Negative pole piece, secondary battery and electronic device

Abstract

The invention provides a negative electrode plate, a secondary battery and an electronic device, the negative electrode plate comprises a negative electrode current collector, and a first active material layer and a second active material layer arranged on at least one surface of the negative electrode current collector, and the second active material layer is arranged between the negative electrode current collector and the first active material layer along the thickness direction of the negative electrode plate; the first active substance layer comprises a first active material and a first binder, the first active material comprises a first silicon material, and the first binder comprises a PAA binder; the second active substance layer comprises a second active material and a second binder, the second active material comprises a second silicon material, and the second binder comprises an SBR binder; the mass percentage content of the first silicon material in the first active material layer is P1, the mass percentage content of the second silicon material in the second active material layer is P2, and P2 is less than P1; the first silicon material Dv50 is D1, the second silicon material Dv50 is D2, and D1 is smaller than D2. The secondary battery provided by the invention has good dynamic performance and cycling stability.

Description

Technical Field

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

[0002] Secondary batteries, such as lithium-ion batteries, have the advantages of high energy storage 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 smart watches.

[0003] With the increasing demand for the energy density of lithium-ion batteries, silicon negative electrodes have gradually become an important direction for the development of lithium-ion batteries. While silicon materials, as the negative electrode active material of lithium-ion batteries, improve the energy density, due to the poor electronic conductivity and ion transport properties of silicon materials themselves, lithium-ion batteries have problems such as insufficient kinetics, narrow charging window, increased impedance and thus large discharge temperature rise. On the other hand, silicon materials will cause large volume changes during the process of lithium ion insertion and deinsertion, resulting in large volume expansion of lithium-ion batteries, poor silicon negative electrode adhesion performance, and rapid lithium-ion battery cycle attenuation. Summary of the Invention

[0004] The purpose of this application is to provide a negative electrode plate, a secondary battery, and an electronic device to improve the dynamic performance and cycle stability of the secondary battery. The specific technical solution is as follows:

[0005] The first aspect of the present application provides a negative electrode plate, which includes a negative electrode collector and a first active material layer and a second active material layer arranged on at least one surface of the negative electrode collector, and the second active material layer is arranged between the negative electrode collector and the first active material layer along the thickness direction of the negative electrode plate; the first active material layer includes a first active material and a first binder, the first active material includes a first silicon material, and the first binder includes a polyacrylic acid (PAA) type binder; the second active material layer includes a second active material and a second binder, the second active material includes a second silicon material, and the second binder includes a styrene-butadiene rubber (SBR) type binder; the mass percentage of the first silicon material in the first active material layer is P1, and the mass percentage of the second silicon material in the second active material layer is P2, P2<P1; the particle size Dv50 of the first silicon material is D1, and the particle size Dv50 of the second silicon material is D2, D1<D2. The present application arranges a first active material layer and a second active material layer on at least one surface of the negative electrode current collector. The first active material layer away from the negative electrode current collector adopts a silicon material with a higher content and smaller particle size and a PAA-type binder with high bonding performance, which can improve the ion transmission efficiency and the kinetic performance of the active material of the first active material layer. At the same time, the PAA-type binder can form a strong hydrogen bond with the silicon material, provide high adhesion, and take into account the stability of the first active material layer during the cycle; the second active material layer close to the negative electrode current collector adopts a silicon material with a lower content and larger particle size and an SBR-type binder with high kinetic performance, which can improve the ability of the active material of the second active material layer to participate in the insertion and extraction of lithium ions, effectively improve the transmission efficiency of lithium ions in the negative electrode of the secondary battery, and at the same time improve the problem of large expansion of the silicon-containing negative electrode plate, so that the secondary battery has both good kinetic performance and cycle stability.

[0006] In one embodiment of the present application, 0.1%≤P2<P1≤30%. The present application regulates the content of the first silicon material in the first active material layer within the above range, which can effectively improve the ion transmission efficiency, thereby enhancing the kinetic performance of the secondary battery. At the same time, the PAA-type binder can form a strong hydrogen bond with the silicon material, providing high adhesion, which can take into account the stability of the first active material layer during the cycle. The content of the second silicon material in the second active material layer is regulated within the above range. The low-content silicon material in the second active material layer is used in combination with the SBR-type binder, which improves the kinetic performance of the silicon material in the second active material layer away from the electrolyte, so that the secondary battery takes into account both the kinetic performance and the stability of the negative electrode during the cycle, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0007] In one embodiment of the present application, 1 μm ≤ D1 < D2 ≤ 30 μm. By regulating the particle size Dv50 of the first silicon material of the first active material layer and the particle size Dv50 of the second silicon material of the second active material layer within the range of the present application, the ion transport efficiency can be further improved, thereby improving the kinetic performance of the secondary battery while also taking into account the cycle stability of the secondary battery.

[0008] In one embodiment of the present application, the first active material layer further includes a first dispersant, the first dispersant including a first carboxymethyl cellulose (CMC)-based dispersant, the first dispersant comprising a mass percentage F1 of 0.1% to 5% of the first active material layer; and / or the second active material layer further includes a second dispersant, the second dispersant including a second carboxymethyl cellulose-based dispersant, the second dispersant comprising a mass percentage F2 of 0.1% to 5% of the second active material layer. By using the above-mentioned types and amounts of dispersants, and combining a PAA-based binder and a CMC-based dispersant in the first active material layer, the stability of the first active material layer during cycling can be improved, thereby improving the kinetic performance of the secondary battery while also taking into account the cyclic stability of the secondary battery. By using the above-mentioned types and amounts of dispersants, and combining a low-content silicon material with an SBR-based binder and a CMC-based dispersant with good kinetic performance in the second active material layer, the kinetic performance of the silicon material in the second active material layer away from the electrolyte can be improved, thereby improving the kinetic performance of the secondary battery. Simultaneously using the first dispersant and the second dispersant within the above types and content ranges can better exert the synergistic effect of the first active material layer and the second active material layer, thereby further improving the kinetic performance and cycle stability of the secondary battery.

[0009] In one embodiment of the present application, 1μm≤D1≤8μm, and / or, 8μm<D2≤30μm. The present application regulates the particle size Dv50 of the first silicon material of the first active material layer within the above range. The silicon material with a smaller particle size has a larger specific surface area and more reactive sites, thereby accelerating ion transport and improving the kinetic performance of the secondary battery. At the same time, it is matched with a PAA-type binder with high bonding performance to reduce the expansion of the silicon material, thereby improving the expansion performance of the silicon-containing negative electrode plate and improving the cycle stability of the secondary battery. By regulating the particle size Dv50 of the second silicon material of the second active material layer within the above range and matching it with an SBR-type binder with high kinetic performance, the kinetic performance of the second active material can be improved, thereby improving the kinetic performance of the secondary battery. By simultaneously regulating the particle size Dv50 of the first silicon material and the particle size Dv50 of the second silicon material within the above range, the synergistic effect of the first active material layer and the second active material layer can be better exerted, further improving the kinetic performance and cycle stability of the secondary battery.

[0010] In one embodiment of the present application, the ratio of the particle size Dv50 to Dv90 of the first silicon material is G1, satisfying: 0.55<G1≤0.7; and / or, the ratio of the particle size Dv50 to Dv90 of the second silicon material is G2, satisfying: 0.5≤G2≤0.55. The present application, by regulating the particle size Dv50 / Dv90 of the first silicon material within the above range and simultaneously combining it with a PAA-type binder with high bonding performance, can better exert the ion transport performance of the silicon material and the bonding performance of the binder, thereby taking into account the kinetic performance and expansion performance of the negative electrode sheet, thereby improving the kinetic performance and cycle stability of the secondary battery. By regulating the particle size Dv50 / Dv90 of the second silicon material within the above range and simultaneously combining it with an SBR-type binder with high kinetic performance, the ability of the second active material to intercalate and deintercalate lithium ions can be improved, reducing the impact on the expansion volume of the second silicon material, thereby improving the kinetic performance and cycle stability of the secondary battery. By regulating the particle size Dv50 / Dv90 of the first silicon material and the particle size Dv50 / Dv90 of the second silicon material within the above range, the synergistic effect of the first silicon material and PAA binder and the second silicon material and SBR binder can be better exerted, thereby further improving the kinetic performance and cycle stability of the secondary battery.

[0011] In one embodiment of the present application, the first active material further comprises a first graphite, wherein, based on the total mass of the first active material, the mass percentage A1 of the first silicon material is 1% to 100%, and the mass percentage B1 of the first graphite is 0% to 99%; and / or, the second active material further comprises a second graphite, wherein, based on the total mass of the second active material, the mass percentage A2 of the second silicon material is 0% to 99%, and the mass percentage B2 of the second graphite is 1% to 100%. In one embodiment of the present application, the mass percentage A1 of the first silicon material is 15% to 100%, and the mass percentage B1 of the first graphite is 0% to 85%; and / or, the mass percentage A2 of the second silicon material is 1% to 15%, and the mass percentage B2 of the second graphite is 85% to 99%. By regulating the content A1 of the first silicon material and the content B1 of the first graphite within the above range, the ion transmission efficiency at the end of discharge can be effectively improved, thereby improving the kinetic performance of the secondary battery. At the same time, the silicon material will increase the cycle expansion rate of the negative electrode plate. By using a PAA-type binder, the first active material layer has good stability during the cycle, thereby making the secondary battery have good kinetic performance and cycle stability. By regulating the content A2 of the second silicon material and the content B2 of the second graphite within the above range, and using a high-kinetic performance SBR-type binder, the kinetic performance of the silicon material in the second active material layer away from the electrolyte can be improved, thereby improving the kinetic performance of the secondary battery. By simultaneously regulating the content A1 of the first silicon material and the content B1 of the first graphite, the content A2 of the second silicon material and the content B2 of the second graphite within the above range, the synergistic effect of the first active material layer and the second active material layer can be exerted to improve the kinetic performance and cycle stability of the secondary battery.

[0012] In one embodiment of the present application, the mass percentage N1 of the first binder in the first active material layer is 1% to 10%; and / or the mass percentage N2 of the second binder in the second active material layer is 1% to 10%. By regulating the content of the first binder within the scope of the present application, the synergistic effect of the first silicon material and the first binder can be better exerted, so that the negative electrode plate can take into account both the dynamic performance and the cycle expansion performance, thereby enabling the secondary battery to have both good dynamic performance and cycle stability. By regulating the content of the second binder within the scope of the present application, the synergistic effect of the second silicon material and the second binder can be better exerted, further improving the dynamic performance of the negative electrode plate, thereby further improving the dynamic performance of the secondary battery. By regulating the content of the first binder and the second binder within the above range, the synergistic effect of the first active material layer and the second active material layer can be further exerted, further improving the dynamic performance and cycle stability of the secondary battery.

[0013] In one embodiment of the present application, the first active material layer further includes a first conductive agent, and the mass percentage C1 of the first conductive agent in the first active material layer is 0.1% to 5%; and / or, the second active material layer further includes a second conductive agent, and the mass percentage C2 of the second conductive agent in the second active material layer is 0.1% to 5%. The present application can improve the kinetic performance of the first active material layer and thus improve the kinetic performance of the secondary battery by regulating the content of the first conductive agent within the above range. By regulating the content of the second conductive agent within the above range, the kinetic performance of the second active material layer can be improved, and thus improve the kinetic performance of the secondary battery. By simultaneously regulating the content of the first conductive agent and the content of the second conductive agent within the above range, the synergistic effect of the first active material layer and the second active material layer can be further exerted, thereby further improving the kinetic performance of the secondary battery.

[0014] In one embodiment of the present application, the polyacrylic acid binder includes at least one of polyacrylic acid or polymethacrylic acid. Using this PAA binder in the first active material layer facilitates synergistic effects between the first silicon material and the PAA binder, thereby improving the kinetic performance and cyclic expansion performance of the first silicon material, thereby improving the kinetic performance and cyclic stability of the secondary battery.

[0015] In one embodiment of the present application, the styrene-butadiene rubber binder includes at least one of a styrene-butadiene rubber emulsion, a styrene-acrylic emulsion, or a pure acrylic emulsion. Selecting the aforementioned SBR binder in the second active material layer facilitates synergistic effects between the second silicon material and the SBR binder, thereby improving the kinetic performance of the second silicon material, thereby facilitating improved kinetic performance of the secondary battery.

[0016] In one embodiment of the present application, the first silicon material and the second silicon material each independently include at least one of pure silicon, a silicon alloy, a silicon-carbon composite material, or a silicon oxide. By employing the aforementioned silicon materials, the present application facilitates the synergistic effect of the silicon materials with the PAA-based binder and the SBR-based binder, thereby improving the kinetic performance and cyclic expansion performance of the negative electrode sheet, and thereby improving the kinetic performance and cyclic stability of the secondary battery.

[0017] The second aspect of the present application provides a secondary battery, which includes the negative electrode sheet according to any one of the aforementioned embodiments. The secondary battery provided in the second aspect of the present application has good dynamic performance and cycle stability.

[0018] A third aspect of the present application provides an electronic device comprising the secondary battery according to any one of the aforementioned embodiments.

[0019] Beneficial effects of this application:

[0020] The present application provides a negative electrode plate, a secondary battery and an electronic device, wherein the negative electrode plate includes a negative electrode current collector and a first active material layer and a second active material layer arranged on at least one surface of the negative electrode current collector, and the second active material layer is arranged between the negative electrode current collector and the first active material layer along the thickness direction of the negative electrode plate; the first active material layer includes a first active material and a first binder, the first active material includes a first silicon material, and the first binder includes a polyacrylic acid binder; the second active material layer includes a second active material and a second binder, the second active material includes a second silicon material, and the second binder includes a styrene-butadiene rubber binder; the mass percentage of the first silicon material in the first active material layer is P1, the mass percentage of the second silicon material in the second active material layer is P2, and P2<P1; the particle size Dv50 of the first silicon material is D1, the particle size Dv50 of the second silicon material is D2, and D1<D2. The present application arranges a first active material layer and a second active material layer on the negative electrode current collector. The first active material layer away from the negative electrode current collector adopts a silicon material with a higher content and a smaller particle size and a PAA-type binder with high bonding performance, and the second active material layer close to the negative electrode current collector adopts a silicon material with a lower content and a larger particle size and an SBR-type binder with high kinetic performance. This can effectively improve the transmission efficiency of lithium ions in the negative electrode of the secondary battery, and at the same time improve problems such as the large expansion of the silicon-containing negative electrode plate. While improving the kinetic performance of the secondary battery, it also takes into account the cycle stability of the secondary battery, so that the secondary battery has both good kinetic performance and cycle stability.

[0021] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0023] Figure 1 This is a negative electrode sheet according to an embodiment of the present application. DETAILED DESCRIPTION

[0024] The following will be combined with the accompanying drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0025] It should be noted that, in the specific embodiments of the present application, lithium-ion batteries are used as an example of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries.

[0026] The first aspect of the present application provides a negative electrode plate, which includes a negative electrode current collector and a first active material layer and a second active material layer arranged on at least one surface of the negative electrode current collector. Along the thickness direction of the negative electrode plate, the second active material layer is arranged between the negative electrode current collector and the first active material layer; the first active material layer includes a first active material and a first binder, and the second active material layer includes a second active material and a second binder. The above-mentioned "arranged on at least one surface of the negative electrode current collector" means that it can be arranged on one surface of the negative electrode current collector along the thickness direction of itself, or it can be arranged on two surfaces of the negative electrode current collector along the thickness direction of itself. It should be noted that the "surface" here can be the entire area of ​​the surface of the negative electrode current collector, or it can be a partial area of ​​the surface of the negative electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved. For example, Figure 1 This is a negative electrode sheet according to one embodiment of the present application, comprising a negative electrode current collector 30, a first active material layer 10, and a second active material layer 20, the first active material layer 10 and the second active material layer 20 being disposed on both surfaces of the negative electrode current collector 30. Along the thickness direction of the negative electrode sheet, the second active material layer 20 is disposed between the negative electrode current collector 30 and the first active material layer 10; the first active material layer 10 comprises a first active material 11 and a first binder (not shown in the figure), and the second active material layer 20 comprises a second active material 21 and a second binder (not shown in the figure).

[0027] The first active material includes a first silicon material, and the first binder includes a polyacrylic acid binder; the second active material includes a second silicon material, and the second binder includes a styrene-butadiene rubber binder; the mass percentage of the first silicon material in the first active material layer is P1, and the mass percentage of the second silicon material in the second active material layer is P2, and P2 is less than P1; the particle size Dv50 of the first silicon material is D1, and the particle size Dv50 of the second silicon material is D2, and D1 is less than D2. The present application provides a first active material layer and a second active material layer on at least one surface of a negative electrode current collector. The first active material layer away from the negative electrode current collector uses a silicon material with a high content and a small particle size and a PAA-type binder with high bonding performance. The silicon material with a small particle size has a large specific surface area and more reactive sites, which can improve the ion transmission efficiency and the kinetic performance of the active material in the first active material layer. At the same time, the PAA-type binder can form strong hydrogen bonds with the silicon material, providing high adhesion, while taking into account the stability of the first active material layer during cycling. The second active material layer close to the negative electrode current collector uses a silicon material with a low content and a large particle size and an SBR-type binder with high kinetic performance, which can improve the ability of the active material in the second active material layer to participate in the insertion and extraction of lithium ions, effectively improve the transmission efficiency of lithium ions in the negative electrode of the secondary battery, and at the same time improve the problem of large expansion of the silicon-containing negative electrode plate. While improving the kinetic performance of the secondary battery, it also takes into account the cycling stability of the secondary battery, so that the secondary battery has both good kinetic performance and cycling stability. In the present application, Dv50 means the particle size at which the cumulative volume reaches 50% when measured from the smallest particle size in a volume-based particle size distribution.

[0028] In one embodiment of the present application, 0.1%≤P2<P1≤30%. For example, P1 can be 0.11%, 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 consisting of any two values ​​therein, and P2 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%, 29.9% or a range consisting of any two of the values, and P2<P1. The present application regulates the content of the first silicon material in the first active material layer within the above range, which can effectively improve the ion transmission efficiency, thereby improving the kinetic performance of the secondary battery. At the same time, the high-content silicon material in the first active material layer is used in combination with a PAA-type binder. The PAA-type binder can form a strong hydrogen bond with the silicon material, provide high adhesion, and take into account the stability of the first active material layer during the cycle. The content of the second silicon material in the second active material layer is regulated within the above range. The low-content silicon material in the second active material layer is used in combination with an SBR-type binder. The SBR-type binder has good kinetic performance, which improves the kinetic performance of the silicon material in the second active material layer away from the electrolyte, so that the secondary battery takes into account both the kinetic performance and the stability of the negative electrode plate during the cycle, thereby improving the kinetic performance and cycle stability of the secondary battery.

[0029] In one embodiment of the present application, 1 μm ≤ D1 < D2 ≤ 30 μm. For example, D1 can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 12 μm, 13 μm, 15 μm, 16 μm, 18 μm, 20 μm, 22 μm, 23 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 29.9 μm, or a range consisting of any two of these values, and D2 The particle size Dv50 of the first silicon material of the first active material layer and the particle size Dv50 of the second silicon material of the second active material layer can be adjusted within the scope of the present application to further improve the ion transmission efficiency, thereby improving the kinetic performance of the secondary battery, and taking into account the cycle stability of the secondary battery.

[0030] In one embodiment of the present application, the first active material layer also includes a first dispersant, and the first dispersant includes a first carboxymethyl cellulose dispersant. The mass percentage F1 of the first dispersant in the first active material layer is 0.1% to 5%. For example, F1 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% or a range consisting of any two of these values. By adopting the above-mentioned types and contents of dispersants, a PAA-type binder and a CMC-type dispersant are used in combination in the first active material layer. The PAA-type binder has low swelling and high elastic modulus, and has a high carboxyl content, which can form strong hydrogen bonds with silicon materials, providing high adhesion. It has high rigidity and is combined with the CMC material with self-healing hydrogen bonds. The stability of the first active material layer during the cycle can be improved, thereby improving the dynamic performance of the secondary battery while taking into account the cycle stability of the secondary battery.

[0031] In one embodiment of the present application, the second active material layer further includes a second dispersant, the second dispersant includes a second carboxymethyl cellulose dispersant, and the mass percentage content F2 of the second dispersant in the second active material layer is 0.1% to 5%. 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% or a range consisting of any two values ​​therein. By adopting the above-mentioned type and content of dispersant, in the second active material layer, the low-content silicon material is used in combination with the SBR binder and CMC dispersant with better kinetic properties, which can improve the kinetic properties of the silicon material in the second active material layer away from the electrolyte, thereby improving the kinetic performance of the secondary battery.

[0032] The present application has no particular limitation on the carboxymethyl cellulose dispersant, as long as the purpose of the present application can be achieved. 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.

[0033] In one embodiment of the present application, the first active material layer further includes a first dispersant, the first dispersant including a first carboxymethyl cellulose dispersant, and the first dispersant accounts for a mass percentage F1 of 0.1% to 5% of the first active material layer; the second active material layer further includes a second dispersant, the second dispersant including a second carboxymethyl cellulose dispersant, and the second dispersant accounts for a mass percentage F2 of 0.1% to 5% of the second active material layer. By simultaneously using the first dispersant and the second dispersant within the above-mentioned type and content range, the synergistic effect of the first active material layer and the second active material layer can be better exerted, thereby further improving the dynamic performance and cycle stability of the secondary battery.

[0034] In one embodiment of the present application, 1 μm ≤ D1 ≤ 8 μm. For example, D1 can be 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.5 μm, 1.6 μm, 1.8 μm, 2 μm, 2.2 μm, 2.5 μm, 2.6 μm, 2.8 μm, 3 μm, 3.2 μm, 3.5 μm, 3.8 μm, 4 μm, 4.2 μm, 4.5 μm, 4.8 μm, 5 μm, 5.1 μm, 5.2 μm, 5.5 μm, 5.8 μm, 6 μm, 6.2 μm, 6.5 μm, 6.8 μm, 7 μm, 7.2 μm, 7.3 μm, 7.5 μm, 7.6 μm, 7.7 μm, 7.8 μm, 8 μm, or a range consisting of any two of these values. In this application, by regulating the particle size Dv50 of the first silicon material of the first active material layer within the above range, the silicon material with a smaller particle size has a larger specific surface area and more reactive sites, thereby accelerating ion transport and improving the kinetic performance of the secondary battery. At the same time, it is combined with a PAA-type binder with high bonding performance to reduce the expansion of the silicon material, thereby improving the expansion performance of the silicon-containing negative electrode sheet, improving the cycle stability of the secondary battery, and improving the cycle life of the secondary battery.

[0035] In one embodiment of the present application, 8 μm < D2 ≤ 30 μm. For example, D2 can be 8.1 μm, 8.2 μm, 8.3 μm, 8.5 μm, 8.6 μm, 8.8 μm, 9 μm, 9.2 μm, 9.3 μm, 9.5 μm, 9.8 μm, 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 20 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, or a range consisting of any two of these values. The present application improves the kinetic performance of the second active material and thus the kinetic performance of the secondary battery by regulating the particle size Dv50 of the second silicon material of the second active material layer within the above range and combining it with an SBR-based binder with high kinetic performance.

[0036] In one embodiment of the present application, 1 μm ≤ D1 ≤ 8 μm, and 8 μm < D2 ≤ 30 μm. By simultaneously regulating the particle size Dv50 of the first silicon material and the particle size Dv50 of the second silicon material within the above range, the present application can better utilize the synergistic effect of the first active material layer and the second active material layer, further improving the kinetic performance and cycle stability of the secondary battery.

[0037] In one embodiment of the present application, the ratio of the particle size Dv50 to Dv90 of the first silicon material is G1, which satisfies: 0.55<G1≤0.7. For example, G1 can be 0.56, 0.57, 0.58, 0.59, 0.6, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, or a range consisting of any two of these values. The present application regulates the particle size Dv50 / Dv90 of the first silicon material of the first active material layer within the above range, and at the same time uses a PAA-type binder with high bonding performance to better exert the ion transport performance of the silicon material and the bonding performance of the binder, thereby taking into account the dynamic performance and expansion performance of the negative electrode sheet, thereby improving the dynamic performance and cycle stability of the secondary battery. In the present application, Dv90 represents the particle size measured from the smallest particle size to the 90% cumulative volume in the volume-based particle size distribution. In this application, the Dv50 and Dv90 values ​​of the silicon material can be controlled by methods known to those skilled in the art. For example, the Dv50 and Dv90 values ​​of the silicon material can be controlled by adjusting the parameters of grading, shaping, and screening, thereby adjusting the values ​​of G1 or G2. This application does not particularly limit the particle size Dv90 of the first silicon material, as long as it can achieve the objectives of this application. For example, the particle size Dv90 of the first silicon material can be 1.5 μm to 14.5 μm.

[0038] In one embodiment of the present application, the ratio of the particle size Dv50 to Dv90 of the second silicon material is G2, satisfying: 0.5≤G2≤0.55. For example, G2 can be 0.5, 0.51, 0.52, 0.53, 0.54, 0.55 or a range consisting of any two values ​​therein. The present application regulates the particle size Dv50 / Dv90 of the second silicon material of the second active material layer within the above range, and at the same time uses an SBR-type binder with high dynamic performance to enhance the ability of the second active material to deintercalate and deintercalate lithium ions, reduce the impact on the expansion volume of the second silicon material, and thus enhance the kinetic performance and cycle stability of the secondary battery. The present application has no special restrictions on the particle size Dv90 of the second silicon material, as long as the purpose of the present application can be achieved. For example, the particle size Dv90 of the second silicon material can be 15μm to 60μm.

[0039] In one embodiment of the present application, the ratio of the particle size Dv50 to Dv90 of the first silicon material is G1, satisfying: 0.55<G1≤0.7, and the ratio of the particle size Dv50 to Dv90 of the second silicon material is G2, satisfying: 0.5≤G2≤0.55. By regulating the particle size Dv50 / Dv90 of the first silicon material and the particle size Dv50 / Dv90 of the second silicon material within the above range, the present application can better utilize the synergistic effect of the first silicon material and PAA-type binder and the second silicon material and SBR-type binder, thereby further improving the dynamic performance and cycle stability of the secondary battery.

[0040] In one embodiment of the present application, the first active material further includes a first graphite. Based on the total mass of the first active material, the mass percentage A1 of the first silicon material is 1% to 100%, and the mass percentage B1 of the first graphite is 0% to 99%. For example, the mass percentage A1 of the first silicon material can be 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%, 86%, 87%, 88%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, 101%, 102%, 103%, 104%, 105%, 106%, 107%, 108%, 109%, 110%, 111%, 112%, 113%, 114%, 115%, 116%, 117%, 118%, 119%, 120%, 121%, 122%, 123%, 124%, 125%, 126%, 127%, 128%, 129%, 130%, 131%, 132%, 133%, 134%, 135%, 136%, 137%, 138%, 139%, 240%, 241%, 242%, 243%, 244%, 245%, 246%, 247%, 248%, 249%, 250%, 251%, 252 %, 85%, 90%, 92%, 95%, 98%, 100% or a range consisting of any two of the values, the mass percentage content B1 of the first graphite can be 0%, 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%, 98%, 99% or a range consisting of any two of the values. In one embodiment of the present application, the mass percentage content A1 of the first silicon material is 15% to 100%, and the mass percentage content B1 of the first graphite is 0% to 85%. When the secondary battery is discharged, the active material graphite in the active material layer will be discharged first, and the silicon material will start to discharge at the end of the discharge, which extends the ion transmission distance between the lithium ions and the silicon material. Therefore, by regulating the content A1 of the first silicon material and the content B1 of the first graphite within the above range, the ion transmission efficiency at the end of the discharge can be effectively improved, thereby improving the kinetic performance of the secondary battery. At the same time, the silicon material will increase the cycle expansion rate of the negative electrode plate. By using a PAA-type binder, the PAA-type binder has low swelling and high elastic modulus, and a high carboxyl content, which can form a strong hydrogen bond with the silicon material, providing high adhesion, so that the first active material layer has good stability in the cycle, thereby making the secondary battery have good kinetic performance and cycle stability at the same time.

[0041] In one embodiment of the present application, the second active material further includes a second graphite. Based on the total mass of the second active material, the mass percentage A2 of the second silicon material is 0% to 99%, and the mass percentage B2 of the second graphite is 1% to 100%. For example, the mass percentage A2 of the second silicon material can be 0%, 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%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, 101%, 102%, 103%, 104%, 105%, 106%, 107%, 108%, 109%, 110%, 111%, 112%, 113%, 114%, 115%, 116%, 117%, 118%, 119%, 120%, 121%, 122%, 123%, 124%, 125%, 126%, 127%, 128%, 129%, 130%, 131%, 132%, 133%, 134%, 135%, 136%, 137%, 138%, 139%, 140%, 141%, 142%, 143%, 144%, 145%, 146%, 147%, 148%, 149%, 150%, 151%, 152%, 153 %, 80%, 85%, 90%, 92%, 95%, 98%, 99% or a range consisting of any two of the values, the mass percentage content B2 of the second graphite can be 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%, 98%, 99%, 100% or a range consisting of any two of the values. In one embodiment of the present application, the mass percentage content A2 of the second silicon material is 1% to 15%, and the mass percentage content B2 of the second graphite is 85% to 99%. The present application improves the kinetic performance of the silicon material in the second active material layer away from the electrolyte by regulating the content A2 of the second silicon material and the content B2 of the second graphite within the above range and simultaneously using an SBR-type binder with high kinetic performance, thereby improving the kinetic performance of the secondary battery.

[0042] In one embodiment of the present application, the first active material further comprises a first graphite. Based on the total mass of the first active material, the mass percentage A1 of the first silicon material is 1% to 100%, and the mass percentage B1 of the first graphite is 0% to 99%. The second active material further comprises a second graphite. Based on the total mass of the second active material, the mass percentage A2 of the second silicon material is 0% to 99%, and the mass percentage B2 of the second graphite is 1% to 100%. In one embodiment of the present application, the mass percentage A1 of the first silicon material is 15% to 100%, and the mass percentage B1 of the first graphite is 0% to 85%. The mass percentage A2 of the second silicon material is 1% to 15%, and the mass percentage B2 of the second graphite is 85% to 99%. When the secondary battery is discharged, the active material graphite in the active material layer discharges preferentially, and the silicon material begins to discharge at the end of discharge, extending the ion transmission distance between lithium ions and the silicon material. By regulating the content A1 of the first silicon material and the content B1 of the first graphite within the above-mentioned ranges, the ion transmission efficiency at the end of discharge can be effectively improved, thereby enhancing the kinetic performance of the secondary battery. At the same time, the silicon material increases the cyclic expansion rate of the negative electrode. By combining the first silicon material with a PAA-type binder, the PAA-type binder has low swelling and high elastic modulus, and a high carboxyl group content, which can form strong hydrogen bonds with the silicon material, providing high adhesion, thus ensuring good stability of the first active material layer during cycling. At the same time, in the second active material layer, regulating the content A2 of the second silicon material and the content B2 of the second graphite within the above-mentioned ranges and using it in combination with an SBR-type binder can improve the kinetic performance of the silicon material in the second active material layer, thereby leveraging the synergistic effect of the first and second active material layers to enhance the kinetic performance and cyclic stability of the secondary battery.

[0043] In one embodiment of the present application, the mass percentage N1 of the first binder in the first active material layer is 1% to 10%. For example, N1 can be 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.2%, 5.5%, 5.8%, 6%, 6.2%, 6.5%, 6.8%, 7%, 7.2%, 7.5%, 7.8%, 8%, 8.2%, 8.5%, 8.8%, 9%, 9.2%, 9.5%, 9.8%, 10% or a range consisting of any two of the values. By regulating the content of the first binder within the scope of this application, the synergistic effect of the first silicon material and the first binder can be better exerted, so that the negative electrode plate can take into account both dynamic performance and cycle expansion performance, thereby enabling the secondary battery to have both good dynamic performance and cycle stability.

[0044] In one embodiment of the present application, the mass percentage N2 of the second binder in the second active material layer is 1% to 10%. For example, N2 can be 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.2%, 5.5%, 5.8%, 6%, 6.2%, 6.5%, 6.8%, 7%, 7.2%, 7.5%, 7.8%, 8%, 8.2%, 8.5%, 8.8%, 9%, 9.2%, 9.5%, 9.8%, 10% or a range consisting of any two of the values. By regulating the content of the second binder within the scope of this application, the synergistic effect of the second silicon material and the second binder can be better exerted, further improving the dynamic performance of the negative electrode sheet, thereby further improving the dynamic performance of the secondary battery.

[0045] In one embodiment of the present application, the first binder accounts for a mass percentage N1 of 1% to 10% of the first active material layer, and the second binder accounts for a mass percentage N2 of 1% to 10% of the second active material layer. By regulating the contents of the first binder and the second binder within the above ranges, the present application can further enhance the synergistic effect of the first active material layer and the second active material layer, further improving the dynamic performance and cycle stability of the secondary battery.

[0046] In one embodiment of the present application, the first active material layer further includes a first conductive agent, and the mass percentage content C1 of the first conductive agent in the first active material layer is 0.1% to 5%. For example, C1 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%, or a range consisting of any two values ​​therein. By regulating the content of the first conductive agent within the above range, the present application can improve the kinetic performance of the first active material layer, thereby improving the kinetic performance of the secondary battery.

[0047] In one embodiment of the present application, the second active material layer further includes a second conductive agent, and the mass percentage C2 of the second conductive agent in the second active material layer is 0.1% to 5%. 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% or a range consisting of any two values ​​therein. By regulating the content of the second conductive agent within the above range, the present application can improve the kinetic performance of the second active material layer, thereby improving the kinetic performance of the secondary battery.

[0048] In one embodiment of the present application, the first active material layer further includes a first conductive agent, the first conductive agent accounting for a mass percentage C1 of 0.1% to 5% of the first active material layer, and the second active material layer further includes a second conductive agent, the second conductive agent accounting for a mass percentage C2 of 0.1% to 5% of the second active material layer. By simultaneously regulating the content of the first conductive agent and the content of the second conductive agent within the above range, the present application can further enhance the synergistic effect of the first active material layer and the second active material layer, thereby further improving the dynamic performance of the secondary battery.

[0049] In one embodiment of the present application, the polyacrylic acid binder includes at least one of polyacrylic acid or polymethacrylic acid. The weight-average molecular weight (Mw) of the polyacrylic acid or polymethacrylic acid is not particularly limited, as long as it can achieve the objectives of the present application. For example, the Mw of polyacrylic acid is 3,000 to 4,000,000, and the Mw of polymethacrylic acid is 5,000 to 100,000. The use of the aforementioned PAA binder in the first active material layer facilitates the synergistic effect between the first silicon material and the PAA binder, thereby improving the kinetic performance and cyclic expansion performance of the first silicon material, thereby improving the kinetic performance and cyclic stability of the secondary battery.

[0050] In one embodiment of the present application, the styrene-butadiene rubber binder includes at least one of styrene-butadiene rubber emulsion, styrene-acrylic emulsion or pure acrylic emulsion. In the present application, styrene-acrylic emulsion (styrene-acrylate emulsion) is obtained by emulsion copolymerization of styrene and acrylate monomers, wherein the molar ratio of styrene and acrylate monomers is 1 to 4. The present application has no particular restrictions on the model of the above-mentioned emulsion, as long as the purpose of the present application can be achieved, and can be obtained by purchasing the above-mentioned emulsion of different models available on the market. The present application has no particular restrictions on the solid content of the above-mentioned emulsion, as long as the purpose of the present application can be achieved, such as the emulsion solid content is 20wt% to 50wt%. By selecting the above-mentioned SBR binder in the second active material layer, it is conducive to the synergistic effect of the second silicon material and the SBR binder, and then improve the kinetic performance of the second silicon material, thereby helping to improve the kinetic performance of the secondary battery.

[0051] The present application has no particular restrictions on silicon materials, as long as the purpose of the present application can be achieved; in one embodiment of the present application, the first silicon material and the second silicon material each independently include at least one of pure silicon, silicon alloy materials, silicon-carbon composite materials or silicon oxides. In the present application, pure silicon may include, but is not limited to, at least one of silicon nanoparticles, silicon nanowires, and micron 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 represented by SiOx, where x is 0 to 2. By adopting the above-mentioned types of silicon materials, the present application is conducive to the synergistic effect of silicon materials with PAA-type binders and SBR-type binders, which is conducive to improving the kinetic performance and cycle expansion performance of the negative electrode sheet, and further conducive to improving the kinetic performance and cycle stability of the secondary battery.

[0052] This application has no particular restrictions on the mass percentage of the first active material in the first active material layer, as long as the purpose of this application can be achieved. In some embodiments, the mass percentage of the first active material in the first active material layer is 80% to 98%. This application has no particular restrictions on the mass percentage S1 of the first graphite in the first active material layer, as long as the purpose of this application can be achieved. This application has no particular restrictions on the mass percentage of the second active material in the second active material layer, as long as the purpose of this application can be achieved. In some embodiments, the mass percentage of the second active material in the second active material layer is 80% to 98%. This application has no particular restrictions on the mass percentage S2 of the second graphite in the second active material layer, as long as the purpose of this application can be achieved.

[0053] The present application does not particularly limit the types of the first conductive agent and the second conductive agent, as long as they can achieve the purpose of the present application. For example, the first conductive agent and the second conductive agent each independently include, but are not limited to, at least one of a carbon-based material, a metal-based material, or a conductive polymer. For example, the carbon-based material may include at least one of natural graphite, artificial graphite, acetylene black, Ketjen black, or carbon fiber; the metal-based material may include, but is not limited to, at least one of metal powder, metal fiber, copper, nickel, aluminum, or silver; and the conductive polymer may include, but is not limited to, a polyphenylene derivative.

[0054] The present application has no special restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector may include copper foil, aluminum foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel or foam copper, etc. In the present application, there is no special restriction on the thickness of the negative electrode current collector, the first active material layer, and the second active material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4μm to 12μm, the thickness of the single-sided first active material layer is 30μm to 130μm, and the thickness of the single-sided second active material layer is 30μm to 130μm. The present application has no special restrictions on the thickness of the negative electrode sheet, as long as the purpose of the present application can be achieved, for example, the thickness of the negative electrode sheet is 50μm to 280μm.

[0055] The second aspect of the present application provides a secondary battery, which includes the negative electrode sheet according to any one of the aforementioned embodiments. The secondary battery provided in the second aspect of the present application has good dynamic performance and cycle stability.

[0056] The secondary battery of the present application also includes a positive electrode sheet. The present application has no special restrictions on the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the positive electrode sheet includes a positive electrode collector and a positive electrode active material layer provided on at least one surface of the positive electrode collector. In the present application, the positive electrode active material layer can be provided on one surface of the positive electrode collector along its own thickness direction, or on two surfaces in the thickness direction of the positive electrode collector. It should be noted that the "surface" here can be the entire area of ​​the positive electrode collector or a partial area of ​​the positive electrode collector. The present application has no special restrictions, as long as the purpose of the present application can be achieved.

[0057] The present application has no special restrictions on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, it may include but is not limited to aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector). The present application has no special restrictions on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 6μm to 12μm, and the thickness of the positive electrode active material layer is 30μm to 120μm. The present application has no special restrictions on the thickness of the positive electrode sheet, as long as the purpose of the present application can be achieved, for example, the thickness of the positive electrode sheet is 50μm to 250μm.

[0058] The positive electrode active material layer of the present application includes a positive electrode active material, and the positive electrode active material includes a substance that can reversibly embed and extract active ions such as lithium ions. The positive electrode active material layer can be one layer or more layers, and each layer in the multi-layer positive electrode active material layer can contain the same or different positive electrode active materials. The present application has no particular restrictions on the positive electrode active material, as long as the purpose of the present application can be achieved. For example, the positive electrode active material may include but is not limited to lithium nickel cobalt manganese oxide (such as 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 iron manganese phosphate or lithium titanate.

[0059] The positive electrode active material layer may also include a conductive agent and a binder. This application does not specifically limit the types of conductive agents and binders, as long as they can achieve the objectives 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, polyvinyl pyrrolidone, 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 a carbon-based material, a metal-based material, or a conductive polymer. For example, the carbon-based material may include at least one of natural graphite, artificial graphite, acetylene black, Ketjen black, or carbon fiber. The metal-based material may include, but is not limited to, at least one of metal powder, metal fiber, copper, nickel, aluminum, or silver. The conductive polymer may include, but is not limited to, a polyphenylene derivative. This application does not specifically limit the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode active material layer, and can be selected according to actual needs, as long as it can achieve the objectives of this application.

[0060] In the present application, the secondary battery also includes a separator to separate the positive electrode plate and the negative electrode plate, prevent internal short circuits in the secondary battery, allow electrolyte ions to pass freely, and do not affect the electrochemical charge and discharge process. The present application has no special restrictions on the separator, as long as it can achieve the purpose of the present application. For example, the material of the separator may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (such as polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid; the type of separator may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane. For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a membrane or a 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 film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film can be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. For example, the inorganic layer includes inorganic particles and a binder, and the application is not particularly limited to the inorganic particles, for example, it can include at least one of aluminum oxide, 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. The application is not particularly limited to the binder, for example, it can be at least one of the above-mentioned binders. Polymers are included in the polymer layer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl ether or polyvinylidene fluoride or poly (vinylidene fluoride-hexafluoropropylene). In the present application, there is no particular limitation on the thickness of the isolation film, as long as the purpose of the present application can be achieved. For example, the thickness of the isolation film can be 5 μm to 500 μm.

[0061] In the present application, the secondary battery also includes an electrolyte. In some embodiments, the electrolyte includes a lithium salt. The present 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 can include but is not limited to lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2, LiTFSI), lithium bis(fluorosulfonyl)imide (Li(N(SO2F)2), LiFSI), lithium difluorophosphate (LiPO2F2), lithium bis(oxalatoborate) (LiB(C2O4)2, LiBOB) or lithium difluorooxalatoborate (LiBF2(C2O4), LiDFOB). The present application does not particularly limit the content of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved.

[0062] In some embodiments, the electrolyte includes a non-aqueous organic solvent. The present application has no particular restrictions on the non-aqueous organic solvent, as long as the purpose of the present application can be achieved. For example, the non-aqueous organic solvent may include at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvents. The above-mentioned carbonate compound may include but is not limited to at least one of a linear carbonate compound, a cyclic carbonate compound or a fluorinated carbonate compound. The above-mentioned linear carbonate compound may include but is not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC) or ethyl methyl carbonate (EMC). The above-mentioned cyclic carbonate compound may include but is not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). The above-mentioned fluorinated carbonate compound may include but is not limited to 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 at least one of trifluoromethylethylene carbonate. The above-mentioned carboxylate compound may include but is 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 above-mentioned ether compound may include but is 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 above-mentioned other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate or phosphate ester.

[0063] In some embodiments, the secondary battery further includes a shell for accommodating a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the above-mentioned other components. This application does not particularly limit the shell, and it can be a shell known in the art, as long as the purpose of this application can be achieved. For example, the shell can be a hard shell or a flexible shell. The material of the hard shell can be metal. This application does not limit the type of metal, and a metal hard shell known in the art can be used, as long as the purpose of this application can be achieved. The flexible shell can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0064] The present application does not particularly limit the type of secondary battery, which may include any device that undergoes an electrochemical reaction. 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).

[0065] The preparation process of the secondary battery of the present application is well known to those skilled in the art and is not particularly limited in the present application. For example, it may include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a shell, injecting the electrolyte into the shell and sealing it to obtain a secondary battery; or stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a shell, injecting the electrolyte into the shell and sealing it to obtain a secondary battery. In addition, as needed, overcurrent protection elements, guide plates, etc. can also be placed in the shell to prevent pressure rise and overcharge and discharge inside the secondary battery.

[0066] The third aspect of the present application provides an electronic device, which includes the secondary battery in any of the aforementioned embodiments. The electronic device of the present application is not particularly limited, and it can be any electronic device known in the prior art. In some embodiments, the electronic device can include, but is not limited to, a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD TV, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery and a lithium ion capacitor, etc.

[0067] Example

[0068] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0069] Test methods and equipment:

[0070] Particle size test:

[0071] Add approximately 0.02g of the negative electrode active material powder sample to a clean 50ml beaker, followed by 20ml of deionized water. A surfactant was then added dropwise to completely disperse the powder in the water. Ultrasonic cleaning was then performed in a 120W ultrasonic cleaner for 5 minutes. The particle size distribution was then measured using a laser particle size analyzer (MasterSizer 2000). In the volume-based particle size distribution of the material, starting from the smallest particle size, the particle size at which the cumulative volume reaches 50% is designated Dv50, and the particle size at which the cumulative volume reaches 90% is designated Dv90.

[0072] 4C discharge maximum temperature test:

[0073] Wrap the lithium-ion battery in insulation. Place a temperature-sensing wire at the center of the intersection of the diagonal lines on the battery's surface. Use a multi-channel thermometer to continuously monitor the battery's surface discharge temperature. At 25°C, charge the battery at a constant current of 1.5C to 4.3V. Then, charge it at a constant voltage of 4.3V until the current reaches 0.05C, then wait for 15 minutes. Then, discharge it at a constant current of 4C to 2.5V. Wait for 30 minutes, and read the maximum discharge temperature at 4C on the multi-channel thermometer.

[0074] Cyclic performance test:

[0075] Under 25℃ environment, the lithium-ion battery is charged at a constant current of 1.5C to 4.3V, and then charged at a constant voltage of 4.3V until the current is less than 0.05C, so that the lithium-ion battery is fully charged and left to stand for 15 minutes. Then, it is discharged at a constant current of 4C to 2.5V and left to stand for 30 minutes. This step process is considered as one circle, and the cycle test is 400 circles. The discharge capacity of the third circle is Q0, and the discharge capacity of the 400th circle is Q N , calculate the cycle capacity retention rate of lithium-ion batteries = Q N / Q0×100%. The thickness of the lithium-ion battery at the first cycle of discharge to 30% SOC (i.e., 3.6V) is T0 (PPG battery thickness gauge, thickness measurement pressure 50g), and the thickness of the lithium-ion battery at the 400th cycle of full charge is tested and recorded as T N , calculate the cycle expansion rate of lithium-ion batteries = (T N -T0) / T0×100%.

[0076] Lithium deposition test:

[0077] At 5°C, charge the lithium-ion battery to 4.30V at a constant current at a specific charge rate X, then charge it to 0.05C at a constant voltage of 4.30V and let it rest for 5 minutes. Then discharge it to 2.0V at a constant current of 0.5C and let it rest for 5 minutes. This is considered a cycle. After repeating 10 cycles, disassemble the lithium-ion battery that has been fully charged to 4.30V at a charge rate X and obtain the negative electrode for observation. If any part of the negative electrode is ≥2mm2 If lithium deposition occurs in the area of ​​the negative electrode, it is determined to be lithium deposition on the negative electrode sheet. The maximum charge rate without lithium deposition is defined as the maximum non-lithium deposition rate of the lithium-ion battery, that is, the lithium deposition level, where X can be 1C, 1.5C, 2C, etc., each time increasing by 0.5C.

[0078] Example 1-1

[0079] <Preparation of negative electrode sheet>

[0080] The first active material: the first silicon material pure silicon (Dv50 is 5.1 μm, Dv50 / Dv90 is 0.58) and the first graphite artificial graphite, the first binder polyacrylic acid (PAA, weight average molecular weight Mw is 4000), the first conductive agent acetylene black, and the first dispersant carboxymethyl cellulose lithium are dissolved in deionized water in a mass ratio of 30:66:3:0.5:0.5, and after being fully stirred, a first active slurry with a solid content of 75 wt% is prepared.

[0081] The second active material: the second silicon material pure silicon (Dv50 is 9.3μm, Dv50 / Dv90 is 0.50) and the second graphite artificial graphite, the second binder styrene-butadiene rubber latex (SBR, model LB-S420, solid content is 46wt%), the second conductive agent acetylene black, and the second dispersant carboxymethyl cellulose lithium are dissolved in deionized water in a mass ratio of 5:91:2.8:0.5:0.7, and after being fully stirred, a second active slurry with a solid content of 75wt% is obtained.

[0082] The first active slurry and the second active slurry prepared above were simultaneously coated on one surface of a 6 μm thick negative electrode current collector copper foil using a coating machine with a double-layer coating die at a speed of 15 m / min. The second active slurry was in contact with the copper foil to form a second active material layer, and the first active slurry was in contact with the second active slurry to form a first active material layer. The surface density of the first active material layer and the second active material layer was 3.76 mg / cm 2 After drying at 80°C, a negative electrode sheet coated on one side with the first and second active material layers is obtained. The above steps are then repeated on the other side of the copper foil to obtain a negative electrode sheet coated on both sides with the first and second active material layers. This sheet is then cold pressed, cut, and the tabs are welded to obtain a 76mm x 867mm negative electrode sheet ready for use. The thickness of the first active material layer on one side is 30μm, and the thickness of the second active material layer is 30μm.

[0083] <Preparation of positive electrode sheet>

[0084] The positive electrode active material lithium nickel cobalt manganese oxide (molecular formula LiNi 0.5 Co 0.2 Mn0.3 O2, abbreviated as NCM523), acetylene black as a positive electrode conductor, and polyvinylidene fluoride (PVDF) as a positive electrode binder were mixed in a mass ratio of 94:3:3, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a positive electrode slurry with a solid content of 75wt%. The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 6μm, with a coating weight of 0.13g / cm 2 The aluminum foil was then dried at 90°C to obtain a single-sided positive electrode sheet coated with a positive electrode active material layer. The above steps were then repeated on the other side of the aluminum foil to obtain a double-sided positive electrode sheet coated with a positive electrode active material layer. The sheet was then cold pressed, cut, and welded to the tabs to obtain a 74mm x 851mm positive electrode sheet ready for use. The compacted density of the positive electrode sheet was 3.45g / cm 3 .

[0085] <Preparation of Electrolyte>

[0086] In an argon atmosphere 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 ethyl methyl carbonate (EMC) are mixed in a weight ratio of 20:30:40:10, and then lithium salt lithium hexafluorophosphate (LiPF6) is added to the above non-aqueous organic solvents, dissolved and mixed uniformly to obtain an electrolyte, wherein the mass percentage of LiPF6 in the electrolyte is 12.5%.

[0087] <Preparation of Separator>

[0088] A polyethylene (PE) porous film with a thickness of 7 μm (provided by Celgard) was used as a separator.

[0089] <Preparation of lithium-ion batteries>

[0090] The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide insulation. The electrodes are then wound to form an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dehydrated at 80°C, and then injected with the aforementioned electrolyte and packaged. The lithium-ion battery is then produced through formation, degassing, and trimming. The upper formation voltage limit is 4.15V, the formation temperature is 70°C, and the formation rest time is 2 hours.

[0091] Example 1-2 to Example 1-16

[0092] Except that in <Preparation of Negative Electrode Sheet>, the relevant preparation parameters are adjusted according to Table 1, 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 Example 1-1.

[0093] Example 2-1 to Example 2-18

[0094] Except for adjusting the relevant preparation parameters according to Table 1 in <Preparation of Negative Electrode Sheet>, the rest is the same as Example 1-1.

[0095] Comparative Example 1-1 to Comparative Example 1-3

[0096] Except for adjusting the relevant preparation parameters according to Table 1 in <Preparation of Negative Electrode Sheet>, the rest is the same as Example 1-1.

[0097] Comparative Example 1-4 to Comparative Example 1-5

[0098] Except that in <Preparation of Negative Electrode Sheet>, only the first active material layer or the second active material layer was coated as shown in Table 1, the rest was the same as Example 1-1.

[0099] Comparative Examples 1-6

[0100] Except that in <Preparation of Negative Electrode Sheet>, only the first active material layer was coated as shown in Table 1, the rest was the same as in Example 1-13.

[0101] Comparative Examples 1-7

[0102] Except that in <Preparation of Negative Electrode Sheet>, only the second active material layer was coated as shown in Table 1, the rest was the same as in Example 1-12.

[0103] The preparation parameters and performance tests of various embodiments and comparative examples are shown in Tables 1 and 2.

[0104]

[0105]

[0106] It can be seen from Examples 1-1 to 1-16 and Comparative Examples 1-1 to 1-7 that in the lithium-ion batteries of the embodiments of the present application, the negative electrode plate includes a first active material layer and a second active material layer located between the negative electrode current collector and the first active material layer, the first active material layer includes a PAA-type binder, the second active material layer includes an SBR-type binder, and the first silicon material of the first active material layer and the second silicon material of the second active material layer satisfy that the first silicon material particle size D1 is less than the second silicon material particle size D2, and the first silicon material content P1 is greater than the second silicon material content P2, so that the lithium-ion battery has a lower 4C discharge maximum temperature, better lithium plating performance and cycle capacity retention performance, and at the same time has a lower cycle expansion rate. As can be seen from Example 1-1 and Comparative Examples 1-1 to 1-5, Examples 1-12 to 1-13, and Comparative Examples 1-6 to 1-7, when the first silicon material particle size D1 is larger than the second silicon material particle size D2, or when the first active material layer uses an SBR-based binder and the second active material layer uses a PAA-based binder, or when only a single layer of the first active material layer or a single layer of the second active material layer is applied, the lithium-ion battery has a higher maximum 4C discharge temperature, a lower lithium plating level and a lower cycle capacity retention rate, and a higher cycle expansion rate. These results indicate that lithium-ion batteries using the negative electrode sheet of the present application have good kinetic performance and cycle stability.

[0107] Silicon material particle size generally affects the kinetic performance and cycling stability of lithium-ion batteries. As can be seen from Examples 1-1 to 1-5, and 1-12 to 1-13, lithium-ion batteries with controlled first silicon material particle sizes D1 and G1 and second silicon material particle sizes D2 and G2 within the scope of this application exhibit lower maximum 4C discharge temperatures, better lithium deposition performance and cycling capacity retention, and lower cycling expansion rates, indicating that the lithium-ion batteries have better kinetic performance and cycling stability.

[0108] The silicon material content generally affects the kinetic performance and cycling stability of lithium-ion batteries. As can be seen from Examples 1-1, 1-6, and 1-15, lithium-ion batteries with controlled first silicon material contents P1 and A1 and second silicon material contents P2 and A2 within the scope of this application exhibit a lower maximum 4C discharge temperature, better lithium plating performance, and better cycling capacity retention, as well as a lower cycling expansion rate, indicating that the lithium-ion batteries have better kinetic performance and cycling stability.

[0109] The types of the first binder, first silicon material, second binder, and second silicon material generally affect the kinetic performance and cycling stability of lithium-ion batteries. As can be seen from Examples 1-1 and 1-16, lithium-ion batteries using the first binder, first silicon material, second binder, and second silicon material within the scope of this application exhibit a low maximum 4C discharge temperature, good lithium deposition performance, and cycle capacity retention, as well as a low cycle expansion rate, indicating that the lithium-ion batteries have good kinetic performance and cycling stability.

[0110] Table 2

[0111]

[0112] The mass percentages of the dispersant, binder, and conductive agent in the negative electrode active material layer generally also affect the kinetic performance and cycle stability of the lithium-ion battery. Referring to Table 2, it can be seen from Examples 2-1 to 2-18 that by synergistically regulating the above parameters within the scope of this application, it is advantageous to obtain a lithium-ion battery with a low maximum 4C discharge temperature, good lithium deposition performance, and cycle capacity retention. Simultaneously, the lithium-ion battery has a low cycle expansion rate, indicating that the lithium-ion battery has good kinetic performance and cycle stability.

[0113] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article comprising a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, or article.

[0114] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A negative electrode sheet comprising a negative electrode current collector and a first active material layer and a second active material layer disposed on at least one surface of the negative electrode current collector, wherein the second active material layer is disposed between the negative electrode current collector and the first active material layer along the thickness direction of the negative electrode sheet; The first active material layer includes a first active material and a first binder, the first active material includes a first silicon material, and the first binder includes a polyacrylic binder; The second active material layer includes a second active material and a second binder, the second active material includes a second silicon material, and the second binder includes a styrene-butadiene rubber-based binder; The mass percentage of the first silicon material in the first active material layer is P1, the mass percentage of the second silicon material in the second active material layer is P2, and P2<P1; The particle size Dv50 of the first silicon material is D1, the particle size Dv50 of the second silicon material is D2, and D1<D2.

2. The negative electrode sheet according to claim 1, wherein: 0.1%≤P2<P1≤30%.

3. The negative electrode sheet according to claim 1, wherein: 1μm≤D1<D2≤30μm.

4. The negative electrode sheet according to claim 1, wherein: The first active material layer further includes a first dispersant, the first dispersant includes a first carboxymethyl cellulose dispersant, and the mass percentage F1 of the first dispersant in the first active material layer is 0.1% to 5%; and / or, The second active material layer further includes a second dispersant, which includes a second carboxymethyl cellulose-based dispersant. The mass percentage F2 of the second dispersant in the second active material layer is 0.1% to 5%.

5. The negative electrode sheet according to claim 1, wherein: 1μm≤D1≤8μm, and / or, 8μm<D2≤30μm.

6. The negative electrode sheet according to claim 1, wherein: The ratio of the particle size Dv50 to Dv90 of the first silicon material is G1, which satisfies: 0.55<G1≤0.7; and / or, The ratio of the particle sizes Dv50 and Dv90 of the second silicon material is G2, which satisfies: 0.5≤G2≤0.

55.

7. The negative electrode sheet according to claim 1, wherein: The first active material further includes first graphite, and based on the total mass of the first active material, the mass percentage A1 of the first silicon material is 1% to 100%, and the mass percentage B1 of the first graphite is 0% to 99%; and / or, The second active material further includes second graphite. Based on the total mass of the second active material, the mass percentage A2 of the second silicon material is 0% to 99%, and the mass percentage B2 of the second graphite is 1% to 100%.

8. The negative electrode sheet according to claim 7, wherein: The mass percentage A1 of the first silicon material is 15% to 100%, and the mass percentage B1 of the first graphite is 0% to 85%; and / or, The mass percentage A2 of the second silicon material is 1% to 15%, and the mass percentage B2 of the second graphite is 85% to 99%.

9. The negative electrode sheet according to claim 1, wherein: The mass percentage N1 of the first binder in the first active material layer is 1% to 10%; and / or, The mass percentage N2 of the second binder in the second active material layer is 1% to 10%.

10. The negative electrode sheet according to claim 1, wherein: The first active material layer further includes a first conductive agent, and the mass percentage C1 of the first conductive agent in the first active material layer is 0.1% to 5%; and / or, The second active material layer further includes a second conductive agent, and the mass percentage C2 of the second conductive agent in the second active material layer is 0.1% to 5%.

11. The negative electrode sheet according to claim 1, wherein: The negative electrode plate satisfies at least one of the following characteristics: (1) The polyacrylic acid binder includes at least one of polyacrylic acid or polymethacrylic acid; (2) The styrene-butadiene rubber adhesive comprises at least one of styrene-butadiene rubber emulsion, styrene-acrylic emulsion or pure acrylic emulsion; (3) The first silicon material and the second silicon material each independently include at least one of pure silicon, a silicon alloy material, a silicon-carbon composite material, or a silicon oxide. 12 . A secondary battery comprising the negative electrode sheet according to claim 1 . 13 . An electronic device comprising the secondary battery according to claim 12 .

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