Electrolyte and lithium-ion battery

Abstract

PURPOSE: To provide an electrolyte and a lithium-ion battery.SOLUTION: An electrolyte includes a solvent that is a fluorinated solvent, a lithium salt, and an additive. The additive includes 1-propene 1,3-sultone (PST) and diethyl 2-thienylmethylphosphonate (DTYP). The electrolyte provided by embodiments of the present invention can withstand high voltages. In addition, since the electrolyte forms a stable protective film at an interface between an electrode of a battery and the electrolyte and ensures stability of the interface between the electrode and the electrolyte at a high temperature, the high-temperature stability of a high-voltage battery can be significantly improved. According to other embodiments of the present invention, a lithium ion battery is also provided.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to the field of batteries, and in particular to electrolytes and lithium-ion batteries. [Background technology]

[0002] In recent years, the rapid development of the new energy vehicle market has led to an explosive growth in the market value of power batteries, primarily secondary alkali metal ion batteries. However, the energy density of currently commercially available secondary lithium ion batteries is already approaching its theoretical limit. To fundamentally address the concerns of electric vehicle buyers about driving range, increasing the battery energy density and battery voltage has become an intuitive and feasible solution. Furthermore, as battery voltages increase, the requirements for high-voltage resistance and high-temperature stability at high voltages are also increasing. Summary of the Invention [Problem to be solved by the invention]

[0003] In view of the problems existing in the prior art, the present invention aims to provide an electrolyte that can withstand high voltages and that can form a stable protective film at the interface between the battery electrodes and the electrolyte, thereby ensuring the stability of the interface between the electrodes and the electrolyte at high temperatures, thereby significantly improving the high-temperature stability of high-voltage batteries. [Means for solving the problem]

[0004] To achieve the above object, one embodiment of the present invention provides an electrolyte solution, the electrolyte solution including a fluorinated solvent, a lithium salt, and an additive, the additive including 1-propene 1,3-sultone (PST) and diethyl (thiophen-2-ylmethyl)phosphonate (DTYP).

[0005] In some embodiments, the mass of diethyl 2-thienylmethylphosphonate is 0.01% to 1% of the total mass of the electrolyte, and the mass of 1-propene 1,3-sultone is 0.01% to 1% of the total mass of the electrolyte.

[0006] In some embodiments, the fluorinated solvent is a fluorinated carbonate solvent.

[0007] In some embodiments, the fluorinated carbonate solvent comprises fluoroethyl methyl carbonate (FEMC) and fluoroethylene carbonate (FEC), and the mass ratio of fluoroethyl methyl carbonate to fluoroethylene carbonate is 7:3 to 10:0.

[0008] In some embodiments, the content of the lithium salt is 12% to 20% of the total mass of the electrolyte.

[0009] In some embodiments, the lithium salt is one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium hexafluoroarsenate, lithium hexafluoroantimonate, lithium perchlorate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, and lithium difluorophosphate.

[0010] Another object of the present invention is to provide a lithium-ion battery including an electrolyte capable of withstanding high voltages. The electrolyte also forms a stable protective film at the interface between the battery's electrodes and the electrolyte, ensuring the stability of the electrode-electrolyte interface at high temperatures, thereby significantly improving the high-temperature stability of the high-voltage battery. Another aspect of the present invention provides a lithium-ion battery including a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and the electrolyte of any one of the above-described embodiments. In some embodiments, the positive electrode includes a positive electrode current collector and a positive electrode material layer positioned on the positive electrode current collector, the positive electrode material layer including lithium manganese nickel oxide, the chemical formula of which is Li a Ni x Mn y O 4-z M z where 0.90≦a≦1.10, 0.4≦x≦0.6, 1.4≦y≦1.6, 0≦z≦0.1, and element M is one or more of Cl, Br, I, S, Se, Te, or F. In some embodiments, the negative electrode includes a negative electrode current collector and a negative electrode material layer located on the negative electrode current collector, and the negative electrode material layer includes a graphite material. [Effects of the Invention]

[0011] The beneficial technical effects of the present invention are as follows: The electrolyte solvent is a fluorinated solvent, and when the fluorinated solvent is used in combination with 1-propene-1,3-sultone additive and 2-thienylmethylphosphonic acid diethyl additive, a stable protective film is formed at the interface between the electrode and the electrolyte, ensuring the stability of the interface between the electrode and the electrolyte at high temperatures, thereby significantly improving the high-temperature stability of the high-voltage battery.

[0012] By simultaneously adding 1-propene-1,3-sultone additive and 2-thienylmethylphosphonic acid diethyl additive to the fluorinated solvent system, the electrolyte can form a stable protective film at the interface between the electrode and the electrolyte of a lithium-ion battery, ensuring the stability of the electrode-electrolyte interface at high temperatures, thereby significantly improving the high-temperature stability of high-voltage batteries. DETAILED DESCRIPTION OF THE INVENTION

[0013] Although exemplary embodiments are described in detail below, these exemplary embodiments may be implemented in different ways and should not be construed as being limited to the embodiments described herein. Rather, the purpose of providing these embodiments is to thoroughly and completely disclose the present invention and fully convey the scope of the present invention to those skilled in the art. Unless specific techniques or conditions are specified in the embodiments, they are carried out based on the techniques or conditions described in the literature in the field or the product instructions. Unless the manufacturers of reagents or equipment used are specified, they are all general products that can be purchased through official sales channels.

[0014] A currently proposed electrolyte design concept is to partially or completely replace the conventional carbonate ester-based electrolyte solvent with a fluorinated solvent, such as fluoroethyl methyl carbonate (FEMC). Because fluorine atoms have a relatively high electron-withdrawing ability, they can better improve the oxidation stability of common carbonate esters. Therefore, the large-scale application of fluorinated solvents as the primary solvent in high-voltage battery systems can significantly improve the high-voltage resistance of lithium battery electrolytes, making them an important solvent component in high-voltage battery systems. In other words, as an important solvent component in high-voltage battery systems, fluorinated solvents have excellent high-voltage stability and can ensure the stability of the electrolyte's positive electrode interface.

[0015] However, at high temperatures, fluorinated solvents are prone to losing F atoms, which, in turn, combine with residual H atoms in the electrolyte to produce the highly acidic HF. HF attacks the interface between the electrolyte and the active material layers on the positive and negative electrodes, destroying the interfaces and causing continuous side reactions. This ultimately manifests as a series of problems, such as gas generation at high temperatures, capacity loss at high temperatures, cycle degradation, and increased direct current resistance (DCR), which leads to deterioration of the battery during cycling.

[0016] The inventors of the present invention have discovered that when a fully fluorinated solvent (i.e., a solvent in which all solvents are fluorinated) is used as the solvent in an electrolyte solution, and at the same time, high-voltage resistant 1-propene-1,3-sultone and 2-thienylmethylphosphonic acid diethyl are used as additives, when this type of electrolyte solution is applied to a lithium ion battery, the 1-propene-1,3-sultone and 2-thienylmethylphosphonic acid diethyl are effectively involved in the formation of a stable solid-liquid interfacial film at high temperatures, thereby significantly improving the stability of the electrolyte solution in high-temperature cycles. This also reduces the battery's DC resistance, and improves the battery's high-temperature cycle stability and high-temperature storage capacity retention rate.

[0017] The present invention provides an electrolyte solution, which includes a fluorinated solvent, a lithium salt, and an additive, the additive including 1-propene-1,3-sultone and diethyl 2-thienylmethylphosphonate.

[0018] Here, the structural formula of diethyl 2-thienylmethylphosphonate is as follows: [ka]

[0019] The solvent in the electrolyte solution according to the embodiment of the present invention is a fluorinated solvent, i.e., a fully fluorinated solvent, which has better high voltage stability and can ensure the stability of the interface of the electrolyte solution on the positive electrode side.

[0020] The additives in the electrolyte of the present invention include 1-propene-1,3-sultone and 2-thienylmethylphosphonic acid diethyl ester. The 1-propene-1,3-sultone and 2-thienylmethylphosphonic acid diethyl ester additives effectively contribute to the formation of a stable solid-liquid interfacial film, significantly improving the stability of the electrolyte during high-temperature cycling. For example, after the formation of the solid-liquid interfacial film, the electrolyte can exhibit a certain degree of reduction resistance stability on the negative electrode side of the battery. Specifically, the thiophene moiety in the molecular structure of the 2-thienylmethylphosphonic acid diethyl ester additive is preferentially oxidized on the positive electrode side of the battery, forming a stable CEI and preventing the electrolyte on the positive electrode side from undergoing continuous oxidation side reactions. The phosphate ester structure in the molecular structure of the 2-thienylmethylphosphonic acid diethyl ester additive can capture PF5 ions floating in the electrolyte, thereby suppressing the increase in electrolyte acidity and mitigating problems such as gas generation and increased DCR at high temperatures. The 1-propene 1,3-sultone additive is a sulfonic acid ester containing an unsaturated hydrocarbon, and the 1-propene 1,3-sultone additive participates in the film-forming reaction to mainly produce an S-containing SEI.

[0021] The present invention can improve the high-voltage stability of the electrolyte and ensure a certain level of anode reduction resistance. The solvent of the electrolyte of the present invention is a fluorinated solvent. When the fluorinated solvent is used in combination with the 1-propene-1,3-sultone additive and the 2-thienylmethylphosphonic acid diethyl additive, a stable protective film is formed at the interface between the electrode and the electrolyte, ensuring the stability of the electrode-electrolyte interface at high temperatures, thereby significantly improving the high-temperature stability of the high-voltage battery.

[0022] In some embodiments, the fluorinated solvent is a fluorinated carbonate ester solvent, which has better high-voltage stability and can ensure the stability of the electrolyte's positive electrode side interface. In some embodiments, the fluorinated carbonate ester solvent includes fluoroethyl methyl carbonate (FEMC) and fluoroethylene carbonate (FEC).

[0023] The structural formula of fluoroethyl methyl carbonate is as follows: [ka]

[0024] In a further embodiment, the mass ratio of fluoroethyl methyl carbonate to fluoroethylene carbonate is 7:3 to 10:0. In another embodiment, fluoroethylene carbonate can be replaced with other fluorinated carbonate esters. In a further embodiment, the mass ratio of fluoroethyl methyl carbonate to fluoroethylene carbonate is 7:3. As the proportion of fluoroethyl methyl carbonate in the solvent increases and the proportion of fluoroethylene carbonate decreases, the probability of fluoroethyl methyl carbonate decomposing at the interface between the electrode and the electrolyte increases, resulting in the production of large amounts of by-products that are unstable at high temperatures. Therefore, the optimal solvent ratio is a mass ratio of fluoroethyl methyl carbonate to fluoroethylene carbonate of 7:3.

[0025] In some embodiments, the mass of 2-thienylmethyldiethylphosphonate is 0.01% to 1% of the total mass of the electrolyte, and the mass of 1-propene-1,3-sultone is 0.01% to 1% of the total mass of the electrolyte. The 1-propene-1,3-sultone additive in the additive mixture is a sulfonate ester containing an unsaturated hydrocarbon. Therefore, using the 1-propene-1,3-sultone additive alone results in a high concentration of sulfonate ester groups after polymerization of 1-propene-1,3-sultone. Because carbonate esters and sulfonate esters have a strong structural correlation, the structure of the 1-propene-1,3-sultone polymer is easily swollen by the carbonate ester. When the electrolyte penetrates the 1-propene-1,3-sultone polymer, the SEI volume expands, exposing new interfaces that further react with the electrolyte, resulting in insufficient protection of the negative electrode interface. Among the additives used in combination, the 2-thienylmethylphosphonate diethyl additive forms a film preferentially on the positive electrode side, while on the negative electrode side, the film formation is relatively weak. Furthermore, because the SEI component is primarily comprised of oligomers that lack high-temperature resistance, simply increasing the amount of the 2-thienylmethylphosphonate diethyl additive alone cannot compensate for this. Therefore, the present invention combines an excellent negative electrode film-forming additive with an excellent positive electrode film-forming additive, namely, the 1-propene-1,3-sultone additive and the 2-thienylmethylphosphonate diethyl additive. Furthermore, by adjusting the appropriate ratio of the 2-thienylmethylphosphonate diethyl additive and the 1-propene-1,3-sultone additive, the present invention can ensure that the two additives form a copolymer and form a stable protective film. This allows the concentration ratio of the 2-thienylmethylphosphonate diethyl additive and the 1-propene-1,3-sultone polymerization monomers in the polymer to be reduced, thereby reducing the swelling capacity and optimizing the dynamic performance of the system.

[0026] In a preferred embodiment, the most suitable ratio is when the mass of 2-thienylmethylphosphonic acid diethyl ester is 0.3% of the total mass of the electrolyte, and the mass of 1-propene-1,3-sultone is 0.5% of the total mass of the electrolyte. This is because using too much 1-propene-1,3-sultone or 2-thienylmethylphosphonic acid diethyl ester additive results in an excessively thick interfacial protective film, which impedes the transport rate of lithium ions at the solid-liquid interface, resulting in a decrease in the dynamics of the battery and, therefore, an impact on the high-temperature cycle capacity. In some embodiments, the additive may further include a common negative electrode film-forming electrolyte additive, such as a vinylene carbonate (VC) additive.

[0027] In some embodiments, the content of the lithium salt is 12% to 20% of the total mass of the electrolyte. In further embodiments, the lithium salt is one or more of an inorganic lithium salt and an organic lithium salt. In further embodiments, the lithium salt may be one or more of lithium hexafluorophosphate (LiPF), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF), lithium methanesulfonate (LiCHSO), lithium trifluoromethanesulfonate (LiCFSO), lithium hexafluoroarsenate (LiAsF), lithium hexafluoroantimonate (LiSbF), lithium perchlorate (LiClO), Li[BF(CO)], Li[PF(CO)], Li[N(CFSO)], Li[C(CFSO)], lithium difluoro(oxalato)borate (LiODFB), lithium bis(oxalato)borate (LiBOB), and lithium difluorophosphate (LiPOF).

[0028] In some embodiments, the lithium salts can include LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSI), where LiPF6 is the primary lithium salt and LiFSI is the secondary lithium salt, and the primary lithium salt comprises 8% to 20% by mass of the total electrolyte.

[0029] The present invention further provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and the electrolyte of the above-described embodiment. In some embodiments, the positive electrode comprises a positive electrode current collector and a positive electrode material layer disposed on the positive electrode current collector, the positive electrode material layer comprising lithium manganese nickel oxide, the chemical formula of which is Li a Ni x Mn y O 4-z M z where 0.90≦a≦1.10, 0.4≦x≦0.6, 1.4≦y≦1.6, and 0≦z≦0.1, and element M is one or more of Cl, Br, I, S, Se, Te, or F. In some embodiments, the negative electrode includes a negative electrode current collector and a negative electrode material layer disposed on the negative electrode current collector, the negative electrode material layer including a graphite material. The lithium-ion battery of the present invention employs an electrolyte containing a fluorinated solvent in combination with a 1-propene-1,3-sultone additive and a 2-thienylmethylphosphonic acid diethyl additive. This allows the electrolyte to form a protective film at the interface between the electrodes (including the positive electrode and negative electrode) of the lithium-ion battery and the electrolyte, even at high temperatures, ensuring the stability of the interface between the electrodes and the electrolyte at high temperatures. This significantly improves the high-temperature stability of the high-voltage battery. In one embodiment, the graphite material may be artificial graphite.

[0030] positive electrode The positive electrode of the lithium-ion battery provided by the present invention includes a positive electrode current collector and a positive electrode material layer disposed on the positive electrode current collector. The positive electrode material layer includes a positive electrode active material capable of absorbing and releasing lithium (Li) (hereinafter, also referred to as a "positive electrode material capable of absorbing / releasing lithium Li"). Examples of positive electrode active materials capable of absorbing / releasing lithium (Li) include lithium manganese nickel oxide, lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxygen phosphate, lithium iron phosphate, lithium titanate, and lithium-rich manganese-based materials. The positive electrode current collector can be, for example, aluminum foil or nickel foil, although other positive electrode current collectors commonly used in the field may also be used. In some embodiments, the positive electrode material layer further includes a positive electrode conductive agent and a positive electrode binder. In some embodiments, the positive electrode conductive agent can include a mixture of one or more of conductive carbon black (Super P), acetylene black, nanometal powder, carbon nanotubes, and graphene. In some embodiments, the positive electrode binder can be a mixture of one or more of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropene, polytetrafluoroethylene, sodium carboxymethylcellulose, and styrene butadiene rubber.

[0031] electrolyte The electrolyte solution for a lithium-ion battery provided by the present invention comprises a lithium salt, a solvent, and an additive. Here, the solvent is a fluorinated solvent, and the additive comprises 1-propene-1,3-sultone and 2-thienylmethylphosphonic acid diethyl ester. Preferably, the mass of 2-thienylmethylphosphonic acid diethyl ester is 0.01% to 1% of the total mass of the electrolyte solution, and the mass of 1-propene-1,3-sultone is 0.01% to 1% of the total mass of the electrolyte solution. Preferably, the additive further comprises vinylene carbonate (VC).

[0032] The fluorinated solvent may be a common fluorinated solvent in this field, for example, the fluorinated solvent includes fluoroethyl methyl carbonate. Preferably, the fluorinated solvent further includes fluoroethylene carbonate. Preferably, the mass ratio of fluoroethyl methyl carbonate to fluoroethylene carbonate is 7:3 to 10:0. Preferably, the mass of 2-thienylmethylphosphonate diethyl is 0.01% to 1% of the total mass of the electrolyte solution, and the mass of 1-propene-1,3-sultone is 0.01% to 1% of the total mass of the electrolyte solution. More preferably, the mass of 2-thienylmethylphosphonate diethyl is 0.3% of the total mass of the electrolyte solution, and the mass of 1-propene-1,3-sultone is 0.5% of the total mass of the electrolyte solution.

[0033] The lithium salt may be any lithium salt commonly used in the art. The lithium salt may be one or more of lithium hexafluorophosphate (LiPF), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF), lithium methanesulfonate (LiCHSO), lithium trifluoromethanesulfonate (LiCFSO), lithium hexafluoroarsenate (LiAsF), lithium hexafluoroantimonate (LiSbF), lithium perchlorate (LiClO), Li[BF(CO)], Li[PF(CO)], Li[N(CFSO)], Li[C(CFSO)], lithium difluoro(oxalato)borate (LiODFB), lithium bis(oxalato)borate (LiBOB), and lithium difluorophosphate (LiPOF). Preferably, the lithium salt may include LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSI), the mass of the lithium salt is 12% to 20% of the total mass of the electrolyte, and the mass of LiPF6 is 8% to 20% of the total mass of the electrolyte.

[0034] negative electrode The negative electrode of the lithium-ion battery provided by the present invention includes a negative electrode current collector and a negative electrode material layer disposed on the negative electrode current collector. In some embodiments, the negative electrode current collector may be copper foil. In some embodiments, the negative electrode material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a negative electrode thickener.

[0035] The negative electrode active material includes a negative electrode material capable of absorbing and releasing lithium (Li) (hereinafter, also referred to as "a negative electrode material capable of absorbing / releasing lithium Li"). Examples of the negative electrode material capable of absorbing / releasing lithium (Li) include carbon materials, metal compounds, oxides, sulfides, lithium nitrides such as LiN3, lithium metal, metals that form alloys with lithium, and polymer materials.

[0036] Carbon materials can include low-graphitizable carbon, easily graphitizable carbon, artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, hard carbon, pyrolytic carbon, coke, glassy carbon, sintered organic polymer compounds, carbon fiber, and activated carbon. Among these, coke can include pitch coke, needle coke, and petroleum coke. Sintered organic polymer compounds refer to materials obtained by carbonizing polymer materials such as phenolic resins and furan resins at appropriate temperatures. Some of these materials are classified as low-graphitizable carbon or easily graphitizable carbon. Examples of polymer materials include polyacetylene and polypyrrole.

[0037] Among these negative electrode materials capable of absorbing / desorbing lithium (Li), materials with charge and discharge voltages close to those of lithium metal are selected. This is because the lower the charge and discharge voltage of the negative electrode material, the easier it is to increase the energy density of the electrochemical device (e.g., secondary battery). Carbon materials can be selected as the negative electrode material because their crystalline structure changes only slightly during charge and discharge, resulting in excellent cycle characteristics and large charge and discharge capacities. Graphite, in particular, can be selected because it can provide a large electrochemical equivalent and high energy density.

[0038] Furthermore, negative electrode materials capable of absorbing / desorbing lithium (Li) can include elemental lithium metal, metal elements and metalloid elements capable of forming alloys with lithium (Li), alloys and compounds containing such elements, etc. In particular, they are used in combination with carbon materials because in such cases, excellent cycle characteristics and high energy density can be obtained. In addition to alloys containing two or more metal elements, the term "alloy" as used herein also includes alloys containing one or more metal elements and one or more metalloid elements. The alloy may be in the form of a solid solution, a eutectic crystal (eutectic mixture), an intermetallic compound, or a mixture thereof.

[0039] Examples of metallic and semi-metallic elements include tin (Sn), lead (Pb), aluminum (Al), indium (In), silicon (Si), zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Examples of the above alloys and compounds include those having the chemical formula Ma. s Mb t Li u The material and chemical formula of p Mc q Md rIn these chemical formulas, Ma represents at least one element selected from metal elements and metalloid elements that can form an alloy with lithium, Mb represents at least one element selected from metal elements and metalloid elements other than lithium and Ma, Mc represents at least one element selected from non-metal elements, Md represents at least one element selected from metal elements and metalloid elements other than Ma, and s, t, u, p, q, and r satisfy s>0, t≧0, u≧0, p>0, q>0, and r≧0.

[0040] In addition, inorganic compounds that do not contain lithium (Li) in the negative electrode, such as MnO2, V2O5, and V6O 13 , NiS, and MoS can be used.

[0041] In some embodiments, the negative electrode conductive agent is a mixture of one or more of Super P, acetylene black, nanosilver powder, carbon nanotubes, and graphene. In some embodiments, the negative electrode binder is a mixture of one or more of vinylidene fluoride, polyvinylidene fluoride-hexafluoropropene, polytetrafluoroethylene, and styrene butadiene rubber (SBR). In some embodiments, the negative electrode thickener may be one or more of polyvinylpyrrolidone (PVP), sodium carboxymethyl cellulose (CMC-Na), and polyvinyl alcohol (PVA).

[0042] Separator The separator of the lithium-ion battery provided by the present invention is located between the positive electrode and the negative electrode. For example, the separator includes a substrate layer and a surface treatment layer. The substrate layer is a porous nonwoven fabric, membrane, or composite membrane, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene (PP), polyethylene terephthalate, and polyimide. Specifically, polypropylene porous membrane, polyethylene porous membrane, polypropylene nonwoven fabric, polyethylene nonwoven fabric, and polypropylene-polyethylene-polypropylene porous composite membrane can be selected. At least one surface of the substrate layer is provided with a surface treatment layer, which may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic material.

[0043] Lithium-ion battery The positive electrode, negative electrode, electrolyte, and separator can be assembled into a lithium ion battery using a method commonly used in this field. Here, the positive electrode, separator, and negative electrode are wound or stacked in order to form a bare battery, which is then packaged in an aluminum plastic film or the like, and the electrolyte is injected. After undergoing chemical formation, packaging, and testing, the assembled lithium ion battery is subjected to an electrochemical performance test and a cycle performance test.

[0044] The lithium ion battery of the present invention may be a primary lithium battery or a secondary lithium battery, and the following describes a method for fabricating a secondary lithium battery. It should be understood that the fabrication method described below is merely an example, and other methods commonly used in the art may be adopted without departing from the disclosure of the present invention.

[0045] Preparation of the positive electrode A positive electrode active material, a positive electrode conductive agent, a positive electrode binder, and a solvent, N-methylpyrrolidone (NMP), are mixed in a vacuum mixer at a specific mass ratio until the mixture becomes uniform and transparent, yielding a positive electrode slurry. The mass ratios of the positive electrode active material, positive electrode conductive agent, and positive electrode binder are 93% to 98.5%, 0.5% to 3%, and 0.5% to 4%. NMP is added so that the positive electrode material accounts for 45% to 70% of the total mass of the positive electrode slurry. The positive electrode slurry is then uniformly applied to a positive electrode current collector, followed by drying, roll pressing, cutting, and other processes to obtain a positive electrode.

[0046] Preparation of the negative electrode After mixing the components of the negative electrode material, deionized water is added and the mixture is thoroughly stirred and mixed in a vacuum mixer to obtain a uniform mixture, resulting in a negative electrode slurry. The mass ratio of the components of the negative electrode material is 94% to 98% negative electrode active material, 0.2% to 1.5% negative electrode conductive agent, 0.01% to 1.5% negative electrode thickener, and 1% to 3% negative electrode binder. Deionized water is added based on the proportion of the negative electrode material in the total mass of the negative electrode slurry being 45% to 70%. Next, the negative electrode slurry is uniformly applied onto a negative electrode current collector, and the negative electrode is obtained through drying, roll pressing, and cutting.

[0047] Preparation of electrolyte In a glove box with an argon atmosphere and a moisture content of <10 ppm, battery-grade fluoroethyl methyl carbonate and fluoroethylene carbonate are mixed to form a solvent. Lithium salt and appropriate additives are then added to the solvent and mixed uniformly to obtain the electrolyte. Optionally, lithium salt, 1-propene-1,3-sultone, and 2-thienylmethylphosphonic acid diethyl ester can be added.

[0048] Preparation of separator The separator includes a substrate layer and a surface treatment layer. The substrate layer is a porous nonwoven fabric, membrane, or composite membrane, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane can be selected. A surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer may be a polymer layer or an inorganic layer, or may be a layer formed by mixing a polymer and an inorganic material.

[0049] Fabrication of a Lithium-ion Battery The above-mentioned positive electrode, negative electrode, electrolyte, and separator can be assembled into a lithium ion battery using a method commonly used in this field. Here, the positive electrode, separator, and negative electrode are wound or stacked in order to form a bare battery, which is then packaged in an aluminum laminate film or the like, and the electrolyte is injected. After undergoing chemical formation, packaging, and testing, the assembled lithium ion battery is subjected to electrochemical performance tests and cycle performance tests.

[0050] The present invention will be explained in more detail below by giving some specific examples and comparative examples. [Example]

[0051] Example 1 (1) Preparation of the positive electrode In the present embodiment, lithium manganese nickel oxide is used as the positive electrode active material. The lithium manganese nickel oxide is obtained by mixing two components and then performing a secondary sintering process. Here, the chemical formula of the first component is Li a Ni x Mn y O 4-z M zThe formula is: 0.90≦a≦1.10, 0.4≦x≦0.6, 1.4≦y≦1.6, 0≦z≦0.1, and the element M is one or more of Cl, Br, I, S, Se, Te, or F. In this embodiment, a=1, x=0.4, y=1.6, and z=0. Next, the carbon source is pre-sintered under a nitrogen atmosphere to obtain a second component. The carbon source may be one or more of fructose, polyethylene glycol, galactose, polyvinylpyrrolidone, or tannic acid. In this embodiment, fructose is selected as the carbon source. The second component covers at least a portion of the surface of the first component. The second component contains carbon, and the mass of the second component is 1.0 wt% to 3.0 wt% of the mass of the first component. In this embodiment, the mass of the second component is 2.0 wt% of the mass of the first component. The positive electrode active material prepared above, polyvinylidene fluoride as a binder, and conductive carbon black (Super P) as a conductive agent were mixed in a weight ratio of 98:1:1, N-methylpyrrolidone (NMP) was added, and the mixture was stirred under a vacuum mixer until the mixture became uniform and transparent, yielding a positive electrode slurry. The positive electrode slurry was then evenly applied to aluminum foil. The aluminum foil was then dried at room temperature and transferred to a drying oven, where it was then cold-pressed and cut to obtain the positive electrode. (2) Preparation of the negative electrode Artificial graphite was used as the negative electrode active material, conductive carbon black (Super P) as the conductive agent, sodium carboxymethyl cellulose (CMC-Na) as the thickener, and styrene butadiene rubber (SBR) as the binder. These materials were mixed in a mass ratio of 96:1:1:2, and deionized water was added to obtain a negative electrode slurry under the action of a vacuum mixer. The negative electrode slurry was then evenly coated onto the copper foil of the negative electrode current collector. The copper foil was then dried at room temperature and transferred to a drying oven, after which it was cold pressed and cut to obtain the negative electrode. (3) Preparation of electrolyte In a glove box with an argon gas atmosphere and a moisture content of <10 ppm, battery-grade fluoroethyl methyl carbonate and fluoroethylene carbonate were mixed in a 7:3 ratio to form a solvent. Next, a lithium salt and additives were added and mixed uniformly to obtain the electrolyte. Here, the lithium salt was LiPF6, with the mass of the lithium salt accounting for 13% of the total mass of the electrolyte. The additives were 1-propene-1,3-sultone and 2-thienylmethylphosphonic acid diethyl ester, with the mass of the 2-thienylmethylphosphonic acid diethyl ester accounting for 0.3% of the total mass of the electrolyte, and the mass of the 1-propene-1,3-sultone accounting for 0.01% of the total mass of the electrolyte. (4) Preparation of separator A 12 μm thick polypropylene (PP) membrane is used as the separator. (5) Fabrication of lithium-ion batteries The positive electrode, separator, and negative electrode prepared above are stacked in this order, with the separator positioned between the positive and negative electrodes to act as an insulator. The resulting battery is then wrapped in an aluminum laminate film, transferred to a vacuum dryer, dried at 120°C, and filled with 3.0 g / Ah of the electrolyte prepared above. The battery is then sealed, and the electrolyte is converted to form a soft-pack battery (i.e., a lithium-ion battery) with a capacity of 1 Ah.

[0052] Example 2 The preparation method is the same as that of Example 1, except that the mass of diethyl 2-thienylmethylphosphonate in Example 2 is 0.3% of the total mass of the electrolyte, and the mass of 1-propene-1,3-sultone is 0.5% of the total mass of the electrolyte.

[0053] Example 3 The preparation method is the same as that of Example 1, except that the mass of diethyl 2-thienylmethylphosphonate in Example 3 is 0.3% of the total mass of the electrolyte, and the mass of 1-propene-1,3-sultone is 1% of the total mass of the electrolyte.

[0054] Example 4 The preparation method is the same as that of Example 2, but differs in that the mass ratio of fluoroethyl methyl carbonate to fluoroethylene carbonate in Example 4 is 6:4.

[0055] Example 5 The preparation method is the same as that of Example 2, but differs in that the mass ratio of fluoroethyl methyl carbonate to fluoroethylene carbonate in Example 5 is 8:2.

[0056] Example 6 The preparation method is the same as that of Example 2, but differs in that the mass ratio of fluoroethyl methyl carbonate to fluoroethylene carbonate in Example 6 is 9:1.

[0057] Example 7 The preparation method is the same as that of Example 2, but differs in that the mass ratio of fluoroethyl methyl carbonate to fluoroethylene carbonate in Example 7 is 10:0.

[0058] Comparative Example 1 The preparation method is the same as that of Example 1, but the difference is that the electrolyte of Comparative Example 1 does not contain 1-propene-1,3-sultone, and the additive is 2-thienylmethylphosphonic acid diethyl ester, and the mass of 2-thienylmethylphosphonic acid diethyl ester is 0.01% of the total mass of the electrolyte.

[0059] Comparative Example 2 The preparation method is the same as that of Example 1, but the difference is that the electrolyte of Comparative Example 2 does not contain 1-propene-1,3-sultone, and the additive is 2-thienylmethylphosphonic acid diethyl ester, and the mass of 2-thienylmethylphosphonic acid diethyl ester is 0.3% of the total mass of the electrolyte.

[0060] Comparative Example 3 The preparation method is the same as that of Example 1, but the difference is that the electrolyte of Comparative Example 3 does not contain 1-propene-1,3-sultone, and the additive is 2-thienylmethylphosphonic acid diethyl ester, and the mass of 2-thienylmethylphosphonic acid diethyl ester is 1% of the total mass of the electrolyte.

[0061] Comparative Example 4 The preparation method is the same as that of Example 1, but the difference is that the electrolyte of Comparative Example 4 does not contain diethyl 2-thienylmethylphosphonate, and the additive is 1-propene-1,3-sultone, and the mass of 1-propene-1,3-sultone is 0.01% of the total mass of the electrolyte.

[0062] Comparative Example 5 The preparation method is the same as that of Example 1, but the difference is that the electrolyte of Comparative Example 5 does not contain diethyl 2-thienylmethylphosphonate, and the additive is 1-propene-1,3-sultone, and the mass of 1-propene-1,3-sultone is 0.3% of the total mass of the electrolyte.

[0063] Comparative Example 6 The preparation method is the same as that of Example 1, but the difference is that the electrolyte of Comparative Example 6 does not contain diethyl 2-thienylmethylphosphonate, and the additive is 1-propene-1,3-sultone, and the mass of 1-propene-1,3-sultone is 1% of the total mass of the electrolyte.

[0064] Comparative Example 7 The preparation method is the same as that of Example 1, but the difference is that the electrolyte of Comparative Example 7 does not contain diethyl 2-thienylmethylphosphonate.

[0065] Comparative Example 8 The preparation method is the same as that of Example 1, but the difference is that the electrolyte of Comparative Example 8 does not contain diethyl 2-thienylmethylphosphonate, and at the same time, the additive is vinylene carbonate, and the mass of vinylene carbonate is 0.5% of the total mass of the electrolyte.

[0066] Comparative Example 9 The preparation method is the same as that of Example 1, but the difference is that the electrolyte of Comparative Example 9 does not contain 2-thienylmethylphosphonic acid diethyl ester, and the additives are vinylene carbonate (VC) and ethylene sulfate (DTD), with the mass of vinylene carbonate being 0.5% of the total mass of the electrolyte, and the mass of ethylene sulfate being 0.5% of the total mass of the electrolyte.

[0067] The lithium ion battery of the present invention can be tested by the following method. (1) 25°C DC resistance (DCR) test for lithium-ion batteries At the specified temperature, the battery is discharged at 1C current to 50% SOC (State of Charge, reflecting the remaining capacity of the battery), then the current is increased to 4C and maintained for 30 seconds. The difference between the updated stable voltage and the original plateau voltage is measured, and the ratio of this value to the 4C current value is the battery's DC resistance. The DCR test result after the battery is fully charged for the first time is the initial DCR of the battery. (2) Capacity retention rate of lithium-ion batteries when stored at high temperatures (60°C) After fully charging the lithium-ion battery, store it in a constant temperature bath at 60°C for 15 days, and after cooling it down sufficiently, discharge it at 1C to the cut-off voltage, and compare the capacity with the percentage of the initial discharge capacity. (3) High temperature 45℃ cycle test The battery is cycled at 45°C. The charge / discharge voltage range is 3.4V to 4.85V, and the charge / discharge ratio is 0.5C / 1C. The discharge capacity of each cycle is recorded. The test is terminated when the remaining battery capacity is 80% SOC, and the number of cycles actually achieved is recorded. For lithium manganese nickel oxide / graphite batteries, the charge / discharge cutoff voltage is 3.4V to 4.85V.

[0068] Electrolyte composition table of examples and comparative examples of different groups [Table 1]

[0069] Performance test results of different groups of examples and comparative examples [Table 2]

[0070] Analyzing the above data, the following conclusions can be drawn: As can be seen from Comparative Examples 1 to 3, the overall performance of the battery was best when the DTYP additive content was 0.3% of the total mass of the electrolyte. When the DTYP content increased from 0.01% to 0.3%, the high-temperature storage capacity retention rate of the battery increased, which explains that the DTYP additive better maintains thermodynamic stability and reduces reactions at the solid-liquid interface even under high-temperature conditions. However, when the DTYP content in the electrolyte reached 1% or more, some deterioration in the high-temperature cycle performance of the battery was observed. Therefore, in the present invention, the DTYP content was limited to 0.01% to 1% of the total mass of the electrolyte, with the optimal ratio being 0.3%. DTYP itself is a film-forming additive that forms a stable protective film at the positive and negative electrode interfaces. When used in appropriate amounts, it can suppress electrolyte side reactions at the interface. However, excessive use of the film-forming additive results in an excessively thick protective film at the interface, impeding the lithium ion transport rate at the solid-liquid interface, resulting in reduced battery dynamics and consequently impacting high-temperature cycling capacity. Furthermore, DTYP film formation occurs preferentially on the positive electrode side, while on the negative electrode side, film formation is relatively weak and the protective film is primarily composed of oligomers that lack high-temperature resistance. Therefore, increasing the amount of a single additive cannot compensate for this.

[0071] As can be seen from Comparative Examples 4 to 6, the overall battery performance was best when the PST additive content was 0.5% of the total mass of the electrolyte. When the PST content increased from 0.01% to 0.5%, the battery's high-temperature storage capacity retention rate increased, indicating that the PST additive better maintained thermodynamic stability and reduced reactions at the solid-liquid interface, even under high-temperature conditions. However, when the PST content reached 1%, some deterioration in the battery's high-temperature cycle performance was observed. Therefore, in this invention, the PST content was limited to 0.01% to 1% of the total mass of the electrolyte, with 0.5% being the optimal ratio. PST itself is a film-forming additive that forms a stable protective film at the positive and negative electrode interfaces. When used in an appropriate amount, it can suppress electrolyte side reactions at the interface. However, when too much film-forming additive is used, the thickness of the interfacial protective film becomes too thick, hindering the transport rate of lithium ions at the solid-liquid interface. This results in a decrease in battery kinetics and, therefore, an impact on the high-temperature cycle capacity. Furthermore, when PST is used alone, if the PST content is too high, the sulfonate ester group concentration after PST polymerization will be high. However, because carbonate esters and sulfonate esters have a strong structural correlation, the PST polymer structure will easily swell with the carbonate ester, allowing the electrolyte to penetrate into the PST polymer. This causes the SEI volume to expand, exposing new interfaces that can further react with the electrolyte, resulting in insufficient protection of the negative electrode interface.

[0072] As can be seen from Examples 1 to 3, when the solvent system was kept constant and the LiPF concentration was 13% wt., the combined use of PST and DTYP significantly improved both the high-temperature storage capacity retention and the high-temperature cycle count of the corresponding batteries. When the DTYP content was limited to 0.3% and the PST content was limited to 0.01% to 1%, the optimal content was 0.5% wt. This is because the thiophene moiety in the DTYP additive used in this invention was preferentially oxidized on the cathode side, forming a stable CEI and protecting the cathode electrolyte from further oxidation side reactions. The phosphate ester structure in the DTYP additive's molecular structure can capture PF5 ions released in the electrolyte, thereby suppressing the increase in electrolyte acidity and mitigating problems such as gas generation and increased DCR at high temperatures. At the same time, when used in combination with an excellent anode film-forming additive, i.e., the other additive, PST, also participates in the film-forming reaction in the battery, further supporting the formation of a stable interfacial passivation layer, producing a predominantly sulfur-containing SEI. Furthermore, the combined use of DTYP and PST prevents the concentration of sulfonate ester groups from becoming too high after PST polymerization, eliminating the problem of the PST polymer structure swelling with carbonate esters and the problem of excessive addition resulting in an excessively thick interfacial protective film. By adjusting the appropriate DTYP to PST ratio, the present invention ensures that the two additives form a copolymer at the electrode / electrolyte interface, reducing the concentration ratio of DTYP and PST polymerization monomers in the polymer, thereby reducing swelling capacity and optimizing the system's dynamic performance.

[0073] As can be seen from Examples 2 and 4 to 7, when the additive blending ratio is optimal, adjusting the solvent also significantly changes the battery performance. Further increasing the FEMC ratio in the solvent and decreasing the FEC ratio increases the battery's DCR. This is because FEMC decomposes at the interface, generating large amounts of by-products that are unstable at high temperatures. This unstable SEI ultimately reduces the high-temperature performance of the battery. Therefore, in this invention, the optimal solvent ratio is specified as FEMC:FEC = 7:3.

[0074] As can be seen from Comparative Examples 7 to 9, when no additives were used or when other additives were used alone, the battery performance was significantly worse. This indicates that DTYP and PST play a very important role in the battery and that their combined use can optimize performance. When the common additives VC and DTD were used in combination, the lithium manganese nickel oxide high-voltage system had an extremely large DCR, making it impossible to cycle at room temperature and unable to demonstrate its capacity after high-temperature storage. This indicates that the combined use of DTYP and PST is necessary to ensure normal operation of the high-voltage system. [Industrial Applicability]

[0075] As described above, the present invention utilizes a combination of 1-propene-1,3-sultone and 2-thienylmethylphosphonic acid diethyl ester in a fully fluorinated solvent-based electrolyte. The addition of the 1-propene-1,3-sultone and 2-thienylmethylphosphonic acid diethyl ester additives to the electrolyte in an optimal ratio effectively contributes to the formation of a stable solid-liquid interfacial film at the electrode-electrolyte interface, ensuring the stability of the electrode-electrolyte interface at high temperatures and significantly improving the stability of the electrolyte during high-temperature cycling, thereby achieving an overall improvement in the high-temperature performance of the battery. For example, the DCR of the battery can be reduced, and the high-temperature cycling stability and high-temperature storage capacity retention rate of the battery can be improved.

[0076] As will be understood by those skilled in the art, the above-described embodiments are merely exemplary embodiments, and various changes, substitutions, and alterations can be made thereto without departing from the spirit and scope of the present invention.

Claims

1. a solvent that is a fluorinated solvent; A lithium salt, an additive comprising 1-propene 1,3-sultone and diethyl 2-thienylmethylphosphonate; an electrolyte solution comprising:

2. 2. The electrolyte solution according to claim 1, wherein the mass of the diethyl 2-thienylmethylphosphonate is 0.01% to 1% of the total mass of the electrolyte solution, and the mass of the 1-propene 1,3-sultone is 0.01% to 1% of the total mass of the electrolyte solution.

3. The electrolyte solution according to claim 1 , wherein the fluorinated solvent is a fluorinated carbonate solvent.

4. 4. The electrolyte solution according to claim 3, wherein the fluorinated carbonate ester solvent contains fluoroethyl methyl carbonate and fluoroethylene carbonate ester, and the mass ratio of the fluoroethyl methyl carbonate to the fluoroethylene carbonate ester is 7:3 to 10:

0.

5. 2. The electrolyte solution according to claim 1, wherein the content of the lithium salt is 12% to 20% of the total mass of the electrolyte solution.

6. 2. The electrolyte solution according to claim 1, wherein the lithium salt is one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium hexafluoroarsenate, lithium hexafluoroantimonate, lithium perchlorate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, and lithium difluorophosphate.

7. A lithium ion battery comprising a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and the electrolyte solution according to any one of claims 1 to 6.

8. The positive electrode includes a positive electrode current collector and a positive electrode material layer located on the positive electrode current collector, the positive electrode material layer including lithium manganese nickel oxide, and the lithium manganese nickel oxide has a chemical formula of Li ａ Ni ｘ Mn ｙ O ４－ｚ M ｚ 8. The lithium ion battery of claim 7, wherein 0.90≦a≦1.10, 0.4≦x≦0.6, 1.4≦y≦1.6, 0≦z≦0.1, and element M is one or more of Cl, Br, I, S, Se, Te, or F.

9. 9. The lithium-ion battery of claim 8, wherein the negative electrode comprises a negative electrode current collector and a negative electrode material layer located on the negative electrode current collector, the negative electrode material layer comprising a graphite material.

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