Negative electrode sheet, secondary battery, and electronic device

Abstract

The present application provides a negative electrode sheet, a secondary battery, and an electronic device. The negative electrode sheet comprises a negative electrode current collector, and a first substance layer and a second substance layer which are arranged on at least one surface of the negative electrode current collector; in the thickness direction of the negative electrode sheet, the second substance layer is arranged between the negative electrode current collector and the first substance layer; the first substance layer comprises a first active material, a first aqueous binder and a second aqueous binder; the first active material comprises a first silicon material; the functional group of the first aqueous binder includes at least one of a, b and c, a is a carboxyl group, b comprises a carboxyl group and a phenyl group, and c comprises a carboxyl group and a hydroxyl group; the second aqueous binder comprises a carbon-carbon double bond and a phenyl group; on the basis of the total mass of the first substance layer, the content of the first aqueous binder is D1, and the content of the second aqueous binder is E1, wherein E1<D1; the second substance layer comprises a second active material and a third aqueous binder; and the third aqueous binder comprises a carbon-carbon double bond and a phenyl group. The secondary battery of the present application has good kinetic performance and cycle stability.

Description

Negative electrode plate, secondary battery and electronic device Technical Field

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

[0002] Secondary batteries, such as lithium-ion batteries, have advantages such as high energy density, high open-circuit voltage, low self-discharge rate, long cycle life, and good safety. They are now widely used as power sources in electronic products such as cameras, mobile phones, drones, laptops, and smartwatches.

[0003] With the increasing demand for energy density in lithium-ion batteries, silicon anodes have gradually become an important direction for the development of lithium-ion batteries. While silicon materials, as the active material of the anode in lithium-ion batteries, improve energy density, their poor electronic conductivity and ion transport properties lead to problems such as insufficient kinetics, narrow charging window, increased impedance, and large discharge temperature rise. On the other hand, the process of lithium-ion insertion and extraction in silicon materials causes a large volume change, resulting in a large volume expansion of the lithium-ion battery. Silicon anodes have poor adhesion performance, and lithium-ion batteries experience rapid cycle degradation.

[0004] Summary of the Invention

[0005] The purpose of this application is to provide a negative electrode sheet, a secondary battery, and an electronic device to improve the kinetic performance and cycle stability of the secondary battery. The specific technical solution is as follows:

[0006] It should be noted that the invention description in this application uses lithium-ion batteries as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0007] The first aspect of this application provides a negative electrode sheet, including a negative electrode current collector and a first material layer and a second material layer disposed on at least one surface of the negative electrode current collector. Along the thickness direction of the negative electrode sheet, the second material layer is disposed between the negative electrode current collector and the first material layer. The first material layer includes a first active material, a first aqueous binder, and a second aqueous binder. The first active material includes a first silicon material. The first aqueous binder has groups including at least one of a, b, and c, wherein a is a carboxyl group, b is a carboxyl group and a phenyl group, and c is a carboxyl group and a hydroxyl group. The second aqueous binder has groups including carbon-carbon double bonds and phenyl groups. Based on the total mass of the first material layer, the mass percentage of the first aqueous binder is D1, and the mass percentage of the second aqueous binder is E1. The second material layer includes a second active material and a third aqueous binder. The third aqueous binder has groups including carbon-carbon double bonds and phenyl groups. Wherein, E1 < D1. This application involves forming a first material layer and a second material layer on at least one surface of the negative electrode current collector. The first material layer, located away from the negative electrode current collector, employs a first aqueous binder with carboxyl groups and a second aqueous binder with carbon-carbon double bonds and phenyl groups. The carboxyl groups of the first aqueous binder can form strong hydrogen bonds with active materials containing hydroxyl groups on their surface, promoting the dispersion stability of the active material and effectively suppressing the volume expansion of the silicon-containing negative electrode sheet, thus improving cycle expansion performance. The second aqueous binder with carbon-carbon double bonds and phenyl groups exhibits good toughness and high elongation, resulting in good kinetic performance. By controlling the content of the first aqueous binder to be greater than that of the second aqueous binder, the effective adhesion and expansion suppression of the silicon-containing first material layer can be improved. The third aqueous binder with carbon-carbon double bonds and phenyl groups enables the second material layer to have good adhesion to both the first material layer and the current collector, reducing the risk of delamination and also helping to improve the overall toughness of the negative electrode sheet. This fully utilizes the interaction between the silicon material and the various binders in the negative electrode sheet, enabling the secondary battery to simultaneously possess good kinetic performance and cycle stability.

[0008] In one embodiment of this application, the mass percentage of the first silicon material is P1 based on the total mass of the first material layer; the second active material includes a second silicon material, and the mass percentage of the second silicon material is P2 based on the total mass of the second material layer, where P2 < P1 ≤ 97%. In this application, the silicon material content in the first material layer is higher than that in the second material layer, and the first material layer is closer to the surface of the negative electrode, thus shortening the transport path of active ions (e.g., lithium ions) involving Si and alleviating the kinetic problems caused by the poor conductivity of silicon materials. With low silicon content and a low volume expansion rate, and with the second material layer closer to the negative electrode current collector, the low expansion rate reduces the risk of the second material layer demolding, improving the overall structural stability of the negative electrode sheet. Simultaneously, the use of a first and second aqueous binder for the first material layer, and a third aqueous binder for the second material layer, effectively enhances the kinetic performance of the silicon-containing negative electrode sheet with dual material layers. This reduces the risk of external lithium plating in the secondary battery at lower operating temperatures, improving the safety of the secondary battery. It also ensures balanced adhesion and toughness between the first and second material layers, reducing the risk of secondary battery deformation and improving the cycle expansion performance of the secondary battery, resulting in better kinetic performance and cycle stability.

[0009] In one embodiment of this application, 0% < P2 < P1 ≤ 30%. By controlling the content of the first silicon material P1 in the first material layer and the content of the second silicon material P2 in the second material layer within the above range, this application can effectively improve ion transport efficiency, thereby enhancing the kinetic performance of the secondary battery. Simultaneously, the high silicon content in the first material layer is used in combination with the first and second aqueous binders, while the low silicon content in the second material layer is used in combination with the third aqueous binder. The first aqueous binder, which has carboxyl groups, can form strong hydrogen bonds with the silicon material, fully utilizing the effective coating and adhesion of the first aqueous binder to the surface of the first silicon material. The second and third aqueous binders form a series effect between the first and second material layers, effectively suppressing the expansion of the secondary battery and reducing the risk of deformation, enabling the secondary battery to achieve both good kinetic performance and cycle stability.

[0010] In one embodiment of this application, 0.1% ≤ D1-E1 ≤ 9.9%. In another embodiment of this application, 1% ≤ D1-E1 ≤ 5%. By controlling the difference between the content D1 of the first aqueous binder and the content E1 of the second aqueous binder within the above-mentioned range, this application can better leverage the synergistic effect of the first silicon material, the first aqueous binder, and the second aqueous binder, which is beneficial for improving ion transport efficiency, enhancing kinetic performance, suppressing the volume expansion of the silicon-containing negative electrode sheet, and thus improving the kinetic performance and cycle stability of the secondary battery.

[0011] In one embodiment of this application, the mass percentage of the third aqueous binder is E2, where E2 > E1, based on the total mass of the second material layer. This application controls the content of the third aqueous binder (E2) in the second material layer to be higher than the content of the second aqueous binder (E1) in the first material layer. This improves the adhesion between the second material layer and the first material layer and the negative electrode current collector, resulting in a balanced adhesive force and toughness between the first and second material layers. Consequently, the secondary battery exhibits both good kinetic performance and cycle stability.

[0012] In one embodiment of this application, the second material layer further includes a fourth aqueous binder. The fourth aqueous binder has groups including at least one of a, b, and c, wherein a is a carboxyl group, b is a carboxyl group and a phenyl group, and c is a carboxyl group and a hydroxyl group. Based on the total mass of the second material layer, the mass percentage of the fourth aqueous binder is D2, where D2 < E2. The second material layer of this application further includes a fourth aqueous binder with carboxyl groups, and by controlling the content of the fourth aqueous binder, D2, to be less than the content of the third aqueous binder, E2, it is possible to achieve better adhesion between the second material layer and the first material layer and the current collector, thereby improving the flexibility of the negative electrode sheet, enhancing the kinetic performance of the negative electrode sheet, and enabling the secondary battery to have both good cycle stability and good kinetic performance.

[0013] In one embodiment of this application, the fourth aqueous binder includes at least one of an acrylic polymer or sodium alginate. In this application, the acrylic polymer includes at least one of polyacrylic acid or polymethacrylic acid. By selecting the aforementioned type of fourth aqueous binder in the second material layer, this application can leverage the synergistic effect of the active material and the binder in the first and second material layers, promoting the dispersion stability of the active material, effectively suppressing the volume expansion of the silicon-containing negative electrode sheet, improving cycle expansion performance, and thus improving the cycle stability and kinetic performance of the secondary battery.

[0014] In one embodiment of this application, the weight-average molecular weight (Mw) of the fourth aqueous binder is 600,000 to 800,000, and the glass transition temperature (Tg) of the fourth aqueous binder is 130°C to 150°C. By selecting a fourth aqueous binder with the aforementioned weight-average molecular weight and glass transition temperature in the second material layer, this application can leverage the synergistic effect of the second active material, the third aqueous binder, and the fourth aqueous binder in the second material layer, thereby improving the kinetic performance of the silicon-containing negative electrode sheet, mitigating the cycle expansion of the negative electrode sheet, and ultimately improving the cycle stability and kinetic performance of the secondary battery.

[0015] In one embodiment of this application, the fourth aqueous binder has a weight-average molecular weight of 32,000 to 400,000 and a glass transition temperature (Tg) of 150°C to 180°C. By selecting a fourth aqueous binder with the aforementioned weight-average molecular weight and glass transition temperature in the second material layer, this application can leverage the synergistic effect of the second active material, the third aqueous binder, and the fourth aqueous binder in the second material layer to improve the kinetic performance of the silicon-containing negative electrode sheet, reduce the cycle expansion of the negative electrode sheet, and thereby improve the cycle stability and kinetic performance of the secondary battery.

[0016] In one embodiment of this application, 0.1% ≤ E2-D2 ≤ 9.9%. In another embodiment of this application, 1% ≤ E2-D2 ≤ 5%. By controlling the difference between the content of the third aqueous binder (E2) and the content of the fourth aqueous binder (D2) within the above-mentioned range, this application can improve the adhesion between the second material layer and the first material layer and the current collector, giving the negative electrode sheet better flexibility. This improves the kinetic performance of the negative electrode sheet, enabling the secondary battery to have both good cycle stability and good kinetic performance.

[0017] In one embodiment of this application, 0.1% ≤ D1-D2 ≤ 9.9%. In another embodiment of this application, 1% ≤ D1-D2 ≤ 5%. By controlling the difference between the content D1-D2 of the first aqueous binder (both containing carboxyl groups) and the content D2 of the fourth aqueous binder to be within the above-mentioned range, this application can effectively suppress the volume expansion of the silicon-containing negative electrode sheet, while ensuring that the first and second material layers have balanced adhesion and toughness, improving the kinetic performance of the negative electrode sheet, thereby enabling the secondary battery to have both good cycle stability and kinetic performance.

[0018] In one embodiment of this application, 0.5% ≤ D1 ≤ 10%. In another embodiment of this application, 1% ≤ D1 ≤ 10%. By controlling the content D1 of the first aqueous binder within the above range, this application can better leverage the synergistic effect of the first silicon material, the first aqueous binder, and the second aqueous binder, further suppressing the volume expansion of the silicon-containing negative electrode sheet, improving the kinetic performance of the negative electrode sheet, and thus improving the kinetic performance and cycle stability of the secondary battery.

[0019] In one embodiment of this application, 0.1% ≤ E1 < 10%. In another embodiment of this application, 0.5% ≤ E1 ≤ 5%. By controlling the content E1 of the second aqueous binder within the above range, this application can better leverage the synergistic effect of the first silicon material, the first aqueous binder, and the second aqueous binder, further enhancing the effective adhesion and expansion suppression of the silicon-containing first material layer, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0020] In one embodiment of this application, 0.5% ≤ E2 ≤ 10%. In another embodiment of this application, 1% ≤ E2 ≤ 10%. By controlling the content E2 of the third aqueous binder within the above range, this application can improve the adhesion between the second material layer and the first material layer and the current collector, giving the negative electrode sheet better flexibility, improving the dynamic performance of the negative electrode sheet, and enabling the secondary battery to have good cycle stability while also having good dynamic performance.

[0021] In one embodiment of this application, 0% ≤ D2 < 10%. In another embodiment of this application, 0% ≤ D2 ≤ 5%. By controlling the content D2 of the fourth aqueous binder within the above range, this application facilitates the balanced adhesion and toughness of the first and second material layers, suppresses the cyclic expansion performance of the negative electrode sheet, and improves the kinetic performance of the negative electrode sheet, thereby enabling the secondary battery to simultaneously possess good cyclic stability and kinetic performance.

[0022] In one embodiment of this application, the weight-average molecular weight of the first aqueous binder is 600,000 to 800,000, and the glass transition temperature (Tg) of the first aqueous binder is 130°C to 150°C. By selecting a first aqueous binder with the above-mentioned weight-average molecular weight and glass transition temperature in the first material layer, this application can leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer to suppress the volume expansion of the silicon-containing negative electrode sheet, improve the kinetic performance of the negative electrode sheet, and thus improve the cycle stability and kinetic performance of the secondary battery.

[0023] In one embodiment of this application, the weight-average molecular weight of the first aqueous binder is 32,000 to 400,000, and the glass transition temperature (Tg) of the first aqueous binder is 150°C to 180°C. By selecting a first aqueous binder with the above-mentioned weight-average molecular weight and glass transition temperature in the first material layer, this application can leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer to suppress the volume expansion of the silicon-containing negative electrode sheet, improve the kinetic performance of the negative electrode sheet, and thus improve the cycle stability and kinetic performance of the secondary battery.

[0024] In one embodiment of this application, the particle size Dv50 of the second aqueous binder is between 80 nm and 160 nm. By controlling the particle size Dv50 of the second aqueous binder within the above range, this application can better leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer, thereby improving the effective adhesion and expansion suppression of the silicon-containing first material layer, and thus improving the cycle stability and kinetic performance of the secondary battery.

[0025] In one embodiment of this application, the glass transition temperature (Tg) of the second aqueous binder is -5°C to 10°C. By selecting a second aqueous binder with the aforementioned glass transition temperature in the first material layer, this application can leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer, thereby improving the effective adhesion and expansion suppression of the silicon-containing first material layer and enhancing the cycle stability and kinetic performance of the secondary battery.

[0026] In one embodiment of this application, the particle size Dv50 of the third aqueous binder is between 80 nm and 160 nm. By controlling the particle size Dv50 of the third aqueous binder within the above range, this application facilitates the improvement of the adhesion between the second material layer and the first material layer and the current collector, giving the negative electrode sheet better flexibility, improving the dynamic performance of the negative electrode sheet, and enabling the secondary battery to have both good cycle stability and good dynamic performance.

[0027] In one embodiment of this application, the glass transition temperature (Tg) of the third aqueous binder is -5°C to 10°C. By selecting a third aqueous binder with the aforementioned glass transition temperature in the second material layer, this application improves the adhesion between the second material layer and the first material layer and the current collector, giving the negative electrode sheet better flexibility and enhancing its kinetic performance. This results in the secondary battery exhibiting both good cycle stability and good kinetic performance.

[0028] In one embodiment of this application, the first active material may or may not include first graphite. Based on the total mass of the first active material, the mass percentage of the first silicon material is A1, where 3% ≤ A1 ≤ 100%, and the mass percentage of the first graphite is B1, where 0% ≤ B1 ≤ 97%. In another embodiment of this application, 10% ≤ A1 ≤ 80%, and 20% ≤ B1 ≤ 90%. When the secondary battery discharges, the active material graphite in the negative electrode layer discharges preferentially, while the silicon material only begins to discharge at the end of the discharge. This prolongs the ion transport distance between lithium ions and silicon. Therefore, by controlling the content of the first silicon material A1 and the content of the first graphite B1 within the above range, the ion transport efficiency at the end of the discharge can be effectively improved, thereby enhancing the kinetic performance of the secondary battery. At the same time, the silicon material increases the cycle expansion rate of the negative electrode sheet. By using the first silicon material, the first aqueous binder, and the second aqueous binder in combination, and with the first aqueous binder having a high carboxyl content, it can form strong hydrogen bonds with the silicon material, providing high adhesion. This makes the first material layer have good stability during cycling, thus enabling the secondary battery to have both good kinetic performance and cycle stability.

[0029] In one embodiment of this application, the second active material includes second graphite and may or may not include a second silicon material. Based on the total mass of the second active material, the mass percentage of the second silicon material is A2, 0% ≤ A2 ≤ 97%, and the mass percentage of the second graphite is B2, 3% ≤ B2 ≤ 100%. In another embodiment of this application, 0% ≤ A2 ≤ 50%, and 50% ≤ B2 ≤ 100%. By controlling the content of the second silicon material A2 and the content of the second graphite B2 within the above ranges, and simultaneously using a third aqueous binder including carbon-carbon double bonds and phenyl groups, this application can improve the kinetic performance of the silicon material in the second material layer away from the electrolyte, thereby enhancing the kinetic performance of the secondary battery.

[0030] In one embodiment of this application, the first active material further includes a first graphite. Based on the total mass of the first active material, the mass percentage of the first silicon material is A1, where 3% ≤ A1 < 100%, and the mass percentage of the first graphite is B1, where 0% < B1 ≤ 97%. The second active material includes a second silicon material and a second graphite. Based on the total mass of the second active material, the mass percentage of the second silicon material is A2, where 0% < A2 ≤ 97%, and the mass percentage of the second graphite is B2, where 3% ≤ B2 < 100%. In one embodiment of this application, 10% ≤ A1 ≤ 80%, 20% ≤ B1 ≤ 90%, 0% < A2 ≤ 50%, and 50% ≤ B2 < 100%. By controlling the contents of the first silicon material A1 and the first graphite B1, the second silicon material A2 and the second graphite B2 within the above ranges, the synergistic effect of the first and second material layers can be achieved, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0031] In one embodiment of this application, the second silicon material includes at least one of pure silicon, silicon alloy material, silicon-carbon composite material, or silicon oxide. By employing the above-mentioned types of second silicon materials, this application can leverage the synergistic effect of the second silicon material with the third and fourth aqueous binders, reduce the risk of demolding of the second material layer, and improve the overall structural stability of the negative electrode sheet. Furthermore, by using the first and second aqueous binders of the first material layer and the third aqueous binder of the second material layer in combination, the kinetic performance of the silicon-containing negative electrode sheet with a double material layer can be effectively improved, thereby enhancing the kinetic performance and cycle stability of the secondary battery.

[0032] In one embodiment of this application, the second active material does not include silicon and includes graphite. By employing the aforementioned type of second active material, this application can leverage the synergistic effect of the second active material with the third and fourth aqueous binders, reduce the risk of demolding of the second material layer, and improve the overall structural stability of the negative electrode sheet. Furthermore, the combined use of the first and second aqueous binders of the first material layer and the third aqueous binder of the second material layer effectively enhances the kinetic performance of the silicon-containing negative electrode sheet with a double-layered structure, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0033] In one embodiment of this application, the first material layer further includes a first conductive agent, the mass percentage of which is C1 based on the total mass of the first material layer; the second material layer further includes a second conductive agent, the mass percentage of which is C2 based on the total mass of the second material layer; 0.1% ≤ C1-C2 ≤ 3%. By controlling the difference between the contents of the first and second conductive agents, C1-C2, within the above-mentioned range, this application can leverage the synergistic effect of the first and second material layers to improve the conductivity of the negative electrode, thereby enhancing the dynamic performance of the secondary battery.

[0034] In one embodiment of this application, 0.2% ≤ C1 ≤ 10%. By controlling the content C1 of the first conductive agent within the above range, this application can further improve the conductivity of the first material layer, improve the kinetic performance of the first material layer, and thus improve the kinetic performance of the secondary battery.

[0035] In one embodiment of this application, 0.1% ≤ C2 ≤ 9.9%. In another embodiment of this application, 0.1% ≤ C2 ≤ 5%. By controlling the content of the second conductive agent C2 within the above range, this application can further improve the kinetic performance of the second material layer, thereby improving the kinetic performance of the secondary battery.

[0036] In one embodiment of this application, the first material layer further includes a first dispersant, which includes a first carboxymethyl cellulose dispersant, and the mass percentage of the first dispersant is F1 based on the total mass of the first material layer; the second material layer further includes a second dispersant, which includes a second carboxymethyl cellulose dispersant, and the mass percentage of the second dispersant is F2 based on the total mass of the second material layer; 0.1% ≤ F1-F2 ≤ 3%. By controlling the content difference between the first and second dispersants, F1-F2, within the above range, this application can assist the binder in enhancing the adhesion of the first material layer, reducing the risk of delamination and powdering of the first material layer. Simultaneously, the hydrogen bonds of the carboxymethyl cellulose dispersant have a self-repairing function, which can better adapt to volume expansion during cycling, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0037] In one embodiment of this application, 0.2% ≤ F1 ≤ 10%. By adjusting the content F1 of the first dispersant within the above range, the adhesion of the first material layer can be further improved, and it can better adapt to the volume expansion during cycling, which is beneficial to further improving the kinetic performance and cycle stability of the secondary battery.

[0038] In one embodiment of this application, 0.1% ≤ F2 ≤ 9.9%. In another embodiment of this application, 0.1% ≤ F2 ≤ 5%. By adjusting the content of the second dispersant F2 within the above range, the adhesion of the second material layer can be further improved, and it can better adapt to the volume expansion during cycling, which is beneficial to further improving the kinetic performance and cycle stability of the secondary battery.

[0039] In one embodiment of this application, the first aqueous binder includes at least one of an acrylic polymer or sodium alginate, and the acrylic polymer includes at least one of polyacrylic acid or polymethacrylic acid. By selecting the above-mentioned types of first aqueous binders in the first material layer, this application can better leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer, thereby improving the cycle expansion and kinetic performance of the negative electrode sheet, and thus improving the cycle stability and kinetic performance of the secondary battery.

[0040] In one embodiment of this application, the second aqueous binder and the third aqueous binder each independently comprise at least one of styrene-butadiene rubber, styrene-acrylic emulsion, or pure acrylic emulsion. By employing a second aqueous binder of the aforementioned type in the first material layer, and / or employing a third aqueous binder of the aforementioned type in the second material layer, it is beneficial to leverage the synergistic effect of the first and second material layers, improve the kinetic performance and cycle expansion performance of the negative electrode sheet, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0041] In one embodiment of this application, the first silicon material includes at least one of pure silicon, silicon alloy material, silicon-carbon composite material, or silicon oxide. By employing the above-mentioned types of first silicon materials, this application can leverage the synergistic effect of the first silicon material with the first aqueous binder and the second aqueous binder, thereby improving the kinetic performance and cycle expansion performance of the negative electrode sheet, and thus improving the kinetic performance and cycle stability of the secondary battery.

[0042] In one embodiment of this application, the thickness of the first material layer is H1, the thickness of the second material layer is H2, and 0.2 ≤ H1 / H2 ≤ 4. By adjusting H1 / H2 within the above range, it is beneficial to leverage the synergistic effect of the first and second material layers, improve the kinetic performance and cycle expansion performance of the negative electrode sheet, and thus improve the kinetic performance and cycle stability of the secondary battery.

[0043] A second aspect of this application provides a secondary battery comprising the negative electrode sheet of any of the foregoing embodiments. The secondary battery provided by the second aspect of this application exhibits good kinetic performance and cycle stability.

[0044] A third aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments.

[0045] The beneficial effects of this application are:

[0046] This application provides a negative electrode sheet, a secondary battery, and an electronic device. The negative electrode sheet includes a negative current collector and a first material layer and a second material layer disposed on at least one surface of the negative current collector. Along the thickness direction of the negative electrode sheet, the second material layer is disposed between the negative current collector and the first material layer. The first material layer includes a first active material, a first aqueous binder, and a second aqueous binder. The first active material includes a first silicon material. The first aqueous binder has groups including at least one of a, b, and c, wherein a is a carboxyl group, b is a carboxyl group and a phenyl group, and c is a carboxyl group and a hydroxyl group. The second aqueous binder has groups including carbon-carbon double bonds and phenyl groups. Based on the total mass of the first material layer, the mass percentage of the first aqueous binder is D1, and the mass percentage of the second aqueous binder is E1. The second material layer includes a second active material and a third aqueous binder. The third aqueous binder has groups including carbon-carbon double bonds and phenyl groups. Wherein, E1 < D1. This application employs a first aqueous binder with carboxyl groups and a second aqueous binder with carbon-carbon double bonds and phenyl groups in the first material layer far from the negative electrode current collector. The carboxyl groups of the first aqueous binder can form strong hydrogen bonds with active materials containing hydroxyl groups on their surface, promoting the dispersion stability of the active material and effectively suppressing the volume expansion of the silicon-containing negative electrode sheet, thus improving cycle expansion performance. The second aqueous binder with carbon-carbon double bonds and phenyl groups has good toughness and high elongation, exhibiting good kinetic performance. By controlling the content of the first aqueous binder to be greater than that of the second aqueous binder, the effective adhesion and expansion suppression of the silicon-containing first material layer can be improved. The third aqueous binder with carbon-carbon double bonds and phenyl groups enables the second material layer to have good adhesion to both the first material layer and the current collector, reducing the risk of delamination and also helping to improve the overall toughness of the negative electrode sheet. This fully utilizes the interaction between the silicon material and the various binders in the negative electrode sheet, enabling the secondary battery to simultaneously possess good kinetic performance and cycle stability.

[0047] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0048] The accompanying drawings, which are provided to further illustrate this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application.

[0049] Figure 1 shows the negative electrode sheet of one embodiment of this application;

[0050] Figure 2 shows the negative electrode sheet of another embodiment of this application. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0052] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0053] This application provides a negative electrode sheet, including a negative electrode current collector and a first material layer and a second material layer disposed on at least one surface of the negative electrode current collector. Along the thickness direction of the negative electrode sheet, the second material layer is disposed between the negative electrode current collector and the first material layer. The first material layer includes a first active material, a first aqueous binder, and a second aqueous binder; the second material layer includes a second active material and a third aqueous binder. The phrase "disposed on at least one surface of the negative electrode current collector" means that it can be disposed on one surface of the negative electrode current collector along its own thickness direction, or it can be disposed on two surfaces of the negative electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or it can be a partial surface area of ​​the negative electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved. For example, Figure 1 shows a negative electrode sheet according to one embodiment of this application, which includes a negative electrode current collector 30, a first material layer 10, and a second material layer 20, with the first material layer 10 and the second material layer 20 disposed on two surfaces of the negative electrode current collector 30. Along the thickness direction of the negative electrode sheet, the second material layer 20 is disposed between the negative electrode current collector 30 and the first material layer 10; the first material layer 10 includes a first active material (not shown in the figure), a first aqueous binder 11 and a second aqueous binder 12, and the second material layer 20 includes a second active material (not shown in the figure) and a third aqueous binder 21.

[0054] The first active material includes a first silicon material. The first aqueous binder has groups including at least one of a, b, and c, wherein a is a carboxyl group, b is a carboxyl group and a phenyl group, and c is a carboxyl group and a hydroxyl group. The second aqueous binder has groups including carbon-carbon double bonds and phenyl groups. Based on the total mass of the first material layer, the mass percentage of the first aqueous binder is D1, and the mass percentage of the second aqueous binder is E1, wherein E1 < D1. The third aqueous binder has groups including carbon-carbon double bonds and phenyl groups. This application provides a first material layer and a second material layer on at least one surface of the negative electrode current collector. The first material layer away from the negative electrode current collector uses a first aqueous binder with carboxyl groups and a second aqueous binder with carbon-carbon double bonds and phenyl groups. The carboxyl groups of the first aqueous binder can form strong hydrogen bonds with the active material containing hydroxyl groups on the surface, which can promote the dispersion stability of the active material and effectively suppress the volume expansion of the silicon-containing negative electrode sheet, improving the cycle expansion performance. The second aqueous binder with carbon-carbon double bonds and phenyl groups has good toughness, high elongation, and good kinetic performance. By adjusting the content of the first aqueous binder to be greater than that of the second aqueous binder, the effective adhesion and expansion suppression of the silicon-containing first material layer can be improved. The third aqueous binder, which has carbon-carbon double bonds and phenyl groups, enables the second material layer to have good adhesion to the first material layer and the current collector, respectively, reducing the risk of delamination and peeling. It also helps to improve the overall toughness of the negative electrode sheet, thereby giving full play to the interaction between silicon materials and various binders in the negative electrode sheet, so that the secondary battery can have good kinetic performance and cycle stability at the same time.

[0055] In one embodiment of this application, the mass percentage of the first silicon material is P1 based on the total mass of the first material layer; the second active material includes a second silicon material, and the mass percentage of the second silicon material is P2 based on the total mass of the second material layer, where P2 < P1 ≤ 97%. For example, P1 can be 0.1%, 0.5%, 1%, 3%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 93%, 97%, or a range of any two of these values, and P2 can be 0.01%, 0.05%, 0.1%, 0.5%, 1%, 3%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 93%, 96.9%, or a range of any two of these values, and P2 < P1. In this application, the silicon content in the first material layer is higher than that in the second material layer. The first material layer is closer to the surface of the negative electrode, thus shortening the transport path of active ions (e.g., lithium ions) involving Si and alleviating the kinetic problems caused by the poor conductivity of silicon. The low silicon content results in a low volume expansion rate, and the second material layer's proximity to the negative electrode current collector further reduces the risk of demolding of the second material layer, improving the overall structural stability of the negative electrode. Simultaneously, the use of a first and second aqueous binder for the first material layer, and a third aqueous binder for the second material layer, effectively enhances the kinetic performance of the silicon-containing negative electrode with a double material layer. This reduces the risk of external lithium plating in the secondary battery at lower operating temperatures, improving battery safety. The first and second material layers possess balanced adhesion and toughness, reducing the risk of battery deformation and improving the cycle expansion performance of the secondary battery, resulting in better kinetic performance and cycle stability.

[0056] In one embodiment of this application, 0% < P2 < P1 ≤ 30%. For example, P1 can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 3%, 4%, 5%, 6%, 8%, 10%, 12%, 15%, 17%, 18%, 20%, 22%, 25%, 26%, 27%, 28%, 29%, 30%, or a range of any two of these values, and P2 can be 0.01%. The values ​​are %, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 3%, 4%, 5%, 6%, 8%, 10%, 12%, 15%, 17%, 18%, 20%, 22%, 25%, 26%, 27%, 28%, 29%, 29.9%, or a range consisting of any two of these values, and P2 < P1. By controlling the content of the first silicon material P1 in the first material layer and the content of the second silicon material P2 in the second material layer within the aforementioned range, this application can effectively improve ion transport efficiency, thereby enhancing the kinetic performance of the secondary battery. Simultaneously, the high silicon content in the first material layer is used in combination with the first and second aqueous binders, while the low silicon content in the second material layer is used in combination with the third aqueous binder. The first aqueous binder, which has carboxyl groups, can form strong hydrogen bonds with the silicon material, fully leveraging the effective coating and adhesion of the first aqueous binder to the surface of the first silicon material. The second and third aqueous binders form a series effect between the first and second material layers, effectively suppressing the expansion of the secondary battery and reducing the risk of deformation, thus enabling the secondary battery to achieve both good kinetic performance and cycle stability.

[0057] In one embodiment of this application, 0.1% ≤ D1-E1 ≤ 9.9%. For example, D1-E1 can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.5%, 9.7%, 9.9%, or a range consisting of any two of these values. In one embodiment of this application, 1% ≤ D1-E1 ≤ 5%. This application, by controlling the difference between the content D1 of the first aqueous binder and the content E1 of the second aqueous binder, D1-E1, within the range of 0.1% to 9.9%, can better leverage the synergistic effect of the first silicon material, the first aqueous binder, and the second aqueous binder. This is beneficial for improving ion transport efficiency, enhancing kinetic performance, suppressing the volume expansion of the silicon-containing negative electrode sheet, and thus improving the kinetic performance and cycle stability of the secondary battery.

[0058] In one embodiment of this application, the mass percentage of the third aqueous binder is E2, where E2 > E1, based on the total mass of the second material layer. This application controls the content of the third aqueous binder (E2) in the second material layer to be higher than the content of the second aqueous binder (E1) in the first material layer. This improves the adhesion between the second material layer and the first material layer and the negative electrode current collector, resulting in a balanced adhesive force and toughness between the first and second material layers. Consequently, the secondary battery exhibits both good kinetic performance and cycle stability.

[0059] In one embodiment of this application, the second material layer includes a second active material, a third aqueous binder, and a fourth aqueous binder. The fourth aqueous binder has groups including at least one of a, b, and c, wherein a is a carboxyl group, b is a carboxyl group and a phenyl group, and c is a carboxyl group and a hydroxyl group. Based on the total mass of the second material layer, the mass percentage of the fourth aqueous binder is D2, where D2 < E2. Exemplarily, FIG2 shows a negative electrode sheet according to another embodiment of this application, which includes a negative electrode current collector 30, a first material layer 10, and a second material layer 20. The first material layer 10 and the second material layer 20 are disposed on two surfaces of the negative electrode current collector 30. Along the thickness direction of the negative electrode sheet, the second material layer 20 is disposed between the negative electrode current collector 30 and the first material layer 10; the first material layer 10 includes a first active material (not shown in the figure), a first aqueous binder 11, and a second aqueous binder 12; the second material layer 20 includes a second active material (not shown in the figure), a third aqueous binder 21, and a fourth aqueous binder 22. The second material layer of this application also includes a fourth aqueous binder with carboxyl groups, and the content D2 of the fourth aqueous binder is controlled to be less than the content E2 of the third aqueous binder. This enables the second material layer to have better adhesion to the first material layer and the current collector, which can improve the flexibility of the negative electrode sheet, improve the dynamic performance of the negative electrode sheet, and enable the secondary battery to have good cycle stability and good dynamic performance.

[0060] In one embodiment of this application, the fourth aqueous binder includes at least one of an acrylic polymer or sodium alginate. In this application, the acrylic polymer includes at least one of polyacrylic acid or polymethacrylic acid. By selecting the aforementioned type of fourth aqueous binder in the second material layer, this application can leverage the synergistic effect of the active material and the binder in the first and second material layers, promoting the dispersion stability of the active material, effectively suppressing the volume expansion of the silicon-containing negative electrode sheet, improving cycle expansion performance, and thus improving the cycle stability and kinetic performance of the secondary battery.

[0061] In one embodiment of this application, the weight-average molecular weight (Mw) of the fourth waterborne adhesive is between 600,000 and 800,000, and the glass transition temperature (Tg) of the fourth waterborne adhesive is between 130°C and 150°C. For example, the weight-average molecular weight of the fourth waterborne adhesive can be 600,000, 610,000, 620,000, 630,000, 640,000, 650,000, 660,000, 680,000, 700,000, 720,000, 730,000, 750,000, 760,000, 780,000, 790,000, 800,000, or a range consisting of any two of these values. The glass transition temperature (Tg) of the fourth aqueous binder can be 130℃, 131℃, 132℃, 133℃, 135℃, 136℃, 138℃, 140℃, 142℃, 143℃, 145℃, 146℃, 148℃, 150℃, or a range of any two of these values. By selecting a fourth aqueous binder with the aforementioned weight-average molecular weight and glass transition temperature in the second material layer, this application can leverage the synergistic effect of the second active material, the third aqueous binder, and the fourth aqueous binder in the second material layer to improve the kinetic performance of the silicon-containing negative electrode sheet, reduce the cycle expansion of the negative electrode sheet, and thus improve the cycle stability and kinetic performance of the secondary battery.

[0062] In one embodiment of this application, the weight-average molecular weight of the fourth waterborne adhesive is 32,000 to 400,000, and the glass transition temperature (Tg) of the fourth waterborne adhesive is 150°C to 180°C. For example, the weight-average molecular weight of the fourth waterborne adhesive can be 32,000, 33,000, 35,000, 37,000, 39,000, 40,000, 42,000, 45,000, 48,000, 50,000, 52,000, 55,000, 58,000, 60,000, 63,000, 65,000, 67,000, 70,000, 75,000, 80,000, or 850. 00, 90000, 100000, 120000, 130000, 150000, 160000, 180000, 200000, 230000, 250000, 270000, 300000, 320000, 330000, 350000, 360000, 380000, 400000, or a range consisting of any two of these values. The glass transition temperature (Tg) of the fourth aqueous binder can be 150℃, 152℃, 153℃, 155℃, 157℃, 158℃, 160℃, 161℃, 163℃, 165℃, 166℃, 168℃, 170℃, 172℃, 173℃, 175℃, 177℃, 178℃, 180℃, or a range of any two of these values. By selecting a fourth aqueous binder with the aforementioned weight-average molecular weight and glass transition temperature in the second material layer, this application can leverage the synergistic effect of the second active material, the third aqueous binder, and the fourth aqueous binder in the second material layer to improve the kinetic performance of the silicon-containing negative electrode sheet, reduce the cycle expansion of the negative electrode sheet, and thus improve the cycle stability and kinetic performance of the secondary battery.

[0063] In one embodiment of this application, 0.1% ≤ E2-D2 ≤ 9.9%. In another embodiment of this application, 1% ≤ E2-D2 ≤ 5%. For example, E2-D2 can be 0.1%, 0.2%, 0.5%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.7%, 3%, 3.3%, 3.5%, 3.6%, 3.8%, 4%, 4.2%, 4.3%, 4.5%, 4.6%, 4.8%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 9.9%, or a range consisting of any two of these values. This application improves the adhesion between the second material layer and the first material layer and the current collector by adjusting the difference between the content of the third aqueous binder E2 and the content of the fourth aqueous binder D2 within the above range, thereby giving the negative electrode sheet better flexibility, improving the dynamic performance of the negative electrode sheet, and enabling the secondary battery to have good cycle stability and good dynamic performance.

[0064] In one embodiment of this application, 0.1% ≤ D1-D2 ≤ 9.9%. In another embodiment of this application, 1% ≤ D1-D2 ≤ 5%. For example, D1-D2 can be 0.1%, 0.2%, 0.5%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.7%, 3%, 3.3%, 3.5%, 3.6%, 3.8%, 4%, 4.2%, 4.3%, 4.5%, 4.6%, 4.8%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 9.9%, or a range consisting of any two of these values. This application effectively suppresses the volume expansion of silicon-containing negative electrode sheets by controlling the difference between the content D1 of the first aqueous binder and the content D2 of the fourth aqueous binder, which both have carboxyl groups, within the above-mentioned range. At the same time, it enables the first and second material layers to have balanced adhesion and toughness, improves the kinetic performance of the negative electrode sheet, and thus enables the secondary battery to have both good cycle stability and kinetic performance.

[0065] In one embodiment of this application, 0.5% ≤ D1 ≤ 10%. For example, D1 can be 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.5%, 9.7%, 9.9%, 10%, or a range of any two of these values. In one embodiment of this application, 1% ≤ D1 ≤ 10%. By controlling the content D1 of the first aqueous binder within the above range, this application can better leverage the synergistic effect of the first silicon material with the first and second aqueous binders, further suppressing the volume expansion of the silicon-containing negative electrode sheet, improving the kinetic performance of the negative electrode sheet, and thus improving the kinetic performance and cycle stability of the secondary battery.

[0066] In one embodiment of this application, 0.1% ≤ E1 < 10%. For example, E1 can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.5%, 9.7%, 9.9%, or a range of any two of these values. In one embodiment of this application, 0.5% ≤ E1 ≤ 5%. By controlling the content E1 of the second aqueous binder within the above range, this application can better leverage the synergistic effect of the first silicon material, the first aqueous binder, and the second aqueous binder, further enhancing the effective adhesion and expansion suppression of the silicon-containing first material layer, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0067] In one embodiment of this application, 0.5% ≤ E2 ≤ 10%. For example, E2 can be 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.5%, 9.7%, 9.9%, 10%, or a range of any two of these values. In one embodiment of this application, 1% ≤ E2 ≤ 10%. By controlling the content E2 of the third aqueous binder within the above range, this application helps to improve the adhesion between the second material layer and the first material layer and the current collector, giving the negative electrode sheet better flexibility, improving the dynamic performance of the negative electrode sheet, and enabling the secondary battery to have good cycle stability and good dynamic performance.

[0068] In one embodiment of this application, 0% ≤ D2 < 10%. For example, D2 can be 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.5%, 9.7%, 9.9%, or a range of any two of these values. In one embodiment of this application, 0% ≤ D2 ≤ 5%. By controlling the content D2 of the fourth aqueous binder within the above range, this application facilitates balanced adhesion and toughness between the first and second material layers, suppresses the cyclic expansion performance of the negative electrode, and improves the kinetic performance of the negative electrode, thereby enabling the secondary battery to simultaneously possess good cycle stability and kinetic performance.

[0069] In one embodiment of this application, the weight-average molecular weight of the first aqueous adhesive is between 600,000 and 800,000, and the glass transition temperature (Tg) of the first aqueous adhesive is between 130°C and 150°C. For example, the weight-average molecular weight of the first aqueous adhesive can be 600,000, 610,000, 620,000, 630,000, 640,000, 650,000, 660,000, 680,000, 700,000, 720,000, 730,000, 750,000, 760,000, 780,000, 790,000, 800,000, or a range consisting of any two of these values. The glass transition temperature (Tg) of the first aqueous binder can be 130℃, 131℃, 132℃, 133℃, 135℃, 136℃, 138℃, 140℃, 142℃, 143℃, 145℃, 146℃, 148℃, 150℃, or a range of any two of these values. By selecting a first aqueous binder with the aforementioned weight-average molecular weight and glass transition temperature in the first material layer, this application can leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer to suppress the volume expansion of the silicon-containing negative electrode sheet, improve the kinetic performance of the negative electrode sheet, and thus improve the cycle stability and kinetic performance of the secondary battery.

[0070] In one embodiment of this application, the weight-average molecular weight of the first aqueous adhesive is 32,000 to 400,000, and the glass transition temperature (Tg) of the first aqueous adhesive is 150°C to 180°C. For example, the weight-average molecular weight of the first aqueous adhesive can be 32,000, 33,000, 35,000, 37,000, 39,000, 40,000, 42,000, 45,000, 48,000, 50,000, 52,000, 55,000, 58,000, 60,000, 63,000, 65,000, 67,000, 70,000, 75,000, 80,000, or 850,000. 00, 90000, 100000, 120000, 130000, 150000, 160000, 180000, 200000, 230000, 250000, 270000, 300000, 320000, 330000, 350000, 360000, 380000, 400000, or a range consisting of any two of these values. The glass transition temperature (Tg) of the first aqueous binder can be 150℃, 152℃, 153℃, 155℃, 157℃, 158℃, 160℃, 161℃, 163℃, 165℃, 166℃, 168℃, 170℃, 172℃, 173℃, 175℃, 177℃, 178℃, 180℃, or a range of any two of these values. By selecting a first aqueous binder with the aforementioned weight-average molecular weight and glass transition temperature in the first material layer, this application can leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer to suppress the volume expansion of the silicon-containing negative electrode sheet, improve the kinetic performance of the negative electrode sheet, and thus improve the cycle stability and kinetic performance of the secondary battery.

[0071] In one embodiment of this application, the particle size Dv50 of the second aqueous binder is from 80 nm to 160 nm. For example, the particle size Dv50 of the second aqueous binder can be 80 nm, 82 nm, 85 nm, 88 nm, 90 nm, 93 nm, 95 nm, 97 nm, 100 nm, 105 nm, 110 nm, 115 nm, 120 nm, 125 nm, 130 nm, 135 nm, 140 nm, 145 nm, 148 nm, 150 nm, 152 nm, 153 nm, 155 nm, 156 nm, 158 nm, 160 nm, or a range of any two of these values. In this application, Dv50 represents the particle size that, measured from the smallest particle size, reaches 50% of the total volumetric particle size in a volumetric particle size distribution. By adjusting the particle size Dv50 of the second aqueous binder within the aforementioned range, this application can better leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer, thereby improving the effective adhesion and expansion suppression of the silicon-containing first material layer, and thus improving the cycle stability and kinetic performance of the secondary battery.

[0072] In one embodiment of this application, the glass transition temperature (Tg) of the second aqueous binder is between -5°C and 10°C. For example, the glass transition temperature (Tg) of the second aqueous binder can be -5°C, -4°C, -3°C, -2°C, -1°C, 0°C, 1°C, 2°C, 3°C, 4°C, 5°C, 6°C, 7°C, 8°C, 9°C, 10°C, or a range consisting of any two of these values. By selecting a second aqueous binder with the aforementioned glass transition temperature in the first material layer, this application can leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer, thereby improving the effective adhesion and expansion suppression of the silicon-containing first material layer and enhancing the cycle stability and kinetic performance of the secondary battery.

[0073] In one embodiment of this application, the particle size Dv50 of the third waterborne binder is from 80 nm to 160 nm. For example, the particle size Dv50 of the third waterborne binder can be 80 nm, 82 nm, 85 nm, 88 nm, 90 nm, 93 nm, 95 nm, 97 nm, 100 nm, 105 nm, 110 nm, 115 nm, 120 nm, 125 nm, 130 nm, 135 nm, 140 nm, 145 nm, 148 nm, 150 nm, 152 nm, 153 nm, 155 nm, 156 nm, 158 nm, 160 nm, or a range of any two of these values. This application improves the adhesion between the second material layer and the first material layer and the current collector by adjusting the particle size Dv50 of the third aqueous binder within the above-mentioned range, thereby giving the negative electrode sheet better flexibility, improving the dynamic performance of the negative electrode sheet, and enabling the secondary battery to have good cycle stability and good dynamic performance.

[0074] In one embodiment of this application, the glass transition temperature (Tg) of the third aqueous binder is between -5°C and 10°C. For example, the glass transition temperature (Tg) of the third aqueous binder can be -5°C, -4°C, -3°C, -2°C, -1°C, 0°C, 1°C, 2°C, 3°C, 4°C, 5°C, 6°C, 7°C, 8°C, 9°C, 10°C, or a range consisting of any two of these values. By selecting a third aqueous binder with the aforementioned glass transition temperature in the second material layer, this application improves the adhesion between the second material layer and the first material layer and the current collector, giving the negative electrode sheet better flexibility, improving the dynamic performance of the negative electrode sheet, and enabling the secondary battery to have both good cycle stability and good dynamic performance.

[0075] In one embodiment of this application, the first active material may or may not include first graphite. Based on the total mass of the first active material, the mass percentage of the first silicon material is A1, where 3% ≤ A1 ≤ 100%, and the mass percentage of the first graphite is B1, where 0% ≤ B1 ≤ 97%. For example, the mass percentage A1 of the first silicon material can be 3%, 5%, 6%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 92%, 95%, 98%, 99%, 100%, or a range consisting of any two of these values. The mass percentage of the first graphite... The mass percentage content B1 can be 0%, 0.1%, 0.5%, 1%, 2%, 3%, 5%, 6%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 92%, 95%, 96%, 97%, or a range of any two of these values. In one embodiment of this application, 10% ≤ A1 ≤ 80%, and 20% ≤ B1 ≤ 90%. When the secondary battery discharges, the active material graphite in the negative electrode layer discharges preferentially, while the silicon material only begins to discharge at the end of the discharge. This prolongs the ion transport distance between lithium ions and silicon. Therefore, by controlling the content of the first silicon material A1 and the content of the first graphite B1 within the above range, the ion transport efficiency at the end of the discharge can be effectively improved, thereby enhancing the kinetic performance of the secondary battery. At the same time, the silicon material increases the cycle expansion rate of the negative electrode sheet. By using the first silicon material, the first aqueous binder, and the second aqueous binder in combination, and with the first aqueous binder having a high carboxyl content, it can form strong hydrogen bonds with the silicon material, providing high adhesion. This makes the first material layer have good stability during cycling, thus enabling the secondary battery to have both good kinetic performance and cycle stability.

[0076] In one embodiment of this application, the second active material includes second graphite and may or may not include a second silicon material. Based on the total mass of the second active material, the mass percentage of the second silicon material is A2, where 0% ≤ A2 ≤ 97%, and the mass percentage of the second graphite is B2, where 3% ≤ B2 ≤ 100%. For example, the mass percentage A2 of the second silicon material can be 0%, 0.1%, 0.5%, 1%, 2%, 3%, 5%, 6%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 92%, 95%, 96%, 97%, or any two of these. The mass percentage content B2 of the second graphite can be 3%, 5%, 6%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 92%, 95%, 97%, 99%, 100%, or any two of these values. In one embodiment of this application, 0% ≤ A2 ≤ 50%, and 50% ≤ B2 ≤ 100%. By controlling the content A2 of the second silicon material and the content B2 of the second graphite within the above ranges, and simultaneously using a third aqueous binder comprising carbon-carbon double bonds and phenyl groups, this application can improve the kinetic performance of the silicon material in the second material layer away from the electrolyte, thereby enhancing the kinetic performance of the secondary battery.

[0077] In one embodiment of this application, the first active material further includes a first graphite. Based on the total mass of the first active material, the mass percentage of the first silicon material is A1, where 3% ≤ A1 < 100%, and the mass percentage of the first graphite is B1, where 0% < B1 ≤ 97%. The second active material includes a second silicon material and a second graphite. Based on the total mass of the second active material, the mass percentage of the second silicon material is A2, where 0% < A2 ≤ 97%, and the mass percentage of the second graphite is B2, where 3% ≤ B2 < 100%. In one embodiment of this application, 10% ≤ A1 ≤ 80%, 20% ≤ B1 ≤ 90%; 0% < A2 ≤ 50%, and 50% ≤ B2 < 100%. When the secondary battery discharges, the active material graphite in the negative electrode layer discharges preferentially, while the silicon material only begins to discharge at the end of the discharge. This prolongs the ion transport distance between lithium ions and silicon. By controlling the content of the first silicon material A1 and the content of the first graphite B1 within the above range, the ion transport efficiency at the end of the discharge can be effectively improved, thereby enhancing the kinetic performance of the secondary battery. At the same time, the silicon material increases the cycle expansion rate of the negative electrode sheet. By using the first silicon material, the first aqueous binder, and the second aqueous binder in combination, and with the first aqueous binder having a high carboxyl content, it can form strong hydrogen bonds with the silicon material, providing high adhesion and ensuring good stability of the first material layer during cycling. Meanwhile, in the second material layer, by controlling the content of the second silicon material A2 and the content of the second graphite B2 within the above range, and using it in combination with the third aqueous binder including carbon-carbon double bonds and phenyl groups, the kinetic performance of the silicon material in the second material layer can be improved. This, in turn, leverages the synergistic effect of the first and second material layers to enhance the kinetic performance and cycle stability of the secondary battery.

[0078] This application does not impose any particular limitation on the second silicon material, as long as it can achieve the purpose of this application. In one embodiment of this application, the second silicon material includes at least one of pure silicon, silicon alloy material, silicon-carbon composite material, or silicon oxide. By using the above-mentioned types of second silicon materials, this application can leverage the synergistic effect of the second silicon material with the third and fourth aqueous binders, reduce the risk of demolding of the second material layer, and improve the overall structural stability of the negative electrode sheet. At the same time, by using the first and second aqueous binders of the first material layer and the third aqueous binder of the second material layer, the kinetic performance of the silicon-containing negative electrode sheet with a double material layer can be effectively improved, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0079] In one embodiment of this application, the first material layer further includes a first conductive agent, the mass percentage of which is C1 based on the total mass of the first material layer; the second material layer further includes a second conductive agent, the mass percentage of which is C2 based on the total mass of the second material layer; 0.1% ≤ C1 - C2 ≤ 3%. For example, C1-C2 can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.7%, 3%, or a range consisting of any two of these values. By controlling the difference between the content of the first conductive agent and the content of the second conductive agent, C1-C2, within the above range, this application can leverage the synergistic effect of the first and second material layers, improving the conductivity of the negative electrode and thus enhancing the kinetic performance of the secondary battery.

[0080] In one embodiment of this application, 0.2% ≤ C1 ≤ 10%. For example, C1 can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.5%, 9.7%, 10%, or a range of any two of these values. By controlling the content C1 of the first conductive agent within the above range, this application can further improve the conductivity of the first material layer, enhance the kinetic performance of the first material layer, and thus improve the kinetic performance of the secondary battery.

[0081] In one embodiment of this application, 0.1% ≤ C2 ≤ 9.9%. For example, C2 can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.5%, 9.7%, 9.9%, or a range of any two of these values. In one embodiment of this application, 0.1% ≤ C2 ≤ 5%. By controlling the content of the second conductive agent C2 within the above range, this application can further improve the kinetic performance of the second material layer, thereby improving the kinetic performance of the secondary battery.

[0082] This application does not impose any particular limitation on the types of the first and second conductive agents, as long as they can achieve the purpose of this application. For example, the first and second conductive agents may independently include, but are not limited to, at least one of carbon-based materials, metal-based materials, or conductive polymers. For example, carbon-based materials may include at least one of natural graphite, artificial graphite, acetylene black, carbon nanotubes, Ketjen black, or carbon fiber; metal-based materials may include, but are not limited to, at least one of metal powder, metal fiber, copper, nickel, aluminum, or silver; and conductive polymers may include, but are not limited to, polyphenylene derivatives.

[0083] In one embodiment of this application, the first material layer further includes a first dispersant, which comprises a first carboxymethyl cellulose dispersant, and the mass percentage of the first dispersant is F1 based on the total mass of the first material layer; the second material layer further includes a second dispersant, which comprises a second carboxymethyl cellulose dispersant, and the mass percentage of the second dispersant is F2 based on the total mass of the second material layer; 0.1% ≤ F1 - F2 ≤ 3%. For example, F1-F2 can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.7%, 3%, or a range consisting of any two of these values. This application, by controlling the difference between the contents of the first dispersant and the second dispersant, F1-F2, within the aforementioned range, can assist the binder in enhancing the adhesion of the first material layer, reducing the risk of delamination and powdering of the first material layer. At the same time, the hydrogen bonds of carboxymethyl cellulose dispersants have a self-repairing function, which can better adapt to the volume expansion during cycling, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0084] In one embodiment of this application, 0.2% ≤ F1 ≤ 10%. For example, F1 can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.5%, 9.7%, 10%, or a range of any two of these values. By controlling the content F1 of the first dispersant within the above range, the adhesion of the first material layer can be further improved, and it can better adapt to the volume expansion during cycling, which is beneficial to further improving the kinetic performance and cycle stability of the secondary battery.

[0085] In one embodiment of this application, 0.1% ≤ F2 ≤ 9.9%. For example, F2 can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.2%, 9.5%, 9.7%, 9.9%, or a range of any two of these values. In one embodiment of this application, 0.1% ≤ F2 ≤ 5%. By controlling the content of the second dispersant F2 within the above range, the adhesion of the second material layer can be further improved, and it can better adapt to the volume expansion during cycling, which is beneficial to further improving the kinetic performance and cycle stability of the secondary battery.

[0086] This application does not impose any particular limitation on carboxymethyl cellulose dispersants, as long as they can achieve the purpose of this application. In some embodiments, the first carboxymethyl cellulose dispersant and the second carboxymethyl cellulose dispersant each independently include at least one of lithium carboxymethyl cellulose or sodium carboxymethyl cellulose (CMC).

[0087] In one embodiment of this application, the first aqueous binder includes at least one of an acrylic polymer or sodium alginate, and the acrylic polymer includes at least one of polyacrylic acid or polymethacrylic acid. By selecting the above-mentioned types of first aqueous binders in the first material layer, this application can better leverage the synergistic effect of the first active material, the first aqueous binder, and the second aqueous binder in the first material layer, thereby improving the cycle expansion and kinetic performance of the negative electrode sheet, and thus improving the cycle stability and kinetic performance of the secondary battery.

[0088] This application does not impose any particular limitation on the weight-average molecular weight (Mw) of acrylic polymers, as long as it achieves the purpose of this application. For example, the Mw of acrylic polymers can be between 600,000 and 800,000. This application also does not impose any particular limitation on the weight-average molecular weight (Mw) of sodium alginate, as long as it achieves the purpose of this application. For example, the weight-average molecular weight of sodium alginate can be between 32,000 and 400,000.

[0089] In one embodiment of this application, the second and third aqueous binders each independently comprise at least one of styrene-butadiene rubber (SBR), styrene-acrylic emulsion, or pure acrylic emulsion. In this application, the SBR is a SBR emulsion. The styrene-acrylic emulsion (styrene-acrylate emulsion) is obtained by emulsion copolymerization of styrene and acrylate monomers, wherein the molar ratio of styrene to acrylate monomers is 1 to 4. This application does not impose any particular limitation on the type of emulsion, as long as it achieves the purpose of this application, and can be obtained by purchasing commercially available emulsions of different types. This application does not impose any particular limitation on the solid content of the emulsion, as long as it achieves the purpose of this application, for example, the solid content of the emulsion is 20 wt% to 50 wt%. By using the above-mentioned type of second aqueous binder in the first material layer, and / or using the above-mentioned type of third aqueous binder in the second material layer, it is beneficial to leverage the synergistic effect of the first and second material layers, improve the kinetic performance and cycle expansion performance of the negative electrode sheet, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0090] In one embodiment of this application, the first silicon material includes at least one of pure silicon, silicon alloy material, silicon-carbon composite material, or silicon oxide. By employing the above-mentioned types of first silicon materials, this application can leverage the synergistic effect of the first silicon material with the first aqueous binder and the second aqueous binder, thereby improving the kinetic performance and cycle expansion performance of the negative electrode sheet, and thus improving the kinetic performance and cycle stability of the secondary battery.

[0091] In this application, pure silicon may include, but is not limited to, at least one of silicon nanoparticles, silicon nanowires, and micron-sized silicon. Silicon alloy materials may include, but are not limited to, at least one of silicon-tin alloys, silicon-magnesium alloys, silicon-iron alloys, silicon-aluminum alloys, silicon-nickel alloys, or silicon-iron-aluminum alloys. Silicon-carbon composite materials refer to materials composed of silicon and carbon, such as SiC. Silicon oxides include materials denoted as SiOx, where x is 0 to 2.

[0092] In one embodiment of this application, the thickness of the first material layer is H1 μm, the thickness of the second material layer is H2 μm, and 0.2 ≤ H1 / H2 ≤ 4. For example, H1 / H2 can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1, 1.2, 1.5, 2, 2.3, 2.5, 2.7, 3, 3.2, 3.5, 3.8, 4, or a range consisting of any two of these values. By adjusting H1 / H2 within the above range, it is beneficial to leverage the synergistic effect of the first and second material layers, improve the kinetic performance and cycle expansion performance of the negative electrode, and thus improve the kinetic performance and cycle stability of the secondary battery.

[0093] This application does not impose any particular limitation on the mass percentage of the first active material in the first material layer, as long as the purpose of this application is achieved. In some embodiments, the mass percentage of the first active material in the first material layer is 61% to 99%. This application does not impose any particular limitation on the mass percentage S1 of the first graphite in the first material layer, as long as the purpose of this application is achieved. This application does not impose any particular limitation on the mass percentage of the second active material in the second material layer, as long as the purpose of this application is achieved. In some embodiments, the mass percentage of the second active material in the second material layer is 61% to 99%. This application does not impose any particular limitation on the mass percentage S2 of the second graphite in the second material layer, as long as the purpose of this application is achieved.

[0094] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector may include copper foil, aluminum foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, or foamed copper, etc. In this application, there are no particular limitations on the thickness of the negative electrode current collector, the first material layer, and the second material layer, as long as they achieve the purpose of this application. For example, the thickness of the negative electrode current collector may be 4 μm to 12 μm, the thickness H1 of the first material layer on one side may be 30 μm to 130 μm, and the thickness H2 of the second material layer on one side may be 30 μm to 130 μm. This application does not impose any particular limitation on the thickness of the negative electrode sheet, as long as it achieves the purpose of this application; for example, the thickness of the negative electrode sheet may be 50 μm to 280 μm.

[0095] A second aspect of this application provides a secondary battery comprising the negative electrode sheet of any of the foregoing embodiments. The secondary battery provided by the second aspect of this application exhibits good kinetic performance and cycle stability.

[0096] The secondary battery of this application also includes a positive electrode sheet. This application does not impose any particular limitation on the positive electrode sheet, as long as it achieves the purpose of this application. For example, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. In this application, the positive active material layer can be disposed on one surface of the positive current collector along its thickness direction, or it can be disposed on two surfaces of the positive current collector along its thickness direction. It should be noted that "surface" here can be the entire area of ​​the positive current collector or a part of the positive current collector; this application does not impose any particular limitation, as long as it achieves the purpose of this application.

[0097] This application does not impose any particular limitation on the positive electrode current collector, as long as it achieves the purpose of this application. For example, it may include, but is not limited to, aluminum foil, aluminum alloy foil, or composite current collectors (e.g., aluminum-carbon composite current collectors). This application does not impose any particular limitation on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as it achieves the purpose of this application. For example, the thickness of the positive electrode current collector may be 6 μm to 12 μm, and the thickness of the positive electrode active material layer may be 30 μm to 120 μm. This application does not impose any particular limitation on the thickness of the positive electrode sheet, as long as it achieves the purpose of this application; for example, the thickness of the positive electrode sheet may be 50 μm to 250 μm.

[0098] The positive electrode active material layer of this application includes a positive electrode active material, which comprises a substance capable of reversibly inserting and de-intercalating active ions such as lithium ions. The positive electrode active material layer can be one or more layers, and each layer in a multilayer positive electrode active material layer can contain the same or different positive electrode active materials. This application does not impose any particular limitation on the positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material can include, but is not limited to, at least one of lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate.

[0099] The positive electrode active material layer may further include a conductive agent and a binder. This application does not impose any particular limitation on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, the binder may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon. The conductive agent may include, but is not limited to, at least one of carbon-based materials, metal-based materials, or conductive polymers. For example, carbon-based materials may include at least one of natural graphite, artificial graphite, acetylene black, Ketjen black, or carbon fiber; metal-based materials may include, but are not limited to, at least one of metal powder, metal fiber, copper, nickel, aluminum, or silver; and conductive polymers may include, but are not limited to, polyphenylene derivatives. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode active material layer; they can be selected according to actual needs, as long as they achieve the purpose of this application.

[0100] In this application, the secondary battery also includes a separator membrane to separate the positive and negative electrode plates, prevent internal short circuits, allow electrolyte ions to pass freely, and not affect the electrochemical charging and discharging process. This application does not impose any particular limitations on the separator membrane, as long as it achieves the purpose of this application. For example, the material of the separator membrane may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid; the type of separator membrane may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. For example, the separator membrane may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane can be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. For example, the inorganic layer includes inorganic particles and a binder. This application does not have any particular limitation on the inorganic particles, and may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not have any particular limitation on the binder, and may include at least one of the binders mentioned above. The polymer layer contains a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene). In this application, there is no particular limitation on the thickness of the separator, as long as it can achieve the purpose of this application. For example, the thickness of the separator can be from 5 μm to 500 μm.

[0101] In this application, the secondary battery also includes an electrolyte. In some embodiments, the electrolyte includes a lithium salt. This application does not particularly limit the type of lithium salt, and lithium salts known in the art can be used. For example, the lithium salt may include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiN(CF3SO2)2, LiTFSI), lithium bis(fluorosulfonyl)imide (Li(N(SO2F)2), LiFSI), lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (LiB(C2O4)2, LiBOB), or lithium difluorooxalato)borate (LiBF2(C2O4), LiDFOB). This application does not particularly limit the content of lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0102] In some embodiments, the electrolyte includes a non-aqueous organic solvent. This application does not impose particular limitations on the non-aqueous organic solvent, as long as it achieves the purpose of this application. For example, the non-aqueous organic solvent may include at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorinated carbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (EMC). The aforementioned cyclic carbonate compounds may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC). The aforementioned fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, or propyl propionate. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The other organic solvents mentioned above may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, or phosphate esters.

[0103] In some embodiments, the secondary battery further includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be any casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use any known metal rigid casing, as long as it achieves the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0104] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. For example, secondary batteries may include, but are not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), sodium-ion secondary batteries (sodium-ion batteries), lithium polymer secondary batteries, and lithium-ion polymer secondary batteries (lithium-ion polymer batteries).

[0105] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, it may include, but is not limited to, the following steps: stacking the positive electrode, separator, negative electrode, and separator in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery; or stacking the separator, positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0106] A third aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0107] Example

[0108] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0109] Test methods and equipment:

[0110] Particle size testing:

[0111] Scanning electron microscope (SEM) was used to take SEM images of the negative electrode sheet obtained from the disassembled lithium-ion battery to observe the particles of the second aqueous binder. Then, using image analysis software, 30 particles were randomly selected from the SEM images to calculate the area of ​​each particle. Next, assuming that the particles are spherical, the particle diameter D was calculated using the following formula: D = 2 × (S1 / π). 1 / 2 Where S1 is the area of ​​the particle; and the particle size of the obtained 30 particles is calculated by arithmetic average to obtain the average particle size of the particles, which is the particle size Dv50 of the second water-based binder; the first material layer is scraped off, and the particle size Dv50 of the third water-based binder in the second material layer is measured in the same way.

[0112] Tests for P1 and P2:

[0113] Scanning electron microscopy combined with argon-ion polishing (CP-SEM) was used to capture SEM images of the cross-section of the negative electrode sheet obtained from the dismantling of lithium-ion batteries. Areas S within the CP field of view were randomly selected. 总 Area S of silicon particles within the field of view si Graphite particle area S Gr S Gr =S 总 -S Si Given the silicon density ρ Si And graphite density ρ Gr That is, the mass percentage of the first silicon material in the first material layer, P1 = ρ Si ×S si / (ρ Si ×S si +ρ Gr ×S Gr ); after scraping off the first material layer, the mass percentage P2 of the second silicon material in the second material layer can be measured using the same method.

[0114] Tests for D1, E1, D2, and E2:

[0115] First, the upper and lower material layers need to be separated and sampled. The thickness of the upper and lower material layers is determined according to the cross-section CP of the negative electrode sheet. The electrode sheet is polished with sandpaper to expose the different material layers in sequence. The powder that is polished off by the sandpaper is sampled for testing. The thickness is measured with vernier calipers during the polishing process. The material layers of the upper and lower layers are sampled separately by thickness subtraction.

[0116] Thermogravimetric analysis was used to first determine the mass percentage of all binders in each material layer. The powder was placed in a thermogravimetric analyzer, the temperature range was set to 20–500℃, the heating rate was 5℃ / min, and the gas atmosphere was air. The equipment was then run, and a weight reduction occurred between 200 and 400℃. The percentage of this weight reduction to the weight of the powder is the mass percentage of all binders.

[0117] Quantitative characterization was performed using a Labram-010 laser Raman spectrometer. For example, for the adhesive SBR, 1674 cm⁻¹... -1 and 1647cm -1 The peak at 1612 cm⁻¹ is the stretching absorption vibration peak of C=C. For the adhesive PAA, this peak is 1612 cm⁻¹. -1 and 1421cm -1 These are the antisymmetric stretching vibration absorption peak and the symmetric stretching vibration absorption peak of -COO-, respectively, which can be determined based on 1674 cm⁻¹. -1 and 1421cm -1 To distinguish between SBR and PAA, the absorption intensity of characteristic peaks of different substances in Raman spectroscopy is directly proportional to the substance content. Therefore, the content of each binder in the material layer can be quantitatively determined by the absorption intensity of characteristic peaks. This is based on the 1421 cm⁻¹ characteristic peak of Raman spectroscopy. -1 and 1612cm -1 The mass percentage of PAA is obtained by combining the absorption intensity of the characteristic peaks with the mass percentage of all binders. The mass percentage of SBR is obtained by subtracting the mass percentage of PAA from the mass percentage of binders. When the binder is another substance (such as one containing phenyl or hydroxyl groups), its mass percentage is determined using a similar method.

[0118] Tests for A1, B1, A2, and B2:

[0119] Using the above method, powders with different material layers are obtained. Based on the mass m1 and P1, P2 of the powder, the silicon content of each material layer is calculated. Then, the binder and conductive agent in the powder are washed away or burned off to obtain the corresponding powder mass m2. Based on the silicon content of each material layer and m2, A1, B1, A2, and B2 are calculated.

[0120] Tests for C1 and C2:

[0121] Conductive agents such as carbon nanotubes exhibit Raman spectra primarily at 1350 cm⁻¹. -1 and 1590cm -1 The mass percentage of carbon nanotubes in a material layer can be quantified by the intensity of characteristic peaks.

[0122] F1 and F2 tests:

[0123] Dispersants such as CMC have Raman spectra at 1612 cm⁻¹. -1 and 1421cm -1 The peaks at 1421 cm⁻¹ represent the antisymmetric and symmetric stretching vibration absorption peaks of -COO-, respectively. Following the aforementioned method for testing binder content, a Raman spectrometer was used, based on the 1421 cm⁻¹ value. -1 and 1674cm -1Characteristic peaks are used to distinguish CMC from SBR and PAA. The mass percentage of CMC in the corresponding material layer is quantified by the intensity of the characteristic peaks.

[0124] Weight-average molecular weight test:

[0125] Referring to the method described above for testing the mass percentage of the binder, powders from both the upper and lower layers were taken and heated and stirred in a suitable dispersion medium (e.g., N-methylpyrrolidone) to dissolve the binder. The negative electrode active material was then removed by centrifugation. After filtering the supernatant, the binder was extracted by evaporation or reprecipitation in water. The weight-average molecular weight of the binder was then measured using a mass spectrometer.

[0126] Glass transition temperature test:

[0127] Referring to the method described above for testing the mass percentage of the binder, powder from both the upper and lower material layers was taken and heated and stirred in a suitable dispersion medium (e.g., N-methylpyrrolidone) to dissolve the binder. Afterward, the negative electrode active material was removed by centrifugation, and the supernatant was filtered. The binder was then extracted by evaporation or reprecipitation in water. A differential scanning calorimeter (DSC, such as Rigaku Corporation's Rigaku Thermo plus DSC8230) was used to heat the sample from several mg to tens of mg at a heating rate of 1 °C / min to 10 °C / min. As the temperature gradually increased and passed the glass transition temperature of the polymer, the baseline on the DSC curve shifted towards endothermic heating. This point of change is identified as the glass transition temperature.

[0128] Testing of the thickness H1 of the first material layer and the thickness H2 of the second material layer:

[0129] Under an environment of (25±3)℃, the negative electrode sheet was removed from the lithium-ion battery and the residual electrolyte on the surface was wiped off. The negative electrode sheet was cut under plasma to obtain its cross-section. The cross-section was analyzed under a scanning electron microscope, and the thickness H1 of the first material layer and the thickness H2 of the second material layer were tested. The adjacent test points were spaced 2 mm to 3 mm apart. At least 15 different points were tested for each material layer. The average of all test points for the thickness of each material layer was recorded as the thickness of that material layer.

[0130] 4C discharge external temperature rise test:

[0131] The lithium-ion battery was wrapped in insulating cotton. A temperature sensing wire was attached to the center of the diagonal intersection of the largest surface area of ​​the lithium-ion battery. A multi-channel temperature measuring instrument was used to continuously monitor the surface discharge temperature of the lithium-ion battery. At 25℃, the lithium-ion battery was charged at a constant current of 1.5C to 4.3V, then charged at a constant voltage of 4.3V until the current reached 0.05C and stopped. After resting for 15 minutes, it was discharged at a constant current of 4C to 2.5V and rested for 30 minutes. The highest temperature of 4C discharge was recorded by the multi-channel temperature measuring instrument. The external temperature rise during 4C discharge was equal to the highest temperature of 4C discharge minus 25℃.

[0132] Cyclic performance test:

[0133] At 25℃, the lithium-ion battery was charged at a constant current of 1.5C to 4.3V, then charged at a constant voltage of 4.3V to 0.05C and stopped, allowing the lithium-ion battery to reach a fully charged state. It was then allowed to rest for 15 minutes, followed by a constant current discharge of 4C to 2.5V, and a rest period of 30 minutes. This process constituted one cycle, and 400 cycles were performed. The discharge capacity of the third cycle was defined as Q0, and the discharge capacity of the Nth cycle was defined as Q. N Calculate the cycle capacity retention rate of the lithium-ion battery in the Nth cycle = Q N / Q0×100%. The thickness of the lithium-ion battery at 30% SOC (3.6V) during the first discharge cycle is defined as T0 (using a PPG battery thickness gauge with a measuring pressure of 50g). The thickness of the lithium-ion battery at full charge during the Nth cycle is measured and recorded as T. N Calculate the cycle expansion rate of the lithium-ion battery in the Nth cycle = (T N -T0) / T0×100%.

[0134] Lithium plating test:

[0135] At 5℃, a lithium-ion battery is charged to 4.30V with a constant current at a specific charging rate X. Then, it is charged to 0.05C with a constant voltage of 4.30V and allowed to rest for 5 minutes. Next, it is discharged to 2.0V with a constant current of 0.5C and allowed to rest for 5 minutes. This constitutes one cycle. After repeating 10 cycles, the lithium-ion battery is fully charged to 4.30V at the charging rate X again. The negative electrode is then disassembled for observation. If any part of the negative electrode is found to be ≥2mm... 2 If lithium plating occurs in a certain area, it is determined to be lithium plating on the negative electrode. The maximum charging rate without lithium plating is defined as the maximum non-lithium plating rate of the lithium-ion battery, which is the lithium plating level. Here, X can be 1C, 1.5C, 2C, etc., increasing by 0.5C each time.

[0136] Example 1-1

[0137] <Preparation of Negative Electrode Sheets>

[0138] The first silicon material (pure silicon), the first graphite (artificial graphite), the first water-based binder (polyacrylic acid (PAA, Mw = 700000, Tg = 140℃), the second water-based binder (styrene-butadiene rubber emulsion, SBR, model LB-S420, solid content = 46wt%, Dv50 = 120nm, Tg = 0℃), the first conductive agent (carbon nanotubes), and the first dispersant (sodium carboxymethyl cellulose (CMC)) were dissolved in deionized water at a mass ratio of 18.9:75.6:2.5:1.0:1.0:1.0. After thorough stirring, a first material layer slurry with a solid content of 75wt% was obtained.

[0139] The second silicon material (pure silicon), the second graphite (artificial graphite), the third water-based binder (styrene-butadiene rubber emulsion, model LB-S420, solid content 46wt%, Dv50 120nm, Tg 0℃), the fourth water-based binder (polyacrylic acid, Mw 700000, Tg 140℃), the second conductive agent (carbon nanotubes), and the second dispersant (sodium carboxymethyl cellulose) were dissolved in deionized water at a mass ratio of 5.0:91.0:2.5:0.5:0.6:0.4. After thorough stirring, a second material layer slurry with a solid content of 75wt% was obtained.

[0140] Using a coating machine equipped with a double-layer coating die, the prepared first and second material layer slurries are simultaneously coated on one surface of a 6 μm thick negative electrode current collector copper foil at a speed of 15 m / min. The second material layer slurry contacts the copper foil to form the second material layer, and the first material layer slurry contacts the second material layer slurry to form the first material layer. The areal density of both the first and second material layers is 3.76 mg / cm³. 2 After drying at 80℃, a negative electrode sheet with a first and second material layer coated on one side is obtained. Then, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a first and second material layer coated on both sides. After cold pressing, cutting, and welding of tabs, a negative electrode sheet with dimensions of 76mm × 867mm is obtained for use. The thickness H1 of the first material layer on one side is 30μm, and the thickness H2 of the second material layer is 30μm.

[0141] <Preparation of the positive electrode>

[0142] Lithium nickel cobalt manganese oxide (molecular formula LiNi) is used as the positive electrode active material. 0.5 Co 0.2 Mn 0.3O2 (abbreviated as NCM523), acetylene black (positive electrode conductive agent), and polyvinylidene fluoride (PVDF) (positive electrode binder) were mixed in a mass ratio of 94:3:3. N-methylpyrrolidone (NMP) was added as a solvent to prepare a positive electrode slurry with a solid content of 75 wt%. The positive electrode slurry was then uniformly coated onto one surface of a 6 μm thick aluminum foil current collector, with a coating weight of 0.13 g / cm³. 2 The aluminum foil is dried at 90℃ to obtain a positive electrode sheet with a single-sided coating of positive active material. The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive active material. After cold pressing, cutting, and welding of tabs, a positive electrode sheet with dimensions of 74mm × 851mm is obtained for use. The compacted density of the positive electrode sheet is 3.45 g / cm³. 3 .

[0143] <Preparation of Electrolyte>

[0144] In an argon-atmospheric glove box with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a weight ratio of 20:30:40:10. Then, fluoroethylene carbonate (FEC) and lithium salt lithium hexafluorophosphate (LiPF6) were added to the above non-aqueous organic solvents, dissolved, and mixed evenly to obtain an electrolyte. The mass percentage of FEC in the electrolyte was 2.5%, and the concentration of LiPF6 in the electrolyte was 1.1 mol / L.

[0145] <Preparation of the separating membrane>

[0146] A 7μm thick porous polyethylene (PE) film (provided by Celgard) was used as the separator.

[0147] <Preparation of Lithium-ion Batteries>

[0148] The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to form the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, and after dehydration at 80°C, the electrolyte is injected and the assembly is sealed. Following formation, degassing, and edge trimming processes, a lithium-ion battery is obtained. The upper limit of the formation voltage is 4.15V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0149] Examples 1-2 to 1-41

[0150] Except for the following in <Preparation of Negative Electrode Sheet>, where the relevant preparation parameters are adjusted according to Table 1, and the total content of the first graphite, the first aqueous binder, and the second aqueous binder remains unchanged, and the total content of the second graphite, the third aqueous binder, and the fourth aqueous binder remains unchanged, the rest is the same as in Example 1-1.

[0151] Examples 2-1 to 2-17

[0152] Except for the preparation of the negative electrode sheet, in which the relevant preparation parameters are adjusted according to Table 2 and the total content of the first silicon material and the first graphite remains unchanged, and the total content of the second silicon material and the second graphite remains unchanged, the rest is the same as in Example 1-1.

[0153] Examples 3-1 to 3-14

[0154] Except for adjusting the relevant preparation parameters according to Table 3 in the <Preparation of Negative Electrode Sheet>, the rest is the same as in Example 1-1.

[0155] Examples 4-1 to 4-27

[0156] Except for the preparation parameters in <Preparation of Negative Electrode Sheet>, which are adjusted according to Table 4, and the total content of the first graphite, the first conductive agent and the first dispersant remains unchanged, and the total content of the second graphite, the second conductive agent and the second dispersant remains unchanged, the rest is the same as in Example 1-1.

[0157] Comparative Examples 1-1 to 1-5

[0158] Except for the following in <Preparation of Negative Electrode Sheet>, where the relevant preparation parameters are adjusted according to Table 1, and the total content of the first graphite, the first aqueous binder, and the second aqueous binder remains unchanged, and the total content of the second graphite, the third aqueous binder, and the fourth aqueous binder remains unchanged, the rest is the same as in Example 1-1.

[0159] The preparation parameters and performance tests of each embodiment and comparative example are shown in Tables 1 to 4.

[0160] As can be seen from Examples 1-1 to 1-41 and Comparative Examples 1-1 to 1-5, in the lithium-ion battery of this application, the negative electrode sheet includes a first material layer and a second material layer located between the negative electrode current collector and the first material layer. The first material layer includes a first aqueous binder and a second aqueous binder, and the second material layer includes a third aqueous binder. The content D1 of the first aqueous binder is greater than the content E1 of the second aqueous binder, which makes the lithium-ion battery have a lower external temperature rise during 4C discharge, better lithium plating performance and cycle capacity retention performance, and a lower cycle expansion rate. As can be seen from Examples 1-1 and Comparative Examples 1-1 to 1-5, when the first material layer uses only one of the first aqueous binder and the second aqueous binder, or when the content D1 of the first aqueous binder is less than the content E1 of the second aqueous binder, or when only the first material layer uses the first aqueous binder and the second aqueous binder, and no third aqueous binder is added to the second material layer, the lithium-ion battery exhibits a higher external temperature rise during 4C discharge, lower lithium plating level and cycle capacity retention, and a higher cycle expansion rate. These results indicate that the lithium-ion battery using the negative electrode sheet of this application has good kinetic performance and cycle stability.

[0161] The content of the first, second, third, and fourth aqueous binders typically affects the kinetic performance and cycle stability of lithium-ion batteries. As can be seen from Examples 1-1 to 1-41, adjusting the content of the first aqueous binder (D1), the second aqueous binder (E1, D1-E1), the third aqueous binder (E2), and the fourth aqueous binder (D2, E2-D2, D1-D2) within the scope of this application results in lithium-ion batteries with lower 4C discharge external temperature rise, better lithium plating performance, and better cycle capacity retention, while also exhibiting a lower cycle expansion rate, indicating that the lithium-ion batteries possess good kinetic performance and cycle stability.

[0162] The content of silicon and graphite typically affects the kinetic performance and cycle stability of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-17, the lithium-ion batteries within the scope of this application, by adjusting the contents of the first silicon material P1 and A1, the first graphite content B1, the second silicon material P2 and A2, and the second graphite content B2, exhibit lower external temperature rise during 4C discharge, better lithium plating performance, and better cycle capacity retention, while also showing a lower cycle expansion rate. This indicates that the lithium-ion batteries possess good kinetic performance and cycle stability.

[0163] The types of the first silicon material, first aqueous binder, second aqueous binder, second silicon material, third aqueous binder, and fourth aqueous binder typically affect the kinetic performance and cycle stability of lithium-ion batteries. As can be seen from Examples 1-1, 3-1 to 3-14, lithium-ion batteries using the selected types of the first silicon material, first aqueous binder, second aqueous binder, second silicon material, third aqueous binder, and fourth aqueous binder within the scope of this application exhibit lower external temperature rise during 4C discharge, better lithium plating performance, and better cycle capacity retention, while also showing a lower cycle expansion rate, indicating that the lithium-ion battery possesses better kinetic performance and cycle stability.

[0164] The content of conductive agents and dispersants typically affects the kinetic performance and cycle stability of lithium-ion batteries. As can be seen from Examples 1-1, 4-1 to 4-22, by synergistically controlling C1-C2, C1, C2, F1-F2, F1, and F2 within the scope of this application, it is beneficial to obtain lithium-ion batteries with lower external temperature rise during 4C discharge, better lithium plating performance, and better cycle capacity retention. Simultaneously, the lithium-ion batteries exhibit a lower cycle expansion rate, indicating that they possess good kinetic performance and cycle stability.

[0165] The thickness ratio H1 / H2 of the first and second material layers typically affects the kinetic performance and cycle stability of lithium-ion batteries. As can be seen from Examples 1-1, 4-23 to 4-27, by adjusting H1 / H2 within the scope of this application, it is beneficial to obtain lithium-ion batteries with lower external temperature rise during 4C discharge, better lithium plating performance, and better cycle capacity retention. Simultaneously, the lithium-ion batteries exhibit a lower cycle expansion rate, indicating that they possess good kinetic performance and cycle stability.

[0166] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

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

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

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a first material layer and a second material layer disposed on at least one surface of the negative electrode current collector, wherein the second material layer is disposed between the negative electrode current collector and the first material layer along the thickness direction of the negative electrode sheet; The first material layer comprises a first active material, a first aqueous binder, and a second aqueous binder. The first active material comprises a first silicon material. The first aqueous binder comprises at least one group selected from a, b, and c. a is a carboxyl group, b is a carboxyl group and phenyl group, c is a carboxyl group and hydroxyl group; the groups of the second water-based adhesive include carbon-carbon double bonds and phenyl groups, and based on the total mass of the first material layer, the mass percentage of the first water-based adhesive is D1, and the mass percentage of the second water-based adhesive is E1. The second material layer comprises a second active material and a third water-based binder, wherein the groups of the third water-based binder include carbon-carbon double bonds and phenyl groups; Where E1 < D1.

2. The negative electrode sheet according to claim 1, wherein, Based on the total mass of the first material layer, the mass percentage of the first silicon material is P1; The second active material includes a second silicon material, and the mass percentage of the second silicon material is P2, where P2 < P1 ≤ 97%, based on the total mass of the second material layer.

3. The negative electrode sheet according to claim 2, wherein, 0% < P2 < P1 ≤ 30%.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein, 0.1% ≤ D1 - E1 ≤ 9.9%.

5. The negative electrode sheet according to claim 4, wherein, 1% ≤ D1 - E1 ≤ 5%.

6. The negative electrode sheet according to any one of claims 1 to 5, wherein, Based on the total mass of the second material layer, the mass percentage of the third water-based adhesive is E2, where E2 > E1.

7. The negative electrode sheet according to claim 6, wherein, The second material layer further includes a fourth water-based adhesive, the fourth water-based adhesive having at least one of groups a, b, and c, wherein a is a carboxyl group, b is a carboxyl group and a phenyl group, and c is a carboxyl group and a hydroxyl group. Based on the total mass of the second material layer, the mass percentage of the fourth water-based adhesive is D2, where D2 < E2.

8. The negative electrode sheet according to claim 7, wherein, The fourth water-based adhesive includes at least one of an acrylic polymer or sodium alginate.

9. The negative electrode sheet according to claim 7 or 8, wherein, The fourth water-based binder has a weight-average molecular weight of 600,000 to 800,000 and a glass transition temperature (Tg) of 130°C to 150°C; or, The fourth water-based adhesive has a weight-average molecular weight of 32,000 to 400,000 and a glass transition temperature (Tg) of 150°C to 180°C.

10. The negative electrode sheet according to any one of claims 7 to 9, wherein, The negative electrode sheet satisfies at least one of the following characteristics: a) 0.1% ≤ E2-D2 ≤ 9.9%, preferably 1% ≤ E2-D2 ≤ 5%; b) 0.1% ≤ D1 - D2 ≤ 9.9%, preferably 1% ≤ D1 - D2 ≤ 5%; c) 0.5% ≤ D1 ≤ 10%, preferably 1% ≤ D1 ≤ 10%; d) 0.1% ≤ E1 < 10%, preferably 0.5% ≤ E1 ≤ 5%; e) 0.5% ≤ E2 ≤ 10%, preferably 1% ≤ E2 ≤ 10%; f) 0% ≤ D2 < 10%, preferably 0% ≤ D2 ≤ 5%.

11. The negative electrode sheet according to any one of claims 1 to 10, wherein, The negative electrode sheet satisfies at least one of the following characteristics: g) The weight-average molecular weight of the first water-based adhesive is 600,000 to 800,000, and the glass transition temperature Tg of the first water-based adhesive is 130°C to 150°C. h) The weight-average molecular weight of the first water-based adhesive is 32,000 to 400,000, and the glass transition temperature Tg of the first water-based adhesive is 150°C to 180°C. i) The particle size Dv50 of the second water-based binder is 80 nm to 160 nm; j) The glass transition temperature Tg of the second water-based adhesive is -5°C to 10°C; k) The particle size Dv50 of the third water-based binder is 80 nm to 160 nm; The glass transition temperature (Tg) of the third water-based adhesive is -5°C to 10°C.

12. The negative electrode sheet according to any one of claims 1 to 11, wherein, The first active material may or may not include the first graphite, and based on the total mass of the first active material, the mass percentage of the first silicon material is A1, 3% ≤ A1 ≤ 100%, and the mass percentage of the first graphite is B1, 0% ≤ B1 ≤ 97%; and / or, The second active material includes second graphite and may or may not include second silicon material. Based on the total mass of the second active material, the mass percentage of the second silicon material is A2, 0% ≤ A2 ≤ 97%, and the mass percentage of the second graphite is B2, 3% ≤ B2 ≤ 100%.

13. The negative electrode sheet according to claim 12, wherein, 10% ≤ A1 ≤ 80%, 20% ≤ B1 ≤ 90%; and / or, 0% ≤ A2 ≤ 50%, 50% ≤ B2 ≤ 100%.

14. The negative electrode sheet according to claim 12 or 13, wherein, The second silicon material includes at least one of pure silicon, silicon alloy materials, silicon-carbon composite materials, or silicon oxides.

15. The negative electrode sheet according to any one of claims 1 to 14, wherein, The first material layer further includes a first conductive agent, and the mass percentage of the first conductive agent is C1 based on the total mass of the first material layer; The second material layer also includes a second conductive agent, the second conductive agent being based on the total mass of the second material layer. The mass percentage content is C2; 0.1% ≤ C1 - C2 ≤ 3%.

16. The negative electrode sheet according to claim 15, wherein, The negative electrode sheet satisfies at least one of the following characteristics: p) 0.2% ≤ C1 ≤ 10%; q) 0.1% ≤ C2 ≤ 9.9%, preferably 0.1% ≤ C2 ≤ 5%.

17. The negative electrode sheet according to any one of claims 1 to 16, wherein, The first material layer further includes a first dispersant, which includes a first carboxymethyl cellulose dispersant, and the mass percentage of the first dispersant is F1 based on the total mass of the first material layer; The second material layer further includes a second dispersant, which includes a second carboxymethyl cellulose dispersant, and the mass percentage of the second dispersant is F2 based on the total mass of the second material layer; 0.1% ≤ F1 - F2 ≤ 3%.

18. The negative electrode sheet according to claim 17, wherein, The negative electrode sheet satisfies at least one of the following characteristics: r)0.2％≤F1≤10％； s)0.1%≤F2≤9.9%, preferably 0.1%≤F2≤5%.

19. The negative electrode sheet according to any one of claims 1 to 18, wherein, The negative electrode sheet satisfies at least one of the following characteristics: t) The first water-based adhesive comprises at least one of an acrylic polymer or sodium alginate, wherein the acrylic polymer comprises at least one of polyacrylic acid or polymethacrylic acid; x) The second water-based adhesive and the third water-based adhesive each independently comprise at least one of styrene-butadiene rubber, styrene-acrylic emulsion, or pure acrylic emulsion; y) The first silicon material includes at least one of pure silicon, silicon alloy material, silicon-carbon composite material, or silicon oxide; z) The thickness of the first material layer is H1 μm, the thickness of the second material layer is H2 μm, and 0.2 ≤ H1 / H2 ≤ 4.

20. A secondary battery comprising a negative electrode sheet according to any one of claims 1 to 19.

21. An electronic device comprising the secondary battery of claim 20.

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