Secondary battery and electronic device

Abstract

The present application provides a secondary battery and an electronic device. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, and an electrolyte; the positive electrode sheet comprises a positive electrode current collector and a positive electrode material layer provided on the surface of the positive electrode current collector; the positive electrode material layer comprises a positive electrode material, and the positive electrode material comprises a nickel element, a cobalt element, and a manganese element; the positive electrode material includes at least two positive electrode materials having different cobalt contents; based on the total number of moles of non-lithium metal elements in each positive electrode material, the molar percentage content of the cobalt element in each positive electrode material is N, wherein the maximum value of N is Nmax, the minimum value of N is Nmin, and 5％≤Nmax-Nmin≤55％; the positive electrode material has a Dv50 of 1 um to 10 um; the electrolyte comprises an additive, and the additive comprises a boron-containing lithium salt and / or vinyl sulfate; and based on the mass of the electrolyte, the additive has a mass percentage W of 0.1% to 5%. The secondary battery provided by the present application is conducive to achieving both low-temperature discharge performance and high-temperature storage performance.

Description

A secondary battery and electronic device 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, have advantages such as high energy density, long cycle life, low self-discharge rate, and environmental friendliness, and are widely used in various fields. Currently, commonly used cathode materials include lithium cobalt oxide and lithium nickel cobalt manganese oxide. Lithium cobalt oxide has a higher cobalt content than lithium nickel cobalt manganese oxide, and it exhibits good discharge performance at low temperatures, such as below or equal to 0°C. However, cobalt metal resources are relatively scarce, resulting in higher costs. Lithium nickel cobalt manganese oxide, as the mainstream cathode material, is cheaper than lithium cobalt oxide, but it struggles to balance low-temperature discharge performance with high-temperature storage performance. Therefore, how to achieve both low-temperature discharge performance and high-temperature storage performance in secondary batteries has become a pressing technical problem to be solved. Summary of the Invention

[0003] The purpose of this application is to provide a secondary battery and electronic device that balances the low-temperature discharge performance and high-temperature storage performance of the secondary battery.

[0004] It should be noted that while this application uses lithium-ion batteries as an example of secondary batteries to explain the invention, the secondary batteries in this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0005] The first aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode material layer disposed on the surface of the positive current collector. The positive electrode material layer includes a positive electrode material comprising nickel, cobalt, and manganese. The positive electrode material includes at least two positive electrode materials with different cobalt contents. Based on the total number of moles of non-lithium metal elements in each positive electrode material, the molar percentage of cobalt in each positive electrode material is N, where the maximum value of N is N0. max The minimum value of N is N min 5% ≤ N max -N min ≤55%; the Dv50 of the positive electrode material is 1μm to 10μm; the electrolyte includes additives, the additives including boron-containing lithium salts and / or vinyl sulfate; based on the mass of the electrolyte, the mass percentage W of the additives is 0.1% to 5%. The positive electrode in the secondary battery of this application includes at least two positive electrode materials with different cobalt contents, and the difference N between the maximum and minimum cobalt contents is... max -N minWithin the aforementioned range, the Dv50 of the cathode material is within the aforementioned range. At the same time, when combined with an electrolyte containing the aforementioned types and amounts of additives, it is beneficial to improve the low-temperature discharge performance of the secondary battery, while also taking into account high-temperature storage performance and cost.

[0006] In some embodiments of this application, the secondary battery satisfies at least one of the following characteristics: (1) 20% ≤ N max -N min (1) ≤40%; (2) The Dv50 of the positive electrode material is 4μm to 5μm; (3) 1% ≤ W ≤ 3%. The secondary battery meets at least one of the above characteristics, which is beneficial to improving the low-temperature discharge performance of the secondary battery, while taking into account the high-temperature storage performance and cost.

[0007] In some embodiments of this application, the boron-containing lithium salt includes at least one of lithium tetrafluoroborate, lithium bis(oxalate-borate), lithium difluoroborate oxalate, lithium tetracyanoborate, or lithium tetra(trifluoromethyl)borate. Based on the mass of the electrolyte, the mass percentage of the boron-containing lithium salt is W1, and the secondary battery satisfies one of the following characteristics: (1) 0.1% ≤ W1 ≤ 3%; (2) 1% ≤ W1 ≤ 2%. The electrolyte containing the above-mentioned types of boron-containing lithium salts while controlling their content within the above range is beneficial for mitigating the dissolution of transition metals in the positive electrode material, improving the structural stability of the positive electrode material, and forming a solid electrolyte film of suitable thickness on the surface of the negative electrode sheet, thus improving the low-temperature discharge performance of the secondary battery while also considering its high-temperature storage performance.

[0008] In some embodiments of this application, based on the mass of the electrolyte, the mass percentage of ethylene sulfate is W2, and the secondary battery satisfies one of the following characteristics: (1) 0.01% ≤ W2 ≤ 2%; (2) 0.2% ≤ W2 ≤ 1.5%. The electrolyte containing the above-mentioned amount of ethylene sulfate is beneficial for forming a solid electrolyte film of suitable thickness on the surface of the negative electrode, improving the low-temperature discharge performance of the secondary battery while also ensuring high-temperature storage performance.

[0009] In some embodiments of this application, the cathode material layer includes a first cathode material and a second cathode material; based on the total molar amount of non-lithium metal elements in the first cathode material, the molar percentage of cobalt in the first cathode material is N1, 15% ≤ N1 ≤ 50%; based on the total molar amount of non-lithium metal elements in the second cathode material, the molar percentage of cobalt in the second cathode material is N2, 0% < N2 ≤ 10%. The first cathode material and the second cathode material are located in the same cathode material layer, and the first cathode material and the second cathode material are uniformly distributed in the cathode material layer. The particles of the first cathode material and the second cathode material are in contact with each other, which is more conducive to the synergistic effect between the first cathode material and the second cathode material, improves the low-temperature discharge performance of the secondary battery, and takes cost into consideration.

[0010] In some embodiments of this application, based on the mass of the cathode material, the mass percentage m1 of the first cathode material is 10% to 80%, and the mass percentage m2 of the second cathode material is 20% to 90%. By adjusting the mass percentage m1 of the first cathode material and the mass percentage m2 of the second cathode material within the above ranges, it is beneficial for the first cathode material and the second cathode material to cooperate with each other, improve the low-temperature discharge performance of the secondary battery, and take cost into consideration.

[0011] In some embodiments of this application, the positive electrode material layer includes a first material layer and a second material layer. The first material layer includes a first positive electrode material, and the second material layer includes a second positive electrode material. The cobalt content in the first positive electrode material is greater than the cobalt content in the second positive electrode material. The first material layer and the second material layer work together to improve the low-temperature discharge performance of the secondary battery while also considering cost.

[0012] In some embodiments of this application, the second material layer is located between the positive electrode current collector and the first material layer. Based on the total molar amount of non-lithium metal elements in the first positive electrode material, the molar percentage of cobalt in the first positive electrode material is N1, 12% ≤ N1 ≤ 50%; based on the total molar amount of non-lithium metal elements in the second positive electrode material, the molar percentage of cobalt in the second positive electrode material is N2, 0% < N2 ≤ 10%. By controlling the molar percentage of cobalt in the first positive electrode material to be N1 and the molar percentage of cobalt in the second positive electrode material to be N2 within the above ranges, the resulting positive electrode material layer has a suitable overall cobalt content. Thus, the first material layer and the second material layer work together, which is beneficial for improving the low-temperature discharge performance of the secondary battery while also considering cost.

[0013] In some embodiments of this application, the first material layer is located between the positive electrode current collector and the second material layer. Based on the total molar amount of non-lithium metal elements in the first positive electrode material, the molar percentage of cobalt in the first positive electrode material is N1, 18% ≤ N1 ≤ 55%; based on the total molar amount of non-lithium metal elements in the second positive electrode material, the molar percentage of cobalt in the second positive electrode material is N2, 1% ≤ N2 ≤ 13%. By controlling the molar percentage of cobalt in the first positive electrode material to be N1 and the molar percentage of cobalt in the second positive electrode material to be N2 within the above ranges, the resulting positive electrode material layer has a suitable overall cobalt content. Thus, the first material layer and the second material layer work together to improve the low-temperature discharge performance of the secondary battery while also considering cost.

[0014] In some embodiments of this application, the coating mass per unit area of ​​the first material layer is CW1, and the coating mass per unit area of ​​the second material layer is CW2, where 5% ≤ CW1 / (CW1+CW2)×100% ≤ 70%, and 30% ≤ CW2 / (CW1+CW2)×100% ≤ 95%. The first and second material layers have suitable thicknesses, thus complementing each other to improve the low-temperature discharge performance of the secondary battery while also considering cost.

[0015] In some embodiments of this application, 150mg / 1540.24mm 2 ≤CW1+CW2≤350mg / 1540.24mm 2 By adjusting CW1+CW2 within the aforementioned range, the resulting cathode material layer exhibits suitable overall coating quality, and the secondary battery possesses a suitable energy density. Furthermore, the cooperation between the first and second material layers is beneficial for improving the low-temperature discharge performance of the secondary battery while also considering cost.

[0016] In some embodiments of this application, the cathode material layer includes a first cathode material and a second cathode material, each of which independently includes a dopant element, which includes at least one of aluminum, magnesium, tungsten, zirconium, yttrium, or lanthanum; the molar percentage of the dopant element in the first cathode material is C1 based on the total molar amount of non-lithium metal elements in the first cathode material; the molar percentage of the dopant element in the second cathode material is C2 based on the total molar amount of non-lithium metal elements in the second cathode material; 0.01% ≤ C1 ≤ 5%, and / or 0.01% ≤ C2 ≤ 5%. Introducing the above-mentioned dopant elements into the first cathode material and / or the second cathode material can improve the low-temperature discharge performance of the secondary battery while further improving its high-temperature storage performance.

[0017] In some embodiments of this application, the secondary battery satisfies at least one of the following characteristics: (1) the first positive electrode material comprises one positive electrode material with a cobalt content or at least two positive electrode materials with different cobalt contents; (2) the second positive electrode material comprises one positive electrode material with a cobalt content or at least two positive electrode materials with different cobalt contents; (3) the first positive electrode material and the second positive electrode material each independently comprise lithium nickel cobalt manganese oxide. A secondary battery satisfying at least one of the above characteristics is beneficial for improving the low-temperature discharge performance of the secondary battery, while also considering high-temperature storage performance and cost.

[0018] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments.

[0019] The beneficial effects of this application are:

[0020] This application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode material layer disposed on the surface of the positive current collector. The positive electrode material layer includes a positive electrode material, which includes nickel, cobalt, and manganese. The positive electrode material includes at least two positive electrode materials with different cobalt contents. Based on the total number of moles of non-lithium metal elements in each positive electrode material, the molar percentage of cobalt in each positive electrode material is N, and the maximum value of N is N0. max The minimum value of N is N min 5% ≤ N max -N min ≤55%; the Dv50 of the positive electrode material is 1μm to 10μm; the electrolyte includes additives, including boron-containing lithium salts and / or vinyl sulfate; based on the mass of the electrolyte, the mass percentage W of the additives is 0.1% to 5%. The positive electrode in the secondary battery of this application includes at least two positive electrode materials with different cobalt contents, and the difference N between the maximum and minimum cobalt contents is... max -N min Within the aforementioned range, the Dv50 of the cathode material is within the aforementioned range. At the same time, when combined with an electrolyte containing the aforementioned types and amounts of additives, it is beneficial to improve the low-temperature discharge performance of the secondary battery, while also taking into account high-temperature storage performance and cost. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this application clearer, the following provides a more detailed description. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0022] It should be noted that, in the specific embodiments of this application, a lithium-ion battery is used as an example of a secondary battery to explain this application; however, the secondary battery in this application is not limited to lithium-ion batteries. The specific technical solution is as follows:

[0023] Currently, lithium nickel cobalt manganese oxide ternary materials are the mainstream cathode materials for secondary batteries. However, their discharge performance at low temperatures is poor, especially at low temperature and low state of charge (SOC). Improving low-temperature discharge performance usually affects high-temperature storage performance. Therefore, this application provides a secondary battery and electronic device that balances both low-temperature discharge performance and high-temperature storage performance. The aforementioned low SOC refers to a secondary battery with an SOC less than or equal to 20%.

[0024] The first aspect of this application provides a secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode material layer disposed on the surface of the positive current collector. The positive electrode material layer includes a positive electrode material comprising nickel, cobalt, and manganese. The positive electrode material includes at least two positive electrode materials with different cobalt contents. Based on the total number of moles of non-lithium metal elements in each positive electrode material, the molar percentage of cobalt in each positive electrode material is N, and the maximum value of N is N0. max The minimum value of N is N min 5% ≤ N max -N min ≤55%. In some embodiments of this application, 20% ≤N max -N min ≤40%. For example, N max -N min The cobalt content can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or a range of any two of these values. The positive electrode material layer simultaneously contains at least two positive electrode materials with different cobalt contents. The positive electrode material with a relatively higher cobalt content can improve the low-temperature discharge performance of the secondary battery, while the material with a relatively lower cobalt content can balance the cost of the secondary battery. When the difference N between the maximum and minimum cobalt contents... max -N min If the cobalt content is too low, for example less than 5%, meaning that the differences between different cobalt contents in cathode materials are not significant, it is not conducive to balancing the low-temperature discharge performance and cost of secondary batteries. When the difference between the maximum and minimum cobalt content N... max -N min When the concentration is too high, for example, greater than 55%, it means that the differences in cobalt content among different cathode materials are too large, which is not conducive to the synergy between cathode materials with different cobalt contents to improve the low-temperature discharge performance of the secondary battery. Therefore, the cathode material contains at least two cathode materials with different cobalt contents, and the N content is controlled. max -N minWithin the aforementioned range, it is beneficial to improve the low-temperature performance of secondary batteries while taking into account their cost. In this application, the above-mentioned at least two cathode materials with different cobalt contents refer to two, three, four, or more cathode materials with different solid contents.

[0025] In some embodiments of this application, the Dv50 of the cathode material is from 1 μm to 10 μm. In some embodiments of this application, the Dv50 of the cathode material is from 4 μm to 5 μm. For example, the Dv50 of the cathode material can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or a range of any two of these values. When the Dv50 of the cathode material is too small, for example, less than 1 μm, side reactions are prone to occur, consuming the cathode material and electrolyte. This problem is more pronounced at high temperatures, thus affecting the low-temperature discharge performance and high-temperature storage performance of the secondary battery. When the Dv50 of the cathode material is too large, for example, greater than 10 μm, active ions such as lithium ions inside the cathode material particles are not easily intercalated or deintercalated. The difficulty of intercalation and deintercalation further increases under low temperature and low SOC conditions, thus affecting the low-temperature discharge performance of the secondary battery. Therefore, by controlling the Dv50 of the cathode material within the aforementioned range, the obtained cathode material is less prone to side reactions with electrolysis and the active ion insertion / extraction is relatively easy, thereby improving the low-temperature discharge performance of the secondary battery while also ensuring high-temperature storage performance. In this application, Dv50 refers to the particle size corresponding to 50% of the volume accumulation from the small particle size side; high temperature refers to a temperature greater than or equal to 40°C.

[0026] In some embodiments of this application, the electrolyte includes additives, including boron-containing lithium salts and / or ethylene sulfate. Including at least two cathode materials with different cobalt contents in the cathode material layer can improve the low-temperature discharge performance of the secondary battery, but it also affects the high-temperature storage performance, such as the problem of gas generation during high-temperature storage. Meanwhile, matching the electrolyte with the aforementioned additives can improve the high-temperature gas generation problem of the secondary battery, thus resulting in a secondary battery that balances low-temperature discharge performance and high-temperature storage performance. In some embodiments of this application, the mass percentage W of the additive is 0.1% to 5% based on the mass of the electrolyte. In some embodiments of this application, 1% ≤ W ≤ 3%. For example, the mass percentage W of the additive can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range of any two of these values. When the mass percentage of the additive is too small, for example less than 0.1%, the improvement in high-temperature storage performance is not significant. When the mass percentage of the additive is too high, for example, greater than 5%, a thick solid electrolyte film can easily form on the surface of the negative electrode, affecting the low-temperature discharge performance of the secondary battery. Therefore, including the above-mentioned additives in the electrolyte and controlling the mass percentage of the additives within the above-mentioned range is beneficial to improving the low-temperature discharge performance of the secondary battery while also taking into account its high-temperature storage performance.

[0027] Therefore, the positive electrode in the secondary battery of this application includes at least two positive electrode materials with different cobalt contents, and the difference N between the maximum and minimum cobalt contents is... max -N min Within the aforementioned range, the Dv50 of the cathode material is within the aforementioned range. At the same time, when combined with an electrolyte containing the aforementioned types and amounts of additives, it is beneficial to improve the low-temperature discharge performance of the secondary battery, while also taking into account high-temperature storage performance and cost.

[0028] In some embodiments of this application, the boron-containing lithium salt includes at least one of lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium difluoroborate oxalate (LiDFOB), lithium tetracyanoborate (LiTCB), or lithium tetra(trifluoromethyl)borate (LiB(CF3)4). In some embodiments of this application, the mass percentage of the boron-containing lithium salt is W1, based on the mass of the electrolyte, where 0.1% ≤ W1 ≤ 3%. In some embodiments of this application, 1% ≤ W1 ≤ 2%. For example, the mass percentage of the boron-containing lithium salt, W1, can be 0.1%, 0.3%, 0.5%, 0.7%, 1%, 1.2%, 1.26%, 1.3%, 1.5%, 1.7%, 2%, 2.2%, 2.5%, 2.7%, 3%, or a range consisting of any two of these values. The electrolyte contains the aforementioned boron-containing lithium salts, and their content is controlled within the aforementioned range. This helps to alleviate the dissolution of transition metals in the positive electrode material, improve the structural stability of the positive electrode material, and form a solid electrolyte film of suitable thickness on the surface of the negative electrode sheet. This improves the low-temperature discharge performance of the secondary battery while also taking into account its high-temperature storage performance.

[0029] In some embodiments of this application, the mass percentage of vinyl sulfate is W2, based on the mass of the electrolyte, where 0.01% ≤ W2 ≤ 2%. In some embodiments of this application, 0.2% ≤ W2 ≤ 1.5%. For example, the mass percentage of vinyl sulfate W2 can be 0.01%, 0.05%, 0.1%, 0.3%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.7%, 2%, or a range of any two of these values. The electrolyte containing the above-mentioned amount of vinyl sulfate is beneficial for forming a solid electrolyte film of suitable thickness on the surface of the negative electrode, improving both the low-temperature discharge performance and high-temperature storage performance of the secondary battery.

[0030] In some embodiments of this application, the electrolyte contains boron-containing lithium salt and vinyl sulfate, with 0.1% ≤ W1 ≤ 3% and 0.01% ≤ W2 ≤ 2%, which is more conducive to forming a solid electrolyte film of suitable thickness on the surface of the negative electrode, improving the low-temperature discharge performance of the secondary battery while taking into account the high-temperature storage performance.

[0031] In some embodiments of this application, the cathode material layer includes a first cathode material and a second cathode material, meaning the first cathode material and the second cathode material are located in the same cathode material layer. Based on the total molar amount of non-lithium metal elements in the first cathode material, the molar percentage of cobalt in the first cathode material is N1, where 15% ≤ N1 ≤ 50%. For example, the molar percentage of cobalt in the first cathode material, N1, can be 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range of any two of these values. Based on the total molar amount of non-lithium metal elements in the second cathode material, the molar percentage of cobalt in the second cathode material is N2, where 0% < N2 ≤ 10%. For example, the molar percentage of cobalt (N2) in the second cathode material can be 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination of two of these values. The first and second cathode materials are located in the same cathode material layer. During the preparation of the cathode material layer slurry, the first and second cathode materials are uniformly mixed in the slurry. The resulting cathode material layer also exhibits uniform distribution of the first and second cathode materials, with particles of both materials in contact with each other. This facilitates synergistic effects between the first and second cathode materials, improving the low-temperature discharge performance of the secondary battery while also considering cost.

[0032] In some embodiments of this application, based on the mass of the cathode materials, the mass percentage m1 of the first cathode material is 10% to 80%, and the mass percentage m2 of the second cathode material is 20% to 90%. For example, the mass percentage m1 of the first cathode material can be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or a range of any two values ​​therein. Similarly, the mass percentage m2 of the second cathode material can be 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or a range of any two values ​​therein. By controlling the mass percentage m1 of the first cathode material and the mass percentage m2 of the second cathode material within the above ranges, it is beneficial for the first and second cathode materials to work together, improving the low-temperature discharge performance of the secondary battery while also considering cost.

[0033] In some embodiments of this application, the cathode material layer includes a first cathode material and a second cathode material, meaning the first cathode material and the second cathode material are located in the same cathode material layer. The first cathode material includes one cathode material with a cobalt content or at least two cathode materials with different cobalt contents, and the second cathode material includes one cathode material with a cobalt content or at least two cathode materials with different cobalt contents. The number of cathode materials with different cobalt contents in the first and second cathode materials can be selected according to actual conditions, as long as the purpose of this application is achieved. In some embodiments of this application, the coating quality of the cathode material layer is 150 mg / 1540.24 mm. 2 Up to 350mg / 1540.24mm 2 For example, the coating quality of the positive electrode material layer can be 150mg / 1540.24mm. 2 170mg / 1540.24mm 2 200mg / 1540.24mm 2 225mg / 1540.24mm 2 250mg / 1540.24mm 2 275mg / 1540.24mm 2 300mg / 1540.24mm 2 325mg / 1540.24mm 2 350mg / 1540.24mm 2 Or it can be a range consisting of any two of these values.

[0034] In some embodiments of this application, the positive electrode material layer includes a first material layer and a second material layer. The first material layer includes a first positive electrode material, and the second material layer includes a second positive electrode material. That is, the first positive electrode material and the second positive electrode material are located in different material layers, and the cobalt content in the first positive electrode material is greater than the cobalt content in the second positive electrode material. As a result, the cobalt content in the first material layer and the second material layer are also different, and the cobalt content in the first material layer is greater than the cobalt content in the second material layer. Thus, the first material layer and the second material layer cooperate with each other, which is beneficial to improving the low-temperature discharge performance of the secondary battery while taking cost into consideration.

[0035] In some embodiments of this application, the second material layer is located between the positive electrode current collector and the first material layer. That is, the second material layer with a relatively lower cobalt content is closer to the positive electrode current collector than the first material layer with a relatively higher cobalt content. The inclusion of a first material layer with a relatively higher cobalt content in the first material layer is more conducive to the intercalation of active ions during discharge, and the relatively high cobalt content itself is also beneficial to improving the low-temperature discharge performance of the secondary battery. Therefore, the positive electrode sheet with the above structure is more conducive to improving the low-temperature discharge performance of the secondary battery. Based on the total molar amount of non-lithium metal elements in the first positive electrode material, the molar percentage of cobalt in the first positive electrode material is N1, where 12% ≤ N1 ≤ 50%. For example, the molar percentage of cobalt in the first positive electrode material, N1, can be 12%, 15%, 18%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range of any two of these values. Based on the total molar amount of non-lithium metal elements in the second positive electrode material, the molar percentage of cobalt in the second positive electrode material is N2, where 0% < N2 ≤ 10%. For example, the molar percentage of cobalt in the second cathode material, N2, can be 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination of two of these values. By controlling the molar percentage of cobalt in the first cathode material, N1, and the molar percentage of cobalt in the second cathode material, N2, within the aforementioned range, the resulting cathode material layer has a suitable overall cobalt content. This allows the first and second material layers to work together, thereby improving the low-temperature discharge performance of the secondary battery while also considering cost.

[0036] In some embodiments of this application, the second material layer is located between the positive electrode current collector and the first material layer. The first material layer includes a first positive electrode material, and the second material layer includes a second positive electrode material. The first positive electrode material includes one positive electrode material with a cobalt content or at least two positive electrode materials with different cobalt contents. The second positive electrode material includes one positive electrode material with a cobalt content or at least two positive electrode materials with different cobalt contents. The number of different types of positive electrode materials with different cobalt contents in the first and second positive electrode materials can be selected according to actual conditions, as long as the purpose of this application is achieved.

[0037] In some embodiments of this application, the first material layer is located between the positive electrode current collector and the second material layer. That is, the first material layer, with a relatively high cobalt content, is closer to the positive electrode current collector than the second material layer, which has a relatively low cobalt content. When the secondary battery is in a low-temperature, low-SOC state, the first material layer will preferentially discharge, thus ensuring good discharge performance of the entire positive electrode material layer under low-temperature, low-SOC conditions, thereby improving the low-temperature discharge performance of the secondary battery. Based on the total molar amount of non-lithium metal elements in the first positive electrode material, the molar percentage of cobalt in the first positive electrode material is N1, where 18% ≤ N1 ≤ 55%. For example, the molar percentage of cobalt in the first positive electrode material, N1, can be 18%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or a range consisting of any two of these values. Based on the total molar amount of non-lithium metal elements in the second positive electrode material, the molar percentage of cobalt in the second positive electrode material is N2, where 1% ≤ N2 ≤ 13%. For example, the molar percentage of cobalt in the second cathode material, N2, can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, or any combination of two of these values. By controlling the molar percentage of cobalt in the first cathode material (N1) and the molar percentage of cobalt in the second cathode material (N2) within the above ranges, the resulting cathode material layer has a suitable overall cobalt content. This allows the first and second material layers to work together, which is beneficial for improving the low-temperature discharge performance of the secondary battery while also considering cost.

[0038] In some embodiments of this application, a first material layer is located between the positive electrode current collector and the second material layer. The first material layer includes a first positive electrode material, and the second material layer includes a second positive electrode material. The first positive electrode material includes one positive electrode material with a cobalt content or at least two positive electrode materials with different cobalt contents. The second positive electrode material includes one positive electrode material with a cobalt content or at least two positive electrode materials with different cobalt contents. The number of different types of positive electrode materials with different cobalt contents in the first and second positive electrode materials can be selected according to actual conditions, as long as the purpose of this application is achieved.

[0039] In some embodiments of this application, the coating mass per unit area of ​​the first material layer is CW1, and the coating mass per unit area of ​​the second material layer is CW2, where 5% ≤ CW1 / (CW1+CW2)×100% ≤ 70%, and 30% ≤ CW2 / (CW1+CW2)×100% ≤ 95%. For example, CW1 / (CW1+CW2)×100% can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or a range of any two of these values. For example, CW2 / (CW1+CW2)×100% can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or any combination of two of these values. By adjusting CW1 / (CW1+CW2)×100% and CW2 / (CW1+CW2)×100% within the above ranges, the first and second material layers have suitable thicknesses, thus allowing the first and second material layers to work together, which is beneficial for improving the low-temperature discharge performance of the secondary battery while also considering cost.

[0040] In some embodiments of this application, CW1 is smaller than CW2, which is beneficial for improving the low-temperature discharge performance of the secondary battery while taking cost into account.

[0041] In some embodiments of this application, 150mg / 1540.24mm 2 ≤CW1+CW2≤350mg / 1540.24mm 2 For example, CW1+CW2 can produce 150mg / 1540.24mm. 2 170mg / 1540.24mm 2 200mg / 1540.24mm 2 225mg / 1540.24mm 2 250mg / 1540.24mm 2 275mg / 1540.24mm 2 300mg / 1540.24mm 2 325mg / 1540.24mm 2 350mg / 1540.24mm 2 Or it can be a range consisting of any two of these values. By adjusting CW1+CW2 within the above range, the resulting cathode material layer has a suitable overall coating quality, the secondary battery has a suitable energy density, and the first and second material layers work together to improve the low-temperature discharge performance of the secondary battery while also considering cost.

[0042] In some embodiments of this application, the cathode material layer includes a first cathode material and a second cathode material, each independently comprising a dopant element, including at least one of aluminum, magnesium, tungsten, zirconium, yttrium, or lanthanum. Introducing the aforementioned dopant elements independently into the first and second cathode materials improves their structural stability and enhances the high-temperature storage performance of the secondary battery. Therefore, introducing these dopant elements into the first and / or second cathode materials not only improves the low-temperature discharge performance of the secondary battery but also further enhances its high-temperature storage performance.

[0043] In some embodiments of this application, based on the total molar amount of non-lithium metal elements in the first cathode material, the molar percentage of dopant elements in the first cathode material is C1, where 0.01% ≤ C1 ≤ 5%. For example, the molar percentage of dopant elements in the first cathode material, C1, can be 0.01%, 0.05%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range of any two of these values. By controlling the molar percentage of dopant elements in the first cathode material, C1, within the above range, the obtained first cathode material exhibits good structural stability at high temperatures, thereby improving the low-temperature discharge performance of the secondary battery while also facilitating high-temperature cycle performance.

[0044] In some embodiments of this application, based on the total molar amount of non-lithium metal elements in the second cathode material, the molar percentage of dopant elements in the second cathode material is C2, where 0.01% ≤ C2 ≤ 5%. For example, the molar percentage of dopant elements in the second cathode material, C2, can be 0.01%, 0.05%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range of any two of these values. By controlling the molar percentage of dopant elements in the second cathode material, C2, within the above range, the obtained second cathode material exhibits good structural stability at high temperatures, thereby improving the low-temperature discharge performance of the secondary battery while also facilitating high-temperature cycle performance.

[0045] In some embodiments of this application, the first cathode material and the second cathode material each independently comprise lithium nickel cobalt manganese oxide. When lithium nickel cobalt manganese oxide does not contain the aforementioned doping elements, its chemical formula is Li. n1 Ni x1 Co y1 Mn z1 O2, where 0.95≤n1≤1.05, x1+y1+z1=1, 0.3≤x1≤0.5, 0<y1≤0.6, 0.1≤z1≤0.49. When lithium nickel cobalt manganese oxide contains the above doping elements, its chemical formula is Li. n2 Nix2 Co y2 Mn z2 M m O2, where 0.95≤n2≤1.05, x2+y2+z2+m=1, 0.25≤x2≤0.45, 0<y2≤0.6, 0.1≤z2≤0.49, 0.0001≤m≤0.05.

[0046] In some embodiments of this application, when lithium nickel cobalt manganese oxide does not contain the aforementioned doping elements, the first cathode material and the second cathode material each independently include LiNi. 0.33 Co 0.33 Mn 0.33 O2(NCM111), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.5 Co 0.01 Mn 0.49 O2, LiNi 0.5 Co 0.05 Mn 0.45 O2, LiNi 0.5 Co 0.07 Mn 0.43 O2, LiNi 0.5 Co 0.1 Mn 0.4 O2, LiNi 0.5 Co 0.12 Mn 0.38 O2, LiNi 0.5 Co 0.13 Mn 0.37 O2, Ni 0.5 Co 0.15 Mn 0.35 O2, LiNi 0.5 Co 0.18 Mn 0.32 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.5 Co 0.25 Mn 0.25 O2, LiNi 0.5 Co 0.3 Mn 0.2 O2(NCM532), LiNi 0.5 Co 0.4 Mn 0.1 O2, LiNi 0.5 Co 0.45 Mn 0.05 O2, LiNi 0.4Co 0.5 Mn 0.1 O2, LiNi 0.3 Co 0.55 Mn 0.15 O2 or LiNi 0.3 Co 0.6 Mn 0.1 At least one of O2. In some embodiments of this application, when lithium nickel cobalt manganese oxide contains the above-mentioned doping elements, the first cathode material and the second cathode material each independently include LiNi. 0.499 Co 0.05 Mn 0.45 Al 0.001 O2, LiNi 0.49 Co 0.05 Mn 0.45 Al 0.01 O2, LiNi 0.47 Co 0.05 Mn 0.45 Al 0.03 O2, LiNi 0.46 Co 0.05 Mn 0.45 Al 0.04 O2, LiNi 0.45 Co 0.05 Mn 0.45 Al 0.05 O2, LiNi 0.49 Co 0.05 Mn 0.45 Mg 0.01 O2, LiNi 0.5 Co 0.1 Mn 0.4 O2, LiNi 0.499 Co 0.1 Mn 0.4 Al 0.001 O2, LiNi 0.49 Co 0.1 Mn 0.4 Al 0.01 O2, LiNi 0.47 Co 0.1 Mn 0.4 Al 0.03 O2, LiNi 0.47 Co 0.1 Mn 0.4 Al 0.03 O2, LiNi 0.46 Co 0.1 Mn 0.4 Al 0.04 O2, LiNi 0.49 Co 0.1 Mn 0.4Mg 0.01 O2, LiNi 0.5 Co 0.3 Mn 0.2 O2, LiNi 0.499 Co 0.3 Mn 0.2 Al 0.001 O2, LiNi 0.49 Co 0.3 Mn 0.2 Al 0.01 O2, LiNi 0.47 Co 0.3 Mn 0.2 Al 0.03 O2, LiNi 0.46 Co 0.3 Mn 0.2 Al 0.04 O2, LiNi 0.45 Co 0.3 Mn 0.2 Al 0.05 O2, LiNi 0.49 Co 0.3 Mn 0.2 Mg 0.01 O2, LiNi 0.5 Co 0.4 Mn 0.1 O2, LiNi 0.499 Co 0.4 Mn 0.1 Al 0.001 O2, LiNi 0.49 Co 0.4 Mn 0.1 Al 0.01 O2, LiNi 0.47 Co 0.4 Mn 0.1 Al 0.03 O2, LiNi 0.46 Co 0.4 Mn 0.1 Al 0.04 O2, LiNi 0.45 Co 0.4 Mn 0.1 Al 0.05 O2 or LiNi 0.49 Co 0.4 Mn 0.1 Mg 0.01 At least one of O2.

[0047] In this application, cathode materials with different cobalt contents can be purchased, and their cobalt contents can be tested using an inductively coupled plasma mass spectrometer to select the cathode material with the desired cobalt content.

[0048] In this application, cathode materials with different Dv50s can be purchased, and their Dv50 can be tested using a laser particle size analyzer. The cathode material with the required Dv50 can then be selected.

[0049] When the positive electrode material layer includes a first positive electrode material and a second positive electrode material, that is, when the first positive electrode material and the second positive electrode material are located in the same positive electrode material layer, this application does not particularly limit the preparation method of the positive electrode sheet, as long as the purpose of this application can be achieved. For example, the preparation method of the positive electrode sheet may include, but is not limited to, the following steps: adding the first positive electrode material, the second positive electrode material, a conductive agent, and a binder to a solvent and mixing them evenly to obtain a positive electrode material layer slurry; depositing the positive electrode material layer slurry on one surface of the positive electrode current collector; and drying it to obtain a positive electrode sheet with a positive electrode material layer on one side. Repeating the above operation on the other surface of the positive electrode current collector yields a positive electrode sheet with a positive electrode material layer on both sides. This application does not particularly limit the solvent mentioned above, as long as the purpose of this application can be achieved.

[0050] When the second material layer is located between the positive current collector and the first material layer, and the first material layer includes a first positive electrode material and the second material layer includes a second positive electrode material, this application does not impose any particular limitation on the preparation method of the positive electrode sheet, as long as the purpose of this application can be achieved. For example, the preparation method of the positive electrode sheet may include, but is not limited to, the following steps: adding the first positive electrode material, conductive agent, and binder to a solvent and mixing them evenly to obtain a first material layer slurry; adding the second positive electrode material, conductive agent, and binder to a solvent and mixing them evenly to obtain a second material layer slurry; placing the second material layer slurry on one surface of the positive current collector and drying it to obtain a positive electrode sheet with a second material layer on one side; then placing the first material layer slurry on the surface of the second material layer and drying it to obtain a positive electrode sheet with both the first and second material layers on one side. Repeating the above operation on the other surface of the positive current collector to obtain a positive electrode sheet with both the first and second material layers on both sides. This application does not impose any particular limitation on the above solvents, as long as the purpose of this application can be achieved.

[0051] When the first material layer is located between the positive current collector and the second material layer, and the first material layer includes a first positive electrode material and the second material layer includes a second positive electrode material, this application does not impose any particular limitation on the preparation method of the positive electrode sheet, as long as the purpose of this application can be achieved. For example, the preparation method of the positive electrode sheet may include, but is not limited to, the following steps: adding the first positive electrode material, conductive agent, and binder to a solvent and mixing them evenly to obtain a first material layer slurry; adding the second positive electrode material, conductive agent, and binder to a solvent and mixing them evenly to obtain a second material layer slurry; placing the first material layer slurry on one surface of the positive current collector and drying it to obtain a positive electrode sheet with the first material layer on one side; then placing the second material layer slurry on the surface of the first material layer and drying it to obtain a positive electrode sheet with both the first and second material layers on one side. Repeating the above operation on the other surface of the positive current collector to obtain a positive electrode sheet with both the first and second material layers on both sides. This application does not impose any particular limitation on the above solvents, as long as the purpose of this application can be achieved.

[0052] In this application, the aforementioned positive electrode material layer, first material layer, and second material layer may each independently include a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent, as long as it achieves the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The 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 polymers may include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. This application does not impose any particular limitation on the binder, as long as it can achieve the purpose of this application. For example, the binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, 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 limitation on the mass ratio of the positive electrode material, conductive agent, and binder in the positive electrode material layer, the first material layer, and the second material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0053] This application does not impose any particular limitation on the positive electrode current collector, as long as it achieves the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collectors (such as aluminum-carbon composite current collectors). This application also does not impose any particular limitation on the thickness of the positive electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the positive electrode current collector may be from 8 μm to 20 μm.

[0054] In this application, the electrolyte includes electrolyte salts and other non-aqueous solvents.

[0055] This application does not impose any particular limitation on the electrolyte salt, as long as it achieves the purpose of this application. For example, the electrolyte salt may include, but is not limited to, at least one of LiPF6, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, or lithium difluoroborate. This application 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. For example, based on the mass of the electrolyte, the mass percentage content of the electrolyte salt is 8% to 15%. Exemplarily, the mass percentage content of the electrolyte salt may be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range consisting of any two of these values.

[0056] This application does not impose any particular limitation on other non-aqueous solvents, as long as they achieve the purpose of this application. For example, other 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. This application also does not impose any particular limitation on the content of other non-aqueous solvents in the electrolyte, as long as they achieve the purpose of this application. For example, based on the mass of the electrolyte, the mass percentage of other non-aqueous solvents may be 80% to 91.9%. Exemplarily, the mass percentage of other non-aqueous solvents may be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 91.9%, or a range consisting of any two of these values.

[0057] 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 carbonates 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). 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.

[0058] In this application, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the term "surface" here can refer to the entire surface area of ​​the negative electrode current collector, or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved.

[0059] This application does not impose any particular restrictions on the negative electrode current collector, as long as it can achieve 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 composite current collector. For example, the composite current collector may be lithium copper composite current collector, carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.

[0060] The negative electrode material layer includes a negative electrode active material. This application does not impose any particular limitation on the negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, natural graphite, artificial graphite, mesophase micro carbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 Or at least one of Li-Al alloys.

[0061] In some embodiments of this application, the negative electrode material layer may further include a conductive agent and a binder. This application does not impose any particular restrictions on the types of conductive agents and binders, as long as they achieve the purpose of this application. For example, it may be at least one of the aforementioned conductive agents and binders. This application does not impose any particular restrictions on the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0062] In this application, the secondary battery also includes a separator. There are no particular limitations on the separator, as long as it achieves the purpose of this application. For example, the separator material 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 separator type may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. In this application, there are no particular limitations on the separator thickness, as long as it achieves the purpose of this application; for example, the separator thickness may be from 3 μm to 30 μm.

[0063] In some embodiments of this application, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used.

[0064] 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.

[0065] In some embodiments of this application, the inorganic layer comprises inorganic particles and a binder. This application does not particularly limit the inorganic particles; for example, the inorganic particles may include at least one selected from alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the binder; for example, the binder may be at least one of the binders described above. In some embodiments of this application, the polymer layer comprises a polymer, the polymer material of which includes at least one selected from polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0066] 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.

[0067] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0068] In some embodiments of this application, the secondary battery may include, but is not limited to, lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries. In some embodiments of this application, the secondary battery includes lithium-ion batteries.

[0069] A second aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. Therefore, the electronic device provided by this application has good performance in use.

[0070] This application does not specifically limit the type of electronic device; it can be any electronic device known in the prior art. In some embodiments of this application, 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.

[0071] Example

[0072] 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.

[0073] Test methods and equipment:

[0074] sampling:

[0075] The lithium-ion battery was disassembled at an ambient temperature of 25°C, the positive electrode was removed, cleaned with dimethyl carbonate, and then placed in an oven and dried at 80°C for 12 hours to obtain a positive electrode sample.

[0076] Unless otherwise specified, the above samples were used for testing.

[0077] Dv50 testing:

[0078] The cross-section of the positive electrode sheet along its thickness direction is ion-polished. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are used to determine whether the positive electrode material layer is a single-layer or double-layer coating. If the cobalt content distribution throughout the entire positive electrode material layer is relatively uniform, and there is no significant difference in cobalt content between the regions near and away from the current collector, it is considered a single-layer coating. If the cobalt content differs between the regions near and away from the current collector and there is stratification, it is considered a double-layer coating. The thickness of each layer is determined using an elemental distribution map. The boundary where the elemental content changes abruptly is the dividing line between the first and second material layers; the layer with higher cobalt content is the first material layer, and the other is the second material layer.

[0079] When the positive electrode material layer is a single-layer coating, the positive electrode material layer on the surface of the positive electrode sheet is scraped off and then calcined in air at 600℃ for 5 hours to obtain positive electrode material powder. Then, the Dv50 of the positive electrode material is tested using a laser particle size analyzer.

[0080] When the positive electrode material layer is a double coating, according to the thickness of the first material layer and the second material layer determined above, the first material layer and the second material layer are scraped off respectively, and calcined in air at 600°C for 5 hours to obtain the first positive electrode material powder and the second positive electrode material powder. The first positive electrode material powder and the second positive electrode material powder are mixed according to the ratio of CW1 / (CW1+CW2)×100%:CW2 / (CW1+CW2)×100% in each embodiment and comparative example, and then the Dv50 is tested by a laser particle size analyzer, which is the Dv50 of the positive electrode material.

[0081] Test of molar percentage of elements:

[0082] The cross-section of the positive electrode sheet along the thickness direction is ion polished, and the cross-section of the obtained positive electrode sheet is observed under a scanning electron microscope to confirm whether the positive electrode material layer is a single layer or a double layer.

[0083] If the cathode material layer is a single layer, scanning electron microscopy combined with energy dispersive spectroscopy (EDS) is used at 1000x magnification to measure the molar content of non-lithium metal elements in all particles of the first and second cathode materials within the field of view. The molar percentage of cobalt in the first cathode material (N1) and the molar percentage of cobalt in the second cathode material (N2) are then calculated. The material with the higher cobalt content is designated as the first cathode material, and the material with the lower cobalt content is designated as the second cathode material. The average value is taken as the final result. Non-lithium metal elements include nickel, cobalt, manganese, and dopant elements.

[0084] If the positive electrode material layer is a double layer, use energy dispersive spectroscopy to confirm that the layer with higher cobalt content is the first material layer, and the layer with lower cobalt content is the second material layer. The boundary where the cobalt content changes abruptly is the dividing line between the first and second material layers. Then measure the thickness of the first material layer and record it as h1, and the thickness of the second material layer as h2. When the second material layer is located between the positive electrode current collector and the first material layer, based on the measured thickness h1, scrape off the portion of the positive electrode material layer with a thickness less than h1. The resulting powder is the first material layer powder. Then scrape off the portion of the positive electrode material layer with a thickness greater than h1, and continue scraping off the positive electrode material layer. The resulting powder is the second material layer powder. When the first material layer is located between the positive electrode current collector and the second material layer, the same steps can be used to obtain the first and second material layer powders.

[0085] The powder of the first material layer was calcined at 600℃ in a nitrogen atmosphere for 2 hours to obtain the first cathode material sample. Then, 0.4g of the sample was dissolved in 10mL of aqua regia (concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3) in a volume ratio of 3:1), dissolved at 180℃ for 30min, cooled to room temperature, and the sample was brought to a final volume of 100mL.

[0086] Standard curves: A series of nickel, cobalt, and manganese standard solutions of different concentrations, along with dopant elements, were prepared. The mass numbers of the standard solutions were measured using inductively coupled plasma mass spectrometry (ICP-MS), and standard curves for nickel, cobalt, manganese, and the dopant elements were plotted. When the type and number of dopant elements can be confirmed using EDS, the number of standard curves for each dopant element corresponds to the number of dopant element types.

[0087] Sample testing: The prepared sample solution was introduced into an ICP-MS instrument to measure the contents of nickel, cobalt, and manganese. Based on the standard curve, the mass concentrations of nickel, cobalt, and manganese were obtained. The masses of nickel, cobalt, manganese, and the dopant element were then calculated, leading to the molar amounts of nickel (n1), cobalt (n2), manganese (n3), and the dopant element (n4). The molar percentage of cobalt in the first cathode material was N1 = n2 / (n1+n2+n3+n4)×100%, and the molar percentage of the dopant element in the first cathode material was C1 = n4 / (n1+n2+n3+n4)×100%.

[0088] By replacing the powder of the first material layer with the powder of the second material layer, and following the steps described above, the molar percentage of cobalt in the second cathode material N2 and the molar percentage of doped elements in the second cathode material C2 can be obtained.

[0089] Testing of coating quality per unit area:

[0090] A piece with an area of ​​1540.24 mm was cut out. 2 For the positive electrode sample, the entire positive electrode material layer was scraped off, and its mass was measured as mg using an analytical balance. If the positive electrode material layer is applied to only one side of the positive electrode current collector, then the coating mass per unit area of ​​the positive electrode material layer is mg / 1540.24 mm. 2 If the positive electrode current collector has positive electrode material layers on both sides, then the coating mass per unit area of ​​the positive electrode material layer is m / 2mg / 1540.24mm. 2 .

[0091] The cross-section of the positive electrode sheet along its thickness direction was ion-polished. The resulting cross-section was observed under a scanning electron microscope (SEM) and, using energy dispersive spectroscopy (EDS), confirmed that the layer with higher cobalt content was the first material layer, and the layer with lower cobalt content was the second material layer. The thicknesses of the first and second material layers were measured and recorded as h1 and h2, respectively. The boundary where the cobalt content abruptly changes was identified as the dividing line between the first and second material layers. When the second material layer was located between the positive current collector and the first material layer, based on the measured thickness h1, the first material layer was scraped off, and its mass was measured as m1 mg using an analytical balance. Then, the second material layer was scraped off, and its mass was measured as m2 mg using an analytical balance. If the positive current collector had both the first and second material layers on one side, the coating mass per unit area of ​​the first material layer, CW1, was m1 mg / 1540.24 mm. 2 The coating mass per unit area of ​​the second material layer, CW1, is m1-m2 mg / 1540.24 mm. 2 If a first material layer and a second material layer are disposed on both sides of the positive electrode current collector, then the coating mass per unit area of ​​the first material layer is m1 / 2mg / 1540.24mm. 2 The coating mass per unit area of ​​the second material layer is (m1-m2) / 2mg / 1540.24mm. 2 Then, CW1+CW2, CW1 / (CW1+CW2)×100%, and CW2 / (CW1+CW2)×100% are calculated. When the first material layer is located between the positive electrode current collector and the second material layer, the above steps can also be followed for testing.

[0092] Testing of the mass percentage of the first and second cathode materials:

[0093] If the cathode material layer is a single layer, the molar content of nickel, cobalt, manganese, and dopant elements in the first and second cathode materials is obtained according to the "test of molar percentage content of elements". The total molar content of nickel, cobalt, manganese, and dopant elements in the first cathode material is recorded as the number of moles of the first cathode material. Each 1 mol of the first cathode material contains 1 mol of lithium and 2 mol of oxygen, and the mass x1 of the first cathode material is then calculated. Similarly, the mass x2 of the second cathode material is obtained. The mass percentage content of the first cathode material m1 = x1 / (x1+x2)×100%, and the mass percentage content of the second cathode material m2 = x2 / (x1+x2)×100%.

[0094] Testing of additive types and content:

[0095] First, discharge the lithium-ion battery to 3V at a constant current of 0.2C at 25℃. Then, remove the packaging bag, cut off the tabs, wind up the remaining electrode assembly and put it into a centrifuge tube. Let it stand to collect the electrolyte sample, centrifuge the electrolyte sample in a centrifuge, and take the supernatant as the electrolyte test sample.

[0096] The types and contents of additives in the electrolyte test sample were determined using gas chromatography-mass spectrometry (GC-MS), with the types of additives being determined by comparison with standard cards.

[0097] Low-temperature discharge performance test:

[0098] A lithium-ion battery was discharged to 3.0V at a discharge rate of 0.2C, then charged at a rate of 0.2C for 0.5 hours. The battery was then placed at 0°C and discharged to 3.0V at a discharge rate of 1C. The time taken to discharge to 3.0V was recorded as the low-temperature discharge time. The low-temperature discharge time is used to evaluate the low-temperature discharge performance of lithium-ion batteries. A shorter low-temperature discharge time indicates better performance, and vice versa.

[0099] High-temperature gas production performance:

[0100] The lithium-ion battery was charged at a constant current rate of 0.2C to 4.4V, and then charged at a constant voltage of 4.4V to 0.05C. The lithium-ion battery was placed at 80℃ for 6 hours. The thickness of the lithium-ion battery before and after high-temperature storage was measured using a flat plate thickness gauge with a 1000g counterweight, and the thicknesses were H0 and H1, respectively. The storage thickness expansion rate (%) = (H1-H0) / H0×100%.

[0101] Example 1-1

[0102] <Preparation of the positive electrode>

[0103] The first cathode material LiNi 0.5 Co0.4 Mn 0.1 O2, second cathode material LiNi 0.5 Co 0.05 Mn 0.45 O2, conductive carbon black (Super P) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 48.5:48.5:2:1. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%. After vacuum stirring, the positive electrode slurry was obtained. The Dv50 of both the first and second positive electrode materials was the same, at 4 μm.

[0104] The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector, and then dried at 120°C to obtain a positive electrode sheet with a single-sided coating of the positive electrode material layer. The coating mass of the positive electrode material layer was 250 mg / cm³. 2 The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying at 120℃ and cold pressing, the sheet is cut and tabs are welded to obtain a positive electrode sheet with dimensions of 74mm × 867mm for later use. The compaction density of the positive electrode material layer is 3.4 g / cm³. 3 .

[0105] <Preparation of Negative Electrode Sheets>

[0106] Artificial graphite (negative electrode active material), styrene-butadiene rubber (binder), and acetylene black (conductive agent) were mixed in a mass ratio of 97.4:1.4:1.2. Deionized water was added as a solvent to prepare a slurry with a solid content of 45 wt%. The slurry was stirred evenly in a vacuum mixer to obtain the negative electrode slurry. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil used as a negative electrode current collector and dried at 120 °C to obtain a negative electrode sheet with a single-sided negative electrode material layer. The coating mass of the negative electrode material layer was 142 mg / 1540 mm². 2 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 negative electrode material. After drying at 120℃ and cold pressing, the sheet is cut and tabs are welded to obtain a negative electrode sheet with dimensions of 78mm × 875mm for later use. The compaction density of the negative electrode material layer is 1.7g / cm³. 3 .

[0107] <Preparation of Electrolyte>

[0108] In an environment with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Then, an electrolyte salt (LiPF6), a boron-containing lithium salt (LiBF4), and ethylene sulfate were added to the organic solvent and mixed thoroughly to obtain an electrolyte solution. Based on the mass of the electrolyte solution, the mass percentage of the electrolyte salt was 12.5%, the mass percentage of the boron-containing lithium salt (W1) was 1.5%, the mass percentage of ethylene sulfate (W2) was 1%, and the remainder was the organic solvent.

[0109] <Septum>

[0110] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0111] <Preparation of Lithium-ion Batteries>

[0112] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The electrolyte prepared above is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming to obtain a lithium-ion battery. The formation upper limit voltage is 4.15V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0113] Examples 1-2 to Examples 1-13

[0114] Except for adjusting the relevant preparation parameters according to Table 1-1, everything else is the same as in Example 1-1. When the mass percentage of the first and second cathode materials changes, the content of conductive agent and binder remains unchanged, and the total mass of the first and second cathode materials is the same as in Example 1-1.

[0115] Examples 1-14 to Examples 1-27

[0116] Except for adjusting the relevant preparation parameters according to Table 1-2, the rest is the same as in Example 1-1. When the mass percentage of boron-containing lithium salt and / or vinyl sulfate changes, the content of organic solvent changes accordingly, while the content of electrolyte salt remains unchanged.

[0117] Examples 1-28 to Examples 1-37

[0118] Except for adjusting the relevant preparation parameters according to Table 1-3, the rest is the same as in Example 1-1.

[0119] Example 2-1

[0120] Except for the preparation of the positive electrode sheet according to the following preparation method, the rest is the same as in Example 1-1:

[0121] <Preparation of the positive electrode>

[0122] The first cathode material LiNi 0.5 Co 0.4 Mn 0.1 O2, Super P, and PVDF were mixed in a mass ratio of 97:2:1, and NMP was added as a solvent to prepare a slurry with a solid content of 75wt%. After being stirred evenly under vacuum, the first positive electrode slurry was obtained.

[0123] The second cathode material LiNi 0.5 Co 0.05 Mn 0.45 O2, Super P, and PVDF were mixed in a mass ratio of 97:2:1, and NMP was added as a solvent to prepare a slurry with a solid content of 75wt%. After being stirred evenly under vacuum, the second positive electrode slurry was obtained.

[0124] A second positive electrode slurry is uniformly coated onto one surface of a 10μm thick aluminum foil current collector, and dried at 120℃ to obtain a positive electrode sheet with a second material layer on one side. Then, a first material layer slurry is applied to the surface of the second material layer and dried to obtain a positive electrode sheet with both first and second material layers on one side. The above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with both first and second material layers on both sides. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a positive electrode sheet with dimensions of 74mm × 867mm for later use. CW1 + CW2 is 250mg / 1540.24mm. 2 The percentage of CW1 / (CW1+CW2)×100% is 40%, and the percentage of CW2 / (CW1+CW2)×100% is 60%. The overall compaction density of the first and second material layers on one side is 3.4 g / cm³. 3 .

[0125] Examples 2-2 to 2-17

[0126] Except for adjusting the relevant preparation parameters according to Table 2-1, the rest is the same as in Example 2-1.

[0127] Examples 2-18 to 2-31

[0128] Except for adjusting the relevant preparation parameters according to Table 2-2, the rest is the same as in Example 2-1. When the mass percentage of boron-containing lithium salt and / or vinyl sulfate changes, the content of organic solvent changes accordingly, while the content of electrolyte salt remains unchanged.

[0129] Examples 2-32 to 2-41

[0130] Except for adjusting the relevant preparation parameters according to Table 2-3, the rest is the same as in Example 2-1.

[0131] Example 3-1

[0132] Except for the preparation of the positive electrode sheet according to the following preparation method, the rest is the same as in Example 1-1:

[0133] <Preparation of the positive electrode>

[0134] The first cathode material LiNi 0.5 Co 0.1 Mn 0.4 O2, Super P, and PVDF were mixed in a mass ratio of 97:2:1, and NMP was added as a solvent to prepare a slurry with a solid content of 75wt%. After being stirred evenly under vacuum, the first positive electrode slurry was obtained.

[0135] The second cathode material LiNi 0.5 Co 0.3 Mn 0.2 O2, Super P, and PVDF were mixed in a mass ratio of 97:2:1, and NMP was added as a solvent to prepare a slurry with a solid content of 75wt%. After being stirred evenly under vacuum, the second positive electrode slurry was obtained.

[0136] A first positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil current collector, and dried at 120°C to obtain a positive electrode sheet with a first material layer on one side. Then, a second material layer slurry was applied to the surface of the first material layer and dried to obtain a positive electrode sheet with both first and second material layers on one side. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with both first and second material layers on both sides. 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 CW1 + CW2 ratio was 250 mg / 1540.24 mm. 2 The percentage of CW1 / (CW1+CW2)×100% is 40%, and the percentage of CW2 / (CW1+CW2)×100% is 60%. The overall compaction density of the first and second material layers on one side is 3.4 g / cm³. 3 .

[0137] Examples 3-2 to 3-15

[0138] Except for adjusting the relevant preparation parameters according to Table 3-1, the rest is the same as in Example 3-1.

[0139] Examples 3-16 to 3-29

[0140] Except for adjusting the relevant preparation parameters according to Table 3-2, the rest is the same as in Example 3-1. When the mass percentage of boron-containing lithium salt and / or vinyl sulfate changes, the content of organic solvent changes accordingly, while the content of electrolyte salt remains unchanged.

[0141] Examples 3-30 to 3-39

[0142] Except for adjusting the relevant preparation parameters according to Table 3-3, the rest is the same as in Example 3-1.

[0143] Comparative Examples 1-1 to 1-3

[0144] Except for adjusting the relevant preparation parameters according to Table 1-1, the rest is the same as in Example 1-1.

[0145] Comparative Examples 1-4 to 1-7

[0146] Except for adjusting the relevant preparation parameters according to Table 1-2, the rest is the same as in Example 1-1. When the mass percentage of boron-containing lithium salt and / or vinyl sulfate changes, the content of organic solvent changes accordingly, while the content of electrolyte salt remains unchanged.

[0147] Comparative Examples 2-1 to 2-3

[0148] Except for adjusting the relevant preparation parameters according to Table 2-1, the rest is the same as in Example 2-1.

[0149] Comparative Examples 2-4 to 2-7

[0150] Except for adjusting the relevant preparation parameters according to Table 2-2, the rest is the same as in Example 2-1. When the mass percentage of boron-containing lithium salt and / or vinyl sulfate changes, the content of organic solvent changes accordingly, while the content of electrolyte salt remains unchanged.

[0151] Comparative Examples 3-1 to 3-3

[0152] Except for adjusting the relevant preparation parameters according to Table 3-1, the rest is the same as in Example 3-1.

[0153] Comparative Examples 3-4 to 3-7

[0154] Except for adjusting the relevant preparation parameters according to Table 3-2, the rest is the same as in Example 3-1. When the mass percentage of boron-containing lithium salt and / or vinyl sulfate changes, the content of organic solvent changes accordingly, while the content of electrolyte salt remains unchanged.

[0155] The preparation parameters and performance tests of each embodiment and comparative example are shown in Tables 1-1 to 1-3, 2-1 to 2-3, and 3-1 to 3-3.

[0156] Table 1-1

[0157] Table 1-2

[0158] Note: " / " in Table 1-2 indicates that there is no corresponding parameter. Dv50 refers to the Dv50 of the cathode material, D1 refers to the Dv50 of the first cathode material, and D2 refers to the Dv50 of the second cathode material.

[0159] As can be seen from Examples 1-1 to 1-27 and Comparative Examples 1-1 to 2-7, the first cathode material and the second cathode material are in the same material layer. In the examples, N... max -N min The Dv50 of the positive electrode material is within the scope of this application, and the electrolyte contains additives, the types and amounts of which are also within the scope of this application. The resulting secondary battery exhibits a longer discharge time and a lower storage thickness expansion rate, indicating that the lithium-ion battery of this application can improve low-temperature discharge performance while also maintaining high-temperature storage performance. N in Comparative Examples 1-1 to 1-3 max -N min The discharge times of the comparative examples (Comparative Examples 1-4 and 1-5) are all shorter than those of the examples, as they are outside the scope of this application. The Dv50 of the cathode material in Comparative Examples 1-4 is not within the scope of this application, and their discharge times are all shorter than those of the examples. Although the discharge time of Comparative Examples 1-4 is longer than that of Comparative Examples 1-5, its storage thickness expansion rate is also greater. Although the storage thickness expansion rate of Comparative Examples 1-5 is smaller, its discharge time is shorter. Comparative Examples 1-6, which do not contain additives, have a longer discharge time but the largest storage thickness expansion rate. The mass percentage of additives in Comparative Examples 1-7 is outside the scope of this application; although the storage thickness expansion rate is smaller, the discharge time is also shorter. Therefore, the lithium-ion batteries in the above comparative examples cannot simultaneously improve low-temperature discharge performance and high-temperature storage performance.

[0160] The cobalt content in the first and second cathode materials affects N. max -N min This affects the low-temperature discharge performance of lithium-ion batteries, while having a relatively small impact on high-temperature storage performance. As can be seen from Examples 1-1 to 1-10, when the molar percentage of cobalt in the first and second cathode materials is within the range of this application, the resulting lithium-ion batteries have a longer discharge time, thus indicating that the low-temperature discharge performance of lithium-ion batteries is improved.

[0161] The mass percentage of the first and second cathode materials affects the low-temperature discharge performance of lithium-ion batteries, while having a relatively small impact on high-temperature storage performance. As can be seen from Examples 1-1, 1-11 to 1-13, when the mass percentages of the first and second cathode materials are within the range specified in this application, the resulting lithium-ion batteries exhibit longer discharge times, thus demonstrating improved low-temperature discharge performance.

[0162] The type and content of additives in the electrolyte affect the low-temperature discharge performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-17 to 1-27, when the type and content of additives in the electrolyte are within the range of this application, the resulting lithium-ion batteries have a longer discharge time and a lower storage thickness expansion rate, thus demonstrating that lithium-ion batteries can improve low-temperature discharge performance while also maintaining high-temperature storage performance.

[0163] Table 1-3

[0164] Note: " / " in Table 1-3 indicates that the corresponding parameter does not exist.

[0165] As can be seen from Examples 1-1, 1-28 to 1-37, when the first cathode material and the second cathode material contain doped elements, the storage thickness expansion rate of the lithium-ion battery can be further reduced, indicating that the high-temperature storage performance is further improved. At the same time, the lithium-ion battery has a longer discharge time, indicating that the lithium-ion battery with doped elements can better balance low-temperature discharge performance and high-temperature storage performance.

[0166] As can be seen from Examples 1-28 to Examples 1-37, when the types and contents of doped elements in the first cathode material and the second cathode material are within the scope of this application, the resulting lithium-ion battery has a longer discharge time and a lower storage thickness expansion rate, thus demonstrating that the lithium-ion battery can improve low-temperature discharge performance while also taking into account high-temperature storage performance.

[0167] Table 2-1

[0168] Table 2-2

[0169] Note: In Table 2-2, “ / ” indicates that there is no corresponding parameter. Dv50 refers to the Dv50 of the cathode material, D1 refers to the Dv50 of the first cathode material, and D2 refers to the Dv50 of the second cathode material.

[0170] As can be seen from Examples 2-1 to 2-31 and Comparative Examples 2-1 to 2-7, the second material layer containing a second cathode material with a relatively low cobalt content is closer to the cathode current collector, while the first material layer containing a first cathode material with a relatively high cobalt content is located on the surface of the cathode sheet. In the examples, N... max -N min The Dv50 of the positive electrode material is within the scope of this application, and the electrolyte contains additives, the types and amounts of which are also within the scope of this application. The resulting secondary battery exhibits a longer discharge time and a lower storage thickness expansion rate, indicating that the lithium-ion battery of this application can improve low-temperature discharge performance while also maintaining high-temperature storage performance. N in Comparative Examples 2-1 to 2-3 max -N min The discharge times of Comparative Examples 2-4 and 2-5, which are not within the scope of this application, are all shorter than those of the Examples. While the discharge time of Comparative Example 2-4 is longer than that of Comparative Example 2-5, its storage thickness expansion rate is also greater. Although the storage thickness expansion rate of Comparative Example 2-5 is smaller, its discharge time is shorter. Comparative Example 2-6, which does not contain additives, has a longer discharge time but the largest storage thickness expansion rate. The mass percentage of additives in Comparative Examples 2-7 is not within the scope of this application; although the storage thickness expansion rate is smaller, the discharge time is also shorter. Therefore, the lithium-ion batteries in the above comparative examples cannot simultaneously improve low-temperature discharge performance and high-temperature storage performance.

[0171] The cobalt content in the first and second cathode materials affects N. max -N min This affects the low-temperature discharge performance of lithium-ion batteries, while having a relatively small impact on high-temperature storage performance. As can be seen from Examples 2-1 to 2-11, when the molar percentage of cobalt in the first and second cathode materials is within the range of this application, the resulting lithium-ion batteries have a longer discharge time, thus indicating that the low-temperature discharge performance of lithium-ion batteries is improved.

[0172] The coating quality ratio of the first and second material layers, as well as the coating quality of the positive electrode material layer, affects the low-temperature discharge performance of lithium-ion batteries, while having a relatively small impact on high-temperature storage performance. As can be seen from Examples 2-1, 2-12 to 2-17, when the above parameters are within the range of this application, the resulting lithium-ion battery has a longer discharge time, thus indicating that the low-temperature discharge performance of the lithium-ion battery is improved.

[0173] The type and content of additives in the electrolyte affect the low-temperature discharge performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 2-1, 2-21 to 2-31, when the type and content of additives in the electrolyte are within the range of this application, the resulting lithium-ion batteries have a longer discharge time and a lower storage thickness expansion rate, thus demonstrating that lithium-ion batteries can improve low-temperature discharge performance while also maintaining high-temperature storage performance.

[0174] Table 2-3

[0175] Note: " / " in Table 2-3 indicates that the corresponding parameter does not exist.

[0176] As can be seen from Examples 2-1, 2-32 to 2-41, when the first cathode material and the second cathode material contain doped elements, the storage thickness expansion rate of the lithium-ion battery can be further reduced, indicating that the high-temperature storage performance is further improved. At the same time, the lithium-ion battery has a longer discharge time, indicating that the lithium-ion battery with doped elements can better balance low-temperature discharge performance and high-temperature storage performance.

[0177] As can be seen from Examples 2-32 to 2-41, when the types and contents of doped elements in the first cathode material and the second cathode material are within the scope of this application, the resulting lithium-ion battery has a longer discharge time and a lower storage thickness expansion rate, thus demonstrating that the lithium-ion battery can improve low-temperature discharge performance while also taking into account high-temperature storage performance.

[0178] Table 3-1

[0179] Table 3-2

[0180] Note: In Table 3-2, “ / ” indicates that there is no corresponding parameter. Dv50 refers to the Dv50 of the cathode material, D1 refers to the Dv50 of the first cathode material, and D2 refers to the Dv50 of the second cathode material.

[0181] As can be seen from Examples 3-1 to 3-29 and Comparative Examples 3-1 to 3-7, the second material layer containing a second cathode material with a relatively low cobalt content is located on the surface of the cathode sheet, while the first material layer containing a first cathode material with a relatively high cobalt content is closer to the cathode current collector. In the examples, N... max -N minThe Dv50 of the positive electrode material is within the scope of this application, and the electrolyte contains additives, the types and amounts of which are also within the scope of this application. The resulting secondary battery exhibits a longer discharge time and a lower storage thickness expansion rate, indicating that the lithium-ion battery of this application can improve low-temperature discharge performance while also maintaining high-temperature storage performance. N in Comparative Examples 3-1 to 3-3 max -N min The discharge times of Comparative Examples 3-4 and 3-5, which are outside the scope of this application, are all shorter than those of the Examples. The Dv50 of the cathode material in Comparative Examples 3-4 is not within the scope of this application, and their discharge times are all shorter than those of the Examples. Although the discharge time of Comparative Example 3-4 is longer than that of Comparative Example 3-5, its storage thickness expansion rate is also greater. Although the storage thickness expansion rate of Comparative Example 3-5 is smaller, its discharge time is shorter. Comparative Example 3-6, which does not contain additives, has a longer discharge time but the largest storage thickness expansion rate. The mass percentage of additives in Comparative Examples 3-7 is outside the scope of this application; although the storage thickness expansion rate is smaller, the discharge time is also shorter. It is evident that the lithium-ion batteries in the above comparative examples cannot simultaneously improve low-temperature discharge performance and high-temperature storage performance.

[0182] The cobalt content in the first and second cathode materials affects N. max -N min This affects the low-temperature discharge performance of lithium-ion batteries, while having a relatively small impact on high-temperature storage performance. As can be seen from Examples 3-1 to 3-9, when the molar percentage of cobalt in the first and second cathode materials is within the range of this application, the resulting lithium-ion batteries have a longer discharge time, thus indicating that the low-temperature discharge performance of lithium-ion batteries is improved.

[0183] The coating quality ratio of the first and second material layers, as well as the coating quality of the positive electrode material layer, affects the low-temperature discharge performance of lithium-ion batteries, while having a relatively small impact on high-temperature storage performance. As can be seen from Examples 3-1, 3-10 to 3-15, when the above parameters are within the range of this application, the resulting lithium-ion battery has a longer discharge time, thus indicating that the low-temperature discharge performance of the lithium-ion battery is improved.

[0184] The type and content of additives in the electrolyte affect the low-temperature discharge performance and high-temperature storage performance of lithium-ion batteries. As can be seen from Examples 3-1, 3-19 to 3-29, when the type and content of additives in the electrolyte are within the range of this application, the resulting lithium-ion batteries have a longer discharge time and a lower storage thickness expansion rate, thus demonstrating that lithium-ion batteries can improve low-temperature discharge performance while also maintaining high-temperature storage performance.

[0185] Table 3-3

[0186] Note: " / " in Table 3-3 indicates that the corresponding parameter does not exist.

[0187] As can be seen from Examples 3-1, 3-30 to 3-39, when the first cathode material and the second cathode material contain doped elements, the storage thickness expansion rate of the lithium-ion battery can be further reduced, indicating that the high-temperature storage performance is further improved. At the same time, the lithium-ion battery has a longer discharge time, indicating that the lithium-ion battery with doped elements can better balance low-temperature discharge performance and high-temperature storage performance.

[0188] As can be seen from Examples 3-30 to 3-39, when the types and contents of doped elements in the first cathode material and the second cathode material are within the scope of this application, the resulting lithium-ion battery has a longer discharge time and a lower storage thickness expansion rate, thus demonstrating that the lithium-ion battery can improve low-temperature discharge performance while also taking into account high-temperature storage performance.

[0189] 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.

[0190] The element connected by the terms "one of," "among," "a kind of," or other similar terms refers to any one of the listed elements. For example, "one of A or B" means only A or only B; similarly, "one of A, B, and C" means only A, only B, or only C. The element connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms refers to any combination of the listed elements. For example, "at least one of A or B" means only A, only B, A and B; similarly, "at least one of A, B, or C" means only A, only B, only C, only A and B, only A and C, only B and C, A and B and C.

[0191] 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

A secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode material layer disposed on the surface of the positive current collector. The positive electrode material layer includes a positive electrode material, which includes nickel, cobalt, and manganese. The cathode material includes at least two cathode materials with different cobalt contents. Based on the total number of moles of non-lithium metal elements in each cathode material, the molar percentage of cobalt in each cathode material is N, where the maximum value of N is N0. max The minimum value of N is N min 5% ≤ N max -N min ≤55%; the Dv50 of the cathode material is 1μm to 10μm; The electrolyte includes additives, which include boron-containing lithium salts and / or vinyl sulfate; the mass percentage W of the additives is 0.1% to 5% based on the mass of the electrolyte. The secondary battery according to claim 1, wherein it satisfies at least one of the following characteristics: (1) 20% < N < 30% max - N < 40% min ≤ 40% (2) The Dv50 of the positive electrode material is 4μm to 5μm; (3)1％≤W≤3％。 According to claim 1, the secondary battery, wherein, The boron-containing lithium salt includes at least one of lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluoroborate oxalate, lithium tetracyanoborate, or lithium tetra(trifluoromethyl)borate, and the mass percentage of the boron-containing lithium salt is W1, based on the mass of the electrolyte, where 0.1% ≤ W1 ≤ 3%. The secondary battery according to claim 3, wherein 1％≤W1≤2％。 The secondary battery according to claim 1, wherein Based on the mass of the electrolyte, the mass percentage of ethylene sulfate is W2, where 0.01% ≤ W2 ≤ 2%. The secondary battery according to claim 5, wherein 0.2％≤W2≤1.5％。 The secondary battery according to any one of claims 1 to 6, wherein The positive electrode material layer includes a first positive electrode material and a second positive electrode material; Based on the total number of moles of non-lithium metal elements in the first cathode material, the molar percentage of cobalt in the first cathode material is N1, 15% ≤ N1 ≤ 50%. Based on the total number of moles of non-lithium metal elements in the second cathode material, the molar percentage of cobalt in the second cathode material is N2, where 0% < N2 ≤ 10%. The secondary battery according to claim 7, wherein Based on the mass of the cathode material, the mass percentage m1 of the first cathode material is 10% to 80%, and the mass percentage m2 of the second cathode material is 20% to 90%. The secondary battery according to any one of claims 1 to 6, wherein The cathode material layer includes a first material layer and a second material layer. The first material layer includes a first cathode material, and the second material layer includes a second cathode material. The cobalt content in the first cathode material is greater than the cobalt content in the second cathode material. The secondary battery according to claim 9, wherein The second material layer is located between the positive electrode current collector and the first material layer. Based on the total number of moles of non-lithium metal elements in the first positive electrode material, the molar percentage of cobalt in the first positive electrode material is N1, 12% ≤ N1 ≤ 50%; based on the total number of moles of non-lithium metal elements in the second positive electrode material, the molar percentage of cobalt in the second positive electrode material is N2, 0% < N2 ≤ 10%. The secondary battery according to claim 9, wherein The first material layer is located between the positive electrode current collector and the second material layer. Based on the total number of moles of non-lithium metal elements in the first positive electrode material, the molar percentage of cobalt in the first positive electrode material is N1, 18% ≤ N1 ≤ 55%; based on the total number of moles of non-lithium metal elements in the second positive electrode material, the molar percentage of cobalt in the second positive electrode material is N2, 1% ≤ N2 ≤ 13%. The secondary battery according to claim 9, wherein The coating mass per unit area of ​​the first material layer is CW1, and the coating mass per unit area of ​​the second material layer is CW2, where 5% ≤ CW1 / (CW1+CW2)×100% ≤ 70%, and 30% ≤ CW2 / (CW1+CW2)×100% ≤ 95%. The secondary battery according to claim 12, wherein 150mg / 1540.24mm 2 ≤CW1+CW2≤350mg / 1540.24mm 2 。 The secondary battery according to any one of claims 7 to 13, wherein The cathode material layer includes a first cathode material and a second cathode material, each of which independently includes a doping element, and the doping element includes at least one of aluminum, magnesium, tungsten, zirconium, yttrium or lanthanum; Based on the total number of moles of non-lithium metal elements in the first cathode material, the molar percentage of the doping element in the first cathode material is C1; based on the total number of moles of non-lithium metal elements in the second cathode material, the molar percentage of the doping element in the second cathode material is C2. 0.01% ≤ C1 ≤ 5%, and / or, 0.01% ≤ C2 ≤ 5%. The secondary battery according to any one of claims 7 to 14, wherein it satisfies at least one of the following characteristics: (1) The first cathode material includes one cathode material with a cobalt content or at least two cathode materials with different cobalt contents; (2) The second cathode material includes one cathode material with a cobalt content or at least two cathode materials with different cobalt contents; (3) The first cathode material and the second cathode material each independently include lithium nickel cobalt manganese oxide. An electronic device comprising a secondary battery as described in any one of claims 1 to 15.

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