A chromium-doped lithium iron phosphate material, its preparation method and application

Abstract

This invention discloses a chromium-doped lithium iron phosphate material, its preparation method, and its application, belonging to the field of lithium-ion battery technology. The method first constructs a self-assembled chromium two-dimensional nanosheet-coated core-shell structure using a template method. Subsequently, hollow composite microspheres are prepared through a series of processes including polymer coating, pore formation, and template removal. Finally, iron phosphate is deposited on the surface of these hollow composite microspheres, ultimately transforming them into a double-layered hollow structure active chromium-doped lithium iron phosphate material. This product not only exhibits good conductivity but also excellent structural stability, resulting in good rate performance when applied in lithium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a chromium-doped lithium iron phosphate material, its preparation method, and its application. Background Technology

[0002] Lithium iron phosphate is a lithium-ion battery cathode material with high safety performance. However, due to its structural limitations, the material has low conductivity, low energy density, low tap density, and a narrow range of applications.

[0003] To improve the performance of lithium iron phosphate (LFP) materials, researchers have focused on the morphology and composition of its precursor, iron phosphate. LFP materials prepared through nano-sizing, morphology control, and ion doping have shown improved electrochemical performance. However, current LFP preparation processes suffer from uneven component dispersion, severe particle agglomeration, and large particle size. Modification may not only fail to improve performance but could even have negative consequences. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing chromium-doped lithium iron phosphate materials. This method first constructs a self-assembled chromium two-dimensional nanosheet-coated core-shell structure using a template method. Then, hollow composite microspheres are prepared through a series of processes such as polymer coating, pore formation, and template removal. Finally, iron phosphate is deposited on the surface of the hollow composite microspheres, ultimately transforming them into an active chromium-doped lithium iron phosphate material with a double-layer hollow structure. This product not only has good conductivity but also excellent structural stability, resulting in good rate performance when applied in lithium-ion batteries.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing chromium-doped lithium iron phosphate material includes the following steps:

[0007] Chromium aluminum carbide was subjected to a peeling reaction in hydrofluoric acid solution, and the resulting bulk chromium nanosheets were ultrasonically dispersed in the liquid phase to obtain a chromium two-dimensional nanosheet dispersion.

[0008] Silica microspheres were dispersed in water, and then a cationic surfactant and the chromium two-dimensional nanosheet dispersion were added and stirred. After solid-liquid separation, washing and drying, chromium two-dimensional nanosheet-coated silica composite particles were obtained.

[0009] The chromium two-dimensional nanosheet-coated silica composite particles were placed in a dispersion containing polymeric monomers, an initiator was added and a monomer polymerization reaction was carried out. After solid-liquid separation, washing and drying, the resulting solid product was kept at 200-300℃ for 0.8-1.2h in an air atmosphere. Finally, after alkali soaking treatment and solid-liquid separation, washing and drying again, hollow composite microspheres were obtained.

[0010] The hollow composite microspheres, an aqueous solution of soluble iron source, and an aqueous solution of soluble phosphoric acid source are mixed, and then an alkaline solution is added to carry out a precipitation reaction. After aging, the resulting solid product is washed, dried, and kept at 500-750℃ for 4-10 hours under an inert atmosphere to obtain chromium-doped iron phosphate material.

[0011] The chromium-doped iron phosphate material is mixed evenly with a lithium source, spray-dried, and kept at 650–750°C for 6–10 hours under an inert atmosphere to obtain the chromium-doped lithium iron phosphate material.

[0012] In the preparation method of chromium-doped lithium iron phosphate material of the present invention, firstly, chromium two-dimensional nanosheets with a thin-film structure are prepared by exfoliation dispersion method. Then, these nanosheets are mixed with silica microspheres as a template. Under the action of a cationic surfactant, the chromium two-dimensional nanosheets and silica microspheres self-assemble due to surface charge to form a coating structure, resulting in chromium two-dimensional nanosheet-coated silica composite particles. Subsequently, a polymer is used to coat these particles, and sintering is employed to transform the polymer into a porous carbon coating layer. Finally, the silica microsphere template is removed, resulting in composite microspheres with a hollow structure that do not exhibit significant agglomeration. Further coating with iron phosphate and ultimately converting the material into lithium iron phosphate is then performed. This product not only possesses a double-layered structure but also exhibits a hollow interior, resulting in a large overall surface area and a short lithium-ion migration and diffusion path, which is highly conducive to the rapid insertion and extraction of lithium ions. Simultaneously, during the product preparation process, chromium elements penetrate the porous carbon coating layer through the pores of the chromium two-dimensional nanosheets, contacting the iron and phosphorus sources and doping into the lattice of iron phosphate to form a continuous solid solution. Therefore, the lithium-ion channels in the final lithium iron phosphate product are expanded, significantly reducing the resistance to lithium-ion insertion and extraction. Furthermore, the chromium two-dimensional nanosheets, coated with a polymer layer, exhibit high stability and do not undergo significant surface oxidation. Consequently, the product boasts high activity purity and high component uniformity, ultimately demonstrating excellent conductivity and rate performance.

[0013] In some embodiments, the average thickness of the chromium two-dimensional nanosheets in the chromium two-dimensional nanosheet dispersion is 1 to 3 nm.

[0014] In some embodiments, the cationic surfactant is hexadecyltrimethylammonium bromide.

[0015] In some embodiments, the molar ratio of chromium to silica microspheres in the chromium two-dimensional nanosheets is 2:(4-6).

[0016] In some embodiments, the mass ratio of the cationic surfactant to the silica microspheres is (0.01 to 0.1):1.

[0017] In some embodiments, the particle size D50 of the silica microspheres is 200–800 nm.

[0018] More preferably, the particle size D50 of the silica microspheres is 300-500 nm.

[0019] In some embodiments, the polymeric monomer is at least one of aniline and pyrrole.

[0020] The polymer prepared using the above-mentioned monomers not only has good stability and can effectively suppress the oxidation of chromium two-dimensional nanosheets during product preparation, but also has abundant amino groups on its surface, which can effectively complex iron ions and improve the bonding efficiency and degree during the subsequent deposition of iron phosphate. On the other hand, aniline and pyrrole will form a nitrogen-containing doped carbon layer after polymerization and calcination. This carbon layer can effectively improve the overall conductivity and structural stability of the material.

[0021] In some embodiments, the initiator includes hydrochloric acid and ammonium persulfate.

[0022] In some embodiments, the molar concentration of the polymeric monomer in the dispersion is 0.2 to 0.6 mol / L.

[0023] More preferably, the solvent in the dispersion containing the polymeric monomer is an alcohol, and more preferably at least one of ethanol, ethylene glycol, propanol, and glycerol.

[0024] More preferably, the molar concentration of the polymeric monomer in the dispersion is 0.3 to 0.5 mol / L, and the volume ratio of the ammonium persulfate to the dispersion is 1:(8 to 12).

[0025] In some embodiments, the soluble iron source is at least one of ferric nitrate and ferric chloride, and the soluble phosphoric acid source is at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, and sodium hydrogen phosphate.

[0026] More preferably, the molar concentration of the aqueous solution of the soluble iron source is 0.4–0.6 mol / L, and the molar concentration of the aqueous solution of the soluble phosphoric acid source is 0.4–0.6 mol / L.

[0027] In some embodiments, the molar ratio of chromium to iron in the chromium-doped lithium iron phosphate material is (0.1-0.2):1.

[0028] In some embodiments, the molar ratio of phosphorus to iron in the chromium-doped lithium iron phosphate material is (0.8–1.2):(0.8–1.2).

[0029] In some embodiments, after the chromium-doped iron phosphate material and the lithium source are mixed evenly and spray-dried, a pretreatment of holding at 350-450°C for 1-2 hours is required before holding at 650-750°C.

[0030] Another object of the present invention is to provide a method for preparing chromium-doped lithium iron phosphate material to obtain chromium-doped lithium iron phosphate material.

[0031] The chromium-doped lithium iron phosphate material of this invention has a special hollow double-shell coating structure, a large specific surface area, abundant lithium insertion and extraction sites, and high overall conductivity. When applied as a cathode material for lithium-ion batteries, it can achieve a discharge specific capacity of over 150 mAh / g at a rate of 0.5C, with good cycle stability and excellent rate performance, resulting in good overall performance.

[0032] Another object of the present invention is to provide a lithium-ion battery, including a positive electrode, said positive electrode comprising the chromium-doped lithium iron phosphate material described in the present invention.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] This invention provides a method for preparing chromium-doped lithium iron phosphate material. The method first constructs a self-assembled chromium two-dimensional nanosheet-coated core-shell structure using a template method. Then, hollow composite microspheres are prepared through a series of processes such as polymer coating, pore formation, and template removal. Finally, iron phosphate is deposited on the surface of the hollow composite microspheres, ultimately transforming them into an active chromium-doped lithium iron phosphate material with a double-layer hollow structure. This product not only has good conductivity but also excellent structural stability, resulting in good rate performance when applied in lithium-ion batteries. Attached Figure Description

[0035] Figure 1 This is a scanning electron microscope image of the chromium-doped iron phosphate material prepared by the method described in Example 1 of the present invention.

[0036] Figure 2 The image shows a transmission electron microscope (TEM) image of the chromium-doped iron phosphate material prepared by the method described in Example 1 of this invention, where NC represents the nitrogen-containing carbon layer and Cr-FePO4 represents chromium-doped iron phosphate.

[0037] Figure 3 The image shows the XPS results of the chromium-doped lithium iron phosphate material prepared by the method described in Example 1 of this invention.

[0038] Figure 4The image shows a comparison of the XRD patterns of the chromium-doped iron phosphate material prepared by the methods described in Example 1 and Comparative Example 2 of this invention with pure iron phosphate. Detailed Implementation

[0039] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0040] Unless otherwise specified, all materials used in the embodiments and comparative examples are commercially available.

[0041] Example 1

[0042] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method, and its application according to the present invention includes the following steps:

[0043] (1) 0.01 mol of chromium aluminum carbide Cr2AlC was stirred and exfoliated in hydrofluoric acid solution for 24 h. The solid and liquid were separated, and the solid was washed with water until the pH was neutral. Then it was washed with ethanol and dried. The resulting blocky chromium nanosheets were stirred in dimethyl sulfoxide for 12 h and centrifuged to precipitate. The resulting solid product was placed in 100 mL of deionized water and ultrasonically dispersed for 3 h. Then it was centrifuged at 3500 rpm for 1 h. The supernatant was collected to obtain a chromium two-dimensional nanosheet dispersion with a concentration of about 0.1 mol / L. The hydrofluoric acid solution was prepared by stirring 0.03 mol of sodium fluoride in 50 mL of 0.6 mol / L hydrochloric acid. The average thickness of the chromium two-dimensional nanosheets was about 2 nm.

[0044] (2) 0.005 mol of silica microspheres with a particle size D50 of 400 nm and a purity of 99.5% were dispersed in 50 mL of water. Then, 5 wt% of the mass of silica microspheres of cationic surfactant cetyltrimethylammonium bromide and 10 mL of the chromium two-dimensional nanosheet dispersion obtained in step (1) were added and stirred for 4 h. After solid-liquid separation, washing with water / ethanol in sequence, and drying, chromium two-dimensional nanosheet-coated silica composite particles were obtained.

[0045] (3) Chromium two-dimensional nanosheets coated with silica composite particles were placed in 50 mL of a dispersion containing 0.3 mol / L aniline monomer. 3 mL of 1 mol / L hydrochloric acid and 5 mL of 1 mg / mL ammonium persulfate solution were added sequentially and stirred to induce monomer polymerization for 3 h. After solid-liquid separation, washing, and drying, the resulting solid product was placed in a muffle furnace and heated to 250 °C at 5 °C / min in air atmosphere and held for 1 h. Finally, it was placed in 50 mL of 1 mol / L sodium hydroxide solution and soaked for 5 h. After solid-liquid separation, washing, and drying again, hollow composite microspheres were obtained. The solvent of the dispersion containing aniline monomer was ethanol.

[0046] (4) After mixing and stirring the hollow composite microspheres and 50 mL of 0.4 mol / L ferric chloride aqueous solution for 2 h, 50 mL of 0.4 mol / L ammonium dihydrogen phosphate aqueous solution was added to the resulting mixture and stirred until homogeneous. Then, sodium hydroxide solution was added dropwise to control the pH of the system to 1.5. The resulting mixture was then heated to 70 °C and kept at that temperature for 4 h for precipitation reaction and aged for 3 h. The resulting solid product was washed and dried, and then heated to 500 °C at 5 °C / min under a nitrogen atmosphere and kept at that temperature for 4 h to obtain chromium-doped iron phosphate material.

[0047] (5) The chromium-doped iron phosphate material and lithium carbonate (with a molar ratio of iron to lithium in the chromium-doped iron phosphate material of 1:1) are mixed in ethanol and ball-milled for 3 hours until homogeneous. The mixture is then spray-dried and the resulting solid powder is heated to 400°C at 5°C / min and held for 1.5 hours under an inert atmosphere. The temperature is then increased to 700°C and held for 8 hours to obtain the chromium-doped lithium iron phosphate material.

[0048] Example 2

[0049] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method and application described in this invention differs from Example 1 only in that the concentration of the aniline monomer dispersion in step (3) is 0.4 mol / L.

[0050] Example 3

[0051] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method and application described in this invention differs from Example 1 only in that the concentration of the aniline monomer dispersion in step (3) is 0.2 mol / L.

[0052] Example 4

[0053] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method and application described in this invention differs from Example 1 only in that the concentration of the aniline monomer dispersion in step (3) is 0.6 mol / L.

[0054] Example 5

[0055] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method and application described in this invention differs from Embodiment 1 only in that the particle size D50 of the silica microspheres in step (2) is 300 nm.

[0056] Example 6

[0057] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method and application described in this invention differs from Embodiment 1 only in that the particle size D50 of the silica microspheres in step (2) is 500 nm.

[0058] Example 7

[0059] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method and application described in this invention differs from Embodiment 1 only in that the particle size D50 of the silica microspheres in step (2) is 200 nm.

[0060] Example 8

[0061] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method and application described in this invention differs from Embodiment 1 only in that the particle size D50 of the silica microspheres in step (2) is 800 nm.

[0062] Example 9

[0063] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method, and its application according to the present invention includes the following steps:

[0064] (1) 0.01 mol of chromium aluminum carbide Cr2AlC was stirred and exfoliated in hydrofluoric acid solution for 24 h. The solid and liquid were separated, and the solid was washed with water until the pH was neutral. Then it was washed with ethanol and dried. The resulting blocky chromium nanosheets were stirred in dimethyl sulfoxide for 12 h and centrifuged to precipitate. The resulting solid product was placed in 100 mL of deionized water and ultrasonically dispersed for 3 h. Then it was centrifuged at 3500 rpm for 1 h. The supernatant was collected to obtain a chromium two-dimensional nanosheet dispersion with a concentration of about 0.1 mol / L. The hydrofluoric acid solution was prepared by stirring 0.03 mol of sodium fluoride in 50 mL of 0.6 mol / L hydrochloric acid. The average thickness of the chromium two-dimensional nanosheets was about 2 nm.

[0065] (2) 0.008 mol of silica microspheres with a particle size D50 of 400 nm and a purity of 99.5% were dispersed in 50 mL of water. Then, 5 wt% of the mass of silica microspheres of cationic surfactant cetyltrimethylammonium bromide and 16 mL of the chromium two-dimensional nanosheet dispersion obtained in step (1) were added and stirred for 4 h. After solid-liquid separation, washing with water / ethanol in sequence, and drying, chromium two-dimensional nanosheet-coated silica composite particles were obtained.

[0066] (3) Chromium two-dimensional nanosheets coated with silica composite particles were placed in 50 mL of a dispersion containing 0.3 mol / L aniline monomer. 3 mL of 1 mol / L hydrochloric acid and 5 mL of 1 mg / mL ammonium persulfate solution were added sequentially and stirred to induce monomer polymerization for 3 h. After solid-liquid separation, washing, and drying, the resulting solid product was placed in a muffle furnace and heated to 250 °C at 5 °C / min in air atmosphere and held for 1 h. Finally, it was placed in 50 mL of 1 mol / L sodium hydroxide solution and soaked for 5 h. After solid-liquid separation, washing, and drying again, hollow composite microspheres were obtained. The solvent of the dispersion containing aniline monomer was ethanol.

[0067] (4) After mixing and stirring the hollow composite microspheres and 50 mL of 0.4 mol / L ferric chloride aqueous solution for 2 h, 50 mL of 0.4 mol / L ammonium dihydrogen phosphate aqueous solution was added to the resulting mixture and stirred until homogeneous. Then, sodium hydroxide solution was added dropwise to control the pH of the system to 1.8. The resulting mixture was then heated to 80 °C and kept at that temperature for 8 h for precipitation reaction and aged for 5 h. After washing and drying, the resulting solid product was heated to 650 °C at 5 °C / min and kept at that temperature for 6 h under a nitrogen atmosphere to obtain chromium-doped iron phosphate material.

[0068] (5) The chromium-doped iron phosphate material and lithium carbonate (with a molar ratio of iron to lithium in the chromium-doped iron phosphate material of 1:1) are mixed in ethanol and ball-milled for 3 hours until homogeneous. The mixture is then spray-dried and the resulting solid powder is heated to 400°C at 5°C / min and held for 1.5 hours under an inert atmosphere. The temperature is then increased to 700°C and held for 8 hours to obtain the chromium-doped lithium iron phosphate material.

[0069] Example 10

[0070] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method, and its application according to the present invention includes the following steps:

[0071] (1) 0.01 mol of chromium aluminum carbide Cr2AlC was stirred and exfoliated in hydrofluoric acid solution for 24 h. The solid and liquid were separated, and the solid was washed with water until the pH was neutral. Then it was washed with ethanol and dried. The resulting blocky chromium nanosheets were stirred in dimethyl sulfoxide for 12 h and centrifuged to precipitate. The resulting solid product was placed in 100 mL of deionized water and ultrasonically dispersed for 3 h. Then it was centrifuged at 3500 rpm for 1 h. The supernatant was collected to obtain a chromium two-dimensional nanosheet dispersion with a concentration of about 0.1 mol / L. The hydrofluoric acid solution was prepared by stirring 0.03 mol of sodium fluoride in 50 mL of 0.6 mol / L hydrochloric acid. The average thickness of the chromium two-dimensional nanosheets was about 2 nm.

[0072] (2) 0.01 mol of silica microspheres with a particle size D50 of 400 nm and a purity of 99.5% were dispersed in 50 mL of water. Then, 5 wt% of the mass of silica microspheres of cationic surfactant cetyltrimethylammonium bromide and 20 mL of the chromium two-dimensional nanosheet dispersion obtained in step (1) were added and stirred for 4 h. After solid-liquid separation, washing with water / ethanol in sequence, and drying, chromium two-dimensional nanosheet-coated silica composite particles were obtained.

[0073] (3) Chromium two-dimensional nanosheets coated with silica composite particles were placed in 50 mL of a dispersion containing 0.5 mol / L aniline monomer. 3 mL of 1 mol / L hydrochloric acid and 5 mL of 1 mg / mL ammonium persulfate solution were added sequentially and stirred to induce monomer polymerization for 3 h. After solid-liquid separation, washing, and drying, the resulting solid product was placed in a muffle furnace and heated to 250 °C at 5 °C / min in air atmosphere and held for 1 h. Finally, it was placed in 50 mL of 1 mol / L sodium hydroxide solution and soaked for 5 h. After solid-liquid separation, washing, and drying again, hollow composite microspheres were obtained. The solvent of the dispersion containing aniline monomer was ethanol.

[0074] (4) After mixing and stirring the hollow composite microspheres and 50 mL of 0.5 mol / L ferric chloride aqueous solution for 2 h, 50 mL of 0.5 mol / L ammonium dihydrogen phosphate aqueous solution was added to the resulting mixture and stirred until homogeneous. Then, sodium hydroxide solution was added dropwise to control the pH of the system to 1.8. The resulting mixture was then heated to 80 °C and kept at that temperature for 8 h for precipitation reaction and aged for 5 h. The resulting solid product was washed and dried, and then heated to 650 °C at 5 °C / min under a nitrogen atmosphere and kept at that temperature for 6 h to obtain chromium-doped iron phosphate material.

[0075] (5) The chromium-doped iron phosphate material and lithium carbonate (with a molar ratio of iron to lithium in the chromium-doped iron phosphate material of 1:1) are mixed in ethanol and ball-milled for 3 hours until homogeneous. The mixture is then spray-dried and the resulting solid powder is heated to 400°C at 5°C / min and held for 1.5 hours under an inert atmosphere. The temperature is then increased to 700°C and held for 8 hours to obtain the chromium-doped lithium iron phosphate material.

[0076] Example 11

[0077] An embodiment of the chromium-doped lithium iron phosphate material, its preparation method, and its application according to the present invention includes the following steps:

[0078] (1) 0.01 mol of chromium aluminum carbide Cr2AlC was stirred and exfoliated in hydrofluoric acid solution for 24 h. The solid and liquid were separated, and the solid was washed with water until the pH was neutral. Then it was washed with ethanol and dried. The resulting blocky chromium nanosheets were stirred in dimethyl sulfoxide for 12 h and centrifuged to precipitate. The resulting solid product was placed in 100 mL of deionized water and ultrasonically dispersed for 3 h. Then it was centrifuged at 3500 rpm for 1 h. The supernatant was collected to obtain a chromium two-dimensional nanosheet dispersion with a concentration of about 0.1 mol / L. The hydrofluoric acid solution was prepared by stirring 0.03 mol of sodium fluoride in 50 mL of 0.6 mol / L hydrochloric acid. The average thickness of the chromium two-dimensional nanosheets was about 2 nm.

[0079] (2) 0.0125 mol of silica microspheres with a particle size D50 of 400 nm and a purity of 99.5% were dispersed in 50 mL of water. Then, 5 wt% of the mass of silica microspheres of cationic surfactant cetyltrimethylammonium bromide and 25 mL of the chromium two-dimensional nanosheet dispersion obtained in step (1) were added and stirred for 4 h. After solid-liquid separation, washing with water / ethanol in sequence, and drying, chromium two-dimensional nanosheet-coated silica composite particles were obtained.

[0080] (3) Chromium two-dimensional nanosheets coated with silica composite particles were placed in 50 mL of a dispersion containing 0.5 mol / L aniline monomer. 3 mL of 1 mol / L hydrochloric acid and 5 mL of 1 mg / mL ammonium persulfate solution were added sequentially and stirred to induce monomer polymerization for 3 h. After solid-liquid separation, washing, and drying, the resulting solid product was placed in a muffle furnace and heated to 250 °C at 5 °C / min in air atmosphere and held for 1 h. Finally, it was placed in 50 mL of 1 mol / L sodium hydroxide solution and soaked for 5 h. After solid-liquid separation, washing, and drying again, hollow composite microspheres were obtained. The solvent of the dispersion containing aniline monomer was ethanol.

[0081] (4) After mixing and stirring the hollow composite microspheres and 50 mL of 0.5 mol / L ferric chloride aqueous solution for 2 h, 50 mL of 0.5 mol / L ammonium dihydrogen phosphate aqueous solution was added to the resulting mixture and stirred until homogeneous. Then, sodium hydroxide solution was added dropwise to control the pH of the system to 1.8. The resulting mixture was then heated to 80 °C and kept at that temperature for 8 h for precipitation reaction and aged for 5 h. The resulting solid product was washed and dried, and then heated to 650 °C at 5 °C / min under a nitrogen atmosphere and kept at that temperature for 6 h to obtain chromium-doped iron phosphate material.

[0082] (5) The chromium-doped iron phosphate material and lithium carbonate (with a molar ratio of iron to lithium in the chromium-doped iron phosphate material of 1:1) are mixed in ethanol and ball-milled for 3 hours until homogeneous. The mixture is then spray-dried and the resulting solid powder is heated to 400°C at 5°C / min and held for 1.5 hours under an inert atmosphere. The temperature is then increased to 700°C and held for 8 hours to obtain the chromium-doped lithium iron phosphate material.

[0083] Comparative Example 1

[0084] A lithium iron phosphate material, its preparation method, and its application, wherein the preparation method includes the following steps:

[0085] (1) 0.005 mol of silica microspheres with a particle size D50 of 400 nm and a purity of 99.5% were placed in 50 mL of a dispersion containing 0.3 mol / L aniline monomer. 3 mL of 1 mol / L hydrochloric acid and 5 mL of 1 mg / mL ammonium persulfate solution were added sequentially and stirred to induce monomer polymerization for 3 h. After solid-liquid separation, washing, and drying, the mixture was placed in a muffle furnace and heated to 250 °C at 5 °C / min in an air atmosphere and held for 1 h. Finally, the mixture was immersed in 50 mL of 1 mol / L sodium hydroxide solution for 5 h and then subjected to solid-liquid separation, washing, and drying again to obtain hollow microspheres. The solvent of the dispersion containing aniline monomer was ethanol.

[0086] (2) Hollow microspheres and 50 mL of 0.4 mol / L ferric chloride aqueous solution were mixed and stirred for 2 h. Then, 50 mL of 0.4 mol / L ammonium dihydrogen phosphate aqueous solution was added and stirred evenly. Then, sodium hydroxide solution was added dropwise to control the pH of the system to 1.5. Then, the temperature was raised to 70 °C and kept at the temperature for 4 h for precipitation reaction. After aging for 3 h, the obtained solid product was washed and dried. Then, the temperature was raised to 500 °C at 5 °C / min and kept at the temperature for 4 h under nitrogen atmosphere to obtain ferric phosphate material.

[0087] (5) Mix the iron phosphate material and lithium carbonate (with a molar ratio of iron in the iron phosphate material to lithium in the lithium carbonate of 1:1) in ethanol and ball mill for 3 hours until uniform. Spray dry the mixture and heat the resulting solid powder to 400°C at 5°C / min under an inert atmosphere and hold for 1.5 hours. Then continue to heat to 700°C and hold for 8 hours to obtain the lithium iron phosphate material.

[0088] Comparative Example 2

[0089] A lithium iron phosphate material, its preparation method, and its application, wherein the preparation method includes the following steps:

[0090] (1) Stir 50 mL of 0.4 mol / L ferric chloride aqueous solution and 50 mL of 0.4 mol / L ammonium dihydrogen phosphate aqueous solution evenly, then add sodium hydroxide solution dropwise to control the pH of the system to 1.5, then heat to 70℃ and keep warm for 4 h for precipitation reaction, then age for 3 h, the obtained solid product is washed and dried, and then heated to 500℃ at 5℃ / min and kept warm for 4 h under nitrogen atmosphere to obtain ferric phosphate material;

[0091] (5) Mix the iron phosphate material and lithium carbonate (with a molar ratio of iron in the iron phosphate material to lithium in the lithium carbonate of 1:1) in ethanol and ball mill for 3 hours until uniform. Spray dry the mixture and heat the resulting solid powder to 400°C at 5°C / min under an inert atmosphere and hold for 1.5 hours. Then continue to heat to 700°C and hold for 8 hours to obtain the lithium iron phosphate material.

[0092] Comparative Example 3

[0093] A chromium-doped lithium iron phosphate material, its preparation method, and its application, wherein the preparation method includes the following steps:

[0094] (1) Stir 50 mL of 0.4 mol / L ferric chloride aqueous solution and 50 mL of 0.4 mol / L ammonium dihydrogen phosphate aqueous solution evenly, then add sodium hydroxide solution dropwise to control the pH of the system to 1.5, then heat to 70℃ and keep warm for 4 h for precipitation reaction, then age for 3 h, the obtained solid product is washed and dried, and then heated to 500℃ at 5℃ / min and kept warm for 4 h under nitrogen atmosphere to obtain ferric phosphate material;

[0095] (2) Lithium carbonate, iron phosphate, chromium oxide and glucose were dispersed in ethanol and mixed and ball-milled for 3 hours according to the molar ratio of lithium, iron, chromium and carbon source of 1:1:0.1:0.08. The mixture was spray-dried and the resulting solid powder was heated to 400℃ at 5℃ / min and held for 1.5 hours under an inert atmosphere. Then the temperature was increased to 700℃ and held for 8 hours to obtain the chromium-doped lithium iron phosphate material.

[0096] Comparative Example 4

[0097] A chromium-doped lithium iron phosphate material, its preparation method, and its application, wherein the preparation method includes the following steps:

[0098] (1) 0.01 mol of chromium aluminum carbide Cr2AlC was stirred and exfoliated in hydrofluoric acid solution for 24 h. The solid and liquid were separated, and the solid was washed with water until the pH was neutral. Then it was washed with ethanol and dried. The resulting blocky chromium nanosheets were stirred in dimethyl sulfoxide for 12 h and centrifuged to precipitate. The resulting solid product was placed in 100 mL of deionized water and ultrasonically dispersed for 3 h. Then it was centrifuged at 3500 rpm for 1 h. The supernatant was collected to obtain a chromium two-dimensional nanosheet dispersion with a concentration of about 0.1 mol / L. The hydrofluoric acid solution was prepared by stirring 0.03 mol of sodium fluoride in 50 mL of 0.6 mol / L hydrochloric acid.

[0099] (2) 0.005 mol of silica microspheres with a particle size D50 of 400 nm and a purity of 99.5% were dispersed in 50 mL of water. Then, 5 wt% of the mass of silica microspheres of cationic surfactant cetyltrimethylammonium bromide and 10 mL of the chromium two-dimensional nanosheet dispersion obtained in step (1) were added and stirred for 4 h. After solid-liquid separation, washing with water / ethanol in sequence, and drying, chromium two-dimensional nanosheet-coated silica composite particles were obtained.

[0100] (3) Chromium two-dimensional nanosheets coated with silica composite particles were placed in 50 mL of a dispersion containing 0.3 mol / L aniline monomer. 3 mL of 1 mol / L hydrochloric acid and 5 mL of 1 mg / mL ammonium persulfate solution were added sequentially and stirred to induce monomer polymerization for 3 h. After solid-liquid separation, washing, and drying, the resulting solid product was placed in a muffle furnace and heated to 250 °C at 5 °C / min in air atmosphere and held for 1 h. Finally, it was placed in 50 mL of 1 mol / L sodium hydroxide solution and soaked for 5 h. After solid-liquid separation, washing, and drying again, hollow composite microspheres were obtained. The solvent of the dispersion containing aniline monomer was ethanol.

[0101] (4) Hollow composite microspheres, lithium carbonate, and commercially available 99.5% iron phosphate powder were dispersed in ethanol according to the molar ratio of chromium, lithium, and iron of 0.1:1:1 and ball-milled for 3 hours. The mixture was then spray-dried and the resulting solid powder was heated to 400°C at 5°C / min under an inert atmosphere and held for 1.5 hours. The temperature was then increased to 700°C and held for 8 hours to obtain the chromium-doped lithium iron phosphate material.

[0102] Example of effect 1

[0103] To verify the effectiveness of the chromium-doped lithium iron phosphate material described in this invention, the products obtained from each embodiment and comparative example were used as positive electrode active materials to assemble coin cells. The specific steps are as follows:

[0104] (1) Preparation of positive electrode sheet: The materials obtained in each example or comparative example are mixed with conductive agent acetylene black and binder polyvinylidene fluoride in N-methylpyrrolidone at a mass ratio of 92:4:4 to prepare a slurry, which is then coated on aluminum foil and dried to obtain a positive electrode sheet.

[0105] (2) Assembly of button cells: Using lithium foil as the counter electrode, polypropylene porous membrane as the separator, and 1 mol / L LiPF6-EC:DMC (volume ratio of 1:1) as the electrolyte, button cells were assembled in a glove box together with the positive electrode. After standing, multiple parallel samples were set for each sample.

[0106] The button cells prepared from the products of each embodiment and comparative example were subjected to charge-discharge tests at operating voltages of 2.5–4.5V and rates of 0.1 and 0.5C. The discharge specific capacity at these rates and the coulombic efficiency of the first charge-discharge at 0.1C were calculated. Then, parallel samples were subjected to 100 charge-discharge cycles at 0.5C, and the capacity retention rate of each sample was calculated (the retention rate was calculated based on the initial discharge specific capacity at 0.5C). Finally, the parallel samples were subjected to rate tests: 10 charge-discharge cycles were performed sequentially at rates of 0.1C, 0.5C, 2C, and 5C, and the discharge specific capacity retention rate at 5C was calculated (the retention rate was calculated based on the initial discharge specific capacity at 0.1C).

[0107] The test results are shown in Table 1.

[0108] Table 1

[0109]

[0110]

[0111] As shown in Table 1, the chromium-doped lithium iron phosphate material of this invention exhibits excellent electrochemical activity and structural stability when used as a cathode material for lithium-ion batteries. It not only has high initial discharge specific capacity and high coulombic efficiency, but also high capacity retention during cycle testing and rate testing. The chromium-doped lithium iron phosphate material prepared in Example 1 was observed using scanning electron microscopy and transmission electron microscopy, and the final prepared chromium-doped lithium iron phosphate material was tested using XPS. Subsequently, XRD tests were performed simultaneously with and compared with the pure iron phosphate prepared in Comparative Example 2. It was found that... Figures 1-4 As shown, the chromium-doped iron phosphate material particles prepared by the method of the present invention have good morphology and exhibit the expected hollow structure in the middle. Furthermore, there are no additional impurity characteristic peaks in the XRD spectrum, indicating that chromium doping did not affect the phase structure of iron phosphate itself during the entire preparation process, and the product has high purity. At the same time, the XPS spectrum also confirms that no element in the final product exhibits a multiphase valence state, and the reaction degree of each component is complete.

[0112] As can be seen from Examples 1-4, the concentration of polymer monomers added during product preparation has varying effects on the improvement of product structural stability and conductivity. When the molar concentration of polymer monomers in the dispersed phase is 0.3-0.5 mol / L, the electrochemical performance of the prepared product is superior. A comparison of the performance of the products prepared in Examples 1 and 5-8 shows that products prepared using silica microspheres of different sizes as templates exhibit different performance characteristics due to variations in the internal hollow structure size. When the particle size D50 of the silica microspheres is 300-500 nm, the product exhibits superior performance.

[0113] In contrast, Comparative Example 2 directly prepared solid lithium iron phosphate material using traditional methods. This resulted in low initial capacity, low coulombic efficiency, and unsatisfactory cycle and rate performance. The main reasons are the low conductivity of lithium iron phosphate itself and the inability to achieve high particle dispersion and uniformity during the preparation of the precursor lithium iron phosphate material using traditional processes. Comparative Example 1, based on Comparative Example 2, constructed a hollow structure and introduced a carbon layer coating, resulting in some performance improvement. However, due to the lack of chromium doping and the absence of a double-layered structure, the product's performance was still inferior to the products in the examples. Comparative Example 3, based on Comparative Example 2, used a traditional solid-state method to introduce a chromium source and carbon layer coating. However, the morphology of this product was not uniform, so its performance was similar to that of Comparative Example 1 without chromium doping. In the preparation of Comparative Example 4, after the hollow composite microspheres were prepared, iron phosphate was directly loaded using a solid-state method and the final product was prepared in one step. In this process, the iron phosphate may not have been pre-coated, the morphology of the hollow composite microspheres was destroyed, and the final lithium iron phosphate did not form a coating layer structure, resulting in unsatisfactory product performance.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a chromium-doped lithium iron phosphate material, characterized in that, The method comprises the following steps: The chromium-aluminum carbide is treated by exfoliation reaction in a hydrofluoric acid solution, and the obtained bulk chromium nanosheets are dispersed in a liquid phase by ultrasonic dispersion to obtain a chromium two-dimensional nanosheet dispersion liquid; The silica microspheres are dispersed in water, and then a cationic surfactant and the chromium two-dimensional nanosheet dispersion liquid are added and stirred to mix, and then solid-liquid separation, washing and drying are performed to obtain chromium two-dimensional nanosheet-coated silica composite particles; The chromium two-dimensional nanosheet-coated silica composite particles are placed in a dispersion liquid containing a polymerization monomer, an initiator is added, and monomer polymerization reaction occurs, and then solid-liquid separation, washing and drying are performed, and then the obtained solid-phase product is kept at 200-300°C for 0.8-1.2h in an air atmosphere, and finally the obtained product is immersed in a lye solution and then subjected to solid-liquid separation, washing and drying again to obtain hollow composite microspheres; The hollow composite microspheres, an aqueous solution of a soluble iron source, and an aqueous solution of a soluble phosphoric acid source are mixed, and then a lye solution is added to perform a precipitation reaction, and then aging is performed, and then the obtained solid-phase product is subjected to washing and drying, and then kept at 500-750°C for 4-10h in an inert atmosphere to obtain a chromium-doped iron phosphate material; The chromium-doped iron phosphate material is mixed with a lithium source, spray dried, and kept at 650-750°C for 6-10h in an inert atmosphere to obtain the chromium-doped lithium iron phosphate material.

2. The method for preparing chromium-doped lithium iron phosphate material as described in claim 1, characterized in that, The average thickness of the chromium two-dimensional nanosheets in the chromium two-dimensional nanosheet dispersion liquid is 1-3nm.

3. The method for preparing chromium-doped lithium iron phosphate material as described in claim 1, characterized in that, The molar ratio of chromium elements in the chromium two-dimensional nanosheets to silica microspheres is 2:(4-6).

4. The method for preparing chromium-doped lithium iron phosphate material as described in claim 1, characterized in that, The cationic surfactant is hexadecyl trimethyl ammonium bromide, and the mass ratio of the cationic surfactant to silica microspheres is (0.01-0.1):

1.

5. The method for preparing chromium-doped lithium iron phosphate material as described in claim 1, characterized in that, The particle size D50 of the silica microspheres is 200-800nm.

6. The method for preparing chromium-doped lithium iron phosphate material as described in claim 1, characterized in that, The polymerization monomer is at least one of aniline and pyrrole, and the molar concentration of the polymerization monomer in the dispersion liquid is 0.2-0.6mol / L.

7. The method for preparing chromium-doped lithium iron phosphate material as described in claim 1, characterized in that, The soluble iron source is at least one of ferric nitrate and ferric chloride, and the soluble phosphoric acid source is at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, and sodium hydrogen phosphate, and the molar concentration of the aqueous solution of the soluble iron source is 0.4-0.6mol / L, and the molar concentration of the aqueous solution of the soluble phosphoric acid source is 0.4-0.6mol / L.

8. The method for preparing chromium-doped lithium iron phosphate material as described in claim 1, characterized in that, The molar ratio of chromium elements to iron elements in the chromium-doped lithium iron phosphate material is (0.1-0.2):

1.

9. The chromium-doped lithium iron phosphate material prepared by the method according to any one of claims 1-8.

10. A lithium-ion battery, characterized by, The positive electrode sheet comprises the chromium-doped lithium iron phosphate material according to claim 9.

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