Positive electrode sheet and secondary battery

Abstract

The application discloses a positive pole piece and a secondary battery. The positive pole piece comprises a positive material layer, the positive material layer comprises an active material, first carbon nanotubes and second carbon nanotubes, the first carbon nanotubes are long straight and / or have a certain curvature, the outer diameter of the first carbon nanotubes is 20-200 nm, the inner diameter of the first carbon nanotubes is 3-15 nm, three points on the same root first carbon nanotube are connected along the length direction to form an included angle alpha, 30 DEG <= alpha <= 180 DEG, and a plurality of second carbon nanotubes are aggregated and / or wound to form a coating layer arranged on the surface of the active material. For the positive pole piece, the number of the first carbon nanotubes is m in any 25 mu m*25 mu m range during scanning electron microscope (SEM) test, and 2 <= m <= 115. The secondary battery provided by the application has better rate performance and cycle performance.

Description

Technical Field

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

[0002] As rechargeable batteries (such as lithium-ion batteries) gradually become the mainstream batteries in mobile phones, laptops, electric vehicles, energy storage devices, and other fields, higher requirements are being placed on their dynamic performance. During the use of rechargeable batteries, the positive electrode sheet continuously expands and contracts during cyclic charging and discharging, causing damage to part of the conductive network, resulting in deterioration of battery rate performance, charging performance, and rapid lifespan decay. Therefore, there is an urgent need to provide a positive electrode sheet with good conductivity to improve the dynamics of rechargeable batteries. Summary of the Invention

[0003] In view of this, this application provides a positive electrode and a secondary battery to improve the rate performance and cycle performance of the secondary battery.

[0004] In a first aspect, this application provides a positive electrode sheet, comprising a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer comprises an active material, a first carbon nanotube, and second carbon nanotubes. The first carbon nanotube is a long, straight carbon nanotube and / or a carbon nanotube with a certain curvature. The outer diameter of the first carbon nanotube is 20 nm to 200 nm, and the inner diameter is 3 nm to 15 nm. Three points are randomly selected on the same first carbon nanotube, and a line is drawn along its length to form an included angle α, where 30° ≤ α ≤ 180°. Multiple second carbon nanotubes aggregate and / or entwine to form a coating layer disposed on the surface of the active material. For the positive electrode sheet, during scanning electron microscopy testing, within any 25 μm × 25 μm range, the number of first carbon nanotubes is m, where 2 ≤ m ≤ 115, preferably 15 ≤ m ≤ 50. The cathode material layer contains both first carbon nanotubes and second carbon nanotubes. The first carbon nanotubes are rigid and not easily bent. An appropriate amount of first carbon nanotubes can connect the active material particles in series, while the second carbon nanotubes are flexible and can wrap around the surface of the active material particles. Through the synergistic cooperation between the first carbon nanotubes, the second carbon nanotubes and the active material particles, a good conductive network can be constructed, thereby improving the dynamics of the secondary battery.

[0005] The inventors speculate that because the first carbon nanotube has a high degree of graphitization, it has greater rigidity. Furthermore, its larger outer diameter and smaller inner diameter make it difficult to bend, resulting in a generally straight and unentangled state. Therefore, the first carbon nanotube can connect multiple active materials in series, forming a long-range conductive path, and it is less prone to "breaking the circuit" after the electrode expands cyclically. The second carbon nanotube, being more flexible, can entangle and adhere to the surface of the active material particles. This serves two purposes: providing short-range conductivity and reducing direct contact between the active material and the electrolyte, thus reducing side reactions and improving the rate performance and cycle performance of the secondary battery, especially its high-temperature cycle performance.

[0006] In some embodiments, for the first carbon nanotube, the Raman spectrum at 1150 cm⁻¹ -1 Up to 1450cm -1 A characteristic peak D exists within the range, with a peak intensity of I. D At 1450cm -1 Up to 1750cm -1 A characteristic peak G exists within the range, with a peak intensity of I. G Satisfying: 0 < I D / I G With a strength ≤0.7, its XRD diffraction pattern shows a diffraction peak A in the range of 10° to 40°, with a 2θ peak width greater than 0 and less than or equal to 15°. Thus, the first carbon nanotube has small defects and a high degree of graphitization and order, which is more conducive to forming long-range conductive pathways. Furthermore, it is less prone to "circuit breakage" after electrode cyclic expansion, thus improving the rate performance and high-temperature cycling performance of the secondary battery.

[0007] In some embodiments, the outer diameter of the first carbon nanotube is 30 nm to 80 nm, the inner diameter is 4 nm to 8 nm, and the length is L1 μm, where 2 ≤ L1 ≤ 50. Three points are randomly selected on the same first carbon nanotube, and lines drawn along its length form an angle α, where 90° ≤ α ≤ 180°. By adjusting the parameters of the first carbon nanotube within the above ranges, the rate performance and high-temperature cycling performance of the secondary battery can be further improved. Preferably, the outer diameter of the first carbon nanotube is 30–50 nm, the inner diameter is 4–7 nm, and 5 ≤ L1 ≤ 30.

[0008] In some embodiments, the outer diameter of the second carbon nanotube is 1 nm to 30 nm, the inner diameter is 0.05 nm to 20 nm, the length is 1 μm to 20 μm, and the mass percentage of the second carbon nanotube is 0.1% to 2%. By adjusting the parameters of the second carbon nanotube within the above range, it is beneficial for it to wrap around and adhere to the surface of the active material particles, which can further reduce the contact between the active material and the electrolyte, thereby improving the rate performance and high-temperature cycling performance of the secondary battery. Preferably, the outer diameter of the second carbon nanotube is 5–20 nm, the inner diameter is 1–15 nm, the length is 3–10 μm, and the content of the second carbon nanotube is 0.3% to 0.8%.

[0009] In some embodiments, the particle size Dv10 of the active material is D μm, and L1 ≥ 0.5D. This facilitates the tandem coordination between the first carbon nanotube and multiple active materials, better forming a long-range conductive path. It also promotes synergistic coordination among the active material particles, the first carbon nanotube, and the second carbon nanotube, further improving the rate performance and high-temperature cycling performance of the secondary battery. Preferably, 1 ≤ D ≤ 10. More preferably, 4 ≤ D ≤ 6.

[0010] In some embodiments, the positive electrode material layer further includes conductive carbon particles with a particle size Dv50 of 10 nm to 100 nm. Based on the mass of the positive electrode material layer, the mass percentage of conductive carbon particles is T, where T ≤ 2%. The conductive carbon particles are typically mixed with second carbon nanotubes and distributed on the surface of the active material particles. The presence of conductive carbon particles can increase the looseness of the conductive agent layer on the surface of the active particles, which is more conducive to the adsorption of more electrolyte and improves ion conductivity.

[0011] In some embodiments, the particle size Dv50 of the conductive carbon particles is 10-60 nm, and the mass ratio of the second carbon nanotube, the first carbon nanotube, the conductive carbon particles, and the positive electrode active material is 1:(0.02-8.0):(0-10):(47-980). With a suitable particle size of the conductive carbon particles and the mass ratio of the first carbon nanotube, the second carbon nanotube, the conductive carbon particles, and the active material within the above range, a conductive network combining long and short ranges can be constructed in the electrode, improving the cell rate and cycle performance. Preferably, the mass ratio of the second carbon nanotube, the first carbon nanotube, the conductive carbon particles, and the positive electrode active material is 1:(0.8-6.67):(0.2-1):(195-320).

[0012] In some embodiments, the porosity of the positive electrode material layer is P%, 10≤P≤30, and the proportion of pores with a diameter greater than or equal to 200nm in the total pore volume is R, satisfying: R≥60%, (0.05*m*L1+50)%≤R≤(0.2*m*L1+60)%. Preferably, 20≤P≤30. More preferably, 82%≤R≤94%. The inventors speculate that because the first carbon nanotube connects multiple active material particles in series, the pores between the particles can be connected, resulting in multiple pores combined into one pore, i.e., an increase in pore size. This means that the tortuosity of the pores in the electrode is reduced, the diffusion path length of lithium ions during transport in the electrolyte can be reduced, improving ion transport efficiency and increasing the rate performance of the cell. In addition, the series connection of multiple pores can also connect closed pores and open pores, guiding the electrolyte into the original closed pores, i.e., turning closed pores into open pores, increasing porosity, increasing the number of lithium ion transport paths, and increasing the rate performance of the cell.

[0013] In some embodiments, the positive electrode material layer further includes substance A, which includes at least one of polyacrylic acid, polyvinylidene fluoride (PVDF), polytetrafluoroethylene-hexafluoropropylene, sodium polyacrylate, nitrile rubber, and polyacrylate. The mass percentage of substance A is 0.8% to 3.5% based on the mass of the positive electrode material layer. This facilitates the construction of a more stable conductive network and improves the desired effect.

[0014] In some embodiments, in the electrochemical impedance spectroscopy of the positive electrode, the intersection point S0 of the curve with the real axis (X-axis) is ≤5000mΩ, the length X of the first semicircle along the real axis is ≤50000mΩ, and the height Y along the imaginary axis (Y-axis) is ≤20000mΩ. Having the intersection point S0, length X, and height Y all within these ranges in the electrochemical impedance spectroscopy is beneficial for improving the rate performance and high-temperature cycling performance of lithium-ion batteries. Preferably, S0 ≤2200mΩ, X ≤19000mΩ, and Y ≤10000mΩ.

[0015] Secondly, this application provides a secondary battery, which includes the aforementioned positive electrode plate. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this application clearer, the following provides a more detailed description. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0017] For simplicity, this paper only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range, just as any upper limit can be combined with any other upper limit to form an undefined range. Furthermore, although not explicitly stated, every point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can serve as its own lower or upper limit and be combined with any other point or individual value, or with other lower or upper limits, to form an undefined range.

[0018] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C.

[0019] Positive electrode sheet

[0020] The first aspect of this application provides a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes an active material, a first carbon nanotube, and a second carbon nanotube. The first carbon nanotube is a long, straight carbon nanotube and / or a carbon nanotube with a certain curvature. The outer diameter of the first carbon nanotube is 20-200 nm, the inner diameter of the first carbon nanotube is 3-15 nm, and the length of the first carbon nanotube is L1 μm, where 2 ≤ L1 ≤ 50. Three points are randomly selected on the same first carbon nanotube and connected along its length to form an included angle α, where 30° ≤ α ≤ 180°. Multiple second carbon nanotubes are aggregated and / or entangled to form a coating layer disposed on the surface of the active material. For the positive electrode sheet, during scanning electron microscopy testing, the number of first carbon nanotubes is m within a range of 25 μm × 25 μm, where 2 ≤ m ≤ 115.

[0021] The inventors have simultaneously incorporated the aforementioned first carbon nanotube and second carbon nanotube into the cathode material layer. The first carbon nanotube is rigid, has a large outer diameter and a small inner diameter, making it difficult to bend. It is typically long and straight in the cathode material layer and does not entangle itself, allowing it to connect multiple active materials in series. The second carbon nanotube is more flexible and can entangle and attach to the surface of the active material particles. Through the synergistic cooperation between the first carbon nanotube, the second carbon nanotube, and the active material particles, this application can construct a good conductive network, thereby improving the dynamics of the secondary battery.

[0022] For example, the outer diameter of the first carbon nanotube is 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 55nm, 60nm, 80nm, 100nm, 120nm, 140nm, 160nm, 180nm, 200nm or any two of the above values.

[0023] For example, the inner diameter of the first carbon nanotube is 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm or any two of the above values.

[0024] For example, the length of the first carbon nanotube is 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm or any two of the above values.

[0025] Exemplary examples include, in some embodiments, 40° ≤ α ≤ 180°. In some embodiments, 60° ≤ α ≤ 180°. In some embodiments, 90° ≤ α ≤ 180°. In some embodiments, 120° ≤ α ≤ 180°. In some embodiments, 150° ≤ α ≤ 180°. In some embodiments, 160° ≤ α ≤ 180°. In some embodiments, 170° ≤ α ≤ 180°. In some embodiments, 175° ≤ α ≤ 180°. In some embodiments, 178° ≤ α ≤ 180°.

[0026] Exemplary examples include, in some embodiments, 4 ≤ m ≤ 110. In some embodiments, 8 ≤ m ≤ 100. In some embodiments, 10 ≤ m ≤ 80. In some embodiments, 13 ≤ m ≤ 60. In some embodiments, 15 ≤ m ≤ 50. In some embodiments, 20 ≤ m ≤ 30. In some embodiments, 25 ≤ m ≤ 28.

[0027] In some embodiments, for the first carbon nanotube, its Raman spectrum at 1150 cm⁻¹ -1 Up to 1450cm -1 A characteristic peak D exists within the range, with a peak intensity of I. D At 1450cm -1 Up to 1750cm -1 A characteristic peak G exists within the range, with a peak intensity of I. G Satisfying: 0 < I D / I G ≤0.7. The first carbon nanotube has small defects and a high degree of graphitization, resulting in a high degree of order, which is more conducive to forming long-range conductive paths and is less prone to "break-off" after the electrode expands cyclically.

[0028] For example, in some embodiments, 0 < I D / I G ≤0.6. In some embodiments, I D / I G ≤0.4. In some embodiments, 0 < I D / I G ≤0.3. In some embodiments, 0 < I D / I G ≤0.2. In some embodiments, 0 < I D / I G ≤0.1. In some embodiments, 0 < I D / I G ≤0.05. In some embodiments, 0 < I D / I G ≤0.001.

[0029] In some embodiments, for the first carbon nanotube, its XRD diffraction pattern shows a diffraction peak A in the range of 10° to 40°, with a 2θ peak width greater than 0 and less than or equal to 15°. The first carbon nanotube has a high degree of graphitization and high degree of order, which is beneficial for forming long-range conductive pathways with active material particles.

[0030] Exemplary examples include: in some embodiments, the 2θ peak width is greater than 0 and less than or equal to 13°; in some embodiments, the 2θ peak width is greater than 0 and less than or equal to 11°; in some embodiments, the 2θ peak width is greater than 0 and less than or equal to 9°; in some embodiments, the 2θ peak width is greater than 0 and less than or equal to 7°; in some embodiments, the 2θ peak width is greater than 0 and less than or equal to 5°; in some embodiments, the 2θ peak width is greater than 0 and less than or equal to 3°; and in some embodiments, the 2θ peak width is greater than 0 and less than or equal to 1°.

[0031] In some embodiments, the outer diameter of the second carbon nanotube is 1–30 nm, the inner diameter is 0.05–20 nm, and the length is 1–20 μm. Based on the mass of the cathode material layer, the mass percentage of the second carbon nanotube is 0.1% to 2%. This facilitates the second carbon nanotubes to wrap around and adhere to the surface of the active material particles, forming short-range pathways. Through the cooperation of the first and second carbon nanotubes, a conductive network combining long and short ranges is constructed, thereby improving the rate performance and high-temperature cycling performance of the secondary battery.

[0032] For example, the outer diameter of the second carbon nanotube is 1 nm, 3 nm, 5 nm, 8 nm, 10 nm, 13 nm, 15 nm, 18 nm, 20 nm, 25 nm, 30 nm or any two of the above values.

[0033] For example, the inner diameter of the second carbon nanotube is 0.05nm, 0.08nm, 1nm, 3nm, 5nm, 8nm, 10nm, 13nm, 15nm, 18nm, 20nm or any two of the above values.

[0034] For example, the mass percentage of the second carbon nanotube is 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.3%, 1.5%, 1.8%, 2%, or any combination of two of the above values.

[0035] In some embodiments, the particle size Dv10 of the active material is Dμm, and L1 ≥ 0.5D. This facilitates the formation of a long-range conductive pathway between the first carbon nanotube and the active material.

[0036] In some embodiments, the positive electrode material layer further includes conductive carbon particles with a particle size Dv50 of 10–100 nm. Based on the mass of the positive electrode material layer, the mass percentage of conductive carbon particles is T, where T ≤ 2%. The presence of conductive carbon particles increases the looseness of the conductive agent layer on the surface of the active particles, which is more conducive to the adsorption of more electrolyte and improves ion conductivity.

[0037] For example, the particle size Dv50 of the conductive carbon particles is 10nm, 15nm, 20nm, 35nm, 40nm, 55nm, 60nm, 80nm, 90nm, 100nm or any two of the above values.

[0038] Exemplary examples include, in some embodiments, 0.1% ≤ T ≤ 2%. In some embodiments, 1% ≤ T ≤ 2%. In some embodiments, 1.5% ≤ T ≤ 2%. In some embodiments, 1.8% ≤ T ≤ 2%.

[0039] In some embodiments, the mass ratio of the second carbon nanotube, the first carbon nanotube, the conductive carbon particles, and the active material is 1:(0.02–8.0):(0–10):(47–980). An appropriate amount of the first carbon nanotube easily forms bridges between active material particles within a suitable mass range, ensuring sufficient conductive pathways after the cyclic electrode expands. Simultaneously, an appropriate amount of the second carbon nanotube, together with a suitable proportion of conductive carbon particles, easily adheres to the surface of the active material particles (e.g., LCO), forming short-range pathways. The combination of these three elements constructs a good conductive network, improving cell dynamics.

[0040] In some embodiments, the porosity of the positive electrode material layer is P%, where 10 ≤ P ≤ 30. Exemplarily, the porosity of the positive electrode material layer is 10%, 13%, 15%, 18%, 20%, 25%, 28%, 30%, or any combination of two of the above values. A value of P within the above range is beneficial for improving the rate performance and high-temperature cycling performance of lithium-ion batteries.

[0041] In some embodiments, the proportion of pores with a diameter greater than or equal to 200 nm in the total pore volume is R, where R ≥ 60% and (0.05*m*L1+50)% ≤ R ≤ (0.2*m*L1+60)%. A value of R within the above range is beneficial for improving the rate performance and high-temperature cycling performance of lithium-ion batteries.

[0042] In some embodiments, the positive electrode material layer further includes substance A, which includes at least one selected from polyacrylic acid, polyvinylidene fluoride (PVDF), polytetrafluoroethylene-hexafluoropropylene, sodium polyacrylate, nitrile rubber, and polyacrylate. The mass percentage of substance A is 0.8% to 3.5% based on the mass of the positive electrode material layer. Exemplarily, the mass percentage of substance A is 0.8%, 1.0%, 1.5%, 1.8%, 2%, 2.5%, 3%, 3.5%, or a range consisting of any two of the above values.

[0043] In some embodiments, in the electrochemical impedance spectroscopy of the positive electrode, the intersection point S0 of the curve with the real axis (X-axis) is ≤5000mΩ, the length X of the first semicircle along the real axis is ≤50000mΩ, and the height Y along the imaginary axis (Y-axis) is ≤20000mΩ. This is beneficial for improving the rate performance and high-temperature cycling performance of lithium-ion batteries.

[0044] Secondary batteries

[0045] The secondary battery of this application includes a positive electrode, a negative electrode, a separator, and an electrolyte. The separator is disposed between the positive electrode and the negative electrode, and the positive electrode can be any of the above-mentioned positive electrode types.

[0046] The negative electrode includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. This application does not impose a particular limitation on the thickness of the negative electrode active layer, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode active layer is 30 μm to 120 μm. In some embodiments, the negative electrode active material may include at least one of carbon materials or silicon-based materials. In some embodiments, the carbon materials include, but are not limited to, at least one of natural graphite, artificial graphite, mesophase microcarbon spheres, hard carbon, or soft carbon. In some embodiments, the silicon-based materials include, but are not limited to, at least one of silicon, silicon-oxygen composite materials, or silicon-carbon composite materials. This application does not impose a particular limitation on the negative current collector, as long as it achieves the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector (e.g., a composite current collector with a metal layer disposed on the surface of a polymer layer). This application does not impose a particular limitation on the thickness of the negative current collector, as long as it achieves the purpose of this application. For example, the thickness of the negative current collector is 5 μm to 12 μm. The negative electrode active layer may also include a binder and a thickener. This application does not impose any particular limitation on the types of binders and thickeners, as long as they achieve the purpose of this application. For example, the binder may include, but is not limited to, at least one of polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, styrene-butadiene rubber, or acrylated styrene-butadiene rubber; the thickener may include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. The negative electrode active layer may also include a conductive agent. This application does not impose any particular limitation on the types of conductive agents, as long as they achieve the purpose of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, Ketjen black, graphene, metallic materials, or conductive polymers. This application does not impose any particular limitation on the mass ratio of the negative electrode active material, conductive agent, binder, and thickener in the negative electrode active layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved. Optionally, the negative electrode sheet may also include a conductive layer located between the negative electrode current collector and the negative electrode active layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer includes a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer, which can be at least one of the conductive agent and binder described above in the negative electrode active layer.

[0047] The separating membrane may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a surface treatment layer may be provided on at least one surface of the substrate layer, and the surface treatment layer may be an adhesive layer or a heat-resistant layer. For example, the adhesive layer includes an adhesive, and the adhesive material may include at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or polyvinylidene fluoride-hexafluoropropylene copolymer. The heat-resistant layer includes inorganic particles and an adhesive. The inorganic particles are not particularly limited, and may include, for example, at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The adhesive is not particularly limited, and may be, for example, at least one of the adhesives described in the adhesive layer above.

[0048] The electrolyte includes an organic solvent, a lithium electrolyte salt, and additives. This application does not specifically limit the types of solvents used; they can be selected according to actual needs. Exemplarily, the aforementioned organic solvent includes one or more of the following: ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butenyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), or diethyl sulfone (ESE), preferably two or more. Exemplarily, the aforementioned electrolyte lithium salt includes one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), or LiTFOP (lithium tetrafluorooxalate phosphate). The electrolyte may also optionally include other additives, which can be any additive that can be used in lithium-ion secondary batteries. This invention does not impose specific limitations and can be selected according to actual needs. As an example, the additive may be one or more of the following: vinylene carbonate (VC), ethylene ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone (PS), succinic acid (SN), adiponitrile (ADN), 1,3-propenesulfonate lactone (PST), tris(trimethylsilane) phosphate (TMSP), or tris(trimethylsilane) borate (TMSB).

[0049] The secondary battery can be prepared according to conventional methods in the art. For example, the above-mentioned positive electrode, separator and negative electrode are stacked in sequence, with the separator acting as a separator between the positive electrode and the negative electrode to obtain an electrode assembly. Alternatively, the electrode assembly can be obtained by winding. The electrode assembly is placed in a packaging shell, electrolyte is injected and the shell is sealed to obtain a secondary battery.

[0050] There are no particular limitations on the structure of lithium batteries; coin-shaped batteries, cylindrical batteries, prismatic batteries, or pouch batteries with single or multiple separators can be used. The application of the lithium-ion battery in this application is not particularly limited; it can be used in any electronic device known in the prior art. In some embodiments, the lithium-ion battery of this application can be used in, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0051] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Unless otherwise stated, all parts, percentages and ratios listed below are by weight, and all raw materials used are commercially available or synthesized by conventional methods.

[0052] Example 1-1

[0053] (I) Preparation of Lithium-ion Batteries

[0054] <Preparation of the positive electrode>

[0055] 97.9% of the positive electrode active material LiCoO2 (parameters shown in Table 1), 0.4% of the first carbon nanotube (parameters shown in Table 1), 0.5% of the second carbon nanotube (parameters shown in Table 1), and 1.2% of the positive electrode binder polyvinylidene fluoride (PVDF) were mixed, and N-methylpyrrolidone (NMP) was added. The mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was uniformly coated onto one surface of a 9 μm thick aluminum foil current collector and dried at 85°C for 4 hours to obtain a single-sided positive electrode sheet with a coating thickness of 110 μm and a width of 74 mm. The above steps were repeated on the other surface of the aluminum foil current collector to obtain a double-sided positive electrode sheet. After drying under vacuum at 85°C for 4 hours, the sheet was cold-pressed, cut, and slit to obtain a positive electrode sheet with a size of 74 mm × 867 mm.

[0056] The parameters of the first carbon nanotube, the second carbon nanotube, and the active material in Examples 1-2 to 1-23 and Comparative Examples 1-1 to 1-3 can be found in Table 1. All other parameters are the same as in Example 1-1.

[0057] <Preparation of Negative Electrode Sheets>

[0058] Silicon carbon material and artificial graphite were mixed uniformly at a mass ratio of 10:90 to form the negative electrode active material. The above negative electrode active material, styrene-butadiene rubber (SBR) as the negative electrode binder, carbon nanotubes (CNTs) as the negative electrode conductive agent, and carboxymethyl cellulose (CMC) as the negative electrode dispersant were mixed at a mass ratio of 95.8:2.9:0.5:0.8. Deionized water was then added as a solvent and stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 45 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil used as the negative electrode current collector. The copper foil was dried at 85°C for 4 hours to obtain a negative electrode sheet with a single-sided coating of the negative electrode mixture. After cold pressing (at a pressure of 20 t), cutting, and slitting, the sheet was dried under vacuum at 120°C for 12 hours to obtain a negative electrode sheet with dimensions of 76.6 mm × 875 mm.

[0059] <Preparation of Electrolyte>

[0060] In a dry argon atmosphere glove box, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed thoroughly at a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was dissolved in the above organic solvent, and then vinylene carbonate (VC) was added and mixed thoroughly to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, and the mass percentage of vinylene carbonate was 3%.

[0061] <Septum>

[0062] A porous polyethylene film with a thickness of 15 μm (provided by Celgard) was used as the diaphragm.

[0063] <Preparation of Lithium-ion Batteries>

[0064] The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. They are then wound into a bare cell, which is placed in an aluminum-plastic film, injected with electrolyte, and sealed. After further processing, including settling, formation, and shaping, a lithium-ion battery is obtained. The formation process is as follows: The first charge-discharge cycle is performed at 45°C, following this procedure: First, constant current charging at 0.1C for 10 minutes; then constant current charging at 0.5C until a specified voltage Q = 4.6V is reached; next, constant voltage charging is performed until the current is less than or equal to 0.05C; finally, constant current discharging at 0.5C until 2.5V is reached.

[0065] (II) Test Methods

[0066] (1) Measurement of the diameter and length of carbon nanotubes:

[0067] 1) Disassemble the finished battery cell to obtain the positive electrode plate;

[0068] 2) Soak the electrode sheet from 1) in DMC (dimethyl carbonate) at room temperature for 60 minutes, remove it, and air dry it at room temperature;

[0069] 3) Obtain the cross-section of the active layer on the electrode by using liquid nitrogen brittle fracture method after drying in 2).

[0070] 4) Observe the cross section obtained in 3) under SEM, test at least 5 different locations, and measure the diameter, length and diameter of the target material in a total of no less than 15 samples. Take the average value as the target value.

[0071] (2) State testing of carbon nanotubes

[0072] 1) Disassemble the finished battery cell to obtain the positive electrode sheet; 2) Immerse the electrode sheet from 1) in DMC (dimethyl carbonate) at room temperature for 60 minutes, take it out and air dry at room temperature; 3) Obtain the cross-section of the active layer on the electrode sheet by liquid nitrogen brittle fracture method; 4) Observe the cross-section obtained in 3) under SEM, test at least 5 different positions, and a total of no less than 15 target materials.

[0073] If the conductive fiber is in a stretched state and three points are randomly selected on the same conductive agent and connected along the length of the conductive agent, the smaller angle is taken as the smaller side. If the measured data is 30°≤α≤180°, then it is the first carbon nanotube.

[0074] If the conductive fibers are mostly present in groups and have mutual entanglement and self-folding phenomena, then it is a second conductive agent.

[0075] (3) Method for determining the number m of the first carbon nanotubes

[0076] 1) Disassemble the finished battery cell to obtain the positive electrode plate;

[0077] 2) Soak the electrode sheet from 1) in DMC (dimethyl carbonate) at room temperature for 60 minutes, remove it, and air dry it at room temperature;

[0078] 3) Obtain the cross-section of the active layer on the electrode by using liquid nitrogen brittle fracture method after drying in 2).

[0079] 4) Select a continuous field of view of 25μm×25μm on the SEM to observe the cross section and determine the number m of the first carbon nanotubes.

[0080] (4) Testing of electrode porosity

[0081] a) Discharge the battery cell to the cutoff voltage (e.g., 3.0V for LCO material battery cells) and disassemble to obtain the positive electrode plate;

[0082] b) Immerse the positive electrode sheet from a) in DMC (dimethyl carbonate) at room temperature for 60 minutes, remove it, and air dry it at room temperature;

[0083] c) Use mercury porosimetry to test the porosity and pore size distribution data of the electrode in b), and then calculate the percentage R of the diameter ≥200nm in the total pore volume.

[0084] (5) EIS curve

[0085] a. Charge the cell to the design voltage at 0.2C, then discharge it to 50% SOC at 0.2C. After disassembling the positive electrode in an environment with humidity ≤15%, immediately pack it into a bag and seal it. b. Make the electrode from a into a symmetrical battery and test its EIS curve.

[0086] (6) 1C discharge rate

[0087] Take the finished battery cells and perform the following tests at 25±2℃:

[0088] 1) Let stand for 2 hours, then discharge at 0.7C to the cutoff voltage (3.0V for LCO positive electrode active material), and let stand for 5 minutes.

[0089] 2) Charge at 1.5C to the cutoff voltage, then charge at the cutoff voltage to 0.05C; let stand for 5 minutes; discharge at 0.2C to the cutoff voltage, and record the discharge capacity as C1.

[0090] 3) Let stand for 10 minutes, then discharge at 0.7C to the cutoff voltage (3.0V for LCO positive electrode active material), and let stand for 5 minutes.

[0091] 4) Charge at 1.5C to the cutoff voltage, then charge at the cutoff voltage to 0.05C; let stand for 5 minutes; discharge at 1C to the cutoff voltage, and record the discharge capacity as C2.

[0092] 5) C2 / C1 is the 1C discharge rate.

[0093] (7) Capacity retention rate during 45℃ cycling

[0094] Take the finished battery cells and perform the following tests at 45±1℃:

[0095] Let stand for 2 hours, then discharge at 0.7C to the cutoff voltage (3.0V for LCO positive electrode active material), and let stand for 5 minutes.

[0096] [Charge at 1.5C to the cutoff voltage, then charge at the cutoff voltage to 0.05C; let stand for 5 minutes; discharge at 0.7C to the cutoff voltage, and record the discharge capacity as C1; let stand for 5 minutes;]

[0097] The process in brackets [ ] is repeated 49 times, with the capacities recorded sequentially as C1 / C2...C49; in cycle 50, the battery is charged at 0.5C to the cutoff voltage, then charged at the cutoff voltage to 0.05C; the battery is allowed to stand for 5 minutes; and then discharged at 0.7C to the set value, with the discharge capacity recorded as C50.

[0098] The process in the loop {} is repeated 10 times + [] once. The capacity retention rate after 500 cycles is C501 / C1*100%.

[0099] In Table 2, the input parameters of the first carbon nanotube, the second carbon nanotube, and the active material in Examples 2-1 to 2-14 and Comparative Examples 2-1 to 2-2, excluding the content, are as follows: for the first carbon nanotube, these include ID1 / IG1, 2θ peak width, outer diameter / inner diameter, length L1, and number of nanotubes m; for the second carbon nanotube, these include outer diameter / inner diameter and length. The lithium cobalt oxide DV10, etc., are the same as in Example 1-1. The difference is that conductive carbon particles are further added in Examples 2-1 to 2-14 during the preparation of the positive electrode sheet in Example 1-1. The parameters of the conductive carbon particles are shown in Table 2, and the mass ratios of the first carbon nanotube, the second carbon nanotube, the conductive carbon particles, and the active material are also shown in Table 2.

[0100]

[0101]

[0102] Data Analysis

[0103] Referring to Table 1, compared with Example 1-1, the parameters of the first carbon nanotubes in Comparative Examples 1-1 to 1-3 are unsuitable. Although the parameters of the second carbon nanotubes are all within the scope of this application, the parameters of the first carbon nanotubes prevent the first carbon nanotubes, second carbon nanotubes, and active material particles from cooperating, thus failing to construct the conductive network expected in this application. The 1C discharge rate of the lithium-ion batteries prepared in Comparative Examples 1-1 to 1-3 is no more than 75%, far lower than the 95% 1C discharge rate of Example 1-1, by about 20%. The capacity retention rate of the lithium-ion batteries prepared in Comparative Examples 1-1 to 1-3 after 500 cycles at 45°C is no more than 73%, far lower than the 90% capacity retention rate of Example 1-1, by about 17%.

[0104] In particular, further adjusting the parameters of the first carbon nanotube, the second carbon nanotube, and the active material lithium cobalt oxide to a suitable range is beneficial for further improving the rate performance and high-temperature cycling performance of lithium-ion batteries. Especially when the parameters of the first carbon nanotube, the second carbon nanotube, and the active material lithium cobalt oxide are within the preferred range, the effect of improving the rate performance and high-temperature cycling performance of lithium-ion batteries is significant.

[0105] As can be seen from Table 2, when conductive carbon particles of appropriate size are further added to the cathode material layer, and the mass ratio of the first carbon nanotube, the second carbon nanotube, the conductive carbon particles, and the active material lithium cobalt oxide is also appropriate, the effect of improving the rate performance and high-temperature cycle performance of lithium-ion batteries is better.

[0106] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A positive electrode plate, characterized in that, The positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, wherein the positive electrode material layer includes an active material, a first carbon nanotube, and a second carbon nanotube. The first carbon nanotube is a long straight carbon nanotube and / or a carbon nanotube with a certain curvature. The outer diameter of the first carbon nanotube is 20nm to 200nm, and the inner diameter of the first carbon nanotube is 3nm to 15nm. Three points are randomly selected on the same first carbon nanotube and connected along its length to form an included angle α, where 30°≤α≤180°. Multiple second carbon nanotubes are intertwined to form a coating layer on the surface of the active material; For the positive electrode, during scanning electron microscopy testing, the number of the first carbon nanotubes is m within any 25μm×25μm range, where 2≤m≤115; The first carbon nanotube, in Raman spectroscopy, at 1150 cm⁻¹ -1 Up to 1450cm -1 A characteristic peak D exists within the range, with a peak intensity of I. D At 1450cm -1 Up to 1750cm -1 A characteristic peak G exists within the range, with a peak intensity of I. G Satisfying: 0 < I D / I G ≤0.7; In the XRD diffraction pattern of the first carbon nanotube, there is a diffraction peak A in the range of 10° to 40°, and its 2θ angle peak width is greater than 0 and less than or equal to 15°; The outer diameter of the second carbon nanotube is 8 nm to 30 nm, and the inner diameter of the second carbon nanotube is 3 nm to 20 nm.

2. The positive electrode sheet according to claim 1, characterized in that, The first carbon nanotube satisfies at least one of the following conditions: (1) The outer diameter of the first carbon nanotube is 30 nm to 80 nm, the inner diameter of the first carbon nanotube is 4 nm to 8 nm, and the length of the first carbon nanotube is L1 μm, 2 ≤ L1 ≤ 50. (2) Take any three points on the same first carbon nanotube and connect them along its length to form an included angle α, where 90°≤α≤180°; （3）15≤m≤50。 3. The positive electrode sheet according to claim 2, characterized in that, The second carbon nanotube satisfies at least one of the following conditions: (1) The length of the second carbon nanotube is 1 μm to 20 μm; (2) Based on the mass of the cathode material layer, the mass percentage of the second carbon nanotube is 0.1% to 2%.

4. The positive electrode sheet according to claim 3, characterized in that, The particle size Dv10 of the active material is Dμm, and the positive electrode sheet satisfies at least one of the following conditions: (1) L1 ≥ 0.5D; (2) The length of the second carbon nanotube is 3 μm to 10 μm; (3) The content of the second carbon nanotube is 0.3% to 0.8%.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that, The positive electrode material layer also includes conductive carbon particles, the particle size Dv50 of which is 10nm to 100nm; Based on the mass of the positive electrode material layer, the mass percentage of the conductive carbon particles is T, where T≤2%.

6. The positive electrode sheet according to claim 5, characterized in that, The particle size Dv50 of the conductive carbon particles is 10~60nm; The mass ratio of the second carbon nanotube, the first carbon nanotube, the conductive carbon particles, and the active material is 1:(0.02~8.0):(0~10):(47~980).

7. The positive electrode sheet according to any one of claims 2 to 4, characterized in that, The porosity of the positive electrode material layer is P%, 10≤P≤30; The proportion of pores with a diameter greater than or equal to 200 nm in the total pore volume is R, where R ≥ 60% and (0.05*m*L1+50) % ≤ R ≤ (0.2*m*L1+60) %.

8. The positive electrode sheet according to any one of claims 1 to 4, characterized in that, The positive electrode material layer also includes substance A; The substance A includes at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene-hexafluoropropylene, sodium polyacrylate, nitrile rubber, and polyacrylate. Based on the mass of the cathode material layer, the mass percentage of substance A is 0.8% to 3.5%.

9. The positive electrode sheet according to any one of claims 1 to 4, characterized in that, In the electrochemical impedance spectrum of the positive electrode, the intersection point S0 of the curve with the real axis (X-axis) is ≤5000mΩ, the length X of the first semicircle along the real axis is ≤50000 mΩ, and the height Y along the imaginary axis (Y-axis) is ≤20000 mΩ.

10. A secondary battery, characterized in that, The secondary battery includes the positive electrode sheet according to any one of claims 1 to 9.

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