Secondary battery and electric device

Abstract

This application belongs to the field of battery technology, specifically relating to a secondary battery and an electrical device. The secondary battery includes a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative electrode active material, which in turn includes graphitized carbon material. The negative electrode active material satisfies the following condition: (1.1) The specific surface area of ​​the negative electrode active material is 0.5 m². 2 / g~3.0m 2 / g; (1.2) The volume distribution particle size Dv1 of the negative electrode active material is ≥1.5μm; and the electrolyte contains carboxylic acid ester solvents and film-forming stabilizers. The design of this application is beneficial to improving battery performance.

Description

Secondary batteries and electrical devices

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application No. 2024103388474, filed on March 22, 2024, entitled “Secondary Battery and Electrical Device,” the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application belongs to the field of batteries, and specifically relates to a secondary battery and an electrical device. Background Art

[0004] Secondary batteries are widely used in various consumer electronics and electric vehicles due to their outstanding features such as light weight, no pollution, and no memory effect. Among them, lithium-ion batteries are widely used in portable electronic devices, electric vehicles and other fields.

[0005] As the application scope of secondary batteries becomes wider and wider, the requirements for battery performance are also getting higher and higher. Summary of the Invention

[0006] The present application provides a secondary battery and an electrical device. The secondary battery designed in the present application is used to balance the comprehensive performance of the battery. For example, to a certain extent, it takes into account both cycle stability and fast charging performance.

[0007] A first aspect of the present application is to provide a secondary battery, comprising:

[0008] Negative electrode sheet: Contains negative electrode active material, which meets the following conditions:

[0009] (1.1) The specific surface area of ​​the negative electrode active material is 0.5 m 2 / g~3.0m 2 / g;

[0010] (1.2) The volume distribution particle size of the negative electrode active material Dv1 ≥ 1.5 μm;

[0011] The negative electrode active material comprises a graphitized carbon material;

[0012] Electrolyte: Contains carboxylic acid ester solvent and film-forming stabilizer.

[0013] The graphitized carbon material in this application has the characteristics of a small specific surface area and a small amount of small-size particles as a negative electrode active material. The negative electrode active material with this characteristic is used in conjunction with an electrolyte containing a carboxylic acid ester solvent and a film-forming stabilizer. On the premise of improving the rate performance of the secondary battery, it can also reduce the impact on the battery cycle performance. Specifically, the specific surface area of ​​the negative electrode active material provided in this application is relatively small, which can effectively reduce the side reactions between it and the electrolyte, and prepare for the use of carboxylic acid ester solvents. This is because carboxylic acid ester solvents have the advantages of low viscosity and low surface tension. They are easier to diffuse to the positive and negative electrode surfaces, and by improving the wettability of the positive and negative electrode active materials to the electrolyte to accelerate the diffusion rate of lithium ions, thereby improving the rate performance of the battery. However, it also has obvious disadvantages: the probability of side reactions occurring on the positive and negative electrode surfaces is high, and gas production is high, which is not conducive to the cyclability of the battery. This application chooses to use a negative electrode active material with a relatively small specific surface area + a film-forming stabilizer to reduce the probability of side reactions. At the same time, the present application also controls the content of small-particle negative electrode active materials, and further reduces side reactions by reducing their usage. On the other hand, in the original negative electrode active material, small-particle materials will occupy the internal pores, causing the lithium ion transmission path to become curved, thereby increasing the actual path length of the lithium ions. The choice of controlling the content of small-particle negative electrode active materials in the present application also plays a positive role in improving the rate performance of the battery. Therefore, the design method provided by the present application is conducive to taking into account both the fast charging performance and the cycle performance of the secondary battery.

[0014] In some embodiments of the present application, the specific surface area of ​​the negative electrode active material is 0.5 m 2 / g~2.0m 2 / g;

[0015] and / or;

[0016] The volume distribution particle size Dv1 of the negative electrode active material is 1.9 μm to 3.1 μm.

[0017] In some embodiments of the present application, the graphitized carbon material satisfies at least one of the following:

[0018] (2.1) The orientation index OI value of the graphitized carbon material is ≤ 24; the orientation index OI value is the peak intensity value of the plane (110) and the plane (004) in the X-ray powder diffraction pattern of the graphitized carbon material;

[0019] (2.2) The volume distribution particle size of the graphitized carbon material satisfies the following requirements: 7.5 μm ≤ Dv50 ≤ 20 μm, Dv10 ≥ 3.5 μm, and Dv90 ≤ 40 μm.

[0020] In some embodiments of the present application, the graphitized carbon material satisfies at least one of the following:

[0021] (3.1) The orientation index of graphitized carbon materials satisfies the following conditions: 10 ≤ OI value ≤ 22;

[0022] (3.2) The volume distribution particle size of the graphitized carbon material satisfies the following requirements: 10 μm ≤ Dv50 ≤ 18.5 μm, 3.5 μm ≤ Dv10 ≤ 7.0 μm, and Dv90 ≤ 40 μm.

[0023] In some embodiments of the present application, the negative electrode active material further comprises amorphous carbon; the amorphous carbon comprises one or both of hard carbon and soft carbon;

[0024] and / or;

[0025] The volume distribution particle size of amorphous carbon satisfies: 3.5μm≤Dv50≤7.0μm;

[0026] and / or;

[0027] The graphitized carbon material includes one or more of natural graphite, artificial graphite, and composite graphite.

[0028] In some embodiments of the present application, the graphitized carbon material has a core-shell structure, the core of the core-shell structure comprises one or more of natural graphite, artificial graphite, and composite graphite, and the shell of the core-shell structure comprises an amorphous carbon coating layer.

[0029] In some embodiments of the present application, the mass percentage of the graphitized carbon material is greater than 75% based on the total mass of the negative electrode active material.

[0030] In some embodiments of the present application, the carboxylate solvent satisfies at least one of the following:

[0031] (4.1) Carboxylic acid esters have the following structural formula:

[0032] R1-COO-R2;

[0033] wherein R1 and R2 each independently comprise a substituted and / or unsubstituted C1-C5 alkyl group;

[0034] (4.2) Based on the mass of the electrolyte, the mass percentage of the carboxylate is 5% to 70%;

[0035] In some embodiments of the present application, the carboxylate solvent satisfies at least one of the following:

[0036] (5.1) Carboxylic acid esters include one or more of ethyl acetate, methyl acetate, ethyl propionate, propyl acetate, methyl propionate, and methyl butyrate;

[0037] (5.2) Based on the mass of the electrolyte, the mass percentage content of the carboxylic acid ester is 10% to 60%.

[0038] In some embodiments of the present application, the electrolyte further comprises a carbonate solvent, and the carbonate solvent comprises one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0039] In some embodiments of the present application, the film-forming stabilizer satisfies at least one of the following:

[0040] (6.1) Based on the total mass of the electrolyte, the mass percentage content of the film-forming stabilizer is 0.02% to 6%.

[0041] (6.2) The film-forming stabilizer comprises one or more of a boron-containing lithium salt, a phosphorus-containing lithium salt, and a sulfur-containing lithium salt;

[0042] and / or;

[0043] The boron-containing lithium salt includes one or more of lithium tetrafluoroborate, lithium bisoxalatoborate and lithium bisfluorooxalatoborate;

[0044] and / or;

[0045] The phosphorus-containing lithium salt includes one or more of lithium difluorophosphate, lithium fluorophosphate and lithium phosphate;

[0046] and / or;

[0047] The sulfur-containing lithium salt includes one or more of lithium fluorosulfonate, lithium sulfate, and lithium aminosulfonate.

[0048] In some embodiments of the present application, the film-forming stabilizer comprises lithium fluorosulfonate and lithium difluorophosphate. Based on the total mass of the electrolyte, the mass percentage content of lithium difluorophosphate is ≤1%, and the mass percentage content of lithium fluorosulfonate is greater than or equal to 0.1% and less than 2%.

[0049] In some embodiments of the present application, a positive electrode sheet is also included, the positive electrode sheet contains a positive electrode active material, and the specific surface area of ​​the positive electrode active material is 8m 2 / g~17m 2 / g.

[0050] In some embodiments of the present application, the positive electrode active material comprises a lithium-containing transition metal phosphate; the lithium-containing transition metal phosphate comprises olivine-type lithium manganese iron phosphate, and the chemical formula of olivine-type lithium manganese iron phosphate is Li (1-x1) Fe x2 Mn (1-x2) PO4, where 0≤x1<1, 0≤x2≤1.

[0051] The second aspect of the present application is to provide an electrical device comprising the secondary battery described in the first aspect.

[0052] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present application. The same reference numerals are used throughout the drawings to denote the same components. In the drawings:

[0054] FIG1 is a schematic diagram of the structure of lithium ion diffusion paths in negative electrode active materials in the prior art;

[0055] FIG2 is a schematic diagram of a battery structure according to some embodiments of the present application;

[0056] FIG3 is a schematic diagram of the exploded structure of batteries according to some embodiments of the present application;

[0057] FIG4 is a schematic diagram of a vehicle structure according to some embodiments of the present application;

[0058] FIG5 is a schematic diagram of the battery pack structure of some embodiments of the present application;

[0059] FIG6 is a schematic structural diagram of a negative electrode sheet according to some embodiments of the present application;

[0060] FIG7 is a schematic structural diagram of another negative electrode sheet according to some embodiments of the present application.

[0061] The figure numbers in the specific implementation manner are as follows: 10000, vehicle; 1000, battery; 2000, controller; 3000, motor; 100, battery cell; 200, casing; 210, first part; 220, second part; 10, secondary battery; 101, shell; 102, electrode assembly; 103, cover plate; 1, negative electrode plate; 11, negative electrode current collector; 12, negative electrode film layer; Coordinate axis x direction: length or width direction of negative electrode plate; Coordinate axis z direction: thickness direction of negative electrode plate. DETAILED DESCRIPTION

[0062] Below, the embodiments of the secondary battery and the electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0063] The "ranges" disclosed herein are defined in terms of lower and upper limits, where a given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise indicated, the numerical range "a to b" is a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0064] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0065] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0066] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0067] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0068] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0069] Unless otherwise specified, in this application, the terms "first", "second", etc. are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0070] Unless otherwise specified, in this application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0071] The term "alkyl" is intended to be a straight chain saturated hydrocarbon structure with 1 to 20 carbon atoms. "Alkyl" is also intended to be a branched or cyclic hydrocarbon structure with 3 to 20 carbon atoms. When specifying an alkyl group with a specific carbon number, it is intended to encompass all geometric isomers with that carbon number; therefore, for example, "butyl" means including n-butyl, sec-butyl, isobutyl, tert-butyl and cyclobutyl; "propyl" includes n-propyl, isopropyl and cyclopropyl. Alkyl examples include, but are not limited to methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, cyclobutyl, n-pentyl, isopentyl, neopentyl, cyclopentyl, methylcyclopentyl, ethylcyclopentyl, n-hexyl, isohexyl, cyclohexyl, n-heptyl, octyl, cyclopropyl, cyclobutyl, norbornyl, etc.

[0072] The term "substituted" includes the replacement of hydrogen atoms in the alkyl group by other groups, and the number of substitutions is determined by the number of hydrogen atoms.

[0073] Secondary batteries have been widely used in various products due to their advantages such as high energy density, long cycle life, safety and reliability. In recent years, with the significant increase in demand for secondary batteries as energy sources, higher requirements have been placed on the performance of secondary batteries, such as dynamic performance and cycle performance.

[0074] Graphite is the most commonly used negative electrode active material for secondary batteries, but graphite has poor kinetic performance, especially when discharged at a high rate, the capacity of the secondary battery decays rapidly.

[0075] Existing technologies have proposed various approaches to address these issues, such as increasing the secondary particle content in graphite, reducing graphite particle size to shorten the lithium ion diffusion path, and improving electrolyte viscosity. However, these improvements in kinetic performance can also introduce new challenges. The technical challenge addressed by this application is to balance the rate capability and cyclability of secondary batteries.

[0076] Based on the above considerations, in order to solve the problem that the comprehensive performance of secondary batteries needs to be improved, a secondary battery and an electrical device are obtained according to the above design concept and relevant experimental research.

[0077] The secondary battery provided in this application comprises a negative electrode plate and an electrolyte, wherein the negative electrode plate comprises a negative electrode active material, and the negative electrode active material comprises a graphitized carbon material, and the negative electrode active material satisfies the following conditions: (1.1) the specific surface area of ​​the negative electrode active material is 0.5 m 2 / g~3.0m 2 / g; (1.2) the volume distribution particle size Dv1 of the negative electrode active material is ≥1.5μm; and the electrolyte contains a carboxylic acid ester solvent and a film-forming stabilizer.

[0078] The graphitized carbon material in this application has the characteristics of a small specific surface area and a small amount of small-size particles as a negative electrode active material. The negative electrode active material with this characteristic is used in conjunction with an electrolyte containing a carboxylic acid ester solvent and a film-forming stabilizer. On the premise of improving the rate performance of the secondary battery, it can also reduce the impact on the battery cycle performance. Specifically, the specific surface area of ​​the negative electrode active material provided in this application is relatively small, which can effectively reduce the side reactions between it and the electrolyte, and prepare for the use of carboxylic acid ester solvents. This is because carboxylic acid ester solvents have the advantages of low viscosity and low surface tension. They are easier to diffuse to the positive and negative electrode surfaces, and by improving the wettability of the positive and negative electrode active materials to the electrolyte to accelerate the diffusion rate of lithium ions, thereby improving the rate performance of the battery. However, it also has obvious disadvantages: the probability of side reactions occurring on the positive and negative electrode surfaces is high, and gas production is high, which is not conducive to the cyclability of the battery. This application chooses to use a negative electrode active material with a relatively small specific surface area + a film-forming stabilizer to reduce the probability of side reactions. At the same time, the present application also controls the content of small-particle negative electrode active materials, and further reduces side reactions by reducing their usage. On the other hand, in the original negative electrode active material, the small-particle material will occupy the internal pores, making the lithium ion transmission path become tortuous, thereby increasing the actual path length of the lithium ions. Specifically, please refer to Figure 1 of the accompanying drawings of the specification, where Lt is the actual path length of lithium ions in the negative electrode active material layer. Combined with Figure 1, it can be seen that due to the presence of small-particle materials, Lt increases. The choice of controlling the content of small-particle negative electrode active materials in this application also plays a positive role in improving the rate performance of the battery. Therefore, the design method provided by this application is conducive to taking into account both the fast charging performance and the cycle performance of the secondary battery.

[0079] The secondary battery provided in the present application has improved rate capability and can be used for high current charge and discharge. For example, the secondary battery has high cycle stability, which can ultimately improve the user experience of the battery. The secondary battery may include an outer packaging. The outer packaging can be used to encapsulate electrode assemblies and electrolytes. The outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic. As plastics, polypropylene, polybutylene terephthalate, and polybutylene succinate can be listed.

[0080] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG2 shows a secondary battery 10 with a square structure as an example.

[0081] According to some embodiments of the present application, referring to Figure 3, the outer packaging may include a shell 101 and a cover plate 103. Among them, the shell 101 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 101 has an opening connected to the receiving cavity, and the cover plate 103 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 102 through a winding process or a lamination process. The electrode assembly 102 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 102. The number of electrode assemblies 102 contained in the secondary battery 10 can be one or more, and those skilled in the art can select according to specific actual needs.

[0082] The secondary battery provided in the present application is beneficial to improving battery performance. The secondary battery can be used as a power source for an electrical device or as an energy storage unit for an electrical device. The electrical device is used in the power field, such as mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited to the above-mentioned fields.

[0083] For ease of explanation, some embodiments of this application are described using a vehicle as an example. The negative electrode plate provided in this application is used in a vehicle battery. Due to the battery's high power performance, the vehicle starts quickly. Furthermore, the battery's high energy density allows for a longer driving range within the available space.

[0084] Please refer to Figure 4, which is a schematic structural diagram of the vehicle 10000 provided in some embodiments of the present application. The vehicle 10000 can be a fuel vehicle, a gas vehicle or a new energy vehicle. The new energy vehicle can be a pure electric vehicle, a hybrid vehicle or an extended-range vehicle, etc. A battery 1000 is provided inside the vehicle 10000, and the battery 1000 can be provided at the bottom, head or tail of the vehicle 10000. The battery 1000 can be used to power the vehicle 10000. For example, the battery 1000 can serve as an operating power source for the vehicle 10000. The vehicle 10000 may also include a controller 2000 and a motor 3000. The controller 2000 is used to control the battery 1000 to power the motor 3000, for example, for starting, navigating and driving the vehicle 10000.

[0085] In some embodiments of the present application, the battery 1000 can serve not only as an operating power source for the vehicle 10000, but also as a driving power source for the vehicle 10000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 10000.

[0086] Please refer to Figure 5, which is an exploded view of a battery 1000 provided in some embodiments of the present application. Battery 1000 includes a housing 200 and a battery cell 100. Conventional battery cells include primary or secondary batteries, but this application specifically protects secondary batteries. Battery cell 100 is housed within housing 200. Housing 200 is used to accommodate battery cell 100 and can adopt a variety of structures.

[0087] In some embodiments, the housing 200 may include a first portion 210 and a second portion 220. The first portion 210 and the second portion 220 overlap each other, and together define a storage space for accommodating the secondary battery 100. The second portion 220 may be a hollow structure with one end open, and the first portion 210 may be a plate-like structure. The first portion 210 overlaps the open side of the second portion 220, so that the first portion 210 and the second portion 220 together define the storage space. The first portion 210 and the second portion 220 may also be hollow structures with one end open, with the open side of the first portion 210 overlapping the open side of the second portion 220. Of course, the housing 200 formed by the first portion 210 and the second portion 220 may have various shapes, such as a cylinder, a rectangular parallelepiped, etc.

[0088] In the battery 1000, there may be multiple battery cells 100, and the multiple battery cells 100 may be connected in series, in parallel, or in a hybrid connection. A hybrid connection refers to a combination of series and parallel connections among the multiple battery cells 100. The multiple battery cells 100 may be directly connected in series, in parallel, or in a hybrid connection, and then the entire battery 100 structure may be housed within the housing 200. Of course, the battery 1000 may also be a battery module formed by first connecting multiple battery cells 100 in series, in parallel, or in a hybrid connection, and then the multiple battery modules 1000 are further connected in series, in parallel, or in a hybrid connection to form an entire battery 1000 structure, and then housed within the housing 200. The battery 1000 may also include other structures, for example, the battery 1000 may also include a busbar component for electrically connecting the multiple battery cells 100.

[0089] secondary batteries

[0090] The present application discloses, in some embodiments, a secondary battery comprising a negative electrode plate and an electrolyte, wherein the negative electrode plate comprises a negative electrode active material, the negative electrode active material comprises a graphitized carbon material, and the negative electrode active material satisfies the following conditions: (1.1) the specific surface area of ​​the negative electrode active material is 0.5 m 2 / g~3.0m 2 / g; (1.2) the volume distribution particle size Dv1 of the negative electrode active material is ≥1.5μm; and the electrolyte contains a carboxylic acid ester solvent and a film-forming stabilizer.

[0091] The graphitized carbon material of this application is a carbon material that primarily contains carbon. The carbon material includes graphite, which can be a single type of graphite, a mixture of multiple types of graphite, or a mixture of graphite and other carbon materials. Graphitized carbon materials have good electrical conductivity and stability, so this application selects them as the negative electrode active material.

[0092] The specific surface area of ​​this application includes any conventional meaning in the art, and can be obtained by testing using instruments or methods known in the art, such as using a gas adsorption method to test the specific surface area, specifically referring to the standard test of GB / T19587-2017.

[0093] The specific surface area of ​​the negative electrode active material selected in this application is 0.5 m 2 / g~3.0m 2 / g, compared with conventional carbon materials in the field, the specific surface area is reduced, which may be affected by the type and properties of the carbon material selected in this application. This application will discuss the types and preparation methods of carbon materials in detail below. This application discloses in these embodiments that the specific surface area of ​​the negative electrode active material can be 0.5m 2 / g, 0.8m 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.5m 2 / g, 3.0m 2 / g or any of the above range values. This application selects a specific surface area of ​​0.5m 2 / g~3.0m 2 / g of negative electrode active material can be conveniently used in combination with carboxylic acid ester solvents and film-forming stabilizers to improve the overall performance of the battery of the present application.

[0094] The volume distribution particle size Dv1 used in this application has the same meaning as the conventional Dv50 in the art, specifically referring to the volume of particles with a particle size smaller than this size that accounts for 1%. The measurement method for Dv1 can refer to Dv50, using conventional measurement methods in the art, such as using a particle size analyzer to measure the particle size distribution and then obtaining it statistically. In these examples, this application chooses to refer to the laser diffraction particle size analysis method for measurement, specifically referring to the standard GB / T19077-2016 to obtain a particle size distribution diagram, and then obtains it through statistics and calculation. In these examples, this application discloses that the volume of particles with a particle size less than 1.5μm accounts for 1%. This application controls the content of small-particle negative electrode active material by reducing its usage to further reduce side reactions. On the other hand, in the existing negative electrode active material, small-particle material will occupy the internal pores, making the lithium ion transmission path become curved, thereby increasing the actual path length of the lithium ions. Therefore, controlling the content of small-particle negative electrode active material also has a positive effect on improving the battery's rate capability.

[0095] The carboxylate solvents of the present application include those with carboxylate groups. Carboxylate solvents have the advantages of low viscosity and small surface tension. They are easier to diffuse to the surface of the positive and negative electrodes, and improve the wettability of the positive and negative electrode active materials to the electrolyte to accelerate the diffusion rate of lithium ions, thereby improving the rate capability of the battery. However, it also has obvious disadvantages: the probability of side reactions occurring on the positive and negative electrode surfaces is high, and a lot of gas is produced, which is not conducive to the cyclability of the battery. The present application selects carboxylate solvents and negative electrode active materials with relatively small specific surface area for use in combination, which is beneficial to taking into account both the fast charging performance and the cyclability of the secondary battery.

[0096] The film-forming stabilizer of the present application includes an additive that facilitates improving the reaction between the carboxylic acid ester solvent and the positive and negative electrode active materials, thereby improving the cycle stability of the battery and being used for high-rate charge and discharge by improving the stability of the interface film.

[0097] In summary, the graphitized carbon material in the present application has the characteristics of small specific surface area and small amount of small-size particles as a negative electrode active material. The negative electrode active material with this characteristic is used in combination with an electrolyte containing a carboxylic acid ester solvent and a film-forming stabilizer. On the premise of improving the rate performance of the secondary battery, it can also reduce the impact on the battery cycle performance.

[0098] In some embodiments of the present application, the specific surface area of ​​the negative electrode active material is 0.5 m 2 / g~2.0m 2 / g.

[0099] In these embodiments, the present application further adjusts the specific surface area of ​​the negative electrode active material to better match the electrolyte.

[0100] In some embodiments of the present application, the volume distribution particle size Dv1 of the negative electrode active material is 1.9 μm to 3.1 μm.

[0101] In these embodiments, the present application further clarifies the upper limit of the volume distribution particle size Dv1, that is, the volume or number of particles with a particle size less than or equal to 3.1 μm accounts for only 1%, and the volume or number of negative electrode materials with smaller particle sizes is controlled. On the one hand, it is used to match the specific value of the specific surface area of ​​the above-mentioned negative electrode active material, and on the other hand, it also prepares for improving the overall performance of the battery.

[0102] In some embodiments of the present application, the graphitized carbon material satisfies at least one of the following:

[0103] (2.1) The orientation index (OI) value of graphitized carbon materials is ≤ 24;

[0104] (2.2) The volume distribution particle size of the graphitized carbon material satisfies the following requirements: 7.5 μm ≤ Dv50 ≤ 20 μm, Dv10 ≥ 3.5 μm, and Dv90 ≤ 40 μm.

[0105] The orientation index OI value of the graphitized carbon material in this application includes the graphite orientation degree, which is mainly used to characterize the overall isotropic degree of graphite. By performing X-ray diffraction on the graphitized carbon material, the peak intensity values ​​of the face (110) and face (004) of the graphite crystal can be obtained, and the orientation index OI value can be obtained by calculating the peak integral area ratio of the two crystal faces. In general, the graphite orientation degree is affected by the structural characteristics of the material itself, and the smaller the value of the graphite orientation degree, the more conducive it is for lithium ions to diffuse inside it, and the problems such as the cyclic expansion of the battery will be better improved, but the orientation index OI value will also affect the compaction density and capacity of the battery. If it is too low, it will cause low compaction density and low capacity. Compared with the conventional graphitized carbon material used to improve the battery rate, the orientation index OI value of the material of this application is relatively higher. This is because the application also takes into account other performance of the battery, and it is necessary to take into account the actual improvement degree of the rate by the carboxylic acid ester solvent. The present application discloses in these embodiments that the orientation index OI value of the graphitized carbon material can be any one of 24, 22, 20, 18, 16, 14, 12, 10, 8 or any one of the values ​​within the above ranges.

[0106] The meanings of the volume distribution particle sizes Dv50, Dv10 and Dv90 of the graphitized carbon material of this application have the same meaning as the volume distribution particle size Dv1 of the above-mentioned negative electrode active material, wherein Dv50 includes 50% of the volume of particles with a particle size larger than it and 50% of the volume of particles with a particle size smaller than it, also known as the median diameter, which is usually used to represent the average particle size of the particles. Dv10 includes 10% of the volume of particles with a particle size smaller than it, and Dv90 includes 90% of the volume of particles with a particle size smaller than it. Whether it is Dv50, Dv10 or Dv90, the measurement and calculation methods are the same as above, such as using a particle size analyzer to measure the particle size distribution of the graphitized carbon material and then obtain it statistically. In these embodiments, this application chooses to refer to the laser diffraction particle size analysis method for determination, specifically referring to the standard GB / T19077-2016 to obtain the particle size distribution diagram, and then obtains it by calculation. In these embodiments, the present application discloses that the volume distribution particle size of the graphitized carbon material satisfies: 7.5μm≤Dv50≤20μm, Dv10≥3.5μm, Dv90≤40μm, that is, the average particle size of the graphitized carbon material is 7.5μm to 20μm. Compared with the conventionally used carbon materials, the particle size of the graphitized carbon material provided in this application is increased. The graphitized carbon material with this particle size conveniently meets the above-mentioned requirements for negative electrode active materials with relatively small specific surface areas. In these embodiments, the present application discloses that the Dv50 of the graphitized carbon material is any one of 7.5μm, 10μm, 12μm, 14μm, 16μm, 18μm, and 20μm or satisfies any of the above-mentioned range values.

[0107] In some embodiments of the present application, the graphitized carbon material satisfies at least one of the following:

[0108] (3.1) The orientation index of the graphitized carbon material satisfies: 10≤OI value≤22.

[0109] (3.2) The volume distribution particle size of the graphitized carbon material satisfies the following requirements: 10 μm ≤ Dv50 ≤ 18.5 μm, 3.5 μm ≤ Dv10 ≤ 7.0 μm, and Dv90 ≤ 40 μm.

[0110] The orientation index of the graphitized carbon material of the present application should not be too small. The present application selects an OI value of 10≤OI value≤22 for better matching with the electrolyte.

[0111] At the same time, the present application discussed above the control of the volume or quantity of small-particle negative electrode active materials, so the particle size uniformity of the graphitized carbon material has been relatively greatly improved, and this particle size improvement is obviously beneficial for reducing side reactions and improving rate capability.

[0112] In some embodiments of the present application, the negative electrode active material further includes amorphous carbon; the amorphous carbon includes either hard carbon or soft carbon.

[0113] The present application has discussed above that the negative electrode active material includes a graphitized carbon material, which may only include a graphitized carbon material, and the graphitized carbon material meets the above-mentioned requirements of having a small specific surface area and particle size. At the same time, the negative electrode active material may also include amorphous carbon, which has a relatively low degree of graphitization, such as hard carbon. On the one hand, hard carbon is combined with graphitized carbon material to meet the above-mentioned requirements for a small specific surface area and particle size. On the other hand, the use of hard carbon can improve the current distribution of the negative electrode, thereby improving the rate performance and cyclability of the graphitized material.

[0114] In some embodiments of the present application, the volume distribution particle size of the amorphous carbon satisfies: 3.5 μm≤Dv50≤7.0 μm.

[0115] The volume distribution particle size Dv50 of the amorphous carbon in this application has the same meaning as the volume distribution particle size Dv50 of the above-mentioned graphitized carbon material. Among them, Dv50 includes 50% of the volume of particles with a particle size greater than it and 50% of the volume of particles smaller than it, which is also called the median diameter and is usually used to represent the average particle size of the particles. The measurement and calculation methods are the same as above, for example, a particle size analyzer is used to measure the particle size distribution of the graphitized carbon material and then obtain it by statistics. In these embodiments, this application chooses to refer to the laser diffraction particle size analysis method for determination, specifically refers to the standard GB / T19077-2016 to obtain the particle size distribution diagram, and then obtains it by calculation. This application selects the volume distribution particle size of amorphous carbon to meet: 3.5μm≤Dv50≤7.0μm. Compared with the average particle size of the graphitized carbon material, the particle size of the amorphous carbon selected for use in this application is relatively small, but it cannot be too small, because it is consistent with the idea of ​​controlling the content of small-particle-size negative electrode materials in this application. At the same time, the relatively small particle size of amorphous carbon is combined inside the graphitized carbon material, which can average the porosity of the graphitized carbon material to improve the current distribution, thereby helping to further improve the rate capability of the battery.

[0116] In some embodiments of the present application, the graphitized carbon material includes one or more of natural graphite, artificial graphite, and composite graphite.

[0117] The composite graphite described herein can also be referred to as modified graphite, which is obtained by further modifying artificial graphite. In some embodiments, this application focuses on the graphitized carbon material being artificial graphite. The preparation method for this artificial graphite comprises: providing raw materials, performing crushing and shaping, granulation, graphitization, and surface roughening to obtain the artificial graphite material. The raw materials used in these embodiments can be one or more of green coke and calcined coke; preferably, the raw materials include one or more of needle-shaped green petroleum coke, non-needle-shaped green petroleum coke, needle-shaped coal-based green coke, non-needle-shaped coal-based green coke, calcined needle coke, and calcined petroleum coke. The crushing process described herein can be performed using devices and methods known in the art, such as a jet mill, mechanical mill, or roller mill. The crushing process typically produces a large number of undersized particles, and sometimes oversized particles. Therefore, after crushing, classification can be performed as needed to remove undersized and oversized particles from the crushed powder. Classification can produce granules with a well-defined particle size distribution, facilitating subsequent forming and / or granulation. Classification can be carried out by devices and methods known in the art, such as grading screens, gravity classifiers or centrifugal classifiers. The shaping of the present application can be carried out using equipment (such as a molding machine or other molding equipment) and methods known in the art. For example, the edges and corners of the obtained granular product are polished to facilitate subsequent operations and to make the obtained product have higher stability. The granulation of the present application includes using devices known in the art such as a granulator to carry out granulation. The granulator generally includes a stirred reactor and a temperature control module for the reactor. In addition, the volume median particle size of the obtained product can be controlled by adjusting the process conditions such as stirring speed, heating rate, granulation temperature and cooling rate during the granulation process. The graphitization treatment of the present application includes high-temperature graphitization treatment and low-temperature graphitization treatment. In some embodiments, any one or two of the high-temperature graphitization treatment and the low-temperature graphitization treatment can be appropriately selected for treatment according to actual specific needs. Alternatively, the high-temperature graphitization treatment and / or the low-temperature graphitization treatment can be repeated. Graphite with an appropriate degree of graphitization and graphite interlayer spacing can be obtained by high-temperature graphitization treatment. Graphite prepared at an appropriate graphitization temperature can obtain an appropriate degree of graphitization and graphite interlayer spacing, thereby enabling the composite artificial graphite to obtain higher structural stability and gram capacity. The surface roughening treatment of the present application includes conventional treatment methods in the art, such as physical methods.

[0118] This application discloses, in some embodiments, a method for preparing artificial graphite, comprising the following steps:

[0119] S1: The raw materials are fed to the crusher through a vibrating feeder for coarse crushing, and then the qualified materials are sent to the mechanical mill for fine crushing. The particles with a Dv50 of less than 1.5μm are removed by a grading screen, and the obtained particles are then polished to obtain a precursor with a Dv50 of 5μm to 25μm;

[0120] S2: Add the precursor obtained in step S1 to a reactor, and add 8% to 15% of a binder pitch (Dv50 of 5 μm to 8 μm) relative to the weight of the precursor for granulation for granulation. The stirring speed is 1000 r / min to 1500 r / min. The temperature is raised to 550°C to 570°C at a rate of 10°C / min at room temperature. The temperature is then maintained at this temperature for 5 to 10 hours. Granulation is performed until the Dv50 is 10 μm to 30 μm to obtain Intermediate 1.

[0121] S3: The intermediate 1 obtained in step S2 is added to a graphitization furnace, heated to 2100° C. to 2200° C. for graphitization treatment, and sieved with a 200-mesh sieve to obtain an artificial graphite having a volume distribution particle size satisfying the following conditions: 7.5 μm ≤ Dv50 ≤ 20 μm, Dv10 ≥ 3.5 μm, and Dv90 ≤ 40 μm.

[0122] In some embodiments of the present application, the graphitized carbon material has a core-shell structure, the core of the core-shell structure comprises one or more of natural graphite, artificial graphite, and composite graphite, and the shell of the core-shell structure comprises an amorphous carbon coating layer.

[0123] The present application discloses in these embodiments that the artificial graphite obtained in the above embodiments is further mixed with a coating agent in a certain mass ratio or preheated and then carbonized to obtain a graphitized carbon material with an amorphous carbon coating layer coated on the surface. The present application discloses in these embodiments that the coating agent includes but is not limited to conventional carbon source gas (volume percentage is 10% to 15%), such as methane, ethylene, acetylene, etc. The carbonization treatment of the present application is preferably carried out in an inert atmosphere furnace, and the temperature is controlled between 700°C and 1800°C. The graphitized carbon material with an amorphous carbon coating layer coated on the surface provided by the present application has a lower specific surface area and a more uniform particle size distribution than the uncoated carbon material, and the probability of side reactions is also reduced due to the isolation of the amorphous carbon coating layer.

[0124] In some embodiments of the present application, the mass percentage content of the graphitized carbon material is greater than 75% based on the total mass of the negative electrode active material. The present application discloses in these embodiments that the mass percentage content of the graphitized carbon material can be any one of 78%, 80%, 85%, 90%, 95%, and 100%.

[0125] In some embodiments of the present application, it is disclosed that the negative electrode active material only contains artificial graphite, and the weight percentage content of the artificial graphite is 100%.

[0126] In some embodiments, the present application discloses that the negative electrode active material includes artificial graphite and hard carbon, wherein the weight percentage of artificial graphite in the negative electrode active material is greater than 75%, and the rest is hard carbon.

[0127] The present application discloses, in some embodiments, a negative electrode active material having a core-shell structure, wherein the core of the core-shell structure comprises artificial graphite, and the shell of the core-shell structure comprises an amorphous carbon coating layer. The core-shell structure negative electrode active material finally formed is modified graphite, in which the weight percentage of artificial graphite in the negative electrode active material is greater than 75%, and the remainder is an amorphous carbon coating layer. The thickness of the amorphous carbon coating layer can be obtained by obtaining a transmission electron microscopy image of the negative electrode active material, and the content of graphitized carbon material in the negative electrode active material can be obtained based on the thickness of the amorphous carbon coating layer and in combination with the X-ray photoelectron spectroscopy analysis spectrum of the negative electrode active material. Among them, the method for obtaining the transmission electron microscopy image and the X-ray photoelectron spectroscopy analysis spectrum includes using conventional instruments and testing methods in the field.

[0128] In some embodiments of the present application, it is disclosed that the thickness of the amorphous carbon coating layer is between 10 nm and 20 nm. An amorphous carbon coating layer having such a thickness is beneficial for reducing the probability of side reactions.

[0129] The negative electrode active material of the present application is mixed with a conductive agent, a thickener, a binder, etc. in a certain mass ratio with a solvent to form a negative electrode slurry. After defoaming, the negative electrode slurry is evenly coated on one or both surfaces of the negative electrode current collector and dried, as shown in Figures 6 and 7. This results in a negative electrode sheet including a negative electrode film layer. The solvent herein includes, but is not limited to, water.

[0130] The conductive agent in this application includes but is not limited to one or a combination of two of graphite, superconducting carbon, carbon black (such as acetylene black, Ketjen black, Super P, etc.), carbon dots, carbon nanotubes, graphene and carbon nanofibers; the thickener includes cellulose and its sodium salt, cellulose includes methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, etc.; the binder includes but is not limited to polyvinyl alcohol, polyethylene glycol, sodium carboxymethyl cellulose, polyethylene oxide, polyacrylic acid, polyacrylamide, sodium alginate, styrene-butadiene rubber (SBR), etc.

[0131] The present application discloses in some embodiments a mass ratio of the negative electrode active material, the conductive agent, the thickener, and the binder: (90-97): (0.5-3): (0.5-3): (0.5-3).

[0132] In some embodiments, the present application discloses that the negative electrode current collector includes, but is not limited to, a metal foil or a composite current collector. The metal foil may be a copper foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material, such as copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys, on a polymer material substrate, such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0133] In some embodiments of the present application, the carboxylate solvent satisfies at least one of the following conditions:

[0134] (4.1) Carboxylic acid esters have the following structural formula:

[0135] R1-COO-R2;

[0136] wherein R1 and R2 each independently comprise a substituted and / or unsubstituted C1-C5 alkyl group;

[0137] (4.2) Based on the mass of the electrolyte, the mass percentage content of the carboxylic acid ester is 5% to 70%.

[0138] The solvents of this application include carboxylic acid esters, which are relatively stable, resistant to oxidation and reduction, and highly compatible with the aforementioned lithium salts and additives. Furthermore, the inclusion of carboxylic acid esters in the solvent can result in lower viscosity and surface tension in the electrolyte. Combined with lithium-containing transition metal phosphates, which possess relatively more lithium ion diffusion channels, these carboxylic acid esters facilitate effective contact between the lithium-containing transition metal phosphate and the electrolyte, thereby improving the battery's rate capability. The carboxylic acid esters selected in these embodiments are primarily linear carboxylic acid esters, as they have lower melting points and viscosities, allowing for better compatibility with the lithium-containing transition metal phosphates used as the positive electrode active material. These embodiments also disclose a carboxylic acid ester content of 5% to 70% by weight, wherein the carboxylic acid ester content can be any of 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%, or any value within the aforementioned ranges.

[0139] In some embodiments of the present application, the carboxylate solvent satisfies at least one of the following conditions:

[0140] (5.1) Carboxylic acid esters include one or more of ethyl acetate, methyl acetate, ethyl propionate, propyl acetate, methyl propionate, and methyl butyrate;

[0141] (5.2) Based on the mass of the electrolyte, the mass percentage content of the carboxylic acid ester is 10% to 60%.

[0142] Examples of carboxylic acid esters include one or more of ethyl acetate, methyl acetate, ethyl propionate, propyl acetate, methyl propionate, and methyl butyrate. Carboxylic acid ester solvents have relatively low viscosities, and when used in combination, multiple carboxylic acid ester solvents can result in an electrolyte having a lower surface tension.

[0143] In some embodiments of the present application, the electrolyte further includes a carbonate solvent, and the carbonate solvent includes one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0144] The lithium salt selected in this application has better solubility in carbonates than in carboxylates. Therefore, this application chooses to use carboxylates and carbonates in combination to reduce the viscosity and melting point of the electrolyte without affecting the solubility of the lithium salt, thereby facilitating use with lithium-containing transition metal phosphates.

[0145] As examples of carbonates, carbonates include one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Selecting the carbonates exemplified is beneficial for better coordination with the carboxylate.

[0146] In some embodiments of the present application, the film-forming stabilizer satisfies at least one of the following:

[0147] (6.1) In some embodiments of the present application, the mass percentage content of the film-forming stabilizer is 0.02% to 6% based on the total mass of the electrolyte;

[0148] (6.2) The film-forming stabilizer comprises one or more of a boron-containing lithium salt, a phosphorus-containing lithium salt, and a sulfur-containing lithium salt;

[0149] (6.3) boron-containing lithium salts including one or more of lithium tetrafluoroborate (LiBF4), lithium bis(oxalatoborate) (LiBOB), and lithium bis(fluorooxalatoborate) (LiDFOB);

[0150] (6.4) Phosphorus-containing lithium salts include one or more of lithium difluorophosphate (LiPO2F2), lithium fluorophosphate (Li2PO3F), and lithium phosphate (Li3PO4);

[0151] (6.5) The sulfur-containing lithium salt includes one or more of lithium fluorosulfonate (LiFSO3), lithium sulfate (Li2SO4), and lithium aminosulfonate (LiSO3NH2).

[0152] The content of the film-forming stabilizer in this application is controlled to be 0.02% to 6% based on the solubility of various types and the actual degree of improvement in film formation in the electrolyte. This application discloses in these embodiments that the mass percentage content of the film-forming stabilizer can be any one of 0.02%, 0.05%, 0.08%, 0.1%, 0.15%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, 3%, 4%, 5%, 6%, or any one of the above ranges.

[0153] In the present application, boron-containing lithium salt is a lithium salt with B atom as the central atom, which can be coordinated with alkoxy, o-diphenol, o-hydroxyl, carboxylic acid to form anionic complexes, and the anionic complexes are mainly large π conjugated structures, and the negative charge distribution of the central ion is relatively dispersed, and its charge is delocalized. At the same time, the anionic radius is relatively large, making it difficult for the anion to form an ion pair with a strong binding force with the lithium ion in an organic solvent, and its solubility is relatively good. The more electron-withdrawing groups there are in the anionic complex, the more stable the anionic structure, and the higher the solubility of the lithium ion in the electrolyte, the more conducive to improving the electrical conductivity of the electrolyte. And boron-containing lithium salt can form an SEI film with excellent performance on the surface of the negative electrode active material, and the SEI film has organic solvent insolubility, can stably exist in organic electrolyte, can effectively reduce the solvent molecules in the electrolyte and embed them into the negative electrode active material, so as to ensure the structural stability of the negative electrode active material, and then improve the cycle performance of the secondary battery.

[0154] The phosphorus-containing lithium salt additive in this application has a relatively large anionic group and a high ionic conductivity, which is conducive to further improving the kinetic performance of the electrolyte. The phosphorus-containing lithium salt additive can form a CEI film on the surface of the positive electrode active material. The CEI film formed has high lithium ion conductivity, which can significantly inhibit the continuous decomposition of the electrolyte and reduce the dissolution of transition metal ions in the positive electrode active material, thereby improving the cycle performance of the secondary battery. In addition, the phosphorus-containing lithium salt additive can also form an SEI film on the surface of the negative electrode active material, which has a low interfacial impedance, thereby significantly improving the cycle performance of the battery.

[0155] The sulfur-containing lithium salt additive in this application has good antioxidant properties, high thermal stability, is insensitive to water in the electrolyte, and is not prone to side reactions; and the sulfur-containing lithium salt additive has a relatively high conductivity, which is beneficial to improving the migration rate of lithium ions, thereby improving the kinetic performance of the electrolyte.

[0156] As examples of boron-containing lithium salts, the boron-containing lithium salts may include one or more of lithium tetrafluoroborate (LiBF4), lithium bis(oxalatoborate) (LiB(C2O4)2, abbreviated as LiBOB), and lithium bis(oxalatoborate) (LiBC2O4F2, abbreviated as LiDFOB). Similarly, as examples of phosphorus-containing lithium salts, the phosphorus-containing lithium salts may include one or more of lithium difluorophosphate (LiPO2F2), lithium fluorophosphate (Li2PO3F), and lithium phosphate (Li3PO4). As examples of sulfur-containing lithium salts, the sulfur-containing lithium salts may include one or more of lithium fluorosulfonate (LiFSO3), lithium sulfate (Li2SO4), and lithium sulfamate (LiSO3NH2).

[0157] In some embodiments of the present application, the film-forming stabilizer comprises lithium fluorosulfonate and lithium difluorophosphate. Based on the total mass of the electrolyte, the mass percentage content of lithium difluorophosphate is ≤1%, and the mass percentage content of lithium fluorosulfonate is greater than or equal to 0.1% and less than 2%.

[0158] The present application selects lithium fluorosulfonate and lithium difluorophosphate as film-forming stabilizers and adds them to the electrolyte to better match the above-mentioned lithium salts. On the other hand, the solubility of each film-forming stabilizer in the solvent is certain. For example, the solubility of lithium difluorophosphate in organic solvents is not high. For example, its content in carbonate solvents is about 1%, and its content in carboxylates such as ethyl acetate is about 3% to 4%. Continuing to increase its content will easily cause the electrolyte to become turbid. In these embodiments, the present application controls the mass percentage content of lithium difluorophosphate to be ≤1%, and the mass percentage content of lithium fluorosulfonate to be greater than or equal to 0.1% and less than 2%.

[0159] In addition to the above-mentioned carboxylic acid ester solvent and film-forming stabilizer, the electrolyte of the present application also contains a lithium salt, which includes but is not limited to one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0160] The specific components and contents of the electrolyte of the present application can be determined by methods known in the art. For example, they can be determined by gas chromatography, gas chromatography-mass spectrometry (GC-MS), ion chromatography (IC), liquid chromatography (LC), nuclear magnetic resonance spectroscopy (NMR), inductively coupled plasma optical emission spectrometry (ICP-OES), etc.

[0161] In some embodiments of the present application, the secondary battery further comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode active material, and the specific surface area of ​​the positive electrode active material is 8m 2 / g~17m 2 / g.

[0162] The specific surface area in this application includes any conventional meaning in the art, and can be obtained by testing using instruments or methods known in the art, such as using a gas adsorption method to test the specific surface area, specifically referring to the standard test of GB / T19587-2017.

[0163] The specific surface area of ​​the positive electrode active material selected in this application is relatively large. The positive electrode active material with this specific surface area is used in combination with the negative electrode active material protected in this application. Under the premise of reducing gas production, the rate performance and cyclability of the battery can be taken into account. In these embodiments, this application discloses that the specific surface area of ​​the positive electrode active material can be 8m 2 / g、8.5m 2 / g、9m 2 / g, 9.5m 2 / g、10m 2 / g、12m 2 / g、14m 2 / g、16m 2 / g、18m 2 / g or any of the values ​​satisfying the above ranges.

[0164] In some embodiments of the present application, the positive electrode active material includes a lithium-containing transition metal phosphate.

[0165] The positive electrode active material selected in this application includes a lithium-containing transition metal phosphate, chosen to meet the aforementioned specific surface area requirements. However, lithium-containing transition metal phosphates also suffer from the significant drawback of high yield rates when used as positive electrode active materials in secondary batteries. Using the negative electrode and electrolyte in combination with this application can help alleviate this issue.

[0166] The lithium-containing transition metal phosphate in this application comprises olivine-type lithium manganese iron phosphate, the chemical formula of which is Li (1-x1) Fe x2 Mn (1-x2) PO4, where 0≤x1<1, 0≤x2≤1. The battery formed by assembling the positive electrode active material having the above chemical formula in the present application is accompanied by lithium deintercalation and consumption during the charge and discharge process. The molar content of lithium in the battery varies when discharged to different states. The above definition includes the lithium content of the battery in different charge and discharge states at a voltage of 2V to 5V.

[0167] The positive electrode sheet of the present application includes a positive electrode current collector and a positive electrode film layer, and the positive electrode film layer can be formed on one or both surfaces of the positive electrode sheet. In addition to the above-mentioned positive electrode active material, the positive electrode film layer also includes a positive electrode conductive agent and a positive electrode binder. The present application discloses in these embodiments that the mass ratio satisfies: lithium-containing transition metal phosphate: positive electrode conductive agent: positive electrode binder is (90% to 98%): (0.5% to 5%): (0.5% to 5%). Among them, the positive electrode conductive agent includes but is not limited to one or more combinations of graphite, superconducting carbon, carbon black (such as acetylene black, Ketjen black, Super P, etc.), carbon dots, carbon nanotubes, graphene and carbon nanofibers. The positive electrode binder includes but is not limited to polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorine-containing acrylate resin, styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid, carboxymethyl chitosan, etc., or a combination of two or more.

[0168] In some embodiments of the present application, a method for preparing a positive electrode sheet comprises: mixing each component with a solvent in a certain mass ratio to obtain a positive electrode slurry, coating the positive electrode slurry on one or both surfaces of a positive electrode current collector, heating, drying, and cooling to obtain a positive electrode sheet comprising a positive electrode film layer. In these embodiments, the present application discloses that the heating and baking temperature is 90°C to 120°C, for example, it can be 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, etc. By regulating the baking temperature, the positive electrode active material and additives can be prevented from thermal decomposition due to overheating while promoting the volatilization of the solvent and solidification of the slurry. At the same time, the solvent used to prepare the positive electrode slurry includes one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), dimethyl sulfoxide (DMSO), pyridine, and tetrahydrofuran (THF). By selecting these solvents, the stability and uniformity of the positive electrode active material in the slurry can be improved. At the same time, the solubility of additives in the slurry can be increased, solving problems such as slurry instability, easy stratification, and sedimentation. The selected solvent is a non-aqueous system, which can solve the problem of additive hydrolysis caused by heat.

[0169] The present application also discloses in some embodiments that the positive electrode current collector can be a metal foil or a composite current collector, wherein the metal foil can be an aluminum foil, and the composite current collector can include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector can be formed by forming a metal material such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy on a polymer material substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0170] Isolation film

[0171] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0172] In some embodiments, the isolation membrane includes a base material layer and a coating provided on the surface of the base material layer; the base material of the base material layer includes one or more of polyethylene, polypropylene, poly(p-phenylene terephthalamide), polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, and polyamide; the coating includes a ceramic coating and / or a polymer coating. The base material layer has a good permeability to lithium ions, which is conducive to the migration of lithium ions; the surface of the base material layer is provided with a coating, which can further improve the mechanical properties of the isolation membrane. Optionally, the ceramic particles in the ceramic coating include one or more of SiO2, Al2O3, AlOOH, CaO, TiO2, MgO, ZnO, ZrO2, Mg(OH)2, and BaSO4. Optionally, the polymer material of the polymer coating includes one or more of polyethylene (PE), polypropylene (PP), poly(p-phenylene terephthalamide) (PPTA), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyimide (PI), and polyamide (PA). The polymer coating layer and the substrate layer may be made of the same or different materials, and the thickness of the polymer coating layer and the substrate layer may be different. Optionally, the thickness of the polymer coating layer is less than the thickness of the substrate layer.

[0173] In other embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0174] Example

[0175] The following examples describe the present disclosure in more detail. These examples are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing. The instruments used in the examples are commercially available.

[0176] Example 1

[0177] 1. Preparation of negative electrode active material - artificial graphite

[0178] S1: The needle-shaped green petroleum coke raw material is fed to the air flow crusher through a vibrating feeder for coarse crushing. The qualified material is then sent to a mechanical mill for fine crushing. A grading screen is used to remove particles with a Dv50 of less than 1.1 μm. The resulting particles are then polished to obtain a precursor with a Dv50 of 15 μm.

[0179] S2: Add the precursor obtained in step S1 to a reactor, and add 10% of a binder pitch (Dv50 of 7.5 μm) relative to the weight of the precursor for granulation for granulation. The stirring speed is 1200 r / min, and the temperature is increased to 560°C at a rate of 10°C / min at room temperature. The temperature is then maintained for 8 hours, and granulation is performed until the Dv50 is 20 μm to obtain intermediate 1.

[0180] S3: The intermediate 1 obtained in step S2 is added to a graphitization furnace, heated to 2100°C to 2150°C for graphitization, and sieved with a 200-mesh sieve to obtain artificial graphite with a Dv50 of 18.5 μm, a Dv90 of 30 μm, a Dv10 of 6.5 μm, and a Dvl of 3 μm. The specific surface area of ​​the artificial graphite is 0.52 m 2 / g, and the orientation index OI value of the artificial graphite is shown in Table 1.

[0181] 2. Preparation of negative electrode sheet:

[0182] The artificial graphite prepared above was dispersed in deionized water with a mass ratio of 94.5:1.5:1.5:2.5 with a conductive agent, conductive carbon, stabilizer sodium carboxymethyl cellulose and binder SBR to form a negative electrode slurry. The negative electrode slurry was evenly coated on both sides of the negative electrode current collector copper foil, and dried in a nine-section oven with the temperature settings of 100°C / 100°C / 95°C / 85°C / 85°C / 80°C / 80°C / 80°C / 60°C. The mixture was then compacted using a cold press to make the single-sided coating weight of the negative electrode film layer 0.15g / 1540.25mm 2 .

[0183] 3. Preparation of electrolyte:

[0184] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate and dimethyl carbonate were mixed in a volume ratio of 1:1 to obtain a first solvent, and ethyl acetate and propyl acetate were mixed in a volume ratio of 2:1 to obtain a second solvent. The first solvent and the second solvent were mixed in a certain amount to form a non-aqueous solvent, lithium hexafluorophosphate was added to the non-aqueous solvent, and then film-forming stabilizers (lithium fluorosulfonate and lithium difluorophosphate) were added to form an electrolyte. The content of each component in the electrolyte is shown in Table 2.

[0185] 4. Preparation of positive electrode sheet:

[0186] Lithium iron manganese phosphate (chemical formula LiFe 0.5 Mn 0.5 PO4, with a BET surface area of ​​16.7 m 2 / g): positive electrode conductive agent carbon black: positive electrode binder polyvinylidene fluoride (PVDF) are dispersed in an appropriate amount of solvent NMP at a ratio of 98:0.9:1.1 and stirred thoroughly to form a uniform positive electrode slurry; the positive electrode slurry is coated on both surfaces of the positive electrode current collector, heated and dried in a multi-section oven with the temperature settings of 120℃ / 100℃ / 90℃, and then compacted using a cold press to obtain a positive electrode sheet with a compaction density of 2.7g / cm 3 The positive electrode sheet has a single-sided coating weight of 0.34g / 1540.25mm 2 .

[0187] 5. Isolation film;

[0188] A porous polyethylene (PE) film is used as the separator.

[0189] 6. Preparation of secondary batteries:

[0190] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive electrode sheet and the negative electrode sheet to serve as an isolation, and then wound to obtain an electrode assembly; the electrode assembly is placed in an outer packaging shell, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0191] Example 2

[0192] A secondary battery is provided. The difference between the secondary battery and Example 1 lies in the different selection of the particle size and specific area of ​​the artificial graphite, which is ultimately reflected in the different preparation method of the artificial graphite, as follows:

[0193] S1: Feed the needle-shaped coal-based raw tar raw material through a vibrating feeder to a jet crusher for coarse crushing. Then, the qualified material is sent to a mechanical mill for fine crushing. A grading screen is used to remove particles with a Dv50 of less than 1.0 μm. The obtained particles are then polished to obtain a precursor with a Dv50 of 13 μm.

[0194] S2: The precursor obtained in step S1 was added to a reactor, and 10% of a binder pitch (Dv50 of 8 μm) relative to the weight of the precursor for granulation was added for granulation. The stirring speed was 1000 r / min, and the temperature was increased to 550°C at a rate of 10°C / min at room temperature. The temperature was maintained at this temperature for 8 hours, and granulation was performed until the Dv50 was 17.1 μm to obtain Intermediate 1.

[0195] S3: The intermediate 1 obtained in step S2 is added to a graphitization furnace, heated to 2150°C to 2180°C for graphitization, and sieved with a 200-mesh sieve to obtain artificial graphite with a Dv50 of 15.4 μm, a Dv90 of 35.6 μm, a Dv10 of 3.5 μm, and a Dvl of 2.5 μm. The specific surface area of ​​the artificial graphite is 1.83 m 2 The orientation index OI value of the artificial graphite is shown in Table 1.

[0196] Example 3

[0197] A secondary battery is provided. The difference between the secondary battery and Example 1 lies in the different selection of the particle size and specific area of ​​the artificial graphite, which is ultimately reflected in the different preparation method of the artificial graphite, as follows:

[0198] S1: The needle-shaped green petroleum coke raw material is fed to the air flow crusher through a vibrating feeder for coarse crushing. The qualified material is then sent to a mechanical mill for fine crushing. A grading screen is used to remove particles with a Dv50 of less than 0.8μm. The obtained particles are then polished to obtain a precursor with a Dv50 of 8.5μm.

[0199] S2: The precursor obtained in step S1 was added to a reactor, and 12% of a binder pitch (Dv50 of 7.5 μm) relative to the weight of the precursor for granulation was added for granulation. The stirring speed was 1300 r / min, and the temperature was increased to 570°C at a rate of 10°C / min at room temperature. The temperature was maintained at this temperature for 8 hours, and granulation was performed until the Dv50 was 10.4 μm to obtain intermediate 1;

[0200] S3: The intermediate 1 obtained in step S2 is added to a graphitization furnace, heated to 2100°C to 2150°C for graphitization, and sieved with a 200-mesh sieve to obtain artificial graphite with a Dv50 of 7.8 μm, a Dv90 of 22.3 μm, a Dv10 of 3.5 μm, and a Dvl of 1.9 μm. The specific surface area of ​​the artificial graphite is 2.5 m2 The orientation index OI value of the artificial graphite is shown in Table 1.

[0201] Example 4-1

[0202] A secondary battery is provided, which differs from Example 1 in that the negative electrode active material comprises the artificial graphite of Example 1 and further comprises hard carbon. Particles with a volume distribution particle size of less than 1.5 μm are removed from the hard carbon, and then the hard carbon is mixed with the artificial graphite.

[0203] The Dv50 of the hard carbon is 7.0 μm. The mass percentage of the hard carbon in the negative electrode active material is 20%, and the remaining 80% is the artificial graphite of Example 1. The specific surface area of ​​the negative electrode active material is 0.83 m 2 / g.

[0204] Example 4-2

[0205] A secondary battery is provided, which is different from Example 4-1 in that the mass percentage of hard carbon in the negative electrode active material is 10%, and the remaining 90% is the artificial graphite of Example 1, and the specific surface area of ​​the negative electrode active material is 0.68m 2 / g.

[0206] Example 4-3

[0207] A secondary battery is provided, which is different from Example 4-1 in that the mass percentage of hard carbon in the negative electrode active material is 5%, and the remaining 95% is the artificial graphite of Example 1, and the specific surface area of ​​the negative electrode active material is 0.60m 2 / g.

[0208] Example 4-4

[0209] A secondary battery is provided, which is different from Example 4-1 in that the Dv50 of the hard carbon in the negative electrode active material is 5.0 μm, and the specific surface area of ​​the negative electrode active material is 0.90 m 2 / g.

[0210] Examples 4-5

[0211] A secondary battery is provided, which is different from Example 4-1 in that the Dv50 of the hard carbon in the negative electrode active material is 3.5 μm, and the specific surface area of ​​the negative electrode active material is 1.0 m 2 / g.

[0212] Example 5

[0213] A secondary battery is provided, which differs from Example 1 in that the negative electrode active material comprises the artificial graphite of Example 1 as a raw material, is placed in an inert atmosphere furnace, methane gas is introduced into the inert atmosphere (the volume percentage of methane gas is 10%, and the gas flow rate is 1.0 L / min), the temperature is controlled to be about 1000° C., and the reaction is carried out for 3 to 4 hours, thereby forming an amorphous carbon coating layer on the surface of the artificial graphite. The thickness of the carbon coating layer is nanometer-scale, about 10 nm, and the content of artificial graphite is about 80%, forming a carbon material with a core-shell structure, and the volume distribution particle size of the carbon material with the core-shell structure satisfies: Dv50 is 19 μm, Dv90 is 30.2 μm, Dv10 is 6.8 μm, and Dvl is 3 μm. The specific surface area of ​​the carbon material is 0.50 m 2 / g, and the orientation index OI value of the carbon material is shown in Table 1.

[0214] Embodiments 6 to 12 provide a secondary battery, wherein the negative electrode active material of the secondary battery is the same as that of embodiment 1.

[0215] Table 1-1 Negative electrode active material properties

[0216] Table 1-2 Graphitized carbon material properties

[0217] Examples 6 to 10

[0218] A secondary battery is provided. This secondary battery differs from Example 1 in that the electrolyte is different, as detailed in Table 2. Specifically, the carboxylate content and carbonate content in Examples 6 and 7 are different from those in Examples 1 to 5, and the content and type of film-forming stabilizer are different from those in Examples 1 to 5. The type of carboxylate in Examples 8 and 9 is different from that in Examples 1 to 5. The type of film-forming stabilizer in Example 10 is different from that in Examples 1 to 5.

[0219] Table 2 Electrolyte performance list

[0220] Example 11

[0221] A secondary battery is provided, which is different from Example 1 in that the specific surface area of ​​the positive electrode active material is 10 m 2 / g, see Table 2 for details.

[0222] Example 12

[0223] A secondary battery is provided, which is different from Example 1 in that the specific surface area of ​​the positive electrode active material is 8 m 2 / g, see Table 2 for details.

[0224] Comparative Example 1

[0225] A secondary battery is provided, which is different from that of Example 1 in that a battery with a specific surface area of ​​5.0 m 2 / g of artificial graphite is used as the negative electrode active material, and the small particle size materials in the artificial graphite are not controlled, so that it contains particles with a particle size of less than 1.5μm, and the volume content of this part of particles accounts for about 1%, see Tables 1-1, 1-2 and 2 for details.

[0226] Comparative Example 2

[0227] A secondary battery is provided. The difference between the secondary battery and Example 1 is that the electrolyte contains only carbonate solvents and no carboxylate solvents are used. See Tables 1-1, 1-2 and 2 for details.

[0228] Comparative Example 3

[0229] A secondary battery is provided. The difference between the secondary battery and Example 1 is that the electrolyte does not contain a film-forming stabilizer. See Tables 1-1, 1-2 and 2 for details.

[0230] [Performance test of negative electrode active materials]

[0231] ①Test volume distribution particle size:

[0232] With reference to GB / T 19077-2016, obtain the volume particle size distribution curve of the negative electrode active material. Take the particle size corresponding to the cumulative volume distribution percentage reaching 50% as the average particle size Dv50, the particle size corresponding to the volume distribution percentage reaching 90% as Dv90, the particle size corresponding to the volume distribution percentage reaching 10% as Dv10, and the particle size corresponding to the volume distribution percentage reaching 1% as Dv1. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0233] Specifically, a laser particle size analyzer (Malvern 3000, MasterSizer 3000) is used for testing, and a helium-neon red light source is used as the main light source. Take a clean small beaker and add 1g of the sample to be tested, add 20mL of deionized water (the sample concentration ensures that the light shielding is 8-12%), add a drop of surfactant to reduce the surface tension of the water to facilitate the infiltration of the particles, and ultrasonicate at 53KHz / 120W for 5 minutes to ensure that the sample is completely dispersed. Turn on the laser particle size analyzer, clean the optical path system, and automatically test the background. Stir the ultrasonicated solution to be tested to make it evenly dispersed, put it into the sample cell as required, and start measuring the particle size. The measurement results can be read from the instrument.

[0234] ②Test specific surface area;

[0235] The specific surface area of ​​the negative electrode active material and / or positive electrode active material is determined by nitrogen adsorption surface area analysis according to GB / T 19587-2004, and the specific surface area is calculated using the Brunauer Emmett Teller (BET) method. The testing instrument may be a TRISTAR II 3020 Surface Area and Porosity Analyzer from Micromeritics, Inc., USA.

[0236] ③Test orientation index OI value;

[0237] The graphitized carbon material is placed on an X-ray powder diffractometer for detection to obtain the peak intensity values ​​of the surface (110) and the surface (004) of the graphite crystal. By calculating the peak integral area ratio of the two crystal planes, the orientation index OI value can be obtained.

[0238] [Battery performance test]

[0239] ④High temperature gas production test:

[0240] The prepared battery was charged to 100% SOC at a rate of 0.1C and stored at 60°C. The internal pressure of the battery was monitored online using a barometer. When the cutoff pressure reached 0.35 MPa, the number of days stored at 60°C was recorded. The test results are shown in Table 3.

[0241] ⑤ Fast charging capability test:

[0242] The batteries of the above embodiments and comparative examples were charged and discharged for the first time at a current of 1C (i.e., the current value at which the theoretical capacity is completely discharged within 1 hour). Specifically, at 35°C, the batteries were charged at a constant current rate of 1C to a voltage of 4.4V, then charged at a constant voltage to a current of ≤0.05C, allowed to stand for 5 minutes, and then discharged at a constant current rate of 0.33C to a voltage of 2.8V. The actual capacity was recorded as C0. Then the battery is charged with a constant current of 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, 3.0C0, 3.5C0, 4C0, 4.5C0, and 5C0 in sequence to the full battery charge cut-off voltage of 4.4V or the negative electrode cut-off potential of 0V (whichever is reached first). After each charge is completed, it is necessary to discharge with 1C0 to the full battery discharge cut-off voltage of 2.8V. Record the state of charge (SOC) at different charge rates to 10%, 20%, 30%, ..., 80%. Charge, state of charge, when "SOC = 0" means the battery is fully discharged, when "SOC = 100%" means the battery is fully charged) the corresponding negative electrode potential, draw the charge rate negative electrode potential curve under different SOC states, after linear fitting, the charge rate corresponding to the negative electrode potential of 0V under different SOC states is obtained, and the charge rate is the charging window under the SOC state, which is recorded as C10% SOC, C20% SOC, C30% SOC, C At 40% SOC, C50% SOC, C60% SOC, C70% SOC, and C80% SOC, the charging time T (in minutes) for charging the battery from 10% SOC to 80% SOC is calculated using the formula (60 / C20% SOC + 60 / C30% SOC + 60 / C40% SOC + 60 / C50% SOC + 60 / C60% SOC + 60 / C70% SOC + 60 / C80% SOC) × 10%. The shorter this time, the better the battery's fast charging performance.

[0243] ⑥Cycling performance test:

[0244] At 45°C, the secondary batteries prepared in the Examples and Comparative Examples were charged to 3.75V at a constant current of 1C (i.e., the current value that completely discharges the theoretical capacity within 1 hour). They were then charged at a constant voltage of 3.75V to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 1C to 2.5V, allowed to stand for 30 minutes. This constitutes one charge-discharge cycle, and the battery capacity at this point is recorded as C0. The battery was charged and discharged n times in this manner, and the battery capacity after n cycles was recorded as C1. The battery's cycle capacity retention at 25°C is then C1 / C0 × 100%. The number of cycles n corresponding to the measured cycle capacity retention of 80% was recorded.

[0245] Table 3 Battery performance list

[0246] Combined with the embodiment of Table 3 and comparative example 1, if the specific surface area of ​​the negative electrode active material is not controlled, and the active material content of the small particles is not controlled, the cycle stability and fast charging performance of the secondary battery cannot be taken into account. Combined with comparative examples 2 and 3, it can be seen that when the electrolyte does not contain a carboxylic acid ester solvent, the battery's rate capability is poor, and when it does not contain a film-forming stabilizer, the battery's cycle stability deteriorates. Therefore, the present application controls the negative electrode active material so that the graphitized carbon material as the negative electrode active material has the characteristics of a small specific surface area and a small amount of small-particle matter. The negative electrode active material with this characteristic is used in conjunction with an electrolyte containing a carboxylic acid ester solvent and a film-forming stabilizer. On the premise of improving the rate capability of the secondary battery, it can also reduce the impact on the battery cycle performance.

[0247] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application, and they should all be included in the scope of the claims and specification of the present application. In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments can be combined in any way. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

1. A secondary battery, characterized in that: include: Negative electrode sheet: contains negative electrode active material, and the negative electrode active material meets the following conditions: (1.1) The specific surface area of ​​the negative electrode active material is 0.5 m 2 / g~3.0m 2 / g; (1.2) The volume distribution particle size Dv1 of the negative electrode active material is ≥ 1.5 μm; The negative electrode active material comprises a graphitized carbon material; Electrolyte: Contains carboxylic acid ester solvent and film-forming stabilizer.

2. The secondary battery according to claim 1, wherein: The specific surface area of ​​the negative electrode active material is 0.5 m 2 / g~2.0m 2 / g; and / or; The volume distribution particle size Dv1 of the negative electrode active material is 1.9 μm to 3.1 μm.

3. The secondary battery according to any one of claims 1 to 2, characterized in that: The graphitized carbon material satisfies at least one of the following requirements: (2.1) The orientation index OI value of the graphitized carbon material is ≤24; the orientation index OI value is the peak intensity value of the plane (110) and the plane (004) in the X-ray powder diffraction pattern of the graphitized carbon material; (2.2) The volume distribution particle size of the graphitized carbon material satisfies the following requirements: 7.5 μm ≤ Dv50 ≤ 20 μm, Dv10 ≥ 3.5 μm, and Dv90 ≤ 40 μm.

4. The secondary battery according to any one of claims 1 to 3, characterized in that: The graphitized carbon material satisfies at least one of the following requirements: (3.1) The orientation index of the graphitized carbon material satisfies: 10≤OI value≤22; (3.2) The volume distribution particle size of the graphitized carbon material satisfies the following requirements: 10 μm ≤ Dv50 ≤ 18.5 μm, 3.5 μm ≤ Dv10 ≤ 7.0 μm, and Dv90 ≤ 40 μm.

5. The secondary battery according to any one of claims 1 to 4, characterized in that: The negative electrode active material further comprises amorphous carbon; the amorphous carbon comprises one or both of hard carbon and soft carbon; and / or; The volume distribution particle size of the amorphous carbon satisfies: 3.5 μm≤Dv50≤7.0 μm; and / or; The graphitized carbon material includes one or more of natural graphite, artificial graphite, and composite graphite.

6. The secondary battery according to any one of claims 1 to 5, characterized in that: The graphitized carbon material has a core-shell structure, the core of the core-shell structure comprises one or more of natural graphite, artificial graphite, and composite graphite, and the shell of the core-shell structure comprises an amorphous carbon coating layer.

7. The secondary battery according to any one of claims 1 to 6, characterized in that: Based on the total mass of the negative electrode active material, the mass percentage content of the graphitized carbon material is greater than 75%.

8. The secondary battery according to any one of claims 1 to 7, characterized in that: The carboxylate solvent satisfies at least one of the following requirements: (4.1) The carboxylate has the following structural formula: R1-COO-R2; wherein R1 and R2 each independently comprise a substituted and / or unsubstituted C1-C5 alkyl group; (4.2) Based on the mass of the electrolyte, the mass percentage content of the carboxylate is 5% to 70%.

9. The secondary battery according to any one of claims 1 to 8, characterized in that: The carboxylate solvent satisfies at least one of the following requirements: (5.1) The carboxylic acid ester comprises one or more of ethyl acetate, methyl acetate, ethyl propionate, propyl acetate, methyl propionate, and methyl butyrate; (5.2) Based on the mass of the electrolyte, the mass percentage content of the carboxylate is 10% to 60%.

10. The secondary battery according to any one of claims 1 to 9, characterized in that: The electrolyte further comprises a carbonate solvent, and the carbonate solvent comprises one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

11. The secondary battery according to any one of claims 1 to 10, characterized in that: The film-forming stabilizer satisfies at least one of the following requirements: (6.1) Based on the total mass of the electrolyte, the mass percentage content of the film-forming stabilizer is 0.02% to 6%; (6.2) The film-forming stabilizer comprises one or more of a boron-containing lithium salt, a phosphorus-containing lithium salt, and a sulfur-containing lithium salt; The boron-containing lithium salt includes one or more of lithium tetrafluoroborate, lithium bisoxalatoborate and lithium bisfluorooxalatoborate; and / or; The phosphorus-containing lithium salt includes one or more of lithium difluorophosphate, lithium fluorophosphate and lithium phosphate; and / or; The sulfur-containing lithium salt includes one or more of lithium fluorosulfonate, lithium sulfate, and lithium aminosulfonate.

12. The secondary battery according to any one of claims 1 to 11, characterized in that: The film-forming stabilizer comprises lithium fluorosulfonate and lithium difluorophosphate. Based on the total mass of the electrolyte, the mass percentage content of the lithium difluorophosphate is ≤1%, and the mass percentage content of the lithium fluorosulfonate is greater than or equal to 0.1% and less than 2%.

13. The secondary battery according to any one of claims 1 to 12, characterized in that: The positive electrode sheet is also included, and the positive electrode sheet contains a positive electrode active material, and the specific surface area of ​​the positive electrode active material is 8m 2 / g~17m 2 / g.

14. The secondary battery according to any one of claims 1 to 13, characterized in that: The positive electrode active material comprises a lithium-containing transition metal phosphate; The lithium-containing transition metal phosphate comprises olivine-type lithium manganese iron phosphate, and the chemical formula of the olivine-type lithium manganese iron phosphate is Li (1-x1) Fe x2 Mn (1-x2) PO4, where 0≤x1<1, 0≤x2≤1.

15. An electrical device, characterized in that: A secondary battery according to any one of claims 1 to 14.

Images