Ultralow-temperature lithium iron phosphate battery and preparation method of positive active material of ultralow-temperature lithium iron phosphate battery

Abstract

The invention relates to an ultralow-temperature lithium iron phosphate battery and a preparation method of a positive active material of the ultralow-temperature lithium iron phosphate battery. Belongs to the technical field of lithium ion batteries. The positive electrode active material with low-temperature stability and electrochemical performance is adopted and matched with the electrolyte containing the LiTFSI and LiPF6 composite system, so that the stable electrochemical performance in a low-temperature scene is realized; according to the positive active material, the surface of the second particle is coated with a graphene-sulfide electrolyte composite layer to form a three-dimensional conductive network by mixing the first particle and the second particle and cooperating with a multi-stage composite coating technology, and the outermost layer of the second particle is coated with a lithium metal oxide, so that the problem that the low-temperature performance of the lithium iron phosphate battery is severely degraded is solved; the LiTFSI and LiPF6 composite system electrolyte salt is adopted in the electrolyte, and the LiTFSI with high thermal stability and good low-temperature dissociation performance is utilized to cooperate with the LiPF6 with a film forming characteristic to compensate the interface inertia of the LiTFSI, so that the low-temperature conductivity and interface stability of the electrolyte are improved, and the low-temperature performance of the battery is greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to an ultra-low temperature lithium iron phosphate battery and a method for preparing a positive electrode active material thereof. Background Art

[0002] Lithium iron phosphate batteries are widely used in the field of new energy vehicle-grade energy storage due to their high safety, long cycle life, and low cost. However, in low-temperature scenarios below -20°C, their performance is severely attenuated, becoming a bottleneck restricting their application. Studies have shown that lithium iron phosphate batteries are hindered in electron and ion transmission at low temperatures, and the electrolyte decomposes at low temperatures to form LiF and Li2CO3 passivation layers, which increase the interface impedance. The Fe in the active material 2+ Due to dissolution and other reasons, the discharge capacity will drop below 60% at room temperature, and the internal resistance will increase significantly. The existing technology improves low-temperature performance through carbon coating, metal doping or electrolyte modification, but there are still limitations. For example, China's invention patent CN119009070A proposes to improve conductivity by niobium, molybdenum doping and secondary carbon coating, but its single carbon coating layer cannot simultaneously solve the problems of ion transport and interface stability, and after mixing large and small particles, although the compaction density can be improved, the ionic conductivity and electronic conductivity of small particle materials under low temperature conditions are quite different from those of large particles, resulting in a large difference in volume change of the material system during low-temperature charging and discharging, resulting in reduced battery cycle performance and serious performance degradation under low temperature conditions. Summary of the invention

[0003] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide an ultra-low temperature lithium iron phosphate battery and a method for preparing a positive electrode active material thereof, which improves the compaction density of the material while reducing the performance difference between the first particles and the second particles by mixing the first particles and the second particles and coordinating the multi-stage composite coating technology, thereby solving the problem of serious low-temperature performance attenuation of lithium iron phosphate batteries in the prior art.

[0004] In order to solve the above technical problems, the present invention is implemented through the following technical solutions.

[0005] A first aspect of the present invention provides an ultra-low temperature lithium iron phosphate battery, comprising a positive electrode plate, wherein the positive electrode plate comprises a positive electrode active material, wherein the positive electrode active material comprises a first particle and a second particle, wherein the surface of the first particle is coated with a carbon material, and the surface of the second particle is sequentially coated with a graphene-sulfide electrolyte composite layer and a lithium metal oxide coating layer, wherein the iron position of the second particle is doped with one or more divalent metal ions, the D50 of the first particle is less than the D50 of the second particle, and the mass ratio of the first particle to the second particle is 3:7-1:9.

[0006] Furthermore, the graphene material is dot-coated on the surface of the second particle, and the sulfide electrolyte is distributed between the graphene oxides to form the graphene-sulfide electrolyte composite layer.

[0007] By coating the graphene-sulfide electrolyte composite layer on the surface of the second particle, on the one hand, graphene serves as a two-dimensional conductive framework. After being combined with the sulfide electrolyte, a three-dimensional conductive network is formed. The graphene significantly improves the electron mobility of the material, and the sulfide electrolyte improves the ionic conductivity of the material. Moreover, the lattice intrinsic defects of the graphene can provide more active sites for the sulfide electrolyte, which can further enhance the ionic conductivity of the material, thereby forming an electron-ion co-transport channel, overcoming the problem of low ionic transport efficiency due to the large particle size of the second particle while increasing the electron conductivity. On the other hand, the sulfide electrolyte can form a stable LiF / Li2S interfacial passivation layer during charge and discharge, which can inhibit the dissolution of Fe 2+ dissolution and oxygen release, reduce side reactions under high or low temperature conditions, improve the interfacial stability of the material, and the sulfide electrolyte has a flexible lattice structure, which can adapt to the volume change of the cathode active material during charge and discharge and reduce the material expansion rate. In addition, the lithium metal oxide coated on the outermost layer of the second particle forms a continuous lithium ion channel with the sulfide electrolyte, significantly reducing the interfacial impedance. Moreover, the hydrophobicity of the lithium metal oxide and the graphene cooperate to reduce the hygroscopicity of the sulfide electrolyte and improve the interfacial reliability.

[0008] Furthermore, the graphene is dot-coated on the surface of the second particle, and the sulfide electrolyte and the lithium metal oxide are sequentially distributed between the graphene along the direction from the center to the outside of the second particle. The graphene penetrates the coating layer of the second particle, further enhancing the electronic conductivity of the material, and at the same time using its hydrophobicity to reduce the water absorption rate on the outer surface of the material.

[0009] Furthermore, the carbon content of the first particle accounts for 0.01%-5% of the total weight of the first particle, the graphene accounts for 0.1%-5% of the total weight of the second particle, the sulfide electrolyte accounts for 0.1%-3% of the total weight of the second particle, and the lithium metal oxide accounts for 0.01%-3% of the total weight of the second particle.

[0010] Furthermore, the carbon content of the first particle accounts for 0.1%-1% of the total weight of the first particle, the graphene accounts for 0.1%-3% of the total weight of the second particle, the sulfide electrolyte accounts for 0.1%-1% of the total weight of the second particle, and the lithium metal oxide accounts for 0.01%-1% of the total weight of the second particle.

[0011] Further, the carbon content of the first particles accounts for 0.1%, 0.3%, 0.5%, 1% of the total weight of the first particles, the graphene accounts for 0.1%, 0.15%, 0.3%, 1%, 3% of the total weight of the second particles, the sulfide electrolyte accounts for 0.1%, 0.2%, 0.35%, 0.5%, 1% of the total weight of the second particles, and the lithium metal oxide accounts for 0.01%, 0.1%, 0.3%, 0.5%, 1% of the total weight of the second particles.

[0012] Further, the mass ratio of the graphene to the sulfide electrolyte is 1:1 - 5:1.

[0013] Further, the lithium metal oxide is one or more of LiAlO2, LiTiO2, Li2SiO3, LiMgO2, LiV3O8, LiNiO2.

[0014] Further, the difference between the D50 of the first particles and the D50 of the second particles is 0.3 μm - 1.2 μm, the D50 of the first particles < 0.5 μm, the D50 of the second particles is 0.5 μm - 1.8 μm, and the tap density of the positive electrode active material is 2.0 - 2.8 g / cm 3 . By controlling the particle size between the first particles and the second particles, the electrochemical property difference between the first particles and the second particles is reduced, and the activity of the positive electrode is improved.

[0015] Further, the difference between the D50 of the first particles and the D50 of the second particles is 0.3 μm - 1 μm. The consistency of the material is further improved, and the low-temperature electrochemical performance of the material is improved.

[0016] Further, the D50 of the first particles is 0.2 μm - 0.5 μm.

[0017] Further, the tap density of the positive electrode active material is 2.5 - 2.8 g / cm 3 .

[0018] Further, the divalent metal ion doped at the iron site in the second particles is Ca 2+ .

[0019] By doping Ca at the iron site of the second particles 2+ , the olivine structure can be stabilized, the volume expansion caused by phase change can be reduced, strong chemical bonds are formed by Ca - O, a fast channel for Li+ diffusion can be formed, and the Fe - O bond energy is weakened, the degree of electron delocalization increases, and the electronic conductivity of the material is improved.

[0020] Further, the positive electrode sheet includes a positive electrode current collector, a positive electrode active material, and a binder coated on one side of the positive electrode current collector. The chemical formula of the first particle is: LiFePO4@C; the chemical formula of the matrix of the second particle is: Li 1- x Ca x / 2 Fe 1-y Ca y PO4, where 0 ≤ x ≤ 0.05 and 0 < y ≤ 0.1. By doping a trace amount of Ca at the Li site 2+ an asymmetric olivine structure is formed to promote the rapid migration of lithium ions at low temperatures.

[0021] Further, the carbon material on the surface of the first particle is one or more of carbon nanotubes, graphene, and carbon black.

[0022] Further, it also includes a negative electrode sheet, an electrolyte, and a separator. The negative electrode sheet includes a negative electrode active material and a binder coated on the negative electrode current collector. The negative electrode active material includes graphite, silicon nanoparticles, and conductive carbon nanotubes. The electrolyte includes an electrolyte salt and a solvent.

[0023] Further, the electrolyte salt is a composite system of LiTFSI and LiPF6, and the molar ratio of LiTFSI to LiPF6 is 7:3 - 9:1.

[0024] Further, the electrolyte salt is a composite system of LiTFSI and LiPF6, and the molar ratio of LiTFSI to LiPF6 is 8:2.

[0025] Further, the concentration of the electrolyte salt in the electrolyte can be 1 mol / L - 1.8 mol / L.

[0026] Further, the solvent includes one or more of carboxylic ester solvents, nitrile solvents, and carbonate solvents.

[0027] Further, the carboxylic ester solvents are selected from one or more of ethyl acetate, methyl acetate, propyl acetate, and butyl acetate.

[0028] Further, the nitrile solvents include one or several of acetonitrile, monofluoroacetonitrile, difluoroacetonitrile, and trifluoroacetonitrile.

[0029] Further, the carbonate solvents include one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and butylene carbonate.

[0030] Furthermore, the electrolyte further includes an additive, and the additive is selected from one or more of a film-forming additive, a low-temperature film-forming agent, a corrosion inhibitor, and a conductive aid.

[0031] Furthermore, the separator is disposed between the positive electrode plate and the negative electrode plate.

[0032] Furthermore, the separator is selected from one or more of polyethylene, glass fiber, non-woven fabric, polypropylene, polyvinylidene fluoride, glass fiber, and non-woven fabric.

[0033] Furthermore, the thickness of the separator is 10 μm to 30 μm.

[0034] The present invention also provides a method for preparing a positive electrode active material, comprising the following steps: the positive electrode active material includes a first particle and a second particle; (1) The first particle preparation step includes: Mixing a lithium source, an iron source, a phosphorus source, a carbon source with a solvent, ball-milling into nanoparticles with a size of 15 nm to 250 nm, drying and then pulverizing to form a lithium iron phosphate precursor, sintering, cooling, pulverizing, and sieving to obtain the first particle; The second particle preparation step includes: a: Mixing a lithium source, an iron source, a phosphorus source, a calcium source, a dispersant, a graphene solution with a solvent, adjusting the pH, drying, and sintering to obtain a second particle precursor; b: Immersing the second particle precursor in a sulfide electrolyte solution, drying, and sintering to obtain a first sintered product of the second particle; c: Mixing the first sintered product of the second particle with a metal source and a lithium source, sintering, cooling, pulverizing, and sieving to obtain the second particle.

[0035] Furthermore, the method for preparing a positive electrode active material includes: the positive electrode active material includes a first particle and a second particle; (1) The first particle preparation step includes: Mixing a lithium source, an iron source, a phosphorus source, a carbon source in a molar ratio with a solvent, ball-milling the mixture into nanoparticles with a size of 15 nm to 250 nm after mixing, then spray-drying and pulverizing to form a lithium iron phosphate precursor, sintering the lithium iron phosphate precursor at 500 °C to 800 °C for 6 h to 10 h, cooling, pulverizing, and sieving to obtain the first particle with D50 < 0.5 μm; (2) The second particle preparation step includes: a: Mixing a lithium source, an iron source, a phosphorus source, a calcium source, a dispersant, a graphene solution with a solvent, and adjusting the pH to alkaline, drying, and sintering at 500 °C to 800 °C for 6 h to 10 h to obtain a second particle precursor; b: Immerse the second particle precursor into a sulfide electrolyte solution, dry it, and keep it at 300 - 500 °C for 1 - 3 h to obtain the first sintered product of the second particles; c: Mix the first sintered product of the second particles with a metal source and a lithium source, keep it at 300 - 500 °C for 1 - 3 hours, then cool it to room temperature, pulverize and screen it to obtain the second particles with a D50 of 0.5 μm - 1.8 μm. Further, the lithium source is at least one of lithium hydroxide, lithium chloride, lithium nitrite, lithium nitrate, lithium carbonate, lithium acetate, lithium oxalate, lithium phosphate, lithium dihydrogen phosphate, and lithium hydrogen phosphate, and the phosphorus source is at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, iron phosphate, and lithium dihydrogen phosphate.

[0036] Further, the iron source is at least one of ferrous oxalate, iron phosphate, iron acetate, iron(III) oxide, and iron nitrate.

[0037] Further, the calcium source is at least one of calcium carbonate, calcium nitrate, calcium oxide, and calcium chloride, the dispersant is at least one of acetone, N,N - dimethylformamide, and N,N - dimethylacetamide, and the sulfide electrolyte includes at least one of Li6PS5Cl and Li3PS4.

[0038] Further, the metal source is one or more of Al2O3, TiO2, SiO2, MgO2, V2O5, and NiO.

[0039] The present invention has the following beneficial effects compared with the prior art: The cathode active material of the present invention adopts the mixture of the first particles and the second particles, and combines with the multi - level composite coating technology to improve the tap density of the material while reducing the performance difference between the first particles and the second particles, and solves the problem of serious attenuation of the low - temperature performance of lithium iron phosphate batteries in the prior art. By coating the graphene - sulfide electrolyte composite layer on the surface of the second particles, a three - dimensional conductive network is formed, and an electron - ion co - transport channel is formed. While improving the electronic conductivity, the problem that the ion transport efficiency of the second particles is low due to their large particle size is overcome; the sulfide electrolyte can form a stable LiF / Li2S interfacial passivation layer during charge and discharge, which can inhibit the dissolution of Fe 2+ and the release of oxygen, reduce side reactions under high - temperature or low - temperature conditions, improve the interface stability of the material, and the sulfide electrolyte has a flexible lattice structure, which can adapt to the volume change of the cathode active material during charge and discharge and reduce the material expansion rate. In addition, the lithium metal oxide coated on the outermost layer of the second particles forms a continuous lithium - ion channel with the sulfide electrolyte, which, combined with the hydrophobicity of the graphene, reduces the hygroscopicity of the sulfide electrolyte and improves the interface reliability.

[0040] In the electrolyte of the present invention, a composite system of LiTFSI and LiPF6 is used as the electrolyte salt. LiTFSI has the advantages of high thermal stability and good low-temperature dissociation performance. In cooperation with LiPF6, the film-forming property of LiPF6 is utilized to compensate for the interfacial inertness of LiTFSI, thereby improving the low-temperature conductivity and interfacial stability of the electrolyte.

[0041] The preparation method of the positive electrode active material of the present invention is simple and suitable for large-scale production. Description of the Drawings

[0042] Figure 1 It is the charge and discharge curve of the ultra-low temperature lithium iron phosphate battery of Example 1 of the present invention at -20°C.

[0043] Figure 2 It is the discharge curve of the ultra-low temperature lithium iron phosphate battery of Example 1 of the present invention at -40 and -45°C. Detailed Description of the Invention

[0044] To make the purpose, technical solutions and effects of the present invention clearer and more definite, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0045] Example 1 Preparation of the positive electrode active material: (1) Preparation of the first particles: Lithium carbonate, ferrous oxalate, and diammonium hydrogen phosphate are weighed according to the molar ratio of Li:Fe:P of 1:1:1. In addition, 5% of sucrose (the carbon content accounts for 0.2% of the weight of the finally obtained primary particles) based on the total mass of lithium carbonate, ferrous oxalate, and diammonium hydrogen phosphate is added and dispersed in absolute ethanol (the volume ratio of the above raw materials to absolute ethanol is 2:8). After mixing, it is ball-milled into nano-particles with an average particle size of 200 nm, and then spray-dried and pulverized to form a lithium iron phosphate precursor. The lithium iron phosphate precursor is heated to 700°C at a heating rate of 6°C / min and then held for 6 h, cooled, pulverized, and sieved to obtain the first particles with a D50 of 0.3 μm; (2) The preparation steps of the second particles include: a: Weigh lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate according to the molar ratio of Li:Ca:Fe:Ca:P of 0.95:0.025 (molar ratio of Ca in Li site): 0.9:0.1 (molar ratio of Ca in transition metal site):1. Additionally, add 8% graphene (graphene content accounts for 0.35% of the weight of the finally obtained primary particles) based on the total mass of lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate, and 2% N,N-dimethylformamide based on the total mass. Add them to anhydrous ethanol (the volume ratio of the above raw materials to anhydrous ethanol is 2:8), stir and disperse at 1000 rpm for 0.6 hours, adjust the pH to 12, dry, and then heat at a heating rate of 6 °C / min to 700 °C and hold for 6 h to obtain the second particle precursor; b: Immerse the second particle precursor in the Li6PS5Cl solution. The Li6PS5Cl accounts for 1% of the mass of the second particle precursor (Li6PS5Cl accounts for 0.15% of the mass of the finally obtained second particle), dry, heat at a heating rate of 5 °C / min to 300 °C and hold for 3 h, and cool to room temperature to obtain the second particle primary sintering product; c: Weigh Al2O3 according to 1% of the mass of the second particle primary sintering product, and lithium oxide accounting for 0.88% of the mass of the second particle primary sintering product (so that LiAlO2 accounts for 0.12% of the mass of the finally obtained second particle). Mix the two, heat at a heating rate of 5 °C / min to 300 °C and hold for 3 h, then cool to room temperature and perform crushing and screening to obtain the second particle with D50 of 1.0 μm. The matrix chemical formula of the obtained second particle is: Li 0.95 Ca 0.25 Fe 0.9 Ca 0.1 PO4.

[0046] Preparation of the positive electrode sheet: Mix the above first particles and second particles according to a mass ratio of 2:8 as the positive electrode active material. Mix the active material with the conductive agent carbon black (Super P) and the binder polyvinylidene fluoride (PVDF) according to a mass ratio of 100:3:1.5 in an appropriate solvent N-methylpyrrolidone (NMP) to obtain the positive electrode slurry.

[0047] Coat the positive electrode slurry on the positive electrode current collector aluminum foil, and perform processes such as drying, cold pressing, slitting, and cutting to obtain the positive electrode sheet.

[0048] Preparation of the negative electrode sheet: Mix artificial graphite, conductive agent carbon black (Super P), binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) evenly at a weight percentage of 96:1.0:1.5:1.5, and add deionized water. After stirring and dispersing, a negative electrode slurry is obtained. The negative electrode slurry is coated on a substrate copper foil, and after drying, cold pressing, slitting, and sheet making, a negative electrode sheet is obtained.

[0049] Separator Select a 10-μm-thick polyethylene separator.

[0050] Preparation of electrolyte: Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a mass ratio of 3:3:4 to obtain an organic solvent.

[0051] Dissolve vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), and fully dried LiTFSI, LiPF6 (molar ratio of LiTFSI:LiPF6 is 8:2) in the above organic solvent to prepare an electrolyte with a concentration of LiTFSI and LiPF6 of 1.2 mol / L. In the total mass of the electrolyte, the mass content of vinylene carbonate (VC) is 2.8 wt%, the mass content of fluoroethylene carbonate (FEC) is 1.2 wt%, and the mass content of ethylene sulfate (DTD) is 0.5 wt%.

[0052] Preparation of battery: Stack the positive electrode sheet, separator, and negative electrode sheet in sequence and obtain an electrode assembly by winding; place the electrode assembly in an outer package, inject the electrolyte after drying, and finally obtain a lithium-ion battery through processes such as vacuum packaging, standing, forming, and degassing. The injection coefficient of the electrolyte is 2.4 g / Ah.

[0053] Example 2 The difference from Example 1 is that the D50 of the first particle is 0.5 μm, and the D50 of the second particle is 1.0 μm.

[0054] Example 3 The difference from Example 1 is that the D50 of the first particle is 0.3 μm, and the D50 of the second particle is 1.5 μm.

[0055] Example 4 The difference from Example 1 is as follows: Preparation of the first particle: Add sucrose accounting for 8% of the total mass of lithium carbonate, ferrous oxalate, and diammonium hydrogen phosphate, and the carbon content of the obtained first particle accounts for 0.3% of the weight of the finally obtained primary particle.

[0056] Example 5 The difference from Example 1 lies in the preparation of the second particles: Another 10% of graphene based on the total mass of lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate was added, and finally the graphene content in the second particles accounted for 0.5% of the weight of the finally obtained primary particles.

[0057] Example 6 The difference from Example 1 lies in the preparation of the second particles: b: Immerse the precursor of the second particles in a Li6PS5Cl solution, where Li6PS5Cl accounts for 2% of the mass of the precursor of the second particles, and finally the Li6PS5Cl in the second particles accounts for 0.3% of the mass of the second particles.

[0058] Example 7 The difference from Example 1 lies in the preparation of the second particles: Weigh Al2O according to 2% of the mass of the first sintering product of the second particles 3， and weigh lithium oxide according to 1.76% of the mass of the first sintering product of the second particles. Finally, the LiAlO2 in the second particles accounts for 0.2% of the mass of the second particles.

[0059] Example 8 The difference from Example 1 lies in the preparation of the second particles: Weigh SiO2 according to 1% of the mass of the first sintering product of the second particles. Finally, the Li2SiO3 in the second particles accounts for 0.1% of the mass of the second particles.

[0060] Example 9 The difference from Example 1 lies in that the mass ratio of the first particles to the second particles is 3:7.

[0061] Example 10 The difference from Example 1 lies in that the mass ratio of the first particles to the second particles is 9:1.

[0062] Comparative Example 1 The difference from Example 1 lies in the preparation of the second particles: a: Mix lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate according to the molar ratio of Li:Ca:Fe:Ca:P of 0.95:0.025 (the molar ratio of Ca in the Li position): 0.9:0.1 (the molar ratio of Ca in the transition metal position): 1. Another 8% of graphene based on the total mass of lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate (the graphene content accounts for 0.35% of the weight of the finally obtained primary particles) and 2% of the total mass of N,N-dimethylformamide were added to 500 ml of absolute ethanol. The pH was adjusted to 12 by adding ammonia water, dried, and then heated to 700 °C at a heating rate of 6 °C / min and held for 6 h to obtain the precursor of the second particles; b: Heat the precursor of the second particles to 300 °C at a heating rate of 5 °C / min and hold for 3 h, and then cool to room temperature to obtain the first sintering product of the second particles; c: Heat the first sintering product of the second particles at a heating rate of 5 °C / min to 300 °C, hold for 3 h, then cool to room temperature, and then crush and screen to obtain the second particles with a D50 of 1.0 μm.

[0063] Comparative Example 2 The difference from Example 1 is as follows: Preparation of the second particles: a: Lithium carbonate, iron oxalate, diammonium hydrogen phosphate, and calcium carbonate are mixed in a molar ratio of Li:Ca:Fe:Ca:P of 0.95:0.025 (molar ratio of Ca in the Li site): 0.9:0.1 (molar ratio of Ca in the transition metal site): 1. Additionally, 8% graphene (the graphene content accounts for 0.35% of the weight of the finally obtained primary particles) based on the total mass of lithium carbonate, iron oxalate, diammonium hydrogen phosphate, and calcium carbonate and 2% N,N-dimethylformamide based on the total mass are added to 500 ml of absolute ethanol, and ammonia water is added to adjust the pH to 12, followed by drying. Then, it is heated to 700 °C at a heating rate of 6 °C / min and held for 6 h to obtain the second particle precursor. b: Immerse the second particle precursor in a Li6PS5Cl solution, where Li6PS5Cl accounts for 1% of the mass of the second particle precursor (Li6PS5Cl accounts for 0.15% of the mass of the finally obtained second particles), dry, heat to 300 °C at a heating rate of 5 °C / min, hold for 3 h, and cool to room temperature to obtain the first sintering product of the second particles. c: Heat the first sintering product of the second particles at a heating rate of 5 °C / min to 300 °C, hold for 3 h, then cool to room temperature, and then crush and screen to obtain the second particles with a D50 of 1.0 μm.

[0064] Comparative Example 3: The difference from Example 1 is that the electrolyte in the electrolyte is only LiPF6.

[0065] Comparative Example 4: The difference from Example 1 is that the electrolyte in the electrolyte is only LiTFSI.

[0066] Comparative Example 5: The difference from Example 1 is that only the second particles are prepared.

[0067] The detection of the carbon mass content is measured with reference to GB / T 33822-2017.

[0068] The conductivity and resistivity of the positive electrode active material powder are tested by the four-probe method.

[0069] After standard charging, the battery is placed in a room temperature environment for 24 h, and then discharged at a constant current of 1C to 1.8V, and the first-cycle discharge capacity per gram at room temperature is recorded.

[0070] After standard charging, the battery was placed in environments of -20°C, -40°C, and -45°C for 24 hours respectively, and then discharged at a constant current of 1C to 1.8V. Record the first-cycle discharge specific capacity at -20°C, -40°C, and -45°C. The first-discharge efficiency at (-20°C, -40°C, -45°C) = the first-cycle discharge specific capacity at (-20°C, -40°C, -45°C) / the first-cycle discharge specific capacity at room temperature × 100%.

[0071] Standard charging: First, charge at a constant current of 1C to 3.7V, and then charge at a constant voltage of 3.7V until the current drops to 0.05C.

[0072] The test results are shown in Table 1.

[0073] Table 1 Electrochemical performance of Examples 1-10 and Comparative Examples 1-5 <![CDATA[Room temperature first-cycle discharge capacity in mAh / g -1 > Initial discharge efficiency at room temperature (%) Capacity retention rate after 100 cycles at 1C rate at room temperature (%) Initial discharge efficiency at -20°C (%) Initial discharge efficiency at -40°C (%) Initial discharge efficiency at -45°C (%) Capacity retention rate after 100 cycles at 1C rate at -40°C (%) <![CDATA[Resistivity of the positive electrode active material powder (Ω·cm 2 )]]> Example 1 158.2 92.3 93.5 89.4 84.1 83.6 89.2 3.2 Example 2 156.8 91.8 92.8 88.5 83.4 81.8 88.5 3.8 Example 3 155.4 91.5 92.1 87.8 82.3 80.9 87.3 4.5 Example 4 157.1 92 93 88.9 84.5 82.0 88.8 3.5 Example 5 159.5 92.5 93.8 89.8 84.2 82.1 89.7 3.0 Example 6 158.9 92.2 93.4 89.3 84.5 83.0 89.1 3.3 Example 7 157.6 91.9 92.9 88.7 83.6 81.4 88.4 3.6 Example 8 158.3 92.1 93.2 88.9 83.4 81.6 88.9 3.4 Example 9 156.9 91.7 92.5 87.5 82.5 81.0 87.6 4.1 Example 10 155.7 91.4 91.8 86.9 81.5 78.9 86.9 4.7 Comparative Example 1 154.3 90.8 88.6 70.3 65.1 60.3 62.4 12.5 Comparative Example 2 153.8 90.5 87.9 69.1 62.1 59.3 61.1 14.8 Comparative Example 3 152.1 89.7 84.3 75.6 64.3 59.8 70.8 3.2 Comparative Example 4 151.9 89.4 83.7 80.9 70.9 62.8 60.3 3.2 Comparative Example 5 145.6 90.1 85.2 76.4 69.8 60.9 51.7 15.1 From the electrochemical performance data in Table 1, compared with Comparative Examples 1-5, the batteries prepared from the positive active material and electrolyte provided by Examples 1-10 have higher specific capacity, energy density, and cycle stability under low-temperature test conditions. In Example 1 of the present invention, the active material mixed with the first particle (coated with 0.2 wt% carbon, D50 = 0.3 μm) and the second particle (coated with 0.2 wt% graphene, 0.15 wt% Li6PS5Cl, 0.12% LiAlO2, D50 = 1 μm) can significantly reduce the interfacial resistance, and the first-discharge efficiency is higher than that of the simple carbon coating in Comparative Example 1. Especially when at -20°C, the first-discharge efficiency of Comparative Example 1 is only 62.4%; in Comparative Example 2, LiAlO2 was not coated on the surface of the second particle, resulting in the exposure of the sulfide electrolyte to the electrolyte, which intensified the interfacial reaction and interrupted the electron-ion transport channel, and the capacity dropped sharply by 28.4% at -40°C; in Comparative Example 3, only a single LiPF6 electrolyte was used, resulting in a sharp increase in the viscosity of the electrolyte and hindered ion migration at -20°C and below low temperatures, directly triggering capacity decay; in Comparative Example 4, only a single LiTFSI electrolyte was used, resulting in a weak self-film-forming ability at -20°C and below low temperatures, making it difficult to form a stable SEI film, resulting in serious attenuation of low-temperature cycle performance; in Comparative Example 5, only the second particle was used, resulting in a decrease in the utilization rate of the active material. At -40°C, the first-discharge efficiency at 1C decreased by 20.3%, the electrode structure was loose, the ion diffusion path was extended, and the generation of pores led to the inability to form a continuous electron transport channel, resulting in a decline in comprehensive performance. In summary, the present invention realizes the stability of electrochemical performance in low-temperature scenarios and greatly improves the low-temperature performance of lithium iron phosphate batteries by using a positive active material with low-temperature stability and electrochemical performance and cooperating with an electrolyte containing a LiTFSI and LiPF6 composite system.

[0074] The above specific implementation manners have specifically introduced the analysis method involved in the present invention. It should be noted that the above introduction is only to help those skilled in the art better understand the method and idea of the present invention, rather than a limitation on the relevant content. Without departing from the principle of the present invention, those skilled in the art can also make appropriate adjustments or modifications to the present invention, and the above adjustments and modifications should also fall within the protection scope of the present invention.

Claims

1. An ultra-low temperature lithium iron phosphate battery, characterized in that, It includes a positive electrode plate, the positive electrode plate includes a positive electrode active material, the positive electrode active material includes a first particle and a second particle, the surface of the first particle is coated with a carbon material, the surface of the second particle is sequentially coated with a graphene-sulfide electrolyte composite layer and a lithium metal oxide coating layer, one or more divalent metal ions are doped at the iron site in the second particle, the D50 of the first particle is less than the D50 of the second particle, and the mass ratio of the first particle to the second particle is 3:7 - 1:

9.

2. The lithium iron phosphate battery according to claim 1, wherein The graphene material is dot-coated on the surface of the second particle, and the sulfide electrolyte is distributed between the graphene oxides to form the graphene-sulfide electrolyte composite layer.

3. The lithium iron phosphate battery according to claim 1 or 2, characterized in that, The carbon content of the first particle accounts for 0.01% - 5% of the total weight of the first particle, the graphene accounts for 0.1% - 5% of the total weight of the second particle, the sulfide electrolyte accounts for 0.1% - 3% of the total weight of the second particle, and the lithium metal oxide accounts for 0.01% - 3% of the total weight of the second particle.

4. The lithium iron phosphate battery according to claim 1, wherein The thickness of the lithium metal oxide coating layer is 1 - 3 nm, and the lithium metal oxide is one or more of LiAlO2, LiTiO2, Li2SiO3, LiMgO2, LiV3O8, LiNiO2.

5. The lithium iron phosphate battery according to claim 1, wherein, The difference between the D50 of the first particle and the D50 of the second particle is 0.3 μm - 1.2 μm, and the D50 of the first particle < 0.5 μm, the D50 of the second particle is 0.5 μm - 1.8 μm, and the tap density of the positive electrode active material is 2.0 - 2.8 g / cm 3 .

6. The lithium iron phosphate battery according to claim 1, wherein The divalent metal ion doped with iron in the second particle is Ca 2+ .

7. The lithium iron phosphate battery according to claim 1, characterized in that, The positive electrode plate includes a positive current collector, a positive active material and a binder coated on one side of the positive current collector. The chemical formula of the first particle is: LiFePO4@C; the chemical formula of the matrix of the second particle is: Li 1-x Ca x / 2 Fe 1-y Ca y PO4, where 0 ≤ x ≤ 0.05 and 0 < y ≤ 0.

1.

8. The lithium iron phosphate battery according to claim 1, characterized in that, It also includes a negative electrode plate, an electrolyte, and a separator. The negative electrode plate includes a negative electrode active material and a binder coated on a negative electrode current collector. The negative electrode active material includes graphite, silicon nanoparticles, and conductive carbon nanotubes. The electrolyte includes an electrolyte salt and a solvent.

9. The lithium iron phosphate battery according to claim 8, wherein the electrolyte salt is a composite system of LiTFSI and LiPF6, and the molar ratio of LiTFSI to LiPF6 is 7:3 - 9:

1.

10. A method for preparing a positive electrode active material, characterized in that, It includes the following steps: The positive electrode active material includes a first particle and a second particle; (1) The preparation step of the first particle includes: Mix a lithium source, an iron source, a phosphorus source, a carbon source, and a solvent, ball mill them into nanoparticles with a size of 15 nm to 250 nm, dry and then crush to form a lithium iron phosphate precursor, and sinter, cool, crush, and screen to obtain the first particle; The preparation step of the second particle includes: a: Mix a lithium source, an iron source, a phosphorus source, a calcium source, a dispersant, a graphene solution, and a solvent, adjust the pH, dry, and sinter to obtain a second particle precursor; b: Immerse the second particle precursor in a sulfide electrolyte solution, dry, and sinter to obtain a first sintered product of the second particle; c: Mix the first sintered product of the second particle with a metal source and a lithium source, sinter, cool, crush, and screen to obtain the second particle.

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