Carbon-coated lithium iron phosphate anode material and preparation method thereof

Abstract

The application provides a carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof. The preparation method of the carbon-coated lithium iron phosphate positive electrode material comprises the following steps: S1, performing a hydrothermal reaction on a first carbon source and a metal salt solution to obtain carbon-coated metal hydroxide; S2, performing calcination treatment on the carbon-coated metal hydroxide in a protective atmosphere to obtain carbon-coated metal oxide; S3, mixing and reacting the carbon-coated metal oxide with an iron salt solution to obtain carbon-coated iron hydroxide, and then performing dehydration on the carbon-coated iron hydroxide to obtain carbon-coated iron oxide; and S4, mixing a lithium source, a phosphorus source, a second carbon source and the carbon-coated iron oxide in a solvent, and then sequentially performing spray drying, sintering and crushing treatment to obtain the carbon-coated lithium iron phosphate positive electrode material. The carbon source has a high graphitization degree, and the generation of impurities is avoided. The carbon-coated lithium iron phosphate positive electrode material has uniform carbon coating and high conductivity.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery cathode material technology, and in particular relates to a carbon-coated lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium iron phosphate (LFP) batteries are widely used in the power battery field due to their advantages such as high safety, long cycle life, and low cost. However, their inherent low electronic conductivity and slow ion diffusion rate require carbon coating modification to improve performance. Traditional carbon coating methods often use organic carbon sources such as sucrose, glucose, and polyethylene glycol, which are sintered at 700–800℃. At this temperature, the carbon source has a low degree of graphitization, resulting in amorphous carbon with poor conductivity, which is insufficient for high-rate batteries. Furthermore, increasing the sintering temperature can lead to excessive decomposition of the organic carbon source, producing reducing gases such as CO and CH4, which can cause Fe3+ in the lithium iron phosphate to degrade. 2+ The lithium iron phosphate (LFP) is reduced to elemental Fe, which then reacts with the phosphorus source to form iron phosphide impurities, severely reducing battery capacity and cycle stability. Simultaneously, during the high-temperature solid-state reaction, LFP particles tend to agglomerate, exhibiting a wide size distribution, and there are no effective means to directly and precisely grade and control the thickness and uniformity of the carbon coating layer. This results in difficulty in controlling particle size and compaction density, high inter-particle porosity, and a compaction density typically below 2.6 g / cm³. 3 This limits the improvement of battery volumetric energy density.

[0003] While existing technologies have attempted to modify lithium iron phosphate using highly conductive carbon sources such as graphene and carbon nanotubes, these methods mostly involve physical mixing. Patent application CN117558903A discloses a method for preparing graphene-coated lithium iron phosphate, which involves mixing and grinding a lithium iron phosphate precursor with graphite powder, followed by calcination to obtain graphene-coated lithium iron phosphate. However, the interface between the carbon source and lithium iron phosphate is weak, making it difficult for the lithium iron phosphate precursor to distribute uniformly on the carbon source surface, resulting in uneven distribution of the final lithium iron phosphate particles. Furthermore, it fails to resolve the core contradiction between impurity phase formation and interface control during high-temperature sintering. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a carbon-coated lithium iron phosphate cathode material and its preparation method. Addressing the problems of low graphitization of existing carbon sources, easy generation of impurities during high-temperature sintering, and uneven carbon coating, this invention achieves uniform carbon coating and high conductivity in lithium iron phosphate while avoiding the formation of impurities.

[0005] To achieve the above objectives, the present invention provides a method for preparing a carbon-coated lithium iron phosphate cathode material, comprising the following steps:

[0006] S1. The first carbon source and the metal salt solution are subjected to a hydrothermal reaction to obtain carbon-coated metal hydroxide;

[0007] S2. Under a protective atmosphere, the carbon-coated metal hydroxide is calcined to obtain a carbon-coated metal oxide.

[0008] S3. The carbon-coated metal oxide is mixed and reacted with an iron salt solution to obtain carbon-coated iron hydroxide; the carbon-coated iron hydroxide is dehydrated to obtain carbon-coated iron oxide.

[0009] S4. After mixing the lithium source, phosphorus source, second carbon source and the carbon-coated iron oxide in a solvent, spray drying, sintering and crushing are performed sequentially to obtain carbon-coated lithium iron phosphate cathode material.

[0010] In step S1, the first carbon source is a carbon nanotube; the metal salt solution includes a magnesium salt solution and / or a calcium salt solution.

[0011] This invention uses a highly graphitized carbon source as a substrate to in-situ coat metal oxides. The metal oxides hydrate to form metal hydroxides, which react with an iron salt solution to generate ferric hydroxide, which is then loaded onto the carbon layer, resulting in carbon-coated ferric hydroxide. After dehydration, the carbon-coated ferric hydroxide yields carbon-coated ferric oxide. During this reaction, the highly graphitized carbon layer does not participate in the reaction but remains as a rigid framework, ensuring that the Fe2O3 particle size is consistent with that of the metal oxide, and that the integrity and uniformity of the carbon layer are not compromised.

[0012] The carbon source of this invention has a high degree of graphitization and stable metal oxide performance, avoiding the formation of impurity phases; the deposition of metal oxides on the carbon source surface is easy to control, the lithium iron phosphate cathode material particles are easy to adjust, and the carbon coating is uniform.

[0013] Another aspect of the present invention provides a carbon-coated lithium iron phosphate cathode material, which is prepared by the method for preparing the carbon-coated lithium iron phosphate cathode material;

[0014] The carbon-coated lithium iron phosphate cathode material includes a lithium iron phosphate core and a carbon layer coated on the surface of the lithium iron phosphate core.

[0015] The thickness of the carbon layer is 5 nm to 20 nm;

[0016] The particle size of the carbon-coated lithium iron phosphate cathode material must meet the following conditions: D10≥0.3μm, D50 is 1μm~1.5μm, and D100≤20μm.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] This invention uses a highly graphitized carbon source as a substrate. Metal oxides are hydrated to form metal hydroxides. These metal hydroxides react with an iron salt solution to generate ferric hydroxide, which is then loaded onto a carbon layer, resulting in carbon-coated ferric hydroxide. The carbon-coated ferric hydroxide is then dehydrated to obtain carbon-coated ferric oxide. During this reaction, the highly graphitized carbon layer does not participate in the reaction itself, but remains as a rigid framework, ensuring that the Fe₂O₃ particle size is consistent with that of the metal oxide, and that the integrity and uniformity of the carbon layer are not compromised.

[0019] The carbon source of this invention has a high degree of graphitization and stable metal oxide performance, avoiding the formation of impurity phases; the deposition of metal oxides on the carbon source surface is easy to control, the lithium iron phosphate cathode material particles are easy to adjust, and the carbon coating is uniform.

[0020] The carbon layer of this invention is a highly graphitized conductive carbon layer, and the conductivity of the lithium iron phosphate cathode material reaches 10. -1 S / cm~10 0 The S / cm ratio is 1 to 2 orders of magnitude higher than that of traditional amorphous carbon coating, resulting in excellent rate performance. Attached Figure Description

[0021] Figure 1 This is a scanning electron microscope (SEM) image, magnified 10,000 times, of the carbon-coated lithium iron phosphate cathode material prepared in Example 1 of the present invention.

[0022] Figure 2 This is a scanning electron microscope (SEM) image, magnified 10,000 times, of the carbon-coated lithium iron phosphate cathode material prepared in Example 2 of the present invention.

[0023] Figure 3 This is a transmission electron microscope (TEM) image of the carbon-coated lithium iron phosphate cathode material prepared in Example 1 of the present invention;

[0024] Figure 4 This is a scanning electron microscope (SEM) image, magnified 10,000 times, of the carbon-coated lithium iron phosphate cathode material prepared in Comparative Example 1 of this invention. Detailed Implementation

[0025] The present invention will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention.

[0026] Unless otherwise defined, all technical terms used in this invention have the same meaning as commonly understood by those skilled in the art. The technical terms used in this invention are for the purpose of describing particular embodiments only and are not intended to limit the scope of protection of this invention.

[0027] In the following examples, unless otherwise specified, all experimental instruments and materials involved are commercially available products.

[0028] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0029] D50 represents the particle size value corresponding to when the cumulative volumetric particle size distribution number of the material reaches 50%; D10 and D100 have similar definitions to D50. Further details are omitted here, and those skilled in the art should not construe this as a limitation of the invention.

[0030] This invention provides a method for preparing carbon-coated lithium iron phosphate cathode material, comprising the following steps:

[0031] S1. The first carbon source and the metal salt solution are subjected to a hydrothermal reaction to obtain carbon-coated metal hydroxide;

[0032] S2. Under a protective atmosphere, the carbon-coated metal hydroxide is calcined to obtain a carbon-coated metal oxide, denoted as C@MO, where MO is MgO / CaO.

[0033] S3. The carbon-coated metal oxide is mixed and reacted with an iron salt solution to obtain carbon-coated iron hydroxide; the carbon-coated iron hydroxide is dehydrated to obtain carbon-coated iron oxide (C@Fe2O3).

[0034] S4. After mixing the lithium source, phosphorus source, second carbon source and the carbon-coated iron oxide in a solvent, spray drying, sintering and crushing are performed sequentially to obtain carbon-coated lithium iron phosphate cathode material.

[0035] In step S1, the first carbon source is a carbon nanotube; the metal salt solution includes a magnesium salt solution and / or a calcium salt solution.

[0036] This invention uses a highly graphitized carbon source as a substrate to in-situ coat metal oxides. The metal oxides hydrate to form metal hydroxides, which react with an iron salt solution to generate ferric hydroxide, which is then loaded onto the carbon layer, resulting in carbon-coated ferric hydroxide. After dehydration, the carbon-coated ferric hydroxide yields carbon-coated ferric oxide. During this reaction, the highly graphitized carbon layer does not participate in the reaction but remains as a rigid framework, ensuring that the Fe2O3 particle size is consistent with that of the metal oxide, and that the integrity and uniformity of the carbon layer are not compromised.

[0037] The carbon source of this invention has a high degree of graphitization and stable metal oxide performance, avoiding the formation of impurity phases; the deposition of metal oxides on the carbon source surface is easy to control, the lithium iron phosphate cathode material particles are easy to adjust, and the carbon coating is uniform.

[0038] The carbon layer of this invention is a highly graphitized conductive carbon layer, and the conductivity of the lithium iron phosphate cathode material reaches 10. -1 S / cm~10 0 The S / cm ratio is 1-2 orders of magnitude higher than that of traditional amorphous carbon coating, resulting in excellent rate performance.

[0039] According to a preferred embodiment of the present invention, the magnesium salt solution comprises one or more of magnesium nitrate solution, magnesium sulfate solution, and magnesium chloride solution. The calcium salt solution comprises one or more of calcium nitrate solution, calcium sulfate solution, and calcium chloride solution.

[0040] Furthermore, the particle size of the metal oxide particles in the carbon-coated metal oxide is 100 nm to 1000 nm.

[0041] Further, in step S1, the hydrothermal reaction temperature is 120℃~200℃, and the reaction time is 6h~12h. The mass ratio of the first carbon source to the dry weight of the metal salt solution is 1:(5~20), and the dry weight of the metal salt solution is based on the mass of the metal oxide. The concentration of the metal salt solution is 0.1mol / L~1mol / L.

[0042] Further, in step S2, the calcination temperature is 600℃~800℃, the heating rate is 2~10℃ / min, and the holding time is 2h~4h. The protective atmosphere is at least one of nitrogen, helium, or argon.

[0043] According to a preferred embodiment of the present invention, in step S3, the temperature of the mixing reaction is 25℃~80℃, and the reaction time is 1h~6h. The mass ratio of the carbon-coated metal oxide to the iron salt solution is 1:(10~50). The concentration of the iron salt solution is 0.1mol / L~1mol / L; the pH value of the iron salt solution is 2~5. The iron salt solution comprises one or more of ferric nitrate solution, ferric sulfate solution, and ferric chloride solution.

[0044] According to a preferred embodiment of the present invention, in step S3, the dehydration temperature is 300℃~600℃ and the dehydration time is 4h~6h.

[0045] According to a preferred embodiment of the present invention, in step S4, the molar ratio of lithium, iron, and phosphorus in the lithium source, the carbon-coated iron oxide, and the phosphorus source is (1.02-1.05):1:1. The phosphorus source includes at least one of phosphoric acid and diammonium phosphate, but is not limited thereto. The lithium source includes at least one of lithium carbonate and lithium hydroxide, but is not limited thereto.

[0046] According to a preferred embodiment of the present invention, in step S4, based on the total mass of the lithium source, the carbon-coated iron oxide, and the phosphorus source, the amount of the second carbon source added is 0.5% to 1%. The second carbon source comprises one or more of citric acid, glucose, sucrose, ascorbic acid, polyaniline, and polypyrrolidone.

[0047] According to a preferred embodiment of the present invention, in step S4, the solvent is at least one selected from water, ethanol, isopropanol, n-butanol, ethylene glycol, propylene glycol, acetone, and butanone.

[0048] According to a preferred embodiment of the present invention, in step S4, the inlet temperature of the spray dryer is 200℃~300℃, the outlet temperature is 90℃~120℃, and the processing time is 0.5h~4h.

[0049] According to a preferred embodiment of the present invention, in step S4, the sintering is carried out under a protective atmosphere; the sintering temperature is 650℃~750℃, the heating rate is 2℃ / min~10℃ / min, and the holding time is 6h~12h.

[0050] According to a preferred embodiment of the present invention, in step S4, the crushing is airflow crushing.

[0051] Another aspect of the present invention provides a carbon-coated lithium iron phosphate cathode material, which is prepared by the method for preparing the carbon-coated lithium iron phosphate cathode material;

[0052] The carbon-coated lithium iron phosphate cathode material includes a lithium iron phosphate core and a carbon layer coated on the surface of the lithium iron phosphate core.

[0053] The thickness of the carbon layer is 5 nm to 20 nm;

[0054] The particle size of the carbon-coated lithium iron phosphate cathode material must meet the following conditions: D10≥0.3μm, D50 is 1μm~1.5μm, and D100≤20μm.

[0055] Example 1

[0056] This embodiment provides a method for preparing a carbon-coated lithium iron phosphate cathode material, comprising the following steps:

[0057] (1) Carbon nanotubes were mixed with magnesium nitrate (Mg(NO3)2·6H2O) solution, ultrasonically dispersed, and then transferred to a reaction vessel. The mixture was subjected to a hydrothermal reaction at 120℃ for 12 h, causing magnesium ions to be deposited in situ on the surface of the carbon nanotubes as magnesium hydroxide, thus obtaining carbon-coated magnesium hydroxide. The concentration of the magnesium nitrate solution was 0.1 mol / L, and the mass ratio of the dry weight of the carbon nanotubes to the magnesium nitrate solution was 1:5. The dry weight of the magnesium nitrate solution was calculated based on the mass of magnesium oxide. The parameters of the carbon nanotubes were: diameter 2 nm, conductivity 78 S / cm.

[0058] (2) The carbon-coated magnesium hydroxide was placed in an atmosphere furnace and heated to 600°C at a rate of 3°C / min under a nitrogen atmosphere, and then calcined at a constant temperature for 4 hours. The magnesium hydroxide was dehydrated to form magnesium oxide (MgO), while the carbon source maintained a highly graphitized structure, forming carbon-coated magnesium oxide (C@MgO). The particle size of the magnesium oxide particles in the carbon-coated magnesium oxide was 267 nm.

[0059] (3) Prepare a 0.1 mol / L ferric nitrate (Fe(NO3)3·9H2O) solution and adjust the pH to 2. Add carbon-coated magnesium oxide to the ferric nitrate solution, with a mass ratio of 1:10. Stir the reaction at 25℃ for 6 h. MgO hydrates to form Mg(OH)2. Utilize the difference in solubility product between Mg(OH)2 and Fe(OH)3 to make Fe... 3+ Spontaneous substitution of Mg in Mg(OH)2 2+ Fe(OH)3 is generated and loaded onto the carbon layer. After the reaction is complete, the mixture is centrifuged and washed three times with deionized water to obtain carbon-coated ferric hydroxide. The carbon-coated ferric hydroxide is then dehydrated at 400℃ for 6 h to obtain carbon-coated ferric oxide (C@Fe2O3).

[0060] (4) Lithium carbonate, carbon-coated iron oxide, diammonium phosphate, and glucose were added to ethanol and mixed to obtain a mixture. The amounts of lithium carbonate, carbon-coated iron oxide, and diammonium phosphate added were such that the molar ratio of lithium, iron, and phosphorus was 1.02:1:1, and the mass of carbon-coated iron oxide was 1000g. The amount of glucose added was 1% of the sum of the masses of lithium carbonate, carbon-coated iron oxide, and diammonium phosphate. The solid content of the mixture was 30%.

[0061] The mixture is spray-dried in a spray drying tower to obtain dry material. The inlet temperature of the spray dryer is 200℃, the outlet temperature is 90℃, and the spray drying time is 2.5h.

[0062] The dry material was placed in an atmosphere furnace and heated to 650°C at a rate of 3°C / min under a nitrogen atmosphere. The material was then sintered at a constant temperature for 12 hours to obtain lithium iron phosphate sintered material.

[0063] After the lithium iron phosphate sinter was naturally cooled, it was pulverized in an air jet mill to obtain carbon-coated lithium iron phosphate cathode material. The performance parameters of the carbon-coated lithium iron phosphate cathode material are shown in Table 1.

[0064] Example 2

[0065] This embodiment provides a method for preparing a carbon-coated lithium iron phosphate cathode material, comprising the following steps:

[0066] (1) Carbon nanotubes were mixed with calcium chloride (CaCl2·2H2O) solution, ultrasonically dispersed, and then transferred to a reaction vessel. The mixture was subjected to hydrothermal reaction at 160℃ for 9 h, allowing calcium ions to be deposited in situ on the surface of the carbon nanotubes as calcium hydroxide, thus obtaining carbon-coated calcium hydroxide. The concentration of the calcium chloride solution was 0.5 mol / L; the mass ratio of the dry weight of the carbon nanotubes to the calcium chloride solution was 1:12, and the dry weight of the calcium chloride solution was based on the mass of calcium oxide. The parameters of the carbon nanotubes were: diameter 3 nm, conductivity 56 S / cm.

[0067] (2) The carbon-coated calcium hydroxide was placed in an atmosphere furnace and heated to 700°C at a rate of 5°C / min under a nitrogen atmosphere, and then calcined at a constant temperature for 3 hours. The calcium hydroxide was dehydrated to form calcium oxide (CaO), while the carbon source maintained a highly graphitized structure, forming carbon-coated calcium oxide (C@CaO). The particle size of the calcium oxide particles in the carbon-coated calcium oxide was 324 nm.

[0068] (3) Prepare a 0.5 mol / L ferric chloride (FeCl3·6H2O) solution and adjust the pH to 3. Add carbon-coated calcium oxide to the ferric chloride solution, with a mass ratio of 1:30. Stir the reaction at 50℃ for 3 h. CaO hydrates to form Ca(OH)2. Utilize the difference in solubility product between Ca(OH)2 and Fe(OH)3 to make Fe... 3+ Spontaneous substitution of Ca in Ca(OH)2 2+ Fe(OH)3 is generated and loaded onto the carbon layer. After the reaction is complete, the mixture is centrifuged and washed four times with deionized water to obtain carbon-coated ferric hydroxide. The carbon-coated ferric hydroxide is then dehydrated at 450℃ for 5 h to obtain carbon-coated ferric oxide (C@Fe2O3).

[0069] (4) Lithium carbonate, carbon-coated iron oxide, diammonium phosphate, and citric acid were added to ethanol and mixed to obtain a mixture. The amounts of lithium carbonate, carbon-coated iron oxide, and diammonium phosphate added were such that the molar ratio of lithium, iron, and phosphorus was 1.03:1:1, and the mass of carbon-coated iron oxide was 1000g. The amount of citric acid added was 0.8% of the sum of the masses of lithium carbonate, carbon-coated iron oxide, and diammonium phosphate. The solid content of the mixture was 35%.

[0070] The mixture is spray-dried in a spray drying tower to obtain dry material. The inlet temperature of the spray dryer is 200℃, the outlet temperature is 90℃, and the spray drying time is 2 hours.

[0071] The dry material was placed in an atmosphere furnace and heated to 700°C at a rate of 5°C / min under a nitrogen atmosphere. The material was then sintered at a constant temperature for 9 hours to obtain lithium iron phosphate sintered material.

[0072] After the lithium iron phosphate sinter was naturally cooled, it was pulverized in an air jet mill to obtain carbon-coated lithium iron phosphate cathode material. The performance parameters of the carbon-coated lithium iron phosphate cathode material are shown in Table 1.

[0073] Example 3

[0074] This embodiment provides a method for preparing a carbon-coated lithium iron phosphate cathode material, comprising the following steps:

[0075] (1) Carbon nanotubes were mixed with magnesium sulfate (MgSO4·7H2O) solution, ultrasonically dispersed, and then transferred to a reaction vessel. The mixture was subjected to hydrothermal reaction at 200℃ for 6 h, allowing magnesium ions to be deposited in situ on the surface of the carbon nanotubes as magnesium hydroxide, thus obtaining carbon-coated magnesium hydroxide. The concentration of the magnesium sulfate solution was 1 mol / L; the mass ratio of the dry weight of the carbon nanotubes to the magnesium sulfate solution was 1:20, and the dry weight of the magnesium sulfate solution was based on the mass of magnesium oxide. The parameters of the carbon nanotubes were: diameter 5 nm, conductivity 43 S / cm.

[0076] (2) The carbon-coated magnesium hydroxide was placed in an atmosphere furnace and heated to 800°C at a rate of 7°C / min under a nitrogen atmosphere, and then calcined at a constant temperature for 2 hours. The magnesium hydroxide was dehydrated to form magnesium oxide (MgO), while the carbon source maintained a highly graphitized structure, forming carbon-coated magnesium oxide (C@MgO). The particle size of the magnesium oxide particles in the carbon-coated magnesium oxide was 272 nm.

[0077] (3) Prepare a 1 mol / L ferric sulfate (Fe2(SO4)3) solution and adjust the pH to 5. Add carbon-coated magnesium oxide to the ferric sulfate solution, with a mass ratio of 1:50. Stir the reaction at 80℃ for 1 h. MgO hydrates to form Mg(OH)2. Utilize the difference in solubility product between Mg(OH)2 and Fe(OH)3 to make Fe... 3+ Spontaneous substitution of Mg in Mg(OH)2 2+ Fe(OH)3 is generated and loaded onto the carbon layer. After the reaction is complete, the mixture is centrifuged and washed five times with deionized water to obtain carbon-coated ferric hydroxide. The carbon-coated ferric hydroxide is then dehydrated at 500℃ for 4 h to obtain carbon-coated ferric oxide (C@Fe2O3).

[0078] (4) Lithium carbonate, carbon-coated iron oxide, diammonium phosphate, and polyaniline were added to ethanol and mixed to obtain a mixture. The amounts of lithium carbonate, carbon-coated iron oxide, and diammonium phosphate added were such that the molar ratio of lithium, iron, and phosphorus was 1.05:1:1, and the mass of carbon-coated iron oxide was 1000g. The amount of polyaniline added was 0.5% of the sum of the masses of lithium carbonate, carbon-coated iron oxide, and diammonium phosphate, and the solid content of the mixture was 40%.

[0079] The mixture is spray-dried in a spray drying tower to obtain dry material. The inlet temperature of the spray dryer is 200℃, the outlet temperature is 90℃, and the spray drying time is 1 hour.

[0080] The dry material was placed in an atmosphere furnace and heated to 750°C at a rate of 6°C / min under a nitrogen atmosphere. The material was then sintered at a constant temperature for 6 hours to obtain lithium iron phosphate sintered material.

[0081] After the lithium iron phosphate sinter was naturally cooled, it was pulverized in an air jet mill to obtain carbon-coated lithium iron phosphate cathode material. The performance parameters of the carbon-coated lithium iron phosphate cathode material are shown in Table 1.

[0082] Example 4

[0083] The method for preparing carbon-coated lithium iron phosphate cathode material in this embodiment differs from that in Example 2 in that: in step (3), a 1.3 mol / L ferric chloride (FeCl3·6H2O) solution is prepared and the pH is adjusted to 3.

[0084] Comparative Example 1

[0085] The comparative example describes a method for preparing a carbon-coated lithium iron phosphate cathode material, comprising the following steps:

[0086] (1) Lithium carbonate, iron oxide, diammonium phosphate, and carbon nanotubes were mixed and ground in ethanol to obtain a slurry. The molar ratio of lithium, iron, and phosphorus in the addition of lithium carbonate, iron oxide, and diammonium phosphate was 1.02:1:1, and the mass of iron oxide was 1000g. The amount of carbon nanotubes added was 8% of the sum of the masses of lithium carbonate, iron oxide, and diammonium phosphate. The parameters of the carbon nanotubes were: diameter 3nm, electrical conductivity 56S / cm.

[0087] The mixed grinding process includes a sequential mixing process and a grinding process. The grinding process is carried out in a sand mill. The zirconium balls used in the sand mill have a particle size of 0.1µm. The particle size distribution in the slurry meets the following conditions: Dv50 is 0.3µm and Dv100 is 0.8µm. The solid content of the slurry is 35%.

[0088] (2) The slurry is spray-dried in a spray drying tower to obtain dry material. The inlet temperature of the spray drying is 200℃, the outlet temperature is 90℃, and the spray drying time is 2h.

[0089] (3) Place the dry material in an atmosphere furnace and heat it to 700°C at a rate of 5°C / min under a nitrogen atmosphere. Sinter at a constant temperature for 9 hours to obtain lithium iron phosphate sintered material.

[0090] (4) After the lithium iron phosphate sinter is naturally cooled, it is pulverized in an air jet mill to obtain carbon-coated lithium iron phosphate cathode material. The performance parameters of the carbon-coated lithium iron phosphate cathode material are shown in Table 1.

[0091] Comparative Example 2

[0092] The comparative example describes a method for preparing a carbon-coated lithium iron phosphate cathode material, comprising the following steps:

[0093] (1) Carbon nanotubes were mixed with ferric nitrate (Fe(NO3)3·9H2O) solution, ultrasonically dispersed, and then transferred to a reaction vessel. The mixture was subjected to hydrothermal reaction at 160℃ for 9 h to obtain carbon-coated ferric hydroxide. The concentration of the ferric nitrate solution was 0.5 mol / L; the mass ratio of the dry weight of carbon nanotubes to the ferric nitrate solution was 1:12, and the dry weight of the ferric nitrate solution was based on the mass of iron oxide. The parameters of the carbon nanotubes were: diameter 3 nm, conductivity 56 S / cm.

[0094] (2) The carbon-coated ferric hydroxide was placed in an atmosphere furnace and heated to 700°C at a rate of 5°C / min under a nitrogen atmosphere, and then calcined at a constant temperature for 3 hours. The ferric hydroxide dehydrated to form ferric oxide, which was then carbon-coated ferric oxide. The particle size of the ferric oxide particles in the carbon-coated ferric oxide was 242 nm.

[0095] (3) Lithium carbonate, carbon-coated iron oxide, diammonium phosphate, and citric acid were added to ethanol and mixed to obtain a mixture. The amounts of lithium carbonate, carbon-coated iron oxide, and diammonium phosphate added were such that the molar ratio of lithium, iron, and phosphorus was 1.03:1:1, and the mass of carbon-coated iron oxide was 1000g. The amount of citric acid added was 0.8% of the sum of the masses of lithium carbonate, carbon-coated iron oxide, and diammonium phosphate. The solid content of the mixture was 35%.

[0096] The mixture is spray-dried in a spray drying tower to obtain dry material. The inlet temperature of the spray dryer is 200℃, the outlet temperature is 90℃, and the spray drying time is 2 hours.

[0097] The dry material was placed in an atmosphere furnace and heated to 700°C at a rate of 5°C / min under a nitrogen atmosphere. The material was then sintered at a constant temperature for 9 hours to obtain lithium iron phosphate sintered material.

[0098] After the lithium iron phosphate sinter was naturally cooled, it was pulverized in an air jet mill to obtain carbon-coated lithium iron phosphate cathode material. The performance parameters of the carbon-coated lithium iron phosphate cathode material are shown in Table 1.

[0099] Battery Assembly and Performance Testing: Positive electrode sheets were prepared using the carbon-coated lithium iron phosphate cathode materials obtained in the various examples and comparative examples. Battery-grade lithium sheets were used as negative electrode sheets, and lithium hexafluorophosphate was used as the electrolyte to simulate battery assembly. The charge / discharge electrochemical window was set to 2.0–3.8 V. The specific capacity of the battery corresponding to the obtained carbon-coated lithium iron phosphate cathode material at the 0.1C rate initial charge, 0.1C rate initial discharge, and 1C rate initial discharge were tested.

[0100] Test method for compaction density: Take 5g of carbon-coated lithium iron phosphate cathode material samples prepared in each example and comparative example, pour them into a steel mold with a diameter (Φ) of 10mm, and compact them at 10MPa. -1 The pressure was uniformly increased to 100 MPa and held for 30 s. After depressurization, the thickness h and mass m of the compacted blank were measured, and the compaction density ρ2 = 4m / (πΦ²h) was calculated.

[0101] Conductivity testing method: The conductivity of the carbon-coated lithium iron phosphate cathode materials obtained in each embodiment and comparative example was tested using the four-probe resistance measurement method.

[0102] The performance test results of the lithium iron phosphate cathode materials prepared in each embodiment and comparative example are shown in Table 1. The discharge specific capacity test results of each simulated battery assembled from the cathode materials prepared in each embodiment and comparative example as battery components are shown in Table 2.

[0103]

[0104]

[0105] As can be seen from Tables 1 and 2, compared with the carbon-coated lithium iron phosphate cathode material prepared in the comparative example, the carbon-coated lithium iron phosphate cathode material prepared in the embodiment of the present invention has higher capacity, higher compaction, and better rate performance.

[0106] Figure 1 This is a scanning electron microscope (SEM) image of the carbon-coated lithium iron phosphate cathode material prepared in Example 1 of this invention. Figure 2 This is a scanning electron microscope (SEM) image of the carbon-coated lithium iron phosphate cathode material prepared in Example 2 of the present invention. Figure 3 This is a transmission electron microscope (TEM) image of the carbon-coated lithium iron phosphate cathode material prepared in Example 1 of this invention. Figure 4 This is a scanning electron microscope (SEM) image of the carbon-coated lithium iron phosphate cathode material prepared in Comparative Example 1 of this invention. From... Figure 1 , Figure 2 , Figure 3 , Figure 4 It can be seen that, compared with the carbon-coated lithium iron phosphate cathode material prepared in the comparative example, the carbon-coated lithium iron phosphate cathode material prepared in the embodiment of the present invention has more uniform carbon coating and has the characteristics of large and small particle gradation.

[0107] The above embodiments should be understood as being used only to illustrate the present invention more clearly, and not to limit the scope of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art fall within the scope defined by the appended claims.

Claims

1. A method for preparing a carbon-coated lithium iron phosphate anode material, characterized in that, Includes the following steps: S1. The first carbon source and the metal salt solution are subjected to a hydrothermal reaction to obtain carbon-coated metal hydroxide; S2. Under a protective atmosphere, the carbon-coated metal hydroxide is calcined to obtain a carbon-coated metal oxide. S3. The carbon-coated metal oxide is mixed and reacted with an iron salt solution to obtain carbon-coated iron hydroxide; the carbon-coated iron hydroxide is dehydrated to obtain carbon-coated iron oxide. S4. After mixing the lithium source, phosphorus source, second carbon source and the carbon-coated iron oxide in a solvent, spray drying, sintering and crushing are performed sequentially to obtain carbon-coated lithium iron phosphate cathode material. In step S1, the first carbon source is a carbon nanotube; the metal salt solution includes a magnesium salt solution and / or a calcium salt solution.

2. The method of claim 1, wherein the carbon-coated lithium iron phosphate cathode material is prepared by the steps of: mixing lithium phosphate, iron phosphate, and a carbon source to form a mixture; and heating the mixture to a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. The magnesium salt solution comprises one or more of magnesium nitrate solution, magnesium sulfate solution, and magnesium chloride solution; And / or, the calcium salt solution comprises one or more of calcium nitrate solution, calcium sulfate solution, and calcium chloride solution.

3. The method of claim 1, wherein the carbon-coated lithium iron phosphate cathode material is prepared by the steps of: mixing lithium phosphate, iron phosphate, and a carbon source to form a mixture; and heating the mixture to a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. In step S2, the particle size of the metal oxide particles in the carbon-coated metal oxide is 100 nm to 1000 nm.

4. The method for preparing carbon-coated lithium iron phosphate cathode material according to claim 3, characterized in that, In step S1, the mass ratio of the first carbon source to the dry weight of the metal salt solution is 1:(5-20), and the dry weight of the metal salt solution is based on the mass of the metal oxide. And / or, the concentration of the metal salt solution is 0.1 mol / L to 1 mol / L; And / or, the temperature of the hydrothermal reaction is 120℃~200℃, and the reaction time is 6h~12h.

5. The method for preparing carbon-coated lithium iron phosphate cathode material according to claim 3, characterized in that, In step S2, the calcination temperature is 600℃~800℃, and the holding time is 2h~4h.

6. The method for preparing carbon-coated lithium iron phosphate cathode material according to any one of claims 1 to 5, characterized in that, In step S3, the temperature of the mixing reaction is 25℃~80℃, and the reaction time is 1h~6h; And / or, the mass ratio of the carbon-coated metal oxide to the iron salt solution is 1:(10-50). And / or, the concentration of the iron salt solution is 0.1 mol / L to 1 mol / L, and the pH value is 2 to 5; And / or, the iron salt solution comprises one or more of ferric nitrate solution, ferric sulfate solution, and ferric chloride solution.

7. The method for preparing carbon-coated lithium iron phosphate cathode material according to any one of claims 1 to 5, characterized in that, In step S3, the dehydration temperature is 300℃~600℃, and the dehydration time is 4h~6h.

8. The method for preparing carbon-coated lithium iron phosphate cathode material according to any one of claims 1 to 5, characterized in that, In step S4, the molar ratio of lithium, iron, and phosphorus in the lithium source, carbon-coated iron oxide, and phosphorus source is (1.02-1.05):1:

1. And / or, the second carbon source comprises one or more of citric acid, glucose, sucrose, ascorbic acid, polyaniline, and polypyrrolidone; And / or, based on the total mass of the lithium source, the carbon-coated iron oxide, and the phosphorus source, the amount of the second carbon source added is 0.5% to 1%.

9. The method for preparing carbon-coated lithium iron phosphate cathode material according to any one of claims 1 to 5, characterized in that, In step S4, the sintering temperature is 650℃~750℃, and the holding time is 6h~12h.

10. A carbon-coated lithium iron phosphate cathode material, characterized in that, It is prepared by the method for preparing carbon-coated lithium iron phosphate cathode material according to any one of claims 1 to 9; The carbon-coated lithium iron phosphate cathode material includes a lithium iron phosphate core and a carbon layer coated on the surface of the lithium iron phosphate core. The thickness of the carbon layer is 5 nm to 20 nm; The particle size of the carbon-coated lithium iron phosphate positive electrode material at least satisfies the following conditions: D10≥0.3 μm, D50 is 1 μm-1.5 μm, and D100≤20 μm.

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