Positive electrode material, positive electrode sheet, and secondary battery

Abstract

Disclosed in the present application are a positive electrode material, a positive electrode sheet, and a secondary battery. The positive electrode material comprises secondary particles formed by the agglomeration of primary particles. The secondary particle comprises an inner layer and an outer layer coated on the surface of the inner layer; and the primary particles are at least partially arranged in an ordered manner along the radial direction of the secondary particles. By means of the above method, the present application can reduce the breakage rate of a positive electrode sheet, make it easier for lithium ions to transport, and improve the performance of a secondary battery. Moreover, the positive electrode material has a simple structure and is highly achievable, thereby reducing the process difficulty in preparing the positive electrode material and balancing the compaction and power performance.

Description

Positive electrode materials, positive electrode sheets and secondary batteries

[0001] This application claims priority to Chinese Patent Application No. 202510984131.6, filed with the Chinese Patent Office on July 16, 2025, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of battery technology, such as a positive electrode material, a positive electrode sheet, and a secondary battery. Background Technology

[0003] In recent years, with the rapid development of electric vehicles, portable electronic devices, and other fields, rechargeable batteries have been widely used due to their advantages such as high energy density and long cycle life. Among them, cylindrical batteries occupy an important position in the field of power batteries due to their advantages such as simple structure, high production efficiency, and relatively low cost.

[0004] Technical issues

[0005] However, cylindrical batteries still face several technical bottlenecks that need to be addressed during long-term use. Among these, the uneven electrolyte distribution caused by electrode expansion is particularly prominent. During charging and discharging, lithium ions repeatedly insert and extract between the positive and negative electrode materials, causing the electrodes to expand and contract. Due to the limited internal space of a cylindrical battery, the expansion of the electrodes is restricted, leading to changes in the gap between the electrodes and the separator. This affects the uniform distribution of the electrolyte on the electrode surface, accelerates the degradation of the battery's main materials, and impacts the battery's long-term reliability.

[0006] Therefore, how to effectively alleviate the defect of uneven electrolyte distribution caused by the expansion of cylindrical battery electrode sheets, improve the uniformity of battery chemical reaction, delay the decay of main materials, and improve the long-term reliability of batteries has become a technical problem that urgently needs to be solved by those skilled in the art.

[0007] Technical solutions

[0008] This application provides a positive electrode material, a positive electrode sheet, and a secondary battery, which can reduce the breakage rate of the positive electrode sheet, make lithium ions easier to transport, and improve the performance of the secondary battery; moreover, the structure is simple, highly feasible, reduces the process difficulty of preparing the positive electrode material, and balances compaction and power performance.

[0009] One technical solution adopted in this application is to provide a positive electrode material; the positive electrode material includes secondary particles formed by the aggregation of primary particles, the secondary particles include an inner layer and an outer layer covering the surface of the inner layer; and the primary particles are at least partially arranged in an ordered manner along the radial direction of the secondary particles.

[0010] The average porosity of the secondary particles is 0.5%-50%, the porosity of the inner layer is 10%-100%, and the porosity of the outer layer is 0-10%.

[0011] The inner and outer layers of the secondary particle are equally divided along the radial direction of the secondary particle.

[0012] Among them, the degree of order of the arrangement of primary particles in the inner layer is lower than that of the arrangement of primary particles in the outer layer.

[0013] Among them, the proportion of primary particles that satisfy L>D is 30%-99%, where L is the size of the primary particle along the direction from the center of the secondary particle to the surface, and D is the size of the primary particle along the direction perpendicular to the center of the secondary particle to the surface.

[0014] The aspect ratio η = L / D of the primary particle gradually increases from the inner layer to the outer layer, with an average value of 1-50. In the inner layer, η is 0.4 ≤ η ≤ 2, and in the outer layer, η is 2.0 ≤ η ≤ 50. L is the dimension of the primary particle along the direction from the center of the secondary particle to the surface, and D is the dimension of the primary particle along the direction perpendicular to the center of the secondary particle to the surface.

[0015] The D50 of the secondary particles is 3μm-15μm; the equivalent radius of the primary particles is 0.001-0.96 of that of the secondary particles.

[0016] Specifically, the shape of a primary particle is at least one of the following: cuboid, cube, sphere, ellipse, or irregular shape.

[0017] The positive electrode material has the chemical formula A. x Ni a Co b M c N d O y Wherein, A is at least one of Li and Na, and 0≤x≤2; 0.1≤a≤1; 0≤b≤0.6; M is at least one of Mn and Al, and 0≤c≤0.9; N is at least one of Al, Zr, W, Ti, Co, Nb, Y, Mg, Sb, La, Os, Pr, Re, Ru, Sr, Sm, Ta, Sc, Cr, B, F, P, and S, and 0≤d≤0.1; O is an anion, and 1.5≤y≤4.

[0018] Specifically, the cathode material is prepared by high-temperature calcination of the precursor. The precursor has a porous structure, and the primary grains or whiskers in the precursor are distributed radially, with the porosity gradually decreasing from the center to the outside.

[0019] The precursor has an average porosity of 1%-60%, an internal porosity of 20%-60%, and an external porosity of <15%.

[0020] Another technical solution adopted in this application is to provide a positive electrode sheet, which includes a positive current collector and a positive electrode material as described above, wherein the positive electrode material is located on the surface of the positive current collector.

[0021] Another technical solution adopted in this application is to provide a secondary battery, which includes a positive electrode, a negative electrode, a separator and an electrolyte as described above.

[0022] Beneficial effects

[0023] The beneficial effects of this application are as follows: Unlike related technologies, the cathode material of this application includes secondary particles formed by the aggregation of primary particles. The secondary particles include an inner layer and an outer layer covering the surface of the inner layer; and the primary particles are arranged in an ordered manner along the radial direction of the secondary particles. In the above-mentioned method, the layered arrangement of primary particles with different characteristics can improve the performance of the secondary battery. Moreover, the two-layer encapsulation scheme, compared with three or more layers, can reduce the process difficulty of cathode material preparation, and the structure is simpler, more feasible, and can balance compaction and power performance. Furthermore, since the cathode sheet will expand and contract in volume during battery use, if the primary particles are arranged in an ordered manner along the radial direction of the secondary particles, the cathode material has a better orientation. At this time, the stress concentration will be along the radial direction, thereby reducing the breakage rate of the cathode sheet. Moreover, such an arrangement also makes lithium ion transport easier. Furthermore, the ordered arrangement of primary particles facilitates smoother diffusion paths for lithium ions within the crystal structure, reducing resistance to lithium ion transport and improving the battery's rate performance, resulting in superior performance under high-current charge and discharge conditions. It also aids in electron conduction within the material, reducing scattering and energy loss during electron transport. This not only improves battery conductivity but also reduces internal resistance, thereby enhancing power density. Additionally, highly oriented cathode materials possess a more stable crystal structure, better resisting volume changes and stresses caused by lithium ion insertion and extraction during charge and discharge, thus improving cycle life, reducing capacity decay, and lowering the risk of material cracking and pulverization during cycling. In terms of material density, the ordered arrangement of primary particles results in a denser particle packing, contributing to higher tap and compaction densities of the cathode sheet, thereby increasing the battery's volumetric energy density, improving the mechanical strength of the electrode sheet, and reducing defects during electrode fabrication.

[0024] After reading and understanding the accompanying diagrams and detailed descriptions, other aspects can be understood. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0026] Figure 1. SEM image of secondary particles of the cathode material in Embodiment 1 of this application;

[0027] Figure 2 is a cross-sectional SEM image of the secondary particles of the cathode material in Embodiment 1 of this application;

[0028] Figure 3 is an Avizo 3D image processing diagram of the secondary particles of the cathode material in Embodiment 1 of this application;

[0029] Figure 4 is a cross-sectional SEM image of the precursor of the cathode material in Embodiment 1 of this application;

[0030] Figure 5 is a SEM image of the cathode material in Embodiment 2 of this application;

[0031] Figure 6 is a SEM image of the secondary particles of the cathode material in Embodiment 3 of this application;

[0032] Figure 7 is a SEM image of the secondary particles of the cathode material in Embodiment 3 of this application;

[0033] Figure 8 is a SEM image of the secondary particles of the cathode material in Embodiment 4 of this application. Detailed Implementation

[0034] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0035] This application provides a cathode material that can be used in secondary batteries, which can be power batteries for new energy vehicles, such as electric vehicles, hybrid vehicles, and range-extended vehicles. The power battery can provide power for the driving of new energy vehicles, specifically such as lithium-ion batteries and sodium-ion batteries.

[0036] The cathode material comprises secondary particles formed by the aggregation of primary particles. Primary particles, as the basic building blocks of the cathode material, are typically single-crystal particles at the nanometer or micrometer scale. Secondary particles are formed by the aggregation of multiple primary particles through physical or chemical processes.

[0037] Specifically, the structure of secondary particles includes an inner layer and an outer layer covering the surface of the inner layer. The distinction between the inner and outer layers can be based on the porosity of the secondary particles, the arrangement, size, morphology, chemical composition, orientation, etc. of the primary particles.

[0038] In one embodiment, the inner and outer layers can be obtained by bisecting along the cross-section of the secondary particle, that is, by bisecting along the radial direction of the secondary particle. In a specific embodiment, the inner and outer layers are obtained by equal division, but this is not a limitation.

[0039] It should be noted that in the secondary particle layering encapsulation scheme, the performance of the secondary battery is improved by arranging primary particles with different characteristics in layers. Moreover, the two-layer encapsulation scheme, compared with three or more layers, can reduce the process difficulty of cathode material preparation, has a simpler structure, higher feasibility, and can balance compaction and power performance.

[0040] Furthermore, in the cathode material, at least some of the primary particles are arranged in an ordered manner along the radial direction of the secondary particles.

[0041] It should be noted that during battery use, the positive electrode sheet will expand and contract in volume. If the primary particles are arranged in an ordered manner along the radial direction of the secondary particles, the positive electrode material will have a better degree of orientation. In this case, the stress concentration will be along the radial direction, thereby reducing the breakage rate of the positive electrode sheet. Moreover, such an arrangement will also make it easier for lithium ions to be transported.

[0042] It should be noted that the ordered arrangement of primary particles makes the diffusion path of lithium ions in the crystal structure smoother, reducing the resistance to lithium ion transport, improving the rate performance of the battery, and making it perform better under high current charge and discharge conditions. It also facilitates electron conduction within the material, reducing scattering and energy loss during electron transport. This not only improves the battery's conductivity but also reduces its internal resistance, thereby improving the battery's power density. Furthermore, highly oriented cathode materials have a more stable crystal structure, better resisting volume changes and stress caused by lithium ion insertion and extraction during charge and discharge, thus improving the battery's cycle life, reducing capacity decay, and lowering the risk of material cracking and pulverization during cycling. In terms of material density, the ordered arrangement of primary particles makes the particles more compact, helping to increase the tap density and compaction density of the cathode sheet, thereby increasing the battery's volumetric energy density, improving the mechanical strength of the electrode sheet, and reducing defects in the electrode fabrication process.

[0043] In one embodiment, the secondary particles are spherical particles formed by the aggregation of primary particles. The secondary particles have a hollow, porous internal structure with an average porosity γ of 0.5%-50%, specifically 0.5%, 1%, 5%, 10%, 20%, 30%, 40%, 50%, etc.

[0044] Specifically, the porosity of the outer layer of secondary particles is lower than that of the inner layer, showing a trend of porosity gradually decreasing from the inside to the outside.

[0045] In one embodiment, the porosity of the inner layer of the secondary particles is 10%-100%, specifically 10%, 20%, 40%, 60%, 80%, 100%, etc., and the porosity of the outer layer is 0-10%, specifically 0%, 1%, 2%, 4%, 6%, 8%, 10%.

[0046] It should be noted that the aforementioned pore gradient distribution structure of the secondary particles can balance fast charging and energy density performance. On one hand, the high internal porosity accelerates electrolyte penetration, ensuring uniform Li⁺ distribution, while the dense external layer improves electron conductivity and reduces the risk of local overcharging / lithium plating. Furthermore, the low external porosity increases the proportion of active material, balancing the material's power and energy density. Simultaneously, the pore gradient design enhances structural stability, buffers volume expansion, and the high internal porosity region provides a buffer space for volume changes in the cathode particles during charging and discharging, reducing particle breakage and preventing cracks from extending from the surface inwards.

[0047] In one embodiment, the degree of order of the primary particles in the inner layer is lower than that in the outer layer, that is, the arrangement in the outer layer is more ordered.

[0048] In this way, combined with the different pore distribution of secondary particles in the inner and outer layers, the absorption and retention of electrolyte by the positive electrode can be promoted to a certain extent, and the absorbed electrolyte is not easily lost from the positive electrode.

[0049] In one embodiment, in the outer layer, primary particles are arranged in an ordered manner along the radial direction of secondary particles; in the inner layer, primary particles are arranged in a disordered manner.

[0050] In one embodiment, the proportion of primary particles satisfying L>D in the cathode material is 30%-99%, specifically 30%, 40%, 60%, 80%, and 99%. Here, L is the dimension of the primary particle along the direction from the center of the secondary particle to the surface, and D is the dimension of the primary particle along the direction perpendicular to the center of the secondary particle to the surface.

[0051] In one embodiment, the aspect ratio η = L / D of the primary particle gradually increases from the inner layer to the outer layer, with an average value of 1-50; and in the inner layer, η is 0.4≤η≤2.0, and in the outer layer, η is 2.0≤η≤50; where L is the dimension of the primary particle along the direction from the center of the secondary particle to the surface, and D is the dimension of the primary particle along the direction perpendicular to the center of the secondary particle to the surface.

[0052] In one embodiment, the D50 of the secondary particles is 3μm-15μm, specifically 3μm, 6μm, 9μm, 12μm, 15μm, etc. The equivalent radius of the primary particles is 0.001-0.96 of that of the secondary particles, specifically 0.001, 0.01, 0.1, 0.3, 0.5, 0.7, 0.9, 0.96, etc.

[0053] In one embodiment, the shape of a primary particle is at least one of a cuboid, cube, sphere, ellipse, or irregular shape. Specifically, different primary particles may have the same or different shapes.

[0054] In one embodiment, the cathode material is a transition metal oxide cathode material. The chemical formula of this transition metal oxide cathode material is A. x Ni a Co b M c N d O y Wherein, A is at least one of Li and Na, and 0≤x≤2; 0.1≤a≤1; 0≤b≤0.6; M is at least one of Mn and Al, and 0≤c≤0.9; N is a doping or coating modification element, including at least one of Al, Zr, W, Ti, Co, Nb, Y, Mg, Sb, La, Os, Pr, Re, Ru, Sr, Sm, Ta, Sc, Cr, B, F, P, and S, and 0≤d≤0.1; O is an anion, and 1.5≤x≤4.

[0055] Specifically, x can be 0, 0.5, 1, 1.4, 1.5, 2, etc.; a can be 0.1, 0.3, 0.5, 0.6, 0.8, 1, etc.; b can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, etc.; c can be 0, 0.1, 0.3, 0.5, 0.7, 0.9, etc.; d can be 0, 0.01, 0.03, 0.05, 0.06, 0.08, 0.1, etc.; and y can be 1.5, 2, 2.5, 3, 3.5, 4, etc., without specific limitations here.

[0056] In one embodiment, the cathode material is prepared by high-temperature calcination of a precursor. Specifically, it can be prepared by mixing the precursor with lithium salt or sodium salt and doping / coating modification elements, followed by high-temperature calcination.

[0057] The precursor has a porous structure, and the primary grains or whiskers in the precursor are distributed radially, with the porosity gradually decreasing from the center to the outside.

[0058] Specifically, the precursor has an average porosity of 1%-60%, an internal porosity of 20%-60%, and an external porosity of <15%. For example, the average porosity of the precursor can be 1%, 5%, 10%, 20%, 40%, 60%, etc., the internal porosity can be 20%, 30%, 40%, 50%, 60%, etc., and the external porosity can be 14%, 13%, 12%, 11%, 10%, etc.

[0059] It should be further explained that the positive electrode material in the above embodiments of this application has an electrolyte affinity structure, which can significantly improve the liquid retention capacity and electrolyte distribution uniformity of the positive electrode material, effectively prevent the electrolyte from being squeezed out and lost after the positive electrode sheet is squeezed, thereby not only improving the power performance of the battery, but also enhancing the long-term reliability of the battery and extending the cycle life.

[0060] This application also provides a positive electrode sheet, which includes a positive current collector and a positive electrode material formed on the surface of the positive current collector.

[0061] Specifically, aluminum foil is typically chosen as the positive electrode current collector due to its excellent conductivity, mechanical strength, and chemical stability. The positive electrode material can be any of the positive electrode materials described in the foregoing embodiments of this application. For detailed information, please refer to the aforementioned embodiments; further elaboration is omitted here.

[0062] In this embodiment, a positive electrode slurry can be prepared by uniformly mixing a positive electrode material as the positive electrode active substance with a conductive agent, binder, solvent, etc., and then uniformly coating the obtained positive electrode slurry onto a positive electrode current collector, for example, by blade coating, roller coating, or spray coating. The coated positive electrode slurry is then dried and cold-pressed to ensure good bonding with the positive electrode current collector, thus obtaining the positive electrode sheet. Of course, this embodiment does not limit the method of preparing the positive electrode, as long as the positive electrode material used is consistent with that in the aforementioned embodiments of the positive electrode material in this application.

[0063] This application also provides a secondary battery. The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode is the same as the positive electrode described in the above embodiments of this application, and it contains the positive electrode material described in the aforementioned embodiments.

[0064] The battery can be one of cylindrical, prismatic, or pouch cells, and features long cycle life and fast charging characteristics. Furthermore, batteries using the aforementioned transition oxide cathode material exhibit a capacity retention rate increase of >10% and an initial DCIR reduction of >5% after cycling. Relevant testing methods are described below and will not be repeated here.

[0065] In this embodiment, the positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then wound to obtain a bare cell. The bare cell is placed inside a casing, and electrolyte is injected and the casing is then completed to obtain a secondary battery.

[0066] The technical solutions of this application will be further illustrated below through specific embodiments. These embodiments are only for the purpose of understanding this application and should not be regarded as specific limitations on this application.

[0067] Example 1:

[0068] In this embodiment, the cathode material is prepared by mixing a precursor with a lithium salt and then calcining it at high temperature. Its chemical formula is LiNi. 0.8 Co 0.1 Mn 0.1 O2.

[0069] In this embodiment, the SEM images, cross-sectional SEM images, and Avizo 3D image processing images of the secondary particles of the cathode material, as well as the cross-sectional SEM image of the precursor used to prepare the cathode material, are shown in Figures 1-4, respectively.

[0070] The cathode material was tested using the following method. SEM images of the cross-section of the cathode material were processed using Avizo 3D imaging software. The equivalent particle size of the primary particles was calculated to be approximately 200 nm, and the particle size D50 of the secondary particles was approximately 12 μm. Using image processing software, the average porosity of the cathode material was calculated to be 3%, with an inner layer porosity of 15% and an outer layer porosity of 0.5%. In the outer layer, the primary particles were radially arranged; the proportion of primary particles with L>D was approximately 80%; and the average L / D ratio was approximately 15.

[0071] Example 2:

[0072] In this embodiment, the cathode material is prepared by mixing a precursor with a lithium salt and then calcining it at high temperature. Its chemical formula is LiNi. 0.6 Co 0.2 Mn 0.2 O2.

[0073] In this embodiment, the SEM image of the secondary particles of the cathode material is shown in Figure 5.

[0074] The cathode material was tested using the following method. SEM images of the cross-section of the cathode material were processed using Avizo 3D imaging software. The equivalent particle size of the primary particles was calculated to be approximately 100 nm, and the particle size D50 of the secondary particles was approximately 10 μm. Using image processing software, the average porosity of the cathode material was calculated to be 6%, with an inner layer porosity of 10% and an outer layer porosity of 2%. In the outer layer, the primary particles were radially arranged; the proportion of primary particles with L>D was approximately 60%; and the average L / D ratio was approximately 11.

[0075] Example 3:

[0076] In this embodiment, the positive electrode material is prepared by mixing a precursor with a lithium salt and then calcining it at high temperature. Its chemical formula is Li. 1.4 Ni 0.3 Mn 0.7 O2.

[0077] In this embodiment, the SEM images and cross-sectional SEM images of the secondary particles of the cathode material are shown in Figures 6 and 7, respectively.

[0078] The cathode material was tested using the following method. SEM images of the cross-section of the cathode material were processed using Avizo 3D imaging software. The equivalent particle size of the primary particles was calculated to be approximately 100 nm, and the particle size D50 of the secondary particles was approximately 6 μm. Using image processing software, the average porosity of the cathode material was calculated to be 7%, with an inner layer porosity of 15% and an outer layer porosity of 1%. In the outer layer, the primary particles were radially arranged; the proportion of primary particles with L>D was approximately 40%; and the average L / D ratio was approximately 5.

[0079] Example 4:

[0080] In this embodiment, the cathode material is prepared by mixing a precursor with a lithium salt and then calcining it at high temperature. Its chemical formula is LiNi. 0.3 Mn 0.7 O2.

[0081] In this embodiment, the SEM image of the secondary particles of the cathode material is shown in Figure 8.

[0082] The cathode material was tested using the following method. SEM images of the cross-section of the cathode material were processed using Avizo 3D imaging software. The equivalent particle size of the primary particles was calculated to be approximately 100 nm, and the particle size D50 of the secondary particles was approximately 7 μm. Using image processing software, the average porosity of the cathode material was calculated to be 15%, with an inner layer porosity of 60% and an outer layer porosity of 1%. In the outer layer, the primary particles were radially arranged; the proportion of primary particles with L>D was approximately 70%; and the average L / D ratio was approximately 7.

[0083] Comparative Example 1

[0084] The cathode material in this comparative example was prepared by mixing a precursor with a lithium salt and then calcining it at high temperature. Its chemical formula is LiNi. 0.8 Co 0.1 Mn 0.1 O2.

[0085] The cathode material exhibits no radial or pore distribution structure. SEM images of the cross-section of this cathode material were processed using Avizo 3D imaging software, calculating that the equivalent particle size of primary particles is approximately 200 nm, and the particle size D50 of secondary particles is approximately 12 μm. Image processing software also determined that the porosity of both the inner and outer layers of the cathode material is approximately 0.1%, and the proportion of primary particles with a density greater than their diameter (L>D) is approximately 2%.

[0086] Comparative Example 2

[0087] The cathode material in this comparative example was prepared by mixing a precursor with a lithium salt and then calcining it at high temperature. Its chemical formula is LiNi. 0.6 Co 0.2 Mn 0.2 O2.

[0088] The cathode material exhibits no radial or pore distribution structure. SEM images of the cross-section of this cathode material were processed using Avizo 3D imaging software, calculating that the equivalent particle size of primary particles is approximately 100 nm, and the particle size D50 of secondary particles is approximately 10 μm. Image processing software also determined that the porosity of both the inner and outer layers of the cathode material is approximately 0.5%, and the proportion of primary particles with a density greater than the diameter (L>D) is approximately 7%.

[0089] Comparative Example 3

[0090] The cathode material in this comparative example was prepared by mixing a precursor with a lithium salt and then calcining it at high temperature. Its chemical formula is Li. 1.4 Ni 0.3 Mn 0.7 O2.

[0091] The cathode material exhibits no radial or pore distribution structure. SEM images of the cross-section of this cathode material were processed using Avizo 3D imaging software, calculating that the equivalent particle size of primary particles is approximately 100 nm, and the particle size D50 of secondary particles is approximately 6 μm. Image processing software also determined that the porosity of both the inner and outer layers of the cathode material is approximately 1%, and the proportion of primary particles with a density (L>D) is approximately 4%.

[0092] Comparative Example 4

[0093] The cathode material in this comparative example was prepared by mixing a precursor with a lithium salt and then calcining it at high temperature. Its chemical formula is LiNi. 0.3 Mn 0.7 O2.

[0094] The cathode material exhibits no radial or pore distribution structure. SEM images of the cross-section of this cathode material were processed using Avizo 3D imaging software, calculating that the equivalent particle size of primary particles is approximately 100 nm, and the particle size D50 of secondary particles is approximately 7 μm. Image processing software also determined that the porosity of both the inner and outer layers of the cathode material is approximately 0.7%, and the proportion of primary particles with a density greater than their diameter (L>D) is approximately 6%.

[0095] The porosity and radial distribution of the cathode materials involved in the above embodiments and comparative examples were obtained by testing according to the following test methods.

[0096] Porosity γ test:

[0097] 1) After the cathode material is treated with CP (Coating and Pressing), the cross-section SEM of secondary particles in the cathode material is taken using a Thermo Fisher scanning electron microscope. The electron microscope uses an accelerating voltage of 1KV, the lens is in backscatter mode, the magnification is 6Kx, and the number of images taken is ≥3.

[0098] 2) Select a cross-sectional SEM of secondary particles with a particle size close to D50. Use Avizo 3D image software to identify the pore area (S0) and total cross-sectional area (S1) in the cross-sectional SEM. Calculate the porosity of the secondary particles according to the formula γ=S0 / S1×100%.

[0099] 3) Repeat the above method to calculate the porosity of at least 3 secondary particles and take the average value.

[0100] Gradient porosity calculation:

[0101] 1) After the cathode material is CP treated, the cross-section SEM of the secondary particles is taken using a Thermo Fisher scanning electron microscope. The electron microscope uses an accelerating voltage of 1KV, the lens is in backscatter mode, the magnification is 6Kx, the secondary particles with a particle size close to D50 are selected, and the number of images taken is ≥3.

[0102] 2) Select a cross-sectional SEM of secondary particles with a particle size close to D50. Treat the secondary particles as spheres and calculate based on the circular cross-section of the sphere. Divide the circle into two concentric circles.

[0103] 3) Using Avizo 3D imaging software, calculate the area and pore area of ​​each concentric circle, and calculate the porosity of the inner and outer layers;

[0104] 4) Repeat the above method to calculate the porosity of the inner and outer layers of at least 3 secondary particles and take the average value.

[0105] Radial distribution characterization:

[0106] 1) After the cathode material is CP treated, the cross-section of the secondary particles is captured by a Thermo Fisher scanning electron microscope. The electron microscope uses an accelerating voltage of 1KV, the lens is in backscatter mode, the magnification is 6Kx, and the number of images taken is ≥3.

[0107] 2) Select a cross-sectional SEM of secondary particles with a particle size close to D50, use EBSD to identify the size of primary particles, collect length and width data, and calculate L / D;

[0108] 3) Repeat the above method to calculate the L / D value of the primary particle among at least 3 secondary particles, and take the average value.

[0109] In addition, for the cathode materials in the above embodiments and comparative examples, the corresponding cylindrical cells were assembled according to the following methods, and then their capacity retention rate and initial DCIR were measured.

[0110] Battery assembly

[0111] The aforementioned ternary cathode material, used as the positive electrode active material, is uniformly dispersed with a conductive agent and binder in an N-methylpyrrolidone solvent to obtain a positive electrode slurry. This slurry is then coated onto aluminum foil, dried, and cold-pressed to obtain a positive electrode sheet. The negative electrode material, conductive agent, and binder are thoroughly mixed in a deionized water solvent system, coated onto copper foil, dried, and cold-pressed to obtain a negative electrode sheet. The positive electrode sheet, separator, and negative electrode sheet are stacked sequentially, with the separator positioned between the positive and negative electrodes for isolation, and then wound to obtain a bare battery cell. The bare battery cell is placed inside a cylindrical steel casing, injected with electrolyte, and encapsulated to form a battery.

[0112] Battery testing methods:

[0113] Battery internal resistance DCIR test:

[0114] Let the fresh battery stand at room temperature for 10 minutes, then use a 0.2C current to adjust the battery SOC to 50%, discharge at 2C for 10 seconds, record the current I, the voltage V1 before discharge, and the voltage V2 after 10 seconds of discharge, and calculate DCIR = (V1 - V2) / I.

[0115] Battery room temperature cycle performance test:

[0116] Under a constant temperature environment of 25℃, at a voltage of 2.8V-4.25V, the battery is charged to 4.2V according to a 15-minute steep CC regime, then kept constant at 0.2C current, left to stand for 5 minutes, and then discharged to 2.8V at 1C. The capacity of the first cycle is D1. The process is repeated 500 times and the capacity Dn (n=1,2……) is recorded. The capacity retention rate after 500 cycles is calculated as: (D500-D1) / D0×100%.

[0117] Based on the above battery assembly method and battery testing method, the test results are as follows: Compared with Comparative Example 1, the capacity retention rate of the battery in Example 1 is increased by 20%, and the initial DCIR is reduced by 15%; compared with Comparative Example 2, the capacity retention rate of the battery in Example 2 is increased by 16%, and the initial DCIR is reduced by 10%; compared with Comparative Example 3, the capacity retention rate of the battery in Example 3 is increased by 11%, and the initial DCIR is reduced by 15%; compared with Comparative Example 4, the capacity retention rate of the battery in Example 4 is increased by 12%, and the initial DCIR is reduced by 16%.

[0118] As can be seen from the above embodiments and comparative examples, the battery using the cathode material satisfying the technical solution of this application has a high capacity retention rate after cycling and a low initial DCIR, meaning that the corresponding battery has better electrochemical performance and lower internal resistance. Furthermore, the technical solution of this application can be applied to cathode materials of different oxides, thus having a wide range of applications.

Claims

1. A cathode material comprising secondary particles formed by the aggregation of primary particles, the secondary particles comprising an inner layer and an outer layer covering the surface of the inner layer; and the primary particles being at least partially arranged in an ordered manner along the radial direction of the secondary particles.

2. The sample collection device of claim 1, wherein, The average porosity of the secondary particles is 0.5%-50%, the porosity of the inner layer is 10%-100%, and the porosity of the outer layer is 0-10%.

3. The positive electrode material of claim 1, wherein, The inner and outer layers of the secondary particle are equally divided along the radial direction of the secondary particle.

4. The sample collection device of claim 1, wherein, The degree of order in the arrangement of primary particles in the inner layer is lower than that in the arrangement of primary particles in the outer layer.

5. The cathode material of claim 1, wherein, The proportion of primary particles satisfying L>D is 30%-99%, where L is the dimension of the primary particle along the direction from the center of the secondary particle to the surface, and D is the dimension of the primary particle along the direction perpendicular to the center of the secondary particle to the surface.

6. The cathode material of claim 1, wherein, The aspect ratio η = L / D of the primary particle gradually increases from the inner layer to the outer layer, with an average value of η ranging from 1 to 50. In the inner layer, η is 0.4 ≤ η ≤ 2, and in the outer layer, η is 2 ≤ η ≤ 50. Wherein, L is the dimension of the primary particle along the direction from the center of the secondary particle to the surface, and D is the dimension of the primary particle along the direction perpendicular to the center of the secondary particle to the surface.

7. The cathode material of claim 1, wherein, The D50 of the secondary particle is 3μm-15μm; the equivalent radius of the primary particle is 0.001-0.96 of that of the secondary particle.

8. The cathode material of claim 1, wherein, The chemical formula of the cathode material is A x Ni a Co b M c N d O y Wherein, A is at least one of Li and Na, and 0≤x≤2; 0.1≤a≤1; 0≤b≤0.6; M is at least one of Mn and Al, and 0≤c≤0.9; N is a doping or coating modification element, including at least one of Al, Zr, W, Ti, Co, Nb, Y, Mg, Sb, La, Os, Pr, Re, Ru, Sr, Sm, Ta, Sc, Cr, B, F, P, and S, and 0≤d≤0.1; O is an anion, and 1.5≤y≤4.

9. The cathode material of claim 1, wherein, The cathode material is prepared by high-temperature calcination of a precursor. The precursor has a porous structure, and the primary grains or whiskers in the precursor are distributed radially, with the porosity gradually decreasing from the center to the outside.

10. The cathode material of claim 9, wherein, The precursor has an average porosity of 1%-60%, an internal porosity of 20%-60%, and an external porosity of <15%.

11. A positive electrode sheet comprising a positive electrode current collector and the positive electrode material of any one of claims 1-10, wherein, The positive electrode material is located on the surface of the positive electrode current collector.

12. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; wherein The positive electrode is the positive electrode as described in claim 11.

Images