Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device. The secondary battery comprises a negative electrode piece and a positive electrode piece, the negative electrode piece comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, the negative electrode material layer comprises a first negative electrode material layer and a second negative electrode material layer which are arranged in a stacked mode, and the first negative electrode material layer is located between the second negative electrode material layer and the negative electrode current collector; the first negative electrode material layer comprises a first negative electrode active material, the second negative electrode material layer comprises a second negative electrode active material, the first negative electrode active material and the second negative electrode active material are graphite materials with different end face ratios, the end face ratio of the first negative electrode active material is I1, the end face ratio of the second negative electrode active material is I2, and I1 is greater than I2. The secondary battery provided by the invention has good cycle performance.

Description

Technical Field

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

[0002] Secondary batteries, such as lithium-ion batteries, are widely used in smartphones, wearable devices, consumer drones, and electric vehicles due to their high energy density, long cycle life, and lack of memory effect. Among these, manganese-containing positive electrode active materials (such as lithium manganese oxide and lithium iron manganese phosphate) have attracted significant attention due to their low cost and high plateau voltage. However, manganese-containing positive electrode active materials experience manganese ion dissolution during charge / discharge and storage. These dissolved manganese ions penetrate the separator and deposit on the surface of the negative electrode, severely damaging the solid electrolyte interphase (SEI) film. This leads to significant capacity decay in lithium-ion batteries and negatively impacts their cycle performance. Summary of the Invention

[0003] The purpose of this application is to provide a secondary battery and an electronic device to reduce capacity decay and improve cycle performance of the secondary battery. The specific technical solution is as follows:

[0004] The first aspect of this application provides a secondary battery, which includes a negative electrode sheet and a positive electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer stacked thereon. The first negative electrode material layer is located between the second negative electrode material layer and the negative current collector. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode material layer includes a second negative electrode active material. The first negative electrode active material and the second negative electrode active material are graphite materials with different end-face ratios. The end-face ratio of the first negative electrode active material is I1, and the end-face ratio of the second negative electrode active material is I2, where I1 > I2. This application designs the structure of the negative electrode sheet and adjusts I1 and I2 to satisfy the above relationship. Compared with the first negative electrode active material, the second negative electrode active material has a smaller end face ratio, which can improve the manganese ion tolerance on the surface of the negative electrode sheet, reduce the amount of manganese ions dissolved from the positive electrode active material entering the first negative electrode material layer, reduce the risk of the SEI film on the surface of the negative electrode graphite material being damaged by manganese ions, reduce the probability of gas generation reaction, thereby reducing the capacity decay of the secondary battery and improving the cycle performance of the secondary battery.

[0005] In one or more embodiments of this application, the secondary battery satisfies at least one of the following characteristics: (1) 0.12≤I1≤0.3, preferably 0.15≤I1≤0.25; (2) 0.05≤I2≤0.15, preferably 0.05≤I2≤0.12. The secondary battery satisfying the above characteristics is beneficial for further reducing the capacity decay of the secondary battery and improving its cycle performance.

[0006] In one or more embodiments of this application, the Dv50 of the first negative electrode active material is D1 μm, the Dv50 of the second negative electrode active material is D2 μm, 0.5≤D2 / D1≤1, and 5≤D2≤20. By adjusting the values ​​of D2 / D1 and D2 within the above range, it is beneficial to reduce the capacity decay of the secondary battery and improve the cycle performance of the secondary battery.

[0007] In one or more embodiments of this application, the secondary battery satisfies at least one of the following characteristics: (1) 0.7 ≤ D2 / D1 ≤ 1; (2) 8 ≤ D2 ≤ 15. By adjusting the values ​​of D2 / D1 and D2 within the above range, it is beneficial to further reduce the capacity decay of the secondary battery and improve its cycle performance.

[0008] In one or more embodiments of this application, the positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material containing Mn, and the Mn-containing positive electrode active material includes at least one of lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium manganese iron phosphate, or lithium-rich manganese-based materials. Using the above-mentioned positive electrode active material in a secondary battery results in low manufacturing cost, high plateau voltage, less capacity decay, and good cycle performance.

[0009] In one or more embodiments of this application, the thickness of the first negative electrode material layer is H1 μm, the thickness of the second negative electrode material layer is H2 μm, 2≤H1 / H2≤20, and 5≤H2≤40. By adjusting the values ​​of H1 / H2 and H2 within the above ranges, it is beneficial to reduce the capacity decay of the secondary battery, improve the cycle performance of the secondary battery, and at the same time, the kinetic performance of the secondary battery is good.

[0010] In one or more embodiments of this application, 8 ≤ H1 / H2 ≤ 15, and 10 ≤ H2 ≤ 20. By adjusting the values ​​of H1 / H2 and H2 within the above ranges, it is beneficial to further reduce the capacity decay of the secondary battery, improve the cycle performance of the secondary battery, and at the same time, the kinetic performance of the secondary battery is good.

[0011] In one or more embodiments of this application, the second negative electrode material layer includes a negative electrode additive, which includes at least one of a carboxylate, a sulfonate, or a metal-organic framework compound. The carboxylate includes at least one of lithium oxalate, sodium citrate, sodium ethylenediaminetetraacetate, or sodium oxalate. The sulfonate includes at least one of sodium dodecylbenzenesulfonate, sulfonated polystyrene, or sulfonate-functionalized montmorillonite. The metal-organic framework compound includes at least one of ZIF-8, MIL-101, or UiO-66. Based on the mass of the second negative electrode material layer, the mass percentage content of the negative electrode additive is 0.1% to 2%. The inclusion of the above-mentioned negative electrode additive in the second negative electrode material layer, and the control of its mass percentage within the above range, is beneficial for reducing the penetration of manganese ions into the first negative electrode material layer, reducing the capacity decay of the secondary battery, improving the cycle performance of the secondary battery, and simultaneously achieving a higher energy density in the secondary battery.

[0012] In one or more embodiments of this application, the mass percentage of the negative electrode additive is 0.5% to 1% based on the mass of the second negative electrode material layer. A mass percentage of the negative electrode additive within this range is beneficial for further reducing the capacity decay of the secondary battery, improving the cycle performance of the secondary battery, and simultaneously resulting in a higher energy density for the secondary battery.

[0013] In one or more embodiments of this application, after the secondary battery is stored at 30% charge and 60°C for 30 days, the manganese content in the first negative electrode material layer is W1 ppm based on the mass of the first negative electrode material layer, and the manganese content in the second negative electrode material layer is W2 ppm based on the mass of the second negative electrode material layer, where 0 < W1 / W2 ≤ 0.2 and 0 < W1 ≤ 100. The values ​​of W1 / W2 and W1 are within the above ranges, indicating a lower manganese content in the first negative electrode material layer, which is beneficial for reducing the capacity decay of the secondary battery and improving its cycle performance.

[0014] In one or more embodiments of this application, along the thickness direction of the negative electrode sheet, the first negative electrode material layer includes a first region located on the surface of the first negative electrode material layer away from the negative electrode current collector, and the thickness of the first region is 10 μm. After the secondary battery is stored at 30% charge and 60°C for 30 days, based on the mass of the first region, the mass content of manganese in the first region is less than 100 ppm. The mass content of manganese in the first region being within the aforementioned range is beneficial for reducing the capacity decay of the secondary battery and improving its cycle performance.

[0015] In one or more embodiments of this application, the secondary battery satisfies at least one of the following characteristics: (1) the first negative electrode active material includes at least one of artificial graphite or natural graphite; (2) the second negative electrode active material includes at least one of artificial graphite or natural graphite. Using the above-mentioned first negative electrode active material and / or second negative electrode active material is beneficial for reducing the capacity decay of the secondary battery and improving its cycle performance.

[0016] The second aspect of this application provides an electronic device that includes the secondary battery provided in the first aspect of this application. The electronic device of this application has a long service life and good performance.

[0017] The beneficial effects of this application are:

[0018] This application provides a secondary battery and an electronic device. The secondary battery includes a negative electrode and a positive electrode. The negative electrode includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer stacked thereon. The first negative electrode material layer is located between the second negative electrode material layer and the negative current collector. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode material layer includes a second negative electrode active material. The first negative electrode active material and the second negative electrode active material are graphite materials with different end-face ratios. The end-face ratio of the first negative electrode active material is I1, and the end-face ratio of the second negative electrode active material is I2, where I1 > I2. This application designs the structure of the negative electrode sheet and adjusts I1 and I2 to satisfy the above relationship. Compared with the first negative electrode active material, the second negative electrode active material has a smaller end face ratio, which can improve the manganese ion tolerance on the surface of the negative electrode sheet, reduce the amount of manganese ions dissolved from the positive electrode active material entering the first negative electrode material layer, reduce the risk of the SEI film on the surface of the negative electrode graphite material being damaged by manganese ions, reduce the probability of gas generation reaction, thereby reducing the capacity decay of the secondary battery and improving the cycle performance of the secondary battery.

[0019] 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

[0020] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0021] Figure 1 This is a schematic diagram of the negative electrode sheet along its thickness and length directions according to one embodiment of this application;

[0022] Figure 2 This is a schematic diagram of the negative electrode sheet along its thickness and length directions, representing another embodiment of this application.

[0023] Reference numerals: negative electrode 100, negative current collector 110, first negative electrode material layer 120, first region 121, second region 122, second negative electrode material layer 130. Detailed Implementation

[0024] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only 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.

[0025] 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.

[0026] The first aspect of this application provides a secondary battery, which includes a negative electrode sheet and a positive electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer stacked thereon. The first negative electrode material layer is located between the second negative electrode material layer and the negative current collector. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode material layer includes a second negative electrode active material. The first negative electrode active material and the second negative electrode active material are graphite materials with different end-face ratios. The end-face ratio of the first negative electrode active material is I1, and the end-face ratio of the second negative electrode active material is I2, where I1 > I2.

[0027] In this application, the graphite material includes end faces and basal faces. End faces refer to the edge cross-sections (perpendicular to the carbon atom layer direction) of the graphite material's layered crystalline structure. They are rich in dangling bonds and defects, serving as the main channels for lithium ion insertion / extraction between graphite layers and also as electrochemical active sites. Basal faces refer to the parallel surfaces (parallel to the carbon atom layer direction) within the graphite material's crystalline layers. They exhibit a highly ordered structure and high chemical stability, but ion diffusion is relatively difficult. The end-face ratio I refers to the ratio of the end-face surface area (Ae) to the basal surface area (Ab), i.e., I = Ae / Ab. Therefore, the end-face ratio can characterize the proportion of active sites on the graphite material surface; a higher end-face ratio indicates stronger reactivity of the graphite material.

[0028] This application designs the structure of the negative electrode sheet and adjusts I1 and I2 to satisfy the above relationship. When the secondary battery includes a positive electrode active material containing manganese, the second negative electrode active material has a smaller end face ratio, fewer surface defects, and lower reactivity compared to the first negative electrode active material. Therefore, it is less affected by the manganese ions dissolved from the positive electrode active material. The second negative electrode material layer, located away from the negative electrode current collector, comprises a second negative electrode active material with a low aspect ratio. This reduces the probability of manganese ions dissolved from the positive electrode active material damaging the active sites on the surface of the negative electrode sheet, decreases the number of active sites damaged by manganese ions on the surface of the negative electrode sheet, and improves the manganese ion tolerance of the negative electrode sheet surface. It also acts as an isolation layer to protect the first negative electrode material layer, causing manganese ions to concentrate in the manganese ion-tolerant second negative electrode material layer, reducing the amount of manganese ions dissolved from the positive electrode active material entering the first negative electrode material layer, reducing the risk of manganese ion damage to the SEI film on the surface of the negative electrode graphite material, reducing the probability of gas generation reaction, reducing the impedance of the secondary battery, reducing the capacity decay of the secondary battery, and extending the cycle life of the secondary battery. At the same time, the first negative electrode material layer, comprising a first negative electrode active material with a high aspect ratio, provides more active sites, shortens the lithium ion diffusion path, reduces polarization, improves the capacity and rate performance of the secondary battery, and improves the kinetic performance of the secondary battery, thereby improving the cycle performance of the secondary battery. The secondary battery also exhibits good kinetic performance.

[0029] If I1≤I2, the end-face ratio of the first negative electrode active material is too small, and the end-face ratio of the second negative electrode active material is too large. The reactivity of the second negative electrode active material is too high, and the tolerance of the second negative electrode material layer to manganese ions decreases. At the same time, the amount of manganese ions adsorbed and deposited on the surface of the negative electrode increases, and the amount of manganese ions dissolved from the positive electrode active material entering the first negative electrode material layer also increases. The risk of the SEI film on the surface of the negative electrode graphite material being damaged by manganese ions increases, and the side reactions between the negative electrode sheet and the electrolyte increase. The capacity decay of the secondary battery is severe during the cycle, and the interfacial impedance deteriorates, thereby affecting the cycle performance of the secondary battery and the kinetic performance of the secondary battery is also poor.

[0030] Therefore, by designing the structure of the negative electrode sheet and adjusting I1 and I2 to satisfy the above relationship, this application can simultaneously leverage the advantages of the first and second negative electrode active materials, reduce the capacity decay of the secondary battery, improve the cycle performance of the secondary battery (including high-temperature cycle performance, where high temperature refers to a temperature greater than or equal to 45°C), and simultaneously achieve good kinetic performance of the secondary battery.

[0031] For ease of understanding, in this application, the length direction of the negative electrode sheet is defined as X, and the thickness direction as Y. The unfolded negative electrode sheet has a long side and a short side, and the aforementioned length direction refers to the extension direction of the long side of the negative electrode sheet. It should be understood that the above definitions of directions are for the purpose of conveniently describing this application. Figure 1As shown, the negative electrode sheet 100 includes a negative electrode current collector 110 and negative electrode material layers disposed on two surfaces of the negative electrode current collector 110. The negative electrode material layers include a first negative electrode material layer 120 and a second negative electrode material layer 130 stacked together, with the first negative electrode material layer 120 located between the second negative electrode material layer 130 and the negative electrode current collector 110. In this application, the negative electrode material layers can be disposed on one surface of the negative electrode current collector along its own thickness direction, or on both surfaces of the negative electrode current collector along its own thickness direction. 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 part of the surface of the negative electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0032] In one or more embodiments of this application, 0.12 ≤ I1 ≤ 0.3, and optionally, 0.15 ≤ I1 ≤ 0.25. For example, the value of I1 can be 0.12, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.28, 0.30, or a range consisting of any two of these values. The value range of I1 can be 0.12 to 0.30, 0.14 to 0.28, 0.15 to 0.25, 0.15 to 0.23, 0.15 to 0.20, 0.16 to 0.18, and all such ranges and sub-ranges. By adjusting the value of I1 within the above range, the first negative electrode active material can provide more active sites, which is beneficial to shorten the diffusion path of lithium ions, reduce polarization, improve the capacity and rate performance of the secondary battery, and improve the kinetic performance of the secondary battery. At the same time, the amount of manganese ions dissolved from the positive electrode active material entering the first negative electrode material layer is small, resulting in less capacity decay and good cycle performance of the secondary battery.

[0033] In one or more embodiments of this application, 0.05 ≤ I2 ≤ 0.15, and optionally, 0.05 ≤ I2 ≤ 0.12. For example, the value of I2 can be 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, or a range consisting of any two of these values. The value range of I2 can be 0.05 to 0.15, 0.05 to 0.12, 0.05 to 0.11, 0.05 to 0.10, 0.05 to 0.09, 0.06 to 0.08, and all such ranges and sub-ranges. By adjusting the value of I2 within the aforementioned range, the SEI film formed on the surface of the second negative electrode active material is more stable due to the relatively intact basal structure. This improves the manganese ion tolerance of the negative electrode surface, reduces the risk of manganese ion damage to the SEI film on the negative electrode graphite material surface, and lowers the probability of gas generation reactions. Simultaneously, the second negative electrode active material adsorbs a larger amount of manganese ions, which helps concentrate these ions in the manganese-tolerant second negative electrode material layer, reducing the amount of manganese ions dissolved from the positive electrode active material entering the first negative electrode material layer. This, in turn, helps reduce the capacity decay of the secondary battery and improves its cycle performance. Furthermore, because the basal structure of the second negative electrode active material is relatively intact, the structure of the negative electrode sheet changes less at high temperatures, which is beneficial for improving the thermal stability of the secondary battery.

[0034] In one or more embodiments, 0.01 ≤ I1-I2 ≤ 0.25. For example, the value of I1-I2 can be 0.01, 0.03, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, 0.22, 0.24, 0.25, or a range consisting of any two of these values. The value range of I1-I2 can be 0.01 to 0.25, 0.03 to 0.25, 0.06 to 0.22, 0.08 to 0.20, 0.1 to 0.18, 0.12 to 0.16, and all such ranges and subranges.

[0035] In one or more embodiments of this application, the Dv50 of the first negative electrode active material is D1 μm, the Dv50 of the second negative electrode active material is D2 μm, 0.5≤D2 / D1≤1, 5≤D2≤20, and optionally, 0.7≤D2 / D1≤1, 8≤D2≤15. For example, the value of D2 / D1 can be any two values ​​from 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, to 1, and the range of D2 / D1 can be 0.5 to 1, 0.6 to 1, 0.7 to 1, 0.8 to 1, 0.9 to 1, and all such ranges and sub-ranges. For example, the value of D2 can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or any two of these values. The range of D2 can be 5 to 20, 8 to 18, 8 to 15, 9 to 14, 10 to 13, and all of these ranges, as well as subranges. By adjusting the values ​​of D2 / D1 and D2 within the aforementioned range, the second negative electrode active material has a smaller Dv50 and a larger specific surface area compared to the first negative electrode active material. This results in a stronger adsorption capacity for manganese ions, allowing for the adsorption of a greater number of manganese ions. The manganese ions are concentrated in the manganese-resistant second negative electrode material layer, which helps reduce the impact of manganese ions dissolved from the positive electrode active material on the first negative electrode material layer. It also helps reduce side reactions between the electrolyte and the negative electrode sheet, extending the cycle life of the secondary battery. Furthermore, it reduces the risk of agglomeration of the first and second negative electrode active materials during the preparation of the negative electrode sheet, thereby reducing the capacity decay of the secondary battery, improving its cycle performance, and enhancing its high-temperature storage performance.

[0036] In one or more embodiments, 5 ≤ D1 ≤ 40, and optionally, 8 ≤ D1 ≤ 21. For example, the value of D1 can be 5, 8, 10, 12.5, 15, 17.5, 20, 21, 25, 27.5, 30, 32.5, 35, 37.5, 40, or a range of any two of these values. The range of D1 can be 5 to 40, 8 to 30, 8 to 25, 8 to 21, 10 to 18, 12 to 16, and all of these ranges, as well as subranges.

[0037] In one or more embodiments of this application, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material containing Mn, and the positive electrode active material containing Mn includes at least one of lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium manganese iron phosphate, or lithium-rich manganese-based materials. The aforementioned lithium nickel cobalt manganese oxide may include, but is not limited to, LiNi. 0.95 Co 0.03 Mn 0.02 O2 (Ni95), LiNi0.91 Co 0.03 Mn 0.06 O2 (Ni91), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523) or LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 At least one of O2 (NCM111). When the above-mentioned positive electrode active material is used in a secondary battery, the secondary battery has low manufacturing cost, high plateau voltage, less capacity decay, and good cycle performance.

[0038] In one or more embodiments of this application, the thickness of the first negative electrode material layer is H1 μm, the thickness of the second negative electrode material layer is H2 μm, 2≤H1 / H2≤20, 5≤H2≤40, and optionally, 8≤H1 / H2≤15, 10≤H2≤20. For example, the value of H1 / H2 can be 2, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 20, or a range consisting of any two of these values. The range of H1 / H2 can be 2 to 20, 4 to 15, 6 to 15, 8 to 15, 10 to 12, and all such ranges and sub-ranges. For example, the value of H2 can be 5, 8, 10, 12, 14, 16, 18, 20, 25, 30, 35, 40, or any range of two values ​​within these ranges. The value range of H2 can be 5 to 40, 10 to 35, 10 to 30, 10 to 25, 10 to 20, 12 to 18, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of H1 / H2 and H2 within the above ranges, the thickness of the second negative electrode material layer is relatively small, while the thickness of the first negative electrode material layer is relatively large. This is beneficial for providing more active sites, shortening the lithium ion diffusion path, reducing polarization, improving the capacity and rate performance of the secondary battery, and improving the kinetic performance of the secondary battery. At the same time, the second negative electrode material layer can play a role in reducing the amount of manganese ions dissolved from the positive electrode active material entering the first negative electrode material layer, reducing the risk of the SEI film on the surface of the negative electrode graphite material being damaged by manganese ions, and reducing the probability of gas generation reactions. This is beneficial for reducing the capacity decay of the secondary battery, improving the cycle performance of the secondary battery, and ensuring good kinetic performance of the secondary battery. In this application, H1 μm refers to the thickness of the first negative electrode material layer on one side, and H2 μm refers to the thickness of the second negative electrode material layer on one side.

[0039] In one or more embodiments, 10 ≤ H1 ≤ 800, and optionally, 80 ≤ H1 ≤ 300. For example, the value of H1 can be 10, 30, 40, 60, 80, 100, 120, 140, 160, 180, 200, 300, 400, 500, 600, 700, 800, or a range of any two of these values. The range of H1 can be 10 to 800, 30 to 600, 40 to 400, 60 to 300, 80 to 300, 80 to 200, 80 to 150, 80 to 120, and all of these ranges, as well as subranges.

[0040] In one or more embodiments, 1 ≤ H1 / D1 ≤ 160. For example, the value of H1 / D1 can be 1, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160 or a range of any two of these values. The range of H1 / D1 can be 1 to 160, 5 to 100, 5 to 80, 5 to 50, 5 to 30 and all of these ranges, as well as subranges.

[0041] In one or more embodiments, 1 ≤ H2 / D2 ≤ 8. For example, the value of H2 / D2 can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, or a range of any two values ​​therein. The range of H2 / D2 can be 1 to 8, 2 to 7, 3 to 6, 4 to 5, and all such ranges and subranges.

[0042] In one or more embodiments of this application, the second negative electrode material layer includes a negative electrode additive, which includes at least one of a carboxylate, a sulfonate, or a metal-organic framework compound. The carboxylate includes at least one of lithium oxalate, sodium citrate, sodium ethylenediaminetetraacetate, or sodium oxalate. The sulfonate includes at least one of sodium dodecylbenzenesulfonate, sulfonated polystyrene, or sulfonate-functionalized montmorillonite. The metal-organic framework compound includes at least one of ZIF-8, MIL-101, or UiO-66. Based on the mass of the second negative electrode material layer, the mass percentage of the negative electrode additive is 0.1% to 2%, and optionally, the mass percentage of the negative electrode additive is 0.5% to 1%. For example, the mass percentage content of the negative electrode additive can be 0.1%, 0.3%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or any two of these values. The mass percentage content of the negative electrode additive can be within the range of 0.1% to 2%, 0.3% to 1.8%, 0.5% to 1.5%, 0.5% to 1.2%, 0.5% to 1%, 0.6% to 0.8%, and all such ranges and sub-ranges. The second negative electrode material layer includes the aforementioned negative electrode additive, and its mass percentage content is controlled within the aforementioned range. The aforementioned negative electrode additive can adsorb and capture manganese ions dissolved from the positive electrode active material, reduce the penetration of manganese ions into the first negative electrode material layer, reduce the mass content of manganese in the first negative electrode material layer, reduce the risk of the SEI film on the surface of the negative electrode graphite material being damaged by manganese ions, and reduce the probability of gas generation reaction. At the same time, the content of the negative electrode additive in the second negative electrode material layer is low, while the content of the second negative electrode active material is high, resulting in a higher energy density of the secondary battery. This is beneficial for reducing the capacity decay of the secondary battery and improving the cycle performance of the secondary battery.

[0043] In one or more embodiments of this application, after the secondary battery is stored at 30% charge and 60°C for 30 days, the mass content of manganese in the first negative electrode material layer is W1 ppm based on the mass of the first negative electrode material layer, and the mass content of manganese in the second negative electrode material layer is W2 ppm based on the mass of the second negative electrode material layer, where 0 < W1 / W2 ≤ 0.2 and 0 < W1 ≤ 100. For example, the value of W1 / W2 can be 0.01, 0.03, 0.05, 0.08, 0.10, 0.12, 0.15, 0.18, 0.2, or a range consisting of any two of these values. The range of W1 / W2 can be 0.01 to 0.2, 0.01 to 0.18, 0.03 to 0.15, 0.05 to 0.12, 0.08 to 0.10, and all such ranges and sub-ranges. For example, the value of W1 can be 0.1, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, or a range of any two of these values. The value range of W1 can be 0.1 to 100, 1 to 90, 10 to 80, 20 to 70, 30 to 60, 40 to 50, and all of these ranges, as well as sub-ranges. When the secondary battery includes a positive electrode active material containing manganese, graphite is selected as the negative electrode active material. The capacity decay of the secondary battery is relatively fast when stored at high temperatures. In particular, after storage at 60°C, the storage capacity loss of the secondary battery near 30% state of charge (SOC) is greater than that at other states of charge, and the capacity retention rate is lower. Furthermore, according to the inductively coupled plasma (ICP) test results of the negative electrode sheet, the manganese content of the negative electrode sheet is also relatively high after storage at 60°C when the secondary battery is near 30% SOC. When the values ​​of W1 / W2 and W1 are within the above range, it indicates that after high-temperature storage, the second negative electrode material layer has a higher manganese content than the first negative electrode material layer. The thinner second negative electrode material layer can intercept most of the manganese ions, concentrating them in the second negative electrode material layer. Very few manganese ions are deposited in the first negative electrode material layer. Therefore, the first negative electrode material layer, which provides most of the capacity, is less affected by manganese ions. This is beneficial for protecting the first negative electrode active material, reducing the risk of the SEI film on the surface of the negative electrode graphite material being damaged by manganese ions, reducing the probability of gas generation reactions, and allowing the secondary battery to perform normally. This helps to reduce the capacity decay of the secondary battery, improve the cycle performance of the secondary battery, and at the same time, the secondary battery has good high-temperature storage performance.

[0044] In one or more embodiments of this application, along the thickness direction of the negative electrode sheet, the first negative electrode material layer includes a first region. The first region is located on the surface of the first negative electrode material layer away from the negative electrode current collector, and the thickness of the first region is 10 μm. After the secondary battery is stored at 30% charge and 60°C for 30 days, based on the mass of the first region, the mass content of manganese in the first region is less than 100 ppm. For example, the mass content of manganese in the first region can be 0 ppm, 10 ppm, 20 ppm, 30 ppm, 40 ppm, 50 ppm, 60 ppm, 70 ppm, 80 ppm, 99 ppm, or a range consisting of any two of these values. The range of the mass content of manganese in the first region can be less than 100 ppm, less than 90 ppm, less than 80 ppm, less than 70 ppm, less than 60 ppm, less than 50 ppm, less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm, and all such ranges and sub-ranges. When a secondary battery includes a positive electrode active material containing manganese, and graphite is chosen as the negative electrode active material, the capacity of the secondary battery decays rapidly during high-temperature storage. Specifically, after storage at 60°C, the capacity loss near 30% state of charge (SOC) is greater than at other SOC states, resulting in lower capacity retention. Furthermore, according to inductively coupled plasma (ICP) testing results of the negative electrode, the manganese content of the negative electrode is also relatively high after storage at 60°C when the secondary battery is near 30% SOC. The first region is located in the part of the first negative electrode material layer close to the second negative electrode material layer. The mass content of manganese in the first region is within the above-mentioned range, indicating that after the secondary battery is stored at high temperature, the mass content of manganese in the first negative electrode material layer is low. The second negative electrode material layer, which is resistant to manganese ions, can isolate and adsorb most of the manganese ions. It is difficult for manganese ions to pass through the second negative electrode material layer to reach the first negative electrode material layer. This helps to reduce the risk of the SEI film on the surface of the negative electrode graphite material being damaged by manganese ions, and reduces the probability of gas generation reaction. This helps to reduce the capacity decay of the secondary battery, improve the cycle performance of the secondary battery, and at the same time, the secondary battery has good high-temperature storage performance.

[0045] like Figure 2 As shown, along the thickness Y direction of the negative electrode sheet 100, the first negative electrode material layer 120 includes a first region 121 and a second region 122. The first region 121 is located on the surface of the first negative electrode material layer 120 away from the negative electrode current collector 110, and the thickness of the first region 121 is H3, where H3 = 10 μm. The second region 122 is located on the surface of the first negative electrode material layer 120 close to the negative electrode current collector 110.

[0046] In one or more embodiments of this application, the first negative electrode active material includes at least one of artificial graphite or natural graphite. Using the above-mentioned first negative electrode active material is beneficial for reducing the capacity decay of the secondary battery and improving its cycle performance.

[0047] In one or more embodiments of this application, the second negative electrode active material includes at least one of artificial graphite or natural graphite. Using the above-mentioned second negative electrode active material is beneficial for reducing the capacity decay of the secondary battery and improving its cycle performance.

[0048] In this application, the first negative electrode material layer includes a first negative electrode active material, and may also include a first negative electrode binder, a first negative electrode conductive agent, and a first dispersant. Based on the mass of the first negative electrode material layer, the mass percentage content of the first negative electrode active material can be 93% to 97%, the mass percentage content of the first negative electrode binder can be 1.5% to 3%, the mass percentage content of the first negative electrode conductive agent can be 0.3% to 4%, and the mass percentage content of the first dispersant can be 0% to 4%. For example, the mass percentage of the first negative electrode active material can be 93%, 94%, 95%, 96%, 97%, or any two of these values. The mass percentage range of the first negative electrode active material can be 93% to 97%, 94% to 97%, 95% to 97%, and all of these ranges, as well as sub-ranges. The mass percentage of the first negative electrode binder can be 1.5%, 1.7%, 1.9%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, or any two of these values. The mass percentage range of the first negative electrode binder can be 1.5% to 3%, 1.6% to 2.8%, 1.7% to 2.6%, 1.8% to 2.4%, 2% to 2.2%, and all of these ranges, as well as sub-ranges. The mass percentage of the first negative electrode conductive agent can be 0.3%, 0.7%, 1 ... The mass percentage of the first negative electrode conductive agent can be 0.3% to 4%, 0.5% to 3.5%, 0.8% to 3.0%, 1% to 2.8%, 1.2% to 2.5%, 1.5% to 2.2%, and all of these ranges, as well as sub-ranges; the mass percentage of the first dispersant can be 0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or any two of these ranges, and the mass percentage of the first dispersant can be 0% to 4%, 0.2% to 3.5%, 0.5% to 3.0%, 0.8% to 2.5%, 1% to 2.2%, 1.5% to 2.0%, and all of these ranges, as well as sub-ranges.

[0049] In this application, the second negative electrode material layer includes a second negative electrode active material, and may also include a second negative electrode binder, a second negative electrode conductive agent, a second dispersant, or may further include a second negative electrode binder, a second negative electrode conductive agent, a second dispersant, and a negative electrode additive. Based on the mass of the second negative electrode material layer, the mass percentage content of the second negative electrode active material may be 93% to 97%, the mass percentage content of the second negative electrode binder may be 1.5% to 3%, the mass percentage content of the second negative electrode conductive agent may be 0.5% to 4%, the mass percentage content of the second dispersant may be 0% to 4%, and the mass percentage content of the negative electrode additive is as described above. For example, the mass percentage of the second negative electrode active material can be 93%, 94%, 95%, 96%, 97%, or any two of these values. The mass percentage range of the second negative electrode active material can be 93% to 97%, 94% to 97%, 95% to 97%, and all of these ranges, as well as sub-ranges. The mass percentage of the second negative electrode binder can be 1.5%, 1.7%, 1.9%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, or any two of these values. The mass percentage range of the second negative electrode binder can be 1.5% to 3%, 1.6% to 2.8%, 1.7% to 2.6%, 1.8% to 2.4%, 2% to 2.2%, and all of these ranges, as well as sub-ranges. The mass percentage of the second negative electrode conductive agent can be 0.5%, 0.7%, 1 ... The mass percentage of the second negative electrode conductive agent can be 0.5% to 4%, 0.5% to 3.5%, 0.8% to 3.0%, 1% to 2.8%, 1.2% to 2.5%, 1.5% to 2.2%, and all of these ranges, as well as sub-ranges; the mass percentage of the second dispersant can be 0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or any two of these ranges, and the mass percentage of the second dispersant can be 0% to 4%, 0.2% to 3.5%, 0.5% to 3.0%, 0.8% to 2.5%, 1% to 2.2%, 1.5% to 2.0%, and all of these ranges, as well as sub-ranges.

[0050] This application does not impose any particular restrictions on the types of the first negative electrode binder, the first negative electrode conductive agent, the first dispersant, the second negative electrode binder, the second negative electrode conductive agent, and the second dispersant, as long as they can achieve the purpose of this application. For example, the first negative electrode binder and the second negative electrode binder may each independently include, but are not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. The first negative electrode conductive agent and the second negative electrode conductive agent may each independently include, but are not limited to, at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. Conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The first dispersant and the second dispersant may each independently include, but are not limited to, at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, or lithium carboxymethyl cellulose.

[0051] 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, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. For example, the composite current collector may be a lithium copper composite current collector, a carbon copper composite current collector, a nickel copper composite current collector, or a titanium copper composite current collector, etc. This application does not impose any particular limitation on the thickness of the negative electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector may be from 4 μm to 15 μm.

[0052] In one or more embodiments, the preparation of the first negative electrode active material may include, but is not limited to, the following steps: selecting flake natural graphite with a purity of 94% to 98% as a precursor, obtaining graphite powder after crushing and passivation, and subjecting the purified graphite powder to low-temperature graphitization treatment in a high-temperature furnace at 2000°C to 2800°C for 2 to 4 hours, and obtaining the first negative electrode active material after crushing.

[0053] In one or more embodiments, the preparation of the second negative electrode active material may include, but is not limited to, the following steps: selecting flake natural graphite with a purity of 97% to 99.9% as a precursor, obtaining graphite powder after crushing and passivation, and subjecting the purified graphite powder to high-temperature graphitization treatment in a high-temperature furnace at 2800°C to 3000°C for 2 to 8 hours, and obtaining the second negative electrode active material after crushing.

[0054] In one or more embodiments, the preparation of the negative electrode sheet may include, but is not limited to, the following steps: (1) preparing a first negative electrode slurry and a second negative electrode slurry; (2) coating the first negative electrode slurry onto one surface of the negative electrode current collector, drying it to form a first negative electrode material layer on one surface of the negative electrode current collector, and then coating the second negative electrode slurry onto the surface of the first negative electrode material layer, drying it to form a first negative electrode material layer and a second negative electrode material layer on one surface of the negative electrode current collector in sequence; or, using a dual extrusion spray nozzle coating machine to extrude and coat one surface of the negative electrode current collector, drying it to form a first negative electrode material layer and a second negative electrode material layer on one surface of the negative electrode current collector in sequence; (3) repeating the above steps on the other surface of the negative electrode current collector to form a first negative electrode material layer and a second negative electrode material layer on both surfaces of the negative electrode current collector; (4) obtaining the negative electrode sheet by cold pressing, cutting, and welding the negative electrode tabs.

[0055] In this application, the end-face ratio I1 of the first negative electrode active material can be controlled by adjusting the purity of the precursor, the temperature of the graphitization treatment, and the time of the graphitization treatment during the preparation process. For example, when other conditions remain unchanged, increasing the purity of the precursor decreases I1, and decreasing the purity of the precursor increases I1. When other conditions remain unchanged, increasing the temperature of the graphitization treatment decreases I1, and decreasing the temperature of the graphitization treatment increases I1. When other conditions remain unchanged, extending the time of the graphitization treatment decreases I1, and shortening the time of the graphitization treatment increases I1.

[0056] In this application, the end-face ratio I2 of the second negative electrode active material can be controlled by adjusting the purity of the precursor, the temperature of the graphitization treatment, and the time of the graphitization treatment during the preparation process. For example, when other conditions remain unchanged, increasing the purity of the precursor decreases I2, and decreasing the purity of the precursor increases I2. When other conditions remain unchanged, increasing the temperature of the graphitization treatment decreases I2, and decreasing the temperature of the graphitization treatment increases I2. When other conditions remain unchanged, extending the time of the graphitization treatment decreases I2, and shortening the time of the graphitization treatment increases I2.

[0057] In this application, Dv50 represents the particle size that reaches 50% of the volumetric particle size distribution of the material, starting from the smallest particle size. Anode active materials with different Dv50s can be obtained through mechanical crushing (e.g., ball milling). For example, the Dv50 of the anode active material, i.e., the D1 and D2 values, can be controlled by adjusting the ball milling time. When other conditions remain constant, extending the ball milling time of the first anode active material decreases the D1 value; shortening the ball milling time increases the D1 value. When other conditions remain constant, extending the ball milling time of the second anode active material decreases the D2 value; shortening the ball milling time increases the D2 value.

[0058] In one or more embodiments, the first negative electrode active material and the second negative electrode active material can be purchased, and the desired first negative electrode active material and the second negative electrode active material can be selected as needed.

[0059] In this application, the thickness H1 μm of the first negative electrode material layer can be controlled by adjusting the single-sided coating quality and the cold pressing pressure of the first negative electrode material layer. For example, when other conditions remain unchanged, increasing the single-sided coating quality of the first negative electrode material layer increases the H1 value; decreasing the single-sided coating quality decreases the H1 value. When other conditions remain unchanged, increasing the cold pressing pressure of the first negative electrode material layer decreases the H1 value; decreasing the cold pressing pressure increases the H1 value.

[0060] In this application, the thickness H2 μm of the second negative electrode material layer can be controlled by adjusting the single-sided coating quality and the cold pressing pressure of the second negative electrode material layer. For example, when other conditions remain unchanged, increasing the single-sided coating quality of the second negative electrode material layer increases the H2 value; decreasing the single-sided coating quality decreases the H2 value. When other conditions remain unchanged, increasing the cold pressing pressure of the second negative electrode material layer decreases the H2 value; decreasing the cold pressing pressure increases the H2 value.

[0061] In this application, the secondary battery includes a positive electrode sheet. The positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The phrase "positive electrode material layer disposed on at least one surface of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the positive current collector along its thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that the "surface" here can be the entire area of ​​the positive current collector or only a portion of it; this application does not have any particular limitation, as long as the purpose of this application is achieved.

[0062] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0063] The positive electrode material layer of this application further includes a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitations on the positive electrode conductive agent and positive electrode binder in the positive electrode material layer, as long as the purpose of this application can be achieved. For example, the positive electrode conductive agent can be at least one of the aforementioned first negative electrode conductive agents. For example, the positive electrode binder can be, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. This application does not impose any particular limitations on the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved. For example, the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer can be (95 to 98): (0.5 to 2.5): (1.5 to 3.4).

[0064] This application does not impose any particular limitation on the thickness of the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector can be from 6 μm to 25 μm. This application also does not impose any particular limitation on the thickness of the positive electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of a single-sided positive electrode material layer can be from 50 μm to 120 μm.

[0065] Optionally, the positive electrode may further include a conductive layer located between the positive current collector and the positive electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer; for example, the conductive agent may be at least one of the aforementioned positive electrode conductive agents, and the binder may be at least one of the aforementioned positive electrode binders.

[0066] In this application, the secondary battery includes a separator. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the material of the separator 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 may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0067] In some embodiments, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used.

[0068] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0069] In some embodiments, the inorganic layer comprises ceramic particles and an inorganic layer binder. This application does not particularly limit the ceramic particles; for example, the ceramic particles may include at least one selected from silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the inorganic layer binder; for example, the inorganic layer binder may be at least one of the above-mentioned positive electrode binders. In some embodiments, the polymer layer comprises a polymer, and the polymer material may include, but is not limited to, at least one selected from polyamide, polyacrylonitrile, acrylate polymers, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0070] 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 3 μm to 20 μm.

[0071] In this application, the secondary battery also includes an electrolyte, which includes lithium salts and non-aqueous solvents.

[0072] This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application also does not impose any particular limitation on the content of the lithium salt in the electrolyte, as long as it achieves the purpose of this application.

[0073] This application does not impose any particular restrictions on non-aqueous solvents, as long as they can achieve the purpose of this application. For example, non-aqueous solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents.

[0074] The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonate compounds may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). 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, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0075] The secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0076] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. In some embodiments, the secondary battery may include, but is not limited to, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0077] The preparation process of the secondary battery in this application is well known to those skilled in the art, and this application has no particular limitations. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, negative electrode, and separator in sequence, and winding, folding, etc., as needed to obtain a wound electrode assembly; placing the electrode assembly into the housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery; or stacking the positive electrode, separator, negative electrode, and separator 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 the housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery.

[0078] The second aspect of this application provides an electronic device that includes the secondary battery provided in the first aspect of this application. The electronic device of this application has a long service life and good performance.

[0079] 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 input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0080] Example

[0081] 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.

[0082] Test methods and equipment:

[0083] End face ratio test

[0084] The lithium-ion battery was placed in an environment of 25℃ and discharged at a constant current of 0.2C to 3.0V. The negative electrode was then removed from the lithium-ion battery. The negative electrode was cleaned with dimethyl carbonate (DMC) and dried at 60℃ to obtain a negative electrode sample. Powders of the first and second negative electrode material layers were scraped off with a scraper, and Raman spectrometry was used to test the powders of the first and second negative electrode active materials, respectively, to obtain their Raman spectra. In the Raman spectrum, the Raman shift was at 1350 cm⁻¹. -1 The peak at position I represents peak D, and its intensity is denoted as I. D Raman shift at 1580cm -1 The peak at that location represents the G peak, and its intensity is denoted as I. G , end face ratio I=I D / I G Thus, the end-face ratio I1 of the first negative electrode active material and the end-face ratio I2 of the second negative electrode active material were obtained by testing.

[0085] Dv50 Test

[0086] According to the national standard "Particle Size Distribution Laser Diffraction Method" (GB / T19077-2016), the Dv50 of the first and second negative electrode active materials was tested using a laser particle size analyzer (model MS3000) to obtain D1 and D2.

[0087] Thickness test

[0088] The lithium-ion battery was discharged at a constant current of 0.2C to 3.0V, and the negative electrode was removed after disassembly. The negative electrode was cleaned with dimethyl carbonate (DMC) and dried at 60°C to obtain a negative electrode sample. The cross-section of the negative electrode sample along its thickness direction was polished with argon ions. Then, the cross-section of the negative electrode sample was observed using a scanning electron microscope (SEM), and the thicknesses of the first and second negative electrode material layers at five locations were measured. The average values ​​were calculated as the thickness H1 of the first negative electrode material layer and the thickness H2 of the second negative electrode material layer.

[0089] Manganese content test

[0090] A lithium-ion battery is placed in a 25°C environment and charged at a constant current of 0.2C to 4.2V. Then, it is charged at a constant voltage of 4.2V to 0.05C. After resting for 5 minutes, it is discharged at a constant current of 0.2C to 3V. This constitutes one charge-discharge cycle, which is the first cycle. The discharge capacity of the first cycle is denoted as the initial discharge capacity R. The ratio of the remaining capacity of the lithium-ion battery after discharge to the initial discharge capacity is defined as the state of charge (SOC).

[0091] The lithium-ion battery was placed in a 25°C environment and discharged at a constant current of 0.2C to 3V. After resting for 5 minutes, it was charged at a constant current of 0.2C for 1.5 hours to bring the lithium-ion battery to 30% SOC. It was then stored in a 60°C oven for 30 days before being removed. The lithium-ion battery was then discharged at a constant current of 0.2C to 3.0V, and the negative electrode was removed. The negative electrode was cleaned with dimethyl carbonate (DMC) and dried at 60°C to obtain a negative electrode sample.

[0092] The negative electrode sheet was ion-beam dicing along its thickness direction using an ion beam cutter to obtain a smooth interface along the thickness direction. The interface was then observed and the manganese content determined using a scanning electron microscope-energy dispersive spectroscopy (EDS). The first negative electrode material layer, the second negative electrode material layer, and the first region of the first negative electrode material layer were located using the scanning electron microscope. The manganese content (W1 ppm) in the first negative electrode material layer, the manganese content (W2 ppm) in the second negative electrode layer, and the manganese content in the first region of the first negative electrode material layer were measured using EDS.

[0093] High-temperature cycling performance test

[0094] The lithium-ion battery was placed in a 45°C environment and charged at a constant current of 0.5C to 4.2V. Then, it was charged at a constant voltage of 4.2V to 0.05C. After resting for 5 minutes, it was discharged at a constant current of 0.5C to 3V. This constitutes one charge-discharge cycle, which is the first cycle. The discharge capacity of the first cycle is recorded as the first cycle discharge capacity. The lithium-ion battery was cycled using the above method, and the discharge capacity of each cycle was recorded. The ratio of the discharge capacity of each cycle to the discharge capacity of the first cycle was used to obtain the cycle capacity decay curve. When the ratio of the lithium-ion battery's discharge capacity to the first cycle discharge capacity in the cycle capacity decay curve reached 80%, the number of charge-discharge cycles at this point was recorded as the 45°C cycle number.

[0095] Example 1-1

[0096] <Preparation of Negative Electrode Sheets>

[0097] (1) Preparation of the first negative electrode active material: 96% pure flake natural graphite was selected as the precursor. After crushing and passivation, graphite powder was obtained. The purified graphite powder was subjected to low-temperature graphitization treatment in a high-temperature furnace at 2300℃ for 3 hours. After crushing, the first negative electrode active material was obtained.

[0098] Preparation of the second negative electrode active material: 99% pure flake natural graphite was selected as the precursor. After crushing and passivation, graphite powder was obtained. The purified graphite powder was subjected to high-temperature graphitization treatment in a high-temperature furnace at 2800℃ for 4 hours. After crushing, the second negative electrode active material was obtained.

[0099] (2) The first negative electrode active material artificial graphite, the first negative electrode conductive agent acetylene black, the first negative electrode binder styrene-butadiene rubber, and the first dispersant sodium carboxymethyl cellulose are mixed in a mass ratio of 96.8:0.5:1.7:1, and deionized water is added as a solvent to prepare a slurry with a solid content of 45wt%. The slurry is then stirred evenly in a vacuum mixer to obtain the first negative electrode slurry.

[0100] (3) The second negative electrode active material artificial graphite, the second negative electrode conductive agent acetylene black, the second negative electrode binder styrene-butadiene rubber, and the second dispersant sodium carboxymethyl cellulose are mixed in a mass ratio of 96.8:0.5:1.7:1, and deionized water is added as a solvent to prepare a slurry with a solid content of 45wt%. The slurry is then stirred evenly in a vacuum mixer to obtain the second negative electrode slurry.

[0101] (4) The first negative electrode slurry is uniformly coated onto one surface of a copper foil with a negative electrode current collector thickness of 8 μm, and dried at 90°C to obtain a negative electrode sheet with a single-sided coating of the first negative electrode material layer. Then, the second negative electrode slurry is uniformly coated onto the surface of the first negative electrode material layer away from the negative electrode current collector, and dried at 90°C to obtain a negative electrode sheet with a single-sided coating of the first and second negative electrode material layers. The above steps are then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of the first and second negative electrode material layers. After cold pressing, cutting, and welding of tabs, a negative electrode sheet with dimensions of 78 mm × 875 mm is obtained for later use. The compaction density of the first negative electrode material layer is 1.5 g / cm³. 3 The compaction density of the second negative electrode material layer is 1.6 g / cm³. 3 The end face ratios I1 and I2 of the first negative electrode active material are shown in Table 1, and the D1, D2, thickness H1, and thickness H2 of the first negative electrode active material and the second negative electrode active material are shown in Table 2.

[0102] <Preparation of the positive electrode>

[0103] Lithium manganese oxide (positive electrode active material), conductive carbon black (Super P) (positive electrode conductive agent), and polyvinylidene fluoride (PVDF) (positive electrode binder) were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%. After vacuum stirring, the slurry was obtained. The positive electrode slurry was uniformly coated onto one surface of a 15 μm thick aluminum foil current collector and dried at 120°C to obtain a positive electrode sheet with a single-sided coating of positive electrode material. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying at 120°C, the sheet was cold-pressed, cut, and had tabs welded to obtain a positive electrode sheet with dimensions of 74 mm × 867 mm for later use. The thickness of the single-sided positive electrode material layer was 230 μm, and the compaction density of the positive electrode material layer was 2.7 g / cm³. 3 .

[0104] <Isolation membrane>

[0105] A 7μm thick porous polyethylene polymer film (manufacturer: Celgard Diaphragm Company, USA) was used as the separator.

[0106] <Preparation of Electrolyte>

[0107] In an argon-atmospheric glove box with a water content of less than 10 ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were uniformly mixed at a mass ratio of EC:EMC:DEC = 3:5:2 to obtain a base solvent. Then, lithium hexafluorophosphate (LiPF6) was added to the base solvent and stirred until homogeneous to obtain the electrolyte. The total mass of the electrolyte contained 12.5% ​​LiPF6, with the remainder being the base solvent.

[0108] <Preparation of Lithium-ion Batteries>

[0109] The positive electrode, separator, negative electrode, and separator are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is then wound to form the electrode assembly. The electrode assembly is placed in an aluminum-plastic film and dehydrated at 80°C. A prepared electrolyte is then injected. After vacuum sealing, settling, formation, and shaping processes, a lithium-ion battery is obtained. The formation upper limit voltage is 4.15V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0110] Examples 1-2 to Examples 1-15

[0111] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Example 1-1. Specifically, the end-face ratio I1 of the first negative electrode active material is controlled by adjusting the purity of the precursor, the temperature and time of graphitization treatment during the preparation of the first negative electrode active material, and the end-face ratio I2 of the second negative electrode active material is controlled by adjusting the purity of the precursor, the temperature and time of graphitization treatment during the preparation of the second negative electrode active material.

[0112] Examples 2-1 to 2-17

[0113] Except for adjusting the corresponding preparation parameters according to Table 2, the rest is the same as in Examples 1-1. Specifically, the D1 value is controlled by adjusting the ball milling time of the first negative electrode active material, the D2 value is controlled by adjusting the ball milling time of the second negative electrode active material, the H1 value is controlled by adjusting the single-sided coating quality of the first negative electrode material layer, and the H2 value is controlled by adjusting the single-sided coating quality of the second negative electrode material layer; when the thickness of the negative electrode material layer changes, the thickness of the positive electrode material layer changes accordingly, keeping the N / P ratio constant.

[0114] Example 3-1

[0115] Except for step (3) of <Preparation of Negative Electrode Sheet>, in which the second negative electrode active material artificial graphite, the negative electrode additive lithium oxalate, the second negative electrode conductive agent acetylene black, the second negative electrode binder styrene-butadiene rubber, and the second dispersant sodium carboxymethyl cellulose are mixed in a mass ratio of 96.7:0.1:0.5:1.7:1, the rest is the same as in Example 1-1.

[0116] Examples 3-2 to 3-7

[0117] Except for adjusting the corresponding preparation parameters according to Table 3, the rest is the same as in Example 3-1. Specifically, when the mass percentage of the negative electrode additive changes, the mass percentage of the second negative electrode active material changes accordingly, while the mass percentages of the second negative electrode conductive agent, the second negative electrode binder, and the second dispersant remain unchanged. The sum of the mass percentages of the second negative electrode active material, the negative electrode additive, the second negative electrode conductive agent, the second negative electrode binder, and the second dispersant is 100%.

[0118] Comparative Example 1

[0119] Except for the preparation method of the negative electrode sheet, the rest is the same as in Example 1-1.

[0120] <Preparation of Negative Electrode Sheets>

[0121] Preparation of the first negative electrode active material: 96% pure flake natural graphite was selected as the precursor. After crushing and passivation, graphite powder was obtained. The purified graphite powder was subjected to low-temperature graphitization treatment in a high-temperature furnace at 2300℃ for 3 hours. After crushing, the first negative electrode active material was obtained.

[0122] The first negative electrode active material, artificial graphite, the first negative electrode conductive agent, acetylene black, the first negative electrode binder, styrene-butadiene rubber, and the first dispersant, sodium carboxymethyl cellulose, were mixed in a mass ratio of 96.8:0.5:1.7:1. Deionized water was added as a solvent to prepare a slurry with a solid content of 45wt%. The slurry was then stirred evenly in a vacuum mixer to obtain the first negative electrode slurry.

[0123] The first negative electrode slurry was uniformly coated onto one surface of an 8μm thick copper foil current collector and dried at 90℃ to obtain a negative electrode sheet with a single-sided coating of the first negative electrode material layer. The above steps were then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of the first negative electrode material layer. After cold pressing, cutting, and welding of tabs, a negative electrode sheet with dimensions of 78mm × 875mm was obtained for later use. The compaction density of the first negative electrode material layer was 1.5 g / cm³. 3 The end face ratio I1 of the first negative electrode active material is shown in Table 1. The D1 of the first negative electrode active material is the same as in Example 1-1, and the thickness H1 of the first negative electrode material layer is 195 μm.

[0124] Comparative Example 2

[0125] Except for the preparation method of the negative electrode sheet, the rest is the same as in Example 1-1.

[0126] <Preparation of Negative Electrode Sheets>

[0127] Preparation of the second negative electrode active material: 99% pure flake natural graphite was selected as the precursor. After crushing and passivation, graphite powder was obtained. The purified graphite powder was subjected to high-temperature graphitization treatment in a high-temperature furnace at 2800℃ for 4 hours. After crushing, the second negative electrode active material was obtained.

[0128] The second negative electrode active material, artificial graphite, the second negative electrode conductive agent, acetylene black, the second negative electrode binder, styrene-butadiene rubber, and the second dispersant, sodium carboxymethyl cellulose, were mixed in a mass ratio of 96.8:0.5:1.7:1. Deionized water was added as a solvent to prepare a slurry with a solid content of 45wt%. The slurry was then stirred evenly in a vacuum mixer to obtain the second negative electrode slurry.

[0129] The second negative electrode slurry was uniformly coated onto one surface of an 8μm thick copper foil current collector and dried at 90℃ to obtain a negative electrode sheet with a single-sided coating of the second negative electrode material layer. The above steps were then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of the second negative electrode material layer. After cold pressing, cutting, and welding of tabs, a negative electrode sheet with dimensions of 78mm × 875mm was obtained for later use. The compaction density of the second negative electrode material layer was 1.6 g / cm³. 3 The end face ratio I2 of the second negative electrode active material is shown in Table 1. The D2 of the second negative electrode active material is the same as in Example 1-1, and the thickness H2 of the second negative electrode material layer is 195 μm.

[0130] Comparative Examples 3 to 5

[0131] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Example 1-1. Specifically, the end-face ratio I1 of the first negative electrode active material is controlled by adjusting the purity of the precursor, the temperature and time of graphitization treatment during the preparation of the first negative electrode active material, and the end-face ratio I2 of the second negative electrode active material is controlled by adjusting the purity of the precursor, the temperature and time of graphitization treatment during the preparation of the second negative electrode active material.

[0132] Comparative Example 6

[0133] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as Comparative Example 1.

[0134] Comparative Example 7

[0135] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as Comparative Example 2.

[0136] Comparative Examples 8 to 10

[0137] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Examples 1-15. Specifically, the end-face ratio I1 of the first negative electrode active material is controlled by adjusting the purity of the precursor, the temperature and time of graphitization treatment during the preparation of the first negative electrode active material, and the end-face ratio I2 of the second negative electrode active material is controlled by adjusting the purity of the precursor, the temperature and time of graphitization treatment during the preparation of the second negative electrode active material.

[0138] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0139] Table 1

[0140] Note: " / " in Table 1 indicates that there is no corresponding parameter.

[0141] Table 2

[0142] As can be seen from Examples 1-1 to 1-15 and Comparative Examples 1 to 10, this application designs the structure of the negative electrode sheet and adjusts I1 and I2 to satisfy the above relationship, resulting in a larger number of 45°C cycle cycles for the lithium-ion battery, indicating that the lithium-ion battery of this application has good high-temperature cycle performance. However, the negative electrode sheets of Comparative Examples 1 to 2 and Comparative Examples 6 to 7 do not simultaneously include a first negative electrode material layer and a second negative electrode material layer, and the I1 and I2 of Comparative Examples 3 to 5 and Comparative Examples 8 to 10 do not satisfy I1 > I2. Consequently, the lithium-ion batteries of Comparative Examples 1 to 10 have a smaller number of 45°C cycle cycles, indicating poor high-temperature cycle performance.

[0143] The types of I1, I2, the first negative electrode active material, the second negative electrode active material, and the positive electrode active material affect the high-temperature cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-15, within the scope of this application, the lithium-ion battery exhibits a relatively large number of cycles at 45°C, indicating that the lithium-ion battery of this application has good high-temperature cycle performance.

[0144] The values ​​of D2 / D1, D2, H1 / H2, and H2 affect the high-temperature cycling performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-17, when the values ​​of D2 / D1, D2, H1 / H2, and H2 are within the range of this application, the lithium-ion battery has a higher number of cycles at 45°C, indicating good high-temperature cycling performance. Among these, a higher H1 value results in relatively poorer cycling performance, but increased energy density (e.g., Example 2-11).

[0145] Table 3

[0146] Note: " / " in Table 3 indicates that there is no corresponding parameter.

[0147] The type and mass percentage of the negative electrode additive affect the high-temperature cycle performance of the lithium-ion battery. As can be seen from Examples 1-1, 3-1 to 3-7, within the scope of this application, the type and mass percentage of the negative electrode additive result in a higher number of cycles at 45°C compared to the lithium-ion battery without the addition of a negative electrode additive, indicating that the lithium-ion battery of this application has good high-temperature cycle performance.

[0148] 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.

[0149] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0150] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising a negative electrode and a positive electrode, wherein the negative electrode includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector, the negative electrode material layer comprising a first negative electrode material layer and a second negative electrode material layer stacked thereon, the first negative electrode material layer being located between the second negative electrode material layer and the negative current collector, the first negative electrode material layer comprising a first negative electrode active material, and the second negative electrode layer comprising a second negative electrode active material. The first negative electrode active material and the second negative electrode active material are graphite materials with different end-face ratios. The end-face ratio of the first negative electrode active material is I1, and the end-face ratio of the second negative electrode active material is I2, where I1 > I2.

2. The secondary battery according to claim 1, wherein, The secondary battery satisfies at least one of the following characteristics: (1) 0.12≤I1≤0.3, preferably, 0.15≤I1≤0.25; (2) 0.05≤I2≤0.15, preferably 0.05≤I2≤0.

12.

3. The secondary battery according to claim 1 or 2, wherein, The Dv50 of the first negative electrode active material is D1 μm, and the Dv50 of the second negative electrode active material is D2 μm, where 0.5≤D2 / D1≤1 and 5≤D2≤20.

4. The secondary battery according to claim 3, wherein, The secondary battery satisfies at least one of the following characteristics: (1) 0.7≤D2 / D1≤1; （2）8≤D2≤15。 5. The secondary battery according to claim 1, wherein, The positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material containing Mn element. The positive electrode active material containing Mn element includes at least one of lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium manganese iron phosphate, or lithium-rich manganese-based material.

6. The secondary battery according to claim 1, wherein, The thickness of the first negative electrode material layer is H1 μm, the thickness of the second negative electrode material layer is H2 μm, 2≤H1 / H2≤20, and 5≤H2≤40.

7. The secondary battery according to claim 6, wherein, 8≤H1 / H2≤15, 10≤H2≤20.

8. The secondary battery according to any one of claims 1 to 7, wherein, The second negative electrode material layer includes a negative electrode additive, which includes at least one of a carboxylate, a sulfonate, or a metal-organic framework compound. The carboxylate includes at least one of lithium oxalate, sodium citrate, sodium ethylenediaminetetraacetate, or sodium oxalate. The sulfonate includes at least one of sodium dodecylbenzenesulfonate, sulfonated polystyrene, or sulfonate-functionalized montmorillonite. The metal-organic framework compound includes at least one of ZIF-8, MIL-101, or UiO-66. Based on the mass of the second negative electrode material layer, the mass percentage of the negative electrode additive is 0.1% to 2%.

9. The secondary battery according to claim 8, wherein, Based on the mass of the second negative electrode material layer, the mass percentage of the negative electrode additive is 0.5% to 1%.

10. The secondary battery according to any one of claims 1 to 9, wherein, After the secondary battery is stored at 30% charge and 60°C for 30 days, the mass content of manganese in the first negative electrode material layer is W1 ppm based on the mass of the first negative electrode material layer, and the mass content of manganese in the second negative electrode material layer is W2 ppm based on the mass of the second negative electrode material layer, where 0 < W1 / W2 ≤ 0.2 and 0 < W1 ≤ 100.

11. The secondary battery according to any one of claims 1 to 10, wherein, Along the thickness direction of the negative electrode sheet, the first negative electrode material layer includes a first region, which is located on the surface of the first negative electrode material layer away from the negative electrode current collector, and the thickness of the first region is 10 μm; After the secondary battery is stored at 30% charge and 60°C for 30 days, based on the mass of the first region, the mass content of manganese in the first region is less than 100 ppm.

12. The secondary battery according to any one of claims 1 to 11, wherein, The secondary battery satisfies at least one of the following characteristics: (1) The first negative electrode active material includes at least one of artificial graphite or natural graphite; (2) The second negative electrode active material includes at least one of artificial graphite or natural graphite.

13. An electronic device comprising a secondary battery according to any one of claims 1 to 12.

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