Secondary battery and electronic device

Abstract

This application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a positive active material, a positive conductive agent, and a positive binder. The positive active material includes lithium nickel cobalt manganese oxide. The positive conductive agent includes conductive carbon black, whisker carbon nanotubes, and at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes. The diameter of the whisker carbon nanotubes is d1 nm, 50 ≤ d1 ≤ 200. The length of the whisker carbon nanotubes is L1 nm, 5000 ≤ L1 ≤ 20000. The porosity of the positive electrode sheet is 25% to 45%. With the above configuration, the cycle performance and temperature rise control capability of the secondary battery during high-rate discharge can be improved.

Description

Technical Field

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

[0002] With the rapid development of new energy technologies, ternary lithium-ion batteries have achieved a penetration rate of over 65% in drones and electric vehicles due to their high energy density, good high-rate charging capability, low-temperature resistance, and safety performance. However, high-end applications are placing increasingly higher demands on the performance of lithium-ion batteries. Among these, the cycle performance and temperature rise control of ternary lithium-ion batteries during high-rate discharge have always been the focus of research.

[0003] Existing technologies attempt to control the temperature rise of lithium-ion batteries by reducing the compaction density of the positive electrode sheet. However, reducing the compaction density often leads to insufficient content of conductive agent in the positive electrode, resulting in a series of problems such as incomplete conductive network and deteriorated cycle performance of lithium-ion batteries. Therefore, how to solve the problems of excessive temperature rise and insufficient cycle performance of ternary lithium-ion batteries during high-rate discharge has become an urgent technical problem to be solved in this field. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and an electronic device to improve the cycle performance and temperature rise control capability of the secondary battery during high-rate discharge.

[0005] 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:

[0006] The first aspect of this application provides a secondary battery, comprising a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a positive active material, a positive conductive agent, and a positive binder. The positive active material includes lithium nickel cobalt manganese oxide. The positive conductive agent includes conductive carbon black, whisker carbon nanotubes (VGCF), and at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes. The diameter of the whisker carbon nanotubes is d1 nm, 50 ≤ d1 ≤ 200; the length of the whisker carbon nanotubes is L1 nm, 5000 ≤ L1 ≤ 20000; and the porosity of the positive electrode sheet is 25% to 45%. The secondary battery of this application, by controlling the porosity of the positive electrode sheet within the range of this application, enhances the wettability and electrolyte retention of the electrolyte, and can alleviate the heat generation problem of the secondary battery during charge-discharge cycles. Therefore, the ternary material system secondary battery of this application, by selecting conductive carbon black, whisker carbon nanotubes, and single-walled carbon nanotubes and / or multi-walled carbon nanotubes as positive electrode conductive agents and controlling the diameter and length of whisker carbon nanotubes within the range of this application, and simultaneously controlling the porosity of the positive electrode sheet within the range of this application, forms a three-dimensional conductive network with good support, stability and conductivity in the positive electrode active material layer. It also enhances the wettability and liquid retention of the electrolyte, enabling good ion and electron transport. The secondary battery reduces heat generation during charge and discharge cycles, thereby improving the cycle performance and temperature rise control capability of the secondary battery at high-rate discharge.

[0007] In some embodiments of this application, the aspect ratio of the whisker carbon nanotubes is L1 / d1, where 25 ≤ L1 / d1 ≤ 400. Controlling the aspect ratio of the whisker carbon nanotubes within this range facilitates the formation of a stable and highly conductive three-dimensional conductive network, optimizes electrolyte wettability and ion transport channels, and improves the cycle performance and temperature rise control capability of the secondary battery.

[0008] In some embodiments of this application, the mass percentage of the positive electrode conductive agent is W1% based on the mass of the positive electrode active material layer, where 1.5 ≤ W1 ≤ 5. Controlling the mass percentage of the positive electrode conductive agent within the above range is beneficial for enabling the secondary battery to have good cycle performance and temperature rise control capability.

[0009] In some embodiments of this application, the sum of the mass percentages of whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes is W2%, where 1 ≤ W2 ≤ 4, based on the mass of the positive electrode active material layer. Controlling the sum of the mass percentages of whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes within the above range is beneficial for enabling the secondary battery to have good cycle performance and temperature rise control capability.

[0010] In some embodiments of this application, the positive electrode sheet satisfies at least one of the following characteristics: (1) 50≤d1≤100; (2) 8000≤L1≤20000; (3) 80≤L1 / d1≤400; (4) 2≤W2≤3.5. Satisfying these characteristics in the positive electrode sheet is beneficial for enabling the secondary battery to have good cycle performance and temperature rise control capability. In some embodiments of this application, the diameter of the single-walled carbon nanotubes is d2 nm, and 0.5≤d2≤2. Controlling the diameter of the single-walled carbon nanotubes within the above range is beneficial for the synergistic effect of single-walled carbon nanotubes and whisker carbon nanotubes to obtain a conductive network with good conductivity, and also beneficial for the positive electrode sheet to have higher porosity, thereby improving the cycle performance and temperature rise control capability of the secondary battery during high-rate discharge.

[0011] In some embodiments of this application, the length of the single-walled carbon nanotubes is L² nm, and the aspect ratio of the single-walled carbon nanotubes is L² / d², where 1000 ≤ L² / d² ≤ 8000. Controlling the aspect ratio of the single-walled carbon nanotubes within this range is beneficial for the single-walled carbon nanotubes and whisker carbon nanotubes to jointly construct a stable three-dimensional conductive network, thereby improving the temperature rise control capability of the secondary battery while maintaining good cycle performance at high discharge rates.

[0012] In some embodiments of this application, in the Raman spectra of single-walled carbon nanotubes, 1350 cm⁻¹ -1 The intensity of the characteristic peak at that location is I. D1 1580cm -1 The intensity of the characteristic peak at that location is I. G1 ,40≤I G1 / I D1 ≤200. Peak intensity ratio I of G peak and D peak in single-walled carbon nanotubes. G1 / I D1 The value being within the above range indicates that single-walled carbon nanotubes have good crystalline order and few structural defects.

[0013] In some embodiments of this application, the diameter of the multi-walled carbon nanotubes is d³ nm, where 3 ≤ d³ ≤ 20. Controlling the diameter of the multi-walled carbon nanotubes within this range can improve the cycle performance and temperature rise control capability of the secondary battery during high-rate discharge.

[0014] In some embodiments of this application, the length of the multi-walled carbon nanotubes is L3 nm, the aspect ratio of the multi-walled carbon nanotubes is L3 / d3, and 75 ≤ L3 / d3 ≤ 500. Controlling the length of the multi-walled carbon nanotubes within the above range helps to reduce the contact resistance of the positive electrode, thereby reducing heat generation during charge-discharge cycles of the secondary battery. This improves the temperature rise control capability of the secondary battery while maintaining good cycle performance at high discharge rates.

[0015] In some embodiments of this application, in the Raman spectra of multi-walled carbon nanotubes, 1350 cm⁻¹ -1 The intensity of the characteristic peak at that location is I. D2 1580cm -1 The intensity of the characteristic peak at that location is I. G2 , 1≤I G2 / I D2 ≤10. Peak intensity ratio of G peak to D peak in multi-walled carbon nanotubes (I) G2 / I D2 The value being within the above range indicates that multi-walled carbon nanotubes have good crystalline order and few structural defects.

[0016] In some embodiments of this application, the length of single-walled carbon nanotubes is L2 nm, and the length of multi-walled carbon nanotubes is L3 nm, satisfying: 4000≤L2≤8000, 1000≤L3≤2000, and 0.125≤L3 / L2≤0.5. By controlling the lengths of single-walled and multi-walled carbon nanotubes and their ratio within the above ranges, the heat generation of the secondary battery during charge-discharge cycles is reduced. This improves the temperature rise control capability of the secondary battery while maintaining good cycle performance at high discharge rates.

[0017] In some embodiments of this application, the surface energy difference between single-walled carbon nanotubes and multi-walled carbon nanotubes is γ mJ / m. 2 5≤y≤30. When the positive electrode conductive agent contains both single-walled carbon nanotubes and multi-walled carbon nanotubes, controlling the difference in surface energy between single-walled carbon nanotubes and multi-walled carbon nanotubes within the above range is beneficial to the bonding of single-walled carbon nanotubes and multi-walled carbon nanotubes, and also beneficial to the uniform dispersion of single-walled carbon nanotubes and multi-walled carbon nanotubes in the positive electrode slurry, thereby improving the conductivity of the three-dimensional conductive network.

[0018] In some embodiments of this application, the thickness of the positive electrode sheet is from 50 μm to 200 μm. When the thickness of the positive electrode sheet is within the above range, the secondary battery exhibits good cycle performance and temperature rise control capability during high-rate discharge, as well as high capacity.

[0019] In some embodiments of this application, the compaction density of the positive electrode sheet is 2.4 g / cm³. 3 Up to 3.2 g / cm 3 By controlling the compaction density of the positive electrode sheet within the above range, the secondary battery exhibits good cycle performance and temperature rise control capability during high-rate discharge.

[0020] A second aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device has good performance characteristics.

[0021] The beneficial effects of the embodiments of this application are as follows:

[0022] This application provides a secondary battery and an electronic device. The ternary material system secondary battery of this application, by selecting conductive carbon black, whisker carbon nanotubes, and single-walled carbon nanotubes and / or multi-walled carbon nanotubes as positive electrode conductive agents and controlling the diameter and length of the whisker carbon nanotubes within the range specified in this application, and simultaneously controlling the porosity of the positive electrode sheet within the range specified in this application, forms a three-dimensional conductive network with good support, stability, and conductivity in the positive electrode active material layer. This also enhances the wettability and electrolyte retention of the electrolyte, enabling good ion and electron transport. The secondary battery reduces heat generation during charge-discharge cycles, thereby improving the cycle performance and temperature rise control capability of the secondary battery at high-rate discharge. Of course, implementing any product or method of this application does not necessarily require achieving all of the above-described advantages simultaneously. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0024] Figure 1 This is a schematic cross-sectional view of the positive electrode sheet along its thickness and length directions according to one embodiment of this application.

[0025] Figure 2 This is a schematic diagram illustrating the mechanism of the three-dimensional conductive network in this application;

[0026] Figure 3 Raman spectra of single-walled carbon nanotubes in Examples 2-2 of this application;

[0027] Figure 4 The images show the Raman spectra of the multi-walled carbon nanotubes in Examples 2-12 of this application.

[0028] Figure label:

[0029] 100 - Positive electrode sheet; 101 - Positive current collector; 10 - Positive active material layer; 21 - VGCF; 22 - CNT; 23 - Conductive carbon black particles; 24 - Positive active material particles. Detailed Implementation

[0030] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0031] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0032] The first aspect of this application provides a secondary battery, comprising a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including a positive active material, a positive conductive agent, and a positive binder. The positive active material includes lithium nickel cobalt manganese oxide. The positive conductive agent includes conductive carbon black, whisker carbon nanotubes, and at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes. The diameter of the whisker carbon nanotubes is d1 nm, 50 ≤ d1 ≤ 200; the length of the whisker carbon nanotubes is L1 nm, 5000 ≤ L1 ≤ 20000; the porosity of the positive electrode sheet is 25% to 45%.

[0033] For ease of understanding, in this application, the length direction of the positive electrode sheet is defined as X, and its thickness direction as Z. It should be understood that the above definitions of direction are for the purpose of describing this application, and the directions defined in this application can be understood based on the relative positions of the elements in the accompanying drawings and actual products. Furthermore, the length and thickness directions of the positive current collector and the positive active material layer are the same as those of the positive electrode sheet. The aforementioned "positive active material layer disposed on at least one surface of the positive current collector" refers to the positive active material layer being disposed on one or both surfaces of the positive current collector. Figure 1 As shown, the positive electrode 100 includes a positive current collector 101 and a positive active material layer 10, the positive active material layer 10 being disposed on two surfaces of the positive current collector 101. It is understood that in some embodiments of this application, the positive active material layer 10 is disposed on only one surface of the positive current collector 101. In some embodiments, the positive conductive agent includes conductive carbon black, whisker carbon nanotubes, and single-walled carbon nanotubes; in other embodiments, the positive conductive agent includes conductive carbon black, whisker carbon nanotubes, and multi-walled carbon nanotubes; and in still other embodiments, the positive conductive agent includes conductive carbon black, whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0034] For example, the diameter d1 nm of the whisker carbon nanotubes can be 50 nm, 60 nm, 72 nm, 85 nm, 90 nm, 100 nm, 110 nm, 130 nm, 150 nm, 162 nm, 170 nm, 182 nm, 200 nm, or any value between any two of the above ranges. If the diameter of the whisker carbon nanotubes is less than 50 nm, the diameter is too small, making it difficult to construct a three-dimensional conductive network. If the diameter is greater than 200 nm, the diameter is too large, making it difficult for conductive carbon black, single-walled carbon nanotubes, and multi-walled carbon nanotubes to adhere to the whisker carbon nanotubes, thus affecting the conductivity of the conductive network and reducing the content of the positive electrode active material.

[0035] For example, the length L1 nm of the whisker carbon nanotubes can be 5000 nm, 6000 nm, 7000 nm, 8000 nm, 9000 nm, 10000 nm, 11000 nm, 12000 nm, 13000 nm, 14000 nm, 15000 nm, 16000 nm, 17000 nm, 18000 nm, 19000 nm, 20000 nm, or any value between any two of the above ranges. If the length of the whisker carbon nanotubes is less than 5000 nm, it is too short and not conducive to building a three-dimensional conductive network; if the length of the whisker carbon nanotubes is greater than 20000 nm, it is too long, placing excessively high demands on the fabrication process, resulting in high production costs or being too difficult to achieve.

[0036] For example, the porosity of the positive electrode sheet can be 25%, 27%, 29%, 30%, 33%, 35%, 37%, 39%, 41%, 42%, 45%, or any value within any two of the above ranges. If the porosity of the positive electrode sheet is less than 25%, the electrolyte will not adequately wet the positive electrode active material layer, resulting in insufficient electrolyte retention and hindered ion and electron transport. This can also lead to excessive heat generation during battery cycling, causing excessive temperature rise in the secondary battery. If the porosity of the positive electrode sheet is greater than 45%, the structural stability of the positive electrode active material layer will be affected, and the content of positive electrode active material in the layer will also decrease, thereby reducing the energy density of the secondary battery.

[0037] When lithium nickel cobalt manganese oxide ternary material is selected as the positive electrode active material in the secondary battery of this application, whisker carbon nanotubes with diameters and lengths within the range of this application are added to the positive electrode sheet, and conductive carbon black and single-walled carbon nanotubes and / or multi-walled carbon nanotubes are selected as a combination of whisker carbon nanotubes (VGCF) as positive electrode conductive agents. The geometric characteristics of whisker carbon nanotubes, conductive carbon black and single-walled carbon nanotubes and / or multi-walled carbon nanotubes (hereinafter referred to as CNTs) are complementary and the interface synergistic effect can construct a conductive network with a three-dimensional skeleton support and micro-region (vacuum between positive electrode active material particles) permeation composite structure. Specifically, firstly, VGCF, with its unique fibrous crystal structure, forms a macroscopic framework network in the positive electrode active material layer. The fibers of this framework structure form physical connection points through van der Waals forces, constructing the main channels for electron transport. The graphitized edge defects on the fiber surface provide additional electron transition sites. The pore size of the framework network is typically 0.5 μm to 2 μm, reserving channels for CNT penetration. During the coating of the positive electrode slurry, the rigid fiber structure of VGCF aligns under fluid shear force, forming a three-dimensional support framework that spans the entire thickness of the positive electrode active material layer. Secondly, CNTs, as secondary conductive units, fill the mesoscopic pores of the framework formed by VGCF. The degree of carboxyl functionalization in T (-COOH content 1.2 mmol / g to 2.8 mmol / g) enables it to form a π-π conjugation with the VGCF surface, establishing a low-barrier electron transport channel at the interface. This structure reduces contact resistance by approximately 40%. Simultaneously, the nanoscale diameter of CNTs allows them to penetrate into the submicron-level gaps (e.g., 200 nm to 500 nm) between the positive electrode active material particles, forming locally conductive micro-regions. Furthermore, after the positive electrode slurry is coated and dried, the conductive network exhibits a "coral reef" hierarchical structure, with VGCF forming the main branches, CNTs forming secondary distributions, and zero-dimensional conductive carbon black uniformly dispersed. All three components collectively coat the surface of the positive electrode active material particles. In the resulting three-dimensional conductive network, the mechanical support of VGCF suppresses particle breakage during secondary battery cycling, the micro-region penetration of CNTs ensures uniform charge distribution, and the high coverage of zero-dimensional conductive carbon black enhances conductivity. The synergistic effect of these three components at the interface results in a conductive network with excellent conductivity for the positive electrode, reducing the contact resistance of the positive electrode. For example, Figure 2 This is a schematic diagram of the mechanism of the three-dimensional conductive network in this application. Figure 2As can be seen, VGCF 21 constitutes the main branches, CNT 22 forms a secondary distribution, and conductive carbon black particles 23 are uniformly dispersed, all three coating the surface of the positive electrode active material particles 24. The secondary battery of this application enhances the wettability and liquid retention of the electrolyte by controlling the porosity of the positive electrode sheet within the range specified in this application, thus alleviating the heat generation problem during charge-discharge cycles. Therefore, the ternary material system secondary battery of this application, by selecting conductive carbon black, whisker carbon nanotubes, and single-walled carbon nanotubes and / or multi-walled carbon nanotubes as positive electrode conductive agents and controlling the diameter and length of the whisker carbon nanotubes within the range specified in this application, while simultaneously controlling the porosity of the positive electrode sheet within the range specified in this application, forms a three-dimensional conductive network with good support, stability, and conductivity in the positive electrode active material layer. It also enhances the wettability and liquid retention of the electrolyte, enabling good ion and electron transport. This reduces heat generation during charge-discharge cycles, thereby improving the cycle performance and temperature rise control capability of the secondary battery at high-rate discharge. In this application, when the positive electrode active material is lithium nickel cobalt manganese oxide with a nickel content of 80% or more, the high rate of the secondary battery refers to a rate of 4C or more; when the positive electrode active material is lithium nickel cobalt manganese oxide with a nickel content of less than 80%, the high rate of the secondary battery refers to a rate of 10C or more.

[0038] This application does not impose any particular restrictions on the method of controlling the diameter and length of the whisker carbon nanotubes, as long as the purpose of this application can be achieved. For example, it can be achieved by purchasing commercially available whisker carbon nanotubes and selecting whisker carbon nanotubes of the required length and diameter, or by purchasing commercially available whisker carbon nanotubes and then using sand milling and sieving methods.

[0039] This application does not impose any particular restrictions on the method of controlling the porosity of the positive electrode sheet, as long as the purpose of this application can be achieved. For example, it can be achieved by controlling at least one of the compaction density of the positive electrode sheet or the content of the positive electrode conductive agent.

[0040] In some embodiments of this application, lithium nickel cobalt manganese oxide includes, but is not limited to, Ni62(LiNi) 0.62 Co 0.1 Mn 0.28 O2), Ni71(LiNi) 0.71 Co 0.14 Mn 0.15 O2), Ni91(LiNi 0.91 Co 0.04 Mn 0.05 O2), Ni92(LiNi) 0.92 Co 0.03 Mn 0.05 O2), Ni93(LiNi) 0.93 Co 0.02 Mn0.05 O2), Ni94(LiNi) 0.94 Co 0.01 Mn 0.05 O2), Ni95(LiNi) 0.95 Co 0.01 Mn 0.04 O2) or NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 At least one of O2).

[0041] In some embodiments of this application, 50 ≤ d1 ≤ 100. For example, the diameter d1 nm of the whisker carbon nanotubes is 50 nm, 60 nm, 72 nm, 85 nm, 90 nm, 100 nm, or any value between any two of the above ranges. By controlling the diameter of the whisker carbon nanotubes within the above range, the whisker carbon nanotubes can provide a well-supporting framework in the pores of the positive electrode sheet, and can also form a stable three-dimensional electron conduction path, giving the positive electrode sheet a complete three-dimensional conductive network. This also results in a higher porosity for the positive electrode sheet, optimizing the wettability of the electrolyte and the ion transport channels, reducing heat generation during the secondary battery cycle, and improving the cycle performance and temperature rise control capability of the secondary battery.

[0042] In some embodiments of this application, 8000 ≤ L1 ≤ 20000. For example, the length L1 nm of the whisker carbon nanotubes is 8000 nm, 9000 nm, 10000 nm, 11000 nm, 12000 nm, 13000 nm, 14000 nm, 15000 nm, 16000 nm, 17000 nm, 18000 nm, 19000 nm, 20000 nm, or any value between any two of the above ranges. By controlling the length of the whisker carbon nanotubes within the above range, the whisker carbon nanotubes can provide a framework with good support in the pores of the positive electrode sheet, and can also form a stable three-dimensional electron conduction path, so that the positive electrode sheet has a complete three-dimensional conductive network, and also makes the positive electrode sheet have high porosity, optimize the wettability of the electrolyte and the ion transport channels, reduce the heat generation during the secondary battery cycle, and improve the cycle performance and temperature rise control capability of the secondary battery.

[0043] In some embodiments of this application, the aspect ratio of the whisker carbon nanotubes is L1 / d1, where 25 ≤ L1 / d1 ≤ 400. For example, L1 / d1 can be 25, 50, 60, 90, 120, 130, 159, 180, 200, 230, 246, 270, 300, 326, 349, 360, 370, 400, or any value between any two of the above ranges. Controlling the aspect ratio of the whisker carbon nanotubes within the above range facilitates the formation of a supportive framework, provides channels for the penetration and distribution of single-walled carbon nanotubes, multi-walled carbon nanotubes, and conductive carbon black, forms a stable and highly conductive three-dimensional conductive network, and provides higher porosity for the positive electrode sheet. This optimizes the wettability of the electrolyte and the ion transport channels, reduces heat generation during secondary battery cycling, and improves the cycle performance and temperature rise control capability of the secondary battery.

[0044] In some embodiments of this application, 80 ≤ L1 / d1 ≤ 400. For example, L1 / d1 is 80, 90, 120, 130, 159, 180, 200, 230, 246, 270, 300, 326, 349, 360, 370, 400, or any value between any two of the above ranges. Controlling the aspect ratio of the whisker carbon nanotubes within the above range is beneficial for further forming a three-dimensional conductive network with good support, stability, and conductivity. This results in a higher porosity for the positive electrode, thereby enhancing the wettability and liquid retention of the electrolyte, reducing heat generation during secondary battery cycling, and further improving the cycle performance and temperature rise control capability of the secondary battery.

[0045] In some embodiments of this application, the mass percentage of the positive electrode conductive agent is W1%, where 1.5 ≤ W1 ≤ 5, based on the mass of the positive electrode active material layer. For example, the mass percentage W1% of the positive electrode conductive agent is 1.5%, 1.7%, 2.0%, 2.5%, 3.0%, 3.3%, 3.5%, 3.7%, 4.0%, 4.1%, 4.3%, 4.5%, 4.7%, 5.0%, or any value between any two of the above ranges. Controlling the mass percentage of the positive electrode conductive agent within the above range is beneficial for preparing a positive electrode active material layer with good conductivity, thereby enabling the secondary battery to have good cycle performance and temperature rise control capability.

[0046] In some embodiments of this application, the sum of the mass percentages of whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes is W2%, where 1 ≤ W2 ≤ 4, based on the mass of the positive electrode active material layer. For example, the sum of the mass percentages W2% of whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes is 1%, 1.5%, 1.7%, 2.0%, 2.5%, 3.0%, 3.3%, 3.5%, 3.7%, 4.0%, or any value between any two of the above ranges. Controlling the sum of the mass percentages of whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes within the above range is beneficial for forming a three-dimensional conductive network with good support, stability, and conductivity in the positive electrode active material layer, thereby enabling the secondary battery to have good cycle performance and temperature rise control capability.

[0047] In some embodiments of this application, 2 ≤ W2 ≤ 3.5. For example, the sum of the mass percentages W2% of whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes is 2.0%, 2.5%, 3.0%, 3.3%, 3.5%, or any value between any two of the above ranges. Controlling the sum of the mass percentages of whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes within the above range is beneficial for further forming a three-dimensional conductive network with good support, stability, and conductivity in the positive electrode active material layer, thereby enabling the secondary battery to have good cycle performance and temperature rise control capability.

[0048] This application does not impose any particular limitation on the content of whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes, as long as the purpose of this application is achieved. This application also does not impose any particular limitation on the content of conductive carbon black, as long as the purpose of this application is achieved. For example, based on the mass of the positive electrode active material layer, the mass percentage content of whisker carbon nanotubes is 0.2% to 2%. For example, based on the mass of the positive electrode active material layer, the mass percentage content of single-walled carbon nanotubes is 0% to 0.6%. For example, based on the mass of the positive electrode active material layer, the mass percentage content of multi-walled carbon nanotubes is 0% to 2%. For example, based on the mass of the positive electrode active material layer, the mass percentage content of conductive carbon black is 0.8% to 2%.

[0049] In some embodiments of this application, the diameter of the single-walled carbon nanotube is d² nm, where 0.5 ≤ d² ≤ 2. For example, the diameter d² nm of the single-walled carbon nanotube is 0.5 nm, 0.6 nm, 0.7 nm, 0.9 nm, 1.0 nm, 1.1 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.8 nm, 1.9 nm, 2.0 nm, or any value between any two of the above ranges. Controlling the diameter of single-walled carbon nanotubes within the aforementioned range facilitates their penetration into the framework formed by whisker carbon nanotubes, enabling secondary distribution of the single-walled carbon nanotubes. This allows the single-walled carbon nanotubes and whisker carbon nanotubes to work synergistically to create a conductive network with excellent conductivity, thereby reducing the contact resistance of the positive electrode. Furthermore, it helps to achieve a higher porosity in the positive electrode, enhancing the wettability and liquid retention of the electrolyte. This results in better ion and electron transport, reducing the probability of heat generation during charge-discharge cycles and improving the cycle performance and temperature rise control capability of the secondary battery at high discharge rates.

[0050] In some embodiments of this application, the length of the single-walled carbon nanotube is L² nm, and the aspect ratio of the single-walled carbon nanotube is L² / d², where 1000 ≤ L² / d² ≤ 8000. For example, the aspect ratio L² / d² of the single-walled carbon nanotube is 1000, 2100, 3200, 4300, 5140, 6330, 7020, 8000, or any value between any two of the above ranges. Controlling the aspect ratio of the single-walled carbon nanotube within the above range is beneficial for the single-walled carbon nanotube and whisker carbon nanotube to jointly construct a stable three-dimensional conductive network, improve the ion and electron transport rate, reduce the contact resistance of the positive electrode, and reduce heat generation during charge-discharge cycles of the secondary battery. This allows the secondary battery to maintain good cycle performance at high discharge rates while improving its temperature rise control capability.

[0051] This application does not impose any particular restrictions on the method of controlling the diameter and length of single-walled carbon nanotubes, as long as the purpose of this application can be achieved. For example, it can be achieved by purchasing commercially available single-walled carbon nanotubes and selecting those with the required length and diameter, or by purchasing commercially available single-walled carbon nanotubes and then using methods such as sand milling and sieving.

[0052] In some embodiments of this application, in the Raman spectra of single-walled carbon nanotubes, 1350 cm⁻¹ -1 The intensity of the characteristic peak at that location is I. D1 1580cm -1 The intensity of the characteristic peak at that location is I. G1 ,40≤I G1 / I D1 ≤200. Preferably, 50≤I G1 / ID1 ≤100. For example, I G1 / I D1 The value is 40, 50, 55, 60, 70, 90, 100, 110, 120, 135, 150, 160, 170, 180, 200, or any value within any two of the above ranges. The peak intensity ratio I of the G peak and D peak of single-walled carbon nanotubes. G1 / I D1 The value being within the above range indicates that single-walled carbon nanotubes have good crystalline order and fewer structural defects, which is conducive to penetrating into the framework formed by whisker carbon nanotubes to form a stable and continuous three-dimensional conductive network. This results in a conductive network with good conductivity in the positive electrode active material layer, improving the transport rate of ions and electrons, reducing the contact resistance of the positive electrode sheet, and reducing the heat generation of the secondary battery during charge and discharge cycles. As a result, the secondary battery has good cycle performance at high discharge rates, while also improving its temperature rise control capability.

[0053] This application relates to I G1 / I D1 There are no particular restrictions on how the value of I can be adjusted, as long as the purpose of this application can be achieved. For example, it can be achieved by adjusting the diameter of the single-walled carbon nanotubes. Generally, the larger the diameter of the single-walled carbon nanotubes, the better I can be adjusted. G1 / I D1 The smaller the value, the better.

[0054] In some embodiments of this application, the diameter of the multi-walled carbon nanotubes is d3 nm, where 3 ≤ d3 ≤ 20. For example, the diameter d3 nm of the multi-walled carbon nanotubes is 3 nm, 5 nm, 8 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm, or any value between any two of the above ranges. Controlling the diameter of the multi-walled carbon nanotubes within the above range facilitates the penetration of multi-walled carbon nanotubes into the framework formed by whisker carbon nanotubes, thereby completing the secondary distribution of multi-walled carbon nanotubes. This allows the multi-walled carbon nanotubes and whisker carbon nanotubes to work synergistically to form a conductive network with good conductivity, thus reducing the contact resistance of the positive electrode and enabling the positive electrode to have higher porosity. This enhances the wettability and liquid retention of the electrolyte, improves the transport properties of ions and electrons, and reduces the probability of heat generation during charge-discharge cycles. Consequently, it improves the cycle performance and temperature rise control capability of the secondary battery during high-rate discharge.

[0055] In some embodiments of this application, the length of the multi-walled carbon nanotubes is L³ nm, and the aspect ratio of the multi-walled carbon nanotubes is L³ / d³, where 75 ≤ L³ / d³ ≤ 500. For example, the aspect ratio L³ / d³ of the multi-walled carbon nanotubes can be 75, 100, 120, 160, 200, 260, 300, 350, 400, 420, 500, or any value between any two of the above ranges. Controlling the aspect ratio of the multi-walled carbon nanotubes within the above range is beneficial for the multi-walled carbon nanotubes and whisker carbon nanotubes to jointly construct a stable three-dimensional conductive network, improving the ion and electron transport rate, reducing the contact resistance of the positive electrode, and reducing heat generation during charge-discharge cycles of the secondary battery. This allows the secondary battery to maintain good cycle performance at high discharge rates while improving its temperature rise control capability.

[0056] This application does not impose any particular restrictions on the method of controlling the diameter and length of multi-walled carbon nanotubes, as long as the purpose of this application can be achieved. For example, it can be achieved by purchasing commercially available multi-walled carbon nanotubes and selecting multi-walled carbon nanotubes of the required length and diameter, or by purchasing commercially available multi-walled carbon nanotubes and then using sand milling and sieving methods.

[0057] In some embodiments of this application, the number of multi-walled carbon nanotubes is N layers, where 8 ≤ N ≤ 57. For example, N can be 8, 9, 10, 12, 15, 17, 20, 22, 25, 28, 30, 40, 45, 57, or any combination of two of the above values. Selecting multi-walled carbon nanotubes with a layer count within the above range is beneficial for constructing a three-dimensional conductive network with good support, stability, and conductivity in the positive electrode active material layer. This improves the ion and electron transport rate, reduces the contact resistance of the positive electrode sheet, and decreases heat generation during charge-discharge cycles. Consequently, while maintaining good cycle performance at high discharge rates, the temperature rise control capability of the secondary battery is also improved.

[0058] In some embodiments of this application, in the Raman spectra of multi-walled carbon nanotubes, 1350 cm⁻¹ -1 The intensity of the characteristic peak at that location is I. D2 1580cm -1 The intensity of the characteristic peak at that location is I. G2 , 1≤I G2 / I D2 ≤10. For example, I G2 / I D2 The value is 1, 1.2, 1.4, 1.7, 2.0, 2.2, 2.3, 2.5, 2.8, 3, 4, 4.5, 5, 5.6, 7, 7.3, 8, 8.5, 9, 10, or any value within any two of the above ranges. The peak intensity ratio I of the G and D peaks of multi-walled carbon nanotubes. G2 / I D2The value being within the above range indicates that multi-walled carbon nanotubes have good crystalline order and fewer structural defects, which is conducive to penetrating into the framework formed by whisker carbon nanotubes to form a stable and continuous three-dimensional conductive network. This results in a conductive network with good conductivity in the positive electrode active material layer, improving the transport rate of ions and electrons, reducing the contact resistance of the positive electrode, and reducing the heat generation of the secondary battery during charge and discharge cycles. As a result, the secondary battery has good cycle performance at high discharge rates, while also improving its temperature rise control capability.

[0059] This application relates to I G2 / I D2 There are no particular restrictions on how the value of I can be adjusted, as long as the purpose of this application can be achieved. For example, it can be achieved by adjusting the diameter of the multi-walled carbon nanotubes. Generally, the larger the diameter of the multi-walled carbon nanotubes, the better I can be adjusted. G2 / I D2 The smaller the value, the better.

[0060] In some embodiments of this application, the length of single-walled carbon nanotubes is L2 nm, and the length of multi-walled carbon nanotubes is L3 nm, satisfying: 4000≤L2≤8000, 1000≤L3≤2000, and 0.125≤L3 / L2≤0.5. For example, the length L2 nm of single-walled carbon nanotubes is 4000nm, 4300nm, 4500nm, 4600nm, 4800nm, 5000nm, 5100nm, 5400nm, 5500nm, 5800nm, 6000nm, 6200nm, 6400nm, 6700nm, 7000nm, 7200nm, 7400nm, 7700nm, 7800nm, 8000nm, or any value between any two of the above ranges. For example, the length L3 nm of the multi-walled carbon nanotube can be 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, 2000 nm, or any value between any two of the above ranges. For example, L3 / L2 can be 0.125, 0.15, 0.17, 0.21, 0.26, 0.3, 0.32, 0.37, 0.40, 0.42, 0.45, 0.5, or any value between any two of the above ranges. Controlling the length of single-walled carbon nanotubes, the length of multi-walled carbon nanotubes, and their ratio within the aforementioned range is beneficial for forming a three-dimensional interpenetrating conductive network. This further improves the electronic and ionic conductivity of the positive electrode, reduces the contact resistance of the positive electrode, and decreases the heat generation of the secondary battery during charge-discharge cycles. As a result, the secondary battery not only has good cycle performance at high discharge rates but also improves its temperature rise control capability.

[0061] In some embodiments of this application, the positive electrode conductive agent includes conductive carbon black, whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes, wherein the surface energy difference between single-walled carbon nanotubes and multi-walled carbon nanotubes is γ mJ / m. 2 5 ≤ y ≤ 30. Preferably, 10 ≤ y ≤ 22. It should be noted that the surface energy difference mentioned above can also be understood as the absolute value of the surface energy difference between single-walled carbon nanotubes and multi-walled carbon nanotubes. For example, y can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 20, 22, 25, 27, 30, or any value within any two of the above ranges. When the positive electrode conductive agent contains both single-walled and multi-walled carbon nanotubes, controlling the surface energy difference between them within the above range is beneficial for the bonding of single-walled and multi-walled carbon nanotubes, and also beneficial for their uniform dispersion in the positive electrode slurry, thereby improving the conductivity of the three-dimensional conductive network and reducing the resistance of the positive electrode sheet. This improves the cycle performance and temperature rise control capability of the secondary battery during high-rate discharge.

[0062] This application does not impose any particular restrictions on the surface energy of single-walled carbon nanotubes and multi-walled carbon nanotubes, as long as the objective of this application can be achieved. For example, the surface energy of a single-walled carbon nanotube is 40 mJ / m². 2 Up to 70mJ / m 2 The surface energy of multi-walled carbon nanotubes is 30 mJ / m. 2 Up to 50mJ / m 2 In some embodiments, the surface energy of single-walled carbon nanotubes is greater than that of multi-walled carbon nanotubes.

[0063] This application does not impose any particular restrictions on the method of controlling the surface energy of single-walled carbon nanotubes, as long as the purpose of this application can be achieved. For example, it can be achieved by controlling the diameter of the single-walled carbon nanotubes. This application also does not impose any particular restrictions on the method of controlling the surface energy of multi-walled carbon nanotubes, as long as the purpose of this application can be achieved. For example, it can be achieved by controlling the number of layers of multi-walled carbon nanotubes.

[0064] In some embodiments of this application, such as Figure 1 As shown, the thickness H of the positive electrode 100 is... 100 The thickness ranges from 50 μm to 200 μm. For example, the thickness of the positive electrode sheet can be 50 μm, 60 μm, 82 μm, 90 μm, 100 μm, 110 μm, 130 μm, 150 μm, 162 μm, 180 μm, 190 μm, 200 μm, or any value between any two of the above ranges. When the thickness of the positive electrode sheet is within the above range, the secondary battery exhibits good cycle performance and temperature rise control during high-rate discharge. Furthermore, the positive electrode sheet contains a relatively high amount of positive electrode active material, which can provide a higher capacity for the secondary battery.

[0065] In some embodiments of this application, the compaction density of the positive electrode sheet is 2.4 g / cm³. 3 Up to 3.2 g / cm 3 For example, the compaction density of the positive electrode sheet is 2.4 g / cm³. 3 2.5g / cm 3 2.6g / cm 3 2.7g / cm 3 2.8g / cm 3 2.9g / cm 3 3.0g / cm 3 3.1g / cm 3 3.2g / cm 3 Or any value between any two of the above ranges. Controlling the compaction density of the positive electrode sheet within the above range is beneficial because it allows the positive electrode sheet to have a high porosity while minimizing the probability of breakage of the positive electrode active material, thus enabling the positive electrode active material to fully utilize its capacity. The positive electrode sheet also exhibits high electrolyte wettability and liquid retention, good ion and electron transport within the secondary battery, lower heat generation during charge-discharge cycles, and good cycle performance and temperature rise control during high-rate discharge.

[0066] This application does not impose any particular restrictions on the method of controlling the compaction density of the positive electrode sheet, as long as the purpose of this application can be achieved. For example, it can be achieved by controlling the cold pressing pressure of the positive electrode sheet and the particle size of the positive electrode active material particles.

[0067] This application does not impose any particular limitation on the content of the positive electrode active material and the positive electrode binder, as long as the purpose of this application can be achieved. For example, based on the mass of the positive electrode active material layer, the mass percentage content of the positive electrode active material is 94% to 97%, and the mass percentage content of the positive electrode binder is 1% to 2.5%. This application does not impose any particular limitation on the type of positive electrode binder, as long as the purpose of this application can be achieved. For example, the positive electrode binder includes, 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, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0068] 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, the positive electrode current collector may contain aluminum foil or aluminum alloy foil. In this application, there is no 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 5 μm to 20 μm.

[0069] In this application, the secondary battery further includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode active material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode active 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 "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the negative electrode current collector, as long as the purpose of this application is achieved. For example, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. Exemplarily, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc. The negative electrode active 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 microcarbon 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.

[0070] In some embodiments of this application, the negative electrode active material layer may further include a negative electrode conductive agent and a negative electrode binder. This application does not particularly limit the types of negative electrode conductive agents and negative electrode binders, as long as they achieve the purpose of this application. For example, the negative electrode conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes, carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. Conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers; specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. For example, the negative electrode 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, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. This application does not impose any particular limitation on the mass ratio of the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder in the negative electrode active material layer; those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0071] This application does not impose any particular limitation on the thickness of the negative electrode active material layer, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode active material layer is 30 μm to 120 μm. This application also does not impose any particular limitation on the thickness of the negative electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector is 4 μm to 15 μm.

[0072] In this application, the secondary battery also includes an electrolyte, which comprises lithium salts and non-aqueous solvents. This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application does not impose any particular limitation on the content of lithium salts in the electrolyte, as long as it achieves the purpose of this application. This application does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or 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). Fluorinated carbonate 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. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-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.

[0073] In this application, 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 or rigid plastic. This application does not limit the type of metal; the rigid casing can be a metal casing known in the art, as long as it achieves the purpose of this application. The flexible casing can be a metal-plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0074] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. For example, a secondary battery may include, but is not limited to, a lithium metal secondary battery, a lithium-ion secondary battery (lithium-ion battery), a lithium polymer secondary battery, or a lithium-ion polymer secondary battery.

[0075] This application does not impose any particular restrictions on the preparation method of the positive electrode sheet, as long as it can achieve the purpose of this application. For example, in some embodiments, the preparation method of the positive electrode sheet includes, but is not limited to, the following steps: 1) dispersing whisker carbon nanotubes in a solution of N-methylpyrrolidone (NMP) or polyvinylidene fluoride (PVDF) at a high speed of 4500 rpm to 5500 rpm for 25 min to 35 min to form a pre-dispersed framework network, then adding single-walled carbon nanotubes and / or multi-walled carbon nanotubes and mixing at a low speed of 1500 rpm to 2500 rpm to obtain a dispersion; mixing the positive electrode active material, conductive carbon black and positive electrode binder at a low speed of 45 rpm to 55 rpm for 25 min to 35 min, adding NMP solvent and dispersing for 55 min to 65 min, then adding the aforementioned dispersion for final dispersion to obtain a positive electrode slurry with a solid content of 60 wt% to 70 wt% and a viscosity of 4000 mPa·s to 6000 mPa·s; 2) coating the positive electrode slurry on one surface of the positive electrode current collector, and drying and cold pressing to form a positive electrode sheet with a positive electrode active material layer on one side. In some other embodiments, the preparation method of the positive electrode sheet includes the following steps: (1) dispersing whisker carbon nanotubes in NMP or PVDF solution at a high speed of 4500 rpm to 5500 rpm for 25 min to 35 min, adding single-walled carbon nanotubes and / or multi-walled carbon nanotubes and mixing at a low speed of 1500 rpm to 2500 rpm to obtain a dispersion; mixing the positive electrode active material, conductive carbon black and positive electrode binder at a low speed of 45 rpm to 55 rpm for 25 min to 35 min. n, after adding NMP solvent, disperse for 55 min to 65 min, then add the aforementioned dispersion for final dispersion to obtain a positive electrode slurry with a solid content of 60 wt% to 70 wt% and a viscosity of 4000 mPa·s to 6000 mPa·s; (2) the positive electrode slurry coated on one surface of the positive electrode current collector is dried to form a positive electrode active material layer; (3) repeat step (2) on the other surface of the positive electrode current collector, and obtain a positive electrode sheet with positive electrode active material layers on both sides by cold pressing and cutting. In steps 1) and (1) above, the whisker carbon nanotubes are dispersed to form a pre-dispersed framework network. The single-walled carbon nanotubes and / or multi-walled carbon nanotubes are dispersed at the above low speed, which promotes the spontaneous adsorption of single-walled carbon nanotubes and / or multi-walled carbon nanotubes on the surface of the whisker carbon nanotubes. This preparation method increases the entanglement density of the whisker carbon nanotube-single-walled carbon nanotube / multi-walled carbon nanotube composite by 2 to 3 times. After the positive electrode slurry is coated and dried, the conductive network presents a "coral reef" hierarchical structure, that is, VGCF constitutes the main branches, CNTs form the secondary distribution, and zero-dimensional conductive carbon black is uniformly dispersed. The three together coat the surface of the positive electrode active material particles to improve the conductivity of the positive electrode sheet.

[0076] This application does not impose any particular limitation on the preparation method of the secondary battery. Any preparation method known in the art can be used, as long as it can achieve the purpose of this application. For example, the preparation method of the secondary battery includes, but is not limited to, the following steps: stacking the separator, positive electrode, separator and negative electrode in sequence, and winding, folding or other operations as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery; or stacking the separator, positive electrode, separator and negative electrode in sequence, and then fixing the four corners of the entire stacked structure to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery.

[0077] A second aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device has good performance characteristics.

[0078] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to: laptop computers, 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, household large-capacity batteries, and lithium-ion capacitors.

[0079] Example

[0080] 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.

[0081] Test methods and equipment:

[0082] Sampling method for positive electrode:

[0083] The lithium-ion batteries of each embodiment and comparative example were discharged to 3.0V at a constant current of 0.2C, and then disassembled to obtain the positive electrode sheet. The surface of the positive electrode sheet was cleaned with dimethyl carbonate (DMC) to obtain the positive electrode sheet sample.

[0084] Unless otherwise specified, the positive electrode plates in the following test methods are obtained using the methods described above.

[0085] Testing of diameter, length, and aspect ratio:

[0086] (1) Scrape off the positive electrode sheet to obtain the positive electrode active material layer, take 5g of positive electrode active material layer powder and dissolve it in 10ml of NMP, sonicate at 60℃ for 30min and let stand for 20min, discard the supernatant and keep the precipitate (the purpose of this step is to remove the positive electrode binder, in order to prepare for the subsequent acquisition of dispersed single-walled carbon nanotubes, multi-walled carbon nanotubes and whisker carbon nanotubes).

[0087] (2) Disperse the precipitate from step (1) into 10 ml of NMP, sonicate at room temperature for 30 min, let stand for 20 min, and take the supernatant.

[0088] (3) Drop the supernatant obtained in step (2) onto aluminum foil, dry it, and obtain the sample to be tested;

[0089] (4) Take the sample to be tested obtained in step (3). Use a scanning electron microscope (SEM) at 50,000x magnification to test the diameter and length of the sample. The aspect ratio is calculated using the following formula: Aspect ratio = Length / Diameter. For both diameter and length tests, 20 samples are randomly selected, and the average value is taken as the final value. For example, when measuring the diameter of single-walled carbon nanotubes, 20 single-walled carbon nanotubes are randomly selected to obtain 20 diameter values, and the average value is taken as the final diameter value. The same applies to the testing of other parameters.

[0090] The diameter, length, and aspect ratio of whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes were obtained using the methods described above. It should be noted that whisker carbon nanotubes, single-walled carbon nanotubes, and multi-walled carbon nanotubes can be directly distinguished by their morphology under SEM.

[0091] Porosity testing of the positive electrode sheet:

[0092] Fifty positive electrode sheets with radius d were punched using the same die. The thickness h of each positive electrode sheet was measured using a micrometer, and the sheets were placed into the sample chamber of an AccuPyc 1340 instrument. Helium (He) gas was used to fill the sealed sample chamber, and Bohr's law PV = nRT was used to determine the true volume V of the positive electrode sheet. After testing, the number of small discs was counted, and the apparent volume πd of the sample was calculated. 2 ×50×h. Finally, the porosity α of the positive electrode sheet is obtained by the following formula: α(%)=(1-V / πd 2 (×50×h)×100%.

[0093] Testing of the compaction density of the positive electrode sheet:

[0094] A small circular sheet with an area of ​​S is punched from the positive electrode sheet using a punching machine. Its mass is measured as M1, and its thickness is measured as T1 using a micrometer. A positive electrode current collector with the same area S is punched using the same punching machine, and its mass is measured as M2. Its thickness is measured as T2 using a micrometer. The compaction density Cd of the positive electrode sheet is... 3 )=(M1-M2) / (T1-T2) / S.

[0095] Raman spectroscopy testing:

[0096] 10 μL of the test slurry was coated onto a silicon wafer, dried, and then a 532 nm laser Raman spectrometer (compliant with GB / T36065-2018) was used with a laser power set to 0.5 mW / μm. 2 Integration time: 30 seconds. At 1350cm... -1 (D peak) and 1580cm -1 Data was collected at peak G, and after baseline correction, peak fitting was performed to calculate the intensity ratio: G / D = I. G / I D The average value of 5 test points is taken, and the relative standard deviation (RSD) is ≤5%.

[0097] Preparation of the test slurry of single-walled carbon nanotubes: Single-walled carbon nanotubes and polyvinylidene fluoride were mixed at a mass ratio of 1:2 and dispersed in NMP solvent to obtain a test slurry with a solid content of 0.5 wt%.

[0098] Preparation of the test slurry of multi-walled carbon nanotubes: Multi-walled carbon nanotubes and polyvinylidene fluoride were mixed at a mass ratio of 1:1.5 and dispersed in NMP solvent to obtain a test slurry with a solid content of 0.5 wt%.

[0099] Raman spectra of both single-walled carbon nanotubes and multi-walled carbon nanotubes were obtained using the above method.

[0100] Surface energy testing:

[0101] Pure single-walled carbon nanotubes (CNTs) were extracted from the slurry by thermogravimetric analysis (TGA) and centrifugation. A standard sample, designated Sample 1, was prepared by pressing (10 MPa pressure, surface roughness Ra < 0.1 μm). Simultaneously, the original slurry was coated onto a glass substrate and vacuum-dried at 80°C to form a film, designated Sample 2. The contact angles θ1 and θ2 of both samples with deionized water (polar) and diiodomethane (non-polar) were measured using a contact angle meter (5 points were measured at each point, and the average was taken). When the difference in contact angle between Sample 2 and Sample 1, Δθ < 5°, the effect of the dispersant on the surface energy of single-walled carbon nanotubes was deemed negligible, and the data from Sample 1 was directly adopted (γ_single-walled carbon nanotubes = γ_slurry). If Δθ ≥ 5°, a correction was made using the empirical formula γ_carbon nanotubes ≈ γ_slurry + 0.8 × Δθ (Δθ is in degrees, and 0.8 is the empirical correction parameter).

[0102] Similarly, the surface energy of multi-walled carbon nanotubes can be obtained using the above method. The surface energy difference y is the absolute value of the difference between the surface energy of single-walled carbon nanotubes and the surface energy of multi-walled carbon nanotubes.

[0103] Testing of high-rate discharge cycle performance:

[0104] When selecting Ni6 system cathode active materials, such as Ni62 and Ni71, the following methods are used for testing:

[0105] (1) Before the formal test cycle, the procedure is as follows: 1) Test temperature is 25℃; 2) Let stand for 30 minutes; 3) Charge at 6.5C constant current to 4.35V, then charge at 4.35V constant voltage to 0.05C; 4) Let stand for 5 minutes; 5) Discharge at 0.2C constant current to 3V.

[0106] (2) Cyclic process: 1) Test temperature is 25℃; 2) Let stand for 20 min; 3) Charge at 6.5C constant current to 4.35V, charge at 4.35V constant voltage to 0.05C (set the cutoff capacity of this step as variable parameter C2); 4) Let stand for 5 min; 5) Discharge at 11.5C constant current to 3V (set the cutoff capacity of this step as variable parameter C3); 6) Let stand for 1 min (sampling mode is 1s); 7) If C2 / C3>1.02, then jump to step 9); 8) If C2 / C3≤1.02, then jump to step 10); 9) Pause; 10) Let stand for 30 min; 11) Cycle from step 3) to step 10) 1000 times (set the cutoff capacity of 1000 times as variable parameter C4); Capacity retention rate (%) = C4 / C2×100%.

[0107] When selecting positive electrode active materials based on Ni8 and Ni9 systems, such as NCM811, Ni91, Ni92, and Ni93, the following methods are used for testing:

[0108] 1) Test temperature: 25℃; 2) Let stand for 30 minutes; 3) Charge at 3C constant current to 4V (set the cutoff capacity of this step as variable parameter C1); 4) Charge at 1.2C constant current to 4.3V, then charge at 4.3V constant voltage to 0.05C (set the cutoff capacity of this step as variable parameter C2); 5) Let stand for 15 minutes; 6) Discharge at 5C constant current to 2.5V (set the cutoff capacity of this step as variable parameter C3); 7) 8) If (C1+C2) / C3>1.01, then jump to step 10); 9) If (C1+C2) / C3≤1.01, then jump to step 11); 10) Pause; 11) Let stand for 30 minutes; 12) Repeat steps 3) to 11) 300 times (set the cutoff capacity for 300 cycles to variable parameter C4); Capacity retention rate (%) = C4 / C2 × 100%.

[0109] Capacity retention rate is used to characterize the cycle performance of a secondary battery at high discharge rates. Specifically, a higher capacity retention rate indicates better cycle performance at high discharge rates, while a lower capacity retention rate indicates worse cycle performance at high discharge rates.

[0110] Temperature rise control capability test:

[0111] During the aforementioned "high-rate discharge cycle performance test", the surface temperature change of the lithium-ion battery was detected at the head of the lithium-ion battery using a temperature sensing line. The temperature difference was obtained by subtracting the temperature before discharge from the temperature at the end of the first charge-discharge cycle, and recorded as the temperature rise value, which was used to evaluate the temperature rise control capability of the lithium-ion battery.

[0112] Temperature rise value is used to characterize the temperature rise control capability of a secondary battery during high-rate discharge. Specifically, the smaller the temperature rise value, the better the temperature rise control capability of the secondary battery during high-rate discharge; the larger the temperature rise value, the worse the temperature rise control capability of the secondary battery during high-rate discharge.

[0113] Example 1-1

[0114] <Preparation of the positive electrode>

[0115] Whiskers of carbon nanotubes were dispersed in NMP solution at a high-speed shear rate of 5000 rpm for 30 min. Single-walled carbon nanotubes and multi-walled carbon nanotubes were then added and mixed at a low speed of 2000 rpm to obtain a dispersion. The positive electrode active material Ni93 (LiNi) was then added. 0.93 Co 0.02 Mn 0.05 O2), conductive carbon black, and positive electrode binder PVDF (weight-average molecular weight of 5×10⁻⁶). 5The mixture was stirred at a low speed of 50 rpm for 30 min, then NMP solvent was added and dispersed for 60 min. The previously obtained dispersion was then added for final dispersion, yielding a positive electrode slurry with a solid content of 65 wt% and a viscosity of 5000 mPa·s. The 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 single-sided coating of the positive active material layer. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of the positive active material layer. After drying at 120 °C, the electrode sheet was cold-pressed, then cut and had tabs welded to obtain a positive electrode sheet with dimensions of 74 mm × 867 mm for later use.

[0116] Among them, based on the mass of the positive electrode active material layer, the mass percentage of the positive electrode active material is 96.4%, the mass percentage of the positive electrode binder is 1.4%, and the mass percentages of the positive electrode conductive agent, whisker carbon nanotubes and multi-walled carbon nanotubes are shown in Table 1.

[0117] <Preparation of Negative Electrode Sheets>

[0118] Artificial graphite (negative electrode active material), styrene-butadiene rubber (negative electrode binder), and acetylene black (negative electrode 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 4.5 μm thick copper foil current collector and dried at 120°C to obtain a negative electrode sheet with a single-sided coating of negative electrode active material. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode active material. After drying at 120°C, the sheet was cold-pressed, cut, and had tabs welded to obtain a negative electrode sheet with a size of 78 mm × 875 mm for later use. The compaction density of the negative electrode sheet was 1.4 g / cm³. 3 The thickness of the negative electrode sheet is 135μm.

[0119] <Preparation of Electrolyte>

[0120] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Lithium salt LiPF6 was then added to the organic solvent and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the lithium salt comprised 12.5% ​​by mass, with the remainder being organic solvent.

[0121] <Isolation membrane>

[0122] A porous polyethylene film with a thickness of 7μm (provided by Celgard) was used as the separator.

[0123] <Preparation of Lithium-ion Batteries>

[0124] The separator, positive electrode, separator, and negative electrode prepared above are stacked in sequence, with the separator positioned between the positive and negative electrode to provide isolation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film housing and dehydrated at 80°C. The electrolyte prepared above is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, degassing, and edge trimming.

[0125] Examples 1-2 to 1-23

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

[0127] When the mass percentage of the positive electrode conductive agent changes, the mass percentage of the positive electrode active material changes accordingly, while the mass percentage of the positive electrode binder remains unchanged.

[0128] Examples 2-1 to 2-20

[0129] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-16.

[0130] Examples 3-1 to 3-4

[0131] Except for adjusting the thickness of the positive and negative electrode sheets according to Table 3, the rest is the same as in Example 1-1.

[0132] The thickness of the positive electrode sheet is changed by adjusting the thickness of a single layer of the positive active material layer, and the thickness of the negative electrode sheet is changed by adjusting the thickness of a single layer of the negative active material layer.

[0133] Comparative Examples 1 to 10

[0134] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0135] The preparation parameters and performance data of each embodiment and comparative example are shown in Tables 1 to 3.

[0136] Table 1

[0137]

[0138]

[0139]

[0140] Note: In Table 1, “\” indicates no corresponding parameter; “W3” in Table 1 represents the mass percentage of conductive carbon black based on the mass of the positive electrode active material layer; “W4” in Table 1 represents the mass percentage of whisker carbon nanotubes based on the mass of the positive electrode active material layer; “W5” represents the mass percentage of single-walled carbon nanotubes based on the mass of the positive electrode active material layer; and “W6” represents the mass percentage of multi-walled carbon nanotubes based on the mass of the positive electrode active material layer.

[0141] As can be seen from Examples 1-1 to 1-15 and Comparative Examples 1 to 10, when lithium nickel cobalt manganese oxide is selected as the positive electrode active material in the secondary battery of this application, by selecting conductive carbon black, whisker carbon nanotubes, and single-walled carbon nanotubes and / or multi-walled carbon nanotubes as positive electrode conductive agents and controlling the diameter and length of whisker carbon nanotubes within the range of this application, and controlling the porosity of the positive electrode sheet within the range of this application, the secondary battery has a higher capacity retention rate at high discharge rate and a smaller temperature rise during high discharge rate cycling, indicating that the cycling performance and temperature rise control capability of the secondary battery at high discharge rate are improved. In contrast, the secondary batteries in the comparison have diameters or lengths of whisker carbon nanotubes that are not within the scope of this application, or the positive electrode conductive agent does not include whisker carbon nanotubes or conductive carbon black, or the positive electrode conductive agent does not include single-walled carbon nanotubes or multi-walled carbon nanotubes, or the porosity of the positive electrode sheet is not within the scope of this application. The secondary batteries have lower capacity retention rates at high discharge rates and greater temperature rises during high discharge rate cycles, indicating that the cycle performance and temperature rise control capabilities of the secondary batteries at high discharge rates have not been improved.

[0142] The diameter, length, and aspect ratio of whisker carbon nanotubes affect the cycle performance and temperature rise control capability of secondary batteries during high-rate discharge. As can be seen from Examples 1-1 to 1-10 and Comparative Examples 5 to 8, secondary batteries using whisker carbon nanotubes with diameters, lengths, and aspect ratios within the range of this application exhibit higher capacity retention during high-rate discharge and lower temperature rise during high-rate cycling, indicating that the secondary batteries possess good cycle performance and temperature rise control capability during high-rate discharge.

[0143] The porosity and compaction density (Cd) of the positive electrode sheet affect the cycle performance and temperature rise control capability of the secondary battery during high-rate discharge. As can be seen from Examples 1-1, 1-11 to 1-13, secondary batteries using positive electrode sheets with porosity and compaction density (Cd) within the range of this application exhibit higher capacity retention during high-rate discharge and lower temperature rise during high-rate cycling, indicating that the secondary battery has good cycle performance and temperature rise control capability during high-rate discharge.

[0144] The type of positive electrode active material affects the cycle performance and temperature rise control capability of a secondary battery during high-rate discharge. As can be seen from Examples 1-1, 1-14, and 1-15, secondary batteries using positive electrode active materials within the scope of this application exhibit higher capacity retention during high-rate discharge and lower temperature rise during high-rate cycle operation, indicating that the secondary batteries have good cycle performance and temperature rise control capability during high-rate discharge.

[0145] W1% and W2% affect the cycle performance and temperature rise control capability of secondary batteries during high-rate discharge. As can be seen from Examples 1-1, 1-16 to 1-23, secondary batteries with W1% and W2% within the scope of this application exhibit higher capacity retention during high-rate discharge and lower temperature rise during high-rate cycle, indicating that the secondary batteries have good cycle performance and temperature rise control capability during high-rate discharge.

[0146] Table 2

[0147]

[0148]

[0149] Note: In Table 2, "y1" represents the surface energy of single-walled carbon nanotubes and "y2" represents the surface energy of multi-walled carbon nanotubes.

[0150] The diameter and aspect ratio of single-walled carbon nanotubes affect the cycle performance and temperature rise control capability of secondary batteries during high-rate discharge. As can be seen from Examples 1-16 and Examples 2-1 to 2-7, secondary batteries using single-walled carbon nanotubes with diameters, lengths, and aspect ratios within the range of this application exhibit higher capacity retention during high-rate discharge and lower temperature rise during high-rate cycling, indicating good cycle performance and temperature rise control capability during high-rate discharge. When the diameter and aspect ratio of the single-walled carbon nanotubes change, I... G1 / I D1 The value changes accordingly. Figure 3 The Raman spectra of the single-walled carbon nanotubes in Example 2-2 are shown. Figure 3 It can be seen that I G1 / I D1 The value is 50.

[0151] The diameter and aspect ratio of multi-walled carbon nanotubes affect the cycle performance and temperature rise control capability of secondary batteries during high-rate discharge. As can be seen from Examples 1-16 and Examples 2-8 to 2-19, secondary batteries using multi-walled carbon nanotubes with diameters, lengths, and aspect ratios within the range of this application exhibit higher capacity retention during high-rate discharge and lower temperature rise during high-rate cycling, indicating good cycle performance and temperature rise control capability during high-rate discharge. When the diameter and aspect ratio of the multi-walled carbon nanotubes change, I... G2 / I D2 The value changes accordingly. Figure 4 Raman spectra of the multi-walled carbon nanotubes in Examples 2-12 are shown. Figure 4 It can be seen that I G2 / I D2 The value is 1.4. The lengths of single-walled carbon nanotubes and multi-walled carbon nanotubes, and their length ratio L3 / L2, affect the cycle performance and temperature rise control capability of the secondary battery during high-rate discharge. As can be seen from Examples 1-16, Examples 2-4 to 2-7, and Examples 2-14 to 2-20, secondary batteries using single-walled carbon nanotubes and multi-walled carbon nanotubes, and their length ratio L3 / L2 within the scope of this application, exhibit higher capacity retention during high-rate discharge and lower temperature rise during high-rate cycling, indicating that the secondary battery has good cycle performance and temperature rise control capability during high-rate discharge.

[0152] Table 3

[0153]

[0154]

[0155] The thickness of the positive electrode sheet affects the cycle performance and temperature rise control capability of the secondary battery during high-rate discharge. As can be seen from Examples 1-1, 3-1 to 3-4, secondary batteries with positive electrode sheet thicknesses within the range of this application exhibit higher capacity retention during high-rate discharge and lower temperature rise during high-rate cycle operation, indicating good cycle performance and temperature rise control capability during high-rate discharge. Among these, compared to Examples 1-1, 3-2, and 3-3, the thickness of the positive electrode active material layer in Example 3-1 is smaller, resulting in a lower temperature rise. However, due to the smaller thickness of the positive electrode active material layer, the positive electrode sheet of Example 3-1 may present processing difficulties.

[0156] 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.

[0157] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0158] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising a positive active material, a positive conductive agent and a positive binder; The positive electrode active material includes lithium nickel cobalt manganese oxide; The positive electrode conductive agent includes conductive carbon black, whisker carbon nanotubes, and at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes. The diameter of the whisker carbon nanotubes is d1 nm, and 50≤d1≤200; The length of the whisker carbon nanotubes is L1 nm, and 5000≤L1≤20000; The porosity of the positive electrode sheet is 25% to 45%.

2. The secondary battery according to claim 1, wherein, The aspect ratio of the whisker carbon nanotubes is L1 / d1, where 25 ≤ L1 / d1 ≤ 400.

3. The secondary battery according to claim 1, wherein, Based on the mass of the positive electrode active material layer, the mass percentage of the positive electrode conductive agent is W1%, where 1.5 ≤ W1 ≤ 5.

4. The secondary battery according to claim 3, wherein, Based on the mass of the positive electrode active material layer, the sum of the mass percentages of the whisker carbon nanotubes, the single-walled carbon nanotubes, and the multi-walled carbon nanotubes is W2%, where 1 ≤ W2 ≤ 4.

5. The secondary battery according to claim 4, wherein, The positive electrode sheet satisfies at least one of the following characteristics: (1)50≤d1≤100； (2)8000≤L1≤20000； (3) 80≤L1 / d1≤400; (4)2≤W2≤3.5。 6. The secondary battery according to claim 1, wherein, The diameter of the single-walled carbon nanotube is d2 nm, and 0.5≤d2≤2.

7. The secondary battery according to claim 6, wherein, The length of the single-walled carbon nanotube is L2 nm, and the aspect ratio of the single-walled carbon nanotube is L2 / d2, where 1000≤L2 / d2≤8000.

8. The secondary battery according to claim 1, wherein, In the Raman spectrum of the single-walled carbon nanotubes, 1350 cm⁻¹ -1 The intensity of the characteristic peak at that location is I. D1 1580cm -1 The intensity of the characteristic peak at that location is I. G1 ,40≤I G1 / I D1 ≤200.

9. The secondary battery according to claim 1, wherein, The diameter of the multi-walled carbon nanotube is d3 nm, where 3 ≤ d3 ≤ 20.

10. The secondary battery according to claim 8, wherein, The length of the multi-walled carbon nanotube is L3 nm, and the aspect ratio of the multi-walled carbon nanotube is L3 / d3, where 75≤L3 / d3≤500.

11. The secondary battery according to claim 1, wherein, In the Raman spectrum of the multi-walled carbon nanotubes, 1350 cm⁻¹ -1 The intensity of the characteristic peak at that location is I. D2 1580cm -1 The intensity of the characteristic peak at that location is I. G2 , 1≤I G2 / I D2 ≤10.

12. The secondary battery according to claim 1, wherein, The length of the single-walled carbon nanotube is L2 nm, and the length of the multi-walled carbon nanotube is L3 nm, satisfying: 4000≤L2≤8000, 1000≤L3≤2000, and 0.125≤L3 / L2≤0.

5.

13. The secondary battery according to claim 1, wherein, The surface energy difference between the single-walled carbon nanotubes and the multi-walled carbon nanotubes is y mJ / m 2 , 5≤y≤30.

14. The secondary battery according to claim 1, wherein, The thickness of the positive electrode sheet is 50 μm to 200 μm.

15. The secondary battery according to claim 1, wherein, The compaction density of the positive electrode sheet is 2.4 g / cm³. 3 Up to 3.2 g / cm 3 .

16. An electronic device, wherein, The electronic device includes a secondary battery as described in any one of claims 1 to 15.

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