Lithium iron phosphate positive electrode material with low interface impedance, preparation method of lithium iron phosphate positive electrode material and lithium battery

Abstract

The invention discloses a lithium iron phosphate positive electrode material with low interface impedance, a preparation method thereof and a lithium battery, and belongs to the technical field of lithium batteries. The lithium iron phosphate positive electrode material is composed of a core material and a coating layer, wherein the core material is lithium iron phosphate particles; the coating layer comprises an inner layer and an outer layer, the inner layer is a titanium-magnesium-nitrogen co-doped carbon layer, and the outer layer is a Ti3SiC2 composite carbon layer; the Ti3SiC2 composite carbon layer is obtained by mixing and coating Ti3SiC2 and a polyvinyl alcohol solution and then carbonizing. The inner layer coating and the outer layer coating are combined to form double-layer coating, the titanium-magnesium-nitrogen co-doped carbon layer inhibits decomposition after electrolyte permeation by reducing the surface electronegativity, the interface impedance is reduced, a lithium ion diffusion channel is ensured, and meanwhile, the structural stability is improved. And the Ti3SiC2 composite carbon layer has high conductivity and temperature resistance, is relatively good in corrosion resistance, and can inhibit the corrosion effect of electrolyte on the positive electrode material and reduce the interface impedance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular relates to a lithium iron phosphate positive electrode material with low interfacial impedance, a preparation method thereof, and a lithium battery. Background Art

[0002] Compared to previous lead-acid, nickel-cadmium, and nickel-metal hydride batteries, lithium-ion power batteries offer advantages such as high operating voltage, light weight, compact size, no memory effect, low self-discharge, and long cycle life. They are gaining increasing popularity in applications such as cars and buses. Currently, among the various lithium-ion battery cathode materials, lithium iron phosphate batteries are the most popular. They offer a wide range of raw material sources, a stable structure, a long cycle life, high battery safety, and low cost.

[0003] Currently, the electrolyte used in lithium iron phosphate batteries is typically a mixture of an organic solvent and a lithium salt. Due to the relatively flat surface of lithium iron phosphate, the electrolyte has poor wettability on the cathode surface, resulting in increased resistance to charge transfer and, consequently, increased interfacial impedance. Furthermore, the cathode is prone to rupture during cycling, and continuous side reactions occur between the cathode and the electrolyte. The formation of a resistive surface film gradually increases interfacial impedance. Surface coating can reduce stress, increase the wettability of the liquid electrolyte, and lower interfacial charge transfer resistance, minimizing side reactions and thus effectively optimizing lithium iron phosphate cathode materials.

[0004] While surface coating can effectively inhibit side reactions between cathode materials and electrolytes, the stability of the coating layer remains a key issue. Existing coating materials will gradually deteriorate under the continuous erosion of the electrolyte during recycling, thus affecting the interface stability. Summary of the Invention

[0005] The present invention provides a lithium iron phosphate positive electrode material with low interfacial impedance, a preparation method thereof, and a lithium battery, which can solve the problems of high interfacial impedance and poor interfacial stability of lithium iron phosphate positive electrode materials in the prior art.

[0006] The purpose of the present invention can be achieved through the following technical solutions: A low interfacial impedance lithium iron phosphate positive electrode material, consisting of a core material and a coating layer, wherein the core material is lithium iron phosphate particles, the coating layer includes an inner layer and an outer layer, the inner layer is a titanium, magnesium, and nitrogen co-doped carbon layer, and the outer layer is a Ti3SiC2 composite carbon layer; The Ti3SiC2 composite carbon layer is obtained by mixing and coating Ti3SiC2 and polyvinyl alcohol solution and then carbonizing them.

[0007] In the titanium-magnesium-nitrogen co-doped carbon layer, on the one hand, Ti 4+ The ionic radius of Fe 2+It can stabilize the olivine structure and reduce lithium vacancy defects. On the other hand, Ti doping can reduce the electronegativity of the lithium iron phosphate (LiFePO4) particle interface. Polar molecules in the electrolyte tend to adsorb on the high electronegative surface. After Ti doping reduces the surface electronegativity, it reduces the adsorption force of the electrolyte on the surface after penetration, reducing the PF6 - Mg accumulation on the cathode surface inhibits the formation of PF5, a decomposition product of the electrolyte (LiPF6), thereby reducing HF production, lowering the rate of interfacial side reactions, and reducing the deposition of electrolyte decomposition products, thereby lowering interfacial impedance. Mg doping broadens lithium ion diffusion channels and reduces the activation energy for lithium ion migration. Co-doping with Mg and Ti balances charge, reduces oxygen vacancy formation, and enhances structural stability. The presence of nitrogen strengthens the interfacial bonding between the carbon layer and the core material, preventing delamination of the coating layer.

[0008] Ti3SiC2 is a ternary layered ceramic material with high conductivity of metal (resistivity of about 10 -6 Ω·m) and the high-temperature resistance of ceramics (melting point > 3000°C). Its layered structure effectively buffers volume expansion and inhibits particle cracking. It possesses the properties of a metal, with excellent thermal and electrical conductivity at room temperature, a relatively low Vickers hardness, and a high elastic modulus. Simultaneously, it possesses the properties of a ceramic material, with high yield strength, a high melting point, high thermal stability, and excellent oxidation resistance. It maintains high strength at high temperatures and exhibits excellent corrosion resistance to hydrofluoric acid, inhibiting electrolyte corrosion of the positive electrode material and reducing interfacial impedance.

[0009] A method for preparing the lithium iron phosphate positive electrode material with low interfacial impedance as described above comprises the following steps: S1. Synthesis of lithium iron phosphate particles: weigh a lithium source, an iron source, and a phosphorus source, mix them by ball milling, and calcine them at a high temperature of 650-750° C. in an inert atmosphere for 8-12 hours to obtain lithium iron phosphate particles.

[0010] S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=(0.5-1):1:(2-5), and ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol, mixed evenly, and coated particles are formed by spray drying to obtain primary coated particles.

[0011] Tetrabutyl titanate undergoes hydrolysis reaction in ethanol medium to generate Ti(OH)4 and form a sol. Magnesium nitrate is a metal salt and melamine is an organic molecule. The interaction between melamine and metal ions can enhance the stability of the sol and promote the uniform distribution of lithium iron phosphate particles after mixing.

[0012] S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500-1600℃ for 3-6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles, Ti3SiC2 nanoparticles and polyvinyl alcohol aqueous solution are mixed to form a coating liquid, the primary coated particles are placed in the coating liquid and mixed evenly, spray dried to form secondary coated particles, and sintered at 700-750℃ for 3-6h under an inert atmosphere to obtain lithium iron phosphate positive electrode material.

[0013] After Ti3SiC2 nanoparticles are mixed with polyvinyl alcohol aqueous solution, the polyvinyl alcohol in the outer layer will be carbonized during calcination. Since the inner layer in S2 is only simply dried and not calcined and carbonized in advance, the inner and outer layers will be carbonized simultaneously. The Ti atoms of Ti3SiC2 in the outer layer can covalently bond with the N in the Ti-N bond in the inner layer (Ti-N-Ti), thereby enhancing the interface bonding strength. The Mg doped inner layer 2+ The Si-C groups in the outer layer Ti3SiC2 are attracted by electrostatic action to form a local charge compensation area. The charge compensation area balances the interface potential distribution, reduces the energy barrier for lithium ion migration, reduces the polarization adsorption of the electrolyte at the interface, and inhibits the generation of HF.

[0014] Ti3SiC2 adheres tightly to the inner layer through chemical bonding, forming a physical barrier that reduces direct contact between the electrolyte and the core material, lowering interfacial impedance. The combination of the outer and inner layers inhibits particle pulverization and phase transitions, increasing the material's compaction density while also raising the thermal decomposition temperature to prevent thermal runaway.

[0015] Furthermore, the lithium source is at least one of lithium oxalate, lithium nitrate, lithium carbonate, and lithium hydroxide; The iron source is at least one of ferric oxalate, ferric nitrate, and ferric citrate; The phosphorus source is at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0016] Furthermore, the particle size of the lithium iron phosphate particles is 1-2 μm.

[0017] Furthermore, the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol in the sol is 2-5:5.

[0018] Furthermore, the mass ratio of the lithium iron phosphate particles to the sol is 0.3-1:1.

[0019] Furthermore, the particle size of the Ti3SiC2 nanoparticles is 0.1-0.2 nm.

[0020] Furthermore, the concentration of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 10-20 wt %.

[0021] Furthermore, the mass concentration of Ti3SiC2 nanoparticles in the coating solution is 4-10wt%.

[0022] Furthermore, the mass ratio of the primary coated particles to the coating liquid is 0.3-1:1.

[0023] Furthermore, the particle size of the lithium iron phosphate positive electrode material is 2-2.5 μm.

[0024] A lithium battery comprises a positive electrode, a negative electrode, an electrolyte and a separator, wherein the positive electrode is made of the lithium iron phosphate positive electrode material with low interfacial impedance as described above.

[0025] Beneficial effects of the present invention: (1) The lithium iron phosphate cathode material provided by the present invention is a double-layer coating formed by combining an inner layer and an outer layer. The titanium, magnesium, and nitrogen co-doped carbon layer suppresses decomposition after electrolyte penetration by reducing surface electronegativity, lowering interfacial impedance, ensuring lithium ion diffusion channels, and improving structural stability. The Ti3SiC2 composite carbon layer has both high conductivity and temperature resistance, and good corrosion resistance, which can inhibit the corrosion of the electrolyte on the cathode material and reduce interfacial impedance.

[0026] (2) In the process of preparing the lithium iron phosphate positive electrode material of the present invention, the inner layer structure is first simply dried, and the outer layer is coated and then calcined in one step. During calcination, the components in the inner and outer layer structures will produce chemical bonds at the bonding interface due to thermodynamic migration. The Ti atoms of Ti3SiC2 can covalently bond with the N in the Ti-N bond in the inner layer (Ti-N-Ti), and the Mg doped in the inner layer 2+ The Si-C groups in the outer layer Ti3SiC2 are attracted by electrostatic action to enhance the interface bonding strength. The inner and outer layer structures are tightly bonded, not easy to fall off, and have good stability. The presence of N elements in the inner layer can enhance the interface bonding strength between the carbon layer and the core material. The coating layer structure of the lithium iron phosphate positive electrode material has excellent stability. DETAILED DESCRIPTION

[0027] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0028] Example 1 This embodiment provides a lithium iron phosphate positive electrode material with low interfacial impedance, which is composed of a core material and a coating layer. The core material is lithium iron phosphate particles, and the coating layer includes an inner layer and an outer layer. The inner layer is a carbon layer co-doped with titanium, magnesium and nitrogen, and the outer layer is a Ti3SiC2 composite carbon layer. The Ti3SiC2 composite carbon layer is obtained by carbonizing a mixture of Ti3SiC2 and polyvinyl alcohol solution.

[0029] The steps for preparing the above-mentioned positive electrode material are as follows: S1. Synthesis of lithium iron phosphate particles: lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate were weighed in a molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, and calcined at 650°C in a nitrogen atmosphere for 12 h to obtain lithium iron phosphate particles, which were crushed and sieved to control the particle size to 1-2 μm.

[0030] S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=0.5:1:3, and the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 2:5. Ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol with a mass ratio of lithium iron phosphate particles to sol of 1:1. The mixture is evenly mixed and coated particles are formed by spray drying to obtain primary coated particles.

[0031] S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500℃ for 6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, and the Ti3SiC2 nanoparticles are mixed with a polyvinyl alcohol aqueous solution with a concentration of 10wt% to form a coating liquid. The mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 4wt%. The primary coated particles are placed in the coating liquid and mixed evenly. The mass ratio of the primary coated particles to the coating liquid is 1:1. Spray drying is used to form secondary coated particles, which are sintered at 750℃ for 3h under an inert atmosphere to obtain lithium iron phosphate positive electrode materials with a particle size of 2-2.2μm.

[0032] Example 2 The only difference from Example 1 is that tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=0.7:1:3.

[0033] The steps for preparing the positive electrode material in this embodiment are as follows: S1. Synthesis of lithium iron phosphate particles: lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate were weighed in a molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, and calcined at 650°C in a nitrogen atmosphere for 12 h to obtain lithium iron phosphate particles, which were crushed and sieved to control the particle size to 1-2 μm.

[0034] S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=0.7:1:3, and the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 2:5. Ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol with a mass ratio of lithium iron phosphate particles to sol of 1:1. The mixture is evenly mixed and coated particles are formed by spray drying to obtain primary coated particles.

[0035] S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500℃ for 6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, and the Ti3SiC2 nanoparticles are mixed with a polyvinyl alcohol aqueous solution with a concentration of 10wt% to form a coating liquid. The mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 4wt%. The primary coated particles are placed in the coating liquid and mixed evenly. The mass ratio of the primary coated particles to the coating liquid is 1:1. Spray drying is used to form secondary coated particles, which are sintered at 750℃ for 3h under an inert atmosphere to obtain lithium iron phosphate positive electrode materials with a particle size of 2-2.2μm.

[0036] Example 3 The only difference from Example 1 is that tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=1:1:3.

[0037] The steps for preparing the positive electrode material in this embodiment are as follows: S1. Synthesis of lithium iron phosphate particles: lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate were weighed in a molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, and calcined at 650°C in a nitrogen atmosphere for 12 h to obtain lithium iron phosphate particles, which were crushed and sieved to control the particle size to 1-2 μm.

[0038] S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=1:1:3, and the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 2:5. Ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol at a mass ratio of lithium iron phosphate particles to sol of 1:1. The mixture is evenly mixed and coated particles are formed by spray drying to obtain primary coated particles.

[0039] S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500℃ for 6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, and the Ti3SiC2 nanoparticles are mixed with a polyvinyl alcohol aqueous solution with a concentration of 10wt% to form a coating liquid. The mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 4wt%. The primary coated particles are placed in the coating liquid and mixed evenly. The mass ratio of the primary coated particles to the coating liquid is 1:1. Spray drying is used to form secondary coated particles, which are sintered at 750℃ for 3h under an inert atmosphere to obtain lithium iron phosphate positive electrode materials with a particle size of 2-2.2μm.

[0040] Example 4 The only difference from Example 2 is that the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 4:5.

[0041] The steps for preparing the positive electrode material in this embodiment are as follows: S1. Synthesis of lithium iron phosphate particles: lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate were weighed in a molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, and calcined at 650°C in a nitrogen atmosphere for 12 h to obtain lithium iron phosphate particles, which were crushed and sieved to control the particle size to 1-2 μm.

[0042] S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=0.7:1:3, and the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 4:5. Ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol with a mass ratio of lithium iron phosphate particles to sol of 1:1. The mixture is evenly mixed and coated particles are formed by spray drying to obtain primary coated particles.

[0043] S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500℃ for 6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, and the Ti3SiC2 nanoparticles are mixed with a polyvinyl alcohol aqueous solution with a concentration of 10wt% to form a coating liquid. The mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 4wt%. The primary coated particles are placed in the coating liquid and mixed evenly. The mass ratio of the primary coated particles to the coating liquid is 1:1. Spray drying is used to form secondary coated particles, which are sintered at 750℃ for 3h under an inert atmosphere to obtain lithium iron phosphate positive electrode materials with a particle size of 2.2-2.4μm.

[0044] Example 5 The only difference from Example 2 is that the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 5:5.

[0045] The steps for preparing the positive electrode material in this embodiment are as follows: S1. Synthesis of lithium iron phosphate particles: lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate were weighed in a molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, and calcined at 650°C in a nitrogen atmosphere for 12 h to obtain lithium iron phosphate particles, which were crushed and sieved to control the particle size to 1-2 μm.

[0046] S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=0.7:1:3, and the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 5:5. Ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol with a mass ratio of lithium iron phosphate particles to sol of 1:1. The mixture is evenly mixed and coated particles are formed by spray drying to obtain primary coated particles.

[0047] S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500℃ for 6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, and the Ti3SiC2 nanoparticles are mixed with a polyvinyl alcohol aqueous solution with a concentration of 10wt% to form a coating liquid. The mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 4wt%. The primary coated particles are placed in the coating liquid and mixed evenly. The mass ratio of the primary coated particles to the coating liquid is 1:1. Spray drying is used to form secondary coated particles, which are sintered at 750℃ for 3h under an inert atmosphere to obtain lithium iron phosphate positive electrode materials with a particle size of 2.3-2.5μm.

[0048] Example 6 The only difference from Example 4 is that the mass concentration of Ti3SiC2 nanoparticles in the coating solution is 8 wt%.

[0049] The steps for preparing the positive electrode material in this embodiment are as follows: S1. Synthesis of lithium iron phosphate particles: lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate were weighed in a molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, and calcined at 650°C in a nitrogen atmosphere for 12 h to obtain lithium iron phosphate particles, which were crushed and sieved to control the particle size to 1-2 μm.

[0050] S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=0.7:1:3, and the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 4:5. Ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol with a mass ratio of lithium iron phosphate particles to sol of 1:1. The mixture is evenly mixed and coated particles are formed by spray drying to obtain primary coated particles.

[0051] S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500℃ for 6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, and the Ti3SiC2 nanoparticles are mixed with a polyvinyl alcohol aqueous solution with a concentration of 10wt% to form a coating liquid. The mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 8wt%. The primary coated particles are placed in the coating liquid and mixed evenly. The mass ratio of the primary coated particles to the coating liquid is 1:1. Spray drying is used to form secondary coated particles, which are sintered at 750℃ for 3h under an inert atmosphere to obtain lithium iron phosphate positive electrode materials with a particle size of 2.3-2.5μm.

[0052] Example 7 The only difference from Example 4 is that the mass concentration of Ti3SiC2 nanoparticles in the coating solution is 10 wt%.

[0053] The steps for preparing the positive electrode material in this embodiment are as follows: S1. Synthesis of lithium iron phosphate particles: lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate were weighed in a molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, and calcined at 650°C in a nitrogen atmosphere for 12 h to obtain lithium iron phosphate particles, which were crushed and sieved to control the particle size to 1-2 μm.

[0054] S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=0.7:1:3, and the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 4:5. Ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol with a mass ratio of lithium iron phosphate particles to sol of 1:1. The mixture is evenly mixed and coated particles are formed by spray drying to obtain primary coated particles.

[0055] S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500℃ for 6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, and the Ti3SiC2 nanoparticles are mixed with a polyvinyl alcohol aqueous solution with a concentration of 10wt% to form a coating liquid. The mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 10wt%. The primary coated particles are placed in the coating liquid and mixed evenly. The mass ratio of the primary coated particles to the coating liquid is 1:1. Spray drying is used to form secondary coated particles, which are sintered at 750℃ for 3h under an inert atmosphere to obtain lithium iron phosphate positive electrode materials with a particle size of 2.3-2.5μm.

[0056] Comparative Example 1 The difference from Example 1 is that magnesium nitrate is not added to the inner coating of this comparative example.

[0057] The steps for preparing the positive electrode material are as follows: S1. Synthesis of lithium iron phosphate particles: lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate were weighed in a molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, and calcined at 650°C in a nitrogen atmosphere for 12 h to obtain lithium iron phosphate particles, which were crushed and sieved to control the particle size to 1-2 μm.

[0058] S2. Inner layer coating: Tetrabutyl titanate and melamine are dissolved in ethanol at a molar ratio of Ti:N=0.5:3, and the mass ratio of the total mass of tetrabutyl titanate and melamine to ethanol is 2:5. Ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol at a mass ratio of lithium iron phosphate particles to sol of 1:1. The mixture is evenly mixed and coated particles are formed by spray drying to obtain primary coated particles.

[0059] S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500℃ for 6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, and the Ti3SiC2 nanoparticles are mixed with a polyvinyl alcohol aqueous solution with a concentration of 10wt% to form a coating liquid. The mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 4wt%. The primary coated particles are placed in the coating liquid and mixed evenly. The mass ratio of the primary coated particles to the coating liquid is 1:1. Spray drying is used to form secondary coated particles, which are sintered at 750℃ for 3h under an inert atmosphere to obtain lithium iron phosphate positive electrode materials with a particle size of 2-2.2μm.

[0060] Comparative Example 2 The difference from Example 1 is that Ti3SiC2 is not added to the outer coating of this comparative example.

[0061] The steps for preparing the positive electrode material are as follows: S1. Synthesis of lithium iron phosphate particles: lithium carbonate, ferrous oxalate and ammonium dihydrogen phosphate were weighed in a molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, and calcined at 650°C in a nitrogen atmosphere for 12 h to obtain lithium iron phosphate particles, which were crushed and sieved to control the particle size to 1-2 μm.

[0062] S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol at a molar ratio of Ti:Mg:N=0.5:1:3, and the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol is 2:5. Ultrasonic dispersion is performed to form a sol. Lithium iron phosphate particles are added to the sol with a mass ratio of lithium iron phosphate particles to sol of 1:1. The mixture is evenly mixed and coated particles are formed by spray drying to obtain primary coated particles.

[0063] S3. Outer layer coating: prepare a polyvinyl alcohol aqueous solution with a concentration of 10wt% to form a coating liquid, place the primary coated particles in the coating liquid and mix evenly, the mass ratio of the primary coated particles to the coating liquid is 1:1, spray dry to form secondary coated particles, sinter at 750℃ for 3h under an inert atmosphere to obtain a lithium iron phosphate positive electrode material with a particle size of 2-2.2μm.

[0064] The lithium iron phosphate positive electrode materials obtained in Examples 1-7 and Comparative Examples 1-2 were assembled into batteries. The lithium iron phosphate positive electrode material, PVDF, and carbon black were weighed in a mass ratio of 90:6:4, and then an appropriate amount of NMP solvent was added for dispersion. The mixture was evenly coated on aluminum foil and dried in an oven at 100°C for 8 hours. The mixture was rolled and cut into positive electrode sheets. In a vacuum glove box, a metal lithium sheet was used as the negative electrode, a composite film of PE and PP was used as the separator, the electrolyte solute was LiPF6 with a concentration of 1 mol / L, and the electrolyte solvent was ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1. The batteries were assembled into CR2430 button batteries.

[0065] The interface impedance test and rate performance test of the assembled button cell were performed, and the results are shown in Table 1.

[0066] Table 1

[0067] As can be seen from Table 1, the lithium iron phosphate positive electrode material provided in the embodiment of the present invention has a low interface impedance, and the capacity retention rate after 1000 cycles of 1C can reach up to 90.3%, with excellent performance and good stability. Example 1-Example 3 optimizes the ratio of Ti, Mg, and N in the inner layer coating. When the Ti content increases, its ability to suppress side reactions after electrolyte penetration is enhanced, and the interface impedance is reduced. Among them, the cycle capacity retention rate of Example 2 reaches 89.4%. In Example 4 and Example 5, the concentration of the sol gradually increases during the inner layer coating. When the concentration increases, the inner layer coating becomes denser, and the interface impedance will decrease at this time. However, when the coating is too dense, the mass proportion of the coating layer in the positive electrode material increases, resulting in a decrease in the active component content and a slight decrease in the battery specific capacity. Example 6 and Example 7 further optimize the parameters of the outer layer coating on the basis of Example 4. The greater the concentration of Ti3SiC2 nanoparticles, the better its conductivity and corrosion resistance, and the lower the interface impedance. Combining the results of the comparative examples and examples, it can be seen that when the inner layer is not doped with magnesium, on the one hand, the lithium ion migration rate will be reduced, and on the other hand, the interfacial bonding strength of the inner and outer layers will be weakened, the interface stability will decrease, and the electrochemical performance will be reduced. The outer layer coating is not doped with Ti3SiC2 and is only a carbon coating. The corrosion resistance of the positive electrode material is reduced, the interfacial impedance is high, and the cycle performance is poor.

[0068] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0069] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A lithium iron phosphate cathode material with low interfacial impedance, characterized in that: It is composed of a core material and a coating layer, wherein the core material is lithium iron phosphate particles; the coating layer includes an inner layer and an outer layer, the inner layer is a titanium, magnesium and nitrogen co-doped carbon layer, and the outer layer is a Ti3SiC2 composite carbon layer; The Ti3SiC2 composite carbon layer is obtained by mixing and coating Ti3SiC2 and polyvinyl alcohol solution and then carbonizing them.

2. A method for preparing a lithium iron phosphate positive electrode material with low interfacial impedance, characterized in that: The method for preparing the lithium iron phosphate positive electrode material with low interfacial impedance as claimed in claim 1 comprises the following steps: S1. Synthesis of lithium iron phosphate particles: Weigh a lithium source, an iron source, and a phosphorus source, mix them by ball milling, and calcine them at 650-750°C in an inert atmosphere for 8-12 hours to obtain lithium iron phosphate particles; S2. Inner layer coating: Tetrabutyl titanate, magnesium nitrate, and melamine are dissolved in ethanol at a molar ratio of (0.5-1):1:(2-5), and ultrasonically dispersed to form a sol. Lithium iron phosphate particles are added to the sol, mixed uniformly, and spray-dried to form coated particles to obtain primary coated particles. S3. Outer layer coating: Titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio of TiH2:Si:TiC=1:1:2, calcined at 1500-1600℃ for 3-6h, crushed and ball-milled to obtain Ti3SiC2 nanoparticles, Ti3SiC2 nanoparticles and polyvinyl alcohol aqueous solution are mixed to form a coating liquid, the primary coated particles are placed in the coating liquid and mixed evenly, spray dried to form secondary coated particles, and sintered at 700-750℃ for 3-6h under an inert atmosphere to obtain lithium iron phosphate positive electrode material.

3. The method for preparing a lithium iron phosphate positive electrode material with low interfacial impedance according to claim 2, characterized in that: The lithium source is at least one of lithium oxalate, lithium nitrate, lithium carbonate, and lithium hydroxide; The iron source is at least one of ferric oxalate, ferric nitrate, and ferric citrate; The phosphorus source is at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

4. The method for preparing a lithium iron phosphate positive electrode material with low interfacial impedance according to claim 2, characterized in that: The particle size of the lithium iron phosphate particles is 1-2 μm.

5. The method for preparing a lithium iron phosphate positive electrode material with low interfacial impedance according to claim 2, characterized in that: The mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol in the sol is 2-5:5; The mass ratio of the lithium iron phosphate particles to the sol is 0.3-1:

1.

6. The method for preparing a lithium iron phosphate positive electrode material with low interfacial impedance according to claim 2, characterized in that: The particle size of the Ti3SiC2 nanoparticles is 0.1-0.2 nm.

7. The method for preparing a lithium iron phosphate positive electrode material with low interfacial impedance according to claim 2, characterized in that: The concentration of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 10-20 wt %.

8. The method for preparing a lithium iron phosphate positive electrode material with low interfacial impedance according to claim 2, characterized in that: The mass concentration of Ti3SiC2 nanoparticles in the coating solution is 4-10wt%; The mass ratio of the primary coating particles to the coating liquid is 0.3-1:

1.

9. The method for preparing a lithium iron phosphate positive electrode material with low interfacial impedance according to claim 2, characterized in that: The particle size of the lithium iron phosphate positive electrode material is 2-2.5 μm.

10. A lithium battery comprising a positive electrode, a negative electrode, an electrolyte and a separator, characterized in that: The positive electrode is made of the lithium iron phosphate positive electrode material with low interfacial resistance according to claim 1.

Images