Positive electrode active material, method for preparing the same, secondary battery, and electric device

Abstract

The application provides a positive electrode active material and a preparation method thereof, a secondary battery and an electric device, and relates to the technical field of lithium batteries. The secondary battery comprises a positive electrode sheet, and the positive electrode sheet comprises a positive electrode active material. The positive electrode active material comprises a lithium-containing transition metal oxide, and the lithium-containing transition metal oxide comprises a nickel element, a cobalt element and a first element. The first element is a manganese element and / or an aluminum element, and the content of the nickel element is higher than the content of the cobalt element. The lithium-containing transition metal oxide comprises single particles and / or quasi-single particles. In a primary particle in the single particles and / or quasi-single particles, a single primary particle satisfies that the distribution of the element concentration of the cobalt element is more uniform than the distribution of the element concentration of the nickel element. The secondary battery provided by the application can effectively improve the cycle retention rate while maintaining a high initial discharge specific capacity by introducing the improved lithium-containing transition metal oxide.

Description

[0001] Cross-referencing This application claims priority to WIPO (International Bureau) Patent Application No. PCT / CN2025 / 126148, filed on September 30, 2025, entitled "Positive Electrode Active Material and Method for Preparation Thereof, Secondary Battery and Electrical Device", the entire contents of which are incorporated herein by reference. This application also claims priority to WIPO (International Bureau) Patent Application No. PCT / CN2025 / 129114, filed on October 21, 2025, entitled "Positive Electrode Active Material and Method for Preparation Thereof, Secondary Battery and Electrical Device", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium battery technology, and in particular to a positive electrode active material and its preparation method, a secondary battery, and an electrical device. Background Technology

[0003] Lithium-containing transition metal oxides are currently the mainstream cathode active materials. However, current lithium-containing transition metal oxide battery systems cannot simultaneously achieve high energy density and long cycle performance. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode active material and its preparation method, a secondary battery and an electrical device. The secondary battery provided by this application introduces an improved lithium-containing transition metal oxide, which enables the secondary battery to maintain a high initial discharge specific capacity while effectively improving the cycle retention rate. That is, the lithium-containing transition metal oxide battery system takes into account both high energy density and long cycle performance.

[0005] The first aspect of this application provides a secondary battery, which includes a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector, the positive electrode film layer including a positive electrode active material, the positive electrode active material including a lithium-containing transition metal oxide, the lithium-containing transition metal oxide including nickel, cobalt and a first element, the first element being manganese and / or aluminum, and the content of nickel being higher than the content of cobalt.

[0006] Lithium-containing transition metal oxides include single particles and / or quasi-single particles. Among the primary particles of single particles and / or quasi-single particles, each primary particle satisfies the following condition: the elemental concentration distribution of cobalt is more uniform than that of nickel.

[0007] The secondary battery provided in this application, by introducing an improved lithium-containing transition metal oxide and controlling the nickel content to be higher than the cobalt content, achieves high energy density. However, nickel kinetics are lower than cobalt, so when the nickel content is higher than the cobalt content, a nickel-rich region is easily formed. This nickel-rich region is highly active but unstable, leading to uneven local reactions. Therefore, in the primary particles of single particles and / or near-single particles, the primary particles must satisfy the following condition: the distribution of cobalt element concentration is more uniform than the distribution of nickel element concentration. That is, the distribution of cobalt element concentration in at least a majority of the primary particles must be more uniform than the distribution of nickel element concentration, so that most or all of the nickel-rich region contains cobalt. This can suppress oxygen loss caused by transition delithiation in the nickel-rich region (cobalt also undergoes redox reactions, the reaction order being nickel, cobalt, oxygen), thus improving cycle performance.

[0008] In other words, the secondary battery provided in this application achieves both high energy density and long cycle performance by introducing an improved lithium-containing transition metal oxide.

[0009] In any embodiment, within a single first particle, the standard deviation of the cobalt element concentration distribution is A1, and the standard deviation of the nickel element concentration distribution is B1; A1 < B1. A lower standard deviation indicates a more uniform distribution of the element within the primary particle. Therefore, by ensuring that the standard deviation of the cobalt element concentration distribution in a single particle and / or a near-single particle is A1, and the standard deviation of the nickel element concentration distribution is B1, with A1 < B1, it can be concluded that the cobalt element concentration distribution in at least some primary particles is more uniform than the nickel element concentration distribution. This ensures that most or all of the nickel-enriched regions contain cobalt, which can suppress oxygen loss caused by excessive delithiation in the nickel-enriched regions and improve cycle performance.

[0010] In any implementation, A1 ranges from 0.3% to 1.0%; and / or, The range of B1 is 0.5%-4%. Controlling A1 within this range results in a more uniform distribution of cobalt element concentration, which is beneficial for stabilizing the structural stability of lithium-containing transition metal oxides during cycling and improving cycle performance. And / or, controlling B1 within this range keeps the nickel concentration in the nickel-rich region at a lower level, which helps reduce nickel-rich cation mixing and stress, suppresses cycle cracking, and effectively improves the cycle performance and initial discharge capacity of the secondary battery.

[0011] In any embodiment, among the multiple primary particles, the average standard deviation of the cobalt element concentration is A2, and the average standard deviation of the nickel element concentration is B2, where A2 < B2. A lower average standard deviation indicates a more uniform overall distribution of the element within the primary particle. Therefore, controlling the average standard deviation of the cobalt element concentration A2 to be less than the average standard deviation of the nickel element concentration B2 indicates that the overall distribution of cobalt element concentration among the multiple primary particles is more uniform than that of nickel. This is beneficial for the presence of cobalt in nickel-rich regions, which can suppress oxygen loss caused by excessive delithiation in these regions and improve cycle performance.

[0012] In any implementation, A2 ranges from 0.3% to 1.0%; and / or, The range of B2 is 0.5%-4%. Controlling the average standard deviation A2 of the cobalt element concentration distribution in multiple primary particles within this range results in a more uniform distribution of cobalt element concentration across the primary particles. This is beneficial for stabilizing the structural stability of lithium-containing transition metal oxides during cycling, thus improving cycle performance. And / or, controlling the average standard deviation B2 of the nickel element concentration distribution in multiple primary particles within this range helps reduce nickel-enriched cation mixing and stress, suppresses cycle cracking, and effectively improves the cycle performance and initial discharge capacity of the secondary battery.

[0013] In any embodiment, among the plurality of primary particles, at least 90% of the primary particles have a cobalt element concentration distribution standard deviation of A1 and a nickel element concentration distribution standard deviation of B1, where A1 < B1. The lower the standard deviation of the element concentration distribution, the more uniform the distribution of that element within the primary particle. Therefore, by ensuring that at least 90% of the primary particles satisfy the condition that the cobalt element concentration distribution standard deviation A1 is less than the nickel element concentration distribution standard deviation B1, it can be concluded that at least 90% of the primary particles satisfy the condition that the cobalt element concentration distribution is more uniform than the nickel element concentration distribution. This ensures that most or all of the nickel-enriched areas contain cobalt, which can suppress oxygen loss caused by excessive delithiation in the nickel-enriched areas and improve cycle performance.

[0014] In any embodiment, the first element is manganese, the content of nickel is higher than that of manganese, and the individual primary particles satisfy the following: the elemental concentration distribution of cobalt is more uniform than that of manganese. Because Ni 2+ and Mn 4+There is a strong electrostatic repulsion between them. When the nickel content is higher than the manganese content, manganese easily forms a manganese-enriched region. This manganese-enriched region is inert but stable, which can lead to uneven local reactions in the primary particles. Therefore, it is necessary to control the distribution of cobalt element concentration in at least some of the primary particles to be more uniform than that of manganese element concentration, so that the manganese-enriched region contains cobalt. This can compensate for the insufficient electron conduction in the manganese-enriched region (cobalt has good conductivity, while manganese has poor conductivity), thereby improving the cycle performance of the secondary battery.

[0015] In any embodiment, within a single first particle, the standard deviation of the cobalt element concentration distribution is A1, and the standard deviation of the manganese element concentration distribution is C1; A1 < C1. A lower standard deviation indicates a more uniform distribution of the element within the primary particle. Therefore, by ensuring that the standard deviation of the cobalt element concentration distribution in a single particle and / or a near-single particle is A1, and the standard deviation of the manganese element concentration distribution is C1, with A1 < C1, it can be concluded that the cobalt element concentration distribution in at least some primary particles is more uniform than the manganese element concentration distribution. This ensures that most or all of the nickel-enriched regions contain cobalt, which can suppress oxygen loss caused by excessive delithiation in the manganese-enriched regions and improve cycle performance.

[0016] In any embodiment, C1 ranges from 0.5% to 4%. Controlling C1 within this range helps to keep the nickel concentration in the manganese-rich region at a lower level, which is beneficial for reducing manganese enrichment and inertness, thereby making the reaction of the primary particles more uniform and effectively improving the cycle performance and initial discharge capacity of the secondary battery.

[0017] In any embodiment, the average standard deviation of the cobalt element concentration in the multiple primary particles is A2, and the average standard deviation of the manganese element concentration is C2, where A2 < C2. A lower average standard deviation indicates a more uniform distribution of the element within the primary particle. Therefore, this application uses a standard deviation of A2 for cobalt element concentration that is less than the standard deviation of C2 for manganese element concentration. This indicates that the overall distribution of cobalt element concentration in the multiple primary particles is more uniform than that of manganese element concentration. As described above, this allows the manganese-rich region to contain cobalt, compensating for insufficient electron conduction in the manganese-rich region (cobalt has good conductivity, while manganese has poor conductivity), thereby improving the cycle performance of the secondary battery.

[0018] In any embodiment, C2 ranges from 0.5% to 4%. Controlling the distribution standard deviation of manganese element concentration C2 within the above range and keeping the manganese concentration in the manganese enrichment region within a lower range is beneficial to the reversibility and cycle stability of the structure, while ensuring that the region still contains a considerable proportion of active nickel (and cobalt), thereby resulting in a higher overall reversible capacity of the material.

[0019] In any embodiment, at least a portion of the primary particles, based on the total number of single particles and / or near-single particles, contain pores with an average pore size of 50 nm to 500 nm. Controlling the presence of pores in at least a portion of the primary particles, with the average pore size within the 50 nm to 500 nm range, minimizes capacity loss while maintaining a low discharge discharge rate (DCR), allowing the secondary battery to achieve a high initial discharge capacity.

[0020] In any embodiment, the average perimeter of the pores is 400nm-2000nm; optionally, it is 600nm-1000nm. Controlling the average perimeter of the pores within the above range can effectively reduce the impact on the specific capacity of the material while facilitating lithium-ion diffusion, which is beneficial for improving the energy density of the secondary battery and reducing the discharge density-reduction ratio (DCR).

[0021] In any embodiment, under a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median R1 of the pore wall roughness is calculated based on the cumulative proportion of the pore area. A50 The value is 0.949-0.975; it can be selected as 0.957-0.970. The concentration value of the pore wall roughness is close to 1, which indicates that the pore walls of the primary particles are highly uniform and smooth, which is conducive to lithium-ion diffusion, improves kinetics, and reduces discharge DCR.

[0022] In any embodiment, under a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median L1 of the pore wall roundness is based on the cumulative proportion of the pore area. A50 The median L of the roundness of the hole wall is 0.728-0.836; it can be selected as 0.757-0.798. A50 Within the aforementioned range, the hole walls (cross-sections) are approximately circular, resulting in uniform stress distribution, reduced stress concentration, lower risk of cracking during cold pressing, improved electrode compaction density, and increased energy density of the secondary battery.

[0023] In any embodiment, the proportion of primary particles with internal pores is 1%-20% based on the total number of primary particles. Since the presence of pores affects the specific capacity of the material, controlling the proportion of primary particles with internal pores within the above range ensures that the vast majority of primary particles are free of pores, thus preventing the specific capacity of the vast majority of primary particles from being affected. The presence of pores in a small number of primary particles can effectively reduce the impact on the specific capacity of the material while facilitating lithium-ion diffusion, which is beneficial for secondary batteries to achieve both high energy density and low discharge DCR.

[0024] Optionally, based on the total number of primary particles, the proportion of primary particles with pores having a diameter >300nm is 0%-2%. Controlling the pore diameter of most or all of the pores in the primary particles with pores to ≤300nm can effectively reduce the adverse effects of excessively large pores on the specific capacity and compaction density of the material, while facilitating lithium-ion diffusion. This can effectively improve the energy density of the secondary battery while maintaining a low discharge DCR.

[0025] In any embodiment, under an electron microscope image at 3000x magnification, the area ratio of the total area of ​​pores in the cross-section of the positive electrode film along the thickness direction of the positive electrode sheet is 0.02%-0.5% based on the total area of ​​the primary particles; optionally, it is 0.1%-0.3%. Controlling the total area of ​​pores to a small proportion of the total area of ​​the primary particles can effectively reduce the material specific capacity loss caused by pores while facilitating lithium-ion diffusion, which is beneficial for the secondary battery to achieve both high energy density and low discharge DCR.

[0026] In any embodiment, the average particle size of the primary particles is 1 μm-4 µm. Controlling the average particle size of the primary particles in single particles and / or near-single particles to be between 1 μm and 4 µm can result in higher stability of lithium-containing transition metal oxides and fewer side reactions, which is beneficial for improving cycle performance.

[0027] In any embodiment, under a cross-sectional electron microscope (SEM) image of the positive electrode film along the thickness direction of the positive electrode sheet, the primary particles include a first particle with a particle size D1 and a second particle with a particle size D2, where 1 μm ≤ D1 ≤ 3 μm and 3 μm < D2 ≤ 10 μm. The ratio of the total area of ​​the first particle to the total area of ​​the second particle is (0.2-0.5):1. Controlling the ratio of the total area of ​​the first particle to the total area of ​​the second particle under the cross-sectional SEM image of the positive electrode film along the thickness direction of the positive electrode sheet within the above range helps to construct a reasonable particle size distribution, improve the electrode sheet compaction density, improve the battery energy density, and also ensure a low battery internal resistance. Optionally, the ratio of the total area of ​​the first particle to the total area of ​​the second particle is (0.32-0.38):1.

[0028] In any embodiment, the ratio of the first particle to the second particle is (1.2-3):1. Controlling the first and second particles to be compounded as described above helps to construct a reasonable particle size distribution, improve electrode compaction density, and increase the energy density of the secondary battery. Simultaneously, controlling the proportion of the larger second particles helps to reduce the internal resistance of the secondary battery and improve kinetics. Optionally, the ratio of the first particle to the second particle is (1.5-2):1.

[0029] In any embodiment, under a cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet, the median L2 of the sphericity of the primary particles is calculated based on the cumulative area ratio of the primary particles. A50 The median L of sphericity is 0.6-0.85. A50 Within the aforementioned range, the primary particles are approximately spherical, which helps them maintain good slidability during stacking, making it easier to fill the gaps between them. This further improves the compaction density of the electrode and increases the energy density of the secondary battery.

[0030] In any embodiment, under a cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet, the median roughness R2 of the primary particles is calculated based on the cumulative area ratio of the primary particles. A50 The median roughness R is 0.92-0.97. 50 Within the aforementioned range, the surface of the primary particles is relatively smooth, and the friction between particles is relatively small. Under the action of external force, it is easy to slip, which can further improve the compaction density of the electrode and increase the energy density of the battery.

[0031] In any embodiment, the molar ratio of nickel, cobalt, and the first element is (5-9):(0.5-2.5):(0.5-2.5). By controlling the molar ratio of nickel, cobalt, and the first element within the above range, the lithium-containing transition metal oxide exhibits high energy density.

[0032] In any embodiment, the first element is manganese, and the molar ratio of nickel, cobalt, and manganese is (5-9):(0.5-2.5):(0.5-2.5). Choosing manganese as the first element and controlling the molar ratio within the above range is advantageous because manganese is low in cost, thus reducing the cost of lithium-containing transition metal oxides. Simultaneously, manganese ions (Mn)... 4+ It hardly participates in the reaction during lithium ion desorption (electrochemical inertness), which can effectively stabilize the three-dimensional layered crystal structure of the material and improve the thermal stability and cycle life of the above-mentioned high energy density lithium-containing transition metal oxides.

[0033] In any embodiment, the lithium-containing transition metal oxide further contains a dopant element, including at least one selected from Ti, Al, Zr, W, Nb, Mo, B, Ce, and La. The presence of these dopant elements in the lithium-containing transition metal oxide improves its structural stability and / or thermal stability, thereby enhancing the cycle performance of the secondary battery.

[0034] In any embodiment, the lithium-containing transition metal oxide contains W, and the W content is 500 ppm to 2500 ppm based on the mass of the lithium-containing transition metal oxide. By doping with W, W reacts with oxygen ions (O²⁻). -Strong WO bonds will form between them. WO bonds can suppress lattice distortion and oxygen loss in nickel-rich areas, which is beneficial to improving the cycle performance of secondary batteries. At the same time, the content of W element is controlled at 500 ppm-2500 ppm based on the mass of lithium transition metal oxides, which has little or no impact on the first discharge capacity of secondary batteries.

[0035] In any embodiment, the lithium-containing transition metal oxide surface is coated with a coating layer; wherein the coating layer is rich in cobalt; and / or, the coating layer is rich in a second element, the second element including at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce and La.

[0036] However, due to the low sphericity of the primary particles, the side reactions at the edges are large and the cycle performance is poor. Therefore, the physical barrier of the coating layer is used to isolate the lithium-containing transition metal oxide from the electrolyte, effectively suppressing the interfacial side reactions. When the coating layer is rich in cobalt, it is beneficial to improve surface kinetics. When the coating layer is rich in the second element, it can stabilize the cobalt, reduce cobalt dissolution, and reduce the side reactions at the edges.

[0037] In any embodiment, the coating layer contains cobalt, with a cobalt content of 5000 ppm to 15000 ppm. Controlling the cobalt content in the coating layer within this range results in high surface kinetics of the positive electrode active material.

[0038] In any embodiment, the coating layer contains a second element, the content of which is 1000 ppm to 5000 ppm. Controlling the content of the second element within this range can effectively suppress cobalt leaching and optimize cycle performance.

[0039] In any embodiment, the sodium content in the lithium-containing transition metal oxide is less than 500 ppm. Controlling the sodium content in the lithium-containing transition metal oxide to be low does not substantially affect the structural stability of the lithium-containing transition metal oxide.

[0040] In any embodiment, the sodium content in the lithium-containing transition metal oxide is 0 ppm ≤ 20 ppm. This allows the lithium-containing transition metal oxide to be essentially free of sodium, resulting in higher material purity.

[0041] In any embodiment, the sodium content in the lithium-containing transition metal oxide is 20 ppm < 100 ppm. Including a small amount of sodium in the lithium-containing transition metal oxide can widen the interlayer spacing and make it more stable, which is beneficial for improving the cycle performance of the battery.

[0042] In any embodiment, the sulfur content in the lithium-containing transition metal oxide is less than 500 ppm. Controlling the sulfur content in the lithium-containing transition metal oxide to be less than 500 ppm means that the content of impurity sulfur in the lithium-containing transition metal oxide is low, reducing the negative impact on the lithium-containing transition metal oxide (negative impacts include increased impedance, reduced battery discharge capacity and rate of return, etc.).

[0043] In any embodiment, the sulfur content in the lithium-containing transition metal oxide is 0 ppm ≤ 300 ppm. This allows the lithium-containing transition metal oxide to be essentially sulfur-free, resulting in higher material purity.

[0044] In any embodiment, the areal density of the single-sided positive electrode film is 0.255 g / 1540.25 mm. 2 -0.28 g / 1540.25mm 2 A cathode film with an areal density within the above range can help improve the energy density of a secondary battery.

[0045] In any embodiment, the compaction density of the single-sided positive electrode film is 3.30 g / cm³. 3 -3.55 g / cm 3 A compaction density of the positive electrode film within the above-mentioned range is beneficial for improving the energy density of lithium-ion secondary batteries.

[0046] In any embodiment, the positive electrode film layer also contains a conductive agent, and the conductive agent accounts for 1%-3% of the mass of the positive electrode film layer.

[0047] In any embodiment, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0048] In any embodiment, the positive electrode film layer also contains a binder, and the binder accounts for 0.5%-3.0% of the mass of the positive electrode film layer.

[0049] In any embodiment, the binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0050] In any embodiment, the specific discharge capacity of the lithium-containing transition metal oxide is 170 mAh / g - 205 mAh / g. The high specific discharge capacity of the lithium-containing transition metal oxide is beneficial for improving the energy density of the secondary battery.

[0051] A second aspect of this application also provides a positive electrode active material, which includes a lithium-containing transition metal oxide, wherein the lithium-containing transition metal oxide includes nickel, cobalt and a first element, wherein the first element is manganese and / or aluminum, and the content of nickel is higher than the content of cobalt.

[0052] Lithium-containing transition metal oxides include single particles and / or quasi-single particles. Among the primary particles of single particles and / or quasi-single particles, each primary particle satisfies the following condition: the elemental concentration distribution of cobalt is more uniform than that of nickel.

[0053] The lithium-containing transition metal oxide provided in this application achieves high energy density in secondary batteries by controlling the nickel content to be higher than the cobalt content. However, nickel kinetics are lower than cobalt, so when the nickel content is higher than the cobalt content, nickel-rich regions are easily formed. These nickel-rich regions are highly active but unstable, leading to uneven local reactions. Therefore, in the primary particles of single particles and / or near-single particles, each primary particle satisfies the following condition: the cobalt element concentration distribution is more uniform than the nickel element concentration distribution. That is, controlling the cobalt element concentration distribution in most or all of the primary particles to be more uniform than the nickel element concentration distribution ensures that most or all of the nickel-rich regions contain cobalt. This can suppress oxygen loss caused by transition delithiation in the nickel-rich regions and improve cycle performance.

[0054] In any embodiment, within a single first particle, the standard deviation of the cobalt element concentration distribution is A1, and the standard deviation of the nickel element concentration distribution is B1; A1 < B1. A lower standard deviation indicates a more uniform distribution of the element within the primary particle. Therefore, by ensuring that the standard deviation of the cobalt element concentration distribution in a single particle and / or a near-single particle is A1, and the standard deviation of the nickel element concentration distribution is B1, with A1 < B1, it can be concluded that the cobalt element concentration distribution in at least some primary particles is more uniform than the nickel element concentration distribution. This ensures that most or all of the nickel-enriched regions contain cobalt, which can suppress oxygen loss caused by excessive delithiation in the nickel-enriched regions and improve cycle performance.

[0055] In any implementation, A1 ranges from 0.3% to 1.0%; and / or, The range of B1 is 0.5%-4%. Controlling A1 within this range results in a more uniform distribution of cobalt element concentration, which is beneficial for stabilizing the structural stability of lithium-containing transition metal oxides during cycling and improving cycle performance. And / or, controlling B1 within this range keeps the nickel concentration in the nickel-rich region at a lower level, which helps reduce nickel-rich cation mixing and stress, suppresses cycle cracking, and effectively improves the cycle performance and initial discharge capacity of the secondary battery.

[0056] In any embodiment, the first element is manganese, the content of nickel is higher than that of manganese, and the individual primary particles satisfy the following: the elemental concentration distribution of cobalt is more uniform than that of manganese. Because Ni 2+ and Mn 4+ There is a strong electrostatic repulsion between them. When the nickel content is higher than the manganese content, manganese easily forms a manganese-enriched region. This manganese-enriched region is inert but stable, which can lead to uneven local reactions in the primary particles. Therefore, it is necessary to control the distribution of cobalt element concentration in at least some of the primary particles to be more uniform than that of manganese element concentration, so that the manganese-enriched region contains cobalt. This can compensate for the insufficient electron conduction in the manganese-enriched region (cobalt has good conductivity, while manganese has poor conductivity), thereby improving the cycle performance of the secondary battery.

[0057] A third aspect of this application provides a method for preparing a positive electrode active material, comprising: The cobalt source and solid dispersant are dry-mixed to obtain a premix.

[0058] The premix, nickel source, first element source, lithium salt, flux, and organic carbon source are dry-milled and mixed to obtain a first mixed powder.

[0059] The first mixed powder and the remaining premix are dry-milled and mixed to obtain the second mixed powder.

[0060] The second mixed powder was sintered at 750℃-1000℃ in an oxygen-containing atmosphere to obtain a lithium-containing transition metal oxide.

[0061] Among them, the nickel source, cobalt source and the first element source are all hydroxides and / or oxides; the first element is manganese and / or aluminum, the content of nickel is higher than that of cobalt, the volume ratio of oxygen in the oxygen-containing atmosphere is ≥90%, and the solid dispersant includes at least one of citric acid, trisodium phosphate, sodium silicate and magnesium carbonate.

[0062] The preparation method provided in this application uses nickel, cobalt, and the first element source, all of which are hydroxides and / or oxides, to dry prepare and obtain lithium-containing transition metal oxides, including single particles and / or near-single particles. These single-particle and / or near-single-particle structures exhibit high stability. Because the nickel content is higher than the cobalt content, the resulting lithium-containing transition metal oxide-based battery system has high energy density. However, nickel kinetics are lower than cobalt kinetics, so when the nickel content is higher than the cobalt content, nickel-rich regions are easily formed. These nickel-rich regions are highly active but unstable, leading to uneven local reactions. Therefore, by pre-mixing the cobalt source and the solid dispersant using a dry method, a uniform distribution is obtained. The premix is ​​then partially dry-milled and mixed with other raw materials. The resulting first mixed powder and the remaining premix are then further dry-milled and mixed. This process facilitates the full and more uniform dispersion of the cobalt source in the final second mixed powder. Simultaneously, the distribution mixing and solid dispersant also help the nickel source and the first element source to be more uniformly dispersed in the final second mixed powder. As a result, after subsequent sintering, it is beneficial to obtain at least some of the primary particles in single particles and / or near-single particles where the distribution of cobalt element concentration is more uniform than that of nickel element concentration. This allows the nickel-enriched region to contain cobalt element, which can suppress oxygen loss caused by excessive delithiation in the nickel-enriched region and improve cycle performance.

[0063] In any embodiment, the mass ratio between a portion of the premix and the remainder of the premix is ​​0.5:1 to 1.5:1; and / or, The amount of solid dispersant added is 0.1%-2.3% by mass of the total mass of the mixed powder; and / or, The volumetric particle size distribution (Dv50) of the cobalt source is 3 μm - 5 μm.

[0064] Maintaining a mass ratio of 0.5:1 to 1.5:1 between the premixed portion and the remaining premixed portion is beneficial for thorough and uniform mixing, and facilitates the preparation of primary particles in which the cobalt element concentration distribution is more uniform than the nickel element concentration distribution in at least a portion of the single particles and / or near-single particles. Controlling the added mass of the solid dispersant within the above range results in excellent dispersion and simultaneously suppresses the deterioration of the performance of lithium-containing transition metal oxides due to the introduction of impurities.

[0065] The fourth aspect of this application provides an electrical device including the secondary battery of the first aspect of this application. Attached Figure Description

[0066] Figure 1 This is a cross-sectional EDS sampling marker diagram of the primary particles in this application.

[0067] Figure 2 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0068] Figure 3 yes Figure 2 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0069] Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application.

[0070] Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0071] Figure 6 yes Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown.

[0072] Figure 7 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0073] Explanation of reference numerals in the attached figures: 1-Battery pack; 2-Upper housing; 3-Lower housing; 4-Battery module; 5-Battery cell; 51-Housing; 52-Electrode assembly; 53-Top cover assembly. Detailed Implementation

[0074] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its preparation method, secondary battery, and power application device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0075] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0076] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0077] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0078] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0079] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0080] Lithium-containing transition metal oxides are currently the mainstream cathode active materials. However, current lithium-containing transition metal oxide battery systems cannot simultaneously achieve high energy density and long cycle performance.

[0081] Based on this, the first aspect of the present application provides a secondary battery, which includes a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector, the positive electrode film layer including a positive electrode active material, the positive electrode active material including a lithium-containing transition metal oxide, the lithium-containing transition metal oxide including nickel, cobalt and a first element, the first element being manganese and / or aluminum, and the content of nickel being higher than the content of cobalt.

[0082] Lithium-containing transition metal oxides include single particles and / or quasi-single particles. Among the primary particles of single particles and / or quasi-single particles, each primary particle satisfies the following condition: the elemental concentration distribution of cobalt is more uniform than that of nickel.

[0083] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0084] The main elemental types and their proportions in lithium-containing transition metal oxides, as shown below, can be determined by inductively coupled plasma (ICP) spectroscopy. Specifically, a lithium-containing transition metal oxide sample is digested with aqua regia and HF hydrofluoric acid. 15 ml of the completely digested solution is then subjected to ICP testing to obtain the elemental types and proportions of the lithium-containing transition metal oxides. For secondary batteries, a discharged secondary battery can be disassembled, and the positive electrode sheet can be soaked in DMC (ethylene carbonate) for 4 hours to remove residual electrolyte and lithium salts. Afterward, it is dried in an oven, scraped off, and the scraped powder is heat-treated in a tube furnace at 600°C for 2 hours under argon protection to remove the binder adhering to the surface of the positive electrode active material (PVDF decomposition). The obtained positive electrode active material is then used as a sample for ICP testing. Alternatively, energy dispersive spectroscopy (EDS) can be combined with scanning and analysis of the positive electrode film cross-section to obtain the elemental types and proportions of the lithium-containing transition metal oxides. EDS operation can be performed according to relevant techniques.

[0085] In this application, "single particle" refers to a particle consisting of a single primary particle. "Single-particle-like" refers to a particle formed by a small number of primary particles, typically two or more, or 49 or fewer. The primary particles within a single particle or single-particle-like particle usually have irregular shapes. The size of the primary particles within a single particle or single-particle-like particle is typically less than 1 μm. A "primary particle" is defined as a single particle observed under an electron microscope, possessing a complete boundary. Defects may exist within the particle, but there are no complete boundaries within the particle sufficient to divide it into two or more particles.

[0086] It is understood that, unless otherwise specified in this application, a primary particle refers to a single particle and / or a primary particle in the form of a single particle or a particle-like particle.

[0087] The identification and elemental distribution of single particles and / or single-particle-like primary particles can be obtained by marking and testing under the cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet.

[0088] It is understood that in this application, the magnification of the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet is not required, as long as the field of view can clearly see the primary particles containing lithium transition metal oxide and the pores shown later. Furthermore, if multiple cross-sections are used, the magnification should be uniform. Non-limitingly, the cross-section of the positive electrode sheet can be obtained using instruments or equipment including but not limited to a focused electron beam (FIB) microscope (non-limiting examples such as the FEIScios2HiVac device), an ion section polisher (non-limiting examples such as the IB-09010CP argon ion section polisher and IB-19500CP ion section polisher from JEOL Corporation of Japan), or by using plasma quenching. The electron microscope image field of view can be obtained using instruments or equipment including but not limited to scanning electron microscopy (SEM) technology. In particular, a high-resolution field emission scanning electron microscope can be used. Examples of non-limiting SEM instruments include the Sigma 300 scanning electron microscope and the Apreo 2SEM field emission scanning electron microscope from ZEISS GmbH, Germany.

[0089] For example, in this embodiment of the application, the discharged secondary battery is taken out, disassembled, and the positive electrode is soaked in DMC (ethylene carbonate) for 4 hours to remove residual electrolyte and lithium salt. Then it is placed in an oven to dry, and the processed positive electrode is obtained. The positive electrode film is cut along the thickness direction of the electrode by argon ion beam (for example, the equipment model: Leica EM TIC 3X CP, working voltage: 6kV, working time: 6h). After exposing the cut surface, the cross-section of the positive electrode film along the thickness direction of the electrode is observed by scanning electron microscope (for example, the equipment model: Hitachi SU8230, working voltage: 3kV, beam current: high, probe model: U (LA100), working distance <5mm), and the cross-section of the positive electrode film along the thickness direction of the electrode is obtained.

[0090] The identification and labeling of single particles and / or single-particle-like primary particles can be achieved by using a field emission scanning electron microscope (FESEM) to acquire images in secondary electron mode at non-edge locations within the cross-section of the positive electrode film (after observing the electrode edge under the SESEM, the field of view is adjusted to the center of the sample). Electron micrographs are then taken at 3000x magnification, and ImageJ software (1.46r, Win64 version) is used to analyze the single particles and / or single-particle-like primary particles in the electron micrographs. Primary particles that are not labeled by the software, are not fully labeled by the software, or have labeling errors are manually labeled, thus completing the identification and labeling of single particles and / or single-particle-like primary particles in the image.

[0091] The nickel content is positively correlated with the specific capacity of lithium-containing transition metal oxides. By having a higher nickel content than cobalt content, i.e., a high-nickel, low-cobalt configuration, the secondary battery has a high energy density.

[0092] However, nickel has lower kinetics than cobalt. Therefore, when the nickel content is higher than the cobalt content, a nickel enrichment zone is easily formed. This nickel enrichment zone is highly active but unstable, leading to uneven local reactions. Therefore, by controlling the primary particles in the lithium transition metal oxide particles, specifically single particles and / or near-single particles, to ensure that the distribution of cobalt element concentration is more uniform than that of nickel element concentration, the nickel enrichment zone contains cobalt. This can suppress oxygen loss caused by transition delithiation in the nickel enrichment zone (cobalt also undergoes redox reactions, with the reaction sequence being nickel, cobalt, and oxygen), thus improving cycle performance.

[0093] The secondary battery provided in this application, by introducing an improved lithium-containing transition metal oxide and controlling the nickel content to be higher than the cobalt content, achieves high energy density. However, nickel kinetics are lower than cobalt, so when the nickel content is higher than the cobalt content, a nickel-rich region is easily formed. This nickel-rich region is highly active but unstable, leading to uneven local reactions. Therefore, in the primary particles of single particles and / or near-single particles, each primary particle satisfies the following: the distribution of cobalt element concentration is more uniform than the distribution of nickel element concentration. That is, controlling the distribution of cobalt element concentration in most or all primary particles to be more uniform than the distribution of nickel element concentration ensures that the nickel-rich region contains cobalt. This can suppress oxygen loss caused by transition delithiation in the nickel-rich region (cobalt also undergoes redox reactions, the reaction sequence being nickel, cobalt, oxygen), thus improving cycle performance.

[0094] In other words, the secondary battery provided in this application achieves both high energy density and long cycle performance by introducing an improved lithium-containing transition metal oxide.

[0095] In some embodiments, the standard deviation of the cobalt element concentration distribution in a single first particle is A1, and the standard deviation of the nickel element concentration distribution is B1; A1 < B1.

[0096] Methods for testing the standard deviation of elemental concentration distribution in primary particles include: such as... Figure 1 As shown, under a 3000x electron microscope image of the cross-section of the positive electrode film along the thickness direction of the positive electrode sheet, EDS was used to scan the cross-section of the identified primary particles. A 200nm depth region from the surface of the primary particle was used as the boundary to exclude interference from the surface coating layer. Then, four positions were taken at equal intervals along any line from the center of the primary particle cross-section (in this application, the center of the primary particle cross-section refers to the center of the smallest circumcircle of the projected outline of the primary particle cross-section) to this boundary (from the inside out, these positions were used as spectra 8, 9, 10, and 11). Figure 7 Spectrum Figure 6 and spectrum Figure 5 The molar percentage of the same element (nickel, cobalt, and the first element, calculated based on 100% of the total moles of nickel, cobalt, and the first element) at each position was obtained. Then, the sample standard deviation of the element concentration in that primary particle was calculated based on the element concentration at the four positions as the distribution standard deviation, thus obtaining the distribution standard deviation of the element concentration in a single primary particle. It should be noted that... Figure 1 A point scan diagram illustrating the standard deviation test method for the distribution of elemental concentrations in particles, performed only once.

[0097] The formula for calculating the sample standard deviation s is: , X is the sample mean, which is the average of n (n=4) samples (the sample represents the molar percentage of each element at each position). i Let i represent the i-th sample point.

[0098] In the sampling of a single primary particle, five cross-sections at 3000x magnification were randomly selected at different positions along the thickness direction of the positive electrode film. The average particle size of the primary particles in the five cross-sections was statistically analyzed. Then, a primary particle with an average particle size of ±20% was selected from the five cross-sections to test the standard deviation of the element concentration distribution of nickel, cobalt and the first element in a single primary particle.

[0099] The lower the standard deviation of the element concentration distribution, the more uniform the distribution of that element in the primary particle. Therefore, if the standard deviation of the cobalt element concentration distribution in a single particle and / or a near-single particle is A1 and the standard deviation of the nickel element concentration distribution is B1, and A1 < B1, it indicates that the cobalt element concentration distribution in at least some primary particles is more uniform than the nickel element concentration distribution. This ensures that most or all of the nickel-enriched areas contain cobalt, which can suppress oxygen loss caused by excessive delithiation in the nickel-enriched areas and improve cycle performance.

[0100] In some embodiments, A1 ranges from 0.3% to 1.0%. Controlling A1 within this range results in a more uniform distribution of cobalt element concentration, which is beneficial for stabilizing the structural stability of lithium-containing transition metal oxides during cycling and improving cycling performance.

[0101] For example, A1 is any value among 0.3%, 0.4%, 0.42%, 0.43%, 0.44%, 0.47%, 0.49%, 0.5%, 0.53%, 0.59%, 0.6%, 0.62%, 0.64%, 0.7%, 0.8%, 0.9%, and 1.0%, or between any two values.

[0102] In some embodiments, B1 ranges from 0.5% to 4%. Controlling B1 within this range helps to keep the nickel concentration in the nickel-rich region at a lower level, which is beneficial for reducing nickel-rich cation mixing and stress, suppressing cycle cracking, and effectively improving the cycle performance and initial discharge capacity of the secondary battery.

[0103] For example, B1 is any value among 0.5%, 0.62%, 0.63%, 0.67%, 0.71%, 0.77%, 0.78%, 0.84%, 0.86%, 1%, 1.5%, 2%, 2.5%, 3%, 3.1%, 3.5%, and 4%, or between any two values.

[0104] In some implementations, the average standard deviation of the cobalt element concentration in multiple primary particles is A2, and the average standard deviation of the nickel element concentration is B2, where A2 < B2. A lower average standard deviation indicates a more uniform overall distribution of the element within the primary particle. Therefore, controlling the average standard deviation of the cobalt element concentration A2 to be less than the average standard deviation of the nickel element concentration B2 indicates that the overall distribution of cobalt element concentration in the multiple primary particles is more uniform than that of nickel. This is beneficial for the presence of cobalt in nickel-rich regions, which can suppress oxygen loss caused by excessive delithiation in these regions and improve cycle performance.

[0105] In some implementations, A2 ranges from 0.3% to 1.0%. By controlling the average standard deviation A2 of the cobalt element concentration distribution in multiple primary particles to be within the above range, the overall distribution of cobalt element concentration in multiple primary particles is more uniform, which is beneficial to stabilizing the structural stability of lithium-containing transition metal oxides during cycling and improving cycling performance.

[0106] For example, A2 is any value among 0.3%, 0.4%, 0.45%, 0.46%, 0.48%, 0.5%, 0.52%, 0.57%, 0.6%, 0.62%, 0.63%, 0.67%, 0.7%, 0.8%, 0.9%, and 1.0%, or between any two values.

[0107] In some implementations, B2 ranges from 0.5% to 4%. Controlling the average standard deviation B2 of the nickel element concentration distribution in multiple primary particles within the above range is beneficial for reducing nickel-enriched cation mixing and stress, suppressing cycle cracking, and effectively improving the cycle performance and initial discharge capacity of the secondary battery.

[0108] For example, B2 is any value among 0.5%, 0.67%, 0.68%, 0.69%, 0.73%, 0.74%, 0.79%, 0.84%, 0.85%, 0.88%, 0.9%, 1%, 1.5%, 2%, 2.5%, 3%, 3.3%, 3.5%, and 4%, or between any two values.

[0109] The method for testing the average distribution standard deviation of elemental concentrations in multiple primary particles includes: randomly selecting five cross-sections at different positions along the thickness direction of the positive electrode film; randomly selecting five single particles with an average particle size of ±20% from different regions of each cross-section as samples; and testing the distribution standard deviation of elemental concentrations of nickel, cobalt, and the first element in each single primary particle. Then, calculating the average distribution standard deviation of the same element in the 25 primary particle samples selected from the five cross-sections. The sum of the distribution standard deviations of nickel elemental concentration in the 25 primary particle samples is N3, and the average distribution standard deviation of nickel elemental concentration in multiple primary particles is N3 / 25. Similarly, the sum of the distribution standard deviations of cobalt elemental concentration in the 25 primary particle samples is N4, and the average distribution standard deviation of cobalt elemental concentration in primary particles is N4 / 25. Similarly, the sum of the distribution standard deviations of the first elemental concentration in the 25 primary particle samples is N5, and the average distribution standard deviation of the first elemental concentration in primary particles is N5 / 25. In some implementations, at least 90% of the primary particles in a plurality of primary particles satisfy the following conditions: the standard deviation of the cobalt element concentration distribution is A1, the standard deviation of the nickel element concentration distribution is B1, and A1 < B1.

[0110] It should be noted that the multiple primary particles mentioned here refer to multiple primary particles that have obtained the average standard deviation of the above element concentration distribution.

[0111] In other words, most or all of the primary particles in a plurality of primary particles satisfy A1 < B1, ensuring that most or all of the nickel-enriched regions contain cobalt. This can suppress oxygen loss caused by excessive delithiation in the nickel-enriched regions and improve cycle performance. For example, in a plurality of primary particles, 90%, 91%, 93%, 95%, 98%, and 100% of the primary particles satisfy A1 < B1.

[0112] In some embodiments, the first element is manganese, the content of nickel is higher than that of manganese, and in the primary particles of single particles and / or quasi-single particles, the individual primary particles satisfy the following: the distribution of cobalt element concentration is more uniform than that of manganese element concentration.

[0113] Due to Ni 2+ and Mn 4+ There is a strong electrostatic repulsion between them. When the nickel content is higher than the manganese content, manganese easily forms a manganese-enriched region. This manganese-enriched region is inert but stable, which can lead to uneven local reactions in the primary particles. Therefore, it is necessary to control the distribution of cobalt element concentration in at least a portion of the primary particles to be more uniform than that of manganese element concentration, so that the manganese-enriched region contains cobalt. This can compensate for the insufficient electron conduction in the manganese-enriched region (cobalt has good conductivity, while manganese has poor conductivity), thereby improving the cycle performance of the secondary battery.

[0114] In some embodiments, within a single first particle, the standard deviation of the cobalt element concentration distribution is A1, and the standard deviation of the manganese element concentration distribution is C1; A1 < C1. A lower standard deviation indicates a more uniform distribution of the element within the primary particle. Therefore, by ensuring that the standard deviation of the cobalt element concentration distribution in a single particle and / or a near-single particle is A1, and the standard deviation of the manganese element concentration distribution is C1, with A1 < C1, it can be concluded that the cobalt element concentration distribution in at least some primary particles is more uniform than the manganese element concentration distribution. This ensures that most or all of the nickel-enriched regions contain cobalt, which can suppress oxygen loss caused by excessive delithiation in the manganese-enriched regions and improve cycle performance.

[0115] In some implementations, C1 ranges from 0.5% to 4%. By controlling C1 within this range while ensuring A1 < C1, the nickel concentration in the manganese-rich region is kept at a lower level. This helps reduce manganese enrichment and inertness, resulting in a more uniform reaction of the primary particles and effectively improving the cycle performance and initial discharge capacity of the secondary battery.

[0116] For example, C1 is any value among 0.5%, 0.64%, 0.65%, 0.67%, 0.68%, 0.73%, 0.79%, 0.81%, 0.87%, 0.9%, 1%, 1.5%, 2%, 2.5%, 3%, 3.15%, 3.5%, and 4%, or between any two values.

[0117] In some embodiments, the average standard deviation of the cobalt element concentration in the multiple primary particles is A2, and the average standard deviation of the manganese element concentration is C2, where A2 < C2. A lower average standard deviation indicates a more uniform distribution of the element within the primary particle. Therefore, this application uses a standard deviation of A2 for cobalt element concentration that is less than the average standard deviation of C2 for manganese element concentration. This indicates that the overall distribution of cobalt element concentration in the multiple primary particles is more uniform than that of manganese element concentration. As described above, this allows the manganese-rich region to contain cobalt, compensating for insufficient electron conduction in the manganese-rich region (cobalt has good conductivity, while manganese has poor conductivity), thereby improving the cycle performance of the secondary battery.

[0118] In some implementations, C2 ranges from 0.5% to 4%. Based on A2 < C2, controlling the distribution standard deviation C2 of the manganese element concentration within the above range, and controlling the manganese concentration in the manganese enrichment region to be within a lower range, is beneficial to the reversibility and cycle stability of the structure, while ensuring that the region still contains a considerable proportion of active nickel (and cobalt), thereby resulting in a higher overall reversible capacity of the material.

[0119] For example, C2 is any value among 0.5%, 0.7%, 0.71%, 0.72%, 0.74%, 0.75%, 0.8%, 0.85%, 0.88%, 0.92%, 0.93%, 1%, 1.5%, 2%, 2.5%, 3%, 3.4%, 3.5%, and 4%, or between any two values.

[0120] In some implementations, at least a portion of the primary particles, based on the total number of single particles and / or quasi-single particles, contain pores with an average pore size of 50 nm to 500 nm.

[0121] The identification of single particles and / or single-particle-like primary particles, as well as pores, can be achieved by acquiring images in secondary electron mode at non-edge positions in the cross-section of the positive electrode film using a field emission scanning electron microscope (FET). Images are taken at 3000x magnification, and ImageJ software (1.46r, Win64 version) is used to analyze the single particles and / or single-particle-like primary particles and pores in the FET images. Primary particles that are not identified by the software, are not fully identified, or have errors in identification are manually identified, thus completing the determination and identification of single particles and / or single-particle-like primary particles and pores in the images.

[0122] The aperture of a hole refers to the maximum distance between any two points on the outer periphery (outline) of the hole in a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, since the shape of the hole may be irregular.

[0123] The average pore size refers to the average pore size calculated from multiple pores. To make the average pore size test more accurate, multiple cross-sections at different positions along the thickness direction of the positive electrode film can be randomly selected, such as 5 cross-sections. All the pores in the primary particles identified in the 5 cross-sections are counted. The total number of all identified pores is M2, and the sum of the pore sizes of all identified pores is B. The average particle size of the pores = B / M2.

[0124] By controlling at least some primary particles to contain pores with an average pore size in the range of 50nm-500nm, the loss of material capacity can be minimized while maintaining a low discharge DCR, thus enabling the secondary battery to achieve a high initial discharge capacity. For example, the average aperture of the hole is any value of 50nm, 100nm, 150nm, 200nm, 219nm, 220nm, 225nm, 240nm, 250nm, 260nm, 300nm, 350nm, 400nm, 450nm, or 500nm, or between any two values.

[0125] In some implementations, the average perimeter of the hole is 400nm-2000nm.

[0126] The average perimeter of the pores refers to: five cross-sections of the positive electrode film layer at different positions along the thickness direction of the electrode sheet are randomly selected. Under the electron microscope image of the cross-section of the positive electrode film layer along the thickness direction of the positive electrode sheet, the primary particles with pores inside are identified using ImageJ software. The perimeter of the pores in all identified primary particles in the five cross-sections is counted. The total number of identified pores is M3, and the sum of the perimeters of all identified pores is C. The average perimeter of the pores = C / M3.

[0127] Since the shape of the pores may be irregular, further controlling the average perimeter of the pores within the above range, combined with the average pore diameter, to control the size of the pores within a suitable range, can effectively reduce the impact on the initial discharge capacity of the material while facilitating lithium-ion diffusion. This is beneficial for improving the energy density of the secondary battery and reducing the discharge DCR.

[0128] For example, the average perimeter of the hole is any value of 400nm, 450nm, 500nm, 550nm, 600nm, 650nm, 700nm, 750nm, 800nm, 850nm, 900nm, 1000nm, 1050nm, 1100nm, 1150nm, 1200nm, 1250nm, 1300nm, 1350nm, 1400nm, 1450nm, 1500nm, 1550nm, 1600nm, 1650nm, 1700nm, 1750nm, 1800nm, 1850nm, 1900nm, 1950nm, and 2000nm, or between any two values.

[0129] In some implementations, the average perimeter of the hole is 600nm-1000nm.

[0130] Controlling the average perimeter of the holes within the above range is beneficial to improving the energy density of the secondary battery and reducing the discharge density-reduction ratio (DCR).

[0131] In some implementations, the median R1 of the pore wall roughness is calculated based on the cumulative proportion of the pore area in a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet. A50 The value is 0.949-0.975.

[0132] Median roughness R A50The specific testing method is as follows: Under cross-sectional electron microscopy (SEM) images, single particles and / or primary particles (single-particle-like particles) within the positive electrode film layer are identified. The "Shape Description" analysis function in ImageJ is used to analyze the pore wall morphology of the primary particles. According to the software manual (ImageJ User Guide IJ 1.46r), the analyzed "Solidity" parameter represents the ratio of the pixel area to the convex area of ​​the pore wall within the primary particle. Therefore, the "Solidity" parameter of the analyzed particles is used to characterize the roughness of the pore wall within the primary particle. By definition, the closer the roughness is to 1, the smoother the pore wall. All identified primary particles with internal pores under the cross-sectional view are arranged in ascending order of pore wall roughness. The cumulative distribution curve of the pore wall roughness of the primary particles within the positive electrode film layer is obtained by plotting roughness on the horizontal axis and the cumulative area ratio on the vertical axis. R A50 This is the roughness R value when the cumulative area ratio of the vertical axis in the cumulative distribution curve of the roughness R value is 50%.

[0133] By definition, the closer the roughness is to 1, the smoother the hole walls. Since the median roughness R1... A50 A value close to 1 indicates that the pore walls within the particles are smooth, reducing transport resistance and increasing the lithium-ion diffusion rate, thus improving kinetics and reducing discharge DCR.

[0134] For example, in the cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet, the median R1 of the pore wall roughness is calculated based on the cumulative proportion of the pore area. A50 It is any one of the following values: 0.949, 0.950, 0.952, 0.955, 0.957, 0.960, 0.962, 0.965, 0.968, 0.970, 0.972, 0.975, or between any two values.

[0135] Optionally, in the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median R1 of the pore wall roughness is calculated based on the cumulative proportion of the pore area. A50 It is 0.957-0.97.

[0136] In some implementations, the median L1 of the pore wall roundness is determined by the cumulative proportion of the pore area in a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet. A50 The range is 0.728-0.836.

[0137] In a cross-section of the positive electrode film along the thickness direction of the electrode sheet, the median L of the pore wall roundness is... A50The specific testing method is as follows: Following the method described above in this application, pores in single particles and / or primary particles (including those resembling single particles) of lithium transition metal oxide in the cross-section of the positive electrode film are identified. The morphology and area of ​​the pores in the cross-section along the electrode thickness direction of ImageJ are analyzed using the "Shape Description" and "Area" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the analyzed "Area" parameter represents the pixel area of ​​the pore, and the "Round" parameter represents the ratio of the pixel area of ​​the pore to the area of ​​a circle with the fitted major axis as its diameter. The closer the cross-section of the pore is to a circle, the closer the ratio of the pixel area to the area of ​​the circle with the fitted major axis as its diameter is to 1. Therefore, the "Round" parameter of the pore obtained from the analysis characterizes the roundness of the pore's inner wall. Among all identified primary particles with internal pores in the cross-sectional view, they are arranged in ascending order of the roundness of the pore walls. A cumulative distribution curve of the roundness area of ​​the pore walls in the primary particles of the positive electrode film is obtained by plotting roundness on the horizontal axis and the cumulative area percentage on the vertical axis. A50 The L-value represents the roundness L value when the cumulative area along the vertical axis of the cumulative distribution curve of roundness L is 50%.

[0138] The median L of the roundness of the hole walls A50 Within the aforementioned range, the hole walls (cross-sections) are approximately circular, resulting in uniform stress distribution, reduced stress concentration, lower risk of cracking during cold pressing, improved electrode compaction density, and increased energy density of the secondary battery.

[0139] For example, in the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median L1 of the pore wall roundness is calculated based on the cumulative proportion of the pore area. A50 It is any value among 0.728, 0.730, 0.735, 0.740, 0.745, 0.750, 0.755, 0.760, 0.765, 0.780, 0.785, 0.786, 0.790, 0.795, 0.800, 0.803, 0.805, 0.813, 0.810, 0.817, 0.820, 0.823, 0.825, 0.827, 0.830, 0.833, 0.835, and 0.836, or between any two values.

[0140] Optionally, in the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median L1 of the pore wall roundness is calculated based on the cumulative proportion of the pore area. A50 It ranges from 0.757 to 0.798.

[0141] In some implementations, the proportion of primary particles with internal pores is 1%-20% of the total number of primary particles.

[0142] In this process, multiple cross-sections at different positions along the thickness direction of the positive electrode film can be randomly selected, such as 5 cross-sections. The number of all primary particles marked in the 5 cross-sections is counted to obtain the total number of primary particles. The number of primary particles marked with holes in each cross-section is also counted to obtain the proportion of the number of primary particles with holes in the total number of primary particles.

[0143] By controlling the proportion of primary particles with internal pores within the above range, and ensuring that the vast majority of primary particles are free of pores, the specific capacity of the vast majority of primary particles is not affected. The presence of pores in a small number of primary particles can effectively reduce the impact on specific capacity while facilitating lithium-ion diffusion, thus enabling secondary batteries to achieve both high energy density and low discharge DCR.

[0144] For example, the percentage of primary particles with internal pores, based on the total number of primary particles, is any one of 1.0%, 2.0%, 2.5%, 3.0%, 3.2%, 4.0%, 5.0%, 5.5%, 6.0%, 7.0%, 8.0%, 9.0%, 9.5%, 10.0%, 11.0%, 12.0%, 13.0%, 14.0%, 15.0%, 16.0%, 17.0%, 18.0%, 19.0%, or 20.0%, or between any two of these values.

[0145] In some implementations, the proportion of primary particles with pores larger than 300 nm that have internal pores is 0%-2% of the total number of primary particles.

[0146] Since, in terms of the total number of primary particles, the proportion of primary particles with pores larger than 300nm is 0%-2%, that is, the vast majority or all of the pores have a pore size ≤300nm.

[0147] By controlling the pore size of most or all of the pores in primary particles with internal pores to ≤300nm, the adverse effects of excessively large pores on specific capacity and compaction density can be effectively reduced while facilitating lithium-ion diffusion. This can effectively improve the energy density of secondary batteries while maintaining low discharge DCR.

[0148] For example, based on the total number of primary particles, the percentage of primary particles with pores larger than 300 nm that have internal pores is any one of 2.0%, 1.7%, 1.5%, 1.3%, 1.0%, 0.7%, 0.5%, 0.3%, 0, or between any two values.

[0149] In some embodiments, when viewed under an electron microscope at 3000x magnification, the area ratio of the total area of ​​pores in a cross-section of the positive electrode film along the thickness direction of the positive electrode sheet is 0.02%-0.5% based on the total area of ​​primary particles.

[0150] It should be noted that the area of ​​a primary particle refers to the area of ​​the outer periphery (outline) of a two-dimensional projection of the primary particle under an electron microscope at 3000x magnification, representing a cross-section of the positive electrode film along its thickness direction. In other words, for a primary particle containing internal pores, its area includes the area of ​​the pores. The total area of ​​a primary particle here refers to the sum of the areas of all identified primary particles within the cross-section under the electron microscope image, and the total area of ​​pores refers to the sum of the areas of all identified pores within that cross-section.

[0151] Controlling the total area of ​​pores to account for ≤0.5% of the total area of ​​primary particles is a good practice. This small percentage of pore area can effectively reduce the specific capacity loss caused by pores while facilitating lithium-ion diffusion. This is beneficial for secondary batteries to achieve both high energy density and low discharge DCR.

[0152] For example, under an electron microscope image with a magnification of 3000x, the area percentage of the total area of ​​the pores, based on the total area of ​​the primary particles, is any one of 0.02%, 0.036%, 0.05%, 0.065%, 0.08%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, or 0.50%, or between any two values, when the cross-section of the positive electrode film along the thickness direction of the positive electrode sheet is viewed.

[0153] Optionally, in an electron microscope image with a magnification of 3000x, the area ratio of the total area of ​​the pores in the cross-section of the positive electrode film along the thickness direction of the positive electrode sheet is 0.1%-0.3% based on the total area of ​​the primary particles.

[0154] Further reducing the proportion of the total area of ​​the control pores in the total area of ​​the primary particles is beneficial to further improving the energy density of the secondary battery.

[0155] In some implementations, the average particle size of the primary particles is 1 μm-4 µm.

[0156] The particle size of a primary particle refers to the particle size and area of ​​a single particle in a cross-section along the thickness direction of the positive electrode film. This is achieved by importing images of primary particles after identification and labeling into ImageJ software for analysis. The scale is set based on the scanning electron microscope (SEM) image, and the particle size and area of ​​each particle in the cross-section are analyzed using the "Feret Diameter," "Area," "Round," and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter represents the maximum distance between any two points on the outer periphery (outline) of the two-dimensional projection of a primary particle, thus characterizing the particle size.

[0157] The average particle size of a primary particle is defined as follows: Select multiple cross-sections with a magnification of 3000x at different positions along the thickness direction of the positive electrode film, for example, 5 cross-sections, and count the particle size of all identified primary particles in the 5 cross-sections. The total number of identified primary particles is M1, and the sum of the particle sizes of all identified primary particles is A. The average particle size of a primary particle is A / M1.

[0158] Controlling the average particle size of primary particles in single particles and / or single-particle-like particles to between 1 μm and 4 µm can improve the stability of lithium-containing transition metal oxides and reduce side reactions, which is beneficial to improving cycle performance.

[0159] For example, the average particle size of the primary particles is any value among 1.0µm, 1.1µm, 1.4µm, 1.8µm, 2.0µm, 2.1µm, 2.2µm, 2.3µm, 2.4µm, 2.45µm, 3.0µm, 3.91µm, and 4.0µm, or between any two values. Specifically, the average particle size of the primary particles is between 1µm and 4µm, and the particle size of a single primary particle can be greater than 4µm.

[0160] In some embodiments, under a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the primary particles include a first particle with a particle size of D1 and a second particle with a particle size of D2, where 1μm≤D1≤3μm, 3μm<D2≤10μm, and the ratio of the total area of ​​the first particle to the total area of ​​the second particle is (0.2-0.5):1.

[0161] The total area of ​​the first particle refers to the total area of ​​the first particles identified in a cross-section of the positive electrode film along the thickness direction of the positive electrode sheet. Similarly, the total area of ​​the second particle refers to the total area of ​​the second particles identified in a cross-section of the positive electrode film along the thickness direction of the positive electrode sheet. It is understood that the first and second particles are distinguished by a particle size of 3 μm. It is also understood that the particle size here refers to the particle size of each primary particle, not the average particle size.

[0162] Controlling the ratio of the total area of ​​the first particle to the total area of ​​the second particle within the above range helps to construct a reasonable particle size distribution (the first particle of 1μm-3μm can fill the gap between the second particle of 3μm-10μm), improve the electrode compaction density, improve the battery energy density, and at the same time take into account the low DC internal resistance of the battery.

[0163] Optionally, in the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median R1 of the pore wall roughness is calculated based on the cumulative proportion of the pore area. A50 It is 0.957-0.97.

[0164] In some implementations, the median L1 of the pore wall roundness is determined by the cumulative proportion of the pore area in a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet. A50 The range is 0.728-0.836.

[0165] In a cross-section of the positive electrode film along the thickness direction of the electrode sheet, the median L of the pore wall roundness is... A50 The specific testing method is as follows: Following the method described above in this application, pores in single particles and / or primary particles (including those resembling single particles) of lithium transition metal oxide in the cross-section of the positive electrode film are identified. The morphology and area of ​​the pores in the cross-section along the electrode thickness direction of ImageJ are analyzed using the "Shape Description" and "Area" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the analyzed "Area" parameter represents the pixel area of ​​the pore, and the "Round" parameter represents the ratio of the pixel area of ​​the pore to the area of ​​a circle with the fitted major axis as its diameter. The closer the cross-section of the pore is to a circle, the closer the ratio of the pixel area to the area of ​​the circle with the fitted major axis as its diameter is to 1. Therefore, the "Round" parameter of the pore obtained from the analysis characterizes the roundness of the pore's inner wall. Among all identified primary particles with internal pores in the cross-sectional view, they are arranged in ascending order of the roundness of the pore walls. A cumulative distribution curve of the roundness area of ​​the pore walls in the primary particles of the positive electrode film is obtained by plotting roundness on the horizontal axis and the cumulative area percentage on the vertical axis. A50 The L-value represents the roundness L value when the cumulative area along the vertical axis of the cumulative distribution curve of roundness L is 50%.

[0166] The median L of the roundness of the hole walls A50 Within the aforementioned range, the hole walls (cross-sections) are approximately circular, resulting in uniform stress distribution, reduced stress concentration, lower risk of cracking during cold pressing, improved electrode compaction density, and increased energy density of the secondary battery.

[0167] For example, in the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median L1 of the pore wall roundness is calculated based on the cumulative proportion of the pore area. A50 It is any value among 0.728, 0.730, 0.735, 0.740, 0.745, 0.750, 0.755, 0.760, 0.765, 0.780, 0.785, 0.786, 0.790, 0.795, 0.800, 0.803, 0.805, 0.813, 0.810, 0.817, 0.820, 0.823, 0.825, 0.827, 0.830, 0.833, 0.835, and 0.836, or between any two values.

[0168] Optionally, in the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median L1 of the pore wall roundness is calculated based on the cumulative proportion of the pore area. A50 It ranges from 0.757 to 0.798.

[0169] In some implementations, the proportion of primary particles with internal pores is 1%-20% of the total number of primary particles.

[0170] In this process, multiple cross-sections at different positions along the thickness direction of the positive electrode film can be randomly selected, such as 5 cross-sections. The number of all primary particles marked in the 5 cross-sections is counted to obtain the total number of primary particles. The number of primary particles marked with holes in each cross-section is also counted to obtain the proportion of the number of primary particles with holes in the total number of primary particles.

[0171] By controlling the proportion of primary particles with internal pores within the above range, and ensuring that the vast majority of primary particles are free of pores, the specific capacity of the vast majority of primary particles is not affected. The presence of pores in a small number of primary particles can effectively reduce the impact on specific capacity while facilitating lithium-ion diffusion, thus enabling secondary batteries to achieve both high energy density and low discharge DCR.

[0172] For example, the percentage of primary particles with internal pores, based on the total number of primary particles, is any one of 1.0%, 2.0%, 2.5%, 3.0%, 3.2%, 4.0%, 5.0%, 5.5%, 6.0%, 7.0%, 8.0%, 9.0%, 9.5%, 10.0%, 11.0%, 12.0%, 13.0%, 14.0%, 15.0%, 16.0%, 17.0%, 18.0%, 19.0%, or 20.0%, or between any two of these values.

[0173] In some implementations, the proportion of primary particles with pores larger than 300 nm that have internal pores is 0%-2% of the total number of primary particles.

[0174] Since, in terms of the total number of primary particles, the proportion of primary particles with pores larger than 300nm is 0%-2%, that is, the vast majority or all of the pores have a pore size ≤300nm.

[0175] By controlling the pore size of most or all of the pores in primary particles with internal pores to ≤300nm, the adverse effects of excessively large pores on specific capacity and compaction density can be effectively reduced while facilitating lithium-ion diffusion. This can effectively improve the energy density of secondary batteries while maintaining low discharge DCR.

[0176] For example, based on the total number of primary particles, the percentage of primary particles with pores larger than 300 nm that have internal pores is any one of 2.0%, 1.7%, 1.5%, 1.3%, 1.0%, 0.7%, 0.5%, 0.3%, 0, or between any two values.

[0177] In some embodiments, when viewed under an electron microscope at 3000x magnification, the area ratio of the total area of ​​pores in a cross-section of the positive electrode film along the thickness direction of the positive electrode sheet is 0.02%-0.5% based on the total area of ​​primary particles.

[0178] It should be noted that the area of ​​a primary particle refers to the area of ​​the outer periphery (outline) of a two-dimensional projection of the primary particle under an electron microscope at 3000x magnification, representing a cross-section of the positive electrode film along its thickness direction. In other words, for a primary particle containing internal pores, its area includes the area of ​​the pores. The total area of ​​a primary particle here refers to the sum of the areas of all identified primary particles within the cross-section under the electron microscope image, and the total area of ​​pores refers to the sum of the areas of all identified pores within that cross-section.

[0179] Controlling the total area of ​​pores to account for ≤0.5% of the total area of ​​primary particles is a good practice. This small percentage of pore area can effectively reduce the specific capacity loss caused by pores while facilitating lithium-ion diffusion. This is beneficial for secondary batteries to achieve both high energy density and low discharge DCR.

[0180] For example, under an electron microscope image with a magnification of 3000x, the area percentage of the total area of ​​the pores, based on the total area of ​​the primary particles, is any one of 0.02%, 0.036%, 0.05%, 0.065%, 0.08%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, or 0.50%, or between any two values, when the cross-section of the positive electrode film along the thickness direction of the positive electrode sheet is viewed.

[0181] Optionally, in an electron microscope image with a magnification of 3000x, the area ratio of the total area of ​​the pores in the cross-section of the positive electrode film along the thickness direction of the positive electrode sheet is 0.1%-0.3% based on the total area of ​​the primary particles.

[0182] Further reducing the proportion of the total area of ​​the control pores in the total area of ​​the primary particles is beneficial to further improving the energy density of the secondary battery.

[0183] In some implementations, the average particle size of the primary particles is 1 μm-4 µm.

[0184] The particle size of a primary particle refers to the particle size and area of ​​a single particle in a cross-section along the thickness direction of the positive electrode film. This is achieved by importing images of primary particles after identification and labeling into ImageJ software for analysis. The scale is set based on the scanning electron microscope (SEM) image, and the particle size and area of ​​each particle in the cross-section are analyzed using the "Feret Diameter," "Area," "Round," and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter represents the maximum distance between any two points on the outer periphery (outline) of the two-dimensional projection of a primary particle, thus characterizing the particle size.

[0185] The average particle size of a primary particle is defined as follows: Select multiple cross-sections with a magnification of 3000x at different positions along the thickness direction of the positive electrode film, for example, 5 cross-sections, and count the particle size of all identified primary particles in the 5 cross-sections. The total number of identified primary particles is M1, and the sum of the particle sizes of all identified primary particles is A. The average particle size of a primary particle is A / M1.

[0186] Controlling the average particle size of primary particles in single particles and / or single-particle-like particles to between 1 μm and 4 µm can improve the stability of lithium-containing transition metal oxides and reduce side reactions, which is beneficial to improving cycle performance.

[0187] For example, the average particle size of the primary particles is any one of 1.0µm, 1.1µm, 1.4µm, 1.8µm, 2.0µm, 2.1µm, 2.2µm, 2.3µm, 2.4µm, 2.45µm, 3.0µm, 3.91µm, 4.0µm or between any two values.

[0188] In some embodiments, under a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the primary particles include a first particle with a particle size of D1 and a second particle with a particle size of D2, where 1μm≤D1≤3μm, 3μm<D2≤10μm, and the ratio of the total area of ​​the first particle to the total area of ​​the second particle is (0.2-0.5):1.

[0189] The total area of ​​the first particle refers to the total area of ​​the first particles identified in a cross-section of the positive electrode film along the thickness direction of the positive electrode sheet. Similarly, the total area of ​​the second particle refers to the total area of ​​the second particles identified in a cross-section of the positive electrode film along the thickness direction of the positive electrode sheet. It is understood that the first and second particles are distinguished by a particle size of 3 μm. It is also understood that the particle size here refers to the particle size of each primary particle, not the average particle size.

[0190] Controlling the ratio of the total area of ​​the first particle to the total area of ​​the second particle within the above range helps to construct a reasonable particle size distribution (the first particle of 1μm-3μm can fill the gap between the second particle of 3μm-10μm), improve the electrode compaction density, improve the battery energy density, and at the same time take into account the low DC internal resistance of the battery.

[0191] For example, the ratio of the total area of ​​the first particle to the total area of ​​the second particle is any one of 0.20:1, 0.25:1, 0.30:1, 0.32:1, 0.35:1, 0.37:1, 0.40:1, 0.43:1, 0.45:1, 0.50:1 or between any two values.

[0192] Optionally, in the cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet, the ratio of the total area of ​​the first particle to the total area of ​​the second particle is (0.32-0.38):1.

[0193] The above settings help to further optimize particle size distribution, increase electrode compaction density, and improve battery energy density, while also ensuring low battery internal resistance.

[0194] In some embodiments, under a cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet, the primary particles include a first particle with a particle size of D1 and a second particle with a particle size of D2, where 1μm≤D1≤3μm, 3μm<D2≤10μm, and the ratio of the number of the first particle to the number of the second particle is (1.2-3):1.

[0195] Controlling the first and second particles to be compounded as described above helps to construct a reasonable particle size distribution (the first particles of 1μm-3μm can fill the gaps between the second particles of 3μm-10μm), improve the electrode compaction density, and improve the energy density of the secondary battery. At the same time, controlling the proportion of the second particles with larger particle sizes to be small helps to reduce the internal resistance of the secondary battery and improve the kinetics.

[0196] For example, D1 is any value among 1.0μm, 1.3μm, 1.5μm, 1.8μm, 2.0μm, 2.3μm, 2.5μm, 2.8μm, and 3.0μm, or between any two values.

[0197] For example, D2 is any value among 3.1μm, 3.5μm, 4.0μm, 4.5μm, 5.0μm, 5.5μm, 6.0μm, 6.5μm, 7.0μm, 7.5μm, 8.0μm, 8.5μm, 9.0μm, 9.5μm, and 10.0μm, or between any two values.

[0198] For example, the ratio of the number of the first particle to the number of the second particle is any one of 1.20:1, 1.25:1, 1.30:1, 1.35:1, 1.40:1, 1.50:1, 1.55:1, 1.60:1, 1.65:1, 1.66:1, 1.70:1, 1.80:1, 1.83:1, 1.90:1, 2.00:1, 2.10:1, 2.20:1, 2.30:1, 2.40:1, 2.50:1, 2.60:1, 2.70:1, 2.80:1, 2.90:1, or 3.00:1, or between any two of these values.

[0199] Optionally, in the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the ratio of the number of the first particle to the number of the second particle is (1.5-2):1.

[0200] The above settings help to further optimize particle size distribution, increase electrode compaction density, and improve battery energy density, while also ensuring low battery internal resistance.

[0201] In some implementations, the median L2 of the sphericity of the primary particles, based on the cumulative area ratio of the primary particles, is obtained from a cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet. A50 It ranges from 0.6 to 0.85.

[0202] In a cross-section of the positive electrode film along the thickness direction of the electrode, the median L of the sphericity of the primary particles is... A50The specific testing method is as follows: Primary particles in the cross-section of the positive electrode film layer are identified using the method described above in this application. The morphology and area of ​​the primary particles in the cross-section along the electrode thickness direction of the positive electrode film layer are analyzed using the "Shape Description" and "Area" analysis functions in ImageJ. According to the software manual (ImageJ User Guide IJ 1.46r), the "Area" parameter obtained from the analysis represents the pixel area of ​​the primary particle, and the "Round" parameter represents the ratio of the pixel area of ​​the primary particle to the area of ​​a circle with the fitted major axis as its diameter. The closer the primary particle is to a sphere, the closer the ratio of the pixel area to the area of ​​the circle with the fitted major axis as its diameter is to 1. Therefore, the "Round" parameter of the primary particle obtained from the analysis characterizes the sphericity of the primary particle. The sphericity of all identified primary particles in the cross-sectional view is arranged in ascending order, and the cumulative distribution curve of the sphericity area of ​​the primary particles in the positive electrode film layer is obtained with sphericity as the horizontal axis and the cumulative area ratio as the vertical axis. L A50 This is the L-value of sphericity when the cumulative area under the vertical axis of the cumulative distribution curve of L-values ​​accounts for 50%.

[0203] Those skilled in the art can control the sphericity of particles using any known process. For example, the sphericity of particles can be adjusted through processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, and adding surfactants, as well as by adjusting the parameters of each process.

[0204] Median L of sphericity A50 Within the aforementioned range, the primary particles are approximately spherical, which helps them maintain good slidability during stacking, making it easier to fill the gaps between them. This further improves the compaction density of the electrode and increases the energy density of the secondary battery.

[0205] For example, in the cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet, the median L2 of the spheroidality of the primary particles is calculated based on the cumulative area ratio of the primary particles. A50 It is any value among 0.60, 0.63, 0.65, 0.68, 0.70, 0.73, 0.75, 0.78, 0.80, 0.83, and 0.85, or between any two values.

[0206] In some implementations, the median roughness R2 of the primary particles is calculated based on the cumulative area of ​​the primary particles in a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet. A50 It is 0.92-0.97.

[0207] In the cumulative roughness area distribution curve of primary particles obtained from a cross-section of the positive electrode film along the electrode thickness direction, the median R of the roughness of the primary particles is...A50 The specific testing method is as follows: Primary particles in the cross-section of the positive electrode film layer are identified using the method described above in this application. The morphology of the primary particles in the cross-section along the electrode thickness direction of the positive electrode film layer is analyzed using the "Shape Description" analysis function in ImageJ. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained from the analysis represents the ratio of the pixel area to the convex area of ​​the primary particle. Therefore, the "Solidity" parameter of the primary particle obtained from the analysis is used to characterize the roughness of the particle. By definition, the closer the roughness is to 1, the smoother the primary particle. The roughness of all identified primary particles in the cross-sectional view is arranged in ascending order, and the cumulative area ratio is plotted on the horizontal axis to obtain the cumulative roughness area distribution curve of the primary particles in the positive electrode film layer. A50 This is the roughness R value when the cumulative area ratio of the vertical axis in the cumulative distribution curve of the roughness R value is 50%.

[0208] Those skilled in the art can control the sphericity of particles using any known process. For example, the sphericity of particles can be adjusted through processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, and adding surfactants, as well as by adjusting the parameters of each process.

[0209] Median roughness R 50 Within the aforementioned range, the surface of the primary particles is relatively smooth, and the friction between particles is relatively small. Under the action of external force, it is easy to slip, which can further improve the compaction density of the electrode and increase the energy density of the battery.

[0210] For example, in the cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet, the median roughness R2 of the primary particles is calculated based on the cumulative area ratio of the primary particles. A50 It is any value among 0.92, 0.93, 0.94, 0.95, 0.96, and 0.97, or between any two values.

[0211] In some embodiments, the molar ratio of nickel, cobalt, and the first element is (5-9): (0.5-2.5):(0.5-2.5).

[0212] It should be noted that the molar ratio of nickel, cobalt, and the first element refers to the molar ratio of each element in a unit molar amount of lithium-containing transition metal oxide.

[0213] By controlling the molar ratio of nickel, cobalt, and the first element within the above range, lithium-containing transition metal oxides with high energy density can be obtained.

[0214] For example, the molar ratio of nickel, cobalt, and the first element (manganese and / or aluminum) is any one of 5:2.5:2.5, 5:2:3, 6:2:2, 7:1.5:1.5, 8:1:1, 9:0.5:0.5, 6:1.5:2.5, 6:2.5:1.5, 7:2:1, 7:1:2, 8:0.5:1.5, 8:1.5:0.5, or 8.8:0.7:0.5, or between any two of these values.

[0215] For example, lithium-containing transition metal oxides include, but are not limited to, LiNi. 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 LiNi 0.8 Co 0.15 Al 0.05 At least one of O2, etc.

[0216] During the charging and discharging process, Li will be deintercalated and consumed. When the positive electrode material is used in the battery system, the molar content of Li will change after charge and discharge cycles.

[0217] In the examples of cathode materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0218] In some embodiments, the first element is manganese, and the molar ratio of nickel, cobalt and manganese is (5-9): (0.5-2.5):(0.5-2.5).

[0219] Choosing manganese as the first element and controlling the molar ratio of the three elements within the above range is beneficial because manganese is inexpensive, thus reducing the cost of lithium-containing transition metal oxides. Simultaneously, manganese ions (Mn)... 4+ It hardly participates in the reaction during lithium ion desorption (electrochemical inertness), which can effectively stabilize the three-dimensional layered crystal structure of the material and improve the thermal stability and cycle life of the above-mentioned high energy density lithium-containing transition metal oxides.

[0220] It should be noted that when the first element is manganese, aluminum can be used as a bulk dopant in lithium-containing transition metal oxides, and / or aluminum can be used as a coating element to coat the surface of lithium-containing transition metal oxides.

[0221] In some embodiments, the lithium-containing transition metal oxide also contains a dopant element, including at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce, and La.

[0222] Understandably, the doping element differs from the primary element. For example, the main elements of lithium-containing transition metals are nickel, cobalt, and manganese, and the doping element includes at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce, and La. (This is repeated three times in the original text.)

[0223] Lithium-containing transition metal oxides also contain doping elements, meaning that these doping elements are introduced into the crystal lattice of the host material (lithium-containing transition metal oxide) to replace some existing atomic positions, thereby changing the physical and chemical properties of the material without significantly altering its main structure. In other words, the bulk phase of lithium-containing transition metal oxides is doped with doping elements.

[0224] Including the aforementioned doping elements in lithium-containing transition metal oxides can improve the structural stability and / or thermal stability of lithium-containing transition metal oxides, thereby enhancing the cycle performance of secondary batteries.

[0225] In some embodiments, the lithium-containing transition metal oxide contains W, and the W content is 500 ppm to 2500 ppm based on the mass of the lithium-containing transition metal oxide.

[0226] In some embodiments, the lithium-containing transition metal oxide surface is coated with a coating layer, wherein the coating layer is rich in cobalt; and / or, The coating layer is rich in a second element, which includes at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce and La.

[0227] In other words, the surface of the lithium-containing transition metal oxide is coated with a coating layer. This coating layer may be rich in cobalt alone and contain no second element, or it may be rich in the second element alone and contain no cobalt, or it may be rich in both cobalt and the second element simultaneously. It should be noted that the coating layer is a part of the composition of the lithium-containing transition metal oxide.

[0228] The doping elements and their contents in the bulk phase of the lithium transition metal oxide, as well as the elements and their contents in the coating layer, as shown above, can all be tested using an electron probe microanalyzer (EPMA). Specifically, an argon ion beam is used to cut the positive electrode film along the thickness direction of the positive electrode sheet. The cut surface of the positive electrode active material is located through the grain boundaries of the lithium transition metal oxide under the backscattered electron (BSE) image of the scanning electron microscope (SEM) of the EPMA, thus obtaining the interface position between the coating layer and the lithium transition metal oxide. Subsequently, the wave spectroscopy (WDS) spectrometer in the EPMA is used to perform line scan analysis on the coating layer region and the lithium transition metal oxide, which can obtain the element content in the coating layer and the doping element content in the lithium transition metal oxide, respectively.

[0229] The physical barrier of the coating layer isolates the lithium-containing transition metal oxide from the electrolyte, effectively suppressing interfacial side reactions. When the coating layer is rich in cobalt, it is beneficial to improve surface kinetics. When the coating layer is rich in a second element, it can stabilize cobalt and reduce cobalt dissolution.

[0230] For example, the cobalt in the coating layer exists in the form of cobalt oxide, such as lithium cobalt oxide. In this case, the cobalt-rich coating layer, in addition to having good ionic and electronic conductivity and providing physicochemical protection, can also contribute additional specific capacity, thereby increasing the energy density.

[0231] It should be noted that doping and coating can use the same element. Generally, it can be considered that the dopant is inside the bulk phase of the particle and the coating is on the surface of the particle.

[0232] In some embodiments, the coating layer contains cobalt, with the cobalt content in the coating layer ranging from 5000 ppm to 15000 ppm.

[0233] By controlling the cobalt content in the coating layer within the above-mentioned range, the surface dynamics of the positive electrode active material are high.

[0234] For example, the cobalt content in the coating layer is any value of 5000ppm, 5500ppm, 6000ppm, 6500ppm, 7000ppm, 7500ppm, 8000ppm, 8500ppm, 9000ppm, 9500ppm, 10000ppm, 10500ppm, 11000ppm, 11500ppm, 12000ppm, 12500ppm, 13000ppm, 13500ppm, 14000ppm, 14500ppm, or 15000ppm, or between any two values.

[0235] In some embodiments, the coating layer contains a second element, the content of which is 1000ppm-5000ppm.

[0236] Controlling the content of the second element within the above range can effectively suppress cobalt leaching and optimize cycle performance.

[0237] For example, the content of the second element in the coating layer is any value of 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, or 5000ppm, or between any two values.

[0238] In some embodiments, the sodium content in the lithium transition metal oxide is less than 500 ppm.

[0239] The sodium content can be tested using the ICP method, as follows: Take a lithium transition metal oxide as a sample, digest the sample with aqua regia and hydrofluoric acid HF, take 15 ml of the completely digested solution for ICP testing, and obtain the sodium content of the lithium transition metal oxide.

[0240] For dry preparation processes, sodium is mainly an impurity present in the raw materials such as lithium salts, nickel sources, cobalt sources, and manganese sources. A sodium content below 500 ppm includes cases where the sodium content is zero. Controlling the sodium content in lithium-containing transition metal oxides to below 500 ppm means that the sodium impurity content in lithium-containing transition metal oxides is low and has virtually no impact on the structural stability of lithium-containing transition metal oxides.

[0241] Those skilled in the art can control the sodium content using any known process. For example, sodium content can be adjusted by using raw materials such as higher-purity lithium salts, nickel sources, cobalt sources, and manganese sources; regularly cleaning the mixing tank; using an oxygen-based sintering atmosphere; adjusting the heating program and holding time; and modifying the parameters of each process.

[0242] For example, the sodium content in the lithium-containing transition metal oxide is any value or between any two of the following: 0 ppm, 10 ppm, 20 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, 350 ppm, 400 ppm, 450 ppm, and 500 ppm. Optionally, in the lithium-containing transition metal oxide, the sodium content is 0 ppm ≤ 20 ppm. This allows the lithium-containing transition metal oxide to be essentially free of sodium, resulting in higher material purity.

[0243] In another embodiment, the sodium content in the lithium-containing transition metal oxide is 20 ppm < 100 ppm. Including a small amount of sodium in the lithium-containing transition metal oxide can widen the interlayer spacing and make it more stable, which is beneficial for improving the cycle performance of the battery.

[0244] In some embodiments, the sulfur content in the lithium transition metal oxide is less than 500 ppm.

[0245] The sulfur content can be tested using the ICP method, as follows: Take a lithium-containing transition metal oxide as a sample, digest the sample with aqua regia and hydrofluoric acid HF, take 15 ml of the completely digested solution for ICP testing, and obtain the sulfur content of the lithium-containing transition metal oxide.

[0246] The sulfur content below 500 ppm includes cases where the sulfur content is 0. Sulfur can lead to increased impedance, reduced specific capacity and rate capability in lithium-containing transition metal oxides. Therefore, controlling the sulfur content in lithium-containing transition metal oxides to below 500 ppm, i.e., a low content of sulfur impurities, reduces the negative impact on lithium-containing transition metal oxides. In traditional co-precipitation methods, sulfur is mainly introduced during the preparation of lithium-containing transition metal oxides because the nickel, cobalt, and manganese sources are usually sulfates of the corresponding elements, which are not completely removed during sintering. This results in a sulfur content in the generated lithium-containing transition metal oxides that is much higher than 500 ppm. In this application, as can be seen from the context, a dry preparation method is used, where the raw materials such as nickel, cobalt, manganese, and aluminum are non-sulfate sources. The sulfur content is mainly introduced because the purity of the raw materials such as nickel, cobalt, manganese, and aluminum is not 100%, and some impurities are present, which may contain sulfur and sodium, and are not completely removed during sintering.

[0247] Those skilled in the art can control the sulfur content using any known process. For example, the sulfur content can be adjusted by using raw materials such as lithium salts, nickel sources, cobalt sources, manganese sources, and aluminum sources with higher purity, regularly cleaning the mixing tank, controlling the sintering temperature, controlling the sintering atmosphere to be oxygen, and adjusting the parameters of each process.

[0248] For example, the sulfur content in the lithium transition metal oxide is any value of 0ppm, 10ppm, 20ppm, 50ppm, 100ppm, 150ppm, 200ppm, 250ppm, 300ppm, 350ppm, 400ppm, 450ppm, or 500ppm, or between any two values.

[0249] Optionally, in lithium-containing transition metal oxides, the sulfur content is 0 ppm ≤ 300 ppm. This allows the lithium-containing transition metal oxides to be essentially sulfur-free, resulting in higher material purity.

[0250] In some embodiments, the areal density of the single-sided positive electrode film is 0.255 g / 1540.25 mm. 2 -0.280 g / 1540.25mm 2 .

[0251] In this application, the unilateral density of the positive electrode film layer has a meaning known in the art and can be tested using methods known in the art. For example, take a positive electrode sheet that has been coated on one side and compacted (if it is a double-sided coated positive electrode sheet, the positive electrode film layer on one side can be wiped off first), cut it into a small circular piece with an area of ​​S1, weigh it, and record its weight as M1. Then wipe off the positive electrode film layer of the above-weighed positive electrode sheet, weigh the current collector, and record it as M0. The unilateral density of the positive electrode film layer = (M1-M0) / S1. To ensure the accuracy of the test results, multiple sets (e.g., 10 sets) of samples can be tested, and the average value can be calculated as the test result.

[0252] A cathode film with an areal density within the above range can help improve the energy density of a secondary battery.

[0253] For example, the areal density of the single-sided positive electrode film is 0.255 g / 1540.25 mm. 2 0.260 g / 1540.25mm 2 0.265g / 1540.25mm 2 0.270 g / 1540.25mm 2 0.275 g / 1540.25mm 2 0.280 g / 1540.25mm 2 It can be any value in the range or any two values ​​in between.

[0254] In some embodiments, the compaction density of the single-sided positive electrode film is 3.30 g / cm³. 3 -3.55 g / cm 3 .

[0255] The compaction density of the positive electrode film can be tested using methods known in the art. As an example, the battery is discharged to 2.8V, disassembled, and the positive electrode sheet is obtained. The residual electrolyte is treated with dimethyl carbonate solvent, the electrode sheet is dried, and it is cut into small circular pieces with an area of ​​S, yielding a mass of W1. The thickness T1 of the positive electrode sheet is measured using a micrometer. Then, the positive electrode film layer of the weighed electrode sheet is wiped off, and the mass of the current collector is weighed and recorded as W2. The thickness T2 of the current collector is measured using a micrometer. The compaction density PD of the positive electrode film layer is then calculated as follows: PD = (W1...) W2) / [(T1-T2)×S。

[0256] A compaction density of the positive electrode film within the above-mentioned range is beneficial to improving the energy density of lithium-ion secondary batteries.

[0257] For example, the compaction density of the single-sided positive electrode film is 3.30 g / cm³. 3 3.33 g / cm 3 3.35 g / cm 3 3.38g / cm 3 3.40 g / cm 3 3.43 g / cm 3 3.45 g / cm 3 3.48 g / cm 3 3.50 g / cm 3 3.53 g / cm 3 3.55 g / cm 3 It can be any value in the range or any two values ​​in between.

[0258] In some embodiments, the positive electrode film layer also contains a conductive agent, which accounts for 1%-3% of the mass of the positive electrode film layer.

[0259] When the mass content of the conductive agent is within the above range, it can provide a conductive network, improve the electronic conductivity of the electrode, and also help to increase the loading of the positive electrode active material and improve the energy density of the lithium-ion secondary battery.

[0260] For example, the conductive agent accounts for any one of the following mass percentages in the positive electrode film layer: 1.0%, 1.2%, 1.5%, 1.7%, 2.0%, 2.2%, 3.0%, or between any two of these values.

[0261] In some embodiments, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0262] All of the above-mentioned conductive agents can effectively improve the electronic conductivity of the electrode.

[0263] For example, the conductive agent can be any one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers, and also includes conductive agents obtained by compounding zero-dimensional conductive agents (superconducting carbon, acetylene black, carbon black, Ketjen black, and carbon dots) and one-dimensional conductive agents (carbon nanotubes and carbon nanofibers).

[0264] In some embodiments, the positive electrode film layer also contains a binder, and the binder accounts for 0.5%-3.0% of the mass of the positive electrode film layer.

[0265] When the mass content of the binder is within the above range, it can maintain good internal adhesion, reduce the probability of powder shedding, expansion and cracking, and improve the energy density of the secondary battery while taking into account safety performance.

[0266] For example, the mass percentage of the binder in the positive electrode film layer is any one of 0.5%, 0.7%, 1.0%, 1.2%, 1.5%, 1.7%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0% or between any two values.

[0267] In some embodiments, the binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0268] All of the above binders are suitable for high-energy-density battery systems. They can maintain good internal adhesion, reduce the probability of powder shedding, expansion and cracking, and improve the energy density of secondary batteries while taking into account safety performance.

[0269] In some embodiments, the discharge specific capacity of the lithium transition metal oxide is 170 mAh / g - 205 mAh / g.

[0270] The discharge specific capacity of lithium-containing transition metal oxides is within the above range, which helps to improve the energy density of secondary batteries.

[0271] For example, the discharge specific capacity of the lithium transition metal oxide is any one of 170 mAh / g, 192.0 mAh / g, 190.0 mAh / g, 186.0 mAh / g, 193.0 mAh / g, 191.1 mAh / g, 189.9 mAh / g, 189.1 mAh / g, 187.0 mAh / g, 190.8 mAh / g, 191.2 mAh / g, 191.7 mAh / g, 189.2 mAh / g, 190.1 mAh / g, 183.0 mAh / g, 184.1 mAh / g, 185.8 mAh / g, 175 mAh / g, 180 mAh / g, 185.0 mAh / g, 195.0 mAh / g, 201.0 mAh / g, or 205.0 mAh / g, or between any two of these values.

[0272] The second aspect of this application provides a positive electrode active material, which includes a lithium-containing transition metal oxide. The lithium-containing transition metal oxide includes nickel, cobalt, and a first element, wherein the first element is manganese and / or aluminum, and the content of nickel is higher than the content of cobalt.

[0273] Lithium-containing transition metal oxides include single particles and / or quasi-single particles. Among the primary particles of single particles and / or quasi-single particles, each primary particle satisfies the following condition: the elemental concentration distribution of cobalt is more uniform than that of nickel.

[0274] Understandably, the testing method is as follows: Take the discharged secondary battery, disassemble it, remove the positive electrode sheet and soak it in DMC (ethylene carbonate) for 4 hours to remove residual electrolyte and lithium salt. Then place it in an oven to dry, and obtain the processed positive electrode sheet. Cut the positive electrode film along the thickness direction of the electrode sheet with an argon ion beam (for example, the equipment model can be: Leica EM TIC 3X CP, working voltage: 6kV, working time: 6h). After exposing the cut surface, use a scanning electron microscope (for example, the equipment model can be: Hitachi SU8230, working voltage: 3kV, beam current: high, probe model: U (LA100), working distance <5mm) to observe the cross-section of the positive electrode film along the thickness direction of the electrode sheet, and obtain the field of view of the electron microscope image of the cross-section of the positive electrode film along the thickness direction of the positive electrode sheet. The magnification of the cross-sectional electron microscope image is not required, as long as the field of view can clearly see the lithium-containing transition metal oxide and pores. If five cross-sections are used, the magnification can be uniform, for example, all of them can be 3000x.

[0275] The definitions and testing methods for the types and proportions of elements in lithium transition metal oxides, identification of primary particles, standard deviation of element concentration distribution and average standard deviation of element concentration, identification of pores, average particle size of primary particles, pore diameter and average pore diameter, etc., are all referred to above and will not be repeated here.

[0276] The lithium-containing transition metal oxide provided in this application achieves high energy density in secondary batteries by controlling the nickel content to be higher than the cobalt content. However, nickel kinetics are lower than cobalt, so when the nickel content is higher than the cobalt content, nickel-rich regions are easily formed. These nickel-rich regions are highly active but unstable, leading to uneven local reactions. Therefore, in the primary particles of single particles and / or near-single particles, each primary particle satisfies the following condition: the distribution of cobalt element concentration is more uniform than the distribution of nickel element concentration. That is, the distribution of cobalt element concentration in at least some primary particles is controlled to be more uniform than the distribution of nickel element concentration, so that most or all of the nickel-rich regions contain cobalt. This can suppress oxygen loss caused by transition delithiation in the nickel-rich regions (cobalt also undergoes redox reactions, the reaction order being nickel, cobalt, oxygen), thus improving cycle performance.

[0277] In other words, the secondary battery provided in this application achieves both high energy density and long cycle performance by introducing an improved lithium-containing transition metal oxide.

[0278] In some embodiments, within a single first particle, the standard deviation of the cobalt element concentration distribution is A1, and the standard deviation of the nickel element concentration distribution is B1; A1 < B1. A lower standard deviation indicates a more uniform distribution of the element within the primary particle. Therefore, by ensuring that the standard deviation of the cobalt element concentration distribution in a single particle and / or a near-single particle is A1, and the standard deviation of the nickel element concentration distribution is B1, with A1 < B1, it can be concluded that the cobalt element concentration distribution in at least some primary particles is more uniform than the nickel element concentration distribution. This ensures that most or all of the nickel-enriched regions contain cobalt, which can suppress oxygen loss caused by excessive delithiation in the nickel-enriched regions and improve cycle performance.

[0279] In some embodiments, A1 ranges from 0.3% to 1.0%. Controlling A1 within this range results in a more uniform distribution of cobalt element concentration, which is beneficial for stabilizing the structural stability of lithium-containing transition metal oxides during cycling and improving cycling performance.

[0280] For example, A1 is any value among 0.3%, 0.4%, 0.42%, 0.43%, 0.44%, 0.47%, 0.49%, 0.5%, 0.53%, 0.59%, 0.6%, 0.62%, 0.64%, 0.7%, 0.8%, 0.9%, and 1.0%, or between any two values.

[0281] In some embodiments, B1 ranges from 0.5% to 4%. Controlling B1 within this range helps to keep the nickel concentration in the nickel-rich region at a lower level, which is beneficial for reducing nickel-rich cation mixing and stress, suppressing cycle cracking, and effectively improving the cycle performance and initial discharge capacity of the secondary battery.

[0282] For example, B1 is any value among 0.5%, 0.62%, 0.63%, 0.67%, 0.71%, 0.77%, 0.78%, 0.84%, 0.86%, 1%, 1.5%, 2%, 2.5%, 3%, 3.1%, 3.5%, and 4%, or between any two values.

[0283] In some implementations, the average standard deviation of the cobalt element concentration in multiple primary particles is A2, and the average standard deviation of the nickel element concentration is B2, where A2 < B2. A lower average standard deviation indicates a more uniform overall distribution of the element within the primary particle. Therefore, controlling the average standard deviation of the cobalt element concentration A2 to be less than the average standard deviation of the nickel element concentration B2 indicates that the overall distribution of cobalt element concentration in the multiple primary particles is more uniform than that of nickel. This is beneficial for the presence of cobalt in nickel-rich regions, which can suppress oxygen loss caused by excessive delithiation in these regions and improve cycle performance.

[0284] In some implementations, A2 ranges from 0.3% to 1.0%. By controlling the average standard deviation A2 of the cobalt element concentration distribution in multiple primary particles to be within the above range, the overall distribution of cobalt element concentration in multiple primary particles is more uniform, which is beneficial to stabilizing the structural stability of lithium-containing transition metal oxides during cycling and improving cycling performance.

[0285] For example, A2 is any value among 0.3%, 0.4%, 0.45%, 0.46%, 0.48%, 0.5%, 0.52%, 0.57%, 0.6%, 0.62%, 0.63%, 0.67%, 0.7%, 0.8%, 0.9%, and 1.0%, or between any two values.

[0286] In some implementations, B2 ranges from 0.5% to 4%. Controlling the average standard deviation B2 of the nickel element concentration distribution in multiple primary particles within the above range is beneficial for reducing nickel-enriched cation mixing and stress, suppressing cycle cracking, and effectively improving the cycle performance and initial discharge capacity of the secondary battery.

[0287] For example, B2 is any value among 0.5%, 0.67%, 0.68%, 0.69%, 0.73%, 0.74%, 0.79%, 0.84%, 0.85%, 0.88%, 0.9%, 1%, 1.5%, 2%, 2.5%, 3%, 3.3%, 3.5%, and 4%, or between any two values.

[0288] In some implementations, at least 90% of the primary particles in a plurality of primary particles satisfy the following conditions: the standard deviation of the cobalt element concentration distribution is A1, the standard deviation of the nickel element concentration distribution is B1, and A1 < B1.

[0289] It should be noted that the multiple primary particles mentioned here refer to multiple primary particles that have obtained the average standard deviation of the above element concentration distribution.

[0290] In other words, most or all of the primary particles in a plurality of primary particles satisfy A1 < B1, ensuring that most or all of the nickel-enriched regions contain cobalt. This can suppress oxygen loss caused by excessive delithiation in the nickel-enriched regions and improve cycle performance. For example, in a plurality of primary particles, 90%, 91%, 93%, 95%, 98%, and 100% of the primary particles satisfy A1 < B1.

[0291] In some embodiments, the first element is manganese, and the content of nickel is higher than that of manganese. In the primary particles of single particles and / or near-single particles, the distribution of cobalt element concentration is more uniform than that of manganese. Due to the strong electrostatic repulsion between Ni²⁺ and Mn⁴⁺, when the nickel content is higher than the manganese content, manganese easily forms manganese-enriched regions. These manganese-enriched regions are inert but stable, leading to uneven local reactions in the primary particles. Therefore, controlling the distribution of cobalt element concentration in at least a portion of the primary particles to be more uniform than that of manganese, ensuring that the manganese-enriched regions contain cobalt, can compensate for insufficient electron conduction in these regions (cobalt has good conductivity, while manganese has poor conductivity), thereby improving the cycle performance of the secondary battery.

[0292] In some embodiments, within a single first particle, the standard deviation of the cobalt element concentration distribution is A1, and the standard deviation of the manganese element concentration distribution is C1; A1 < C1. A lower standard deviation indicates a more uniform distribution of the element within the primary particle. Therefore, by ensuring that the standard deviation of the cobalt element concentration distribution in a single particle and / or a near-single particle is A1, and the standard deviation of the manganese element concentration distribution is C1, with A1 < C1, it can be concluded that the cobalt element concentration distribution in at least some primary particles is more uniform than the manganese element concentration distribution. This ensures that most or all of the nickel-enriched regions contain cobalt, which can suppress oxygen loss caused by excessive delithiation in the manganese-enriched regions and improve cycle performance.

[0293] In some implementations, C1 ranges from 0.5% to 4%. By controlling C1 within this range while ensuring A1 < C1, the nickel concentration in the manganese-rich region is kept at a lower level. This helps reduce manganese enrichment and inertness, resulting in a more uniform reaction of the primary particles and effectively improving the cycle performance and initial discharge capacity of the secondary battery.

[0294] For example, C1 is any value among 0.5%, 0.64%, 0.65%, 0.67%, 0.68%, 0.73%, 0.79%, 0.81%, 0.87%, 0.9%, 1%, 1.5%, 2%, 2.5%, 3%, 3.15%, 3.5%, and 4%, or between any two values.

[0295] In some embodiments, the average standard deviation of the cobalt element concentration in the multiple primary particles is A2, and the average standard deviation of the manganese element concentration is C2, where A2 < C2. A lower average standard deviation indicates a more uniform distribution of the element within the primary particle. Therefore, this application uses a standard deviation of A2 for cobalt element concentration that is less than the average standard deviation of C2 for manganese element concentration. This indicates that the overall distribution of cobalt element concentration in the multiple primary particles is more uniform than that of manganese element concentration. As described above, this allows the manganese-rich region to contain cobalt, compensating for insufficient electron conduction in the manganese-rich region (cobalt has good conductivity, while manganese has poor conductivity), thereby improving the cycle performance of the secondary battery.

[0296] In some implementations, C2 ranges from 0.5% to 4%. Based on A2 < C2, controlling the distribution standard deviation C2 of the manganese element concentration within the above range, and controlling the manganese concentration in the manganese enrichment region to be within a lower range, is beneficial to the reversibility and cycle stability of the structure, while ensuring that the region still contains a considerable proportion of active nickel (and cobalt), thereby resulting in a higher overall reversible capacity of the material.

[0297] For example, C2 is any value among 0.5%, 0.7%, 0.71%, 0.72%, 0.74%, 0.75%, 0.8%, 0.85%, 0.88%, 0.92%, 0.93%, 1%, 1.5%, 2%, 2.5%, 3%, 3.4%, 3.5%, and 4%, or between any two values.

[0298] A third aspect of this application provides a method for preparing a positive electrode active material, comprising: The cobalt source and solid dispersant are dry-mixed to obtain a premix.

[0299] The premix, nickel source, first element source, lithium salt, flux, and organic carbon source are dry-milled and mixed to obtain a first mixed powder.

[0300] The first mixed powder and the remaining premix are dry-milled and mixed to obtain the second mixed powder.

[0301] The second mixed powder was sintered at 750℃-1000℃ in an oxygen-containing atmosphere to obtain a lithium-containing transition metal oxide. Among them, the nickel source, cobalt source and the first element source are all hydroxides and / or oxides; the first element is manganese and / or aluminum, the content of nickel is higher than that of cobalt, the volume ratio of oxygen in the oxygen-containing atmosphere is ≥90%, and the solid dispersant includes at least one of citric acid, trisodium phosphate, sodium silicate and magnesium carbonate.

[0302] The dry preparation method refers to the mechanical mixing of nickel source, cobalt source, first element source, lithium source and other additives, followed by direct high-temperature solid-state sintering, which causes chemical reactions between solid particles to directly generate lithium-containing transition metal oxides.

[0303] In this application, the positive electrode active material is obtained by dry grinding and mixing the raw materials step by step and then sintering them directly in an oxygen-containing atmosphere. That is, the preparation process of the positive electrode active material in this application does not involve the use of solvent chemistry to prepare the precursor. In other words, the lithium-containing transition metal oxide is prepared by dry method in this application.

[0304] In dry preparation, since the reaction starts from a solid mixed powder and the nucleation sites are dispersed, it is easier for atoms to form independent grains when they directly react and nucleate during sintering through solid-phase diffusion. In addition, the high resistance of solid-phase mass transfer kinetics in high-temperature solid-phase sintering makes grain growth take precedence over grain merging, which is ultimately conducive to the formation of single particles and / or quasi-single particles of lithium-containing transition metal oxides.

[0305] The nickel source, cobalt source, and first element source are each independently a hydroxide and / or oxide. Taking the nickel source as an example, it can be nickel oxide and / or nickel hydroxide, and the cobalt source can be cobalt oxide and / or cobalt hydroxide. Both the nickel and cobalt sources can be oxides, or one can be an oxide and the other a hydroxide, depending on the actual needs. By using nickel, cobalt, and the first element source as hydroxides and / or oxides, a dry process can be used to prepare lithium-containing transition metal oxides. It is understood that the nickel source, cobalt source, and first element source exist independently and are mixed using a dry grinding method.

[0306] In the preparation method adopted in this application, a uniformly distributed premix is ​​obtained by dry mixing the cobalt source and the solid dispersant in advance. Then, a portion of the premix is ​​dry-ground and mixed with other raw materials. The resulting first mixed powder and the remaining premix are then further dry-ground and mixed. This facilitates the full and more uniform dispersion of the cobalt source in the final second mixed powder. At the same time, the distribution mixing and solid dispersant also facilitate the relatively uniform dispersion of the nickel source and the first element source in the final second mixed powder. Thus, after subsequent sintering, it is beneficial to obtain a more uniform distribution of cobalt element concentration in most or all (at least 90% of the primary particles) of single particles and / or near-single particles than in the distribution of nickel element concentration.

[0307] In this application, a solid dispersant is used, including at least one of citric acid, trisodium phosphate, sodium silicate, and magnesium carbonate, which has a good dispersing effect.

[0308] The preparation method provided in this application uses nickel, cobalt, and the first element source, all of which are hydroxides and / or oxides, to dry prepare and obtain lithium-containing transition metal oxides, including single particles and / or near-single particles. These single-particle and / or near-single-particle structures exhibit high stability. Because the nickel content is higher than the cobalt content, the resulting lithium-containing transition metal oxide-based battery system has high energy density. However, nickel kinetics are lower than cobalt kinetics, so when the nickel content is higher than the cobalt content, nickel-rich regions are easily formed. These nickel-rich regions are highly active but unstable, leading to uneven local reactions. Therefore, by pre-mixing the cobalt source and the solid dispersant using a dry method, a uniform distribution is obtained. The premix is ​​then partially dry-milled and mixed with other raw materials. The resulting first mixed powder and the remaining premix are then further dry-milled and mixed. This process facilitates the full and more uniform dispersion of the cobalt source in the final second mixed powder. Simultaneously, the distribution mixing and solid dispersant also help the nickel source and the first element source to be more uniformly dispersed in the final second mixed powder. As a result, after subsequent sintering, it is beneficial to obtain at least some of the primary particles in single particles and / or near-single particles where the distribution of cobalt element concentration is more uniform than that of nickel element concentration. This allows the nickel-enriched region to contain cobalt element, which can suppress oxygen loss caused by excessive delithiation in the nickel-enriched region and improve cycle performance.

[0309] In some implementations, the molar percentage of nickel is 80%-95% based on the total molar amount of nickel in the nickel source, cobalt in the cobalt source, and the first element in the first element source, and the sintering temperature is 750℃-830℃.

[0310] For example, based on the total molar amount of nickel in the nickel source, cobalt in the cobalt source, and the first element in the first element source, the molar percentage of nickel is any one of 80%, 82%, 85%, 88%, 90%, 92%, or 95%, or between any two of these values, and the sintering temperature is any one of 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, 810℃, 820℃, or 830℃, or between any two of these values.

[0311] In some implementations, when the molar percentage of nickel is greater than 60% and less than 80% based on the total molar amount of nickel in the nickel source, cobalt in the cobalt source, and the first element in the first element source, the sintering temperature is 830℃-870℃.

[0312] For example, based on the total molar amount of nickel in the nickel source, cobalt in the cobalt source, and the first element in the first element source, the molar percentage of nickel is any one of 61%, 65%, 68%, 70%, 73%, 75%, 78%, 79%, or between any two values, and the sintering temperature is any one of 830℃, 835℃, 840℃, 845℃, 850℃, 855℃, 860℃, 870℃ or between any two values.

[0313] In some implementations, the molar percentage of nickel is 50%-60% based on the total molar amount of nickel in the nickel source, cobalt in the cobalt source, and the first element in the first element source, and the sintering temperature is 850℃-1000℃.

[0314] For example, based on the total molar amount of nickel in the nickel source, cobalt in the cobalt source, and the first element in the first element source, the molar percentage of nickel is any value of 50%, 53%, 55%, 57%, 60%, or between any two values, and the sintering temperature is any value of 850℃, 855℃, 860℃, 870℃, 880℃, 890℃, 900℃, 910℃, 920℃, 935℃, 940℃, 950℃, 955℃, 960℃, 965℃, 970℃, 980℃, 988℃, 990℃, and 1000℃, or between any two values.

[0315] Selecting the appropriate sintering temperature based on different nickel contents is beneficial for obtaining lithium-containing transition metal oxides. In some embodiments, the lithium-containing transition metal oxides also contain doping elements. In this case, in the preparation method of the positive electrode active material, the nickel source, cobalt source, first element source, lithium salt, doping element source, organic carbon source, solid dispersant, and flux are dry-milled and mixed according to the molar ratio of each element in the lithium-containing transition metal oxide to obtain a mixed powder.

[0316] In some implementations, the mass ratio between a portion of the premix and the remainder of the premix is ​​0.5:1 to 1.5:1.

[0317] Controlling the mass ratio between the premixed portion and the remaining premixed portion to be 0.5:1-1.5:1 is beneficial for thorough and uniform mixing, and for preparing primary particles in which the distribution of cobalt element concentration is more uniform than that of nickel element concentration in at least a portion of the primary particles in single particles and / or near-single particles.

[0318] For example, the mass ratio between a portion of the premix and the remaining premix is ​​any one of 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1.0:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1 or between any two values.

[0319] In some embodiments, the volumetric particle size distribution Dv50 of the cobalt source is 3 μm to 5 μm. Exemplarily, the volumetric particle size distribution Dv50 of the cobalt source is any value of 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or between any two values.

[0320] In some implementations, the organic carbon source accounts for 0.02%-0.3% of the mass of the mixed powder.

[0321] Furthermore, by controlling the amount of organic carbon source added, and utilizing its full decomposition during the subsequent high-temperature sintering process in an oxidizing atmosphere, pores with smaller average pore sizes are introduced into the interior of some primary particles containing lithium transition metal oxide particles and / or quasi-single particles. This can minimize the loss of material capacity while maintaining a low discharge DCR, enabling the secondary battery to achieve a high initial discharge capacity.

[0322] For example, the mass percentage of the organic carbon source in the mixed powder is any one of 0.02%, 0.05%, 0.07%, 0.10%, 0.12%, 0.15%, 0.17%, 0.20%, 0.22%, 0.25%, 0.27%, or 0.30%, or between any two of these values.

[0323] Optionally, the organic carbon source accounts for 0.1%-0.2% of the mass of the mixed powder.

[0324] By optimizing the mass ratio of organic carbon sources in the mixed powder, the pore size and average pore size can be further optimized, which helps the secondary battery to have high energy density and low discharge DCR.

[0325] In some embodiments, the organic carbon source includes at least one of glucose, sucrose, and polyethylene glycol.

[0326] Controlling the organic carbon source, including the aforementioned organic carbon source, is beneficial for decomposition and the formation of the aforementioned pores within the primary particles.

[0327] In some implementations, the sintering time is 8-15 hours.

[0328] Dry grinding and mixing methods include, but are not limited to, ball milling and sand milling.

[0329] In some implementations, the dry grinding and mixing method is ball milling.

[0330] The ball milling parameters can be selected according to actual needs. For example, the ball milling speed is 300 rpm-1000 rpm and the ball milling time is 30 min-12 h.

[0331] In some embodiments, the lithium salt includes, but is not limited to, lithium hydroxide and / or lithium carbonate.

[0332] In some implementations, the amount of solid dispersant added is 0.1%-2.3% by mass of the total mass of the mixed powder.

[0333] For example, the added mass of solid dispersant is any one of 0.1%, 0.2%, 0.5%, 0.7%, 1.0%, 1.2%, 1.5%, 1.7%, 2.0%, 2.1%, 2.2%, 2.3% or between any two of these values, based on the total mass of the mixed powder.

[0334] By controlling the addition quality of the solid dispersant within the above range, the dispersion effect is excellent, while suppressing the deterioration of the performance of lithium-containing transition metal oxides due to the introduction of impurities.

[0335] It is understood that the flux includes at least one of strontium oxide, calcium oxide, magnesium oxide, calcium fluoride, and boric acid. The aforementioned flux includes an oxophilic element; for example, strontium oxide includes the oxophilic element strontium, calcium oxide includes the oxophilic element calcium, magnesium oxide includes the oxophilic element magnesium, calcium fluoride includes the oxophilic element calcium, and boric acid includes the oxophilic element boron.

[0336] In some embodiments, the flux includes an oxyphilic element, and the amount of flux added is controlled such that the total content of the oxyphilic element in the lithium-containing transition metal oxide is 500 ppm to 5000 ppm.

[0337] It is understandable that when there are multiple fluxes, such as strontium oxide and calcium oxide, the total content of oxyphilic elements in lithium-containing transition metal oxides refers to the sum of the contents of calcium and strontium in lithium-containing transition metal oxides. When there is only one flux, such as strontium oxide, the total content of oxyphilic elements in lithium-containing transition metal oxides refers to the content of strontium in lithium-containing transition metal oxides.

[0338] For example, the amount of flux added is controlled such that the content of the oxyphilic element in the lithium-containing transition metal oxide is any one of 500 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, or 5000 ppm, or between any two values. Controlling the amount of flux added within the above range is beneficial for promoting solid-phase reactions between raw materials and reducing the introduction of impurities.

[0339] Understandably, the sintering temperature of lithium-containing transition metal oxides is usually 750℃-1000℃, and the specific temperature can be selected according to the nickel content.

[0340] The doping element includes at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce and La, and the doping element source includes oxides and hydroxides of the doping element.

[0341] By adopting the above-mentioned settings, the doping of lithium-containing transition metal oxides with the aforementioned doping elements is beneficial to improving the structural stability and / or thermal stability of lithium-containing transition metal oxides and enhancing the cycle performance of secondary batteries.

[0342] Understandably, the doping element differs from the primary element. For example, the main elements of lithium-containing transition metals are nickel, cobalt, and manganese, and the doping element includes at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce, and La. (This is repeated three times in the original text.)

[0343] In some embodiments, the preparation method further includes: mixing the lithium-containing transition metal oxide and the second element oxide obtained after sintering, and sintering them at 400°C-600°C in an oxygen-containing atmosphere to form a coating layer rich in the second element on the surface of the lithium-containing transition metal oxide, wherein the second element includes at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce and La.

[0344] The physical barrier of the coating layer isolates the lithium-containing transition metal oxide from the electrolyte, effectively suppressing interfacial side reactions. At the same time, the coating layer is rich in a second element, which can suppress cobalt dissolution and optimize cycle performance.

[0345] For example, the lithium-containing transition metal oxide and the second element oxide obtained after sintering are mixed and sintered in an oxygen-containing atmosphere at any temperature of 400°C, 420°C, 450°C, 470°C, 500°C, 530°C, 550°C, 580°C, or 600°C, or between any two of these temperatures.

[0346] The content of the second element in the coating layer is 1000 ppm - 5000 ppm, and the amount of the second element oxide added can be selected according to the content of the second element.

[0347] In some embodiments, the preparation method further includes: mixing the lithium-containing transition metal oxide obtained after sintering with a cobalt coating source (cobalt hydroxide and / or cobalt oxide), and sintering at 700°C-800°C in an oxygen-containing atmosphere to form a cobalt-rich coating layer on the surface of the lithium-containing transition metal oxide.

[0348] By introducing a coating layer, the contact between lithium-containing transition metal oxides and the electrolyte is isolated, effectively suppressing interfacial side reactions. At the same time, the coating layer is rich in cobalt, resulting in high surface kinetics of the positive electrode active material.

[0349] For example, the lithium-containing transition metal oxide obtained after sintering is mixed with a cobalt-coated source and sintered in an oxygen-containing atmosphere at any temperature of 700°C, 720°C, 750°C, 770°C, or 800°C, or between any two of these temperatures.

[0350] The cobalt content in the coating layer is 5000 ppm to 15000 ppm. The amount of cobalt oxide added can be selected according to the cobalt content.

[0351] In some embodiments, the preparation method further includes: first mixing the lithium-containing transition metal oxide obtained after sintering with a cobalt source (cobalt hydroxide and / or cobalt oxide), sintering at 700°C-800°C in an oxygen-containing atmosphere, then mixing the sintered material with a second element oxide, sintering at 400°C-600°C in an oxygen-containing atmosphere, and finally forming a coating layer rich in a second element and cobalt on the surface of the lithium-containing transition metal oxide, wherein the second element includes at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce and La.

[0352] A third aspect of this application provides an electrical device that includes the secondary battery provided in the first aspect of this application. Furthermore, the secondary battery and electrical device of this application will be described below with appropriate reference to the accompanying drawings.

[0353] The fourth aspect of this application provides an electrical device that includes the secondary battery provided in the first aspect of this application.

[0354] [Rechargeable Battery] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0355] This application does not impose any particular restriction on the type of electrolyte, which can be selected according to actual needs. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions). This applies to secondary batteries using electrolyte solutions, as well as some secondary batteries using solid electrolytes.

[0356] [Positive electrode plate] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The arrangement of the positive electrode film layer is as shown in the arrangement in the secondary battery provided in the first aspect of this application. Exemplarily, the positive electrode film layer includes the positive electrode active material of the second aspect of this application.

[0357] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0358] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0359] [Negative electrode plate] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector.

[0360] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0361] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0362] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material may be any negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0363] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0364] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0365] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0366] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0367] [Electrolytes] The electrolyte plays a role in conducting ions between the positive and negative electrode plates.

[0368] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0369] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0370] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0371] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0372] [Isolation membrane] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0373] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0374] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0375] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0376] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0377] A secondary battery is a battery cell that can be recharged after it has been discharged, allowing the active materials to be activated and the battery to continue to be used.

[0378] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 The example shown is a square-structured battery cell 5.

[0379] In some implementations, refer to Figure 3 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0380] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and specific capacity of the battery module.

[0381] Figure 4 This is battery module 4, used as an example. (See reference...) Figure 4 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0382] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0383] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0384] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0385] In addition, this application also provides an electrical device, which includes a secondary battery (at least one of a battery cell, battery module, or battery pack) provided in this application. The secondary battery can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0386] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0387] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0388] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0389] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0390] Example 1 Preparation of positive electrode active materials (1) The cobalt source Co3O4 (Dv50 is 3.7 μm, BET is 5 μm) 2 The mixture of citric acid (98% purity) and solid dispersant (g) was stirred to obtain a premix.

[0391] (2) Add 1 / 2 mass of the premix and the nickel source Ni(OH)2 (Dv50 is 5μm, BET is 5m) 2 / g), the first element source MnO2 (Dv50 is 4μm, BET is 6m) 2 / g, purity 99%; lithium carbonate (Dv50 6μm, BET 3μm) 2 / g, purity 99.7%), organic carbon source glucose (purity 95%), and flux strontium oxide (Dv50 0.5μm, BET 4m) 2 / g) is dry ball milled in a high-energy ball mill to obtain a first mixed powder, wherein the ball milling speed is 500 rpm and the ball milling time is 5 h.

[0392] (3) The remaining 1 / 2 mass of the premix and the first mixed powder are dry ball milled in a high-energy ball mill, wherein the ball milling speed is 500 rpm and the ball milling time is 5 h, to obtain the second mixed powder.

[0393] In the second mixed powder, the nickel source, cobalt source, first element source, and lithium salt are present in a molar ratio of Ni, Co, Mn, and Li of 6:1:3:10.5. The organic carbon source glucose has a mass content of 0.1% in the second mixed powder. The solid dispersant has a mass content of 2% in the second mixed powder.

[0394] (4) The second mixed powder is placed in a roller kiln and sintered at 930°C for 8 hours in a mixed atmosphere of nitrogen and oxygen (oxygen volume ratio of 90%) to obtain the first lithium-containing transition metal oxide.

[0395] (5) In a high-energy ball mill, the first lithium-containing transition metal oxide obtained in step (4) is mixed and ball-milled with cobalt hydroxide and lithium hydroxide (wherein the cobalt hydroxide and lithium hydroxide are weighed according to the molar ratio of lithium element in lithium hydroxide to cobalt element in cobalt hydroxide is 2:1) to obtain a third mixed powder. The ball milling speed is 400 rpm and the ball milling time is 8 h.

[0396] (6) The second mixed powder is placed in a roller kiln and sintered at 750°C for 8 hours in a mixed atmosphere of nitrogen and oxygen (oxygen volume ratio of 80%) to obtain a second lithium-containing transition metal oxide.

[0397] (7) In a high-energy ball mill, the product obtained in step (4) is mixed and ball-milled with tungsten oxide to obtain a third mixed powder, wherein the ball milling speed is 400 rpm and the ball milling time is 5 h.

[0398] (8) The third mixed powder is placed in a rotary kiln and sintered at 500°C for 6 hours in a mixed atmosphere of nitrogen and oxygen (oxygen volume ratio of 80%) to obtain the third lithium-containing transition metal oxide. The third lithium-containing transition metal oxide consists of lithium-containing transition metal oxide and a coating layer on the surface (hereinafter, the third lithium-containing transition metal oxide is simply referred to as lithium-containing transition metal oxide, where the coating layer is part of the composition of lithium-containing transition metal oxide).

[0399] The lithium-containing transition metal oxide obtained in step (8) was used as a sample. The sample was digested with aqua regia and hydrofluoric acid HF. The completely digested solution was subjected to ICP testing. The sulfur content in the material was 200 ppm, the sodium content was 50 ppm (due to the impurities of the raw materials such as nickel, cobalt, and manganese not being 100%, some impurities were present, which contained sulfur and sodium, which were introduced during the sintering process and were not completely removed), and the Sr content in the flux was 500 ppm. Assuming the total molar amount of nickel, cobalt, and manganese is 100%, the molar proportion of nickel is about 60%, the molar proportion of cobalt is about 10%, and the molar proportion of manganese is about 30% (based on the combined effect of inherent variations in the production process and inherent errors in the analysis and testing process, the amount of feed and the ICP test data have certain fluctuations. Here, the ICP test fluctuation is ≤0.3%, which is basically consistent with the amount of feed).

[0400] Preparation of the positive electrode sheet The above-mentioned positive electrode active material, binder polyvinylidene fluoride, and conductive agent acetylene black were mixed in a weight ratio of 95:2.5:2.5 and dissolved in the solvent N-methylpyrrolidone (NMP) to prepare a positive electrode slurry. The mixture was stirred under vacuum until homogeneous to obtain the positive electrode slurry. The positive electrode slurry was then uniformly coated onto both surfaces of the positive electrode current collector aluminum foil. After drying, it underwent cold pressing, edge trimming, cutting, and slitting processes to obtain the positive electrode sheet. The areal density of the single-sided positive electrode film was 0.27 g / 1540.25 mm². 2 The compaction density of the single-sided positive electrode film is 3.45 g / cm³. 3 .

[0401] Preparation of the negative electrode sheet A negative electrode slurry was prepared by mixing artificial graphite (anode active material), acetylene black (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC-Na) (thickener) in a weight ratio of 92:3:3:2 and dissolving them in deionized water. The slurry was then coated onto copper foil (current collector), dried, and subsequently cold-pressed, trimmed, cut, and slit to produce the negative electrode sheet for the secondary battery. The areal density of the single-sided negative electrode film was 0.168 g / 1540.25 mm². 2 The compaction density of the single-sided negative electrode film is 1.65 g / cm³. 3 .

[0402] Preparation of Electrolyte Preparation of electrolyte: In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed uniformly at a volume ratio of 3:3:4 as an organic solvent. Then, LiPF6 is uniformly dissolved in the above solution. The concentration of LiPF6 in the electrolyte is 1mol / L.

[0403]

Isolation Film

[0404] [Battery Manufacturing] Preparation of secondary battery: The above positive electrode sheet, separator and negative electrode sheet are stacked and wound in sequence to obtain electrode assembly. The electrode assembly is placed in aluminum shell, the electrolyte prepared above is added, and after encapsulation, standing, formation and aging processes, the battery is obtained.

[0405] [Positive Electrode Parameter Test] (1) Slicing: Take the discharged secondary battery, disassemble it and take out the positive electrode sheet. Soak it in DMC (ethylene carbonate) for 4 hours to remove the residual electrolyte and lithium salt. Then dry it in an oven to obtain the processed positive electrode sheet. Cut the positive electrode film layer of the positive electrode sheet along the thickness direction of the electrode sheet with an argon ion beam (equipment model: Leica EM TIC 3X CP, working voltage: 6kV, working time: 6h). After exposing the cut surface, use a scanning electron microscope (equipment model: Hitachi SU8230, working voltage: 3kV, beam current: high, probe model: U (LA100), working distance <5mm) to observe the cut surface of the positive electrode film layer along the thickness direction of the electrode sheet and obtain the electron microscope image of the cut surface of the positive electrode film layer along the thickness direction of the positive electrode sheet.

[0406] (2) Identification of primary particles and pores in single particles and / or single-particle-like particles: Images were acquired in secondary electron mode at non-edge positions in the cross-section of the positive electrode film using a field emission scanning electron microscope (after observing the edge of the electrode under the scanning electron microscope, the field of view was adjusted to the center of the sample). Electron micrographs were taken at 3000x magnification. The primary particles and pores in the single particles and / or single-particle-like particles in the electron micrographs were analyzed using ImageJ software (1.46r, win64 version). Primary particles that were not identified by the software, were not fully identified by the software, or had errors in identification were manually identified. This completed the identification and marking of primary particles and pores in single particles and / or single-particle-like particles in the images.

[0407] (3) Determination of the average particle size of primary particles: The five cross-sectional images after the primary particle identification and labeling were imported into ImageJ software for analysis. The scale was set according to the scanning electron microscope images. The particle size of primary particles in the cross-section along the thickness direction of the positive electrode film was analyzed using the "Feret diameter", "Area", "Round" and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained from the analysis represents the maximum distance between any two points on the outer periphery (outline) of the two-dimensional projection of the primary particle, which is used to characterize the particle size of the primary particle. The particle size of all labeled primary particles in the five cross-sections and the total number of labeled primary particles were counted as M1. The sum of the particle sizes of all labeled primary particles was A. The average particle size of the primary particles = A / M1.

[0408] The test results are as follows: Under the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the average particle size of the primary particles is 2.2 μm.

[0409] (4) Average pore diameter: The five cross-sectional images after the primary particle identification and labeling were imported into ImageJ software for analysis. ImageJ software was used to label primary particles with pores inside. The maximum distance between any two points on the outer periphery (outline) of the pore was taken as the pore diameter. The sum of the pore diameters B of all labeled primary particles in the five cross-sections and the total number of labeled pores M2 were calculated. The average particle diameter of the pores = B / M2.

[0410] The test results show that the average pore size of the pores in the primary particles containing pores in the positive electrode film is 235 nm.

[0411] (5) Average perimeter of pores in primary particles: After the primary particles are identified and labeled, the five cross-sectional images are imported into ImageJ software for analysis. ImageJ software is used to label primary particles with pores inside, and the pores are analyzed to obtain the perimeter of a single pore. Following the steps (1) and (2) above, five cross-sections are selected, and the perimeter of pores in all labeled primary particles in the five cross-sections is counted. The total number of labeled pores is M3, and the sum of the perimeters of all labeled pores is C. The average perimeter of the pores = C / M3.

[0412] The test results show that the average perimeter of the pores in the primary particles containing pores in the positive electrode film is 872 nm.

[0413] (6) Pore wall roughness of primary particles: The five cross-sectional images obtained after the primary particle identification and labeling were imported into ImageJ software for analysis. The "shape description" analysis function in ImageJ was used to analyze the morphology of the pore walls in the primary particles. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained from the analysis represents the ratio of the pixel area to the convex area of ​​the pore wall in the primary particle. Therefore, the "Solidity" parameter of the particle obtained from the analysis is used to characterize the roughness of the pore wall in the primary particle. Following the steps (1) and (2) above, five cross-sections were selected. Among all the primary particles with pores in the cross-sectional view, the pore wall roughness of the pores was arranged in ascending order. The cumulative distribution curve of the roughness area of ​​the pore wall in the primary particles in the positive electrode film layer was obtained with roughness as the horizontal axis and cumulative area ratio as the vertical axis. R A50 This is the roughness R value when the cumulative area ratio of the vertical axis in the cumulative distribution curve of the roughness R value is 50%.

[0414] The test results are as follows: In the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median R1 of the pore wall roughness is calculated based on the cumulative proportion of the pore area. A50 It is 0.965.

[0415] (7) Pore wall roundness of primary particles: The five cross-sectional images obtained after the primary particles were identified and labeled were imported into ImageJ software for analysis. The "shape description" analysis function in ImageJ was used to analyze the morphology of the pore walls in the primary particles. According to the software manual (ImageJ User Guide IJ 1.46r), the "Area" parameter obtained from the analysis represents the pixel area of ​​the pore, and the "Round" parameter represents the ratio of the pixel area of ​​the pore to the area of ​​a circle with the fitted major axis as the diameter. Therefore, the "Round" parameter of the particle obtained from the analysis is used to characterize the roundness of the pore walls in the primary particles. Following the steps (1) and (2) above, five cross-sections were selected. Among all the labeled primary particles with pores in the cross-sectional view, the roundness of the pore walls was arranged in ascending order. The cumulative distribution curve of the roundness area of ​​the pore walls of the primary particles in the positive electrode film was obtained with roundness as the horizontal axis and the cumulative area ratio as the vertical axis. L A50 The L-value represents the roundness L value when the cumulative area along the vertical axis of the cumulative distribution curve of roundness L is 50%.

[0416] The test results are as follows: In the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, based on the cumulative proportion of the pore area, the median L1 of the pore wall roundness is... A50It is 0.794.

[0417] (8) The percentage of primary particles with internal pores and the percentage of the total area of ​​pores based on the total area of ​​primary particles: Select five cross-sections according to the steps (1) and (2) above, and count the total number of primary particles D and the number of primary particles with internal pores E in each of the five cross-sections. E / D represents the percentage of primary particles with internal pores based on the total number of primary particles. The total area of ​​primary particles refers to the sum of the areas of all primary particles marked in the cross-section under the electron microscope image. The total area of ​​pores refers to the sum of the areas of all pores marked in the cross-section. Calculate the percentage of the total area of ​​pores based on the total area of ​​primary particles.

[0418] The test results are as follows: In the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the proportion of primary particles with pores is 2.9% based on the total number of primary particles.

[0419] Based on the total area of ​​the particles, the area of ​​the pores accounts for 0.34% of the total area.

[0420] (9) Area ratio of the first and second particles in the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet: After step (2) above, the five cross-sectional images after the primary particle identification and labeling are imported into ImageJ software for analysis. The scale is set according to the scanning electron microscope image. The particle size of the primary particles in the cross-section along the thickness direction of the positive electrode film is analyzed by the analysis functions of "Feret diameter", "Area", "Round" and "Solidity". According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained by the analysis represents the maximum distance between any two points on the outer periphery (outline) of the two-dimensional projection of the primary particle, which is used to characterize the particle size of the primary particle. 1μm≤D1≤3μm is recorded as the first particle, and 3μm<D2≤10μm is recorded as the second particle. The total area of ​​the first particle and the total area of ​​the second particle are obtained respectively, and the ratio of the total area of ​​the first particle to the total area of ​​the second particle is obtained.

[0421] The test results are as follows: In the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the area ratio of the first particle to the second particle is 0.32:1.

[0422] (10) Ratio of the number of first and second particles: After step (2) above, the five cross-sectional images of the primary particles after identification and labeling are imported into ImageJ software for analysis. The scale is set according to the scanning electron microscope image. The particle size of the primary particles in the cross-section along the thickness direction of the positive electrode film is analyzed by the analysis functions of "Feret diameter", "Area", "Round" and "Solidity". According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained by the analysis represents the maximum distance between any two points on the outer periphery (outline) of the two-dimensional projection of the primary particle, which is used to characterize the particle size of the primary particle. 1μm≤D1≤3μm is recorded as the first particle, and 3μm<D2≤10μm is recorded as the second particle. The number of the first particles and the number of the second particles in the five cross-sections are counted respectively to obtain the ratio of the total number of the first particles to the total number of the second particles.

[0423] The test results are as follows: In the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the ratio of the number of the first particle to the number of the second particle is 1.8:1.

[0424] (11) Sphericity of primary particles: The images after primary particle identification and labeling were imported into ImageJ software for analysis. The "Shape Description" and "Area" analysis functions in ImageJ were used to analyze the morphology and area of ​​primary particles in the cross-section along the thickness direction of the cathode film. According to the software manual (ImageJ User Guide IJ 1.46r), the "Area" parameter obtained from the analysis represents the pixel area of ​​the primary particle, and the "Round" parameter represents the ratio of the pixel area of ​​the primary particle to the area of ​​a circle with the fitted major axis as the diameter. The "Round" parameter of the primary particle is used to characterize the sphericity of the primary particle. The sphericity of all identified primary particles in the five cross-sectional views is arranged in ascending order. The cumulative distribution curve of the sphericity area of ​​primary particles in the cathode film is obtained with sphericity as the horizontal axis and cumulative area ratio as the vertical axis. L A50 This is the L-value of sphericity when the cumulative area under the vertical axis of the cumulative distribution curve of L-values ​​accounts for 50%.

[0425] The test results are as follows: Based on the cumulative area ratio of primary particles, the median L2 of the spheroidality of the primary particles is calculated from the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet. A50 It is 0.73.

[0426] (12) Roughness of primary particles: The images after primary particle identification and labeling were imported into ImageJ software for analysis. The "Shape Description" analysis function in ImageJ was used to analyze the morphology of primary particles in the cross-section along the thickness direction of the positive electrode film. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained from the analysis represents the ratio of the pixel area to the convex area of ​​the primary particle. The obtained "Solidity" parameter of the primary particle characterizes the roughness of the particle. The roughness of all identified primary particles in the five cross-sectional views was arranged in ascending order. The cumulative distribution curve of the roughness area of ​​primary particles in the positive electrode film was obtained with roughness as the horizontal axis and cumulative area ratio as the vertical axis. A50 This is the roughness R value when the cumulative area ratio of the vertical axis in the cumulative distribution curve of the roughness R value is 50%.

[0427] The test results are as follows: Under the cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median roughness R² of the primary particles is calculated based on the cumulative area ratio of the primary particles. A50 It is 0.92.

[0428] (13) Compacted density of the positive electrode film: Cut the positive electrode sheet into small circular pieces with an area of ​​S, obtain its mass as W1, and measure the thickness T1 of the positive electrode sheet using a micrometer. Then wipe off the positive electrode film of the weighed electrode sheet, weigh the current collector and record it as W2, and measure the thickness T2 of the current collector using a micrometer. Then the compacted density PD of the positive electrode film is (W1) / (W2) = W1 ... W2) / [(T1-T2)×S。

[0429] The test results are as follows: after 5 repetitions (excluding the highest and lowest values, the average value was calculated), the compaction density of the positive electrode film is 3.45 g / cm³. 3 .

[0430] (14) One-sided surface density of the positive electrode film: First, wipe off the positive electrode film on one side of the positive electrode sheet, cut it into a small circular piece with an area of ​​S1, weigh it, and record its weight as M1. Then wipe off the positive electrode film of the positive electrode sheet after weighing, weigh the current collector, and record it as M0. One-sided surface density of the positive electrode film = (M1-M0) / S1.

[0431] The test results are as follows: After 5 repetitions (excluding the highest and lowest values, the average value was calculated), the areal density of the positive electrode film is 0.27 g / 1540.25 mm. 2 .

[0432] (15) In the cross section obtained in step (1), the cross section of the positive electrode active material is located through the grain boundary of the lithium-containing transition metal oxide under the backscattered electron (BSE) image of the scanning electron microscope (SEM) of the EPMA to obtain the interface position between the coating layer and the lithium-containing transition metal oxide. Then, the line scan analysis of the coating layer region and the lithium-containing transition metal oxide is performed by the wave spectrometer (WDS) in the EPMA to obtain the element content in the coating layer and the doping element content in the lithium-containing transition metal oxide, respectively.

[0433] The test results showed that the cobalt content in the coating layer was 8000 ppm, and the tungsten content was 3000 ppm.

[0434] (16) Standard deviation of element concentration distribution of nickel, cobalt and first element in a single primary particle: Select five cross sections at 3000x magnification at different positions along the thickness direction of the positive electrode film layer, and count the average particle size of the primary particles in the five cross sections. Then select a primary particle with an average particle size of ±20% in the five cross sections to test the standard deviation of element concentration distribution of nickel, cobalt and first element in a single primary particle.

[0435] Methods for testing the standard deviation of elemental concentration distribution in primary particles include: such as... Figure 1 As shown, under a 3000x electron microscope image of the cross-section of the positive electrode film along the thickness direction of the positive electrode sheet, EDS was used to scan the cross-section of the identified primary particles. A 200nm depth region from the surface of the primary particle was used as the boundary to exclude interference from the surface coating layer. Then, four positions were taken at equal intervals along any line from the center of the primary particle cross-section (in this application, the center of the primary particle cross-section refers to the center of the smallest circumcircle of the projected outline of the primary particle cross-section) to this boundary (from the inside out, these positions were used as spectra 8, 9, 10, and 11). Figure 7 Spectrum Figure 6 and spectrum Figure 5 The content of the same element (nickel, cobalt, and the first element) at each location was obtained. Then, the sample standard deviation of the element concentration in that primary particle was calculated based on the element concentration at the four locations as the distribution standard deviation, thus obtaining the distribution standard deviation of the element concentration in a single primary particle. It should be noted that... Figure 1 A point scan diagram illustrating the standard deviation test method for the distribution of elemental concentrations in particles, performed only once.

[0436] The test results are as follows: In a single primary particle, the standard deviation of the cobalt element concentration distribution A1 is 0.8%, the standard deviation of the cobalt element concentration distribution B1 is 3.1%, and the standard deviation of the manganese element concentration distribution C1 is 0.64%.

[0437] (17) The test method for the average distribution standard deviation of the element concentrations of nickel, cobalt and the first element in multiple primary particles includes: randomly selecting 5 cross-sections at different positions along the thickness direction of the positive electrode film, randomly selecting 5 primary particles with an average particle size of ±20% in different regions of each cross-section as samples, and testing the distribution standard deviation of the element concentrations of nickel, cobalt and the first element in each sample according to (15), and then calculating the average distribution standard deviation of the same element in the 25 primary particle samples selected in the 5 cross-sections, where 2 The sum of the standard deviations of the nickel element concentration distribution in 5 primary particle samples is N3, and the average standard deviation of the nickel element concentration distribution in multiple primary particles is N3 / 25. Similarly, the sum of the standard deviations of the cobalt element concentration distribution in 25 primary particle samples is N4, and the average standard deviation of the cobalt element concentration distribution in multiple primary particles is N4 / 25. Similarly, the sum of the standard deviations of the first element concentration distribution in 25 primary particle samples is N5, and the average standard deviation of the first element concentration distribution in multiple primary particles is N5 / 25.

[0438] Test results: Among multiple primary particles, the average standard deviation of the cobalt element concentration was A2 of 0.9%, the average standard deviation of the nickel element concentration was B2 of 3.3%, and the average standard deviation of the manganese element concentration was C2 of 0.72%.

[0439] Example 2-11 The differences between the various embodiments are shown in Table 1.

[0440] The difference between Example 2 and Example 1 is that the amount of glucose added in step (1) is 0.05%, the amount of flux strontium oxide added is 2000ppm, the amount of other raw materials added remains unchanged, and the sintering conditions in step (2) are different as shown in Table 1.

[0441] The only difference between Examples 3-4 and Example 2 is that the amount of glucose added in step (1) is different as shown in Table 1, while the addition of other raw materials remains unchanged.

[0442] The only difference between Example 5 and Example 3 is that titanium oxide is added in step (1) so that the Ti element content in the first mixed powder is 1500 ppm as shown in Table 1, while the amount of other raw materials added remains unchanged.

[0443] The only difference between Examples 6-9 and Example 3 is that the sintering conditions in step (2) are different as shown in Table 1. The only difference between Examples 6-7 and Example 3 is that the sintering conditions in step (2) are different as shown in Table 1. The only difference between Examples 8-9 and Example 3 is that the ratio of nickel, cobalt and manganese in step (1) is different, the amount of glucose added is different, the amount of flux strontium oxide added is different, and the sintering conditions in step (2) are different as shown in Table 1.

[0444] The difference between Examples 8-9 and Example 1 is only that: in step (1), the nickel source, cobalt source, and first element source are weighed according to the molar ratio of Ni, Co, and Mn elements of 5:2:3, and the amount of flux, organic carbon source added and the sintering conditions in step (2) are different as shown in Table 1.

[0445] The only difference between Example 10 and Example 3 is that in step (1), the nickel source Ni(OH)2 (Dv50 is 5 μm, BET is 5 μm) is used. 2 / g), cobalt source Co3O4 (Dv50 is 3.7μm, BET is 5 μm) 2 / g), the first element source Al2O3 (Dv50 is 4μm, BET is 7m) 2 / g, purity 99%; lithium carbonate (Dv50 6μm, BET 3μm) 2 / g, purity 99.7%), organic carbon source glucose (purity 95%), solid dispersant citric acid (purity 98%), flux strontium oxide (Dv50 0.5μm, BET 4m) 2 / g) is dry ball milled in a high-energy ball mill to obtain the first mixed powder, wherein the ball milling speed is 500 rpm and the ball milling time is 10 h.

[0446] The nickel source, cobalt source, and first element source were weighed according to a molar ratio of Ni, Co, and Al of 8.8:0.7:0.5. The lithium salt was weighed according to a molar ratio of total nickel, cobalt, and aluminum to lithium of 1:1.04 (i.e., lithium was in excess to compensate for lithium consumption during sintering; therefore, the molar ratio of total nickel, cobalt, and the first element to lithium was not 1). The organic carbon source had a mass content of 0.10% in the first mixed powder, and citric acid had a mass content of 2%. The amount of flux added was adjusted as shown in Table 1 to control the Sr content (introduced by the flux) in the lithium-containing transition metal oxide to 2000 ppm.

[0447] In step (2), the sintering conditions are 810℃ for 10h.

[0448] The only difference between Example 11 and Example 10 is that, in step (1), the first element source is Al2O3 (Dv50 is 4μm, BET is 7m). 2 / g, purity 99%) and MnO2 (Dv50 4μm, BET 6m) 2 / g (purity 99%), wherein the nickel source, cobalt source, and first element source are weighed according to the molar ratio of Ni, Co, Al, and Mn of 0.88:0.7:0.1:0.4. The organic carbon source has a mass content of 0.10% in the first mixed powder.

[0449] Comparative Example 1 The only difference between it and Example 1 is that no dispersant or flux is added, no dispersant premixing is performed, and the amount of organic carbon source glucose added is 0.2%.

[0450] Table 1

[0451] In Table 1, the molar ratio of nickel, cobalt, manganese and lithium is 6:1:3:10.5, which means that in step (1), Ni(OH)2, Co3O4, MnO2 and lithium carbonate are weighed according to the molar ratio of Ni, Co, Mn and Li elements in the order of 6:1:3:10.5. Similarly, the molar ratio of nickel, cobalt, manganese and lithium is 5:2:3:10.5, which means that in step (1), Ni(OH)2, Co3O4, MnO2 and lithium carbonate are weighed according to the molar ratio of Ni, Co, Mn and Li elements in the order of 5:2:3:10.5. Similarly, the molar ratio of nickel, cobalt, manganese, and lithium being 8.8:0.7:0.5:10.4 means that in step (1), Ni(OH)2, Co3O4, Al2O3, and lithium carbonate are weighed according to the molar ratio of Ni, Co, Mn, and Li elements in the order of 8.8:0.7:0.5:10.4. Likewise, the molar ratio of nickel, cobalt, aluminum, manganese, and lithium being 8.8:0.7:0.1:0.4:10.4 means that in step (1), Ni(OH)2, Co3O4, Al2O3, MnO2, and lithium carbonate are weighed according to the molar ratio of Ni, Co, Al, Mn, and Li elements in the order of 8.8:0.7:0.1:0.4:10.4.

[0452] The batteries prepared in Examples 1-11 and Comparative Example 1 were tested as follows, and the method for measuring the relevant parameters in the positive electrode sheet is described in the specific implementation method.

[0453] (1) Battery discharge specific capacity test methods for Examples 1-9 and Comparative Example 1: The specific capacity of the battery is tested by placing the lithium-ion battery at a constant temperature of 25°C for 2 hours, then charging it to 4.4V at 1 / 3C from 2.8V to 4.4V, then charging it at 4.4V at a constant voltage until the current is ≤0.05mA, letting it stand for 5 minutes, and then discharging it to 2.8V at 1 / 3C. The specific capacity C of the battery is recorded.

[0454] Battery discharge capacity = battery discharge capacity C_discharge (mAh) / mass of positive electrode active material (g).

[0455] (2) Battery discharge specific capacity test method in Examples 10-11: The specific capacity of the battery is tested by placing the lithium-ion battery at a constant temperature of 25°C for 2 hours, then charging it to 4.25V at 1 / 3C under a range of 2.8V to 4.25V, then charging it at a constant voltage of 4.25V until the current is ≤0.05mA, letting it stand for 5 minutes, and then discharging it to 2.8V at 1 / 3C. The specific capacity C of the battery is recorded.

[0456] Battery discharge capacity = battery discharge capacity C_discharge (mAh) / mass of positive electrode active material (g).

[0457] (3) Cycle count test methods for batteries in Examples 1-9 and Comparative Example 1: Place the battery in a 25℃ oven and let it stand for 2 hours. Once the battery temperature remains at 25℃, conduct a charge-discharge test. Charge the battery at a constant current of 1C to 4.4V, then continue charging at a constant voltage until the charging current is less than 0.05C, then stop; pause for 10 minutes; discharge the battery at a constant current of 1C to 2.8V; pause for 10 minutes. This constitutes one charge-discharge cycle. Repeat this process continuously until the battery capacity decreases to 75% of its initial value, and record the number of cycles.

[0458] (4) Cycle count test method for batteries in Examples 10-11: Place the battery in a 25℃ oven and let it stand for 2 hours. Once the battery temperature remains at 25℃, conduct a charge-discharge test. Charge the battery at a constant current of 1C to 4.25V, then continue charging at a constant voltage until the charging current is less than 0.05C, then stop; pause for 10 minutes; discharge the battery at a constant current of 1C to 2.8V; pause for 10 minutes. This constitutes one charge-discharge cycle. Repeat this process continuously until the battery capacity decreases to 75% of its initial value, and record the number of cycles.

[0459] The test results are shown in Table 2.

[0460] Table 2

[0461] As can be seen from Tables 1 and 2, compared with Comparative Example 1, the embodiments of this application, in which the primary particles in the single particles and / or near-single particles satisfy the condition that the elemental concentration distribution of the cobalt element is more uniform than that of the nickel element, effectively improve the cycle performance of the secondary battery while maintaining a high initial discharge capacity.

[0462] As can be seen from Examples 1-9, the range of A1 for a single primary particle is 0.3%-1.0%, and the range of B1 is 0.5%-4%, with A1 < B1. Further, the range of A1 is 0.3%-0.5%, and the range of B1 is 0.5%-1%, which is beneficial for further optimizing the cycle number and discharge capacity of the secondary battery. The A1 and B1 values ​​in a single primary particle can be controlled by adjusting the parameters of the dry preparation process and the amount of raw materials added.

[0463] As can be seen from Examples 1-9, the range of A2 in the multiple primary particles is 0.3%-1.0%, and the range of B2 is 0.5%-4%, with A2 < B2. Further, the range of A2 is 0.3%-0.6%, and the range of B2 is 0.6%-1%, which is beneficial for further optimizing the cycle number and discharge capacity of the secondary battery. The proportions of A2 and B2 in the multiple primary particles can be achieved by controlling the parameters of the dry preparation process and the amount of raw materials added.

[0464] As can be seen from Examples 1-9, the range of A1 for a single primary particle is 0.3%-1.0%, and the range of C1 is 0.5%-4%, with A1 < C1. Further, the range of A1 is 0.3%-0.5%, and the range of C1 is 0.5%-1%, which is beneficial for further optimizing the cycle number and discharge capacity of the secondary battery. The A1 and C1 values ​​in a single primary particle can be controlled by adjusting the parameters of the dry preparation process and the amount of raw materials added.

[0465] As can be seen from Examples 1-9, the range of A2 in the multiple primary particles is 0.3%-1.0%, and the range of C2 is 0.5%-4%, with A2 < C2. Further, the range of A2 is 0.3%-0.6%, and the range of C2 is 0.6%-1%, which is beneficial for further optimizing the cycle number and discharge capacity of the secondary battery. The A2 and C2 values ​​in the multiple primary particles can be controlled by adjusting the parameters of the dry preparation process and the amount of raw materials added.

[0466] A comparison of Examples 3 and 5 shows that, based on satisfying the following conditions: A1 ranges from 0.3% to 0.5%, B1 ranges from 0.5% to 1%, C1 ranges from 0.5% to 1%, A2 ranges from 0.3% to 0.6%, B2 ranges from 0.6% to 1%, and C2 ranges from 0.6% to 1%, further controlling the doping of titanium in the lithium-containing transition metal oxide can maintain the battery's initial discharge specific capacity while significantly improving its cycle performance.

[0467] As can be seen from Comparative Example 1, in the lithium-containing transition metal oxides prepared in Examples 1-11 of this application, the dispersant was premixed and added in two steps, which is beneficial to control the distribution of nickel, cobalt and the first element, so that the discharge capacity of the secondary battery is basically maintained while the cycle performance is significantly improved.

[0468] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, The device includes a positive electrode sheet, which includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes a lithium-containing transition metal oxide. The lithium-containing transition metal oxide includes nickel, cobalt, and a first element, wherein the first element is manganese and / or aluminum, and the content of nickel is higher than the content of cobalt. The lithium-containing transition metal oxide includes single particles and / or quasi-single particles, wherein in the primary particles of the single particles and / or quasi-single particles, each primary particle satisfies the following condition: the elemental concentration distribution of cobalt is more uniform than the elemental concentration distribution of nickel.

2. The secondary battery according to claim 1, characterized in that, In a single first particle, the standard deviation of the cobalt element concentration distribution is A1, and the standard deviation of the nickel element concentration distribution is B1; A1 < B1.

3. The secondary battery according to claim 2, characterized in that, The range of A1 is 0.3%–1.0%; and / or, The range of B1 is 0.5%-4%.

4. The secondary battery according to claim 2 or 3, characterized in that, In the plurality of primary particles, the average standard deviation of the cobalt element concentration is A2, and the average standard deviation of the nickel element concentration is B2, where A2 < B2.

5. The secondary battery according to claim 4, characterized in that, The range of A2 is 0.3%-1.0%; and / or, The range for B2 is 0.5%-4%.

6. The secondary battery according to claims 1-5, characterized in that, In the plurality of primary particles, at least 90% of the primary particles have a standard deviation of A1 for the distribution of cobalt element concentration and a standard deviation of B1 for the distribution of nickel element concentration, where A1 < B1.

7. The secondary battery according to any one of claims 1-6, characterized in that, The first element is manganese, the content of nickel is higher than the content of manganese, and a single primary particle satisfies the following: the elemental concentration distribution of cobalt is more uniform than the elemental concentration distribution of manganese. Optionally, in a single first particle, the standard deviation of the cobalt element concentration distribution is A1, and the standard deviation of the manganese element concentration distribution is C1; A1 < C1; Optionally, C1 ranges from 0.5% to 4%.

8. The secondary battery according to claim 7, characterized in that, In the plurality of primary particles, the average standard deviation of the cobalt element concentration is A2, and the average standard deviation of the manganese element concentration is C2, where A2 < C2. Optionally, C2 ranges from 0.5% to 4%.

9. The secondary battery according to any one of claims 1-7, characterized in that, Based on the total number of primary particles among the single particles and / or similar single particles, at least a portion of the primary particles contain pores, and the average pore diameter of the pores is 50nm-500nm.

10. The secondary battery according to claim 9, characterized in that, The average perimeter of the hole is 400nm-2000nm; optionally, it is 600nm-1000nm.

11. The secondary battery according to any one of claims 9-10, characterized in that, In a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median R1 of the pore wall roughness is calculated based on the cumulative proportion of the pore area. A50 The range is 0.949-0.975; the range is 0.957-0.

970.

12. The secondary battery according to any one of claims 9-11, characterized in that, In a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, based on the cumulative proportion of the area of ​​the pores, the median L1 of the pore wall roundness is calculated. A50 The range is 0.728-0.836; the range is 0.757-0.

798.

13. The secondary battery according to any one of claims 9-12, characterized in that, Based on the total number of primary particles, the proportion of primary particles with internal pores is 1%-20%; Optionally, based on the total number of primary particles, the proportion of primary particles with pores having a diameter >300nm is 0%-2%.

14. The secondary battery according to any one of claims 9-13, characterized in that, In a cross-section of the positive electrode film along the thickness direction of the positive electrode sheet, under an electron microscope image at 3000x magnification, the area of ​​the pores accounts for 0.02%-0.5% of the total area of ​​the primary particles. The value can be selected as 0.1%-0.3%.

15. The secondary battery according to any one of claims 1-14, characterized in that, The average particle size of the primary particles is 1μm-4µm.

16. The secondary battery according to any one of claims 1-15, characterized in that, In the cross-sectional electron microscope image of the positive electrode film layer along the thickness direction of the positive electrode sheet, the primary particles include a first particle with a particle size of D1 and a second particle with a particle size of D2, where 1μm≤D1≤3μm and 3μm<D2≤10μm. The ratio of the total area of ​​the first particle to the total area of ​​the second particle is (0.2-0.5):1, which can be optionally (0.32-0.38):

1.

17. The secondary battery according to claim 16, characterized in that, The ratio of the number of the first particle to the number of the second particle is (1.2-3):1, and can be optionally (1.5-2):

1.

18. The secondary battery according to any one of claims 1-17, characterized in that, In a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median L2 of the sphericity of the primary particles is calculated based on the cumulative area ratio of the primary particles. A50 It ranges from 0.6 to 0.

85.

19. The secondary battery according to any one of claims 1-18, characterized in that, In a cross-sectional electron microscope image of the positive electrode film along the thickness direction of the positive electrode sheet, the median roughness R2 of the primary particles is calculated based on the cumulative area ratio of the primary particles. A50 It is 0.92-0.

97.

20. The secondary battery according to any one of claims 1-19, characterized in that, The molar ratio of the nickel, cobalt, and the first element is (5-9): (0.5-2.5): (0.5-2.5).

21. The secondary battery according to claim 20, characterized in that, The first element is manganese, and the molar ratio of the nickel, cobalt and manganese is (5-9): (0.5-2.5):(0.5-2.5).

22. The secondary battery according to any one of claims 1-21, characterized in that, The lithium-containing transition metal oxide also contains doping elements, including at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce, and La.

23. The secondary battery according to claim 22, characterized in that, The lithium-containing transition metal oxide contains W, and the content of W is 500 ppm to 2500 ppm based on the mass of the lithium-containing transition metal oxide.

24. The secondary battery according to any one of claims 1-23, characterized in that, The lithium-containing transition metal oxide is coated with a coating layer on its surface. Wherein, the coating layer is rich in cobalt; and / or, The coating layer is rich in a second element, which includes at least one of Ti, Al, Zr, W, Nb, Mo, B, Ce, and La.

25. The secondary battery according to claim 24, characterized in that, The coating layer contains cobalt, and the cobalt content in the coating layer is 5000 ppm - 15000 ppm.

26. The secondary battery according to claim 24 or 25, characterized in that, The coating layer contains a second element, and the content of the second element in the coating layer is 1000 ppm - 5000 ppm.

27. The secondary battery according to any one of claims 1-26, characterized in that, The sodium content in the lithium-containing transition metal oxide is 0 ppm to 500 ppm.

28. The secondary battery according to claim 27, characterized in that, 0 ppm ≤ Sodium content in the lithium-containing transition metal oxide ≤ 20 ppm; or, 20ppm < Sodium content in the lithium-containing transition metal oxide ≤ 100ppm.

29. The secondary battery according to any one of claims 1-28, characterized in that, The sulfur content in the lithium-containing transition metal oxide is 0 ppm-500 ppm; Optionally, the sulfur content in the lithium-containing transition metal oxide is ≤300ppm, with a concentration of 0 ppm or less.

30. The secondary battery according to any one of claims 1-29, characterized in that, The areal density of the positive electrode film layer on one side is 0.255 g / 1540.25 mm. 2 -0.28 g / 1540.25mm 2 .

31. The secondary battery according to any one of claims 1-30, characterized in that, The compaction density of the positive electrode film layer on one side is 3.30 g / cm³. 3 -3.55 g / cm 3 .

32. The secondary battery according to any one of claims 1-31, characterized in that, The positive electrode film also contains a conductive agent; Wherein, the conductive agent comprises 1%-3% by mass in the positive electrode film layer; and / or, The conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

33. The secondary battery according to any one of claims 1-32, characterized in that, The positive electrode film layer also contains a binder; Wherein, the binder comprises 0.5%-3.0% by mass in the positive electrode film layer; and / or, The adhesive includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

34. The secondary battery according to any one of claims 1-33, characterized in that, The discharge specific capacity of the lithium-containing transition metal oxide is 170 mAh / g - 205 mAh / g.

35. A positive electrode active material, characterized in that, The positive electrode active material includes a lithium-containing transition metal oxide, which includes nickel, cobalt, and a first element, wherein the first element is manganese and / or aluminum, and the content of nickel is higher than the content of cobalt. The lithium-containing transition metal oxide includes single particles and / or quasi-single particles, wherein in the primary particles of the single particles and / or quasi-single particles, each primary particle satisfies the following condition: the elemental concentration distribution of cobalt is more uniform than the elemental concentration distribution of nickel.

36. The positive electrode active material according to claim 35, characterized in that, In a single first particle, the standard deviation of the cobalt element concentration distribution is A1, and the standard deviation of the nickel element concentration distribution is B1; A1 < B1.

37. The positive electrode active material according to claim 36, characterized in that, The range of A1 is 0.3%–1.0%; and / or, The range of B1 is 0.5%-4%.

38. The positive electrode active material according to any one of claims 35-37, characterized in that, The first element is manganese, the content of nickel is higher than the content of manganese, and a single primary particle satisfies the following: the elemental concentration distribution of cobalt is more uniform than the elemental concentration distribution of manganese.

39. A method for preparing a positive electrode active material, characterized in that, include: The cobalt source and solid dispersant are dry-mixed to obtain a premix; The premix, nickel source, first element source, lithium salt, flux, and organic carbon source are dry-milled and mixed to obtain a first mixed powder. The first mixed powder and the remaining premix are dry-milled and mixed to obtain the second mixed powder; The second mixed powder was sintered at 750℃-1000℃ in an oxygen-containing atmosphere to obtain a lithium-containing transition metal oxide; Wherein, the nickel source, the cobalt source, and the first element source are all hydroxides and / or oxides; the first element is manganese and / or aluminum; the content of nickel is higher than the content of cobalt; the volume percentage of oxygen in the oxygen-containing atmosphere is ≥90%; and the solid dispersant includes at least one of citric acid, trisodium phosphate, sodium silicate, and magnesium carbonate.

40. The preparation method according to claim 39, characterized in that, The mass ratio of the premixed component to the remaining premixed component is 0.5:1 to 1.5:1; and / or, The solid dispersant is added at a mass of 0.1%-2.3% based on the total mass of the mixed powder; and / or, The volumetric particle size distribution (Dv50) of the cobalt source is 3 μm - 5 μm.

41. An electrical appliance, characterized in that, This includes the secondary battery according to any one of claims 1-34, the positive electrode active material according to any one of claims 35-38, or the positive electrode active material prepared by the preparation method according to any one of claims 39-40.

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