Preparation method and application of iron phosphate and lithium iron phosphate material

Abstract

The application provides a preparation method of a ferric phosphate material, and comprises the following steps: S1: mixing a first monomer, a second monomer, an initiator and a first phosphorus source in water to obtain an aqueous phase solution, and adjusting the pH to 1.3-1.5 by adding acid; S2: mixing a third monomer, a fourth monomer and an emulsifier in an organic solvent to obtain an oil phase solution; S3: mixing the aqueous phase solution and the oil phase solution, performing ultrasonic treatment, and then performing a polymerization reaction to obtain a polymer capsule solution containing a phosphorus source water core; S4: mixing an iron source and a second phosphorus source in water, adjusting the pH to 1.5-2.2 to obtain a phosphorus-iron solution, mixing the phosphorus-iron solution with the polymer capsule solution, and performing a heating reaction to obtain a ferric phosphate precursor. The application solves the problem that the diffusion path of lithium ions in large particles is too long, and increases the rate capability; meanwhile, the grading problem is solved from the source, and the problem of 'overburning' in the calcination process is avoided.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of positive electrode materials, and particularly relates to a preparation method and application of a phosphorus iron and lithium iron phosphate material. BACKGROUND

[0002] Lithium iron phosphate is an olivine-type crystal structure, belonging to the orthorhombic space group Pnmb, and the structure includes FeO6 octahedron, LiO6 octahedron and PO4 tetrahedron. Lithium iron phosphate is one of the most important research topics in the field of lithium ion battery positive electrode materials due to its high safety, good cycle performance, widely available raw materials and environmental friendliness. However, lithium iron phosphate also has the disadvantages of low tap density and low actual specific capacity, which restricts its further development.

[0003] Researches on the tap density of lithium iron phosphate material show that the tap density of large particle products is generally high, but the diffusion path of lithium ions in the solid material is also lengthened, which makes the electrochemical performance of the material worse. At present, the tap density of the material is also improved by size grading of raw materials, but the size grading of raw materials may cause the problems of “over-burning” or “insufficient calcination” in the preparation of lithium iron phosphate from phosphorus iron. SUMMARY

[0004] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application provides a preparation method and application of a phosphorus iron and lithium iron phosphate material.

[0005] According to a first aspect of the present application, a preparation method of a phosphorus iron material is provided, comprising the following steps:

[0006] S1: mixing a first monomer, a second monomer, an initiator and a first phosphorus source in water to obtain an aqueous solution, and adjusting the pH to 1.3-1.5 by adding acid;

[0007] S2: mixing a third monomer and a fourth monomer in an organic solvent to obtain an oil phase solution;

[0008] S3: mixing, ultrasonicating and then polymerizing the aqueous solution, the oil phase solution and an emulsifier to obtain a polymer capsule solution containing a phosphorus source water core;

[0009] S4: mixing a ferric source and a second phosphorus source in water to obtain a phosphorus-iron solution, adjusting the pH to 1.5-2.2, mixing the phosphorus-iron solution with the polymer capsule solution, and heating to obtain a phosphorus iron precursor.

[0010] Preferably, in step S1, the first monomer is selected from N-isopropyl acrylamide; and / or, the second monomer is selected from N,N'-methylenebisacrylamide.

[0011] Preferably, in step S1, the mass percentage concentration of the first monomer in the aqueous solution is 8-12wt%; and / or, the mass percentage concentration of the second monomer in the aqueous solution is 0.1-0.15wt%; and / or, the molar concentration of the first phosphorus source in the aqueous solution is 0.8-1.2mol / L.

[0012] Preferably, in step S1, the initiator is ammonium persulfate; and / or, the mass percentage concentration of the initiator in the aqueous solution is 0.4wt%.

[0013] Preferably, in step S1, the acid is 1mol / L hydrochloric acid.

[0014] Preferably, in step S2, the third monomer is selected from methyl methacrylate; and / or, the fourth monomer is selected from divinylbenzene.

[0015] Preferably, in step S2, the organic solvent is at least one of liquid paraffin, n-hexane, vegetable oil, petroleum ether, n-heptane, toluene, xylene, dichloromethane, trichloromethane, and acrylic ester.

[0016] Preferably, in step S2, the mass percentage concentration of the third monomer in the oil phase solution is 6-12wt%; and / or, the mass percentage concentration of the fourth monomer in the oil phase solution is 1.5-2.5wt%.

[0017] Preferably, the first phosphorus source and the second phosphorus source are independently selected from at least one of (NH4)2HPO4, NH4H2PO4, (NH4)3PO4, NaH2PO4, and Na2HPO4.

[0018] Preferably, in step S3, the volume ratio of the aqueous solution to the oil phase solution is 1:(1-4); and / or, the amount of the emulsifier is 3-8wt% of the mass of the oil phase solution.

[0019] Preferably, in step S3, the emulsifier is at least one of sodium alkyl sulfonate, sodium alkylbenzenesulfonate, sodium oleate, lauryl alcohol polyoxyethylene (120) ether, zinc stearate, cetyl PEG / PPG-10 / 1 dimethicone, Tween-80, and Span-80.

[0020] Preferably, the HLB value of the emulsifier is 3-6. When the emulsifier is a compound emulsifier, the HLB value can be obtained by the following formula: HLB=(W1*HLB1+W2*HLB2+...+W n *HLB n ) / (W1+W2+...+W n), wherein W1, W2...Wn are the mass of each emulsifier, HLB1, HLB2...HLBn are the HLB values of each emulsifier. n are the HLB values of each emulsifier.

[0021] Preferably, the ultrasonic treatment in step S3 is as follows: intermittent ultrasonic treatment is performed, and the total ultrasonic treatment time is 10-20 min.

[0022] Further preferably, in the ultrasonic treatment, the single ultrasonic time is 1 min, and the intermittent time is 40 s.

[0023] Preferably, the temperature of the polymerization reaction in step S3 is 69-75°C, and the time is 22-28 h.

[0024] Preferably, in step S4, the iron source is at least one of ferric nitrate or ferric chloride.

[0025] Preferably, in step S4, the molar concentration of the iron source in the phosphorus-iron solution is 1-1.5 mol / L; and / or, the molar concentration of the second phosphorus source in the phosphorus-iron solution is 0.5-0.8 mol / L.

[0026] Preferably, the volume ratio of the aqueous solution to the phosphorus-iron solution is (0.166-0.625):1.

[0027] Preferably, in step S4, the organic solvent is removed before the heating reaction. The organic solvent in the mixed solution can be removed by using the density difference with the aqueous phase.

[0028] Preferably, in step S4, the heating reaction is performed under the following conditions: reaction temperature 60-80°C, stirring speed 400-600 rpm, and reaction time 5-10 h.

[0029] According to a second aspect of the present application, a preparation method of lithium iron phosphate is provided, comprising the following steps:

[0030] The preparation raw materials including a lithium source and a phosphoric iron precursor are calcined, wherein the phosphoric iron precursor is prepared by the preparation method according to the first aspect of the present application.

[0031] Preferably, the lithium source is at least one of lithium carbonate, lithium hydroxide or lithium acetate.

[0032] Preferably, the preparation raw materials further include a carbon source.

[0033] Preferably, the preparation method of lithium iron phosphate comprises the following steps: the preparation raw materials including a lithium source, a carbon source and a phosphoric iron precursor are mixed, ground, dried and then calcined, and the lithium iron phosphate is obtained.

[0034] Preferably, the mass of the carbon source is 8% to 12% of the mass of the iron phosphate precursor.

[0035] Preferably, the carbon source is at least one of glucose, sucrose, stearic acid, cyclodextrin, citric acid, aniline, cellulose acetate, or polyvinylpyrrolidone (PVP).

[0036] Preferably, the molar ratio of the iron phosphate precursor to the lithium source is 1:1.

[0037] Preferably, the grinding time is 2 to 6 hours.

[0038] Preferably, the calcination temperature is 600 to 850℃, and the holding time is 6 to 15 hours.

[0039] According to a third aspect of the present application, the preparation method according to the first aspect of the present application is applied in the preparation of lithium ion batteries.

[0040] The present application mixes the water phase and the oil phase, obtains a stable emulsion through ultrasonic treatment, then increases the temperature, the initiator in the water phase of the dispersed phase undergoes thermal decomposition to generate primary free radicals, N-isopropyl acrylamide (NIPAM) in the water phase captures the free radicals and polymerizes into PNIPAM, when the polymerization temperature is higher than the phase transition temperature (32℃) of PNIPAM, the PNIPAM chain becomes hydrophobic, thus gathers to the oil-water interface, and N,N'-methylene bisacrylamide acts as a crosslinking agent and participates in the polymerization reaction. Meanwhile, methyl methacrylate and the crosslinking agent divinylbenzene in the continuous phase of the oil phase diffuse on the water interface, capture the free radicals and participate in the polymerization reaction at the interface, thereby forming a polymer layer at the oil-water interface, and obtaining a polymer capsule with a phosphate (phosphorus source) solution as a water core.

[0041] A certain concentration of a mixed solution of iron salt and phosphate is added to the polymer capsule solution with a phosphate solution water core, during the growth of iron phosphate crystals, part of the iron phosphate crystals are generated in the solution environment, and a small part of the crystals adhere to the polymer capsule and grow, as the phosphate in the solution environment is continuously consumed, the phosphate in the polymer capsule continuously diffuses out of the polymer capsule due to the concentration difference, the iron ions continuously diffuse into the polymer capsule, and grow near the interface of the polymer capsule, further increasing the thickness of the iron phosphate layer inside and outside the polymer capsule. After sufficient reaction, filtration, washing, and drying, large-particle-size hollow iron phosphate and small-particle-size solid iron phosphate are finally obtained.

[0042] According to an embodiment of the present application, at least the following beneficial effects are achieved:

[0043] The present application solves the problem of long diffusion path of lithium ion in large particles by preparing large-particle hollow iron phosphate and small-particle solid iron phosphate, increases the electrolyte infiltration area, and increases the rate capability; meanwhile, the grading problem is solved from the source, and the "over-burning" problem in the calcination process is avoided. Specifically, in the calcination process, the large-particle hollow iron phosphate does not need to be fully calcined because only the shell layer has the iron phosphate active material; at the same time, the small-particle solid iron phosphate also does not need to be excessively calcined because of its small size, so that the calcination degree of the two particle sizes of materials is the same in the same calcination time. BRIEF DESCRIPTION OF DRAWINGS

[0044] The present application will be further described below in conjunction with the drawings and examples, in which:

[0045] Figure 1 TEM image of the iron phosphate prepared in Example 1 of the present application;

[0046] Figure 2 Particle size distribution graph of the iron phosphate prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0047] The concept and technical effects of the present application will be described below in conjunction with examples to fully understand the purpose, features and effects of the present application.

[0048] The raw materials, reagents or devices used in the following examples and comparative examples can be obtained from conventional commercial channels or can be obtained by existing known methods unless otherwise specified.

[0049] Example 1

[0050] The present embodiment provides a preparation method of lithium iron phosphate material, comprising the following steps:

[0051] (1) N-isopropyl acrylamide, N, N'-methylene bisacrylamide, ammonium persulfate and ammonium hydrogen phosphate are dissolved in deionized water to obtain an aqueous solution, the mass percentage concentration of N-isopropyl acrylamide in the aqueous solution is 10wt%, the mass percentage concentration of N, N'-methylene bisacrylamide in the aqueous solution is 0.12wt%, the mass percentage concentration of ammonium persulfate in the aqueous solution is 0.4wt%, and the concentration of ammonium hydrogen phosphate in the aqueous solution is 1mol / L, and the pH of the solution is adjusted to 1.5 with 1mol / L HCl.

[0052] (2) Methyl methacrylate and divinylbenzene are dissolved in liquid paraffin to obtain an oil phase solution, the mass percentage concentration of methyl methacrylate is 8wt%, and the mass percentage concentration of divinylbenzene is 2wt%. The oil phase solution and the water phase solution are mixed at a volume ratio of 4:6, emulsifiers Tween-80 and Span-80 are added, the HLB value of the mixture is 5, the total content of the emulsifiers is 5wt% of the mass of the oil phase solution, ultrasonic treatment is performed for 15min with a pause of 40s per minute, and after the treatment, the emulsion is placed in a water bath heating environment at 72°C for polymerization reaction for 24h to obtain a polymer capsule solution containing a phosphate solution water core.

[0053] (3) A mixed solution of ammonium hydrogen phosphate and ferric chloride salt is configured, the concentration of the ferric chloride is 1mol / L, the solution concentration of the ammonium hydrogen phosphate is 0.7mol / L, the volume ratio of the mixed solution to the aforementioned water phase solution is 10:3, and the pH of the solution is adjusted to 1.5. The mixed solution is added to the polymer capsule solution, and the liquid paraffin is removed by using the density difference of the solution. After reaction at a speed of 500rpm and a temperature of 70°C for 9h, the ferric phosphate precursor is obtained through filtration, washing and drying. The transmission electron microscope image of the ferric phosphate precursor is shown in FIG. 2, which includes hollow ferric phosphate with a large particle shell structure and small particle solid ferric phosphate around. Figure 1 Figure 2 is a corresponding particle size distribution diagram, and it can be seen that the large particle hollow ferric phosphate and the small particle solid ferric phosphate correspond to two different particle size distribution peaks.

[0054] (4) The aforementioned ferric phosphate precursor and lithium hydroxide are mixed in a molar ratio of 1:1, then glucose is added, the mass of the glucose is 8% of the mass of the ferric phosphate precursor, and ethanol is used as a dispersant to obtain a mixture, the mass fraction of the ethanol is 2.5wt% of the total mass of the mixture, the mixture is put into a grinder for grinding and stirring for 5h, and then dried; the dried mixture is put into a tube furnace, and is kept at 800°C for 12h in an inert atmosphere to finally obtain lithium iron phosphate, which inherits the morphology of the ferric phosphate precursor.

[0055] Example 2

[0056] The embodiment provides a preparation method of a lithium iron phosphate material, including the following steps:

[0057] ​(1) N-isopropyl acrylamide, N, N'-methylene bisacrylamide, ammonium persulfate and ammonium hydrogen phosphate are dissolved in deionized water to obtain an aqueous solution, the mass percentage concentration of N-isopropyl acrylamide in the aqueous solution is 10 wt%, the mass percentage concentration of N, N'-methylene bisacrylamide in the aqueous solution is 0.12 wt%, the mass percentage concentration of ammonium persulfate in the aqueous solution is 0.4 wt%, and the concentration of ammonium hydrogen phosphate in the aqueous solution is 1.2 mol / L, and the pH of the solution is adjusted to 1.5 by using 1 mol / L HCl.

[0058] (2) Methyl methacrylate and divinylbenzene are dissolved in liquid paraffin to obtain an oil phase solution, the mass percentage concentration of methyl methacrylate is 8 wt%, and the mass percentage concentration of divinylbenzene is 2 wt%. The aqueous solution and the oil phase solution are mixed at a volume ratio of 3:7, emulsifiers Tween-80 and Span-80 are added, the HLB value of the mixture is 5, the total content of the emulsifiers is 5 wt% of the mass of the oil phase solution, ultrasonic treatment is performed for 15 min with a pause of 40 s per minute, and after the treatment, the emulsion is placed in a water bath heating environment at 72°C for polymerization reaction for 24 h to obtain a polymer capsule solution containing a phosphate solution water core.

[0059] (3) A mixed solution of ammonium hydrogen phosphate and ferric chloride salt is configured, the concentration of ferric chloride is 1 mol / L, the solution concentration of ammonium hydrogen phosphate is 0.7 mol / L, the volume ratio of the mixed solution to the aforementioned aqueous solution is 8:5, and the pH of the solution is adjusted to 1.5, the mixed solution is added to the aforementioned polymer capsule solution, and the liquid paraffin is removed by using the density difference of the solution. After reaction at a speed of 500 rpm and a temperature of 70°C for 9 h, the ferric phosphate precursor is obtained by filtration, washing and drying.

[0060] (4) The aforementioned ferric phosphate precursor and lithium hydroxide are mixed in a molar ratio of 1:1, then glucose is added, the mass of the glucose is 8% of the mass of the ferric phosphate precursor, and ethanol is used as a dispersant to obtain a mixture, the mass fraction of the ethanol in the total mass of the mixture is 2.5 wt%, the mixture is put into a grinder for grinding and stirring for 5 h, and then dried; the dried mixture is put into a tube furnace, and finally the lithium iron phosphate is obtained by keeping the temperature at 800°C for 12 h in an inert atmosphere.

[0061] The concentration of phosphate in the aqueous solution is increased (1.2 mol / L) in this embodiment, and the volume ratio of the aqueous solution to the phosphorus-iron mixed solution (5:8) is adjusted so that the molar amount of phosphate contained in the two solutions is similar. However, due to the high concentration of phosphate in the water core of the polymer capsule, the phosphate is immediately released outside the capsule due to the concentration difference after the two solutions are mixed. The phosphate released outside the capsule reacts with iron ions at the interface to form an iron phosphate shell outside the interface, so the thickness of the hollow iron phosphate shell increases, and the proportion of solid small-particle-size iron phosphate materials decreases.

[0062] Example 3

[0063] The embodiment provides a preparation method of a lithium iron phosphate material, which comprises the following steps:

[0064] (1) N-isopropyl acrylamide, N,N'-methylenebisacrylamide, ammonium persulfate and ammonium hydrogen phosphate are dissolved in deionized water to obtain an aqueous solution, the mass percentage concentration of N-isopropyl acrylamide in the aqueous solution is 10 wt%, the mass percentage concentration of N,N'-methylenebisacrylamide in the aqueous solution is 0.12 wt%, the mass percentage concentration of ammonium persulfate in the aqueous solution is 0.4 wt%, and the concentration of ammonium hydrogen phosphate in the aqueous solution is 0.8 mol / L, and the pH of the solution is adjusted to 1.5 by using 1 mol / L HCl.

[0065] (2) Methyl methacrylate and divinylbenzene are dissolved in liquid paraffin to obtain an oil phase solution, the mass percentage concentration of methyl methacrylate is 8 wt%, and the mass percentage concentration of divinylbenzene is 2 wt%. The aqueous solution and the oil phase solution are mixed in a volume ratio of 3:7, and Tween-80 and Span-80 are added, the HLB value of the complex is 5, the total content of the emulsifier is 5 wt% of the mass of the oil phase solution, ultrasonic treatment is carried out for 15 min, and the treatment is stopped for 40 s every minute. After the treatment is completed, the emulsion is placed in a water bath heating environment at 72℃ for polymerization reaction for 24 h to obtain a polymer capsule solution containing a phosphate solution water core.

[0066] (3) A mixed solution of ammonium hydrogen phosphate and ferric chloride salt is configured, the concentration of ferric chloride is 1 mol / L, the solution concentration of ammonium hydrogen phosphate is 0.8 mol / L, the volume ratio of the mixed solution to the aforementioned aqueous solution is 4:1, and the pH of the solution is adjusted to 1.5. The mixed solution is added to the above-mentioned polymer capsule solution, and the liquid paraffin is removed by using the density difference of the solution. After reaction at a speed of 500 rpm and a temperature of 70℃ for 9 h, the phosphorus iron precursor is obtained through filtration, washing and drying.

[0067] (4) The above iron phosphate precursor and lithium hydroxide are mixed in a molar ratio of 1:1, then glucose is added, the mass of the glucose is 8% of the mass of the iron phosphate precursor, a mixture is obtained with ethanol as a dispersant, the mass fraction of the ethanol is 2.5wt% of the total mass of the mixture, the mixed material is put into a grinder for grinding and stirring for 5h, then drying treatment; the dried mixed material is put into a tube furnace, in an inert atmosphere, 800℃ for 12h, finally lithium iron phosphate is obtained.

[0068] In this embodiment, the volume ratio of the aqueous solution and the phosphorus-iron mixed solution is reduced (1:4), and the concentration of the phosphate in the two solutions is changed (both 0.8mol / L), so that the phosphate solution in the polymer can be released after a period of time, and the total amount of the salt solution in the polymer is less and the concentration is lower, so that a thin layer of iron phosphate shell is obtained, the particle size of the solid iron phosphate is large, and the tap density and the rate performance are affected.

[0069] Example 4

[0070] The embodiment provides a preparation method of lithium iron phosphate material, comprising the following steps:

[0071] (1) N-isopropyl acrylamide, N,N'-methylene bisacrylamide, ammonium persulfate and ammonium hydrogen phosphate are dissolved in deionized water to obtain an aqueous solution, the mass percentage concentration of N-isopropyl acrylamide in the aqueous solution is 12wt%, the mass percentage concentration of N,N'-methylene bisacrylamide in the aqueous solution is 0.12wt%, the mass percentage concentration of ammonium persulfate in the aqueous solution is 0.4wt%, and the concentration of ammonium hydrogen phosphate in the aqueous solution is 1mol / L, and the pH of the solution is adjusted to 1.5 by using 1mol / L HCl.

[0072] (2) Methyl methacrylate and divinylbenzene are dissolved in liquid paraffin to obtain an oil phase solution, the mass percentage concentration of methyl methacrylate is 12wt%, and the mass percentage concentration of divinylbenzene is 2.5wt%. The aqueous solution and the oil phase solution are mixed in a volume ratio of 5:5, Tween-80 and Span-80 are added, the HLB value of the compound is 5, the total content of the emulsifier is 5wt% of the mass of the oil phase solution, ultrasonic treatment is carried out for 15min, and the treatment is stopped for 40s every minute, after the treatment, the emulsion is placed in a water bath heating environment at 72℃ for polymerization reaction for 24h, and a polymer capsule solution containing a phosphate solution water core is obtained.

[0073] (3) configuring a mixed solution of ammonium hydrogen phosphate and ferric chloride salt, wherein the concentration of ferric chloride is 1 mol / L, the solution concentration of ammonium hydrogen phosphate is 0.7 mol / L, the volume ratio of the mixed solution to the aforementioned aqueous solution is 10:3, and the pH of the solution is adjusted to 1.5, and the mixed solution is added to the aforementioned polymer capsule solution, and the liquid paraffin is removed by using the density difference of the solution. After reaction at a speed of 500 rpm and a temperature of 70°C for 9h, the ferric phosphate precursor is obtained by filtration, washing and drying.

[0074] (4) mixing the aforementioned ferric phosphate precursor and lithium hydroxide according to a molar ratio of 1:1, then adding glucose, the mass of the glucose being 8% of the mass of the ferric phosphate precursor, and obtaining a mixture by taking ethanol as a dispersant, the mass fraction of the ethanol being 2.5wt% of the total mass of the mixture, grinding and stirring the mixture in a grinder for 5h, and then drying; placing the dried mixture in a tube furnace, and finally obtaining lithium ferric phosphate by keeping the temperature at 800°C for 12h in an inert atmosphere.

[0075] Since the N-isopropyl acrylamide monomer captures most of the water-soluble radicals and polymerizes, the concentration of N-isopropyl acrylamide is increased in this embodiment, more PNIPAM chains are incorporated into the polymer layer, which to some extent reduces the mechanical properties of the polymer microspheres and easily leads to the collapse of the polymer microspheres, which can cause the polymer sphericity to be poor and the rate performance to be reduced; at the same time, the content of methyl methacrylate and divinylbenzene is increased, the polymer wall thickness is increased, and the shell structure is stabilized.

[0076] Example 5

[0077] The embodiment provides a preparation method of a lithium ferric phosphate material, and comprises the following steps:

[0078] (1) dissolving N-isopropyl acrylamide, N,N'-methylenebisacrylamide, ammonium persulfate and ammonium hydrogen phosphate in deionized water to obtain an aqueous solution, the mass percentage concentration of N-isopropyl acrylamide in the aqueous solution is 8wt%, the mass percentage concentration of N,N'-methylenebisacrylamide in the aqueous solution is 0.12wt%, the mass percentage concentration of ammonium persulfate in the aqueous solution is 0.4wt%, and the concentration of ammonium hydrogen phosphate in the aqueous solution is 1 mol / L, and the pH of the solution is adjusted to 1.5 by using 1 mol / L HCl.

[0079] (2) Methyl methacrylate and divinylbenzene are dissolved in liquid paraffin to obtain an oil phase solution, the mass percentage concentration of methyl methacrylate is 6wt%, and the mass percentage concentration of divinylbenzene is 1.5wt%. The aqueous phase solution and the oil phase solution are mixed at a volume ratio of 5:5, and Tween-80 and Span-80 are added, the HLB value of the mixture is 5, the total content of the emulsifier is 5wt% of the mass of the oil phase solution, ultrasonic treatment is performed for 15min with a pause of 40s per minute, and after the treatment is completed, the emulsion is placed in a water bath heating environment at 72°C for polymerization reaction for 24h to obtain a polymer capsule solution containing a phosphate solution water core.

[0080] (3) A mixed solution of ammonium hydrogen phosphate and ferric chloride salt is configured, the concentration of ferric chloride is 1mol / L, the solution concentration of ammonium hydrogen phosphate is 0.7mol / L, the volume ratio of the mixed solution to the aforementioned aqueous phase solution is 10:3, and the pH of the solution is adjusted to 1.5. The mixed solution is added to the above-mentioned polymer capsule solution, and the liquid paraffin is removed by using the density difference of the solution. After reaction at a speed of 500rpm and a temperature of 70°C for 9h, the ferric phosphate precursor is obtained through filtration, washing and drying.

[0081] (4) The above-mentioned ferric phosphate precursor and lithium hydroxide are mixed in a molar ratio of 1:1, then glucose is added, the mass of the glucose is 8% of the mass of the ferric phosphate precursor, ethanol is used as a dispersant to obtain a mixture, the mass fraction of the ethanol is 2.5wt% of the total mass of the mixture, the mixture is put into a grinder for grinding and stirring for 5h, and then dried; the dried mixture is put into a tube furnace, and finally the lithium iron phosphate is obtained under an inert atmosphere at 800°C for 12h.

[0082] The concentration of N-isopropyl acrylamide is reduced in the embodiment, the wall thickness of the polymer microspheres is small, the stability is reduced and the polymer microspheres are easily deformed, which can cause the spherical degree of the polymer to be poor and the rate performance to be reduced; at the same time, the content of methyl methacrylate and divinylbenzene is reduced, the wall thickness of the polymer is reduced, the spherical degree is affected, the phosphate is quickly released due to the too thin wall layer, and the hollow ferric phosphate cannot be formed, so the rate performance is reduced.

[0083] Comparative Example 1

[0084] The comparative example provides a preparation method of lithium iron phosphate material, which is different from the embodiment 1 in that the aqueous phase solution in step (1) does not contain phosphate, and the preparation method comprises the following steps:

[0085] (1) N-isopropyl acrylamide, N, N'-methylene bisacrylamide, ammonium persulfate were dissolved in deionized water to obtain an aqueous solution, the mass percentage concentration of N-isopropyl acrylamide in the aqueous solution was 10 wt%, the mass percentage concentration of N, N'-methylene bisacrylamide in the aqueous solution was 0.12 wt%, and the mass percentage concentration of ammonium persulfate in the aqueous solution was 0.4 wt%, and the pH of the solution was adjusted to 1.5 with 1 mol / L HCl.

[0086] (2) Methyl methacrylate and divinylbenzene were dissolved in liquid paraffin to obtain an oil phase solution, the mass percentage concentration of methyl methacrylate was 8 wt%, and the mass percentage concentration of divinylbenzene was 2 wt%. The aqueous solution and the oil phase solution were mixed at a volume ratio of 4:6, and Tween-80 and Span-80 were added, the HLB value of the complex was 5, the total content of the emulsifier was 5 wt% of the mass of the oil phase solution, ultrasonic treatment was carried out for 15 min, and the treatment was stopped for 40 s every minute, after the treatment, the emulsion was placed in a water bath heating environment at 72°C for polymerization reaction for 24 h, to obtain a polymer capsule solution containing water core.

[0087] (3) A mixed solution of ammonium hydrogen phosphate and ferric chloride salt was configured, the concentration of ferric chloride was 1 mol / L, the solution concentration of ammonium hydrogen phosphate was 0.7 mol / L, the volume ratio of the mixed solution to the aforementioned aqueous solution was 10:3, and the pH of the solution was adjusted to 1.5, the mixed solution was added to the above-mentioned polymer capsule solution, and the liquid paraffin was removed by using the density difference of the solution. After reaction at a speed of 500 rpm and a temperature of 70°C for 9 h, the ferric phosphate precursor was obtained by filtration, washing and drying.

[0088] (4) The above-mentioned ferric phosphate precursor and lithium hydroxide were mixed in a molar ratio of 1:1, then glucose was added, the mass of the glucose was 8% of the mass of the ferric phosphate precursor, and ethanol was used as a dispersant to obtain a mixed material, the mass fraction of the ethanol was 2.5 wt% of the total mass of the mixed material, the mixed material was put into a grinder for grinding and stirring for 5 h, and then dried; the dried mixed material was put into a tube furnace, and the final lithium iron phosphate was obtained under an inert atmosphere at 800°C for 12 h.

[0089] In the present comparative example, no phosphate solution was added in the polymer, resulting in a low proportion of spherical ferric phosphate, a thin shell, instability, and easy breaking into irregular ferric phosphate, and a decrease in rate performance and tap density.

[0090] Comparative Example 2

[0091] The present comparative example provides a preparation method of a lithium iron phosphate material, comprising the following steps:

[0092] (1) Configuration of a mixed solution of ammonium hydrogen phosphate and ferric chloride salt, wherein the concentration of ferric chloride is 1 mol / L, the concentration of ammonium hydrogen phosphate is 1 mol / L, and the pH of the solution is adjusted to 1.5. After reaction at a speed of 500 rpm and a temperature of 70°C for 9h, filtration, washing and drying, the iron phosphate precursor is obtained.

[0093] (2) The above iron phosphate precursor and lithium hydroxide are mixed in a molar ratio of 1:1, then glucose is added, the mass of the glucose is 13% of the mass of the iron phosphate precursor, and ethanol is used as a dispersant to obtain a mixed material, the mass fraction of the ethanol is 2.5wt% of the total mass of the mixture, the mixed material is put into a grinder for grinding and stirring for 5h, and then dried; the dried mixed material is put into a tube furnace, heated at 800°C for 12h in an inert atmosphere, and finally the lithium iron phosphate is obtained.

[0094] The comparative example does not use a polymer, and the rate performance and tap density of the prepared iron phosphate are greatly reduced.

[0095] Test example

[0096] The lithium iron phosphate materials obtained in Examples 1-5 and Comparative Examples 1-2 are characterized as follows:

[0097] 1. Test of tap density:

[0098] The mass of the dry cylinder is m1(g), a certain amount of solid sample is added to the cylinder (about 5mL), the cylinder is vertically vibrated until the volume of the sample in the cylinder no longer decreases, and the volume of the sample is recorded as V(mL). The total mass of the cylinder and the sample is m2(g); the tap density of the sample is calculated using the formula ρ=(m2-m1) / V(g / cm3), and the results are shown in Table 1.

[0099] 2. Test of specific surface area:

[0100] The specific surface area of the lithium iron phosphate materials of the examples and comparative examples is measured using a specific surface area tester, and the results are shown in Table 1.

[0101] Table 1

[0102] Specific surface area (m 2 / g) Tap density (g / cm 3 )]]> Example 1 28.6 1.22 Example 2 25.3 1.16 Example 3 23.5 1.05 Example 4 25.4 1.12 Example 5 21.1 0.98 Comparative Example 1 19.5 0.90 Comparative Example 2 18.2 0.81

[0103] As can be seen from Examples 2-3, by changing the concentration of the solution in the polymer microspheres and the concentration of the iron phosphate solution, the proportion of hollow large particle lithium iron phosphate and solid small particle lithium iron phosphate can be adjusted and controlled, thereby affecting the size of the specific surface area and the tap density; as can be seen from Examples 4-5, changing the content of N-isopropyl acrylamide, methyl methacrylate and divinylbenzene will affect the sphericity and the wall thickness of the polymer microspheres, thereby affecting the specific surface area and the tap density of the lithium iron phosphate.

[0104] 3. Rate and cycle test of simulated battery:

[0105] A slurry was prepared by mixing the positive electrode material, acetylene black and PVDF in a mass ratio of 75:15:10, and then uniformly coated on an aluminum foil substrate to serve as the positive electrode of the simulated battery. Lithium sheet was used as the negative electrode of the simulated battery, a polypropylene porous membrane was used as the separator, and the electrolyte was 1 mol LiPF6 dissolved in 1 L of a mixed solvent of EC and DMC (volume ratio 1:1). The positive electrode, negative electrode, electrolyte and separator were assembled into a battery in an argon glove box.

[0106] First, constant current charging was performed to 4.2 V, then discharge was performed at a rate current to 2.0 V, and the discharged capacity was the discharge capacity at the rate, and after the discharge ended, constant current discharge was performed to 2.0 V. Then, the next rate test was performed. The test results of the simulated battery are shown in Table 2.

[0107] Table 2

[0108]

[0109] It can be seen that the performance of the lithium iron phosphate material prepared in the examples of the present application is better than that of the comparative examples, and the performance of the material of Example 1 is more excellent.

[0110] It is apparent to those skilled in the art that the present application is not limited to the details of the foregoing exemplary embodiments, and that the present application can be implemented in other specific forms without departing from the spirit or essential characteristics of the present application. Therefore, the embodiments should be considered in all respects as illustrative and not restrictive, and the scope of the present application should be defined by the appended claims rather than the above description, and all changes coming within the meaning and range of equivalency of the claims are intended to be embraced therein. Any reference signs in the claims should not be construed as limiting the claims to the figures in which the reference signs are used.

Claims

1. A method for producing a ferric phosphate material, characterized by, The method comprises the following steps: S1: mixing a first monomer, a second monomer, an initiator and a first phosphorus source in water to obtain an aqueous solution, and adjusting the pH to 1.3-1.5 by adding acid; S2: mixing a third monomer and a fourth monomer in an organic solvent to obtain an oil phase solution; S3: mixing the aqueous solution, the oil phase solution and an emulsifier, and then performing ultrasonic treatment and polymerization to obtain a polymer capsule solution containing a phosphorus source water core; S4: mixing an iron source and a second phosphorus source in water, adjusting the pH to 1.5-2.2 to obtain a phosphorus-iron solution, mixing the phosphorus-iron solution with the polymer capsule solution, and heating to obtain an iron phosphate precursor; In step S1, the first monomer is selected from N-isopropyl acrylamide, and the second monomer is selected from N,N'-methylene bisacrylamide. In step S2, the third monomer is selected from methyl methacrylate, and the fourth monomer is selected from divinylbenzene; the mass percentage concentration of the third monomer in the oil phase solution is 6-12 wt%; and the mass percentage concentration of the fourth monomer in the oil phase solution is 1.5-2.5 wt%.

2. The production method according to claim 1, characterized by, In step S1, the mass percentage concentration of the first monomer in the aqueous solution is 8-12 wt%; and / or, the mass percentage concentration of the second monomer in the aqueous solution is 0.1-0.15 wt%; and / or, the molar concentration of the first phosphorus source in the aqueous solution is 0.8-1.2 mol / L.

3. The preparation method according to claim 1, characterized in that, In step S3, the volume ratio of the aqueous solution to the oil phase solution is 1:(1-4); and / or, the amount of the emulsifier is 3-8 wt% of the mass of the oil phase solution.

4. The production method according to claim 1, characterized by, In step S4, the molar concentration of the iron source in the phosphorus-iron solution is 1-1.5 mol / L; and / or, the molar concentration of the second phosphorus source in the phosphorus-iron solution is 0.5-0.8 mol / L.

5. The preparation method according to claim 1, characterized in that, The volume ratio of the aqueous solution to the phosphorus-iron solution is 0.166-0.625:

1.

6. A method of producing lithium iron phosphate, characterized by, The method comprises the following steps: The preparation raw materials comprising a lithium source and an iron phosphate precursor are calcined, wherein the iron phosphate precursor is prepared by the method according to any one of claims 1-5.

7. Use of the method according to any one of claims 1-6 in the preparation of a lithium ion battery.

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