Modified positive electrode material and preparation method therefor, and positive electrode sheet

Abstract

A modified positive electrode material and a preparation method therefor, a positive electrode sheet, and a lithium-ion battery. The modified positive electrode material comprises a positive electrode material serving as a matrix, and a composite coating layer generated in situ on the surface of the matrix. The composite coating layer comprises a spinel phase coating layer disposed on the inner side and a composite nitride coating layer disposed on the outer side, wherein the chemical formula of the composite nitride coating layer is selected from αANβOγ·(1-α)DδEε. The thermal conductivity and electrical conductivity of the modified positive electrode material are effectively improved. Moreover, when used in a lithium-ion battery, the modified positive electrode material has excellent thermal safety and electrochemical performance.

Description

Modified positive electrode material and preparation method thereof, positive electrode sheet

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Chinese patent application 202311552496.9 filed on November 20, 2023, the contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to the technical field of lithium-ion batteries, and in particular to a modified positive electrode material and a preparation method thereof, a positive electrode sheet containing the modified positive electrode material, and a lithium-ion battery containing the positive electrode sheet. Background Art

[0004] Lithium-ion batteries are widely used in energy storage due to their high operating voltage, energy density, long lifespan, environmental friendliness, and lack of memory effect. With the rapid development of new energy electric vehicles, higher demands are being placed on the energy density and safety of lithium-ion batteries, particularly regarding thermal safety under extreme abuse conditions such as overcharging, high-temperature storage, hot boxes, and needle punctures. In these situations, heat can accumulate and diffuse within the battery cell due to diaphragm shrinkage, internal short circuits, or SEI membrane decomposition, further triggering electrolyte decomposition, gassing of the positive and negative electrodes, and even failure. In severe cases, this can lead to battery fires and explosions.

[0005] Modification of the positive electrode material and design of the positive electrode plate structure can help improve the thermal safety and structural stability of lithium-ion batteries. An effective modification method is to coat the surface of the positive electrode material with a thermally conductive material to improve the thermal stability of the positive electrode material. Nitride ceramics have high thermal conductivity and dielectric constant, a thermal expansion coefficient that matches silicon, and excellent high-temperature electrical insulation, and are widely used in the fields of integrated circuits and electronics. Although nitride ceramics currently have stable synthesis processes and preparation methods, their application in lithium-ion batteries is limited due to their high sintering temperature and secondary removal of residual carbon. In addition, the choice of some coating materials may hinder the kinetic transport of lithium ions at the interface, reducing the capacity of the active material to a certain extent. Therefore, how to balance the thermal safety and electrochemical performance of positive electrode materials and positive electrode plates under harsh operating conditions has received widespread attention from industry and academia.

[0006] CN115377383A uses a wet coating method to contact and mix the ternary cathode material matrix, nitride nanoparticle coating agent, and organic combustion aid in a dispersant to obtain a mixed slurry; then ball milling and drying are performed to obtain the pre-coated material; the pre-coated material is then sintered to form a nitride coating layer on the surface of the ternary cathode material matrix. This technical solution adopts a non-in-situ coating method, directly using nitride particles as the coating material for mixed material sintering, which can only form an island-like coating layer and has the disadvantage of a locally too thick coating layer; the subsequent use of an air burning method easily causes the organic combustion aid to be oxidized and decomposed first, resulting in partial loss, thereby failing to fully exert its role in lowering the reaction temperature and promoting the forward progress of the coating reaction.

[0007] The high-temperature resistant high-nickel ternary positive electrode material disclosed in CN108400301A includes a substrate and a coating layer on the substrate. The chemical formula of the substrate material is: LiNi 0.8 Co 0.1 Mn 0.1 O2 or LiNi 0.8 Co 0.15 Al 0.05 O2; a modified positive electrode material obtained by in-situ generation of a BN coating layer on a base material; the preparation method comprises: adding a nitrogen source and a boron source to a solvent, heating and stirring to fully dissolve, and obtaining a mixed solution; adding a certain amount of base material to the mixed solution, stirring, drying, and grinding into powder; sintering in an ammonia atmosphere and then ball milling to obtain a positive electrode material. In this technical solution, the sintering temperature is 400-500°C, and it is difficult for the nitrogen source and the boron source to fully react to generate the target product. In addition, impurity phases such as boron oxide are easily introduced, making it difficult to obtain a pure boron nitride coating layer. The thermal decomposition temperature of the nitrogen source is low. For example, ammonia water, ammonium fluoride, ammonium carbonate, etc. are easily decomposed into ammonia volatilization during the sintering process, which reduces the reaction nitrogen source. On the one hand, it causes a waste of raw materials, and on the other hand, it reduces the yield of the coating material.

[0008] CN108336315A employs in-situ coating, mixing the cathode material, a titanium-containing compound, and water in a dispersion medium to produce a precursor. This precursor is then sintered under a nitrogen atmosphere to yield a titanium nitride-coated lithium nickel manganese oxide cathode material. The titanium-containing compound employed in this technical solution readily decomposes during subsequent heat treatment to produce titanium dioxide, which introduces coating impurities. The nitride coating typically requires a relatively high sintering temperature, resulting in high energy consumption. At higher temperatures, this can affect the stability of the cathode material's lattice structure.

[0009] Summary of the Invention

[0010] The purpose of the present invention is to overcome the above-mentioned technical problems and provide a modified positive electrode material and a preparation method thereof, a positive electrode plate and a lithium-ion battery. The modified positive electrode material generates a composite coating layer in situ on the surface of the substrate, so that the modified positive electrode material has both high electrical conductivity and thermal conductivity; at the same time, the modified positive electrode material is used in lithium-ion batteries to effectively improve the electrochemical performance and thermal safety of the battery.

[0011] In order to achieve the above-mentioned object, the first aspect of the present invention provides a modified positive electrode material, wherein the modified positive electrode material comprises a positive electrode material as a substrate, and a composite coating layer generated in situ on the surface of the substrate; the composite coating layer comprises an inner spinel phase coating layer and an outer composite nitride coating layer;

[0012] Wherein, the chemical formula of the composite nitride coating is selected from αAN β O γ ·(1-α)D δ E ε , wherein, 0.7≤α<1, 0.7≤β<1, 0.1≤γ≤0.3, 1≤δ≤2, 1≤ε≤3; A is selected from one of aluminum Al, boron B and titanium Ti, D is selected from at least one of yttrium Y, calcium Ca, lithium Li, barium Ba, magnesium Mg, strontium Sr, lanthanum La, hafnium Hf, zirconium Zr and cerium Ce, and E is selected from at least one of carbon C, nitrogen N, oxygen O and fluorine F.

[0013] Preferably, the composite coating layer is composed of a reactant containing an organic carbon source, A and a general formula D δ E ε The mixture of the fluxing and sintering agent and the matrix is ​​generated in situ through a nitriding reaction in a nitrogen atmosphere.

[0014] Preferably, the composite nitride coating layer is composed of a reactant containing an organic carbon source, A and a general formula D δ E ε The mixture of fluxing and sintering agents is generated in situ during the nitriding reaction.

[0015] Preferably, the spinel phase coating layer is a new phase synthesized in situ during the process of coating the composite nitride coating layer.

[0016] A second aspect of the present invention provides a method for preparing a modified positive electrode material, the method comprising:

[0017] (1) ultrasonically dispersing an organic carbon source and an A-containing reactant in a mixed solvent, and heating the obtained dispersion until the mixed solvent is completely volatilized to obtain a first mixture;

[0018] (2) The first mixture, the positive electrode material as the matrix and the general formula D δ Eε and a fluxing and sintering agent to obtain a second mixture;

[0019] (3) contacting the second mixture with nitrogen and performing a nitridation reaction to in-situ generate a composite coating layer on the surface of the substrate to obtain a modified positive electrode material;

[0020] Wherein, the composite coating layer comprises a spinel phase coating layer arranged inside and a composite nitride coating layer arranged outside; wherein, the chemical formula of the composite nitride coating layer is selected from αAN β O γ ·(1-α)D δ E ε , wherein, 0.7≤α<1, 0.7≤β<1, 0.1≤γ≤0.3, 1≤δ≤2, 1≤ε≤3; A is selected from one of aluminum Al, boron B and titanium Ti, D is selected from at least one of yttrium Y, calcium Ca, lithium Li, barium Ba, magnesium Mg, strontium Sr, lanthanum La, hafnium Hf, zirconium Zr and cerium Ce, and E is selected from at least one of carbon C, nitrogen N, oxygen O and fluorine F.

[0021] The third aspect of the present invention provides a positive electrode plate, which includes a current collector and a positive electrode active material layer, and the positive electrode active material layer includes: the modified positive electrode material provided by the first aspect, or the modified positive electrode material prepared by the preparation method provided by the second aspect.

[0022] Preferably, the positive electrode active material layer further comprises: a first additive, a second additive, a binder and an optional dispersant.

[0023] Preferably, the first additive is selected from thermally conductive graphite and / or graphene; the second additive is selected from single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0024] A fourth aspect of the present invention provides a lithium-ion battery, comprising the positive electrode sheet provided in the third aspect.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] (1) The modified positive electrode material provided by the present invention generates a composite coating layer by in-situ coating on the surface of the positive electrode material, and the composite coating layer is limited to include a spinel phase coating layer and a composite nitride coating layer, thereby effectively improving the thermal conductivity and electrical conductivity of the positive electrode material. This is attributed to the synergistic effect between the spinel phase coating layer and the composite nitride coating layer. Experimental results show that the thermal conductivity of the base material and the electronic conductivity of the electrode are greatly improved; at the same time, the modified positive electrode material is used in lithium-ion batteries and has excellent thermal safety and electrochemical performance;

[0027] (2) The preparation method provided by the present invention adopts dry in-situ coating to construct a composite nitride coating layer with high thermal conductivity on the surface of the positive electrode material, which can alleviate the heat accumulation problem of the positive electrode material during the charge and discharge process. Specifically, by using an organic carbon source and an A-containing reactant to form a first mixture that is uniformly mixed at the molecular level, it is ensured that the organic carbon source preferentially forms a uniform coating on the surface of the A-containing reactant, which can effectively avoid the secondary removal of carbon powder during the nitride coating process and ensure that the reaction proceeds in the forward direction; at the same time, by adding a fluxing agent, on the one hand, the nitride coating heat treatment temperature is reduced, and on the other hand, the fluxing agent is introduced into the coating layer on the surface of the positive electrode material to reduce impurities and purify the lattice, which can further improve the thermal conductivity of the composite nitride coating layer, thereby increasing the decomposition temperature of the positive electrode material during thermal runaway and reducing the heat, avoiding local heat concentration and accumulation under thermal abuse conditions, and further improving the safety of lithium-ion batteries;

[0028] At the same time, during the reduction process, the organic carbon source can form a spinel surface structure in situ on the surface of the positive electrode material, effectively broadening the three-dimensional channel for lithium ion transmission on the surface of the positive electrode material, thereby achieving the effect of improving the discharge performance of the positive electrode material and avoiding the negative impact of capacity sacrifice caused by nitride coating, thereby effectively improving the electrochemical performance and safety of the battery.

[0029] (3) The positive electrode provided by the present invention, in particular, by selecting a first additive (for example, two-dimensional graphene) and a second additive (for example, linear carbon nanotubes) with high electronic conductivity and high thermal conductivity to replace the traditional acetylene black conductive agent, and in conjunction with the specific modified positive electrode material of the present invention, can not only ensure the rapid transmission of electrons between the positive electrode and the current collector, but also can form an effective point, line, and surface heat conduction network at the positive electrode level. Therefore, from the two aspects of the positive electrode material and the positive electrode structure, it ensures excellent kinetic transmission and heat conduction process during the charge and discharge process, showing excellent electrical performance and safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG1 is a TEM image of the modified cathode material S1 obtained in Example 1;

[0031] FIG2 is a 0.1C first-cycle charge-discharge curve of a lithium-ion battery assembled from the positive electrode sheet P1 of Example 1 and the positive electrode sheet DP1 of Comparative Example 1, respectively. DETAILED DESCRIPTION

[0032] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0033] In the present invention, unless otherwise specified, the terms "first" and "second" do not indicate a sequential order or limit the materials or steps involved. They are used only to distinguish or indicate that they are not the same material or step. For example, the terms "first" and "second" in "first additive" and "second additive" are used only to indicate that they are not the same additive.

[0034] A first aspect of the present invention provides a modified positive electrode material, comprising a positive electrode material as a substrate, and a composite coating layer generated in situ on the surface of the substrate; the composite coating layer comprises an inner spinel phase coating layer and an outer composite nitride coating layer;

[0035] Wherein, the chemical formula of the composite nitride coating is selected from αAN β O γ ·(1-α)D δ E ε , wherein, 0.7≤α<1, 0.7≤β<1, 0.1≤γ≤0.3, 1≤δ≤2, 1≤ε≤3; A is selected from one of aluminum Al, boron B and titanium Ti, D is selected from at least one of yttrium Y, calcium Ca, lithium Li, barium Ba, magnesium Mg, strontium Sr, lanthanum La, hafnium Hf, zirconium Zr and cerium Ce, and E is selected from at least one of carbon C, nitrogen N, oxygen O and fluorine F.

[0036] The inventors of the present invention have found that by in-situ coating the surface of the positive electrode material with a composite nitride material having high thermal conductivity and high temperature electrical insulation properties, that is, the general formula is αAN β O γ ·(1-α)D δ E ε By introducing fluxing and sintering agents, especially alkaline earth metal and rare earth metal compounds, the lattice oxygen is induced to form a new phase at the grain boundary, the lattice is purified to reduce the content of impurity elements and the heat-carrying phonon scattering cross-section, and the thermal conductivity of the positive electrode material surface is greatly improved; at the same time, the use of organic carbon source as a reducing agent can form a spinel structure on the surface of the positive electrode material. Its three-dimensional lithium ion transmission channel is conducive to accelerating the electrode process kinetics, slowing down the capacity utilization problem caused by the introduction of the coating layer, and achieving improved electrochemical performance.

[0037] In addition, by coating the surface of the positive electrode material with a thermally conductive composite nitride, heat accumulation at the positive electrode material level under thermal runaway conditions can be avoided, thereby achieving excellent heat transfer between active material particles. Furthermore, in particular, by selecting a first additive (e.g., two-dimensional graphene) and a second additive (e.g., linear carbon nanotubes) as a thermal conductor and a conductive agent in the positive electrode sheet structure, a heat transfer network is constructed at the positive electrode sheet level to achieve the purpose of improving the thermal safety of lithium-ion batteries.

[0038] In the present invention, unless otherwise specified, the composite coating layer includes an inner spinel phase coating layer and an outer composite nitride coating layer, which means that the inner surface of the positive electrode material is provided with a spinel phase coating layer, and the outer surface is provided with a composite nitride coating layer.

[0039] In the present invention, unless otherwise specified, the chemical formula of the composite nitride coating is αAN β O γ ·(1-α)D δ E ε The “·” in the formula represents compound AN. β O γ With compound D δ E ε Compound AN β O γ With compound D δ E ε The molar ratio is α:(1-α).

[0040] In some embodiments of the present invention, preferably, the composite coating layer is composed of a reactant containing an organic carbon source, A and a general formula D δ E ε The mixture of the fluxing and sintering agent and the matrix is ​​generated in situ through a nitriding reaction in a nitrogen atmosphere.

[0041] In some embodiments of the present invention, preferably, the composite nitride coating layer is composed of a reactant containing an organic carbon source, A and a general formula D δ E ε The mixture of fluxing and sintering agents is generated in situ during the nitriding reaction.

[0042] In some embodiments of the present invention, the chemical formula of the composite nitride coating is selected from αAN β O γ ·(1-α)D δ E ε, wherein, 0.7≤α<1, preferably 0.7≤α≤0.9, for example, 0.7, 0.8, 0.85, 0.9, and any value in a range consisting of any two values; 0.7≤β<1, preferably 0.7≤α≤0.933, for example, 0.7, 0.8, 0.9, 0.933, and any value in a range consisting of any two values; 0.1≤γ≤0.3, for example, 0.1, 0.2, 0.3, and any value in a range consisting of any two values; 1≤δ≤2, for example, 1, 2; 1≤ε≤3, for example, 1, 2, 3; A is selected from one of aluminum Al, boron B and titanium Ti; D is selected from at least one of yttrium Y, calcium Ca, lithium Li, barium Ba, magnesium Mg, strontium Sr, lanthanum La, hafnium Hf, zirconium Zr and cerium Ce; E is selected from at least one of C, N, O and F, preferably selected from C and / or O.

[0043] In the present invention, the positive electrode material has a wide range of choices. Preferably, the positive electrode material is selected from LiNi 1-y1-z1 Co y1 Mn z1 O2 (y1 is selected from 0<y1≤0.33; z1 is selected from 0<z1≤0.33), LiNi 1-y2-z2 Co y2 Al z2 O2 (y2 is selected from 0<y2≤0.33; z2 is selected from 0<z2≤0.33), xLiMO2·(1-x)Li2MnO3 (x is selected from 0≤x<1; M is selected from at least one of nickel Ni, cobalt Co, manganese Mn, iron Fe, chromium Cr and copper Cu), lithium cobaltate and lithium nickelate.

[0044] In one embodiment of the present invention, the positive electrode material is selected from LiNi 0.9 Co 0.05 Mn 0.05 O2、LiNi 0.6 Co 0.2 Mn 0.2 O2、LiNi 0.8 Co 0.1 Al 0.1 O2, LiCoO2, LiNi 0.9 Co 0.05 Al 0.05 O2、Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O2、Li 1.2 Ni 0.2 Mn 0.6 O2.

[0045] In some embodiments of the present invention, preferably, the average particle size D of the positive electrode material is 50 Selected from 100-20000nm, preferably 1000-10000nm.

[0046] In some embodiments of the present invention, preferably, based on the total weight of the modified positive electrode material, the content of the composite coating layer is 0.01-10wt%, for example, 0.01wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.3wt%, 0.5wt%, 0.8wt%, 1wt%, 2wt%, 5wt%, and any value in a range consisting of any two values, preferably 0.1-5wt%. A composite coating layer having the above content range is more conducive to improving the thermal conductivity and electrical conductivity of the modified positive electrode material.

[0047] In some embodiments of the present invention, preferably, the thickness of the composite coating layer is 0.2-220 nm, for example, 0.2 nm, 1.5 nm, 2 nm, 3 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 50 nm, 55 nm, 60 nm, 80 nm, 100 nm, 150 nm, 200 nm, 220 nm, and any value in the range consisting of any two values, preferably 1.5-55 nm.

[0048] In the present invention, when the thickness of the composite coating layer is too thin, the effect of improving the thermal conductivity of the positive electrode material and enhancing thermal safety is limited; when the thickness of the composite coating layer is too thick, it is easy to cause a higher interface charge transfer impedance and affect the capacity of the positive electrode material.

[0049] In some embodiments of the present invention, it is further preferred that the thickness of the spinel phase coating layer is 0.1-20nm, for example, 0.1nm, 0.5nm, 1nm, 1.5nm, 2nm, 3nm, 4nm, 5nm, 10nm, 15nm, 20nm, and any value in the range consisting of any two numerical values, preferably 0.5-5nm; the thickness of the composite nitride coating layer is 0.1-200nm, for example, 0.1nm, 1nm, 2nm, 5nm, 10nm, 20nm, 30nm, 40nm, 50nm, 80nm, 100nm, 150nm, 200nm, and any value in the range consisting of any two numerical values, preferably 1-50nm.

[0050] In some embodiments of the present invention, preferably, the spinel phase coating layer is a new phase synthesized in situ during the process of coating the complex nitride coating layer.

[0051] In the present invention, the composition of the spinel phase coating layer depends on the composition of the positive electrode material. Further preferably, the chemical formula of the spinel phase coating layer is selected from Li x’ M' y’ O4, wherein 0.5≤x'≤1, 1≤y'≤2, and M' is selected from one of nickel Ni, cobalt Co, manganese Mn, iron Fe and chromium Cr.

[0052] In some embodiments of the present invention, preferably, in the composite nitride coating layer, AN β O γ Selected from AlN 0.3 O 0.3 、AlN 0.933 O 0.1 , BN 0.8 O 0.3 At least one of the D δ E ε At least one selected from Y2O3, CaO, Li2O, BaO, MgO, SrO, La2O3, HfO2, CeO2, CaC2, YC2 and ZrO2.

[0053] According to the present invention, preferably, the thermal conductivity of the modified positive electrode material is ≥10W / (m·K), preferably 10-20W / (m·K), for example, 10W / (m·K), 12W / (m·K), 14W / (m·K), 15W / (m·K), 18W / (m·K), 20W / (m·K), and any value in the range consisting of any two numerical values.

[0054] A second aspect of the present invention provides a method for preparing a modified positive electrode material, the method comprising:

[0055] (1) ultrasonically dispersing an organic carbon source and an A-containing reactant in a mixed solvent, and heating the obtained dispersion until the mixed solvent is completely volatilized to obtain a first mixture;

[0056] (2) The first mixture, the positive electrode material as the matrix and the general formula D δ E ε and a fluxing and sintering agent to obtain a second mixture;

[0057] (3) contacting the second mixture with nitrogen and performing a nitridation reaction to in-situ generate a composite coating layer on the surface of the substrate to obtain a modified positive electrode material;

[0058] Wherein, the composite coating layer comprises a spinel phase coating layer arranged inside and a composite nitride coating layer arranged outside; wherein, the chemical formula of the composite nitride coating layer is selected from αAN β Oγ ·(1-α)D δ E ε , wherein, 0.7≤α<1, 0.7≤β<1, 0.1≤γ≤0.3, 1≤δ≤2, 1≤ε≤3; A is selected from one of aluminum Al, boron B and titanium Ti, D is selected from at least one of yttrium Y, calcium Ca, lithium Li, barium Ba, magnesium Mg, strontium Sr, lanthanum La, hafnium Hf, zirconium Zr and cerium Ce, and E is selected from at least one of carbon C, nitrogen N, oxygen O and fluorine F.

[0059] In the present invention, the organic carbon source acts as a reducing agent, firstly forms a first mixture with the reactant containing A to form a molecular level coating, and then reacts with the fluxing agent D to form a first mixture. δ E ε A composite nitride coating with high thermal conductivity and high-temperature electrical insulation properties is in situ generated at the grain boundary by nitridation and carbothermal reduction reaction. δ E ε It can not only lower the sintering temperature, but also further reduce the impurity element content and heat-carrying phonon scattering cross section of the nitride coating layer by purifying the lattice, thereby greatly improving the thermal conductivity of the positive electrode material surface; at the same time, the organic carbon source reacts with the lattice oxygen on the surface of the positive electrode material to produce oxygen vacancies, forming a spinel structure on the surface of the positive electrode material. Its three-dimensional lithium ion transmission channel is conducive to accelerating the electrode process kinetics, slowing down the capacity utilization problem caused by the introduction of the coating layer, and achieving improved electrochemical performance.

[0060] Therefore, the present invention adopts dry coating, which not only avoids the limitation of nitride ceramics' application in lithium-ion batteries due to their high sintering temperature and secondary removal of residual carbon, but also effectively improves the thermal conductivity and electrical conductivity of the modified positive electrode material, thereby effectively improving the electrochemical performance and safety of the battery.

[0061] In some embodiments of the present invention, preferably, in step (1), the molar ratio of the organic carbon source to the A-containing reactant is 2-5:1, for example, 2:1, 3:1, 4:1, 5:1, and any two values ​​in the range of 2:1. The molar ratio meeting the above range not only allows the A-containing reactant to generate AN in situ, but also allows the A-containing reactant to generate AN in situ. β O γ , and a spinel structure phase can also be generated on the surface of the positive electrode material.

[0062] Compared to the prior art that uses inorganic carbon sources, the present invention uses an organic carbon source dissolved in a mixed solvent to form an in-situ coating. Preferably, the organic carbon source is selected from at least one of glucose, sucrose, maltose, soluble starch, cellulose, lignin, polydopamine, PVP, formaldehyde resin, and tannic acid.

[0063] In the present invention, the reactant containing A has a wide range of choices, as long as the reactant contains element A. Preferably, the reactant containing A is selected from one of an aluminum source, a boron source, and a titanium source.

[0064] In some embodiments of the present invention, preferably, the aluminum source is selected from at least one of α-Al2O3, γ-Al2O3, A1(OH)3, AlOOH, boehmite and pseudoboehmite; the boron source is selected from at least one of B2O3, borax, trimethyl borate and ammonium borate; and the titanium source is selected from at least one of TiO2, titanium tetrachloride, tetrabutyl titanate and titanium isopropoxide.

[0065] In some embodiments of the present invention, preferably, the average particle size D of the reactant containing A is 50 is 10-1000 nm. That is, the average particle size D of the aluminum source, boron source and titanium source is 50 Each independently 10-1000 nm.

[0066] In the present invention, a wide range of choices is available for the type of mixed solvent, as long as the organic carbon source and the reactant containing A are fully dissolved in the mixed solvent. Preferably, the mixed solvent is selected from water and an organic solvent, and the volume ratio of water to organic solvent is 1-5:1. Further preferably, the organic solvent is selected from at least one of methanol, ethanol, ethylene glycol, glycerol, butanol, isopropanol, methyl ether, ethyl ether, acetone, formaldehyde, acetaldehyde, ethyl acetate, and N-methylpyrrolidone.

[0067] In a specific embodiment of the present invention, the mixed solvent is selected from water and an organic solvent in a volume ratio of 1-5:1, and the organic solvent is selected from at least one of methanol, ethanol, ethylene glycol, glycerol, butanol, isopropanol, methyl ether, ethyl ether, acetone, formaldehyde, acetaldehyde, ethyl acetate and N-methylpyrrolidone.

[0068] In the present invention, the ultrasonic dispersion is intended to uniformly mix the organic carbon source and the A-containing reactant in a mixed solvent to obtain a uniform dispersion. Preferably, the ultrasonic dispersion process includes: first uniformly mixing the organic carbon source and the mixed solvent, and then adding the A-containing reactant to carry out the ultrasonic dispersion.

[0069] In some embodiments of the present invention, preferably, the ultrasonic dispersion conditions include: a time of 5-30 min, and an ultrasonic frequency of 10-20 KHz.

[0070] In the present invention, the heating is intended to remove the mixed solvent in the dispersion to obtain a first mixture that is uniform at the molecular level. Preferably, the heating temperature is selected from 50-200° C. In the present invention, the first mixture refers to an organic carbon source and an A-containing reactant.

[0071] In some embodiments of the present invention, preferably, in step (2), the mass ratio of the positive electrode material to the first mixture is 10-10000: 1, preferably 100-5000: 1. In the present invention, the above mass ratio range is met so that the first mixture is uniformly coated on the surface of the positive electrode material.

[0072] In some embodiments of the present invention, preferably, the molar ratio of the first mixture to the fluxing and sintering agent, calculated as A, is ≥ 7:3. Meeting the above molar ratio range not only helps reduce the temperature of the nitridation reaction, but also forms a specific complex nitride, resulting in a complex nitride coating layer with excellent thermal conductivity and electrical conductivity.

[0073] In some embodiments of the present invention, preferably, the mixing process includes: first mixing at a rotation speed of 50-500 r / min for 1-5 min, and then mixing at a rotation speed of 500-3000 r / min for 5-30 min.

[0074] In some embodiments of the present invention, preferably, the fluxing agent is selected from at least one of Y2O3, CaO, Li2O, BaO, MgO, SrO, La2O3, HfO2, CeO2, CaC2, YC2 and ZrO2.

[0075] In some embodiments of the present invention, preferably, the average particle size D of the fluxing agent is 50 10-500nm.

[0076] In some embodiments of the present invention, preferably, in step (3), the conditions of the nitridation reaction include: temperature of 400-1200°C, preferably 600-1000°C; pressure of 0.1-2MPa, preferably 0.5-2MPa; time of 0.1-5h, preferably 0.5-3h.

[0077] In some embodiments of the present invention, further preferably, the heating rate of the nitridation reaction is 1-5°C / min.

[0078] In some embodiments of the present invention, the nitrogen gas preferably has a flow rate of 1-50 L / min, preferably 2-20 L / min. Meeting the above flow rate range allows for the regulation of the content of the in-situ generated complex nitride on the surface of the cathode material, thereby regulating the thermal conductivity and electrical conductivity of the modified cathode material.

[0079] The third aspect of the present invention provides a positive electrode plate, the positive electrode plate current collector and the positive electrode active material layer, and the positive electrode active material layer includes: the modified positive electrode material provided by the first aspect, or the modified positive electrode material prepared by the preparation method provided by the second aspect.

[0080] In some embodiments of the present invention, preferably, the positive electrode active material layer further comprises: a first additive, a second additive, a binder and an optional dispersant.

[0081] In a specific embodiment of the present invention, the positive electrode plate includes a current collector and a positive electrode active material layer supported on the current collector, and the positive electrode active material layer is composed of a modified positive electrode material, a first additive, a second additive, a binder and a dispersant.

[0082] In another specific embodiment of the present invention, the positive electrode plate includes a current collector and a positive electrode active material layer supported on the current collector, and the positive electrode active material layer is composed of a modified positive electrode material, a first additive, a second additive and a binder.

[0083] In some embodiments of the present invention, preferably, based on the total weight of the positive electrode active material layer, the content of the modified positive electrode material is 80-99wt%, the content of the first additive is 0.2-5wt%, the content of the second additive is 0.2-5wt%, the content of the binder is 0.4-5wt%, and the content of the dispersant is 0-5wt%.

[0084] In the present invention, in order to further improve the thermal conductivity and electrical conductivity of the positive electrode plate, a first additive (for example, thermally conductive graphite and / or graphene) and a second additive (for example, single-walled carbon nanotubes and / or multi-walled carbon nanotubes) with high electronic conductivity and high thermal conductivity are selected to replace the traditional acetylene black conductive agent. This not only ensures the rapid transmission of electrons between the positive electrode plate and the current collector, but also forms an effective point, line, and surface thermal transmission network at the positive electrode plate level. Therefore, from the two aspects of the positive electrode material and the positive electrode structure, excellent kinetic transmission and heat conduction process are guaranteed during the charging and discharging process, showing excellent electrical performance and safety.

[0085] In some embodiments of the present invention, preferably, the thermal conductivity of the first additive and the second additive are each independently ≥1500 W / (m·K), preferably ≥2000 W / (m·K); further preferably, the first additive is selected from thermally conductive graphite and / or graphene; and the second additive is selected from single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0086] In some embodiments of the present invention, preferably, the size of the first additive is 1-10 μm, and the size refers to the average value of the lateral size.

[0087] In some embodiments of the present invention, preferably, the aspect ratio of the second additive is 200-50,000, preferably 500-10,000; further preferably, the length of the second additive is 0.5-20 μm and the diameter is 0.5-30 nm.

[0088] In some embodiments of the present invention, the binder is preferably selected from at least one of PVDF, PVDF-HFP, polyvinyl alcohol, sodium carboxymethyl cellulose, polyacrylonitrile, sodium alginate, polyimide, polyethyleneimine, and polymethyl methacrylate. In the present invention, PVDF is polyvinylidene fluoride; PVDF-HFP is polyvinylidene fluoride-hexafluoropropylene copolymer.

[0089] In the present invention, the dispersant is primarily used to adjust the state of the positive electrode slurry, ensuring a more uniform dispersion. Preferably, the dispersant is at least one of a titanate coupling agent, PVP, sodium dodecylbenzenesulfonate, a silane coupling agent, and disodium methylene dinaphthyl sulfate. In the present invention, PVP is polyvinyl pyrrolidone.

[0090] In some embodiments of the present invention, preferably, the positive electrode plate is prepared by the following method:

[0091] (1) uniformly mixing the modified positive electrode material, the first additive, the second additive, the binder, and the dispersant with an organic solvent in a mass ratio of 80-99:0.2-5:0.2-5:0.4-5:0.2-5 to obtain a positive electrode material;

[0092] (2) The positive electrode slurry is coated on aluminum foil and dried at 70-90° C. to obtain a positive electrode sheet.

[0093] In the present invention, when the modified positive electrode material provided by the present invention is used in the positive electrode sheet, the electronic conductivity of the positive electrode sheet can be effectively improved, especially when combined with the specific first additive and the second additive, which is more conducive to improving the electronic conductivity of the positive electrode sheet. Preferably, the electronic conductivity of the positive electrode sheet is ≥1.1S·cm -1 , for example, 1.1 S·cm -1 , 1.2S·cm -1 , 1.3S·cm -1 , 1.5S·cm -1 , 1.8S·cm -1 , 2S·cm -1 , and any value in the range of any two values, preferably selected from 1.1-2S·cm -1 .

[0094] A fourth aspect of the present invention provides a lithium-ion battery, comprising the positive electrode sheet provided in the third aspect.

[0095] According to a particularly preferred embodiment of the present invention, a modified positive electrode material comprises a positive electrode material as a substrate, and a composite coating layer generated in situ on the surface of the substrate; the composite coating layer comprises an inner spinel phase coating layer and an outer composite nitride coating layer;

[0096] Wherein, the chemical formula of the composite nitride coating is selected from αAN β O γ ·(1-α)D δ E ε , wherein 0.7≤α<1, 0.7≤β<1, 0.1≤γ≤0.3, 1≤δ≤2, 1≤ε≤3; A is selected from one of aluminum Al, boron B and titanium Ti, D is selected from at least one of yttrium Y, calcium Ca, lithium Li, barium Ba, magnesium Mg, strontium Sr, lanthanum La, hafnium Hf, zirconium Zr and cerium Ce, and E is selected from at least one of carbon C, nitrogen N, oxygen O and fluorine F;

[0097] The modified positive electrode material is prepared by the following method: (1) ultrasonically dispersing an organic carbon source and a reactant containing A in a mixed solvent, and heating the obtained dispersion until the mixed solvent is completely volatilized to obtain a first mixture; (2) mixing the first mixture, the positive electrode material as a matrix, and a general formula D δ E ε and a fluxing and sintering agent to obtain a second mixture; (3) contacting the second mixture with nitrogen and performing a nitriding reaction to in-situ generate a composite coating layer on the surface of the substrate to obtain the modified positive electrode material.

[0098] According to another particularly preferred embodiment of the present invention, a positive electrode sheet includes a current collector and a positive electrode active material layer, and the positive electrode active material layer includes: a modified positive electrode material, a first additive, a second additive, a binder, and an optional dispersant;

[0099] The modified positive electrode material includes a positive electrode material as a substrate and a composite coating layer generated in situ on the surface of the substrate; the composite coating layer includes a spinel phase coating layer arranged inside and a composite nitride coating layer arranged outside; wherein the chemical formula of the composite nitride coating layer is selected from αAN β O γ ·(1-α)D δ E ε, 0.7≤α<1, 0.7≤β<1, 0.1≤γ≤0.3, 1≤δ≤2, 1≤ε≤3; A is selected from one of aluminum Al, boron B and titanium Ti, D is selected from at least one of yttrium Y, calcium Ca, lithium Li, barium Ba, magnesium Mg, strontium Sr, lanthanum La, hafnium Hf, zirconium Zr and cerium Ce, and E is selected from at least one of carbon C, nitrogen N, oxygen O and fluorine F;

[0100] Wherein, the first additive is selected from thermal conductive graphite and / or graphene; the second additive is selected from single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0101] The present invention will be described in detail below through examples.

[0102] Conductivity test method for positive electrode: Cut out the disc from the dried electrode with a sampler, measure the thickness of the disc, align the center of the disc with the probe, place it on a four-probe tester, set the thickness and diameter and other parameters on the computer, and you can get the set current. Adjust the knob to adjust the actual current to the set current value, and you can get the conductivity of the electrode at the center point. The same method can be used to get the conductivity of the four positions of the disc, up, down, left and right.

[0103] The process parameters for preparing the modified positive electrode materials and positive electrode sheets in the examples and comparative examples are listed in Table 1, and the physical properties of the prepared modified positive electrode materials and positive electrode sheets are listed in Table 2.

[0104] Example 1

[0105] (1) The organic carbon source (glucose) and the mixed solvent (water and ethanol with a volume ratio of 1:1) were mixed evenly, and then the reactant containing A (α-Al2O3, average particle size D 50 The obtained dispersion was heated at 150° C. until the mixed solvent was completely evaporated to obtain a first mixture;

[0106] Wherein, the molar ratio of the above organic carbon source to the reactant calculated as A is 2:1;

[0107] (2) The first mixture and the positive electrode material (LiNi 0.9 Co 0.05 Mn 0.05 O2, average particle size D 50 10000nm) and flux sintering agent (Y2O3, average particle size D 50 The mixture was stirred at 200 r / min for 1 min and then at 1000 r / min for 10 min to obtain a second mixture;

[0108] Wherein, the mass ratio of the above-mentioned positive electrode material to the first mixture is 1000:1;

[0109] The molar ratio of the first mixture to the fluxing agent is 8:2.

[0110] (3) contacting the second mixture with nitrogen and performing a nitridation reaction (heating rate of 3°C / min, temperature of 600°C, pressure of 0.5 MPa, time of 2 h) to obtain a modified positive electrode material S1; wherein the nitrogen flow rate is 2 L / min;

[0111] (4) The modified positive electrode material S1, the first additive (graphene, size 5 μm), the second additive (single-walled carbon nanotube, aspect ratio 10000, length 10 μm, diameter 1 nm), binder (PVDF) and dispersant (sodium dodecylbenzenesulfonate) were mixed with solvent NMP in a mass ratio of 90:3:2:4:1. The obtained positive electrode slurry was coated on an aluminum foil current collector and dried at 80°C to obtain a positive electrode sheet P1.

[0112] Among them, the TEM image of the above-mentioned modified positive electrode material S1 is shown in Figure 1. It can be seen from Figure 1 that the above-mentioned modified positive electrode material S1 includes: a positive electrode material and a composite coating layer coated on the surface of the positive electrode material, and the composite coating layer includes an inner spinel phase coating layer and an outer composite nitride coating layer.

[0113] At the same time, the modified cathode material S1 was subjected to elemental analysis, and the spinel phase layer composed of LiMn2O4 and the spinel phase layer composed of 0.8AlN 0.8 O 0.3 0.2Y2O3 composite compound coating layer.

[0114] Example 2-22

[0115] According to the method of Example 1, the difference is that

[0116] According to the process parameters in Table 1, modified positive electrode materials S2-S22 were prepared respectively, and the physical properties of the prepared modified positive electrode materials S2-S22 are listed in Table 2.

[0117] At the same time, according to the process parameters in Table 1, positive electrode sheets P2-P22 were prepared respectively.

[0118] Comparative Example 1

[0119] According to the method of Example 1, the difference is that

[0120] In step (1), the reactant containing A is replaced with SiO2;

[0121] The other conditions are the same, and the modified positive electrode material DS1 and the positive electrode plate DP1 are obtained.

[0122] Comparative Example 2

[0123] According to the method of Example 1, the difference is that

[0124] In step (2), no fluxing agent (Y2O3, average particle size D 50 50nm);

[0125] The other conditions are the same, and the modified positive electrode material DS2 and the positive electrode plate DP2 are obtained.

[0126] Comparative Example 3

[0127] According to the method of Example 1, the difference is that

[0128] In step (1), no reactant containing A (α-Al2O3, average particle size D 50 10nm);

[0129] In step (2), no fluxing agent (Y2O3, average particle size D 50 50nm);

[0130] The remaining conditions were the same, and a modified positive electrode material DS3 and a positive electrode plate DP3 containing only a spinel phase coating layer were obtained.

[0131] Comparative Example 4

[0132] According to the method of Example 1, the difference is that

[0133] In step (1), the molar ratio of the organic carbon source to the reactant calculated as A is changed to 0.5:1;

[0134] The remaining conditions were the same, and a modified positive electrode material DS4 and a positive electrode piece DP4 containing only a composite nitride coating layer were obtained.

[0135] Comparative Example 5

[0136] According to the method of Example 1, the difference is that

[0137] There is no step (1), and in step (2), the first mixture is directly replaced by AlN;

[0138] The other conditions were the same, and the modified positive electrode material DS5 and the positive electrode piece DP5 were obtained.

[0139] Comparative Example 6

[0140] According to the method of Example 1, the difference is that

[0141] In step (1), the organic carbon source glucose is replaced by carbon black;

[0142] The other conditions were the same, and the modified positive electrode material DS6 and the positive electrode piece DP6 were obtained.

[0143] Comparative Example 7

[0144] According to the method of Example 1, the difference is that

[0145] There are no steps (1)-(3), that is, the positive electrode material (LiNi 0.9 Co 0.05 Mn 0.05 O2, average particle size D 50 10000nm) as the positive electrode material DS7;

[0146] The other conditions are the same, and the positive electrode material DS7 and the positive electrode sheet DP7 are obtained.

[0147] Table 1 Note: 1- volume ratio of water and organic solvent in the mixed solvent; 2- molar ratio of organic carbon source and reactant (A).

[0148] Table 1 Note: 3-mass ratio of the positive electrode material to the first mixture; 4-molar ratio of the first mixture to the fluxing and sintering agent in terms of A.

[0149] Table 1

[0150] Table 1

[0151] Table 1

[0152] Table 2

[0153] Table 2 Note: 5- thermal conductivity of modified cathode material, W / (m·K); 6- electronic conductivity of cathode electrode, S·cm -1 .

[0154] It can be seen from the results in Table 1-2 that compared with Comparative Examples 1-7, Examples 1-22 adopt the method provided by the present invention to in-situ synthesize the modified positive electrode material of the composite coating layer, wherein the composite coating layer includes a spinel phase coating layer disposed inside and a coating layer disposed outside that satisfies the general formula αAN β O γ ·(1-α)D δ E ε composite nitride coating layer; at the same time, the modified positive electrode material has a higher thermal conductivity, and the positive electrode sheet prepared from the modified positive electrode material has a higher electronic conductivity.

[0155] Comparative Example 1 uses SiO2 to replace Al2O3 in Example 1. The intrinsic thermal conductivity of the generated composite nitride 0.8Si3N4·0.2Y2O3 is lower than that of 0.8AlN 0.8 O 0.3 0.2Y2O3, so the modified positive electrode material DS1 shows a lower thermal conductivity than that of Example 1.

[0156] In Comparative Example 2, no sintering flux was added, and the resulting AlN cladding layer had a higher impurity phase content, and thus its thermal conductivity was also lower than that of Example 1.

[0157] Since Comparative Example 3 only contains a spinel phase coating layer, it cannot achieve a heat transfer effect, and thus its thermal conductivity is much lower than that of Example 1.

[0158] Since Comparative Example 4 only contains a composite nitride coating layer, the lithium ion transmission path is blocked, resulting in its conductivity being lower than that of Example 1.

[0159] In Comparative Example 5, a non-in-situ method is used to form a composite coating layer, and the thermal conductivity of the modified positive electrode material obtained is lower than that of Example 1, and the electronic conductivity of the positive electrode sheet is also lower than that of Example 1.

[0160] Comparative Example 6 uses inorganic carbon black, which cannot form a mixture with uniform molecular-level coating with Al2O3. It only performs simple physical mixing. In the subsequent nitriding process, the forward direction of the reaction cannot be guaranteed, and it is easy to cause the spinel phase to account for too high a proportion and the composite aluminum nitride coating to contain a large amount of oxide impurity phase, thereby significantly reducing the thermal conductivity and electrical conductivity of the coating, resulting in the thermal conductivity of the modified positive electrode material being lower than that of Example 1, and the conductivity of the positive electrode sheet is also lower than that of Example 1.

[0161] In Comparative Example 7, since the surface of the base material is not coated with any material, the thermal conductivity of the modified positive electrode material is much lower than that of Example 1, and the electronic conductivity of the positive electrode sheet is also much lower than that of Example 1.

[0162] Test Case

[0163] Assemble the battery: Use the positive electrode sheets prepared in the above embodiments and comparative examples, use the lithium sheet as the negative electrode, and drip a commercial carbonate electrolyte (1MLiPF6 dissolved in EC and DMC solvents with a volume ratio of 1:1) onto the surface of the diaphragm coated with an alumina coating. Lithium-ion batteries are assembled in the order of negative electrode shell, negative electrode sheet, diaphragm, positive electrode sheet, gasket, spring, and positive electrode shell.

[0164] The above lithium-ion batteries were tested for electrochemical performance (0.1C discharge capacity, 1C cycle 100-cycle capacity retention rate) and safety performance (DSC exothermic peak temperature, thermal conductivity). The test results are listed in Table 3.

[0165] The test conditions include: placing the assembled button cell on the clamp of the blue electric test stand, setting the test voltage range to 2.8-4.25V, and performing constant current charge and discharge at 0.1C and 1C rates to obtain electrochemical data.

[0166] The DSC test method is to place the assembled button cell on the blue electric test stand clamp, set the test voltage range to 2.8-4.25V, and the test procedure is 0.1C rate constant current charge and discharge for two cycles, and then charge to full charge at 0.1C. Place the fully charged battery in the glove box, disassemble the battery and remove the positive electrode sheet. After cleaning the electrode sheet with DMC, place it in the small compartment of the glove box and drain it. After drying, use a ceramic knife to scrape the positive electrode powder. Weigh a certain amount of positive electrode powder and electrolyte and put them into a high-pressure crucible. After the powder is soaked, place the high-pressure crucible in the differential scanning calorimeter, set the instrument temperature range to RT-350℃, the heating rate to 5℃ / min, and the atmosphere to N2. After the test is completed, obtain the test data.

[0167] Among them, the 0.1C first-week charge and discharge curves of the lithium-ion battery assembled by the positive electrode sheet P1 of Example 1 and the positive electrode sheet DP1 of Comparative Example 1 are shown in Figure 2. It can be seen from Figure 2 that compared with Comparative Example 1, the lithium-ion battery assembled by the positive electrode sheet P1 of Example 1 has a higher 0.1C first-week charge and discharge capacity.

[0168] Table 3

[0169] The results in Table 3 show that, compared to Comparative Examples 1-7, the lithium-ion batteries assembled from the positive electrode sheets prepared in Examples 1-22 simultaneously exhibited higher 0.1C first-cycle discharge capacity, 1C 100-cycle capacity retention, and DSC exothermic peak temperature, as well as lower thermal conductivity. Therefore, the lithium-ion batteries prepared from the modified positive electrode materials provided by the present invention exhibit both excellent electrochemical performance and safety.

[0170] It can be seen from Example 1 and Examples 9-11 that since Example 9 does not contain the first additive graphene and the second additive single-walled carbon nanotubes, Example 9 cannot form point, line, and surface contact with the modified positive electrode material, so that the thermal stability and electrochemical performance are lower than those of Example 1 and Examples 10-11; Example 10 only contains the first additive graphene, and Example 11 only contains the second additive single-walled carbon nanotubes, and Examples 10-11 can only form point-surface and point-line contacts with the modified positive electrode material; and Example 1, since it contains both graphene and single-walled carbon nanotubes, can form a point, line, and surface three-dimensional contact transmission network with the modified positive electrode material, showing a good synergistic improvement effect, so that the thermal stability and electrochemical performance are better than Examples 9-11. Therefore, the present invention adopts a specific modified positive electrode material, and cooperates with the first additive and the second additive in the electrode sheet, which can not only ensure the rapid transmission of electrons between the positive electrode sheet and the current collector, but also form an effective point, line and surface heat conduction transmission network at the positive electrode sheet level. Therefore, from the two aspects of the positive electrode material and the positive electrode sheet structure, excellent kinetic transmission and heat conduction process are guaranteed during the charging and discharging process, showing excellent electrical performance and safety performance.

[0171] Comparative Example 1 uses SiO2 as a reactant, and the thermal conductivity of the Si3N4 nitride finally formed is much lower than that of AlN. Therefore, its effect as a coating layer in improving the heat dissipation performance of the positive electrode material is limited, and the thermal stability of the positive electrode material is lower than that of Example 1.

[0172] In comparative example 2, under the condition of no flux, the lattice impurity content in the generated nitride coating is higher, the heat-carrying phonon scattering cross-section is small, and a higher reaction temperature is required. Compared with the composite nitride coating, the heat dissipation coefficient is lower, so its DSC exothermic peak temperature is lower than that of Example 1, and the heat release is higher.

[0173] Comparative Example 3 only has a spinel coating layer on the surface of the base material, while Comparative Example 4 only has a composite nitride coating layer on the surface of the base material. The thermal conductivity, electronic conductivity, first-week discharge capacity, 1C cycle 100-week capacity retention rate, and DSC exothermic peak temperature of Comparative Example 3 are significantly lower than those of Example 1, and the thermal conductivity is significantly higher than that of Comparative Example 1. This is because Comparative Example 3 lacks the key composite nitride coating layer. However, the electronic conductivity and discharge capacity of Example 1 are better than those of Comparative Example 3. From a theoretical perspective, the composite nitride coating contained in Example 1 will reduce the lithium ion migration rate and electrode process kinetics to a certain extent. Therefore, it is speculated that the first-week discharge capacity of Comparative Example 3 should be smaller than that of Example 1. A reasonable explanation may be that compared with the base material that simply adopts the spinel phase coating layer or the composite nitride coating layer, the positive electrode material uniformly coated with the spinel phase coating layer and the composite nitride coating layer shows better thermal stability and electrochemical cycling effect. This may be that, on the one hand, the lack of the composite nitride coating layer in comparative example 3 will cause the heat accumulation generated by the positive electrode material during the charging and discharging process to be difficult to transmit, and thus the thermal stability of the positive electrode material deteriorates; on the other hand, the lack of the spinel phase coating layer in comparative example 4 will block the lithium ion transmission channel, which will lead to a higher interfacial charge transfer impedance, thereby affecting the first-week discharge capacity and cycle performance of the positive electrode material. The present invention greatly improves the thermal stability and electrochemical performance of the base material through the internal and external synergistic effect of the two.

[0174] The composite nitride coating layer synthesized by the non-in situ method in Comparative Example 5 is difficult to form a relatively uniform coating layer on the surface of the positive electrode material particles, but is distributed in an island-like manner. Therefore, its heat dissipation effect is lower than that of Example 1. This uneven coating will also affect the interface side reaction to a certain extent, resulting in poor electrochemical performance.

[0175] Comparative Example 6 uses inorganic carbon source carbon black instead of organic carbon source graphene, which cannot form a first mixture with uniform molecular level coating on the Al2O3 surface, affecting the forward progress of the subsequent nitridation reaction, resulting in a higher content of impurity phase in the synthesized composite nitride, thereby worsening the thermal stability and electrochemical performance of the material compared with Example 1.

[0176] Comparative Example 7 uses a base material without coating any material on the surface. The first-week discharge capacity, 1C cycle 100-week capacity retention rate, and DSC exothermic peak temperature of the assembled battery are significantly lower than those in Example 1, while the thermal conductivity is significantly higher than that in Example 1. This result confirms that the technical solution of the present invention can effectively solve the thermal stability problem of the base material and improve the electrochemical properties of the base material.

[0177] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as the contents disclosed by the present invention and fall within the scope of protection of the present invention.

Claims

1. A modified positive electrode material, characterized in that: The modified positive electrode material includes a positive electrode material as a substrate, and a composite coating layer generated in situ on the surface of the substrate; the composite coating layer includes an inner spinel phase coating layer and an outer composite nitride coating layer; Wherein, the chemical formula of the composite nitride coating is selected from αAN β O γ ·(1-α)D δ E ε , wherein 0.7≤α<1, 0.7≤β<1, 0.1≤γ≤0.3, 1≤δ≤2, 1≤ε≤3; A is selected from one of aluminum Al, boron B and titanium Ti, D is selected from at least one of yttrium Y, calcium Ca, lithium Li, barium Ba, magnesium Mg, strontium Sr, lanthanum La, hafnium Hf, zirconium Zr and cerium Ce, and E is selected from at least one of carbon C, nitrogen N, oxygen O and fluorine F.

2. The modified positive electrode material according to claim 1, wherein The positive electrode material is selected from LiNi 1-y1-z1 Co y1 Mn z1 O2、LiNi 1-y2-z2 Co y2 Al z2 At least one of O2, xLiMO2·(1-x)Li2MnO3, lithium cobalt oxide and lithium nickel oxide; Wherein, 0<y1≤0.33; 0<z1≤0.33; 0<y2≤0.33; 0<z2≤0.33; 0≤x<1; M is selected from at least one of nickel Ni, cobalt Co, manganese Mn, iron Fe, chromium Cr and copper Cu; And / or, the average particle size D of the positive electrode material 50 Selected from 100-20000nm, preferably 1000-10000nm.

3. The modified positive electrode material according to claim 1 or 2, wherein Based on the total weight of the modified positive electrode material, the content of the composite coating layer is 0.01-10wt%, preferably 0.1-5wt%; And / or, the thickness of the composite coating layer is 0.2-220 nm, preferably 1.5-55 nm; And / or, the thickness of the spinel phase coating layer is 0.1-20 nm, preferably 0.5-5 nm; And / or, the thickness of the composite nitride coating layer is 0.1-200 nm, preferably 1-50 nm; And / or, the spinel phase coating layer is a new phase synthesized in situ during the process of coating the composite nitride coating layer; And / or, the chemical formula of the spinel phase coating layer is selected from Li x’ M' y’ O4, wherein 0.5≤x'≤1, 1≤y'≤2, and M' is selected from one of nickel Ni, cobalt Co, manganese Mn, iron Fe and chromium Cr; And / or, in the composite nitride coating layer, the D δ E ε At least one selected from Y2O3, CaO, Li2O, BaO, MgO, SrO, La2O3, HfO2, CeO2, CaC2, YC2 and ZrO2.

4. The modified positive electrode material according to any one of claims 1 to 3, wherein The thermal conductivity of the modified positive electrode material is ≥10 W / (m·K), preferably 10-20 W / (m·K).

5. A method for preparing a modified positive electrode material, characterized in that: The preparation method comprises: (1) The organic carbon source and the reactant containing A are ultrasonically dispersed in a mixed solvent, and the obtained dispersion is heated until The mixed solvent is completely volatilized to obtain a first mixture; (2) The first mixture, the positive electrode material as a matrix and the general formula D δ E ε and a fluxing and sintering agent to obtain a second mixture; (3) contacting the second mixture with nitrogen and performing a nitridation reaction to in-situ generate a composite coating layer on the surface of the substrate to obtain a modified positive electrode material; Wherein, the composite coating layer comprises a spinel phase coating layer disposed inside and a composite nitride coating layer disposed outside; wherein the chemical formula of the composite nitride coating layer is selected from αAN β O γ ·(1-α)D δ E ε , wherein 0.7≤α<1, 0.7≤β<1, 0.1≤γ≤0.3, 1≤δ≤2, 1≤ε≤3; A is selected from one of aluminum Al, boron B and titanium Ti, D is selected from at least one of yttrium Y, calcium Ca, lithium Li, barium Ba, magnesium Mg, strontium Sr, lanthanum La, hafnium Hf, zirconium Zr and cerium Ce, and E is selected from at least one of carbon C, nitrogen N, oxygen O and fluorine F.

6. The preparation method according to claim 5, wherein: In step (1), The molar ratio of the organic carbon source to the A-containing reactant is 2-5:1; and / or, the organic carbon source is selected from at least one of glucose, sucrose, maltose, soluble starch, cellulose, lignin, polydopamine, PVP, formaldehyde resin and tannic acid; And / or, the reactant containing A is selected from one of an aluminum source, a boron source and a titanium source; Preferably, the aluminum source is selected from at least one of α-Al2O3, γ-Al2O3, A1(OH)3, AlOOH, boehmite and pseudoboehmite; the boron source is selected from at least one of B2O3, borax, trimethyl borate and ammonium borate; the titanium source is selected from at least one of TiO2, titanium tetrachloride, tetrabutyl titanate and titanium isopropoxide; And / or, the average particle size D of the reactant containing A 50 10-1000nm; And / or, the mixed solvent is selected from water and an organic solvent, and the volume ratio of water to the organic solvent is 1-5:1; Preferably, the organic solvent is selected from at least one of methanol, ethanol, ethylene glycol, glycerol, butanol, isopropanol, methyl ether, ethyl ether, acetone, formaldehyde, acetaldehyde, ethyl acetate and N-methylpyrrolidone; And / or, the ultrasonic dispersion process includes: firstly mixing the organic carbon source and the mixed solvent uniformly, and then adding the A-containing reactant to perform the ultrasonic dispersion; And / or, the conditions of the ultrasonic dispersion include: time of 5-30 min, ultrasonic frequency of 10-20 KHz; And / or, the heating temperature is selected from 50-200°C.

7. The preparation method according to claim 5 or 6, wherein: In step (2), The mass ratio of the positive electrode material to the first mixture is 10-10000:1, preferably 100-5000:1; and / or, the molar ratio of the first mixture to the fluxing and sintering agent, calculated as A, is ≥7:3; And / or, the mixing process includes: first mixing at a speed of 50-500 r / min for 1-5 min, and then mixing at a speed of 500-3000 r / min for 5-30 min; and / or, the fluxing agent is selected from at least one of Y2O3, CaO, Li2O, BaO, MgO, SrO, La2O3, HfO2, CeO2, CaC2, YC2 and ZrO2; And / or, the average particle size D of the fluxing agent 50 10-500nm.

8. The preparation method according to any one of claims 5 to 7, wherein: In step (3), The conditions of the nitridation reaction include: temperature of 400-1200°C, preferably 600-1000°C; pressure of 0.1-2MPa, preferably 0.5-2MPa; time of 0.1-5h, preferably 0.5-3h; And / or, the heating rate of the nitridation reaction is 1-5°C / min; And / or, the flow rate of the nitrogen is 1-50 L / min, preferably 2-20 L / min.

9. A positive electrode sheet, characterized in that: The positive electrode sheet comprises a current collector and a positive electrode active material layer, and the positive electrode active material layer comprises: the modified positive electrode material according to any one of claims 1 to 4, or the modified positive electrode material prepared by the preparation method according to any one of claims 5 to 8; Preferably, the positive electrode active material layer further comprises: a first additive, a second additive, a binder and an optional dispersant.

10. The positive electrode sheet according to claim 9, wherein: Based on the total weight of the positive electrode active material layer, the content of the modified positive electrode material is 80-99wt%, the content of the first additive is 0.2-5wt%, the content of the second additive is 0.2-5wt%, the content of the binder is 0.4-5wt%, and the content of the dispersant is 0-5wt%; and / or, the thermal conductivity of the first additive and the second additive are each independently ≥1500 W / (m·K), preferably ≥2000 W / (m·K); And / or, the first additive is selected from thermally conductive graphite and / or graphene; the second additive is selected from single-walled carbon nanotubes and / or multi-walled carbon nanotubes; and / or, the size of the first additive is 1-10 μm, wherein the size refers to the average value of the lateral size; and / or, the aspect ratio of the second additive is 200-50000, preferably 500-10000; and / or, the second additive has a length of 0.5-20 μm and a diameter of 0.5-30 nm; And / or, the binder is selected from at least one of PVDF, PVDF-HFP, polyvinyl alcohol, sodium carboxymethyl cellulose, polyacrylonitrile, sodium alginate, polyimide, polyethyleneimine, and polymethyl methacrylate; And / or, the dispersant is at least one of a titanate coupling agent, PVP, sodium dodecylbenzene sulfonate, a silane coupling agent and disodium methylene dinaphthyl sulfate.

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