Composite phosphate-based positive electrode material, and preparation method therefor and use thereof

Abstract

The present invention belongs to the technical field of battery materials, and particularly relates to a composite phosphate-based positive electrode material, and a preparation method therefor and the use thereof. The composite phosphate-based positive electrode material comprises a phosphate-based active core, wherein the outer surface of the phosphate-based active core is sequentially coated with a carbon coating layer, a nitrogen-doped carbon coating layer and a nitrogen-phosphorus co-doped carbon coating layer from inside to outside, wherein the carbon coating layer can improve the electronic and ionic conductivity of the composite positive electrode material, and optimize the particle size and morphology of the composite positive electrode material. The nitrogen-doped carbon coating layer improves the consistency and integrity of coating; and nitrogen-atom-doped carbon further enhances the electronic conductivity and lithium-ion diffusion of the carbon coating layer. Electron-rich clouds of P and N atoms in the nitrogen-phosphorus co-doped carbon coating layer can provide a rich conductive network, mutual contact is formed among different particles to form a series network, thus promoting the interfacial electron transport of the composite material during a charging and discharging process, and improving the cycling and rate performance of the composite phosphate-based positive electrode material during the charging and discharging process.

Description

Composite phosphate-based cathode materials, their preparation methods and applications

[0001] This application claims priority to Chinese Patent Application No. 202410546848.8, filed on May 6, 2024, entitled "Composite Phosphate-Based Cathode Material and its Preparation Method and Application", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of battery materials technology, and in particular to a composite phosphate-based cathode material, its preparation method, and its application. Background Technology

[0003] The technology of power batteries in the new energy vehicle market has been continuously upgraded. However, as people's demands for the driving range of electric vehicles increase, their operating voltage and energy density can no longer meet the requirements. Lithium manganese iron phosphate (LMFP) batteries are a new type of power battery. Compared with traditional ternary lithium batteries and lithium iron phosphate (LFP) batteries, they have better safety performance and higher energy density. As phosphate-based cathode materials with the same olivine structure, lithium manganese iron phosphate (LMFP) and other phosphate-based cathode materials are solid solution cathode materials formed by combining the high energy density of lithium manganese phosphate, lithium manganese iron phosphate, and lithium iron phosphate.

[0004] However, one of the biggest challenges for phosphate-based cathode materials such as lithium manganese iron phosphate in large-scale applications is their extremely low electronic / ionic conductivity, which results in poor fast charge and discharge capabilities.

[0005] Application content

[0006] One of the objectives of this application is to provide a composite phosphate-based cathode material, its preparation method, and its application, aiming to solve, to some extent, the problem of low electronic / ionic conductivity of phosphate-based cathode materials such as lithium manganese iron phosphate.

[0007] The technical solution adopted in the embodiments of this application is:

[0008] In a first aspect, a composite phosphate-based cathode material is provided, comprising a phosphate-based active core, wherein a carbon coating layer, a nitrogen-doped carbon coating layer, and a nitrogen-phosphorus co-doped carbon coating layer are sequentially coated from the inside to the outside on the outer surface of the phosphate-based active core.

[0009] Secondly, a method for preparing a composite phosphate-based cathode material is provided, comprising the following steps:

[0010] A phosphate-based solid-phase precursor material containing a first carbon source is prepared, and the solid-phase precursor material is subjected to a first sintering treatment to obtain a primary crystallization product comprising a phosphate-based active core and a carbon coating layer.

[0011] The primary crystallization product is mixed with a second carbon source, a nitrogen source and a solvent, dried and pulverized, and then subjected to a second sintering process to form a nitrogen-doped carbon coating layer on the surface of the carbon coating layer, thereby obtaining a secondary crystallization product.

[0012] The secondary crystallization product is dispersed in a nitrogen, phosphorus and carbon-containing hydrogel and subjected to a third sintering treatment to form a nitrogen-phosphorus co-doped carbon coating layer on the surface of the nitrogen-doped carbon coating layer, thereby obtaining a composite phosphate-based cathode material.

[0013] Thirdly, a positive electrode sheet is provided, comprising a current collector and a positive electrode active layer formed on the surface of the current collector, wherein the positive electrode active layer contains the aforementioned composite phosphate-based positive electrode material.

[0014] Fourthly, a secondary battery is provided, which includes the aforementioned positive electrode plate.

[0015] The beneficial effects of the composite phosphate-based cathode material provided in this application embodiment are as follows: it includes a phosphate-based active core, and a carbon coating layer, a nitrogen-doped carbon coating layer, and a nitrogen-phosphorus co-doped carbon coating layer sequentially coated on the outer surface of the phosphate-based active core. The carbon coating layer directly coated on the outer surface of the phosphate-based active core can improve the electronic and ionic conductivity of the composite cathode material, and optimize the particle size and morphology of the composite cathode material, which is beneficial for obtaining a composite phosphate-based cathode material with small particle size, high uniformity, and complete morphology. The nitrogen-doped carbon coating layer coated on the outer surface of the carbon coating layer can not only increase the uniformity and integrity of the coating, but also enhance the electronic conductivity and lithium-ion diffusion of the carbon coating layer through non-metallic atom doping of carbon. Furthermore, the nitrogen-doped carbon coating layer also helps to increase the contact density with the nitrogen-phosphorus co-doped carbon coating layer. A CNCNPC-like conductive network is formed between the nitrogen-doped carbon coating layer and the nitrogen-phosphorus co-doped carbon layer, ensuring the stability of the three coating layers and further improving the electrochemical performance of the composite cathode material. In the nitrogen-phosphorus co-doped carbon coating layer on the outer surface of the nitrogen-doped carbon coating layer, the electron-rich clouds of the doped P and N atoms provide a rich conductive network. Moreover, the nitrogen-phosphorus co-doped carbon coating layer can form mutual contacts between different composite phosphate-based cathode material particles, acting as a series network between them, providing continuous electron channels and layered porous channels for Li ion transport. As a framework element of phosphate-based active materials, phosphorus (P) can enhance the connection between phosphate-based active materials and carbon coating layers, improve the coating effect of the three types of carbon coating layers, promote interfacial electron transport in composite phosphate-based cathode materials during charging and discharging, and improve the cycle and rate performance of composite phosphate-based cathode materials during charging and discharging.

[0016] The beneficial effects of the method for preparing composite phosphate-based cathode materials provided in this application are as follows: After preparing a phosphate-based solid-phase precursor material containing a first carbon source, a first sintering treatment is performed to allow the phosphate-based precursor material in the solid-phase precursor material to undergo primary crystallization to form a phosphate-based active material. The carbon material generated by the first carbon source is in situ coated on the outer surface of the phosphate-based active core, optimizing the particle size and morphology of the active core, and obtaining a core-shell primary crystallization product. Then, it is mixed with a second carbon source, a nitrogen source, and a solvent, dried, and pulverized to allow the second carbon source and nitrogen source to coat the surface of the primary crystallization product. A second sintering treatment is then performed to optimize the crystal structure in the phosphate-based active core, and the second carbon source and nitrogen source react at high temperature to form a nitrogen-doped carbon coating layer in situ on the surface of the carbon coating layer, increasing the uniformity and integrity of the coating. The electronic conductivity and lithium-ion diffusion of the carbon coating layer are enhanced by nitrogen atom doping of carbon. After obtaining the secondary crystallization product, it is dispersed in a nitrogen-, phosphorus-, and carbon-containing hydrogel and subjected to a third sintering treatment. This process dehydrates and carbonizes the hydrogel on the surface of the nitrogen-doped carbon coating layer, forming a nitrogen-phosphorus co-doped carbon coating layer. High-temperature sintering then creates a CNCNPC-like conductive network between the nitrogen-doped carbon coating layer and the nitrogen-phosphorus co-doped carbon layer. The electron-rich clouds of P and N atoms in the nitrogen-phosphorus co-doped carbon coating layer provide a rich conductive network, forming interconnected series networks between different composite phosphate-based cathode material particles. This provides continuous electron channels and layered porous channels for Li ion transport. Furthermore, it enhances the connection between the phosphate-based active material and the carbon coating layer, improves the coating effect of the three carbon coating layers, and enhances the cycle and rate performance of the composite cathode material during charge and discharge.

[0017] The beneficial effects of the positive electrode sheet provided in this application embodiment are as follows: Since the positive electrode active layer adopts the above-mentioned composite phosphate-based positive electrode material, which includes three coating layers: carbon coating layer, nitrogen-doped carbon coating layer, and nitrogen-phosphorus co-doped carbon coating layer, the ionic conductivity and electronic conductivity of the composite phosphate-based positive electrode material are significantly improved. At the same time, it has the characteristics of good structural stability, high capacity, and excellent conductivity. Therefore, the stability, energy density, rate performance, cycle performance and other electrochemical performance of the positive electrode sheet are improved.

[0018] The beneficial effects of the secondary battery provided in this application embodiment are as follows: because it contains a positive electrode sheet with excellent electrochemical performance such as good stability, high energy density, good rate performance, and good cycle stability, the energy density, cycle stability, and other electrochemical performance of the secondary battery are improved. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or exemplary technologies will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 is a schematic flowchart of the preparation method of the composite phosphate-based cathode material provided in the embodiments of this application;

[0021] Figure 2 is a schematic flowchart of the preparation method of the composite phosphate-based cathode material provided in Example 1 of this application;

[0022] Figure 3 shows the XRD patterns of C@LMFP-Ti (labeled a) and N / PC@NC@C@LMFP-Ti (labeled b) composite phosphate cathode materials provided in Example 1 of this application;

[0023] Figure 4 is a TEM image of the composite phosphate-based cathode material C@LMFP-Ti (labeled as (a)) and the composite phosphate-based cathode material N / PC@NC@C@LMFP-Ti (labeled as (b)) provided in Embodiment 1 of this application;

[0024] Figure 5 is a SEM image of the composite phosphate cathode material C@LMFP-Ti (labeled as (a)) and the composite phosphate cathode material N / PC@NC@C@LMFP-Ti (labeled as (b)) provided in Embodiment 1 of this application;

[0025] Figure 6 is an EDS diagram of the composite phosphate-based cathode material C@LMFP-Ti (labeled as (a)) and the composite phosphate-based cathode material N / PC@NC@C@LMFP-Ti (labeled as (b)) provided in Embodiment 1 of this application. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the scope of this application.

[0027] It should be noted that when a component is referred to as "fixed to" or "set on" another component, it can be directly or indirectly attached to that other component. When a component is referred to as "connected to" another component, it can be directly or indirectly connected to that other component. The terms "upper," "lower," "left," "right," etc., indicate orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, and are for ease of description only, not to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances. The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features. "A plurality" means two or more, unless otherwise explicitly defined.

[0028] To illustrate the technical solutions described in this application, the following detailed description is provided in conjunction with specific accompanying drawings and embodiments.

[0029] The first aspect of this application provides a composite phosphate-based cathode material, including a phosphate-based active core, on the outer surface of which a carbon coating layer, a nitrogen-doped carbon coating layer, and a nitrogen-phosphorus co-doped carbon coating layer are sequentially coated from the inside to the outside.

[0030] The first aspect of this application provides a composite phosphate-based cathode material, comprising a phosphate-based active core and, from the inside out, a carbon coating layer, a nitrogen-doped carbon coating layer, and a nitrogen-phosphorus co-doped carbon coating layer sequentially coated on the outer surface of the phosphate-based active core. The carbon coating layer directly coated on the outer surface of the phosphate-based active core improves the electronic and ionic conductivity of the composite cathode material and optimizes its particle size and morphology, resulting in a composite phosphate-based cathode material with small particle size, high uniformity, and complete morphology. The nitrogen-doped carbon coating layer on the outer surface of the carbon coating layer not only increases the uniformity and integrity of the coating but also enhances the electronic conductivity and lithium-ion diffusion of the carbon coating layer through non-metallic atom doping of carbon. Furthermore, the nitrogen-doped carbon coating layer also helps to increase the contact density with the nitrogen-phosphorus co-doped carbon coating layer. A CNCNPC-like conductive network is formed between the nitrogen-doped carbon coating layer and the nitrogen-phosphorus co-doped carbon layer, ensuring the stability of the three coating layers and further improving the electrochemical performance of the composite cathode material. In the nitrogen-phosphorus co-doped carbon coating layer on the outer surface of the nitrogen-doped carbon coating layer, the electron-rich clouds of the doped P and N atoms provide a rich conductive network. Moreover, the nitrogen-phosphorus co-doped carbon coating layer can form mutual contacts between different composite phosphate-based cathode material particles, acting as a series network between them, providing continuous electron channels and layered porous channels for Li ion transport. As a framework element of phosphate-based active materials, phosphorus (P) can enhance the connection between phosphate-based active materials and carbon coating layers, improve the coating effect of the three types of carbon coating layers, promote interfacial electron transport in composite phosphate-based cathode materials during charging and discharging, and improve the cycle and rate performance of composite phosphate-based cathode materials during charging and discharging.

[0031] In some possible implementations, taking the composite phosphate-based cathode material as 100% by mass, the carbon coating layer contains 0.3% to 0.8% carbon by mass. This can improve the electronic and ionic conductivity of the composite cathode material, as well as optimize the particle size and morphology of the composite cathode material.

[0032] For example, taking the mass of the composite phosphate-based cathode material as 100%, the mass percentage of carbon in the carbon coating layer can be any typical but non-limiting point value or an interval between any two point values, such as 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%.

[0033] In some possible implementations, taking the composite phosphate-based cathode material as 100% by mass, the nitrogen-doped carbon coating layer contains 0.3% to 0.8% carbon and 1.5% to 3.0% nitrogen by mass. In the embodiments of this application, with this doping level, on the one hand, the electron-rich cloud of N atoms provides a rich conductive network, which is beneficial for enhancing the electronic conductivity and ion diffusion performance of the carbon layer. On the other hand, this doping level ensures the graphitization degree of the nitrogen-doped carbon coating layer, thus ensuring its electronic and ionic conductivity.

[0034] For example, taking the mass of the composite phosphate-based cathode material as 100%, the mass percentage of carbon in the nitrogen-doped carbon coating layer can be any typical but non-limiting point value or a range between any two point values, such as 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, and 0.8%; the mass percentage of nitrogen can be any typical but non-limiting point value or a range between any two point values, such as 1.5%, 1.8%, 2.0%, 2.5%, 2.8%, and 3.0%.

[0035] In some possible implementations, nitrogen elements in the nitrogen-doped carbon coating layer form chemical bonds with carbon elements and / or are doped in a free form. In the nitrogen-doped carbon coating layer of this application, nitrogen elements can be doped into the carbon coating layer in a free form; or they can form chemical bonds with the carbon material, with nitrogen elements suspended on the outer surface of the nitrogen-doped carbon coating layer. This is beneficial for improving the electronic and ionic conductivity of the composite phosphate-based cathode material, and also for increasing the contact density between the nitrogen-doped carbon coating layer and the nitrogen-phosphorus co-doped material.

[0036] In some possible implementations, taking the composite phosphate-based cathode material as 100% by mass, the nitrogen-phosphorus co-doped carbon coating layer contains 0.3%–0.8% carbon, 0.6%–1.5% nitrogen, and 1.2%–3.2% phosphorus by mass. The nitrogen and phosphorus doping content in the nitrogen-phosphorus co-doped carbon coating layer of this application provides a rich electron cloud, offering an abundant conductive network. This facilitates the formation of interconnected series network structures between different composite phosphate-based cathode material particles, providing continuous and abundant electron channels for ion transport. Simultaneously, this doping content also enhances the connection between the phosphate-based active material and the carbon coating layer, improves the coating effect of the three carbon coating layers, promotes interfacial electron transport, and enhances the cycle and rate performance of the composite phosphate-based cathode material during charge and discharge.

[0037] For example, taking the mass of the composite phosphate-based cathode material as 100%, the mass percentage of carbon in the nitrogen-phosphorus co-doped carbon coating layer can be any typical but non-limiting value or a range between any two values, such as 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, and 0.8%; the mass percentage of nitrogen can be any typical but non-limiting value or a range between any two values, such as 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, and 1.6%; and the mass percentage of phosphorus can be any typical but non-limiting value or a range between any two values, such as 1.2%, 1.5%, 2.0%, 2.5%, 3.0%, and 3.2%.

[0038] In some possible implementations, the nitrogen-phosphorus co-doped carbon coating is prepared by sintering a nitrogen-, phosphorus-, and carbon-containing hydrogel, resulting in a three-dimensional porous network structure. In the embodiments of this application, the nitrogen-phosphorus co-doped carbon coating is prepared by sintering a nitrogen-, phosphorus-, and carbon-containing hydrogel. The hydrogel possesses a three-dimensional network structure that retains its loose three-dimensional porous network structure even after losing moisture and carbonizing during the drying sintering process. Therefore, the nitrogen-phosphorus co-doped carbon coating with its three-dimensional porous network structure can form mutual contacts between particles of different composite phosphate-based cathode materials, while simultaneously acting as a conductive network connecting the various composite phosphate-based cathode material particles in series. Furthermore, a CNCNPC-like conductive network is formed between the nitrogen-doped carbon coating and the nitrogen-phosphorus co-doped carbon layer, ensuring the stability of the three coating layers—the carbon coating, the nitrogen-doped carbon coating, and the nitrogen-phosphorus co-doped carbon coating—and further improving the electrochemical performance of the composite cathode material.

[0039] In some possible implementations, the phosphate-based active core includes the chemical formula Li m Mn x Fe y A n The phosphate-based cathode material of PO4, wherein A is selected from at least one doping element from Ti, Mg, V, Co, In, Ni, Zr, and Nb, with 0.95≤m≤1.05, 0≤x≤1, 0≤y≤1, 0≤n≤0.015, and x+y+n=1. The phosphate-based active core in this application embodiment can contain lithium manganese iron phosphate, lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate-based materials doped with other metal elements, lithium iron phosphate-based materials doped with other metal elements, lithium manganese phosphate-based materials doped with other metal elements, etc. Doping with metal elements can improve the electrochemical performance of the phosphate-based cathode material. The phosphate-based active core in this application embodiment has broad selectivity for phosphate-based cathode materials, strong practicality, and can meet different application requirements.

[0040] In some possible implementations, taking the mass of the composite phosphate-based cathode material as 100%, the coating layer of the composite phosphate-based cathode material contains 0.9%–2.4% carbon, 2.1%–4.5% nitrogen, and 1.2%–3.2% phosphorus by mass. In this case, the total carbon, total nitrogen, and total phosphorus content in the coating layer of the composite phosphate-based cathode material ensures both the capacity and other electrochemical properties of the composite phosphate-based cathode material, while also improving its ionic and electronic conductivity. This, in turn, enhances the cycle stability, rate performance, and other electrochemical properties of the composite phosphate-based cathode material during charge and discharge processes.

[0041] For example, taking the mass of the composite phosphate-based cathode material as 100%, the mass percentage of carbon in the coating layer of the composite phosphate-based cathode material can be any typical but non-limiting point value or a range between any two point values, such as 0.9%, 1.0%, 1.5%, 2.0%, and 2.4%; the mass percentage of nitrogen can be any typical but non-limiting point value or a range between any two point values, such as 2.1%, 2.5%, 3.0%, 3.5%, 4.0%, and 4.5%; and the mass percentage of phosphorus can be any typical but non-limiting point value or a range between any two point values, such as 1.2%, 1.5%, 2.0%, 2.5%, 3.0%, and 3.2%.

[0042] In some possible implementations, the total thickness of the carbon coating layer, nitrogen-doped carbon coating layer, and nitrogen-phosphorus co-doped carbon coating layer in the composite phosphate-based cathode material is 1 nm to 9 nm. At this thickness, the overall coating layer on the outer surface of the phosphate-based active core can effectively improve the electronic and ionic conductivity of the composite cathode material, promote interfacial electron transport during charge and discharge, and enhance the cycle and rate performance of the composite phosphate-based cathode material during charge and discharge. For example, the total thickness of the carbon coating layer, nitrogen-doped carbon coating layer, and nitrogen-phosphorus co-doped carbon coating layer in the composite phosphate-based cathode material can be any typical but non-limiting value such as 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, or 9 nm, or a range between any two values. In some specific embodiments, the total thickness of the carbon coating layer, nitrogen-doped carbon coating layer, and nitrogen-phosphorus co-doped carbon coating layer in the composite phosphate-based cathode material is 2 nm to 3 nm.

[0043] The composite phosphate-based cathode material described in the above embodiments of this application can be prepared by the methods described in the following embodiments.

[0044] Secondly, embodiments of this application provide a method for preparing a composite phosphate-based cathode material, as shown in Figure 1, including the following steps:

[0045] S10. Prepare a phosphate-based solid-phase precursor material containing a first carbon source, and perform a first sintering treatment on the solid-phase precursor material to obtain a primary crystallization product including a phosphate-based active core and a carbon coating layer.

[0046] S20. The primary crystallization product is mixed with a second carbon source, a nitrogen source and a solvent, dried and pulverized, and then subjected to a second sintering treatment to form a nitrogen-doped carbon coating layer on the surface of the carbon coating layer, thereby obtaining a secondary crystallization product.

[0047] S30. The secondary crystallization product is dispersed into a hydrogel containing nitrogen, phosphorus and carbon, and a third sintering treatment is performed to form a nitrogen-phosphorus co-doped carbon coating layer on the surface of the nitrogen-doped carbon coating layer, thereby obtaining a composite phosphate-based cathode material.

[0048] The method for preparing composite phosphate-based cathode materials provided in this application involves preparing a phosphate-based solid-phase precursor material containing a first carbon source, followed by a first sintering treatment. This allows the phosphate-based precursor material to undergo primary crystallization, forming a phosphate-based active material. The carbon material generated from the first carbon source is in situ coated onto the outer surface of the phosphate-based active core, optimizing the particle size and morphology of the active core and obtaining a core-shell primary crystallization product. Then, the product is mixed with a second carbon source, a nitrogen source, and a solvent, dried, and pulverized. This allows the second carbon source and nitrogen source to coat the surface of the primary crystallization product, followed by a second sintering treatment. This optimizes the crystal structure in the phosphate-based active core and allows the second carbon source and nitrogen source to react at high temperature on the surface of the carbon coating layer, forming a nitrogen-doped carbon coating layer in situ. This increases the uniformity and integrity of the coating, and the nitrogen atom doping enhances the electronic conductivity and lithium-ion diffusion of the carbon coating layer. After obtaining the secondary crystallization product, it is dispersed in a nitrogen-, phosphorus-, and carbon-containing hydrogel and subjected to a third sintering treatment. This process dehydrates the hydrogel and carbonizes it onto the surface of the nitrogen-doped carbon coating layer, forming a nitrogen-phosphorus co-doped carbon coating layer. High-temperature sintering then creates a CNCNPC-like conductive network between the nitrogen-doped carbon coating layer and the nitrogen-phosphorus co-doped carbon layer. The electron-rich clouds of P and N atoms in the nitrogen-phosphorus co-doped carbon coating layer provide a rich conductive network, forming interconnected series networks between different composite phosphate-based cathode material particles. This provides continuous electron channels and layered porous channels for Li ion transport. Furthermore, it enhances the connection between the phosphate-based active material and the carbon coating layer, improves the coating effect of the three carbon coating layers, and enhances the cycle and rate performance of the composite cathode material during charge and discharge.

[0049] In step S10 above:

[0050] In some possible implementations, the preparation of solid-phase precursor materials includes the steps of: [the following steps are described in the original text, but the provided text is incomplete and requires further context to translate accurately.] m Mn x Fe y An The stoichiometric ratio of each element in PO4 can be either the molar ratio of the raw material components or the mass ratio calculated based on the molar ratio. Iron, phosphorus, manganese, lithium, and A sources are obtained, mixed with a first carbon source and a solvent, and then dried and crushed to obtain a solid-phase precursor material. A is selected from at least one doping element from Ti, Mg, V, Co, In, Ni, Zr, and Nb, with the following properties: 0.95≤m≤1.05, 0≤x≤1, 0≤y≤1, 0≤n≤0.015, and x+y+n=1. In this case, the proportions of each raw material component are ensured to conform to the Li... m Mn x Fe y A n The stoichiometry of the PO4 active material is carefully controlled to avoid imbalances that could lead to a large amount of impurity phases, while ensuring the stability of the Li content. m Mn x Fe y A n Electrochemical performance of PO4 active materials.

[0051] In some possible implementations, the phosphate-based cathode material contained in the phosphate-based active core can be lithium manganese iron phosphate, lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate-based material doped with other metal elements, lithium iron phosphate-based material doped with other metal elements, lithium manganese phosphate-based material doped with other metal elements, etc.

[0052] In some possible implementations, the conditions for the first sintering treatment include: heating to 300℃-400℃ in an inert atmosphere at a heating rate of 1℃ / min-5℃ / min, followed by holding at that temperature for 5h-10h. Under these sintering conditions, the phosphate-based precursor material in the solid-phase precursor material can undergo primary crystallization to form a phosphate-based active material. The carbon material generated from the first carbon source is in situ coated on the outer surface of the phosphate-based active core, optimizing the particle size and morphology of the active core, resulting in a core-shell primary crystallization product.

[0053] In some specific embodiments, the inert atmosphere can be at least one of nitrogen, argon, helium, etc.; the specific heating rate can be any typical but non-limiting point value or a range between any two point values, such as 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min; the sintering temperature can be any typical but non-limiting point value or a range between any two point values, such as 300℃, 320℃, 350℃, 380℃, 400℃; and the holding time can be any typical but non-limiting point value or a range between any two point values, such as 5h, 6h, 7h, 8h, 9h, 10h.

[0054] In some possible implementations, the amount of the first carbon source added is 2 wt% to 8 wt% of the theoretical yield of the composite phosphate-based cathode material. In this case, the addition of the first carbon source can form a carbon coating layer in situ on the outer surface of the phosphate-based active core during sintering, thereby optimizing the shape and size of the phosphate-based active core particles. For example, the amount of the first carbon source added can be any typical but non-limiting value, such as 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, or 8%, or a range between any two values, representing the theoretical yield of the composite phosphate-based cathode material.

[0055] In some possible implementations, the first carbon source includes one or more of glucose, sucrose, citric acid, and fructose. These organic carbon sources can be carbonized during high-temperature sintering in an inert atmosphere to form amorphous carbon, which is then deposited in situ onto the outer surface of the phosphate-based active core, forming a carbon coating layer.

[0056] In some possible implementations, the lithium source includes one or more of lithium carbonate, lithium oxalate, and lithium acetate.

[0057] In some possible implementations, the phosphorus source includes one or more of phosphoric acid, ammonium phosphate, and ammonium dihydrogen phosphate.

[0058] In some possible implementations, the manganese source includes one or more of manganese carbonate, manganese oxalate, and manganese acetate.

[0059] In some possible implementations, the iron source includes one or more of ferrous oxalate, ferrous acetate, ferric phosphate, and ferric nitrate.

[0060] The raw material components used in the above embodiments of this application all have a purity of ≥99%, and all of the above raw material components have good solubility, which is beneficial for the preparation of phosphate-based active materials.

[0061] In step S20 above:

[0062] In some possible implementations, the amount of the second carbon source added is 2 wt% to 5 wt% of the theoretical yield of the composite phosphate-based cathode material. In some possible implementations, the amount of the nitrogen source added is 1 wt% to 10 wt% of the theoretical yield of the composite phosphate-based cathode material. In this case, the ratio of carbon and nitrogen sources is beneficial for mutual doping of the carbon and nitrogen sources during high-temperature sintering, forming a nitrogen-doped carbon coating layer, optimizing the consistency and integrity of the carbon coating layer, improving the electronic and ionic conductivity of the carbon coating layer, and ensuring that the carbon material has high graphitization performance, thus ensuring its conductivity.

[0063] For example, the amount of the second carbon source added can be any typical but non-limiting point value or an interval between any two point values, such as 2wt%, 3wt%, 4wt%, 5wt%, etc., which is the theoretical yield of the composite phosphate-based cathode material.

[0064] For example, the amount of nitrogen source added can be any typical but non-limiting point value or an interval between any two point values, such as 1wt%, 3wt%, 5wt%, 7wt%, 9wt%, 10wt%, etc., which is the theoretical yield of composite phosphate-based cathode materials.

[0065] In some possible implementations, the conditions for the second sintering process include: heating to 400℃-500℃ in an inert atmosphere at a heating rate of 1℃ / min-5℃ / min, followed by holding at that temperature for 2h-3h. Under these second sintering conditions, the crystal structure of the phosphate-based active core can be optimized, allowing the second carbon source and nitrogen source to react at high temperatures to form a nitrogen-doped carbon coating layer in situ on the surface of the carbon coating layer. This increases the uniformity and integrity of the coating, and the nitrogen atom doping of carbon enhances the electronic conductivity and lithium-ion diffusion of the carbon coating layer, resulting in a secondary crystallization product.

[0066] In some specific embodiments, the inert atmosphere can be at least one of nitrogen, argon, helium, etc., and the specific heating rate can be any typical but non-limiting point value or a range between any two point values, such as 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, etc.; the sintering temperature can be any typical but non-limiting point value or a range between any two point values, such as 300℃, 420℃, 450℃, 480℃, 500℃, etc.; and the holding time can be any typical but non-limiting point value or a range between any two point values, such as 2h, 2.5h, 3h, etc.

[0067] In some possible implementations, the second carbon source includes one or more of glucose, sucrose, citric acid, and fructose; these organic carbon sources can be carbonized during high-temperature sintering in an inert atmosphere and react with the nitrogen source to generate carbon-nitrogen bonds and / or nitrogen atoms are doped into the carbon coating layer in a free form, forming a nitrogen-doped carbon coating layer on the surface of the carbon coating layer in situ.

[0068] In some possible implementations, the nitrogen source includes one or more of amino acids, urea, acetonitrile, trimethylamine, methylimine, and melamine; these nitrogen sources can react with the carbon source or be directly doped into the carbon coating layer in a free form as nitrogen atoms during high-temperature sintering in an inert atmosphere, thereby enhancing the electronic conductivity and lithium-ion diffusion of the carbon coating layer through nitrogen atom doping of carbon.

[0069] In some possible implementations, the solvent includes one or more of deionized water and anhydrous ethanol; these solvents have good solubility for carbon and nitrogen sources and good dispersibility for the primary crystallization product. After the primary crystallization product is mixed with the second carbon source, nitrogen source, and solvent, the second carbon source and nitrogen source can coat the surface of the primary crystallization product. In some embodiments, the amount of solvent can be 5-10 times the weight of the finished phosphate-based cathode material.

[0070] In some embodiments, the method of mixing the primary crystallization product with the second carbon source, nitrogen source and solvent can be wet ball milling, such as ball milling in a ball mill for 4 to 10 hours, to ensure that the components are fully mixed and uniform, and after drying, the second carbon source and nitrogen source are coated on the surface of the primary crystallization product.

[0071] In step S30 above:

[0072] In some possible implementations, the step of dispersing the secondary crystallization product into a nitrogen-, phosphorus-, and carbon-containing hydrogel includes: preparing a mixed solution of phytic acid, aniline, and the secondary crystallization product with water; adding an initiator solution to carry out a polymerization reaction to form a nitrogen-, phosphorus-, and carbon-containing hydrogel encapsulating the secondary crystallization product, thereby obtaining the polymerized product. In the embodiments of this application, phytic acid is also known as inositol hexaphosphate or cyclohexanehexaphosphate. Phytic acid provides the phosphorus and carbon sources, while aniline provides the nitrogen and carbon sources. After preparing a mixed solution of phytic acid, aniline, and the secondary crystallization product with water, an initiator is used to promote the cross-linking polymerization of phytic acid and aniline to form a three-dimensional network structure hydrogel, while simultaneously encapsulating the secondary crystallization product in situ within the hydrogel.

[0073] In some possible implementations, the polymerization reaction is carried out under ice bath conditions of -20°C to -5°C. Adding an initiator under these conditions helps to avoid safety issues caused by the subsequent large exothermic reaction.

[0074] In some possible implementations, the mixed solution contains 10%–20% phytic acid by mass, 0.1 mol / L–0.2 mol / L aniline, and 0.2 g / mL–0.6 g / mL of the secondary crystallization product. In this case, the concentration of the secondary crystallization product ensures its stability in the polymer product, while the concentrations of phytic acid and aniline ensure sufficient cross-linking polymerization between them via subsequent initiator catalysis, forming a nitrogen-, phosphorus-, and carbon-containing hydrogel coating the secondary crystallization product.

[0075] For example, in the mixed solution, the mass fraction of phytic acid is any typical but non-limiting point value or any range between any two point values, such as 10%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%; the concentration of aniline is any typical but non-limiting point value or any range between any two point values, such as 0.1mol / L, 0.12mol / L, 0.13mol / L, 0.15mol / L, 0.16mol / L, 0.18mol / L, 0.20mol / L; and the concentration of the secondary crystallization product is any typical but non-limiting point value or any range between any two point values, such as 0.2g / mL, 0.3g / mL, 0.4g / mL, 0.5g / mL, 0.6g / mL.

[0076] In some possible implementations, the concentration of the initiator solution is 0.05 mol / L to 0.15 mol / L; under this concentration condition, the crosslinking polymerization reaction between phytic acid and aniline can be initiated well. For example, the concentration of the initiator solution can be any typical but non-limiting point value or a range between any two point values, such as 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.09 mol / L, 0.1 mol / L, 0.12 mol / L, 0.13 mol / L, and 0.15 mol / L.

[0077] In some possible implementations, the initiator solution contains one or more of ammonium persulfate, sodium persulfate, potassium persulfate, benzoyl peroxide, and azobisisobutyronitrile. These initiators can all initiate a cross-linking polymerization reaction between aniline and phytic acid to form a nitrogen-, phosphorus-, and carbon-containing hydrogel.

[0078] In some possible implementations, the conditions for the third sintering process include: heating to 650℃–850℃ in an inert atmosphere at a heating rate of 1℃ / min–5℃ / min, holding at that temperature for 5h–10h, and then annealing at 400℃–500℃ for 2h–3h. Under these sintering conditions, the crystal structure of the phosphate-based active core in the polymer product is optimized, and the nitrogen, phosphorus, and carbon-containing hydrogel is dehydrated and carbonized to form a nitrogen-phosphorus co-doped carbon coating layer. Through high-temperature sintering, a CNCNPC-like conductive network is formed between the nitrogen-doped carbon coating layer and the nitrogen-phosphorus co-doped carbon layer. Simultaneously, the secondary crystallization product is in situ coated within the nitrogen-phosphorus co-doped carbon coating layer, which has a three-dimensional porous network structure. The electron-rich clouds of P and N atoms doped in the nitrogen-phosphorus co-doped carbon coating layer provide abundant conductive networks, forming interconnected series networks between different composite phosphate-based cathode material particles, providing continuous electron channels and layered porous channels for Li ion transport. It also enhances the connection between phosphate-based active materials and carbon coating layers, improves the coating effect of the three types of carbon coating layers, and improves the cycle and rate performance of composite cathode materials during charge and discharge processes.

[0079] In some specific embodiments, the inert atmosphere can be at least one of nitrogen, argon, helium, etc.; the specific heating rate can be any typical but non-limiting point value or a range between any two point values, such as 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min; the sintering temperature can be any typical but non-limiting point value or a range between any two point values, such as 650℃, 700℃, 850℃; the holding time can be any typical but non-limiting point value or a range between any two point values, such as 5h, 6h, 7h, 8h, 9h, 10h; the temperature can be lowered to any typical but non-limiting point value or a range between any two point values, such as 400℃, 450℃, 500℃; and the annealing can be performed for 2h or 3h, which are typical but non-limiting point values ​​or a range between any two point values.

[0080] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector and a positive electrode active layer formed on the surface of the current collector, wherein the positive electrode active layer contains the aforementioned composite phosphate-based positive electrode material.

[0081] The positive electrode sheet of this application uses the above-mentioned composite phosphate-based positive electrode material in the positive electrode active layer. This composite phosphate-based positive electrode material contains three types of coating layers: a carbon coating layer, a nitrogen-doped carbon coating layer, and a nitrogen-phosphorus co-doped carbon coating layer. This significantly improves the ionic conductivity and electronic conductivity of the composite phosphate-based positive electrode material. At the same time, it has the characteristics of good structural stability, high capacity, and excellent conductivity. Therefore, it improves the stability, energy density, rate performance, cycle performance, and other electrochemical performance of the positive electrode sheet.

[0082] In some possible implementations, the preparation of the positive electrode active layer includes the following steps: mixing the above-mentioned composite phosphate-based positive electrode material, conductive agent and binder to form an electrode slurry, coating the electrode slurry onto the current collector, and then preparing the positive electrode sheet through steps such as drying, rolling and die cutting.

[0083] In some possible implementations, the mass percentage of the composite phosphate-based cathode material in the cathode active layer of the cathode sheet is 90% to 95%. Specifically, the mass percentage of the composite phosphate-based cathode material in the cathode active material layer can be 90%, 91%, 92%, 93%, 94%, 95%, etc.

[0084] In some possible implementations, the current collector of the positive electrode includes, but is not limited to, any one of copper foil or aluminum foil.

[0085] In some possible implementations, the binder content in the positive electrode active material layer is 2wt% to 5wt%. In specific embodiments, the binder content can be typical but not limited to 2wt%, 3wt%, 4wt%, 5wt%, etc.

[0086] In some possible implementations, the binder includes one or more of the following: polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0087] In some possible implementations, the conductive agent content in the positive electrode active material layer is 1 wt% to 5 wt%. In specific embodiments, the conductive agent content can be a typical but not limited content such as 3 wt%, 4 wt%, or 5 wt%.

[0088] In some possible implementations, the conductive agent includes graphite, carbon black, acetylene black, graphene, carbon fiber, and C. 60 And one or more of carbon nanotubes.

[0089] Fourthly, embodiments of this application provide a secondary battery, which includes the aforementioned positive electrode sheet.

[0090] The secondary battery provided in this application embodiment has improved the energy density, cycle stability and other electrochemical performance of the secondary battery because it contains a positive electrode sheet with excellent electrochemical performance such as good stability, high energy density, good rate performance and good cycle stability.

[0091] This application does not specifically limit the negative electrode, electrolyte, separator, etc. in the secondary battery of the embodiments, and can be applied to any battery system.

[0092] In some possible implementations, the negative electrode active material of the secondary battery includes, but is not limited to, carbon materials such as graphite, soft carbon (e.g., coke), and hard carbon, or nitrides, tin-based oxides, tin alloys, and nano-anode materials. The current collector includes, but is not limited to, any one of copper foil and aluminum foil.

[0093] In some possible implementations, the steps for making the negative electrode sheet include: mixing the negative electrode active material with conductive agents such as conductive carbon black, binders such as carboxymethyl cellulose and styrene-butadiene rubber, and solvents such as water in a mass ratio of (80-99):(1-5):(2-10):100 to make a positive electrode mixed slurry, then degassing under vacuum, discharging the material, coating it on a coating machine, and obtaining the negative electrode sheet after rolling, slitting, and die-cutting.

[0094] In some possible implementations, the membrane is capable of blocking electrons while allowing ions to pass through. Exemplary membranes include, but are not limited to, at least one material selected from polypropylene fibers, polyacrylonitrile fibers, polyvinyl formal fibers, poly(ethylene glycol terephthalate), polyethylene terephthalate, polyamide fibers, and poly(p-phenylene terephthalamide).

[0095] In some possible implementations, the electrolyte comprises at least one soluble metal salt. In some specific embodiments, the metal salt includes LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiTDI, Li[(CF3SO2)2N], Li[(FSO2)2N], Li[(C m F 2m+1 SO2)(C n F 2n+1 At least one of SO2(N)[m, n], where m and n are natural numbers. These electrolytic salts can ensure high ionic conductivity of the electrolyte and do not undergo harmful side reactions with electrode materials, electrolyte, diaphragm, etc., and have good chemical stability.

[0096] In some possible implementations, the secondary battery includes at least one of a battery cell, a battery module, and a battery pack.

[0097] In some possible implementations, the battery cell types include lithium-ion batteries, as well as novel batteries such as lithium-air batteries and lithium metal batteries.

[0098] In some possible implementations, the battery cells of this application can be assembled into a battery module. The battery module can contain multiple battery cells, the specific number of which can be adjusted according to the application and capacity of the battery module. The battery module may also include a housing with a receiving space in which multiple battery cells are received.

[0099] In one possible implementation, battery cells and / or battery modules can also be assembled into a battery pack, and the number of battery cells or battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0100] To enable those skilled in the art to clearly understand the above-described implementation details and operations, and to demonstrate the significant advancements in the performance of the composite phosphate-based cathode material, its preparation method, and its application in the embodiments of this application, the following examples illustrate the above technical solutions.

[0101] Example 1

[0102] A composite phosphate-based cathode material, the preparation process of which is shown in Figure 2, includes the following steps:

[0103] 1. Take 1 mol LiNO3, 0.4 mol Fe(NO3)3 (the amount added is based on the theoretical value in the experiment), 1 mol NH4H2PO4, 0.6 mol Mn(NO3)2, 0.003 mol titanium chloride (the dopant element), 6.296 g of glucose (4% of the theoretical weight of 1 mol lithium manganese iron phosphate), and 1500 mL of water, mix them evenly to form a solution, and then heat and stir until the water evaporates to dryness to obtain a solid precursor material, which is then crushed.

[0104] 2. The precursor was placed in a sintering furnace and heated to 320°C in a nitrogen atmosphere at a heating rate of 3°C / min for 6 hours. After cooling to room temperature, the primary crystallization product C@LMFP-Ti material was obtained.

[0105] 3. The above primary crystallization product (theoretical weight of 1 mol lithium manganese iron phosphate), 4.7 g glucose (3% of the theoretical weight of 1 mol lithium manganese iron phosphate), 6 g urea (1.78% of the theoretical weight of 1 mol lithium manganese iron phosphate), and 1000 mL water are mixed in a certain proportion, ball-milled in a ball mill for 6 hours, dried and pulverized, and then carbonized by heating to 480℃ at 3℃ / min and holding for 2.5 hours; the obtained sample is then ground to obtain the secondary crystallization product NC@C@LMFP-Ti material.

[0106] 4. Dissolve 80g of 10% phytic acid (P doping amount approximately 1.43%, relative to the theoretical weight of 1 mol of lithium manganese iron phosphate) and 160mL of 0.5mol / L aniline (0.69% of the theoretical weight of 1 mol of lithium manganese iron phosphate) in 500mL of aqueous solution (this mixture is now solution A). Disperse the above secondary crystallization product (1 mol of the theoretical weight of lithium manganese iron phosphate) in solution A and stir for 30min. Prepare another 500mL of 0.1mol / L ammonium persulfate solution B for later use.

[0107] 5. Place solution A in an ice bath environment, then quickly add solution B to solution A. The two undergo a polymerization reaction. After the reaction is complete, dry the water at 120°C to obtain phytic acid-doped polyaniline hydrogel of lithium manganese iron phosphate.

[0108] 6. The final product obtained in step 5 (lithium manganese iron phosphate encapsulated by phytic acid-doped polyaniline hydrogel) is placed in a sintering furnace and heated to 740℃ for 8 hours under a nitrogen atmosphere at a heating rate of 2℃ / min. Then, it is cooled to 420℃ for annealing for 2 hours and finally cooled to room temperature to obtain the composite phosphate-based cathode material N / PC@NC@C@LMFP-Ti.

[0109] Example 2

[0110] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that the doping element in step 1 is changed to cobalt nitrate.

[0111] Example 3

[0112] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that the nitrogen source in step 3 is changed to melamine.

[0113] Example 4

[0114] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that the amount of urea added in step 3 is changed to 10.12g (content is about 3.0%).

[0115] Example 5

[0116] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that the amount of urea added in step 3 is 20g (content is about 5.93%).

[0117] Example 6

[0118] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that the amount of aniline added in step 4 is 500 mL of 0.5 mol / L (content is about 2.22%).

[0119] Example 7

[0120] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that the phytic acid in step 4 is replaced with 250g of phytic acid with a mass fraction of 10% (content approximately 4.47%).

[0121] Example 8

[0122] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that: in step 4, ammonium persulfate, an initiator, is not added for polymerization, and phytic acid and aniline only form a mixture.

[0123] Comparative Example 1

[0124] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that urea is not added in step 3.

[0125] Comparative Example 2

[0126] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that steps 4 and 5 are not performed.

[0127] Comparative Example 3

[0128] A composite phosphate-based cathode material, comprising the following steps:

[0129] 1. Take 1 mol LiNO3, 0.4 mol Fe(NO3)3 (the amount added is based on the theoretical value in the experiment), 1 mol NH4H2PO4, 0.6 mol Mn(NO3)2, 0.003 mol titanium chloride (the dopant element), 6.296 g of glucose (4% of the theoretical weight of 1 mol lithium manganese iron phosphate), and 1500 mL of water, mix them evenly to form a solution, and then heat and stir until the water evaporates to dryness to obtain a solid precursor material, which is then crushed.

[0130] 2. The precursor was placed in a sintering furnace and heated to 320°C in a nitrogen atmosphere at a heating rate of 3°C / min for 6 hours. After cooling to room temperature, the primary crystallization product C@LMFP-Ti material was obtained.

[0131] 3. Dissolve 80g of phytic acid (10% by mass, with a phosphorus doping amount of approximately 1.43%, relative to the theoretical weight of 1 mol of lithium manganese iron phosphate) and 160mL of 0.5mol / L aniline (0.69% of the theoretical weight of 1 mol of lithium manganese iron phosphate) in 500mL of aqueous solution (this mixture is now solution A). Disperse the above primary crystallization product (1 mol of the theoretical weight of lithium manganese iron phosphate) in solution A and stir for 30min. Prepare another 500mL of 0.1mol / L ammonium persulfate solution B for later use.

[0132] 4. Place solution A in an ice bath environment, then quickly add solution B to solution A. The two undergo a polymerization reaction. After the reaction is complete, dry the water at 120°C to obtain lithium manganese iron phosphate doped with phytic acid polyaniline hydrogel.

[0133] 5. The phytic acid-doped polyaniline hydrogel obtained in step 4 is placed in a sintering furnace and heated to 740℃ for 8 hours under a nitrogen atmosphere at a heating rate of 2℃ / min. Then, it is cooled to 420℃ for annealing for 2 hours and finally cooled to room temperature to obtain the secondary crystallized product N / PC@C@LMFP-Ti.

[0134] 6. The above-mentioned secondary crystallized product N / PC@C@LMFP-Ti (theoretical weight of 1 mol lithium manganese iron phosphate), 4.7 g glucose (3% of the theoretical weight of 1 mol lithium manganese iron phosphate), 6 g urea (1.78% of the theoretical weight of 1 mol lithium manganese iron phosphate), and 1000 mL of water are mixed in a certain proportion, ball-milled in a ball mill for 6 hours, dried and pulverized, and then carbonized by heating to 480℃ at 3℃ / min and holding at that temperature for 2.5 hours; the obtained sample is then ground to obtain the secondary crystallized product NC@N / PC@C@LMFP-Ti material.

[0135] Comparative Example 4

[0136] A composite phosphate-based cathode material is prepared differently from that in Example 1 in that glucose is not added in step 1.

[0137] Test section

[0138] To verify the progressiveness of the embodiments of this application, the following performance tests were performed on the above embodiments and comparative examples:

[0139] 1. X-ray diffraction (XRD) analysis was performed on the primary crystallization product C@LMFP-Ti (labeled a) and the composite phosphate-based cathode material N / PC@NC@C@LMFP-Ti (labeled b) obtained in Example 1, as shown in Figure 3. The test results show that the composite phosphate-based cathode material prepared in this application is doped with nitrogen and phosphorus elements. LFP / LMP are the standard peaks of lithium iron phosphate and lithium manganese phosphate, respectively. Since lithium manganese iron phosphate (LMFP) does not have its own characteristic XRD crystal diffraction peaks, its peaks are mainly between those of lithium manganese phosphate (LMP) and lithium iron phosphate (LFP).

[0140] 2. The primary crystallization product C@LMFP-Ti (labeled as (a)) and the composite phosphate cathode material N / PC@NC@C@LMFP-Ti (labeled as (b)) prepared in Example 1 were subjected to transmission electron microscopy (TEM) and scanning electron microscopy (SEM) tests, respectively. The TEM image is shown in Figure 4, and the SEM image is shown in Figure 5. The test results show that after the nitrogen-doped carbon coating layer and the nitrogen-phosphorus co-doped carbon coating layer are applied, there are flocculent substances on the particle surface and between the particles of the composite phosphate cathode material. These flocculent substances are coating layers with a three-dimensional porous network structure.

[0141] 4. The primary crystallization product C@LMFP-Ti (labeled as (a)) and the composite phosphate cathode material N / PC@NC@C@LMFP-Ti (labeled as (b)) prepared in Example 1 were subjected to EDS (Energy Dispersive Spectrometer) analysis. The EDS spectrum is shown in Figure 6. It can be seen from the test structure that the composite phosphate cathode material prepared in this application example is doped with nitrogen and phosphorus elements.

[0142] 5. The above-described embodiments and comparative examples were applied to lithium-ion batteries, and their electrochemical performance was tested. The preparation process of the lithium-ion batteries is as follows:

[0143] ① Preparation of the positive electrode sheet: According to the mass ratio of the composite phosphate-based positive electrode material in the examples or comparative examples: SP:PVDF:NMP = 93.5:2.5:4:100, the mixture was stirred in a ball mill at 360 rpm for 1 hour to obtain a uniform positive electrode slurry. The prepared positive electrode slurry was poured onto aluminum foil and evenly spread with a scraper. The positive electrode sheet was dried in an oven at 120°C for 20 minutes, and then rolled under a pressure of 10 MPa to obtain a rolled electrode sheet. A 12 mm diameter circular sheet was cut from the middle area of ​​the electrode sheet using a punching machine, weighed, and its thickness was measured. The compaction density was calculated to obtain the positive electrode sheet for later use.

[0144] ② The battery assembly process is as follows: The prepared positive electrode sheet is attached to the positive metal shell with conductive adhesive and then dried; a lithium metal sheet is used as the negative electrode, a Celgard 2400 microporous membrane is used as the battery separator, and a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) containing 1.0 M LiPF6 is used as the electrolyte (EC / DEC volume ratio is 1:1). The coin cell is assembled in a glove box to obtain a lithium-ion coin cell.

[0145] The testing equipment involved in the above-mentioned tests includes the following:

[0146] ① Element content testing: Inductively coupled plasma optical emission spectrometer (ICP-OES);

[0147] ② Coating thickness: Transmission electron microscopy (TEM);

[0148] ③ Electronic conductivity: resistivity tester;

[0149] ④ The electrochemical performance of the coin cells was tested using a LAND electrochemical analyzer.

[0150] Discharge rate capacity: At room temperature, the charge / discharge rate of the coin cell battery is 0.1C / 1C / 2C / 3C.

[0151] Cyclic performance: At room temperature, the specific capacity of a button cell after 500 cycles of 1C charge / 1C discharge is equal to the initial specific capacity of the first cycle multiplied by 100%.

[0152] The elemental content tests are shown in Table 1 below:

[0153] Table 1

[0154] The test results for coating thickness, electronic conductivity, rate discharge capacity, and cycle performance are shown in Table 2 below.

[0155] Table 2

[0156] As can be seen from the above test results, the phosphate-based cathode material prepared in this application embodiment includes a phosphate-based active core and carbon coating layers, nitrogen-doped carbon coating layers and nitrogen-phosphorus co-doped carbon coating layers that are sequentially coated from the inside out. All of these materials exhibit better electronic conductivity, cycle stability and rate performance.

[0157] Comparing Examples 1 and Examples 4-6, it can be seen that the nitrogen doping amount of the intermediate coating layer and the outer carbon coating layer has a significant impact on conductivity and coating uniformity.

[0158] Comparing Examples 1 and 7, it can be seen that when the phosphorus doping content in the outer carbon coating layer is too high, it not only affects the coating uniformity, but also leads to a decrease in electronic conductivity. The free carbon doping actually hinders the rapid migration of lithium ions, resulting in a significant decrease in fast charging capability and cycle performance.

[0159] Comparing Examples 1 and 8, it can be seen that the nitrogen-phosphorus co-doped carbon coating layer prepared by nitrogen, phosphorus and carbon hydrogel has a three-dimensional porous structure. During lithium ion insertion and extraction, it can quickly conduct electrons to conduct electricity, thereby reducing polarization and improving the diffusion kinetics of lithium manganese iron phosphate.

[0160] Comparing Example 1 with Comparative Examples 1-4, it can be seen that the doping of nitrogen and phosphorus elements at different positions affects the electronic conductivity, cycle stability, and rate performance of the composite material.

[0161] The above are merely optional embodiments of this application and are not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A composite phosphate-based cathode material, characterized in that, It includes a phosphate-based active core, and the outer surface of the phosphate-based active core is coated with a carbon coating layer, a nitrogen-doped carbon coating layer and a nitrogen-phosphorus co-doped carbon coating layer from the inside to the outside.

2. The composite phosphate-based cathode material as described in claim 1, characterized in that, Based on the mass of the composite phosphate-based cathode material as 100%, the carbon coating layer contains 0.3% to 0.8% carbon by mass.

3. The composite phosphate-based cathode material as described in claim 1, characterized in that, Based on the mass of the composite phosphate-based cathode material as 100%, the nitrogen-doped carbon coating layer contains 0.3% to 0.8% carbon and 1.5% to 3.0% nitrogen by mass.

4. The composite phosphate-based cathode material as described in claim 1, characterized in that, Based on the mass of the composite phosphate-based cathode material as 100%, the nitrogen-phosphorus co-doped carbon coating layer contains 0.3% to 0.8% carbon, 0.6% to 1.5% nitrogen, and 1.2% to 3.2% phosphorus.

5. The composite phosphate-based cathode material as described in claim 1, characterized in that, In the nitrogen-doped carbon coating layer, nitrogen elements form chemical bonds with carbon elements and / or nitrogen elements are doped in a free form.

6. The composite phosphate-based cathode material as described in claim 1, characterized in that, The nitrogen-phosphorus co-doped carbon coating is prepared by sintering a hydrogel containing nitrogen, phosphorus and carbon, and has a three-dimensional porous network structure.

7. The composite phosphate-based cathode material according to any one of claims 1 to 6, characterized in that, The phosphate-based active core includes Li m Mn x Fe y A n PO4 is a phosphate-based cathode material, wherein A is selected from at least one doping element selected from Ti, Mg, V, Co, In, Ni, Zr, and Nb, with 0.95≤m≤1.05, 0≤x≤1, 0≤y≤1, 0≤n≤0.015, and x+y+n=1.

8. The composite phosphate-based cathode material according to any one of claims 1 to 6, characterized in that, Based on the mass of the composite phosphate-based cathode material as 100%, the coating layer of the composite phosphate-based cathode material contains 0.9% to 2.4% carbon, 2.1% to 4.5% nitrogen, and 1.2% to 3.2% phosphorus.

9. The composite phosphate-based cathode material according to any one of claims 1 to 6, characterized in that, In the composite phosphate-based cathode material, the total thickness of the carbon coating layer, the nitrogen-doped carbon coating layer, and the nitrogen-phosphorus co-doped carbon coating layer is 1 nm to 9 nm.

10. A method for preparing a composite phosphate-based cathode material, characterized in that, Includes the following steps: A phosphate-based solid-phase precursor material containing a first carbon source is prepared, and the solid-phase precursor material is subjected to a first sintering treatment to obtain a primary crystallization product comprising a phosphate-based active core and a carbon coating layer. The primary crystallization product is mixed with a second carbon source, a nitrogen source and a solvent, dried and pulverized, and then subjected to a second sintering process to form a nitrogen-doped carbon coating layer on the surface of the carbon coating layer, thereby obtaining a secondary crystallization product. The secondary crystallization product is dispersed in a nitrogen, phosphorus and carbon-containing hydrogel and subjected to a third sintering treatment to form a nitrogen-phosphorus co-doped carbon coating layer on the surface of the nitrogen-doped carbon coating layer, thereby obtaining a composite phosphate-based cathode material.

11. The method for preparing the composite phosphate-based cathode material as described in claim 10, characterized in that, The preparation of the solid precursor material includes the following steps: according to the chemical formula of the phosphate-based active core, Li m Mn x Fe y A n The stoichiometric ratio of each element in PO4 is determined, and iron, phosphorus, manganese, lithium, and A sources are obtained. These sources are then mixed with the first carbon source and solvent, dried, and crushed to obtain the solid-phase precursor material. Among them, A is selected from at least one doping element among Ti, Mg, V, Co, In, Ni, Zr, and Nb, with 0.95≤m≤1.05, 0≤x≤1, 0≤y≤1, 0≤n≤0.015, and x+y+n=1.

12. The method for preparing the composite phosphate-based cathode material as described in claim 10, characterized in that, The conditions for the first sintering treatment include: heating to 300℃ to 400℃ in an inert atmosphere at a heating rate of 1℃ / min to 5℃ / min, and then holding at that temperature for 5h to 10h.

13. The method for preparing the composite phosphate-based cathode material as described in claim 10, characterized in that, The conditions for the second sintering treatment include: heating to 400℃ to 500℃ in an inert atmosphere at a heating rate of 1℃ / min to 5℃ / min, and then holding at that temperature for 2h to 3h.

14. The method for preparing the composite phosphate-based cathode material as described in claim 10, characterized in that, The step of dispersing the secondary crystallization product into a nitrogen-, phosphorus-, and carbon-containing hydrogel includes: preparing a mixed solution of phytic acid, aniline, and the secondary crystallization product with water; adding an initiator solution to carry out a polymerization reaction to form the nitrogen-, phosphorus-, and carbon-containing hydrogel coating the secondary crystallization product, thereby obtaining a polymerized product.

15. The method for preparing the composite phosphate-based cathode material as described in claim 10, characterized in that, The conditions for the third sintering treatment include: heating to 650℃ to 850℃ in an inert atmosphere at a heating rate of 1℃ / min to 5℃ / min, holding at that temperature for 5h to 10h, and then annealing at 400℃ to 500℃ for 2h to 3h.

16. The method for preparing the composite phosphate-based cathode material as described in claim 14, characterized in that, In the mixed solution, the phytic acid has a mass fraction of 10% to 20%, the aniline has a concentration of 0.1 mol / L to 0.2 mol / L, and the secondary crystallization product has a concentration of 0.2 g / mL to 0.6 g / mL. And / or, the concentration of the initiator solution is 0.05 mol / L to 0.15 mol / L; And / or, the polymerization reaction is carried out under ice bath conditions of -20°C to -5°C; And / or, the initiator solution contains one or more of ammonium persulfate, sodium persulfate, potassium persulfate, benzoyl peroxide, and azobisisobutyronitrile.

17. The method for preparing the composite phosphate-based cathode material according to any one of claims 11 to 16, characterized in that, The amount of the first carbon source added is 2 wt% to 8 wt% of the theoretical yield of the composite phosphate-based cathode material; And / or, the amount of the second carbon source added is 2 wt% to 5 wt% of the theoretical yield of the composite phosphate-based cathode material; And / or, the amount of nitrogen source added is 1 wt% to 10 wt% of the theoretical yield of the composite phosphate-based cathode material.

18. The method for preparing the composite phosphate-based cathode material as described in claim 11, characterized in that, And / or, the lithium source includes one or more of lithium carbonate, lithium oxalate, and lithium acetate; And / or, the phosphorus source includes one or more of phosphoric acid, ammonium phosphate, and ammonium dihydrogen phosphate; And / or, the manganese source includes one or more of manganese carbonate, manganese oxalate, and manganese acetate; And / or, the iron source includes one or more of ferrous oxalate, ferrous acetate, ferric phosphate, and ferric nitrate; And / or, the first carbon source and the second carbon source each independently include one or more of glucose, sucrose, citric acid, and fructose; And / or, the solvent includes one or more of deionized water and anhydrous ethanol; And / or, the nitrogen source includes one or more of amino acids, urea, acetonitrile, trimethylamine, methylimine, and melamine.

19. A positive electrode plate, characterized in that, It includes a current collector and a positive electrode active layer formed on the surface of the current collector, wherein the positive electrode active layer contains a composite phosphate-based positive electrode material as described in any one of claims 1 to 9, or a composite phosphate-based positive electrode material prepared by the method described in any one of claims 10 to 18.

20. A secondary battery, characterized in that, The secondary battery includes the positive electrode as described in claim 19.

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