Water-free electrolyte and lithium ion battery

Abstract

The invention provides a water-free electrolyte and a lithium ion battery, and particularly relates to the technical field of electrolytes. The water-free electrolyte comprises an organic solvent, a lithium salt and an additive, the additive comprises lithium difluoro-bis (oxalate) phosphate and a compound # imgabs0 #, R1, R2, R3 and R4 in the formula are independently selected from substituent groups with the carbon atom number of 1-6, the unsaturation degree of 0-4 and the heteroatom number of 0-3, and the compound # imgabs0 # is one or more of lithium difluoro-bis (oxalate) phosphate and a compound # imgabs0 #. The heteroatom is selected from any one of a nitrogen atom, a sulfur atom, an oxygen atom, a boron atom and a phosphorus atom. The non-aqueous electrolyte can improve the low-temperature cycle performance of the lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of electrolytes, and in particular to an anhydrous electrolyte and a lithium ion battery. Background Art

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

[0003] The electrolyte is an important component of lithium-ion batteries, and plays a role in conducting lithium ions between the positive and negative electrodes. However, in low-temperature environments, the electrolyte viscosity is high, the ion conductivity is reduced, and the impedance of the solid interface electrolyte membrane increases, resulting in an increase in the diffusion resistance of lithium ions at low temperatures, which seriously deteriorates the performance of lithium-ion batteries at low temperatures. Therefore, providing a low-temperature electrolyte is crucial to improving the low-temperature performance of lithium-ion batteries. Summary of the invention

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

[0005] To achieve the above-mentioned object and other related objects, the present invention provides an anhydrous electrolyte, which comprises: an organic solvent, a lithium salt and an additive; wherein the additive comprises lithium difluorobis(oxalate)phosphate and a compound shown in formula I:

[0006]

[0007] R1, R2, R3, and R4 in formula I are each independently selected from a substituent having 1-6 carbon atoms, 0-4 unsaturations, and 0-3 heteroatoms, and the heteroatom is selected from any one of a nitrogen atom, a sulfur atom, an oxygen atom, a boron atom, and a phosphorus atom.

[0008] In one example of the present invention, the content of lithium difluorobis(oxalyl)phosphate 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 an alkyl group, an alkenyl group, an alkynyl group, a carbonyl group, an ester group, an amino group, or a heterocycle, and the heterocycle is selected from any one of pyrridine, 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 an example of the present invention, the additive further includes one or more of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, propenyl-1,3-sultone, and tetravinylsilane.

[0014] In one example of the present invention, the organic solvent includes carbonates and carboxylates, the carbonates include one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate and fluoroethylene carbonate; the carboxylates 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 also includes: an ether solvent and / or a nitrile solvent, the ether solvent includes one or two of ethylene glycol dimethyl ether and diethanol diethyl ether, and the nitrile solvent includes one or more of acetonitrile, propionitrile, butyronitrile, and valeronitrile.

[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] Another aspect of the present invention provides a lithium-ion battery, which includes a positive electrode plate, a negative electrode plate, a separator and any of the above-mentioned anhydrous electrolytes; the positive electrode plate includes a lithium iron phosphate material.

[0018] The present invention simultaneously introduces lithium difluorobisoxalate phosphate and the compound shown in formula I as additives in an anhydrous electrolyte, wherein lithium difluorobisoxalate phosphate can reduce impedance and improve the low-temperature performance of the battery, but in an acidic environment, it will have a reverse effect; and the tertiary amino group in the compound molecule shown in formula I can interact with the acidic substance in the electrolyte (such as the acidic substance produced by the decomposition of the electrolyte lithium salt), adjust the pH of the electrolyte, and make the chemical environment of the electrolyte more stable, thereby inhibiting the negative impact of lithium difluorobisoxalate phosphate in an acidic environment, and realizing the effect of lithium difluorobisoxalate phosphate in improving low-temperature impedance and improving low-temperature cycle capacity. In addition, the compound shown in formula I can also participate in the formation process of the membrane with the salt substance generated by the acidic substance, increasing the ionic conductivity and stability of the membrane.

[0019] In addition, the unsaturated double bonds of the compound shown in Formula I can undergo polymerization in the electrolyte. During the battery charging process, the electric field and oxidative environment on the electrode surface will cause the double bonds to open, and then copolymerize with other components in the electrolyte (such as solvent molecules, other additives, etc.) to form a polymer film. This polymer film can cover the electrode surface, protect the electrode, reduce the side reactions between the electrode and the electrolyte, prevent the direct contact between the electrolyte and the negative electrode, and reduce the irreversible consumption of lithium ions, thereby improving the battery's cycle performance and coulomb efficiency.

[0020] In addition, the compound shown in Formula I can be adsorbed on the electrode surface by physical adsorption 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, making it better able to resist the volume change and mechanical stress generated during battery charging and discharging, thereby protecting the integrity of the electrode structure. DETAILED DESCRIPTION

[0021] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0023] In this article, when it comes to numerical ranges, unless otherwise specified, the distribution of optional values ​​within the numerical range is considered continuous and includes the two numerical endpoints (i.e., the minimum value and the maximum value) of the numerical range, as well as every value between the two numerical endpoints. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be combined.

[0024] In this document, "multiple", "multiple", "multiple times", etc., unless otherwise specified, refer to a number greater than 2 or equal to 2. For example, "one or more" means one or greater than or equal to two. "Further", "further", "particularly", etc. are used for descriptive purposes to indicate differences in content, but should not be understood as limiting the scope of protection of the present invention.

[0025] In this document, unless otherwise specified, “%” refers to mass percentage.

[0026] In this article, substituents refer to atoms or groups of atoms that replace the main chain or ring in organic compounds. They can replace a hydrogen atom or other atoms in the molecule and affect the chemical and physical properties of the molecule. The type and position of the substituent have an important influence on the reactivity, polarity, solubility and other properties of the molecule.

[0027] Unsaturation, also known as the hydrogen deficiency index or the ring double bond index, is a quantitative indicator of the degree of unsaturation of organic compound molecules. n H m Hydrocarbons with molecular formula C n H m O x If m<2n+2, 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 in the organic compound molecule, the unsaturation of the organic compound increases by 1 for every 2 hydrogen atoms reduced.

[0028] Number of heteroatoms: In organic chemistry, non-carbon atoms are collectively referred to as heteroatoms. The most common heteroatoms are nitrogen atoms, sulfur atoms, and oxygen atoms.

[0029] Lithium iron phosphate 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 temperature), the battery life of electric vehicles has been greatly reduced, which has become one of the constraints on the development of electric vehicles. Therefore, the development of an electrolyte that can improve the low-temperature performance of lithium-ion batteries is crucial for the development of lithium-ion batteries and even the electric vehicle industry.

[0030] The inventors of the present application have found in their research that lithium difluorobis(oxalate) phosphate (LiODFP) is a good positive electrode film-forming additive that can reduce battery impedance and improve the low-temperature performance of the battery, but it is rarely used in lithium-ion batteries, especially in lithium iron phosphate (LiFePO4, referred to as LFP) systems. The reason is that organic solvents, lithium salts and other substances in the electrolyte will react to produce acid, causing the acidity of the electrolyte to increase, and lithium difluorobis(oxalate) phosphate will in turn cause the battery impedance to increase in an acidic environment, thereby offsetting its positive effect. Therefore, the inventors propose to introduce a substance that can inhibit the increase in electrolyte acidity and use it in combination with lithium difluorobis(oxalate) phosphate, so as to achieve the effect of lithium difluorobis(oxalate) phosphate in improving low-temperature impedance and enhancing low-temperature cycle capacity.

[0031] The first aspect of the present invention provides an anhydrous electrolyte, which comprises an organic solvent, a lithium salt and an additive. The additive comprises lithium difluorobis(oxalate)phosphate and a compound represented by formula I:

[0032]

[0033] R1, R2, R3, and R4 in Formula I are independently selected from substituents having 1-6 carbon atoms, 0-4 unsaturation, and 0-3 heteroatoms. That is, the types of R1, R2, R3, and R4 substituents in Formula I do not affect each other, and can be completely identical substituents, or partially identical substituents, such as R1 and R2 are the same or R1, R2, and R3 are the same, etc., or completely different substituents. The number of carbon atoms in the above substituents can be any value from 1 to 6, such as 1, 3, 5, or 6, and the degree of unsaturation can be any value from 0 to 4, such as 0, 1, 3, or 4. Heteroatoms are non-carbon atoms, and heteroatoms are selected from any one of nitrogen (N) atoms, sulfur (S) atoms, oxygen (O) atoms, boron (B) atoms, and phosphorus (P) atoms. For example, heteroatoms can be nitrogen atoms, sulfur atoms, or oxygen atoms, but are not limited thereto. The number of heteroatoms can be any value from 0 to 3. For example, 0, 1, 2 or 3, etc.

[0034] The compound molecule shown in Formula I contains a tertiary amino group (R3N, R represents an alkyl group). The tertiary amino group exhibits alkalinity due to the lone pair of electrons on the nitrogen atom. 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, so that lithium difluorobisoxalatoborate can play a role in improving low-temperature impedance and enhancing low-temperature cycle performance, thereby improving the low-temperature performance of lithium-ion batteries.

[0035] In addition, the salt substances generated by the amine group and the acidic substance can participate in the formation process of the membrane, increasing the ionic conductivity and stability of the membrane. In addition, the compound molecule shown in Formula I also contains unsaturated double bonds. During the battery charging process, the electric field and oxidative environment on the electrode surface will cause the double bonds to open, and then copolymerize with other components in the electrolyte (such as solvent molecules, other additives, etc.) to form a polymer film. This polymer film can cover the surface of the electrode, protect the electrode, reduce the side reactions between the electrode and the electrolyte, prevent the direct contact between the electrolyte and the negative electrode, and reduce the irreversible consumption of lithium ions, thereby improving the cycle performance and coulomb efficiency of the battery.

[0036] In addition, the compound molecules shown in Formula I can also be adsorbed on the electrode surface by physical adsorption or chemical adsorption, and cross-linking reactions can occur between the molecules or between the 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 the volume changes and mechanical stresses generated during the battery charging and discharging process, thereby protecting the integrity of the electrode structure.

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

[0038] In some embodiments, R1, R2, R3, and R4 in Formula I are independently selected from alkyl, alkenyl, alkynyl, carbonyl, ester, amino, or heterocyclic. Wherein, heterocyclic means that the atoms constituting the ring include at least one non-carbon atom in addition to carbon atoms, and these non-carbon atoms are heteroatoms. As an example, heterocyclic includes but is not limited to any one of pyridine, pyrrole, thiophene, thiazole, and furan. For example, the heterocyclic can be pyridine, thiophene, or furan, etc. That is, R1, R2, R3, and R4 can all be selected from any one of alkyl, alkenyl, alkynyl, carbonyl, ester, amino, and heterocyclic, and can be all the same, partially the same, or not the same. As an example, R1, R2, R3, and R4 can all be alkyl; or R1 and R2 are alkyl, R3 is alkenyl, and R4 is alkynyl; or R1 is alkyl, R2 is ester, R3 is amino, and R4 is alkenyl; and so on.

[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 additives may also include one or more of vinylene carbonate (VC), dithiothreitol (DTD), fluoroethylene carbonate (FEC), propenyl-1,3-sultone (PST), and tetravinylsilane (TVSI). Among them, VC can form a solid electrolyte interface film (SEI film) by electrochemical reaction on the surface of the negative electrode during the initial charge and discharge of the lithium battery, thereby improving the first efficiency of the battery and the cycle performance of the battery; DTD can improve the cycle performance of the battery and reduce capacity decay and internal resistance; FEC forms fluoride ions after decomposition, and the negative ions can react with lithium salts in the solvent to produce LiF that is not easy to decompose and has good insulation, thereby forming an SEI film; PST can form a solid electrolyte phase interface film on the surface of the battery electrode, which can inhibit the co-embedding and reduction decomposition of solvent molecules in the negative electrode, thereby improving the cycle performance and high temperature performance of the lithium-ion battery; TVSI can significantly improve the performance of lithium-ion batteries under high temperature and high pressure conditions, etc. The specific selection can be based on actual production needs. For example, in addition to lithium difluorobis(oxalate)phosphate and the compound shown in Formula I, the additives may also include VC, or VC and FEC, or VC, DTD and FEC, etc., so as to make the battery performance more fully exerted. It should be noted that the addition amount of the above additives can be set according to the conventional proportions in the art, and no specific limitation is made here.

[0043] As the main part of the electrolyte, the organic solvent can be selected from the solvents conventionally used in lithium-ion batteries in the art. In one embodiment, the organic solvent includes carbonates and carboxylates, wherein the carbonates include cyclic carbonates and linear carbonates, cyclic carbonates have a higher dielectric constant, which is conducive to the dissociation of lithium salts and improves the ionic conductivity of the electrolyte, linear carbonates have a lower melting point and viscosity, which is conducive to improving the performance of the electrolyte at low temperatures, and carboxylates have lower melting points and viscosity than linear carbonates. After adding low-viscosity carboxylates, the conductivity of the electrolyte will increase significantly, and the bond length between carboxylates and lithium is greater than that of carbonates, indicating that it is easier to desolvate, which helps to improve the kinetic performance of the battery cell. Mixing carbonates and carboxylates can exert a synergistic effect and improve the comprehensive 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 carboxylate 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 carboxylates listed above, or any two or a combination of the two. For example, the organic solvent is a combination of ethylene carbonate, dimethyl carbonate, and methyl formate, or a combination of ethyl formate and ethylene carbonate, and the like.

[0045] In other embodiments, the organic solvent may also include ether solvents and / or nitrile solvents. Ether solvents can reduce battery impedance and inhibit the low-temperature growth of lithium dendrites. Nitrile solvents can enable the electrolyte to maintain good ion conductivity at a lower temperature, as well as higher ion embedding and de-embedding capabilities, effectively improving battery efficiency. Among them, ethers are selected from one or two of ethylene glycol dimethyl ether and diethanol diethyl ether, but are not limited thereto; nitrile solvents include but are not limited to acetonitrile, propionitrile, butyronitrile or valeronitrile. Nitrile solvents can be selected from a single type, such as butyronitrile, etc.; 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 generally 70% to 80%, for example, 70%, 80% or 75%, etc.

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

[0047] In some embodiments, the lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonyl imide (LiFSI), lithium bis(trifluoromethyl)sulfonyl imide (LiTFSI), lithium acetate (CH3COOLi), lithium methane sulfonate (CH3SO3Li), and lithium trifluoromethyl sulfonate (CF3SO3Li). The lithium salt may be a single lithium salt or a mixed lithium salt. For example, the lithium salt may be lithium hexafluorophosphate, or a mixed lithium salt of lithium hexafluorophosphate and lithium bisfluorosulfonyl imide, or a mixed lithium salt of lithium hexafluorophosphate and lithium bis(trifluoromethyl)sulfonyl imide and lithium acetate, but is not limited thereto. Preferably, the lithium salt is a mixed lithium salt of lithium hexafluorophosphate and lithium bisfluorosulfonyl imide. Lithium hexafluorophosphate has high safety and strong comprehensive performance: it has a moderate ion migration number, a moderate dissociation constant, good antioxidant performance and good aluminum foil passivation ability. Lithium bisfluorosulfonyl imide has high electrical conductivity, good thermal stability, good ion migration rate and interface stability. The comprehensive performance of the electrolyte can be improved by using lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide together, wherein the mass ratio of lithium hexafluorophosphate to lithium bis(fluorosulfonyl)imide is 14:1 to 1:16, for example, 14:1, 1:1, 1:16, etc.

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

[0049] The anhydrous electrolyte of the present invention can be prepared according to a conventional preparation method. As an example, in a glove box with an argon atmosphere having a water content of <10 ppm, the organic solvent is mixed evenly according to a set ratio, and then the fully dried lithium salt and additive are added to the organic solvent, mixed evenly, and prepared into an electrolyte. Among them, the content of each component in the electrolyte, except the organic solvent, is the weight percentage calculated based on the total weight of the electrolyte.

[0050] The second aspect of the present invention provides a lithium ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and the above-mentioned anhydrous electrolyte of the present invention. The separator is arranged between the positive electrode sheet and the negative electrode sheet to play a role of isolation; during the battery charging and discharging process, lithium ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet, and the anhydrous electrolyte plays a role of conducting lithium ions between the positive electrode sheet and the negative electrode sheet.

[0051] Specifically, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one side surface of the positive electrode current collector. The positive electrode current collector is, for example, a foil formed after surface treatment of nickel, titanium, aluminum, silver, stainless steel or carbon. In addition to the foil, the positive electrode current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam or non-woven fabric. Among them, the thickness of the positive electrode current collector is, for example, 8μm to 15μm. In this embodiment, the positive electrode current collector is, for example, aluminum foil, and the thickness of the aluminum foil is, for example, 13μm. The positive electrode active material layer can be disposed on one side surface of the positive electrode current collector, or it can be disposed on both sides. The positive electrode active material layer includes a positive electrode active material, a conductive agent and a binder. In the present invention, the positive electrode active material is a lithium iron phosphate (LiFePO4) material, but is not limited to this, and can also be a ternary positive electrode material, etc. The binder of the positive electrode is, for example, selected from any one or more of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinylether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene or styrene butadiene rubber (SBR). The conductive agent of the positive electrode is, for example, selected from one or at least two of conductive carbon black (Super P), acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc.

[0052] The negative electrode plate 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 is selected from, for example, a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foam copper current collector, or a stainless steel current collector, etc. 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 disposed on one surface of the negative current collector, or can also be disposed on both surfaces. The negative active material layer includes a negative active material, a conductive agent, a binder, and a thickener. Here, the specific types of the negative active material, the conductive agent, and the binder are not specifically limited, and materials known in the art that can be used in lithium-ion batteries can be adopted, and 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 oxide 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 kinds 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 several compositions mixed in any proportion; the thickener 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, polyethylene film (Polyethylene, PE), polypropylene film (Polypropylene, PP), glass fiber film, polyethylene film, or 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 exemplified below:

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

[0057] Disperse the above 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 ratio between the components in the positive electrode slurry can be set with reference to the conventional ratio and is not limited here.

[0058] (2) Negative electrode sheet preparation

[0059] The above 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 coated on the negative electrode current collector, and after drying, cold pressing and other processes, a negative electrode sheet is obtained. Among them, the ratio between the components in the negative electrode slurry can be set according to the conventional ratio and is not limited here.

[0060] (3) Battery assembly

[0061] The prepared positive electrode sheet, separator, and negative electrode sheet are placed in sequence, so that the separator is placed between the positive and negative electrode sheets to play an isolating role, and a bare cell is obtained by winding or stacking. The bare cell is placed in a battery casing, and a lithium-ion battery is obtained after assembly, injection, formation, and capacity division.

[0062] The lithium-ion battery of the present invention can be used in the form of a single cell, a battery module or a battery pack for electronic devices to power them. Electronic devices include but are not limited to mobile phones, tablets, laptops, electric toys, battery cars, new energy vehicles, ships, spacecraft, etc. Among them, electric toys can include fixed or mobile electric toys, for example, game consoles, electric car toys, electric ship toys and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles and spacecraft, etc. New energy vehicles can be pure electric vehicles, hybrid vehicles or extended-range vehicles, etc.

[0063] The technical scheme of the present invention is described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the examples are all commercially available.

[0064] Example 1

[0065] The present embodiment provides an anhydrous electrolyte, which includes an organic solvent, a lithium salt and an additive, wherein the organic solvent is a composition of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl propionate (EA), and propylene carbonate (PC), and the mass ratio EC:DMC:EA:PC=2.5:3:4:0.5, the lithium salt is 11% lithium hexafluorophosphate (LiPF6) and 4% lithium bis(fluorosulfonyl)imide (LiFSI), and the additive includes 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 remaining components are all weight percentages calculated based on the total weight of the electrolyte.

[0066] The preparation process of the electrolyte 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 salt lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI) and additives are added to the organic solvent in the above ratio, mixed evenly, and an electrolyte is obtained.

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

[0068] Preparation of positive electrode sheets: 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 the action of a vacuum mixer until the system becomes uniform and transparent to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on an aluminum foil, dried at room temperature, and then transferred to an oven for drying, and then the positive electrode sheets are obtained through cold pressing, slitting and other processes.

[0069] Preparation of negative electrode sheet: Mix 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) in a mass ratio of 96:1:1:2, add deionized water, and stir and mix thoroughly under the action of a vacuum mixer to obtain a negative electrode slurry; evenly coat the negative electrode slurry on a copper foil, dry it at room temperature, and then transfer it to an oven for drying, and then obtain a negative electrode sheet through cold pressing, slitting and other processes.

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

[0071] Battery assembly: Place the positive electrode sheet, separator, and negative electrode sheet in sequence, with the separator placed between the positive and negative electrode sheets to serve as an isolation, and wind them into a bare cell. Then, after steel shell assembly, liquid injection, and capacity division, a lithium-ion battery is obtained.

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

[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 respectively. The test results are shown in Table 1. The test methods are as follows:

[0076] (1) Beginning of Life Direct Current Resistance (BOL DCR) test:

[0077] Adjust the temperature of the thermostat to -20℃, let it stand for 10min; charge to 3.8V at 0.33C constant current, then charge to 0.05C at 3.8V, let it stand for 30min; discharge to 2.0V at 0.33C constant current, let it stand for 10min, repeat twice, and take the last discharge capacity as C0. Charge to 3.8V at 0.33C constant current, then charge to 0.05C at constant voltage, let it stand for 30min, then discharge to 50% C0 at 0.33C constant current, let it stand for 1h, record the terminal voltage V0; discharge at 4C0 constant current for 30s, record the voltage V1. Then BOL DCR=(V0-V1) / 4C0.

[0078] (2) Low temperature cycle test:

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

[0080] Referring to Table 1, by comparing Examples 1-6 and Comparative Examples 1 and 2, it can be concluded that, under the premise of other conditions being the same, the addition of LiODFP (Comparative Example 2) alone can improve the low-temperature cycle performance of lithium-ion batteries to a certain extent compared to the addition of LiODFP (Comparative Example 1), but the DCR value will also be improved. This is because the acidic substances produced by the decomposition of lithium salts cause the acidity of the electrolyte to increase, and LiODFP reverses in an acidic environment to increase the battery impedance. In Examples 1 to 6, with the addition of compound I-1, the battery impedance gradually decreases, and the low-temperature cycle performance gradually increases. When the addition amount of compound I-1 exceeds 0.5%, the battery impedance rebounds, and the low-temperature cycle performance significantly decreases. 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 in the acidity of the electrolyte and keep the chemical environment of the electrolyte stable. LiODFP plays a role in reducing the battery impedance, thereby improving the low-temperature cycle performance of the battery; and when compound I-1 exceeds 0.5%, a thicker SEI film will be formed at the electrode interface, which will have an adverse effect on the battery dynamics and interface stability, and will affect the battery cycle performance. Therefore, when compound Ⅰ-1 is used in combination with LiODFP, an addition amount of 0.05%-3% is reasonable, and an addition amount of 0.1%-0.5% has the best effect.

[0081] By comparing Examples 3, 7, 8 and 9, it can be concluded that the use of Compound I-1, Compound I-2, Compound I-3 and Compound I-4 in combination with LiODFP can achieve the effect of improving the low-temperature cycle performance of the battery.

[0082] By comparing Example 3 and Examples 10 to 11, it can be seen that when the same amount of Compound I-1 is added, the low-temperature DCR value of the battery will first decrease and then increase as the amount of LiODFP added increases, and the low-temperature cycle performance will first increase and then decrease. 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, and when the amount of LiODFP added is too much, 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 the low-temperature cycle performance.

[0083] In summary, the present invention simultaneously introduces lithium difluorobisoxalate phosphate and the compound shown in formula I as additives in an anhydrous electrolyte, and uses the compound shown in formula I to adjust the pH of the electrolyte, so that the chemical environment of the electrolyte is more stable, and a stable use environment is provided for lithium difluorobisoxalate phosphate, so that lithium difluorobisoxalate phosphate can effectively play its role in reducing battery impedance and improving low-temperature cycle performance, thereby improving the low-temperature cycle performance of lithium-ion batteries. Therefore, the present invention effectively overcomes some practical problems in the prior art and has high utilization value and use significance.

[0084] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A non-aqueous electrolyte, characterized in that: include: An organic solvent, a lithium salt and an additive; wherein the additive comprises lithium difluorobis(oxalate)phosphate and a compound shown in formula I: R1, R2, R3, and R4 in formula I are each independently selected from a substituent having 1-6 carbon atoms, 0-4 unsaturations, and 0-3 heteroatoms, and the heteroatom is selected from any one of a nitrogen atom, a sulfur atom, an oxygen atom, a boron atom, and a phosphorus atom.

2. The anhydrous electrolyte according to claim 1, characterized in that The content of lithium difluorobisoxalate phosphate 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%.

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

4. The anhydrous electrolyte according to claim 1, characterized in that R1, R2, R3, and R4 in formula I are each independently selected from an alkyl group, an alkenyl group, an alkynyl group, a carbonyl group, an ester group, an amino group, or a heterocycle, and the heterocycle is selected from any one of pyrridine, 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:

6. The anhydrous electrolyte according to claim 1, characterized in that The additives further include one or more of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, propenyl-1,3-sultone, and tetravinylsilane.

7. The anhydrous electrolyte according to claim 1, characterized in that The organic solvent includes carbonates and carboxylates. The carbonates include one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate and fluoroethylene carbonate; the carboxylates 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 comprises: an ether solvent and / or a nitrile solvent, wherein the ether solvent comprises one or both of ethylene glycol dimethyl ether and diethanol diethyl ether, and the nitrile solvent comprises one or more of acetonitrile, propionitrile, butyronitrile and valeronitrile.

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 trifluoromethylsulfonate. The total content of the lithium salt in the electrolyte is 12% to 17%.

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

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