Composite positive electrode material, preparation method thereof, positive electrode sheet and secondary battery

Abstract

The application provides a composite positive electrode material, a preparation method of the composite positive electrode material, a positive electrode sheet and a secondary battery. The composite positive electrode material comprises a shell layer and a core material. The chemical formula of the core material is LiNi x Co y Mn 1‑x‑y‑z Me z O2, 0≤x≤1, 0≤y≤1, 0≤z≤0.2, and x+y+z>0. The chemical formula of the shell layer is Me p N q , 1≤p≤3, and 1≤q≤3. Me comprises at least one of Mn, Co, Ni, Fe, Al, Cu, Ti, Cr, Zr, Zn, Mg and W. In the application, the core material is coated with a metal nitride in the shell layer, and metal cations in the core material are doped. The shell layer and the core material cooperate with each other to reduce phase change of the positive electrode material in a cycle, effectively improve the compaction density and pressure resistance of the composite positive electrode material, and the shell layer also has high electrical conductivity, which helps to improve the electronic transmission performance of the positive electrode material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a composite positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Since commercialization in the 1990s, lithium batteries have been widely used due to their high energy density, high charging and discharging efficiency, small self-discharge, long service life and environmental friendliness. At present, they have been applied to consumer electronics, aerospace, military, electric tools and electric vehicles. With the development of technology, people's requirements for the endurance of lithium ion batteries are getting higher and higher in both consumer and power battery fields. Developing high energy density has become the key to the development of lithium batteries. Generally, two approaches can be taken. One is to develop new high-capacity positive and negative electrode materials. The other is to increase the charging and discharging voltage of lithium batteries. Increasing the charging and discharging voltage of lithium batteries can increase the mass energy density and volume energy density of lithium batteries, and also reduce the cost of lithium batteries, which has become a research hotspot.

[0003] With the increasing demand for energy density of ternary positive electrode materials, increasing the upper limit of the voltage to increase the energy density of ternary positive electrode materials is easy to lead to the decline of the electrical performance of ternary positive electrode materials, and the compaction density and pressure resistance of ternary positive electrode material powder need to be improved to increase the energy density of ternary positive electrode materials.

[0004] Therefore, it is necessary to provide a technical solution to solve the above problems. SUMMARY

[0005] One of the purposes of the present application is to provide a composite positive electrode material to improve the low compaction density and poor pressure resistance of the current ternary positive electrode material, in view of the shortcomings of the prior art.

[0006] To achieve the above purpose, the present application adopts the following technical solutions:

[0007] A composite positive electrode material, comprising an outer shell layer and an inner core material, the outer shell layer comprising a metal nitride; wherein the chemical formula of the inner core material is LiNi x Co y Mn 1-x-y-z Me z O2, 0≤x≤1, 0≤y≤1, 0≤z≤0.2, and x+y+z>0; the chemical formula of the metal nitride is Me p N q , 1≤p≤3, 1≤q≤3; wherein Me comprises at least one of Mn, Co, Ni, Fe, Al, Cu, Ti, Cr, Zr, Zn, Mg and W.

[0008] Preferably, the mass ratio of the shell layer and the core material is (0.1-0.3):100.

[0009] Preferably, the composite positive electrode material comprises secondary particles composed of a plurality of primary particles, the primary particles are polycrystalline particles, and the ratio of the particle size D50 of the primary particles to the particle size D50 of the secondary particles is 1:(5-40).

[0010] Preferably, the particle size D50 of the secondary particles is 8-40 μm, and the particle size D50 of the primary particles is 0.8-2.0 μm.

[0011] The second object of the present application is to provide a preparation method of a composite positive electrode material, comprising the following steps:

[0012] S1, uniformly mixing a lithium compound, a nickel-cobalt-manganese ternary precursor, and a metal nitride, then performing first-stage sintering under a protective atmosphere, continuing to heat for second-stage sintering, then holding, cooling, and grinding in an inert atmosphere to obtain a pre-sintered product;

[0013] S2, adding a metal nitride to the pre-sintered product, then thoroughly mixing, performing first-stage sintering under a protective atmosphere, continuing to heat for second-stage sintering, then holding, cooling, and grinding in an inert atmosphere, and finally obtaining a composite positive electrode material.

[0014] Preferably, in step S1, the molar ratio of the lithium compound, the nickel-cobalt-manganese ternary precursor, and the metal nitride is Li:(Ni+Co+Mn):Me=(1.01-1.10):1:(0.005-0.3).

[0015] Preferably, in step S1, the first-stage sintering temperature is 800-1400℃, the heating rate is 1-5℃ / min, and the sintering time is 2-10h;

[0016] The second-stage sintering temperature is 1200-1800℃, the heating rate is 5-10℃ / min, the sintering time is 1-8h, and the holding temperature is 400-1000℃.

[0017] Preferably, in step S2, the first-stage sintering temperature is 600-1200℃, the heating rate is 1-5℃ / min, and the sintering time is 4-14h;

[0018] The second-stage sintering temperature is 1000-1600℃, the heating rate is 5-10℃ / min, the sintering time is 1-8h, and the holding temperature is 200-800℃. The third object of the present application is to provide a positive electrode sheet comprising the above-mentioned positive electrode material.

[0019] The fourth object of the present application is to provide a secondary battery including a positive electrode sheet, a negative electrode sheet, and a separator interposed between the positive electrode sheet and the negative electrode sheet, the positive electrode sheet being the positive electrode sheet described above.

[0020] Compared with the prior art, the composite positive electrode material provided by the present application has the beneficial effects that: the metal cation doping and metal nitride coating are performed on the inner core material, the metal nitride shell layer can effectively isolate the direct contact between the positive electrode material and the electrolyte, reduce the occurrence of side reactions, thereby improving the chemical stability and cycle stability of the material, and improving the pressure resistance of the material. The introduction of an appropriate amount of metal cation into the inner core ternary material can change the crystal structure and electronic structure of the material, thereby improving the structural stability and electrochemical performance thereof. Under the synergistic action of the two, the particle strength of the composite positive electrode material is significantly improved, and the compaction density and pressure resistance are also improved. This not only enhances the structural stability of the positive electrode material, but also helps to improve the energy density and cycle life of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 SEM image of the composite positive electrode material prepared for an embodiment of the present application. DETAILED DESCRIPTION

[0022] In order to make the technical solutions and advantages of the present application clearer, the following will combine specific embodiments to further describe the present application and its beneficial effects, but the embodiments of the present application are not limited thereto.

[0023] The terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise specifically limited.

[0024] In the first aspect of the present application, a composite positive electrode material is provided, including a shell layer and an inner core material, the shell layer including a metal nitride; wherein the chemical formula of the inner core material is LiNi x Co y Mn 1-x-y-z Me z O2, 0≤x≤1, 0≤y≤1, 0≤z≤0.2, and x+y+z>0; the chemical formula of the metal nitride is Me p N q , 1≤p≤3, 1≤q≤3; wherein Me includes at least one of Mn, Co, Ni, Fe, Al, Cu, Ti, Cr, Zr, Zn, Mg and W.

[0025] The metal nitride is preferably zirconium nitride, which has high hardness and good wear resistance, thus improving the compaction density and pressure resistance of the composite cathode material; meanwhile, the structure of zirconium nitride is stable and changes little in the process of battery charging and discharging, thus improving the cycle life of the battery; as the shell layer, zirconium nitride also has good electron conductivity and electrochemical activity, which can effectively improve the charging and discharging efficiency and cycle stability of the battery.

[0026] In some embodiments, the mass ratio of the shell layer and the core material is (0.1-0.3):100. Specifically, it can be 0.1:100, 0.2:100, 0.3:100, and can include but is not limited to the above-mentioned values; the mass ratio within the above-mentioned range can optimize the particle size and compaction density of the prepared composite cathode material.

[0027] In some embodiments, the composite cathode material includes secondary particles composed of a plurality of primary particles, the primary particles are polycrystalline particles, and the ratio of the average particle size of the primary particles to the average particle size of the secondary particles is 1:5-40. The primary particles have high energy density, large specific surface area, many lithium ion migration channels and short paths, and thus have good rate performance; the secondary particles composed of primary particles can take advantage of both large particles and small particles, and can improve the cycle performance and compaction density of the primary particles and compensate for the safety of the primary particles.

[0028] Specifically, the ratio of the particle size D50 of the primary particles to the particle size D50 of the secondary particles can be 1:5, 1:10, 1:15, 1:20, 1:25, 1:30, 1:35, 1:40, and can include but is not limited to the above-mentioned values.

[0029] In some embodiments, the particle size D50 of the secondary particles is 8-40 μm, and the particle size D50 of the primary particles is 0.8-2.0 μm. Specifically, the particle size D50 of the secondary particles can be 8 μm, 9 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, and can include but is not limited to the above-mentioned values; the particle size D50 of the primary particles can be 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, and can include but is not limited to the above-mentioned values.

[0030] In a second aspect of the present application, a preparation method of the above-mentioned composite cathode material is provided, which includes the following steps:

[0031] S1, the lithium compound, nickel cobalt manganese ternary precursor, metal nitride are uniformly mixed, then first stage sintering is carried out under a protective atmosphere, then second stage sintering is continuously carried out by heating, then temperature is kept, and after cooling, the pre-sintered product is obtained by grinding in an inert atmosphere;

[0032] S2, the pre-sintered product is added with metal nitride, then after being fully mixed, first stage sintering is carried out under a protective atmosphere, then second stage sintering is continuously carried out by heating, then temperature is kept, and after cooling, the composite positive electrode material is finally obtained in an inert atmosphere.

[0033] In the embodiment of the present application, in step S1, the molar ratio of the lithium compound, nickel cobalt manganese ternary precursor and metal nitride is Li:(Ni+Co+Mn):Me=(1.01-1.10):1:(0.005-0.3). Preferably, Li:(Ni+Co+Mn):Me=1.05:1:(0.01-0.2). Specifically, it can be 1.05:1:0.01, 1.05:1:0.01, 1.05:1:0.02, 1.05:1:0.03, 1.05:1:0.04, 1.05:1:0.05, 1.05:1:0.06, 1.05:1:0.07, 1.05:1:0.08, 1.05:1:0.09, 1.05:1:0.1, 1.05:1:0.15, 1.05:1:0.2, which can include but is not limited to the above-mentioned values, and more preferably 1.05:1:0.1.

[0034] In the embodiment of the present application, in step S1, the first stage sintering temperature is 800-1400℃, preferably 1000-1200℃; the heating rate is 1-5℃ / min, the sintering time is 2-10h, preferably 4-8h; the second stage sintering temperature is 1200-1800℃, preferably 1400-1600℃; the heating rate is 5-10℃ / min, the sintering time is 1-8h, preferably 2-6h; and the temperature keeping temperature is 400-1000℃, preferably 600-800℃.

[0035] In the embodiment of the present application, in step S2, the first stage sintering temperature is 600-1200℃, preferably 800-1000℃; the heating rate is 1-5℃ / min, and the sintering time is 4-14h, preferably 6-12h.

[0036] The second stage sintering temperature is 1000-1600℃, preferably 1200-1400℃; the heating rate is 5-10℃ / min, the sintering time is 1-8h, preferably 1-5h; and the temperature keeping temperature is 200-800℃, preferably 400-600℃.

[0037] The present application adopts multiple segment sintering and nitride coating on the material, which can improve the stability of the surface morphology of the core material, i.e. ternary material, and obtain a more dense coating layer, and can improve the particle size of the ternary positive electrode material particles and the primary particle size thereof. At the same time, the present application improves the morphology stability and density of the surface of the ternary material by controlling the temperature and time.

[0038] In step S1, the lithium compound can be at least one of lithium sulfate, lithium nitrate, lithium chloride, and lithium carbonate, and preferably is lithium carbonate; and the metal nitride can be at least one of cobalt nitride, iron nitride, aluminum nitride, copper nitride, titanium nitride, zinc nitride, zirconium nitride, and magnesium nitride, and preferably is zirconium nitride.

[0039] Compared with the traditional single positive electrode active material, the composite positive electrode material in the present application can improve the particle strength of the composite positive electrode material, improve the compaction density and pressure resistance of the material, and improve the energy density of the positive electrode material by doping metal cations in the core material and coating the nitride on the outer surface of the core material.

[0040] 2. Positive electrode sheet

[0041] The third aspect of the present application provides a positive electrode sheet, which comprises the composite positive electrode material described above, and specifically comprises a positive electrode current collector and a positive electrode active material layer coated on at least one surface of the positive electrode current collector, wherein the positive electrode active material is the positive electrode material described in the present application.

[0042] The positive electrode current collector can be various materials suitable for being used as a lithium ion battery positive electrode current collector in the art, for example, the positive electrode current collector can be, but is not limited to, a metal foil, and more specifically can be, but is not limited to, an aluminum foil.

[0043] 3. Secondary battery

[0044] The fourth aspect of the present application aims to provide a secondary battery, which comprises a positive electrode sheet, a negative electrode sheet, and a separator interposed between the positive electrode sheet and the negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet described above.

[0045] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector. The negative electrode active material layer can be one or more of, but not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbead, silicon-based material, tin-based material, lithium titanate, or other metal capable of forming an alloy with lithium. The graphite can be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon alloy; and the tin-based material can be selected from one or more of elemental tin, tin oxide compound, tin alloy. The negative electrode current collector is generally a structure or part that collects current, and can be any material suitable for use as a negative electrode current collector in a lithium ion battery, for example, the negative electrode current collector can be, but not limited to, a metal foil, and more specifically, can be, but not limited to, a copper foil.

[0046] The separator can be any material suitable for use as a separator in a lithium ion battery, for example, can be a combination of one or more of, but not limited to, polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fiber.

[0047] The secondary battery further includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt, and an additive. The electrolyte lithium salt can be LiPF6 and / or LiBOB used in a high-temperature electrolyte, at least one of LiBF4, LiBOB, and LiPF6 used in a low-temperature electrolyte, at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in an overcharge-preventing electrolyte, and at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC, a chain carbonate, including DFC, DMC, or EMC, or a carboxylic acid ester, including MF, MA, EA, and MP. The additive includes, but is not limited to, at least one of a film-forming additive, a conductive additive, a flame-retardant additive, an overcharge-preventing additive, an additive for controlling the contents of H2O and HF in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive.

[0048] To make the technical solutions and advantages of the present application clearer, the following will describe the present application and its beneficial effects in further detail with reference to specific embodiments, but the embodiments of the present application are not limited thereto.

[0049] Example 1

[0050] The preparation method of the composite positive electrode material of this example is as follows:

[0051] S1: Lithium carbonate, nickel cobalt manganese ternary precursor, zirconium nitride, according to the stoichiometric ratio Li:(Ni+Co+Mn):Zr=1.05:1:0.1 proportion is mixed uniformly, then sintered at 1-5 ℃ / min heating rate to 1100 ℃ for 6h, then continue to heat at a rate of 5-10 ℃ / min to 1500 ℃ for 4h, then 700 ℃ for 8h, after cooling, in a nitrogen atmosphere, grinding to no obvious grain feeling, get pre-sintered product.

[0052] S2: In the pre-sintered product, add zirconium nitride, then mix by ball milling, then sintered at 1-5 ℃ / min heating rate to 900 ℃ for 8h, then continue to heat at a rate of 5-10 ℃ / min to 1300 ℃ for 3h, then 500 ℃ for 3h, after cooling, in a nitrogen atmosphere, grinding, finally get nitride modified lithium nickel cobalt manganese ternary composite cathode material.

[0053] Examples 2-3

[0054] The difference between this embodiment and example 1 is that in step S1, the stoichiometric ratio of zirconium nitride, see table 1 for details.

[0055] The rest is the same as example 1, which will not be repeated here.

[0056] Examples 4-5

[0057] The difference between this embodiment and example 1 is that the sintering temperature of step S1; see table 1 for details.

[0058] The rest is the same as example 1, which will not be repeated here.

[0059] Examples 6-7

[0060] The difference between this embodiment and example 1 is that the sintering time of step S1; see table 1 for details.

[0061] The rest is the same as example 1, which will not be repeated here.

[0062] Examples 8-9

[0063] The difference between this embodiment and example 1 is that in step S2, different mass ratio of zirconium nitride is added, so that the mass ratio of the shell layer and the core material is different; see table 2 for details.

[0064] The rest is the same as example 1, which will not be repeated here.

[0065] Examples 10-11

[0066] The difference between this embodiment and example 1 is that the sintering temperature of step S2; see table 2 for details.

[0067] The rest is the same as example 1, which will not be repeated here.

[0068] Examples 12-13

[0069] The difference between this example and example 1 is the sintering time of step S2; see Table 2 for details.

[0070] The rest is the same as example 1, which will not be repeated here.

[0071] Comparative Example 1

[0072] Lithium carbonate, nickel-cobalt-manganese ternary precursor, and zirconium nitride were mixed uniformly in a stoichiometric ratio of Li:(Ni+Co+Mn) = 1.05:1, and then sintered at 1100°C for 6h under a nitrogen atmosphere at a heating rate of 1-5°C / min, and then held at 700°C for 8h. After cooling, the product was ground in a nitrogen atmosphere to obtain a primary sintered ternary material.

[0073] Comparative Example 2

[0074] The difference between this comparative example and comparative example 1 is that the primary sintered product is subjected to secondary calcination; see Tables 1 and 2 for details.

[0075] The rest is the same as comparative example 1, which will not be repeated here.

[0076] Comparative Example 3

[0077] The difference between this comparative example and example 1 is that there is no step S2.

[0078] The rest is the same as example 1, which will not be repeated here.

[0079] Comparative Example 4

[0080] The difference between this comparative example and example 1 is that there is no second-stage temperature rising sintering in step S1.

[0081] The rest is the same as example 1, which will not be repeated here.

[0082] Comparative Example 5

[0083] The difference between this comparative example and example 1 is that there is no second-stage temperature rising sintering in step S2.

[0084] The rest is the same as example 1, which will not be repeated here.

[0085] Comparative Example 6

[0086] The difference between this comparative example and example 1 is that no zirconium nitride is added in step S1.

[0087] The rest is the same as example 1, which will not be repeated here.

[0088] Comparative Example 7

[0089] The difference between this comparative example and Example 1 is that zirconium nitride is not added in step S2.

[0090] The rest is the same as in Example 1, and will not be repeated here.

[0091] The specific preparation parameters are summarized in Tables 1 and 2.

[0092] Table 1

[0093]

[0094]

[0095] Table 2

[0096]

[0097] The ternary materials prepared in Examples 1-13 and the composite cathode materials prepared in Comparative Examples 1-7 were assembled into lithium-ion coin cells. Electrical performance was tested at room temperature under a charge / discharge condition of 0.5C and a charge / discharge range of 3.0V-4.5V. The test results are as follows:

[0098] Table 3

[0099]

[0100] Particle size measurement method: The weighed ternary cathode material is added to the particle size analyzer. The dispersant in the instrument is pure water. The speed of the stirrer is 3000 rpm. By adjusting the refractive index, the test cycle is repeated 3 times. Finally, the particle size data of the particle size analyzer is read.

[0101] Table 4. Test results of secondary and primary particle sizes

[0102]

[0103] Compacted density measurement method: The weighed ternary cathode material is loaded into the measuring cylinder of the compaction device, and the measuring cylinder is fixed on the support. The cam is rotated, causing the directional rod to slide the support up and down, pressing it onto the anvil. Then, a pressure of 1 t per minute is applied, finally increasing to 5 t. The volume of the ternary cathode material in the measuring cylinder is measured; the ratio of the mass of the ternary cathode material to its volume is the compacted density of the ternary cathode material.

[0104] Table 5 Compacted density test results

[0105]

[0106]

[0107] From the above test results, it can be seen that Examples 1-3 and Comparative Examples 1-2 show that doping metal elements in the core material in step S1, combined with segmented multiple sintering, and using 0.1-0.3wt% nitride coating in step S2, combined with segmented multiple sintering again and controlling the sintering temperature and time, are beneficial to improving the comprehensive electrical performance of the modified lithium nickel cobalt manganese ternary material.

[0108] From Examples 1-3 and Comparative Examples 1 and 2, in Examples 1-3, when the mass ratio of the added nitride in step S1 sintering is 0.01-0.2, the particle size, primary particle size and compaction density are all greater than those of the comparative examples, and the mass ratio of the added nitride is optimal when it is 0.1; from Examples 1, 4, 5 and Comparative Examples 1 and 2, changing the sintering temperature, the particle size, primary particle size and compaction density of the prepared composite positive electrode material are all greater than those of the comparative examples, and the particle size and primary particle size increase with the increase of the temperature, but the compaction density is more optimal when the first-stage sintering temperature, second-stage sintering temperature and holding temperature are 1100℃, 1500℃ and 700℃, respectively.

[0109] From Examples 1, 6, 7 and Comparative Examples 1 and 2, when the first-stage sintering time, second-stage sintering time and holding time of the primary sintering are 4-8h, 2-6h and 6-10h, respectively, the particle size, primary particle size and compaction density are all greater than those of the comparative examples, and the particle size increases with the increase of the time, but the primary particle size and compaction density are optimal when the first-stage sintering time, second-stage sintering time and holding time are 6h, 4h and 8h, respectively; from Examples 1, 8, 9 and Comparative Examples 1 and 2, in Examples 1, 8, 9, when the mass ratio of the nitride coating layer is 0.1%-0.3%, the particle size, primary particle size and compaction density are all greater than those of the comparative examples, and the particle size, primary particle size and compaction density increase with the increase of the mass ratio of the added nitride; from Examples 1, Comparative Examples 4-5 and Comparative Examples 1-3, when the second-stage sintering in step S1 is cancelled or the second-stage sintering in step S2 is cancelled, the particle size, primary particle size and compaction density are all greater than those of Comparative Examples 1 and 2, but the particle size and compaction density of Comparative Example 4 are less than those of Comparative Example 3, and the particle size, primary particle size and compaction density of Comparative Examples 4 and 5 are all deteriorated compared with those of Example 1; from Examples 1, Comparative Examples 6-7 and Comparative Examples 1-3, when the added nitride in the primary sintering is cancelled or the added nitride in the secondary sintering is cancelled, the particle size, primary particle size and compaction density are all greater than those of Comparative Examples 1 and 2, but the particle size of Comparative Example 6 is less than that of Comparative Example 3, and the particle size, primary particle size and compaction density of Comparative Examples 6-7 are all deteriorated compared with those of Example 1.

[0110] Those skilled in the art can make various modifications and variations to the above embodiments based on the disclosure and teachings of this specification. Therefore, the present application should not be limited to the above specific embodiments, and any obvious modifications, replacements or variations made by those skilled in the art based on the present application shall fall within the scope of the present application. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation on the present application.

Claims

1. A composite cathode material, characterized in that, It includes a shell layer and a core material. The shell layer includes metal nitride; wherein, the chemical formula of the core material is LiNi x Co y Mn 1-x-y-z Me z O2, 0 < x < 1, 0 < y < 1, 0 < z ≤ 0.2, and x + y + z > 0; the chemical formula of the metal nitride is Me p N q , 1 ≤ p ≤ 3, 1 ≤ q ≤ 3; wherein Me includes at least one of Mn, Co, Ni, Fe, Al, Cu, Cr, Zr, Zn, Mg and W; the preparation method of the composite cathode material includes the following steps: S1. After uniformly mixing lithium compound, nickel-cobalt-manganese ternary precursor and metal nitride, the first stage of sintering is carried out under a protective atmosphere, and then the temperature is increased to carry out the second stage of sintering. After holding at the temperature and cooling, the product is ground in an inert atmosphere to obtain the pre-sintered product. S2. Add metal nitride to the pre-sintered product, mix thoroughly, and then perform the first stage of sintering under a protective atmosphere. Continue to heat up for the second stage of sintering, then keep warm, cool, and grind in an inert atmosphere to finally obtain the composite cathode material. In step S1, the first sintering temperature is 1000-1200℃, the heating rate is 1-5℃ / min, and the sintering time is 4-8h; the second sintering temperature is 1400-1600℃, the heating rate is 5-10℃ / min, the sintering time is 2-6h, and the holding temperature is 600-800℃. In step S2, the first sintering temperature is 800-1000℃, the heating rate is 1-5℃ / min, and the sintering time is 6-12h; the second sintering temperature is 1200-1400℃, the heating rate is 5-10℃ / min, the sintering time is 1-5h, and the holding temperature is 400-600℃.

2. The composite cathode material according to claim 1, characterized in that, The mass ratio of the outer shell material to the core material is (0.1-0.3):

100.

3. The composite cathode material according to claim 1, characterized in that, The composite cathode material includes secondary particles composed of multiple primary particles. The primary particles are polycrystalline particles, and the ratio of the particle size D50 of the primary particles to the particle size D50 of the secondary particles is 1:(5-40).

4. The composite cathode material according to claim 3, characterized in that, The particle size D50 of the secondary particles is 8~40μm; the particle size D50 of the primary particles is 0.8~2.0μm.

5. The composite cathode material according to claim 1, characterized in that, In step S1, the molar ratio of the lithium compound, the nickel-cobalt-manganese ternary precursor, and the metal nitride is Li : (Ni+Co+Mn) : Me = (1.01-1.10): 1 : (0.005-0.3).

6. A positive electrode plate, characterized in that, Includes the composite cathode material as described in any one of claims 1 to 5.

7. A secondary battery, comprising a positive electrode, a negative electrode, and a separator spaced between the positive electrode and the negative electrode, characterized in that, The positive electrode is the positive electrode as described in claim 6.

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