Secondary battery and electric device

Abstract

This application provides a secondary battery and an electrical device. The secondary battery includes a negative electrode sheet, which includes a negative electrode film layer. The negative electrode film layer includes a first active layer and a second active layer. The first active layer and / or the second active layer includes silicon, and the mass percentage of silicon in the second active layer is higher than the mass percentage of silicon in the first active layer. The first active layer includes a first composite binder, and the second active layer includes a second composite binder. The first composite binder and / or the second composite binder include binder A and binder B. Binder A is styrene-butadiene rubber, and binder B is selected from one or more of polyacrylic acid binders, polyacrylonitrile, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and their derivatives. The mass ratio of binder A to binder B in the first composite binder is higher than the mass ratio of binder A to binder B in the second composite binder.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and more particularly to a secondary battery and an electrical device. Background Technology

[0002] In recent years, with the increasingly widespread application of lithium-ion batteries, they have been widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, and many other fields. Due to the significant advancements in lithium-ion battery technology, higher requirements have been placed on their energy density and capacity retention. Summary of the Invention

[0003] This application was made in view of the above-mentioned problems, and its object is to provide a secondary battery and an electrical device. The secondary battery of the present invention can achieve both high energy density and capacity retention.

[0004] To achieve the above objectives, a first aspect of this application provides a secondary battery, comprising a negative electrode sheet, wherein the negative electrode sheet includes a negative current collector and a negative electrode film layer formed on at least one surface of the negative current collector, the negative electrode film layer including a first active layer and a second active layer, wherein the first active layer and / or the second active layer includes silicon.

[0005] The mass percentage of silicon in the second active layer is higher than that in the first active layer;

[0006] The first active layer includes a first composite adhesive, and the second active layer includes a second composite adhesive, wherein the first composite adhesive and / or the second composite adhesive includes adhesive A and adhesive B.

[0007] The adhesive A is styrene-butadiene rubber, and the adhesive B is selected from one or more of the following: polyacrylic acid adhesives, polyacrylonitrile, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and its derivatives.

[0008] The mass ratio of adhesive A to adhesive B in the first composite adhesive is higher than that in the second composite adhesive.

[0009] The dual-layer electrode design facilitates precise control of the negative electrode and enhances its stability, thereby extending the lifespan of the secondary battery. The inclusion of silicon in the first and / or second active layers contributes to the high energy density of the secondary battery. The use of a composite binder comprising binder A and binder B improves the adhesive strength while maintaining the good flexibility of the negative electrode film. Furthermore, a higher proportion of binder A in the first active layer (with a lower silicon content) improves the adhesion of the first composite binder; a higher proportion of binder B in the second active layer (with a higher silicon content) helps suppress volume expansion of the active material during charge and discharge and improves the structural stability of the negative electrode, thus enhancing the capacity retention of the secondary battery.

[0010] In some embodiments, the mass ratio of binder A to binder B in the first composite binder is 50:1 to 1:5; and / or the mass ratio of binder A to binder B in the second composite binder is 10:1 to 1:10. This is beneficial for improving the adhesion between the negative electrode film layer and the negative electrode current collector, suppressing the expansion of the active material in the second active layer with a high silicon content, improving the stability of the negative electrode film layer, and increasing the capacity retention rate of the secondary battery.

[0011] In some embodiments, the first active layer is located between the negative electrode current collector and the second active layer. This is beneficial in two ways: firstly, it provides sufficient adhesion to the negative electrode film and improves the structural stability of the negative electrode sheet, reducing the risk of negative electrode film detachment; secondly, it helps to improve the rate performance of the secondary battery.

[0012] In some embodiments, the difference between the mass percentage of silicon in the second active layer and the mass percentage of silicon in the first active layer is 5%-80%. This is beneficial for improving the structural stability of the negative electrode sheet, thereby improving the capacity retention rate and service life of the secondary battery.

[0013] In some embodiments, the mass percentage of silicon in the first active layer is greater than or equal to 0. This helps reduce the volume expansion of the first active layer, improves the stability of the negative electrode, and increases the capacity retention and lifespan of the negative electrode.

[0014] In some embodiments, the silicon content in the first active layer is 0%-50% by mass. This further improves the stability of the negative electrode, increases its capacity retention, and extends its service life.

[0015] In some embodiments, the silicon content in the second active layer is 5%-80% by mass. This is beneficial for increasing the specific capacity of the negative electrode film and improving the energy density of the secondary battery.

[0016] In some embodiments, the silicon content in the second active layer is 10%-60% by mass. This is beneficial for further improving the energy density of the secondary battery.

[0017] In some embodiments, the first active layer comprises a first silicon-based material and / or the second active layer comprises a second silicon-based material, wherein the first silicon-based material and / or the second silicon-based material are each independently selected from one or more of nano-silicon, silicon-based alloys, silicon oxides, or silicon-carbon composites. This improves the energy density of the secondary battery.

[0018] In some embodiments, the first silicon-based material and / or the second silicon-based material are silicon-carbon composites. This is beneficial for improving the stability of the negative electrode film and increasing the capacity retention and lifespan of the secondary battery.

[0019] In some embodiments, the silicon-carbon composite satisfies one or more of the following characteristics:

[0020] 1) The silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon, wherein the porous carbon is optionally hard carbon;

[0021] 2) The silicon-carbon composite further includes a carbon-containing coating layer, which is located on the surface of the porous carbon and / or silicon material;

[0022] 3) The silicon-carbon composite contains 30%-70% silicon by mass;

[0023] 4) The average particle size of the silicon-carbon composite is 2μm-15μm, optionally 7μm-11μm;

[0024] 5) The powder resistivity of the silicon-carbon composite at 8 MPa is 4 Ω·cm-17 Ω·cm;

[0025] 6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g.

[0026] In some embodiments, the average particle size of the first silicon-based material and / or the second silicon-based material is 2 μm-18 μm. This is beneficial for improving the stability of the negative electrode film and increasing the service life of the secondary battery.

[0027] In some embodiments, the specific surface area of ​​the first silicon-based material and / or the second silicon-based material is 1.0 m². 2 / g-6.7m 2 / g. This improves the capacity retention rate of the secondary battery and extends its service life.

[0028] In some embodiments, the first composite binder accounts for 1%-5% of the mass of the first active layer. This improves the energy density of the secondary battery.

[0029] In some embodiments, the second composite binder accounts for 1%-7% of the mass of the second active layer. This is beneficial for improving the structural stability of the second active layer, thereby improving the capacity retention and service life of the secondary battery.

[0030] In some embodiments, the first active layer comprises a first carbon-based material, and / or the second active layer further comprises a second carbon-based material. This is beneficial for improving the cycle stability of the negative electrode film.

[0031] In some embodiments, the first carbon-based material accounts for 30%-100% of the mass of the first active layer, and / or the second carbon-based material accounts for 5%-90% of the mass of the second active layer. This is beneficial for the secondary battery to achieve both high energy density and good cycle performance and capacity retention.

[0032] In some embodiments, the negative electrode sheet satisfies one or more of the following:

[0033] (1) The average particle size of the first carbon-based material and / or the second carbon-based material is 3-20 μm;

[0034] (2) The specific surface area of ​​the first carbon-based material and / or the second carbon-based material is 1.2-4.5 m². 2 / g;

[0035] (3) The graphitization degree of the first carbon-based material and / or the second carbon-based material is 90%-97%;

[0036] (4) The specific capacity of the first carbon-based material and / or the second carbon-based material is 340-370 mAh / g.

[0037] In some embodiments, the first carbon-based material and / or the second carbon-based material are each independently selected from one or more of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon. This improves the capacity retention and lifespan of the secondary battery.

[0038] In some embodiments, the first carbon-based material and / or the second carbon-based material comprises artificial graphite and natural graphite, wherein the mass ratio of the artificial graphite to the natural graphite is 9:1 to 1:9. This improves the energy density and capacity retention of the secondary battery.

[0039] In some embodiments, the negative electrode further includes a buffer layer located between the negative electrode current collector and the negative electrode film layer. This improves the cycle stability of the secondary battery.

[0040] In some embodiments, the buffer layer comprises a third binder and a conductive agent, wherein the mass ratio of the third binder to the conductive agent is 20:1 to 1:1. This facilitates good adhesion and conductivity between the negative electrode film layer and the negative electrode current collector.

[0041] In some embodiments, the third adhesive is selected from one or more of styrene-butadiene rubber, acrylic adhesives, polyacrylonitrile, polyvinylidene fluoride, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives. Using these adhesives provides good adhesion.

[0042] In some embodiments, the mass ratio of the first active layer to the second active layer in the negative electrode film is 1:9 to 9:1. This is beneficial for the secondary battery to achieve both high energy density and high capacity retention.

[0043] In some embodiments, the areal density of the negative electrode film is 4 mg / cm³. 2 -16 mg / cm 2 This is beneficial for improving the energy density of secondary batteries.

[0044] In some embodiments, the thickness of the negative electrode film is 20 μm-100 μm. This is beneficial for the secondary battery to have a high energy density.

[0045] In some embodiments, the specific capacity of the negative electrode film is 360 mAh / g to 3000 mAh / g. This is beneficial for the secondary battery to have a high energy density.

[0046] A second aspect of this application provides an electrical device including a secondary battery selected from the first aspect of this application. Attached Figure Description

[0047] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0048] Figure 2 yes Figure 1 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0049] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0050] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0051] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown.

[0052] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0053] Explanation of reference numerals in the attached figures:

[0054] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation

[0055] The embodiments of the secondary battery and electrical device of this application are hereby disclosed in detail with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0056] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0057] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0058] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0059] Silicon, due to its extremely high specific capacity, can effectively improve the energy density of secondary batteries when used as a negative electrode active material. However, silicon materials exhibit significant volume expansion. Excessive volume expansion of silicon-based active material negative electrode sheets during cycling causes the binder in the negative electrode sheet to gradually fail, losing its effective bonding effect. The loss of bonding effect in the silicon-based active material particles leads to further expansion. This excessive expansion causes the active material particles to gradually shift relative to each other and detach from the electrical contact, forming electrochemical islands that can no longer intercalate or deintercalate lithium ions. This results in material loss in the negative electrode sheet, leading to a rapid capacity decay of the secondary battery.

[0060] In view of this, this application provides a secondary battery, including a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer formed on at least one surface of the negative current collector, the negative electrode film layer including a first active layer and a second active layer, the first active layer and / or the second active layer including silicon, the mass percentage of silicon in the second active layer being higher than the mass percentage of silicon in the first active layer; the first active layer including a first composite binder, the second active layer including a second composite binder, the first composite binder and / or the second composite binder including binder A and binder B, the binder A being styrene-butadiene rubber, the binder B being selected from one or more of polyacrylic acid binders, polyacrylonitrile, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and their derivatives, the mass ratio of binder A to binder B in the first composite binder being higher than the mass ratio of binder A to binder B in the second composite binder.

[0061] In this application, the negative electrode film layer includes a first active layer and a second active layer. By designing the negative electrode film layer as a double-layer structure, it is beneficial to achieve fine control of the negative electrode sheet. On the other hand, the double-layer design is beneficial to improve the stability of the negative electrode sheet, thereby improving the service life of the secondary battery.

[0062] The first active layer and / or the second active layer include silicon. Due to the ultra-high specific capacity of silicon material, including silicon in the first active layer and / or the second active layer is beneficial to improving the specific capacity of the negative electrode film, thereby enabling the secondary battery to have a high energy density.

[0063] The first and second active layers employ a composite binder comprising binder A and binder B. Binder A is styrene-butadiene rubber, which has a low elastic modulus but good flexibility. Binder B is selected from one or more of polyacrylic acid binders, polyacrylonitrile, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and their derivatives; these binders have high elastic moduli. Using a composite binder comprising binder A and binder B improves the adhesive strength while maintaining good flexibility of the negative electrode film. This helps suppress the expansion of the active material during secondary battery cycling, reduces the relative movement between negative electrode active material particles, and minimizes problems such as tensile stress, breakage, and aging of the binder, thereby extending its service life. In addition, binder B is connected to the free hydroxyl groups and other groups on the surface of the active material particles in the negative electrode film through chemical bonds, hydrogen bonds or electrostatic adsorption, forming a coating layer similar to an SEI film on the surface of the active material. This can reduce the side reactions between the electrolyte and the active material in the secondary battery, thereby helping to further improve the stability of the negative electrode sheet and increase the capacity retention rate and service life of the secondary battery.

[0064] In this application, the mass percentage of silicon in the second active layer is higher than that in the first active layer, and the mass ratio of binder A to binder B in the first composite binder is higher than that in the second composite binder. By increasing the mass percentage of binder A in the first active layer, which has a lower silicon content, the bonding strength of the first composite binder is improved, resulting in better flexibility and lower brittleness in the first active layer. This reduces brittle fracture of the binder caused by compression due to the expansion of the active material, thus enhancing the bonding effect of the binder. By increasing the mass percentage of binder B in the second active layer, which has a higher silicon content, the elastic modulus of the binder is improved. Binder B has a higher aspect ratio than binder A, allowing it to connect more active material particles. Furthermore, the long-chain binder molecules cross-link to form a network spatial structure, which helps to suppress the volume expansion of the active material in the second active layer during charging and discharging and improves the structural stability of the negative electrode sheet, thereby improving the capacity retention rate of the secondary battery.

[0065] In some embodiments, the mass ratio of adhesive A to adhesive B in the first composite adhesive is 50:1 to 1:5. By keeping the mass ratio of adhesive A to adhesive B in the first composite adhesive within this range, it is beneficial to improve the adhesive strength of the first composite adhesive while also providing good flexibility. This allows the first active layer, with its low silicon content, to have good and continuous adhesion to the negative electrode current collector. Even if the first active layer experiences a small amount of relative slippage, it can still maintain a stable bond with the negative electrode current collector. Exemplarily, the mass ratio of adhesive A to adhesive B in the first composite adhesive can be 50:1, 49:1, 45:1, 40:1, 35:1, 30:1, 25:1, 20:1, 15:1, 10:1, 5:1, 3:1, 1:1, 1:3, 1:5, or any two of these ratios. In some embodiments, the mass ratio of adhesive A to adhesive B in the first composite adhesive is 49:1 to 1:4, and optionally 3:1 to 1:4.

[0066] In some embodiments, the mass ratio of binder A to binder B in the second composite binder is 10:1 to 1:10. Maintaining the mass ratio of binder A to binder B in the second composite binder within this range helps to improve the elastic modulus and bonding strength of the second composite binder. This, in turn, suppresses the expansion of the active material in the second active layer with a high silicon content, thus improving the stability of the negative electrode film and enhancing the capacity retention rate of the secondary battery. Exemplarily, the mass ratio of binder A to binder B in the second composite binder can be 10:1, 9:1, 8:1, 6:1, 4:1, 2:1, 1:1, 1:2, 1:4, 1:6, 1:8, 1:9, 1:10, or any two of these values ​​within a range. In some embodiments, the mass ratio of binder A to binder B in the second composite binder is 9:1 to 1:9, optionally 3:2 to 1:9.

[0067] In some embodiments, the first composite binder accounts for 1% to 5% of the mass of the first active layer. By ensuring that the mass percentage of the first composite binder in the first active layer is within this range, it is beneficial to provide sufficient adhesion to the negative electrode film layer and reduce the proportion of inactive materials in the negative electrode film layer, thereby improving the energy density of the secondary battery. Exemplarily, the mass percentage of the first composite binder in the first active layer can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a value within a range of any two of these values.

[0068] In some embodiments, the mass percentage of the second composite binder in the second active layer is 1%-7%. By ensuring the mass percentage of the second composite binder in the second active layer is within this range, it is beneficial to suppress the volume expansion of the silicon material in the second active layer, improve the structural stability of the second active layer, and thus enhance the capacity retention and lifespan of the secondary battery. Exemplarily, the mass percentage of the second composite binder in the second active layer can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, or a value within a range of any two of these values.

[0069] In some embodiments, the first active layer is located between the negative electrode current collector and the second active layer. A higher silicon content in the active layer results in greater volume expansion, which in turn leads to greater shear stress. By placing the first active layer with a lower silicon content closer to the negative electrode current collector and the second active layer with a higher silicon content further away, the shear stress in the negative electrode film caused by the expansion of silicon-containing active material particles decreases gradually from the direction further away from the negative electrode current collector. This helps reduce the shear stress between the first active layer and the negative electrode current collector, lowering the risk of the negative electrode film peeling off. Furthermore, the second active layer further away from the negative electrode current collector has a higher binder B content in the second composite binder, which helps suppress the volume expansion of active particles in the second active layer, reducing shear stress and lowering the absolute shear stress difference between the negative electrode film and the negative electrode current collector, further reducing the risk of the negative electrode film peeling off.

[0070] Furthermore, the first active layer, with its lower silicon content, contains a higher amount of binder A, resulting in better flexibility. Positioning this more flexible first active layer closer to the negative electrode current collector helps reduce binder failure caused by excessive extrusion strength. The second active layer, with its higher silicon content, is located further away from the negative electrode current collector, which helps reduce the liquid phase transport distance of active ions during battery charging, decreases internal polarization, and improves the rate performance of the secondary battery.

[0071] In some embodiments, the difference between the mass percentage of silicon in the second active layer and the mass percentage of silicon in the first active layer is 5%-80%. By ensuring that the difference in the mass percentage of silicon in the second and first active layers is within this range, it is beneficial to reduce the shear stress difference between the negative electrode film and the negative electrode current collector, reduce the risk of detachment of the negative electrode film and the negative electrode current collector, improve the structural stability of the negative electrode sheet, and thus improve the capacity retention rate and service life of the secondary battery. In some embodiments, the difference between the mass percentage of silicon in the second active layer and the mass percentage of silicon in the first active layer can be 5%, 10%, 15%, 25%, 35%, 45%, 55%, 65%, 75%, 80%, or a value within a range of any two of these values. In some embodiments, the difference between the mass percentage of silicon in the second active layer and the mass percentage of silicon in the first active layer is 10%-75%, optionally 10%-40%.

[0072] In some embodiments, the mass percentage of silicon in the first active layer is greater than or equal to 0. By keeping the silicon content in the first active layer within this range, it is beneficial to reduce the volume expansion of the first active layer, thereby reducing shear stress and binder failure in the negative electrode film, reducing the risk of the negative electrode film detaching from the negative electrode current collector, and thus improving the stability of the negative electrode sheet, increasing its capacity retention rate and service life. In some embodiments, the mass percentage of silicon in the first active layer is 0%-50%. This further improves the stability of the negative electrode sheet, increasing its capacity retention rate and service life. Exemplarily, the mass percentage of silicon in the first active layer can be 0%, 2%, 5%, 8%, 10%, 12%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or any two of these values. In some embodiments, the mass percentage of silicon in the first active layer is 0%-25%, optionally 5%-20%.

[0073] In some embodiments, the silicon content in the second active layer is 5%-80% by mass. Maintaining the silicon content in the second active layer within this range helps to increase the specific capacity of the negative electrode film and improve the energy density of the secondary battery. Furthermore, by positioning the second active layer away from the negative electrode current collector and maintaining the silicon content within the aforementioned range, it helps to reduce the liquid phase transport distance of active ions in the negative electrode film, thereby reducing the internal polarization of the secondary battery and improving its rate performance. Exemplarily, the silicon content in the second active layer can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 70%, 80%, or any two of these values. In some embodiments, the silicon content in the second active layer is 10%-60% by mass. This further improves the energy density of the secondary battery. In some embodiments, the silicon content in the second active layer is 20%-40% by mass, optionally 25%.

[0074] In some embodiments, the first active layer comprises a first silicon-based material and / or the second active layer comprises a second silicon-based material, wherein the first silicon-based material and / or the second silicon-based material are each independently selected from one or more of nano-silicon, silicon-based alloys, silicon oxides, or silicon-carbon composites. The silicon element in the first active layer and / or the second active layer is primarily derived from the silicon-based material. By selecting the first silicon-based material and / or the second silicon-based material from the aforementioned materials, it is beneficial to increase the active sites of the negative electrode active material in the negative electrode film layer, increase the specific capacity of the negative electrode film layer, and thereby improve the energy density of the secondary battery.

[0075] In some embodiments, the silicon oxide includes at least one of unpre-lithium silicon oxide, pre-lithium silicon oxide, unpre-magnesium silicon oxide, and pre-magnesium silicon oxide.

[0076] In some embodiments, the first silicon-based material and / or the second silicon-based material is a silicon-carbon composite. By using a silicon-carbon composite for the first and / or second silicon-based materials, it is beneficial to reduce the volume expansion of the active material particles, improve the stability of the negative electrode film, and increase the capacity retention and lifespan of the secondary battery. Furthermore, the silicon-carbon composite helps to reduce the direct contact between silicon nanoparticles and the electrolyte during the charging and discharging process of the secondary battery, reducing the rate of interfacial side reactions and improving the lifespan of the secondary battery.

[0077] The silicon-carbon composite of this application can be prepared using conventional silicon-carbon composites or conventional preparation methods, such as depositing nano-silicon materials on porous carbon by chemical vapor deposition, and further carbon coating, such as using amorphous carbon coating.

[0078] In some embodiments, the silicon-carbon composite satisfies one or more of the following characteristics:

[0079] 1) The silicon-carbon composite includes porous carbon and silicon-containing materials dispersed in the pores of the porous carbon, wherein the porous carbon is optionally hard carbon;

[0080] 2) The silicon-carbon composite also includes a carbon-containing coating layer, which is located on the surface of porous carbon and / or silicon-containing materials;

[0081] 3) The silicon content in the silicon-carbon composite is 30%-70% by mass;

[0082] 4) The average particle size of the silicon-carbon composite is 2μm-15μm, optionally 7μm-11μm;

[0083] 5) The powder resistivity of silicon-carbon composite at 8 MPa is 4 Ω·cm - 17 Ω·cm;

[0084] 6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g.

[0085] In some embodiments, the silicon-carbon composite includes porous carbon and silicon-containing material dispersed in the pores of the porous carbon. The porous carbon, acting as a carrier for the silicon-containing material, provides support while its pores offer expansion space, effectively mitigating stress caused by expansion during charging. Especially when the silicon-containing material has a nanometer-scale particle size, it exhibits higher specific capacity and is better dispersed within the pores of the porous carbon. Furthermore, it allows for more efficient utilization of the porous carbon's buffering effect on expansion. When this silicon-carbon composite is used in wound electrode assemblies, it can significantly alleviate the stretching of the outer negative electrode sheet caused by silicon expansion.

[0086] In some embodiments, the porous carbon may optionally be hard carbon. When the porous carbon is hard carbon, it has stronger support, a more stable pore structure, and is harder, thus providing better porosity for the negative electrode active layer, providing a smoother path for active ion transport, and improving the charging capability of the battery cell.

[0087] In some embodiments, the silicon-containing material includes at least one of elemental silicon, silicon oxides, silicon nitrides, and silicon alloys. In some embodiments, the silicon-containing material includes crystalline silicon, thereby further improving the structural stability of the silicon-containing material and the energy density of the battery cell.

[0088] In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer located on the surface of the porous carbon and the silicon-containing material. This can improve the conductivity of the silicon-carbon composite, reduce the internal impedance of the battery cell, and effectively reduce the probability of direct contact between the silicon-containing material in the porous carbon channels and the external environment, thereby improving the chemical stability of the silicon-carbon composite.

[0089] In some embodiments, the silicon content in the silicon-carbon composite is 30%-70% by mass. This approach, while maximizing the specific capacity of the negative electrode active material by utilizing silicon, also facilitates the full dispersion of silicon in the carbon-containing porous material and helps control the expansion of silicon during charging.

[0090] In this application, the method for testing the silicon content in the silicon-carbon composite can be a method known in the art. As an example, the following method can be used for testing: a certain amount of silicon-carbon composite is taken, and the mass of silicon element in the silicon-carbon composite is obtained by inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentage of silicon element in the silicon-carbon composite can be calculated.

[0091] In addition to providing structural support and buffering for the expansion of silicon materials, the pores in the silicon-carbon composite also form between the particles. To further improve the flow of lithium ions through the intraparticle and interparticle pores, in some embodiments, the average particle size of the silicon-carbon composite is 2μm-15μm. Optionally, the average particle size of the silicon-carbon composite is 7μm-11μm, or 5μm-10μm. This creates a particle size distribution between the average particle size of the silicon-carbon composite and the average particle size of the graphite material, thus facilitating the use of interparticle gaps to increase the compactness of the negative electrode active layer, thereby further improving the energy density of the battery cell.

[0092] The number-average particle size of the aforementioned silicon-carbon composite can be tested using equipment and methods known in the art. For example, a scanning electron microscope (SEM) (e.g., ZEISS Sigma 300) can be used, referring to JY / T010-1996, to obtain SEM images of the negative electrode sheet. As an example, the test can be performed as follows: Randomly select a test sample of length × width = 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5 areas) within the test sample, and at a certain magnification (e.g., 1000x when measuring silicon-carbon composites), read the particle size of each silicon-carbon composite particle in each test area (i.e., take the distance between the two farthest points on the silicon-carbon composite particle as the particle size). Count the number and particle size values ​​of silicon-carbon composite particles in each test area, and take the arithmetic mean of the silicon-carbon composite particles in each test area, which is the number-average particle size of the silicon-carbon composite particles in the test sample. To ensure the accuracy of the test results, multiple test samples (e.g., 10) can be taken and the above test can be repeated. The average value of each test sample can be taken as the final test result.

[0093] In some embodiments, the silicon-carbon composite powder resistivity at 8 MPa is between 4 Ω·cm and 17 Ω·cm. Exemplarily, it is a value within a range of 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11 Ω·cm, 12 Ω·cm, 13 Ω·cm, 14 Ω·cm, 15 Ω·cm, 16 Ω·cm, 17 Ω·cm, or any combination thereof. By controlling the powder resistivity as described above, the conductivity of the silicon-carbon composite is improved, thereby increasing the charging rate of the battery cell.

[0094] In this application, the powder resistivity of silicon-carbon composites can be determined using methods known in the art. As an example, a four-probe method can be used, where two probes apply voltage and the other two probes measure current. The powder resistivity can be calculated by measuring the resistance value. Models of four-probe semiconductor powder resistivity testers include the ST-2722.

[0095] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g.

[0096] In this application, the method for testing the BET specific surface area of ​​the silicon-carbon composite can be a method known in the art. As an example, referring to GB / T 19587-2017, a nitrogen adsorption specific surface area analysis method can be used. The sample tube containing the first graphite material sample is immersed in liquid nitrogen at -196℃, and the amount of nitrogen adsorbed on the surface of the solid sample at different pressures of 0.05 to 0.30 is measured. Based on the BET multilayer adsorption theory and calculation formula, the amount of monolayer adsorption of the sample is obtained, and thus the BET specific surface area is obtained. This test can be performed using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0097] In some embodiments, the average particle size of the first silicon-based material and / or the second silicon-based material is 2-18 μm. By ensuring the average particle size of the first silicon-based material and / or the second silicon-based material is within this range, it is beneficial to reduce the risk of internal cracking due to the large volume expansion of the silicon material during cycling, improve the stability of the negative electrode film, and thus increase the lifespan of the secondary battery. Exemplarily, the average particle size of the first silicon-based material and / or the second silicon-based material can be 2 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, or a value within a range of any two of these values. In some embodiments, the average particle size of the first silicon-based material and / or the second silicon-based material is 5-12 μm.

[0098] In some embodiments, the specific surface area of ​​the first silicon-based material and / or the second silicon-based material is 1.0-6.7 m². 2 / g. By ensuring the specific surface area of ​​the first silicon-based material and / or the second silicon-based material is within the aforementioned range, it is beneficial to reduce interfacial side reactions between the active material particles and the electrolyte, reduce the consumption of active ions, thereby improving the capacity retention rate of the secondary battery and extending its service life. For example, the specific surface area of ​​the first silicon-based material and / or the second silicon-based material can be 1.0 m². 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.5m 2 / g, 2.8m 2 / g, 3.0m 2 / g, 3.5m 2 / g, 4.0m 2 / g, 4.5m 2 / g, 5.0m 2 / g, 5.5m 2 / g, 6.0m 2 / g, 6.5m 2 / g, 6.7m2 / g, or a value within a range of any two of these values. In some alternative embodiments, the specific surface area of ​​the first silicon-based material and / or the second silicon-based material is 1.5m². 2 / g-3.2m 2 / g.

[0099] In some embodiments, the first active layer comprises a first carbon-based material, and / or the second active layer further comprises a second carbon-based material. By including carbon-based materials in the first active layer and / or the second active layer, it is beneficial to improve the cycle stability of the negative electrode film and enhance the capacity retention and cycle performance of the secondary battery under long-term operating conditions.

[0100] In some embodiments, the first carbon-based material accounts for 30%-100% of the mass of the first active layer. Maintaining the mass percentage of the first carbon-based material in the first active layer within this range helps reduce the volume expansion of the first active layer during cycling, improves the stability of the negative electrode film, and thus enhances the cycle performance and capacity retention of the secondary battery. Exemplarily, the mass percentage of the first carbon-based material in the first active layer can be 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, or a value within a range of any two of these values. In some embodiments, the first carbon-based material accounts for 70%-90% of the mass of the first active layer.

[0101] In some embodiments, the second carbon-based material accounts for 5%-90% of the mass of the second active layer. By maintaining the mass percentage of the second carbon-based material in the second active layer within this range, it is beneficial to reduce the volume expansion of the second active layer while maintaining high capacity, thereby enabling the secondary battery to achieve both high energy density and high capacity retention. Exemplarily, the mass percentage of the second carbon-based material in the second active layer can be 0%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or a value within a range of any two of these values. In some embodiments, the mass percentage of the second carbon-based material in the second active layer is 30%-80%.

[0102] In some embodiments, the average particle size of the first carbon-based material and / or the second carbon-based material is 3-20 μm. By ensuring the average particle size of the first carbon-based material and / or the second carbon-based material is within this range, it is beneficial to reduce the volume expansion of the active material during cycling, reduce the risk of internal rupture of the negative electrode film, improve the stability of the negative electrode film, and thus increase the lifespan of the secondary battery. Exemplarily, the average particle size of the first carbon-based material and / or the second carbon-based material can be 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 14 μm, 15 μm, 16 μm, 18 μm, 20 μm, or a value within a range of any two of these values. In some embodiments, the average particle size of the first carbon-based material and / or the second carbon-based material is 4 μm-18 μm.

[0103] In some embodiments, the specific surface area of ​​the first carbon-based material and / or the second carbon-based material is 1.2-4.5 m². 2 / g. By ensuring the specific surface area of ​​the first carbon-based material and / or the second carbon-based material is within the aforementioned range, it is beneficial to reduce interfacial side reactions of the active materials in the first active layer and / or the second active layer, reduce the consumption of active ions, and thereby improve the cycle stability and capacity retention of the secondary battery. For example, the specific surface area of ​​the first carbon-based material and / or the second carbon-based material can be 1.2m². 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.8m 2 / g, 3.2m 2 / g, 3.6m 2 / g, 4.0m 2 / g, 4.2m 2 / g, 4.5m 2 / g or a value within a range of any two of these values. In some embodiments, the specific surface area of ​​the first carbon-based material and / or the second carbon-based material can be 2.5m². 2 / g-3.9m 2 / g.

[0104] In some embodiments, the graphitization degree of the first carbon-based material and / or the second carbon-based material is 90%-97%. Maintaining the graphitization degree of the first carbon-based material and / or the second carbon-based material within this range is beneficial for the energy density and cycle stability of the secondary battery. Exemplarily, the graphitization degree of the first carbon-based material and / or the second carbon-based material can be 90%, 92%, 93%, 94%, 95%, 96%, 97%, or a value within a range consisting of any two of these values.

[0105] In some embodiments, the specific capacity of the first carbon-based material and / or the second carbon-based material is 340-370 mAh / g. By ensuring the specific capacity of the first carbon-based material and / or the second carbon-based material is within the aforementioned range, it is beneficial to improve the energy density of the secondary battery. Exemplarily, the specific capacity of the first carbon-based material and / or the second carbon-based material can be 340 mAh / g, 345 mAh / g, 350 mAh / g, 355 mAh / g, 360 mAh / g, 365 mAh / g, 370 mAh / g, or a value within a range consisting of any two of these values.

[0106] In some embodiments, the first carbon-based material and / or the second carbon-based material are each independently selected from one or more of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon. Using the above materials as the first carbon-based material and / or the second carbon-based material is beneficial for the negative electrode film to have good cycle stability, thereby improving the capacity retention rate and service life of the secondary battery.

[0107] In some embodiments, the first carbon-based material and / or the second carbon-based material comprises artificial graphite and natural graphite, wherein the mass ratio of the artificial graphite to the natural graphite is 9:1 to 1:9. By including both artificial and natural graphite in the first and / or second carbon-based materials, it is beneficial to improve the energy density and cycle stability of the carbon-based material, thereby improving the energy density and capacity retention of the secondary battery. Exemplarily, the mass ratio of artificial graphite to natural graphite can be 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, or any two of these ratios within a range.

[0108] In some embodiments, the negative electrode sheet further includes a buffer layer located between the negative electrode current collector and the negative electrode film layer. The buffer layer helps to improve the adhesion between the negative electrode film layer and the negative electrode current collector, reducing the risk of the negative electrode film layer detaching, and enhancing the bonding between the negative electrode film layer and the negative electrode current collector under long-term operating conditions, thereby improving the cycle stability of the secondary battery.

[0109] In some embodiments, the thickness of the buffer layer is 0.2 μm to 1.5 μm. By keeping the thickness of the buffer layer within this range, it is beneficial to improve the adhesion between the first active layer and the negative electrode current collector, reducing the risk of debonding due to insufficient adhesion between the negative electrode film layer and the negative electrode current collector during cold pressing and charge / discharge processes. Exemplarily, the thickness of the buffer layer can be 0.2 μm, 0.5 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.5 μm, or a value within a range of any two of these values.

[0110] In some embodiments, the buffer layer comprises a third binder and a conductive agent, wherein the mass ratio of the third binder to the conductive agent is 20:1 to 1:1. This facilitates good adhesion and conductivity between the negative electrode film layer and the negative electrode current collector. Exemplarily, the mass ratio of the third binder to the conductive agent in the buffer layer can be 20:1, 18:1, 16:1, 14:1, 12:1, 10:1, 8:1, 6:1, 4:1, 2:1, 1:1, or any two of these ratios within a range.

[0111] In some embodiments, the third adhesive is selected from one or more of styrene-butadiene rubber, acrylic adhesives, polyacrylonitrile, polyvinylidene fluoride, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives. Using these adhesives provides good adhesion.

[0112] In some embodiments, the mass ratio of the first active layer to the second active layer in the negative electrode film is 1:9 to 9:1. This is beneficial for the secondary battery to achieve both high energy density and high capacity retention. For example, the mass ratio of the first active layer to the second active layer can be 1:9, 2:8, 3:7, 4:6, 5:5, 4:6, 3:7, 2:8, 1:9, or any two of these ratios within a range.

[0113] In some embodiments, the areal density of the negative electrode film is 4 mg / cm³. 2 -16 mg / cm 2 By maintaining the areal density of the negative electrode film within the aforementioned range, it is beneficial to improve the energy density of the secondary battery. For example, the areal density of the negative electrode film can be 4 mg / cm³. 2 6mg / cm 2 8mg / cm 2 10mg / cm 2 12mg / cm 2 14mg / cm 2 16mg / cm 2 Or the value between any two of them within a range.

[0114] In some embodiments, the thickness of the negative electrode film is 20 μm-100 μm. By keeping the thickness of the negative electrode film within this range, it is beneficial for the secondary battery to have a high energy density, while simultaneously reducing the diffusion distance of lithium ions during charging and discharging, reducing internal polarization of the secondary battery, and thus improving the kinetic performance of the secondary battery. Exemplarily, the thickness of the negative electrode film can be 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 80 μm, 100 μm, or a value within a range of any two of these values.

[0115] In some embodiments, the specific capacity of the negative electrode film is 360 mAh / g to 3000 mAh / g. By ensuring the specific capacity of the negative electrode film is within this range, it is beneficial for the secondary battery to have a high energy density. Exemplarily, the specific capacity of the negative electrode film can be 360 ​​mAh / g, 400 mAh / g, 640 mAh / g, 800 mAh / g, 1200 mAh / g, 1400 mAh / g, 1600 mAh / g, 1800 mAh / g, 2000 mAh / g, 2500 mAh / g, 3000 mAh / g, or a value within a range of any two of these values.

[0116] In this application, the types and mass percentages of elements (e.g., silicon) in the material can be measured using equipment and methods known in the art. For example, the following method can be used: a certain amount of material is taken, and the types and masses of elements in the material are obtained by inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentage of elements in the material can then be calculated.

[0117] In this application, the types and mass proportions of materials (e.g., binder A, binder B, first composite binder, second composite binder) can be measured using equipment and methods known in the art. For example, they can be analyzed by liquid chromatography or liquid chromatography-mass spectrometry (GC-MS). As an example, the binder in the negative electrode film layer can be quantitatively analyzed by liquid chromatography with reference to standard GB / T9722-2006.

[0118] In this application, the average particle size of the materials (first silicon-based material, second silicon-based material, first carbon-based material, second carbon-based material, silicon-carbon composite) can be tested using equipment and methods known in the art. For example, a scanning electron microscope (e.g., ZEISS Sigma 300) can be used, referring to JY / T010-1996, to obtain a scanning electron microscope (SEM) image of the negative electrode sheet. As an example, the test can be performed as follows: Randomly select a test sample of length × width = 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5) in the test sample, and at a certain magnification (e.g., 1000x when measuring silicon-carbon composite materials), read the particle size of the material particles in each test area (i.e., take the distance between the two farthest points on the material particle as the particle size). Count the number and particle size values ​​of the material particles in each test area, and take the arithmetic mean of the material particles in each test area, which is the average particle size of the silicon-carbon composite material particles in the test sample. To ensure the accuracy of the test results, multiple test samples (e.g., 10) can be taken and the above test can be repeated. The average value of each test sample can be taken as the final test result.

[0119] In this application, the specific surface area of ​​a material (e.g., a first carbon-based material, a second carbon-based material, etc.) has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be tested using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, Inc., USA.

[0120] In this application, the degree of graphitization of a material (e.g., a first carbon-based material, a second carbon-based material, etc.) refers to the proportion of carbon elements in the material existing in the form of a graphite structure, which can be tested using instruments and methods known in the art. For example, an X-ray diffractometer (such as a Bruker D8 Discover) can be used for testing, and the testing can be performed in accordance with JIS K 0131-1996 and JB / T 4220-2011, to obtain the average interlayer spacing d of the C(002) crystal plane in the material's crystal structure. 002 Then, according to the formula g = (0.344 - d) 002 The degree of graphitization is calculated as d / (0.344-0.3354)×100%. In the above formula, d... 002 It is the average interlayer spacing of the C(002) crystal plane in the material crystal structure, expressed in nanometers (nm).

[0121] In this application, the specific capacity of the material (e.g., the first carbon-based material, the second carbon-based material, etc.) is the ratio of the electrical capacity that the active material can release to the mass of the active material, and can be tested using methods known in the art. An exemplary test method is as follows: The sample powder is mixed evenly with a conductive agent, a binder, and optional other additives in a certain mass ratio with a solvent to form a slurry; the prepared slurry is coated on the surface of the negative electrode current collector copper foil and dried in an oven for later use; the electrolytic salt is dissolved in an organic solvent to prepare an electrolyte of a certain concentration; then, using a lithium metal sheet as the counter electrode and a polyethylene (PE) film as the separator, the CR2430 coin cell is assembled with the electrolyte in an argon-protected glove box. After the obtained coin cells were allowed to stand for 12 hours, they were discharged at 25°C with a constant current of 0.15 mA to 0.005 V. After standing for 10 minutes, they were discharged again with a constant current of 50 μA to 0.005 V. After standing for 10 minutes, they were discharged again with a constant current of 10 μA to 0.005 V. Then, they were charged with a constant current of 0.3 mA to 2.0 V, and the charging capacity was recorded. The ratio of charging capacity to sample mass is the specific capacity of the corresponding material (e.g., negative electrode active material, first artificial graphite, second artificial graphite, etc.).

[0122] In this application, the areal density of the negative electrode film can be determined by the following method: after disassembling the secondary battery to obtain the negative electrode sheet, a certain area S (unit: cm²) is punched out. 2 Fifteen negative electrode sheets and 15 current collectors (from the same production batch as the negative electrode sheets) were weighed, and their average mass was calculated. The average mass of the negative electrode sheets was M1 (mg), and the average mass of the current collectors was M2 (mg). When the active material layer was only disposed on one side of the current collector, the areal density was (M1-M2) / S. When the active material layer was disposed on both sides of the current collector, the areal density was (M1-M2) / 2S.

[0123] In this application, the thickness of the negative electrode film has a meaning known in the art and can be tested using methods known in the art. For example, after disassembling a secondary battery to obtain the negative electrode sheet, the thickness of the negative electrode film can be measured using a micrometer (e.g., a Mitutoyo 293-100 model with an accuracy of 0.1 μm). The thickness range given in this application refers to the thickness range of the negative electrode film on one side of the negative electrode current collector.

[0124] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0125] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0126] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0127] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0128] The secondary battery of this application also includes a positive electrode, an electrolyte, and a separator. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator is disposed between the positive and negative electrodes, mainly to prevent short circuits between the positive and negative electrodes, while allowing ions to pass through.

[0129] Positive electrode sheet

[0130] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. During battery charging and discharging, active ions are inserted and extracted back and forth between the positive and negative electrode.

[0131] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0132] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0133] In some embodiments, when the secondary battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0134] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0135] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0136] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0137] electrolytes

[0138] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0139] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0140] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0141] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0142] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0143] Separating membrane

[0144] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0145] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0146] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0147] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0148] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0149] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 This is an example of a square-structured secondary battery 5.

[0150] In some implementations, refer to Figure 2The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 using a winding or stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0151] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0152] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0153] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0154] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0155] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0156] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0157] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0158] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0159] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0160] Example

[0161] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0162] Example 1

[0163] (1) Preparation of the first active layer slurry

[0164] The first artificial graphite, silicon-carbon composite (average particle size 6 μm, silicon content 50% by mass), styrene-butadiene rubber (binder A), polyacrylic acid (binder B), conductive carbon black, conductive carbon nanotubes, and sodium carboxymethyl cellulose were thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 80:15:1.6:0.4:1.5:0.5:1 to form the first active layer slurry. The silicon-carbon composite was obtained by chemical vapor deposition (CVD) using porous carbon as the matrix and silane as the raw material, with a silicon content of 50%.

[0165] (2) Preparation of the second active layer slurry

[0166] The second artificial graphite, silicon-carbon composite (average particle size 6μm, silicon content 50% by mass), styrene-butadiene rubber (binder A), polyacrylic acid (binder B), conductive carbon black, conductive carbon nanotubes, and sodium carboxymethyl cellulose were thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 60:35:0.8:1.2:1.2:0.8:1 to form the second active layer slurry.

[0167] (3) Preparation of negative electrode sheet

[0168] The first and second active layer slurries are simultaneously extruded using a dual-cavity coating device, with a mass ratio of 1:1. The first active layer slurry is coated onto the copper foil of the negative electrode current collector, and the second active layer is coated onto the first active layer slurry. After drying at 85°C, the material is cold-pressed, then die-cut and slit to produce the negative electrode sheet for lithium-ion batteries.

[0169] (4) Preparation of positive electrode sheet

[0170] According to the mass ratio, the ternary material nickel-cobalt-manganese LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811): conductive carbon black: polyvinylidene fluoride (PVDF) = 96.5:2.5:1, then N-methylpyrrolidone solvent is added and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is coated on both surfaces of the positive electrode current collector aluminum foil, dried at 80℃, cold-pressed, and then die-cut and slit to produce lithium-ion battery positive electrode sheets.

[0171] (5) Preparation of lithium-ion batteries

[0172] Using polyethylene microporous film as the porous separator film substrate, inorganic alumina powder, polyvinylpyrrolidone, and acetone solvent are mixed evenly in a weight ratio of 3:1.5:5.5 to form a slurry, which is then coated on one side of the substrate and dried to obtain the separator film.

[0173] Lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 1:2:1 to obtain a lithium-ion battery electrolyte.

[0174] The positive and negative electrode sheets prepared above are placed in sequence, with the separator placed between the positive and negative electrode sheets to provide isolation. Then, the electrode assembly is wound up to obtain the electrode assembly. The electrode assembly is placed in the outer packaging, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, and shaping, a secondary battery is obtained.

[0175] Secondary battery performance test

[0176] (1) Negative electrode sheet delamination test

[0177] The following test steps were performed on the secondary battery in Example 1 at 25°C:

[0178] ① Let stand for 5 minutes;

[0179] ② Charge at a constant current of 0.33C to 4.25V, then charge at a constant voltage of 4.25V to a current of 0.33C;

[0180] ③ Discharge at a constant current of 0.33C to 2.5V, and then discharge at a constant voltage of 2.5V to a current of 0.33C;

[0181] ④ Repeat steps ② and ③ to perform capacity calibration;

[0182] ⑤ Let stand for 5 minutes;

[0183] ⑥ Charge the capacitor to 4.25V at a constant current of 0.5C according to the rated capacity, and then charge it to 0.5C at a constant voltage of 4.25V.

[0184] ⑦ Discharge at a constant current of 0.5C to 2.5V, then discharge at a constant voltage of 2.5V to a current of 0.5C, and record the discharge capacity D1 of the first cycle;

[0185] Repeat steps ⑤ to ⑦ above 1000 times to disassemble the battery cell after 1000 cycles and observe whether the anode diaphragm separates from the substrate or sheds powder.

[0186] (2) Capacity retention test after 1000 cycles

[0187] The following test steps were performed on the secondary battery in Example 1 at 25°C:

[0188] ① Let stand for 5 minutes;

[0189] ② Charge at a constant current of 0.33C to 4.25V, then charge at a constant voltage of 4.25V to a current of 0.33C;

[0190] ③ Discharge at a constant current of 0.33C to 2.5V, and then discharge at a constant voltage of 2.5V to a current of 0.33C;

[0191] ④ Repeat steps ② and ③ to perform capacity calibration;

[0192] ⑤ Let stand for 5 minutes;

[0193] ⑥ Charge the capacitor to 4.25V at a constant current of 0.5C according to the rated capacity, and then charge it to 0.5C at a constant voltage of 4.25V.

[0194] ⑦ Discharge at a constant current of 0.5C to 2.5V, then discharge at a constant voltage of 2.5V to a current of 0.5C, and record the discharge capacity D1 of the first cycle;

[0195] Repeat steps ⑤ to ⑦ above 1000 times, and record the discharge capacity Dn on the 1000th cycle;

[0196] Capacity retention rate after 1000 cycles (%) = Discharge capacity Dn of the 1000th cycle / Discharge capacity D1 of the first cycle × 100%. The test results are shown in Table 2.

[0197] (3) Test method for electrode expansion rate after 1000 cycles

[0198] The following test steps were performed on the secondary battery in Example 1 at 25°C:

[0199] ① Let stand for 5 minutes;

[0200] ② Charge at a constant current of 0.33C to 4.25V, then charge at a constant voltage of 4.25V to a current of 0.33C;

[0201] ③ Discharge at a constant current of 0.33C to 2.5V, and then discharge at a constant voltage of 2.5V to a current of 0.33C;

[0202] ④ Repeat steps ② and ③ to perform capacity calibration; after the second cycle, disassemble the cell that has been discharged to 2.5V and measure the thickness of the anode plate as the initial anode plate thickness;

[0203] ⑤ Let stand for 5 minutes;

[0204] ⑥ Charge the capacitor to 4.25V at a constant current of 0.5C according to the rated capacity, and then charge it to 0.5C at a constant voltage of 4.25V.

[0205] ⑦ Discharge at a constant current of 0.5C to 2.5V, then discharge at a constant voltage of 2.5V to a current of 0.5C;

[0206] Repeat steps ⑤ to ⑦ above 1000 times; then perform two 0.33C / 0.33C charge-discharge cycles on the secondary battery according to steps ② and ③ to calibrate its capacity; after the second cycle, disassemble the cell that has been discharged to 2.5V and measure the thickness of the anode plate as the thickness of the anode plate after the cycle.

[0207] The electrode expansion rate (%) after 1000 cycles = (anode electrode thickness after cycling - initial anode electrode thickness) / initial anode electrode thickness × 100%. The test results are shown in Table 2.

[0208] Example 2

[0209] The first active layer slurry and the second active layer slurry were prepared in the same manner as in Example 1, except that when preparing the negative electrode sheet, the second active layer slurry was coated on the negative electrode current collector, and the first active layer slurry was coated on the second active layer.

[0210] Examples 3-5

[0211] The secondary battery was prepared using a method similar to that in Example 1, except that the first active layer slurry and the second active layer slurry were prepared according to the formulations shown in Table 1.

[0212] Comparative Example 1

[0213] The secondary battery was prepared using a method similar to that in Example 1, except that the first active layer slurry and the second active layer slurry were prepared according to the formulations shown in Table 1.

[0214] Comparative Example 2

[0215] The secondary battery was prepared using a method similar to that in Example 1, except that the first active layer slurry contained only styrene-butadiene rubber as a binder, and the second active layer slurry contained only polyacrylic acid as a binder.

[0216] Comparative Example 3

[0217] The secondary battery was prepared using a method similar to that in Example 1, except that the first active layer slurry contained only polyacrylic acid as a binder, and the second active layer slurry contained only styrene-butadiene rubber as a binder.

[0218] Comparative Example 4

[0219] The secondary battery was prepared using a method similar to that in Example 1, except that the negative electrode film in the negative electrode sheet was a single active layer prepared from the first active layer slurry, and the mass of the single active layer was the sum of the masses of the first active layer and the second active layer in Example 1.

[0220] Comparative Example 5

[0221] The secondary battery was prepared using a method similar to that in Example 1, except that the negative electrode film in the negative electrode sheet was a single active layer prepared from the second active layer slurry, and the mass of the single active layer was the sum of the masses of the first active layer and the second active layer in Example 1.

[0222] Table 1

[0223]

[0224] The secondary batteries prepared in Examples 1-5 and Comparative Examples 1-5 were tested for performance using the secondary battery performance testing method described above. The test results are shown in Table 2.

[0225] Table 2

[0226]

[0227] Based on the above results, it can be seen that the secondary batteries prepared in Examples 1-5 of this application have improved capacity retention and reduced electrode expansion rate by making the mass ratio of binder A to binder B in the first active layer with low silicon content higher than that in the second active layer with high silicon content. Furthermore, the secondary batteries do not exhibit demolding or cracking after long-term cycling.

[0228] Examples 6-10

[0229] The secondary battery was prepared using a method similar to that in Example 1, except that the negative electrode was prepared using the formulations of the first and second active layers shown in Table 3 below.

[0230] The secondary batteries prepared in Examples 6-10 were tested according to the battery performance test method shown in Example 1. The test results are shown in Table 4.

[0231] Table 3

[0232]

[0233]

[0234] Table 4

[0235]

[0236] Based on the above results, it can be seen that in the secondary battery of the embodiment of the present invention, the mass ratio of binder A to binder B in the first composite binder is 49:1-1:4, or the mass ratio of binder A to binder B in the second composite binder is 9:1-1:9, thereby the secondary battery has a high capacity retention rate and a low electrode expansion rate.

[0237] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, The device includes a negative electrode sheet, which comprises a negative current collector and a negative electrode film layer formed on at least one surface of the negative current collector. The negative electrode film layer comprises a first active layer and a second active layer, wherein the first active layer and / or the second active layer comprises silicon. The mass percentage of silicon in the second active layer is higher than that in the first active layer. The first active layer includes a first composite adhesive, and the second active layer includes a second composite adhesive, wherein the first composite adhesive and / or the second composite adhesive includes adhesive A and adhesive B. The adhesive A is styrene-butadiene rubber, and the adhesive B is selected from one or more of the following: polyacrylic acid adhesives, polyacrylonitrile, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and its derivatives. The mass ratio of adhesive A to adhesive B in the first composite adhesive is higher than that in the second composite adhesive.

2. The secondary battery according to claim 1, characterized in that, In the first composite adhesive, the mass ratio of adhesive A to adhesive B is 50:1 to 1:5; and / or In the second composite adhesive, the mass ratio of adhesive A to adhesive B is 10:1 to 1:

10.

3. The secondary battery according to claim 1 or 2, characterized in that, The first active layer is located between the negative electrode current collector and the second active layer.

4. The secondary battery according to any one of claims 1-3, characterized in that, The difference between the mass percentage of silicon in the second active layer and the mass percentage of silicon in the first active layer is 5%-80%.

5. The secondary battery according to any one of claims 1-4, characterized in that, The mass percentage of silicon in the first active layer is greater than or equal to 0.

6. The secondary battery according to claim 5, characterized in that, The mass percentage of silicon in the first active layer is 0%-50%.

7. The secondary battery according to any one of claims 1-6, characterized in that, The silicon content in the second active layer is 5%-80% by mass.

8. The secondary battery according to claim 7, characterized in that, The mass percentage of silicon in the second active layer is 10%-60%.

9. The secondary battery according to any one of claims 1-8, characterized in that, The first active layer comprises a first silicon-based material and / or the second active layer comprises a second silicon-based material, wherein the first silicon-based material and / or the second silicon-based material are each independently selected from one or more of nano-silicon, silicon-based alloy, silicon oxide or silicon-carbon composite.

10. The secondary battery according to claim 9, characterized in that, The first silicon-based material and / or the second silicon-based material are silicon-carbon composites.

11. The secondary battery according to claim 9 or 10, characterized in that, The silicon-carbon composite satisfies one or more of the following characteristics: 1) The silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon, wherein the porous carbon is optionally hard carbon; 2) The silicon-carbon composite further includes a carbon-containing coating layer, which is located on the surface of the porous carbon and / or silicon material; 3) The silicon-carbon composite contains 30%-70% silicon by mass; 4) The average particle size of the silicon-carbon composite is 2μm-15μm, optionally 7μm-11μm; 5) The powder resistivity of the silicon-carbon composite at 8 MPa is 4 Ω·cm-17 Ω·cm; 6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g.

12. The secondary battery according to any one of claims 9-11, characterized in that, The average particle size of the first silicon-based material and / or the second silicon-based material is 2μm-18μm.

13. The secondary battery according to any one of claims 9-12, characterized in that, The specific surface area of ​​the first silicon-based material and / or the second silicon-based material is 1.0 m². 2 / g-6.7m 2 / g.

14. The secondary battery according to any one of claims 1-13, characterized in that, The first composite adhesive accounts for 1%-5% of the mass of the first active layer; and / or The second composite adhesive accounts for 1%-7% of the mass of the second active layer.

15. The secondary battery according to any one of claims 1-14, characterized in that, The first active layer includes a first carbon-based material, and / or the second active layer further includes a second carbon-based material.

16. The secondary battery according to any one of claims 1-15, characterized in that, The first carbon-based material accounts for 30%-100% of the mass of the first active layer, and / or the second carbon-based material accounts for 5%-90% of the mass of the second active layer.

17. The secondary battery according to any one of claims 15 or 16, characterized in that, The negative electrode sheet satisfies one or more of the following conditions: (1) The average particle size of the first carbon-based material and / or the second carbon-based material is 3-20 μm; (2) The specific surface area of ​​the first carbon-based material and / or the second carbon-based material is 1.2-4.5 m². 2 / g; (3) The graphitization degree of the first carbon-based material and / or the second carbon-based material is 90%-97%; (4) The specific capacity of the first carbon-based material and / or the second carbon-based material is 340-370 mAh / g.

18. The secondary battery according to any one of claims 15-17, characterized in that, The first carbon-based material and / or the second carbon-based material are selected from one or more of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, or hard carbon.

19. The secondary battery according to claim 18, characterized in that, The first carbon-based material and / or the second carbon-based material each independently comprise artificial graphite and natural graphite, wherein the mass ratio of the artificial graphite to the natural graphite is 9:1 to 1:

9.

20. The secondary battery according to any one of claims 1-19, characterized in that, The negative electrode sheet also includes a buffer layer, which is located between the negative electrode current collector and the negative electrode film layer.

21. The secondary battery according to claim 20, characterized in that, The buffer layer comprises a third adhesive and a conductive agent, wherein the mass ratio of the third adhesive and the conductive agent is 20:1 to 1:

1.

22. The secondary battery according to claim 21, characterized in that, The third adhesive is selected from one or more of styrene-butadiene rubber, polyacrylic adhesives, polyacrylonitrile, polyvinylidene fluoride, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

23. The secondary battery according to any one of claims 1-22, characterized in that, In the negative electrode film layer, the mass ratio of the first active layer to the second active layer is 1:9-9:

1.

24. The secondary battery according to any one of claims 1-23, characterized in that, The areal density of the negative electrode film is 4 mg / cm³. 2 -16mg / cm 2 .

25. The secondary battery according to any one of claims 1-24, characterized in that, The thickness of the negative electrode film is 20μm-100μm.

26. The secondary battery according to any one of claims 1-25, characterized in that, The specific capacity of the negative electrode film is 360mAh / g-3000mAh / g.

27. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1-26.

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