substances in the electrolyte, which can inhibit the increase of electrolyte acidity and keep the chemical environment of the electrolyte stable. LiODFP plays a role in reducing battery impedance, thereby improving the low-temperature cycle performance of the battery. However, when the compound I-1 exceeds 0.5%, a thicker SEI film will form at the electrode interface, which will have an adverse effect on cell dynamics and interface stability, and will instead affect the cycle performance of the battery. Therefore, when compound I-1 is used in combination with LiODFP, an addition of 0.05%-3% is reasonable, and an addition of 0.1%-0.5% is optimal.

[0081] Comparing Examples 3, 7, 8, and 9, it can be concluded that the combination of Compound I-1, Compound I-2, Compound I-3, and Compound I-4 with LiODFP can all improve the low-temperature cycling performance of the battery.

[0082] Comparing Examples 3 and 10-11, it can be seen that when an equal amount of Compound I-1 is added, the low-temperature DCR value of the battery first decreases and then increases with the increase of LiODFP addition, while the low-temperature cycle performance first improves and then decreases. This is because when the amount of LiODFP added is insufficient, the effect of reducing the low-temperature impedance of the battery is not obvious, while when the amount of LiODFP added is too large, a thicker positive electrode interface film will be formed, resulting in an increase in the low-temperature impedance of the battery and a decrease in low-temperature cycle performance.

[0083] In summary, this invention introduces lithium difluorodioxanol phosphate and the compound shown in Formula I as additives into anhydrous electrolyte. The compound shown in Formula I is used to adjust the pH of the electrolyte, making the chemical environment of the electrolyte more stable. This provides a stable operating environment for lithium difluorodioxanol phosphate, allowing it to effectively reduce battery impedance and improve low-temperature cycle performance, thereby enhancing the low-temperature cycle performance of lithium-ion batteries. Therefore, this invention effectively overcomes some practical problems in the prior art and has high utilization value and practical significance.

[0084] 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 anhydrous electrolyte, characterized in that, include Organic solvents, lithium salts, and additives; wherein the additives include lithium difluorodioxanol phosphate and a compound represented by Formula I, or lithium difluorodioxanol phosphate and a compound represented by Formula I-3: Formula I; Formula I-3; In Formula I, R1, R2, R3, and R4 are each independently selected from substituents having 1-6 carbon atoms, an unsaturation degree of 0-4, and a heteroatom number of 0-3. The heteroatom is selected from any one of nitrogen, sulfur, oxygen, boron, and phosphorus atoms.

2. The anhydrous electrolyte according to claim 1, characterized in that, The anhydrous electrolyte contains 0.1% to 0.8% lithium difluorodioxazophosphate and 0.05% to 3% of the compound represented by Formula I or Formula I-3.

3. The anhydrous electrolyte according to claim 2, characterized in that, The content of the compound represented by Formula I or Formula I-3 in the anhydrous electrolyte is 0.1% to 0.5%.

4. The anhydrous 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, or heterocyclic groups, wherein the heterocyclic group is selected from any one of pyridine, pyrrole, thiophene, thiazole, and furan.

5. The anhydrous electrolyte according to claim 1, characterized in that, The compound represented by Formula I includes any one or more of the following compounds: Ⅰ-1； Ⅰ-2； Ⅰ-4。 6. The anhydrous electrolyte according to claim 1, characterized in that, The additives also include one or more of vinylene carbonate, vinyl sulfate, fluorovinyl carbonate, propylene-1,3-sulfonyl lactone, and tetravinylsilane.

7. The anhydrous electrolyte according to claim 1, characterized in that, The organic solvent includes carbonates and carboxylic acid esters, wherein the carbonates include one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and fluoroethylene carbonate; and the carboxylic acid esters include one or more of ethyl formate, ethyl acetate, propyl acetate, and ethyl propionate.

8. The anhydrous electrolyte according to claim 7, characterized in that, The organic solvent further includes: ether solvents and / or nitrile solvents, wherein the ether solvents include one or two of ethylene glycol dimethyl ether and diethanol diethyl ether, and the nitrile solvents include one or more of acetonitrile, propionitrile, butyronitrile, and valerate.

9. The anhydrous electrolyte according to claim 1, characterized in that, The lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methanesulfonate, and lithium trifluoromethanesulfonate, and the total content of lithium salt in the electrolyte is 12% to 17%.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an anhydrous electrolyte as described in any one of claims 1 to 9, wherein the positive electrode comprises lithium iron phosphate material.

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