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 located on at least one surface of the negative electrode current collector; the negative electrode material layer comprises a first negative electrode material layer and a second negative electrode material layer; and the first negative electrode material layer is located between the second negative electrode material layer and the negative electrode current collector. The first negative electrode material layer comprises a first graphite material and a first negative electrode binder, the second negative electrode material layer comprises a silicon-based material and a second graphite material, the first negative electrode binder comprises a first binder, and the first binder comprises poly(styrene-isoprene-acrylate). By means of the described configuration, the structural stability and cycle performance of the secondary battery are improved while the kinetic performance is maintained.

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, especially lithium-ion batteries, have been widely used in electronic products such as laptops and mobile phones in recent years. As the market demands longer battery life and extended lifespan for electronic products, the requirements for energy density and cycle capacity retention of lithium-ion batteries are becoming increasingly stringent. Silicon has a theoretical specific capacity far exceeding that of graphite, reaching up to 4200 mAh / g, making silicon-based materials the most promising candidate to replace graphite as the next-generation anode material for lithium-ion batteries.

[0003] However, during continuous charge and discharge, silicon-based materials are highly susceptible to volume expansion and contraction, leading to voids between silicon particles. This can ultimately cause the solid electrolyte interphase (SEI) film to rupture, affecting electron and lithium-ion transfer. Furthermore, repairing the SEI film consumes electrolyte and active lithium, resulting in cycle degradation. In addition, silicon particles undergo lithiation and delithiation, causing significant expansion and contraction. This leads to instability at the interface between the material layers and the current collector in the electrode, compromising the structural integrity and stability of the negative electrode. Therefore, improving the cycle stability of silicon-based materials and reducing the expansion of silicon-based negative electrode sheets are crucial. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device that improves the structural stability and cycle performance of the secondary battery while taking into account dynamic performance.

[0005] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0006] Existing technologies primarily suppress the expansion of silicon-based materials by increasing the binder content in the negative electrode material layer, thereby improving cycle stability and reducing electrode expansion. However, when the binder content in the negative electrode material layer is high, the content of active materials decreases, leading to a reduction in the energy density of the secondary battery. Furthermore, since binders often have low conductivity, more binder reduces the charging speed and kinetic performance of the secondary battery. Therefore, this application provides a secondary battery and electronic device that, while maintaining kinetic performance, improves the structural stability and cycle performance of the secondary battery. The specific technical solution is as follows:

[0007] The first aspect of this application provides a secondary battery, comprising a negative electrode sheet, a negative electrode current collector, and a negative electrode material layer located on at least one surface of the negative electrode current collector. The negative electrode material layer comprises a first negative electrode material layer and a second negative electrode material layer, with the first negative electrode material layer located between the second negative electrode material layer and the negative electrode current collector. The first negative electrode material layer comprises a first graphite material and a first negative electrode binder, the second negative electrode layer comprises a silicon-based material and a second graphite material, and the first negative electrode binder comprises a first binder comprising poly(styrene-isoprene-acrylate). By combining the first and second negative electrode material layers and adding poly(styrene-isoprene-acrylate) to the first negative electrode material layer, the interfacial stability between the first and second negative electrode material layers can be improved while maintaining kinetic performance, thereby improving the structural stability of the negative electrode sheet and thus improving the structural stability and cycle performance of the secondary battery.

[0008] In some embodiments of this application, the monomers forming poly(styrene-isoprene-acrylate) include styrene, isoprene, and acrylate. Based on the mass of poly(styrene-isoprene-acrylate), in the repeating units of poly(styrene-isoprene-acrylate), the mass percentage of styrene units is A, the mass percentage of isoprene units is B, and the mass percentage of acrylate units is C, where A > B > C, 2.2 ≤ A / B ≤ 4.8, preferably 2.8 ≤ A / B ≤ 4.1; 1.1 ≤ B / C ≤ 3.2, and 16% ≤ B ≤ 27%. When the monomers forming poly(styrene-isoprene-acrylate) satisfy the above settings, the structural stability of the first negative electrode material layer and the interfacial stability between the first negative electrode material layer and the second negative electrode material layer are improved. The secondary battery of this application improves structural stability and cycle performance while taking into account kinetic performance.

[0009] In some embodiments of this application, the glass transition temperature of the first binder is Tg℃, where -20 ≤ Tg ≤ 30. By controlling the glass transition temperature of the first binder within the above range, the first binder can maintain good flexibility and adhesion within the operating temperature range of the secondary battery. The secondary battery of this application improves structural stability and cycle performance while taking into account kinetic performance.

[0010] In some embodiments of this application, the mass percentage of the first negative electrode binder is W1, based on the mass of the first negative electrode material layer, where 1.5% ≤ W1 ≤ 15%. By controlling the mass percentage of the first negative electrode binder within the above range, the secondary battery of this application improves structural stability and cycle performance while taking into account kinetic performance.

[0011] In some embodiments of this application, the first negative electrode binder further includes a second binder, which includes at least one of polyacrylic acid, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol. Based on the mass of the first negative electrode material layer, the mass percentage content of the second binder is W1', where 0.2% ≤ W1' ≤ 5.2%. By selecting the above-mentioned types of second binders and controlling the mass percentage content of the second binder within the above range, a synergistic effect can be formed between different binders. The secondary battery of this application improves structural stability and cycle performance while taking into account kinetic performance.

[0012] In some embodiments of this application, based on the mass of the first negative electrode material layer, the mass percentage of the first graphite material is W2, where 83% ≤ W2 ≤ 97%. By controlling the mass percentage of the first graphite material within the above range, the secondary battery achieves a high energy density while simultaneously improving kinetic performance, structural stability, and cycle performance.

[0013] In some embodiments of this application, 5.5 ≤ W2 / W1 ≤ 45. By adjusting the value of W2 / W1 within the above range, the secondary battery of this application improves structural stability and cycle performance while taking into account kinetic performance.

[0014] In some embodiments of this application, the second negative electrode material layer includes a second negative electrode binder, which includes at least one of polyacrylic acid, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol. Based on the mass of the second negative electrode material layer, the mass percentage content of the second negative electrode binder is W3, where 0.5% ≤ W3 ≤ 10%. By selecting the above-mentioned types of second negative electrode binders and controlling the mass percentage content of the second negative electrode binder within the above range, the stability of the silicon-based material is increased while the impedance of the negative electrode sheet is reduced, thereby improving the capacity retention rate and charging speed. The secondary battery of this application improves structural stability and cycle performance while taking into account dynamic performance.

[0015] In some embodiments of this application, the mass percentage of silicon-based material is W4, where 5% ≤ W4 ≤ 50%, based on the mass of the second negative electrode material layer. By controlling the mass percentage of silicon-based material within the above range, the secondary battery achieves a high energy density while balancing kinetic performance, improving structural stability and cycle performance.

[0016] In some embodiments of this application, 5 ≤ W4 / W3 ≤ 20. By adjusting the value of W4 / W3 within the above range, the secondary battery of this application further improves structural stability and cycle performance while taking into account kinetic performance.

[0017] In some embodiments of this application, the mass percentage of the second graphite material is W5, based on the mass of the second negative electrode material layer, where 45% ≤ W5 ≤ 90%. By controlling the mass percentage of the second graphite material within the above range, the secondary battery of this application further improves structural stability and cycle performance while taking into account kinetic performance.

[0018] In some embodiments of this application, the silicon-based material includes at least one of elemental silicon, silicon oxide, silicon carbide, or silicon alloy, and the first graphite material and the second graphite material are each independently selected from at least one of hard carbon, soft carbon, artificial graphite, or natural graphite. By selecting the aforementioned silicon-based material, first graphite material, and second graphite material, and by combining the first negative electrode material layer with the second negative electrode material layer, the secondary battery of this application improves structural stability and cycle performance while balancing kinetic performance and energy density.

[0019] In some embodiments of this application, the specific surface area of ​​the silicon-based material is S m 2 / g, 5≤S≤25. By adjusting the specific surface area of ​​the silicon-based material within the above range, the secondary battery of this application improves structural stability and cycle performance while taking into account kinetic performance.

[0020] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device of this application has good performance characteristics.

[0021] The beneficial effects of this application are:

[0022] This application provides a secondary battery and an electronic device. The secondary battery includes a negative electrode sheet, which includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer, with the first negative electrode material layer located between the second negative electrode material layer and the negative current collector. The first negative electrode material layer includes a first graphite material and a first negative electrode binder. The second negative electrode layer includes a silicon-based material and a second graphite material. The first negative electrode binder includes a first binder comprising poly(styrene-isoprene-acrylate). By combining the first and second negative electrode material layers and adding poly(styrene-isoprene-acrylate) to the first negative electrode material layer, the interfacial stability between the first and second negative electrode material layers can be improved while maintaining kinetic performance, thereby improving the structural stability of the negative electrode sheet and consequently improving the structural stability and cycle performance of the secondary battery.

[0023] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0024] 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.

[0025] Figure 1 is a partial cross-sectional view of the negative electrode sheet along its own thickness direction after it is unfolded in one embodiment of this application.

[0026] Reference numerals: negative electrode 10; negative electrode current collector 11; negative electrode material layer 12; first negative electrode material layer 121; second negative electrode material layer 122. Detailed Implementation

[0027] 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.

[0028] 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:

[0029] 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 located on at least one surface of the negative current collector, the negative electrode material layer including a first negative electrode material layer and a second negative electrode material layer, the first negative electrode material layer being located between the second negative electrode material layer and the negative current collector. The first negative electrode material layer includes a first graphite material and a first negative electrode binder, the second negative electrode layer includes a silicon-based material and a second graphite material, and the first negative electrode binder includes a first binder comprising poly(styrene-isoprene-acrylate).

[0030] In this application, the unfolded negative electrode sheet is defined with its length direction as the X direction, its width direction as the Y direction, and its thickness direction as the Z direction. For example, as shown in FIG1, the negative electrode sheet 10 includes a negative electrode current collector 11 and negative electrode material layers 12 located on the two surfaces of the negative electrode current collector. The negative electrode material layers 12 include a first negative electrode material layer 121 and a second negative electrode material layer 122, with the first negative electrode material layer 121 located between the second negative electrode material layer 122 and the negative electrode current collector 11.

[0031] The inventors discovered that a layered design combining silicon-based materials and graphite with an overlay of negative electrode material can effectively reduce the expansion of the negative electrode sheet. In existing technologies, silicon-based materials are generally used as the active negative electrode material in the negative electrode layer closest to the negative electrode current collector. However, since the silicon-based material is in direct contact with the negative electrode current collector, the expansion stress of the silicon-based material may directly act on the negative electrode current collector during the charging and discharging process of the secondary battery. This weakens the bonding force between the negative electrode current collector and the negative electrode material layer, leading to an increase in the internal resistance of the secondary battery. Furthermore, silicon-based materials are prone to structural changes during cycling, and their expansion can directly compress the upper negative electrode material layer, potentially causing delamination between the negative electrode material layers. This reduces the structural stability of the negative electrode sheet, thus affecting the structural stability and cycle performance of the secondary battery. In addition, silicon-based materials have relatively poor conductivity; using silicon-based materials as the active negative electrode material in the negative electrode layer closest to the negative electrode current collector may affect the transport of lithium ions within the negative electrode material layer, thereby affecting the kinetic performance of the secondary battery.

[0032] In this application, the active material of the first negative electrode material layer closest to the negative electrode current collector is graphite. Graphite has good mechanical strength and stability, and can withstand certain pressure and deformation. Using it as a base layer can provide more stable support for the entire negative electrode structure, reduce structural instability factors caused by volume changes and chemical reactions of silicon-based materials, and thus improve the cycle life of the secondary battery. In addition, graphite has better conductivity than silicon-based materials. Graphite has a higher lithium-ion diffusion coefficient and good electronic conductivity. When the first negative electrode material layer including graphite is closer to the negative electrode current collector, it is beneficial to improve the dynamic performance of the secondary battery.

[0033] Furthermore, placing the second negative electrode material layer containing silicon-based material on the surface helps reduce the pressure on the negative electrode current collector caused by its volume expansion, thereby reducing the risk of deformation or damage to the negative electrode current collector due to excessive stress and improving the cycle stability of the secondary battery. Enriching the second negative electrode material layer with silicon-based material utilizes the characteristic of silicon particles expanding during charging, increasing the porosity of the second negative electrode material layer. This increases the channels for electrolyte to enter the negative electrode sheet, adds more ion transport paths, effectively reduces ion transport impedance, and increases energy density while improving charging speed. Therefore, the secondary battery exhibits excellent kinetic performance.

[0034] However, the expansion and contraction of silicon-based materials during charging and discharging may lead to instability at the interface between the first and second negative electrode material layers, or even delamination between them. In this application, the first binder in the first negative electrode material layer comprises poly(styrene-isoprene-acrylate). This binder possesses a certain degree of flexibility and elasticity, acting as a buffer when the silicon-based material expands or contracts, reducing the risk of delamination or peeling of the negative electrode material layer due to the volume expansion of the silicon-based material. This further improves the stability of the negative electrode structure. Furthermore, it exhibits good adhesion; the styrene segments form good adhesion to the current collector surface, while the isoprene and acrylate segments form strong physical and chemical bonds with the graphite particles, tightly bonding the graphite material to the negative electrode current collector. This reduces the risk of active material detachment during cycling, thereby further improving the cycle stability of the secondary battery.

[0035] Therefore, this application improves the structural stability and cycle performance of the secondary battery by layering the negative electrode material layers, placing a second negative electrode material layer including silicon-based material on the surface, and combining it with the first binder poly(styrene-isoprene-acrylate) in the first negative electrode material layer, while taking into account the kinetic performance.

[0036] In some embodiments of this application, the monomers forming poly(styrene-isoprene-acrylate) include styrene, isoprene, and acrylate. Based on the mass of poly(styrene-isoprene-acrylate), in the repeating units of poly(styrene-isoprene-acrylate), the mass percentage of styrene units is A, the mass percentage of isoprene units is B, and the mass percentage of acrylate units is C, where A > B > C, 2.2 ≤ A / B ≤ 4.8, preferably 2.8 ≤ A / B ≤ 4.1; 1.1 ≤ B / C ≤ 3.2, and 16% ≤ B ≤ 27%. For example, the value of A / B can be 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.1, 4.2, 4.5, 4.8, or any range of any two of these values. The range of A / B can be 2.2 to 4.8, 2.5 to 4.5, 2.5 to 4.3, 2.8 to 4.1, 3 to 4, 3.2 to 3.8, 3.5 to 3.8, and all of these ranges and subranges; the value of B / C can be 1.1, 1.3, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, or any two of these values. The range of values ​​for B / C can be 1.1 to 3.2, 1.3 to 3, 1.5 to 2.8, 1.8 to 2.5, 2 to 2.5, 2.2 to 2.5, and all of these ranges, as well as sub-ranges; the value of B can be 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, or any two of these values. The range of values ​​for B can be 16% to 27%, 18% to 25%, 20% to 25%, 22% to 25%, and all of these ranges, as well as sub-ranges. When the monomers forming the poly(styrene-isoprene-acrylate) meet the above-mentioned requirements, the styrene units in the poly(styrene-isoprene-acrylate) binder form good adhesion to the current collector surface. The isoprene units have good elasticity and flexibility, while the acrylate units can enhance the cohesive force of the material layer, thereby improving the adhesion of the poly(styrene-isoprene-acrylate), reducing the expansion of the first negative electrode material layer, improving the structural stability of the first negative electrode material layer and the interfacial stability between the first and second negative electrode material layers, and thus improving the structural stability of the negative electrode sheet. Furthermore, styrene has a rigid benzene ring structure, and a high content of styrene monomer units can form a stable conductive path within the first negative electrode material layer, reducing internal resistance, thereby improving the kinetic performance and cycle performance of the secondary battery. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining good kinetic performance.

[0037] In this application, it is understood that in the repeating unit of poly(styrene-isoprene-acrylate), the sum of the mass percentage of styrene unit, the mass percentage of isoprene unit and the mass percentage of acrylate unit is 100%, i.e., A+B+C=100%.

[0038] In some embodiments of this application, 48.45% ≤ A ≤ 79%, and 5% ≤ C ≤ 24.55%. For example, the value of A can be 48.45%, 49%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, 72%, 75%, 79%, or any range of any two of these values. The value range of A can be 48.45% to 79%, 50% to 75%, 55% to 65%, 60% to 65%, and all of these ranges, as well as sub-ranges. The value of C can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 24%, 24.55%, or any range of any two of these values. The value range of C can be 5% to 24.55%, 8% to 24%, 10% to 20%, 12% to 18%, 15% to 18%, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of A and C within the aforementioned ranges, when the monomers forming the poly(styrene-isoprene-acrylate) meet the above settings, the styrene units in the poly(styrene-isoprene-acrylate) binder form good adhesion to the current collector surface. The isoprene units possess good elasticity and flexibility, while the acrylate units enhance the cohesive force of the material layer, thereby improving the adhesion of the poly(styrene-isoprene-acrylate), reducing the expansion of the first negative electrode material layer, improving the structural stability of the first negative electrode material layer, and enhancing the interfacial stability between the first and second negative electrode material layers, thus improving the structural stability of the negative electrode sheet. Furthermore, styrene has a rigid benzene ring structure, and a high content of styrene monomer units can form a stable conductive path within the first negative electrode material layer, reducing internal resistance and thereby improving the kinetic and cycle performance of the secondary battery. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining kinetic performance.

[0039] In some embodiments of this application, the glass transition temperature of the first binder is Tg℃, where -20 ≤ Tg ≤ 30. For example, the value of Tg can be -20, -15, -10, -5, 0, 5, 10, 15, 20, 25, 30, or a range consisting of any two of these values; the range of Tg values ​​can be -20 to 30, -15 to 25, -10 to 20, -5 to 15, 0 to 15, 0 to 10, 5 to 10, and all such ranges and sub-ranges. By controlling the glass transition temperature of the first binder within the above range, the first binder can maintain good flexibility and adhesion within the operating temperature range of the secondary battery, and the first binder can provide stable structural support during cycling, thereby improving the stability of the entire negative electrode structure. Furthermore, at an appropriate glass transition temperature, the first binder is better able to fill the voids between graphite particles, enhancing the electron transport path between active material particles, thereby improving the kinetic performance and cycle performance of the secondary battery. Therefore, the secondary battery of this application improves structural stability and cycle performance while taking into account dynamic performance.

[0040] In some embodiments of this application, the mass percentage of the first negative electrode binder is W1, based on the mass of the first negative electrode material layer, where 1.5% ≤ W1 ≤ 15%. For example, the value of W1 can be 1.5%, 2%, 4%, 5%, 8%, 10%, 12%, 15%, or a range consisting of any two of these values; the range of W1 can be 1.5% to 15%, 2% to 15%, 5% to 15%, 8% to 12%, 10% to 12%, and all of these ranges, as well as sub-ranges. By controlling the mass percentage of the first negative electrode binder within the above ranges, the first negative electrode material layer exhibits better structural stability, reducing the risk of delamination or peeling of the negative electrode material layer due to the volume expansion of the silicon-based material, thereby further improving the stability of the negative electrode structure. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining kinetic performance.

[0041] In some embodiments of this application, the first negative electrode binder further includes a second binder, which includes at least one of polyacrylic acid, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol. Based on the mass of the first negative electrode material layer, the mass percentage of the second binder is W1', where 0.2% ≤ W1' ≤ 5.2%. For example, the value of W1' can be 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.2%, or a range of any two of these values; the value range of W1' can be 0.2% to 5.2%, 0.5% to 5%, 0.8% to 4.8%, 1% to 4.5%, 1.2% to 4.2%, 1.5% to 4%, 1.8% to 3.8%, 2% to 3.5%, 2.2% to 3.2%, 2.5% to 3%, 2.5% to 2.8%, and all of these ranges, as well as subranges. By selecting the aforementioned types of second binders and controlling their mass percentage within the specified range, a synergistic effect can be achieved between different binders. Poly(styrene-isoprene-acrylate) provides good adhesion and flexibility, while the second binder further enhances the bonding force with the active material, thereby further improving the binder's adhesion to the graphite active material and the current collector. Furthermore, the addition of the second binder can enhance the cohesion of the first negative electrode material layer, reducing the risk of delamination or peeling of the negative electrode material layer due to the volume expansion of the silicon-based material, thus further improving the stability of the negative electrode structure. In addition, the second binder possesses certain conductivity or hydrophilicity, which is beneficial for promoting electron or ion transport, thereby further improving the kinetic and cycle performance of the secondary battery. Moreover, when the second binder includes sodium carboxymethyl cellulose, the risk of active material particle agglomeration can be reduced during the preparation of the first negative electrode slurry, ensuring uniform dispersion of the active material particles in the first negative electrode slurry, which is beneficial for improving the overall performance of the secondary battery. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining kinetic performance.

[0042] In some embodiments of this application, based on the mass of the first negative electrode material layer, the mass percentage content of the first binder is W1”, 1.3% ≤ W1” ≤ 9.8%. For example, the value of W1” can be 1.3%, 1.5%, 1.8%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 9.2%, 9.5%, 9.8%, or a range of any two of these values; the value range of W1” can be 1.3% to 9.8%, 1.5% to 9.5%, 2% to 9%, 3% to 7%, 4% to 6%, and all of these ranges, as well as sub-ranges. By controlling the mass percentage content of the first binder within the above ranges, poly(styrene-isoprene-acrylate) provides good adhesion and flexibility. Combined with the second binder, this further improves the adhesion of the binder to the graphite active material and the current collector, reducing the risk of delamination or peeling of the negative electrode material layer due to the volume expansion of the silicon-based material, thereby further improving the stability of the negative electrode structure. Therefore, the secondary battery of this application improves structural stability and cycle performance while taking into account dynamic performance.

[0043] In some embodiments of this application, based on the mass of the first negative electrode material layer, the mass percentage content of the first graphite material is W2, where 83% ≤ W2 ≤ 97%. For example, the value of W2 can be 83%, 85%, 88%, 90%, 92%, 95%, 97%, or a range consisting of any two of these values; the range of W2 can be 83% to 97%, 85% to 95%, 88% to 95%, 90% to 95%, 92% to 95%, and all of these ranges, as well as sub-ranges. By controlling the mass percentage content of the first graphite material within the above ranges, the volume change of the first graphite material during the charging and discharging process of the secondary battery is small, which is beneficial to improving the structural stability of the negative electrode sheet. Furthermore, the first graphite material has good conductivity, which is beneficial to improving the kinetic performance of the secondary battery. Combined with the high-capacity silicon-based material in the second negative electrode material layer, while taking into account kinetic performance and improving the structural stability and cycle performance of the secondary battery, the secondary battery also has a high energy density.

[0044] In some embodiments of this application, 5.5 ≤ W2 / W1 ≤ 45. For example, the value of W2 / W1 can be 5.5, 6, 8, 10, 12, 15, 18, 20, 22, 25, 28, 30, 32, 35, 38, 40, 42, 45 or a range of any two of these values; the range of W2 / W1 can be 5.5 to 45, 6 to 42, 8 to 40, 10 to 38, 12 to 35, 15 to 32, 18 to 30, 20 to 30, 22 to 28, 25 to 28 and all of these ranges, as well as subranges. By controlling the W2 / W1 value within the aforementioned range, the risk of reduced energy density in the secondary battery due to excessive first negative electrode binder content is reduced, as is the risk of active material detachment during cycling due to insufficient first negative electrode binder content. Furthermore, this facilitates the formation of a more uniform conductive network, improves the conductivity of the first negative electrode material layer, and thus enhances the kinetic and cycle performance of the secondary battery. In addition, it helps maintain the structural stability of the negative electrode sheet during charge-discharge cycles, thereby improving the structural stability of the secondary battery. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining good kinetic performance.

[0045] In some embodiments of this application, the second negative electrode material layer includes a second negative electrode binder, which includes at least one of polyacrylic acid, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol. Based on the mass of the second negative electrode material layer, the mass percentage of the second negative electrode binder is W3, where 0.5% ≤ W3 ≤ 10%. For example, the value of W3 can be 0.5%, 0.8%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range of any two of these values; the range of W3 can be 0.5% to 10%, 0.8% to 10%, 1% to 9%, 2% to 8%, 3% to 7%, 4% to 6%, and all of these ranges, as well as sub-ranges. By selecting the aforementioned types of second negative electrode binders and controlling their mass percentage within the specified range, the surface of the second negative electrode binder contains carboxyl groups. These carboxyl groups can form strong hydrogen bonds with active materials containing hydroxyl groups, thereby uniformly coating the surface of the active material particles. Furthermore, it exhibits minimal swelling in the carbonate solvent of the electrolyte, resulting in a stable negative electrode structure during charge and discharge, reducing the risk of repeated SEI film formation due to excessive expansion and fragmentation of the silicon-based material. This increases the stability of the silicon-based material while reducing the impedance of the negative electrode, improving capacity retention and charging speed. Moreover, when the second negative electrode binder includes sodium carboxymethyl cellulose, the risk of active material particle agglomeration can be reduced during the preparation of the second negative electrode slurry, ensuring uniform dispersion of the active material particles and improving the overall performance of the secondary battery. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining kinetic performance.

[0046] In some embodiments of this application, the mass percentage of silicon-based material is W4, where 5% ≤ W4 ≤ 50%, based on the mass of the second negative electrode material layer. For example, the value of W4 can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, or a range of any two of these values; the range of W4 can be 5% to 50%, 8% to 48%, 10% to 45%, 12% to 42%, 15% to 40%, 18% to 38%, 20% to 35%, 22% to 32%, 25% to 30%, 25% to 28%, and all of these ranges, as well as sub-ranges. By controlling the mass percentage of silicon-based materials within the aforementioned range, the secondary battery achieves a higher energy density and reduces the risk of delamination or peeling of the negative electrode material layer due to excessive silicon-based material volume expansion, thereby improving the stability of the negative electrode structure. Therefore, while balancing kinetic performance and improving the structural stability and cycle performance of the secondary battery, it also achieves a high energy density.

[0047] In some embodiments of this application, 5 ≤ W4 / W3 ≤ 20. For example, the value of W4 / W3 can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range consisting of any two of these values; the range of W4 / W3 can be 5 to 20, 8 to 18, 10 to 15, 12 to 15, and all of these ranges, as well as sub-ranges. By adjusting the value of W4 / W3 within the above ranges, the second negative electrode binder, combined with the silicon-based material, can uniformly coat the surface of the active material particles, reducing the risk of repeated SEI film formation due to excessive expansion and fragmentation of the silicon-based material, and maintaining the structural stability of the second negative electrode material layer during charge and discharge, thereby improving the structural stability of the negative electrode sheet. While increasing the stability of the silicon-based material, the impedance of the negative electrode sheet is reduced, improving capacity retention and charging speed. Therefore, the secondary battery of this application further improves structural stability and cycle performance while taking into account kinetic performance.

[0048] In some embodiments of this application, the mass percentage of the second graphite material is W5, 45% ≤ W5 ≤ 90%, based on the mass of the second negative electrode material layer. For example, the value of W5 can be 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or a range of any two of these values; the range of W5 can be 45% to 90%, 50% to 85%, 55% to 80%, 60% to 75%, 65% to 70%, and all of these ranges, as well as sub-ranges. By controlling the mass percentage of the second graphite material within the above ranges, the combination of the second graphite material and the silicon-based material facilitates the formation of a more uniform conductive network, improves the conductivity of the second negative electrode material layer, and thus improves the kinetic performance of the secondary battery. Furthermore, during the charging and discharging process of the secondary battery, the second graphite material can provide support for the silicon-based material, reducing the impact of its volume changes on the negative electrode structure. Therefore, the secondary battery of this application not only takes into account the dynamic performance, but also further improves the structural stability and cycle performance.

[0049] In some embodiments of this application, the silicon-based material includes at least one of elemental silicon, silicon oxide, silicon carbide, or silicon alloy. The first graphite material and the second graphite material are each independently selected from at least one of hard carbon, soft carbon, artificial graphite, or natural graphite. The silicon carbide is a silicon-carbon composite material, 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 composite material. This application does not impose any particular limitation on the silicon-carbon composite material, as long as it achieves the purpose of this application. For example, the silicon-carbon composite material can be a composite material obtained by deposition. Exemplarily, the silicon-carbon composite material can be silicon material deposited on a carbon skeleton or carbon material deposited on a silicon skeleton. The silicon oxide includes SiOx, where 0 < x < 2. Exemplarily, the silicon oxide can include silicon suboxide (SiO, where the molar ratio of silicon to oxygen is 1:1). By selecting the aforementioned silicon-based material, first graphite material, and second graphite material, and by combining the first negative electrode material layer with the second negative electrode material layer, the structural stability of the negative electrode sheet is improved, while the negative electrode sheet also has a lower impedance. Therefore, the secondary battery of this application improves structural stability and cycle performance while taking into account both kinetic performance and energy density.

[0050] In some embodiments of this application, the specific surface area of ​​the silicon-based material is S m 2 / g, 5≤S≤25. For example, the value of S can be 5, 8, 10, 12, 15, 18, 20, 22, 25, or any two of these values; the range of S values ​​can be 5 to 25, 8 to 22, 10 to 20, 12 to 18, 13 to 15, and all of these ranges, as well as sub-ranges. By controlling the specific surface area of ​​the silicon-based material within the above range, it is beneficial for the negative electrode to maintain good structural stability during cycling, reducing the risk of delamination or peeling of the negative electrode material layer due to volume expansion caused by silicon-based materials with excessively large specific surface areas, thereby improving the structural stability of the secondary battery. Furthermore, silicon-based materials with moderate specific surface areas can form a more uniform conductive network with the second graphite material, improving the conductivity of the second negative electrode material layer and reducing the internal resistance of the secondary battery. Therefore, the secondary battery of this application improves structural stability and cycle performance while taking into account kinetic performance.

[0051] In this application, there are no particular restrictions on the method of controlling the specific surface area S of silicon-based materials, as long as the purpose of this application can be achieved. For example, it can be achieved through crushing, sieving, grinding, etc.

[0052] In some embodiments of this application, the coating mass of the first negative electrode material layer is a mg / cm³. 2 The coating mass of the second negative electrode material layer is b mg / cm³. 2The value of a+b is 6.5 ≤ a+b ≤ 12.5. For example, the value of a+b can be 6.5, 7, 8, 9, 10, 11, 12, 12.5, or any two of these values; the range of a+b can be 6.5 to 12.5, 7 to 12, 8 to 11, 9 to 10, and all of these ranges, as well as sub-ranges. By adjusting the value of a+b within the above range, while maintaining the energy density of the secondary battery, it is beneficial to reduce the risk of reduced kinetic performance of the secondary battery due to excessive coating quality of the negative electrode material layer. Therefore, the secondary battery of this application improves structural stability and cycle performance while balancing kinetic performance and energy density.

[0053] In some embodiments of this application, 3.5 ≤ a ≤ 11; and / or, 1 ≤ b ≤ 4. For example, the value of 'a' can be 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.2, 9.5, 10, 10.5, 11, or any range of any two of these values. The value range of 'a' can be 3.5 to 11, 4.5 to 9.2, 5 to 9, 5.5 to 8.5, 6 to 8, 6.5 to 7.5, and all of these ranges, as well as sub-ranges. The value of 'b' can be 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, or any range of any two of these values. The value range of 'b' can be 1 to 4, 1.2 to 3.5, 1.5 to 3.2, 1.8 to 3, 2 to 2.8, 2.2 to 2.5, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of a and / or b within the aforementioned range, while maintaining the energy density of the secondary battery, it is beneficial to reduce the risk of reduced kinetic performance of the secondary battery due to excessive coating quality of the first and / or second negative electrode material layers. Therefore, the secondary battery of this application improves structural stability and cycle performance while balancing kinetic performance and energy density.

[0054] In some embodiments of this application, the compaction density (PD) of the negative electrode material layer is 1.70 g / cm³. 2 Up to 1.95 g / cm 2 For example, the value of PD can be 1.70, 1.75, 1.80, 1.85, 1.90, 1.95, or a range consisting of any two of these values; the value range of PD can be 1.70 to 1.95, 1.75 to 1.90, 1.80 to 1.90, 1.80 to 1.85, and all such ranges and sub-ranges. By adjusting the value of PD within the above ranges, while maintaining the energy density of the secondary battery, it is beneficial to reduce the risk of reduced kinetic performance of the secondary battery due to excessive compaction density of the negative electrode material layer. Therefore, the secondary battery of this application improves structural stability and cycle performance while balancing kinetic performance and energy density.

[0055] In some embodiments of this application, the first negative electrode material layer further includes a first conductive agent, and the second negative electrode material layer further includes a second conductive agent. The first and second conductive agents are each independently selected from at least one of conductive carbon materials, conductive carbon black (SP), carbon nanotubes (CNTs), vinyl ester glass fiber (VGCF), metal particles, or metal fibers. This arrangement facilitates the formation of a uniform and stable conductive network in both the first and second negative electrode material layers, thereby reducing the internal resistance of the negative electrode sheet. Therefore, the secondary battery of this application improves structural stability and cycle performance while maintaining kinetic performance.

[0056] This application does not impose any particular restrictions on the preparation method of the negative electrode sheet, as long as it can achieve the purpose of this application. For example, the preparation method of the negative electrode sheet includes, but is not limited to, the following steps: (1) mixing the first graphite material, the first negative electrode binder and the first conductive agent evenly to obtain the first negative electrode slurry; (2) mixing the silicon-based material, the second graphite material, the second negative electrode binder and the second conductive agent evenly to obtain the second negative electrode slurry; (3) simultaneously and evenly coating the first material layer slurry and the second material layer slurry obtained above onto one surface of the negative electrode current collector copper foil using a double-layer coating machine, and drying to obtain a negative electrode sheet with the first negative electrode material layer and the second negative electrode material layer on one side; (4) repeating the above steps on the other surface of the negative electrode current collector to obtain a negative electrode sheet with the first negative electrode material layer and the second negative electrode material layer on both sides.

[0057] In this application, the mass percentage of the first graphite material and the mass percentage of the first negative electrode binder in the first negative electrode material layer can be controlled by adjusting the mass ratio of the first graphite material, the first negative electrode binder, and the first conductive agent added to the first negative electrode slurry; the mass percentage of the first binder and the mass percentage of the second binder in the first negative electrode material layer can be controlled by adjusting the mass ratio of the first binder and the second binder added to the first negative electrode binder in the first negative electrode slurry; and the mass percentage of the silicon-based material, the second graphite material, the second negative electrode binder, and the second conductive agent in the second negative electrode slurry can be controlled by adjusting the mass ratio of the silicon-based material, the second graphite material, the second negative electrode binder, and the second conductive agent added to the second negative electrode slurry.

[0058] 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 aforementioned "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 "surface" here can be the entire surface area of ​​the positive current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the positive current collector, as long as the purpose of this application is achieved. For example, the positive current collector can include aluminum foil, aluminum alloy foil, or composite current collectors (e.g., aluminum-carbon composite current collectors). The positive electrode material layer of this application includes a positive electrode active material; this application has no particular limitation on the type of positive electrode active material, as long as the purpose of this application is achieved. For example, the positive electrode active material can include lithium nickel cobalt manganese oxide (LiNi). 0.90 Co 0.05 Mn 0.05At least one of the following: O2 (NCM955), NCM811, NCM622, NCM523, NCM111, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, or lithium manganese iron phosphate. In this application, the positive electrode active material may also contain non-metallic elements, such as at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. In this application, there are no 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 is achieved. In this application, the positive electrode material layer may also include a binder and a conductive agent. In this application, there are no particular limitations on the type of binder in the positive electrode material layer, as long as the purpose of this application is achieved. For example, the binder may include, but is not limited to, at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. This application does not impose any particular limitation on the type of conductive agent in the positive electrode material layer, as long as it can 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, Ketjen black, graphene, metallic materials, or conductive polymers. 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. 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 it can achieve the purpose of this application.

[0059] In this application, the electrolyte includes lithium salts and non-aqueous solvents. 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 (LiBOB), or lithium difluoroborate. This application does not impose any particular limitation on the content of lithium salts in the electrolyte, as long as it achieves the purpose of this application. This application does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. 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). Fluorinated carbonate 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. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 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.

[0060] This application does not impose any particular limitation on the diaphragm, as long as it can achieve the purpose of this application. For example, the diaphragm material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) mainly composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. 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 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. Optionally, at least one surface of the substrate layer is provided with a surface treatment layer, which may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. In some embodiments of this application, the inorganic layer comprises inorganic particles and a binder. This application does not impose any particular limitation on the inorganic particles; for example, the inorganic particles may include at least one selected from alumina, silicon oxides, 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 impose any particular limitation on 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 polymers, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene). In this application, the thickness of the diaphragm is not particularly limited, as long as it achieves the purpose of this application; for example, the thickness of the diaphragm may be from 3 μm to 30 μm.

[0061] 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.

[0062] 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.

[0063] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device of this application has good performance characteristics.

[0064] 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.

[0065] Example

[0066] 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.

[0067] Test methods and equipment:

[0068] Glass transition temperature test:

[0069] The glass transition temperature (Tg) of the first adhesive was measured by differential scanning calorimetry (DSC). 5 mg of the first adhesive sample was weighed and heated from -150 °C to 50 °C at a heating rate of 5 °C / min. The Tg of the first adhesive was obtained by DSC curve, with the unit being °C.

[0070] Tests A, B, and C:

[0071] 1) Material preparation: Disassemble the lithium-ion battery, remove the negative electrode sheet, soak it in dimethyl carbonate (DMC) for 20 minutes, and then rinse it with DMC and acetone in turn. After that, place the negative electrode sheet in an oven and bake it at 80°C for 12 hours to obtain the negative electrode sheet.

[0072] 2) Mechanical crushing: The negative electrode material layer is scraped off from the negative electrode sheet to obtain the powder of the negative electrode material layer. The powder of the negative electrode material layer is ground into fine particles (<1mm) to increase the solvent contact area.

[0073] 3) Soak the fine particles obtained in step 2) in tetrahydrofuran solvent, heat to 60°C, and sonicate for 2 hours to dissolve the binder into the solvent to obtain a mixture.

[0074] 4) Centrifuge the above mixture (5000 rpm, 20 minutes) to separate the supernatant (containing binder) and precipitate (active material + conductive agent).

[0075] 5) Add the precipitant methanol to the supernatant at a rate of 5 mL / min, stirring continuously. As the precipitant is added, the binder will gradually precipitate from the solution. Simultaneously, concentrate the solution containing the binder in a rotary evaporator (80℃, reduced pressure) to accelerate the precipitation of the binder and obtain the polymer.

[0076] 6) The polymer is heated to 450°C and thermally decomposed in an inert argon atmosphere. The high temperature causes the polymer chains to break and generate monomers.

[0077] 7) The monomers in the pyrolysis products are separated by gas chromatography, and quantitative analysis is performed by calibration curves to obtain the mass content of each monomer, i.e., the values ​​of A, B, and C.

[0078] Specific surface area S test of silicon-based materials:

[0079] 1) Refer to steps 1) to 4) of the above "Tests for A, B, and C" and retain the precipitate.

[0080] 2) Disperse the precipitate in deionized water.

[0081] 3) The silicon-based material is obtained by separating it using gravity separation, taking advantage of the density differences between the silicon-based material, the active material graphite, and the conductive agent (carbon nanotubes in this embodiment). Among them, the silicon-based material has the highest density value.

[0082] 4) After drying the obtained silicon-based material in a vacuum drying oven, it was placed into a sample tube. The specific surface area S of the silicon-based material was tested using a specific surface area analyzer (TristarⅡ3020M) via nitrogen adsorption / desorption method. The specific testing was conducted according to GB / T 19587-2017 "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method".

[0083] Cycle performance and structural stability testing of lithium-ion batteries:

[0084] At a test temperature of 25℃, the lithium-ion battery under test was left to stand for 5 minutes, and the initial thickness MMC0 of the lithium-ion battery was recorded. The lithium-ion battery was charged at a constant current of 1.65C to 4.26V, then charged at 1.3C to 4.28V, then charged at 1.1C to 4.32V, then charged at 1.0C to 4.35V, then charged at 0.7C to 4.4V, then charged at 0.4C to 4.52V, and then charged at a constant voltage of 4.52V to 0.05C; left to stand for 5 minutes, and then discharged at a constant current of 0.5C to 3.0V, left to stand for 5 minutes, and the discharge capacity C1 of the lithium-ion battery was recorded. After the above 3C charge / 0.5C discharge cycle was repeated 400 times, the thickness MMC1 and the discharge capacity C2 of the lithium-ion battery were recorded.

[0085] Capacity retention rate (%) after 400 cycles = C2 / C1 × 100%.

[0086] Expansion rate (%) after 400 cycles = (MMC1 - MMC0) / MMC0 × 100%.

[0087] Among them, the higher the capacity retention rate after 400 cycles, the better the cycle performance of the lithium-ion battery; the lower the expansion rate after 400 cycles, the better the structural stability of the lithium-ion battery.

[0088] Dynamic performance testing:

[0089] At a test temperature of 25℃, the lithium-ion battery under test was left to stand for 5 minutes, then charged at a constant current of 0.7C to 4.52V, and then charged at a constant voltage to 0.025C. After standing for 5 minutes, it was discharged at a constant current of 0.2C to 3.0V, yielding a discharge capacity C0.2. The above charging process was then repeated, discharging at a constant current of 2.0C to 3.0V, yielding a discharge capacity C2.0. The formula D = C2.0 / C0.2 was set. The value of D was used to determine the quality of the kinetic performance.

[0090] The larger the D value, the better the kinetic performance of the lithium-ion battery; the smaller the D value, the worse the kinetic performance of the lithium-ion battery.

[0091] Example 1

[0092] <Preparation of Negative Electrode Sheets>

[0093] The first graphite material (artificial graphite), the first binder (poly(styrene-isoprene-acrylate)) of the first negative electrode binder, the second binder (polyacrylic acid) of the first negative electrode binder, and the first conductive agent (carbon nanotubes) were mixed in a mass ratio of 90:5:3:2. Deionized water was added as a solvent, and the mixture was stirred evenly in a vacuum mixer to prepare a first negative electrode slurry with a solid content of 45 wt%. The second graphite material (artificial graphite), the silicon-based material (silicon carbide), the second negative electrode binder (polyacrylic acid), and the second conductive agent (carbon nanotubes) were mixed in a mass ratio of 63:30:5:2. Deionized water was added as a solvent, and the mixture was stirred evenly in a vacuum mixer to prepare a second negative electrode slurry with a solid content of 45 wt%. The first and second negative electrode slurries were simultaneously and evenly coated onto one surface of a 6 μm thick copper foil for negative electrode current collector using a double-layer coating machine. The coatings were then dried at 120°C to obtain a negative electrode sheet with a first negative electrode material layer and a second negative electrode material layer on one side. 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 the first and second negative electrode material layers. This is followed by cold pressing, cutting, and welding of tabs to obtain a negative electrode sheet with dimensions of 78mm × 875mm for later use. The coating mass 'a' of the first negative electrode material layer is 9.3 mg / cm³. 2 The coating mass b of the second negative electrode material layer is 2.5 mg / cm². 2 The compaction density (PD) of the negative electrode material layer is 1.80 g / cm³. 2 Based on the mass of the first negative electrode material layer, the mass percentage of the first negative electrode binder W1 is 8%, the mass percentage of the first binder W1' is 5%, the mass percentage of the second binder W1' is 3%, and the mass percentage of the first graphite material W2 is 90%. Based on the mass of the second negative electrode material layer, the mass percentage of the second negative electrode binder W3 is 5%, the mass percentage of the silicon-based material W4 is 30%, and the mass percentage of the second graphite material W5 is 63%. The specific surface area S of the silicon-based material is 15 m². 2 / g.

[0094] <Preparation of the positive electrode>

[0095] 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 single-sided coated positive electrode sheet with a coating mass of 325 mg / 1540 mm². 2 Then, the above steps are 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 use. The thickness of the positive electrode material layer on one side is 70μm.

[0096] <Septum>

[0097] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0098] <Preparation of Electrolyte>

[0099] 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. Lithium salt LiPF6 was then added to the organic solvent and mixed thoroughly to obtain the electrolyte. The concentration of the lithium salt was 1.2 mol / L, and the remainder was the organic solvent.

[0100] <Preparation of Lithium-ion Batteries>

[0101] 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 upper limit of the formation voltage is 4.2V, and the formation temperature is 70°C.

[0102] Examples 2 to 21

[0103] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1. Specifically, when the values ​​of W1 and W2 change, the mass percentage of the first conductive agent also changes accordingly, and the total mass percentage of the first negative electrode binder, the first graphite material, and the first conductive agent is 100%. When the values ​​of W3, W4, and W5 change, the mass percentage of the second conductive agent also changes accordingly, and the total mass percentage of the second negative electrode binder, the silicon-based material, the second graphite material, and the second conductive agent is 100%. When the value of S changes, the grinding time is adjusted so that its specific surface area is as shown in Table 1.

[0104] Comparative Example 1

[0105] Except for the preparation of the negative electrode sheet according to the following steps, the rest is the same as in Example 1.

[0106] <Preparation of Negative Electrode Sheets>

[0107] Artificial graphite (graphite material), silicon carbide (silicon-based material), polyacrylic acid (negative electrode binder), and carbon nanotubes (conductive agent) were mixed in a mass ratio of 63:30:5:2. Deionized water was added as a solvent, and the mixture was stirred evenly in a vacuum mixer to prepare a negative electrode slurry with a solid content of 45 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil used as a negative electrode current collector and dried at 120°C to obtain a negative electrode sheet with a single-sided negative electrode material layer. The above steps were then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided negative electrode material layer. The sheet was then cold-pressed, cut, and had tabs welded to obtain a negative electrode sheet with dimensions of 78 mm × 875 mm for later use. The coating weight of the negative electrode material layer was 11.8 mg / cm². 2 The compaction density (PD) of the negative electrode material layer is 1.80 g / cm³. 2 Based on the mass of the negative electrode material layer, the negative electrode binder has a mass percentage of 5%, the silicon-based material has a mass percentage of 30%, and the graphite material has a mass percentage of 63%. The specific surface area S of the silicon-based material is 15 m². 2 / g.

[0108] Comparative Example 2

[0109] Except for the fact that the preparation steps of the first negative electrode material layer in <Preparation of the negative electrode sheet> are the same as those of the second negative electrode material layer in Example 1, and the preparation steps of the second negative electrode material layer are the same as those of the first negative electrode material layer in Example 1, and the relevant preparation parameters are adjusted according to Table 1, the rest is the same as in Example 1.

[0110] Comparative Example 3

[0111] Except for adjusting the relevant preparation parameters according to Table 1, everything else is the same as in Example 1.

[0112] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1.

[0113] As can be seen from Examples 1 to 21 and Comparative Examples 1 to 3, by layering the negative electrode material layers and placing the second negative electrode material layer, including silicon-based material, on the surface, and combining it with the first binder poly(styrene-isoprene-acrylate) in the first negative electrode material layer, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D-value. This indicates that the lithium-ion battery of this application has good structural stability and cycle performance while balancing kinetic performance. In Comparative Example 1, the negative electrode material layer is not layered and there is no first binder; in Comparative Example 2, the positions of the first and second negative electrode material layers are exactly opposite to those in Example 1; in Comparative Example 3, there is no first binder in the first negative electrode material layer. In Comparative Examples 1 to 3, the lithium-ion batteries have a low capacity retention rate after 400 cycles, a high expansion rate after 400 cycles, and / or a small D-value, indicating that the lithium-ion batteries in the comparative examples cannot simultaneously achieve kinetic performance, structural stability, and cycle performance. The lithium-ion batteries in Examples 1 to 21 exhibit high capacity retention after 400 cycles, low expansion rate after 400 cycles, and large D-value, indicating that the lithium-ion batteries of this application have good structural stability and cycle performance while taking into account dynamic performance.

[0114] The values ​​of A / B, B / C, and B typically affect the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 7, when the values ​​of A / B, B / C, and B are within the range of this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0115] The glass transition temperature of the first binder typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 7, when the glass transition temperature of the first binder is within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D-value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0116] The value of W1 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 8 to 10, when the value of W1 is within the range of this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value, indicating that the lithium-ion battery of this application has good structural stability and cycle performance while taking into account kinetic performance.

[0117] The type and content of the second binder typically affect the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 8 to 12, and 19, when the type and content of the second binder are within the range of this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D-value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0118] The value of W2 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 8, and 9, when the value of W2 is within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0119] The value of W2 / W1 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 8 to 10, when the value of W2 / W1 is within the range of this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0120] The type and content of the second negative electrode binder typically affect the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 13 to 17, and 19, when the type and content of the second negative electrode binder are within the range of this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D-value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0121] The value of W4 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 13 to 15, when the value of W4 is within the range of this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value, indicating that the lithium-ion battery of this application has good structural stability and cycle performance while taking into account kinetic performance.

[0122] The W4 / W3 ratio typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 13 to 15, when the W4 / W3 ratio falls within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D-value. This indicates that the lithium-ion battery of this application achieves good structural stability and cycle performance while maintaining good kinetic performance.

[0123] The value of W5 typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 13 to 15, when the value of W5 is within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0124] The types of silicon-based materials, first graphite materials, and second graphite materials typically affect the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1 and 18, when the types of silicon-based materials, first graphite materials, and second graphite materials are within the scope of this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D-value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0125] The specific surface area of ​​silicon-based materials typically affects the kinetic performance, structural stability, and cycle performance of lithium-ion batteries. As can be seen from Examples 1, 20, and 21, when the specific surface area of ​​the silicon-based material is within the range specified in this application, the lithium-ion battery exhibits a high capacity retention rate after 400 cycles, a low expansion rate after 400 cycles, and a large D-value. This indicates that the lithium-ion battery of this application possesses good structural stability and cycle performance while maintaining good kinetic performance.

[0126] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, 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.

[0127] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0128] 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 located on at least one surface of the negative current collector, the negative electrode material layer comprising a first negative electrode material layer and a second negative electrode material layer, the first negative electrode material layer being located between the second negative electrode material layer and the negative current collector; The first negative electrode material layer includes a first graphite material and a first negative electrode binder, the second negative electrode material layer includes a silicon-based material and a second graphite material, the first negative electrode binder includes a first binder, and the first binder includes poly(styrene-isoprene-acrylate).

2. The secondary battery according to claim 1, wherein, The monomers forming poly(styrene-isoprene-acrylate) include styrene, isoprene, and acrylate. Based on the mass of poly(styrene-isoprene-acrylate), in the repeating units of poly(styrene-isoprene-acrylate), the mass percentage of styrene units is A, the mass percentage of isoprene units is B, and the mass percentage of acrylate units is C, where A > B > C, 2.2 ≤ A / B ≤ 4.8, 1.1 ≤ B / C ≤ 3.2, and 16% ≤ B ≤ 27%.

3. The secondary battery according to claim 2, wherein, 2.8≤A / B≤4.

1.

4. The secondary battery according to claim 1, wherein, The glass transition temperature of the first adhesive is Tg℃, -20≤Tg≤30.

5. The secondary battery according to claim 1, wherein, Based on the mass of the first negative electrode material layer, the mass percentage of the first negative electrode binder is W1, where 1.5% ≤ W1 ≤ 15%.

6. The secondary battery according to claim 5, wherein, The first negative electrode binder further includes a second binder, which includes at least one of polyacrylic acid, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol. Based on the mass of the first negative electrode material layer, the mass percentage of the second binder is W1', 0.2% ≤ W1' ≤ 5.2%.

7. The secondary battery according to claim 5, wherein, Based on the mass of the first negative electrode material layer, the mass percentage of the first graphite material is W2, 83% ≤ W2 ≤ 97%.

8. The secondary battery according to claim 7, wherein, 5.5≤W2 / W1≤45.

9. The secondary battery according to claim 1, wherein, The second negative electrode material layer includes a second negative electrode binder, which includes at least one of polyacrylic acid, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol. Based on the mass of the second negative electrode material layer, the mass percentage of the second negative electrode binder is W3, where 0.5% ≤ W3 ≤ 10%.

10. The secondary battery according to claim 9, wherein, Based on the mass of the second negative electrode material layer, the mass percentage of the silicon-based material is W4, where 5% ≤ W4 ≤ 50%.

11. The secondary battery according to claim 10, wherein, 5≤W4 / W3≤20.

12. The secondary battery according to claim 10, wherein, Based on the mass of the second negative electrode material layer, the mass percentage of the second graphite material is W5, 45% ≤ W5 ≤ 90%.

13. The secondary battery according to claim 1, wherein, The silicon-based material includes at least one of elemental silicon, silicon oxide, silicon carbide, or silicon alloy, and the first graphite material and the second graphite material are each independently selected from at least one of hard carbon, soft carbon, artificial graphite, or natural graphite.

14. The secondary battery according to claim 1, wherein, The specific surface area of ​​the silicon-based material is Sm 2 / g, 5≤S≤25.

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

Images