Electrolyte solution and secondary battery

Abstract

An electrolyte solution and a secondary battery. The electrolyte solution comprises a solvent, an additive, and an electrolyte, wherein the solvent comprises a fluorinated solvent, the fluorinated solvent comprises a fluorinated ethylene carbonate and a fluorinated chain ester, and the fluorinated chain ester comprises a compound represented by formula I and a compound represented by formula II, wherein R2 and R4 are each independently selected from fluorine-substituted C1-C6 alkyl groups, and R1 and R3 are each independently selected from C1-C6 alkyl groups; the mass ratio of the fluorinated ethylene carbonate to the fluorinated chain ester is 1:(0.33-4), the mass ratio of the compound represented by formula I to the compound represented by formula II is 1:(0.33-3), and based on the mass of the electrolyte solution, the mass percentage of the fluorinated solvent is 20% to 80%; the additive comprises tetravinylsilane, and based on the mass of the electrolyte solution, the mass percentage of the tetravinylsilane is M, where 0.02%≤M≤3%. The fluorinated ethylene carbonate, the compound represented by formula I, the compound represented by formula II, and the tetravinylsilane act synergistically so that the electrolyte solution has appropriate viscosity and higher oxidation resistance, while also improving the negative impact of defluorination of the fluorinated solvent on a positive electrode, thereby improving the high-temperature cycling performance and high-temperature storage performance of the secondary battery in high-voltage conditions.

Description

An electrolyte and a secondary battery

[0001] This application claims priority to Chinese Patent Application No. 202411525371.1, filed on October 30, 2024, entitled "An Electrolyte and a Secondary Battery", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of electrochemical technology, and in particular to an electrolyte and a secondary battery. Background Technology

[0003] Rechargeable batteries (such as lithium-ion batteries) are widely used in electric vehicles and consumer electronics due to their advantages such as high energy density, high output power, long cycle life, and environmental friendliness. As the application scope of rechargeable batteries continues to expand and their usage scenarios become more diverse, the market is placing higher demands on their electrochemical performance. To meet the market's demand for high-capacity batteries, a common technical approach is to increase the upper cutoff voltage. With advancements in materials and technology, people are paying more attention to battery range, portability, and safety, which places even higher demands on battery performance.

[0004] As a key material in rechargeable batteries, the performance of the electrolyte determines the maximum performance of materials such as the positive electrode, negative electrode, and separator. Generally, to improve the cycle performance of rechargeable batteries, conventional film-forming additives can form a protective interfacial film at the positive and negative electrodes, thereby improving the battery's cycle performance. However, under more demanding operating environments (high temperature, high voltage), conventional electrolytes exhibit poor oxidation resistance, and the byproducts generated from excessive decomposition can damage the interfacial film, rendering its protective function ineffective and affecting the cycle performance of the rechargeable battery. Simultaneously, the oxidation stability of the positive electrode material also deteriorates under harsh operating conditions, leading to increased capacity loss. Therefore, improving the high-temperature cycle performance and high-temperature storage performance of rechargeable batteries under high-voltage conditions has become an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this application is to provide an electrolyte and a secondary battery to improve the high-temperature cycling performance and high-temperature storage performance of the secondary battery under high-voltage conditions. The specific technical solution is as follows:

[0006] A first aspect of this application provides an electrolyte comprising a solvent, an additive, and an electrolyte, wherein the solvent comprises a fluorinated solvent, the fluorinated solvent comprising fluoroethylene carbonate and fluorinated chain esters, and the fluorinated chain esters comprising at least one compound of Formula I and at least one compound of Formula II.

[0007] R2 and R4 are each independently selected from fluorinated C1-C6 alkyl groups, and R1 and R3 are each independently selected from C1-C6 alkyl groups; the mass ratio of the fluorinated ethylene carbonate to the fluorinated chain ester is 1:(0.33-4), the mass ratio of the compound shown in Formula I to the compound shown in Formula II is 1:(0.33-3), and the mass percentage of the fluorinated solvent is 20% to 80% based on the mass of the electrolyte; the additive includes tetraethylenesilane, and the mass percentage of the tetraethylenesilane is M, 0.02% ≤ M ≤ 3% based on the mass of the electrolyte.

[0008] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application.

[0009] The beneficial effects of this application are:

[0010] This application provides an electrolyte and a secondary battery. The electrolyte includes a solvent, an additive, and an electrolyte. The solvent includes fluoroethylene carbonate, a compound shown in Formula I, and a compound shown in Formula II. The mass ratio of fluoroethylene carbonate to fluorochain ester is 1:(0.33-4), and the mass ratio of the compound shown in Formula I to the compound shown in Formula II is 1:(0.33-3). Based on the mass of the electrolyte, the mass percentage of the fluorosolvent is 20% to 80%. The additive includes tetraethylenesilane, and based on the mass of the electrolyte, the mass percentage of tetraethylenesilane is M, where 0.02% ≤ M ≤ 3%. Through the above configuration, the fluoroethylene carbonate, the compound shown in Formula I, the compound shown in Formula II, and tetraethylenesilane work synergistically to give the electrolyte a suitable viscosity and high oxidation resistance, while also effectively mitigating the negative impact of defluorination on the positive electrode, thereby improving the high-temperature cycle performance and high-temperature storage performance of the secondary battery under high-voltage conditions.

[0011] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0012] The technical solutions in this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0013] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0014] A first aspect of this application provides an electrolyte comprising a solvent, an additive, and an electrolyte. The solvent includes a fluorinated solvent, which includes fluoroethylene carbonate and fluorinated chain esters. The fluorinated chain esters include at least one compound of Formula I and at least one compound of Formula II.

[0015] R2 and R4 are each independently selected from fluorinated C1-C6 alkyl groups, and R1 and R3 are each independently selected from C1-C6 alkyl groups; the mass ratio of fluoroethylene carbonate to fluorinated chain ester is 1:(0.33-4), preferably 1:(1-3); the mass ratio of the compound shown in Formula I to the compound shown in Formula II is 1:(0.33-3), preferably 1:(1-3); based on the mass of the electrolyte, the mass percentage of the fluorinated solvent is 20% to 80%, preferably 40% to 60%; the additive includes tetraethylenesilane, and based on the mass of the electrolyte, the mass percentage of tetraethylenesilane is M, 0.02% ≤ M ≤ 3%, preferably 1% ≤ M ≤ 2%. For example, the mass ratio of fluoroethylene carbonate to fluorochain ester can be 1:0.33, 1:0.5, 1:0.8, 1:1, 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2, 1:2.2, 1:2.5, 1:2.8, 1:3, 1:3.2, 1:3.5, 1:3.8, 1:4, or a range of any two of these values; the mass ratio of the compound shown in Formula I to the compound shown in Formula II can be 1:0.33, 1:0.5, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2, etc. The ratios are 1:2.2, 1:2.5, 1:2.8, 1:3, or any two of these values; the mass percentage of the fluorinated solvent can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any two of these values; the value of M can be 0.02%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, or any two of these values.

[0016] The inventors discovered that the electrolyte includes a fluorinated solvent, which comprises fluoroethylene carbonate (FEC), the compound shown in Formula I, and the compound shown in Formula II. By controlling the mass ratio of fluoroethylene carbonate to the fluorinated chain ester, the mass ratio of the compound shown in Formula I to the compound shown in Formula II within the fluorinated chain ester, and the mass percentage of the fluorinated solvent within the scope of this application, FEC not only provides a favorable solvation environment for lithium salts, promoting their dissolution and ionic existence in the electrolyte, but also, when combined with a certain proportion of fluorinated chain esters, facilitates the formation of an electrolyte with suitable viscosity. This promotes uniform film formation of FEC at the negative electrode, reduces impedance, and thus improves battery cycle performance. The compound shown in Formula I has a higher viscosity, which leads to increased battery impedance, but it has better oxidation resistance. The compound shown in Formula II has poorer oxidation resistance but lower viscosity, which is beneficial for improving lithium-ion mobility. Controlling the mass ratio of the compound shown in Formula I to the compound shown in Formula II within the scope of this application is beneficial for improving the oxidation resistance of the electrolyte and achieving a suitable viscosity. Therefore, the electrolyte simultaneously includes FEC, the compound shown in Formula I, and the compound shown in Formula II. These three components work synergistically to improve the high-temperature cycling performance of the secondary battery under high-voltage conditions. However, fluorinated solvents in the electrolyte can lead to defluorination, which is accelerated under high voltage. The HF formed by defluorination attacks and corrodes the positive electrode, while also exacerbating the oxidative decomposition of the electrolyte on the positive electrode surface, resulting in gas production. This can cause the battery to experience a drop in performance during high-temperature storage or cycling. Therefore, in addition to including the aforementioned fluorinated solvents in the electrolyte, tetraethylenesilane is further introduced. Tetraethylenesilane can remove the HF generated in the electrolyte, providing excellent dehydration and acid suppression effects. Furthermore, it can form a stable interfacial film (CEI film) on the positive electrode side, effectively reducing the negative impact of defluorination on the positive electrode and thus improving the high-temperature storage and cycling performance of the secondary battery. It should be noted that the high-voltage conditions described in this application refer to a charging upper limit voltage of the secondary battery greater than or equal to 4.5V.

[0017] When the mass ratio of fluoroethylene carbonate to fluorinated chain ester is too large, for example, greater than 1:0.33, the FEC content is high, leading to a higher electrolyte viscosity, which affects lithium-ion transport. Simultaneously, a thicker SEI film is formed, reducing the cycle kinetics of the secondary battery and negatively impacting its cycle performance. When the mass ratio of fluoroethylene carbonate to fluorinated chain ester is too small, for example, less than 1:4, the FEC content is low, and some FEC participates in SEI film formation, resulting in consumption. The remaining FEC is low, weakening its solvation ability for lithium salts, leading to lithium salt precipitation in the electrolyte and affecting the electrochemical performance of the secondary battery. When the mass ratio of the compound shown in Formula I to the compound shown in Formula II is too large, for example, greater than 1:0.33, the electrolyte viscosity is high, which is detrimental to lithium-ion transport, leading to increased impedance and affecting the cycle performance of the secondary battery. When the mass ratio of the compound shown in Formula I to the compound shown in Formula II is too small, for example, less than 1:3, the electrolyte has poor oxidation resistance, which is detrimental to improving the high-temperature cycling and high-temperature storage performance of the secondary battery under high-voltage conditions. When the mass percentage M of tetraethylenesilane is large, such as greater than 3%, the interfacial film formed at the positive and negative electrodes is thicker, resulting in increased impedance and increased resistance to lithium ion insertion / extraction at the interface, which affects the high-temperature storage and high-temperature cycling performance of the secondary battery. When the mass percentage M of tetraethylenesilane is small, such as less than 0.02%, the content of tetraethylenesilane is low, which cannot effectively remove HF generated in the electrolyte, and the protective effect of the CEI film formed at the positive electrode is weak, which is not conducive to improving the high-temperature storage and high-temperature cycling performance of the secondary battery.

[0018] In some embodiments of this application, the compound represented by Formula I has 4 to 6 carbon atoms and 3 to 5 fluorine atoms. The compound represented by Formula I satisfies the above conditions, which is beneficial for further improving the oxidation resistance of the electrolyte and giving the electrolyte a suitable viscosity.

[0019] In some embodiments of this application, the compound represented by Formula II has 4 to 6 carbon atoms and 2 to 4 fluorine atoms. The compound represented by Formula II satisfies the above conditions, which is beneficial for further improving the oxidation resistance of the electrolyte and giving the electrolyte a suitable viscosity.

[0020] In some embodiments of this application, the compound represented by Formula I is selected from at least one of the following compounds:

[0021] The compound shown in Formula II is selected from at least one of the following compounds:

[0022] In some embodiments of this application, the mass percentage of fluoroethylene carbonate (FEC) is A, based on the mass of the electrolyte, where 10% ≤ A ≤ 40%, preferably 10% ≤ A ≤ 30%. For example, the value of A can be 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, or a range of any two of these values. By controlling the mass percentage A of FEC within the above range, a good solvation environment can be provided for lithium salts, promoting their dissolution and ionic existence in the electrolyte. Furthermore, when FEC is combined with a certain proportion of fluorinated chain esters, it is beneficial to obtain an electrolyte with suitable viscosity, which facilitates uniform film formation of FEC at the negative electrode, reduces impedance, and thus further improves the high-temperature cycle performance and high-temperature storage performance of the secondary battery.

[0023] In some embodiments of this application, based on the mass of the electrolyte, the mass percentage content of the compound shown in Formula I is B, where 5% ≤ B ≤ 20%, preferably 10% ≤ B ≤ 20%. For example, the value of B can be 5%, 8%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or a range consisting of any two of these values. By controlling the mass percentage content B of the compound shown in Formula I within the above range, it is beneficial to improve the oxidation resistance of the electrolyte and further improve the high-temperature cycle performance and high-temperature storage performance of the secondary battery under high voltage conditions.

[0024] In some embodiments of this application, based on the mass of the electrolyte, the mass percentage of the compound represented by Formula II is C, where 5% ≤ C ≤ 30%, preferably 10% ≤ C ≤ 30%. For example, the value of C can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, or a range of any two of these values. By controlling the mass percentage C of the compound represented by Formula II within the above range, it is beneficial to reduce the viscosity of the electrolyte, which is beneficial to the transport of lithium ions, and further improves the high-temperature cycle performance and high-temperature storage performance of the secondary battery under high voltage conditions.

[0025] In some embodiments of this application, the solvent includes a non-fluorinated solvent selected from at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, phenyl acetate, 1,4-butylsulfonate lactone, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, and ethylene glycol dimethyl ether. This application does not impose any particular limitation on the content of the non-fluorinated solvent in the electrolyte, as long as the purpose of this application is achieved. For example, based on the mass of the electrolyte, the mass percentage of the non-fluorinated solvent can be from 5.5% to 65.5%. The use of both fluorinated and non-fluorinated solvents in combination is beneficial for further improving the high-temperature cycle performance and high-temperature storage performance of secondary batteries under high-voltage conditions.

[0026] In some embodiments of this application, the additive includes an auxiliary additive selected from 1,3-propanesulfonate lactone, vinyl sulfate, and compounds represented by Formula III:

[0027] Wherein, based on the mass of the electrolyte, the mass percentage of the auxiliary additive is N, 0.1% ≤ N ≤ 5%, preferably 0.5% ≤ N ≤ 2%. For example, the value of N can be 0.1%, 0.5%, 0.8%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.5%, 4%, 4.5%, 5%, or a range of any two of these values. The electrolyte includes the auxiliary additives within the scope of this application. On the one hand, during the secondary battery formation process, it can preferentially form an interface film (SEI film) at the negative electrode compared to tetraethylenesilane, thereby inhibiting the formation of a tetraethylenesilane film at the negative electrode, allowing more tetraethylenesilane to undergo oxidation reaction at the positive electrode to form a CEI film, which then plays a role in repairing the interface film during subsequent cycles of the secondary battery. On the other hand, the auxiliary additives within the scope of this application are beneficial for improving the film-forming quality of the interfacial film and enhancing its oxidation resistance. Furthermore, these auxiliary additives preferentially form a film on the negative electrode compared to FEC, thereby reducing FEC consumption and allowing more FEC to be used for solubilizing the electrolyte, promoting electrolyte dissolution, and improving the capacity and oxidation resistance of the secondary battery. By controlling the mass percentage N of the auxiliary additives within the scope of this application, the synergistic effect between the fluorinated solvent, tetraethylenesilane, and the auxiliary additives can be effectively utilized, further improving the high-temperature cycling and high-temperature storage performance of the secondary battery and increasing its capacity.

[0028] In some embodiments of this application, 0.01 ≤ M / N ≤ 10, preferably 0.5 ≤ M / N ≤ 4. For example, the value of M / N can be 0.01, 0.05, 0.5, 1, 2, 3, 4, 6, 8, 10, or a range consisting of any two of these values. By adjusting the value of M / N within the above range, it is beneficial to further leverage the synergistic effect of tetraethylenesilane and auxiliary additives, further improve the high-temperature cycling and high-temperature storage performance of the secondary battery, and increase the capacity of the secondary battery.

[0029] The electrolyte in this application also includes an electrolyte, and there are no particular limitations on the electrolyte, as long as it achieves the purpose of this application. For example, the electrolyte is selected from at least one of LiPF6, LiAsF6, LiClO4, LiB(C2O4)2, LiBF2C2O4, LiN(SO2F)2, LiN(SO2CF3)2, LiPO2F2, LiPF2(C2O4)2, and LiPF4C2O4. There are no particular limitations on the content of the electrolyte in the electrolyte, as long as it achieves the purpose of this application. For example, based on the mass of the electrolyte, the mass percentage of the electrolyte is 10%-18%.

[0030] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application.

[0031] In this application, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The phrase "positive electrode material layer disposed on at least one surface of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the positive current collector along its thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that "surface" here can refer to the entire surface of the positive current collector or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved. This application does not impose any particular limitation on the positive current collector, as long as the purpose of this application is achieved. For example, the positive current collector can be aluminum foil, aluminum alloy foil, or a composite positive current collector. The aforementioned composite positive current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The material of the aforementioned polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT), and the material of the aforementioned metal layer can be, but is not limited to, at least one of aluminum, aluminum alloy, nickel, or nickel alloy. This application does not impose any particular limitation on the thickness of the positive electrode material layer and the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided positive electrode material layer is 50 μm to 250 μm, and the thickness of the positive electrode current collector is 7 μm to 15 μm.

[0032] In some embodiments of this application, the positive electrode material layer includes a positive electrode active material, which is selected from lithium cobalt oxide, LiNi... x Co y Mn z O2 (0≤x,y,z≤1,x+y+z=1), lithium-rich manganese-based materials (nLi2MnO3·(1-n)LiAO2, where A can be Ni, Co, or Mn, 0<n<1), lithium nickel manganese oxide, and lithium manganese oxide, at least one of them.

[0033] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of positive electrode conductive agents and positive electrode binders, as long as they achieve the purpose of this application. For example, the positive electrode conductive agent may include, but is not limited to, at least one of superconducting carbon black (Super P), acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon fiber. For example, the positive electrode binder may include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0034] In this application, there are no particular limitations on the preparation method of the positive electrode sheet, as long as it can achieve the purpose of this application. For example, it can be prepared by the following method: mixing positive electrode active material, positive electrode conductive agent, and positive electrode binder, adding N-methylpyrrolidone (NMP) and stirring evenly to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated on the surface of the positive electrode current collector, dried to obtain a positive electrode sheet coated with a positive electrode material layer, and then subjected to cold pressing, cutting and other processes to obtain the positive electrode sheet.

[0035] In this application, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the negative electrode current collector, as long as the purpose of this application is achieved; for example, the negative electrode current collector can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel or foamed copper, aluminum foil, or a composite negative electrode current collector. The aforementioned composite negative electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The material of the polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the metal layer can be, but is not limited to, at least one of copper, copper alloy, nickel, or nickel alloy. This application does not impose any particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided negative electrode material layer can be from 50 μm to 180 μm, and the thickness of the negative electrode current collector can be from 3 μm to 10 μm.

[0036] The negative electrode material layer includes a negative electrode active material. This application does not have a particular limitation on the type of negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, lithium metal, etc. Graphite may include, but is not limited to, at least one of natural graphite or artificial graphite; the aforementioned silicon-based materials may include, but are not limited to, at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys; the aforementioned tin-based materials may include, at least one of elemental tin, tin oxide compounds, or tin alloys.

[0037] The negative electrode material layer may further include a negative electrode conductive agent and a negative electrode binder. This application does not impose any particular limitation on the types of negative electrode conductive agents and negative electrode binders, as long as they can achieve the purpose of this application. For example, the negative electrode conductive agent may include, but is not limited to, at least one of superconducting carbon black (Super P), acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon fibers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. For example, the negative electrode binder may include, but is not limited to, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl methacrylate (PMAA), or carboxymethyl chitosan (CMCS). In some embodiments of this application, the negative electrode material layer may also include a thickener, such as sodium carboxymethyl cellulose (CMC-Na). This application does not impose any particular restrictions on the mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder, and thickener in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved.

[0038] In this application, there are no particular restrictions on the preparation method of the negative electrode sheet, as long as it can achieve the purpose of this application. For example, it can be prepared by the following method: adding negative electrode active material, negative electrode conductive agent, negative electrode binder, and thickener to deionized water and stirring evenly to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on the surface of the negative electrode current collector, and after drying, a negative electrode sheet coated with a negative electrode material layer is obtained. Then, the negative electrode sheet is obtained through cold pressing, cutting, and other processes.

[0039] The secondary battery of this application also includes a separator to separate the positive and negative electrode plates, prevent internal short circuits, allow electrolyte ions to pass freely, and not affect the electrochemical charging and discharging process. This application does not impose any particular limitation on the type of separator; any porous structure separator with good chemical and mechanical stability can be selected. For example, the separator material can include at least one of glass fiber, nonwoven fabric, polyolefin membrane, aromatic polyamide membrane, polytetrafluoroethylene membrane, and polyethersulfone membrane. The separator type can include, but is not limited to, at least one of woven membrane, nonwoven membrane (nonwoven fabric), microporous membrane, composite membrane, rolled membrane, or spun membrane. The separator can be a single-layer thin film or a multi-layer composite thin film. This application does not impose any particular limitation on the thickness of the separator, as long as it achieves the purpose of this application; for example, the thickness can be from 5 μm to 20 μm.

[0040] In this application, the secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0041] In some embodiments of this application, the secondary battery may include, but is not limited to: lithium metal secondary battery, lithium-ion secondary battery (lithium-ion battery), lithium polymer secondary battery or lithium-ion polymer secondary battery, etc.

[0042] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0043] The secondary batteries of this application include battery cell form, battery module form, and battery pack form. Battery cells can be assembled into battery modules, and a battery module can contain one or more battery cells; the specific number can be selected by those skilled in the art based on the application and capacity of the battery module. The battery modules of this application can also be assembled into battery packs, and a battery pack can contain one or more battery modules; the specific number can be selected by those skilled in the art based on the application and capacity of the battery pack.

[0044] Example

[0045] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0046] Test methods and equipment:

[0047] High-temperature storage performance test

[0048] After capacity testing, the lithium-ion batteries were placed at 25°C, and their thickness was measured and recorded as D0. Then, the batteries were stored in a 60°C oven for 14 days. After the storage period, the thickness was measured while still hot and recorded as D1. The batteries were then removed and cooled to room temperature. At 25°C, they were discharged at a constant current of 1C to 3V, and the discharge capacity was recorded as C1. The calculation methods for high-temperature storage capacity retention and battery thickness expansion rate are as follows:

[0049] High-temperature storage capacity retention rate (%) = (first discharge capacity after storage C1 / discharge capacity at 1C) × 100%.

[0050] Thickness expansion rate (%) = (D1 - D0) / D0 × 100%

[0051] High-temperature cycling performance test

[0052] The lithium-ion battery was placed in a 45°C constant temperature test chamber and allowed to stand for 5 minutes to reach a constant temperature. It was then charged at a constant current of 1.0C to 4.5V, followed by constant voltage charging at 4.5V until the cutoff current reached 0.05C. After standing for 5 minutes, it was discharged at a constant current of 1.0C to 3V, and the discharge capacity was recorded as Q1. This constitutes one charge-discharge cycle. This charge-discharge cycle was repeated for 500 cycles, and the discharge capacity after 500 cycles was recorded as Q2. The 45°C cycle capacity retention rate of the lithium-ion battery was calculated as follows: 45°C cycle capacity retention rate = Q2 / Q1 × 100%.

[0053] Example 1-1

[0054] <Preparation of Electrolyte>

[0055] In an argon-filled glove box (moisture <10ppm, oxygen <1ppm), the solvent fluoroethylene carbonate (FEC), formula I-1, formula II-1, and propyl propionate were mixed uniformly in a mass ratio of 5:7.5:7.5:65.5. Then, lithium hexafluorophosphate (LiPF6) and tetraethylenesilane were added to the solvent. Based on the mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, the mass percentage of tetraethylenesilane was 2%, the mass percentage of fluoroethylene carbonate (FEC) A was 5%, the mass percentage of formula I-1 B was 7.5%, the mass percentage of formula II-1 C was 7.5%, and the mass percentage of the non-fluorinated solvent propyl propionate was 65.5%.

[0056] <Preparation of the positive electrode>

[0057] Lithium cobalt oxide (LiCoO2), polyvinylidene fluoride (PVDF), acetylene black, and conductive carbon nanotubes were mixed in a mass ratio of 95:2.3:2:0.7. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 55 wt%. The mixture was then vacuum-stirred to obtain the positive electrode slurry. This slurry was uniformly coated onto both surfaces of a 16 μm thick aluminum foil used as a positive electrode current collector. After drying at 85°C, the foil was cold-pressed to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. The coating weight of the single-sided positive electrode material layer was 35 g / m². 2 The thickness of the single-sided positive electrode material layer is 33μm. Then, the edges are cut, the sheets are cut, and the sheets are slit. After slitting, the sheets are dried at 85℃ for 4 hours under vacuum. The tabs are then welded to obtain positive electrode sheets with a size of 540mm×55mm for later use.

[0058] <Preparation of Negative Electrode Sheets>

[0059] Artificial graphite (negative electrode active material), acetylene black (conductive agent), sodium carboxymethyl cellulose (CMC) (thickener), and styrene-butadiene rubber (SBR) (binder) were mixed in a mass ratio of 95:1.5:1.2:2.3. Deionized water was added as a solvent to prepare a slurry with a solid content of 49.0 wt%. The slurry was stirred evenly in a vacuum mixer to obtain the negative electrode slurry. The negative electrode slurry was uniformly coated on both surfaces of a 9 μm thick copper foil current collector. After drying at 85°C, it was cold-pressed to obtain a negative electrode sheet with a double-sided coated negative electrode material layer. The coating weight of the single-sided negative electrode material layer was 20 g / m². 2 The thickness of the single-sided negative electrode material layer is 45.9μm. After cutting, cutting, and slitting, the material is dried at 85℃ for 4 hours under vacuum. Then, the tabs are welded to obtain a negative electrode sheet with a specification of 660mm×59mm for later use.

[0060] <Preparation of the diaphragm>

[0061] A porous polyethylene (PE) membrane with a thickness of 8μm (provided by Shenzhen Xingyuan Material Technology Co., Ltd.) was used as the separator.

[0062] <Preparation of Lithium-ion Batteries>

[0063] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and vacuum-baked at 85°C for 48 hours. The electrolyte prepared above is then injected, and the battery undergoes vacuum sealing, settling, formation, and capacity testing to obtain a lithium-ion battery. The settling time is 24 hours, and the battery is charged to 65% SOC (State of Charge) at a formation current of 0.1C. The upper limit of the formation voltage is 4.5V, the formation temperature is 45°C, and the formation pressure is 3 kg / cm². 2 After charging, the battery is left to stand for at least 24 hours under the same temperature and pressure conditions to fully activate it. After formation, the battery is charged and discharged sequentially at 0.2C, 0.5C, and 1C. The 0.1C charge / discharge cycle is repeated once, the 0.5C cycle once, and the 1C cycle three times, with a voltage window of 3V to 4.5V. Finally, the battery is charged at 1C constant current and constant voltage to 4.5V, with a cutoff current of 0.05C. This yields the capacity-classified battery, and the capacity of the last 1C discharge cycle is taken as the actual capacity C0 of the battery.

[0064] Examples 1-2 to 1-27

[0065] Except for the adjustment of relevant parameters according to Table 1 in <Preparation of Electrolyte>, where the mass percentage of the total mass percentage of FEC, compound shown in Formula 1, compound shown in Formula II and tetraethylenesilane changes accordingly, and the mass percentage of the non-fluorinated solvent propyl propionate remains unchanged, the rest is the same as in Examples 1-1.

[0066] Examples 1-28

[0067] Except in the section on "Preparation of Positive Electrode Sheet", where the positive electrode active material is replaced with LiNi. 0.6 Co 0.2 Mn 0.2 Except for O2, the rest is the same as in Examples 1-3.

[0068] Examples 1-29

[0069] Except in the section on "Preparation of Positive Electrode Sheet", where the positive electrode active material is replaced with LiNi. 0.5 Mn 1.5 Except for O4, the rest is the same as in Examples 1-3.

[0070] Examples 1-30

[0071] Except for replacing the positive electrode active material with LiMn2O4 in the <Preparation of Positive Electrode Sheet>, the rest is the same as in Examples 1-3.

[0072] Examples 2-1 to 2-11

[0073] Except for the adjustment of relevant parameters according to Table 2 in the <Preparation of Electrolyte>, where the mass percentage of the non-fluorinated solvent propyl propionate changes accordingly with changes in the mass percentage of tetraethylenesilane and auxiliary additives, while the mass percentages of FEC, compound 1, compound II, and LiPF6 remain unchanged, the rest are the same as in Examples 1-3. In Table 2, 1,3-propanesulfonate lactone is abbreviated as PS, and vinyl sulfate is abbreviated as DTD.

[0074] Example 2-12

[0075] Except for the section on <Preparation of Electrolyte>, where the non-fluorinated solvent is replaced with propylene carbonate instead of propyl propionate, the rest of the process is the same as in Examples 2-4.

[0076] Comparative Examples 1 to 8

[0077] Except for the adjustment of relevant parameters according to Table 1 in <Preparation of Electrolyte>, where the mass percentage of FEC, compound shown in Formula 1, compound shown in Formula II and tetraethylenesilane changes accordingly, and the mass percentage of other solvents, propyl propionate, remains unchanged, the rest is the same as in Examples 1-1.

[0078] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0079] Table 1

[0080] In Table 1, " / " indicates that the corresponding preparation parameters or substances do not exist.

[0081] As can be seen from Examples 1-1 to 1-30 and Comparative Examples 1 to 8, the electrolyte in the secondary batteries of each embodiment of this application includes fluoroethylene carbonate, the compound shown in Formula I, the compound shown in Formula II, and tetraethylenesilane within the scope of this application. The mass ratio of fluoroethylene carbonate to fluorochain ester, the mass ratio of the compound shown in Formula I to the compound shown in Formula II, the mass percentage content of the fluorinated solvent, and the mass percentage content of tetraethylenesilane are all within the scope of this application. The secondary batteries obtained in the examples have a low high-temperature storage expansion rate, a high high-temperature storage capacity retention rate, and a high 45°C cycle capacity retention rate, indicating that the secondary batteries have good high-temperature cycle performance and high-temperature storage performance under high-voltage conditions. Comparative Example 1 has an excessively low mass percentage of fluorinated solvent; Comparative Example 2 has an excessively high mass percentage of fluorinated solvent; Comparative Example 3 has an excessively high mass ratio of fluoroethylene carbonate to fluorinated chain ester; Comparative Example 4 has an excessively low mass ratio of fluoroethylene carbonate to fluorinated chain ester; Comparative Example 5 has an excessively high mass ratio of the compound shown in Formula I to the compound shown in Formula II; Comparative Example 6 has an excessively low mass ratio of the compound shown in Formula I to the compound shown in Formula II; Comparative Example 7 does not include tetraethylenesilane; and Comparative Example 8 has an excessively high mass percentage of tetraethylenesilane. These secondary batteries exhibit high high-temperature storage expansion rates, low high-temperature storage capacity retention rates, and low 45°C cycle capacity retention rates. This indicates that at least one of the following: the mass ratio of fluoroethylene carbonate to fluorinated chain ester; the mass ratio of the compound shown in Formula I to the compound shown in Formula II; the mass percentage of fluorinated solvent; and the mass percentage of tetraethylenesilane are outside the scope of this application. Consequently, the secondary batteries exhibit poor high-temperature cycle performance and high-temperature storage performance under high-voltage conditions.

[0082] The type of compound shown in Formula I typically affects the high-temperature cycling performance and high-temperature storage performance of secondary batteries. As can be seen from Examples 1-3, 1-13, and 1-16, by selecting the compound shown in Formula I within the scope of this application, the resulting secondary battery exhibits a lower high-temperature storage expansion rate, a higher high-temperature storage capacity retention rate, and a higher 45°C cycle capacity retention rate. This demonstrates that the secondary battery possesses excellent high-temperature cycling performance and high-temperature storage performance under high-voltage conditions.

[0083] The type of compound shown in Formula II typically affects the high-temperature cycling performance and high-temperature storage performance of secondary batteries. As can be seen from Examples 1-3, 1-17, and 1-20, by selecting the compound shown in Formula II within the scope of this application, the resulting secondary battery exhibits a lower high-temperature storage expansion rate, a higher high-temperature storage capacity retention rate, and a higher 45°C cycle capacity retention rate. This demonstrates that the secondary battery possesses excellent high-temperature cycling performance and high-temperature storage performance under high-voltage conditions.

[0084] Table 2

[0085] In Table 2, “ / ” indicates that the corresponding preparation parameters or substances do not exist.

[0086] As can be seen from Examples 1-3 and Examples 2-1 to 2-6, when an auxiliary additive is further introduced into the electrolyte, which includes fluoroethylene carbonate, the compound shown in Formula I, the compound shown in Formula II, and tetraethylenesilane within the scope of this application, and the mass percentage of the auxiliary additive is controlled within the scope of this application, the resulting secondary battery exhibits a lower high-temperature storage expansion rate, a higher high-temperature storage capacity retention rate, and a higher 45°C cycle capacity retention rate. This indicates that the high-temperature cycle performance and high-temperature storage performance of the secondary battery are further improved.

[0087] As can be seen from Examples 2-1 to 2-9, by adjusting the ratio of tetraethylenesilane to auxiliary additives within the range of this application, the obtained secondary battery exhibits a lower high-temperature storage expansion rate, a higher high-temperature storage capacity retention rate, and a higher 45°C cycle capacity retention rate. This indicates that the high-temperature cycle performance and high-temperature storage performance of the secondary battery are further improved.

[0088] The type of auxiliary additives typically affects the high-temperature cycling performance and high-temperature storage performance of secondary batteries. As can be seen from Examples 2-4, 2-10, and 2-11, by selecting auxiliary additives within the scope of this application, the resulting secondary batteries exhibit lower high-temperature storage expansion rates, higher high-temperature storage capacity retention rates, and higher 45°C cycle capacity retention rates. This demonstrates that the secondary batteries possess excellent high-temperature cycling performance and high-temperature storage performance under high-voltage conditions.

[0089] The type of non-fluorinated solvent typically affects the high-temperature cycling and high-temperature storage performance of secondary batteries. As can be seen from Examples 2-4 and 2-12, by selecting non-fluorinated solvents within the scope of this application, the resulting secondary batteries exhibit lower high-temperature storage expansion rates, higher high-temperature storage capacity retention rates, and higher 45°C cycle capacity retention rates. This demonstrates that the secondary batteries possess excellent high-temperature cycling and high-temperature storage performance under high-voltage conditions.

[0090] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An electrolyte comprising a solvent, an additive, and an electrolyte, wherein the solvent comprises a fluorinated solvent, the fluorinated solvent comprising fluoroethylene carbonate and fluorinated chain esters, the fluorinated chain esters comprising at least one compound of Formula I and at least one compound of Formula II: in, R2 and R4 are each independently selected from fluorine-substituted C1-C6 alkyl groups, and R1 and R3 are each independently selected from C1-C6 alkyl groups; The mass ratio of the fluoroethylene carbonate to the fluorochain ester is 1:(0.33-4), the mass ratio of the compound shown in Formula I to the compound shown in Formula II is 1:(0.33-3), and the mass percentage of the fluorosolvent is 20% to 80% based on the mass of the electrolyte. The additive includes tetraethylenesilane, and the mass percentage of the tetraethylenesilane is M, based on the mass of the electrolyte, where 0.02% ≤ M ≤ 3%.

2. The electrolyte according to claim 1, wherein, The electrolyte satisfies at least one of the following conditions: (1) The compound shown in Formula I has 4 to 6 carbon atoms and 3 to 5 fluorine atoms; (2) The compound shown in Formula II has 4 to 6 carbon atoms and 2 to 4 fluorine atoms.

3. The electrolyte according to claim 1, wherein, The compound represented by Formula I is selected from at least one of the following compounds: The compound represented by Formula II is selected from at least one of the following compounds:

4. The electrolyte according to any one of claims 1 to 3, wherein, The electrolyte satisfies at least one of the following conditions: (1) Based on the mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is A, 10% ≤ A ≤ 40%; (2) Based on the mass of the electrolyte, the mass percentage of the compound shown in Formula I is B, where 5% ≤ B ≤ 20%; (3) Based on the mass of the electrolyte, the mass percentage of the compound shown in Formula II is C, where 5% ≤ C ≤ 30%.

5. The electrolyte according to any one of claims 1 to 3, wherein, The electrolyte satisfies at least one of the following conditions: (1) Based on the mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is A, 10% ≤ A ≤ 30%; (2) Based on the mass of the electrolyte, the mass percentage of the compound shown in Formula I is B, 10% ≤ B ≤ 20%; (3) Based on the mass of the electrolyte, the mass percentage of the compound shown in Formula II is C, 10% ≤ C ≤ 30%; (4) Based on the mass of the electrolyte, the mass percentage of the fluorinated solvent is 40% to 60%; (5) The mass ratio of the fluoroethylene carbonate to the fluorochain ester is 1:(1-3), and the mass ratio of the compound shown in Formula I to the compound shown in Formula II is 1:(1-3).

6. The electrolyte according to any one of claims 1 to 3, wherein, The solvent includes a non-fluorinated solvent selected from at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, phenyl acetate, 1,4-butylsulfonate lactone, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, and ethylene glycol dimethyl ether.

7. The electrolyte according to any one of claims 1 to 3, wherein, The additives include auxiliary additives selected from 1,3-propanesulfonate lactone, vinyl sulfate, and compounds of formula III: Wherein, based on the mass of the electrolyte, the mass percentage of the auxiliary additive is N, where 0.1% ≤ N ≤ 5%.

8. The electrolyte according to claim 7, wherein, 0.01≤M / N≤10.

9. The electrolyte according to claim 7, wherein, The electrolyte satisfies at least one of the following conditions: (1)1％≤M≤2％； (2)0.5％≤N≤2％； (3) 0.5 ≤ M / N ≤ 4.

10. A secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte according to any one of claims 1 to 9.

11. The secondary battery according to claim 10, wherein, The positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material selected from lithium cobalt oxide and LiNi oxide. x Co y Mn z O2, lithium-rich manganese-based materials, lithium nickel manganese oxide and lithium manganese oxide, wherein 0≤y≤1, 0≤x≤1, 0≤z≤1, and x+y+z=1.

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