Electrolyte and lithium ion battery

Abstract

The invention relates to an electrolyte and a lithium ion battery. The electrolyte comprises a lithium salt, a solvent and an additive, the lithium salt comprises a first lithium salt and a second lithium salt, the mass ratio of the first lithium salt in the electrolyte is 0.1%-3%, and the mass ratio of lithium hexafluorophosphate in the electrolyte is 10%-20% based on the mass percentage of the electrolyte being 100%; the additive comprises fluoroethylene carbonate, and the mass ratio of the fluoroethylene carbonate in the electrolyte is 5%-25%. In the scheme provided by the invention, the electrolyte can construct a stable and compact composite SEI film, the irreversible consumption of the additive is effectively reduced, and the cycle life of the battery is prolonged.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage device technology, and in particular to an electrolyte and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries are widely used in portable electronic devices, electric vehicles, and energy storage systems due to their advantages such as high energy density, long cycle life, no memory effect, and environmental friendliness. A lithium-ion battery consists of main materials including a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte, as the medium for ion transport, directly affects the battery's interface stability, cycle life, and safety performance.

[0003] Fluorinated ethylene carbonate (FEC), a key functional additive in lithium-ion battery electrolytes, is widely used in silicon-based anode systems. FEC can be preferentially reduced on the silicon surface to form a lithium fluoride (LiF)-rich SEI film, effectively buffering the volume changes of silicon during lithium intercalation, inhibiting the pulverization of active material particles and the repeated rupture and regeneration of the SEI film, thus improving the initial cycle performance of the battery. However, during long-term cycling, especially in high-capacity silicon anode systems, FEC is continuously consumed to repair the interface damaged by volume changes. This leads to a continuous decrease in the concentration of FEC in the electrolyte. Once it falls below the critical level required to maintain a stable interface, the repair ability of the SEI film significantly deteriorates, the battery interfacial impedance rises sharply, and the capacity experiences a precipitous decline, severely limiting the battery's cycle life. To alleviate this problem, sufficient FEC reserves are often maintained by increasing the amount of electrolyte injected (i.e., high electrolyte retention). However, this not only limits the further improvement of battery energy density, but also increases manufacturing costs. In addition, excessively high FEC content is prone to oxidative decomposition on the positive electrode side, causing increased battery gas production and swelling, which deteriorates the battery's high-temperature performance and safety performance.

[0004] On the other hand, lithium hexafluorophosphate (LiPF6) is currently the mainstream lithium salt for commercial lithium-ion batteries due to its good solubility in common organic solvents, high ionic conductivity, and moderate cost. However, LiPF6 has poor thermal stability and is highly sensitive to trace amounts of water, easily undergoing hydrolysis to generate acidic substances such as HF. This corrodes electrode materials, damages the integrity of the SEI / CEI film, and catalyzes further decomposition of the electrolyte, thus deteriorating the battery's long-term cycle life and high-temperature storage performance.

[0005] Therefore, it is necessary to develop a lithium-ion battery that can slow down electrolyte consumption, suppress harmful side reactions, and effectively extend cycle life. Summary of the Invention

[0006] To address or partially address the problems existing in related technologies, this application provides an electrolyte and a lithium-ion battery that can construct a stable and dense composite SEI film, effectively slowing down the irreversible consumption of additives and improving the cycle life of the battery.

[0007] A first aspect of this application provides an electrolyte comprising a lithium salt, a solvent, and an additive, wherein the lithium salt comprises a first lithium salt and a second lithium salt, and the first lithium salt is selected from compounds with the following structural formulas: ; The second lithium salt is lithium hexafluorophosphate, and the additive includes a first additive, which is fluoroethylene carbonate; Based on the electrolyte's mass percentage of 100%, the first lithium salt accounts for 0.1% to 3% of the electrolyte's mass, the lithium hexafluorophosphate accounts for 10% to 20% of the electrolyte's mass, and the fluoroethylene carbonate accounts for 5% to 25% of the electrolyte's mass.

[0008] In some embodiments, the first lithium salt accounts for 0.3% to 2% of the mass of the electrolyte.

[0009] In some embodiments, the fluoroethylene carbonate accounts for 10% to 20% of the mass of the electrolyte.

[0010] In some embodiments, the additive further comprises a second additive, the second additive including at least one selected from: vinylene carbonate, 1,3-propanesulfonate lactone, vinyl sulfate, succinate, adiponitrile, 1,3,6-hexanetrionitrile, propylene sulfonate lactone, methanedisulfonate, ethylene glycol bis(propionitrile) ether, pentafluoroethoxyphosphazene, dicyclohexylcarbonyl, trimethyl imide phosphate, and hexamethylene diisocyanate.

[0011] In some embodiments, the second additive has a mass percentage content of 5% to 15% in the electrolyte.

[0012] In some embodiments, the solvent includes at least one selected from ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl carbonate, ethyl propionate, propyl propionate, ethyl fluorocarbonate, methyl ethyl fluorocarbonate, dimethyl fluorocarbonate, propylene fluorocarbonate, γ-butyrolactone, sulfolane, methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, methyl propionate, methyl butyrate, ethyl butyrate, methyl acrylate, and ethyl acrylate.

[0013] In some embodiments, the total content of the solvent accounts for 55% to 75% of the total mass of the electrolyte.

[0014] A second aspect of this application provides a lithium-ion battery, including a positive electrode, a negative electrode, a separator, and the electrolyte described above.

[0015] In some embodiments, the negative electrode sheet contains a negative electrode active material, and the first lithium salt in the electrolyte and fluoroethylene carbonate satisfy the following relationship: A+k×(A×B)≥α×S+β; Wherein, A represents the mass percentage of fluoroethylene carbonate in the electrolyte, in units of %; B represents the mass percentage of the first lithium salt in the electrolyte, in units of %; and S represents the specific surface area of ​​the negative electrode active material in the negative electrode sheet, in units of m². 2 / g, with a value ranging from 0.5 to 5; k represents the synergistic effect coefficient, with a value ranging from 0.05 to 0.15; α represents the specific surface area influence factor, with a value ranging from 2 to 4; β represents the baseline constant, with a value ranging from 3 to 5.

[0016] In some embodiments, the negative electrode active material includes at least one of graphite, hard carbon, silicon, silicon oxide, and silicon carbide.

[0017] In some embodiments, the negative electrode active material includes silicon carbide compounds.

[0018] In some embodiments, the silicon content in the negative electrode active material is 5% to 25%.

[0019] In some embodiments, the specific surface area of ​​the negative electrode active material is 0.8 m². 2 / g~3.5m 2 / g.

[0020] In some embodiments, the positive electrode sheet contains a positive electrode active material, which includes at least one of lithium cobalt oxide, nickel-cobalt-manganese ternary materials, lithium iron phosphate, and lithium manganese oxide; preferably lithium cobalt oxide.

[0021] A third aspect of this application provides an electronic device including the aforementioned lithium-ion battery. The technical solution provided in this application may include the following beneficial results: Introducing a functional first lithium salt into the electrolyte, which may function through the following dual mechanisms: At the chemical stability level, its boron-containing component can interact with Lewis acidic groups in the electrolyte (such as intermediates generated from the hydrolysis of LiPF6), and its decomposition products can participate in the formation of a denser interfacial layer on the electrode surface, thereby helping to interfere with the chain reaction of "LiPF6 hydrolysis → HF generation → catalytic further hydrolysis," slowing the accumulation of acidic products, and improving the chemical stability of the electrolyte and interface. At the electrochemical film formation level, this functional first lithium salt undergoes reduction decomposition at the negative electrode interface, directly providing LiF, and then synergistically interacts with the necessary additive fluoroethylene carbonate (FEC) to jointly construct a LiF-rich, highly stable solid electrolyte interphase (SEI) film. This synergistic film formation mechanism helps to alleviate the consumption pressure of FEC as the main source of LiF, thus providing a basis for mitigating FEC consumption during long-term cycling and extending the cycle life of the battery cell under low electrolyte retention conditions.

[0022] By establishing a quantitative relationship between the electrolyte formulation and the specific surface area of ​​the negative electrode active material, the required content of film-forming additives fluoroethylene carbonate and the first additive can be calculated scientifically, accurately, and quickly for different specific surface areas of the negative electrode active material in the negative electrode sheet. This achieves the optimal matching between the electrolyte formulation and the electrode structure, avoids the blindness of the traditional "trial and error method," and provides design guidance for high-energy-density batteries.

[0023] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Detailed Implementation

[0024] The embodiments of this application will now be described in more detail. It should be understood that this application may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to make this application more thorough and complete, and to fully convey the scope of this application to those skilled in the art.

[0025] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. While the methods and materials described herein, or any equivalent methods and materials, may also be used in the implementation or testing of the invention, preferred methods and materials are now described.

[0026] It should be understood that although the terms “first,” “second,” “third,” etc., may be used in this application to describe various information, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. Features defined as “first” or “second” may explicitly or implicitly include one or more of that feature. The singular forms “a,” “the,” and “the” used in this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0027] Where numerical ranges are provided, it should be understood that every intermediate value between the upper and lower limits of the range and any other specified or intermediate value within the specified range is covered within the present invention. The upper and lower limits of these smaller ranges may be independently included in the smaller range and are also covered within the present invention, subject to any explicitly excluded limits within the specified range. Where a specified range includes one or two limits, the range excluding any or both of those included limits is also included within the present invention. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0028] This application provides an electrolyte comprising lithium salt, solvent, and additives.

[0029] The lithium salt comprises a first lithium salt and a second lithium salt. The first lithium salt is selected from compounds with the following structural formulas: .

[0030] The first lithium salt is lithium difluoroborate (LiFMDFB). The introduction of this salt likely functions through a dual mechanism: at the chemical stability level, its boron-containing component can interact with Lewis acidic groups in the electrolyte (such as intermediates generated from the hydrolysis of LiPF6), and / or its decomposition products can participate in the formation of a denser interfacial layer on the electrode surface. This helps to disrupt the chain reaction of "LiPF6 hydrolysis → HF generation → further catalytic hydrolysis," slowing the accumulation of acidic products and improving the chemical stability of the electrolyte and interface. At the electrochemical film formation level, LiFMDFB undergoes reductive decomposition at the negative electrode interface, directly providing LiF. This, in turn, synergistically interacts with the necessary additive, fluoroethylene carbonate (FEC), to jointly construct a LiF-rich, highly stable solid electrolyte interphase (SEI) film. This synergistic film formation mechanism helps to alleviate the consumption pressure of FEC as the main source of LiF, thus providing a basis for mitigating FEC consumption during long-term cycling and extending the cycle life of the cell under low electrolyte retention conditions.

[0031] The mass percentage of the first lithium salt in the electrolyte is 0.1% to 3%, preferably 0.3% to 2%. Specifically, the amount of the first lithium salt added can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, etc., or any range between the above values. As a functional auxiliary lithium salt, if the content of the first lithium salt is too low (<0.1%), it will be difficult to fully exert its effects of inhibiting hydrolysis and synergistic film formation; if the content is too high (>3%), it may cause an increase in impedance due to excessive decomposition of itself, which will worsen the interfacial dynamics.

[0032] The second lithium salt is lithium hexafluorophosphate (LiPF6), which has comprehensive advantages such as high ionic conductivity, good passivation effect on electrode materials, and relatively low cost. The first lithium salt has a mass percentage of 10% to 20% in the electrolyte. Specifically, the amount of the second lithium salt added can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or any range between these values. This content range ensures that the electrolyte has suitable ion concentration and conductivity, meeting the normal charge and discharge requirements of the battery.

[0033] In embodiments of this application, the lithium salt may further comprise a third lithium salt. The third lithium salt includes at least one of lithium difluorooxalatoborate (LiODFB), lithium difluorodioxalatophosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorophosphate (LiPO2F2), and lithium bis(oxalatoborate) (LiBOB). The introduction of these third lithium salts can further optimize the performance of the electrolyte, such as improving thermal stability, enhancing high and low temperature performance, and strengthening the density of the interfacial film.

[0034] The third lithium salt has a mass percentage of 0% to 8% in the electrolyte. Specifically, the amount of the third lithium salt added can be 0%, 0.1%, 0.5%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, or any range between the above values. The addition of the third lithium salt can act as a performance regulator, but the total content should not be too high to avoid deteriorating the film impedance and increasing the viscosity of the electrolyte.

[0035] In this embodiment, the additive includes a first additive, which is fluoroethylene carbonate (FEC). FEC can preferentially decompose on the surface of silicon materials to form an initial solid electrolyte interface (SEI) rich in lithium fluoride (LiF). This interface has both high mechanical strength and a wide electrochemical stability window, which can effectively buffer the huge volume expansion of the silicon anode during cycling and suppress particle pulverization during cycling. When FEC is used in combination with the first lithium salt LiFMDFB of this application, the two can produce a significant synergistic film-forming effect. LiFMDFB provides additional LiF through decomposition, sharing the pressure on FEC as the main source of LiF during the construction and repair of the SEI film, thereby effectively slowing down the consumption rate of FEC and extending the cycle life of the cell under low electrolyte conditions.

[0036] The mass percentage of fluoroethylene carbonate in the electrolyte is 5% to 25%; preferably 10% to 20%. Specifically, the amount of fluoroethylene carbonate added can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, or any range between the above values. If the content of fluoroethylene carbonate is too low (<5%), it will be difficult to form a complete and effective initial protective film on the negative electrode surface; if the content is too high (>25%), it will not only increase the cost, but also lead to an increased risk of swelling during high-temperature storage.

[0037] In the embodiments of this application, the additive may further include a second additive. The second additive includes at least one selected from the following: vinylene carbonate (VC), 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), succinic anhydride (SN), adiponitrile (ADN), 1,3,6-hexanetrionitrile (HTCN), propenesulfonate lactone (PST), methylene disulfonate (MMDS), ethylene glycol bis(propionitrile) ether (EGBE), pentafluoroethoxyphosphazene, dicyclohexylcarbonyl, trimethyl imide phosphate, and hexamethylene diisocyanate. The introduction of these second additives can further optimize the performance of the electrolyte, such as SEI film toughness, improved high-temperature stability, and enhanced antioxidant properties. Combined with the first additive FEC and the first lithium salt LiFMDFB, the overall performance of the electrolyte can be further optimized.

[0038] The second additive has a mass percentage of 5% to 15% in the electrolyte. Specifically, the amount of the second additive can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or any range between these values. Excessive content of the second additive can complicate the electrolyte system, potentially introducing uncontrollable side reactions, increasing costs, and possibly affecting the synergistic effect of the main additive system.

[0039] The second additive is preferably succinic anhydride, 1,3,6-hexanetrionitrile and 1,3-propanesulfonate lactone.

[0040] In the embodiments of this application, the solvent is an organic solvent, including at least one of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC), ethyl propionate (EP), propyl propionate (PP), ethyl fluorocarbonate (DFEA), methyl ethyl fluorocarbonate (FEMC), dimethyl fluorocarbonate (FDMC), propylene fluorocarbonate (FPC), γ-butyrolactone, sulfolane, methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isopentyl acetate, methyl propionate (MP), methyl butyrate, ethyl butyrate, methyl acrylate, and ethyl acrylate.

[0041] In one specific embodiment of this application, a combination of high dielectric constant solvents (such as EC, PC) and low viscosity solvents (such as EP, PP) can be used to balance the dissociation of lithium salts and the rapid migration of ions.

[0042] The total solvent content accounts for 55% to 75% of the total electrolyte mass. Specifically, the total solvent content can be 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, or any range between these values. This range ensures that the lithium salt and various additives can be fully dissolved to form a homogeneous and stable solution system, while providing a sufficient medium for lithium ion migration.

[0043] The electrolyte described in this application is suitable for lithium-ion batteries, and especially for silicon-based negative electrode lithium-ion batteries. Therefore, the above-mentioned negative electrode active material is a negative electrode active material containing silicon material.

[0044] The lithium-ion battery of this application embodiment includes the electrolyte described above, as well as main materials such as positive electrode, negative electrode and separator.

[0045] In this embodiment of the application, the negative electrode sheet includes a negative current collector and a negative electrode material layer coated on at least one surface of the negative current collector. The negative electrode material layer is formed by coating the surface of the negative current collector with a negative electrode slurry.

[0046] The negative electrode current collector mentioned in the embodiments of this application is not particularly limited, as long as it is conductive and will not cause adverse chemical changes in the battery. Typical enriched current collectors can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collectors, etc.; copper foil is preferred.

[0047] The negative electrode slurry comprises a negative electrode active material, a conductive agent, and a binder. The negative electrode active material is primarily a compound capable of reversibly inserting / deintercalating lithium ions. The negative electrode active material mentioned in the embodiments of this application includes at least one of graphite, hard carbon, silicon, silicon oxide compounds, and silicon carbide compounds; preferably, a silicon carbide compound; more preferably, a silicon carbide compound with a silicon doping content of 5% to 25%. The silicon doping content refers to the mass percentage of silicon in the silicon carbide compound, which can specifically be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, etc., or any range between the above values. In some embodiments of this application, the conductive agent can be one or more of superconducting carbon black, acetylene black, Ketjen black, natural graphite, artificial graphite, graphene, carbon fiber, carbon nanotubes, etc. The adhesive can be one or more of carboxymethyl cellulose, styrene-butadiene rubber, styrene-acrylic emulsion, lithium polyacrylate, polyacrylic acid, and sodium alginate.

[0048] In this embodiment, the first lithium salt and fluoroethylene carbonate in the electrolyte satisfy the following relationship: A+k×(A×B)≥α×S+β; Where A represents the mass percentage of fluoroethylene carbonate in the electrolyte, in %; B represents the mass percentage of the first lithium salt in the electrolyte, in %; and S represents the specific surface area of ​​the negative electrode active material in the negative electrode sheet, in m². 2 / g, with a value ranging from 0.5 to 5; k represents the synergistic effect coefficient, with a value ranging from 0.05 to 0.15; α represents the specific surface area influence factor, with a value ranging from 2 to 4; β represents the baseline constant, with a value ranging from 3 to 5.

[0049] Specifically, the product of k and (A×B) most accurately reflects the correlation between the synergistic effect of fluoroethylene carbonate and the first lithium salt and battery life, with the range of 0.05–0.15 representing the empirical range where the synergistic contribution can significantly improve performance. The effective range of α, from 2.0 to 4.0, is calculated by testing negative electrodes with different specific surface areas (S) and correlating their cycle performance with additive consumption. The minimum amount of additive required to maintain basic interfacial stability (such as the positive electrode interface) is the range of β, from 3.0 to 5.0.

[0050] (A+k×(A×B)) represents the total effective amount of interface modifier, and (α×S+β) represents the minimum amount of interface modifier required to form a stable SEI film. Through this relationship, the electrolyte formulation for different negative electrode systems (different specific surface areas S) can be designed precisely and quantitatively to ensure that a sufficiently stable SEI film can still be formed under low electrolyte retention, thereby extending cycle life.

[0051] In the embodiments of this application, the specific surface area S m of the negative electrode active material 2 / g satisfies: 0.5 ≤ S ≤ 5. For example, it can be 0.5 m 2 / g、1 m 2 / g, 1.5 m 2 / g、2 m 2 / g, 2.5 m 2 / g、3 m 2 / g, 3.5 m 2 / g、4 m 2 / g, 4.5 m 2 / g、5 m 2 / g, or a range between any of the above values.

[0052] In this embodiment, the positive electrode sheet includes a positive current collector and a positive electrode material layer coated on at least one surface of the positive current collector. The positive electrode material layer is formed by coating the surface of the positive current collector with a positive electrode slurry.

[0053] The positive electrode current collector mentioned in the embodiments of this application is not particularly limited, as long as it is conductive and will not cause adverse chemical changes in the battery, it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector can be a metal material such as aluminum, stainless steel, nickel plating, titanium, tantalum, or carbon material such as carbon cloth or carbon paper; preferably, it is aluminum foil.

[0054] The positive electrode slurry may contain a positive electrode active material, a conductive agent, and a binder. The positive electrode active material primarily provides the source of lithium ions. The positive electrode active material mentioned in the embodiments of this application may be selected from at least one of lithium cobalt oxide, nickel-cobalt-manganese ternary materials, lithium iron phosphate, and lithium manganese oxide; preferably, it is a high-voltage positive electrode material such as lithium cobalt oxide or nickel-cobalt-manganese ternary materials, which have high theoretical capacity and voltage plateau under high voltage. The conductive agent mentioned in the embodiments of this application can improve the conductivity of the electrode and may be selected from superconducting carbon black, acetylene black, Ketjen black, natural graphite, artificial graphite, graphene, carbon fiber, carbon nanotubes, etc. The binder may be at least one of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene ternary copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene ternary copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0055] In some embodiments of this application, the battery separator can be a porous polymer membrane made of polyolefin polymers (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer).

[0056] In the lithium-ion battery mentioned in this application, a separator is disposed between the positive and negative electrodes to prevent short circuits. The battery manufacturing process may include the following steps: overlapping the positive and negative electrode sheets via the separator, and then, as needed, winding, folding, or performing other operations, placing them into a casing; injecting electrolyte into the casing and sealing it; and then performing processes such as settling, formation, capacity testing, and inspection to complete the battery manufacturing. Furthermore, overcurrent protection components, conductive plates, etc., may be placed in the casing as needed to prevent pressure rise and overcharging / discharging within the electrochemical device.

[0057] This application also provides an electronic device that includes the aforementioned lithium-ion battery. This electronic device can be a consumer electronics product, or a product used in fields such as new energy vehicles and energy storage.

[0058] To make the present invention easier to understand, the present application will be further described in detail below with reference to embodiments. These embodiments are for illustrative purposes only and are not limited to the scope of application of the present application. Unless otherwise specified, the raw materials or components used in the present application can be obtained commercially or by conventional methods.

[0059] Example 1 (1) Preparation of positive electrode sheet Lithium cobalt oxide (LCO), carbon nanotubes (CNTs), and polyvinylidene fluoride (PVDF) binder were thoroughly mixed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 97:1.5:1.5 to obtain a homogeneous positive electrode slurry. This slurry was uniformly coated on both sides of an aluminum foil with a safety primer as the positive electrode current collector. After drying, cold pressing, slitting, sheet forming, welding of tabs, and adhesive bonding, a positive electrode sheet meeting the winding requirements was produced.

[0060] (2) Preparation of negative electrode sheet The negative electrode active material (graphite and silicon carbide compounds (5% silicon doping)) is thoroughly mixed with a binder (styrene-butadiene rubber-sodium carboxymethyl cellulose composite binder SBR-CMC) and a conductive agent (carbon black) at a mass ratio of 95:3.5:1.5 in deionized water to obtain a homogeneous negative electrode slurry. This negative electrode slurry is then uniformly coated on both sides of the negative electrode current collector copper foil. After drying, cold pressing, slitting, sheet forming, welding of tabs, and adhesive application, a negative electrode sheet that meets the winding requirements is produced.

[0061] (3) Preparation of electrolyte The organic solvents ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP), and propyl propionate (PP) were mixed and stirred evenly in a mass ratio of 1:1:2.5:5.5 to prepare a mixed solvent. Subsequently, based on the total mass of the electrolyte, 10% fluoroethylene carbonate (FEC), 2% succinate (SN), 2% 1,3,6-hexanetrionitrile (HTCN), 4% 1,3-propanesulfonyl lactone (PS), 13.5% LiPF6, and 1.5% first lithium salt LiFMDFB were added and mixed evenly to obtain the electrolyte.

[0062] (4) Preparation of lithium-ion batteries PE porous polymer film is used as the separator.

[0063] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes. After winding and tab welding, a bare cell is obtained. The bare cell is then placed in a pre-formed aluminum-plastic film to complete the top and side sealing. After high-temperature baking, the prepared electrolyte is injected, followed by processes such as settling, formation, aging, capacity testing, and inspection to obtain a lithium-ion battery.

[0064] Examples 2-8 and Comparative Examples 1-3 use the same method as Example 1, except that the type or content of the first lithium salt LiFMDFB is different, or the content of fluoroethylene carbonate is different, as shown in Table 1.

[0065] The batteries prepared in the above-described examples and comparative examples were subjected to the same lithium-ion battery performance tests, and the test results were recorded in Table 1.

[0066] (1) 45℃ cycle test The lithium-ion battery was left to stand at 45℃ for 2 hours, then charged to 4.55V at a constant current of 0.5C, with a cutoff current of 0.05C. After standing for 10 minutes, the full-charge thickness D0 was measured. Subsequently, it was discharged at 0.5C to 3.0V, and the discharge capacity C0 was recorded as the initial capacity. This cycle was repeated 300 times to obtain the capacity C after 300 cycles. 300 Thickness D 300 Then the capacity retention rate = C 300 / C0, thickness expansion rate = [(D 300 / D0)-1]×100%.

[0067] (2) Storage test at 80℃ The lithium-ion battery was charged at a constant current of 0.5C for 32 minutes and then left to stand for 10 minutes. The cell thickness T0 was measured. The lithium-ion battery was then charged to 4.53V at a constant current and constant voltage of 0.5C in a (25±2)℃ constant temperature chamber, with a cutoff current of 0.05C. The fully charged battery was then stored at (80±2)℃ for 6 hours, and the thermal thickness T0 after storage was measured. 6HCalculate the thickness expansion rate of the lithium-ion battery (T) 6H -T0) / T0.

[0068] Table 1

[0069] According to the comparison between Example 3 and Comparative Example 1, when the electrolyte contains only the first additive, fluoroethylene carbonate (FEC), but not the first lithium salt (2-fluoromalonate) lithium difluoroborate (LiFMDFB) (Comparative Example 1), the lithium-ion battery retains 82.5% of its capacity after 300 cycles at 45°C, has a thickness expansion rate of 12.2%, and a thickness expansion rate of 8.8% after 6 hours of storage at 80°C. However, when the electrolyte contains both FEC and the first lithium salt LiFMDFB (Example 3), under the same test conditions, the battery's cycle capacity retention rate increases to 84%, the thickness expansion rate at 45°C decreases to 10.9%, and the thickness expansion rate at 80°C decreases to 7.5%. This comparative result fully demonstrates that the introduction of the first lithium salt LiFMDFB and FEC produces a significant synergistic effect, effectively balancing the long cycle life and excellent high-temperature storage performance of lithium-ion batteries.

[0070] A comparison of Example 2 and Comparative Example 3 shows that when the first lithium salt added to the electrolyte is lithium difluorooxalate borate (LiODFB) (Comparative Example 3), the capacity retention rate of the lithium-ion battery after 300 cycles at 45°C is 81.4%, the thickness expansion rate is 12.7%, and the thickness expansion rate after 6 hours of storage at 80°C is 9.4%. However, when the electrolyte contains both FEC and the first lithium salt LiFMDFB (Example 2), under the same test conditions, the battery's cycle capacity retention rate increases to 82.9%, the thickness expansion rate at 45°C decreases to 12.0%, and the thickness expansion rate at 80°C decreases to 8.6%. This comparison fully demonstrates that selecting LiFMDFB as the first lithium salt allows for a better synergistic effect with FEC. Therefore, the introduction of LiFMDFB is a specific key component for achieving long cycle life and excellent high-temperature storage performance gains in lithium-ion batteries, and its synergistic effect with FEC is difficult to be easily replaced by other boron-containing lithium salts.

[0071] Based on the comparison of Examples 1 to 8 and Comparative Example 2, it can be seen that when the content of the first additive fluoroethylene carbonate (FEC) is preferably 5% to 20%, the content of the first lithium salt (LiFMDFB) is preferably 0.3% to 3%, and the formula A + k × (A × B) ≥ α × S + β is satisfied, the lithium-ion battery has better cycle stability and high-temperature storage performance, and its performance is superior. Furthermore, through the synergy between LiFMDFB and FEC, better overall performance and safety can be achieved while reducing the amount of FEC used, thereby avoiding the risks of gas generation and swelling caused by blindly increasing the FEC content.

[0072] When the content of the first lithium salt LiFMDFB is too high (as in Comparative Example 2) or the content of the first additive fluoroethylene carbonate FEC is too high (as in Example 8), the high-temperature storage performance of the lithium-ion battery at 80°C will deteriorate, which is different from the trend of energy retention rate and thickness expansion rate of the battery when cycling at 45°C.

[0073] Furthermore, the data from Example 7 showed a significant deterioration in battery performance due to the formula not satisfying the relationship. After 300 cycles at 45°C, the capacity retention rate was 76.2%, the thickness expansion rate was 16.2%, and the thickness expansion rate after 6 hours of storage at 80°C was 10.5%. This result fully demonstrates that the relationship proposed in this application is an effective design criterion and performance prediction tool to ensure that silicon anode lithium-ion batteries achieve acceptable long-term performance under harsh conditions.

[0074] Examples 9-11: The same method as in Example 1 was used, except that the specific surface area of ​​the negative electrode active material was different, and the contents of FEC and LiFMDFB in the electrolyte were adjusted accordingly. The batteries prepared in the above examples were subjected to the same lithium-ion battery performance tests, and the test results are recorded in Table 2.

[0075] Table 2

[0076] Based on the comparison of Examples 2, 3, and 9-11, it can be seen that as the specific surface area (S) of the negative electrode active material changes, when the calculation requirements (relationship) are not met, as shown in Example 11, the capacity retention rate of the lithium-ion battery after 300 cycles at 45°C decreases to 79.4%, the thickness expansion rate increases to 14.5%, and the thickness expansion rate after 6 hours of storage at 80°C is 10.2%, resulting in a "cliff-like" drop in performance. At the same time, for lithium-ion batteries with excessively low specific surface area of ​​the negative electrode active material (such as Example 9), even if their compatibility meets the formula requirements, it will lead to a higher risk of storage expansion (the thickness expansion rate after 6 hours of storage at 80°C is 10.0%). This fully demonstrates that, for silicon anode lithium-ion batteries, the specific surface area of ​​the anode active material is a crucial factor determining the film formation requirements at the anode interface. It is necessary to carefully consider the compatibility between the specific surface area of ​​the anode active material and the electrolyte formulation to achieve a balance between the long-cycle performance and high-temperature storage performance of lithium-ion batteries. Furthermore, the formula described in this application can quantitatively guide the precise compatibility and balance between the anode active material and the electrolyte, avoiding the deterioration of lithium-ion battery performance caused by blindly adding additives or functional lithium salts.

[0077] Examples 12-15: The same method as in Example 1 was used, except that the amount of silicon doped in the negative electrode active material was different, and the contents of FEC and LiFMDFB in the electrolyte were adjusted accordingly. The batteries prepared in the above examples were subjected to the same lithium-ion battery performance tests, and the test results are recorded in Table 3.

[0078] Table 3

[0079] A comparison of Examples 3 and 12-15 shows that as the silicon content in the negative electrode active material increases, the consumption of the solid electrolyte interphase (SEI) film increases significantly. Directly using the electrolyte formulation of the low-silicon content (5%) system will not meet the higher interface construction requirements of the high-silicon system. As shown in Example 14, the parameters of this lithium-ion battery do not meet the calculation requirements (relationships). After 300 cycles at 45°C, the capacity retention rate of the lithium-ion battery decreases to 77.2%, and the thickness expansion rate increases to 14.6%. After storage at 80°C for 6 hours, the thickness expansion rate is 9.5%. Compared with the low-silicon lithium-ion battery of Example 3, its cycle performance shows a significant decline. However, when adjusted to a high-silicon system, as shown in Example 12, it maintains good cycle performance. Therefore, the electrolyte film-forming system composed of FEC and the first lithium salt LiFMDFB in the electrolyte should follow the formula A+k×(A×B)≥α×S+β, and appropriate adjustments should be made according to the silicon content and specific surface area of ​​the negative electrode active material to redefine the effective amount of FEC and LiFMDFB, so as to meet the needs of lithium-ion batteries of different standards and balance energy density and long cycle and high temperature storage performance.

[0080] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. An electrolyte comprising a lithium salt, a solvent, and an additive, characterized in that: The lithium salt comprises a first lithium salt and a second lithium salt, wherein the first lithium salt is selected from compounds shown in the following structural formulas: ； The second lithium salt is lithium hexafluorophosphate, and the additive includes a first additive, which is fluoroethylene carbonate; Based on the electrolyte's mass percentage of 100%, the first lithium salt accounts for 0.1% to 3% of the electrolyte's mass, the lithium hexafluorophosphate accounts for 10% to 20% of the electrolyte's mass, and the fluoroethylene carbonate accounts for 5% to 25% of the electrolyte's mass.

2. The electrolyte according to claim 1, characterized in that, The first lithium salt comprises 0.3% to 2% by mass in the electrolyte; and / or, The fluoroethylene carbonate accounts for 10% to 20% of the mass of the electrolyte.

3. The electrolyte according to claim 1, characterized in that, The additive further comprises a second additive, which includes at least one of the following: vinylene carbonate, 1,3-propanesulfonate lactone, vinyl sulfate, succinate, adiponitrile, 1,3,6-hexanetrionitrile, propylene sulfonate lactone, methanedisulfonate, ethylene glycol bis(propionitrile) ether, pentafluoroethoxyphosphazene, dicyclohexylcarbonyl, trimethyl imide phosphate, and hexamethylene diisocyanate; Preferably, the second additive has a mass percentage content of 5% to 15% in the electrolyte.

4. The electrolyte according to claim 1, characterized in that, The solvent includes at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl carbonate, ethyl propionate, propyl propionate, ethyl fluorocarbonate, methyl ethyl fluorocarbonate, dimethyl fluorocarbonate, propylene fluorocarbonate, γ-butyrolactone, sulfolane, methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, methyl propionate, methyl butyrate, ethyl butyrate, methyl acrylate, and ethyl acrylate. Preferably, the total content of the solvent accounts for 55% to 75% of the total mass of the electrolyte.

5. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte as described in any one of claims 1 to 4.

6. The lithium-ion battery according to claim 5, characterized in that, The negative electrode sheet contains a negative electrode active material, and the first lithium salt and fluoroethylene carbonate in the electrolyte satisfy the following relationship: A+k×(A×B)≥α×S+β; Wherein, A represents the mass percentage of fluoroethylene carbonate in the electrolyte, in units of %; B represents the mass percentage of the first lithium salt in the electrolyte, in units of %; and S represents the specific surface area of ​​the negative electrode active material in the negative electrode sheet, in units of m². 2 / g, with a value ranging from 0.5 to 5; k represents the synergistic effect coefficient, with a value ranging from 0.05 to 0.15; α represents the specific surface area influence factor, with a value ranging from 2 to 4; β represents the baseline constant, with a value ranging from 3 to 5.

7. The lithium-ion battery according to claim 6, characterized in that, The negative electrode active material includes at least one of graphite, hard carbon, silicon, silicon oxide, and silicon carbide.

8. The lithium-ion battery according to claim 7, characterized in that, The negative electrode active material includes silicon-carbon compounds; Preferably, the silicon content in the negative electrode active material is 5% to 25%.

9. The lithium-ion battery according to any one of claims 6 to 8, characterized in that, The specific surface area of ​​the negative electrode active material is 0.8 m². 2 / g~3.5m 2 / g.

10. The lithium-ion battery according to claim 6, characterized in that, The positive electrode sheet contains a positive electrode active material, which includes at least one of lithium cobalt oxide, nickel-cobalt-manganese ternary materials, lithium iron phosphate, and lithium manganese oxide, preferably lithium cobalt oxide.

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