Cathode material, preparation method and application thereof

Abstract

The application relates to a positive electrode material and a preparation method and application thereof, the positive electrode material comprising a core, an inner coating layer and an outer coating layer which are sequentially coated on the surface of the core from inside to outside, the core comprising phosphate particles, the inner coating layer being a nitrogen-doped carbon layer, and the outer coating layer being a layer of a layered structure material; wherein the mass percentage of nitrogen in the nitrogen-doped carbon layer is 0.5% to 8%. According to the scheme, the positive electrode material has high conductivity, interface stability and structural integrity, the transition metal dissolution amount after high-temperature storage is low, and the conductivity of the battery can be improved, so that the battery has excellent high-temperature cycle performance and rate performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to cathode materials, their preparation methods, and applications. Background Technology

[0002] Lithium-ion batteries are widely used in 3C digital products, power tools, aerospace, energy storage, and electric vehicles due to their advantages such as high specific energy, no memory effect, and long cycle life. The rapid development of electronic information technology and consumer products has placed higher demands on the electrochemical performance of lithium-ion batteries.

[0003] Lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP), as olivine-structured cathode materials, are highly favored due to their high safety and long cycle life. However, their shared intrinsic drawbacks limit their application in high-performance batteries: their extremely low electronic and ionic conductivity leads to poor rate performance; the direct contact between phosphate and electrolyte, especially under high voltage (for LMFP) and high temperature conditions, easily causes interfacial side reactions, catalyzing electrolyte decomposition and resulting in cycle life degradation. Furthermore, LFP also carries the risk of manganese dissolution, causing manganese ions to enter the electrolyte, migrate to the negative electrode, damage the SEI film, and ultimately degrade the battery's cycle performance.

[0004] Therefore, there is an urgent need to develop a cathode material with high conductivity and stability to improve the conductivity of the battery and enhance its rate performance and high-temperature cycling performance. Summary of the Invention

[0005] To address or partially address the problems existing in related technologies, this application provides a cathode material, its preparation method, and its application. This cathode material has high conductivity, interface stability, and structural integrity, low transition metal leaching after high-temperature storage, and can improve the conductivity of the battery, enabling the battery to exhibit excellent high-temperature cycle performance and rate performance.

[0006] The first aspect of this application provides a cathode material, wherein the cathode material includes a core and an inner coating layer and an outer coating layer sequentially covering the surface of the core from the inside to the outside, the core includes phosphate particles, the inner coating layer is a nitrogen-doped carbon layer, and the outer coating layer is a layered structure material layer; The nitrogen content in the nitrogen-doped carbon layer is 0.5% to 8% by mass.

[0007] As described in the first aspect, in the cathode material, the nitrogen-doped carbon layer has a mass percentage content of 0.5% to 3% in the cathode material; And / or, the thickness of the nitrogen-doped carbon layer is 2 nm to 10 nm; And / or, the mass percentage of nitrogen in the nitrogen-doped carbon layer is 2% to 6%.

[0008] As described in the first aspect, in the cathode material, the layered structure material comprises ternary nickel-cobalt-manganese oxide and / or binary nickel-manganese oxide; wherein the chemical formula of the ternary nickel-cobalt-manganese oxide is LiNi. x Co y Mn z O2, x+y+z=1; the chemical formula of the binary nickel-manganese oxide is LiNi α Mn β O2, α+β=1; And / or, the layered structure material layer has a mass percentage content of 1% to 8% in the cathode material; And / or, the thickness of the layered structure material layer is 5nm~30nm.

[0009] As described in the first aspect, the cathode material contains phosphate particles comprising lithium iron phosphate and / or lithium manganese iron phosphate; wherein the lithium manganese iron phosphate has the chemical formula LiMn. x Fe 1-x PO4, 0.3 <x<0.8。

[0010] A second aspect of this application provides a method for preparing a cathode material as described in the first aspect, comprising the following steps: S1. The phosphate particles are placed in a nitrogen-containing polymer monomer solution and subjected to in-situ oxidative polymerization to obtain a polymer-coated core; then subjected to carbonization heat treatment to obtain a phosphate material coated with a nitrogen-doped carbon layer; wherein the nitrogen-doped carbon layer forms the inner coating layer on the surface of the core. The carbonization temperature of the carbonization heat treatment is 500℃~750℃, and the carbonization time is 1h~6h; S2. The nitrogen-doped carbon-coated phosphate material is dispersed in a precursor solution containing elements of layered structure materials, and after dispersion treatment and obtaining a dry nitrogen-doped carbon-coated phosphate material containing the precursor, it is sintered in an oxygen atmosphere to obtain the cathode material; wherein the precursor forms an outer coating layer on the surface of the nitrogen-doped carbon layer.

[0011] The method for preparing the cathode material as described in the second aspect, wherein the nitrogen-containing polymer monomer includes at least one of pyrrole, aniline, and dopamine hydrochloride; And / or, the mass ratio of the phosphate particles to the nitrogen-containing polymer monomer is (10~100):1.

[0012] The method for preparing the cathode material as described in the second aspect, wherein the in-situ oxidative polymerization reaction is carried out under the action of an oxidant, while the pH value of the system is adjusted simultaneously; Preferably, the nitrogen-containing polymer monomer is dopamine hydrochloride, the oxidant includes dissolved oxygen and / or ammonium persulfate, and the pH value is 8.2-8.8; more preferably, the mass ratio of dopamine hydrochloride to ammonium persulfate is 1:(1-1.5); even more preferably, the mass ratio of dopamine hydrochloride to dissolved oxygen is 1:(0.05-0.15). Preferably, the nitrogen-containing polymer monomer is pyrrole, the oxidant includes ferric chloride and / or ammonium persulfate, and the pH value is 1-4; more preferably, the mass ratio of pyrrole to ferric chloride is 1:(2-2.5); even more preferably, the mass ratio of pyrrole to ammonium persulfate is 1:(1.2-1.8). Preferably, the nitrogen-containing polymer monomer is aniline, the oxidant includes ammonium persulfate, and the pH value is 1-4; the mass ratio of aniline to the oxidant is 1:(1-1.3). And / or, the in-situ oxidative polymerization reaction is carried out under stirring, with a stirring rate of 300 rpm to 500 rpm and a stirring time of 2 h to 8 h.

[0013] The method for preparing the cathode material as described in the second aspect, wherein the precursor solution containing the constituent elements of the layered structure material is prepared by dissolving a lithium source and a transition metal source in a solvent, wherein the lithium source includes lithium acetate and / or lithium nitrate, and the transition metal source includes the corresponding acetate and / or nitrate; the molar ratio of the lithium source to the transition metal source is based on the stoichiometry of the layered structure material, and the lithium source is in excess by 5% to 10%; And / or, the dispersion treatment includes mechanical stirring and ultrasonic treatment; the stirring rate of the mechanical stirring is 300 rpm to 500 rpm, and the stirring time is 1 h to 4 h; the ultrasonic power of the ultrasonic treatment is 300 W to 500 W, and the ultrasonic time is 30 min to 60 min; And / or, the sintering temperature of the sintering treatment is 450℃~700℃, and the sintering time is 2h~10h.

[0014] A third aspect of this application provides a positive electrode sheet, wherein the positive electrode sheet includes a positive current collector and a positive electrode coating coated on at least one side of the positive current collector, the positive electrode coating including a positive electrode material as described in the first aspect or a positive electrode material prepared by a method for preparing a positive electrode material as described in the second aspect.

[0015] A fourth aspect of this application provides a battery, wherein the battery includes a positive electrode as described in the third aspect.

[0016] The technical solution provided in this application can include the following beneficial effects: The cathode material exhibits a three-layer core-shell structure, with a nitrogen-doped carbon layer containing a specific nitrogen element content and a layered structure material layer working synergistically. On the one hand, the inner nitrogen-doped carbon layer provides an excellent electronic conductivity network, while the outer layered structure material layer provides good ion and electron conductivity channels. The two produce a synergistic effect, improving the electronic conductivity and ionic conductivity of the cathode material, thereby improving the battery's conductivity and thus enabling the battery to exhibit excellent rate performance. On the other hand, the inner nitrogen-doped carbon layer effectively isolates the direct contact between phosphate and electrolyte, inhibiting the electrolyte's erosion of phosphate, reducing the dissolution of transition metals, and also avoiding the outer layered structure... The direct contact between the material layer and the core phosphate avoids the formation of high-resistivity impurity phases due to lattice mismatch and element interdiffusion, thereby improving the structural stability of the cathode material under high temperature or high pressure conditions, reducing the amount of transition metal dissolution, and thus improving the rate performance and high-temperature cycle performance of the battery. In addition, the presence of an inner nitrogen-doped carbon layer with a specific nitrogen content helps to achieve a thinner and more uniform coating of the outer layered structure material layer, which not only optimizes the interface quality but also reduces the amount of layered structure material used. Furthermore, the flexibility and conductivity of the inner nitrogen-doped carbon layer help to alleviate local stress during charging and discharging, maintain the structural integrity of the outer layered structure material layer, thereby ensuring the high stability of the cathode material and ultimately improving the cycle performance of the battery.

[0017] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Detailed Implementation

[0018] To facilitate understanding of this application, it will be described in detail below. However, before describing this application in detail, it should be understood that this application is not limited to the specific embodiments described. It should also be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to be restrictive.

[0019] Where a numerical range is provided, it should be understood that every intermediate value between the upper and lower limits of the range and any other specified or intermediate value within the specified range is covered within this application. The upper and lower limits of these smaller ranges may be independently included in the smaller range and are also covered within this application, subject to any explicitly excluded limits within the specified range. Where the specified range includes one or two limits, the range excluding any or both of those included limits is also included within this application.

[0020] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. While the methods and materials described herein, or any equivalent methods and materials, may also be used in the implementation or testing of this application, preferred methods and materials are now described.

[0021] Lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP), as olivine-structured cathode materials, are highly favored due to their high safety and long cycle life. However, their shared intrinsic drawbacks limit their application in high-performance batteries: their extremely low electronic and ionic conductivity leads to poor rate performance; the direct contact between phosphate and electrolyte, especially under high voltage (for LMFP) and high temperature conditions, easily causes interfacial side reactions, catalyzing electrolyte decomposition and resulting in cycle life degradation. Furthermore, LFP also carries the risk of manganese dissolution, causing manganese ions to enter the electrolyte, migrate to the negative electrode, damage the SEI film, and ultimately degrade the battery's cycle performance.

[0022] Currently, carbon coating is the mainstream strategy for improving the conductivity of cathode materials. However, carbon layers obtained by pyrolysis of traditional carbon sources (such as glucose and sucrose) are usually loosely structured and unevenly dense, offering limited protection to the particle interface and potentially hindering lithium-ion transport, which in turn leads to a decrease in the rate performance and high-temperature cycling performance of the battery.

[0023] Another approach is to use layered materials (such as NCM) for surface coating to enhance conductivity. However, this method faces two key problems: First, there is a significant lattice mismatch between layered materials and olivine-structured LFP / LMFP, and direct coating easily leads to poor interfacial compatibility. During subsequent high-temperature sintering, elemental interdiffusion easily occurs at the interface, generating harmful impurity phases, increasing interfacial impedance, and thus degrading rate and cycle performance. Second, if nickel-rich layered materials are used as the coating layer, they are prone to side reactions with the electrolyte under high voltage, and residual alkaline substances on the surface, if directly exposed to the electrolyte, will accelerate battery performance degradation.

[0024] To address the aforementioned issues, this application provides a cathode material comprising a core and an inner coating layer and an outer coating layer sequentially covering the surface of the core from the inside out. The core comprises phosphate particles, the inner coating layer is a nitrogen-doped carbon layer, and the outer coating layer is a layered structure material layer; wherein the mass percentage of nitrogen in the nitrogen-doped carbon layer is 0.5% to 8%.

[0025] The cathode material of this application has a three-layer core-shell structure, consisting of a core, an inner coating layer covering the surface of the core, and an outer coating layer covering the surface of the inner coating layer, from the inside out.

[0026] The phosphate particles in this application refer to phosphate cathode materials with an olivine structure. This application does not limit the specific selection of phosphate particles, but can select them according to actual needs, such as lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, etc.

[0027] The nitrogen-doped carbon layer in this application refers to a coating layer with a specific electronic structure and chemical activity formed by introducing nitrogen atoms into the framework of a carbon material. The mass percentage of nitrogen in the nitrogen-doped carbon layer is 0.5% to 8%, for example, the mass percentage of nitrogen in the nitrogen-doped carbon layer can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc.

[0028] The layered structure material layer in this application refers to a coating layer composed of compounds with a layered crystal structure. The compounds with a layered crystal structure in this application refer to lithium transition metal oxides with an α-NaFeO2 type structure, such as nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, lithium cobalt oxide, etc.

[0029] According to the above-mentioned scheme provided in this application, the cathode material possesses high conductivity, interfacial stability, and structural integrity. It exhibits low transition metal dissolution after high-temperature storage and enhances battery conductivity, resulting in excellent rate performance and high-temperature cycle performance. The applicant analyzed this principle and believes the reason lies in the three-layer core-shell structure of the cathode material, where a nitrogen-doped carbon layer with a specific nitrogen content and a layered material layer work synergistically. On one hand, the inner nitrogen-doped carbon layer provides an excellent electronic conductivity network, while the outer layered material layer provides good ion and electron conductivity channels. This synergistic effect improves the electronic and ionic conductivity of the cathode material, thereby enhancing battery conductivity and resulting in excellent rate performance. On the other hand, the inner nitrogen-doped carbon layer effectively isolates the phosphate from direct contact with the electrolyte, inhibiting electrolyte erosion of the phosphate, reducing transition metal dissolution, and also preventing damage to the outer layered material layer. The direct contact between the cathode layer and the core phosphate avoids the formation of high-resistivity impurity phases due to lattice mismatch and element interdiffusion, thereby improving the structural stability of the cathode material under high temperature or high pressure conditions, reducing the amount of transition metal dissolution, and thus improving the rate performance and high-temperature cycle performance of the battery. In addition, the presence of an inner nitrogen-doped carbon layer with a specific nitrogen content helps to achieve a thinner and more uniform coating of the outer layered structure material, which not only optimizes the interface quality but also reduces the amount of layered structure material used. Furthermore, the flexibility and conductivity of the inner nitrogen-doped carbon layer help to alleviate local stress during charging and discharging, maintain the structural integrity of the outer layered structure material, thereby ensuring the high stability of the cathode material and ultimately improving the cycle performance of the battery.

[0030] In one specific embodiment, the nitrogen-doped carbon layer has a mass percentage content of 0.5% to 3% in the cathode material. For example, the mass percentage content of the nitrogen-doped carbon layer in the cathode material can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc. When the mass percentage content of the nitrogen-doped carbon layer is within the above range, the nitrogen-doped carbon layer can completely coat the phosphate particles, forming a dense coating layer, thereby providing an excellent electronic conductivity network for the cathode material, improving the conductivity of the cathode material, protecting the phosphate, reducing the dissolution of transition metals, and avoiding the problem of energy density loss caused by excessive nitrogen-doped carbon layer content, thereby achieving improved rate performance and cycle performance.

[0031] Specifically, the mass percentage of the nitrogen-doped carbon layer in the cathode material can be obtained by elemental analysis (EA) or X-ray photoelectron spectroscopy (XPS).

[0032] In one specific embodiment, the thickness of the nitrogen-doped carbon layer is 2nm to 10nm, for example, the thickness of the nitrogen-doped carbon layer can be 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, etc. When the thickness of the nitrogen-doped carbon layer is within the above range, the nitrogen-doped carbon layer can effectively prevent direct contact between the electrolyte and phosphate particles, reduce the occurrence of side reactions, and improve the conductivity of the cathode material. At the same time, it avoids the problems of prolonged lithium-ion diffusion path and increased charge transfer impedance caused by excessively thick nitrogen-doped carbon layers. It can also effectively alleviate local stress during charging and discharging, comprehensively ensuring the high stability and high conductivity of the cathode material, thereby improving the rate performance and cycle performance of the battery.

[0033] Specifically, the thickness of the nitrogen-doped carbon layer in this application can be obtained by transmission electron microscopy (TEM).

[0034] In one specific embodiment, the mass percentage of nitrogen in the nitrogen-doped carbon layer is 2% to 6%, for example, the mass percentage of nitrogen in the nitrogen-doped carbon layer can be 2%, 3%, 4%, 5%, 6%, etc. When the nitrogen content is within the above range, the nitrogen-doped carbon layer can provide a better electronic conductivity network, improve the electronic conductivity of the cathode material, and enhance the stability of the nitrogen-doped carbon layer, thereby improving the stability of the cathode material, thus enabling the battery to exhibit excellent cycle performance and rate performance.

[0035] Specifically, the mass percentage of nitrogen in the nitrogen-doped carbon layer of this application can be obtained by testing with an elemental analyzer (EA) or X-ray photoelectron spectroscopy (XPS).

[0036] In one specific embodiment, the layered structural material in the layered structure material comprises ternary nickel-cobalt-manganese oxide and / or binary nickel-manganese oxide; wherein, the chemical formula of the ternary nickel-cobalt-manganese oxide is LiNi. x Co y Mn z O2, x+y+z=1; the chemical formula of the binary nickel-manganese oxide is LiNi α Mn β O2, α+β=1. When the above-mentioned compounds are selected as the layered structure material, the layered structure material can provide better ion and electron conduction channels for the cathode material, thereby enabling better synergy with the inner nitrogen-doped carbon layer, improving the conductivity of the cathode material, and thus improving the cycle performance and rate performance of the battery.

[0037] In one specific embodiment, the layered structure material layer has a mass percentage of 1% to 8% in the cathode material. For example, the mass percentage of the layered structure material layer in the cathode material can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc. When the mass percentage of the layered structure material layer is within the above range, the layered structure material layer can better coat the nitrogen-doped carbon layer, forming a dense outer shell layer, and can provide better ion and electron conduction channels, improving the conductivity of the cathode material, thereby greatly improving the rate performance of the battery.

[0038] Specifically, the mass percentage of the layered structure material layer in the cathode material can be obtained by testing with an elemental analyzer (EA).

[0039] In one specific embodiment, the thickness of the layered material layer is 5nm to 30nm, for example, the thickness of the layered material layer can be 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, etc. When the thickness of the layered material layer is within the above range, the layered material layer provides better ion and electron conduction channels, resulting in better conductivity of the cathode material. At the same time, it can better protect the phosphate particles in the inner layer, preventing direct contact between the electrolyte and the phosphate particles. It can also avoid the problem of lithium-ion transport obstruction caused by excessive thickness, thereby further improving the rate performance and cycle performance of the battery.

[0040] Specifically, the thickness of the layered structure material layer in this application can be obtained by transmission electron microscopy (TEM).

[0041] In one specific embodiment, the phosphate particles comprise lithium iron phosphate and / or lithium manganese iron phosphate; wherein the chemical formula of lithium manganese iron phosphate is LiMn. x Fe 1-xPO4, where 0.3 < x < 0.8. When the phosphate particles are lithium iron phosphate or lithium manganese iron phosphate, the cathode material has excellent stability and can improve the cycle life of the battery.

[0042] The second aspect of this application provides a method for preparing a cathode material, which includes the following steps: S1. Place the phosphate particles in a nitrogen-containing polymer monomer solution and carry out an in-situ oxidative polymerization reaction to obtain a polymer-coated core; then carry out carbonization heat treatment to obtain a phosphate material coated with a nitrogen-doped carbon layer; wherein, the nitrogen-doped carbon layer forms an inner coating layer on the surface of the core. The carbonization temperature of the carbonization heat treatment is 500°C to 750°C, and the carbonization time is 1h to 6h. S2. Disperse the phosphate material coated with a nitrogen-doped carbon layer in a precursor solution containing the constituent elements of the layered structure material. After carrying out dispersion treatment and obtaining a dried phosphate material coated with a nitrogen-doped carbon layer containing the precursor, carry out sintering treatment in an oxygen atmosphere to obtain the cathode material; wherein, the precursor powder forms an outer coating layer on the surface of the nitrogen-doped carbon layer.

[0043] Specifically, in step S1, the inner nitrogen-doped carbon layer is coated on the surface of the phosphate particles: disperse the phosphate particles in a nitrogen-containing polymer monomer solution, and then carry out an in-situ oxidative polymerization reaction at room temperature, so that the nitrogen-containing polymer monomer polymerizes on the surface of the core phosphate particles to form a nitrogen-containing polymer coating layer, and then carry out separation, washing, and drying to obtain a polymer-coated core; then carry out carbonization heat treatment on the polymer-coated core in an inert atmosphere to carbonize the nitrogen-containing polymer into a nitrogen-doped carbon material, and form a nitrogen-doped carbon layer on the surface of the phosphate, thereby obtaining a phosphate material coated with a nitrogen-doped carbon layer.

[0044] This application does not limit the reaction parameters of the in-situ oxidative polymerization reaction, which can be selected according to actual needs.

[0045] This application does not limit the specific parameters of separation, washing, and drying, which can be selected according to actual needs.

[0046] This application does not limit the specific selection of the inert atmosphere, which can be selected according to actual needs. For example, nitrogen, argon, etc. can be selected.

[0047] The carbonization temperature of the heat treatment in this application is 500℃~750℃, and the carbonization time is 1h~6h. For example, the carbonization temperature can be 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, etc., and the carbonization time can be 1h, 2h, 3h, 4h, 5h, 6h, etc. The nitrogen-containing polymer can fully carbonize to form a dense, nitrogen-doped continuous carbon network, significantly improving the electronic conductivity and interfacial stability of the cathode material, and avoiding direct contact between the core phosphate and the electrolyte. If the carbonization temperature is too low or the carbonization time is too short, incomplete carbonization will result in a loose carbon layer and poor conductivity; while if the carbonization temperature is too high or the carbonization time is too long, a large amount of nitrogen will volatilize, reducing active nitrogen species, and may induce the reduction and decomposition of the phosphate core, destroying structural integrity. Preferably, the carbonization time is 2h~5h.

[0048] Step S2: Coating the surface of the nitrogen-doped carbon-coated phosphate material with an outer layer of layered structure material: Prepare a precursor solution containing the constituent elements of the layered structure material, disperse the nitrogen-doped carbon-coated phosphate material in the precursor solution, and after dispersion treatment, allow the precursor ions to be uniformly adsorbed on the surface of the nitrogen-doped carbon layer. Then, separate, wash, and dry to obtain the nitrogen-doped carbon-coated phosphate material containing the precursor. Sinter it in an oxygen atmosphere to decompose the precursor and crystallize it into a layered structure coating layer, thereby obtaining the cathode material.

[0049] This application does not limit the preparation method of the precursor solution containing the constituent elements of layered structural materials. For example, lithium source, nickel source, cobalt source (if needed), and manganese source can be dissolved in a solvent in stoichiometric ratio, and a complexing agent can be added to form a homogeneous precursor solution. The precursor solution includes lithium ions, nickel ions, cobalt ions (if needed), and manganese ions.

[0050] This application does not limit the specific parameters of distributed processing; they can be selected according to actual needs.

[0051] This application does not limit the specific parameters of the sintering process, which can be selected according to actual needs.

[0052] This application employs a step-by-step coating strategy of "polymerization and carbonization followed by liquid-phase coating and sintering": First, nitrogen-containing polymer monomers are polymerized in situ on the core surface, and after carbonization, an inner nitrogen-doped carbon layer is formed. Subsequently, precursor ions are adsorbed onto the surface of this nitrogen-doped carbon layer, followed by oxidative decomposition and crystallization to construct the outer layered structure. The sintering process in an oxygen atmosphere is crucial: it not only promotes the formation of a highly ordered crystal structure in the outer precursor layer but also, thanks to the protective effect of the dense inner carbon layer, prevents the phosphate core from oxidative decomposition. This method effectively constructs a cathode material with a three-layer core-shell structure, ensuring not only the high uniformity of the inner and outer coating layers but also balancing the density of the inner nitrogen-doped carbon layer with the crystallinity of the outer layered structure. This significantly improves the electronic conductivity, ionic conductivity, and structural stability of the cathode material, thereby greatly enhancing the battery's cycle performance and rate performance. Furthermore, the preparation process is clear, parameters are controllable, requires no complex equipment, is easy to scale up, and is suitable for large-scale production.

[0053] In one specific embodiment, the nitrogen-containing polymer monomer includes at least one of pyrrole, aniline, and dopamine hydrochloride. The nitrogen-containing polymer monomer used in this application has excellent in-situ polymerization ability, which can form a uniform and continuous polymer coating layer on the core surface. After carbonization under an inert atmosphere, it can be transformed into a dense and highly conductive nitrogen-doped carbon layer, thereby ensuring the successful preparation of the cathode material.

[0054] In one specific embodiment, the mass ratio of phosphate particles to nitrogen-containing polymer monomers is (10~100):1, for example, the mass ratio of phosphate particles to nitrogen-containing polymer monomers can be 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, 90:1, 100:1, etc. At this ratio, the nitrogen-containing polymer monomers can fully polymerize in situ on the surface of the phosphate particles, forming a uniform and continuous polymer coating layer. After subsequent carbonization treatment, this is transformed into a nitrogen-doped carbon layer of moderate thickness, density, and good conductivity, thereby improving the electronic conductivity and interface stability of the cathode material. Simultaneously, it avoids hindering lithium-ion diffusion due to an excessively thick carbon layer, thus enabling the battery to exhibit excellent cycle performance and rate performance. Preferably, the mass ratio of phosphate particles to nitrogen-containing polymer monomers is 100:(1.5~3).

[0055] In one specific embodiment, the in-situ oxidative polymerization reaction is carried out under the action of an oxidant, while the pH value of the system is adjusted.

[0056] Preferably, if the nitrogen-containing polymer monomer is dopamine hydrochloride, the oxidant includes dissolved oxygen and / or ammonium persulfate, and the pH value is 8.2-8.8; more preferably, the mass ratio of dopamine hydrochloride to ammonium persulfate is 1:(1-1.5), for example, the mass ratio of dopamine hydrochloride to ammonium persulfate can be 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, etc.; even more preferably, the mass ratio of dopamine hydrochloride to dissolved oxygen is 1:(0.05-0.15), for example, the mass ratio of dopamine hydrochloride to dissolved oxygen can be 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.1, 1:0.11, 1:0.12, 1:0.13, 1:0.14, 1:0.15, etc. Appropriate amounts of dissolved oxygen or ammonium persulfate can effectively oxidize dopamine hydrochloride in a system with a pH of 8.2-8.8, causing it to undergo in-situ oxidative polymerization. This process is highly efficient with few side reactions, enabling the efficient construction of the polymer coating layer and facilitating the subsequent formation of a uniform and dense nitrogen-doped carbon layer.

[0057] Preferably, if the nitrogen-containing polymer monomer is pyrrole, the oxidant includes ferric chloride and / or ammonium persulfate, and the pH value is 1-4; the mass ratio of pyrrole to ferric chloride is 1:(2-2.5), for example, the mass ratio of pyrrole to ferric chloride can be 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, etc.; more preferably, the mass ratio of pyrrole to ammonium persulfate is 1:(1.2-1.8), for example, the mass ratio of pyrrole to ammonium persulfate can be 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, etc. In this acidic environment, pyrrole molecules maintain good reactivity and are easily oxidized by the oxidant to generate cationic free radicals. Ferric chloride has moderate oxidizing power, while ammonium persulfate generates strongly oxidizing sulfate free radicals through thermal decomposition, which can rapidly and effectively initiate polymerization under acidic conditions. A suitable mass ratio of pyrrole to oxidant can ensure that the polymerization reaction proceeds fully, forming polypyrrole with high molecular weight, complete conjugated structure, and high electrical conductivity.

[0058] Preferably, if the nitrogen-containing polymer monomer is aniline, the oxidant includes ammonium persulfate, and the pH value is 1-4; the mass ratio of aniline to oxidant is 1:(1-1.3), and the mass ratio of aniline to oxidant can be 1:1, 1:1.05, 1:1.1, 1:1.15, 1:1.2, 1:1.25, 1:1.3, etc. In an acidic environment, aniline mainly exists in the form of aniline cations, and its amino group is protonated, which is conducive to ordered polymerization. Ammonium persulfate decomposes upon heating in an acidic medium to generate strongly oxidizing sulfate radicals, which can effectively oxidize aniline to generate free radical cations, initiating a chain polymerization reaction to generate polyaniline. When the mass ratio of aniline to oxidant is within the above range, ammonium persulfate is sufficient to fully initiate and maintain the polymerization, forming high molecular weight polyaniline with a complete conjugated structure, which is beneficial for the subsequent preparation of a uniform and dense nitrogen-doped carbon layer.

[0059] In one specific embodiment, the in-situ oxidative polymerization reaction is carried out under stirring, with a stirring rate of 300 rpm to 500 rpm and a stirring time of 2 h to 8 h. For example, the stirring rate can be 300 rpm, 320 rpm, 340 rpm, 360 rpm, 380 rpm, 400 rpm, 420 rpm, 440 rpm, 460 rpm, 480 rpm, 500 rpm, etc., and the stirring time can be 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, etc. When the stirring rate and stirring time are within the above ranges, the nitrogen-containing polymer monomers can fully undergo the polymerization reaction under the action of the oxidant, ensuring the efficient progress of the polymerization reaction and reducing the occurrence of side reactions. If the stirring time is too short, it may lead to incomplete polymerization, uneven carbon layer thickness, and the appearance of free polyaniline homopolymer, affecting the preparation of the cathode material; if the stirring time is too long, the gain on the performance of the cathode material is minimal, but it will significantly increase energy consumption and production costs. Preferably, the stirring time is 3 h to 6 h.

[0060] In one specific embodiment, the precursor solution containing the constituent elements of the layered structure material is prepared by dissolving a lithium source and a transition metal source in a solvent. The lithium source includes lithium acetate and / or lithium nitrate, and the transition metal source includes the corresponding acetate and / or nitrate. This application does not limit the choice of solvent; for example, water, ethanol, or a water-ethanol mixture can be selected. Acetates or nitrates have high solubility, low decomposition temperatures, and few residual impurities after decomposition, which facilitates crystallization after oxidative decomposition to form the outer layered structure.

[0061] In one specific embodiment, the molar ratio of lithium source to transition metal source is based on the stoichiometry of the layered structure material, with lithium source in excess by 5% to 10%. For example, the molar ratio of lithium, nickel, cobalt, and manganese in NCM111 is 1:1 / 3:1 / 3:1 / 3, therefore the molar ratio of lithium source, nickel source, cobalt source, and manganese source is (1.05~1.1):1 / 3:1 / 3:1 / 3. When the molar ratio of lithium source to transition metal source is within the above range, it can compensate for lithium volatilization loss during high-temperature sintering, avoid defects such as lithium vacancies and cation mixing, and ensure the structural order and electrochemical performance of the cathode material.

[0062] In one specific embodiment, the dispersion treatment includes mechanical stirring and ultrasonic treatment. The mechanical stirring rate is 300 rpm to 500 rpm, and the stirring time is 1 h to 4 h; for example, the stirring rate can be 300 rpm, 350 rpm, 400 rpm, 450 rpm, 500 rpm, etc., and the stirring time can be 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, etc. The ultrasonic power of the ultrasonic treatment is 300 W to 500 W, and the ultrasonic time is 30 min to 60 min; for example, the ultrasonic power can be 300 W, 350 W, 400 W, 450 W, 500 W, etc., and the ultrasonic time can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, etc. Through mechanical stirring and ultrasonic treatment, precursor ions in the precursor solution can be fully adsorbed onto the surface of the nitrogen-doped carbon layer, which helps the subsequent decomposition and crystallization of the precursor salt, forming a uniform and dense layered structure. Preferably, the stirring time is 2 to 3 hours.

[0063] In one specific embodiment, the sintering temperature is 450℃~700℃, and the sintering time is 2h~10h. For example, the sintering temperature can be 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, etc., and the sintering time can be 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h. When the sintering temperature and sintering time are within the above range, the precursor salt can be fully decomposed, the transition metal can be completely oxidized, and an ordered layered structure can be formed. This avoids excessively high temperatures or excessively long times from causing increased lithium volatilization, abnormal grain growth, particle agglomeration, or increased surface residual lithium, which would damage the electrochemical performance of the cathode material. Preferably, the sintering time is 4h~8h.

[0064] A third aspect of this application provides a positive electrode sheet, comprising a positive current collector and a positive electrode coating loaded on at least one side of the positive current collector. The positive electrode coating comprises the aforementioned positive electrode material or a positive electrode material prepared by the aforementioned method. The positive electrode material of this application exhibits high conductivity, interfacial stability, and structural integrity, with low transition metal leaching after high-temperature storage, and can improve the conductivity of the battery, enabling the battery to exhibit excellent high-temperature cycle performance and rate performance.

[0065] In this application embodiment, there is no particular limitation on the type of positive electrode current collector; it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper. Preferably, the positive electrode current collector is a metallic material.

[0066] In one specific embodiment, the positive electrode coating further includes a conductive agent and a binder. The conductive agent includes at least one of carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, graphene, and vapor-grown carbon fiber (VGCF). The binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, polyvinylidene fluoride, and polytetrafluoroethylene.

[0067] A fourth aspect of this application provides a battery comprising the aforementioned positive electrode. This battery exhibits excellent cycle performance and rate performance.

[0068] In one specific embodiment, the battery of this application includes a negative electrode sheet, which includes a negative electrode current collector and a negative electrode coating coated on at least one side of the negative electrode current collector. The negative electrode coating includes a negative electrode active material and a conductive agent. This application does not limit the selection of the negative electrode active material, and it can be selected according to actual needs. For example, natural graphite, artificial graphite, hard carbon, soft carbon, silicon-carbon composite materials, elemental silicon, and silicon-oxygen composite materials can be used as the negative electrode active material. The conductive agent in the negative electrode coating of this application can be selected from conventional materials in the art.

[0069] In one specific embodiment, the battery of this application further includes an electrolyte, which includes a lithium salt and an organic solvent. The lithium salt and organic solvent are lithium salts and organic solvents known in the art that can be used in electrolytes to improve the electrochemical performance of the battery, and can be specifically set as needed.

[0070] In one specific embodiment, the lithium-ion battery further includes a separator. The embodiments of this application do not have any particular restrictions on the material and shape of the separator, as long as it does not significantly impair the effect of this application. It may include porous sheet-like or non-woven fabric-like materials with excellent liquid retention. The materials of the resin or glass fiber separator include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc., and can be set according to needs.

[0071] In one embodiment, the battery may include an outer packaging that can be used to encapsulate the electrode assembly and electrolyte.

[0072] In one specific embodiment, the outer packaging of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0073] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0074] This application does not impose any particular restrictions on the application fields of lithium-ion batteries, and they can be used in fields such as consumer batteries, power batteries for new energy vehicles, and energy storage batteries.

[0075] The present application will be further described in detail below through specific embodiments.

[0076] Example 1 1. Preparation of cathode materials 1) Coating with an inner nitrogen-doped carbon layer: 2 g of lithium iron phosphate (LFP) powder was dispersed in 200 mL of Tris-HCl buffer (pH=8.5), 0.2 g of dopamine hydrochloride was added, and the mixture was stirred for 30 min. Then, 0.2 g of ammonium persulfate was added, and the mixture was reacted at room temperature for 12 h with a stirring rate of 400 rpm to obtain polymer-coated lithium iron phosphate. The solid was collected by centrifugation, dried, and then carbonized at 650 °C for 2 h under Ar atmosphere to obtain nitrogen-doped carbon-coated lithium iron phosphate.

[0077] The mass ratio of lithium iron phosphate to dopamine hydrochloride is 10:1; the mass ratio of dopamine hydrochloride to ammonium persulfate is 1:1.

[0078] 2) Weigh lithium acetate, nickel acetate, cobalt acetate, and manganese acetate in a molar ratio of Li:Ni:Co:Mn = 1.05:1:1:1, dissolve them in water, add citric acid complexing agent, and prepare a precursor solution; place the nitrogen-doped carbon-coated lithium iron phosphate in the precursor solution, perform mechanical stirring and ultrasonic treatment, then separate and dry it, and finally sinter it in an oxygen atmosphere to obtain the cathode material.

[0079] The mechanical stirring rate was 300 rpm and the stirring time was 2 h; the ultrasonic power of the ultrasonic treatment was 400 W and the ultrasonic time was 45 min; the sintering temperature of the sintering treatment was 550℃ and the sintering time was 6 h.

[0080] Elemental analysis, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) of the cathode material revealed that the nitrogen-doped carbon layer has a mass percentage of 1.5%, a thickness of 6 nm, and a nitrogen content of 4%. The layered material is a ternary nickel-cobalt-manganese oxide (NCM111), with a mass percentage of 4.5% and a thickness of 18 nm.

[0081] 2. Preparation of the positive electrode sheet The positive electrode material, conductive carbon black (SP), and polyvinylidene fluoride (PVDF) binder were mixed uniformly at a mass ratio of 92:4:4 and thoroughly stirred in N-methylpyrrolidone solvent to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated onto one surface of a 12 μm thick aluminum foil, dried at 90°C, and cold-pressed to obtain a positive electrode sheet with a coating thickness of 110 μm. The above steps were then repeated on the other surface of the same positive electrode sheet to obtain a positive electrode sheet with a double-sided coating. The positive electrode sheet was cut into 76 mm × 851 mm dimensions and tabs were welded on for later use.

[0082] 3. Preparation of electrolyte In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:1:1. Based on the total mass of the electrolyte, lithium hexafluorophosphate (LiPF6) is added to the solvent, dissolved, and mixed evenly. Then, electrolyte additives are added to obtain the electrolyte. The molar concentration of LiPF6 in the electrolyte is 1M, and the electrolyte additives include vinylene carbonate (VC), with a mass percentage of 2%.

[0083] 4. Manufacturing of lithium-ion batteries Using lithium metal sheets as the counter electrode and Celgard2325 as the separator, the electrode assembly is placed in a pre-formed aluminum-plastic film and dehydrated at 80°C. A prepared electrolyte is then injected, followed by vacuum sealing, settling, formation, and shaping processes to obtain the CR2032 button cell.

[0084] Example 2 The battery preparation method in this embodiment is largely the same as that in Example 1, except for the preparation of the positive electrode material: 1) Coating with an inner nitrogen-doped carbon layer: 2 g of lithium manganese iron phosphate (LMFP) powder was dispersed in 200 mL of Tris-HCl buffer (pH=2), 0.2 g of pyrrole was added, and the mixture was stirred for 30 min. Then, 0.44 g of ferric chloride was added, and the mixture was reacted at room temperature for 12 h with a stirring rate of 400 rpm to obtain polymer-coated lithium manganese iron phosphate. The solid was collected by centrifugation, dried, and then carbonized at 650 °C for 2 h under an Ar atmosphere to obtain nitrogen-doped carbon-coated lithium manganese iron phosphate.

[0085] The mass ratio of lithium manganese iron phosphate to pyrrole is 10:1; the mass ratio of pyrrole to ferric chloride is 1:2.2.

[0086] 2) Weigh lithium acetate, nickel acetate, cobalt acetate, and manganese acetate according to the molar ratio of Li:Ni:Co:Mn=1.05:0.6:0.2:0.2, dissolve them in water, add citric acid complexing agent, and prepare a precursor solution; place nitrogen-doped carbon-coated lithium manganese iron phosphate in the precursor solution, perform mechanical stirring and ultrasonic treatment, then separate and dry, and finally sinter in an oxygen atmosphere to obtain the cathode material.

[0087] The mechanical stirring rate was 400 rpm and the stirring time was 2 h; the ultrasonic power of the ultrasonic treatment was 400 W and the ultrasonic time was 45 min; the sintering temperature of the sintering treatment was 500℃ and the sintering time was 5 h.

[0088] Elemental analysis, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) of the cathode material revealed that the nitrogen-doped carbon layer has a mass percentage of 1.5%, a thickness of 6 nm, and a nitrogen content of 4%. The layered material is a ternary nickel-cobalt-manganese oxide (NCM622), with a mass percentage of 4.5% and a thickness of 18 nm.

[0089] Example 3 The battery preparation method in this embodiment is largely the same as that in Example 1, except for the preparation of the positive electrode material: 1) Coating with an inner nitrogen-doped carbon layer: 8g of lithium manganese iron phosphate (LMFP) powder (LiMn) 0.7 Fe 0.3Poly(PO4, D50 = 500 nm) was dispersed in 200 mL of Tris-HCl buffer (pH = 8.5), and 0.4 g of dopamine hydrochloride was added. After stirring for 30 min, 0.48 g of ammonium persulfate was added, and the mixture was reacted at room temperature for 6 h with a stirring rate of 450 rpm to obtain poly(dopamine hydrochloride) coated lithium manganese iron phosphate. The solid was collected by centrifugation, dried, and then carbonized at 600 °C for 2 h under Ar atmosphere to obtain nitrogen-doped carbon-coated lithium iron phosphate.

[0090] The mass ratio of lithium manganese iron phosphate to dopamine hydrochloride is 20:1; the mass ratio of dopamine hydrochloride to ammonium persulfate is 1:1.2.

[0091] 2) Weigh lithium acetate, nickel acetate, and manganese acetate in a molar ratio of Li:Ni:Mn = 1.05:0.5:0.5, dissolve them in water, add citric acid complexing agent, and prepare a precursor solution; place 1.5g of nitrogen-doped carbon-coated lithium manganese iron phosphate in the precursor solution, perform mechanical stirring and ultrasonic treatment, then separate and dry, and finally sinter in an oxygen atmosphere to obtain the cathode material.

[0092] The mechanical stirring rate was 400 rpm and the stirring time was 2 h; the ultrasonic power of the ultrasonic treatment was 400 W and the ultrasonic time was 45 min; the sintering temperature of the sintering treatment was 500℃ and the sintering time was 8 h.

[0093] Elemental analysis, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) of the cathode material revealed that the nitrogen-doped carbon layer comprised 2.0% of the cathode material by mass, with a thickness of 8 nm. The nitrogen content in the nitrogen-doped carbon layer was 4.5% by mass, and the layered structure material was a binary nickel-manganese oxide (LiNi). 0.5 Mn 0.5 O2, the layered structure material layer has a mass percentage of 4% in the cathode material, and the thickness of the layered structure material layer is 15nm.

[0094] The main difference between Examples 4-37 and Example 3 is the different parameters of the cathode material, as shown in Tables 1-4.

[0095] Comparative Example 1 The preparation method of the battery in this comparative example is roughly the same as that in Example 3, except that the positive electrode material is lithium manganese iron phosphate.

[0096] Comparative Example 2 The preparation method of the battery in this comparative example is roughly the same as that in Example 3, except for the preparation of the positive electrode material: 8g of lithium manganese iron phosphate (LMFP) powder (LiMn) 0.7 Fe 0.3Poly(PO4, D50 = 500 nm) was dispersed in 200 mL of Tris-HCl buffer (pH = 8.5), and 0.4 g of dopamine hydrochloride was added. After stirring for 30 min, 0.48 g of ammonium persulfate was added, and the mixture was reacted at room temperature for 6 h with a stirring rate of 450 rpm to obtain poly(dopamine hydrochloride) coated lithium manganese iron phosphate. The solid was collected by centrifugation, dried, and then carbonized at 600 °C for 2 h under Ar atmosphere to obtain the cathode material (nitrogen-doped carbon-coated lithium manganese iron phosphate).

[0097] The mass ratio of lithium manganese iron phosphate to dopamine hydrochloride is 20:1; the mass ratio of dopamine hydrochloride to ammonium persulfate is 1:1.2.

[0098] Elemental analysis, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) of the cathode material revealed that the nitrogen-doped carbon layer has a mass percentage content of 2.0%, a thickness of 8 nm, and a nitrogen content of 4.5%.

[0099] Comparative Example 3 The preparation method of the battery in this comparative example is roughly the same as that in Example 3, except for the preparation of the positive electrode material: Lithium acetate, nickel acetate, and manganese acetate were weighed according to the molar ratio of Li:Ni:Mn = 1.05:0.5:0.5, dissolved in water, and citric acid complexing agent was added to prepare a precursor solution. 1.5g of lithium manganese iron phosphate was placed in the precursor solution, mechanically stirred and ultrasonically treated, then separated and dried, and finally sintered in an oxygen atmosphere to obtain the cathode material.

[0100] The mechanical stirring rate was 400 rpm and the stirring time was 2 h; the ultrasonic power of the ultrasonic treatment was 400 W and the ultrasonic time was 45 min; the sintering temperature of the sintering treatment was 500℃ and the sintering time was 8 h.

[0101] Elemental analysis, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) of the cathode material revealed that the layered structure material is a binary nickel-manganese oxide, LiNi. 0.5 Mn 0.5 O2, the layered structure material layer has a mass percentage of 4% in the cathode material, and the thickness of the layered structure material layer is 15nm.

[0102] Table 1

[0103] Table 2

[0104] Table 3

[0105] Table 4

[0106] Test case The following performance tests were performed on the batteries prepared in the examples and comparative examples: 1. Ratio Performance Test Rate performance test: The lithium-ion batteries were charged to 4.4V at a constant current and constant voltage of 0.5C in a constant temperature chamber at 25±2℃, with a cutoff current of 0.02C, and then discharged to 3.0V at 0.5C. The discharge specific capacity at rates of 0.2C, 0.5C, 1C, 2C and 5C was tested under the above conditions, with 5 batteries in each group.

[0107] 2. Cyclic performance test In a constant temperature chamber at (55±2)℃, the lithium-ion battery was charged to 4.4V at a constant current and constant voltage of 1C, and then discharged to 3.0V at 1C after resting for 5 minutes. The capacity obtained in this step was taken as the initial capacity. Cyclic tests were performed using 1C charge / 1C discharge, and the capacity retention rate of the battery after 300 cycles was calculated.

[0108] Cycle capacity retention (%) = Discharge capacity at 300th cycle (mAh) / Discharge capacity at first cycle (mAh) × 100% The average cycle life of each group of 5 batteries is recorded in Table 5.

[0109] 3. Electrochemical Impedance Spectroscopy (EIS) Test EIS testing was performed using an electrochemical workstation, with a test frequency range of 10. -2 Hz~10 5 The AC signal amplitude was 5mV (sine wave) at Hz, and the battery was in an open-circuit state during the test. After the test, the impedance spectrum (Nyquist plot) and raw data, including key parameters such as high-frequency impedance, mid-frequency charge transfer impedance, and low-frequency diffusion impedance, were saved for subsequent analysis of the conductivity and interface stability of the cathode material.

[0110] 4. Manganese leaching test after high-temperature storage The fully charged battery was stored at 85°C for 24 hours, then the battery was disassembled, the electrolyte was collected, and the content of dissolved manganese ions in the electrolyte was tested using ICP-MS.

[0111] The average value of manganese ion leaching from each group of 5 batteries is recorded in Table 5.

[0112] Table 5

[0113] As shown in Table 5, based on the comparison between Examples 1-3 and Comparative Examples 1-3, it can be seen that when the cathode material includes phosphate particles, a nitrogen-doped carbon inner layer, and a layered structure material outer layer, the conductivity of the battery can be improved, enabling the battery to exhibit excellent high-temperature cycle performance and rate performance, while reducing the amount of manganese leaching.

[0114] According to the comparison of Examples 3 to 7, when the mass ratio of phosphate particles to nitrogen-containing polymer monomers is (10 to 100): 1, the mass percentage of nitrogen-doped carbon layer in the cathode material can be 0.5% to 3%, the thickness of nitrogen-doped carbon layer is 2 nm to 10 nm, and the mass percentage of nitrogen element in nitrogen-doped carbon layer is 2% to 6%, which helps to improve the electrochemical performance of the battery.

[0115] According to the comparison of Examples 3, 8-10, when the nitrogen-containing polymer monomer is dopamine hydrochloride, the oxidant is ammonium persulfate, the pH value is 8.2-8.8, and the mass ratio of dopamine hydrochloride to ammonium persulfate is 1:(1-1.5), the successful preparation of the cathode material can be guaranteed, the high-temperature cycle performance and rate performance of the battery can be improved, and the battery impedance and manganese dissolution can be reduced.

[0116] According to the comparison of Examples 11-13, when the nitrogen-containing polymer monomer is pyrrole, the oxidant is ferric chloride, the pH value is 1-4, and the mass ratio of pyrrole to ferric chloride is 1:(2-2.5), the battery has better high-temperature cycle performance and rate performance, while the battery impedance and manganese dissolution are lower.

[0117] According to the comparison of Examples 14-16, when the nitrogen-containing polymer monomer is aniline, the oxidant includes ammonium persulfate, the pH value is 1-4, and the mass ratio of aniline to oxidant is 1:(1-1.3), the battery exhibits better high-temperature cycle performance and rate performance, lower impedance, and less manganese dissolution.

[0118] According to the comparison of Examples 3 and 17-19, when the stirring rate of the in-situ oxidative polymerization reaction is 300 rpm to 500 rpm and the stirring time is 2 h to 8 h, the performance of the cathode material is better and can further improve the electrochemical performance of the battery.

[0119] According to the comparison of Examples 3, 20, 21 and Comparative Example 4, when the carbonization temperature of the carbonization heat treatment is 500℃~750℃ and the carbonization time is 1h~6h, the nitrogen-containing polymer can be fully carbonized into a nitrogen-doped carbon layer, which is beneficial to the improvement of battery cycle life and rate performance, as well as the reduction of impedance and manganese dissolution.

[0120] As can be seen from Examples 22 and 23, by adjusting the mass ratio of phosphate particles to nitrogen-containing polymer monomers and the parameters of carbonization heat treatment, the mass percentage of nitrogen in the nitrogen-doped carbon layer can be controlled within the range of 2% to 6%, thereby improving the high-temperature cycle performance, rate performance, impedance, and manganese dissolution of the battery.

[0121] As can be seen from Examples 3, 24, and 25, by making the molar ratio of lithium source to transition metal source based on the stoichiometry of the layered structure material and by making the lithium source in excess by 5% to 10%, the successful coating of the outer layer of the layered structure material can be guaranteed, thereby improving the electrochemical performance of the battery.

[0122] According to the comparison of Examples 3, 26-31, when the stirring rate of mechanical stirring is 300rpm-500rpm and the stirring time is 1h-4h, and the ultrasonic power of ultrasonic treatment is 300W-500W and the ultrasonic time is 30min-60min, the battery exhibits better high-temperature cycle performance and rate performance, lower impedance and manganese dissolution.

[0123] According to the comparison of Examples 3 and 32-34, when the sintering temperature is 450℃~700℃ and the sintering time is 2h~10h, the mass percentage content and thickness of the layered structure material layer can be controlled, thereby improving the high-temperature cycle performance and rate performance of the battery, while reducing the battery impedance and manganese dissolution.

[0124] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A positive electrode material, characterized in that, The cathode material includes a core and an inner coating layer and an outer coating layer that are sequentially coated on the surface of the core from the inside out. The core includes phosphate particles, the inner coating layer is a nitrogen-doped carbon layer, and the outer coating layer is a layered structure material layer. The nitrogen content in the nitrogen-doped carbon layer is 0.5% to 8% by mass.

2. The cathode material according to claim 1, characterized in that, The nitrogen-doped carbon layer has a mass percentage content of 0.5% to 3% in the cathode material; And / or, the thickness of the nitrogen-doped carbon layer is 2 nm to 10 nm; And / or, the mass percentage of nitrogen in the nitrogen-doped carbon layer is 2% to 6%.

3. The cathode material according to claim 1, characterized in that, The layered structural material comprises ternary nickel-cobalt-manganese oxide and / or binary nickel-manganese oxide; wherein the chemical formula of the ternary nickel-cobalt-manganese oxide is LiNi. x Co y Mn z O2, x+y+z=1; the chemical formula of the binary nickel-manganese oxide is LiNi α Mn β O2, α+β=1; And / or, the layered structure material layer in the cathode material has a mass percentage content of 1% to 8%; And / or, the thickness of the layered structure material layer is 5nm~30nm.

4. The cathode material according to claim 1, characterized in that, The phosphate particles comprise lithium iron phosphate and / or lithium manganese iron phosphate; wherein the chemical formula of the lithium manganese iron phosphate is LiMn. x Fe 1-x PO4, 0.3 <x<0.8。 5. A method for preparing the cathode material according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1. The phosphate particles are placed in a nitrogen-containing polymer monomer solution and subjected to in-situ oxidative polymerization to obtain a polymer-coated core; then subjected to carbonization heat treatment to obtain a phosphate material coated with a nitrogen-doped carbon layer; wherein the nitrogen-doped carbon layer forms the inner coating layer on the surface of the core. The carbonization temperature of the carbonization heat treatment is 500℃~750℃, and the carbonization time is 1h~6h; S2. The nitrogen-doped carbon-coated phosphate material is dispersed in a precursor solution containing elements of layered structure materials, and after dispersion treatment and obtaining a dry nitrogen-doped carbon-coated phosphate material containing the precursor, it is sintered in an oxygen atmosphere to obtain the cathode material; wherein the precursor forms an outer coating layer on the surface of the nitrogen-doped carbon layer.

6. The method for preparing the cathode material according to claim 5, characterized in that, The nitrogen-containing polymer monomer includes at least one of pyrrole, aniline, and dopamine hydrochloride; And / or, the mass ratio of the phosphate particles to the nitrogen-containing polymer monomer is (10~100):

1.

7. The method for preparing the cathode material according to claim 6, characterized in that, The in-situ oxidative polymerization reaction is carried out under the action of an oxidant, while the pH value of the system is adjusted simultaneously; Preferably, the nitrogen-containing polymer monomer is dopamine hydrochloride, the oxidant includes dissolved oxygen and / or ammonium persulfate, and the pH value is 8.2-8.8; more preferably, the mass ratio of dopamine hydrochloride to ammonium persulfate is 1:(1-1.5); even more preferably, the mass ratio of dopamine hydrochloride to dissolved oxygen is 1:(0.05-0.15). Preferably, the nitrogen-containing polymer monomer is pyrrole, the oxidant includes ferric chloride and / or ammonium persulfate, and the pH value is 1-4; more preferably, the mass ratio of pyrrole to ferric chloride is 1:(2-2.5); even more preferably, the mass ratio of pyrrole to ammonium persulfate is 1:(1.2-1.8). Preferably, the nitrogen-containing polymer monomer is aniline, the oxidant includes ammonium persulfate, and the pH value is 1-4; the mass ratio of aniline to the oxidant is 1:(1-1.3). And / or, the in-situ oxidative polymerization reaction is carried out under stirring, with a stirring rate of 300 rpm to 500 rpm and a stirring time of 2 h to 8 h.

8. The method for preparing the cathode material according to claim 5, characterized in that, A precursor solution containing the constituent elements of a layered structure material is prepared by dissolving a lithium source and a transition metal source in a solvent. The lithium source includes lithium acetate and / or lithium nitrate, and the transition metal source includes the corresponding acetate and / or nitrate. The molar ratio of the lithium source to the transition metal source is based on the stoichiometry of the layered structure material, with the lithium source in 5% to 10% excess. And / or, the dispersion treatment includes mechanical stirring and ultrasonic treatment; the stirring rate of the mechanical stirring is 300 rpm to 500 rpm, and the stirring time is 1 h to 4 h; the ultrasonic power of the ultrasonic treatment is 300 W to 500 W, and the ultrasonic time is 30 min to 60 min; And / or, the sintering temperature of the sintering treatment is 450℃~700℃, and the sintering time is 2h~10h.

9. A positive electrode plate, characterized in that, The positive electrode sheet includes a positive current collector and a positive electrode coating coated on at least one side of the positive current collector, wherein the positive electrode coating includes the positive electrode material according to any one of claims 1 to 4 or the positive electrode material prepared by the method of the preparation of the positive electrode material according to any one of claims 5 to 8.

10. A battery, characterized in that, The battery includes the positive electrode as described in claim 9.

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