Positive electrode material, positive electrode sheet, and secondary battery

Abstract

Provided in the present application are a positive electrode material, a positive electrode sheet, and a secondary battery. The positive electrode material contains an element M, the element M being selected from at least one metal element in groups IA and IIA in the periodic table of elements; the distribution of the element M among particles satisfies: 1.2≤X2 / X1≤20, wherein X1 is the concentration of the element M at a central region of the particles and X2 is the concentration of the element M at a surface layer region of the particles; and the surface of the positive electrode material contains nitrate ions, and the amount of nitrate ions is N1, wherein 10 ppm≤N1≤100 ppm. The positive electrode material of the present application has high structural stability and few microcracks, and a stable interface protection layer can be formed on the surface of the positive electrode material, reducing the occurrence of side reactions between an electrolyte and the surface of the positive electrode material, and thereby comprehensively improving the capacity and cycle performance of a battery prepared using the positive electrode material while also improving gas production performance.

Description

Cathode material, cathode sheet and secondary battery

[0001] The present application claims priority to the Chinese patent application No. 202411981769.6, filed on December 26, 2024, and entitled "Cathode material, cathode sheet and secondary battery", the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of battery cathode materials, in particular to a cathode material, a cathode sheet and a secondary battery. BACKGROUND

[0003] With the continuous popularity and development of electric vehicles, the market demand for lithium ion power batteries is increasing, especially higher requirements are put forward for the aspects of endurance mileage, service life and driving safety.

[0004] At present, people have higher and higher requirements for the energy density, safety and cycle performance of lithium ion batteries, and it is urgently needed to improve the cathode material in the lithium ion battery to improve the capacity loss and cycle performance of the existing lithium ion battery under high voltage. SUMMARY

[0005] Therefore, in order to solve at least one of the above defects, it is necessary to provide a cathode material.

[0006] In addition, it is also necessary to provide a cathode sheet and a secondary battery using the aforementioned cathode material.

[0007] In a first aspect, the embodiments of the present application provide a cathode material, the cathode material contains M elements, the M elements are selected from at least one metal element in group IA and group IIA of the periodic table, the cathode material includes a plurality of particles, the distribution of the M elements in the particles satisfies: 1.2≤X2 / X1≤20, wherein X1 is the concentration of the M elements at the center region of the particles, X2 is the concentration of the M elements at the surface layer region of the particles; and the cathode material contains free nitrate ions, the mass content of the free nitrate ions is N1, wherein 10ppm≤N1≤100ppm.

[0008] In a second aspect, the embodiments of the present application provide a cathode sheet, which includes the cathode material as described above.

[0009] In a third aspect, the embodiments of the present application provide a secondary battery, which includes an electrode assembly, the electrode assembly includes a cathode sheet, a separator and an anode sheet, the separator is arranged between the cathode sheet and the anode sheet, and the cathode sheet includes the cathode material as described above.

[0010] The positive electrode material provided by the embodiments of the present application controls the concentration of the element M in the particles to satisfy 1.2≤X2 / X1≤20, so that the content of the element M near the surface layer region of the particles is relatively high, the element M increases the unit cell volume of the surface layer region of the particles, the content of the element M near the central region of the particles is relatively low, the unit cell volume at the central region of the particles is relatively small, and thus the compressive stress of the particles from the surface to the center is formed. In the process of high-voltage cycle performance, the compressive stress of the particles can relieve the structure collapse caused by the lattice expansion and shrinkage of lithium ions in the process of deintercalation, and the particles are less likely to produce microcracks in the long-term cycle process, and the propagation of the cracks can also be inhibited, thereby improving the high-voltage cycle performance of the material. In addition, during charging and discharging, the unit cell volume is in a shrinkage state, which can balance the compressive stress caused by the deintercalation of lithium ions in the charging and discharging process, so that the structure of the material is more stable, and the positive electrode material can have higher deintercalation lithium capacity and structure recovery capacity in the charging and discharging process, thereby enabling the positive electrode material to exhibit higher capacity in the charging and discharging process.

[0011] At the same time, the positive electrode material contains free nitrate ions, and the content of the free nitrate ions is controlled to satisfy 10ppm≤N1≤100ppm, which improves the structural stability of the positive electrode material, and the nitrate ions can induce the formation of a Li2O stable interface protection layer on the surface of the positive electrode material, thereby reducing the side reaction between the electrolyte and the surface of the positive electrode material, and comprehensively improving the capacity, rate and cycle stability of the battery prepared by using the positive electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0012] FIG. 1 is a schematic diagram of a cross section of a lithium ion battery during charging of the positive electrode material according to the embodiments of the present application.

[0013] FIG. 2 is a schematic diagram of a cross section of a lithium ion battery during discharging of the positive electrode material according to the embodiments of the present application.

[0014] FIG. 3 is a diagram defining the central region and the surface layer region of a single crystal particle when testing the concentration distribution of the element M in the positive electrode material according to the embodiments of the present application.

[0015] FIG. 4 is an SEM image of the positive electrode material prepared in Example 1 according to the embodiments of the present application.

[0016] FIG. 5 is an SEM image of the positive electrode material prepared in Comparative Example 1 according to the embodiments of the present application.

[0017] FIG. 6 is a distribution diagram of the Mg element in the particles of the positive electrode material prepared in Example 1 according to the embodiments of the present application.

[0018] FIG. 7 is a distribution diagram of the Mg element in the particles of the positive electrode material prepared in Comparative Example 1 according to the embodiments of the present application. DETAILED DESCRIPTION

[0019] Embodiments of the present application are described in detail below. The embodiments described below are exemplary only, and are not intended to be limiting of the present application; it being understood that unless otherwise defined, all technical and scientific terms used herein are intended to have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; and that non-limiting embodiments of the present application can be combined together where not inconsistent. Numerous specific details are described below in order to provide a thorough understanding of the present application. The described embodiments are merely part of the present application and other embodiments can be used without departing from the present application.

[0020] The positive electrode material provided by the embodiments of the present application contains element M, which is selected from at least one metal element in Group IA and Group IIA of the periodic table. The positive electrode material comprises a plurality of particles, and the distribution of the element M in the particles satisfies 1.2≤X2 / X1≤20, wherein X1 is the concentration (atom number) of the element M at the center region of the particle, and X2 is the concentration (atom number) of the element M at the surface layer region of the particle. In addition, the positive electrode material contains free nitrate ions (NO 3- ), and the mass content of the free nitrate ions is N1, wherein 10 ppm≤N1≤100 ppm. The positive electrode material provided by the embodiments of the present application has high structural stability at high voltage, and is not prone to micro-cracks in long-term cycle tests, thereby effectively improving the cycle performance of the positive electrode material.

[0021] The embodiment of the present application firstly introduces metal elements (i.e. M elements) in Group IA and Group IIA of the periodic table into the particles, and controls the distribution of the M elements in the central region and the surface layer region of the particles to satisfy 1.2≤X2 / X1≤20, so that the content of the element M near the surface layer region of the particle is relatively high, the element M increases the unit cell volume of the surface layer region of the particle, the content of the element M near the central region of the particle is relatively low, the unit cell volume at the central region of the particle is relatively small, thereby forming compressive stress of the particle from the surface to the center. In the process of high-voltage cycle performance, the compressive stress of the particle can relieve the structure collapse caused by lattice expansion and shrinkage of lithium ions in the process of deintercalation, and the particle is less likely to produce microcracks in the long-term cycle process, and the crack propagation can also be inhibited, thereby improving the high-voltage cycle performance of the positive electrode material. Then, the content of free nitrate ions in the positive electrode material is controlled to satisfy 10ppm≤N1≤100ppm. The presence of an appropriate amount of free nitrate ions in the positive electrode material is beneficial to inhibiting the volume change in the charging and discharging process of the positive electrode material. The inventor speculates that, in the first charging and discharging process, the free nitrate ions in the positive electrode material can induce the formation of a Li2O stable interface protection layer (CEI film) on the surface of the positive electrode material, thereby inhibiting the volume change in the charging and discharging process of the positive electrode material, and further improving the high-voltage cycle performance of the positive electrode material.

[0022] It should be noted that X1 is the concentration of the M element at the central region of the particle, which refers to the number of atoms of the M element in the central region of the particle characterized by an energy dispersive spectrometer (EDS), and is dimensionless. X2 is the concentration of the M element at the surface layer region of the particle, which refers to the number of atoms of the M element in the surface layer region of the particle characterized by an energy dispersive spectrometer (EDS), and is dimensionless. In the cross-sectional view of the particle, the center of the circumscribed circle of the particle is taken as the center, the radius of the circumscribed circle is the radius L of the particle, and the region from the center of the particle to the region at a distance of L / 2 from the center of the particle is the central region, and the region from the position of L / 2 of the particle to the surface of the particle is the surface layer region.

[0023] Exemplarily, the ratio of X2 / X1 can be 1.2, 1.5, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or any value within the range composed of any two of the above values. Further, the distribution of the M element in the particle in the positive electrode material satisfies 6≤X2 / X1≤11, which is beneficial to further improving the capacity, cycle stability and rate performance of the prepared battery, and further reducing the gas production.

[0024] When X2 / X1<1.2, the surface cell volume and the internal cell volume of the particle present uniform distribution, the average cell volume of the particle presents a large state, and the pressure stress is not easy to generate, which is not conducive to improving the long-term cycle performance of the secondary battery. When X2 / X1>20, too much element M will be enriched on the surface layer region of the particle, will occupy the lithium site of the lattice, will affect the transmission and diffusion of lithium ions, and will reduce the rate performance of the secondary battery.

[0025] The embodiment of the present application controls the content of free nitrate ions in the positive electrode material to meet 10ppm≤N1≤100ppm, and the presence of an appropriate amount of free nitrate ions in the positive electrode material is beneficial to inhibit the volume change in the charging and discharging process of the positive electrode material. The inventors speculate that the free nitrate ions in the positive electrode material will induce the formation of a Li2O stable interface protection layer (CEI film) on the surface of the positive electrode material in the first charging and discharging process, and the specific reaction formula is: 2Li + +NO3 - +2e - =Li2O+NO2 - Li2O has high-voltage resistance performance, thereby helping to improve the high-voltage resistance performance of the positive electrode material, inhibit the volume change in the charging and discharging process of the positive electrode material, and further improve the high-voltage cycle performance of the positive electrode material. Too high mass content of free nitrate in the positive electrode material will cause too much Li2O to be formed on the surface of the positive electrode material, and the ion transmission of the positive electrode material will be blocked. When the mass content of free nitrate in the positive electrode material is too low, too little Li2O is formed on the surface of the positive electrode material, and the side reaction of the positive electrode material and the electrolyte is intensified, especially in the high-voltage charging and discharging state, the capacity loss of the positive electrode material is intensified. The present application controls the mass content of free nitrate in the positive electrode material within the above range, which is beneficial to the positive electrode material to have high-voltage resistance performance, excellent cycle stability and high capacity. Further, the content of free nitrate ions in the positive electrode material is 30ppm≤N1≤80ppm, within this range, the formed CEI film can be further optimized, the occurrence of interface side reactions can be further reduced, and the influence on the capacity and electrical performance of the battery and the internal resistance of the battery can be further reduced. Exemplarily, the content of free nitrate ions can be 10ppm, 20ppm, 30ppm, 40ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm, 100ppm or any value within the range composed of any two of the above values.

[0026] In some embodiments, the mass percentage of the M element can be 10 ppm to 400 ppm based on the mass of the positive electrode material. When the concentration of the M element in the particle is within the above range, the concentration of the M element is moderate, which can effectively generate compressive stress on the unit cell to well inhibit the generation and growth of micro-cracks, and the M element does not occupy the lithium site, which is beneficial to further improve the capacity of the prepared battery, further improve the structural stability of the positive electrode material, improve the cycle stability of the battery, and ensure that the electron transmission is not blocked to improve the electrical conductivity. For example, the concentration of the M element can be 10 ppm, 30 ppm, 50 ppm, 80 ppm, 100 ppm, 120 ppm, 150 ppm, 180 ppm, 200 ppm, 220 ppm, 250 ppm, 280 ppm, 300 ppm, 320 ppm, 350 ppm, 380 ppm, 400 ppm, or any value within a range defined by any two of the above values, based on the mass of the positive electrode material. The test method of the mass percentage of the M element is as follows: 0.3 g of the positive electrode material is dissolved with aqua regia, and the volume is cooled and fixed to 100 ml, and then the content of the element M is tested by ICP, which is recorded as ICP(M).

[0027] The concentration (mass content) M1 of the M element in the central region of the particle can be 10 ppm to 400 ppm based on the mass of the positive electrode material. For example, the concentration (mass content) M1 of the M element in the central region of the particle can be 10 ppm, 30 ppm, 50 ppm, 80 ppm, 100 ppm, 120 ppm, 150 ppm, 180 ppm, 200 ppm, 220 ppm, 250 ppm, 280 ppm, 300 ppm, 320 ppm, 350 ppm, 380 ppm, 400 ppm, or any value within a range defined by any two of the above values. The calculation method of M1 is as follows: M1 (ppm) = ICP(M) * X1 / (X1 + X2); wherein ICP(M) is the mass percentage of the M element based on the mass of the positive electrode material, X1 is the atomic number of the central region obtained by EDS characterization, and X2 is the atomic number of the surface layer region obtained by EDS characterization.

[0028] The concentration M2 (mass content) of the M element in the surface layer region of the particle can be 10 ppm to 400 ppm based on the mass of the positive electrode material. Illustratively, the concentration M2 (mass content) of the M element in the surface layer region of the particle can be 10 ppm, 30 ppm, 50 ppm, 80 ppm, 100 ppm, 120 ppm, 150 ppm, 180 ppm, 200 ppm, 220 ppm, 250 ppm, 280 ppm, 300 ppm, 320 ppm, 350 ppm, 380 ppm, 400 ppm, or any value within a range defined by any two of the above values. The mass content of X2 is calculated as follows: M2 (ppm) = ICP(M) * X2 / (X1 + X2); ICP(M) is the mass percentage of the M element based on the mass of the positive electrode material, X1 is the atomic number of the center region obtained by EDS characterization, and X2 is the atomic number of the surface layer region obtained by EDS characterization.

[0029] Further, the concentration M1 of the M element in the center region of the particle is 10 ppm to 50 ppm, the concentration M2 of the M element in the surface layer region of the particle is 300 ppm to 400 ppm, and the concentration of the M element in the center region and the surface layer region of the particle is within the above ranges, which can further reduce the unit cell volume, expand the difference between the inner and outer unit cell volumes, make the contraction state of the unit cell volume of the particle more obvious, further increase the compressive stress on the lattice, thereby making the structure stability of the positive electrode material stronger, reducing the cracks generated in the positive electrode material during long-term cycling, and achieving further optimization and improvement of the capacity, rate performance, and cycle performance of the battery. Illustratively, the concentration X1 of the M element in the center region of the particle can be 10 ppm, 15 ppm, 20 ppm, 25 ppm, 30 ppm, 35 ppm, 40 ppm, 45 ppm, 50 ppm, or any value within a range defined by any two of the above values. Illustratively, the concentration X2 of the M element in the surface layer region of the particle can be 300 ppm, 310 ppm, 320 ppm, 330 ppm, 340 ppm, 350 ppm, 360 ppm, 370 ppm, 380 ppm, 390 ppm, 400 ppm, or any value within a range defined by any two of the above values.

[0030] In some embodiments, the content of free lithium ions in the cathode material is 400 ppm to 600 ppm based on the mass of the cathode material. By controlling the concentration of free lithium ions in the cathode material to be appropriate, the free nitrate ions and the free lithium ions are beneficial to react to form an effective CEI film on the surface of the cathode material during the charging and discharging process. Exemplarily, the concentration of free lithium ions in the cathode material can be 400 ppm, 410 ppm, 420 ppm, 430 ppm, 440 ppm, 450 ppm, 460 ppm, 470 ppm, 480 ppm, 490 ppm, 500 ppm, 510 ppm, 520 ppm, 530 ppm, 540 ppm, 550 ppm, 560 ppm, 570 ppm, 580 ppm, 590 ppm, 600 ppm or any value within the range between any two of the above values.

[0031] Specifically, the general formula of the cathode material in the present application is as follows: LiaNibCocQdM(1-b-c-d-e)NeO2, wherein 0.95≤a≤1.2, 0.5

[0032] When the M element enters the site of the main element in the transition metal layer of the positive electrode material, the M element will expand the spacing between the transition metal layers, thereby increasing the unit cell volume. When the M element is concentrated near the surface of the grain, the unit cell volume of the surface layer region of the grain will be larger than that of the center region of the grain, the stress of the surface layer region of the grain is larger, thereby generating a compressive stress towards the center of the grain, and further making the grain shrink from the surface layer to the center of the grain. For example, the M element is Mg element, since the ionic radius of Mg2+ is 65 pm, while the ionic radius of Ni2+ is 72 pm, the ionic radius of Co2+ is 74 pm, and the ionic radius of Mn2+ is 80 pm, when the Mg element enters the transition metal layer of the positive electrode material to replace the Ni / Co / Mn element, the smaller ionic radius of Mg2+ will make the spacing between the transition metal layers smaller, thereby making the unit cell volume smaller. When the concentration of Mg element in the center region of the particle is less than that in the surface layer region, the surface layer region of the particle is in a surface enrichment state, the degree of reduction of the unit cell volume of the surface layer region of the particle is smaller than that of the center region, thereby the unit cell volume of the particle presents a reduction state from the surface layer to the center, generating a compressive stress in the particle. This compressive stress can inhibit the volume expansion of the center region of the particle, and improve the structural stability of the particle. In some embodiments, the M element includes at least one of Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, and the metal elements M in the above IA group and IIA group can better achieve the effect that when the M element enters the site of the main element in the transition metal layer of the positive electrode material, the M element will expand the spacing between the transition metal layers, thereby increasing the unit cell volume; when the M element is concentrated near the surface of the grain, the unit cell volume of the surface layer region of the grain will be larger than that of the center region of the grain, the stress of the surface layer region of the grain is larger, thereby generating a compressive stress towards the center of the grain, and further making the grain shrink from the surface layer to the center of the grain.

[0033] In some embodiments, the positive electrode material further contains N element, and the N element is selected from at least one element of Zr, Ti, Al, Co, Mg, W, Ce, Y, La, Nb and Mo, etc. By doping the N element into the positive electrode material, the crystal structure of the positive electrode material can be optimized, the stability of the crystal structure is further improved, and the electrochemical performance of the positive electrode material is further improved.

[0034] In some embodiments, based on the mass of the positive electrode material, the concentration X3 of the N element satisfies: 0≤X3≤5000ppm.

[0035] In the above positive electrode material, appropriate doping of N element can further improve the specific capacity and cycle stability of the positive electrode material. The content of the doped N element is within the above suitable range, which is beneficial to grow the unit cell volume of the particles within a suitable range, so as to further improve the specific capacity and cycle stability of the positive electrode material. If the concentration of N element is too large, such as more than 5000 ppm, too much N element will be enriched on the surface layer of the particles, which will affect the growth of the particles, make the unit cell volume of the particles smaller, and easily cause the agglomeration and adhesion of the particles. In the compaction process of preparing the positive electrode sheet using the positive electrode material, the agglomerated particles are prone to cracking, which will increase the side reaction of the electrolyte and the positive electrode material in the battery, and affect the long-term electrochemical performance of the battery. In addition, when the concentration of N element is too high, the internal resistance of the prepared battery will increase, which will affect the diffusion efficiency of lithium ions and reduce the charge and discharge efficiency of the battery. Illustratively, the concentration of the N element can be 0.1 ppm, 10 ppm, 100 ppm, 500 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, or any value within the range formed by any two of the above values. Further, the concentration of N element can be 500 ppm to 3000 ppm. It can be understood that the positive electrode material can also not be doped with N element, i.e. the content of N element is 0.

[0036] In some embodiments, the volume particle size distribution of the positive electrode material satisfies: 1.0≤(D90-D10) / D50≤1.5, L=(D90-D10) / D50 represents the width of the particle size distribution in the positive electrode material. By controlling 1.0≤(D90-D10) / D50≤1.5, the particle size distribution of the positive electrode material is wider, which can improve the compaction density and material activity of the material, and can improve the energy density of the material. Illustratively, (D90-D10) / D50 can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or any value within the range formed by any two of the above values.

[0037] In some embodiments, the lattice strain ε of the positive electrode material is <0.1%. Specifically, the lattice strain ε of the positive electrode material can be 0.095%, 0.09%, 0.05%, 0.01%, 0.005%, 0%, -0.01%, -0.05%, -0.08%, -0.10%, or -0.15%, etc., and of course can also be other values within the above range, which is not limited herein. By controlling the lattice strain ε within <0.1%, the internal micro-cracks of the particles can be reduced, the particles of the positive electrode material are not prone to cracking and failure during the cycle process, and thus the long-term cycle retention rate of the positive electrode material is improved. It should be noted that when measuring the lattice strain of the positive electrode material, the lattice strain generated by the compressive stress caused by the lattice contraction is negative, and the lattice strain generated by the tensile stress caused by the lattice stretching is positive.

[0038] In some embodiments, the specific surface area of the positive electrode material is 0.4 m 2 / g~0.9 m 2 / g. The specific surface area of the positive electrode material is appropriate, which is conducive to reducing the direct contact between the positive electrode material and the electrolyte, thereby reducing the generation of side reactions, and is conducive to improving the cycle performance of the battery prepared by using the positive electrode material, while also ensuring that the battery prepared by using the positive electrode material has good discharge capacity and rate performance. Exemplarily, the specific surface area of the positive electrode material can be 0.5 m 2 / g, 0.6 m 2 / g, 0.7 m 2 / g, 0.8 m 2 / g, 0.9 m 2 / g, or any value within a range consisting of any two of the above values.

[0039] In some embodiments, the tap density p1 of the positive electrode material is 2.0 g / cm 3 ~3.0 g / cm 3 . The tap density of the positive electrode material is within an appropriate range, which is conducive to improving the compaction degree of the electrode sheet containing the positive electrode material and improving the energy density of the battery. Exemplarily, the tap density of the positive electrode material can be 2.1 g / cm 3 , 2.2 g / cm 3 , 2.3 g / cm 3 , 2.4 g / cm 3 , 2.5 g / cm 3 , 2.6 g / cm 3 , 2.7 g / cm 3 , 2.8 g / cm 3 , 2.9 g / cm 3 , 3.0 g / cm 3 , or any value within a range consisting of any two of the above values.

[0040] In some embodiments, the powder compaction density p2 of the positive electrode material is 2.8 g / cm 3 ~4.0 g / cm 3 . The powder compaction density of the positive electrode material is within an appropriate range, which is conducive to further improving the compaction degree of the electrode sheet containing the positive electrode material and improving the energy density of the battery. Exemplarily, the powder compaction density of the positive electrode material can be 2.8 g / cm 3 , 2.9 g / cm 3 , 3.0 g / cm 3 , 3.3 g / cm 3 , 3.5 g / cm 3 , 3.8 g / cm 3 , 4.0 g / cm3 or any value within a range derived from any two of the above.

[0041] In some embodiments, the secondary battery prepared by using the aforementioned positive electrode material has a volume change rate of ≤ 6.0%, further 4.0% to 6.0%, when charged to 50 SOC% and stored in an oven at 60°C for 21 days. Exemplarily, the volume change rate of the battery can be 4.0%, 4.1%, 4.2%, 4.3%, 4.4%, 4.5%, 4.6%, 4.7%, 4.8%, 4.9%, 5.0%, 5.1%, 5.2%, 5.3%, 5.4%, 5.5%, 5.6%, 5.7%, 5.8%, 5.9%, 6.0% or any value within a range derived from any two of the above.

[0042] In some embodiments, the positive electrode material is a single crystal or single crystal-like material, i.e., the smallest particle of the positive electrode material is a primary particle or a secondary particle formed by no more than five primary particles, wherein the average particle size of the primary particle is 1 μm to 5 μm. By controlling the particle size of the primary particle of the positive electrode material within the above range, the primary particle size is moderate, which is conducive to reducing the specific surface area of the particle, reducing the side reaction between the particle and the electrolyte, and further improving the safety and cycle life of the material. Moreover, the primary particle within the above particle size range can shorten the transport path of lithium ions, which is conducive to improving the rate performance of the battery. In addition, the primary particle within the above range can also reduce the internal stress of the particle and reduce the risk of electrochemical polarization of lithium ions inside and outside the particle, thereby improving the capacity of the battery prepared by using the positive electrode material. Exemplarily, the average particle size of the primary particle can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or any value within a range derived from any two of the above.

[0043] It should be noted that the difference between the single-crystal positive electrode material and the polycrystal positive electrode material (i.e. polycrystal secondary particles) is that the minimum particle of the polycrystal secondary particle is a secondary particle formed by agglomeration of nanoscale primary particles. The minimum particle of the single-crystal positive electrode material is usually a micron-scale single primary particle. In general, in addition to the EBSD testing method, the obtained positive electrode product can also be determined to be a single-crystal material by a characterization method such as a scanning electron microscope (SEM). For example, for the single-crystal positive electrode material, the morphology of the single-crystal particle can be characterized by SEM, and it can be seen that the shape of the single-crystal particle generally exhibits regular or irregular spherical shape, and there is no significant particle agglomeration. The orientation of the single-crystal positive electrode material can also be characterized by EBSD. It can be observed by EBSD that the colors in at least one grain are the same, so as to determine that the orientations in at least one grain are the same, and the grains with the same orientation are single crystals. It should be specifically noted that the "single-crystal positive electrode material" known to those skilled in the art is not a "single crystal" in the strict crystallographic sense. In crystallography, an ideal single crystal refers to a crystal with completely the same arrangement and direction. However, due to impurities, strain and crystal defects, ideal single crystals are very rare and difficult to produce in the laboratory. Therefore, the single-crystal positive electrode material known in the art is actually a "single-crystal-like morphology" positive electrode material, which is only different from the polycrystal composed of numerous small primary particles in that it exhibits a large particle size of a single crystal.

[0044] It can be understood that the single grain in the present application can be a single particle composed of one primary particle. The above-mentioned single-crystal positive electrode material can also have a small amount of "secondary particle-like" formed by adhesion of several single particles. The "primary particle" refers to the smallest particle unit identified by observing the positive electrode active material by a scanning electron microscope, and the "secondary particle" refers to a secondary structure formed by agglomeration of a plurality of primary particles, which exhibits a relatively round spherical morphology. The "secondary particle-like" refers to adhesion of several single particles, and the particle size of the single particle in the above-mentioned secondary particle-like is usually between 1 μm and 5 μm. Generally, the roundness of the "secondary particle-like" is lower than that of the above-mentioned conventional "secondary particle".

[0045] Based on the same inventive concept, the present application also provides a preparation method of the above-mentioned positive electrode material, which specifically comprises the following steps:

[0046] In step S1, raw materials containing transition metals are put into nitric acid to prepare a nitrate mixed solution, and a dopant solution containing M elements is added to the nitrate mixed solution to obtain a premixed solution, wherein the M elements are selected from at least one metal element in Group IA and Group IIA of the periodic table.

[0047] In this step, the raw material containing the excess metal can be one or more of a recovered positive electrode material, a pure metal, a hydroxide or a nitrate. The excess metal includes at least one of Ni, Co, Mn and Al, and can specifically be a raw material containing Ni, Co and Mn or a raw material containing Ni, Co and Al. For example, it can be one or more of a recovered positive electrode material containing Ni, Co and Mn / Al, a pure metal of Ni, Co and Mn / Al, a hydroxide of Ni, Co and Mn / Al, and a nitrate of Ni, Co and Mn / Al. When the Ni, Co and Mn / Al come from a recovered ternary positive electrode material, specifically, the recovered ternary positive electrode material is subjected to component detection to confirm the content ratio of the main elements and the types and ratios of the metal elements present in the recovered material, and then the positive electrode material with the confirmed components is mixed with the pure metal of Ni, Co and Mn / Al to prepare a mixed salt solution according to the final target Ni:Co:Mn / Al ratio. It can be understood that in some other embodiments, the raw material of Ni, Co and Mn / Al can be a commercially available metal salt solution.

[0048] In some embodiments, the stoichiometric ratio between Ni:Co:Mn / Al in the mixed salt solution is in the range of 0.5≤Ni<1, 0

[0049] Step S2, preparing the pre-mixed solution into an oxide precursor by a spray pyrolysis method.

[0050] In this step, the distribution of the M element inside the precursor particles can be changed by adjusting the flow rate of the different nozzles in the spray pyrolysis process, thereby changing the size of X2 / X1.

[0051] The specific mechanism is that in the spray pyrolysis cavity, it can be roughly divided into a first region and a second region connected to each other. The first region is the region from the nozzle to the position before the highest temperature point, which is a gradually heating zone. The second region is a region maintaining the highest temperature, which is a constant temperature zone. When the flow rate of the pre-mixed solution flowing out of the nozzle is low, it stays in the first region for a long time, and the drying rate of the droplets is slow, which is more conducive to the diffusion and uniform distribution of the M element inside the droplets; when the flow rate of the nozzle is high, the droplets quickly pass through the first region of the heating zone to the second region of the high temperature, the drying rate of the droplets becomes fast, and the outermost water is quickly evaporated, resulting in a gradient difference in the concentration of the internal and external water, at which time the internal water carries the M element to the surface, thereby causing the M element to be enriched on the surface.

[0052] In this step, the temperature of spray pyrolysis can also be controlled to control the content of NO3-in the positive electrode material. The mechanism is that when the temperature of spray pyrolysis is high, the decomposition degree of nitrate is high, the residual amount of nitrate in the precursor is low, and the content of nitrate in the final positive electrode material is low; on the contrary, when the temperature of spray pyrolysis is low, the decomposition degree of nitrate is low, the residual amount of nitrate in the precursor is high, and the content of nitrate in the final positive electrode material is high.

[0053] In some embodiments, the nozzle flow rate V satisfies: 300 L / h < V < 700 L / h. Controlling the nozzle flow rate within the above range is appropriate, and the droplets pass through the first region to the high-temperature second region at a controllable rate to control the drying rate of the droplets, thereby controlling the moisture concentration difference inside and outside the droplets within an appropriate range, so that the internal moisture carries the M element to the surface and enriches on the surface layer, thereby forming a concentration difference between the central region and the surface layer of the precursor particle, and realizing the distribution of the M element in the particles of the final positive electrode material satisfies: 1.2 ≤ X2 / X1 ≤ 20. Exemplarily, the nozzle flow rate V can be 300 L / h, 350 L / h, 400 L / h, 450 L / h, 500 L / h, 550 L / h, 600 L / h, 650 L / h, 700 L / h, or any value within the range composed of any two of the above values. Further, the nozzle flow rate can be 350 L / h-650 L / h.

[0054] In some embodiments, the temperature of spray pyrolysis is 550-950°C. By controlling the temperature of spray pyrolysis within the above range, the decomposition degree of nitrate can be controlled, and the residual amount of nitrate in the positive electrode material can be controlled, so that the content of free nitrate ions in the positive electrode material prepared using the precursor can be ensured to be 10-100 ppm, so as to meet the requirements of the positive electrode material. Exemplarily, the nitrate content of the oxide precursor is 200 ppm, 500 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, or any value within the range composed of any two of the above values.

[0055] In some embodiments, the temperature of the first region is 60-200°C. In some embodiments, the temperature of the second region is 400-800°C. The temperature of the first region plays a role of preliminary drying, removing a certain amount of moisture; the temperature of the second region is higher, and the drying rate is much faster than that of the first region, and the purpose is to control the moisture migration speed by drying temperature to achieve the purpose of moisture carrying M element migration. Exemplarily, the temperature of the first region can be 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, or any value within a range consisting of any two of the above values.

[0056] In some embodiments, the concentration of nitric acid is 1-3%, and the concentration of nitric acid is controlled, which can regulate the retention amount of nitrate ions in the oxide precursor, and then regulate the content of free nitrate ions in the positive electrode material prepared by the precursor to be 10-100 ppm to meet the requirements of the positive electrode material. Exemplarily, the concentration of nitric acid can be 1%, 2%, 3%, or any value within a range consisting of any two of the above values.

[0057] Step S3, mixing the oxide precursor and lithium salt and sintering to obtain a positive electrode material matrix.

[0058] The oxide precursor is subjected to jet milling, the jet milling pressure is 0.3-0.5 MPa, the classification frequency is 20-160 Hz, and the particle size D50 of the milled particles is 2.0-4.0 μm, which is conducive to reducing the content of fine powder in the subsequently prepared positive electrode material, and the particle surface is relatively round. In the step, a dopant containing N element can also be added to the mixture of the oxide precursor and lithium salt for sintering.

[0059] In some embodiments, the N element can be at least one of Zr, Ti, Al, Co, Mg, W, Ce, Y, La, Nb, and Mo.

[0060] In some embodiments, the lithium salt can be at least one of lithium hydroxide, lithium carbonate, and the like.

[0061] In some embodiments, the lithium salt is added in an amount of 60wt% to 120wt% based on the mass of the cathode material. By controlling the amount of lithium salt added within the aforementioned range, the concentration of Li element in the surface layer of the cathode material can be ensured to be 400ppm to 600ppm. Thus, during the charging and discharging process, nitrate and lithium ions can react in the surface layer of the cathode material to form an effective CEI film. Exemplarily, the lithium salt is added in an amount of 60wt%, 70wt%, 80wt%, 90wt%, 100wt%, 110wt%, 120wt% or any value within the range between any two of the aforementioned values.

[0062] Specifically, the obtained oxide precursor is mixed with a lithium source in a molar ratio (n(Ni+Co+Mn):nLi=1:1). Then, the mixture is placed in an atmosphere furnace and subjected to primary sintering at a corresponding temperature and oxygen atmosphere. Sintering in air or oxygen atmosphere can enable the material to react with oxygen to form the target layered compound.

[0063] The sintering temperature is 880°C to 980°C and the sintering time is 4 to 12 hours. Under this condition, the lithium salt and auxiliary materials can be fully melted and infiltrated into the precursor, improving the uniformity of infiltration. At the same time, during the sintering process, particles are formed and stabilized. Therefore, by controlling the sintering temperature and sintering time of the primary sintering, the crystal can be ensured to grow fully without excessive sintering, thereby controlling the unit cell volume size to achieve the required standard, and further improving the particle strength and structural stability. Exemplarily, the sintering temperature can be 880°C, 890°C, 900°C, 910°C, 920°C, 930°C, 940°C, 950°C, 960°C, 970°C, 980°C or any value within the range between any two of the aforementioned values. Exemplarily, the sintering time can be 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h or any value within the range between any two of the aforementioned values.

[0064] Step S4: crushing and coating the cathode material matrix to obtain the cathode material.

[0065] The crushed cathode matrix material is uniformly mixed with a coating agent and subjected to secondary sintering, which is to improve the electrochemical performance of the cathode matrix material by coating. Then, the secondary sintered cathode material is sieved and magnetically separated to obtain the finished cathode material.

[0066] In some embodiments, the coating element in the coating agent can be selected from at least one of Zr, Ti, Al, Co, Mg, W, Ce, Y, K, Na, etc., and the coating agent can be an oxide of the coating element or a lithium ion conductor.

[0067] In some embodiments, the content of the coating element can be 200 ppm to 5000 ppm. Illustratively, the content of the coating element can be any value in a range defined by any two of 200 ppm, 500 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm or above.

[0068] In some embodiments, the temperature of the secondary sintering can be 400 to 600 DEG C, and the sintering time can be 6 to 12 h. By controlling the temperature and time of the secondary sintering, the coating agent can be uniformly coated on the surface of the substrate in batches, and the surface structure of the positive electrode material is further optimized, so as to further improve the structural stability and electrical conductivity of the positive electrode material. Illustratively, the temperature of the secondary sintering can be any value in a range defined by any two of 400 DEG C, 500 DEG C, 600 DEG C or above, and the sintering time can be any value in a range defined by any two of 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h or above.

[0069] The embodiments of the present application also provide a positive electrode tab using the aforementioned positive electrode material, which comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, and the positive electrode active material layer comprises the aforementioned positive electrode material.

[0070] The positive electrode current collector can use an aluminum foil or a nickel foil, or any composite current collector disclosed in the prior art, such as but not limited to a current collector formed by combining the aforementioned conductive foil and a polymer substrate.

[0071] The embodiments of the present application also provide a secondary battery (such as a lithium ion battery, a sodium ion battery, etc.), which comprises a shell, an electrode assembly and an electrolyte. The electrode assembly and the electrolyte are both located in the shell. The electrode assembly comprises a separator, a negative electrode tab and the aforementioned positive electrode tab, and the separator is arranged between the positive electrode tab and the negative electrode tab.

[0072] In some embodiments, the shell can be a packaging bag obtained by packaging with a packaging film (such as an aluminum plastic film), such as a soft package battery. In other embodiments, the secondary battery can also be a steel shell battery, an aluminum shell battery, etc.

[0073] In some embodiments, the electrode assembly can be a stacked structure formed by alternately stacking the positive electrode tab, the separator and the negative electrode tab in sequence. In other embodiments, the electrode assembly can also be a wound structure formed by winding the positive electrode tab, the separator and the negative electrode tab in sequence.

[0074] In some embodiments, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can use at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or carbon-based current collector, etc., and can also be any composite current collector disclosed in the prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and polymer substrate. The negative electrode active material can comprise at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as battery negative electrode active materials can also be used. These negative electrode active materials can be used alone or in combination with two or more. The battery provided by the embodiments of the present application has the advantages of high capacity, high initial efficiency, long cycle life, excellent rate performance, and low expansion. The battery can be a lithium ion battery, a sodium ion battery, a solid electrolyte battery, etc., without limitation.

[0075] As shown in FIGS. 1 and 2, the schematic diagrams of lithium extraction and lithium intercalation of lithium ion batteries prepared by using the positive electrode material provided by the embodiments of the present application during charging and discharging, respectively. As shown in FIG. 1, when the lithium ion battery is charged, lithium ions are extracted from the positive electrode sheet and intercalated into the negative electrode sheet; as shown in FIG. 2, when the lithium ion battery is discharged, lithium ions are extracted from the negative electrode sheet and intercalated back into the positive electrode sheet.

[0076] By controlling the distribution of M elements in the particles in the positive electrode material to satisfy 1.2≤X2 / X1≤20, and controlling the content of free nitrate ions in the positive electrode material to satisfy 10ppm≤N1≤100ppm, the embodiments of the present application can effectively improve the structural stability of the positive electrode material, reduce the generation of micro-cracks, and form a stable interface protection layer on the surface of the positive electrode material, thereby reducing the side reaction between the electrolyte and the surface of the positive electrode material, and comprehensively improving the capacity and cycle performance of the battery prepared by using the positive electrode material, and improving the gas production performance. Therefore, by using the aforementioned positive electrode material at high voltage, the capacity, rate and cycle stability of the battery can be effectively improved, and the gas production performance can be improved. When the aforementioned positive electrode material is used in a button cell, the capacity is ≥182.6 mAh / g, the cycle performance of the battery is evaluated, after 0.5C charging and 1C discharging for 50 cycles at room temperature (25°C), the cycle retention rate is ≥95.5%, the initial efficiency is ≥87%, and the volume change rate is ≤6.0%. It shows that by using the aforementioned positive electrode material, the capacity and cycle performance of the battery can be effectively improved.

[0077] The solutions of the present application will be explained below with examples. Those skilled in the art will understand that the following examples are only for explaining the present application and cannot be understood as limiting the present application. Unless otherwise indicated, the reagents, software and instruments involved in the following examples, which are not specifically indicated, are conventional commercially available products or open source.

[0078] Example 1

[0079] Step S1, pure metal particles of Ni, Co and Mn were respectively put into nitric acid to form nitrate solutions, and the three nitrate solutions were prepared into a mixed nitrate solution according to the molar ratio Ni:Co:Mn=60:10:30, wherein the total concentration of Ni, Co and Mn in the solution was 5 mol / L, and a nitrate solution of Mg was added to the mixed nitrate solution until the molar ratio of Mg:Ni in the mixed solution was 1:466, to obtain a premix solution.

[0080] Step S2, the aforementioned premix solution was subjected to spray pyrolysis, wherein the nozzle flow rate was 500 L / h, the temperature of the first region was 150°C, and the temperature of the second region was 700°C, to obtain an oxide precursor, and the obtained oxide precursor was subjected to gas crushing.

[0081] Step S3, 500 g of the aforementioned crushed oxide precursor, 233.4 g of lithium carbonate, and 2.25 g of ZrO2 were uniformly mixed. Sintering was carried out at 930°C for 8h under compressed air atmosphere, and the sintered material was crushed to obtain a particle size D50=4.17μm to obtain a positive electrode material matrix.

[0082] Step S4, 400 g of the positive electrode material matrix was preliminarily mixed with 0.67 g of TiO2, and sintering was carried out at 550°C for 8h under oxygen atmosphere, and the obtained material was sieved to obtain a positive electrode material. Among them, the X2 / X1 of Mg element in the positive electrode material was 7.2, and the content of free nitrate was 68 ppm.

[0083] Example 2

[0084] The difference from Example 1 is that in Step S2, the temperature of the first region is increased from 150°C to 160°C, and the temperature of the second region is increased from 700°C to 730°C.

[0085] In Step S3, the particle size D50 of the positive electrode material matrix was 4.2μm.

[0086] In Step S4, the X2 / X1 of Mg element in the obtained positive electrode material was 6.9, and the content of free nitrate was 12 ppm.

[0087] The other steps are basically the same as those of Example 1, please refer to Example 1.

[0088] Example 3

[0089] The difference from Example 1 is that in step S2, the temperature of the first zone is reduced from 150°C to 140°C, and the spray pyrolysis temperature is reduced from 700°C to 670°C.

[0090] In step S3, the particle size D50 of the positive electrode material base is 4.15 μm.

[0091] In step S4, the X2 / X1 of the Mg element in the obtained positive electrode material is 8.7, and the content of free nitrate is 97 ppm.

[0092] Example 4

[0093] The difference from Example 1 is that in step S1, a nitrate solution of Ca is added to the mixed nitrate solution until the molar ratio of Ca:Ni in the mixed solution is 1:466.

[0094] In step S3, the particle size D50 of the positive electrode material base is 4.16 μm.

[0095] In step S4, the X2 / X1 of the Ca element in the obtained positive electrode material is 12.5, and the content of free nitrate is 65 ppm.

[0096] The other steps are basically the same as those of Example 1, please refer to Example 1.

[0097] Example 5

[0098] The difference from Example 1 is that in step S1, a nitrate solution of K is added to the mixed nitrate solution until the molar ratio of K:Ni in the mixed solution is 1:466.

[0099] In step S3, the particle size D50 of the positive electrode material base is 4.16 μm.

[0100] In step S4, the X2 / X1 of the K element in the obtained positive electrode material is 5.6, and the content of free nitrate is 69 ppm.

[0101] The other steps are basically the same as those of Example 1, please refer to Example 1.

[0102] Example 6

[0103] The difference from Example 1 is that in step S2, the nozzle flow rate during spray pyrolysis is reduced from 500 L / h to 300 L / h.

[0104] In step S3, the particle size D50 of the positive electrode material base is 4.13 μm.

[0105] In step S4, the X2 / X1 of the Mg element in the obtained positive electrode material is 1.2, and the content of free nitrate is 72 ppm.

[0106] Other steps are basically the same as those in Example 1, please refer to Example 1.

[0107] Example 7

[0108] The difference from Example 1 is that in step S2, the nozzle flow rate in the spray pyrolysis process is increased from 500 L / h to 700 L / h.

[0109] In step S3, the particle size D50 of the positive electrode material matrix is 4.3 μm.

[0110] In step S4, the X2 / X1 of Mg element in the obtained positive electrode material is 19.2, and the content of free nitrate is 69 ppm.

[0111] Other steps are basically the same as those in Example 1, please refer to Example 1.

[0112] Example 8

[0113] The difference from Example 1 is that in step S1, the recovered positive electrode material containing Ni, Co and Mn is used to replace the pure Ni, Co and Mn metals to form a mixed solution in nitric acid, and Ni(NO3)2, Co(NO3)2 and Mn(NO3)2 are added to make the molar ratio of Ni:Co:Mn in the mixed solution reach the target molar ratio, while the target molar ratio of Ni:Co:Mn is controlled to be 70:10:20.

[0114] In step S3, 500 g of oxide precursor, 252.6 g of lithium hydroxide monohydrate and 2.25 g of ZrO2 are uniformly mixed, sintered at 915°C for 8h in an oxygen atmosphere, and the sintered material is crushed to a particle size D50 of 4.21 μm to obtain a positive electrode material matrix.

[0115] In step S4, the X2 / X1 of Mg element in the obtained positive electrode material is 7.3, and the content of free nitrate is 65 ppm.

[0116] Other steps are basically the same as those in Example 1, please refer to Example 1.

[0117] Example 9

[0118] The difference from Example 1 is that in step S1, the target molar ratio of Ni:Co:Mn in the nitrate mixed solution is controlled to be 80:10:10.

[0119] In step S3, 500 g of oxide precursor, 252.6 g of lithium hydroxide monohydrate and 2.25 g of ZrO2 are uniformly mixed, sintered at 915°C for 8h in an oxygen atmosphere, and the sintered material is crushed to a particle size D50 of 4.21 μm to obtain a positive electrode material matrix.

[0120] In step S4, the X2 / X1 of Mg element in the obtained positive electrode material is 9.3, and the content of free nitrate is 75 ppm.

[0121] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0122] Example 10

[0123] The difference from Example 1 is that in step S3, 2.25 g of ZrO2 is replaced by 0.67 g of WO3, and the particle size D50 of the calcined material is 4.02 μm.

[0124] In step S4, the X2 / X1 of Mg element in the obtained positive electrode material is 10.1, and the content of free nitrate is 71 ppm.

[0125] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0126] Example 11

[0127] The difference from Example 1 is that in step S3, 2.25 g of ZrO2 is replaced by 1.02 g of Y2O3, and the particle size D50 of the calcined material is 4.3 μm.

[0128] In step S4, the X2 / X1 of Mg element in the obtained positive electrode material is 10.5, and the content of free nitrate is 77 ppm.

[0129] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0130] Example 12

[0131] The difference from Example 1 is that in step S3, the sintering temperature after mixing is adjusted from 930 ℃ to 920 ℃.

[0132] In step S4, the X2 / X1 of Mg element in the obtained positive electrode material is 6.8, and the content of free nitrate is 82 ppm.

[0133] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0134] Example 13

[0135] The difference from Example 1 is that in step S3, the sintering temperature after mixing is adjusted from 930 ℃ to 940 ℃.

[0136] In step S4, the X2 / X1 of Mg element in the obtained positive electrode material is 7.4, and the content of free nitrate is 53 ppm.

[0137] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0138] Example 14

[0139] The difference from Example 1 is that the temperature of the first zone in step S2 is raised from 150°C to 155°C, and the temperature of the second zone is raised from 700°C to 720°C.

[0140] In step S3, the particle size D50 of the positive electrode material substrate is 4.02 μm.

[0141] In step S4, the X2 / X1 of the Mg element in the obtained positive electrode material is 5.8, and the content of free nitrate is 32 ppm.

[0142] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0143] Example 15

[0144] The difference from Example 1 is that in step S3, the sintering temperature after mixing is adjusted from 930°C to 915°C.

[0145] In step S4, the X2 / X1 of the Mg element in the obtained positive electrode material is 6.4, and the content of free nitrate is 73 ppm.

[0146] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0147] Example 15

[0148] The difference from Example 1 is that in step S1, the pure metal particles of Ni, Co and Al are respectively put into nitric acid to form nitrate solutions, and the three nitrate solutions are prepared into a mixed nitrate solution according to the molar ratio Ni:Co:Al=83:12:5, wherein the total concentration of Ni, Co and Al in the solution is 5 mol / L, and then the nitrate solution of Mg is added to the mixed nitrate solution until the molar ratio of Mg:Ni in the mixed solution is 1:466, to obtain a pre-mixed solution.

[0149] In step S3, sintering is carried out at 880°C for 8h under an oxygen atmosphere, the sintered material is crushed, the particle size D50 is 4.10 μm, and a positive electrode material substrate is obtained.

[0150] In step S4, the X2 / X1 of the Mg element in the obtained positive electrode material is 11.3, and the content of free nitrate is 80 ppm.

[0151] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0152] Comparative Example 1

[0153] Step S1, prepare a mixed hydrochloride solution by mixing hydrochloride solutions of Ni, Co and Mn in a molar ratio of Ni:Co:Mn = 60:10:30, and then add a hydrochloride solution of Mg to the mixed hydrochloride solution until the molar ratio of Mg:Ni in the mixed solution is 1:1300 to obtain a premix solution.

[0154] Step S2, spray pyrolysis of the premix solution, wherein the nozzle flow rate is 500 L / h, only one temperature zone is set, and the temperature is 700°C to obtain an oxide precursor, and the obtained oxide precursor is subjected to gas crushing.

[0155] Step S3, uniformly mix 500 g of the crushed oxide precursor, 233.4 g of lithium carbonate, and 2.25 g of ZrO2, sinter at 930°C for 8 h under compressed air atmosphere, crush the sintered material, and obtain a positive electrode material substrate with a particle size D50 = 4.18 μm.

[0156] Step S4, preliminarily mix 400 g of the positive electrode material substrate with 0.67 g of TiO2, sinter at 550°C for 8 h under an oxygen atmosphere, and sieve the obtained material to obtain a positive electrode material. The X2 / X1 of Mg element in the positive electrode material is 1.1, and the content of free nitrate is 0 ppm.

[0157] Comparative Example 2

[0158] Step S1, put pure metal particles of Ni, Co and Mn into nitric acid respectively to form nitrate solutions, and prepare a mixed nitrate solution by mixing the three nitrate solutions in a molar ratio of Ni:Co:Mn = 60:10:30.

[0159] Step S2, spray pyrolysis of the mixed nitrate solution, wherein the nozzle flow rate is 900 L / h, only one temperature zone is set, and the temperature is 600°C to obtain an oxide precursor, and the obtained oxide precursor is subjected to gas crushing.

[0160] Step S3, uniformly mix 500 g of the crushed oxide precursor, 233.4 g of lithium carbonate, and 2.25 g of ZrO2 and 1.25 g of MgO, sinter at 550°C for 8 h under an oxygen atmosphere, and sieve the obtained material to obtain a positive electrode material. The X2 / X1 of Mg element in the positive electrode material is 57, and the content of free nitrate is 147 ppm.

[0161] Comparative Example 3

[0162] Step S1, hydroxide precursor with the ratio of Ni:Co:Mn=60:10:30 was synthesized by co-precipitation method. Step S2, 500g of the hydroxide precursor was uniformly mixed with 211.8g of lithium carbonate, 2.25g of ZrO2 and 1.25g of MgO, and sintered at 930℃ for 8h under compressed air atmosphere. The sintered material was crushed to obtain a positive electrode material matrix with a particle size D50=4.15μm.

[0163] Step S4, 400g of the positive electrode material matrix was preliminarily mixed with 0.67g of TiO2, and sintered at 550℃ for 8h under oxygen atmosphere. The obtained material was sieved to obtain a positive electrode material. In the positive electrode material, the X2 / X1 of Mg element was 5.7, and the content of free nitrate was 2ppm.

[0164] Comparative Example 4

[0165] The difference from Example 1 is that in Step S2, the temperature of the second temperature zone is increased from 700℃ to 800℃; in Step S3, the particle size D50 of the positive electrode material matrix is 4.18μm. In Step S4, the X2 / X1 of Mg element in the obtained positive electrode material is 5.7, and the content of free nitrate is 2ppm.

[0166] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0167] Comparative Example 5

[0168] The difference from Example 1 is that in Step S2, the nozzle flow rate in the spray pyrolysis process is reduced from 500L / h to 200L / h; in Step S3, the particle size D50 of the positive electrode material matrix is 4.10μm. The X2 / X1 of Mg element in the obtained positive electrode material is 1.1, and the content of nitrate is 76ppm.

[0169] The other steps are basically the same as those in Example 1, please refer to Example 1.

[0170] The positive electrode materials obtained in Examples 1-16 and Comparative Examples 1-5 were tested for performance by the following method.

[0171] 1. The testing method of the main element and M element concentration in the positive electrode material is as follows:

[0172] Agilent 5110ICP-OES equipment was used. 0.3g of sample was added with aqua regia for digestion, and the mother liquor was cooled and diluted to a volume of 100ml. The mass content of M element was tested by Agilent 5110ICP-OES equipment, and was recorded as ICP(M). 1ml of the mother liquor was diluted 100 times, and the mass content of the main elements Li / Ni / Co / Mn was tested by Agilent 5110ICP-OES equipment.

[0173] 2. The test method of the concentration distribution of element M in the positive electrode material is as follows:

[0174] The test method of the average concentration X1 of element M in the center region and the average concentration X2 of element M in the surface layer region in the positive electrode material particle is as follows: a section sample is prepared by using a Hitachi ion cutter (IM5000), a suitable section field is selected by using a Hitachi S4800 scanning electron microscope, and line scanning or point scanning of the particle section in the 5k field is performed by using EDS. For example, the point scanning mode can be used, the content of element M at 10 positions (generally at least 5 points) in the center region of the particle section is measured, the content of element M is summed and averaged to obtain the center region atomic number (dimensionless) in the particle; the content of element M at 10 positions in the surface layer region of the particle section is measured, the content of element M is summed and averaged to obtain the surface layer region atomic number (dimensionless) of the particle. Then, 10 particles are randomly selected, and the center region atomic number and the surface layer region atomic number in each particle are characterized according to the above method, the center region atomic numbers of the 10 particles are summed and averaged to obtain the average concentration X1 (dimensionless) of element M in the center region, and the surface layer region atomic numbers of the 10 particles are summed and averaged to obtain the average concentration X2 (dimensionless) of element M in the surface layer region.

[0175] As shown in FIG. 3, in the particle section view, the center of the circumscribed circle of the particle is taken as the center, the radius of the circumscribed circle is the radius L of the particle, the region from the center of the particle to the position at a distance of L / 2 from the center of the particle is the center region, and the region from the position at L / 2 to the surface of the particle is the surface layer region. When the positive electrode material contains two or more elements M, the center region atomic number X1 and the surface layer region atomic number X2 of each element M are determined respectively, and the X2 / X1 value of the corresponding element is calculated accordingly.

[0176] For point scanning and line scanning, the following is explained in combination with FIG. 3:

[0177] Point scanning: 5-10 points in the center region are selected for EDS measurement, and the average value after measurement can obtain the center region atomic number X1 value; 5-10 points in the surface layer region are selected for EDS measurement, and the average value after measurement can obtain the surface layer region atomic number X2 value.

[0178] Line scanning: at least three line positions in the center region are randomly selected for line scanning, and the average value after measurement can obtain the center region atomic number X1 value; at least three line positions in the surface layer region are randomly selected for line scanning, and the average value after measurement can obtain the surface layer region atomic number X2 value. In actual testing, point scanning and line scanning can also be used in combination.

[0179] The calculation of the central area element mass content M1: M1 (ppm) = ICP (M) * X1 / (X1 + X2); wherein, ICP (M) is the mass content of element M calculated by the above method (1), X1 is the atomic number of the central area characterized by EDS, and X2 is the atomic number of the surface layer area characterized by EDS;

[0180] The calculation of the surface layer area element mass content M2: M2 (ppm) = ICP (M) * X2 / (X1 + X2); ICP (M) is the mass content of element M calculated by the above method (1), X1 is the atomic number of the central area characterized by EDS, and X2 is the atomic number of the surface layer area characterized by EDS.

[0181] 3. The content of nitrate ions on the surface of the positive electrode material is characterized by using an ICS-6000 type ion chromatograph, and the test method is as follows:

[0182] (1) About 0.7g of sample was weighed in a 70mL beaker, accurate to 0.0001g.

[0183] (2) 50mL of ultrapure water was accurately measured in a 50mL volumetric flask, a small amount of ultrapure water (about 7mL) was first added to the sample, and then the remaining ultrapure water was added after stirring with a glass rod. After stirring and ultrasonic treatment for 3min, the sample was allowed to settle for about 17min.

[0184] (3) The supernatant of the sample was taken with a 1mL sterile syringe and injected into the instrument through a disposable filter head for testing; the ion chromatograph model: TherMo Scientific Dionex ICS-6000.

[0185] (4) Chromatography setting conditions: eluent concentration: 25mmol / L; eluent flow rate: 0.3mL / min; column temperature: 30±1℃; detection cell temperature: 35±1℃; suppressor current: 26mA; chromatographic column: anion exchange column (4*250mm, AS11-HC).

[0186] (5) Data acquisition: after the baseline is stable (signal value change is less than 0.1μS / 20min), 1mL of anion mixed solution (supernatant) is taken with a 1mL syringe, and the concentration Ci of nitrate ions is obtained after analysis, which is mg / L.

[0187] (6) Result calculation: the calculation formula is Xi = Ci*V / M, wherein Xi is the mass content of nitrate ions in the sample, which is mg / kg; Ci is the concentration of nitrate ions in the sample solution, which is mg / L; V is the volume of the sample solution, which is mL; and M is the mass of the sample, which is g.

[0188] 4. Particle size test:

[0189] The particle size distribution of the positive electrode material was tested by a Malvern laser particle size tester MS 3000. Among them, Dv10 represents the particle size corresponding to the cumulative particle size distribution percentage of 10%, Dv50 represents the particle size corresponding to the cumulative particle size distribution percentage of 50%, and Dv90 represents the particle size corresponding to the cumulative particle size distribution percentage of 90%. The particle size distribution span = (Dv90-Dv10) / Dv50.

[0190] 5. The tap density p1 of the positive electrode material is tested as follows:

[0191] The tap density was tested by Dandong Bit BT-303: vibration frequency 3000 times / min, vibration time 1 min, and vibration amplitude 3 mm ± 0.1 mm.

[0192] 6. The powder compaction density p2 of the positive electrode material is tested as follows:

[0193] The compaction density was tested by a U.S. carver 4350 tester: 1 g of sample was placed in a mold and pressed for 30 s with a pressure of 6 t. After pressing, the height was tested to calculate the compaction density. The compaction density is the ratio of the sample mass to the volume after compaction.

[0194] 7. The specific surface area S1 of the positive electrode material is tested as follows: the specific surface area of the material was tested by a Micromeritics 3020 nitrogen adsorption method.

[0195] Equipment: specific surface area and pore structure analyzer.

[0196] Method: weigh the empty sample tube mass m1; take 3 g of sample and add it to the sample tube through a long neck funnel, vacuum degassing at 300°C for 1 h, and weigh the sample tube mass after cooling as m2; the sample mass is m = m2-m1. The sample tube is placed in liquid nitrogen, and the nitrogen adsorption amount V of the sample is measured at a series of relative pressures P / P0 to obtain the adsorption isotherm. Among them, P / P0 is set to 0.05 / 0.1 / 0.15 / 0.20 / 0.25 / 0.30. The isothermal adsorption curve is fitted, and the single-layer saturated adsorption amount Vm is calculated according to the slope and intercept, and then the specific surface area is calculated according to Vm.

[0197] 8. The average particle size of the primary particles is tested as follows:

[0198] The Hitachi S4800 scanning electron microscope is used to take SEM images of the positive electrode material, with a magnification of 3k times, and the number of single particles in each SEM image is not less than 200. The Nano Measure software is selected to open the SEM image, 2 grids in the image ruler are selected as the measurement ruler, the size of the single crystal primary particles in the SEM image is measured by cross marking method, and the number of statistics is not less than 400. The average size in the final output report is the average particle size of the primary particles.

[0199] 9. Battery production and performance test:

[0200] a. Sample preparation for electrochemical performance test:

[0201] Button cell production: The ternary positive electrode material, conductive carbon black and binder PVDF (polyvinylidene fluoride) are mixed in a mass ratio of 80:10:10, then NMP (N-methyl pyrrolidone) is added to make a uniform slurry, which is coated on a copper foil, dried in an oven, and then pressed by a 10Mpa pressure roller. The copper foil is cut into a circular electrode with a diameter of 14mm. The lithium ion battery is assembled according to the industrial CR2025 type button cell, the separator is Cellgard separator, the electrolyte is 1Mol / L LiPF6 solution with EC / PC / DEC as solvent, and the anode is lithium sheet. The whole assembly process is carried out in an argon-filled glove box, and the oxygen content and moisture content in the glove box are controlled below 0.5ppm.

[0202] b. Button cell test:

[0203] After standing for 12h, the battery test is carried out according to the following system:

[0204] 0.1C charge 0.1C discharge for 2 weeks, the charge cut-off voltage is 4.45V, the discharge cut-off voltage is 2.5V, the constant voltage cut-off current is 0.005C, and the first circle capacity, discharge specific capacity and first coulomb efficiency are measured.

[0205] High voltage cycle performance test: 0.5C charge 1C discharge, charge cut-off voltage is 4.45V, discharge cut-off voltage is 2.5V, constant voltage cut-off current is 0.05C, 50 weeks cycle, the ratio of capacity measured after 50 weeks cycle to first circle capacity is 50 weeks cycle capacity retention rate.

[0206] 11. Gas production performance test:

[0207] The storage gas production performance of the positive electrode material is evaluated by using a unit soft package battery. The specific method is as follows: the positive electrode material, PVDF, SP and CNT are weighed according to the mass ratio of 97.0:1.0:1.5:0.5, NMP is added according to the solid content of 50%, and a viscous slurry is prepared. The slurry is uniformly coated on an aluminum foil, dried in an 80°C oven, and then rolled and cut into a positive electrode sheet with a size of 4cmx8cm. The graphite negative electrode, SP, CMC and SBR are weighed according to the mass ratio of 96:1.0:1.2:1.8, dispersed into a slurry, coated on a copper foil, and cut into a negative electrode sheet with a size of 4cmx8cm. The positive electrode sheet and the negative electrode sheet are assembled into a unit soft package battery, charged to 50SOC% after formation, and then stored in a 60°C oven for 21 days. The volume difference before and after storage is tested by the drainage method, and the volume change rate is calculated, which is the gas production rate.

[0208] The corresponding test results of the positive electrode materials of Examples 1-16 and Comparative Examples 1-5 are shown in Tables 1 and 2.

[0209] Table 1

[0210] Table 2

[0211] As shown in FIGS. 4 and 5, the distribution of Mg element in the particles of the positive electrode materials prepared in Example 1 and Comparative Example 1 is shown, respectively. As shown in FIG. 4, the Mg element in Example 1 shows a concentration difference in the center region and the surface region of the single crystal particles. As shown in FIG. 5, the Mg element in Comparative Example 1 shows a uniform distribution in the center region and the surface region of the single crystal particles.

[0212] FIG. 6 is an XPS spectrum of the surface Li element of the positive electrode material in Example 1 after being charged and discharged in a battery for one cycle. As shown in FIG. 6, the positive electrode material contains a large amount of Li2O phase on the surface due to the presence of a proper amount of free nitrate ions in Example 1. FIG. 7 is an XPS spectrum of the surface Li element of the positive electrode material after being charged and discharged in a battery for one cycle. As shown in FIG. 7, the positive electrode material in Comparative Example 1 does not contain free nitrate, and only a small amount of Li2O phase is present on the surface of the positive electrode material.

[0213] As can be seen from Table 1-2, in Examples 1-16, the positive electrode materials all have excellent capacity, initial efficiency and high-pressure cycle retention rate, the capacity is ≥182.6 mAh / g, the initial efficiency is ≥87.3%, after 0.5C charging and 1C discharging for 50 cycles, the high-pressure cycle retention rate is all ≥93%, and the volume change rate is all ≤6%. This is mainly because, in Examples 1-16, the content of free nitrate ions in the positive electrode material is controlled to be 10 ppm-100 ppm, and the distribution of M elements in the positive electrode material satisfies: 1.2≤X2 / X1≤20.

[0214] Compared with Example 1, in the positive electrode material prepared in Comparative Example 1, the concentration X2 / X1 of M elements is <1.2, and the distribution of M elements in the surface particles is relatively uniform; at the same time, the content of free nitrate in the material is too low to form an effective Li2O stable interface protection layer on the surface of the positive electrode material. Therefore, the capacity, initial efficiency and cycle retention rate of the battery prepared by the positive electrode material of Comparative Example 1 are all low.

[0215] Compared with Example 1, in the positive electrode material prepared in Comparative Example 2, the concentration X2 / X1 of M elements is >20, and too much M elements will be enriched on the surface layer of the particles, similar to the conventional coating effect, which has no obvious advantage in improving the performance of the material, and also occupies the lithium site in the lattice, affecting the transmission and diffusion of lithium ions, and reducing the rate performance of the secondary battery; at the same time, the content of free nitrate in the material is too high, which is easy to cause the CEI film layer on the surface of the positive electrode material to be too thick, which increases the internal resistance of the battery and reduces the rate performance. Therefore, the capacity, initial efficiency and cycle retention rate of the battery prepared by the positive electrode material of Comparative Example 2 are all low.

[0216] Compared with Example 1, in Comparative Example 3, the hydroxide precursor is synthesized by coprecipitation method, and the precursor is not doped with M elements during the preparation process, although M elements are doped in the subsequent sintering process, but it is difficult to form the concentration difference of M elements in the inner and outer layers, at the same time, the content of free nitrate in the positive electrode material is 0, therefore, the capacity, initial efficiency and cycle retention rate of the battery prepared by the positive electrode material of Comparative Example 3 are all low.

[0217] Compared with Example 1, in Comparative Example 4, the distribution of M elements in the positive electrode material satisfies 1.2≤X2 / X1≤20, but the content of free nitrate in the material is too low (only 2 ppm), which is difficult to induce the formation of Li2O stable interface protection layer on the surface of the material, therefore, the capacity, initial efficiency, cycle retention rate and gas production performance of the battery prepared by the positive electrode material of Comparative Example 4 are all poor.

[0218] Compared with Example 1, the distribution of M element in the cathode material in Comparative Example 5 is X2 / X1 < 1.2, the distribution of M element is relatively uniform, and does not show the characteristics of different concentration gradient in the inner layer and outer layer of the particle, and the cell pressure stress is not obvious, therefore, the capacity, initial efficiency, cycle retention rate and gas production performance of the battery prepared by the cathode material of Comparative Example 5 are all poor.

[0219] The above embodiments are only used to illustrate the technical solutions of the present application but not limit the present application. Although the present application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application.

Claims

1. A positive electrode material, characterized by, The positive electrode material contains an M element, the M element is selected from at least one metal element in group IA and group IIA in the periodic table of elements, the positive electrode material includes a plurality of particles, the distribution of the M element in the particles satisfies: 1.2≤X2 / X1≤20, wherein X1 is the concentration of the M element at the central region of the particles, X2 is the concentration of the M element at the surface layer region of the particles; and the positive electrode material contains free nitrate ions, the mass content of the free nitrate ions is N1, wherein 10ppm≤N1≤100ppm.

2. The positive electrode material of claim 1, wherein, The positive electrode material satisfies at least one of the following conditions: (1) the X2 / X1 is 1.2, 1.5, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or within a range between any two of the above values; (2) the N1 is 10ppm, 20ppm, 30ppm, 40ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm, 100ppm or within a range between any two of the above values.

3. The positive electrode material of claim 1, wherein, The mass percentage of the M element is 10ppm-400ppm based on the mass of the positive electrode material.

4. The positive electrode material of claim 1, wherein, The mass content of the M element in the central region of the particles M1 is 10ppm-50ppm; and / or, the mass content of the M element in the surface layer region of the particles M2 is 300ppm-400ppm.

5. The cathode material of claim 1, wherein, The distribution of the M element in the particles satisfies 6≤X2 / X1≤11; and / or, the mass content of the free nitrate ions N1 is 30ppm-80ppm.

6. The cathode material of claim 1, wherein, The positive electrode material has the following general formula: Li a Ni b Co c Q d M (1-b-c-d-e) NeO2, wherein 0.95 < a < 1.2, 0.5 < b < 1, 0 < c < 0.25, 0 < d < 0.45, b + c + d + e <1, 0≤e<0.1, Q includes at least one of Mn and Al, the element M is selected from at least one metal element in group IA and group IIA in the periodic table of elements, the element N does not include Li, Ni, Co, Q element and M element.

7. The positive electrode material of claim 6, wherein, The positive electrode material satisfies at least one of the following conditions: (1) the M element is selected from at least one of Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba; (2) the N element is selected from at least one element of Zr, Ti, Al, Co, Mg, W, Ce, Y, La, Nb and Mo.

8. The cathode material of claim 1, wherein, The volume particle size distribution width of the positive electrode material satisfies: 1.0≤(D90-D10) / D50≤1.

5.

9. The cathode material of claim 1, wherein, The lattice strain ε of the positive electrode material is <0.1%.

10. The cathode material of claim 1, wherein, The positive electrode material satisfies at least one of the following conditions: (1) the specific surface area of the positive electrode material is 0.4 m 2 / g ~ 0.9 m 2 / g; (2) the tap density p1 of the positive electrode material is 2.0 g / cm3 3 ~ 3.0 g / cm3 3 .

11. The cathode material of claim 1, wherein, The powder compaction density p2 of the positive electrode material is 2.8 g / cm 3 ~ 4.0 g / cm 3 .

12. The cathode material of claim 1, wherein, The positive electrode material is a single crystal material; and / or the positive electrode material includes primary particles, the average particle size of the primary particles is 1μm-5μm.

13. The cathode material of claim 1, wherein, The free lithium content of the positive electrode material is 400ppm-600ppm.

14. A positive electrode sheet characterized by comprising: The positive electrode material as claimed in any one of claims 1 to 13.

15. A secondary battery characterized by comprising: The positive electrode material as claimed in any one of claims 1 to 13.

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