Secondary battery and electronic device

Abstract

The present application provides a secondary battery and an electronic device. The secondary battery comprises a negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The negative electrode material layer comprises a negative electrode active material, polyurethane and a conductive agent. At least part of a surface of the negative electrode active material is provided with polyurethane, and at least part of the conductive agent is provided in polyurethane. The negative electrode active material comprises a silicon-containing material, and the sphericity of the silicon-containing material is 90% to 100%. The conductive agent comprises carbon nanotubes. Based on the mass of the negative electrode material layer, the mass percentage W1 of polyurethane is 0.5% to 5%. The negative electrode material layer contains the silicon-containing material having the described sphericity, which is used in conjunction with the carbon nanotubes and polyurethane, thereby facilitating the construction of a good conductive network and a good bonding network in the negative electrode material layer, and ameliorating the problem of poor conductivity caused by point-to-point contact between spherical silicon-containing material particles, and thus facilitating improving the first coulombic efficiency and cycle performance of the secondary battery.

Description

A secondary battery and electronic device Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

[0002] Secondary batteries, such as lithium-ion batteries, possess advantages such as high energy density, high power, and long cycle life, and are widely used in consumer electronics, electric bicycles, and electric vehicles. As their application scope continues to expand, the requirements for energy density and cycle performance of lithium-ion batteries are constantly increasing. Currently, the negative electrode active material of commonly used lithium-ion batteries is mostly graphite, but this is no longer sufficient to meet the energy density requirements. Although silicon-carbon and silicon-oxygen negative electrode materials have higher theoretical specific capacities and can replace graphite to improve the energy density of lithium-ion batteries, silicon-containing materials, due to their large volume changes during charge-discharge cycles, significantly impact the bonding or conductive networks in the negative electrode material layer, thus affecting the cycle performance of the secondary battery. Simultaneously, the conductivity of silicon-containing materials is worse than that of graphite, affecting polarization during charge-discharge processes, and consequently affecting the capacity utilization of silicon-containing materials, i.e., impacting the initial coulombic efficiency of the secondary battery. Summary of the Invention

[0003] The purpose of this application is to provide a secondary battery and electronic device to improve the conductivity between silicon-containing material particles and enhance the initial coulombic efficiency and cycle performance of the secondary battery.

[0004] It should be noted that while this application uses lithium-ion batteries as an example of secondary batteries to explain the invention, the secondary batteries in this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0005] A first aspect of this application provides a secondary battery comprising a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector, the negative electrode material layer comprising a negative electrode active material, polyurethane, and a conductive agent; at least a portion of the surface of the negative electrode active material is disposed of polyurethane, and at least a portion of the conductive agent is disposed within the polyurethane; the negative electrode active material comprises a silicon-containing material, the sphericity of the silicon-containing material being 90% to 100%; the conductive agent comprising carbon nanotubes; based on the mass of the negative electrode material layer, the mass percentage W1 of polyurethane is 0.5% to 5%. In some embodiments of this application, W1 is 1% to 3%. The negative electrode material layer comprising the aforementioned silicon-containing material with the aforementioned sphericity, which is compatible with carbon nanotubes and polyurethane, facilitates the construction of a good conductive network and bonding network in the negative electrode material layer, improves the poor conductivity problem caused by point-to-point contact between spherical silicon-containing material particles, thereby improving the initial coulombic efficiency and cycle performance of the secondary battery.

[0006] In some embodiments of this application, the polyurethane coverage on the surface of the negative electrode active material is 50% to 100%. By controlling the coverage within the above range, the polyurethane builds a three-dimensional bonding network between the silicon-containing material particles, improving the adhesion between the negative electrode material layer and the negative electrode current collector. This also helps to reduce stress changes during charging and discharging, and improves the conductive network, thereby further improving the cycle performance and initial coulombic efficiency of the secondary battery.

[0007] In some embodiments of this application, the diameter d of the carbon nanotubes is between 10 nm and 100 nm. By controlling the diameter d of the carbon nanotubes within the above range, their dispersion in polyurethane is better, and they are less prone to agglomeration, which is beneficial for constructing a good conductive network and further improving the initial coulombic efficiency of the secondary battery.

[0008] In some embodiments of this application, the mass percentage content W2 of the conductive agent is 0.5% to 2.5% based on the mass of the negative electrode material layer. By controlling the mass percentage content W2 of the conductive agent within the above range, it is beneficial to construct a good conductive network and less likely to affect the transport of lithium ions, thereby improving the initial coulombic efficiency and cycle performance of the secondary battery.

[0009] In some embodiments of this application, the negative electrode material layer further includes a styrene-acrylate copolymer; based on the mass of the negative electrode material layer, the mass percentage W3 of the styrene-acrylate copolymer is 2% to 10%. In some embodiments of this application, W3 is 4% to 8%. By controlling the mass percentage W3 of the styrene-acrylate copolymer within the above range, on the one hand, the polyurethane and the styrene-acrylate copolymer synergistically construct a relatively stable bonding network within the negative electrode material layer, making it less prone to collapse during charge and discharge; on the other hand, it is less likely to affect lithium-ion transport. Therefore, the cycle performance and initial coulombic efficiency of the secondary battery are further improved.

[0010] In some embodiments of this application, the average particle size D of the styrene-acrylate copolymer is 200 nm to 300 nm. When the average particle size D of the styrene-acrylate copolymer is within this range, it provides suitable bonding sites with the silicon-containing material particles, offering a robust conductive network while minimizing interference with lithium-ion transport, thereby improving the cycle performance and initial coulombic efficiency of the secondary battery.

[0011] In some embodiments of this application, the number of styrene-acrylate copolymer particles in the 12.7 μm × 9.9 μm range of the scanning electron microscope (SEM) image of the negative electrode is between 50 and 300. When the number N of styrene-acrylate copolymer particles in the 12.7 μm × 9.9 μm range of the negative electrode is within the aforementioned range, the styrene-acrylate copolymer provides a robust conductive network while minimizing interference with lithium-ion transport, thereby improving the cycle performance and initial coulombic efficiency of the secondary battery.

[0012] In some embodiments of this application, the silicon-containing material includes at least one of silicon-carbon or silicon-oxygen materials. The aforementioned silicon-containing materials have high specific capacity; by selecting these materials, the resulting secondary battery can improve cycle performance and initial coulombic efficiency while also maintaining adequate energy density.

[0013] In some embodiments of this application, the specific surface area of ​​the negative electrode active material is 0.1 m². 2 / g to 0.7m 2 / g. By adjusting the specific surface area of ​​the negative electrode active material within the above range, it is less likely to react with the electrolyte and has low lithium ion diffusion resistance. When combined with polyurethane and carbon nanotubes, or with polyurethane, carbon nanotubes and styrene-acrylate copolymer, it is beneficial to improve cycle performance and first coulombic efficiency while taking into account kinetic performance.

[0014] In some embodiments of this application, the particle size Dv50 of the negative electrode active material is 6 μm to 10 μm. A particle size Dv50 within this range indicates that the negative electrode active material is less likely to react with the electrolyte and has low lithium-ion diffusion resistance. When combined with polyurethane and carbon nanotubes, or with polyurethane, carbon nanotubes, and styrene-acrylate copolymers, it is beneficial for improving cycle performance and initial coulombic efficiency while also considering kinetic performance.

[0015] In some embodiments of this application, the adhesion force F between the negative electrode material layer and the negative electrode current collector is between 30 N / m and 200 N / m. The adhesion force F between the negative electrode material layer and the negative electrode current collector being within the above range indicates good adhesion between them, resulting in strong structural stability of the negative electrode sheet. It also minimizes interference with lithium-ion transport, which is beneficial for improving the cycle performance of the secondary battery while maintaining good kinetic performance.

[0016] In some embodiments of this application, the secondary battery satisfies at least one of the following characteristics:

[0017] (1) The coating mass (CW) of the negative electrode material layer is 25 mg / 1540.25 mm. 2 Up to 50mg / 1540.25mm 2 ;

[0018] (2) Based on the mass of the negative electrode material layer, the mass percentage content W4 of the negative electrode active material is 82% to 95%;

[0019] (3) The mass ratio of the negative electrode material layer to the negative electrode current collector is 0.15 to 0.30;

[0020] (4) The porosity of the negative electrode sheet is 28% to 42%;

[0021] (5) The specific capacity C of the negative electrode sheet is 1600mAh / g to 2500mAh / g;

[0022] (6) In the dynamic thermomechanical analysis curve of the negative electrode material layer, there are characteristic peaks at 230±1℃, 455±1℃ and 415.7±1℃.

[0023] A secondary battery that meets at least one of the above characteristics is beneficial to improving the cycle performance and initial coulombic efficiency of the secondary battery.

[0024] A second aspect of this application provides an electronic device that includes the electronic device in any of the above embodiments.

[0025] The beneficial effects of this application are:

[0026] This application provides a secondary battery comprising a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a negative electrode active material, polyurethane, and a conductive agent; at least a portion of the surface of the negative electrode active material is disposed of with polyurethane, and at least a portion of the conductive agent is disposed within the polyurethane; the negative electrode active material includes a silicon-containing material with a sphericity of 90% to 100%; the conductive agent includes carbon nanotubes; based on the mass of the negative electrode material layer, the mass percentage W1 of polyurethane is 0.5% to 5%. The negative electrode material layer comprising the aforementioned silicon-containing material with sphericity, which is compatible with the carbon nanotubes and polyurethane, facilitates the construction of a good conductive network and bonding network within the negative electrode material layer, improving the poor conductivity caused by point-to-point contact between spherical silicon-containing material particles, thereby improving the initial coulombic efficiency and cycle performance of the secondary battery. Attached Figure Description

[0027] The accompanying drawings, which are provided to further illustrate this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application.

[0028] Figure 1 is an electron microscope image magnified to 5000x of the negative electrode material layer in Examples 1-15 of this application;

[0029] Figure 2 is an electron microscope image of the negative electrode material layer in Examples 1-15 of this application, magnified to 10,000 times;

[0030] Figure 3 shows the dynamic thermomechanical analysis curves of the negative electrode material layer in Embodiment 1-1 of this application;

[0031] Figure 4 shows the dynamic thermomechanical analysis curves of the negative electrode material layer in Examples 1-24 of this application. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0033] Spherical silicon-containing materials exhibit lower stress during charge-discharge cycles compared to non-spherical silicon-containing materials, resulting in less damage to the bonding and conductive networks of the negative electrode material layer. This offers certain advantages for improving the cycle performance of secondary batteries. However, the contact between particles in spherical silicon materials is point-to-point, leading to poor initial electrical contact between material particles. Furthermore, silicon's conductivity is inherently lower than that of graphite, making polarization issues more pronounced during charge-discharge processes. This negatively impacts the capacity utilization of the silicon-containing material, resulting in a lower initial coulombic efficiency of the secondary battery.

[0034] Based on this, this application provides a secondary battery to improve the conductivity between spherical silicon-containing material particles, alleviate polarization problems caused by poor electrical contact between particles during charging and discharging, promote the utilization of the silicon-containing material's capacity, and thus improve the coulombic efficiency of the secondary battery. Simultaneously, spherical silicon-containing materials, compared to non-spherical silicon-containing materials, can improve the cycle performance of the secondary battery. Therefore, the initial coulombic efficiency and cycle performance of the secondary battery provided by this application are improved.

[0035] It should be noted that, in the specific embodiments of this application, a lithium-ion battery is used as an example of a secondary battery to explain this application; however, the secondary battery in this application is not limited to lithium-ion batteries. The specific technical solution is as follows:

[0036] The first aspect of this application provides a secondary battery comprising a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a negative electrode active material, polyurethane, and a conductive agent. The negative electrode active material includes a silicon-containing material, and at least a portion of the surface of the negative electrode active material is disposed of with polyurethane. The conductive agent includes carbon nanotubes, and at least a portion of the conductive agent is disposed within the polyurethane. The inclusion of carbon nanotubes in the conductive agent is beneficial for constructing a three-dimensional conductive network in the negative electrode material layer. However, carbon nanotubes are prone to agglomeration. The addition of polyurethane can improve the problem of carbon nanotube agglomeration, allowing the carbon nanotubes to be dispersed along with the polyurethane on the surface of the silicon-containing material particles, thereby improving the conductivity between the silicon-containing material particles. Simultaneously, the polyurethane can also adhere to the surface of the silicon-containing material particles, improving the adhesion between the particles and constructing a well-bonded network within the negative electrode material layer. This is beneficial for improving the adhesion between the negative electrode material layer and the negative current collector, enhancing the structural stability of the negative electrode sheet, and thus improving the cycle performance of the secondary battery. Furthermore, polyurethane possesses electrolyte swelling properties, which facilitates lithium-ion transport during the charging and discharging process of secondary batteries. This allows lithium ions to be smoothly intercalated and deintercalated into silicon-containing materials, enabling the silicon-containing materials to fully utilize their capacity, improving the initial coulombic efficiency of the secondary battery, and enhancing its cycle performance. The sphericity of the silicon-containing materials... It ranges from 90% to 100%. For example, the sphericity of silicon-containing materials. The percentage can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, or any two values ​​within this range. That is, the silicon-containing material is spherical. Compared to irregular silicon-containing materials, there are fewer stress concentration points in the silicon-containing material particles during the rolling process of negative electrode preparation and during the charge-discharge cycle of the secondary battery. This results in less stress concentration during rolling and charge-discharge expansion, a larger spherical contact area, and lower pressure, minimizing the impact on the conductive and bonding networks within the negative electrode material layer, thus improving the cycle performance of the secondary battery. Furthermore, the spherical silicon-containing material can interact with carbon nanotubes and polyurethane to form a three-dimensional conductive network with good conductivity uniformity between the spherical silicon-containing material particles. This improves the poor electrical contact caused by point-to-point contact in spherical silicon-containing materials, thereby maximizing the capacity of the silicon-containing material and improving the initial coulombic efficiency of the secondary battery. Based on the mass of the negative electrode material layer, the mass percentage of polyurethane W1 is 0.5% to 5%. In some embodiments of this application, W1 is 1% to 3%. For example, the mass percentage of polyurethane W1 can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range of any two values ​​therein. When the mass percentage of polyurethane W1 is too low, for example below 0.5%, the dispersion effect of polyurethane on carbon nanotubes is limited, and the improvement on the poor electrical contact problem caused by point-to-point contact of spherical silicon-containing materials is limited. When the mass percentage of polyurethane W1 is too high, for example above 5%, the improvement on the dispersion effect of carbon nanotubes is not significant and it will affect the transport of lithium ions, thereby affecting the initial coulombic efficiency and cycle performance of the secondary battery. Therefore, the negative electrode material layer contains the aforementioned spherical silicon-containing material, which is matched with carbon nanotubes and polyurethane. This facilitates the construction of a good conductive network and bonding network in the negative electrode material layer, improves the poor conductivity caused by point-to-point contact between spherical silicon-containing material particles, and thus helps to improve the initial coulombic efficiency and cycle performance of the secondary battery.

[0037] In this application, spherical silicon-containing materials refer to silicon-containing materials with a particle sphericity greater than or equal to 90%.

[0038] In this application, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the term "surface" here can refer to the entire surface area of ​​the negative electrode current collector, or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved.

[0039] In some embodiments of this application, the coverage S of polyurethane on the surface of the negative electrode active material is 50% to 100%. For example, the coverage S can be 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, or any range of two values ​​therein. The polyurethane is disposed on the surface of the silicon-containing material particles, and the carbon nanotubes are disposed within the polyurethane, i.e., the carbon nanotubes are dispersed on the surface of the silicon-containing material particles. This facilitates the construction of a good conductive network, promotes the utilization of the silicon-containing material's capacity, and improves the initial coulombic efficiency of the secondary battery. Furthermore, the polyurethane coating on the surface of the silicon-containing material particles, and the presence of polyurethane between the particles, helps improve the adhesion between particles. Moreover, the silicon-containing material particles are constrained by the polyurethane coating during charge and discharge, which helps reduce stress changes during the charge and discharge process, thereby improving the cycle performance of the secondary battery. By adjusting the coverage S within the above range, the polyurethane constructs a three-dimensional bonding network between the silicon-containing material particles, which improves the bonding force between the negative electrode material layer and the negative electrode current collector. It also helps to reduce stress changes during charging and discharging, and improves the conductive network, thereby further improving the cycle performance and first coulombic efficiency of the secondary battery.

[0040] In some embodiments of this application, the diameter d of the carbon nanotubes is from 10 nm to 100 nm. For example, the diameter d of the carbon nanotubes can be 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any combination of two values ​​in between. By controlling the diameter d of the carbon nanotubes within the above range, their dispersion in polyurethane is better, and they are less prone to aggregation, which is beneficial for constructing a good conductive network and further improving the initial coulombic efficiency of the secondary battery.

[0041] In some embodiments of this application, the mass percentage content W2 of the conductive agent is 0.5% to 2.5% based on the mass of the negative electrode material layer. For example, the mass percentage content W2 of the conductive agent can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, or a range of any two values ​​within this range. By controlling the mass percentage content W2 of the conductive agent within the above range, it is beneficial to construct a good conductive network and less likely to affect the transport of lithium ions, thereby improving the initial coulombic efficiency and cycle performance of the secondary battery.

[0042] In some embodiments of this application, the negative electrode material layer further includes a styrene-acrylate copolymer. The styrene-acrylate copolymer is a particulate binder that can fill the spaces between silicon-containing material particles, further improving the adhesion between particles, stabilizing the bonding network within the material layer, and enhancing the adhesion between the negative electrode material layer and the negative electrode current collector, thereby improving the cycle performance of the secondary battery. Based on the mass of the negative electrode material layer, the mass percentage W3 of the styrene-acrylate copolymer is 2% to 10%. In some embodiments of this application, W3 is 4% to 8%. For example, the mass percentage W3 of the styrene-acrylate copolymer can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any two values ​​within this range. By controlling the mass percentage W3 of the styrene-acrylate copolymer within the above range, on the one hand, the polyurethane and the styrene-acrylate copolymer synergistically construct a relatively stable bonding network within the negative electrode material layer, which is less prone to collapse during charging and discharging; on the other hand, it is less likely to affect lithium-ion transport. Therefore, the cycle performance and initial coulombic efficiency of the secondary battery are further improved.

[0043] In this application, the styrene-acrylate copolymer is a copolymer of styrene, acrylate, and acrylic acid. This application does not impose any particular limitation on the proportions of styrene, acrylate, and acrylic acid in the styrene-acrylate copolymer, as long as the purpose of this application is achieved. The molar percentage of each component can be: styrene 60% to 90%, acrylate 5% to 20%, and acrylic acid 5% to 25%. In this application, the acrylate may include, but is not limited to, methyl methacrylate, vinyl acetate, butyl acrylate, butyl methacrylate, butyl acrylate, ethyl acrylate, ethyl acrylate, etc.

[0044] In some embodiments of this application, the average particle size D of the styrene-acrylate copolymer is between 200 nm and 300 nm. For example, the average particle size D of the styrene-acrylate copolymer can be 200 nm, 210 nm, 220 nm, 230 nm, 240 nm, 250 nm, 260 nm, 270 nm, 280 nm, 290 nm, 300 nm, or any combination of two values ​​in between. When the average particle size D of the styrene-acrylate copolymer is within the above range, it provides suitable bonding sites with the silicon-containing material particles, offering a robust conductive network while minimizing interference with lithium-ion transport, thereby improving the cycle performance and initial coulombic efficiency of the secondary battery.

[0045] In some embodiments of this application, characteristic peaks are present at 230±1℃, 455±1℃, and 415.7±1℃ in the dynamic thermomechanical analysis curve of the negative electrode material layer. Specifically, 230±1℃ and 455±1℃ are the thermal decomposition peaks corresponding to polyurethane, and 415.7±1℃ is the thermal decomposition peak corresponding to the styrene-acrylate copolymer. The presence of all these characteristic peaks in the dynamic thermomechanical analysis curve of the negative electrode indicates that the negative electrode material layer simultaneously contains both polyurethane and styrene-acrylate copolymer.

[0046] This application does not limit the molecular weight of polyurethane and styrene-acrylate copolymer, as long as the purpose of this application can be achieved. For example, the weight-average molecular weight of polyurethane can be 2W to 50W; the weight-average molecular weight of styrene-acrylate copolymer can be 5W to 50W.

[0047] In this application, styrene-acrylate copolymers with different average particle sizes can be purchased, and their average particle sizes can be tested using scanning electron microscopy to select the desired styrene-acrylate copolymer. When testing the average particle size, the average diameter of the largest circumscribed circle of 50 particles is selected as the average particle size.

[0048] In some embodiments of this application, the number N of styrene-acrylate copolymer particles in the 12.7 μm × 9.9 μm range of the scanning electron microscope (SEM) image of the negative electrode is between 50 and 300. For example, the number N of styrene-acrylate copolymer particles can be 50, 60, 70, 80, 90, 100, 120, 140, 150, 160, 180, 200, 220, 240, 250, 260, 280, 300, or any combination of two values ​​in between. When the number N of styrene-acrylate copolymer particles in the 12.7 μm × 9.9 μm range of the negative electrode is within the above range, the styrene-acrylate copolymer provides a stable conductive network while minimizing interference with lithium-ion transport, thereby improving the cycle performance and initial coulombic efficiency of the secondary battery.

[0049] In some embodiments of this application, the mass percentage W4 of the negative electrode active material is 82% to 95% based on the mass of the negative electrode material layer; for example, the mass percentage W4 of the negative electrode active material can be 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, or a range of any two values ​​therein. By controlling the mass percentage W4 of the negative electrode active material within the above range, the obtained secondary battery can improve cycle performance and initial coulombic efficiency while also maintaining energy density.

[0050] In some embodiments of this application, the silicon-containing material includes at least one of silicon-carbon or silicon-oxygen materials. The aforementioned silicon-containing materials have high specific capacity; by selecting these silicon-containing materials, the resulting secondary battery can improve cycle performance and initial coulombic efficiency while also maintaining adequate energy density.

[0051] In some embodiments of this application, the silicon-carbon material may include, but is not limited to, at least one of SiC or silicon-carbon composite materials, wherein the mass percentage of silicon is 30% to 70% and the mass percentage of carbon is 30% to 70% based on the mass of the silicon-carbon material. The silicon-carbon composite material may be a composite material obtained by deposition. Exemplarily, the silicon-carbon composite material may be silicon material deposited on a porous carbon framework, or carbon material deposited on a porous silicon framework. The silicon-oxygen material includes SiOx, where 0 < x < 2. Exemplarily, the silicon-oxygen material may include silicon suboxide (SiO, where the molar ratio of silicon to oxygen is 1:1).

[0052] In some embodiments of this application, the specific surface area (BET) of the negative electrode active material is 0.1 m². 2 / g to 0.7m 2 / g. For example, the specific surface area (BET) of the negative electrode active material can be 0.1m². 2 / g, 0.2m 2 / g, 0.3m 2 / g, 0.4m 2 / g, 0.5m 2 / g, 0.6m 2 / g, 0.7m 2 / g or any two values ​​within this range. By controlling the specific surface area of ​​the negative electrode active material within the above range, it is less likely to react with the electrolyte and has low lithium-ion diffusion resistance. Combining it with polyurethane and carbon nanotubes, or with polyurethane, carbon nanotubes and styrene-acrylate copolymers, is beneficial for improving cycle performance and first coulombic efficiency while also taking into account kinetic performance.

[0053] In some embodiments of this application, the particle size Dv50 of the negative electrode active material is from 6 μm to 10 μm. For example, the particle size Dv50 of the negative electrode active material can be 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, or any combination of two values ​​in between. When the particle size Dv50 of the negative electrode active material is within the above range, it is less likely to react with the electrolyte and has low lithium-ion diffusion resistance. Combining it with polyurethane and carbon nanotubes, or with polyurethane, carbon nanotubes, and styrene-acrylate copolymers, is beneficial for improving cycle performance and first coulombic efficiency while also considering kinetic performance.

[0054] In some embodiments of this application, the negative electrode material layer further includes a dispersant. This application does not impose any particular limitation on the type and content of the dispersant, as long as it can achieve the purpose of this application. For example, the mass percentage W5 of the dispersant can be 0.3% to 3%; the type of dispersant can include, but is not limited to, at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, or carboxymethyl cellulose.

[0055] In some embodiments of this application, the negative electrode material layer further includes a lubricant. This application does not impose any particular limitations on the type and content of the lubricant, as long as it achieves the purpose of this application. For example, the mass percentage content W6 of the lubricant can be 0.5% to 5%; the type of lubricant can include, but is not limited to, flake graphite.

[0056] In some embodiments of this application, the mass ratio X of the negative electrode material layer to the negative electrode current collector is 0.15 to 0.30; for example, the mass ratio X of the negative electrode material layer to the negative electrode current collector can be 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, or a range of any two values ​​in between. The mass of the negative electrode material layer increases with the coating mass of the negative electrode material layer. When the mass of the negative electrode material layer increases, the strain of the negative electrode material layer also increases during the charge and discharge process. The corresponding negative electrode current collector needs to withstand the strain of the negative electrode material layer. Therefore, negative electrode material layers of different masses need to be matched with negative electrode current collectors of different masses, that is, current collectors of different thicknesses. The resulting negative electrode sheet has better structural stability during charge and discharge cycles. By adjusting the mass ratio X of the negative electrode material layer to the negative electrode current collector within the above range, the masses of the two are matched, and the resulting negative electrode sheet has good structural stability during charge and discharge cycles, which is beneficial to improving the cycle performance of the secondary battery.

[0057] In some embodiments of this application, the coating weight (CW) of the negative electrode material layer is 25 mg / 1540.25 mm. 2 Up to 50mg / 1540.25mm 2 For example, the coating quality (CW) of the negative electrode material layer can be 25 mg / 1540.25 mm. 2 26mg / 1540.25mm 2 27mg / 1540.25mm 2 28mg / 1540.25mm 2 29mg / 1540.25mm 2 30mg / 1540.25mm 2 32mg / 1540.25mm 234mg / 1540.25mm 2 35mg / 1540.25mm 2 36mg / 1540.25mm 2 38mg / 1540.25mm 2 40mg / 1540.25mm 2 42mg / 1540.25mm 2 44mg / 1540.25mm 2 45mg / 1540.25mm 2 46mg / 1540.25mm 2 48mg / 1540.25mm 2 50mg / 1540.25mm 2 Or it can be any range consisting of two values ​​within this range. When the coating quality CW of the negative electrode material layer is within the above range, the resulting secondary battery exhibits a high energy density.

[0058] In some embodiments of this application, the thickness H of the negative electrode current collector can be from 7 μm to 22 μm. For example, the thickness H of the negative electrode current collector can be 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, or a range of any two values ​​in between. In this application, negative electrode current collectors of different thicknesses can be purchased, and the required thickness can be selected by combining micrometer measurement.

[0059] The ratio X of the mass of the negative electrode material layer to the mass of the negative electrode current collector varies with the coating mass CW of the negative electrode material layer and / or the thickness H of the negative electrode current collector. For example, when the coating mass CW of the negative electrode material layer remains constant, an increase in the thickness H of the negative electrode current collector will decrease the ratio X, and vice versa; when the thickness H of the negative electrode current collector remains constant, an increase in the coating mass CW of the negative electrode material layer will increase the ratio X, and vice versa.

[0060] In some embodiments of this application, the adhesion force F between the negative electrode material layer and the negative electrode current collector is from 30 N / m to 200 N / m. For example, the adhesion force F between the negative electrode material layer and the negative electrode current collector can be 30 N / m, 40 N / m, 50 N / m, 60 N / m, 70 N / m, 80 N / m, 90 N / m, 100 N / m, 110 N / m, 120 N / m, 130 N / m, 140 N / m, 150 N / m, 160 N / m, 170 N / m, 180 N / m, 190 N / m, 200 N / m, or a range consisting of any two values ​​within this range. An adhesion force F between the negative electrode material layer and the negative electrode current collector within the above range indicates good adhesion between them, resulting in strong structural stability of the negative electrode sheet. It also minimizes the impact on lithium-ion transport, which is beneficial for improving the cycle performance of the secondary battery while also considering kinetic performance.

[0061] In some embodiments of this application, the porosity P of the negative electrode sheet is between 28% and 42%; for example, the porosity P of the negative electrode sheet can be 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, or any range of two values ​​in between. When the porosity P of the negative electrode sheet is within this range, the particles of the negative electrode active material are less likely to break during the rolling process when preparing the negative electrode sheet, which helps to reduce side reactions during the charge and discharge process of the secondary battery and improve cycle performance. At the same time, the negative electrode material layer also has a suitable thickness and good electrolyte wettability, which can balance the kinetic performance and energy density of the secondary battery.

[0062] In some embodiments of this application, the compaction density of the negative electrode material layer is 0.7 g / cm³. 3 Up to 1.2 g / cm 3 For example, the compaction density of the negative electrode material layer can be 0.7 g / cm³. 3 0.8g / cm 3 0.9g / cm 3 1.0g / cm 3 1.1 g / cm³, 1.2 g / cm³ 3 Or it can be a range consisting of any two values. In this application, the compaction density of the negative electrode material layer varies with the coating quality CW of the negative electrode material layer and the cold pressing pressure during the preparation of the negative electrode sheet. Generally, when the coating quality CW of the negative electrode material layer is constant, an increase in cold pressing pressure increases the compaction density of the negative electrode material layer, and vice versa; when the cold pressing pressure is constant, an increase in the coating quality CW of the negative electrode material layer increases the compaction density of the negative electrode layer, and vice versa.

[0063] In some embodiments of this application, the specific capacity C of the negative electrode sheet is between 1600 mAh / g and 2500 mAh / g; for example, the specific capacity C of the negative electrode sheet can be 1600 mAh / g, 1700 mAh / g, 1800 mAh / g, 1900 mAh / g, 2000 mAh / g, 2100 mAh / g, 2200 mAh / g, 2300 mAh / g, 2400 mAh / g, 2500 mAh / g, or a range of any two values ​​within this range. A specific capacity C of the negative electrode sheet within the above range indicates that the negative electrode active material exhibits a high capacity. Therefore, the secondary battery provided by this application not only improves cycle performance and initial coulombic efficiency but also has a high energy density. In this application, the specific capacity of the negative electrode sheet refers to the capacity exhibited by the negative electrode active material.

[0064] In some embodiments of this application, the film resistivity R of the negative electrode is from 30 Ω·cm to 90 Ω·cm. For example, the film resistivity R of the negative electrode is 30 Ω·cm, 40 Ω·cm, 50 Ω·cm, 60 Ω·cm, 70 Ω·cm, 80 Ω·cm, 90 Ω·cm, or a range of any two values ​​therein. This indicates that the negative electrode material layer has a good conductive network, and the resulting secondary battery has good cycle performance and a high initial coulombic efficiency.

[0065] This application does not impose any particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collector. For example, the composite current collector may be lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.

[0066] In this application, the secondary battery further includes a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The phrase "positive electrode material layer disposed on at least one surface of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the positive current collector along its thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that the term "surface" here can refer to the entire surface area of ​​the positive current collector, or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved.

[0067] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0068] The positive electrode material layer includes a positive electrode active material. This application does not impose any particular limitation on the positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material may include, but is not limited to, lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate.

[0069] The positive electrode material layer may also include a conductive agent and a binder. This application does not impose any particular limitation on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymers may include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode material layer; those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0070] This application does not impose any particular limitations on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector can be 5 μm to 20 μm, and the thickness of the positive electrode material layer can be 30 μm to 120 μm.

[0071] In this application, the secondary battery also includes a separator. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the separator material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The separator type may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0072] In some embodiments of this application, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used.

[0073] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0074] In some embodiments of this application, the inorganic layer comprises inorganic particles and a binder. This application does not particularly limit the inorganic particles; for example, the inorganic particles may include at least one selected from alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the binder; for example, the binder may be at least one of the binders described above. In some embodiments of this application, the polymer layer comprises a polymer, the polymer material of which includes at least one selected from polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0075] In this application, there is no particular limitation on the thickness of the diaphragm, as long as it can achieve the purpose of this application. For example, the thickness of the diaphragm can be from 3 μm to 30 μm.

[0076] In this application, the secondary battery also includes an electrolyte, which includes lithium salts and non-aqueous solvents.

[0077] This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (C4BLiO8, abbreviated as LiBOB), or lithium difluoroborate. This application does not impose any particular limitation on the content of the lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0078] This application does not impose any particular restrictions on non-aqueous solvents, as long as they can achieve the purpose of this application. For example, non-aqueous solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents.

[0079] The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0080] The secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0081] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0082] In some embodiments of this application, the secondary battery may include, but is not limited to, lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries. In some embodiments of this application, the secondary battery includes lithium-ion batteries.

[0083] A second aspect of this application provides an electronic device that includes the electronic device in any of the above embodiments. Therefore, the electronic device provided by this application has good performance in use.

[0084] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In some embodiments of this application, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0085] Example

[0086] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0087] Test methods and equipment:

[0088] Negative electrode sampling:

[0089] The lithium-ion battery was discharged at a constant current of 0.5C to the discharge cutoff voltage at 25°C. The lithium-ion battery was then disassembled under an argon atmosphere, and the negative electrode sheet was soaked in dimethyl carbonate solvent for 2 hours and dried at 60°C for 1 hour to obtain the negative electrode sheet. The discharge cutoff voltage of the lithium-ion battery in the embodiments and comparative examples of this application is 3.0V. It can be understood that when the voltage range marked on the battery packaging is 3.0V to 4.45V, the charging cutoff voltage is 4.45V, and the discharging cutoff voltage is 3.0V.

[0090] Unless otherwise specified, the following test methods shall use the negative electrode sheet obtained by the above method for testing.

[0091] Sphericity test method:

[0092] The negative electrode material layer on the surface of the negative electrode sheet is scraped off, and then heated to 1700℃ at 5℃ / min in an inert N2 atmosphere and kept at the temperature for 2h to obtain negative electrode active material powder, that is, silicon-containing material powder. The powder is ultrasonically dispersed in ethanol for 30min, and then the dispersion is dropped onto a glass slide and dried at 50℃.

[0093] Using the BT-1700 scanning image particle size and shape analysis system, the sphericity of silicon-containing materials was calculated using the diameter method. The diameter method calculates sphericity by measuring the diameter of the particles. Particle sphericity is defined as the ratio of the particle's perimeter-equivalent diameter to its area-equivalent diameter; this definition is two-dimensional. Q = d1 / da, where d1 represents the particle's perimeter-equivalent diameter, and the particle's perimeter is L, then L = 2 × π × d1; da represents the particle's area-equivalent diameter, and the particle's area is A, then A = π × da. 2 In particle size and shape analysis, the BT-1700 can analyze every single particle on the slide in one go and determine its sphericity.

[0094] Test for the mass percentage of polyurethane, W1:

[0095] The negative electrode material layer on the surface of the negative electrode sheet is scraped off, and dynamic thermomechanical analysis is performed on the negative electrode material layer. The weight loss peaks at 213℃ to 243℃ and 425℃ to 480℃ (inclusive) are the characteristic peaks of thermal decomposition of polyurethane. The weight loss rate within the above temperature range is the mass percentage content of polyurethane, W1.

[0096] Test for the mass percentage (W3) of styrene-acrylate copolymer:

[0097] The negative electrode material layer on the surface of the negative electrode sheet is scraped off, and dynamic thermomechanical analysis is performed on the negative electrode material layer. The weight loss peak at 390℃ to 425℃ is the characteristic peak of thermal decomposition of styrene-acrylate copolymer. The weight loss rate within the above temperature range is the mass percentage content W3 of styrene-acrylate copolymer.

[0098] Test of the mass percentage content W2 of the conductive agent:

[0099] The negative electrode material layer on the surface of the negative electrode sheet is scraped off, and dynamic thermomechanical analysis is performed on the negative electrode material layer. The weight loss peak at 1500℃ to 1700℃ is the characteristic peak of thermal decomposition of the conductive agent. The weight loss rate within the above temperature range is the mass percentage content W3 of the conductive agent.

[0100] Test of the mass percentage (W4) of the negative electrode active material:

[0101] The negative electrode material layer on the surface of the negative electrode sheet was scraped off, and dynamic thermomechanical analysis was performed on the negative electrode material layer to test the weight loss rate below 1700℃. The mass percentage of the negative electrode active material W4 = 100% - weight loss rate below 1700℃.

[0102] Test of polyurethane coverage S on the surface of the negative electrode active material:

[0103] The lithium-ion battery was disassembled to obtain the negative electrode sheet. Scanning electron microscopy (SEM) images of the negative electrode material layer on the surface of the negative electrode sheet were taken at a magnification of 10,000x. Ten particles of the negative electrode active material (i.e., silicon-containing particles) were randomly selected from the 12.7 μm × 9.9 μm field of view of the SEM image. Using SEM image software, the area of ​​the coated portion of the particle surface was delineated as 'a', and the uncoated portion as 'b'. The coating rate S = a / (a+b) × 100%.

[0104] Testing of carbon nanotube diameter d:

[0105] Transmission electron microscopy (TEM) was used to capture TEM images of the negative electrode material layer in the negative electrode sheet. The diameter of carbon nanotubes was measured at 100 locations in one TEM image. At the same time, the diameter of carbon nanotubes in five TEM images for each sample was counted, and the average value was taken as the diameter d of the carbon nanotubes.

[0106] Tests of the average particle size D and particle number N of styrene-acrylate copolymers:

[0107] Lithium-ion batteries were disassembled to obtain the negative electrode sheet. Scanning electron microscopy (SEM) images of the negative electrode material layer on the surface of the negative electrode sheet were taken at 10,000x magnification. Fifty styrene-acrylate copolymer particles were randomly selected from the 12.7μm × 9.9μm field of view of the SEM images. The diameter of the circumcircle of the particle profile was measured as the particle size. Following the same method, five more SEM images were selected to measure the particle size of the styrene-acrylate copolymer, and the average value was taken as the average particle size D of the styrene-acrylate copolymer. Simultaneously, the number of styrene-acrylate copolymer particles in the field of view of the five SEM images was counted, and the average value was recorded as N. It is understood that there may be errors in the statistical process, typically within ±3%.

[0108] Specific surface area test:

[0109] According to the national standard "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method" (GB / T 19587-2017), the specific surface area of ​​the negative electrode active material was tested by nitrogen adsorption using a specific surface area analyzer (model TristarⅡ3020M).

[0110] Particle size Dv50 test:

[0111] The particle size of the negative electrode active material was measured using a Malvern particle size analyzer (MasterSizer 2000). A layer of the negative electrode material was scraped from the surface of the negative electrode sheet and heated to 1700℃ at a rate of 5℃ / min in an inert N2 atmosphere, then held at that temperature for 2 hours to obtain silicon-containing material powder. 0.02g of the negative electrode active material was added to a 50mL clean beaker, along with 20mL of ethanol as a dispersant. The mixture was then ultrasonicated for 30 minutes in a 120W ultrasonic cleaner to completely disperse the negative electrode active material in the ethanol, yielding the sample dispersion. The above sample dispersion was tested using a Malvern particle size analyzer. First, the sample introduction system was rinsed with water. The refractive index was selected as 1.8, the absorbance as 1, the dispersant name as water, and the dispersant refractive index as 1.333. The analysis model was a general spherical model. When the background light intensity was <200 or the laser intensity was >70%, the sample was added until the occlusion reached 8% to 12%. Then, sample addition was stopped, and the sample was allowed to stand for 30 seconds. After the occlusion stabilized, the particle size distribution was obtained. Dv50 represents the particle size that accumulates to 50% of the volume in the volumetric particle size distribution, starting from the smallest particle size.

[0112] Test of adhesive strength F:

[0113] Cut the negative electrode sheet into strips of 100mm×20mm and attach them to double-sided tape. Roll the strip back and forth four times with a roller. Clamp one end of the strip in the tensile testing machine fixture and stretch it at 180°. Turn on the tensile testing machine and pull the negative electrode sheet at a uniform speed of 50mm / min until the negative electrode material layer peels off from the negative electrode current collector. After the test is completed, the adhesion between the negative electrode material layer and the negative electrode current collector is obtained.

[0114] Diaphragm resistivity testing methods:

[0115] First, the thickness of the negative electrode sheet was measured and recorded. Then, a BER2500 film resistance meter from Yuaneng Technology was used for testing. The holding time was 10 seconds, the pressure was 0.4 tons, and the test area was 153.9 mm². 2 Input the thickness of the negative electrode sheet, test 12 values, output a test report, and take the average of the 12 film resistivity values ​​to obtain the film resistivity R of the negative electrode sheet.

[0116] Testing the porosity of the negative electrode sheet:

[0117] Referring to the national standard GB / T24586-2009 "Determination of Apparent Density, True Density and Porosity of Iron Ore", the equipment used is: The II 1340 fully automatic true density meter uses the gas displacement method to test the porosity of the negative electrode sheet.

[0118] Testing the specific capacity of the negative electrode sheet:

[0119] The obtained negative electrode sheet was used to assemble coin cells. A lithium metal sheet was used as the counter electrode. The electrolyte and separator were the same as in Example 1. The obtained coin cells were subjected to the following testing procedure: constant current discharge at 0.05C to 0.005V, constant current discharge at 50μA to 0.005V, resting for 1 hour, constant current discharge at 10μA to 0.005V, resting for 5 minutes, constant current charging at 0.05C to 1.2V, and resting for 5 minutes. The capacity at 1.2V after constant current charging at 0.05C was taken as the electrode capacity. The specific capacity of the negative electrode sheet, C, was calculated as: electrode capacity / mass percentage of negative electrode active material in the negative electrode sheet, W4. The testing of W4 was performed according to the above-described "Testing of Mass Percentage of Negative Electrode Active Material, W4".

[0120] In this application, the specific capacity C of the negative electrode sheet is used to characterize the capacity utilization of the negative electrode active material. The higher the C value, the higher the capacity utilization of the negative electrode active material.

[0121] Test of the mass ratio X of the negative electrode material layer to the negative electrode current collector:

[0122] The cut area is 1540.25mm. 2The sample of the negative electrode sheet is weighed and recorded as m1. The negative electrode material layer is scraped off to obtain the negative electrode current collector, and its mass is recorded as m2. The ratio of the mass of the negative electrode material layer to the mass of the negative electrode current collector is X = (m1-m2) / m2.

[0123] Dynamic thermomechanical analysis and testing:

[0124] The negative electrode material layer on the surface of the negative electrode sheet was scraped off. Referring to JY T 0589.5-2020 "General Rules for Thermal Analysis Methods", a thermal analyzer was used to test the change in the mass of the negative electrode material layer powder with temperature. The thermal analyzer model was STA449F3, using an inert N2 atmosphere. The test temperature ranged from 35℃ to 1700℃, the heating rate was 5℃ / min, the purge gas flow rate was 60mL / min, and the protective gas flow rate was 20mL / min.

[0125] Loop testing:

[0126] At 25°C, the lithium-ion battery was charged at a constant current of 2C to 4.45V, then charged at a constant voltage of 4.45V to a current of 0.05C, and finally discharged at a constant current of 0.5C to 3.0V. This discharge capacity was recorded as the first cycle discharge capacity. The lithium-ion battery was subjected to 600 cycles under the same conditions, and the discharge capacity after 600 cycles was recorded. The cycle capacity retention rate characterizes the cycle performance of the lithium-ion battery; the higher the cycle capacity retention rate, the better the cycle performance of the lithium-ion battery.

[0127] Cycle capacity retention = (Discharge capacity after 600 cycles / Discharge capacity in the first cycle) × 100%.

[0128] First-time effectiveness test:

[0129] At 80℃ and 1.2MPa, the lithium-ion battery was charged at a constant current of 0.1C for 1200s, then at a constant current of 0.9C for 2400s. The pressure was then reduced to 0.6MPa, and the battery was charged at a constant current of 1C to 4.45V. The battery was then charged at a constant voltage of 4.45V to a current of 0.05C, and the capacity at this point was recorded as C0. The battery was then discharged at a constant current of 0.5C to 3.0V, and the capacity was recorded as D0. The initial coulombic efficiency was calculated as D0 / C0 × 100%.

[0130] Example 1-1

[0131] <Preparation of Negative Electrode Sheets>

[0132] The negative electrode active material SiC, polyurethane, styrene-acrylic emulsion, conductive agent carbon nanotubes, dispersant sodium carboxymethyl cellulose, and lubricant sheet graphite were mixed and mixed with deionized water as a solvent to form a slurry with a solid content of 25 wt%. The mixture was then stirred evenly in a vacuum mixer to obtain the negative electrode slurry. The styrene-acrylic emulsion was a styrene-acrylate copolymer emulsion with a solid content of 48%. The molar ratio of styrene monomer, acrylate monomer, and acrylic acid was 75:15:10. The acrylate monomer was methyl methacrylate, and the weight-average molecular weight of the styrene-acrylate copolymer was 15 W. The weight-average molecular weight of the polyurethane was 10 W. The mass ratio of the negative electrode active material, polyurethane, styrene-acrylate copolymer, conductive agent, dispersant, and lubricant was 87.8:2:6:1.5:0.7:2. In the negative electrode active material SiC, the molar ratio of silicon to carbon was 4:6.

[0133] The negative electrode slurry was uniformly coated onto one surface of a copper foil current collector with a thickness H of 12 μm, and dried at 120℃ to obtain a negative electrode sheet with a single-sided negative electrode material layer. The coating mass CW of the negative electrode material layer was 36 mg / 1540.25 mm. 2 The above steps are then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a negative electrode sheet with dimensions of 78mm × 875mm for later use. The cold-pressing pressure is 30t, and the compaction density of the negative electrode material layer is 1.0g / cm³. 3 .

[0134] <Preparation of the positive electrode>

[0135] LiCoO2 (positive electrode active material), Super P (conductive agent), and polyvinylidene fluoride (PVDF) (binder) were mixed in a mass ratio of 97.9:0.9:1.2. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%. The mixture was then stirred under vacuum until homogeneous to obtain the positive electrode slurry. This positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector. The foil was then dried at 120°C to obtain a positive electrode sheet with a single-sided coating of the positive electrode material layer. The coating mass of the positive electrode material layer was 267.8 mg / 1540 mm². 2 The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a positive electrode sheet with a size of 74mm×867mm for later use. The thickness of the single-sided positive electrode material layer is 42μm.

[0136] <Preparation of Electrolyte>

[0137] In an environment with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Then, the electrolyte salt LiPF6 was added to the organic solvent and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the electrolyte salt comprised 12.5% ​​by mass, with the remainder being the organic solvent.

[0138] <Septum>

[0139] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0140] <Preparation of Lithium-ion Batteries>

[0141] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The electrolyte prepared above is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming to obtain a lithium-ion battery. The formation upper limit voltage is 4.15V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0142] Examples 1-2 to Examples 1-30

[0143] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as in Examples 1-1. Specifically, when at least one of W1, W2, W3, and W4 changes, the contents of the dispersant and lubricant remain unchanged. In the SiC anode active material of Examples 1-28, the molar ratio of silicon to carbon is 1:1. In Examples 1-29 and 1-30, the coating quality CW of the anode material layer remains unchanged, and the pressure during cold pressing is adjusted so that the compaction density of the anode material layer in Examples 1-29 is 1.15 g / cm³. 3 The compaction density of the negative electrode material layer in Examples 1-30 is 0.75 g / cm³. 3 .

[0144] Examples 2-1 to 2-7

[0145] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0146] Comparative Example 1

[0147] Except for replacing the polyurethane and styrene-acrylate copolymer with polyacrylic acid (weight average molecular weight of 15W), the rest is the same as in Examples 1-1. The mass ratio of the negative electrode active material, polyacrylic acid, conductive agent, dispersant, and lubricant is 87.8:8:1.5:0.7:2.

[0148] Comparative Examples 2 to 7

[0149] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0150] The preparation parameters and performance tests for each embodiment and comparative example are shown in Tables 1 and 2.

[0151] Table 1 Note: " / " in Table 1 indicates that there is no corresponding parameter or performance.

[0152] As can be seen from Examples 1-1 to 1-30 and Comparative Examples 1 to 7, the negative electrode material layer of the examples contains polyurethane, and the mass percentage W1 of polyurethane is within the range of this application, as is the sphericity Z of the silicon-containing material. In Comparative Examples 1 to 4, the mass percentage W1 of polyurethane is not within the range of this application; in Comparative Example 5, the sphericity Z of the silicon-containing material is not within the range of this application; and in Comparative Examples 6 and 7, both the mass percentage W1 of polyurethane and the sphericity Z of the silicon-containing material are not within the range of this application. Compared with the comparative examples, the examples show a higher specific capacity C for the negative electrode sheet, indicating that the capacity of the silicon-containing material in the negative electrode sheet is better utilized. At the same time, the resulting secondary battery has a higher initial coulombic efficiency and cycle capacity retention, thus indicating that the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0153] As can be seen from Examples 1-1 to 1-5, Comparative Examples 3 and 4, polyurethane has the function of dispersing carbon nanotubes. Increasing its dosage improves the uniformity of carbon nanotube dispersion, increases the specific capacity of the negative electrode, and enhances the adhesion force F between the negative electrode material layer and the negative electrode current collector. However, with further increases in polyurethane dosage, since the carbon nanotubes are already uniformly dispersed, further increases in dosage do not significantly change the specific capacity C and adhesion force. The increased polyurethane coating thickness on the silicon-containing material surface actually hinders lithium-ion transport, affecting the kinetics of the lithium-ion battery and causing a decrease in initial coulombic efficiency and cycle performance. Therefore, controlling the mass percentage W1 of polyurethane can improve the initial coulombic efficiency and cycle performance of the secondary battery.

[0154] As can be seen from Examples 1-1, 1-21 to 1-23 and Comparative Example 5, when the sphericity of the silicon-containing material is small, the irregularity of the silicon-containing material particles increases, the contact area between particles increases, and the bonding force increases. However, the stress unevenness increases during cycling, resulting in more damage to the bonding network during cycling, which affects the cycling performance of the secondary battery.

[0155] The mass percentage W2 of the conductive carbon nanotubes and the diameter d of the carbon nanotubes affect the initial coulombic efficiency and cycle performance of the secondary battery. As can be seen from Examples 1-1, 1-6 to 1-10, when the diameter d of the carbon nanotubes is within the range of this application, the specific capacity C of the obtained negative electrode is high, and the initial coulombic efficiency and cycle capacity retention rate of the secondary battery are also relatively high. This indicates that the capacity of the silicon-containing material in the negative electrode is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved. When the diameter of the carbon nanotubes is relatively small, such as in Examples 1-6, some carbon nanotubes may be unevenly dispersed, resulting in low conductivity utilization and weak conductivity. The use of polyurethane provides a certain dispersion effect. Although its adhesion, specific capacity, initial coulombic efficiency, and cycle capacity retention rate are slightly worse than those of Examples 1-1, 1-7 to 1-9, they are still better than the comparative example. This indicates that Examples 1-6 still improve the initial coulombic efficiency and cycle performance compared to the comparative example. As can be seen from Examples 1-1, 1-11 to 1-12, when the mass percentage content W2 of carbon nanotubes is within the range of this application, the specific capacity C of the obtained negative electrode sheet is high, and the initial coulombic efficiency and cycle capacity retention of the secondary battery are also high. This indicates that the capacity of the silicon-containing material in the negative electrode sheet is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0156] The mass percentage W3 of styrene-acrylate and its average particle size D affect the initial coulombic efficiency and cycle performance of the secondary battery. As can be seen from Examples 1-1, 1-13 to 1-18, when the mass percentage W3 of styrene-acrylate is within the range of this application, the specific capacity C of the obtained negative electrode is high, and the initial coulombic efficiency and cycle capacity retention of the secondary battery are also relatively high. This indicates that the capacity of the silicon-containing material in the negative electrode is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved. When the amount of styrene-acrylate is low, such as in Examples 1-13, there is a risk of collapse of the bonding network in the negative electrode material layer during charging and discharging, which may affect the contact between the silicon-containing material particles. The initial coulombic efficiency and cycle performance of the lithium-ion battery are slightly lower than those of Examples 1-1, 1-14 to 1-16, but still better than the comparative example. This indicates that Examples 1-13 still improve the initial coulombic efficiency and cycle performance compared to the comparative example. When the amount of styrene-acrylate is too high, such as in Examples 1-18, there is a risk of hindering lithium-ion transport, which may affect the kinetic and cycle performance of the lithium-ion battery. The initial coulombic efficiency and cycle capacity retention are slightly lower than in Examples 1-1, 1-14 to 1-17, but still better than the comparative example, indicating that Examples 1-18 still improve the initial coulombic efficiency and cycle performance compared to the comparative example. As can be seen from Examples 1-1, 1-19 to 1-20, when the average particle size D of the styrene-acrylate is within the range of this application, the specific capacity C of the obtained negative electrode is high, and the initial coulombic efficiency and cycle capacity retention of the secondary battery are also relatively high, indicating that the capacity of the silicon-containing material in the negative electrode is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0157] The coverage of polyurethane on the surface of the negative electrode active material mainly varies with the mass percentage of polyurethane, thus affecting the initial coulombic efficiency and cycle performance of the secondary battery. As can be seen from Examples 1-1 to 1-5, when the mass percentage of polyurethane is within the range of this application, the coverage S obtained is also within the range of this application. Furthermore, when the mass percentage of polyurethane increases to a certain level, the coverage reaches 100% and no longer changes. The resulting negative electrode sheet has a high specific capacity C, and the initial coulombic efficiency and cycle capacity retention rate of the secondary battery are also relatively high. This indicates that the capacity of the silicon-containing material in the negative electrode sheet is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0158] In the scanning electron microscope (SEM) images of the negative electrode sheet, within a range of 12.7 μm × 9.9 μm, the number of styrene-acrylate copolymer particles varies with the mass percentage of styrene-acrylate and its average particle size, thus affecting the initial coulombic efficiency and cycle performance of the secondary battery. As can be seen from Examples 1-1, 1-13 to 1-20, when the number of styrene-acrylate copolymer particles varies with the mass percentage of styrene-acrylate and its average particle size within the range of this application, the number of styrene-acrylate copolymer particles in the obtained negative electrode sheet is also within the range of this application. Furthermore, the specific capacity C of the negative electrode sheet is high, and the initial coulombic efficiency and cycle capacity retention of the secondary battery are also relatively high. This indicates that the capacity of the silicon-containing material in the negative electrode sheet is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0159] The adhesion force F between the negative electrode material layer and the negative electrode sheet, the film resistivity R of the negative electrode sheet, and the porosity P of the negative electrode material layer vary with the sphericity of the silicon-containing material in the negative electrode material layer, the mass percentage of the silicon-containing material, the mass percentage of polyurethane, the diameter of the carbon nanotubes, the mass percentage of the conductive agent, the mass percentage of styrene-acrylate, and the average particle size of styrene-acrylate. When the above parameters are within the range of this application, the adhesion force F between the negative electrode material layer and the negative electrode sheet, the film resistivity R of the negative electrode sheet, and the porosity P of the negative electrode material layer are also within the range of this application. Moreover, the specific capacity C of the negative electrode sheet is high, and the initial coulombic efficiency and cycle capacity retention of the secondary battery are also high. This indicates that the capacity of the silicon-containing material in the negative electrode sheet is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0160] Specifically, Figures 1 and 2 are scanning electron microscope (SEM) images of the negative electrode sheets in Examples 1-15. As can be seen from the figures, the largest spherical particles are silicon-containing material particles, and the film-like substance covering the surface of these particles is polyurethane, with a coverage of 100%. Due to variations in the amount of polyurethane covering different areas of the particle surface, areas with higher polyurethane coverage appear darker, while areas with lower coverage appear lighter. The smaller spherical particles are styrene-acrylate copolymers, and the fine lines are conductive carbon nanotubes. Figure 3 is a dynamic thermomechanical analysis curve of the negative electrode material layer in Examples 1-1, showing characteristic peaks at 230℃, 415.7℃, and 455℃. The characteristic peak at 415.7℃ corresponds to the styrene-acrylate copolymer, while the characteristic peaks at 230℃ and 455℃ correspond to polyurethane. Figure 4 is a dynamic thermomechanical analysis curve of the negative electrode material layer in Examples 1-24, showing characteristic peaks at 230℃ and 455℃, which also correspond to polyurethane. In addition, characteristic peaks also exist at 300℃ in Figures 3 and 4, which correspond to sodium carboxymethyl cellulose dispersant in the negative electrode material layer.

[0161] Table 2

[0162] As can be seen from Examples 1-1, 2-1 to 2-2, when the specific surface area BET and Dv50 of the negative electrode active material are within the range of this application, the specific capacity C of the obtained negative electrode sheet is high, and the initial coulombic efficiency and cycle capacity retention of the secondary battery are also high. This indicates that the capacity of the silicon-containing material in the negative electrode sheet is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0163] As can be seen from Examples 1-1, 2-3 to 2-4, when the coating quality CW of the negative electrode material layer is within the range of this application, the specific capacity C of the obtained negative electrode sheet is high, and the initial coulombic efficiency and cycle capacity retention of the secondary battery are also high. This indicates that the capacity of the silicon-containing material in the negative electrode sheet is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0164] As can be seen from Examples 1-1, 2-5 to 2-6, when the mass ratio X of the negative electrode material layer to the negative electrode current collector is within the range of this application, the specific capacity C of the obtained negative electrode sheet is high, and the initial coulombic efficiency and cycle capacity retention rate of the secondary battery are also high. This indicates that the capacity of the silicon-containing material in the negative electrode sheet is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0165] As can be seen from Examples 1-1 and Examples 2-7, when the type of negative electrode active material is within the scope of this application, the specific capacity C of the obtained negative electrode sheet is high, and the initial coulombic efficiency and cycle capacity retention of the secondary battery are also high, indicating that the capacity of the silicon-containing material in the negative electrode sheet is better utilized, and the initial coulombic efficiency and cycle performance of the secondary battery are improved.

[0166] It should be noted that, in this document, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0167] The element connected by the terms "one of," "among," "a kind of," or other similar terms refers to any one of the listed elements. For example, "one of A or B" means only A or only B; similarly, "one of A, B, and C" means only A, only B, or only C. The element connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms refers to any combination of the listed elements. For example, "at least one of A or B" means only A, only B, A and B; similarly, "at least one of A, B, or C" means only A, only B, only C, only A and B, only A and C, only B and C, A and B and C.

[0168] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery comprising a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector, the negative electrode material layer comprising a negative electrode active material, polyurethane, and a conductive agent; at least a portion of the surface of the negative electrode active material is disposed of polyurethane, and at least a portion of the conductive agent is disposed in the polyurethane; The negative electrode active material includes a silicon-containing material, wherein the sphericity of the silicon-containing material is 90% to 100%. The conductive agent includes carbon nanotubes; Based on the mass of the negative electrode material layer, the mass percentage W1 of the polyurethane is 0.5% to 5%.

2. The secondary battery according to claim 1, wherein W1 ranges from 1% to 3%.

3. The secondary battery according to claim 1, wherein The polyurethane has a coverage rate of 50% to 100% on the surface of the negative electrode active material.

4. The secondary battery according to claim 1, wherein The diameter d of the carbon nanotubes is 10 nm to 100 nm.

5. The secondary battery according to claim 1, wherein Based on the mass of the negative electrode material layer, the mass percentage content W2 of the conductive agent is 0.5% to 2.5%.

6. The secondary battery according to claim 1, wherein The negative electrode material layer further includes a styrene-acrylate copolymer; based on the mass of the negative electrode material layer, the mass percentage W3 of the styrene-acrylate copolymer is 2% to 10%.

7. The secondary battery according to claim 6, wherein W3 is 4% to 8%.

8. The secondary battery according to claim 6, wherein The average particle size D of the styrene-acrylate copolymer is 200 nm to 300 nm.

9. The secondary battery according to claim 6, wherein In the scanning electron microscope image of the negative electrode, the number of styrene-acrylate copolymer particles is between 50 and 300 within a range of 12.7 μm × 9.9 μm.

10. The secondary battery according to any one of claims 1 to 9, wherein The silicon-containing material includes at least one of silicon-carbon material or silicon-oxygen material.

11. The secondary battery according to any one of claims 1 to 9, wherein The specific surface area of the negative active material is 0.1 m 2 / g to 0.7 m 2 / g.

12. The secondary battery according to any one of claims 1 to 9, wherein The particle size Dv50 of the negative electrode active material is 6 μm to 10 μm.

13. The secondary battery according to any one of claims 1 to 9, wherein The bonding force F between the negative electrode material layer and the negative electrode current collector is 30 N / m to 200 N / m.

14. The secondary battery according to any one of claims 1 to 9, wherein it satisfies at least one of the following characteristics: (1) the coating weight CW of the negative electrode material layer is 25 mg / 1540.25 mm 2 to 50 mg / 1540.25 mm 2 ; (2) Based on the mass of the negative electrode material layer, the mass percentage W4 of the negative electrode active material is 82% to 95%; (3) The mass ratio of the negative electrode material layer to the negative electrode current collector is 0.15 to 0.30; (4) The porosity of the negative electrode sheet is 28% to 42%; (5) The specific capacity C of the negative electrode sheet is 1600mAh / g to 2500mAh / g; (6) In the dynamic thermomechanical analysis curve of the negative electrode material layer, there are characteristic peaks at 230±1℃, 455±1℃ and 415.7±1℃.

15. An electronic device comprising a secondary battery according to any one of claims 1 to 14.

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