Positive electrode sheet and secondary battery

Abstract

Disclosed in the present application are a positive electrode sheet and a secondary battery. The positive electrode sheet comprises a positive electrode material layer; the positive electrode material layer comprises an active material, first carbon nanotubes and second carbon nanotubes, the first carbon nanotubes being long straight carbon nanotubes and / or carbon nanotubes having a certain radian, the outer diameter of the first carbon nanotubes being 20 nm to 200 nm, and the inner diameter of the first carbon nanotubes being 3 nm to 15 nm. When three arbitrary points on a same first carbon nanotube are selected and said points are connected in the length direction thereof to form an included angle α, 30°≤α≤180°. The plurality of the second carbon nanotubes are aggregated and / or twisted with each other to form a coating layer that is arranged on the surface of the active material. During scanning electron microscope tests of the positive electrode sheet, in any 25μm×25μm range, the number of the first carbon nanotubes m satisfies 2≤m≤115. The secondary battery provided by the present application exhibits better rate capability and cycle performance.

Description

Positive electrode and secondary battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on March 31, 2024, with application number 202410383426.3 and invention name “Positive Electrode Plate and Secondary Battery”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of battery technology, and in particular to a positive electrode plate and a secondary battery. Background Art

[0003] As secondary 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 secondary batteries, the positive electrode sheet constantly expands and contracts during the cyclic charge and discharge process, which destroys part of the conductive network, resulting in deterioration of the battery's rate performance and charging performance, and rapid life decay. Therefore, there is an urgent need to provide a positive electrode sheet with excellent conductive performance to improve the dynamics of secondary batteries. Summary of the Invention

[0004] In view of this, the present application provides a positive electrode plate and a secondary battery to improve the rate performance and cycle performance of the secondary battery.

[0005] In a first aspect, the present application provides a positive electrode plate, comprising a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode material layer comprises an active material, a first carbon nanotube, and a second carbon nanotube. The first carbon nanotube is a straight carbon nanotube and / or has 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 on the same first carbon nanotube are connected along its length to form an angle α, 30°≤α≤180°. A plurality of 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 plate, when tested by scanning electron microscopy, within any 25μm×25μm area, the number of first carbon nanotubes is m, 2≤m≤115, and preferably 15≤m≤50. The positive electrode material layer contains both first carbon nanotubes and second carbon nanotubes. The first carbon nanotubes are very rigid and not easy to bend. An appropriate amount of the first carbon nanotubes can connect the active material particles in series, while the second carbon nanotubes are very flexible and can be wrapped around the surface of the active material particles. Through the coordinated 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.

[0006] The inventors speculate that the first carbon nanotubes are more rigid due to their high degree of graphitization. Furthermore, their larger outer diameter and smaller inner diameter make them less prone to bending, generally remaining straight and untwisted. Therefore, the first carbon nanotubes can connect multiple active material particles in series, forming a long-range conductive path that is less susceptible to "breakage" after cyclic expansion of the electrode. The second carbon nanotubes, on the other hand, are more flexible and can wrap around and adhere to the surface of the active material particles, providing short-range conductivity while also reducing direct contact between the active material and the electrolyte, minimizing side reactions and ultimately improving the rate capability and cycling performance of the secondary battery, particularly high-temperature cycling performance.

[0007] In some embodiments, for the first carbon nanotube, in the Raman spectrum, at 1150 cm -1 to 1450cm -1 There is a characteristic peak D in the range, and its peak intensity is I D , at 1450cm -1 to 1750cm -1 There is a characteristic peak G in the range, and its peak intensity is I G , satisfying: 0<I D / I G ≤0.7, and its XRD diffraction pattern shows a diffraction peak A within the range of 10° to 40°, with a 2θ angle peak width greater than 0 and less than or equal to 15°. Thus, the first carbon nanotubes have few defects, a high degree of graphitization, and a high degree of order, which is more conducive to forming long-range conductive pathways and is less likely to "break" after cyclic expansion of the electrode sheet, thereby improving the rate performance and high-temperature cycling performance of the secondary battery.

[0008] In some embodiments, the outer diameter of the first carbon nanotube is 30nm to 80nm, the inner diameter of the first carbon nanotube is 4nm to 8nm, the length of the first carbon nanotube is L1μm, 2≤L1≤50, and three points are randomly selected on the same first carbon nanotube and connected along its length direction to form an angle α, 90°≤α≤180°. By adjusting the various parameter values ​​of the first carbon nanotube within the above range, the rate performance and high-temperature cycle performance of the secondary battery can be further improved. Preferably, the outer diameter of the first carbon nanotube is 30-50nm, the inner diameter of the first carbon nanotube is 4-7nm, and 5≤L1≤30.

[0009] In some embodiments, the outer diameter of the second carbon nanotube is 1 nm to 30 nm, the inner diameter of the second carbon nanotube is 0.05 nm to 20 nm, the length of the second carbon nanotube is 1 μm to 20 μm, and the mass percentage of the second carbon nanotube is 0.1% to 2%. By adjusting the various parameter values ​​of the second carbon nanotube within the above range, it is beneficial for the second carbon nanotube to be wrapped around and attached 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 cycle performance of the secondary battery. Preferably, the outer diameter of the second carbon nanotube is 5 to 20 nm, the inner diameter of the second carbon nanotube is 1 to 15 nm, the length of the second carbon nanotube is 3 to 10 μm, and the content of the second carbon nanotube is 0.3% to 0.8%.

[0010] In some embodiments, the active material particle size Dv10 is D μm, and L1 ≥ 0.5D. This facilitates the series coordination between the first carbon nanotube and the multiple active material particles, further facilitating the formation of a long-range conductive pathway. It also facilitates the synergistic coordination between the active material particles, the first carbon nanotube, and the second carbon nanotube, further improving the rate capability and high-temperature cycling performance of the secondary battery. Preferably, 1 ≤ D ≤ 10. More preferably, 4 ≤ D ≤ 6.

[0011] In some embodiments, the positive electrode material layer further includes conductive carbon particles having a particle size Dv50 of 10 nm to 100 nm. The mass percentage of the conductive carbon particles, based on the mass of the positive electrode material layer, is T, where T is ≤ 2%. The conductive carbon particles are typically mixed with the second carbon nanotubes and distributed on the surface of the active material particles. The presence of the conductive carbon particles increases the bulkiness of the conductive layer on the surface of the active particles, facilitating the adsorption of more electrolyte and enhancing ion conductivity.

[0012] In some embodiments, the particle size Dv50 of the conductive carbon particles is 10 to 60 nm, and the mass ratio of the second carbon nanotubes, the first carbon nanotubes, the conductive carbon particles and the positive electrode active material is 1: (0.02 to 8.0): (0 to 10): (47 to 980). The particle size of the conductive carbon particles is appropriate, and the mass ratio of the first carbon nanotubes, the second carbon nanotubes, the conductive carbon particles and the active material is within the above range. A long- and short-range conductive network can be built in the electrode to improve the rate and cycle performance of the battery cell. Preferably, the mass ratio of the second carbon nanotubes, the first carbon nanotubes, the conductive carbon particles and the positive electrode active material is 1: (0.8 to 6.67): (0.2 to 1): (195 to 320).

[0013] In some embodiments, the porosity of the positive electrode material layer is P%, 10≤P≤30, and the proportion of pores with a pore size 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. Preferably, 82%≤R≤94%. The inventor speculates that because the first carbon nanotube connects multiple active material particles in series, that is, the pores between the particles can be connected together, multiple pores are combined into one pore, that is, the pore size is increased. This means that the tortuosity of the pores in the electrode is reduced, and the length of the diffusion path can be reduced when lithium ions are transmitted in the electrolyte, thereby improving the ion transmission efficiency and increasing the battery cell rate performance. In addition, the multiple pores can be connected in series, and the closed pores and open pores can be connected together to lead the electrolyte into the original closed pores, that is, the closed pores are changed into open pores, thereby increasing the porosity, increasing the number of lithium ion transmission paths, and increasing the battery cell rate performance.

[0014] 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 further facilitates the realization of the effect.

[0015] In some embodiments, in the electrochemical impedance spectrum of the positive electrode sheet, 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. In the electrochemical impedance spectrum, the intersection point S0, length X, and height Y are all within the above ranges, which is beneficial to improving the rate performance and high temperature cycle performance of the lithium-ion battery. Preferably, S0 ≤2200mΩ, X ≤19000mΩ, and Y ≤10000mΩ.

[0016] In a second aspect, the present application provides a secondary battery, which includes the above-mentioned positive electrode plate. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical solutions and advantages of this application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0018] For simplicity, only some numerical ranges are explicitly disclosed herein. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, and similarly, any upper limit can be combined with any other upper limit to form an unspecified range. In addition, although not explicitly stated, each point or individual value between the endpoints of a range is included in the range. Thus, each point or individual value can serve as its own lower limit or upper limit and be combined with any other point or individual value, or with other lower limits or upper limits, to form an unspecified range.

[0019] 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, 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, 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.

[0020] Positive electrode

[0021] A first aspect of an embodiment of the present application provides a positive electrode plate, which includes a positive electrode collector and a positive electrode material layer arranged on at least one surface of the positive electrode collector, the positive electrode material layer including an active material, a first carbon nanotube and a second carbon nanotube, the first carbon nanotube being a long straight carbon nanotube and / or having a certain curvature, the outer diameter of the first carbon nanotube being 20 to 200 nm, the inner diameter of the first carbon nanotube being 3 to 15 nm, the length of the first carbon nanotube being L1 μm, 2≤L1≤50, any three points on the same first carbon nanotube and connected along its length direction to form an angle α, 30°≤α≤180°, a plurality of second carbon nanotubes are aggregated and / or entangled with each other to form a coating layer arranged on the surface of the active material, and for the positive electrode plate, when tested by a scanning electron microscope, within the range of 25 μm×25 μm, the number of the first carbon nanotubes is m, 2≤m≤115.

[0022] The inventors simultaneously arrange the above-mentioned first carbon nanotubes and the above-mentioned second carbon nanotubes in the positive electrode material layer. The first carbon nanotubes have high rigidity, large outer diameter and small inner diameter, so they are not easy to bend. They are usually long and straight in the positive electrode material layer and are not entangled themselves. They can connect multiple active materials in series, while the second carbon nanotubes are relatively soft and can be wrapped around and attached to the surface of the active material particles. The present application can construct a good conductive network through the coordinated cooperation between the first carbon nanotubes, the second carbon nanotubes and the active material particles, thereby improving the dynamics of the secondary battery.

[0023] Illustratively, the outer diameter of the first carbon nanotube is 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 55 nm, 60 nm, 80 nm, 100 nm, 120 nm, 140 nm, 160 nm, 180 nm, 200 nm, or a range consisting of any two of the foregoing values.

[0024] Illustratively, the inner diameter of the first carbon nanotube is 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, or a range consisting of any two of the foregoing values.

[0025] Illustratively, 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 a range consisting of any two of the above values.

[0026] For example, 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°.

[0027] For example, 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.

[0028] In some embodiments, for the first carbon nanotube, its Raman spectrum has a wavelength of 1150 cm -1 to 1450cm -1 There is a characteristic peak D in the range, and its peak intensity is I D , at 1450cm -1 to 1750cm -1 There is a characteristic peak G in the range, and its peak intensity is I G , satisfying: 0<I D / I G ≤0.7. First, the carbon nanotubes have small defects and a high degree of graphitization, which means they are highly ordered, more conducive to forming long-range conductive paths, and are less likely to "break" after the electrode cyclically expands.

[0029] 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.

[0030] In some embodiments, the first carbon nanotubes have an XRD diffraction pattern with a diffraction peak A within the range of 10° to 40°, and a 2θ peak width greater than 0 and less than or equal to 15°. The first carbon nanotubes have a high degree of graphitization and order, which facilitates the formation of long-range conductive pathways with the active material particles.

[0031] For example, in some embodiments, the 2θ angle peak width is greater than 0 and less than or equal to 13°. In some embodiments, the 2θ angle peak width is greater than 0 and less than or equal to 11°. In some embodiments, the 2θ angle peak width is greater than 0 and less than or equal to 9°. In some embodiments, the 2θ angle peak width is greater than 0 and less than or equal to 7°. In some embodiments, the 2θ angle peak width is greater than 0 and less than or equal to 5°. In some embodiments, the 2θ angle peak width is greater than 0 and less than or equal to 3°. In some embodiments, the 2θ angle peak width is greater than 0 and less than or equal to 1°.

[0032] In some embodiments, the outer diameter of the second carbon nanotube is 1 to 30 nm, the inner diameter of the second carbon nanotube is 0.05 to 20 nm, the length of the second carbon nanotube is 1 to 20 μm, and the weight percentage of the second carbon nanotube is 0.1% to 2% based on the weight of the positive electrode material layer. This facilitates the second carbon nanotube to wrap around and adhere to the surface of the active material particles to form short-range pathways. Through the coordination of the first carbon nanotube and the second carbon nanotube, a long- and short-range conductive network is established, thereby improving the rate performance and high-temperature cycling performance of the secondary battery.

[0033] Illustratively, 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 a range consisting of any two of the foregoing values.

[0034] Illustratively, the inner diameter of the second carbon nanotube is 0.05 nm, 0.08 nm, 1 nm, 3 nm, 5 nm, 8 nm, 10 nm, 13 nm, 15 nm, 18 nm, 20 nm, or a range consisting of any two of the foregoing values.

[0035] Illustratively, the mass percentage of the second carbon nanotubes is 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.3%, 1.5%, 1.8%, 2%, or a range consisting of any two of the above values.

[0036] In some embodiments, the particle size Dv10 of the active material is D μm, and L1 ≥ 0.5 D. This facilitates the formation of a long-range conductive path between the first carbon nanotubes and the active material.

[0037] In some embodiments, the positive electrode material layer further includes conductive carbon particles having a particle size Dv50 of 10 to 100 nm. Based on the mass of the positive electrode material layer, the mass percentage of the conductive carbon particles is T, where T is ≤ 2%. The presence of the conductive carbon particles can increase the bulkiness of the conductive agent layer on the surface of the active particles, thereby facilitating the adsorption of more electrolyte and improving ion conductivity.

[0038] Illustratively, the particle size Dv50 of the conductive carbon particles is 10 nm, 15 nm, 20 nm, 35 nm, 40 nm, 55 nm, 60 nm, 80 nm, 90 nm, 100 nm, or a range consisting of any two of the foregoing values.

[0039] For example, 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%.

[0040] In some embodiments, the mass ratio of the second carbon nanotubes, the first carbon nanotubes, 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 nanotubes easily forms bridges between active material particles within a suitable mass range, ensuring sufficient conductive pathways after cycling electrode expansion. Simultaneously, an appropriate amount of the second carbon nanotubes, along with an appropriate proportion of the conductive carbon particles, easily adhere to the surface of the active material particles (e.g., LCO), forming short-range pathways. The combination of the three creates a good conductive network and improves the battery cell dynamics.

[0041] In some embodiments, the porosity of the positive electrode material layer is P%, and 10≤P≤30. For example, the porosity of the positive electrode material layer is 10%, 13%, 15%, 18%, 20%, 25%, 28%, 30%, or a range consisting of any two of the foregoing values. A value of P within the foregoing range is beneficial for improving the rate capability and high-temperature cycling performance of the lithium-ion battery.

[0042] In some embodiments, the proportion of pores with a pore diameter greater than or equal to 200 nm in the total pore volume is R, R ≥ 60% and (0.05*m*L1+50)% ≤ R ≤ (0.2*m*L1+60)%. The value of R within the above range is beneficial to improving the rate performance and high-temperature cycling performance of the lithium-ion battery.

[0043] In some embodiments, the positive electrode material layer further includes substance A, wherein substance A includes at least one of polyacrylic acid, polyvinylidene fluoride (PVDF), polytetrafluoroethylene-hexafluoropropylene, sodium polyacrylate, nitrile rubber, and polyacrylate, and 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.

[0044] In some embodiments, in the electrochemical impedance spectrum of the positive electrode sheet, the intersection point S0 of the curve with the real axis (X axis) is ≤ 5000 mΩ, the length X of the first semicircle along the real axis is ≤ 50,000 mΩ, and the height Y along the imaginary axis (Y axis) is ≤ 20,000 mΩ. This helps improve the rate performance and high-temperature cycling performance of the lithium-ion battery.

[0045] secondary batteries

[0046] The secondary battery of the present application includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The separator is arranged between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet is any one of the positive electrode sheets mentioned above.

[0047] The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The present application does not particularly limit the thickness of the negative electrode active layer, as long as the objectives of the present application can be achieved. For example, the thickness of the negative electrode active layer is 30μm to 120μm. In some embodiments, the negative electrode active material may include at least one of a carbon material or a silicon-based material. In some embodiments, the carbon material includes but is not limited to at least one of natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon, or soft carbon. In some embodiments, the silicon-based material includes but is not limited to at least one of silicon, a silicon-oxygen composite material, or a silicon-carbon composite material. The present application does not particularly limit the thickness of the negative electrode current collector, as long as the objectives of the present application can be achieved. 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). The present application does not particularly limit the thickness of the negative electrode current collector, as long as the objectives of the present application can be achieved. For example, the thickness of the negative electrode current collector is 5μm to 12μm. The negative electrode active layer may also include a binder and a thickener. This application does not specifically limit the types of binder and thickener, as long as they can achieve the objectives 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, polyvinyl pyrrolidone, 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 specifically limit the type of conductive agent, as long as it can achieve the objectives 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, a metallic material, or a conductive polymer. This application does not specifically limit 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 may select the ratio based on actual needs, as long as it can achieve the objectives of this application. Optionally, the negative electrode plate may also include a conductive layer, which is located between the negative electrode current collector and the negative electrode active layer. The present application does not particularly limit the composition of the conductive layer, and it may be any conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. The present application does not particularly limit the conductive agent and binder in the conductive layer, and for example, it may be at least one of the conductive agent and binder used in the negative electrode active layer described above.

[0048] The isolation membrane may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a film, or a composite film having 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 is provided on at least one surface of the substrate layer, and the surface treatment layer may be a bonding layer or a heat-resistant layer. For example, the bonding layer contains a binder, and the binder material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer. The heat-resistant layer includes inorganic particles and a binder. The inorganic particles are not particularly limited and may, for example, include at least one of aluminum oxide, 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 binder is not particularly limited and may, for example, be at least one of the binders in the above-mentioned bonding layer.

[0049] The electrolyte includes an organic solvent, an electrolyte lithium salt and an additive. The present application does not specifically limit its type and can be selected according to actual needs. For example, the organic solvent includes 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), butylene 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), cyclopentane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS) or diethyl sulfone (ESE) One or more, preferably two or more. Exemplarily, the 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 difluorooxalatoborate), LiBOB (lithium dioxalatoborate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorooxalatophosphate) or LiTFOP (lithium tetrafluorooxalatophosphate). The electrolyte may optionally include other additives, which may be any additive that can be used as a lithium-ion secondary battery. The present invention is not specifically limited thereto and may be selected according to actual needs. As an example, the additive may be one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), succinonitrile (SN), adiponitrile (ADN), 1,3-propylene sultone (PST), tris(trimethylsilyl) phosphate (TMSP) or tris(trimethylsilyl) borate (TMSB).

[0050] Secondary batteries can be prepared according to conventional methods in the art. For example, the aforementioned positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrode sheets to provide isolation, to form an electrode assembly. Alternatively, the electrode assembly can be wound together. The electrode assembly is then placed in a packaging case, injected with electrolyte, and sealed to form a secondary battery.

[0051] There is no particular limitation on the structure of the lithium battery, and coin-type batteries, cylindrical batteries, square batteries, or soft-pack batteries with single-layer or multi-layer separators can be used. The use of the lithium-ion battery of the present application is not particularly limited, and it can be used in any electronic device known in the prior art. In some embodiments, the lithium-ion battery of the present application can be used for, but not limited to, laptop computers, pen-input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries and lithium-ion capacitors, etc.

[0052] The following examples and comparative examples are given to illustrate the embodiments of the present application in more detail. Unless otherwise stated, the parts, percentages and ratios listed below are all based on weight, and the raw materials used are commercially available or synthesized according to conventional methods.

[0053] Example 1-1

[0054] (1) Preparation of lithium-ion batteries

[0055] <Preparation of positive electrode sheet>

[0056] 97.9% of the positive electrode active material LiCoO2 (parameters see Table 1), 0.4% of the first carbon nanotube (parameters see Table 1), 0.5% of the second carbon nanotube (parameters see Table 1), and 1.2% of the positive electrode binder polyvinylidene fluoride (PVDF) were mixed, N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly in a vacuum mixer to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was evenly coated on one surface of a 9 μm thick positive electrode current collector aluminum foil and dried at 85°C for 4 hours to obtain a single-sided positive electrode material-coated 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 positive electrode current collector aluminum foil to obtain a double-sided positive electrode sheet coated with the positive electrode material layer. After drying at 85°C under vacuum for 4 hours, the sheet was cold pressed, cut, and slit to obtain a positive electrode sheet with a size of 74 mm x 867 mm.

[0057] The parameters of the first carbon nanotubes, second carbon nanotubes, and active materials in Examples 1-2 to 1-23 and Comparative Examples 1-1 to 1-3 can be found in Table 1, and the rest are the same as in Example 1-1.

[0058] <Preparation of negative electrode sheet>

[0059] Silicon-carbon material and artificial graphite were mixed uniformly in a mass ratio of 10:90 to form the negative electrode active material. The negative electrode active material, the negative electrode binder styrene-butadiene rubber (SBR), the negative electrode conductive agent carbon nanotubes (CNT), and the negative electrode dispersant carboxymethyl cellulose (CMC) were mixed in a mass ratio of 95.8:2.9:0.5:0.8, and then deionized water was added as a solvent and stirred uniformly to prepare a negative electrode slurry with a solid content of 45wt%. The negative electrode slurry was evenly coated on one surface of a 6μm thick negative electrode current collector copper foil. The copper foil was dried at 85°C for 4 hours to obtain a negative electrode sheet coated with a negative electrode mixture layer on one side. After cold pressing (cold pressing pressure of 20t), cutting, and slitting, the negative electrode sheet was dried under vacuum conditions at 120°C for 12 hours to obtain a negative electrode sheet with a size of 76.6mm×875mm.

[0060] <Preparation of Electrolyte>

[0061] In a dry argon atmosphere glove box, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was dissolved in the organic solvent, and vinylene carbonate (VC) was added and mixed to obtain an electrolyte solution. The weight percentage of LiPF6, based on the total weight of the electrolyte, was 12.5% ​​and the weight percentage of vinylene carbonate was 3%.

[0062] <Diaphragm>

[0063] A porous polyethylene film with a thickness of 15 μm (supplied by Celgard) was used as a separator.

[0064] <Preparation of lithium-ion batteries>

[0065] The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed between the positive and negative electrode sheets to act as an insulator. The cells are then wound into bare cells, which are then enclosed in aluminum-plastic film. The electrolyte is injected and sealed. The cells then undergo a resting, formation, and shaping process to produce a lithium-ion battery. The formation process is as follows: The first cycle of charge and discharge is performed at 45°C, with the following flow: first, charge at a constant current rate of 0.1C for 10 minutes, then charge at a constant current rate of 0.5C to a specified voltage of Q = 4.6V, then charge at a constant voltage until the current is less than or equal to 0.05C, and then discharge at a constant current rate of 0.5C to 2.5V.

[0066] (2) Test method

[0067] (1) Diameter and length test of carbon nanotubes:

[0068] 1) Disassemble the finished battery cell to obtain the positive electrode sheet;

[0069] 2) Soak the electrode in 1) in DMC (dimethyl carbonate) at room temperature for 60 minutes, take it out, and dry it at room temperature;

[0070] 3) Using liquid nitrogen to fracture the electrode dried in 2) to obtain a cross section of the active layer on the electrode;

[0071] 4) Observe the cross section obtained in 3) under SEM, test the tube diameter, length, and diameter of the target material at at least 5 different locations, and a total of no less than 15 lines, and take the average value as the target value.

[0072] (2) Carbon nanotube status test

[0073] 1) disassembling the finished battery cell to obtain the positive electrode piece; 2) soaking the electrode piece in DMC (dimethyl carbonate) at room temperature for 60 minutes, removing it, and air-drying it at room temperature; 3) fracturing the electrode piece dried in 2) with liquid nitrogen to obtain a cross-section of the active layer on the electrode piece; 4) observing the cross-section obtained in 3) under a scanning electron microscope (SEM), testing at least 5 different locations and a total of no less than 15 target materials;

[0074] 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 direction of the conductive agent, the angle formed is the smaller side, and the measured data is 30°≤α≤180°, then it is the first carbon nanotube;

[0075] If the conductive fibers are mostly present in groups and are entangled with each other and folded on themselves, it is the second conductive agent.

[0076] (3) Method for determining the number m of the first carbon nanotubes

[0077] 1) Disassemble the finished battery cell to obtain the positive electrode sheet;

[0078] 2) Soak the electrode in 1) in DMC (dimethyl carbonate) at room temperature for 60 minutes, take it out, and dry it at room temperature;

[0079] 3) Using liquid nitrogen to fracture the electrode dried in 2) to obtain a cross section of the active layer on the electrode;

[0080] 4) Select a continuous field of view of 25 μm×25 μm on a SEM for cross-sectional observation to determine the number m of the first carbon nanotubes.

[0081] (4) Test of electrode porosity

[0082] a) Discharge the battery cell to the cut-off voltage (e.g. 3.0V for LCO material battery cell) and disassemble it to obtain the positive electrode sheet;

[0083] b) soaking the positive electrode sheet in DMC (dimethyl carbonate) at room temperature for 60 minutes, taking it out, and drying it at room temperature;

[0084] c) Use mercury intrusion porosimetry to test the porosity and pore size distribution data of the electrode in b), and then calculate the percentage R of pores with a diameter ≥ 200 nm in the total pore volume.

[0085] (5) EIS curve

[0086] a. Fully charge the cell at 0.2C to the designed voltage, then discharge it at 0.2C to 50% SOC. Disassemble the cell in an environment with a humidity of ≤15% to obtain the positive electrode, then immediately bag and seal it. b. Make a symmetrical battery using the electrode in step (a) and measure its EIS curve.

[0087] (6) 1C discharge rate

[0088] Take the finished battery cells and perform the following tests at 25±2℃:

[0089] 1) Let it stand for 2 hours, then discharge at 0.7C to the cut-off voltage (for the positive electrode active material, the LCO voltage is 3.0V), and let it stand for 5 minutes.

[0090] 2) Charge at 1.5C to the cut-off voltage, then charge to 0.05C at the cut-off voltage; let stand for 5 minutes; discharge at 0.2C to the cut-off voltage, and record the discharge capacity as C1

[0091] 3) Let it stand for 10 minutes, then discharge at 0.7C to the cut-off voltage (for the positive electrode active material, the LCO voltage is 3.0V), and let it stand for 5 minutes.

[0092] 4) Charge at 1.5C to the cut-off voltage, then charge to 0.05C at the cut-off voltage; let stand for 5 minutes; discharge at 1C to the cut-off voltage, and record the discharge capacity as C2

[0093] 5) C2 / C1 is the 1C discharge rate.

[0094] (7) 45℃ cycle capacity retention

[0095] Take the finished battery cells and perform the following tests at 45±1℃:

[0096] After standing for 2 hours, the battery was discharged at 0.7C to the cut-off voltage (for the positive electrode active material, the LCO voltage is 3.0V), and then stood for 5 minutes.

[0097] {[Charge at 1.5C to the cut-off voltage, charge to 0.05C at the cut-off voltage; let stand for 5 minutes; discharge at 0.7C to the cut-off voltage, record the discharge capacity as C1; let stand for 5 minutes;]

[0098] Cycle the process in [] 49 times, and record the capacity as C1 / C2...C49 in sequence; in the 50th cycle, charge at 0.5C to the cut-off voltage, and charge to 0.05C at the cut-off voltage; let it stand for 5 minutes; discharge at 0.7C to the set value, and record the discharge capacity as C50}

[0099] The process in {} is cycled 10 times + [ ] 1 time, and the capacity retention rate after 500 cycles is C501 / C1*100%.

[0100] The other 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 in Table 2 except for the content, for example: for the first carbon nanotube, including ID1 / IG1, 2θ angle peak width, the first carbon nanotube outer diameter / inner diameter, length L1 and the number of roots m; for the second carbon nanotube, including the second carbon nanotube outer diameter / inner diameter, length, lithium cobalt oxide DV10, etc. are the same as those in Example 1-1, except that, during the preparation process of the positive electrode sheet of Example 1-1, conductive carbon particles are further added to Examples 2-1 to 2-14. The various parameters of the conductive carbon particles are shown in Table 2, and the mass ratios among the first carbon nanotube, the second carbon nanotube, the conductive carbon particles and the active material are also shown in Table 2.

[0101] Table 1

[0102] Table 2

[0103] Data Analysis

[0104] Combined with Table 1, compared with Example 1-1, the parameters of the first carbon nanotubes in Comparative Examples 1-1 to Comparative Examples 1-3 are not suitable. Although the parameters of the second carbon nanotubes are within the scope of this application, the parameters of the first carbon nanotubes result in the first carbon nanotubes, the second carbon nanotubes and the active material particles not being able to cooperate with each other, and thus the conductive network expected by this application cannot be built. The 1C discharge rate of the lithium-ion batteries prepared in Comparative Examples 1-1 to Comparative Examples 1-3 does not exceed 75%, which is much lower than the 1C discharge rate of 95% in Example 1-1, which is about 20% lower. The capacity retention rate of the lithium-ion batteries prepared in Comparative Examples 1-1 to Comparative Examples 1-3 after 500 cycles at 45°C does not exceed 73%, which is much lower than the capacity retention rate of 90% in Example 1-1, which is about 17% lower.

[0105] In particular, further adjusting the parameters of the first carbon nanotubes, the second carbon nanotubes, and the active material lithium cobalt oxide within appropriate ranges can further enhance the rate performance and high-temperature cycling performance of the lithium-ion battery. In particular, when the parameters of the first carbon nanotubes, the second carbon nanotubes, and the active material lithium cobalt oxide are within the preferred ranges, the effect of improving the rate performance and high-temperature cycling performance of the lithium-ion battery is significant.

[0106] From Table 2, it can be seen that when conductive carbon particles of appropriate particle size are further added to the positive electrode material layer, and the mass ratio of the first carbon nanotubes, the second carbon nanotubes, 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 the lithium-ion battery is better.

[0107] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A positive electrode plate, characterized in that: The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode 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 straight carbon nanotube and / or has a certain curvature, the outer diameter of the first carbon nanotube is 20 nm to 200 nm, the inner diameter of the first carbon nanotube is 3 nm to 15 nm, and three points are randomly selected on the same first carbon nanotube and connected along the length direction thereof to form an angle α, 30°≤α≤180°; A plurality of the second carbon nanotubes are aggregated and / or entangled with each other to form a coating layer disposed on the surface of the active material; For the positive electrode plate, when tested by a scanning electron microscope, within any range of 25 μm×25 μm, the number of the first carbon nanotubes is m, and 2≤m≤115.

2. The positive electrode sheet according to claim 1, characterized in that: The first carbon nanotubes satisfy at least one of the following conditions: (1) In Raman spectroscopy, at 1150 cm -1 to 1450cm -1 There is a characteristic peak D in the range, and its peak intensity is I D , at 1450cm -1 to 1750cm -1 There is a characteristic peak G in the range, and its peak intensity is I G , satisfying: 0<I D / I G ≤0.7; (2) In the XRD diffraction pattern, 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°; (3) 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; (4) Select any three points on the same first carbon nanotube and connect them along its length to form an angle α, 90°≤α≤180°; (5)15≤m≤50。 3. The positive electrode sheet according to claim 2, characterized in that: The second carbon nanotubes satisfy at least one of the following conditions: (1) the outer diameter of the second carbon nanotube is 1 nm to 30 nm, the inner diameter of the second carbon nanotube is 0.05 nm to 20 nm, and the length of the second carbon nanotube is 1 μm to 20 μm; (2) Based on the mass of the positive electrode material layer, the mass percentage of the second carbon nanotubes 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 D μm, and the positive electrode sheet satisfies at least one of the following conditions: (1) L1 ≥ 0.5D; (2) the outer diameter of the second carbon nanotube is 5 nm to 20 nm, and the inner diameter of the second carbon nanotube is 1 nm to 15 nm; (3) The length of the second carbon nanotube is 3 μm to 10 μm; (4) The content of the second carbon nanotubes 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 further comprises conductive carbon particles, and the particle size Dv50 of the conductive carbon particles is 10 nm to 100 nm; Based on the mass of the positive electrode material layer, the mass percentage of the conductive carbon particles is T, and T is less than or equal to 2%.

6. The positive electrode sheet according to claim 5, characterized in that: The particle size Dv50 of the conductive carbon particles is 10 to 60 nm; The mass ratio of the second carbon nanotubes, the first carbon nanotubes, 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 pore diameter greater than or equal to 200 nm in the total pore volume is R, 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 further includes substance A; The substance A comprises at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene-hexafluoropropylene, sodium polyacrylate, nitrile rubber, and polyacrylate; Based on the mass of the positive electrode material layer, the mass percentage of the 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 sheet, the intersection S0 of the curve and 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.

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

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