Positive plate and secondary battery

Abstract

The invention belongs to the technical field of new energy batteries, and particularly relates to a positive plate and a secondary battery. The invention provides a positive plate. The positive plate comprises a positive current collector and a positive active layer arranged on at least one side surface of the positive current collector, the positive current collector comprises an aluminum foil, the aluminum foil comprises aluminum grains, the average diameter of the aluminum grains is 0.4-6 microns, and the maximum diameter of the aluminum grains is less than or equal to 15 microns; the positive electrode active layer comprises a chain-shaped conductive material, and at least part of the chain-shaped conductive material is provided with one or more branched chains. The grain size in the current collector is controlled, the deformation problem of the current collector can be effectively resisted, and the mechanical strength and the anti-fatigue performance of the positive electrode current collector are improved; the chain-shaped conductive material can be in bridge connection with a gap between the current collector and a material in the positive electrode active layer, and a branch chain of the chain-shaped conductive material, an active substance in the positive electrode active layer, a conductive agent and a binder form a plurality of contact sites to jointly form a three-dimensional network structure.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy batteries, and particularly relates to a positive electrode sheet and a secondary battery. BACKGROUND

[0002] At present, with the continuous increase of the energy density of lithium ion batteries, higher requirements are also put forward for the rate performance. The capacity retention rate and thermal safety of the battery under high-rate charging and discharging conditions are the key to restricting the rate performance. If the capacity retention rate is poor, it means that the capacity of the battery will decay during the cycle process, resulting in a shortened service life. If the thermal safety is poor, the battery is more prone to heat under high-rate conditions, resulting in thermal runaway, even self-ignition, explosion and other serious accidents. SUMMARY

[0003] Therefore, the technical problem to be solved by the application is to overcome the problems of poor cycle performance and thermal safety of the battery under high-rate charging and discharging conditions in the prior art, so as to provide a positive electrode sheet and a secondary battery.

[0004] To this end, the application provides the following technical solutions.

[0005] In a first aspect, the application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active layer arranged on at least one side surface of the positive electrode current collector. The positive electrode current collector comprises an aluminum foil, the aluminum foil comprises aluminum grains, the average diameter of the aluminum grains is 0.4-6 pm, and the maximum diameter of the aluminum grains is less than or equal to 15 pm. The positive electrode active layer comprises chain-shaped conductive material, and at least part of the chain-shaped conductive material has one or more branches.

[0006] In some embodiments, the chain-shaped conductive material is chain-shaped conductive carbon black, and the positive electrode active layer further comprises spherical conductive carbon black. The oil absorption value of the chain-shaped conductive carbon black and / or the spherical conductive carbon black is denoted as D mL / 100g, the specific surface area of the chain-shaped conductive carbon black and / or the spherical conductive carbon black is denoted as S m 2 / g, and the thickness of the positive electrode sheet is denoted as H pm, and the following formula is satisfied: 1.4≤(0.1S+D) / H≤6.6.

[0007] In some embodiments, the following formula is satisfied: 150≤D≤400; further, 200≤D≤300.

[0008] In some embodiments, the following formula is satisfied: 100≤S≤1400; further, 200≤S≤800.

[0009] In some embodiments, the following formula is satisfied: 80≤H≤160; further, 90≤H≤140.

[0010] In some embodiments, the positive electrode active layer further includes carbon nanotubes, and the mass content of the carbon nanotubes is denoted as m, based on the mass of the positive electrode active layer. CNT The mass content of the conductive carbon black is denoted as m. sp %.

[0011] In some implementations, the following condition is satisfied: 0.4 ≤ m sp / m CNT ≤20.

[0012] In some implementations, 0.3 ≤ m sp ≤4; further, 0.4≤m sp ≤3.

[0013] In some implementations, 0.05 ≤ m CNT ≤1.2.

[0014] In some embodiments, the surface resistivity of the positive electrode is denoted as ρ kΩ·cm, satisfying 1.2≤ρ≤15; further, 1.5≤ρ≤12.

[0015] In some embodiments, the positive electrode active layer is immersed in the electrolyte for less than or equal to 10 minutes. The electrolyte includes an organic solvent, additives, and LiPF6. The organic solvent includes ethylene carbonate, propylene carbonate, ethyl propionate, and propyl propionate, with a mass ratio of propylene carbonate, ethyl propionate, and propyl propionate of 2:6:7. Based on the total mass of the electrolyte, the mass content of ethylene carbonate is 20 wt%. The additives are 10 wt% fluoroethylene carbonate, 5 wt% 1,3-propanesulfonate lactone, and 1 wt% lithium difluorooxalate borate. The mass content of LiPF6 is 15 wt%.

[0016] In some embodiments, the positive electrode active layer further includes a positive electrode active material, which includes a ternary material.

[0017] Furthermore, in some embodiments, the chemical formula of the ternary material is LiNi. x Co y Mn z O2, where x+y+z=1, 0 <x<1,0<y<1,0<z<1。

[0018] Furthermore, in some embodiments, the ternary material comprises single-crystal particles, and the mass content of the single-crystal particles is denoted as m, based on the mass of the positive electrode active material. 单晶 %, satisfying 50≤m 单晶 ≤100.

[0019] Further, in some embodiments, at least part of the surface of the ternary material is coated or doped with at least one of Al, Zr, Mg, Y, Ti, B, Sr, W, Si, La, Nb elements.

[0020] In some embodiments, the following is satisfied: 50≤m 单晶 ≤80.

[0021] In some embodiments, the ternary material further comprises polycrystalline particles, and the mass content of the polycrystalline particles is denoted as m 多晶 %, and the following is satisfied: 0 多晶 ≤50; further, 20≤m 多晶 ≤50.

[0022] In the second aspect, the application provides a battery, comprising: a negative electrode sheet, an electrolyte and the positive electrode sheet of the first aspect.

[0023] In some embodiments, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector, and the negative electrode active layer comprises a negative electrode active material; and the negative electrode active material comprises a silicon-based material and a carbon-based material.

[0024] Further, in some embodiments, the silicon-based material comprises at least one of a silicon-carbon material and a silicon-oxygen material.

[0025] Further, in some embodiments, the volume median particle size of the silicon-based material is 4-15 μm.

[0026] Further, in some embodiments, the sphericity F of the silicon-based material satisfies: 0.8≤F<1.

[0027] In some embodiments, the electrolyte comprises a solvent and a lithium salt, and the solvent comprises ethylene carbonate, and the mass content of the ethylene carbonate is denoted as m EC %, and the following is satisfied: 0.01≤m sp / m EC ≤0.6.

[0028] Further, in some embodiments, 0.3≤m sp ≤4; further, 0.4≤m sp ≤3.

[0029] Further, in some embodiments, 5≤m EC ≤50.

[0030] In some embodiments, the electrolyte further comprises fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate is 5%-25% based on the mass of the electrolyte.

[0031] Further, in some embodiments, the mass content of the fluoroethylene carbonate is 8%-20% based on the mass of the electrolyte.

[0032] The technical scheme provided in the application has the following advantages: The positive electrode sheet provided in the application comprises a positive electrode current collector and a positive electrode active layer arranged on at least one side surface of the positive electrode current collector. The positive electrode current collector comprises an aluminum foil, the aluminum foil comprises aluminum grains, the average diameter of the aluminum grains is 0.4-6 μm, and the maximum diameter of the aluminum grains is less than or equal to 15 μm. The positive electrode active layer comprises chain-shaped conductive materials, and at least part of the chain-shaped conductive materials has one or more branches. By controlling the average diameter (0.5-5 μm) and the maximum diameter (less than or equal to 15 μm) of the aluminum grains in the positive electrode current collector, the following advantages can be achieved: the positive electrode current collector has more grain boundaries, which can effectively resist the deformation of the current collector caused by the stress generated by the volume change of the active material during the cycle process; the stress concentration problem caused by the existence of large grain size in the positive electrode current collector can be avoided, the adverse phenomenon of micro-cracks or even cracks in the positive electrode sheet can be reduced, and the mechanical strength and fatigue resistance of the positive electrode current collector can be improved. Further, the chain-shaped conductive materials can "bridge" the small gaps between the current collector and the materials in the positive electrode active layer, and the branch structure of the chain-shaped conductive materials can form multiple contact sites with the active material, the conductive agent and the binder in the positive electrode active layer, thereby forming a three-dimensional network structure, reducing the contact resistance, reducing the heat generation of the battery during the cycle process, and helping the uniform flow of current and the rapid conduction of electrolyte, which is beneficial to the cycle performance of the battery and effectively improves the temperature rise problem.

[0033] Additional aspects and advantages of the embodiments of the application will be described in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following description and drawings. BRIEF DESCRIPTION OF DRAWINGS

[0034] In order to more clearly illustrate the specific embodiments of the application or the technical scheme in the prior art, the following will briefly introduce the drawings needed to be used in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the application, and those skilled in the art can also obtain other drawings according to these drawings without creating any inventive labor.

[0035] Figure 1 FIG. 1 is a schematic diagram of a scanning electron microscope of the chain-shaped conductive carbon black of the application. Figure 2 This is a schematic diagram of a scanning electron microscope image of the spherical conductive carbon black of this application. Detailed Implementation

[0036] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by any person based on the teachings of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0037] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0038] As described in the background section, existing batteries suffer from poor cycle performance and thermal safety under high-rate charge-discharge conditions. Accordingly, this application provides a positive electrode sheet, comprising a positive current collector and a positive active layer disposed on at least one surface of the positive current collector; The positive current collector includes an aluminum foil, the aluminum foil includes aluminum grains, the average diameter of the aluminum grains is 0.4μm-6μm, and the maximum diameter of the aluminum grains is less than or equal to 15μm; the positive active layer includes a chain-like conductive material, at least a portion of the chain-like conductive material having one or more branches.

[0039] This application research found that the smaller the average diameter of aluminum grains in the positive electrode current collector, the more grain boundaries there are per unit volume of current collector. The more grain boundaries there are, the stronger the resistance to dislocation movement, and the stronger the ability to resist external forces and prevent the current collector from undergoing plastic deformation. At the same time, if there are abnormally large grains in the positive electrode current collector, these excessively large grains have different crystal orientations than smaller grains, and their deformation coordination when resisting external forces is inconsistent. Stress concentration is likely to occur at grain boundaries or inside grains, leading to microcracks or even cracks in the positive electrode sheet, affecting the structural stability of the positive electrode sheet. Therefore, this application controls the average diameter of aluminum grains in the positive electrode current collector (0.The optimal grain size (4μm-6μm) and maximum diameter (≤15μm) ensure a high number of grain boundaries within the positive electrode current collector. This effectively resists deformation caused by stress resulting from volume changes in the active material during cycling. Furthermore, it avoids stress concentration issues caused by large grain sizes in the positive electrode current collector, thus reducing the likelihood of microcracks or even full cracking in the positive electrode sheet. This improves the mechanical strength and fatigue resistance of the positive electrode current collector and enhances its structural stability. While a relatively high number of grain boundaries within the positive electrode current collector, when the average grain diameter is within the above range, can mitigate these issues, a large number of grain boundaries can lead to… Electrons frequently collide and scatter with grain boundaries during their movement, especially when the battery is charged and discharged at high rates. This phenomenon is more pronounced, leading to an increase in the resistance of the positive electrode current collector and increased heat generation. Furthermore, it exacerbates the expansion of the positive electrode active material, conductive agent, and binder materials in the positive electrode sheet, resulting in micro-gaps between the materials in the positive electrode active layer and the positive electrode current collector, as well as poorer contact between different materials in the positive electrode active layer. This increases the contact impedance between the positive electrode current collector and the positive electrode active layer, further triggering more severe battery heat generation problems. This application incorporates a chain-like conductive material into the positive electrode active layer. This chain-like conductive material can "bridge" the micro-gaps between the current collector and the materials in the positive electrode active layer. In addition, at least a portion of the chain-like conductive material has one or more branches. The presence of these branches allows them to form multiple contact sites with the active material, conductive agent, and binder in the positive electrode active layer, collectively forming a three-dimensional network structure. This significantly increases the contact area between the active material, conductive agent, and binder particles, effectively reducing contact resistance, decreasing heat generation during battery cycling, and suppressing battery temperature rise. Secondly, the resilient branches in the chain-like conductive material, under the action of the binder, can physically "hook" and "tether" the active material particles, forming a mechanical interlocking structure that reduces cracking of the positive electrode active layer. The branch structure can also form abundant contact paths with the positive electrode current collector, reducing the positive electrode temperature. The three-dimensional network improves the contact resistance between the current collector and the positive electrode active layer, reducing battery cycle time and heat generation. Thirdly, it helps current flow evenly within the positive electrode, preventing localized heat generation due to excessively high local current density caused by localized conductive path failure. Fourthly, the three-dimensional network facilitates rapid electrolyte conduction, allowing the positive electrode active layer to be quickly wetted, thus providing lithium-ion transport channels for the active material at the bottom of the active layer. This increases the number of charge carriers in the active layer, reduces internal resistance, and consequently reduces heat generation, improving battery cycle performance and reducing excessive temperature rise, thereby enhancing battery thermal safety. This is particularly evident when the battery is charged and discharged at high rates, where its cycle performance and temperature rise are significantly improved.

[0040] For example, the average diameter (in μm) of the aluminum grains can be 0.4, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, or a value within the range of any two of the above values. For example, the maximum diameter (in μm) of the aluminum grains can be 0.4, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or a value within the range of any two of the above values.

[0041] The testing methods for the average and maximum diameter of the aluminum grains include: cutting appropriately sized samples from aluminum foil, embedding and fixing the samples with epoxy or acrylic resin to prevent deformation or chamfering during subsequent polishing; grinding the sample surface with sandpaper of different grits (e.g., from 400# to 2000#) from coarse to fine until the sample surface is smooth and the scratches are uniform; then evenly spraying diamond polishing agent or alumina suspension onto a cloth to finely polish the sample surface to obtain a scratch-free, mirror-like surface, eliminating interference with the microstructure during sample preparation. Finally, etching the polished aluminum foil surface with an etchant (e.g., an etchant solution formed by mixing water and oxalate in a 10:1 ratio) to clearly reveal the grain boundaries. The etched sample surface was observed using a metallographic microscope or scanning electron microscope at appropriate magnifications (e.g., 200x, 500x, or 1000x). Multiple fields of view (e.g., 100) were randomly selected, and high-resolution metallographic photographs were taken. The maximum grain size in each field of view was measured, and the maximum grain size was compared to obtain the maximum grain diameter. According to the international standard ASTM E112, the metallographic photographs of the sample were compared with the standard grain size chart provided in ASTM E112 to quickly determine the grain size level (G) of each grain in the field of view. The average grain diameter was then calculated using a formula as follows: Average diameter = .

[0042] In some embodiments, the chain-like conductive material is chain-like conductive carbon black, and the positive electrode active layer further includes spherical conductive carbon black; the oil absorption value of the chain-like conductive carbon black and / or the spherical conductive carbon black is denoted as D mL / 100g, and the specific surface area of ​​the chain-like conductive carbon black and / or the spherical conductive carbon black is denoted as S m 2 / g, the thickness of the positive electrode is denoted as H μm, which satisfies: 1.4≤(0.1S+D) / H≤6.6.

[0043] Furthermore, this application's research found that as the energy density of batteries continuously increases, the amount of active material in the positive electrode also increases, and the thickness of the positive electrode sheet also increases. This leads to a longer lithium-ion diffusion path during charging and discharging, increased lithium-ion diffusion resistance, increased battery resistance, increased battery heat generation, increased side reactions in the battery, and decreased cycle performance. Simultaneously, a thicker positive electrode sheet may also affect the electrolyte's wetting of the positive electrode active material at the bottom of the positive electrode sheet, preventing some of the bottom positive electrode active material from participating in the reaction, reducing the area of ​​the active layer that can effectively participate in the reaction, and affecting the battery's cycle performance. Therefore, this application controls 1.4≤(0.1S+D) / H≤6.6 to enable conductive carbon black particles to more effectively connect with the positive electrode active material particles filled in the electrode, increasing conductive contact points and reducing resistance. Especially during high-rate charging and discharging, it can effectively reduce battery heat generation and side reactions, further improving the battery's thermal safety and cycle performance. By adjusting the oil absorption value and specific surface area of ​​the conductive carbon black in the positive electrode sheet according to the thickness of the positive electrode sheet, it can effectively help the conductive carbon black... The presence of suitable pores in the positive electrode active layer facilitates rapid wetting and uniform diffusion of the electrolyte, thereby enabling effective ion transport in the positive electrode active material at the bottom of the positive electrode sheet. This ensures sustained capacity utilization, extends battery life, and improves cycle performance. Furthermore, excessive thickness of the positive electrode sheet necessitates a more efficient and robust conductive network to accommodate the longer electron transport path and greater mechanical stress. This ensures electrons can be transported to the bottom of the positive electrode sheet while maintaining the structural stability of the conductive network. High specific surface area conductive carbon black can provide more conductive contact points. Simultaneously, it needs to possess a suitable oil absorption value to prevent ion channel blockage when forming a dense network in the positive electrode active layer. Therefore, when the oil absorption value (D) of chain-like and / or spherical conductive carbon black, the specific surface area (S) of chain-like and / or spherical conductive carbon black, and the thickness (H) of the positive electrode sheet meet the aforementioned ranges, it can increase conductive contact points, reduce resistance, and alleviate the problems of high heat generation and numerous side reactions during high-rate charge and discharge, further improving battery cycle performance and mitigating temperature rise issues. For example, the value of (0.1S+D) / H can be 1.4, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 6.6, etc., or a value within the range of any two of the above values.

[0044] The test method for the oil absorption value D of the conductive carbon black includes: measurement using ASTM D2414, specifically as follows: Under constant stirring conditions, DBP oil (dibutyl phthalate) is added dropwise to a quantitative conductive carbon black sample at a constant rate. The chain-like or spherical conductive carbon black gradually absorbs DBP and fills the gaps between the aggregates. The mixture gradually changes from a free-flowing powder state to a semi-dry agglomerate, eventually reaching a point where the torque increases sharply. At this point, all gaps are filled, the conductive carbon black aggregates are lubricated by DBP, and the system viscosity is at its maximum. The volume of DBP consumed at this point is recorded, and the oil absorption value can be calculated. Specifically, in some embodiments, the oil absorption value D of the conductive carbon black satisfies: 150 ≤ D ≤ 400; particularly 200 ≤ D ≤ 300. This further improves the conductivity stability of the conductive carbon black, reduces side reactions in the battery, lowers internal resistance, improves battery temperature rise, and is beneficial to battery cycle performance and thermal safety. When the oil absorption value of conductive carbon black is too high, it will adsorb a large amount of binder and solvent, making dispersion difficult, resulting in increased local resistance and excessive liquid retention, leading to more side reactions. Conversely, when the oil absorption value of conductive carbon black is too low, its liquid absorption is insufficient, resulting in a sparse network, fewer electron transport channels, increased electrode resistance, and decreased cycle performance. For example, the oil absorption value D (unit mL / 100g) of conductive carbon black can be 150, 200, 250, 300, 350, 400, or a value within any two of the above ranges.

[0045] The method for testing the specific surface area S of the conductive carbon black includes: using the BET nitrogen adsorption method to test the specific surface area, specifically as follows: at liquid nitrogen temperature (-196℃), the conductive carbon black sample is exposed to nitrogen gas at different partial pressures, and nitrogen molecules will physically adsorb onto the surface of the conductive carbon black (including the outer surface and the inner pore surface); the amount of nitrogen adsorbed by the sample under different relative pressures is measured, and an "adsorption-desorption isotherm" is plotted; within a suitable relative pressure range (i.e., P / P0 = 0.05-0.35), the adsorption data conforms to the BET equation, the BET equation is converted into a linear regression equation, and the nitrogen volume required to form a monolayer is calculated; combined with the cross-sectional area of ​​a single nitrogen molecule (0.162 nm), the specific surface area is determined. 2The specific surface area of ​​the sample is calculated. Specifically, in some embodiments, the specific surface area S of the conductive carbon black satisfies: 100≤S≤1400, especially 200≤S≤800. This further improves the stability of the conductive carbon black, ensuring that while providing an appropriate number of conductive sites, its contact with the electrolyte is reduced, reducing the occurrence of side reactions, lowering battery resistance, and reducing heat and gas generation during cycling. This is beneficial to the battery's cycle performance and improves the battery's temperature rise problem, thus enhancing the battery's thermal safety. If the specific surface area S of the conductive carbon black is too small, the particle size is large and the aggregates are simple, making it impossible to form sufficient "point-line-network" contact with the active material. The electron transport channels are scarce, the electrode resistance increases, heat generation increases, battery cycle performance decreases, and temperature rise increases. If the specific surface area S of the conductive carbon black is too large, the surface energy is high and the van der Waals forces are strong, making it easy to form "hard agglomerates" in the positive electrode slurry. Even with high shear dispersion, these agglomerates are difficult to break up, resulting in an uneven conductive network in the positive electrode active layer. The electrode resistance increases, which also deteriorates the battery's cycle performance and increases heat generation. For example, the specific surface area S (in m²) of conductive carbon black 2 The value of / g) can be 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, etc., or a value within the range of any two of the above values.

[0046] The method for testing the thickness H of the positive electrode sheet includes: discharging the battery to 0% SOC, removing the positive electrode sheet, cutting off the edge of the positive electrode sheet sample, preparing a cross-section sample using an argon ion polishing (CP) instrument, observing the cross-section using a scanning electron microscope (SEM), and measuring the thickness of the positive electrode sheet at 10 points sequentially, taking the average value as the thickness H of the positive electrode sheet. Specifically, in some embodiments, the thickness H of the positive electrode sheet satisfies: 80 ≤ H ≤ 160, especially 90 ≤ H ≤ 140; in this way, while ensuring the energy density of the battery, the path of electrolyte wetting of the positive electrode sheet and the path of lithium ion transport can be shortened as much as possible, thereby improving the thermal safety and cycle performance of the battery. For example, the thickness H (unit μm) of the positive electrode sheet can be 80, 90, 100, 110, 120, 130, 140, 150, 160, etc., or a value within any two of the above values.

[0047] The chain-like conductive carbon black is formed by the directional growth of multiple conductive carbon black molecules through chemical bonding, creating an open, extended three-dimensional branched aggregate. The branches are derived from other spherical conductive carbon black molecules on the aggregate. A scanning electron microscope schematic diagram of the chain-like conductive carbon black of this application is shown below. Figure 1 Spherical conductive carbon black is a secondary aggregate formed by the physical aggregation of multiple conductive carbon black particles through random collisions caused by van der Waals forces, resulting in an approximately spherical or grape-like shape. A scanning electron microscope schematic diagram of the spherical conductive carbon black of this application is shown below.Figure 2 .

[0048] In some embodiments, the positive electrode active layer further includes carbon nanotubes, and the mass content of the carbon nanotubes is denoted as m, based on the mass of the positive electrode active layer. CNT The mass content of the conductive carbon black is denoted as m. sp %. Carbon nanotubes can further reduce the internal resistance of the battery, improve the battery's cycle performance, reduce the heat generated inside the battery, and suppress the problem of temperature rise during battery use.

[0049] In some implementations, m CNT and m sp Satisfy: 0.4≤m sp / m CNT ≤20; Carbon nanotubes have a larger specific surface area and more side reaction sites compared to conductive carbon black. Therefore, this application controls the mass ratio of conductive carbon black to carbon nanotubes to fully utilize the properties of carbon nanotubes in reducing internal resistance and enhancing conductivity, while also reducing side reactions, thereby reducing gas and heat generation, which is beneficial to the thermal safety and cycle performance of the battery. For example, m sp / m CNT It can be 0.4, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, etc., or a value within the range of any two of the above values.

[0050] In some embodiments, the mass content m of conductive carbon black sp Satisfy: 0.3≤m sp ≤4; especially 0.4≤m sp ≤3; thus, while ensuring battery energy density, it further improves battery cycle performance and thermal safety. For example, the mass content m of conductive carbon black... sp The value can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.5%, 3%, 3.5%, 4%, etc., or a value within the range of any two of the above values.

[0051] In some embodiments, the mass content m of carbon nanotubes CNT Satisfies: 0.05≤m CNT ≤1.2; This ensures battery energy density while reducing internal resistance and side reactions, further improving cycle performance and thermal safety. For example, the mass content m of carbon nanotubes... CNTThe value can be 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, or a value within the range of any two of the above values.

[0052] In some embodiments, the surface resistivity of the positive electrode is denoted as ρ kΩ·cm, satisfying 1.2 ≤ ρ ≤ 15; particularly 1.5 ≤ ρ ≤ 12. A surface resistivity within this range can reduce the battery's internal resistance, thereby ensuring good capacity retention during battery cycling, reducing heat and gas generation, and improving battery thermal safety. The method for testing the resistivity ρ of the positive electrode includes a four-probe testing method, specifically as follows: a constant current of 0.5-2mA is applied through the outer probe, the voltage signal is detected by the inner probe, and the resistivity value is calculated. For example, the resistivity ρ (unit: kΩ·cm) of the positive electrode can be 1.2, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or a value within any two of the above ranges. It should be noted that the surface resistivity ρ of the positive electrode is affected by the average diameter of the aluminum grains, the maximum diameter of the aluminum grains, the parameters and content of the conductive carbon black, the content of carbon nanotubes, and the content of polycrystalline and single crystals in the ternary active material.

[0053] In some embodiments, the positive electrode active layer is immersed in the electrolyte for a time of less than or equal to 10 minutes. The electrolyte includes an organic solvent, additives, and LiPF6. The organic solvent includes ethylene carbonate, propylene carbonate, ethyl propionate, and propyl propionate, with a mass ratio of propylene carbonate, ethyl propionate, and propyl propionate of 2:6:7. Based on the total mass of the electrolyte, the mass content of ethylene carbonate is 20 wt%. The additives are 10 wt% fluoroethylene carbonate, 5 wt% 1,3-propanesulfonate lactone, and 1 wt% lithium difluorooxalate borate. The mass content of LiPF6 is 15 wt%. Further, by controlling the oil absorption value and specific surface area of ​​the conductive carbon black, or by increasing the number of branches in the chain-like conductive material, the wetting speed of the positive electrode active layer in the electrolyte can be further increased. Further, in some embodiments, the immersion time of the positive electrode active layer in the electrolyte is less than or equal to 8 minutes. The test method for the immersion time of the positive electrode active layer in the electrolyte includes discharging the battery to 0. SOC%, then disassemble the battery, take a positive electrode sheet of a certain size (specifically, the size of the positive electrode sheet can be 2 cm × 2 cm), soak it in 500 ml of DMC (dimethyl carbonate) solution for 2 hours, then rinse it clean with DMC (dimethyl carbonate), place it in an oven at 80℃ and dry it for 10 hours to fully remove the electrolyte or volatile solvent from the positive electrode active layer. Remove the electrode sheet from the oven and place it in a drying room. Using a pipette, titrate the above-mentioned electrolyte onto the positive electrode sheet, with a titration weight of approximately 0.2~1 g of electrolyte. Start timing when the electrolyte is titrated onto the positive electrode sheet. Stop timing when no flowable electrolyte is visible to the naked eye on the surface of the positive electrode sheet. This is the time when the positive electrode active layer is wetted by the electrolyte. If the electrode sheet has a shorter wetting time, it indicates good wettability; conversely, if the electrode sheet has a longer wetting time, it indicates poor wettability. For example, the time (in minutes) for the positive electrode active layer to be immersed in the electrolyte can be 0.5, 0.75, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or a value within any two of the above values.

[0054] In some embodiments, the positive electrode active layer further includes a positive electrode active material, which includes a ternary material.

[0055] Furthermore, in some embodiments, the chemical formula of the ternary material is LiNi. x Co y Mn zO2, where x + y + z = 1, 0 < x < 1, 0 < y < 1, 0 < z < 1; Exemplarily, the value of x can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc. or a value within the range formed by any two of the above values, the value of y can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc. or a value within the range formed by any two of the above values, and the value of z can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc. or a value within the range formed by any two of the above values.

[0056] Further, in some embodiments, the ternary material includes single crystal particles. Based on the mass of the positive electrode active material, the mass content of the single crystal particles is denoted as m 单晶 %, satisfying 50 ≤ m 单晶 ≤ 100; The single crystal particles are independent single crystals without grain boundaries inside, with a dense structure and good stability, having good cycling performance, less cracking problems of the particles during the cycling process, reducing the side reactions between it and the electrolyte, effectively inhibiting the penetration and continuous oxidation of the electrolyte, thereby reducing gas generation and improving the thermal safety of the battery. Exemplarily, the value of the mass content m 单晶 % can be 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, etc. or a value within the range formed by any two of the above values.

[0057] Further, in some embodiments, at least part of the surface of the ternary material is coated or doped with at least one of elements such as Al, Zr, Mg, Y, Ti, B, Sr, W, Si, La, Nb, etc. Although the ternary material has a high energy density, during the cycling process, the lattice oxygen on the surface of the ternary material is likely to escape, resulting in the transformation of its structure from a stable layered structure to a disordered spinel or rock salt phase. The surface of the material will directly contact with the electrolyte to occur side reactions. Transition metal ions (such as nickel ions and cobalt ions) in the ternary material will dissolve and migrate to the surface of the negative electrode, damaging the solid electrolyte interface film (SEI film) on the negative electrode side, resulting in continuous consumption of lithium ions and electrolyte to occur side reactions to regenerate the SEI film, and catalyzing the oxidation and decomposition of the electrolyte to produce gas. The contact between the positive electrode sheet, the separator and the negative electrode sheet is poor, and the internal resistance increases. Further doping or coating the above elements on the surface of the ternary material can strengthen the lattice of the ternary material or form a stable interface on the surface of the ternary material, reduce cracks in the positive electrode material, and its direct contact with the electrolyte, thereby inhibiting side reactions and transition metal dissolution, thus improving the adverse reactions on the negative electrode side and enhancing the cycling performance and thermal safety of the battery.

[0058] In some implementations, the following condition is satisfied: 50 ≤ m 单晶 ≤80.

[0059] In some embodiments, the ternary material further includes polycrystalline particles, and the mass content of the polycrystalline particles is denoted as m, based on the mass of the positive electrode active material. 多晶 %, satisfying 0 < m 多晶 ≤50; especially 20≤m 多晶 ≤50. Further combining single-crystal and polycrystalline particles can fully guarantee capacity utilization. This is because polycrystalline particles have abundant grain boundaries, excellent processability, and anisotropy. Grain boundaries provide more and faster migration paths for lithium ions. During cycling, the cathode material expands, grain boundaries extend, promoting electrolyte wetting. Blending single-crystal and polycrystalline ternary materials can also increase the compaction density of the cathode sheet, thereby effectively improving the battery's energy density. However, if the proportion of polycrystalline particles is too high, the risk of microcracks forming at grain boundaries in the expanded polycrystalline ternary cathode material during cycling increases, leading to more exposed fresh interfaces and increased side reactions between the cathode sheet and electrolyte, resulting in more severe heat and gas generation. Therefore, it is necessary to balance the single-crystal ratio to achieve sufficient wetting while suppressing heat and gas generation from side reactions, while simultaneously increasing the usable active lithium content throughout the cycling process, improving cycle performance and energy density. For example, the mass content m of polycrystalline particles... 多晶 The value of % can be 0.1%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc., or a value within the range of any two of the above values.

[0060] Furthermore, when the ternary material contains both single-crystal and polycrystalline particles, and satisfies 50 ≤ m 单晶 ≤80, 0<m 多晶 ≤50, especially 20≤m 多晶 ≤50; This can achieve full wetting of the electrolyte, increase the available active lithium content, and control the expansion effect of polycrystalline particles to prevent cracking, which leads to increased side reactions and more heat and gas production. Thus, while ensuring the thermal safety of the battery, it can improve the cycle performance and energy density of the battery.

[0061] Secondly, this application provides a battery comprising: a negative electrode, an electrolyte, and the positive electrode described in the first aspect.

[0062] It is understood that the battery provided in this application has good cycle performance and thermal safety because it includes the positive electrode sheet of the first aspect.

[0063] In some embodiments, the negative electrode sheet includes a negative current collector and a negative active layer disposed on at least one side surface of the negative current collector, the negative active layer including a negative active material; the negative active material includes silicon-based materials and carbon-based materials.

[0064] Furthermore, in some embodiments, the silicon-based material includes at least one of silicon-carbon materials and silicon-oxygen materials.

[0065] Furthermore, in some embodiments, the median volumetric particle size of the silicon-based material is 4μm-15μm. Silicon-based materials exhibit a significant volume expansion effect. During cycling, the SEI film on the surface of the negative electrode continuously ruptures due to the volume change of the silicon-based material, leading to side reactions with the electrolyte and the regeneration of the SEI film. This reaction is continuously exothermic, exacerbating heat generation within the battery. If the particle size of the silicon-based material is too small, the specific surface area increases, intensifying the aforementioned side reactions; if the particle size is too large, the lithium-ion diffusion distance increases, kinetics decrease, and expansion stress concentrates. By controlling the median volumetric particle size of the silicon-based material to be between 4μm and 15μm, the degree of expansion of the silicon-based material can be reduced, the contact reaction area between the silicon-based material and the electrolyte can be reduced, thereby reducing gas and heat generation, lowering interfacial impedance, and minimizing the occurrence of side reactions. Furthermore, a shorter lithium-ion diffusion distance results in better kinetics and lower expansion stress, which is beneficial for further improving the battery's cycle performance and thermal safety. The median volumetric particle size Dv50 of the silicon-based material can be tested using conventional testing methods in the art, such as laser particle size analysis. For example, a Malvern particle size analyzer can be used for measurement. For instance, the volume median particle size (in μm) of silicon-based materials can be 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or a value within any two of the above ranges.

[0066] Furthermore, in some embodiments, the sphericity F of the silicon-based material satisfies: 0.8 ≤ F < 1; thus, the stress concentration problem of the silicon-based material during expansion is reduced, thereby reducing the rupture of the SEI film, improving the stability of the SEI film on the negative electrode side, and further improving the cycle performance and thermal safety of the battery. The sphericity of the silicon-based material can be tested by conventional methods in the art. For example, using image processing software (e.g., Image Pro Plus), at least 10 silicon-based material particles are selected from the scanning electron microscope (SEM) image of the silicon-based material at a certain magnification (e.g., 2500x), and the perimeter and area of ​​each particle are measured. The perimeter equivalent radius r1 and area equivalent radius r2 of each particle are calculated respectively, and the sphericity is r2 / r1. The average value is then taken to obtain the sphericity F of the silicon-based material. For example, the sphericity F of the silicon-based material can be 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, or a value within any two of the above ranges.

[0067] In some embodiments, the electrolyte comprises a solvent and a lithium salt, wherein the solvent comprises ethylene carbonate, and the mass content of ethylene carbonate is denoted as m based on the mass of the electrolyte. EC %, satisfying: 0.01≤m sp / m EC ≤0.6; Ethylene carbonate has an extremely high dielectric constant, which can effectively break the ionic bonds of lithium salts and promote their dissociation, thereby providing a sufficient number of free lithium ions to ensure high ionic conductivity. Conductive carbon black forms a three-dimensional conductive network between the positive electrode particles, transporting electrons to the surface of each active material; EC Subsequently, lithium ions are dissociated and simultaneously delivered to the same location, achieving "electron-ion synchronous arrival," which improves battery cycle performance. The conductive carbon black surface has numerous sp2 edges, oxygen-containing functional groups (–OH, –COOH, carbonyl groups, etc.), and topological defects. These sites can adsorb ethylene carbonate molecules and lower their dehydrogenation / ring-opening activation energy at >4.2V. Simultaneously, the conductive carbon black itself has low resistance, allowing it to instantly "transfer" electrons from the positive electrode current collector to the adsorbed ethylene carbonate, accelerating the oxidation of ethylene carbonate and increasing side reactions in the battery. When the ratio of conductive carbon black to ethylene carbonate is within the aforementioned range, a complete conductive network can be formed, increasing the effective reaction area and reducing polarization. It also reduces side reactions of ethylene carbonate on the conductive carbon black surface, reducing heat and gas generation and lowering contact resistance, which is beneficial to battery cycle performance and thermal safety. If m sp / m ECA value greater than 0.6 indicates that a larger amount of conductive carbon black was added compared to ethylene carbonate. The conductive carbon black provides more surface oxidation sites, which, under the influence of current, cause ethylene carbonate to decompose at these sites, generating heat and gas. This leads to separation of the positive electrode from the separator, increasing contact resistance and further generating more Joule heat, thus affecting the battery's thermal safety. If m sp / m EC A value less than 0.01 indicates that the amount of ethylene carbonate added is greater than that of conductive carbon black. The conductive carbon black forms an imperfect network, resulting in a smaller effective reaction area, poor conductivity, and high polarization. This increases the battery's ohmic internal resistance, generates Joule heat, and also affects the battery's cycle performance and thermal safety.

[0068] The mass content of ethylene carbonate m EC The test method can be obtained by conventional methods in the field, such as using gas chromatography-mass spectrometry to separate and quantify the electrolyte sample, and determining the content of ethylene carbonate by standard curve method.

[0069] Specifically, in some embodiments, the mass content of ethylene carbonate is m EC Satisfy: 5≤m EC ≤50; thus, the electrolyte can possess high ionic conductivity and suitable viscosity, further improving the battery's cycle performance and thermal safety. For example, the mass content of ethylene carbonate m EC The value can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc., or a value within the range of any two of the above values.

[0070] In some embodiments, the electrolyte further includes fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate is 5%-25%, particularly 8%-20%, based on the mass of the electrolyte. Furthermore, this application has found that ethylene carbonate can participate in the formation of an SEI film on the surface of the negative electrode. However, when the volume of the silicon-based material changes, the SEI film formed by the ethylene carbonate on the negative electrode side has low strength and is easily broken, leading to continuous reduction of the ethylene carbonate, consuming lithium ions to regenerate the SEI film, and increasing side reactions in the battery. This application further adds fluoroethylene carbonate to the electrolyte, which can generate a lithium fluoride-containing SEI film on the negative electrode side, thereby improving the mechanical strength and chemical stability of the SEI film, suppressing side reactions between the electrolyte and the silicon-based material, and reducing the loss of active lithium. Furthermore, this SEI film can also constrain the volume expansion of the silicon-based material, thereby reducing the destructive effect of the silicon-based material on the SEI film, reducing active lithium loss and battery heat generation, and ensuring the cycle performance and thermal safety of the battery. If the mass content of fluoroethylene carbonate is greater than 25%, excessive fluoroethylene carbonate will form an excessively thick SEI film, increasing initial irreversible capacity loss and interfacial impedance, thus affecting the battery's cycle performance and thermal safety. Conversely, if the mass content of fluoroethylene carbonate is less than 5%, it cannot effectively improve the mechanical strength and chemical stability of the SEI film. The mass content of fluoroethylene carbonate can be determined by conventional methods in the art, such as using gas chromatography-mass spectrometry to separate and quantify the electrolyte sample, and determining the fluoroethylene carbonate content using a standard curve method. For example, the mass content of fluoroethylene carbonate can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, etc., or values ​​within any two of the above ranges.

[0071] In some embodiments, the lithium-ion battery also includes a separator; the separator includes a base film and a coating. This application does not have any particular limitation on the type of base film, and any known base film with good chemical and mechanical stability can be selected.

[0072] In some embodiments, the base film can be made of at least one of polyethylene (PE), polypropylene (PP), or a composite base film of polyethylene and polypropylene. The base film can be a single-layer film or a multi-layer composite film, without particular limitation. When the base film is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0073] In some embodiments, the coating comprises ceramic particles selected from at least one of alumina, boehmite, silicon dioxide, zirconium dioxide, barium sulfate, magnesium hydroxide, fluorapatite, fluorophlogopite, mullite, aluminum titanate, copper oxide, titanium dioxide, and zinc oxide.

[0074] In some embodiments, the coating further includes an adhesive, which includes at least one of polyvinylidene fluoride adhesives and polyacrylate adhesives; the coating is applied according to conventional processes in the art, such as gravure coating, transfer coating, dip coating and spraying, for single-sided or double-sided coating.

[0075] In some embodiments, the conductive agent further includes at least one of acetylene black, conductive carbon black, Ketjen black, conductive graphite, carbon nanotubes, conductive carbon fibers, and graphene.

[0076] In some embodiments, the negative electrode active layer further includes a binder. The binder is selected from at least one of polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyethylene oxide.

[0077] In some embodiments, the negative electrode current collector further comprises a doping element, which includes at least one of Si, Cr, and Mn.

[0078] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorooxalate borate, lithium difluorobis(oxalate) phosphate, lithium tetrafluoroborate, lithium bis(oxalate) borate, lithium hexafluoroantimonyate, lithium hexafluoroarsenate, lithium difluorosulfonylimide, lithium di(pentafluoroethylsulfonyl)imide, and lithium tri(trifluoromethylsulfonyl)methyl.

[0079] The present application is further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Reagents or instruments used, unless otherwise specified, are all commercially available conventional reagent products. In all embodiments and comparative examples of this application, the unit % represents mass percentage.

[0080] Example 1 This embodiment provides a method for preparing a battery, including the following steps: (1) Preparation of positive electrode Ternary active material (lithium nickel cobalt manganese oxide LiNi) 0.6 Co 0.2 Mn 0.2 O2), conductive carbon black (oil absorption value D is 250mL / 100g, specific surface area S is 600m²) 2The following components were mixed in a mass ratio of 98:1:0.5:0.5:N-methylpyrrolidone (NMP) was added, and the mixture was stirred until a homogeneous and fluid positive electrode slurry was formed. The positive electrode slurry was uniformly coated onto a current collector aluminum foil (8 μm thick) and dried in a vacuum oven at 80 °C for 10 h. The resulting positive electrode sheet was then obtained through rolling and slitting. The conductive carbon black was a mixture of chain-like and grape-like conductive carbon black, with each chain-like conductive carbon black having at least one branch. Specific parameters of the positive electrode sheet are shown in Table 1-2. Among the ternary active materials, single-crystal lithium nickel cobalt manganese oxide (LiNi) was used. 0.6 Co 0.2 Mn 0.2 O2 and polycrystalline lithium nickel cobalt manganese oxide (LiNi) 0.6 Co 0.2 Mn 0.2 The mass ratio of O2 is 4:1.

[0081] (2) Preparation of negative electrode Silicon carbon material (volume median particle size 7 μm, sphericity 0.85), artificial graphite, conductive carbon black, sodium carboxymethyl cellulose, and polyacrylic acid were mixed in a mass ratio of 22:73.5:1.5:1:2, and deionized water was added and stirred to form a negative electrode slurry. The negative electrode slurry was coated on both sides of a copper foil current collector (thickness 4 μm), dried in a vacuum oven at 80℃ for 10 h, rolled, and slit to obtain negative electrode sheets.

[0082] (3) Preparation of electrolyte In an argon-filled glove box (H2O < 1 ppm, O2 < 1 ppm), propylene carbonate (PC), ethyl propionate (EP), propyl propionate (PP), and 20 wt% ethylene carbonate (EC) based on the total electrolyte mass were mixed thoroughly. Then, 15 wt% LiPF6 based on the total electrolyte mass was slowly added and dissolved. After dissolution, 10 wt% fluoroethylene carbonate (FEC), 5 wt% 1,3-propanesulfonate lactone, and 1 wt% lithium difluorooxalate borate were added to obtain the desired electrolyte. The mass ratio of propylene carbonate (PC), ethyl propionate (EP), and propyl propionate (PP) was 2:6:7. It should be noted that the content of ethylene carbonate (EC) in the electrolyte can be changed by adjusting the content of propylene carbonate (PC), ethyl propionate (EP), propyl propionate (PP), fluoroethylene carbonate (FEC), and LiPF6 in the electrolyte; the content of fluoroethylene carbonate (FEC) in the electrolyte can be changed by adjusting the content of propylene carbonate (PC), ethyl propionate (EP), propyl propionate (PP), fluoroethylene carbonate (FEC), and LiPF6 in the electrolyte.

[0083] (4) Preparation of the diaphragm The separator is made of 6μm thick polyethylene membrane (provided by Asahi Kasei Corporation). The side of the separator facing the positive electrode is coated with a 4μm thick ceramic layer containing alumina and polyvinylidene fluoride. The ceramic layer slurry is prepared by adding deionized water and stirring for 10 minutes, heating to 50°C to form a mixture, and filtering it through a 400-mesh stainless steel sieve to obtain the ceramic layer slurry. The porosity of the separator is 30%.

[0084] (5) Preparation of lithium-ion batteries The prepared positive electrode, negative electrode and separator are wound to form a bare cell. The bare cell is then placed in an aluminum-plastic film shell. The prepared electrolyte is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping and capacity testing, the desired lithium-ion battery is obtained.

[0085] The preparation methods of Examples 2-24 and Comparative Examples 1-3 are basically the same as those of Example 1, with the differences shown in Table 1. In Table 1, " / " indicates that the item is not present. In Table 1, D mL / 100g is the oil absorption value of conductive carbon black, and S m 2 / g represents the specific surface area of ​​the conductive carbon black, H μm represents the thickness of the positive electrode sheet, and m represents the thickness of the positive electrode sheet. sp % represents the mass content of conductive carbon black, m CNT % represents the mass content of carbon nanotubes, ρ kΩ·cm represents the surface resistivity of the positive electrode, and m represents the mass content of carbon nanotubes. EC The percentage represents the mass content of ethylene carbonate.

[0086] In Example 15, the mass ratio of ternary active material, conductive carbon black, polyvinylidene fluoride (PVDF), and carbon nanotubes was 98.45:0.3:0.5:0.75; in Example 16, the mass ratio was 95.72:3.6:0.5:0.18; in Example 17, the mass ratio was 99.05:0.4:0.5:0.05; in Example 18, the mass ratio was 96.1:3:0.5:0.4; and in Example 19, the mass ratio was 95:4:0.5:0. 0.5; In Example 20, the mass ratio of ternary active material, conductive carbon black, polyvinylidene fluoride, and carbon nanotubes was 99.25:0.2:0.5:0.05; In Example 21, the mass ratio of ternary active material, conductive carbon black, polyvinylidene fluoride, and carbon nanotubes was 94:3.5:0.5:2; In Example 22, the mass ratio of ternary active material, conductive carbon black, polyvinylidene fluoride, and carbon nanotubes was 94:4.5:0.5:1; In Example 23, the mass ratio of ternary active material, conductive carbon black, polyvinylidene fluoride, and carbon nanotubes was 97.9:0.4:0.5:1.2; In Example 24, the mass ratio of ternary active material, conductive carbon black, polyvinylidene fluoride, and carbon nanotubes was 96.9:2.5:0.5:0.1.

[0087] Example 25 This embodiment provides a method for preparing a battery, which differs from Embodiment 1 in that the method used in this embodiment is a single-crystal lithium nickel cobalt manganese oxide (LiNi). 0.6 Co 0.2 Mn 0.2 O2 and polycrystalline lithium nickel cobalt manganese oxide (LiNi) 0.6 Co 0.2 Mn 0.2 The mass ratio of O2 is 1:1, replacing the single-crystal lithium nickel cobalt manganese oxide (LiNi) of Example 1. 0.6 Co 0.2 Mn 0.2 O2 and polycrystalline lithium nickel cobalt manganese oxide (LiNi) 0.6 Co 0.2 Mn 0.2 The mass ratio of O2 is 4:1. In this embodiment, ρ is 14.47 kΩ·cm, and the time for the positive electrode active layer to be wetted by the electrolyte is 9 min.

[0088] Example 26 This embodiment provides a method for preparing a battery. The difference from Embodiment 1 is that, in this embodiment, polycrystalline lithium nickel cobalt manganese oxide is not added; instead, single-crystal lithium nickel cobalt manganese oxide is used by mass. In this embodiment, ρ is 4.82 kΩ·cm, and the time for the positive electrode active layer to be immersed in the electrolyte is 9 min.

[0089] Example 27 This embodiment provides a method for preparing a battery. The difference from Embodiment 1 is that the ternary active material and the silicon-carbon material are different. Specifically, at least part of the surface of the ternary active material in this embodiment has a coating layer, the coating layer contains Al element, and the thickness of the coating layer is 200 nm. Silicon-carbon material (median particle size 15 μm, sphericity 0.99) was used instead of the silicon-carbon material (median particle size 7 μm, sphericity 0.85) in Example 1. In this example, ρ was 2.45 kΩ·cm, and the positive electrode active layer was wetted by the electrolyte for 1 min.

[0090] Example 28 This embodiment provides a method for preparing a battery. The difference from Embodiment 1 is that the ternary active material and the silicon-carbon material are different. Specifically, the ternary active material in this embodiment is doped with W element, and the doping amount of W element is 0.2% based on the mass of the ternary active material. Silicon-carbon material (median particle size 4 μm, sphericity 0.8) was used instead of the silicon-carbon material (median particle size 7 μm, sphericity 0.85) in Example 1. In this example, ρ was 2.45 kΩ·cm, and the positive electrode active layer was wetted by the electrolyte for 1 min.

[0091] Table 1. Parameters of Examples and Comparative Examples 1

[0092] Table 2 Parameters of Examples and Comparative Examples 2

[0093] Test case 1. Cyclic Performance Test: Step 1: At 45℃, discharge at 0.5C to the lower limit voltage, let stand for 30 minutes, charge at 1.8C to the upper limit voltage, maintain constant voltage to 0.05C, let stand for 30 minutes, discharge at 2C to the lower limit voltage, let stand for 30 minutes, charge at 1.8C to the upper limit voltage, maintain constant voltage to 0.05C; Step 2: Let stand for 30 minutes, discharge the battery at 3C to the lower limit voltage, if the temperature exceeds 82℃, switch to 2C discharge; Step 3: Let stand for 60 minutes, then discharge the battery at 1... Step 1: Charge the battery at 8C to the upper limit voltage, maintain constant voltage to 0.05C, and switch to 1C charging if the temperature exceeds 57℃. Step 2: Let the battery rest for 60 minutes, then discharge the battery at 3C to the lower limit voltage, and switch to 2C discharge if the temperature exceeds 82℃. Step 3: Let the battery rest for 60 minutes, then charge the battery at 1.8C to the upper limit voltage, maintain constant voltage to 0.05C, and switch to 1C charging if the temperature exceeds 57℃. Record the initial capacity. Repeat steps 2 to 5 for 400 cycles. The capacity retention rate after 400 cycles is calculated as: (Capacity after 400 cycles / Initial capacity) × 100%.

[0094] 2. 4C Discharge Temperature Rise Test: At 45℃, firstly, after allowing the battery to stand for 2 hours, discharge it at a rate of 0.5C to the lower limit voltage; secondly, after allowing the battery to stand for 60 minutes, charge it at 1.8C to the upper limit voltage, and maintain the voltage at 0.05C; then, after allowing it to stand for 30 minutes, discharge the battery at a rate of 4C to the lower limit voltage. Repeat the entire process, monitoring the cell surface temperature. The 4C discharge temperature rise = cell surface temperature - 45℃.

[0095] 3. Positive electrode breakage: Disassemble the battery that has completed the cycle performance test in step 1. If the positive electrode is not broken, it is considered to have passed. Randomly select 10 batteries and observe whether the positive electrode is broken. 8 / 10 PASS means that 8 out of 10 batteries did not have positive electrode breakage after cycling.

[0096] The specific test results are shown in Table 3.

[0097] Table 3 Test results of the examples and comparative examples

[0098] As can be seen from Tables 1-3, compared with Comparative Examples 1-3, the thermal safety and cycle performance of the positive electrode sheet provided in this application have been improved to varying degrees. However, in Comparative Example 1 (with an excessively large average grain diameter), the current collector has a weaker ability to block external forces and prevent plastic deformation. In Comparative Example 2 (with an excessively large maximum grain diameter), stress concentration occurs locally in the positive electrode current collector, and the positive electrode sheet is prone to microcracks or even cracking, all of which affect the cycle performance and thermal safety of the battery.

[0099] Compared with Comparative Example 3, this application incorporates chain-like conductive materials with a branch number of 1 or more, which "bridge" the tiny gaps between the current collector and the materials in the positive electrode active layer. The branches also form multiple contact sites with the active materials, conductive agents, and binders in the positive electrode active layer, forming a three-dimensional network structure together. This reduces contact resistance, reduces heat generation in the battery during cycling, and helps the current flow evenly and the electrolyte conduct rapidly, thereby improving the battery's cycle performance and thermal safety.

[0100] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A positive electrode sheet, comprising a positive current collector and a positive active layer disposed on at least one surface of the positive current collector; characterized in that, The positive current collector includes an aluminum foil, the aluminum foil includes aluminum grains, the average diameter of the aluminum grains is 0.4μm-6μm, and the maximum diameter of the aluminum grains is less than or equal to 15μm; The positive electrode active layer includes a chain-like conductive material, and at least a portion of the chain-like conductive material has one or more branches.

2. The positive electrode sheet according to claim 1, characterized in that, The chain-like conductive material is chain-like conductive carbon black, and the positive electrode active layer also includes spherical conductive carbon black. The oil absorption value of the chain-like conductive carbon black and / or the spherical conductive carbon black is denoted as D mL / 100g, and the specific surface area of ​​the chain-like conductive carbon black and / or the spherical conductive carbon black is denoted as S m. 2 / g, the thickness of the positive electrode is denoted as H μm, which satisfies: 1.4≤(0.1S+D) / H≤6.

6.

3. The positive electrode sheet according to claim 2, characterized in that, Satisfies: 150≤D≤400; preferably, 200≤D≤300; And / or, 100≤S≤1400; preferably, 200≤S≤800; And / or, 80≤H≤160; preferably, 90≤H≤140.

4. The positive electrode sheet according to claim 2, characterized in that, The positive electrode active layer also includes carbon nanotubes, and the mass content of the carbon nanotubes, based on the mass of the positive electrode active layer, is denoted as m. CNT The mass content of the conductive carbon black is denoted as m. sp %, satisfying: 0.4≤m sp / m CNT ≤20; And / or, 0.3≤m sp ≤4; preferably 0.4≤m sp ≤3; And / or, 0.05≤m CNT ≤1.

2.

5. The positive electrode sheet according to any one of claims 1-4, characterized in that, The surface resistivity of the positive electrode is denoted as ρ kΩ·cm, which satisfies 1.2≤ρ≤15; preferably, 1.5≤ρ≤12. And / or, the positive electrode active layer is wetted by the electrolyte for less than or equal to 10 min, the electrolyte comprising an organic solvent, additives, and LiPF6, the organic solvent comprising ethylene carbonate, propylene carbonate, ethyl propionate, and propyl propionate, the mass ratio of propylene carbonate, ethyl propionate, and propyl propionate being 2:6:7, the mass content of ethylene carbonate being 20 wt% based on the total mass of the electrolyte, the additives being 10 wt% fluoroethylene carbonate, 5 wt% 1,3-propanesulfonate lactone, and 1 wt% lithium difluorooxalate borate, and the mass content of LiPF6 being 15 wt%. And / or, the positive electrode active layer further includes a positive electrode active material, the positive electrode active material including a ternary material; Preferably, the chemical formula of the ternary material is LiNi. x Co y Mn z O2, where x+y+z=1, 0 <x<1,0<y<1,0<z<1; Preferably, the ternary material comprises single-crystal particles, and the mass content of the single-crystal particles is denoted as m, based on the mass of the positive electrode active material. 单晶 %, satisfying 50≤m 单晶 ≤100; Preferably, at least a portion of the surface of the ternary material is coated or doped with at least one of the elements selected from Al, Zr, Mg, Y, Ti, B, Sr, W, Si, La, and Nb.

6. The positive electrode sheet according to claim 5, characterized in that, Satisfy: 50≤m 单晶 ≤80; And / or, the ternary material further includes polycrystalline particles, and the mass content of the polycrystalline particles is denoted as m, based on the mass of the positive electrode active material. 多晶 %, satisfying 0 < m 多晶 ≤50; preferably, 20≤m 多晶 ≤50.

7. A battery, characterized in that, include: The negative electrode, the electrolyte, and the positive electrode according to any one of claims 1-6.

8. The battery according to claim 7, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector, the negative electrode active layer including a negative electrode active material; the negative electrode active material includes silicon-based materials and carbon-based materials; Preferably, the silicon-based material includes at least one of silicon-carbon materials and silicon-oxygen materials; Preferably, the median particle size of the silicon-based material is 4 μm-15 μm; Preferably, the sphericity F of the silicon-based material satisfies: 0.8 ≤ F < 1.

9. The battery according to claim 7 or 8, characterized in that, The electrolyte comprises a solvent and a lithium salt. The solvent includes ethylene carbonate, and the mass content of ethylene carbonate, based on the mass of the electrolyte, is denoted as m. EC %, satisfying: 0.01≤m sp / m EC ≤0.6; Preferably, 0.3 ≤ m sp ≤4; preferably 0.4≤m sp ≤3; Preferably, 5≤m EC ≤50.

10. The battery according to claim 9, characterized in that, The electrolyte also includes fluoroethylene carbonate, and the mass content of the fluoroethylene carbonate is 5%-25% based on the mass of the electrolyte. Preferably, based on the mass of the electrolyte, the mass content of the fluoroethylene carbonate is 8%-20%.

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