Lithium-ion secondary battery and electric device

Abstract

The application relates to the technical field of batteries, in particular to a lithium ion secondary battery and an electric device, which comprise a positive electrode sheet, a negative electrode sheet and an electrolyte; the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector, the porosity of the negative electrode active material layer is 25%-35%, the single-sided coating area density of the negative electrode active material layer is 100 mg / 1540.25 mm 2 -160 mg / 1540.25 mm 2 ; the negative electrode active material layer comprises a first negative electrode active material layer and a second negative electrode active material layer, the first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer, and the porosity of the first negative electrode active material layer is smaller than that of the second negative electrode active material layer; the first negative electrode active material layer comprises a first negative electrode binder, and the first negative electrode binder comprises a modified product of butadiene styrene rubber. The lithium ion secondary battery and the electric device provided by the application can simultaneously improve the fast charging performance and the energy density of the lithium ion secondary battery.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and in particular to lithium-ion secondary batteries and electrical equipment. Background Technology

[0002] Lithium-ion rechargeable batteries possess advantages such as high energy density, environmental friendliness, and long lifespan, and are currently widely used in mobile phones, computers, electric vehicles, power tools, and large-scale energy storage devices. With the rapid development of new energy electric vehicles, consumers are placing higher demands on the fast-charging performance and energy density of lithium-ion rechargeable batteries. Therefore, how to simultaneously improve the fast-charging performance and energy density of lithium-ion rechargeable batteries remains a challenge. Summary of the Invention

[0003] In view of this, the lithium-ion secondary battery and electrical equipment provided in this application can simultaneously improve the fast charging performance and energy density of lithium-ion secondary batteries.

[0004] The first aspect of this application provides a lithium-ion secondary battery, including a positive electrode, a negative electrode, and an electrolyte; the negative electrode includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector, the porosity of the negative active material layer is 25%-35%, and the single-sided coating areal density of the negative active material layer is 100mg / 1540.25mm. 2 -160mg / 1540.25mm 2 The negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer. The porosity of the first negative electrode active material layer is less than that of the second negative electrode active material layer. The first negative electrode active material layer includes a first negative electrode binder, which includes a modified styrene-butadiene rubber.

[0005] In the technical solution of this application embodiment, controlling the porosity and single-sided coating density of the negative electrode active material layer within the aforementioned range enables the negative electrode active material layer to balance energy density and fast-charging performance. A porosity within the aforementioned range results in a relatively richer pore structure in the negative electrode active material layer, making the lithium-ion liquid phase pathway more linear, thereby improving the fast-charging performance of the lithium-ion secondary battery. A coating density within the aforementioned range allows for a greater amount of negative electrode active material per unit area in the negative electrode active material layer, thereby increasing the energy density of the lithium-ion secondary battery. Furthermore, a coating density within the aforementioned range also improves the liquid phase diffusion capability of the negative electrode sheet, thereby enhancing the fast-charging performance of the lithium-ion secondary battery. The porosity of the first negative electrode active material layer is lower than that of the second negative electrode active material layer, which facilitates the rapid penetration of the electrolyte into the negative electrode sheet, improving the wettability of the negative electrode sheet and thus enhancing the fast-charging performance of the lithium-ion secondary battery. In this application, a styrene-butadiene rubber modifier is added to the first negative electrode active material layer. The styrene-butadiene rubber modifier has a high deformation capacity and a swelling property when in contact with the electrolyte. Through swelling and pore formation, the first negative electrode active material layer has a certain porosity, thereby reducing the liquid phase ion impedance of the first negative electrode active material layer and improving fast charging performance while taking into account energy density.

[0006] In any embodiment, the glass transition temperature of the first negative electrode binder is between -70°C and -20°C. When the glass transition temperature of the first negative electrode binder is within the above range, the molecular chains in the first negative electrode binder have greater rigidity, resulting in greater hardness. After being impregnated with the electrolyte, the first negative electrode binder increases the porosity between the particles of the first negative electrode active material, thereby improving the fast-charging performance of the lithium-ion secondary battery. In addition, the increased porosity between the particles of the first negative electrode active material allows for the existence of spaces between the particles, which is beneficial for the release of stress between the particles and thus improves the stability of the first negative electrode active material layer structure.

[0007] In any embodiment, the mass percentage of the first negative electrode binder is 1.5%-3% based on the total mass of the first negative electrode active material layer. A mass percentage of the first negative electrode binder within this range allows it to swell upon contact with the electrolyte, creating pores and thus improving the fast-charging performance of the first negative electrode active material layer. It also reduces the risk of excessive rebound from the first negative electrode binder, which could degrade the energy density of the first negative electrode active material layer.

[0008] In any embodiment, the modified styrene-butadiene rubber (SBR) contains one or both of ester groups and fluorine. Ester groups can improve the affinity of the modified SBR for the electrolyte, i.e., improve its liquid absorption (electrolyte absorption) capacity. Increased liquid absorption helps enhance its swelling performance upon contact with the electrolyte, thereby improving the fast-charging performance of the lithium-ion secondary battery. The modified SBR containing ester groups can be obtained by introducing acrylate monomers into the SBR, including methyl acrylate, ethyl acrylate, and methyl methacrylate. Fluorine is an electronegative element with lone pairs of electrons. Under the influence of an electric field, it continuously undergoes complexation / de-complexation reactions with lithium ions, reducing the liquid-phase diffusion ion impedance of the first negative electrode active material layer, thereby improving the output power and cycle life of the lithium-ion secondary battery.

[0009] In any embodiment, the modified styrene-butadiene rubber includes a polyacrylic acid styrene-butadiene rubber copolymer. Polyacrylic acid is a chain-like polymeric waterborne binder with relatively long molecular chains and a certain degree of flexibility. This chain structure allows polyacrylic acid to move and diffuse freely in the solvent, thus exhibiting good swelling properties. In addition, the polyacrylic acid molecular chains contain a large number of carboxylic acid groups, which can form a hydrogen bond network with solvent molecules. This hydrogen bond network structure facilitates the unwinding and dispersion of the polyacrylic acid molecular chains, thereby further improving its swelling properties.

[0010] In any embodiment, the second negative electrode active material layer includes a second negative electrode binder, which comprises styrene-butadiene rubber and / or a modified styrene-butadiene rubber. When the second negative electrode binder comprises styrene-butadiene rubber and / or a modified styrene-butadiene rubber, the second negative electrode binder also has high deformation capacity and exhibits swelling characteristics upon contact with the electrolyte. Through swelling and pore formation, the porosity of the second negative electrode active material layer is increased, thereby improving the fast-charging capability of the second negative electrode active material layer.

[0011] In any embodiment, the first negative electrode active material layer includes a first negative electrode active material, and the second negative electrode active material layer includes a second negative electrode active material. The powder compaction density of the first negative electrode active material is greater than that of the second negative electrode active material. By controlling the powder compaction density of the first negative electrode active material to be greater than that of the second negative electrode active material, both the fast-charging performance and energy density of the lithium-ion secondary battery can be balanced. Because the powder compaction density of the second negative electrode active material is lower, it is less prone to lateral deformation during compression, resulting in a more abundant porosity in the second negative electrode active material layer and a more linear lithium-conducting liquid phase pathway, thereby improving the fast-charging performance of the lithium-ion secondary battery. The higher powder compaction density of the first negative electrode active material results in a greater amount of negative electrode active material per unit volume in the first negative electrode active material layer, thus increasing the energy density of the lithium-ion secondary battery.

[0012] In any embodiment, the unit area is 1540.25 mm². 2 The first negative electrode active material is at 4.9 × 10⁻⁶. 4 The powder compaction density under N pressure is 1.4 g / cc-2.2 g / cc. When the powder compaction density of the first negative electrode active material is within the above range, there is more negative electrode active material per unit volume in the first negative electrode active material layer, thereby improving the energy density of the lithium-ion secondary battery.

[0013] In any embodiment, the unit area is 1540.25 mm². 2 The second negative electrode active material is at 4.9 × 10⁻⁶. 4 The compaction density of the powder under N pressure is 1.2 g / cc-2.0 g / cc. When the compaction density of the second negative electrode active material is within the above range, it is less prone to lateral deformation during compression, resulting in a more abundant porosity in the second negative electrode active material layer and a more linear lithium-conducting liquid phase pathway, thereby improving the fast-charging performance of the lithium-ion secondary battery.

[0014] In any embodiment, the unit area is 1540.25 mm². 2 The first negative electrode active material is at 4.9 × 10⁻⁶. 4 The powder compaction density under N pressure is 1.5 g / cc-2.1 g / cc. By controlling the powder compaction density of the first negative electrode active material within the above range, a larger amount of negative electrode active material per unit volume can be obtained in the first negative electrode active material layer, thereby improving the energy density of the lithium-ion secondary battery.

[0015] In any embodiment, the unit area is 1540.25 mm². 2 The second negative electrode active material is at 4.9 × 10⁻⁶. 4 The powder compaction density under N pressure is 1.4 g / cc-1.9 g / cc. By controlling the powder compaction density of the second negative electrode active material within the above range, the porosity of the second negative electrode active material layer can be made relatively richer, thereby improving the fast charging performance of lithium-ion secondary batteries.

[0016] In any embodiment, the volume average particle size DV50 of the first negative electrode active material is greater than that of the second negative electrode active material. By adjusting the volume average particle size DV50 of the first negative electrode active material layer to be greater than that of the second negative electrode active material layer, both the fast-charging performance and energy density of the lithium-ion secondary battery are balanced. Because the volume average particle size DV50 of the second negative electrode active material is smaller, it is less prone to lateral deformation under pressure, resulting in relatively richer pores in the second negative electrode active material layer and a more linear lithium-conducting liquid phase pathway, thereby improving the fast-charging performance of the lithium-ion secondary battery. The larger volume average particle size DV50 of the first negative electrode active material, acting as the main load-bearing element under pressure, makes it more easily deformable, resulting in relatively fewer pores in the first negative electrode active material layer. This increases the amount of negative electrode active material per unit volume in the first negative electrode active material layer, thereby improving the energy density of the lithium-ion secondary battery.

[0017] In any embodiment, the volume average particle size DV50 of the first negative electrode active material is 4μm-25μm. By controlling the volume average particle size DV50 of the first negative electrode active material within the above range, the first negative electrode active material can act as the main load-bearing element during the pressure process, making it easy to deform. This results in relatively fewer pores in the first negative electrode active material layer, thereby increasing the amount of negative electrode active material per unit volume in the first negative electrode active material layer, and thus improving the energy density of the lithium-ion secondary battery.

[0018] In any embodiment, the volume average particle size DV50 of the second negative electrode active material is 3μm-24μm. By controlling the volume average particle size DV50 of the second negative electrode active material within the above range, the second negative electrode active material is less prone to lateral deformation during pressure, resulting in relatively richer pores in the second negative electrode active material layer and a more linear lithium-conducting liquid phase pathway, thereby improving the fast-charging performance of the lithium-ion secondary battery.

[0019] In any embodiment, the volume average particle size DV50 of the second negative electrode active material is 3μm-16μm. By controlling the volume average particle size DV50 of the second negative electrode active material within the above range, the porosity in the second negative electrode active material layer is relatively richer, and the lithium-conducting liquid phase pathway tends to be more linear, thereby improving the fast-charging performance of the lithium-ion secondary battery.

[0020] In any embodiment, the particle size distribution of the first negative electrode active material satisfies: 0.8 ≤ (DV90 - DV10) / DV50 ≤ 1.6. When (DV90 - DV10) / DV50 of the first negative electrode active material is within the above range, the particle size distribution of the first negative electrode active material is more concentrated, the consistency of the first negative electrode active material is better, and the influence of relatively small and large particles on the internal pore structure of the negative electrode sheet is smaller, resulting in better pore size uniformity of the internal pore structure of the first negative electrode active material, thereby improving the fast charging performance of the lithium-ion secondary battery.

[0021] In any embodiment, the particle size distribution of the second negative electrode active material satisfies: 0.9 ≤ (DV90 - DV10) / DV50 ≤ 1.6. When (DV90 - DV10) / DV50 of the second negative electrode active material is within the above range, the particle size distribution of the second negative electrode active material is more concentrated, the consistency of the second negative electrode active material is better, and the influence of relatively small and large particles on the internal pore structure of the negative electrode sheet is smaller, thereby improving the fast charging performance of the lithium-ion secondary battery.

[0022] In any embodiment, the particle size distribution of the first negative electrode active material satisfies: 0.8 ≤ (DV90 - DV10) / DV50 ≤ 1.4. By controlling (DV90 - DV10) / DV50 of the first negative electrode active material within the above range, the particle size distribution of the first negative electrode active material can be made more concentrated, thereby improving the fast charging performance of the lithium-ion secondary battery.

[0023] In any embodiment, the particle size distribution of the second negative electrode active material satisfies: 0.9 ≤ (DV90 - DV10) / DV50 ≤ 1.4. By controlling (DV90 - DV10) / DV50 of the second negative electrode active material within the above range, the particle size distribution of the second negative electrode active material can be made more concentrated, thereby improving the fast charging performance of the lithium-ion secondary battery.

[0024] In any embodiment, the OI value of the first negative electrode active material is greater than that of the second negative electrode active material. The second negative electrode active material has a lower OI value, making it less prone to longitudinal (perpendicular to the negative electrode current collector direction) expansion and deformation during charging, resulting in less overall rebound of the negative electrode and thus improving the energy density of the lithium-ion secondary battery. The first negative electrode active material has a higher OI value, making it more susceptible to particle rebound during charging, re-forming pores between particles, reducing the liquid phase ion resistance of the first negative electrode active material layer, and thus improving the fast-charging performance of the lithium-ion secondary battery.

[0025] In any embodiment, the powder OI value of the first negative electrode active material is 3-30. By controlling the powder OI value of the first negative electrode active material within the above range, it is beneficial to reduce the liquid phase ion impedance of the first negative electrode active material layer, thereby improving the fast charging performance of the lithium-ion secondary battery.

[0026] In any embodiment, the OI value of the second negative electrode active material powder is 1.5-15. By controlling the OI value of the second negative electrode active material powder within the above range, the negative electrode sheet is less prone to longitudinal (perpendicular to the direction of the negative electrode current collector) expansion deformation during charging, and the overall rebound of the negative electrode sheet is smaller, thereby improving the energy density of the lithium-ion secondary battery.

[0027] In any embodiment, the OI value of the first negative electrode active material powder is 10-25. By controlling the OI value of the first negative electrode active material powder within the above range, it is beneficial to reduce the liquid phase ion impedance of the first negative electrode active material layer, thereby improving the fast charging performance of the lithium-ion secondary battery.

[0028] In any embodiment, the OI value of the second negative electrode active material powder is 2-10. By controlling the OI value of the second negative electrode active material powder within the above range, the negative electrode sheet is less prone to longitudinal (perpendicular to the direction of the negative electrode current collector) expansion deformation during charging, and the overall rebound of the negative electrode sheet is smaller, thereby improving the energy density of the lithium-ion secondary battery.

[0029] In any embodiment, the tap density of the first negative electrode active material is 0.9 g / cc to 1.2 g / cc. When the tap density of the first negative electrode active material is within the above range, the material itself has better density and surface smoothness, which is more conducive to electrolyte wetting and dispersion of lithium-conducting auxiliary materials on the surface of the negative electrode active material. This also helps reduce the tortuosity of the negative electrode sheet, thereby improving the fast-charging performance of the lithium-ion secondary battery. It should be noted that the tap density originates from the internal compactness of the active material and the smoothness of its surface. The denser the internal structure of the active material and the smaller the surface roughness, the greater the tap density. The greater the tap density, the more stable its structure, resulting in better dispersion of lithium-conducting auxiliary materials, better particle gradation within the negative electrode sheet, and lower ion resistance, thus improving the fast-charging performance of the lithium-ion secondary battery.

[0030] In any embodiment, the tap density of the second negative electrode active material is 0.9 g / cc to 1.2 g / cc. When the tap density of the second negative electrode active material is within the above range, the second negative electrode active material itself has better density and surface smoothness, which is more conducive to electrolyte wetting and dispersion of lithium-conducting auxiliary materials on the surface of the negative electrode active material, and it is easier to reduce the tortuosity of the negative electrode sheet, thereby improving the fast charging performance of the lithium-ion secondary battery.

[0031] In any embodiment, the tap density of the first negative electrode active material is 0.9 g / cc to 1.1 g / cc. By controlling the tap density of the first negative electrode active material within the above range, the density and surface smoothness of the first negative electrode active material can be improved, which is more conducive to electrolyte wetting and the dispersion of lithium-conducting auxiliary materials on the surface of the negative electrode active material, thereby improving the fast-charging performance of the lithium-ion secondary battery.

[0032] In any embodiment, the tap density of the second negative electrode active material is 0.9 g / cc to 1.1 g / cc. By controlling the tap density of the second negative electrode active material within the above range, the density and surface smoothness of the second negative electrode active material can be improved, which is more conducive to electrolyte wetting and dispersion of lithium-conducting auxiliary materials on the surface of the negative electrode active material, thereby improving the fast-charging performance of the lithium-ion secondary battery.

[0033] In any embodiment, the first negative electrode active material includes one or both of first artificial graphite and first natural graphite. First artificial graphite possesses excellent high-rate charge / discharge performance, which can improve the fast-charging performance of lithium-ion secondary batteries. Furthermore, first artificial graphite has good compatibility with the electrolyte, which can reduce the occurrence of side reactions, thereby improving the lifespan of lithium-ion secondary batteries. First natural graphite has high capacity and compaction density, which can improve the energy density of lithium-ion secondary batteries. In addition, first natural graphite has large reserves and low development and processing costs, which can reduce the cost of lithium-ion secondary batteries.

[0034] In any embodiment, the second negative electrode active material includes one or both of second artificial graphite and second natural graphite. In the embodiments of this application, the second artificial graphite has excellent high-rate charge-discharge performance, which can improve the fast-charging performance of lithium-ion secondary batteries; the second natural graphite has high capacity and compaction density, which can improve the energy density of lithium-ion secondary batteries.

[0035] A second aspect of this application provides an electrical device that includes the lithium-ion secondary battery of the first aspect of this application. In embodiments of this application, the electrical device possesses at least the same advantages as the lithium-ion secondary battery of the first aspect.

[0036] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0037] Figure 1 This is a structural schematic diagram of a vehicle according to one embodiment of this application.

[0038] Figure 2 This is an exploded structural diagram of a battery according to one embodiment of this application.

[0039] Figure 3 This is an exploded structural diagram of a battery cell according to one embodiment of this application. Detailed Implementation

[0040] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the lithium-ion secondary battery and electrical device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0041] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60–120 and 80–110 are listed for a specific parameter, it is understood that ranges of 60–110 and 80–120 are also expected. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range "a–b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0042] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0043] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0044] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0045] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0046] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: 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).

[0047] With the rapid development of new energy electric vehicles, consumers have placed higher demands on the fast-charging performance and energy density of lithium-ion rechargeable batteries. One related technology is to improve the fast-charging performance of lithium-ion rechargeable batteries by increasing the porosity of the negative electrode active material layer. However, higher porosity reduces the amount of negative electrode active material per unit volume in the negative electrode active material layer, thereby reducing the energy density of the lithium-ion rechargeable battery.

[0048] Based on this, the first aspect of this application provides a lithium-ion secondary battery, including a positive electrode, a negative electrode, and an electrolyte; the negative electrode includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector, the porosity of the negative active material layer is 25%-35%, and the single-sided coating areal density of the negative active material layer is 100mg / 1540.25mm. 2 -160mg / 1540.25mm 2 The negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer. The porosity of the first negative electrode active material layer is less than that of the second negative electrode active material layer. The first negative electrode active material layer includes a first negative electrode binder, which includes a modified styrene-butadiene rubber.

[0049] In the technical solution of this application embodiment, controlling the porosity and single-sided coating density of the negative electrode active material layer within the aforementioned range enables the negative electrode active material layer to balance energy density and fast-charging performance. A porosity within the aforementioned range results in a relatively richer pore structure in the negative electrode active material layer, making the lithium-ion liquid phase pathway more linear, thereby improving the fast-charging performance of the lithium-ion secondary battery. A coating density within the aforementioned range allows for a greater amount of negative electrode active material per unit area within the negative electrode active material layer, thereby increasing the energy density of the lithium-ion secondary battery. Furthermore, a coating density within the aforementioned range also improves the liquid phase diffusion capability of the negative electrode sheet, thereby enhancing the fast-charging performance of the lithium-ion secondary battery. The lower porosity of the first negative electrode active material layer compared to the second negative electrode active material layer facilitates rapid electrolyte penetration into the negative electrode sheet, improving the wettability of the negative electrode sheet and thus enhancing the fast-charging performance of the lithium-ion secondary battery. In this application, a styrene-butadiene rubber modifier is added to the first negative electrode active material layer. This modifier possesses high deformation capacity and exhibits swelling properties upon contact with the electrolyte. This swelling creates pores, resulting in a certain porosity for the first negative electrode active material layer. This reduces the liquid-phase ion impedance of the first negative electrode active material layer, thereby improving fast-charging performance while maintaining energy density. The porosity of the negative electrode active material layer is 25%, 28%, 30%, 32%, 35%, or any range of two of these values, such as 25%-28%, 28%-30%, 30%-32%, 32%-35%, etc. The single-sided coating areal density of the negative electrode active material layer is 100 mg / 1540.25 mm². 2 115mg / 1540.25mm 2 128mg / 1540.25mm 2 141mg / 1540.25mm 2 152mg / 1540.25mm 2 160mg / 1540.25mm 2 Etc., or a range of values ​​consisting of any two of the above, for example, 100mg / 1540.25mm. 2 -115mg / 1540.25mm 2 115mg / 1540.25mm 2 -128mg / 1540.25mm 2 128mg / 1540.25mm 2 -141mg / 1540.25mm 2 ,

[0050] 141mg / 1540.25mm 2-152mg / 1540.25mm 2 152mg / 1540.25mm 2 -160mg / 1540.25mm 2 wait.

[0051] Porosity and coating density are common knowledge in the field and have common meanings in the field. They can be measured by methods and instruments in the field.

[0052] In any embodiment, the glass transition temperature of the first negative electrode binder is between -70°C and -20°C. When the glass transition temperature of the first negative electrode binder is within the above range, the molecular chains in the first negative electrode binder have greater rigidity, resulting in greater hardness. After being impregnated with the electrolyte, the first negative electrode binder increases the porosity between the particles of the first negative electrode active material, thereby improving the fast-charging performance of the lithium-ion secondary battery. In addition, the increased porosity between the particles of the first negative electrode active material allows for the existence of spaces between the particles, which is beneficial for the release of stress between the particles and thus improves the stability of the first negative electrode active material layer structure. The glass transition temperatures of the first negative electrode binder are -70℃, -60℃, -50℃, -42℃, -34℃, -20℃, etc., or any range of two of the above values, such as -70℃ to -60℃, -60℃ to -50℃, -50℃ to -42℃, -42℃ to -34℃, -34℃ to -20℃, etc.

[0053] In any embodiment, the mass percentage of the first negative electrode binder is 1.5%-3% based on the total mass of the first negative electrode active material layer. A mass percentage of the first negative electrode binder within this range allows it to swell upon contact with the electrolyte, creating pores and thus improving the fast-charging performance of the first negative electrode active material layer. It also reduces the risk of excessive rebound from the first negative electrode binder, which could degrade the energy density of the first negative electrode active material layer. Specifically, the mass percentage of the first negative electrode binder based on the total mass of the first negative electrode active material layer is 1.5%, 2%, 2.3%, 2.6%, 2.8%, 3%, etc., or any range of two of the above values, such as 1.5%-2%, 2%-2.3%, 2.3%-2.6%, 2.6%-2.8%, 2.8%-3%, etc.

[0054] In any embodiment, the modified styrene-butadiene rubber (SBR) contains one or both of ester groups and fluorine. Ester groups can improve the affinity of the modified SBR for the electrolyte, i.e., improve its liquid absorption (electrolyte absorption) capacity. Increased liquid absorption helps enhance its swelling performance upon contact with the electrolyte, thereby improving the fast-charging performance of the lithium-ion secondary battery. The modified SBR containing ester groups can be obtained by introducing acrylate monomers into the SBR, including methyl acrylate, ethyl acrylate, and methyl methacrylate. Fluorine is an electronegative element with lone pairs of electrons. Under the influence of an electric field, it continuously undergoes complexation / de-complexation reactions with lithium ions, reducing the liquid-phase diffusion ion impedance of the first negative electrode active material layer, thereby improving the output power and cycle life of the lithium-ion secondary battery.

[0055] In any embodiment, the modified styrene-butadiene rubber includes a polyacrylic acid styrene-butadiene rubber copolymer. Polyacrylic acid is a chain-like polymeric waterborne binder with relatively long molecular chains and a certain degree of flexibility. This chain structure allows polyacrylic acid to move and diffuse freely in the solvent, thus exhibiting good swelling properties. In addition, the polyacrylic acid molecular chains contain a large number of carboxylic acid groups, which can form a hydrogen bond network with solvent molecules. This hydrogen bond network structure facilitates the unwinding and dispersion of the polyacrylic acid molecular chains, thereby further improving its swelling properties.

[0056] In any embodiment, the second negative electrode active material layer includes a second negative electrode binder, which comprises styrene-butadiene rubber and / or a modified styrene-butadiene rubber. When the second negative electrode binder comprises styrene-butadiene rubber and / or a modified styrene-butadiene rubber, the second negative electrode binder also has high deformation capacity and exhibits swelling characteristics upon contact with the electrolyte. Through swelling and pore formation, the porosity of the second negative electrode active material layer is increased, thereby improving the fast-charging capability of the second negative electrode active material layer.

[0057] In any embodiment, the second negative electrode binder may also be selected from materials with low swelling properties after contact with the electrolyte, such as sodium alginate, carboxymethyl chitosan, etc. When the second negative electrode binder is selected from materials with low swelling properties after contact with the electrolyte, the volume expansion of the second negative electrode active material layer after contact with the electrolyte can be reduced, thereby reducing the risk of deterioration of the energy density of the second negative electrode active material layer.

[0058] In any embodiment, the first negative electrode active material layer includes a first negative electrode active material, and the second negative electrode active material layer includes a second negative electrode active material. The powder compaction density of the first negative electrode active material is greater than that of the second negative electrode active material. By controlling the powder compaction density of the first negative electrode active material to be greater than that of the second negative electrode active material, both the fast-charging performance and energy density of the lithium-ion secondary battery can be balanced. Because the powder compaction density of the second negative electrode active material is lower, it is less prone to lateral deformation during compression, resulting in a more abundant porosity in the second negative electrode active material layer and a more linear lithium-conducting liquid phase pathway, thereby improving the fast-charging performance of the lithium-ion secondary battery. The higher powder compaction density of the first negative electrode active material results in a greater amount of negative electrode active material per unit volume in the first negative electrode active material layer, thus increasing the energy density of the lithium-ion secondary battery.

[0059] In any embodiment, the unit area is 1540.25 mm². 2 The first negative electrode active material is at 4.9 × 10⁻⁶. 4 The powder compaction density under N pressure is 1.4 g / cc-2.2 g / cc. When the powder compaction density of the first negative electrode active material is within the above range, there is a greater amount of negative electrode active material per unit volume in the first negative electrode active material layer, thereby improving the energy density of the lithium-ion secondary battery. The unit area is 1540.25 mm². 2 The first negative electrode active material is at 4.9 × 10⁻⁶. 4 The compacted density of the powder under N pressure is 1.4 g / cc, 1.6 g / cc, 1.8 g / cc, 2.0 g / cc, 2.2 g / cc, etc., or a range of any two of the above values, such as 1.4 g / cc-1.6 g / cc, 1.6 g / cc-1.8 g / cc, 1.8 g / cc-2.0 g / cc, 2.0 g / cc-2.2 g / cc, etc.

[0060] It should be noted here that the unit area is 1540.25 mm². 2 The first negative electrode active material is at 4.9 × 10⁻⁶. 4 The compacted density of powder under nitrogen pressure refers to a density of 1540.25 mm² per unit area. 2 The first negative electrode active material is at 4.9 × 10⁻⁶. 4 Density after treatment under N pressure.

[0061] In any embodiment, the unit area is 1540.25 mm². 2 The second negative electrode active material is at 4.9 × 10⁻⁶. 4The powder compaction density under N pressure is 1.2 g / cc-2.0 g / cc. When the powder compaction density of the second negative electrode active material is within this range, it is less prone to lateral deformation during compression, resulting in a more abundant porosity in the second negative electrode active material layer and a more linear lithium-conducting liquid phase pathway, thereby improving the fast-charging performance of the lithium-ion secondary battery. The unit area is 1540.25 mm². 2 The second negative electrode active material is at 4.9 × 10⁻⁶. 4 The compacted density of the powder under N pressure is 1.2 g / cc, 1.4 g / cc, 1.6 g / cc, 1.8 g / cc, 2.0 g / cc, etc., or a range of any two of the above values, such as 1.2 g / cc-1.4 g / cc, 1.4 g / cc-1.6 g / cc, 1.6 g / cc-1.8 g / cc, 1.8 g / cc-2.0 g / cc, etc.

[0062] It should be noted here that the unit area is 1540.25 mm². 2 The second negative electrode active material is at 4.9 × 10⁻⁶. 4 The compacted density of powder under nitrogen pressure refers to a density of 1540.25 mm² per unit area. 2 The first negative electrode active material is at 4.9 × 10⁻⁶. 4 Density after treatment under N pressure.

[0063] In any embodiment, the unit area is 1540.25 mm². 2 The first negative electrode active material is at 4.9 × 10⁻⁶. 4 The powder compaction density under N pressure is 1.5 g / cc-2.1 g / cc. By controlling the powder compaction density of the first negative electrode active material within this range, a higher concentration of negative electrode active material per unit volume can be achieved in the first negative electrode active material layer, thereby increasing the energy density of the lithium-ion secondary battery. The unit area is 1540.25 mm². 2 The first negative electrode active material is at 4.9 × 10⁻⁶. 4 The compacted density of the powder under N pressure is 1.5 g / cc, 1.7 g / cc, 1.9 g / cc, 2.1 g / cc, etc., or a range of any two of the above values, such as 1.5 g / cc-1.7 g / cc, 1.7 g / cc-1.9 g / cc, 1.9 g / cc-2.1 g / cc, etc.

[0064] In any embodiment, the unit area is 1540.25 mm². 2 The second negative electrode active material is at 4.9 × 10⁻⁶. 4The powder compaction density under N pressure is 1.4 g / cc-1.9 g / cc. By controlling the powder compaction density of the second negative electrode active material within this range, the porosity of the second negative electrode active material layer can be relatively richer, thereby improving the fast-charging performance of the lithium-ion secondary battery. The unit area is 1540.25 mm². 2 The second negative electrode active material is at 4.9 × 10⁻⁶. 4 The compacted density of the powder under N pressure is 1.4 g / cc, 1.5 g / cc, 1.7 g / cc, 1.9 g / cc, etc., or a range of any two of the above values, such as 1.4 g / cc-1.5 g / cc, 1.5 g / cc-1.7 g / cc, 1.7 g / cc-1.9 g / cc, etc.

[0065] In any embodiment, the volume average particle size DV50 of the first negative electrode active material is greater than that of the second negative electrode active material. By adjusting the volume average particle size DV50 of the first negative electrode active material layer to be greater than that of the second negative electrode active material layer, both the fast-charging performance and energy density of the lithium-ion secondary battery are balanced. Because the volume average particle size DV50 of the second negative electrode active material is smaller, it is less prone to lateral deformation under pressure, resulting in relatively richer pores in the second negative electrode active material layer and a more linear lithium-conducting liquid phase pathway, thereby improving the fast-charging performance of the lithium-ion secondary battery. The larger volume average particle size DV50 of the first negative electrode active material, acting as the main load-bearing element under pressure, makes it more easily deformable, resulting in relatively fewer pores in the first negative electrode active material layer. This increases the amount of negative electrode active material per unit volume in the first negative electrode active material layer, thereby improving the energy density of the lithium-ion secondary battery.

[0066] In any embodiment, the volume average particle size DV50 of the first negative electrode active material is 4μm-25μm. By controlling the volume average particle size DV50 of the first negative electrode active material within the above range, the first negative electrode active material can act as the main load-bearing element during the pressure process, making it easily deformable. This results in relatively fewer pores in the first negative electrode active material layer, thereby increasing the amount of negative electrode active material per unit volume in the first negative electrode active material layer, and thus improving the energy density of the lithium-ion secondary battery. Specifically, the volume average particle size DV50 of the first negative electrode active material is 4μm, 7μm, 11μm, 15μm, 19μm, 22μm, 25μm, etc., or a range of any two of the above values, such as 4μm-7μm, 7μm-11μm, 11μm-15μm, 15μm-19μm, 19μm-22μm, 22μm-25μm, etc.

[0067] In any embodiment, the volume average particle size DV50 of the second negative electrode active material is 3μm-24μm. By controlling the volume average particle size DV50 of the second negative electrode active material within the above range, the second negative electrode active material is less prone to lateral deformation during pressure, resulting in relatively richer pores in the second negative electrode active material layer and a more linear lithium-conducting liquid phase pathway, thereby improving the fast-charging performance of the lithium-ion secondary battery. Specifically, the volume average particle size DV50 of the second negative electrode active material is 3μm, 7μm, 11μm, 15μm, 19μm, 22μm, 24μm, etc., or a range of any two of the above values, such as 3μm-7μm, 7μm-11μm, 11μm-15μm, 15μm-19μm, 19μm-22μm, 22μm-24μm, etc.

[0068] In any embodiment, the volume average particle size DV50 of the second negative electrode active material is 3μm-16μm. By controlling the volume average particle size DV50 of the second negative electrode active material within the above range, the porosity in the second negative electrode active material layer is relatively richer, and the lithium-conducting liquid phase pathway tends to be more linear, thereby improving the fast-charging performance of the lithium-ion secondary battery. Specifically, the average particle size DV50 of the second negative electrode active material is 3μm, 6μm, 9μm, 12μm, 16μm, etc., or a range of any two of the above values, such as 3μm-6μm, 6μm-9μm, 9μm-12μm, 12μm-16μm, etc.

[0069] In any embodiment, the particle size distribution of the first negative electrode active material satisfies: 0.8 ≤ (DV90 - DV10) / DV50 ≤ 1.6. When (DV90 - DV10) / DV50 of the first negative electrode active material is within the above range, the particle size distribution of the first negative electrode active material is more concentrated, the consistency of the first negative electrode active material is better, and the influence of relatively small and large particles on the internal pore structure of the negative electrode sheet is smaller, thereby improving the fast charging performance of the lithium-ion secondary battery. Specifically, (DV90 - DV10) / DV50 of the first negative electrode active material is 0.8, 1.0, 1.2, 1.4, 1.6, etc., or a range of any two of the above values, such as 0.8-1.0, 1.0-1.2, 1.2-1.4, 1.4-1.6, etc.

[0070] Among them, the volume average particle size DV10, DV50 and DV90 are common knowledge in the field, have common meanings in the field, and can be measured by methods and instruments in the field.

[0071] In any embodiment, the particle size distribution of the second negative electrode active material satisfies: 0.9 ≤ (DV90 - DV10) / DV50 ≤ 1.6. When (DV90 - DV10) / DV50 of the second negative electrode active material is within the above range, the particle size distribution of the second negative electrode active material is more concentrated, the consistency of the second negative electrode active material is better, and the influence of relatively small and large particles on the internal pore structure of the negative electrode sheet is smaller, thereby improving the fast charging performance of the lithium-ion secondary battery. Specifically, (DV90 - DV10) / DV50 of the second negative electrode active material is 0.9, 1.1, 1.3, 1.5, 1.6, etc., or a range of any two of the above values, such as 0.9-1.1, 1.1-1.3, 1.3-1.5, 1.5-1.6, etc.

[0072] In any embodiment, the particle size distribution of the first negative electrode active material satisfies: 0.8 ≤ (DV90 - DV10) / DV50 ≤ 1.4. By controlling the (DV90 - DV10) / DV50 of the first negative electrode active material within the above range, the particle size distribution of the first negative electrode active material can be made more concentrated, thereby improving the fast-charging performance of the lithium-ion secondary battery. Specifically, the (DV90 - DV10) / DV50 of the first negative electrode active material is 0.8, 0.9, 1.1, 1.3, 1.4, etc., or a range of any two of the above values, such as 0.8-0.9, 0.9-1.1, 1.1-1.3, 1.3-1.4, etc.

[0073] In any embodiment, the particle size distribution of the second negative electrode active material satisfies: 0.9 ≤ (DV90 - DV10) / DV50 ≤ 1.4. By controlling the (DV90 - DV10) / DV50 of the second negative electrode active material within the above range, the particle size distribution of the second negative electrode active material can be made more concentrated, thereby improving the fast-charging performance of the lithium-ion secondary battery. Specifically, the (DV90 - DV10) / DV50 of the second negative electrode active material is 0.9, 1.0, 1.2, 1.4, etc., or a range of any two of the above values, such as 0.9-1.0, 1.0-1.2, 1.2-1.4, etc.

[0074] In any embodiment, the OI value of the first negative electrode active material is greater than that of the second negative electrode active material. The second negative electrode active material has a lower OI value, making it less prone to longitudinal (perpendicular to the negative electrode current collector direction) expansion and deformation during charging, resulting in less overall rebound of the negative electrode and thus improving the energy density of the lithium-ion secondary battery. The first negative electrode active material has a higher OI value, making it easier for the negative electrode active material particles to rebound during charging, re-forming pores between particles, reducing the liquid phase ion resistance of the first negative electrode active material layer, and thus improving the fast-charging performance of the lithium-ion secondary battery. It should be noted that the powder OI value is related to the raw material type, particle type (primary or secondary particles), and coating degree of the active material. It should also be noted that the OI value represents the ratio of the graphite material's end face to its base face; the smaller the OI value, the larger the ratio of the graphite material's end face to its base face, the more exposed end faces, and the stronger the lithium intercalation capability.

[0075] In any embodiment, the powder OI value of the first negative electrode active material is 3-30. By controlling the powder OI value of the first negative electrode active material within the above range, it is beneficial to reduce the liquid phase ion resistance of the first negative electrode active material layer, thereby improving the fast charging performance of the lithium-ion secondary battery. Specifically, the powder OI of the first negative electrode active material is 3, 9, 14, 19, 22, 26, 30, or a range of any two of the above values, such as 3-9, 9-14, 14-19, 19-22, 22-26, 26-30, etc.

[0076] Among them, the powder OI value is common knowledge in the field, has a common meaning in the field, and can be measured by the methods and instruments in the field.

[0077] In any embodiment, the OI value of the second negative electrode active material powder is 1.5-15. By controlling the OI value of the second negative electrode active material powder within the above range, the negative electrode sheet is less prone to longitudinal (perpendicular to the direction of the negative electrode current collector) expansion deformation during charging, and the overall rebound of the negative electrode sheet is smaller, thereby improving the energy density of the lithium-ion secondary battery. Specifically, the OI of the second negative electrode active material powder is 1.5, 5, 9, 12, 15, etc., or a range of any two of the above values, such as 1.5-5, 5-9, 9-12, 12-15, etc.

[0078] In any embodiment, the OI value of the first negative electrode active material powder is 10-25. By controlling the OI value of the first negative electrode active material powder within the above range, it is beneficial to reduce the liquid phase ion resistance of the first negative electrode active material layer, thereby improving the fast charging performance of the lithium-ion secondary battery. The OI values ​​of the first negative electrode active material powder are 10, 13, 16, 20, 23, 25, etc., or any range of two of the above values, such as 10-13, 13-16, 16-20, 20-23, 23-25, etc.

[0079] In any embodiment, the OI value of the second negative electrode active material powder is 2-10. By controlling the OI value of the second negative electrode active material powder within the above range, the negative electrode sheet is less prone to longitudinal (perpendicular to the direction of the negative electrode current collector) expansion and deformation during charging, and the overall rebound of the negative electrode sheet is smaller, thereby improving the energy density of the lithium-ion secondary battery. The OI values ​​of the second negative electrode active material powder are 2, 4, 6, 8, 10, etc., or any range of two of the above values, such as 2-4, 4-6, 6-8, 8-10, etc.

[0080] In any embodiment, the tap density of the first negative electrode active material is 0.9 g / cc to 1.2 g / cc. When the tap density of the first negative electrode active material is within the above range, the material itself has better density and surface smoothness, which is more conducive to electrolyte wetting and dispersion of lithium-conducting auxiliary materials on the surface of the negative electrode active material. This also helps reduce the tortuosity of the negative electrode sheet, thereby improving the fast-charging performance of the lithium-ion secondary battery. It should be noted that the tap density originates from the internal compactness of the active material and the smoothness of its surface. The denser the internal structure of the active material and the smaller the surface roughness, the greater the tap density. The greater the tap density, the more stable its structure, resulting in better dispersion of lithium-conducting auxiliary materials, better particle gradation within the negative electrode sheet, and lower ion resistance, thus improving the fast-charging performance of the lithium-ion secondary battery. The tap density of the first negative electrode active material is 0.9 g / cc, 1.0 g / cc, 1.1 g / cc, 1.2 g / cc, etc., or a range of any two of the above values, such as 0.9 g / cc-1.0 g / cc, 1.0 g / cc-1.1 g / cc, 1.1 g / cc-1.2 g / cc, etc.

[0081] Among them, tap density is common knowledge in the field, has a common meaning in the field, and can be measured by methods and instruments in the field.

[0082] It should be noted that the low tortuosity mainly comes from two aspects. The first aspect is that the ion conduction rate is fast during the lithium ion liquid phase diffusion process (i.e., the ion resistance is low). The second aspect is that the liquid phase diffusion path of lithium ions on the negative electrode is relatively small (the ion conduction tends to be more linear).

[0083] In any embodiment, the tap density of the second negative electrode active material is 0.9 g / cc to 1.2 g / cc. When the tap density of the second negative electrode active material is within the above range, the second negative electrode active material itself has better density and surface smoothness, which is more conducive to electrolyte wetting and dispersion of lithium-conducting auxiliary materials on the surface of the negative electrode active material, and it is easier to reduce the tortuosity of the negative electrode sheet, thereby improving the fast charging performance of the lithium-ion secondary battery. Specifically, the tap density of the second negative electrode active material is 0.9 g / cc, 1.0 g / cc, 1.1 g / cc, 1.2 g / cc, etc., or a range of any two of the above values, such as 0.9 g / cc-1.0 g / cc, 1.0 g / cc-1.1 g / cc, 1.1 g / cc-1.2 g / cc, etc.

[0084] In any embodiment, the tap density of the first negative electrode active material is 0.9 g / cc to 1.1 g / cc. By controlling the tap density of the first negative electrode active material within the above range, the density and surface smoothness of the first negative electrode active material can be improved, which is more conducive to electrolyte wetting and dispersion of lithium-conducting auxiliary materials on the surface of the negative electrode active material, thereby improving the fast-charging performance of the lithium-ion secondary battery. Specifically, the tap density of the first negative electrode active material is 0.9 g / cc, 0.95 g / cc, 0.99 g / cc, 1.05 g / cc, 1.1 g / cc, etc., or a range of any two of the above values, such as 0.9 g / cc-0.95 g / cc, 0.95 g / cc-0.99 g / cc, 0.99 g / cc-1.05 g / cc, 1.05 g / cc-1.1 g / cc, etc.

[0085] In any embodiment, the tap density of the second negative electrode active material is 0.9 g / cc to 1.1 g / cc. By controlling the tap density of the second negative electrode active material within the above range, the density and surface smoothness of the second negative electrode active material can be improved, which is more conducive to electrolyte wetting and dispersion of lithium-conducting auxiliary materials on the surface of the negative electrode active material, thereby improving the fast-charging performance of the lithium-ion secondary battery. Specifically, the tap density of the second negative electrode active material is 0.9 g / cc, 0.95 g / cc, 0.99 g / cc, 1.05 g / cc, 1.1 g / cc, etc., or a range of any two of the above values, such as 0.9 g / cc-0.95 g / cc, 0.95 g / cc-0.99 g / cc, 0.99 g / cc-1.05 g / cc, 1.05 g / cc-1.1 g / cc, etc.

[0086] In any embodiment, the first negative electrode active material includes one or both of first artificial graphite and first natural graphite. First artificial graphite possesses excellent high-rate charge / discharge performance, which can improve the fast-charging performance of lithium-ion secondary batteries. Furthermore, first artificial graphite has good compatibility with the electrolyte, which can reduce the occurrence of side reactions, thereby improving the lifespan of lithium-ion secondary batteries. First natural graphite has high capacity and compaction density, which can improve the energy density of lithium-ion secondary batteries. In addition, first natural graphite has large reserves and low development and processing costs, which can reduce the cost of lithium-ion secondary batteries.

[0087] In any embodiment, the second negative electrode active material includes one or both of second artificial graphite and second natural graphite. In the embodiments of this application, the second artificial graphite has excellent high-rate charge-discharge performance, which can improve the fast-charging performance of lithium-ion secondary batteries; the second natural graphite has high capacity and compaction density, which can improve the energy density of lithium-ion secondary batteries.

[0088] In any embodiment, the degree of graphitization of the first negative electrode active material is greater than that of the second negative electrode active material. In the embodiments of this application, the degree of graphitization of the first artificial graphite and the first natural graphite is greater than that of the second artificial graphite and the second natural graphite, respectively. The high degree of graphitization of the first artificial graphite and the first natural graphite results in smaller volume changes during the charging and discharging process of the lithium-ion secondary battery, which is beneficial for improving the energy density of the lithium-ion secondary battery. The low degree of graphitization of the second artificial graphite and the second natural graphite typically has more microporous structures, which is beneficial for improving the fast-charging performance of the lithium-ion secondary battery.

[0089] In any embodiment, the graphitization degree of the first artificial graphite and the first natural graphite is 93%-98%. By controlling the graphitization degree of the first artificial graphite and the first natural graphite within the above range, their volume change during the charging and discharging process of the lithium-ion secondary battery can be minimized, thereby improving the energy density of the lithium-ion secondary battery. Specifically, the graphitization degree of the first artificial graphite and the first natural graphite is 93%, 94%, 95%, 96%, 97%, 98%, etc., or a range of any two of the above values, such as 93%-94%, 94%-95%, 95%-96%, 96%-97%, 97%-98%, etc.

[0090] Among them, the degree of graphitization is common knowledge in the field, has a common meaning in the field, and can be measured by methods and instruments in the field.

[0091] In any embodiment, the graphitization degree of the second artificial graphite and the second natural graphite is 90%-94%. When the graphitization degree of the second artificial graphite and the second natural graphite is within the above range, the second artificial graphite and the second natural graphite have more microporous structures, which can make the pores in the second negative electrode active material layer relatively richer, and the lithium-conducting liquid phase pathway tends to be more linear, thereby which is beneficial to improving the fast charging performance of the lithium-ion secondary battery. Specifically, the graphitization degree of the second artificial graphite and the second natural graphite is 90%, 91%, 92%, 93%, 94%, etc., or a range of any two of the above values, such as 90%-91%, 91%-92%, 92%-93%, 93%-94%, etc.

[0092] In any embodiment, the negative electrode current collector includes copper foil with a thickness of 4μm-8μm. Copper foil has good conductivity, effectively transferring electrons and reducing the internal resistance of the lithium-ion secondary battery, thereby improving the charge and discharge efficiency of the lithium-ion secondary battery. In this embodiment, a copper foil with a thickness of 4μm-8μm allows for a thinner overall lithium-ion secondary battery, which is beneficial for reducing the battery's volume. High-strength copper foil can be selected, as it can withstand greater tensile forces and is less prone to breakage during use. The thickness of the copper foil can be 4μm, 5μm, 6μm, 7μm, 8μm, or any range of two of these values, such as 4μm-5μm, 5μm-6μm, 6μm-7μm, 7μm-8μm, etc.

[0093] In the embodiments of this application, the electrolyte plays a role in conducting ions between the positive and negative electrode plates. This application does not specifically limit the type of electrolyte; it can be selected according to requirements.

[0094] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0095] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0096] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0097] In some embodiments, the electrolyte may optionally include electrolyte additives. For example, electrolyte additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0098] In some embodiments, the lithium-ion secondary battery further includes a separator membrane disposed between the positive and negative electrode plates. The separator membrane primarily serves to prevent short circuits between the positive and negative electrodes while allowing ions to pass through. This application does not impose any particular limitation on the type of separator membrane; any known porous structure separator membrane with good chemical and mechanical stability can be selected.

[0099] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven 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.

[0100] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0101] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0102] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0103] In some embodiments, the positive electrode active material may be a known positive electrode active material for lithium-ion secondary batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0104] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0105] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0106] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0107] In some embodiments, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0108] In some embodiments, the negative electrode current collector includes copper foil with a thickness of 4μm-8μm. Copper foil has good conductivity, effectively transferring electrons and reducing the internal resistance of the lithium-ion secondary battery, thereby improving the charge and discharge efficiency of the lithium-ion secondary battery. In this embodiment, a copper foil with a thickness of 4μm-8μm allows for a thinner overall lithium-ion secondary battery, which is beneficial for reducing the battery's volume. High-strength copper foil can be selected, as it can withstand greater tensile forces and is less prone to breakage during use. The thickness of the copper foil can be 4μm, 5μm, 6μm, 7μm, 8μm, or any range of two of these values, such as 4μm-5μm, 5μm-6μm, 6μm-7μm, 7μm-8μm, etc.

[0109] In some embodiments, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0110] In some embodiments, the negative electrode active material layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0111] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0112] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into a battery cell assembly using a winding or stacking process.

[0113] The second aspect of this application provides an electrical device including the lithium-ion secondary battery of the first aspect of this application. In embodiments of this application, the electrical device possesses at least the same advantages as the lithium-ion secondary battery of the first aspect. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0114] For ease of explanation, we will take a vehicle 1000 as an example of an electrical device.

[0115] Please refer to Figure 1 , Figure 1 This is a schematic diagram of the structure of one embodiment of the vehicle 1000 of this application. The vehicle 1000 can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. A battery 100 is disposed inside the vehicle 1000, and the battery 100 can be located at the bottom, front, or rear of the vehicle 1000. The battery 100 can be used to power the vehicle 1000; for example, the battery 100 can serve as the operating power source for the vehicle 1000. The vehicle 1000 may also include a controller 200 and a motor 300. The controller 200 is used to control the battery 100 to supply power to the motor 300, for example, to meet the power needs of the vehicle 1000 during starting, navigation, and driving.

[0116] In some embodiments of this application, the battery 100 can not only serve as the operating power source for the vehicle 1000, but also as the driving power source for the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0117] Please refer to Figure 2 , Figure 2This is an exploded structural diagram of an embodiment of the battery 100 of this application. The battery 100 includes a housing 10 and a battery cell 20, with the battery cell 20 housed within the housing 10. The housing 10 provides a space for the battery cell 20 and can have various structures. In some embodiments, the housing 10 may include a first portion 11 and a second portion 12, which overlap each other, jointly defining a space for accommodating the battery cell 20. The second portion 12 may be a hollow structure with one open end, and the first portion 11 may be a plate-like structure, covering the open side of the second portion 12 so that the first portion 11 and the second portion 12 jointly define the space; alternatively, the first portion 11 and the second portion 12 may both be hollow structures with one open side, with the open side of the first portion 11 covering the open side of the second portion 12. Of course, the housing 10 formed by the first portion 11 and the second portion 12 can have various shapes, such as a cylinder, a cuboid, etc.

[0118] In battery 100, there can be multiple battery cells 20. These multiple battery cells 20 can be connected in series, in parallel, or in a mixed manner. A mixed connection means that multiple battery cells 20 are connected in both series and parallel. Multiple battery cells 20 can be directly connected in series, in parallel, or in a mixed manner, and then the whole assembly of multiple battery cells 20 is housed in the housing 10. Alternatively, battery 100 can also be composed of multiple battery cells 20 first connected in series, in parallel, or in a mixed manner to form a battery module, and then multiple battery modules are connected in series, in parallel, or in a mixed manner to form a whole, which is then housed in the housing 10.

[0119] The battery cell 20 includes the lithium-ion secondary battery provided in this application. There can be multiple battery cells 20. Besides the lithium-ion secondary battery provided in this application, the battery cell 20 may also include lithium-sulfur batteries, sodium-ion batteries, or magnesium-ion batteries, but is not limited to these. The battery cell 20 may be cylindrical, flat, cuboid, or other shapes.

[0120] Please refer to Figure 3 , Figure 3 This is an exploded structural diagram of one embodiment of the battery cell 20 of this application. Figure 3 In this context, X, Y, and Z represent the directions of the three-dimensional spatial coordinate axes. A battery cell 20 refers to the smallest unit that makes up the battery 100. The battery cell 20 includes an end cap 21, a casing 22, a cell assembly 23, and other functional components.

[0121] End cap 21 refers to a component that covers the opening of housing 22 to isolate the internal environment of battery cell 20 from the external environment. The shape of end cap 21 can be adapted to the shape of housing 22 to fit it. Optionally, end cap 21 can be made of a material with certain hardness and strength (such as aluminum alloy), so that end cap 21 is not easily deformed under pressure and impact, allowing battery cell 20 to have higher structural strength and improved safety performance. Functional components such as electrode terminals 21a can be provided on end cap 21. Electrode terminals 21a can be used for electrical connection with cell assembly 23 to output or input electrical energy to battery cell 20. In some embodiments, end cap 21 can also be provided with a pressure relief mechanism for releasing internal pressure when the internal pressure or temperature of battery cell 20 reaches a threshold. The material of end cap 21 can also be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and this application embodiment does not impose special limitations on this. In some embodiments, an insulating element may be provided on the inner side of the end cap 21. The insulating element can be used to isolate the electrical connection components within the housing 22 from the end cap 21 to reduce the risk of short circuits. For example, the insulating element may be made of plastic, rubber, etc.

[0122] The housing 22 is a component used to cooperate with the end cap 21 to form the internal environment of the battery cell 20. This internal environment can accommodate the cell assembly 23, electrolyte, and other components. The housing 22 and the end cap 21 can be independent components. An opening can be provided on the housing 22, and the end cap 21 can be used to close the opening to form the internal environment of the battery cell 20. Alternatively, the end cap 21 and the housing 22 can be integrated. Specifically, the end cap 21 and the housing 22 can form a common connecting surface before other components are inserted into the housing. When it is necessary to encapsulate the interior of the housing 22, the end cap 21 closes the housing 22. The housing 22 can be of various shapes and sizes, such as cuboid, cylindrical, hexagonal prism, etc. Specifically, the shape of the housing 22 can be determined according to the specific shape and size of the cell assembly 23. The material of the housing 22 can be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc. This application embodiment does not impose any special limitations on this.

[0123] The cell assembly 23 is the component in the battery cell 20 where the electrochemical reaction occurs. The casing 22 may contain one or more cell assemblies 23. The cell assembly 23 is mainly formed by winding or stacking positive and negative electrode plates, and typically a separator is provided between the positive and negative electrode plates. The portions of the positive and negative electrode plates containing active material constitute the main body of the cell assembly, while the portions of the positive and negative electrode plates without active material each constitute a tab 23a. The positive and negative tabs may be located together at one end of the main body or separately at both ends of the main body. During the charging and discharging process of the battery 100, the positive and negative active materials react with the electrolyte, and the tabs 23a connect to the electrode terminals 21a to form a current loop.

[0124] The beneficial effects of this application are further illustrated below with reference to the embodiments.

[0125] To make the technical problems, technical solutions, and beneficial effects solved by the embodiments of this application clearer, the following will provide a more detailed description in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0126] Example 1

[0127] 1) Preparation of lithium-ion secondary batteries

[0128] 1.1) Preparation of the positive electrode sheet

[0129] Lithium iron phosphate (LiFePO4), the positive electrode active material, carbon black (Super P), and the positive electrode binder, polyvinylidene fluoride (PVDF), were mixed uniformly in N-methylpyrrolidone (NMP) at a mass ratio of 97.5:0.4:2.1 to obtain a positive electrode slurry. The positive electrode slurry was coated onto aluminum foil for the positive electrode current collector, and the positive electrode sheet was obtained through processes such as drying, cold pressing, slitting, and cutting.

[0130] 1.2) Preparation of negative electrode sheet

[0131] The first negative electrode active material, the negative electrode conductive agent carbon black (Super P), the first negative electrode binder, and sodium carboxymethyl cellulose (CMC) are mixed evenly in an appropriate amount of deionized water at a mass ratio of 96.4:0.4:2.5:0.7 to obtain the first negative electrode slurry. The second negative electrode active material, the negative electrode conductive agent carbon black (Super P), the second negative electrode binder, and sodium carboxymethyl cellulose (CMC) are mixed evenly in deionized water at a mass ratio of 97.8:0.7:0.8:0.7 to obtain the second negative electrode slurry. The first negative electrode slurry is coated onto a 5 μm thick copper foil for the negative electrode current collector to form the first negative electrode active material layer. The second negative electrode slurry is coated onto the first negative electrode active material layer to form the second negative electrode active material layer. After drying, cold pressing, slitting, and cutting, the negative electrode sheet is obtained. The first and second negative electrode active materials are both artificial graphite; the OI value of the powder of the first negative electrode active material is 5.5, and the OI value of the powder of the second negative electrode active material is 2.5; the first negative electrode binder is a polyacrylic acid styrene-butadiene rubber copolymer, and the second negative electrode binder is styrene-butadiene rubber; the porosity of the negative electrode active material layer, the single-sided coating density of the negative electrode active material layer, the type of the first negative electrode binder, the glass transition temperature of the first negative electrode binder, the mass percentage of the first negative electrode active material in the first negative electrode active material layer, the type of the second negative electrode binder, and the compacted density of the powder of the first and second negative electrode active materials (unit area 1540.25 mm²) are also specified. 2 The negative electrode active material is at 4.9 × 10 4 The compaction density of powder under N pressure), volume average particle size DV50, tap density, and particle size distribution (DV90-DV10) / DV50 are shown in Table 1.

[0132] 1.3) Separating membrane

[0133] A 7μm thick polypropylene membrane was used as the separator.

[0134] 1.4) Electrolyte

[0135] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 30:70 to obtain an organic solvent. Thoroughly dried LiPF6 was then dissolved in this organic solvent to obtain an electrolyte. The concentration of LiPF6 in this electrolyte was 1 mol / L.

[0136] 1.5) Assembly of lithium-ion secondary batteries

[0137] The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is placed in an outer packaging, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion secondary battery is obtained.

[0138] Examples 2-9 are similar to Example 1. Specific process parameters and performance parameters are detailed in Table 2.

[0139] Comparative Example 1

[0140] The difference between Comparative Example 1 and Example 1 is that in the preparation of the negative electrode sheet, the types of the first negative electrode binder and the second negative electrode binder are different.

[0141] In Comparative Example 1, the first negative electrode binder is styrene-butadiene rubber, and the second negative electrode binder is polyacrylic acid styrene-butadiene rubber copolymer.

[0142] The specific testing methods for the relevant parameters are as follows:

[0143] 1) Porosity test.

[0144] True volume V1 test: Place the sample cup containing the sample into the true density tester, seal the test system, and introduce helium gas according to the procedure. Calculate the true volume V1 by detecting the gas pressure in the sample chamber and expansion chamber, and then using Bohr's law (PV = nRT).

[0145] Apparent volume V² test: Measure the area and thickness of the sample, and calculate the apparent volume V² of the sample using the formula V² = S * H ​​* A. Where S is the area (cm²). 2 H - thickness, cm; A - number of samples.

[0146] Porosity P = (V2 - V1) / V2 * 100%.

[0147] 2) Single-sided coating surface density test.

[0148] The mass of one side of the negative electrode active material layer to be tested is weighed using a balance, and the coating area of ​​one side of the negative electrode active material layer to be tested is measured using a ruler. The surface density of the coating on one side is equal to the mass of one side of the negative electrode active material layer to be tested divided by the coating area of ​​one side of the negative electrode active material layer to be tested.

[0149] 3) Glass transition temperature test.

[0150] Differential scanning calorimetry (DSC) was used for testing, specifically a Netzsch / DSC200F3 / STA449F3 calorimeter.

[0151] Sample preparation: Cut the sample to be tested into thin slices or powder (about 5mg-20mg) and ensure good contact with the crucible.

[0152] Instrument calibration: Temperature and heat flow signals are calibrated using standard substances (such as indium and tin).

[0153] Test conditions: Gently place the crucible containing the sample to be tested on the sample position of the support, heat it to 100°C at a heating rate of 5°C / min, then let it cool naturally to -100°C, and then heat it to 100°C again at the same heating rate.

[0154] Determination of glass transition temperature: The temperature corresponding to the peak value of the first derivative of the heat flow curve is the glass transition temperature of the sample to be tested.

[0155] 4) Powder compaction density test.

[0156] Disassemble the battery cell, remove the negative electrode plate, and punch it to 1540.25mm. 2 Take a small circular sheet, measure its weight (m) and thickness (L), take another negative electrode sheet, remove the film layer from the surface, and cut the remaining empty current collector foil into 1540.25mm pieces. 2 The small round piece is weighed, and the mass of the empty aluminum foil is M0. Then the compaction density PD = (M-M0) / 1.54025 / n / L, where n is the number of film layers coated on the current collector, which is 1 or 2, and the coating is single-sided or double-sided.

[0157] 5) Laser diffraction (LPS) particle size analysis.

[0158] Pretreatment: Take a clean beaker, weigh an appropriate amount of the negative electrode active material to be tested, add surfactant and then add 20 ml of dispersant, sonicate at 120 W for 5 min to ensure that the sample is completely dispersed in the dispersant.

[0159] Test: The pretreated negative electrode active material is poured into the sample column of the laser particle size analyzer (Malvin Company, model: Mastersizer3000) and circulated with the solution to the test optical path system. Under the irradiation of the laser beam, the particle size distribution characteristics (shading degree: 15-20%) can be obtained by receiving and measuring the energy distribution of the scattered light. The corresponding values ​​of DV10, DV50 and DV90 are read, and the particle size distribution (DV90-DV10) / DV50 is calculated.

[0160] 6) Powder OI value test.

[0161] Powder XRD characterization test was performed to calculate the corresponding C004 / C110 (integral ratio) value, where C004 / C110 corresponds to the peak intensity ratio of the crystal plane.

[0162] 7) Tap density test.

[0163] Pretreatment: Take a 25mL graduated cylinder with a known mass, fill it with 21mL-24mL of powder sample, weigh the total weight, and seal the tube opening with a sealing film.

[0164] Test: Fix the graduated cylinder containing powder onto the mechanical vibration device. The motor drives the mechanical vibration device to vibrate vertically up and down, gradually compacting the powder. After reaching the set number of vibrations, stop the vibration and read the volume of the graduated cylinder. Divide the mass of the powder sample by the volume of the compacted powder sample to obtain the compacted density of the powder sample.

[0165] The parameters for determining the tap density are: amplitude of 3.0 ± 0.1 mm, vibration frequency of 250 ± 15 times / min, and vibration count of 5000 times.

[0166] 8) Ion polished cross-sectional morphology (CP) image test.

[0167] Sample preparation: a. Use ceramic scissors to cut the cold-pressed negative electrode sheet into 6mm*6mm samples, attach them to the sample stage coated with paraffin, and make the samples protrude slightly (<1mm) from the edge of the sample stage; b. Set the polishing voltage and time for polishing (100min at 7.5KV).

[0168] Parameter settings: mode is In-lens, voltage is 10KV, aperture is 30um, working distance is 4.5mm.

[0169] Test procedure: Take 2-3 photos at 500x magnification (including the upper and lower active materials and current collector in the field of view) to test the thickness of the upper and lower electrode layers. Take two photos of each of the upper and lower layers at 1000x magnification (including the active material on the current collector side). Take two sets of photos at 10K, 5K, 3K, and 1K focusing on the anode with the material particles.

[0170] 9) Electrochemical performance testing.

[0171] 9.1) Equivalent charging rate test.

[0172] The prepared lithium-ion secondary battery was fully charged and fully discharged at 1C for 10 cycles at an equivalent xC (actually a stepped charging rate). Then, the battery was fully charged at the equivalent xC again, and the negative electrode was disassembled to observe the lithium deposition on the surface. If no lithium deposition occurred on the negative electrode surface, the equivalent charging rate xC was increased in increments of 0.1C until lithium deposition occurred on the negative electrode surface. The test was then stopped, and the charging rate xC at this point was recorded. (x-0.1)C was recorded as the critical charging rate of the battery. The average value of 10% to 80% SOC was taken as the equivalent charging rate, which reflects the relative magnitude of fast charging performance.

[0173] 9.2) Energy density test.

[0174] At 25°C, the secondary battery was fully charged and fully discharged at a rate of 0.33C three times, and the actual discharge energy C0 was recorded at this time. Under the same temperature conditions, the volume of the secondary battery was measured using an electronic balance. The ratio of the actual discharge energy C0 of the secondary battery at 0.33C to the volume V of the lithium-ion battery is the actual volumetric energy density of the lithium-ion battery.

[0175]

[0176]

[0177] 1) The lithium-ion secondary battery prepared in Comparative Example 1 has an equivalent charge rate of 3.8C and an energy density of 410Wh / L. The lithium-ion secondary battery prepared in Example 1 has an equivalent charge rate of 4C and an energy density of 410Wh / L. It can be seen that, compared with Comparative Example 1, the lithium-ion secondary battery prepared in Example 1 has a better equivalent charge rate while taking into account the energy density, that is, better fast charging performance.

[0178] This is because in Example 1, the negative electrode active material layer adopts a two-layer structure. The first negative electrode binder in the first negative electrode active material layer close to the negative electrode current collector is a polystyrene-butadiene rubber copolymer. The polystyrene-butadiene rubber copolymer is added to the first negative electrode active material layer. The polystyrene-butadiene rubber copolymer has a high deformation capacity and will swell when in contact with the electrolyte. Through swelling and pore formation, the first negative electrode active material layer has a certain porosity, thereby reducing the liquid phase ion impedance of the first negative electrode active material layer, and thus improving the fast charging performance of the lithium-ion secondary battery while taking into account the energy density.

[0179] In Comparative Example 1, although the negative electrode active material layer also adopts a two-layer structure, the first negative electrode binder in the first negative electrode active material layer close to the negative electrode current collector is styrene-butadiene rubber. Styrene-butadiene rubber has poor deformation ability and poor swelling performance when in contact with electrolyte, so it cannot take into account both energy density and fast charging performance.

[0180] 2) Data from Examples 2-9 show that the lithium-ion secondary batteries prepared using the method provided in this application can simultaneously achieve both energy density and fast charging performance.

[0181] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium-ion secondary battery, characterized in that, The device includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative current collector and a negative active material layer disposed on the surface of the negative current collector. The porosity of the negative active material layer is 25%-35%, and the single-sided coating areal density of the negative active material layer is 100 mg / 1540.25 mm². 2 -160mg / 1540.25mm 2 ; The negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer is located between the negative electrode current collector and the second negative electrode active material layer. The porosity of the first negative electrode active material layer is less than that of the second negative electrode active material layer. The first negative electrode active material layer includes a first negative electrode binder, which includes a modified styrene-butadiene rubber.

2. The lithium-ion secondary battery according to claim 1, characterized in that, The glass transition temperature of the first negative electrode binder is -70°C to -20°C.

3. The lithium-ion secondary battery according to claim 1 or 2, characterized in that, Based on the total mass of the first negative electrode active material layer, the mass percentage of the first negative electrode binder is 1.5%-3%.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, characterized in that, The modified styrene-butadiene rubber contains one or both of ester groups and fluorine.

5. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized in that, The modified styrene-butadiene rubber includes polyacrylic acid styrene-butadiene rubber copolymer.

6. The lithium-ion secondary battery according to any one of claims 1 to 5, characterized in that, The second negative electrode active material layer includes a second negative electrode binder, which includes the styrene-butadiene rubber and / or a modified version of the styrene-butadiene rubber.

7. The lithium-ion secondary battery according to any one of claims 1 to 6, characterized in that, The first negative electrode active material layer includes a first negative electrode active material, and the second negative electrode active material layer includes a second negative electrode active material. The powder compaction density of the first negative electrode active material is greater than that of the second negative electrode active material.

8. The lithium-ion secondary battery according to claim 7, characterized in that, The unit area is 1540.25 mm². 2 The first negative electrode active material is at 4.9 × 10 4 The compacted density of the powder under N pressure is 1.4 g / cc - 2.2 g / cc; And / or, The unit area is 1540.25 mm². 2 The second negative electrode active material is at 4.9 × 10 4 The compaction density of powder under N pressure is 1.2 g / cc-2.0 g / cc.

9. The lithium-ion secondary battery according to claim 7 or 8, characterized in that, The unit area is 1540.25 mm². 2 The first negative electrode active material is at 4.9 × 10 4 The compacted density of powder under N pressure is 1.5 g / cc - 2.1 g / cc; And / or, The unit area is 1540.25 mm². 2 The second negative electrode active material is at 4.9 × 10 4 The compacted density of powder under N pressure is 1.4 g / cc-1.9 g / cc.

10. The lithium-ion secondary battery according to any one of claims 7 to 9, characterized in that, The volume average particle size DV50 of the first negative electrode active material is greater than that of the second negative electrode active material.

11. The lithium-ion secondary battery according to claim 10, characterized in that, The volume average particle size (DV50) of the first negative electrode active material is 4 μm-25 μm; And / or, The volume average particle size (DV50) of the second negative electrode active material is 3 μm-24 μm.

12. The lithium-ion secondary battery according to claim 10 or 11, characterized in that, The volume average particle size (DV50) of the first negative electrode active material is 4 μm to 25 μm. And / or, The volume average particle size (DV50) of the second negative electrode active material is 3 μm-16 μm.

13. The lithium-ion secondary battery according to any one of claims 8 to 13, characterized in that, The particle size distribution of the first negative electrode active material satisfies: 0.8≤(DV90-DV10) / DV50≤1.6; And / or, The particle size distribution of the second negative electrode active material satisfies: 0.9≤(DV90-DV10) / DV50≤1.

6.

14. The lithium-ion secondary battery according to any one of claims 7 to 13, characterized in that, The particle size distribution of the first negative electrode active material satisfies: 0.8≤(DV90-DV10) / DV50≤1.4; And / or, The particle size distribution of the second negative electrode active material satisfies: 0.9≤(DV90-DV10) / DV50≤1.

4.

15. The lithium-ion secondary battery according to any one of claims 7 to 14, characterized in that, The OI value of the powder of the first negative electrode active material is greater than that of the powder of the second negative electrode active material.

16. The lithium-ion secondary battery according to any one of claims 7 to 15, characterized in that, The OI value of the powder of the first negative electrode active material is 3-30; And / or, The OI value of the second negative electrode active material powder is 1.5-15.

17. The lithium-ion secondary battery according to any one of claims 7 to 16, characterized in that, The OI value of the powder of the first negative electrode active material is 10-25; And / or, The OI value of the powder of the second negative electrode active material is 2-10.

18. The lithium-ion secondary battery according to any one of claims 7 to 17, characterized in that, The tap density of the first negative electrode active material is 0.9 g / cc - 1.2 g / cc; And / or, The tap density of the second negative electrode active material is 0.9 g / cc-1.2 g / cc.

19. The lithium-ion secondary battery according to any one of claims 7 to 18, characterized in that, The tap density of the first negative electrode active material is 0.9 g / cc - 1.1 g / cc; And / or, The tap density of the second negative electrode active material is 0.9 g / cc-1.1 g / cc.

20. The lithium-ion secondary battery according to any one of claims 7 to 19, characterized in that, The first negative electrode active material includes one or two of the following: first artificial graphite and first natural graphite; And / or, The second negative electrode active material includes one or both of the following: second artificial graphite and second natural graphite.

21. An electrical appliance, characterized in that, Includes the lithium-ion secondary battery as described in any one of claims 1 to 20.

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