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 carbon materials with different graphitization degrees, the graphitization degree of the first negative electrode active material is G1%, the graphitization degree of the second negative electrode active material is G2%, G1 is more than or equal to 90 and less than or equal to 98, and G2 is more than or equal to 5 and less than or equal to 78. The secondary battery provided by the invention has good high-temperature storage performance and 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 can penetrate the separator and deposit on the surface of the negative electrode, severely damaging the solid electrolyte interphase (SEI) film and affecting the high-temperature storage and cycle performance of lithium-ion batteries. Summary of the Invention

[0003] The purpose of this application is to provide a secondary battery and electronic device to improve the high-temperature storage performance and 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 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 carbon materials with different degrees of graphitization. The degree of graphitization of the first negative electrode active material is G1%, and the degree of graphitization of the second negative electrode active material is G2%, where 90≤G1≤98 and 5≤G2≤78. This application designs the structure of the negative electrode sheet and controls the values ​​of G1 and G2 within the above range. Compared with the first negative electrode active material, the second negative electrode active material has a lower degree of graphitization, which can reduce the amount of manganese ions dissolved from the positive electrode active material entering the first negative electrode material layer, reduce the probability of SEI film damage and gas generation reaction, reduce the capacity decay of the secondary battery during high-temperature storage, and thus improve the high-temperature storage performance and 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) 93 ≤ G1 ≤ 98; (2) 10 ≤ G2 ≤ ​​50. By adjusting the values ​​of G1 and G2 within the above range, it is beneficial to further improve the high-temperature storage performance and cycle performance of the secondary battery.

[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 ranges, it is beneficial to improve the high-temperature storage performance and 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 ranges, it is beneficial to further improve the high-temperature storage performance and cycle performance of the secondary battery.

[0008] 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 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, and improved electrochemical performance. Simultaneously, the secondary battery exhibits good high-temperature storage performance and 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 improve the high-temperature storage performance and cycle performance of the secondary battery, while the energy density of the secondary battery is relatively high.

[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 improve the high-temperature storage performance and cycle performance of the secondary battery, while also achieving a higher energy density.

[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 content within the above range, is beneficial to reduce the penetration of manganese ions into the first negative electrode material layer, improve the high-temperature storage performance and cycle performance of the secondary battery, and at the same time, the energy density of the secondary battery is high.

[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 improving the high-temperature storage performance and cycle performance of the secondary battery, while also resulting in a higher energy density.

[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 improving the high-temperature storage performance and cycle performance of the secondary battery.

[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 improving the high-temperature storage performance and cycle performance of the secondary battery.

[0015] In one or more embodiments of this application, after the secondary battery is stored at 20% to 50% state of charge and 60°C for 30 days, the capacity retention rate of the secondary battery is C1; after the secondary battery is stored at 70% to 100% state of charge and 60°C for 30 days, the capacity retention rate of the secondary battery is C2, where 0 ≤ C2 - C1 ≤ 3%. The value of C2 - C1 being within the above range indicates that the capacity decay of the secondary battery is small during high-temperature storage, and the high-temperature storage performance of the secondary battery is good.

[0016] 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, natural graphite, soft carbon, hard carbon, or mesophase carbon microspheres; (2) the second negative electrode active material includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, or mesophase carbon microspheres. Using the above-mentioned first and / or second negative electrode active materials is beneficial to improving the high-temperature storage performance and cycle performance of the secondary battery.

[0017] 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.

[0018] The beneficial effects of this application are:

[0019] 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 and second negative electrode active materials are carbon materials with different degrees of graphitization. The degree of graphitization of the first negative electrode active material is G1%, and the degree of graphitization of the second negative electrode active material is G2%, where 90≤G1≤98 and 5≤G2≤78. This application designs the structure of the negative electrode sheet and controls the values ​​of G1 and G2 within the above range. Compared with the first negative electrode active material, the second negative electrode active material has a lower degree of graphitization, which can reduce the amount of manganese ions dissolved from the positive electrode active material entering the first negative electrode material layer, reduce the probability of SEI film damage and gas generation reaction, reduce the capacity decay of the secondary battery during high-temperature storage, and thus improve the high-temperature storage performance and cycle performance of the secondary battery.

[0020] 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

[0021] 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.

[0022] 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;

[0023] Figure 2 This is a schematic diagram of the negative electrode sheet along its thickness and length directions, representing another embodiment of this application.

[0024] 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

[0025] 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.

[0026] 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.

[0027] The first aspect of this application provides a secondary battery, which 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 carbon materials with different degrees of graphitization. The degree of graphitization of the first negative electrode active material is G1%, and the degree of graphitization of the second negative electrode active material is G2%. 90≤G1≤98, 5≤G2≤78, and optionally, 93≤G1≤98, 10≤G2≤50. For example, the value of G1 can be 90, 91, 92, 93, 94, 95, 96, 97, 98, or a range consisting of any two of these values. The value range of G1 can be 90 to 98, 91 to 98, 92 to 98, 93 to 98, 94 to 98, 95 to 97, and all of these ranges, as well as subranges. For example, the value of G2 can be 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 78, or a range consisting of any two of these values. The value range of G2 can be 5 to 78, 7 to 70, 10 to 60, 10 to 50, 10 to 40, 15 to 40, 20 to 35, and all of these ranges, as well as subranges.

[0028] During the charging, discharging, and storage stages, manganese ions can leach out from positive electrode active materials containing manganese. These dissolved manganese ions can pass through the separator and deposit on the surface of the negative electrode, severely damaging the SEI film. This leads to increased impedance, significant capacity loss, and severe gas generation in the secondary battery, thereby affecting its high-temperature storage performance and cycle performance.

[0029] The inventors discovered that when a secondary battery includes a manganese-containing positive electrode active material, using a carbon material with a high degree of graphitization (such as graphite) as the negative electrode active material results in rapid capacity decay during high-temperature storage. Specifically, after storage at 60°C, the capacity loss near 30% state of charge (SOC) is greater and the capacity retention is lower compared to other SOC states. Furthermore, according to inductively coupled plasma (ICP) testing of the negative electrode, the manganese content of the negative electrode is also relatively high after storage at 60°C near 30% SOC. The significant capacity loss near 30% SOC is related to the lithium intercalation state of the negative electrode active material, i.e., to the degree of graphitization of the carbon material.

[0030] Carbon materials with low graphitization (such as soft carbon) primarily store lithium through the reversible adsorption / desorption of lithium ions on their surface and in micropores, without a significant lithium intercalation state. When secondary batteries include manganese-containing positive electrode active materials, using carbon materials with low graphitization as negative electrode active materials results in minimal capacity difference between secondary batteries with 30% SOC and 100% SOC after high-temperature storage. However, carbon materials with low graphitization exhibit lower initial efficiency and lower packing density, which can negatively impact the energy density of secondary batteries when used independently as negative electrode active materials.

[0031] Based on this, this application designs the structure of the negative electrode sheet and controls the values ​​of G1 and G2 within the aforementioned range. Compared to the first negative electrode active material, the second negative electrode active material has a lower degree of graphitization. The second negative electrode active material with a low degree of graphitization is less affected by manganese ions dissolved from the positive electrode active material. The second negative electrode material layer, located far from the negative electrode current collector, can act as an isolation layer to protect the first 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 reduces the probability of SEI film damage and gas generation reactions, and decreases the risk of secondary batteries being damaged during high-temperature storage. The capacity decay is reduced, especially the capacity decay of the secondary battery when stored at high temperature near 30% SOC, thus extending the cycle life of the secondary battery. At the same time, the high graphitization degree of the first negative electrode active material in the first negative electrode material layer is conducive to the efficient insertion and extraction of lithium ions, and also helps to improve the compaction density of the negative electrode material layer, thereby improving the initial coulombic efficiency, rate performance, thermal stability and cycle stability of the secondary battery, reducing the manufacturing cost of the secondary battery, and thus improving the high temperature storage performance and cycle performance of the secondary battery. Meanwhile, the initial coulombic efficiency, rate performance and thermal stability of the secondary battery are good.

[0032] When the value of G1 is too small, for example, less than 90, the graphitization of the first negative electrode active material is too low, affecting the initial coulombic efficiency of the secondary battery, deteriorating rate performance, thermal stability, and cycle stability, thus affecting the cycle life of the secondary battery. The secondary battery cannot simultaneously achieve high-temperature storage performance and cycle performance. When the value of G1 is too large, for example, greater than 98, the preparation cost of the first negative electrode active material is too high, and the process is difficult to implement, making the preparation of the secondary battery difficult and costly. The first negative electrode active material with excessive graphitization may also affect the formation of the SEI film, resulting in insufficient SEI film formation, affecting the high-temperature stability of the secondary battery, and the cycle performance of the secondary battery is poor. When the value of G2 is too small, for example, less than 5, the second negative electrode active material is prone to structural changes at high temperatures, resulting in poor thermal stability and increasing the risk of thermal runaway of the secondary battery. At the same time, the second negative electrode material layer will consume too much active lithium, deteriorating the cycle performance. The secondary battery cannot simultaneously achieve high-temperature storage performance and cycle performance. When the value of G2 is too large, such as greater than 78, the second negative electrode active material is greatly affected by the deposition of manganese due to the change of lithium intercalation state. The ability of the second negative electrode material layer to adsorb and deposit manganese ions is also reduced. Excessive manganese ions enter the first negative electrode material layer, which deteriorates the high-temperature storage performance and cycle performance of the secondary battery.

[0033] Therefore, this application, by designing the structure of the negative electrode sheet and controlling the values ​​of G1 and G2 within the aforementioned range, can simultaneously leverage the advantages of both the first and second negative electrode active materials, improving the high-temperature storage performance and cycle performance of the secondary battery. Simultaneously, the secondary battery exhibits high initial coulombic efficiency, good rate performance and thermal stability, and low manufacturing cost. In this application, high temperature refers to a temperature ≥45℃.

[0034] In one or more embodiments, 12 ≤ G1-G2 ≤ 93. For example, the value of G1-G2 can be 12, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 93 or a range of any two of these values. The value range of G1-G2 can be 12 to 93, 20 to 90, 30 to 85, 40 to 80, 50 to 75, 60 to 70, and all of these ranges, as well as subranges.

[0035] 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.

[0036] 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 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, or a range consisting of any two of these values. 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, a larger specific surface area, more active sites, and a stronger adsorption capacity for manganese ions compared to the first negative electrode active material. This results in a greater number of manganese ions being adsorbed, concentrating in the manganese-resistant second negative electrode material layer. This helps reduce the impact of manganese ions dissolved from the positive electrode active material on the first negative electrode material layer, reducing capacity decay of the secondary battery during high-temperature storage. It also helps reduce side reactions between the electrolyte and the negative electrode sheet, lowering the risk of agglomeration of the first and second negative electrode active materials during negative electrode sheet preparation. Therefore, it improves the high-temperature storage performance of the secondary battery and also enhances its cycle life.

[0037] 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.

[0038] 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), LiNi 0.91 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). Using the above-mentioned positive electrode active material in a secondary battery results in low manufacturing cost, high plateau voltage, and improved electrochemical performance. Simultaneously, the secondary battery exhibits good high-temperature storage performance and cycle performance.

[0039] 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 of these values. 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 results in a higher compaction density of the negative electrode material layer and a higher initial coulombic efficiency of the secondary battery, which is beneficial for improving the energy density 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 probability of SEI film damage and gas generation reactions, and reducing capacity decay of the secondary battery during high-temperature storage. This is beneficial for improving the high-temperature storage performance and cycle performance of the secondary battery, while also resulting in a higher energy density. 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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 probability of SEI film damage and gas generation reaction, and reduce the capacity decay of the secondary battery during high-temperature storage. 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 to improving the high-temperature storage performance and cycle performance of the secondary battery, while also increasing the energy density of the secondary battery.

[0044] 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 any range of two 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 values ​​of W1 / W2 and W1 are within the above ranges, 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, which helps protect the first negative electrode active material and allows the secondary battery to perform normally, thereby improving the high-temperature storage performance and cycle life of the secondary battery.

[0045] 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. 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 can isolate and adsorb most of the manganese ions, and 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 probability of SEI film damage and gas generation reaction, reduce the capacity decay of the secondary battery during high temperature storage, and thus help to improve the high temperature storage performance and cycle performance of the secondary battery.

[0046] 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.

[0047] In one or more embodiments of this application, after the secondary battery is stored at 20% to 50% state of charge and 60°C for 30 days, the capacity retention rate of the secondary battery is C1; after the secondary battery is stored at 70% to 100% state of charge and 60°C for 30 days, the capacity retention rate of the secondary battery is C2, where 0 ≤ C2 - C1 ≤ 3%. For example, the value of C2 - C1 can be 0%, 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, or a range consisting of any two of these values. The value range of C2 - C1 can be 0% to 3%, 0.1% to 2%, 0.2% to 1.8%, 0.2% to 1.5%, 0.5% to 1.5%, 0.5% to 1%, and all such ranges and sub-ranges. The C2-C1 values ​​are within the above range, indicating that the secondary battery has a small capacity decay during high-temperature storage and the difference in the effect of manganese deposition is small when it is in different lithium intercalation states, indicating that the secondary battery has good high-temperature storage performance.

[0048] In one or more embodiments, 50% ≤ C1 ≤ 100%. For example, the value of C1 can be 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, or a range of any two of these values. The range of C1 can be 50% to 100%, 60% to 100%, 70% to 100%, 80% to 100%, 90% to 100%, and all of these ranges, as well as sub-ranges.

[0049] In one or more embodiments, 50% ≤ C2 ≤ 100%. For example, the value of C2 can be 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, or a range of any two of these values. The value range of C2 can be 50% to 100%, 60% to 100%, 70% to 100%, 80% to 100%, 90% to 100%, and all of these ranges, as well as subranges.

[0050] In one or more embodiments of this application, the first negative electrode active material includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, or mesophase carbon microspheres. Selecting the above-mentioned first negative electrode active material is beneficial for G1 to improve the high-temperature storage performance and cycle performance of the secondary battery within the scope of this application.

[0051] In one or more embodiments of this application, the second negative electrode active material includes at least one of artificial graphite, natural graphite, soft carbon, hard carbon, or mesophase carbon microspheres. Selecting the above-mentioned second negative electrode active material is beneficial for G2 to improve the high-temperature storage performance and cycle performance of the secondary battery within the scope of this application.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] In one or more embodiments, the preparation of the first negative electrode active material and the second negative electrode active material may include, but is not limited to, the following steps: pretreating the precursor material for preparing the carbon material (e.g., flake natural graphite, coconut shell, etc.), and then carbonizing it at high temperature in an inert atmosphere, wherein the high temperature carbonization temperature is 1200°C to 3000°C and the time is 1h to 8h, to obtain the first negative electrode active material or the second negative electrode active material.

[0057] 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.

[0058] In this application, the graphitization degree G1 of the first anode active material can be controlled by adjusting the temperature and time of high-temperature carbonization during the preparation process. For example, when other conditions remain unchanged, increasing the high-temperature carbonization temperature increases G1, while decreasing the high-temperature carbonization temperature decreases G1. When other conditions remain unchanged, extending the high-temperature carbonization time increases G1, while shortening the high-temperature carbonization time decreases G1. The type of precursor during the preparation process affects G1, and the type of the first anode active material also affects G1.

[0059] In this application, the graphitization degree G2 of the second anode active material can be controlled by adjusting the temperature and time of high-temperature carbonization during the preparation process. For example, when other conditions remain constant, increasing the high-temperature carbonization temperature increases G2, while decreasing the high-temperature carbonization temperature decreases G2. When other conditions remain constant, extending the high-temperature carbonization time increases G2, while shortening the high-temperature carbonization time decreases G2. The type of precursor during the preparation process affects G2, and the type of the second anode active material also affects G2.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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).

[0066] 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).

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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).

[0073] 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.

[0074] In this application, the secondary battery also includes an electrolyte, which includes lithium salts and non-aqueous solvents.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] Example

[0084] 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.

[0085] Test methods and equipment:

[0086] Graphitization degree test

[0087] 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 first and second negative electrode material layers were scraped off with a scraper to obtain powder samples of the first and second negative electrode material layers.

[0088] The industry standard "Test Methods for Graphite-based Anode Materials Part 1: Determination of Graphitization Degree" (YB / T 6139.1-2023) specifies a method for determining the graphitization degree of carbon materials using X-ray diffraction. The testing of the graphitization degree of the first and second anode active materials in this application was conducted according to the aforementioned standard. The graphitization degree G1 of the first anode active material was obtained using a powder sample of the first anode material layer, and the graphitization degree G2 of the second anode active material was obtained using a powder sample of the second anode material layer.

[0089] Dv50 Test

[0090] 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.

[0091] Thickness test

[0092] 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.

[0093] Manganese content test

[0094] 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).

[0095] 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.

[0096] 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.

[0097] C1 and C2 tests

[0098] 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).

[0099] The lithium-ion batteries of each embodiment and comparative example were placed in an environment of 25°C and discharged at a constant current of 0.2C to 3V. After resting for 5 minutes, they were charged at a constant current of 0.2C for 4.5 hours to bring the lithium-ion batteries to 90% SOC. They were then stored in a 60°C oven for 30 days. After the lithium-ion batteries returned to room temperature, they were discharged at a constant current of 0.2C to 3V, and the discharge capacity at this point was recorded as the residual capacity. The lithium-ion batteries were then charged again at a constant current of 0.2C to 4.2V, and then charged at a constant voltage of 4.2V to 0.05C, reaching 100% SOC. After resting for 5 minutes, they were discharged at a constant current of 0.2C to 3V, and the discharge capacity at this point was recorded as the recovered capacity S. Capacity retention rate C2 (%) = recovered capacity S / initial discharge capacity R × 100%.

[0100] The lithium-ion batteries of each embodiment and comparative example were placed in an environment of 25°C and discharged at a constant current of 0.2C to 3V. After resting for 5 minutes, they were charged at a constant current of 0.2C for 1.5 hours to bring the lithium-ion batteries to 30% SOC. They were then stored in a 60°C oven for 30 days. After the lithium-ion batteries returned to room temperature, they were discharged at a constant current of 0.2C to 3V, and the discharge capacity at this point was recorded as the residual capacity. The lithium-ion batteries were then charged again at a constant current of 0.2C to 4.2V, and then charged at a constant voltage of 4.2V to 0.05C, reaching 100% SOC. After resting for 5 minutes, they were discharged at a constant current of 0.2C to 3V, and the discharge capacity at this point was recorded as the recovered capacity T. Capacity retention rate C1 (%) = recovered capacity T / initial discharge capacity R × 100%. Based on C1 and C2, C2-C1 was calculated.

[0101] The high-temperature storage performance of lithium-ion batteries is evaluated by the C2-C1 ratio. The smaller the C2-C1 value, the better the high-temperature storage performance of the lithium-ion battery; the larger the C2-C1 value, the worse the high-temperature storage performance of the lithium-ion battery.

[0102] High-temperature cycling performance test

[0103] 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.

[0104] Example 1-1

[0105] <Preparation of Negative Electrode Sheets>

[0106] (1) Preparation of the first negative electrode active material: flake natural graphite was selected as the precursor. After acid washing and water washing, graphite powder was obtained. The graphite powder was carbonized at 2800℃ for 3 hours in an argon atmosphere and then crushed to obtain the first negative electrode active material, artificial graphite.

[0107] Preparation of the second negative electrode active material: Coconut shell was selected as the precursor with a purity of 99%. After crushing the precursor, it was first carbonized at a low temperature of 700°C for 3 hours under a nitrogen atmosphere. Then, the material carbonized at low temperature was carbonized at a high temperature of 1300°C for 2 hours. After crushing, the second negative electrode active material, hard carbon, was obtained.

[0108] (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.

[0109] (3) The second negative electrode active material hard carbon, 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.

[0110] (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 1.6 g / cm³. 3 The compaction density of the second negative electrode material layer is 1.1 g / cm³. 3 The graphitization degree G1% of the first negative electrode active material and the graphitization degree G2% of the second negative electrode active material are shown in Table 1. The D1 of the first negative electrode active material, the D2 of the second negative electrode active material, the thickness H1 of the first negative electrode material layer, and the thickness H2 of the second negative electrode material layer are shown in Table 2.

[0111] <Preparation of the positive electrode>

[0112] 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 .

[0113] <Isolation membrane>

[0114] A 5μm thick porous polyethylene polymer film (manufacturer: Celgard Diaphragm Company, USA) was used as the separator.

[0115] <Preparation of Electrolyte>

[0116] 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.

[0117] <Preparation of Lithium-ion Batteries>

[0118] 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.

[0119] Examples 1-2 to Examples 1-10

[0120] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Examples 1-1. Specifically, the graphitization degree G1 of the first negative electrode active material is controlled by adjusting the temperature and time of high-temperature carbonization during the preparation of the first negative electrode active material, and the graphitization degree G2 of the second negative electrode active material is controlled by adjusting the temperature and time of high-temperature carbonization during the preparation of the second negative electrode active material.

[0121] Examples 2-1 to 2-17

[0122] 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.

[0123] Example 3-1

[0124] Except for step (3) of <Preparation of Negative Electrode Sheet>, in which the second negative electrode active material hard carbon, negative electrode additive lithium oxalate, second negative electrode conductive agent acetylene black, second negative electrode binder styrene-butadiene rubber, and 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.

[0125] Examples 3-2 to 3-7

[0126] 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%.

[0127] Comparative Example 1

[0128] Except for the preparation method of the negative electrode sheet, the rest is the same as in Example 1-1.

[0129] <Preparation of Negative Electrode Sheets>

[0130] Preparation of the first negative electrode active material: Natural flake graphite was selected as the precursor. After acid washing and water washing, graphite powder was obtained. The graphite powder was carbonized at 2800℃ for 3 hours in an argon atmosphere and then pulverized to obtain the first negative electrode active material, artificial graphite.

[0131] 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.

[0132] 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 1.6 g / cm³. 3 The graphitization degree G1% 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.

[0133] Comparative Example 2

[0134] Except for the preparation method of the negative electrode sheet, the rest is the same as in Example 1-1.

[0135] <Preparation of Negative Electrode Sheets>

[0136] Preparation of the second negative electrode active material: Coconut shell was selected as the precursor with a purity of 99%. After crushing the precursor, it was first carbonized at a low temperature of 700°C for 3 hours under a nitrogen atmosphere. Then, the material carbonized at low temperature was carbonized at a high temperature of 1300°C for 2 hours. After crushing, the second negative electrode active material, hard carbon, was obtained.

[0137] The second negative electrode active material, hard carbon, 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.

[0138] 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 1.1 g / cm³. 3The graphitization degree G2% 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.

[0139] Comparative Examples 3 to 6

[0140] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Examples 1-1. Specifically, the graphitization degree G1 of the first negative electrode active material is controlled by adjusting the temperature and time of high-temperature carbonization during the preparation of the first negative electrode active material, and the graphitization degree G2 of the second negative electrode active material is controlled by adjusting the temperature and time of high-temperature carbonization during the preparation of the second negative electrode active material.

[0141] Comparative Example 7

[0142] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as Comparative Example 1.

[0143] Comparative Example 8

[0144] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as Comparative Example 2.

[0145] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0146] Table 1

[0147] Note: " / " in Table 1 indicates that there is no corresponding parameter.

[0148] Table 2

[0149] As can be seen from Examples 1-1 to 1-10 and Comparative Examples 1 to 8, this application, by designing the structure of the negative electrode sheet and controlling the values ​​of G1 and G2 within the aforementioned range, achieves a smaller C2-C1 and a larger number of cycles at 45°C in the lithium-ion battery, indicating that the lithium-ion battery of this application exhibits good high-temperature storage performance and cycle performance. However, the negative electrode sheets of Comparative Examples 1, 2, 7, and 8 do not simultaneously include a first negative electrode material layer and a second negative electrode material layer; G1 or G2 in Comparative Examples 3 to 6 are not within the scope of this application; and the lithium-ion batteries of Comparative Examples 1 to 8 have a larger C2-C1 and / or a smaller number of cycles at 45°C, indicating that the lithium-ion battery cannot simultaneously achieve both high-temperature storage performance and cycle performance.

[0150] The G1 value affects the high-temperature storage performance and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-4 and Comparative Examples 3 to 4, when the G1 value is too small, such as in Comparative Example 3, the number of cycles at 45°C is smaller, indicating poor high-temperature cycle performance. When the G1 value is too large, such as in Comparative Example 4, although the number of cycles at 45°C is larger, the preparation cost of the first negative electrode active material is too high, and the processing difficulty is greater, increasing the manufacturing cost of the lithium-ion battery. When the G1 value is within the range of this application, the C2-C1 ratio of the lithium-ion battery is smaller, and the number of cycles at 45°C is larger, indicating that the lithium-ion battery of this application has good high-temperature storage performance and cycle performance, good processing performance, and low manufacturing cost.

[0151] The G2 value affects the high-temperature storage performance and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-5 to 1-8, and Comparative Examples 5 to 6, when the G2 value is too small, such as in Comparative Example 5, the number of cycles at 45°C is smaller; when the G2 value is too large, such as in Comparative Example 6, the C2-C1 ratio of the lithium-ion battery is larger, and the number of cycles at 45°C is smaller, indicating that lithium-ion batteries cannot simultaneously achieve both high-temperature storage performance and cycle performance. When the G2 value is within the range of this application, the C2-C1 ratio of the lithium-ion battery is smaller, and the number of cycles at 45°C is larger, indicating that the lithium-ion battery of this application has good high-temperature storage performance and cycle performance.

[0152] The types of the first negative electrode active material, the second negative electrode active material, and the positive electrode active material affect the high-temperature storage performance and cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-10, when the first negative electrode active material, the second negative electrode active material, and the positive electrode active material within the scope of this application are selected, the C2-C1 ratio of the lithium-ion battery is smaller, and the number of cycles at 45°C is larger, indicating that the lithium-ion battery of this application has good high-temperature storage performance and cycle performance.

[0153] The values ​​of D2 / D1, D2, H1 / H2, and H2 affect the high-temperature storage performance and cycle 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 C2-C1 of the lithium-ion battery is smaller, and the number of cycles at 45°C is larger, indicating that the lithium-ion battery of this application has good high-temperature storage performance and cycle performance. Among these, a larger value of H1, although resulting in relatively poor cycle performance, increases the energy density (e.g., Example 2-11).

[0154] Table 3

[0155] Note: " / " in Table 3 indicates that there is no corresponding parameter.

[0156] The type and mass percentage of negative electrode additives affect the high-temperature storage performance and cycle performance of lithium-ion batteries. 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 negative electrode additives result in lower C2-C1 values ​​and a higher number of cycles at 45°C for lithium-ion batteries compared to batteries without negative electrode additives, indicating that the lithium-ion batteries of this application exhibit good high-temperature storage performance and cycle performance.

[0157] 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.

[0158] The various embodiments in this specification are described in a related manner. For the same or similar parts between the various embodiments, please refer to each other. Each embodiment focuses on describing the differences from other embodiments.

[0159] 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 tab and a positive electrode tab, the negative electrode tab comprising a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, the negative electrode material layer comprising a first negative electrode material layer and a second negative electrode material layer disposed in a stacked manner, the first negative electrode material layer being located between the second negative electrode material layer and the negative electrode current collector, the first negative electrode material layer comprising a first negative electrode active material, and the second negative electrode material layer comprising a second negative electrode active material, wherein the first negative electrode active material and the second negative electrode active material are carbon materials having different degrees of graphitization, the degree of graphitization of the first negative electrode active material is G1%, and the degree of graphitization of the second negative electrode active material is G2%, 90≤G1≤98, and 5≤G2≤78. The secondary battery satisfies at least one of the following characteristics:

2. The secondary battery according to claim 1, 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, 0.5≤D2 / D1≤1, and 5≤D2≤20. （1）93≤G1≤98； （2）10≤G2≤50。 3. The secondary battery according to claim 1 or 2, wherein The secondary battery satisfies at least one of the following characteristics:

4. The secondary battery according to claim 3, wherein (1) 0.7≤D2 / D1≤1. The positive electrode tab comprises a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, the positive electrode material layer comprising a positive electrode active material containing Mn elements, the positive electrode active material containing Mn elements comprising at least one of lithium manganate, lithium nickel cobalt manganate, lithium manganese iron phosphate, or a lithium-rich manganese-based material. （2）8≤D2≤15。 5. The secondary battery according to claim 1, wherein The thickness of the first negative electrode material layer is H1 μm, and the thickness of the second negative electrode material layer is H2 μm, 2≤H1 / H2≤20, and 5≤H2≤40.

6. The secondary battery according to claim 1, wherein 8≤H1 / H2≤15, and 10≤H2≤20.

7. The secondary battery according to claim 6, wherein The second negative electrode material layer comprises a negative electrode additive, the negative electrode additive comprising at least one of a carboxylate salt, a sulfonate salt, or a metal-organic framework compound, the carboxylate salt comprising at least one of lithium oxalate, sodium citrate, sodium ethylenediaminetetraacetate, or sodium oxalate, the sulfonate salt comprising at least one of sodium dodecylbenzenesulfonate, sulfonated polystyrene, or sulfonic-functionalized montmorillonite, and the metal-organic framework compound comprising at least one of ZIF-8, MIL-101, or UiO-66.

8. The secondary battery according to any one of claims 1 to 7, wherein The mass percentage content of the negative electrode additive is 0.1% to 2% based on the mass of the second negative electrode material layer. The mass percentage content of the negative electrode additive is 0.5% to 1% based on the mass of the second negative electrode material layer.

9. The secondary battery according to claim 8, wherein After the secondary battery is stored at 60°C for 30 days at a 30% state of charge, the mass content of manganese elements 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 elements in the second negative electrode material layer is W2 ppm based on the mass of the second negative electrode material layer, 0 10. The secondary battery according to any one of claims 1 to 9, wherein In the thickness direction of the negative electrode tab, the first negative electrode material layer comprises a first region, the first region being 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 being 10 μm.

11. The secondary battery according to any one of claims 1 to 10, wherein ​ The mass content of manganese in the first region is less than 100 ppm based on the mass of the first region after the secondary battery is stored at 60°C for 30 days at a 30% state of charge.

12. The secondary battery according to any one of claims 1 to 10, wherein The capacity retention of the secondary battery is C1 after the secondary battery is stored at 60°C for 30 days at a state of charge of 20% to 50%, and the capacity retention of the secondary battery is C2 after the secondary battery is stored at 60°C for 30 days at a state of charge of 70% to 100%, wherein 0≤C2-C1≤3%.

13. The secondary battery according to any one of claims 1 to 12, wherein The secondary battery satisfies at least one of the following characteristics: (1) the first negative electrode active material comprises at least one of artificial graphite, natural graphite, soft carbon, hard carbon, or mesocarbon microbeads; (2) the second negative electrode active material comprises at least one of artificial graphite, natural graphite, soft carbon, hard carbon, or mesocarbon microbeads.

14. An electronic device comprising the secondary battery of any one of claims 1 to 13.

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