A sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material and its preparation method

Abstract

A sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material belongs to the field of lithium iron phosphate composite cathode materials. It includes zinc-doped lithium iron phosphate material and sulfur-carbon material wrapped outside the zinc-doped lithium iron phosphate material. The preparation method steps include: A. Prepare sulfur-carbon material using a carbon source and phenyl disulfide; B. Prepare zinc oxide by hydrothermal method using zinc acetate solution and sodium hydroxide solution; C. Obtain iron phosphate by hydrothermal method using a mixed solution containing ferrous sulfate and phosphoric acid solution; D. Disperse lithium hydroxide and iron phosphate into an acid solution, then add zinc oxide, and obtain zinc-doped lithium iron phosphate material through stirring and heating decomposition; E. Mix and grind the zinc-doped lithium iron phosphate material and sulfur-carbon material and heat them to obtain the sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material. The sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material prepared by the inventive method has a high lithium ion diffusion rate and excellent electrochemical performance.

Description

Technical Field

[0001] The present invention belongs to the field of lithium iron phosphate composite cathode materials, and particularly relates to a sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material and a preparation method thereof. Background Art

[0002] In recent years, with the increasing impact of fossil energy on the earth's environment, clean energy is being widely used as a substitute. Lithium-ion batteries have been widely recognized for their advantages such as high energy density, light weight, small size, no memory effect, long cycle life, and environmental friendliness.

[0003] In lithium-ion batteries, commercially available products for cathode materials include lithium manganate, lithium cobaltate, ternary materials, and lithium iron phosphate, etc. Among them, olivine-structured lithium iron phosphate has a theoretical specific capacity of 170 mAh / g and a lithium charging platform of 3.5 V. Compared with traditional LiCoO 2 、LiMn2O 4 and ternary materials, the lithium iron phosphate cathode material has the advantages of wide raw material sources, low cost, no environmental pollution, good cycle performance, good thermal stability, and outstanding safety performance. The lithium iron phosphate electrode is an ideal cathode material for power lithium-ion batteries. According to the 1-4th batch of new energy vehicle promotion and application recommended vehicle models catalog released in 2021, a total of 169 passenger cars were selected, among which the models equipped with lithium iron phosphate electrodes accounted for 40.24%.

[0004] However, the poor conductivity, low charge-discharge rate, and low lithium-ion diffusion rate of lithium iron phosphate result in high initial capacity loss and poor rate capacity, seriously restricting the application of lithium iron phosphate in new energy vehicles. Summary of the Invention

[0005] In order to solve the problems existing in the above-mentioned prior art, the present invention provides a sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material and a preparation method thereof. The sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material of the present invention has the advantages of good conductivity, high charge-discharge rate, and high lithium-ion diffusion rate. Moreover, after sulfur-carbon / zinc doping and coating, the tap density of the lithium iron phosphate material is increased, thereby improving the capacity of the battery and solving the problem that restricts the application of lithium iron phosphate in new energy vehicles.

[0006] The specific technical solution adopted by the present invention is as follows:

[0007] A sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material, comprising a zinc-doped lithium iron phosphate material and a sulfur-carbon material coated on the outside of the zinc-doped lithium iron phosphate material. The sulfur-carbon material includes a porous carbon material and sulfur distributed inside and on the surface of the porous carbon material. The sulfur distributed on the surface of the porous carbon material replaces part of the oxygen in the crystal of the zinc-doped lithium iron phosphate material.

[0008] A preparation method of a sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material, comprising the following steps:

[0009] A. Add a carbon source to ethanol for ultrasonic treatment. After ultrasonic treatment, add diphenyl disulfide and stir evenly. Then dry the mixture, and finally anneal the dried mixture to obtain a sulfur-carbon material;

[0010] B. Mix an acetic acid zinc solution and a sodium hydroxide solution and stir evenly. Add polyethylene glycol to obtain a mixed solution A. Prepare zinc oxide from the mixed solution A by a hydrothermal method;

[0011] C. Sequentially add ferrous sulfate and excessive hydrogen peroxide to a CTAB solution, then add a phosphoric acid solution, and stir evenly to obtain a mixed solution B. Prepare iron phosphate from the mixed solution B by a hydrothermal method;

[0012] D. Sequentially add lithium hydroxide and iron phosphate to a chelating agent, adjust the pH value of the solution to 6-7, then add zinc oxide, and stir until a gel is obtained. Heat and decompose the obtained gel in a reducing atmosphere to obtain a zinc-doped lithium iron phosphate material;

[0013] E. Mix and grind the zinc-doped lithium iron phosphate material and the sulfur-carbon material, and heat the ground mixed material in a reducing atmosphere to obtain a sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material.

[0014] The carbon source described in step A is one or more of glucose, sucrose, polyvinyl alcohol, and natural fiber, preferably glucose; the mass ratio of the carbon source to diphenyl disulfide is 4-5:1, preferably 4.3:1.

[0015] The annealing temperature of the annealing treatment described in step A is 900-1100 °C, preferably 950-1050 °C; the annealing time is 20-40 min, preferably 25-35 min.

[0016] The preparation of zinc oxide from the mixed solution A by a hydrothermal method described in step B includes the following steps:

[0017] B1. Add the mixed solution A to a hydrothermal reaction vessel, heat at a temperature of 120-160 °C, preferably 135-145 °C; the heating time is 8-16 h, preferably 10-14 h;

[0018] B2. Naturally cool the reaction vessel to room temperature, perform solid-liquid separation on the solution in the reaction vessel, wash and dry the solid substance to obtain zinc oxide with a particle size of 50-200 nm, and the particle size of the zinc oxide is preferably 100-150 nm.

[0019] Ferrous sulfate in step C can also be replaced by one or more of ferrous sulfate, ferrous acetate, and ferrous chloride, preferably ferrous sulfate.

[0020] The phosphoric acid solution described in step C can also be replaced by one or more of a phosphoric acid solution, a lithium dihydrogen phosphate solution, an ammonium phosphate solution, an ammonium dihydrogen phosphate solution, and a diammonium hydrogen phosphate solution, preferably a phosphoric acid solution.

[0021] The preparation of iron phosphate from the mixed solution B by hydrothermal method in step C includes the following steps:

[0022] C1. Add the mixed solution B into a hydrothermal reaction vessel, heat at a temperature of 160 - 180°C, preferably 165 - 175°C; heat for 1.5 - 2.5 h, preferably 1.8 - 2.2 h;

[0023] C2. Naturally cool the reaction vessel to room temperature, perform solid-liquid separation on the solution in the reaction vessel, wash and dry the solid substance to obtain iron phosphate.

[0024] Lithium hydroxide in step D can also be replaced by one or more of lithium hydroxide, lithium chloride, lithium sulfate, lithium nitrate, lithium acetate, and lithium dihydrogen phosphate, preferably lithium hydroxide.

[0025] The chelating agent in step D is one or several of citric acid, tartaric acid, polyethylene glycol, and polyvinyl alcohol. The preferred chelating agent is citric acid; the heating and decomposition of the obtained gel in a reducing atmosphere means that the gel decomposes in a reducing atmosphere at 300 - 400°C for 4 - 6 h, preferably at 320 - 380°C for 4.5 - 5 h.

[0026] The chemical formula of the zinc-doped lithium iron phosphate material in step D is LiFe x Zn y PO 4 , where x = 0.7 - 0.8 and y = 1 - x.

[0027] The sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material in step E is composed of 10 - 25% carbon-sulfur material and 75 - 90% zinc-doped lithium iron phosphate material by mass percentage, preferably composed of 15 - 20% carbon-sulfur material and 80 - 85% zinc-doped lithium iron phosphate material.

[0028] When heating the ground mixed material in a slightly reducing atmosphere in step E, the heating temperature is 600 - 900°C, preferably 700 - 800°C; the heating time is 8 - 16 h, preferably 10 - 14 h.

[0029] The beneficial effects of the present invention are:

[0030] 1. In the present invention, zinc ions are doped inside the lithium iron phosphate composite cathode material, and a carbon-sulfur coating is wrapped outside the lithium iron phosphate composite cathode material. Through the special structure formed by lithium iron phosphate - zinc - carbon - sulfur and optimizing the ratio of each component in the sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material, the prepared sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material has the advantages of good conductivity, high charge-discharge rate and high lithium ion diffusion rate. Moreover, after sulfur-carbon / zinc doping and coating, the tap density of the lithium iron phosphate material increases, improving the battery capacity.

[0031] 2. The present invention adopts a carbon-sulfur coating. The coating carbon restricts the growth of lithium iron phosphate particles to a certain extent, thereby shortening the diffusion distance of lithium ions. And sulfur distributed on the surface of the porous carbon material replaces part of the oxygen in the crystal of the zinc-doped lithium iron phosphate material, breaking the relatively stable oxygen covalent bond, thereby improving the electronic conductivity of the carbon layer and lithium ion diffusion, further improving the battery performance of the electrode. Among them, the sulfur-doped carbon can easily promote the transport of electrons / ions and inhibit the volume change during the lithium ion transport process, thereby obtaining excellent rate performance. + 5. The volume change during the ion transport process, thereby obtaining excellent rate performance.

[0032] 3. Doping zinc ions in the present invention can effectively stabilize the crystal structure and improve the electron transport. The composite material doped with metal ions has significant high-rate capacity and long-term cycle stability, and its conductivity is also improved. Moreover, when preparing the zinc-doped lithium iron phosphate material, the particle size of the zinc oxide used is 50 - 200 nm, making it easier for zinc oxide to be doped into lithium iron phosphate without generating free zinc oxide, further improving the conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is the SEM schematic diagram of Embodiment 1 of the present invention;

[0034] Figure 2 is the SEM schematic diagram of Comparative Example 1;

[0035] Figure 3 is the SEM schematic diagram of Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION I. Specific Embodiments

[0037] Embodiment 1

[0038] A. Glucose is added to ethanol and subjected to ultrasonic treatment for 15 min. After ultrasonic treatment, diphenyl disulfide is added under stirring. The mass ratio of glucose to diphenyl disulfide is 4.3:1. Then the mixture is dried at 60 °C, and finally the dried mixture is annealed at 1000 °C for 30 min in an argon atmosphere to obtain a sulfur-carbon material;

[0039] B. Mix 0.5 mol / L zinc acetate solution and 1 mol / L sodium hydroxide solution and stir well. Add polyethylene glycol to obtain mixed solution A, where the volume ratio of zinc acetate solution, sodium hydroxide solution, and polyethylene glycol is 20:20:1. Add mixed solution A to a hydrothermal reactor and heat it at 140 °C for 12 h. After the reaction is completed, let the reactor cool naturally to room temperature. Centrifuge the solution in the reactor for 10 min to obtain a precipitate. Wash the precipitate repeatedly with distilled water first, then wash it twice with absolute ethanol, and dry it to obtain zinc oxide with a particle size of 100 nm.

[0040] C. Add CTAB to deionized water and stir to form a uniform and transparent CTAB aqueous solution with a concentration of 0.03 mol / L. Add 0.045 mol of ferrous sulfate to each liter of CTAB aqueous solution and stir until completely dissolved. Then add an excessive amount of hydrogen peroxide to oxidize Fe 2+ to Fe 3+ , and then add phosphoric acid solution with a volume ratio of 3:200 to the CTAB aqueous solution under stirring. Continue to stir for 1 h to obtain mixed solution B. Add mixed solution B to a hydrothermal reactor and heat it at 170 °C for 2 h. After heating, let the reactor cool naturally to room temperature. Centrifuge the solution in the reactor for 10 min to obtain a precipitate. Wash the precipitate repeatedly with distilled water first, then wash it twice with absolute ethanol, and dry it at 100 °C for 4 h to obtain iron phosphate.

[0041] D. Add lithium hydroxide and iron phosphate to citric acid solution in turn. The molar ratio of lithium hydroxide, iron phosphate, and citric acid is 1:1:2. Adjust the pH value of the solution to 7, and then add zinc oxide, where the molar ratio of zinc oxide and iron phosphate is 1:3. Then stir the solution at 70 °C at a stirring rate of 400 rpm until a transparent green gel is obtained. Put the obtained green gel into a tube furnace filled with a reducing atmosphere and heat it at 350 °C for 5 h to obtain LiFe 0.75 Zn 0.25 PO 4 ;

[0042] E. Mix 80% LiFe 075 Zn 0.25 PO 4 and 20% sulfur-carbon material by mass percentage. Ball-mill the mixture in toluene at 400 rpm for 2 h. Heat the milled mixed material in a slightly reducing atmosphere at 750 °C for 12 h to obtain LiFe 075 Zn 0.25 PO 4 / SC composite cathode material.

[0043] Example 2

[0044] A. Add sucrose to ethanol and perform ultrasonic treatment for 15 min. After ultrasonic treatment, add diphenyl disulfide under stirring. The mass ratio of sucrose to diphenyl disulfide is 4:1. Then dry the mixture at 60 °C. Finally, anneal the dried mixture at 1100 °C for 20 min under an argon atmosphere to obtain a sulfur-carbon material;

[0045] B. Mix a 0.5 mol / L zinc acetate solution and a 1 mol / L sodium hydroxide solution and stir evenly. Add polyethylene glycol to obtain a mixed solution A. The volume ratio of the zinc acetate solution, sodium hydroxide solution, and polyethylene glycol is 20:20:1. Add the mixed solution A to a hydrothermal reaction kettle and heat it at 160 °C for 8 h. After the reaction ends, let the reaction kettle cool naturally to room temperature. Centrifuge the solution in the reaction kettle for 10 min to obtain a precipitate. Wash the precipitate repeatedly with distilled water first, and then wash it twice with absolute ethanol. After drying, zinc oxide with a particle size of 50 nm is obtained;

[0046] C. Add CTAB to deionized water and stir to form a homogeneous and transparent CTAB aqueous solution. The concentration of the CTAB aqueous solution is 0.03 mol / L. Add 0.045 mol of ferrous sulfate to each liter of the CTAB aqueous solution and stir until completely dissolved. Then add an excessive amount of hydrogen peroxide to oxidize Fe 2+ to Fe 3+ , and then add a phosphoric acid solution under stirring. The volume ratio of the phosphoric acid solution to the CTAB aqueous solution is 3:200. Continue to stir for 1 h to obtain a mixed solution B. Add the mixed solution B to a hydrothermal reaction kettle and heat it at 160 °C for 2.5 h. After heating, let the reaction kettle cool naturally to room temperature. Centrifuge the solution in the reaction kettle for 10 min to obtain a precipitate. Then wash the precipitate repeatedly with distilled water first, and then wash it twice with absolute ethanol. Dry it at 100 °C for 4 h to obtain iron phosphate;

[0047] D. Add lithium hydroxide and iron phosphate to a tartaric acid solution in sequence. The molar ratio of lithium hydroxide, iron phosphate, and tartaric acid is 1:1:2. Adjust the pH value of the solution to 7, and then add zinc oxide. The molar ratio of zinc oxide to iron phosphate is 1:4. Then stir the solution at a stirring rate of 400 rpm at 70 °C until a transparent green gel is obtained. Put the obtained green gel into a tubular furnace filled with a reducing atmosphere and heat it at 400 °C for 4 h to obtain LiFe 0.8 Zn 0.2 PO 4 ;

[0048] E. By mass percentage, 90% LiFe 0.8 Zn 0.2 PO 4Mix with 10% sulfur-carbon material, ball-mill the mixture in toluene at 500 rpm for 1.5 h, heat the milled mixture in a slightly reducing atmosphere at 600 °C for 16 h to obtain LiFe 0.8 Zn 0.2 PO 4 / SC composite cathode material.

[0049] Example 3

[0050] A. Add cotton to ethanol and perform ultrasonic treatment for 15 min. After ultrasonic treatment, add diphenyl disulfide under stirring. The mass ratio of cotton to diphenyl disulfide is 5:1. Then dry the mixture at 60 °C. Finally, anneal the dried mixture in an argon atmosphere at 900 °C for 40 min to obtain sulfur-carbon material;

[0051] B. Mix a 0.5 mol / L zinc acetate solution and a 1 mol / L sodium hydroxide solution and stir evenly. Add polyethylene glycol to obtain mixed solution A. The volume ratio of the zinc acetate solution, sodium hydroxide solution, and polyethylene glycol is 20:20:1. Add mixed solution A to a hydrothermal reaction kettle and heat it at 120 °C for 16 h. After the reaction ends, let the reaction kettle cool naturally to room temperature. Centrifuge the solution in the reaction kettle for 10 min to obtain a precipitate. Wash the precipitate repeatedly with distilled water first, then wash it twice with absolute ethanol, and dry it to obtain zinc oxide with a particle size of 200 nm;

[0052] C. Add CTAB to deionized water and stir to form a uniform and transparent CTAB aqueous solution with a concentration of 0.03 mol / L. Add 0.045 mol of ferrous sulfate to each liter of the CTAB aqueous solution and stir until completely dissolved. Then add an excessive amount of hydrogen peroxide to oxidize Fe 2+ to Fe 3+ . Then add a phosphoric acid solution under stirring. The volume ratio of the phosphoric acid solution to the CTAB aqueous solution is 3:200. Continue to stir for 1 h to obtain mixed solution B. Add mixed solution B to a hydrothermal reaction kettle and heat it at 180 °C for 1.5 h. After heating, let the reaction kettle cool naturally to room temperature. Centrifuge the solution in the reaction kettle for 10 min to obtain a precipitate. Then wash the precipitate repeatedly with distilled water first, then wash it twice with absolute ethanol, and dry it at 100 °C for 4 h to obtain iron phosphate;

[0053] D. Add lithium hydroxide and iron phosphate to the polyethylene glycol solution in sequence. The molar ratio of lithium hydroxide, iron phosphate, and polyethylene glycol is 1:1:2. Adjust the pH value of the solution to 6, then add zinc oxide, where the molar ratio of zinc oxide to iron phosphate is 3:7. Then, stir the solution at a stirring rate of 400 rpm at 70 °C until a transparent green gel is obtained. Put the obtained green gel into a tubular furnace filled with a reducing atmosphere and heat it at 300 °C for 6 h to obtain LiFe 0.7 Zn 0.3 PO 4 ;

[0054] E. By mass percentage, mix 75% LiFe 0.7 Zn 0.3 PO 4 with 25% sulfur-carbon material. Ball-mill the mixture in toluene at 300 rpm for 2.5 h. Heat the milled mixed material in a slightly reducing atmosphere at 900 °C for 8 h to obtain LiFe 0.7 Zn 0.3 PO 4 / SC composite cathode material.

[0055] Example 4

[0056] A. Add polyvinyl alcohol to ethanol and perform ultrasonic treatment for 15 min. After ultrasonic treatment, add diphenyl disulfide under stirring. The mass ratio of polyvinyl alcohol to diphenyl disulfide is 4.7:1. Then, dry the mixture at 60 °C. Finally, anneal the dried mixture in an argon atmosphere at 950 °C for 35 min to obtain sulfur-carbon material;

[0057] B. Mix a 0.5 mol / L zinc acetate solution and a 1 mol / L sodium hydroxide solution and stir evenly. Add polyethylene glycol to obtain mixed solution A, where the volume ratio of the zinc acetate solution, sodium hydroxide solution, and polyethylene glycol is 20:20:1. Add mixed solution A to a hydrothermal reaction kettle and heat it at 135 °C for 14 h. After the reaction ends, let the reaction kettle cool naturally to room temperature. Centrifuge the solution in the reaction kettle for 10 min to obtain a precipitate. Wash the precipitate repeatedly with distilled water first, and then wash it twice with absolute ethanol. After drying, obtain zinc oxide with a particle size of 150 nm;

[0058] C. Add CTAB to deionized water and stir to form a uniform and transparent CTAB aqueous solution. The concentration of the CTAB aqueous solution is 0.03 mol / L. Add 0.045 mol of ferrous sulfate to each liter of the CTAB aqueous solution and stir until completely dissolved. Then, add an excessive amount of hydrogen peroxide to oxidize Fe 2+ to Fe 3+, then add phosphoric acid solution under stirring. The volume ratio of the phosphoric acid solution to the CTAB aqueous solution is 3:200. Continue stirring for 1 hour to obtain a mixed solution B. Add the mixed solution B into a hydrothermal reaction kettle and heat it at 175 °C for 1.8 h. After heating, naturally cool the reaction kettle to room temperature, centrifuge the solution in the reaction kettle for 10 min to obtain a precipitate. Then wash the precipitate repeatedly with distilled water and then wash it twice with absolute ethanol. Dry it at 100 °C for 4 h to obtain iron phosphate;

[0059] D. Add lithium hydroxide and iron phosphate into the vinyl alcohol solution in turn. The molar ratio of lithium hydroxide, iron phosphate and vinyl alcohol is 1:1:2. Adjust the pH value of the solution to 7, and then add zinc oxide. The molar ratio of zinc oxide to iron phosphate is 1:3. Then stir the solution at 70 °C at a stirring rate of 400 rpm until a transparent green gel is obtained. Put the obtained green gel into a tubular furnace filled with a reducing atmosphere and heat it at 320 °C for 5 h to obtain LiFe 0.75 Zn 0.25 PO 4 ;

[0060] E. By mass percentage, mix 85% LiFe 075 Zn 0.25 PO 4 with 15% sulfur-carbon material. Ball-mill the mixture in toluene at 400 rpm for 2 h. Heat the ground mixed material in a slightly reducing atmosphere at 700 °C for 14 h to obtain LiFe 075 Zn 0.25 PO 4 / SC composite cathode material.

[0061] Example 5

[0062] A. Add glucose and sucrose into ethanol and perform ultrasonic treatment for 15 min. After ultrasonic treatment, add diphenyl disulfide under stirring. The mass ratio of glucose, sucrose and diphenyl disulfide is 3:1.5:1. Then dry the mixture at 60 °C. Finally, anneal the dried mixture at 1050 °C for 25 min in an argon atmosphere to obtain sulfur-carbon material;

[0063] B. Mix a 0.5 mol / L zinc acetate solution and a 1 mol / L sodium hydroxide solution and stir evenly. Add polyethylene glycol to obtain a mixed solution A. The volume ratio of the zinc acetate solution, sodium hydroxide solution and polyethylene glycol is 20:20:1. Add the mixed solution A into a hydrothermal reaction kettle and heat it at 145 °C for 10 h. After the reaction, naturally cool the reaction kettle to room temperature, centrifuge the solution in the reaction kettle for 10 min to obtain a precipitate. Wash the precipitate repeatedly with distilled water and then wash it twice with absolute ethanol. After drying, obtain zinc oxide with a particle size of 100 nm;

[0064] C. Add CTAB to deionized water and stir to form a homogeneous and transparent CTAB aqueous solution with a concentration of 0.03 mol / L. Add 0.045 mol of ferrous sulfate to each liter of the CTAB aqueous solution, stir until completely dissolved, and then add an excessive amount of hydrogen peroxide to oxidize Fe 2+ to Fe 3+ . Then, add a phosphoric acid solution with a volume ratio of the phosphoric acid solution to the CTAB aqueous solution of 3:200 while stirring, continue to stir for 1 hour to obtain a mixed solution B. Add the mixed solution B to a hydrothermal reactor, heat at 165 °C for 2.2 h. After heating, naturally cool the reactor to room temperature, centrifuge the solution in the reactor for 10 min to obtain a precipitate. Then, wash the precipitate repeatedly with distilled water and then wash it twice with absolute ethanol, and dry it at 100 °C for 4 h to obtain iron phosphate;

[0065] D. Add lithium hydroxide and iron phosphate to a citric acid solution in turn. The molar ratio of lithium hydroxide, iron phosphate, and citric acid is 1:1:2. Adjust the pH value of the solution to 7, and then add zinc oxide. The molar ratio of zinc oxide to iron phosphate is 1:3. Then, stir the solution at 70 °C at a stirring rate of 400 rpm until a transparent green gel is obtained. Put the obtained green gel into a tubular furnace filled with a reducing atmosphere and heat and decompose it at 380 °C for 4.5 h to obtain LiFe 0.75 Zn 0.25 PO 4 ;

[0066] E. Mix 80% LiFe 075 Zn 0.25 PO 4 and 20% sulfur-carbon material by mass percentage. Ball-mill the mixture in toluene at 400 rpm for 2 h, and heat the milled mixed material in a slightly reducing atmosphere at 800 °C for 10 h to obtain LiFe 075 Zn 0.25 PO 4 / SC composite cathode material.

[0067] Comparative Example 1

[0068] A. Add glucose to ethanol and perform ultrasonic treatment for 15 min. After ultrasonic treatment, dry at 60 °C, and finally anneal the dried material at 1000 °C for 30 min in an argon atmosphere to obtain a carbon material;

[0069] B. Mix 0.5 mol / L zinc acetate solution and 1 mol / L sodium hydroxide solution and stir well. Add polyethylene glycol to obtain mixed solution A, where the volume ratio of zinc acetate solution, sodium hydroxide solution, and polyethylene glycol is 20:20:1. Add mixed solution A to a hydrothermal reactor and heat it at 140 °C for 12 h. After the reaction is completed, let the reactor cool naturally to room temperature. Centrifuge the solution in the reactor for 10 min to obtain a precipitate. Wash the precipitate repeatedly with distilled water first, then wash it twice with absolute ethanol, and dry it to obtain zinc oxide with a particle size of 100 nm.

[0070] C. Add CTAB to deionized water and stir to form a homogeneous and transparent CTAB aqueous solution with a concentration of 0.03 mol / L. Add 0.045 mol of ferrous sulfate to each liter of CTAB aqueous solution and stir until completely dissolved. Then add an excessive amount of hydrogen peroxide to oxidize Fe 2+ to Fe 3+ . Then add phosphoric acid solution with a volume ratio of 3:200 to the CTAB aqueous solution under stirring and continue stirring for 1 h to obtain mixed solution B. Add mixed solution B to a hydrothermal reactor and heat it at 170 °C for 2 h. After heating, let the reactor cool naturally to room temperature. Centrifuge the solution in the reactor for 10 min to obtain a precipitate. Wash the precipitate repeatedly with distilled water first, then wash it twice with absolute ethanol, and dry it at 100 °C for 4 h to obtain iron phosphate.

[0071] D. Add lithium hydroxide and iron phosphate to citric acid solution in turn, with the molar ratio of lithium hydroxide, iron phosphate, and citric acid being 1:1:2. Adjust the pH value of the solution to 7, then add zinc oxide, where the molar ratio of zinc oxide and iron phosphate is 1:3. Then stir the solution at 70 °C at a stirring rate of 400 rpm until a transparent green gel is obtained. Put the obtained green gel into a tubular furnace filled with a reducing atmosphere and heat it at 350 °C for 5 h to obtain LiFe 0.75 Zn 0.25 PO 4 .

[0072] E. Mix 80% LiFe 075 Zn 0.25 PO 4 and 20% carbon material by mass percentage. Ball-mill the mixture in toluene at 400 rpm for 2 h. Heat the milled mixed material in a slightly reducing atmosphere at 750 °C for 12 h to obtain LiFe 075 Zn 0.25 PO 4 / C composite cathode material.

[0073] Comparative Example 2

[0074] A. Add glucose to ethanol and perform ultrasonic treatment for 15 min. After ultrasonic treatment, add diphenyl disulfide under stirring. The mass ratio of glucose to diphenyl disulfide is 4.3:1. Then dry the mixture at 60 °C, and finally anneal the dried mixture at 1000 °C for 30 min in an argon atmosphere to obtain a sulfur-carbon material;

[0075] B. Add CTAB to deionized water and stir to form a homogeneous and transparent CTAB aqueous solution with a concentration of 0.03 mol / L. Add 0.045 mol of ferrous sulfate to each liter of the CTAB aqueous solution and stir until completely dissolved. Then add an excessive amount of hydrogen peroxide to oxidize Fe 2+ to Fe 3+ , and then add a phosphoric acid solution under stirring. The volume ratio of the phosphoric acid solution to the CTAB aqueous solution is 3:200. Continue to stir for 1 hour to obtain a mixed solution B. Add the mixed solution B to a hydrothermal reaction kettle and heat it at 170 °C for 2 h. After heating, naturally cool the reaction kettle to room temperature, centrifuge the solution in the reaction kettle for 10 min to obtain a precipitate. Then wash the precipitate repeatedly with distilled water and then wash it twice with absolute ethanol, and dry it at 100 °C for 4 h to obtain iron phosphate;

[0076] C. Add lithium hydroxide and iron phosphate to a citric acid solution in turn. The molar ratio of lithium hydroxide, iron phosphate, and citric acid is 1:1:2. Adjust the pH value of the solution to 7, and then continuously stir the solution at 70 °C at a stirring rate of 400 rpm until a transparent green gel is obtained. Put the obtained green gel into a tubular furnace filled with a reducing atmosphere and heat it at 350 °C for 5 h to obtain LiFePO 4 ;

[0077] D. Mix 80% LiFePO 4 and 20% sulfur-carbon material by mass percentage. Ball-mill the mixture in toluene at 400 rpm for 2 h, and heat the milled mixed material in a slightly reducing atmosphere at 750 °C for 12 h to obtain a LiFePO 4 / SC composite cathode material.

[0078] II. Performance Test

[0079] Test the electrochemical performance parameters of the examples and comparative examples according to GB / T 30835-2014.

[0080] The electrochemical performance parameters of the above examples and comparative examples are as follows, as shown in Table 1.

[0081] Table 1

[0082]

[0083] As can be seen from Table 1, compared with Comparative Example 1 without sulfur and Comparative Example 2 without zinc, the sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material prepared by the inventive method has excellent electrochemical performance: the initial charge specific capacity ≥ 162.52 mAh / g at a rate of 0.1C, the initial discharge specific capacity ≥ 154.18 mAh / g, the Coulomb efficiency ≥ 94.74%, the initial discharge specific capacity ≥ 133.05 mAh / g at 1C, after 100 charge-discharge cycles at a rate of 1C, the discharge specific capacity ≥ 126.71 mAh / g, after 100 charge-discharge cycles at a rate of 1C, the discharge specific capacity retention rate ≥ 95.20%, and the tapped density ≥ 1.38 g / cm 3 .

Claims

1. A sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material, characterized in that, it includes a zinc-doped lithium iron phosphate material and a sulfur-carbon material wrapped outside the zinc-doped lithium iron phosphate material. The sulfur-carbon material includes a porous carbon material and sulfur distributed inside and on the surface of the porous carbon material. The sulfur distributed on the surface of the porous carbon material replaces part of the oxygen in the crystal of the zinc-doped lithium iron phosphate material; The preparation method of the sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material includes the following steps: A. Add a carbon source to ethanol for ultrasonic treatment. After ultrasonic treatment, add diphenyl disulfide and stir evenly. Then dry the mixture, and finally anneal the dried mixture to obtain a sulfur-carbon material; B1. Mix and stir evenly an acetic acid zinc solution and a sodium hydroxide solution, add polyethylene glycol to obtain a mixed solution A. Add the mixed solution A to a hydrothermal reaction vessel, heat at a temperature of 120-160 °C for 8-16 h; B2. Naturally cool the reaction vessel to room temperature, perform solid-liquid separation on the solution in the reaction vessel, wash and dry the solid substance to obtain zinc oxide with a particle size of 50-200 nm; C. Add ferrous sulfate and excessive hydrogen peroxide to the CTAB solution in sequence, then add phosphoric acid solution, stir evenly to obtain a mixed solution B, and obtain iron phosphate by hydrothermal method for the mixed solution B; D. Add lithium hydroxide and iron phosphate to a chelating agent in sequence, adjust the pH value of the solution to 6-7, then add zinc oxide, stir until a gel is obtained, and heat and decompose the obtained gel in a reducing atmosphere to obtain a zinc-doped lithium iron phosphate material; E. Mix and grind the zinc-doped lithium iron phosphate material and the sulfur-carbon material, and heat the ground mixed material in a reducing atmosphere to obtain a sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material.

2. The sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material according to claim 1, characterized in that, the carbon source in step A is one or more of glucose, sucrose, polyvinyl alcohol and natural fiber, and the mass ratio of the carbon source to diphenyl disulfide is 4-5:

1.

3. The sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material according to claim 1, characterized in that, the annealing temperature of the annealing treatment in step A is 900-1100 °C, and the annealing time is 20-40 min.

4. The sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material according to claim 1, characterized in that, the obtaining of iron phosphate by hydrothermal method for the mixed solution B in step C includes the following steps: C1. Add the mixed solution B to a hydrothermal reaction vessel, heat at a temperature of 160-180 °C for 1.5-2.5 h; C2. Naturally cool the reaction vessel to room temperature, perform solid-liquid separation on the solution in the reaction vessel, wash and dry the solid substance to obtain iron phosphate.

5. The sulfur-carbon / zinc-doped lithium iron phosphate composite cathode material according to claim 1, characterized in that, The chelating agent described in step D is one or more of citric acid, tartaric acid, polyethylene glycol, and polyvinyl alcohol. The heating and decomposition of the obtained gel in a reducing atmosphere means that the gel is decomposed in a reducing atmosphere at 300-400 °C for 4-6 h.

6. A lithium iron phosphate composite cathode material doped with sulfur carbon / zinc according to claim 1, characterized in that The chemical formula of the zinc-doped lithium iron phosphate material described in step D is LiFe x Zn y PO 4 , where x = 0.7 - 0.8 and y = 1 - x.

7. A lithium iron phosphate composite cathode material doped with sulfur carbon / zinc according to claim 1, characterized in that The sulfur carbon / zinc-doped lithium iron phosphate composite cathode material described in step E is composed of 10-25% carbon-sulfur material and 75-90% zinc-doped lithium iron phosphate material by mass percentage.

8. A lithium iron phosphate composite cathode material doped with sulfur carbon / zinc according to claim 1, characterized in that When the ground mixed material is heated in a slightly reducing atmosphere in step E, the heating temperature is 600-900 °C and the heating time is 8-16 h.

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