Lithium iron phosphate positive electrode material with low interfacial impedance, preparation method thereof and lithium battery

Abstract

The application discloses a lithium iron phosphate anode 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 anode material is composed of a core material and a coating layer, the core material is lithium iron phosphate particles, the coating layer comprises 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 mixing Ti3SiC2 and polyvinyl alcohol solution and then carbonizing. The inner layer and the outer layer are combined to form double-layer coating, the carbon layer co-doped with titanium, magnesium and nitrogen can inhibit the decomposition of the electrolyte after permeation by reducing the surface electronegativity, reduce the interface impedance, ensure the lithium ion diffusion channel and improve the structural stability. The Ti3SiC2 composite carbon layer has high conductivity and temperature resistance, and has good corrosion resistance, can inhibit the corrosion of the electrolyte on the anode material and reduce the interface impedance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium batteries, and particularly relates to a lithium iron phosphate positive electrode material with low interface impedance, a preparation method thereof and a lithium battery. BACKGROUND

[0002] Compared with previous lead-acid, nickel-cadmium and nickel-hydrogen storage batteries, lithium ion power batteries have the advantages of high working voltage, light weight, small size, no memory effect, low self-discharge rate, long cycle life and the like, and have been increasingly widely applied in scenarios such as cars and buses. Among many lithium ion battery positive electrode materials, lithium iron phosphate batteries are the most popular. It has the advantages of wide raw material sources, stable structure, long cycle life, high battery safety and low cost.

[0003] At present, the electrolyte used in lithium iron phosphate batteries is usually a mixture of an organic solvent and a lithium salt. Since the surface of lithium iron phosphate is relatively smooth, the wettability of the electrolyte on the surface of the positive electrode is poor, which increases the resistance of charge transfer, thereby increasing the interface impedance. And the positive electrode is easy to break during the cycle process, and there is a continuous side reaction between the positive electrode and the electrolyte, and the formation of a resistive surface film leads to gradual increase of the interface impedance. Surface coating can reduce stress, increase the wettability of the liquid electrolyte and reduce the interface charge transfer resistance, and reduce side reactions, thereby effectively optimizing the lithium iron phosphate positive electrode material.

[0004] Although surface coating can effectively inhibit the side reaction of the positive electrode material and the electrolyte, the stability of the coating layer is still a key problem. The existing coating material will be gradually destroyed under the continuous erosion of the electrolyte during the cycle process, thereby affecting the interface stability. SUMMARY

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

[0006] The object of the application can be achieved by the following technical solutions:

[0007] A lithium iron phosphate positive electrode material with low interface impedance is composed of a core material and a coating layer, the core material is lithium iron phosphate particles, and 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.

[0008] The Ti3SiC2 composite carbon layer is obtained by mixing Ti3SiC2 and polyvinyl alcohol solution and then carbonizing.

[0009] In the titanium-magnesium-nitrogen co-doped carbon layer, on the one hand, the ionic radius of Ti 4+ is close to that of Fe 2+The similar, stable olivine structure, reduce lithium vacancy defects, on the other hand, Ti-doped can reduce the electronegativity of the interface of lithium iron phosphate (LiFePO4) particles. The polar molecules in the electrolyte tend to be adsorbed on the surface with high electronegativity. After Ti-doping reduces the surface electronegativity, the adsorption force of the electrolyte on the surface after permeation is reduced, and the PF6 - The enrichment on the surface of the positive electrode inhibits the generation of PF5, the decomposition product of the electrolyte (LiPF6), thereby reducing the generation of HF, reducing the interface side reaction rate, reducing the deposition of electrolyte decomposition products, and reducing the interface impedance. Mg doping can widen the lithium ion diffusion channel and reduce the lithium ion migration activation energy. The co-doping of Mg and Ti balances the electric charge, reduces the formation of oxygen vacancies, and improves the structural stability. The presence of N element can enhance the interfacial bonding force between the carbon layer and the core material, preventing the peeling of the coating layer.

[0010] Ti3SiC2 is a ternary layered ceramic material, which has high conductivity of metal (resistivity about 10 -6 Ω·m) and high temperature resistance of ceramic (melting point > 3000℃). Its layered structure can effectively buffer volume expansion and inhibit particle cracking. It has the characteristics of metal, good thermal conductivity and electrical conductivity at room temperature, relatively low Vickers hardness and high elastic modulus. At the same time, it also has the performance of ceramic material, high yield strength, high melting point, high thermal stability and good oxidation resistance, can maintain high strength at high temperature, has good corrosion resistance to hydrofluoric acid, can inhibit the corrosion of electrolyte to positive electrode material, and reduce the interface impedance.

[0011] A preparation method of a low interface impedance lithium iron phosphate positive electrode material as described above, comprising the following steps:

[0012] S1, synthesizing lithium iron phosphate particles: weighing lithium source, iron source and phosphorus source, ball milling, calcining at 650-750℃ in inert atmosphere for 8-12h to obtain lithium iron phosphate particles.

[0013] S2, inner layer coating: dissolving tetrabutyl titanate, magnesium nitrate and melamine in ethanol according to the molar ratio of Ti:Mg:N=(0.5-1):1:(2-5), ultrasonic dispersion to form sol, adding lithium iron phosphate particles into the sol, mixing uniformly, and forming coated particles by spray drying to obtain primary coated particles.

[0014] Tetrabutyl titanate undergoes hydrolysis reaction in ethanol medium to generate Ti(OH)4 to form sol. Magnesium nitrate is a metal salt, and melamine is an organic molecule. Through the interaction between melamine and metal ions, the stability of the sol can be enhanced, and the uniform distribution of lithium iron phosphate particles after mixing can be promoted.

[0015] 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, and then crushed and ball milled to obtain Ti3SiC2nanoparticles. The Ti3SiC2nanoparticles and polyvinyl alcohol aqueous solution are mixed to form a coating liquid. The primary coated particles are mixed uniformly in the coating liquid, and then spray dried to form secondary coated particles. The secondary coated particles are sintered at 700-750℃ for 3-6h in an inert atmosphere to obtain lithium iron phosphate positive electrode material.

[0016] After the Ti3SiC2nanoparticles are mixed in the polyvinyl alcohol aqueous solution, the polyvinyl alcohol in the outer layer will carbonize during calcination. Since the inner layer in S2 is only simply dried and not calcined and carbonized in advance, the inner layer and the outer layer will carbonize at the same time. The Ti atoms in the outer layer Ti3SiC2can covalently bond with the N in the Ti-N bond in the inner layer (Ti-N-Ti), enhancing the interfacial bonding force. The Mg doped in the inner layer 2+ The Si-C groups in the outer layer Ti3SiC2are attracted by electrostatic action to form a local charge compensation zone. The charge compensation zone balances the interfacial potential distribution, reduces the lithium ion migration energy barrier, reduces the polarization adsorption of the electrolyte at the interface, and inhibits the generation of HF.

[0017] Ti3SiC2is tightly attached to the surface of the inner layer by chemical bonding, forming a physical barrier that reduces direct contact between the electrolyte and the core material and reduces interfacial impedance. The combination of the outer layer and the inner layer can inhibit particle pulverization and phase transition, improve material compaction density, and increase thermal decomposition temperature to avoid thermal runaway.

[0018] Further, the lithium source is at least one of lithium oxalate, lithium nitrate, lithium carbonate, and lithium hydroxide;

[0019] The iron source is at least one of iron oxalate, iron nitrate, and iron citrate;

[0020] The phosphorus source is at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0021] Further, the particle size of the lithium iron phosphate particles is 1-2μm.

[0022] Further, the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine in the sol to the mass of ethanol is 2-5:5.

[0023] Further, the mass ratio of the lithium iron phosphate particles to the sol is 0.3-1:1.

[0024] Further, the particle size of the Ti3SiC2nanoparticles is 0.1-0.2nm.

[0025] Further, the concentration of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 10-20 wt%.

[0026] Further, the mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 4-10 wt%.

[0027] Further, the mass ratio of the primary coated particles and the coating liquid is 0.3-1:1.

[0028] Further, the particle size of the lithium iron phosphate positive electrode material is 2-2.5 μm.

[0029] 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 interface impedance as described above.

[0030] The beneficial effects of the present application are:

[0031] (1) The lithium iron phosphate positive electrode material provided by the present application is formed by double-layer coating through the combination of inner layer and outer layer coating, and the carbon layer co-doped with titanium, magnesium and nitrogen can inhibit the decomposition of the electrolyte after the electrolyte permeates by reducing the surface electronegativity, reduce the interface impedance, ensure the lithium ion diffusion channel and improve the structural stability. The Ti3SiC2 composite carbon layer has high conductivity and temperature resistance, and has good corrosion resistance, which can inhibit the corrosion of the electrolyte on the positive electrode material and reduce the interface impedance.

[0032] (2) In the process of preparing the lithium iron phosphate positive electrode material, the inner layer structure is simply dried first, and then the outer layer is coated and calcined in one step. During calcination, the components in the inner and outer layer structures will form chemical bonds at the combination interface due to the thermodynamic migration effect. The Ti atoms in Ti3SiC2 can form covalent bonds (Ti-N-Ti) with the N in the Ti-N bonds in the inner layer, and the Mg 2+ The Si-C groups in the outer layer Ti3SiC2 are attracted by the electrostatic effect, the interface bonding force is enhanced, the inner and outer layer structures are tightly combined and are not easy to fall off, the stability is good, the existence of N element in the inner layer can enhance the interface bonding force between the carbon layer and the core material, and the coating layer structure of the lithium iron phosphate positive electrode material is excellent in stability. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0034] Example 1

[0035] The embodiment provides a low-interface-impedance lithium iron phosphate positive electrode material which is composed of a core material and a coating layer, the core material is lithium iron phosphate particles, the coating layer comprises 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 which is obtained by mixing Ti3SiC2 and polyvinyl alcohol solution and then carbonizing.

[0036] The steps for preparing the positive electrode material are as follows:

[0037] S1, synthesizing lithium iron phosphate particles: lithium carbonate, iron oxalate and ammonium dihydrogen phosphate are weighed according to the molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, calcined at a high temperature of 650 DEG C in a nitrogen atmosphere for 12 hours, to obtain lithium iron phosphate particles, which are crushed and sieved to control the particle size to be 1-2 microns.

[0038] S2, inner layer coating: tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol according to the molar ratio of Ti:Mg:N=0.5:1:3, the mass ratio 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 into the sol, the mass ratio of lithium iron phosphate particles to the sol is 1:1, mixed uniformly, and spray drying is performed to form coated particles, 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 DEG C for 6 hours, crushed and ball-milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2 microns, 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 mixed uniformly in the coating liquid, the mass ratio of the primary coated particles to the coating liquid is 1:1, spray drying is performed to form secondary coated particles, and the secondary coated particles are sintered at 750 DEG C in an inert atmosphere for 3 hours, to obtain lithium iron phosphate positive electrode material with a particle size of 2-2.2 microns.

[0040] Embodiment 2

[0041] The difference from embodiment 1 is that tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol according to the molar ratio of Ti:Mg:N=0.7:1:3.

[0042] The steps for preparing the positive electrode material are as follows:

[0043] S1, synthesizing lithium iron phosphate particles: lithium carbonate, iron oxalate and ammonium dihydrogen phosphate are weighed according to the molar ratio of Li:Fe:PO4=1:1:1, ball-milled and mixed, calcined at a high temperature of 650 DEG C in a nitrogen atmosphere for 12 hours, to obtain lithium iron phosphate particles, which are crushed and sieved to control the particle size to be 1-2 microns.

[0044] S2, inner layer coating: tetrabutyl titanate, magnesium nitrate and melamine were dissolved in ethanol according to the molar ratio of Ti: Mg: N = 0.7: 1: 3, the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol was 2:5, and a sol was formed by ultrasonic dispersion. Lithium iron phosphate particles were added to the sol, the mass ratio of lithium iron phosphate particles to sol was 1:1, and the mixture was uniformly mixed. The coated particles were formed by spray drying to obtain primary coated particles.

[0045] S3, outer layer coating: titanium hydride powder, silicon powder and titanium carbide powder were weighed according to the molar ratio of TiH2: Si: TiC = 1: 1: 2, calcined at 1500℃ for 6h, and then crushed and ball milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm. The Ti3SiC2 nanoparticles were mixed with a 10wt% polyvinyl alcohol aqueous solution to form a coating liquid, and the mass concentration of Ti3SiC2 nanoparticles in the coating liquid was 4wt%. The primary coated particles were mixed uniformly in the coating liquid, the mass ratio of the primary coated particles to the coating liquid was 1:1, and the secondary coated particles were formed by spray drying. The lithium iron phosphate positive electrode material with a particle size of 2-2.2μm was obtained by sintering at 750℃ for 3h in an inert atmosphere.

[0046] Example 3

[0047] The difference from Example 1 is that tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol according to the molar ratio of Ti: Mg: N = 1: 1: 3.

[0048] The steps for preparing the positive electrode material in this example are as follows:

[0049] S1, synthesis of lithium iron phosphate particles: lithium carbonate, iron oxalate and ammonium dihydrogen phosphate were weighed according to the molar ratio of Li: Fe: PO4= 1: 1: 1, ball milled and mixed, and calcined at 650℃ for 12h in a nitrogen atmosphere to obtain lithium iron phosphate particles. The particles were crushed and sieved to control the particle size to 1-2μm.

[0050] S2, inner layer coating: tetrabutyl titanate, magnesium nitrate and melamine were dissolved in ethanol according to the molar ratio of Ti: Mg: N = 1: 1: 3, the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol was 2:5, and a sol was formed by ultrasonic dispersion. Lithium iron phosphate particles were added to the sol, the mass ratio of lithium iron phosphate particles to sol was 1:1, and the mixture was uniformly mixed. The coated particles were formed by spray drying to obtain primary coated particles.

[0051] S3, outer layer coating: titanium hydride powder, silicon powder and titanium carbide powder were 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, the Ti3SiC2 nanoparticles and a 10wt% polyvinyl alcohol aqueous solution were mixed to form a coating liquid, the mass concentration of Ti3SiC2 nanoparticles in the coating liquid was 4wt%, the primary coated particles were mixed uniformly in the coating liquid, the mass ratio of the primary coated particles to the coating liquid was 1:1, spray drying formed secondary coated particles, sintered at 750℃ for 3h in an inert atmosphere to obtain lithium iron phosphate positive electrode material with a particle size of 2-2.2μm.

[0052] Example 4

[0053] The difference from Example 2 is only that the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to the mass of ethanol is 4:5.

[0054] The steps of preparing the positive electrode material in this example are as follows:

[0055] S1, synthesis of lithium iron phosphate particles: lithium carbonate, iron oxalate and ammonium dihydrogen phosphate were weighed according to the molar ratio of Li: Fe: PO4 = 1: 1: 1, ball milled and mixed, calcined at high temperature of 650℃ for 12h in a nitrogen atmosphere to obtain lithium iron phosphate particles, crushed and sieved to control the particle size to 1-2μm.

[0056] S2, inner layer coating: tetrabutyl titanate, magnesium nitrate and melamine were dissolved in ethanol according to the molar ratio of Ti: Mg: N = 0.7: 1: 3, the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to the mass of ethanol was 4:5, ultrasonic dispersion to form a sol, lithium iron phosphate particles were added to the sol, the mass ratio of lithium iron phosphate particles to the sol was 1:1, mixed uniformly, spray drying to form coated particles to obtain primary coated particles.

[0057] S3, outer layer coating: titanium hydride powder, silicon powder and titanium carbide powder were 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, the Ti3SiC2 nanoparticles and a 10wt% polyvinyl alcohol aqueous solution were mixed to form a coating liquid, the mass concentration of Ti3SiC2 nanoparticles in the coating liquid was 4wt%, the primary coated particles were mixed uniformly in the coating liquid, the mass ratio of the primary coated particles to the coating liquid was 1:1, spray drying formed secondary coated particles, sintered at 750℃ for 3h in an inert atmosphere to obtain lithium iron phosphate positive electrode material with a particle size of 2.2-2.4μm.

[0058] Example 5

[0059] The difference from Example 2 is that the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to the mass of ethanol is 5:5.

[0060] The steps for preparing the positive electrode material in this example are as follows:

[0061] S1, synthesis of lithium iron phosphate particles: lithium carbonate, iron oxalate and ammonium dihydrogen phosphate are weighed according to the molar ratio Li:Fe:PO4=1:1:1, ball-milled and mixed, calcined at a high temperature of 650℃ for 12h in a nitrogen atmosphere, to obtain lithium iron phosphate particles, which are crushed and sieved to control the particle size to be 1-2μm.

[0062] S2, inner layer coating: tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol according to the molar ratio Ti:Mg:N=0.7:1:3, the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to the mass of ethanol is 5:5, ultrasonic dispersion is performed to form a sol, lithium iron phosphate particles are added to the sol, the mass ratio of lithium iron phosphate particles to the sol is 1:1, mixed uniformly, and spray drying is performed to form coated particles, to obtain primary coated particles.

[0063] S3, outer layer coating: titanium hydride powder, silicon powder and titanium carbide powder are weighed according to the molar ratio TiH2:Si:TiC=1:1:2, calcined at 1500℃ for 6h, crushed and ball-milled to obtain Ti3SiC2nanoparticles with a particle size of 0.1-0.2nm, the Ti3SiC2nanoparticles are mixed with a 10wt% polyvinyl alcohol aqueous solution to form a coating liquid, the mass concentration of Ti3SiC2nanoparticles in the coating liquid is 4wt%, the primary coated particles are mixed uniformly in the coating liquid, the mass ratio of the primary coated particles to the coating liquid is 1:1, spray drying is performed to form secondary coated particles, and sintering is performed at 750℃ for 3h in an inert atmosphere, to obtain lithium iron phosphate positive electrode material with a particle size of 2.3-2.5μm.

[0064] Example 6

[0065] The difference from Example 4 is that the mass concentration of Ti3SiC2nanoparticles in the coating liquid is 8wt%.

[0066] The steps for preparing the positive electrode material in this example are as follows:

[0067] S1, synthesis of lithium iron phosphate particles: lithium carbonate, iron oxalate and ammonium dihydrogen phosphate are weighed according to the molar ratio Li:Fe:PO4=1:1:1, ball-milled and mixed, calcined at a high temperature of 650℃ for 12h in a nitrogen atmosphere, to obtain lithium iron phosphate particles, which are crushed and sieved to control the particle size to be 1-2μm.

[0068] S2, inner layer coating: tetrabutyl titanate, magnesium nitrate and melamine were dissolved in ethanol according to the molar ratio of Ti: Mg: N = 0.7: 1: 3, the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol was 4:5, and a sol was formed by ultrasonic dispersion. Lithium iron phosphate particles were added to the sol, the mass ratio of lithium iron phosphate particles to the sol was 1:1, and the mixture was uniformly mixed. The coated particles were formed by spray drying to obtain primary coated particles.

[0069] S3, outer layer coating: titanium hydride powder, silicon powder and titanium carbide powder were weighed according to the molar ratio of TiH2: Si: TiC = 1: 1: 2, calcined at 1500℃ for 6h, and then crushed and ball milled to obtain Ti3SiC2nanoparticles with a particle size of 0.1-0.2nm. The Ti3SiC2nanoparticles were mixed with a 10wt% polyvinyl alcohol aqueous solution to form a coating liquid, the mass concentration of Ti3SiC2nanoparticles in the coating liquid was 8wt%, the primary coated particles were mixed uniformly in the coating liquid, the mass ratio of the primary coated particles to the coating liquid was 1:1, and the secondary coated particles were formed by spray drying. The lithium iron phosphate positive electrode material with a particle size of 2.3-2.5μm was obtained by sintering at 750℃ for 3h in an inert atmosphere.

[0070] Example 7

[0071] The difference from Example 4 is that the mass concentration of Ti3SiC2nanoparticles in the coating liquid is 10wt%.

[0072] The steps for preparing the positive electrode material in this example are as follows:

[0073] S1, synthesis of lithium iron phosphate particles: lithium carbonate, iron oxalate and ammonium dihydrogen phosphate were weighed according to the molar ratio of Li: Fe: PO4= 1: 1: 1, ball milled and mixed, and calcined at 650℃ for 12h in a nitrogen atmosphere to obtain lithium iron phosphate particles. The particles were crushed and sieved to control the particle size to 1-2μm.

[0074] S2, inner layer coating: tetrabutyl titanate, magnesium nitrate and melamine were dissolved in ethanol according to the molar ratio of Ti: Mg: N = 0.7: 1: 3, the mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine to ethanol was 4:5, and a sol was formed by ultrasonic dispersion. Lithium iron phosphate particles were added to the sol, the mass ratio of lithium iron phosphate particles to the sol was 1:1, and the mixture was uniformly mixed. The coated particles were formed by spray drying to obtain primary coated particles.

[0075] S3, outer layer coating: titanium hydride powder, silicon powder and titanium carbide powder were weighed according to the molar ratio of TiH2: Si: TiC = 1: 1: 2, calcined at 1500°C for 6h, crushed and ball milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, the Ti3SiC2 nanoparticles and a polyvinyl alcohol aqueous solution with a concentration of 10wt% were mixed to form a coating liquid, the mass concentration of Ti3SiC2 nanoparticles in the coating liquid was 10wt%, the primary coated particles were mixed uniformly in the coating liquid, the mass ratio of the primary coated particles to the coating liquid was 1:1, spray drying formed secondary coated particles, sintered at 750°C for 3h under inert atmosphere to obtain lithium iron phosphate positive electrode material with a particle size of 2.3-2.5μm.

[0076] Comparative Example 1

[0077] The difference from Example 1 is that no magnesium nitrate is added in the inner layer coating of the present comparative example.

[0078] The steps for preparing the positive electrode material are as follows:

[0079] S1, synthesis of lithium iron phosphate particles: lithium carbonate, iron oxalate and ammonium dihydrogen phosphate were weighed according to the molar ratio of Li: Fe: PO4 = 1: 1: 1, ball milled and mixed, calcined at high temperature of 650°C for 12h under nitrogen atmosphere to obtain lithium iron phosphate particles, crushed and sieved to control the particle size of the particles to be 1-2μm.

[0080] S2, inner layer coating: tetrabutyl titanate and melamine were dissolved in ethanol according to the molar ratio of Ti: N = 0.5: 3, the mass ratio of tetrabutyl titanate and melamine to ethanol was 2:5, ultrasonic dispersion to form a sol, lithium iron phosphate particles were added to the sol, the mass ratio of lithium iron phosphate particles to the sol was 1:1, mixed uniformly, spray drying to form coated particles to obtain primary coated particles.

[0081] S3, outer layer coating: titanium hydride powder, silicon powder and titanium carbide powder were weighed according to the molar ratio of TiH2: Si: TiC = 1: 1: 2, calcined at 1500°C for 6h, crushed and ball milled to obtain Ti3SiC2 nanoparticles with a particle size of 0.1-0.2nm, the Ti3SiC2 nanoparticles and a polyvinyl alcohol aqueous solution with a concentration of 10wt% were mixed to form a coating liquid, the mass concentration of Ti3SiC2 nanoparticles in the coating liquid was 4wt%, the primary coated particles were mixed uniformly in the coating liquid, the mass ratio of the primary coated particles to the coating liquid was 1:1, spray drying formed secondary coated particles, sintered at 750°C for 3h under inert atmosphere to obtain lithium iron phosphate positive electrode material with a particle size of 2-2.2μm.

[0082] Comparative Example 2

[0083] The difference from Example 1 is that no Ti3SiC2 is added in the outer layer coating of the present comparative example.

[0084] The steps of preparing the positive electrode material are as follows:

[0085] S1, synthesis of lithium iron phosphate particles: lithium carbonate, iron oxalate and ammonium dihydrogen phosphate are weighed according to the molar ratio Li:Fe:PO4=1:1:1, ball-milled and mixed, calcined at 650℃ in a nitrogen atmosphere for 12h, to obtain lithium iron phosphate particles, which are crushed and sieved to control the particle size to be 1-2μm.

[0086] S2, inner layer coating: tetrabutyl titanate, magnesium nitrate and melamine are dissolved in ethanol according to the molar ratio Ti:Mg:N=0.5:1:3, the mass ratio 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, the mass ratio of lithium iron phosphate particles to sol is 1:1, mixed uniformly, and spray drying is performed to form coated particles, to obtain primary coated particles.

[0087] S3, outer layer coating: a polyvinyl alcohol aqueous solution with a concentration of 10wt% is prepared to form a coating liquid, the primary coated particles are mixed uniformly in the coating liquid, the mass ratio of primary coated particles to coating liquid is 1:1, spray drying is performed to form secondary coated particles, and sintering is performed at 750℃ in an inert atmosphere for 3h, to obtain lithium iron phosphate positive electrode material with a particle size of 2-2.2μm.

[0088] The lithium iron phosphate positive electrode materials obtained in Examples 1-7 and Comparative Examples 1-2 are assembled into batteries, lithium iron phosphate positive electrode material, PVDF and carbon black with a mass ratio of 90:6:4 are weighed, then an appropriate amount of NMP solvent is added for dispersion, uniformly coated on an aluminum foil, dried in an oven at 100℃ for 8h, rolled and cut into positive electrode sheets, a metal lithium sheet is used as the negative electrode in a vacuum glove box, a composite film of PE and PP is used as the separator, LiPF6 is used as the electrolyte solute with a concentration of 1mol / L, ethylene carbonate (EC) and dimethyl carbonate (DMC) are used as the electrolyte solvent with a volume ratio of 1:1, to assemble CR2430 type button cells.

[0089] The interface impedance and rate performance of the assembled button cells are tested, and the results are shown in Table 1.

[0090] Table 1

[0091]

[0092] As can be seen from Table 1, the lithium iron phosphate positive electrode material provided in the embodiments has low interface impedance, and the capacity retention rate after 1C cycle for 1000 times is up to 90.3%, the performance is excellent, and the stability is good. The ratio of Ti, Mg and N in the inner layer coating is optimized in Example 1-Example 3, and the ability to inhibit the side reaction after the electrolyte penetrates is enhanced, and the interface impedance is reduced when the Ti content increases. In Example 2, the cycle capacity retention rate reaches 89.4%. In Example 4 and Example 5, the concentration of the sol gradually increases during the inner layer coating, and when the concentration increases, the inner layer coating is more dense, and at this time the interface impedance is reduced, but when the coating is too dense, the mass ratio of the coating layer in the positive electrode material increases, which reduces the content of the active component and slightly reduces the specific capacity of the battery. In Example 6 and Example 7, the parameters of the outer layer coating are further optimized on the basis of Example 4, and the greater the concentration of Ti3SiC2 nanoparticles, the better the conductivity and corrosion resistance, and the lower the interface impedance. According to the results of the comparative examples and the examples, in the case where magnesium is not doped in the inner layer, on the one hand, the lithium ion migration rate is reduced, and on the other hand, the interface bonding force of the inner and outer layers is weakened, the interface stability is reduced, and the electrochemical performance is reduced. The outer layer coating does not doped Ti3SiC2, only carbon coating, the corrosion resistance of the positive electrode material is reduced, the interface impedance is high, and the cycle performance is poor.

[0093] It should be noted that the relational terms herein such as first and second, and the like are used only to differentiate one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In addition, the terms "comprise", "contain" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or equipment.

[0094] Although the embodiments of the present application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to the embodiments without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A low interfacial impedance lithium iron phosphate cathode material, characterized in that, 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 Ti3SiC2 and a polyvinyl alcohol solution and then carbonizing the mixture; The preparation method of the low-interface-resistance lithium iron phosphate positive electrode material comprises the following steps: S1, synthesizing lithium iron phosphate particles: weighing lithium source, iron source and phosphorus source, ball milling, calcining at 650-750 DEG C in an inert atmosphere for 8-12 hours to obtain lithium iron phosphate particles; S2, inner layer coating: dissolving tetrabutyl titanate, magnesium nitrate and melamine in ethanol according to a molar ratio of (0.5-1):1:(2-5), ultrasonic dispersion to form a sol, adding lithium iron phosphate particles into the sol, mixing uniformly, and then spray drying to form coated particles to obtain primary coated particles; S3, outer layer coating: weighing titanium hydride powder, silicon powder and titanium carbide powder according to a molar ratio of TiH2:Si:TiC=1:1:2, calcining at 1500-1600 DEG C for 3-6 hours, crushing and ball milling to obtain Ti3SiC2 nanoparticles, mixing the Ti3SiC2 nanoparticles and a polyvinyl alcohol aqueous solution to form a coating liquid, mixing the primary coated particles with the coating liquid uniformly, spray drying to form secondary coated particles, and sintering at 700-750 DEG C in an inert atmosphere for 3-6 hours to obtain the lithium iron phosphate positive electrode material; The mass ratio of the total mass of tetrabutyl titanate, magnesium nitrate and melamine in the sol to ethanol is 2-5:5; the mass ratio of the lithium iron phosphate particles to the sol is 0.3-1:

1.

2. The low interfacial impedance lithium iron phosphate cathode material of claim 1, wherein, 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 iron oxalate, iron nitrate and iron citrate; The phosphorus source is at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

3. The low interfacial impedance lithium iron phosphate cathode material of claim 1, wherein, The particle size of the lithium iron phosphate particles is 1-2 μm.

4. The low interfacial impedance lithium iron phosphate cathode material of claim 1, wherein, The particle size of the Ti3SiC2 nanoparticles is 0.1-0.2 nm.

5. The low interfacial impedance lithium iron phosphate cathode material of claim 1, wherein, The concentration of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 10-20 wt%.

6. The low interfacial impedance lithium iron phosphate cathode material of claim 1, wherein, The mass concentration of Ti3SiC2 nanoparticles in the coating liquid is 4-10 wt%; The mass ratio of the primary coated particles to the coating liquid is 0.3-1:

1.

7. The low interfacial impedance lithium iron phosphate cathode material of claim 1, wherein, The particle size of the lithium iron phosphate positive electrode material is 2-2.5 μm.

8. 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 low-interface-resistance lithium iron phosphate positive electrode material according to claim 1.

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