Electrolytes and their preparation methods, lithium-ion secondary batteries, battery modules, battery packs and electrical devices

Abstract

This application discloses an electrolyte and its preparation method, a lithium-ion secondary battery, a battery module, a battery pack, and an electrical device. The electrolyte comprises a lithium salt, a first organic solvent, a second organic solvent, an additive, and a co-solvent; and / or the mass ratio of the additive is 20%-80% relative to the total mass of the first and second organic solvents; and / or the mass ratio of the second organic solvent is 20%-80% relative to the total mass of the first and second organic solvents; and / or the mass ratio of the additive is 0.1%-5% relative to the total mass of the electrolyte. In the technical solution of this application, the addition of an inorganic lithium salt to the electrolyte improves the cycle stability, high-temperature performance, and storage performance of the lithium-ion secondary battery. Furthermore, the use of a co-solvent can further enhance the solubility of the inorganic lithium salt, thereby effectively improving battery performance.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, specifically to an electrolyte and its preparation method, a lithium-ion secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] Electrolytes play a crucial role in transporting ions in lithium-ion batteries. Although they do not contribute to battery energy, their composition directly affects many aspects of lithium-ion battery performance. Extensive experimental data demonstrate that interfacial reactions between the electrodes and the electrolyte, as well as the oxidative decomposition of the electrolyte under high voltage, negatively impact the interfacial stability, electrochemical performance, and safety of lithium-ion batteries, thus limiting their development.

[0003] Therefore, the aforementioned technical issues still need further resolution. Summary of the Invention

[0004] In view of the above problems, this application provides an electrolyte and its preparation method, a lithium-ion secondary battery, a battery module, a battery pack and an electrical device, which improves the cycle stability, high-temperature performance and storage performance of the lithium-ion secondary battery by adding inorganic lithium salt to the electrolyte.

[0005] In a first aspect, this application provides an electrolyte comprising a lithium salt, a first organic solvent, a second organic solvent, an additive, and a co-solvent; wherein the additive is an inorganic lithium salt; and / or

[0006] The mass percentage of the first organic solvent relative to the total mass of the first organic solvent and the second organic solvent is 20%-80%; and / or

[0007] The mass ratio of the second organic solvent to the total mass of the first and second organic solvents is 20%-80%; and / or

[0008] The mass ratio of the additive relative to the total mass of the electrolyte is 0.1%-5%.

[0009] The technical solution of this application aims to improve battery stability by using low-cost inorganic lithium salts as electrolyte additives. Adding inorganic lithium salts as additives can form a stable interfacial film on the surfaces of the positive and negative electrode materials, reducing side reactions, decreasing HF generation, inhibiting the dissolution of transition metal ions, and stabilizing the structure of the electrode materials. Furthermore, the inorganic lithium salts have very high lithium-ion conductivity, which can improve conductivity, reduce polarization, and improve the cycle stability, high-temperature performance, and storage performance of lithium-ion secondary batteries. However, inorganic lithium salts have extremely low solubility in ester electrolytes. By adjusting the solvent ratio of the electrolyte, while stabilizing the high dielectric yield of the electrolyte, reducing the viscosity of the electrolyte can effectively improve the solubility of the inorganic lithium salt additive in the electrolyte. Furthermore, the solubility of inorganic lithium salts can be further improved by using co-solvents.

[0010] In some embodiments, the inorganic lithium salt is selected from at least one of lithium nitrate, lithium sulfate, lithium phosphate, and lithium carbonate. This embodiment, by selecting at least one of lithium nitrate, lithium sulfate, lithium phosphate, and lithium carbonate, enables the final battery to exhibit good cycle stability, high-temperature performance, and storage performance.

[0011] In some embodiments, the co-solvent is selected from at least one of sodium fluoride, potassium fluoride, magnesium fluoride, silver fluoride, and copper fluoride; and / or the mass ratio of the co-solvent is 0.1%-1% relative to the total mass of the electrolyte. This embodiment uses fluorides as co-solvents. The electron-withdrawing ability of metal ions in fluorides is greater than that of lithium ions, and the electron-donating ability of nitrate ions is greater than that of hexafluorophosphate ions. Therefore, the metal ions in the fluoride will form complexes with nitrate ions and dissolve in the electrolyte, thereby increasing the solubility of lithium nitrate. Furthermore, by selecting 0.1%-1% of at least one of sodium fluoride, potassium fluoride, magnesium fluoride, silver fluoride, and copper fluoride, not only can high solubility of inorganic lithium salts be achieved, but other negative effects can also be suppressed while ensuring battery stability.

[0012] In some embodiments, the first organic solvent is selected from at least one of ethylene carbonate and propylene carbonate; and / or the second organic solvent is selected from at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate, and ethyl propionate. This embodiment improves the solubility of inorganic lithium salts in the electrolyte by using the above-mentioned first and second organic solvents in combination.

[0013] In some embodiments, the lithium salt in the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(imide), lithium dioxaborate, lithium difluorooxaborate, and lithium hexafluoroarsenate; and / or the molar concentration of the lithium salt is 0.4 mol / L-1.6 mol / L. This embodiment selects a lithium salt with a concentration of 0.4 mol / L-1.6 mol / L, allowing it to synergistically act with the first and second organic solvents in the aforementioned proportions. This not only stabilizes the high dielectric product of the electrolyte and reduces its viscosity, but also effectively improves the solubility of the inorganic lithium salt in the electrolyte.

[0014] Secondly, this application provides a method for preparing an electrolyte, comprising the following steps:

[0015] In an argon atmosphere with a water content of <10ppm, a first organic solvent and a second organic solvent are mixed to obtain a mixed solvent. Lithium salt is then dissolved in the mixed solvent, and additives and co-solvents are added. After stirring evenly, an electrolyte is obtained.

[0016] In the technical solution of this application embodiment, by adding inorganic lithium salt as an additive, a stable interface film can be formed on the surface of the positive and negative electrode materials, suppressing interface side reactions, reducing the generation of HF, reducing the dissolution of transition metal ions, stabilizing the electrode material structure, and improving the cycle stability, high temperature performance and storage performance of lithium-ion secondary batteries; and by using a co-solvent and further improving the solubility of inorganic lithium salt, the various performances of the battery are effectively improved.

[0017] Thirdly, this application provides a lithium-ion secondary battery, including a negative electrode, a separator, and a positive electrode, wherein the separator is disposed between the negative electrode and the positive electrode; the lithium-ion secondary battery also includes the electrolyte in the above embodiments.

[0018] In the technical solution of this application embodiment, by injecting an electrolyte containing inorganic lithium salt additives into the battery, the cycle stability, high temperature performance and storage performance of lithium-ion secondary batteries can be improved.

[0019] Fourthly, this application provides a battery module including the lithium-ion secondary battery described in the above embodiments. The battery module can serve as a power source or an energy storage device. The number of lithium-ion secondary batteries in the battery module can be adjusted according to the application and capacity of the battery module.

[0020] Fifthly, this application provides a battery pack including the battery modules described in the above embodiments. The number and arrangement of the battery modules used in the battery pack can be determined according to actual needs. The battery pack can serve as a power source or an energy storage device.

[0021] Sixthly, this application provides an electrical device comprising the lithium-ion secondary battery described in the above embodiments, the battery being used to provide electrical energy. This lithium-ion secondary battery has a longer cycle life and higher capacity utilization.

[0022] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, specific embodiments of this application are given below. Attached Figure Description

[0023] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0024] Figure 1 This is a schematic diagram of the vehicle structure according to some embodiments of this application;

[0025] Figure 2 This is a schematic diagram of the structure of a lithium-ion secondary battery according to some embodiments of this application;

[0026] Figure 3 This is a schematic diagram of the structure of a battery module according to some embodiments of this application;

[0027] Figure 4 This is a schematic diagram of the battery pack structure according to some embodiments of this application;

[0028] Among them, top cover-10; housing-11; electrode assembly-12; electrode terminal-14; tab-15; lithium-ion secondary battery-100; vehicle-1000;

[0029] Battery Module-200;

[0030] Upper box-300;

[0031] Lower box-400;

[0032] Battery pack - 500. Detailed Implementation

[0033] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0035] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0036] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0037] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A exists, A and B exist simultaneously, and B exists. In addition, the character " / " in this document generally indicates that the related objects before and after it have an "or" relationship.

[0038] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0039] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0040] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0041] In existing technologies, with the development of clean energy, more and more devices are using electrical energy as their driving force. Consequently, power batteries, such as lithium-ion rechargeable batteries, which can store a large amount of electrical energy and can be repeatedly charged and discharged, are developing rapidly. These power batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, but are also widely used in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace and other fields.

[0042] The inventors have noted that during the charging process of lithium-ion secondary batteries, components such as solvents or additives in the electrolyte undergo reduction decomposition, and the products are deposited on the surface of the negative electrode active material graphite to form an SEI film (negative electrode solid electrolyte interfacial film), or undergo oxidative decomposition, and the products are deposited on the surface of the positive electrode active material to form a CEI film (positive electrode solid electrolyte interfacial film). As the cycle progresses, the interfacial film is continuously generated and decomposed. These interfacial side reactions continuously consume the active lithium ions in the electrolyte and generate by-products, which reduces the cycle performance and storage performance of the battery.

[0043] To address the aforementioned issues, the inventors discovered that adding inorganic lithium salt additives to the electrolyte can form a stable interfacial film on the surface of the positive and negative electrode active materials, reducing interfacial side reactions, decreasing the generation of HF, inhibiting the dissolution of transition metal ions, and stabilizing the crystal structure of the electrode materials. Furthermore, the addition of inorganic lithium salts can improve conductivity, reduce polarization, and enhance the cycle stability, high-temperature performance, and storage performance of lithium-ion secondary batteries.

[0044] Furthermore, the inventors achieved the dissolution of inorganic lithium salts in ester-based electrolytes by adjusting the solvent composition and lithium salt concentration in the electrolyte. Inorganic lithium salts have extremely low solubility in ester-based electrolytes and require the use of ether solvents for dissolution. However, ether solvents have low oxidation potentials and are prone to oxidative decomposition on the surface of the positive electrode active material. Therefore, the key to applying inorganic lithium salts in lithium-ion secondary batteries is achieving their dissolution in ester-based electrolytes. The solubility of inorganic lithium salts in low-viscosity, high-polarity electrolytes can be effectively improved, but a single solvent cannot meet this requirement. It is necessary to adjust the proportion of conventional ester solvents to maintain the high dielectric constant of the electrolyte and reduce ion association to lower viscosity. Simultaneously, increasing the lithium salt concentration in the electrolyte will increase the electrolyte viscosity and reduce the solubility of inorganic lithium salts, but the overall conductivity of the electrolyte must also be considered.

[0045] The electrolyte disclosed in this application can be applied to lithium-ion secondary batteries. The lithium-ion secondary battery includes a negative electrode, an electrolyte, a separator, and a positive electrode, with the electrolyte and separator spaced between the negative and positive electrodes. During battery charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. Injecting an electrolyte containing inorganic lithium salt additives improves the battery's storage performance and cycle life. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0046] According to some embodiments of this application, the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0047] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0048] According to some embodiments of this application, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0049] According to some embodiments of this application, the positive electrode active material may be any positive electrode active material known in the art for use in batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNiO2). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0050] According to some embodiments of this application, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0051] According to some embodiments of this application, the positive electrode film layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0052] According to some embodiments of this application, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0053] According to some embodiments of this application, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0054] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0055] According to some embodiments of this application, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0056] According to some embodiments of this application, the negative electrode active material may be any negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0057] According to some embodiments of this application, the negative electrode film layer may optionally include an adhesive. The adhesive may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0058] According to some embodiments of this application, the negative electrode film layer may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0059] According to some embodiments of this application, the negative electrode film layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0060] According to some embodiments of this application, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0061] According to some embodiments of this application, there are no particular limitations on the type of separator membrane; any known porous structure separator membrane with good chemical and mechanical stability can be selected. The material of the separator membrane can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator membrane can be a single-layer film or a multi-layer composite film, without particular limitations. When the separator membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitations.

[0062] According to some embodiments of this application, the positive electrode sheet, the negative electrode sheet, and the separator can be fabricated into an electrode assembly by a winding process or a stacking process.

[0063] According to some embodiments of this application, a lithium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0064] According to some embodiments of this application, the outer packaging of a lithium-ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of a lithium-ion secondary battery can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0065] This application does not impose any particular restrictions on the shape of the lithium-ion secondary battery, which can be cylindrical, square, or any other arbitrary shape.

[0066] The lithium-ion secondary batteries disclosed in this application can be used, but are not limited to, in electrical devices such as vehicles, ships, or aircraft. A power system for such an electrical device can be constructed using lithium-ion secondary batteries disclosed in this application, thereby solving the problem of interface instability and improving the battery's storage performance and cycle life.

[0067] This application provides an electrical device that uses a lithium-ion secondary battery as a power source. This device can be used, but is not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, electric bicycles, electric scooters, electric golf carts, electric trucks, electric boats, ships, spacecraft, energy storage systems, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc. The lithium-ion secondary battery is used to provide electrical energy.

[0068] For ease of explanation, the following embodiments will be described using a vehicle 1000 as an example of an electrical device according to an embodiment of this application.

[0069] Please refer to Figure 1 , Figure 1 This is a schematic diagram of the structure of a vehicle 1000 provided in some embodiments of this application. The vehicle 1000 can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. The new energy vehicle can be a pure electric vehicle, a hybrid electric vehicle, or a range-extended electric vehicle, etc. A lithium-ion secondary battery 100 is installed inside the vehicle 1000, and the lithium-ion secondary battery 100 can be located at the bottom, front, or rear of the vehicle 1000. The battery 100 can be used to power the vehicle 1000; for example, the lithium-ion secondary battery 100 can serve as the operating power source for the vehicle 1000.

[0070] Please refer to Figure 2 , Figure 2 This is a schematic diagram of the structure of a lithium-ion secondary battery provided in some embodiments of this application. For example... Figure 2 As shown, the lithium-ion secondary battery 100 includes a top cover 10, a housing 11, an electrode assembly 12, and an electrolyte (not shown).

[0071] The top cover 10 refers to a component that closes onto the opening of the housing 11 to isolate the internal environment of the lithium-ion secondary battery 100 from the external environment. Functional components such as electrode terminals 14 may be provided on the top cover 10. The electrode terminals 14 can be electrically connected to the electrode assembly 12 for outputting or inputting electrical energy into the lithium-ion secondary battery 100. In some embodiments, the top cover 10 may also be provided with a pressure relief mechanism for releasing internal pressure when the internal pressure or temperature of the lithium-ion secondary battery 100 reaches a threshold. The material of the end cover 10 can also be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and this application embodiment does not impose any special limitations on this.

[0072] The housing 11 is a component used to cooperate with the top cover 10 to form the internal environment of the lithium-ion secondary battery 100. This internal environment can accommodate the electrode assembly 12, electrolyte, and other components. The housing 11 and the top cover 10 can be independent components. An opening can be provided on the housing 11, and the top cover 10 can be used to close the opening to form the internal environment of the lithium-ion secondary battery 100. Alternatively, the top cover 10 and the housing 11 can be integrated. Specifically, the top cover 10 and the housing 11 can form a common connecting surface before other components are inserted into the housing. When it is necessary to encapsulate the interior of the housing 11, the top cover 10 closes the housing 11. The housing 11 can be made of various materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc. This application embodiment does not impose any special limitations on these materials.

[0073] Electrode assembly 12 is the component in the lithium-ion secondary battery 100 where electrochemical reactions occur. The number of electrode assemblies 12 is not limited; the housing 11 may contain one or more electrode assemblies 12. Electrode assembly 12 is mainly formed by winding or stacking positive and negative electrode sheets, and a separator is typically provided between the positive and negative electrode sheets. The portions of the positive and negative electrode sheets containing active material constitute the main body of electrode assembly 12, while the portions of the positive and negative electrode sheets without active material each constitute tabs 15. The positive and negative tabs may be located together at one end of the main body or separately at both ends of the main body. During the charging and discharging process of the battery, the positive and negative active materials react with the electrolyte, and the tabs 15 connect to the electrode terminals to form a current loop.

[0074] Please refer to Figure 3 , Figure 3 The diagram shows the structure of a battery module provided in some embodiments of this application. Figure 3As shown, the battery module 200 includes multiple lithium-ion secondary batteries 100, which are arranged longitudinally and can be connected in parallel. The battery module 200 can serve as a power source or an energy storage device. The number of lithium-ion secondary batteries 100 in the battery module 200 can be adjusted according to the application and capacity of the battery module 200.

[0075] Please refer to Figure 4 , Figure 4 This is a schematic diagram of the battery pack structure provided in some embodiments of this application. For example... Figure 4 As shown, the battery pack 500 includes a battery module 200, an upper housing 300, and a lower housing 400. The upper housing 300 and the lower housing 400 are assembled together to form a space for housing the battery module 200. The battery module 200 is placed within the space of the assembled upper housing 300 and lower housing 400. The output terminal of the battery module 200 extends from one or both of the upper housing 300 and lower housing 400 to supply power to or charge from the outside. The number and arrangement of the battery modules 200 used in the battery pack 500 can be determined according to actual needs. The battery pack 500 can serve as a power source or an energy storage device.

[0076] According to some embodiments of this application, this application provides an electrolyte comprising a lithium salt, a first organic solvent, a second organic solvent, an additive, and a co-solvent; the additive is an inorganic lithium salt; and / or

[0077] The mass percentage of the first organic solvent relative to the total mass of the first organic solvent and the second organic solvent is 20%-80%; and / or

[0078] The mass ratio of the second organic solvent to the total mass of the first and second organic solvents is 20%-80%; and / or

[0079] The additive accounts for 0.1%-5% of the total mass of the electrolyte.

[0080] By adding inorganic lithium salts as additives to the electrolyte and adjusting the solvent ratio of the electrolyte to increase the solubility of the inorganic lithium salt additive in the electrolyte, the interfacial instability problem is solved, thereby improving the battery's cycle stability, high-temperature performance, and storage performance. Furthermore, adding co-solvents can achieve higher solubility of inorganic lithium salts, for example, increasing the solubility to 3 wt%.

[0081] When the mass ratio of the first organic solvent or the second solvent is less than 20%, the solubility of the inorganic lithium salt additive is low, and the improvement effect of the additive is not obvious. When the mass ratio is greater than 80%, the electrolyte viscosity is too high, which will also reduce the solubility of the inorganic lithium salt. At the same time, the high viscosity will affect the conductivity of the electrolyte and affect the normal recycling of the electrolyte.

[0082] When the mass proportion of inorganic lithium salt is less than 0.1%, the effect of the additive is small and has no significant improvement on the battery's cycle performance. When the mass proportion of inorganic lithium salt is greater than 0.1% and less than 5%, there is a positive correlation between the increase in additive concentration and the improvement in battery cycle performance. However, when the mass proportion of inorganic lithium salt is around 5%, the battery cycle performance tends to stabilize.

[0083] According to some embodiments of this application, optionally, the inorganic lithium salt is selected from at least one of lithium nitrate, lithium sulfate, lithium phosphate, and lithium carbonate. These inorganic lithium salts all contribute to improving the battery's cycle stability, high-temperature performance, and storage performance.

[0084] According to some embodiments of this application, optionally, the co-solvent is selected from at least one of sodium fluoride, potassium fluoride, magnesium fluoride, silver fluoride, and copper fluoride; and / or the mass ratio of the co-solvent is 0.1%-1% relative to the total mass of the electrolyte. When the mass fraction of the co-solvent is less than 0.1%, the co-solvent effect on inorganic lithium salts is small and cannot improve their solubility; when the mass fraction of the co-solvent is greater than 1%, the internal resistance of the battery will be severely deteriorated, affecting the rate performance of the battery.

[0085] According to some embodiments of this application, optionally, the first organic solvent is selected from at least one of ethylene carbonate and propylene carbonate. Ethylene carbonate and propylene carbonate have high dielectric constants and high polarity, which are key to achieving the dissolution of inorganic lithium salts. And / or the second organic solvent is selected from at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate, and ethyl propionate. The second organic solvent is a low-viscosity, low-dielectric-constant solvent, in which inorganic lithium salts cannot dissolve. Its purpose is to stabilize the high dielectric constant of the electrolyte while reducing the viscosity of the electrolyte, thereby achieving the highest solubility of the inorganic lithium salt additive.

[0086] According to some embodiments of this application, optionally, the lithium salt in the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorosulfonylimide, lithium diimide, lithium dioxalate borate, lithium difluorooxalate borate, and lithium hexafluoroarsenate; and / or the molar concentration of the lithium salt is 0.4 mol / L-1.6 mol / L. When the molar concentration of the lithium salt is less than 0.4 mol / L, the conductivity of the electrolyte is too high, the lithium ion diffusion ability is poor and the polarization is large, which cannot meet the normal cycle use of the battery, and may also be accompanied by safety risks such as lithium plating; when the molar concentration of the lithium salt is higher than 1.6 mol / L, the viscosity of the electrolyte is too high, which causes the additive inorganic lithium salt to be unable to dissolve.

[0087] According to some embodiments of this application, this application also provides a method for preparing an electrolyte, comprising the following steps:

[0088] In an argon atmosphere with a water content of <10ppm, a first organic solvent and a second organic solvent are mixed to obtain a mixed solvent. Lithium salt is then dissolved in the mixed solvent, and additives and co-solvents are added. After stirring evenly, an electrolyte is obtained.

[0089] By adding inorganic lithium salts as additives and using co-solvents to improve the solubility of inorganic lithium salts, the cycle stability, high-temperature performance, and storage performance of lithium-ion secondary batteries can be improved.

[0090] The present application will be further described below with reference to specific embodiments.

[0091] The reagents used in the following examples are as follows:

[0092] Cathode materials: lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, lithium iron phosphate, and lithium iron phosphate are all options.

[0093] Anode material: graphite;

[0094] Separating membrane: PE porous polymer film is used as the separating membrane.

[0095] The preparation processes of the lithium-ion secondary batteries (hereinafter referred to as batteries) in Examples 1-16 and Comparative Examples 1-13 are as follows.

[0096] (1) Preparation of negative electrode sheet

[0097] The negative electrode active material graphite, conductive agent acetylene black, binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose were mixed in a mass ratio of graphite:acetylene black:styrene-butadiene rubber:sodium carboxymethyl cellulose = 95:2:2:1. Deionized water was added and the mixture was stirred thoroughly to ensure a slurry viscosity of approximately 15000 mPa·s, forming a uniform negative electrode slurry. The slurry was then coated onto the negative electrode current collector copper foil to a thickness of 150 μm, and subsequently dried and cold-pressed to a density of 1.6 g / cm³. 3 This yields the negative electrode sheet.

[0098] (2) Preparation of positive electrode sheet

[0099] The positive electrode active material (nickel-cobalt-manganese ternary material), conductive agent (acetylene black), and binder (polyvinylidene fluoride) were mixed at a mass ratio of 96:2:2, using N-methylpyrrolidone as a solvent. The mixture was thoroughly stirred to ensure a slurry viscosity of approximately 15000 mPa·s, forming a uniform positive electrode slurry. The slurry was then coated onto the positive electrode current collector aluminum foil to a thickness of 200 μm, followed by drying and cold pressing to a density of 3 g / cm³. 3 This yields the positive electrode sheet.

[0100] (3) Electrolyte preparation

[0101] The electrolytes were all prepared according to the following method:

[0102] In an argon atmosphere glove box with a water content of <0.5ppm and an oxygen content of <0.5ppm, the first organic solvent and the second organic solvent are mixed according to Table 1 or Table 2 in the mass ratio to obtain a mixed solvent. Then, fully dried lithium salt LiPF6 is dissolved in the above mixed solvent according to Table 1 or Table 2. Inorganic lithium salt additives and co-solvents are added to it according to Table 1 or Table 2. After stirring evenly, the electrolyte becomes clear and the color is ≤50 Hazen, thus obtaining the electrolyte.

[0103] (4) Battery manufacturing

[0104] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, they are wound to obtain a bare battery. The bare battery is placed in an outer packaging foil, and the prepared electrolyte is injected into the dried battery. Then, the battery is obtained through vacuum sealing, settling, formation, and shaping processes.

[0105] In Tables 1-2 below, the content of any solvent in the electrolyte is a mass percentage of the total mass of the two solvents, and the content of additives and co-solvents is a mass percentage calculated based on the total mass of the electrolyte.

[0106] The electrolyte formulations for Examples 1-16 and Comparative Examples 1-15 are shown in Table 1.

[0107] Table 1

[0108]

[0109]

[0110]

[0111]

[0112] Test case

[0113] (1) Battery 25 / 45℃ Cyclic Performance Test

[0114] The batteries prepared in Examples 1-16 and Comparative Examples 1-5 were subjected to the following tests:

[0115] At 25 / 45℃, the lithium-ion secondary battery was first charged to 4.3V with a constant current of 1C, then further charged to 0.05C with a constant voltage of 4.3V, and finally discharged to 3V with a constant current of 1C. This constitutes one charge-discharge cycle, and the discharge capacity of this cycle is the discharge capacity of the first cycle. The battery underwent multiple charge-discharge cycles in the above manner, and the discharge capacity of the 200th cycle was measured. The capacity retention rate of the battery after the cycles was calculated using the following formula. The test results are shown in Table 2 below.

[0116] The capacity retention rate (%) of the battery after 200 cycles = [discharge capacity of the 200th cycle / discharge capacity of the 1st cycle] × 100%.

[0117] (2) Battery storage performance test at 60°C

[0118] The batteries prepared in the examples and comparative examples were subjected to the following tests:

[0119] At 25℃, the battery was charged to 4.3V with a constant current of 1C, then charged at a constant voltage of 4.3V until the current was less than 0.05C, and discharged at a constant current of 1C to 3.0V. This process was repeated three times, with the final discharge capacity taken as the recoverable capacity. The test results are shown in Table 2 below.

[0120] Battery recovery capacity percentage (%) = [Battery recovery capacity after high temperature storage / Battery capacity before storage] × 100%.

[0121] (3) Mn metal dissolution test of battery

[0122] Inductively coupled plasma (ICP) was used to characterize the dissolution of Mn metal from the graphite anode after cycling. The test method involved disassembling the cycled battery and digesting the disassembled electrodes with concentrated nitric acid. The elemental content (mass fraction w / w%) was calculated as: element mass / sample weight * 100%. The test results for the battery are shown in Table 2.

[0123] Table 2 Performance test results of Examples 1-16 and Comparative Examples 1-15

[0124]

[0125]

[0126] The test results from Comparative Examples 1-8 show that the solvent ratio and lithium salt concentration in the electrolyte have a significant impact on battery performance. In Comparative Example 1, the low molar concentration of lithium salt in the electrolyte reduces the overall viscosity of the electrolyte, which increases the solubility of inorganic lithium salt. However, the low lithium salt concentration significantly reduces the electrolyte conductivity. When the molar concentration of lithium salt is less than 0.4 mol / L, the ionic conductivity of the electrolyte decreases dramatically, the battery impedance increases sharply, and the battery cannot utilize its full capacity (e.g., only one-third of its normal capacity). The battery's cycle performance; for example, at 25°C, Comparative Example 7 showed DCRs of 3425.67 mΩ, 3146.74 mΩ, and 3674.24 mΩ at 90% SOC (State of Charge, characterizing the remaining capacity of the cell), 50% SOC, and 20% SOC, respectively, significantly higher than the 625.05 mΩ, 597.85 mΩ, and 639.10 mΩ at the same SOC in Comparative Example 8. In Comparative Example 3, increasing the molar concentration of lithium salt in the electrolyte increased the electrolyte viscosity, which reduced the solubility of the inorganic lithium salt additive. The test data from Comparative Examples 1-3 and 6-8 demonstrate that the solubility of the inorganic lithium salt additive decreases with increasing lithium salt concentration in the electrolyte. Comparing Examples 2, 4, and 5, it was found that when either the first or second organic solvent accounted for 90% of the total solvent mass, the solubility of inorganic lithium salts was low, resulting in no significant improvement in battery performance. This is because the proportion of a single solvent is too large, leading to an imbalance between electrolyte viscosity and polarity, which fails to meet the high polarity and low viscosity environment required for electrolyte dissolution. Therefore, it is necessary to coordinate the lithium salt concentration and the ratio of the first and second organic solvents in the electrolyte to achieve the desired dissolution of inorganic lithium salt additives in ester-based electrolytes.

[0127] The test results of Comparative Examples 6-10 and 1-5 show that using inorganic lithium salts such as lithium nitrate can improve the battery's capacity retention after 200 cycles at 25℃ and 45℃, as well as its storage performance at 60℃. This indicates that inorganic lithium salt additives can effectively improve the battery's cycle stability and high-temperature storage performance. The test results of Comparative Examples 7 and 11-13 show that lithium sulfate, lithium carbonate, and lithium phosphate have lower solubility than lithium nitrate in an electrolyte containing 1 mol / L lithium hexafluorophosphate and a 50:50 mass ratio of the first organic solvent to the second organic solvent. However, as additives, they significantly improve the battery's cycle stability and high-temperature storage performance.

[0128] The comparison between Examples 1-4 and Example 6 shows that the solubilizing effect of copper fluoride is also related to the viscosity and polarity of the electrolyte system. The solubilizing effect of copper fluoride varies with the electrolyte solvent formulation and decreases with increasing lithium salt concentration. The test results of Examples 5-8 show that copper fluoride can effectively solubilize lithium nitrate in a suitable electrolyte system. With increasing copper fluoride content, the solubility of lithium nitrate increases, but the solubilizing effect eventually stabilizes. The cycle stability of the battery increases with increasing lithium nitrate solubility, but the effect stabilizes at 3 wt%. Furthermore, adding excessive copper fluoride and lithium nitrate affects other battery performance aspects, such as impedance. Actual measurements show that adding 3 wt% copper fluoride deteriorates the battery's AC impedance DCR by 30%. The test results of Examples 9-11 show that silver fluoride and magnesium fluoride also have a solubilizing effect on silver nitrate, but the effect is slightly lower than that of copper fluoride. The cycle performance of the battery is related to the solubility of lithium nitrate. In Examples 12-16, copper fluoride significantly improved the solubilizing effect on lithium sulfate, lithium carbonate, and lithium phosphate, with the effect increasing with the increase of copper fluoride. This effectively improved the cycle stability and high-temperature storage performance of the battery.

[0129] The results from Comparative Examples 11-13 and Examples 12-16 show that lithium phosphate, lithium carbonate, and lithium phosphate can all improve the cycle stability of the battery. Furthermore, the use of copper fluoride has a significant solubilizing effect on lithium sulfate, lithium carbonate, and lithium phosphate, with solubility increasing with the increase of copper fluoride. The cycle stability and high-temperature storage performance of the battery are effectively improved.

[0130] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. An electrolyte, characterized in that, The electrolyte comprises lithium salt, a first organic solvent, a second organic solvent, an additive, and a co-solvent; the additive is an inorganic lithium salt. The first organic solvent is selected from at least one of ethylene carbonate and propylene carbonate; the second organic solvent is selected from at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate, and ethyl propionate. The inorganic lithium salt is lithium nitrate; The co-solvent is selected from magnesium fluoride; The mass ratio of the first organic solvent to the total mass of the first organic solvent and the second organic solvent is 20%-80%. The mass ratio of the second organic solvent to the total mass of the first organic solvent and the second organic solvent is 20%-80%. The mass ratio of the additive is 0.1%-5% relative to the total mass of the electrolyte.

2. The electrolyte as described in claim 1, characterized in that, The mass ratio of the co-solvent is 0.1%-1% relative to the total mass of the electrolyte.

3. The electrolyte as described in claim 1, characterized in that, In the electrolyte, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorosulfonylimide, lithium diimide, lithium dioxalate borate, lithium difluorooxalate borate, and lithium hexafluoroarsenate; and / or the molar concentration of the lithium salt is 0.4 mol / L to 1.6 mol / L.

4. A method for preparing the electrolyte according to any one of claims 1-3, characterized in that, Includes the following steps: In an argon atmosphere with a water content of <10ppm, a first organic solvent and a second organic solvent are mixed to obtain a mixed solvent. Lithium salt is then dissolved in the mixed solvent, and additives and co-solvents are added. After stirring evenly, an electrolyte is obtained.

5. A lithium-ion secondary battery, characterized in that, It includes a positive electrode, a negative electrode, a separator between the positive and negative electrode, and the electrolyte according to any one of claims 1-3.

6. A battery module, characterized in that, Includes the lithium-ion secondary battery as described in claim 5.

7. A battery pack, characterized in that: The battery pack includes the battery module as described in claim 6.

8. An electrical device, comprising a power supply, characterized in that, The power source is the lithium-ion secondary battery as described in claim 5.

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