Ultra-low temperature lithium iron phosphate battery and method for preparing positive electrode active material thereof

Abstract

The present invention relates to an ultra-low temperature lithium iron phosphate battery and a method for preparing a positive electrode active material thereof; it belongs to the technical field of lithium ion batteries. The present invention adopts a positive electrode active material with low temperature stability and electrochemical performance, and cooperates with an electrolyte containing a LiTFSI and LiPF6 composite system to achieve stable electrochemical performance in low temperature scenarios; the positive electrode active material adopts a first particle and a second particle mixture, and cooperates with a multi-stage composite coating technology to coat a graphene-sulfide electrolyte composite layer on the surface of the second particle to form a three-dimensional conductive network, and coats the outermost layer of the second particle with lithium metal oxide, thereby solving the problem of severe low temperature performance attenuation of the lithium iron phosphate battery; LiTFSI and LiPF6 composite system electrolyte salt are used in the electrolyte, and LiTFSI with high thermal stability and good low temperature dissociation is used to cooperate with LiPF6 with film-forming properties to compensate for the interfacial inertness of LiTFSI, thereby improving the low temperature conductivity and interfacial stability of the electrolyte and significantly improving the low temperature performance of the battery.

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 advantages such as high safety, long cycle life and low cost. However, in low temperature scenarios below -20℃, their performance is severely degraded, 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 to 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, the Chinese 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 it uses large and small particles to mix, 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. By mixing the first particles and the second particles and coordinating the multi-stage composite coating technology, the compaction density of the material is increased while reducing the performance difference between the first particles and the second particles, thereby solving the problem of serious low-temperature performance degradation of lithium iron phosphate batteries in the prior art.

[0004] In order to solve the above technical problems, the present invention is achieved 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 in 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 coated on the surface of the second particle in a dotted manner, and the sulfide electrolyte is distributed between the graphene oxide to form the graphene-sulfide electrolyte composite layer.

[0007] By coating the surface of the second particle with a graphene-sulfide electrolyte composite layer, on the one hand, graphene acts as a two-dimensional conductive skeleton and forms a three-dimensional conductive network after being composited with the sulfide electrolyte. The graphene significantly improves the electron mobility of the material, and the sulfide electrolyte improves the ionic conductivity of the material. Moreover, the lattice defects of the graphene can provide more active sites for the sulfide electrolyte, which can further improve the ionic conductivity of the material, thereby forming an electron-ion cooperative transmission channel, improving the electronic conductivity while overcoming the problem of low ion transmission efficiency of the second particle due to its large particle size; on the other hand, the sulfide electrolyte can form a stable LiF / Li2S interface passivation layer during the charge and discharge process, which can inhibit the Fe 2+ The sulfide electrolyte has a flexible lattice structure that adapts to the volume changes of the positive electrode active material during charge and discharge, reducing the material expansion rate. Furthermore, the lithium metal oxide coating the outermost layer of the second particle forms a continuous lithium ion channel with the sulfide electrolyte, significantly reducing interfacial impedance. The hydrophobicity of the lithium metal oxide and the graphene synergizes to reduce the hygroscopicity of the sulfide electrolyte and improve 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 layers from the center of the second particle outward. The graphene penetrates the second particle coating, further improving the material's electronic conductivity while leveraging its hydrophobicity to reduce water absorption on the material's outer surface.

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

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

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

[0014] Furthermore, the difference between the D50 of the first particle and the D50 of the second particle is 0.3 μm-1.2 μm, the D50 of the first particle is less than 0.5 μm, the D50 of the second particle is 0.5 μm-1.8 μm, and the compaction 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 difference in electrochemical properties between the first particles and the second particles is reduced, thereby improving the positive electrode activity.

[0015] Furthermore, the difference between the first particle D50 and the second particle D50 is 0.3 μm-1 μm, which further improves the consistency of the material and the low-temperature electrochemical performance of the material.

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

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

[0018] Furthermore, the doped divalent metal ion in the iron position of the second particle is Ca 2+ .

[0019] By doping Ca in the iron site of the second particle 2+ It can stabilize the olivine structure, reduce the volume expansion caused by phase change, form a strong chemical bond between Ca and O, form a fast channel for Li+ diffusion, weaken the Fe-O bond energy, increase the degree of electron delocalization, and improve the electronic conductivity of the material.

[0020] Furthermore, the positive electrode sheet includes a positive electrode current collector, a positive electrode active material coated on one side of the positive electrode current collector, and a binder. 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, 0≤x≤0.05, 0<y≤0.1. By doping a trace amount of Ca at the Li position 2+ An asymmetric olivine structure is formed, which promotes the rapid migration of lithium ions at low temperatures.

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

[0022] Furthermore, 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 the negative electrode collector. The negative electrode active material includes graphite, silicon nanoparticles, and conductive carbon nanotubes. The electrolyte includes an electrolyte salt and a solvent.

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

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

[0025] Furthermore, the concentration of the electrolyte salt in the electrolyte solution may be 1 mol / L to 1.8 mol / L.

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

[0027] Furthermore, the carboxylate solvent is selected from one or more of ethyl acetate, methyl acetate, propyl acetate, and butyl acetate.

[0028] Furthermore, the nitrile solvent includes one or more of acetonitrile, monofluoroacetonitrile, difluoroacetonitrile, and trifluoroacetonitrile.

[0029] Furthermore, the carbonate solvent includes 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 solution further comprises additives, and the additives are selected from one or more of film-forming additives, low-temperature film-forming agents, corrosion inhibitors, and conductive additives.

[0031] Furthermore, the separator is arranged between the positive electrode sheet and the negative electrode sheet.

[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 isolation film 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 comprises a first particle and a second particle;

[0035] (1) The first particle preparation step includes:

[0036] The lithium source, iron source, phosphorus source, carbon source and solvent are mixed, ball-milled into nanoparticles of 15nm to 250nm, dried and crushed to form a lithium iron phosphate precursor, sintered, cooled, crushed and sieved to obtain first particles;

[0037] The second particle preparation step includes:

[0038] a) mixing a lithium source, an iron source, a phosphorus source, a calcium source, a dispersant, a graphene solution and a solvent, adjusting the pH, drying and sintering to obtain a second particle precursor;

[0039] b: The second particle precursor is immersed in a sulfide electrolyte solution, dried, and sintered to obtain a second particle primary sintering product;

[0040] c: The primary sintered product of the second particles is mixed with a metal source and a lithium source, sintered, cooled, crushed, and sieved to obtain the second particles.

[0041] Furthermore, a method for preparing a positive electrode active material includes: the positive electrode active material includes a first particle and a second particle;

[0042] (1) The first particle preparation step includes:

[0043] A lithium source, an iron source, a phosphorus source, and a carbon source are mixed with a solvent in a molar ratio, ball-milled to form nanoparticles of 15 nm to 250 nm, spray-dried, and then crushed to form a lithium iron phosphate precursor, and the lithium iron phosphate precursor is sintered at 500° C. to 800° C. for 6 h to 10 h, cooled, crushed, and sieved to obtain the first particles with D50 less than 0.5 μm;

[0044] (2) The second particle preparation step includes:

[0045] a: mixing a lithium source, an iron source, a phosphorus source, a calcium source, a dispersant, a graphene solution and a solvent, 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;

[0046] b: immersing the second particle precursor in a sulfide electrolyte solution, drying, and maintaining the temperature at 300-500° C. for 1-3 hours to obtain a second particle primary sintering product;

[0047] c. Mixing the primary sintered product of the second particles with a metal source and a lithium source, maintaining the mixture at 300-500°C for 1-3 hours, then cooling to room temperature, crushing, and sieving to obtain the second particles having a D50 of 0.5 μm to 1.8 μm. Furthermore, 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 dilithium 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.

[0048] Furthermore, the iron source is at least one of ferrous oxalate, ferric phosphate, ferric acetate, ferric oxide and ferric nitrate.

[0049] Furthermore, 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.

[0050] Furthermore, the metal source is one or more of Al2O3, TiO2, SiO2, MgO2, V2O5, and NiO.

[0051] The present invention has the following beneficial effects compared to the prior art:

[0052] The positive active material of the present invention adopts a mixed first particle and a second particle, and cooperates with a multi-stage composite coating technology to improve the material compaction density while reducing the performance difference between the first particle and the second particle, thereby solving the problem of serious low-temperature performance degradation of lithium iron phosphate batteries in the prior art. By coating the surface of the second particle with the graphene-sulfide electrolyte composite layer, a three-dimensional conductive network is formed to form an electron-ion cooperative transmission channel, thereby improving the electronic conductivity while overcoming the problem of low ion transmission efficiency of the second particle due to its large particle size; the sulfide electrolyte can form a stable LiF / Li2S interface passivation layer during the charge and discharge process, which can inhibit Fe 2+This reduces dissolution and oxygen release, reduces side reactions under high or low temperature conditions, and improves material interface stability. The sulfide electrolyte also possesses a flexible lattice structure that adapts to the volume changes of the positive electrode active material during charge and discharge, reducing material expansion. Furthermore, the lithium metal oxide coating the outermost layer of the second particle forms a continuous lithium ion channel with the sulfide electrolyte. This, in conjunction with the hydrophobicity of the graphene, reduces the hygroscopicity of the sulfide electrolyte and improves interface reliability.

[0053] The electrolyte of the present invention uses an electrolyte salt of a composite system of LiTFSI and LiPF6. The LiTFSI has the advantages of high thermal stability and good low-temperature dissociation. It cooperates with the LiPF6 and utilizes the film-forming properties of the LiPF6 to compensate for the interfacial inertness of the LiTFSI, thereby improving the low-temperature conductivity and interfacial stability of the electrolyte.

[0054] The preparation method of the positive electrode active material of the present invention is simple and suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 This 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.

[0056] Figure 2 This 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

[0057] In order to make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the following examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0058] Example 1

[0059] Preparation of positive electrode active materials:

[0060] (1) Preparation of the first particle:

[0061] Lithium carbonate, ferrous oxalate, and diammonium hydrogen phosphate were weighed at a Li:Fe:P molar ratio of 1:1:1, and sucrose (carbon content accounted for 0.2% of the weight of the final primary particles) accounting for 5% of the total mass of the lithium carbonate, ferrous oxalate, and diammonium hydrogen phosphate was added and dispersed in anhydrous ethanol (the volume ratio of the total volume of the above raw materials to anhydrous ethanol was 2:8). After mixing, the mixture was ball-milled into nanoparticles 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 was heated to 700°C at a heating rate of 6°C / min and then kept at this temperature for 6 hours. It was cooled, pulverized, and sieved to obtain first particles with a D50 of 0.3 μm.

[0062] (2) The second particle preparation step includes:

[0063] a: lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate were weighed according to a molar ratio of Li:Ca:Fe:Ca:P of 0.95:0.025 (molar ratio of Ca at the Li position): 0.9:0.1 (molar ratio of Ca at the transition metal position): 1, and 8% graphene (graphene content accounts for 0.35% of the weight of the final primary particles) and 2% N,N-dimethylformamide (the volume ratio of the total volume of the above raw materials to anhydrous ethanol is 2:8) were added to anhydrous ethanol at 1000 rpm for 0.6 hours, and the pH was adjusted to 12. The mixture was dried, and then heated to 700°C at a heating rate of 6°C / min and kept warm for 6 hours to obtain a second particle precursor;

[0064] b: Immersing the second particle precursor in a Li6PS5Cl solution, wherein the Li6PS5Cl accounts for 1% of the mass of the second particle precursor (Li6PS5Cl accounts for 0.15% of the mass of the final second particle), drying, heating to 300°C at a heating rate of 5°C / min, holding for 3 hours, and cooling to room temperature to obtain a second particle primary sintering product;

[0065] c: Al2O3 is weighed at 1% of the mass of the first sintered product of the second particle, and lithium oxide is weighed at 0.88% of the mass of the first sintered product of the second particle (so that LiAlO2 accounts for 0.12% of the mass of the final second particle). The two are mixed, heated to 300°C at a heating rate of 5°C / min and kept at this temperature for 3 hours. The mixture is then cooled to room temperature and crushed and sieved to obtain the second particle with a 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.

[0066] Positive electrode preparation:

[0067] The first particles and the second particles are mixed at a mass ratio of 2:8 to form a positive electrode active material. The active material is mixed with a conductive agent, carbon black (Super P), and a binder, polyvinylidene fluoride (PVDF), at a mass ratio of 100:3:1.5 in an appropriate amount of solvent, N-methylpyrrolidone (NMP), to obtain a positive electrode slurry.

[0068] The positive electrode slurry is coated on the positive electrode current collector aluminum foil, and then dried, cold pressed, slit, cut and other processes are carried out to obtain the positive electrode sheet.

[0069] Preparation of negative electrode sheet:

[0070] Artificial graphite, conductive carbon black (Super P), binder styrene-butadiene rubber (SBR) and thickener sodium carboxymethyl cellulose (CMC) are mixed evenly in a weight percentage of 96:1.0:1.5:1.5 and deionized water is added. After stirring and dispersion, the negative electrode slurry is obtained. The negative electrode slurry is coated on the base copper foil, and the negative electrode sheet is obtained after drying, cold pressing, slitting and sheeting.

[0071] diaphragm

[0072] A polyethylene diaphragm with a thickness of 10 μm was selected.

[0073] Preparation of electrolyte:

[0074] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl methyl carbonate (DMC) are mixed in a mass ratio of 3:3:4 to obtain an organic solvent.

[0075] Vinylene carbonate (VC), fluoroethylene carbonate (FEC), and diethylene sulfate (DTD), along with fully dried LiTFSI and LiPF6 (LiTFSI:LiPF6 molar ratio of 8:2), were dissolved in the aforementioned organic solvent to prepare an electrolyte solution with a LiTFSI and LiPF6 concentration of 1.2 mol / L. Of the total mass of the electrolyte, the mass content of vinylene carbonate (VC), the mass content of fluoroethylene carbonate (FEC), and the mass content of diethylene sulfate (DTD) was 2.8 wt%, 1.2 wt%, and 0.5 wt%.

[0076] Preparation of the battery:

[0077] The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence and wound to form an electrode assembly. The electrode assembly is placed in an outer packaging, dried, and then injected with electrolyte. After vacuum packaging, static standing, formation, and exhaust, the lithium-ion battery is finally completed. The electrolyte injection coefficient is 2.4g / Ah.

[0078] Example 2

[0079] The difference from Example 1 is that the D50 of the first particles is 0.5 μm, and the D50 of the second particles is 1.0 μm.

[0080] Example 3

[0081] The difference from Example 1 is that the D50 of the first particles is 0.3 μm, and the D50 of the second particles is 1.5 μm.

[0082] Example 4

[0083] The difference from Example 1 is that in the preparation of the first particles, sucrose accounting for 8% of the total weight of lithium carbonate, ferrous oxalate and diammonium hydrogen phosphate is added, and the carbon content of the obtained first particles accounts for 0.3% of the weight of the final primary particles.

[0084] Example 5

[0085] The difference from Example 1 is that in the preparation of the second particles, 10% of graphene is added to the total mass of lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate, and the graphene content of the second particles finally obtained accounts for 0.5% of the weight of the primary particles finally obtained.

[0086] Example 6

[0087] The difference from Example 1 is: Preparation of the second particles: b: Immersing the second particle precursor in a Li6PS5Cl solution, wherein the Li6PS5Cl accounts for 2% of the mass of the second particle precursor, and finally the Li6PS5Cl in the second particle accounts for 0.3% of the mass of the second particle.

[0088] Example 7

[0089] The difference from Example 1 is that: Preparation of the second particle: 2% of the mass of the first sintered product of the second particle is weighed to obtain Al2O 3， Lithium oxide is weighed, which accounts for 1.76% of the mass of the first sintering product of the second particle, and finally LiAlO2 in the second particle accounts for 0.2% of the mass of the second particle.

[0090] Example 8

[0091] The difference from Example 1 is that: Preparation of the second particles: SiO2 is weighed according to 1% of the mass of the first sintering product of the second particles, and finally Li2SiO3 in the second particles accounts for 0.1% of the mass of the second particles.

[0092] Example 9

[0093] The difference from Example 1 is that the mass ratio of the first particles to the second particles is 3:7.

[0094] Example 10

[0095] The difference from Example 1 is that the mass ratio of the first particles to the second particles is 9:1.

[0096] Comparative Example 1

[0097] The difference from Example 1 is as follows: Preparation of the second particles: a: lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate were mixed in a molar ratio of Li:Ca:Fe:Ca:P of 0.95:0.025 (molar ratio of Ca at the Li position): 0.9:0.1 (molar ratio of Ca at the transition metal position): 1. 8% graphene (the graphene content accounted for 0.35% of the total weight of the primary particles obtained) based on the total weight of lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate, and 2% N,N-dimethylformamide were added to 500 ml of anhydrous ethanol. The pH was adjusted to 12 by adding aqueous ammonia, dried, and then heated to 700°C at a heating rate of 6°C / min and maintained for 6 hours to obtain a second particle precursor.

[0098] b: heating the second particle precursor to 300°C at a heating rate of 5°C / min and holding the temperature for 3 hours, and then cooling the precursor to room temperature to obtain a second particle primary sintering product;

[0099] c: The primary sintered product of the second particles was heated to 300° C. at a heating rate of 5° C. / min and kept at that temperature for 3 h. The product was then cooled to room temperature and then crushed and sieved to obtain the second particles with a D50 of 1.0 μm.

[0100] Comparative Example 2

[0101] The difference from Example 1 is as follows: Preparation of second particles: a: lithium carbonate, ferrous oxalate, diammonium hydrogen phosphate, and calcium carbonate were mixed at a molar ratio of Li:Ca:Fe:Ca:P of 0.95:0.025 (molar ratio of Ca at the Li position):0.9:0.1 (molar ratio of Ca at the transition metal position):1, and 8% graphene (the graphene content accounted for 0.35% of the total weight of the primary particles obtained) and 2% N,N-dimethylformamide (the total weight) were added to 500 ml of anhydrous ethanol, and ammonia was added to adjust the pH to 12. The mixture was dried, and then heated to 700°C at a heating rate of 6°C / min and kept warm for 6 hours to obtain a second particle precursor;

[0102] b: Immersing the second particle precursor in a Li6PS5Cl solution, wherein the Li6PS5Cl accounts for 1% of the mass of the second particle precursor (Li6PS5Cl accounts for 0.15% of the mass of the final second particle), drying, heating to 300°C at a heating rate of 5°C / min, holding for 3 hours, and cooling to room temperature to obtain a second particle primary sintering product;

[0103] c: The second particles were sintered once, heated to 300°C at a heating rate of 5°C / min and kept at that temperature for 3 hours, then cooled to room temperature, crushed, and sieved to obtain the second particles with a D50 of 1.0 μm.

[0104] Comparative Example 3: The difference from Example 1 is that the electrolyte in the electrolyte is only LiPF6.

[0105] Comparative Example 4: The difference from Example 1 is that the electrolyte in the electrolyte solution is only LiTFSI.

[0106] Comparative Example 5: The difference from Example 1 is that only the second particles are prepared.

[0107] The carbon mass content is measured in accordance with GB / T 33822-2017.

[0108] The electrical conductivity and resistivity of the cathode active material powder were measured by a four-probe method.

[0109] After standard charging, the battery was placed in a room temperature environment for 24 hours, and then discharged to 1.8V at a constant current of 1C, and the first cycle discharge capacity at room temperature was recorded.

[0110] After standard charging, the battery was placed in -20℃, -40℃, and -45℃ environments for 24h, then discharged to 1.8V at a constant current of 1C, and the first-cycle discharge capacity in grams at -20℃, -40℃, and -45℃ was recorded. The first discharge efficiency (-20℃, -40℃, -45℃) = (-20℃, -40℃, -45℃) first-cycle discharge capacity in grams / first-cycle discharge capacity in grams at room temperature × 100%.

[0111] Standard charge: First charge at 1C constant current to 3.7V, then charge at 3.7V constant voltage until the current drops to 0.05C.

[0112] The test results are shown in Table 1.

[0113] Table 1 Electrochemical performance of Examples 1-10 and Comparative Examples 1-5

[0114] <![CDATA[Room temperature first cycle discharge capacity in mAh / g -1 ).]]> First discharge efficiency at room temperature (%) Capacity retention rate after 100 cycles at 1C rate at room temperature (%) -20℃ first discharge efficiency (%) -40℃ first discharge efficiency (%) -45℃ first discharge efficiency (%) -40℃ 1C rate cycle 100 cycles capacity retention (%) <![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

[0115] From the electrochemical performance data in Table 1, compared with Comparative Examples 1-5, the batteries prepared by the positive electrode active materials and electrolytes provided in Examples 1-10 have higher gram capacity, energy density, and cycle stability under low temperature test conditions. Example 1 of the present invention uses an active material mixed with the first particles (coated with 0.2wt% carbon, D50 is 0.3μm) and the second particles (coated with 0.2wt% graphene, 0.15wt% Li6PS5Cl, 0.12% LiAlO2, D50 is 1μm), which can significantly reduce the interface resistance and the first discharge efficiency is higher than the simple carbon coating of Comparative Example 1, especially when at -20°C, the first discharge efficiency of Comparative Example 1 is only 62.4%; Comparative Example 2 does not coat the surface of the second particles with LiAlO2, resulting in the sulfide electrolyte being exposed to the electrolyte, exacerbating the interfacial reaction, and interrupting the electron-ion transmission channel, and the capacity at -40°C is 0. The capacity dropped sharply by 28.4%; in Comparative Example 3, only a single LiPF6 electrolyte was used, resulting in a sharp increase in the viscosity of the electrolyte at low temperatures of -20°C and below, and the ion migration was hindered, which directly caused capacity attenuation; in Comparative Example 4, only a single LiTFSI electrolyte was used, resulting in a low temperature of -20°C and below. Due to its own weak film-forming ability, it was 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 1C first-cycle discharge efficiency decreased by 20.3%, the electrode structure was loose, the ion diffusion path was extended, and the generation of pores made it impossible to form a continuous electron transmission channel, resulting in a decrease in comprehensive performance. In summary, the present invention achieves electrochemical performance stability in low-temperature scenarios by adopting a positive electrode active material with low-temperature stability and electrochemical properties, and in combination with an electrolyte containing a composite system of LiTFSI and LiPF6, thereby significantly improving the low-temperature performance of lithium iron phosphate batteries.

[0116] The above detailed description of the analytical methods involved in the present invention provides a detailed introduction. It should be noted that the above description is intended solely to help those skilled in the art better understand the methods and concepts of the present invention, and is not intended to limit the relevant content. Without departing from the principles of the present invention, those skilled in the art may make appropriate adjustments or modifications to the present invention, and such adjustments and modifications shall also fall within the scope of protection of the present invention.

Claims

1. An ultra-low temperature lithium iron phosphate battery, characterized in that: The invention comprises 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 smaller than the D50 of the second particle, and the mass ratio of the first particle to the second particle is 3:7-1:9; and the first particle and the second particle are prepared as follows; (1) The first particle preparation step includes: A lithium source, an iron source, a phosphorus source, a carbon source and a solvent are mixed, ball-milled into nanoparticles of 15 nm to 250 nm, dried and crushed to form a lithium iron phosphate precursor, sintered, cooled, crushed and sieved to obtain first particles; 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 and a solvent, adjusting the pH, drying and sintering to obtain a second particle precursor; b: The second particle precursor is immersed in a sulfide electrolyte solution, dried, and sintered to obtain a second particle primary sintering product; c. The primary sintered product of the second particles is mixed with a metal source and a lithium source, sintered, cooled, crushed, and sieved to obtain second particles; The doping divalent metal ion at the iron site of the second particle is Ca 2+ ; The lithium metal oxide is one or more of LiAlO2, LiTiO2, Li2SiO3, LiMgO2, LiV3O8, and LiNiO2.

2. The lithium iron phosphate battery according to claim 1, characterized in that The graphene material is coated on the surface of the second particle in a dotted manner, and the sulfide electrolyte is distributed between the graphenes 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, characterized in that The thickness of the lithium metal oxide coating layer is 1-3 nm.

5. The lithium iron phosphate battery according to claim 1, characterized in that: 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 is less than 0.5 μm, and the D50 of the second particle is 0.5 μm-1.8 μm. The compaction density of the positive active material is 2.0-2.8 g / cm 3 .

6. The lithium iron phosphate battery according to claim 1, wherein: The positive electrode sheet includes a positive electrode current collector, a positive electrode active material coated on one side of the positive electrode current collector, and a binder. 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, 0≤x≤0.05, 0<y≤0.

1.

7. The lithium iron phosphate battery according to claim 1, wherein: 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.

8. The lithium iron phosphate battery according to claim 7, wherein the electrolyte salt is a composite system of LiTFSI and LiPF6, and the molar ratio of the LiTFSI to the LiPF6 is 7:3-9:1.

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