Carbon-coated lithium iron phosphate and preparation method thereof and power lithium-ion battery

Abstract

The present invention provides a carbon-coated lithium iron phosphate and a preparation method thereof and a power lithium-ion battery. A preparation method of carbon-coated lithium iron phosphate comprises the following steps: under the condition of continuous stirring, lithium iron phosphate, N,N'-dimethylacetamide and 4,4'-diaminodiphenyl ether are mixed to obtain a mixed solution; under the condition of 0°C and continuous stirring, pyromellitic dianhydride is added to the mixed solution to obtain a composite glue solution; acetic anhydride and pyridine are added to the composite glue solution for chemical imidization reaction to obtain a reaction slurry, and the reaction slurry is dried to obtain a polyimide-coated lithium iron phosphate powder; under the condition of passing a protective gas, the polyimide-coated lithium iron phosphate powder is heated to 600°C to 900°C, and the temperature is kept and calcined for 1 to 10 hours to obtain carbon-coated lithium iron phosphate. The carbon-coated lithium iron phosphate prepared by the preparation method of the above-mentioned carbon-coated lithium iron phosphate has good electrochemical properties and is relatively simple to operate.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion batteries, and in particular to a carbon-coated lithium iron phosphate and a preparation method thereof, and a power lithium ion battery. Background Art

[0002] Olivine-type lithium iron phosphate (LiFePO 4 ) has good electrochemical properties as a positive electrode material for lithium-ion batteries, with a theoretical capacity of 170mAh / g, a stable charge and discharge platform at around 3.4V, and a stable structure during the charge and discharge process. At the same time, this type of material has the advantages of being non-toxic, pollution-free, having good safety performance, being usable in high temperature environments, having a wide range of raw material sources, and being cheap. It can meet the requirements of automobiles for power batteries of "high safety", "low cost", "high capacity" and "long cycle life", and is currently a hot spot for development and research in the battery industry. However, due to the low electronic conductivity in the pure state and the low diffusion rate of lithium ions in the lattice, LiFePO 4 The poor high-current performance of the material is the main obstacle to its application in power lithium-ion batteries.

[0003] Judging from the current research results, carbon coating is a relatively effective means of material modification. At present, there is a method of mixing iron phosphate, lithium hydroxide and an organic carbon source, ball milling, spray drying, roasting and crushing to obtain a lithium iron phosphate material with carbon coated on the surface. The preparation process is simple and easy to mass produce. However, in the solid phase reaction process, the carbon generated by the pyrolysis of the organic carbon source tends to be distributed between the lithium iron phosphate nanoparticles in an aggregated state, and cannot effectively form a complete coating layer on its surface, which will lead to the degradation of the overall performance of the material; another method is to use chemical vapor deposition to carbon reduce and coat the lithium iron phosphate precursor to prepare lithium iron phosphate / carbon nanocomposite materials, which is more uniform than the traditional solid phase carbon coating process. The performance of lithium iron phosphate can be fully utilized, but this method requires a chemical vapor deposition device, and the preparation process is complicated, which increases the difficulty of industrialization. Therefore, it is necessary to study a method for preparing a carbon-coated lithium iron phosphate positive electrode material that can prepare a carbon-coated lithium iron phosphate positive electrode material with excellent electrochemical performance and relatively simple operation. Summary of the invention

[0004] In view of this, it is necessary to provide a method for preparing carbon-coated lithium iron phosphate that can produce carbon-coated lithium iron phosphate with excellent electrochemical performance and is relatively simple to operate.

[0005] In addition, a carbon-coated lithium iron phosphate and power lithium-ion battery is also provided.

[0006] A method for preparing carbon-coated lithium iron phosphate comprises the following steps:

[0007] Under continuous stirring, lithium iron phosphate, N,N'-dimethylacetamide and 4,4'-diaminodiphenyl ether are mixed to obtain a mixed solution;

[0008] Adding pyromellitic anhydride to the mixed solution at 0° C. and under continuous stirring to obtain a composite glue solution;

[0009] Adding acetic anhydride and pyridine to the composite glue to carry out a chemical imidization reaction to obtain a reaction slurry, and drying the reaction slurry to obtain polyimide-coated lithium iron phosphate powder; and

[0010] Under the condition of introducing protective gas, the polyimide-coated lithium iron phosphate powder is heated to 600° C. to 900° C. and calcined for 1 to 10 hours to obtain the carbon-coated lithium iron phosphate.

[0011] In one embodiment, the step of mixing the lithium iron phosphate, N,N'-dimethylacetamide and 4,4'-diaminodiphenyl ether under continuous stirring is specifically as follows: under the condition of ultrasonic power of 150W to 250W, the lithium iron phosphate is added to the N,N'-dimethylacetamide, ultrasonic stirring is performed for 12 hours to 24 hours, and then the 4,4'-diaminodiphenyl ether is added, and ultrasonic stirring is performed until the 4,4'-diaminodiphenyl ether is dissolved.

[0012] In one of the embodiments, in the step of drying the reaction slurry, a spray drying method is adopted, wherein the process parameters of the spray drying are: a feed rate of 5Kg / h to 15Kg / h, an air inlet temperature of 200°C to 300°C, an air outlet temperature of 90°C to 150°C, and a spray frequency of 250Hz to 350Hz.

[0013] In one of the embodiments, in the step of heating the polyimide-wrapped lithium iron phosphate powder to 600° C. to 900° C., the heating rate is 3 to 10° C. / min.

[0014] In one embodiment, in the step of mixing the lithium iron phosphate, N,N'-dimethylacetamide and 4,4'-diaminodiphenyl ether, the mass ratio of the lithium iron phosphate to the N,N'-dimethylacetamide is 1:30-600, and the mass ratio of the lithium iron phosphate to the 4,4'-diaminodiphenyl ether is 1:2-60.

[0015] In one embodiment, the molar ratio of the 4,4'-diaminodiphenyl ether to pyromellitic anhydride is 1:0.98-1.02; the molar ratio of the pyromellitic anhydride to the acetic anhydride is 1:1-10; and the molar ratio of the acetic anhydride to the pyridine is 1:1-5.

[0016] In one of the embodiments, the preparation step of the lithium iron phosphate is further included, specifically: according to the molar ratio of phosphorus, iron and lithium of 1:1:2 to 3.6, under the condition of introducing protective gas, a phosphorus-containing alcohol solution, an iron-containing alcohol solution and a lithium-containing alcohol solution are mixed in a reaction kettle, the temperature is raised to 120°C to 260°C, and the reaction is kept warm for 4 hours to 15 hours under continuous stirring. After cooling, the nano-scale lithium iron phosphate is obtained.

[0017] In one embodiment, after the phosphorus-containing alcohol solution, the iron-containing alcohol solution and the lithium-containing alcohol solution are mixed in the reactor, in the step of heating the temperature to 120°C to 260°C, the heating rate is 1°C / min to 10°C / min, and the pressure in the reactor is 0.2MPa to 4.7MPa.

[0018] A carbon-coated lithium iron phosphate prepared by the above-mentioned method for preparing carbon-coated lithium iron phosphate.

[0019] A power lithium-ion battery comprises a positive electrode, wherein the material of the positive electrode comprises the above-mentioned carbon-coated lithium iron phosphate.

[0020] The preparation method of the carbon-coated lithium iron phosphate uses a polyimide polymer containing a strong polar functional group to wrap the lithium iron phosphate particles, which will have a polar interaction with the phosphate on the surface of the lithium iron phosphate, and is easy to form a core-shell coating structure, effectively inhibiting the agglomeration of the lithium iron phosphate particles in the composite glue; more importantly, the conductive carbon layer produced by the pyrolysis of the polyimide polymer is evenly covered on the surface of the lithium iron phosphate particles, which enhances the conductivity of the lithium iron phosphate particles and is conducive to improving the rate performance; and because the lithium iron phosphate particles are blocked by the polymer pyrolysis carbon layer coated on the outer layer, the lithium iron phosphate particles can be effectively dispersed, avoiding the agglomeration and growth of the lithium iron phosphate particles, so that the carbon-coated lithium iron phosphate prepared by the above preparation method has good electrochemical performance. And the above preparation method does not require the use of a chemical vapor deposition device, and the operation is relatively simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A flow chart of a method for preparing carbon-coated lithium iron phosphate according to an embodiment;

[0022] Figure 2 This is a scanning electron microscope image of the carbon-coated lithium iron phosphate of Example 1;

[0023] Figure 3 The first charge and discharge curve of a CR2025 button-type half-cell using the carbon-coated lithium iron phosphate of Example 1 as the positive electrode material;

[0024] Figure 4The 18650 full battery composed of the carbon-coated lithium iron phosphate of Example 1 as the positive electrode material is charged at the same current density (0.5C), and the discharge performance curves of different current densities (0.2C, 0.5C, 1C, 2C and 4C) with a voltage range of 2.8 to 3.4V;

[0025] Figure 5 The 0.5C charge-discharge cycle performance curve of a 18650 full battery using the carbon-coated lithium iron phosphate of Example 1 as the positive electrode material. DETAILED DESCRIPTION

[0026] In order to make the above-mentioned purpose, features and advantages of the present invention more obvious and easy to understand, the specific implementation of the present invention is described in detail below in conjunction with the examples. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific implementation disclosed below.

[0027] like Figure 1 As shown, a method for preparing carbon-coated lithium iron phosphate according to an embodiment of the present invention comprises the following steps:

[0028] Step S110: Mix lithium iron phosphate, N,N'-dimethylacetamide and 4,4'-diaminodiphenyl ether under continuous stirring to obtain a mixed solution.

[0029] The specific step of mixing lithium iron phosphate, N,N'-dimethylacetamide and 4,4'-diaminodiphenyl ether under continuous stirring is as follows: adding lithium iron phosphate to N,N'-dimethylacetamide and ultrasonically stirring for 12 to 24 hours under an ultrasonic power of 150W to 250W, then adding 4,4'-diaminodiphenyl ether and ultrasonically stirring until 4,4'-diaminodiphenyl ether is dissolved.

[0030] In the step of mixing lithium iron phosphate, N,N'-dimethylacetamide and 4,4'-diaminodiphenyl ether, the mass ratio of lithium iron phosphate to N,N'-dimethylacetamide is 1:30-600; the mass ratio of lithium iron phosphate to 4,4'-diaminodiphenyl ether is 1:2-60.

[0031] In this embodiment, before step S110, a preparation step of lithium iron phosphate is also included, specifically: according to the molar ratio of phosphorus, iron and lithium of 1:1:2-3.6, under the condition of introducing protective gas, a phosphorus-containing alcohol solution, an iron-containing alcohol solution and a lithium-containing alcohol solution are mixed in a reaction kettle, the temperature is raised to 120°C-260°C, and the reaction is kept warm for 4 hours-15 hours under the condition of continuous stirring, and the temperature is lowered to obtain nano-scale lithium iron phosphate.

[0032] The preparation step of the phosphorus-containing alcohol solution is specifically as follows: dissolving a soluble phosphorus source compound in alcohol to obtain the phosphorus-containing alcohol solution. The soluble phosphorus source compound is selected from at least one of diammonium phosphate, diammonium hydrogen phosphate, phosphoric acid and ammonium phosphate; and the alcohol used to prepare the phosphorus-containing alcohol solution is selected from at least one of ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, butanetriol, n-butanol and isobutanol.

[0033] The mass percentage concentration of the phosphorus-containing alcohol solution is 20% to 30%.

[0034] The preparation step of the iron-containing alcohol solution is specifically as follows: dissolving an iron source compound in alcohol to obtain an iron-containing alcohol solution. The iron source compound is selected from at least one of ferrous oxide, ferrous chloride, ferrous tetroxide, ferric phosphate, ferrous oxalate, ferric nitrate, ferric citrate, ferrous phosphate and ferrous sulfate. The alcohol used to prepare the iron-containing alcohol solution is selected from at least one of ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, butanetriol, n-butanol and isobutanol.

[0035] The mass percentage concentration of the iron-containing alcohol solution is 20% to 30%.

[0036] The steps for preparing the lithium-containing alcohol solution are as follows: dissolving a lithium source compound in alcohol to obtain a lithium-containing alcohol solution. The lithium source compound is selected from at least one of lithium carbonate, lithium dihydrogen phosphate, lithium chloride, lithium acetate, lithium hydroxide, lithium oxalate, lithium nitrate and lithium phosphate. The alcohol used to prepare the lithium-containing alcohol solution is selected from at least one of ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, butanetriol, n-butanol and isobutanol.

[0037] The mass percentage concentration of the lithium-containing alcohol solution is 20% to 30%.

[0038] In this embodiment, after the phosphorus-containing alcohol solution, the iron-containing alcohol solution and the lithium-containing alcohol solution are mixed in a reactor, in the step of heating to 120°C to 260°C, the heating rate is 1°C / min to 10°C / min, and the pressure in the reactor is 0.2MPa to 4.7MPa. When the synthesis temperature is lower than 120°C, the crystallization rate is slow, the particles are formed slowly and in small quantities, and the formed crystals have large particle sizes; when the synthesis temperature is higher than 260°C, the crystallization rate is fast, the crystals have sharp edges and small particle sizes, and agglomeration is easy to occur. When the heating rate is lower than 1°C / min, the crystallization rate is slow, the particles are formed slowly and in small quantities, and the formed crystals have large particle sizes; when the heating temperature is higher than 10°C / min, the crystallization rate is fast, the crystals have sharp edges and small particle sizes, and agglomeration is easy to occur. The pressure in the reactor has a corresponding relationship with the temperature, and is positively correlated.

[0039] Wherein, the reactor is a high-pressure reactor.

[0040] Among them, in the step of preparing lithium iron phosphate, the cooling rate is 1℃ / min~20℃ / min. If the cooling rate is higher than 20℃ / min, the lithium iron phosphate particles are relatively small, but there may be many defects, so the discharge capacity is not ideal; if the cooling rate is lower than 1℃ / min, although the lithium iron phosphate crystals are relatively perfect, the particles are larger, so the discharge capacity is low and the polarization is large.

[0041] The step of mixing the phosphorus-containing alcohol solution, the iron-containing alcohol solution and the lithium-containing alcohol solution in the reactor is specifically as follows: adding the lithium-containing alcohol solution into the reactor, then adding the phosphorus-containing alcohol solution into the reactor under continuous stirring, then introducing protective gas into the reactor, then adding the iron-containing alcohol solution into the reactor under continuous stirring, and finally sealing the reactor.

[0042] In the preparation step of lithium iron phosphate, the protective gas is a non-oxidizing gas; for example, the protective gas may be at least one of an inert gas, nitrogen and hydrogen.

[0043] In this embodiment, in the step of preparing lithium iron phosphate, after the step of cooling, the step of separating and purifying the reaction liquid in the reactor is further included, specifically: subjecting the reaction liquid in the reactor to solid-liquid separation to obtain filter residue, washing the filter residue, and drying the filter residue to obtain purified nano-scale lithium iron phosphate. The method of drying the filter residue is vacuum drying at room temperature.

[0044] The lithium iron phosphate in step S110 can be purchased; preferably, it is nano-scale lithium iron phosphate prepared by the above-mentioned lithium iron phosphate preparation steps.

[0045] Step S120: adding pyromellitic anhydride to the mixed solution at 0° C. and under continuous stirring to obtain a composite adhesive solution.

[0046] Wherein, in the step of adding pyromellitic anhydride to the mixed solution at 0°C and under continuous stirring, the stirring rate of the continuous stirring is 1000 rpm to 2000 rpm. Wherein, pyromellitic anhydride is added to the mixed solution in 4 to 8 times, so that the polymerization reaction is more sufficient.

[0047] Specifically, the step of adding pyromellitic anhydride into the mixed solution is carried out in an ice water bath.

[0048] The molar ratio of 4,4'-diaminodiphenyl ether in step S110 to pyromellitic dianhydride in step S120 is 1:0.98-1.02.

[0049] Step S130: adding acetic anhydride and pyridine to the composite glue to carry out a chemical imidization reaction to obtain a reaction slurry, and drying the reaction slurry to obtain polyimide-coated lithium iron phosphate powder.

[0050] The chemical imidization reaction of adding acetic anhydride and pyridine into the composite glue solution is carried out at room temperature.

[0051] The molar ratio of pyromellitic dianhydride to acetic anhydride is 1:1-10; the molar ratio of acetic anhydride to pyridine is 1:1-5.

[0052] In the step of drying the reaction slurry, a spray drying method is used. The process parameters of the spray drying are: a feed rate of 5Kg / h to 15Kg / h, an air inlet temperature of 200°C to 300°C, an air outlet temperature of 90°C to 150°C, and a spray frequency of 250Hz to 350Hz. The device used for spray drying is a spray dryer.

[0053] Step S140: Under the condition of introducing protective gas, the polyimide-coated lithium iron phosphate powder is heated to 600° C. to 900° C., and calcined for 1 to 10 hours to obtain carbon-coated lithium iron phosphate.

[0054] Wherein, in the step of heating the lithium iron phosphate powder wrapped by polyimide to 600° C. to 900° C., the heating rate is 3 to 10° C. / min.

[0055] Specifically, the protective gas in step S140 is nitrogen or argon, and the flow rate of the protective gas is 0.5 L / min to 5 L / min.

[0056] The calcination of the polyimide-coated lithium iron phosphate powder can be carried out in a rotary kiln, a roller kiln, a pusher kiln or a tube furnace.

[0057] After step S140, the step of crushing the calcined product into particles with a particle size distribution of 2 microns to 15 microns is also included. The crushing method is to crush the coating structure without destroying it and then process it into particles. That is, during the crushing process, the coating carbon layer is peeled off from the surface of the lithium iron phosphate, resulting in the carbon coating layer not being tight enough.

[0058] The device used in the pulverizing step is a turbine pulverizer, an airflow turbine micro-powder machine, a super cyclone vortex mill, an air separation pulverizer or a double-roll pulverizer.

[0059] The preparation method of the above-mentioned carbon-coated lithium iron phosphate uses a polyimide polymer containing a strong polar functional group to wrap the lithium iron phosphate particles. Due to the polar interaction between the polyimide and the phosphate groups on the surface of the lithium iron phosphate, it is easy to form a core-shell coating structure, which effectively inhibits the agglomeration of the lithium iron phosphate particles in the composite colloid; more importantly, the conductive carbon layer produced by the pyrolysis of the polyimide polymer is evenly covered on the surface of the lithium iron phosphate particles, which enhances the conductivity of the lithium iron phosphate particles and is beneficial to the improvement of the rate performance; and due to the obstruction of the polymer pyrolysis carbon layer coated on the outer layer, the lithium iron phosphate particles can be effectively dispersed, avoiding the agglomeration and growth of the lithium iron phosphate particles, so that the carbon-coated lithium iron phosphate prepared by the above-mentioned preparation method has better electrochemical properties.

[0060] Furthermore, the preparation method does not require the use of a chemical vapor deposition device, is simple to operate, and uses inexpensive reagents, thereby reducing the preparation cost and facilitating industrial production.

[0061] A carbon-coated lithium iron phosphate prepared by the above-mentioned method for preparing carbon-coated lithium iron phosphate. Since the carbon-coated lithium iron phosphate is prepared by the above-mentioned method for preparing carbon-coated lithium iron phosphate, the carbon-coated lithium iron phosphate is not easy to agglomerate, has high conductivity, can prevent lithium iron phosphate particles from agglomerating and growing, and has good electrochemical performance.

[0062] A power lithium-ion battery comprises a positive electrode, wherein the material of the positive electrode comprises the carbon-coated lithium iron phosphate. Since the carbon-coated lithium iron phosphate is prepared by the carbon-coated lithium iron phosphate preparation method, the power lithium-ion battery has good initial charge and discharge performance and cycle performance.

[0063] The following is a specific embodiment:

[0064] Example 1

[0065] The preparation method of the carbon-coated lithium iron phosphate of this embodiment is as follows:

[0066] (1) Dissolve ammonium dihydrogen phosphate in ethylene glycol to obtain a phosphorus-containing alcohol solution with a mass percentage concentration of 25%; dissolve ferric oxide in ethylene glycol to obtain an iron-containing alcohol solution with a mass percentage concentration of 25%; dissolve lithium acetate in alcohol to obtain a lithium-containing alcohol solution with a mass percentage concentration of 25%. Add the lithium-containing alcohol solution into an autoclave according to a molar ratio of phosphorus, iron and lithium of 1:1:2, then add the phosphorus-containing alcohol solution into the autoclave under continuous stirring, then introduce nitrogen into the autoclave, then add the iron-containing alcohol solution into the autoclave under continuous stirring, and finally seal the autoclave. The autoclave was heated to 120°C at a heating rate of 1°C / min, and the pressure in the autoclave was set to 0.2 MPa. The reaction was kept warm for 4 hours under continuous stirring, and then cooled to room temperature at a cooling rate of 1°C / min. The reaction liquid in the autoclave was separated into solid and liquid to obtain a filter residue. The filter residue was washed and vacuum dried at room temperature to obtain nano lithium iron phosphate particles.

[0067] (2) Under the conditions of continuous stirring and an ultrasonic power of 150 W, nano-lithium iron phosphate was added to N,N'-dimethylacetamide at a mass ratio of nano-lithium iron phosphate to N,N'-dimethylacetamide of 1:30, and ultrasonic stirring was performed for 12 hours. Then, 4,4'-diaminodiphenyl ether was added at a mass ratio of nano-lithium iron phosphate to 4,4'-diaminodiphenyl ether of 1:2, and ultrasonic stirring was performed until 4,4'-diaminodiphenyl ether was completely dissolved to obtain a mixed solution.

[0068] (3) In an ice water bath and under continuous stirring at a stirring rate of 1000 rpm, pyromellitic anhydride was added to the mixed solution in four portions according to a molar ratio of 4,4'-diaminodiphenyl ether to pyromellitic anhydride of 1:1, and stirring was continued for 12 hours to obtain a composite adhesive solution.

[0069] (4) According to the molar ratio of pyromellitic anhydride to acetic anhydride being 1:1, and the molar ratio of acetic anhydride to pyridine being 1:1, acetic anhydride and pyridine are added to the composite glue at room temperature to carry out a chemical imidization reaction to obtain a reaction solution containing polyimide-wrapped nano-lithium iron phosphate.

[0070] (5) Using a spray dryer, the reaction solution containing polyimide-coated nano-lithium iron phosphate is spray-dried to obtain polyimide-coated nano-lithium iron phosphate powder, wherein the process parameters of the spray drying are: feed rate of 5 Kg / h, air inlet temperature of 200°C, air outlet temperature of 90°C, and spray frequency of 350 Hz.

[0071] (6) In the presence of nitrogen, the dried polyimide-coated nano-lithium iron phosphate powder was heated to 600° C. at a heating rate of 3° C. / min in a rotary kiln, and calcined at 600° C. for 1 hour. The powder was naturally cooled to room temperature to obtain carbon-coated lithium iron phosphate, wherein the nitrogen flow rate was 0.5 L / min.

[0072] (7) Using a turbine pulverizer, the carbon-coated lithium iron phosphate is pulverized into particles with a particle size distribution of 2 μm to 10 μm.

[0073] Figure 2 This is a scanning electron microscope image of the carbon-coated lithium iron phosphate prepared in this example. Figure 2 It can be seen that the carbon-coated lithium iron phosphate of this embodiment has a relatively regular microscopic appearance, all of which are rhombus-shaped flaky crystals with slight agglomeration.

[0074] Assembly of CR2025 button half-cell: The carbon-coated lithium iron phosphate of this embodiment is mixed with the conductive agent acetylene black and the binder PVDF (polyvinylidene fluoride) in a mass ratio of 8:1:1, and NMP (N-methylpyrrolidone) is used as a solvent. After the particles are ground and mixed, they are coated on aluminum foil to prepare a positive electrode. The positive electrode is cut into small pole pieces of 1 cm×1 cm, and vacuum dried at 120°C for 12 hours to obtain a half-cell. A pure Li sheet is used as the negative electrode, Celgard 2300 of the United States is used as the separator, and the electrolyte is: EC:DMC with a volume ratio of 1:1, 1 mol / LLiPF6 (Samsung, South Korea) is assembled in a glove box (M Braun) to form a CR2025 button half-cell.

[0075] The first charge and discharge data (charge specific capacity, discharge specific capacity and first coulombic efficiency) of the CR2025 button half-cell composed of the carbon-coated lithium iron phosphate of this embodiment as the positive electrode material tested using the blue power cabinet (Land) are shown in Table 1.

[0076] in, Figure 3 The first charge and discharge curve of the CR2025 button-type half-cell using the carbon-coated lithium iron phosphate of this embodiment as the positive electrode material. Figure 3 It can be seen that the dotted line is the charging curve. When the voltage rises to 4.0V, it corresponds to the charging capacity, which is 180.0mAh / g; the solid line is the discharge curve. When the voltage drops to 2.5V, it corresponds to the discharge capacity, which is 160.2mAh / g. The ratio of discharge capacity to charging capacity is the first coulomb efficiency, which is 88.5%.

[0077] Assembly and testing of 18650 full battery: The carbon-coated lithium iron phosphate material prepared in this embodiment is used as the positive active material of the lithium-ion battery, the conductive agent acetylene black, and polyvinylidene fluoride PVDF are used as binders to obtain the electrode material; the three are mixed in a mass ratio of active material: acetylene black: PVDF = 97:1.5:1.5. Add an appropriate amount of NMP, mix it into a paste with a slurry mixer, and then use a coating machine to apply it on aluminum foil, and then use it as a lithium-ion full battery positive electrode after vacuum drying. Artificial graphite, natural graphite or composite graphite is used as the negative electrode material; 1M LiPF6 / EC+DMC+EMC is used as the electrolyte; Celgard2300 type PE / PP / PE composite film is used as the diaphragm; the full battery is assembled using the conventional 18650 single cell production process, and the Wuhan Jinnuo LandCT2001A charge and discharge test cabinet is used.

[0078] Figure 4 The 18650 full battery composed of the carbon-coated lithium iron phosphate of this embodiment as the positive electrode material is charged at the same current density (0.5C), and the discharge performance curves of different current densities (0.2C, 0.5C, 1C, 2C and 4C) with a voltage range of 2.8 to 3.6V are shown in the figure. As can be seen from the figure, the discharge capacities at 0.2C, 0.5C, 1C, 2C, and 4C are 1.430Ah / g, 1.316Ah / g, 1.226Ah / g, 1.172Ah / g, and 1.160Ah / g, respectively. The discharge capacity retention rates of 0.5C / 0.2C, 1C / 0.2C, 2C / 0.2C, and 4C / 0.2C are 92.0%, 85.7%, 82.0%, and 81.1%, respectively.

[0079] Figure 5 This is a performance curve of 100 charge and discharge cycles at 0.5C for a 18650 full battery using the carbon-coated lithium iron phosphate of this embodiment as the positive electrode material. It can be seen from the figure that after 100 cycles, the discharge capacity retention rate is 97.2%.

[0080] Example 2

[0081] The preparation method of the carbon-coated lithium iron phosphate of this embodiment is as follows:

[0082] (1) Dissolve lithium nitrate in a mixture of propylene glycol and diethylene glycol to obtain a phosphorus-containing alcohol solution with a mass percentage concentration of 20%; dissolve ferrous chloride and ferroferric oxide in a mixture of propylene glycol and diethylene glycol to obtain an iron-containing alcohol solution with a mass percentage concentration of 25%; dissolve a lithium source compound in a mixture of propylene glycol and diethylene glycol to obtain a lithium-containing alcohol solution with a mass percentage concentration of 30%. Add the lithium-containing alcohol solution to a high-pressure reactor according to a molar ratio of phosphorus, iron and lithium of 1:1:2.2, then add the phosphorus-containing alcohol solution to the high-pressure reactor under continuous stirring, then introduce nitrogen into the high-pressure reactor, then add the iron-containing alcohol solution to the high-pressure reactor under continuous stirring, and finally seal the high-pressure reactor. The autoclave was heated to 140°C at a heating rate of 2°C / min, and the pressure in the autoclave was set to 1MPa. The reaction was kept warm for 6 hours under continuous stirring, and then cooled to room temperature at a cooling rate of 2°C / min. The reaction liquid in the autoclave was separated into solid and liquid to obtain a filter residue. The filter residue was washed and vacuum dried at room temperature to obtain nano lithium iron phosphate particles.

[0083] (2) Under the conditions of continuous stirring and an ultrasonic power of 180 W, nano-lithium iron phosphate was added to N,N'-dimethylacetamide at a mass ratio of nano-lithium iron phosphate to N,N'-dimethylacetamide of 1:100, and ultrasonic stirring was performed for 14 hours. Then, 4,4'-diaminodiphenyl ether was added at a mass ratio of nano-lithium iron phosphate to 4,4'-diaminodiphenyl ether of 1:10, and ultrasonic stirring was performed until 4,4'-diaminodiphenyl ether was completely dissolved to obtain a mixed solution.

[0084] (3) In an ice water bath and under continuous stirring at a stirring rate of 1200 rpm, pyromellitic anhydride was added to the mixed solution in four portions according to a molar ratio of 4,4'-diaminodiphenyl ether to pyromellitic anhydride of 1:1, and stirring was continued for 14 hours to obtain a composite adhesive solution.

[0085] (4) According to the molar ratio of pyromellitic anhydride to acetic anhydride being 1:2, and the molar ratio of acetic anhydride to pyridine being 1:2, acetic anhydride and pyridine are added to the composite glue at room temperature to carry out a chemical imidization reaction to obtain a reaction solution containing polyimide-wrapped nano-lithium iron phosphate.

[0086] (5) Using a spray dryer, the reaction solution containing polyimide-coated nano-lithium iron phosphate is spray-dried to obtain polyimide-coated nano-lithium iron phosphate powder, wherein the process parameters of the spray drying are: feed rate of 6 kg / h, air inlet temperature of 220° C., air outlet temperature of 100° C., and spray frequency of 270 Hz.

[0087] (6) The dried polyimide-coated nano-lithium iron phosphate powder was heated to 650° C. at a heating rate of 10° C. / min in a rotary kiln under nitrogen flow, and calcined at 650° C. for 1.5 hours. The powder was naturally cooled to room temperature to obtain carbon-coated lithium iron phosphate, wherein the nitrogen flow rate was 2.5 L / min.

[0088] (7) Using a turbine pulverizer, the carbon-coated lithium iron phosphate is pulverized into particles with a particle size distribution of 3 μm to 12 μm.

[0089] The same method as in Example 1 was adopted to prepare the carbon-coated lithium iron phosphate of this example into a CR2025 button half-cell, and the testing method of Example 1 was adopted to obtain the first charge and discharge data (charge specific capacity, discharge specific capacity and first coulombic efficiency) of the CR2025 button half-cell using the carbon-coated lithium iron phosphate of this example as the positive electrode material, as shown in Table 1.

[0090] Example 3

[0091] The preparation method of the carbon-coated lithium iron phosphate of this embodiment is as follows:

[0092] (1) Dissolve ammonium dihydrogen phosphate, diammonium hydrogen phosphate and ammonium phosphate in triethylene glycol to obtain a phosphorus-containing alcohol solution with a mass percentage concentration of 25%; dissolve ferric phosphate, ferrous oxalate and ferric nitrate in alcohol to obtain an iron-containing triethylene glycol solution with a mass percentage concentration of 30%; dissolve lithium nitrate and lithium phosphate in alcohol to obtain a lithium-containing triethylene glycol solution with a mass percentage concentration of 25%. Add the lithium-containing alcohol solution into a high-pressure reactor according to a molar ratio of phosphorus, iron and lithium of 1:1:2.4, then add the phosphorus-containing alcohol solution into the high-pressure reactor under continuous stirring, then introduce argon into the high-pressure reactor, then add the iron-containing alcohol solution into the high-pressure reactor under continuous stirring, and finally seal the high-pressure reactor. The autoclave was heated to 170°C at a heating rate of 4°C / min, and the pressure in the autoclave was set to 2MPa. The reaction was kept warm for 8 hours under continuous stirring, and then cooled to room temperature at a cooling rate of 5°C / min. The reaction liquid in the autoclave was separated into solid and liquid to obtain a filter residue. The filter residue was washed and vacuum dried at room temperature to obtain nano lithium iron phosphate particles.

[0093] (2) Under the conditions of continuous stirring and an ultrasonic power of 200 W, nano-lithium iron phosphate was added to N,N'-dimethylacetamide at a mass ratio of nano-lithium iron phosphate to N,N'-dimethylacetamide of 1:200, and ultrasonic stirring was performed for 15 hours. Then, 4,4'-diaminodiphenyl ether was added at a mass ratio of nano-lithium iron phosphate to 4,4'-diaminodiphenyl ether of 1:20, and ultrasonic stirring was performed until 4,4'-diaminodiphenyl ether was completely dissolved to obtain a mixed solution.

[0094] (3) In an ice water bath and under continuous stirring at a stirring rate of 1350 rpm, pyromellitic anhydride was added to the mixture in 5 portions according to a molar ratio of 4,4'-diaminodiphenyl ether to pyromellitic anhydride of 1:1, and stirring was continued for 16 hours to obtain a composite adhesive solution.

[0095] (4) According to the molar ratio of pyromellitic anhydride to acetic anhydride being 1:5, and the molar ratio of acetic anhydride to pyridine being 1:5, acetic anhydride and pyridine are added to the composite glue at room temperature to carry out a chemical imidization reaction to obtain a reaction solution containing polyimide-wrapped nano-lithium iron phosphate.

[0096] (5) Using a spray dryer, the reaction solution containing polyimide-coated nano-lithium iron phosphate is spray-dried to obtain polyimide-coated nano-lithium iron phosphate powder, wherein the process parameters of the spray drying are: feed rate of 6 kg / h, air inlet temperature of 240° C., air outlet temperature of 110° C., and spray frequency of 270 Hz.

[0097] (6) Under the condition of passing argon gas, the dried polyimide-coated nano-lithium iron phosphate powder was heated to 750°C at a heating rate of 5°C / min in a rotary kiln, and calcined at 750°C for 3 hours. After naturally cooling to room temperature, carbon-coated lithium iron phosphate was obtained, wherein the flow rate of argon gas was 1 L / min.

[0098] (7) Using a turbine pulverizer, the carbon-coated lithium iron phosphate is pulverized into particles with a particle size distribution of 5 μm to 15 μm.

[0099] The same method as in Example 1 was adopted to prepare the carbon-coated lithium iron phosphate of this example into a CR2025 button half-cell, and the testing method of Example 1 was adopted to obtain the first charge and discharge data (charge specific capacity, discharge specific capacity and first coulombic efficiency) of the CR2025 button half-cell using the carbon-coated lithium iron phosphate of this example as the positive electrode material, as shown in Table 1.

[0100] Example 4

[0101] The preparation method of the carbon-coated lithium iron phosphate of this embodiment is as follows:

[0102] (1) Dissolve diammonium hydrogen phosphate in a mixture of tetraethylene glycol, butanetriol and n-butanol to obtain a phosphorus-containing alcohol solution with a mass percentage concentration of 20%; dissolve ferric citrate and ferrous phosphate in a mixture of tetraethylene glycol, butanetriol and n-butanol to obtain an iron-containing alcohol solution with a mass percentage concentration of 20%; dissolve lithium oxalate in a mixture of tetraethylene glycol, butanetriol and n-butanol to obtain a lithium-containing alcohol solution with a mass percentage concentration of 20%. Add the lithium-containing alcohol solution into an autoclave according to a molar ratio of phosphorus, iron and lithium of 1:1:2.8, then add the phosphorus-containing alcohol solution into the autoclave under continuous stirring, then introduce a mixed gas of nitrogen and argon into the autoclave, then add the iron-containing alcohol solution into the autoclave under continuous stirring, and finally seal the autoclave. The autoclave was heated to 200°C at a heating rate of 6°C / min, and the pressure in the autoclave was set to 4 MPa. The reaction was kept warm for 10 hours under continuous stirring, and then cooled to room temperature at a cooling rate of 10°C / min. The reaction liquid in the autoclave was separated into solid and liquid to obtain a filter residue. The filter residue was washed and vacuum dried at room temperature to obtain nano lithium iron phosphate particles.

[0103] (2) Under the conditions of continuous stirring and an ultrasonic power of 200 W, nano-lithium iron phosphate was added to N,N'-dimethylacetamide at a mass ratio of nano-lithium iron phosphate to N,N'-dimethylacetamide of 1:300, and ultrasonic stirring was performed for 16 hours. Then, 4,4'-diaminodiphenyl ether was added at a mass ratio of nano-lithium iron phosphate to 4,4'-diaminodiphenyl ether of 1:30, and ultrasonic stirring was performed until 4,4'-diaminodiphenyl ether was completely dissolved to obtain a mixed solution.

[0104] (3) In an ice water bath and under continuous stirring at a stirring rate of 1600 rpm, pyromellitic anhydride was added to the mixed solution in 6 portions according to a molar ratio of 4,4'-diaminodiphenyl ether to pyromellitic anhydride of 1:1, and stirring was continued for 16 hours to obtain a composite adhesive solution.

[0105] (4) According to the molar ratio of pyromellitic anhydride to acetic anhydride being 1:1, and the molar ratio of acetic anhydride to pyridine being 1:1, acetic anhydride and pyridine are added to the composite glue at room temperature to carry out a chemical imidization reaction to obtain a reaction solution containing polyimide-wrapped nano-lithium iron phosphate.

[0106] (5) Using a spray dryer, the reaction solution containing polyimide-coated nano-lithium iron phosphate is spray-dried to obtain polyimide-coated nano-lithium iron phosphate powder, wherein the process parameters of the spray drying are: feed rate of 10 Kg / h, air inlet temperature of 250° C., air outlet temperature of 120° C., and spray frequency of 300 Hz.

[0107] (6) Under the condition of passing argon gas, the dried polyimide-coated nano-lithium iron phosphate powder was heated to 800° C. at a heating rate of 3° C. / min in a rotary kiln, and calcined at 800° C. for 5 hours. After naturally cooling to room temperature, carbon-coated lithium iron phosphate was obtained, wherein the flow rate of the protective gas was 2 L / min.

[0108] (7) Using a turbine pulverizer, the carbon-coated lithium iron phosphate is pulverized into particles with a particle size distribution of 7 μm to 15 μm.

[0109] The same method as in Example 1 was adopted to prepare the carbon-coated lithium iron phosphate of this example into a CR2025 button half-cell, and the testing method of Example 1 was adopted to obtain the first charge and discharge data (charge specific capacity, discharge specific capacity and first coulombic efficiency) of the CR2025 button half-cell using the carbon-coated lithium iron phosphate of this example as the positive electrode material, as shown in Table 1.

[0110] Example 5

[0111] The preparation method of the carbon-coated lithium iron phosphate of this embodiment is as follows:

[0112] (1) Phosphoric acid is dissolved in isobutyl alcohol to obtain a phosphorus-containing alcohol solution with a mass percentage concentration of 30%; ferrous sulfate is dissolved in isobutyl alcohol to obtain an iron-containing alcohol solution with a mass percentage concentration of 25%; lithium chloride, lithium acetate and lithium hydroxide are dissolved in isobutyl alcohol to obtain a lithium-containing alcohol solution with a mass percentage concentration of 30%. The lithium-containing alcohol solution is added to a high-pressure reactor according to a molar ratio of phosphorus, iron and lithium of 1:1:3.0, and then the phosphorus-containing alcohol solution is added to the high-pressure reactor under continuous stirring, and then hydrogen is introduced into the high-pressure reactor, and then the iron-containing alcohol solution is added to the high-pressure reactor under continuous stirring, and finally the high-pressure reactor is sealed. The autoclave was heated to 240°C at a heating rate of 8°C / min, and the pressure in the autoclave was set to 4.2 MPa. The reaction was kept warm for 12 hours under continuous stirring, and then cooled to room temperature at a cooling rate of 15°C / min. The reaction liquid in the autoclave was separated into solid and liquid to obtain a filter residue. The filter residue was washed and vacuum dried at room temperature to obtain nano lithium iron phosphate particles.

[0113] (2) Under the conditions of continuous stirring and an ultrasonic power of 220 W, nano-lithium iron phosphate was added to N,N'-dimethylacetamide at a mass ratio of nano-lithium iron phosphate to N,N'-dimethylacetamide of 1:450, and ultrasonic stirring was performed for 18 hours. Then, 4,4'-diaminodiphenyl ether was added at a mass ratio of nano-lithium iron phosphate to 4,4'-diaminodiphenyl ether of 1:45, and ultrasonic stirring was performed until 4,4'-diaminodiphenyl ether was completely dissolved to obtain a mixed solution.

[0114] (3) In an ice water bath and under continuous stirring at a stirring rate of 1600 rpm, pyromellitic anhydride was added to the mixed solution in 6 portions according to a molar ratio of 4,4'-diaminodiphenyl ether to pyromellitic anhydride of 1:1.02, and stirring was continued for 12 hours to obtain a composite adhesive solution.

[0115] (4) According to the molar ratio of pyromellitic anhydride to acetic anhydride being 1:10 and the molar ratio of acetic anhydride to pyridine being 1:2, acetic anhydride and pyridine are added to the composite glue at room temperature to carry out a chemical imidization reaction to obtain a reaction solution containing polyimide-wrapped nano-lithium iron phosphate.

[0116] (5) Using a spray dryer, the reaction solution containing polyimide-coated nano-lithium iron phosphate is spray-dried to obtain polyimide-coated nano-lithium iron phosphate powder, wherein the process parameters of the spray drying are: feed rate of 12 Kg / h, air inlet temperature of 270°C, air outlet temperature of 125°C, and spray frequency of 300 Hz.

[0117] (6) In the presence of nitrogen, the dried polyimide-coated nano-lithium iron phosphate powder was heated to 800° C. at a heating rate of 8° C. / min in a rotary kiln, and calcined at 800° C. for 6 hours. The powder was naturally cooled to room temperature to obtain carbon-coated lithium iron phosphate, wherein the nitrogen flow rate was 3.5 L / min.

[0118] (7) Using a turbine pulverizer, the carbon-coated lithium iron phosphate is pulverized into particles with a particle size distribution of 8 μm to 12 μm.

[0119] The same method as in Example 1 was adopted to prepare the carbon-coated lithium iron phosphate of this example into a CR2025 button half-cell, and the testing method of Example 1 was adopted to obtain the first charge and discharge data (charge specific capacity, discharge specific capacity and first coulombic efficiency) of the CR2025 button half-cell using the carbon-coated lithium iron phosphate of this example as the positive electrode material, as shown in Table 1.

[0120] Example 6

[0121] The preparation method of the carbon-coated lithium iron phosphate of this embodiment is as follows:

[0122] (1) Dissolve ammonium phosphate in glycerol to obtain a phosphorus-containing alcohol solution with a mass percentage concentration of 25%; dissolve ferric citrate in glycerol to obtain an iron-containing alcohol solution with a mass percentage concentration of 20%; dissolve lithium carbonate and lithium dihydrogen phosphate in glycerol to obtain a lithium-containing alcohol solution with a mass percentage concentration of 20%. Add the lithium-containing alcohol solution into a high-pressure reactor according to a molar ratio of phosphorus, iron and lithium of 1:1:3.6, then add the phosphorus-containing alcohol solution into the high-pressure reactor under continuous stirring, then introduce a mixed gas of hydrogen, nitrogen and argon into the high-pressure reactor, then add the iron-containing alcohol solution into the high-pressure reactor under continuous stirring, and finally seal the high-pressure reactor. The autoclave was heated to 260°C at a heating rate of 10°C / min, and the pressure in the autoclave was set to 4.7MPa. The reaction was kept warm for 15 hours under continuous stirring, and then cooled to room temperature at a cooling rate of 20°C / min. The reaction liquid in the autoclave was separated into solid and liquid to obtain a filter residue. The filter residue was washed and vacuum dried at room temperature to obtain nano lithium iron phosphate particles.

[0123] (2) Under the conditions of continuous stirring and an ultrasonic power of 250 W, nano-lithium iron phosphate was added to N,N'-dimethylacetamide at a mass ratio of nano-lithium iron phosphate to N,N'-dimethylacetamide of 1:600, and ultrasonic stirring was performed for 24 hours. Then, 4,4'-diaminodiphenyl ether was added at a mass ratio of nano-lithium iron phosphate to 4,4'-diaminodiphenyl ether of 1:60, and ultrasonic stirring was performed until 4,4'-diaminodiphenyl ether was completely dissolved to obtain a mixed solution.

[0124] (3) In an ice water bath and under continuous stirring at a stirring rate of 2000 rpm, pyromellitic anhydride was added to the mixed solution in 8 portions according to a molar ratio of 4,4'-diaminodiphenyl ether to pyromellitic anhydride of 1:0.98, and stirring was continued for 24 hours to obtain a composite adhesive solution.

[0125] (4) According to the molar ratio of pyromellitic anhydride to acetic anhydride being 1:1, and the molar ratio of acetic anhydride to pyridine being 1:1, acetic anhydride and pyridine are added to the composite glue at room temperature to carry out a chemical imidization reaction to obtain a reaction solution containing polyimide-wrapped nano-lithium iron phosphate.

[0126] (5) Using a spray dryer, the reaction solution containing polyimide-coated nano-lithium iron phosphate is spray-dried to obtain polyimide-coated nano-lithium iron phosphate powder, wherein the process parameters of the spray drying are: feed rate of 15 Kg / h, air inlet temperature of 300° C., air outlet temperature of 150° C., and spray frequency of 350 Hz.

[0127] (6) Under the condition of introducing argon gas, the dried polyimide-coated nano-lithium iron phosphate powder was heated to 900°C at a heating rate of 6°C / min in a rotary kiln, and calcined at 900°C for 10 hours. After naturally cooling to room temperature, carbon-coated lithium iron phosphate was obtained, wherein the introduction flow rate of argon gas was 5 L / min.

[0128] (7) Using a turbine pulverizer, the carbon-coated lithium iron phosphate is pulverized into particles with a particle size distribution of 10 μm to 15 μm.

[0129] The same method as in Example 1 was adopted to prepare the carbon-coated lithium iron phosphate of this example into a CR2025 button half-cell, and the testing method of Example 1 was adopted to obtain the first charge and discharge data (charge specific capacity, discharge specific capacity and first coulombic efficiency) of the CR2025 button half-cell using the carbon-coated lithium iron phosphate of this example as the positive electrode material, as shown in Table 1.

[0130] Comparative Example 1

[0131] The preparation steps of the carbon-coated lithium iron phosphate of Comparative Example 1 are as follows:

[0132] The iron phosphate, lithium hydroxide and glucose were weighed in a molar ratio of 1:1:2, and the three raw materials were placed in a polyurethane ball mill. Water was added in a volume ratio of 1:2 between the powder and the ball mill medium. The ball mill speed was 250rpm. After 12 hours of ball milling, it was spray dried at 220°C. The mixture was heated to 500°C at a heating rate of 10°C / min, heated to 700°C after 2 hours of heat preservation, and cooled to obtain the active material after 10 hours of heat preservation. The obtained active material, acetylene black and PVDF were mixed evenly in a mass ratio of 75:15:10, an appropriate amount of NMP was added and dispersed evenly, the obtained slurry was coated on aluminum foil, dried at 60°C and flattened with a roller to obtain a carbon-coated lithium iron phosphate positive electrode.

[0133] The carbon-coated lithium iron phosphate of this embodiment was prepared into a CR2025 button half-cell by the same method as in Example 1, and the test method of Example 1 was used to obtain the first charge and discharge data (charge specific capacity, discharge specific capacity and first coulombic efficiency) of the CR2025 button half-cell using the carbon-coated lithium iron phosphate of Comparative Example 1 as the positive electrode material, as shown in Table 1.

[0134] Table 1 shows the first charge and discharge test data (charge specific capacity, discharge specific capacity and first coulombic efficiency) of CR2025 button half-cells composed of carbon-coated lithium iron phosphate of Examples 1 to 6 and Comparative Example 1.

[0135] Table 1

[0136]

[0137]

[0138] It can be seen from Table 1 that the first reversible specific capacity of the button half-cell using the carbon-coated lithium iron phosphate of Examples 1 to 6 is at least 154.4 mAh / g, and the first coulombic efficiency is at least 84.9%, while the first discharge specific capacity of the button half-cell using the carbon-coated battery of Comparative Example 1 is only mAh / g, and the first coulombic efficiency is only 82.1%. Obviously, the battery using the carbon-coated lithium iron phosphate of Examples 1 to 6 has better initial charge and discharge performance, that is, the carbon-coated lithium iron phosphate of Examples 1 to 6 has better electrochemical performance.

[0139] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for preparing carbon-coated lithium iron phosphate, It is characterized in that The steps include: Under continuous stirring, lithium iron phosphate, N,N'-dimethylacetamide and 4,4'-diaminodiphenyl ether are mixed to obtain a mixed solution, wherein the mass ratio of the lithium iron phosphate to the N,N'-dimethylacetamide is 1:30-600, and the mass ratio of the lithium iron phosphate to the 4,4'-diaminodiphenyl ether is 1:2-60; Under the condition of 0° C. and continuous stirring, adding pyromellitic anhydride to the mixed solution to obtain a composite glue solution, wherein the molar ratio of the 4,4'-diaminodiphenyl ether to pyromellitic anhydride is 1:0.98-1.02; Adding acetic anhydride and pyridine to the composite glue to carry out a chemical imidization reaction to obtain a reaction slurry, and drying the reaction slurry to obtain polyimide-coated lithium iron phosphate powder, wherein the molar ratio of the pyromellitic dianhydride to the acetic anhydride is 1:1-10; the molar ratio of the acetic anhydride to the pyridine is 1:1-5; and Under the condition of introducing protective gas, the polyimide-coated lithium iron phosphate powder is heated to 600° C. to 900° C. and calcined for 1 to 10 hours to obtain the carbon-coated lithium iron phosphate.

2. The method for preparing carbon-coated lithium iron phosphate according to claim 1, It is characterized in that The step of mixing the lithium iron phosphate, N,N'-dimethylacetamide and 4,4'-diaminodiphenyl ether under continuous stirring is specifically as follows: adding the lithium iron phosphate to the N,N'-dimethylacetamide under an ultrasonic power of 150W to 250W, ultrasonically stirring for 12 to 24 hours, then adding the 4,4'-diaminodiphenyl ether, and ultrasonically stirring until the 4,4'-diaminodiphenyl ether is dissolved.

3. The method for preparing carbon-coated lithium iron phosphate according to claim 1, It is characterized in that In the step of drying the reaction slurry, a spray drying method is adopted, wherein the process parameters of the spray drying are: a feed rate of 5Kg / h to 15Kg / h, an air inlet temperature of 200°C to 300°C, an air outlet temperature of 90°C to 150°C, and a spray frequency of 250Hz to 350Hz.

4. The method for preparing carbon-coated lithium iron phosphate according to claim 1, It is characterized in that In the step of heating the polyimide-wrapped lithium iron phosphate powder to 600° C. to 900° C., the heating rate is 3 to 10° C. / min.

5. The method for preparing carbon-coated lithium iron phosphate according to claim 1, It is characterized in that The method also includes the steps of preparing the lithium iron phosphate, specifically, mixing a phosphorus-containing alcohol solution, an iron-containing alcohol solution and a lithium-containing alcohol solution in a reaction kettle according to a molar ratio of phosphorus, iron and lithium of 1:1:2 to 3.6 under the condition of introducing a protective gas, heating the mixture to 120°C to 260°C, keeping the mixture warm for 4 to 15 hours under continuous stirring, and cooling the mixture to obtain the nano-scale lithium iron phosphate.

6. The method for preparing carbon-coated lithium iron phosphate according to claim 5, It is characterized in that After the phosphorus-containing alcohol solution, the iron-containing alcohol solution and the lithium-containing alcohol solution are mixed in the reactor, in the step of heating the temperature to 120°C to 260°C, the heating rate is 1°C / min to 10°C / min, and the pressure in the reactor is 0.2MPa to 4.7MPa.

7. A carbon-coated lithium iron phosphate prepared by the method for preparing carbon-coated lithium iron phosphate according to any one of claims 1 to 6.

8. A power lithium-ion battery, comprising a positive electrode, It is characterized in that The material of the positive electrode includes the carbon-coated lithium iron phosphate as claimed in claim 7.

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