Vanadium-sulfur-doped lithium iron phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses a preparation method of a vanadium-sulfur-doped lithium iron phosphate positive electrode material. The preparation method comprises the following steps: (1) doping vanadium into a lithium iron phosphate precursor to obtain a vanadium-doped lithium iron phosphate precursor; (2) doping sulfur into the vanadium-doped lithium iron phosphate precursor to obtain a vanadium-sulfur-doped lithium iron phosphate precursor; and (3) carrying out carbon coating on the vanadium-sulfur-doped lithium iron phosphate precursor obtained in the step (2) by using a carbon nanotube to obtain the vanadium-sulfur-doped lithium iron phosphate positive electrode material. According to the preparation method of the vanadium-sulfur-doped lithium iron phosphate positive electrode material, vanadium is doped in a lithium iron phosphate precursor, then sulfur is doped in the lithium iron phosphate precursor, and then a carbon nanotube is used for coating, so that the sulfur and carbon nanotube double-layer coated vanadium / lithium iron phosphate positive electrode material is prepared. And the capacity retention ratio and the discharge specific capacity of the lithium iron phosphate positive electrode material are remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery cathode materials, and particularly relates to a vanadium and sulfur doped lithium iron phosphate cathode material. BACKGROUND

[0002] Lithium ion batteries are gradually replacing lead-acid batteries and are widely used due to high theoretical specific capacity, safety and no memory effect. A lithium ion battery mainly comprises four main materials, namely, a cathode material, an anode material, a separator and an electrolyte, and auxiliary materials. The cathode material is mainly a ternary cathode material or lithium iron phosphate. Compared with the ternary material, the lithium iron phosphate (LiFePO4) cathode material does not contain precious metals, the raw material is non-toxic and cheap, and the olivine structure is more stable than the layered structure of the ternary material, and thus has the advantages of high working voltage, large energy density, long cycle life, good safety performance, small self-discharge rate and no memory effect, and is expected to be applied on a large scale in the energy storage field. In the charging process of the lithium iron phosphate (LiFePO4) cathode material, part of the lithium ions in the lithium iron phosphate are removed, are transferred to the negative electrode through the electrolyte, and are embedded in the negative electrode carbon material; meanwhile, electrons are released from the positive electrode, reach the negative electrode from the external circuit, and maintain the balance of the chemical reaction; in the discharging process, lithium ions are removed from the negative electrode, reach the positive electrode through the electrolyte, and the negative electrode releases electrons, which reach the positive electrode from the external circuit, and provide energy to the outside. However, the main problem of the lithium iron phosphate is poor electrode ion conductivity, which is not suitable for large-current charging and discharging, and thus limits its large-scale application.

[0003] At present, in order to improve the capacity retention rate and lithium ion migration rate of the lithium iron phosphate cathode material, the Li / Fe / P ratio is usually optimized or a carbon source and a metal ion dopant are selected for doping. The commonly used carbon sources include organic carbon sources, such as low-molecular-weight polymers, glucose and sucrose, and inorganic carbon sources, such as acetylene black, carbon nanotubes and graphene; and the commonly used ion dopants include magnesium, titanium and cerium metal ions. The lithium iron phosphate cathode material also has the problems of low diffusion coefficient, low electronic conductivity and low rate performance caused by the coexistence of two phases. The intrinsic conductivity of the lithium iron phosphate cathode material is about 10 -9 ~ 10 -10 S / cm, and the capacity retention rate significantly decreases with the increase of the discharge rate, the specific capacity at 5C is below 130 mAh / g, and the capacity retention rate is generally below 50%. Therefore, it is of great significance to develop a lithium iron phosphate cathode material with higher capacity retention rate and discharge specific capacity for improving the overall electrochemical performance of the lithium battery. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a lithium iron phosphate cathode material with higher capacity retention rate and discharge specific capacity.

[0005] The technical scheme adopted by the present application to solve its technical problems is: a preparation method of vanadium-sulfur doped lithium iron phosphate positive electrode material, the preparation method comprises the following steps:

[0006] (1) vanadium is doped into lithium iron phosphate precursor to obtain vanadium-doped lithium iron phosphate precursor;

[0007] (2) sulfur is doped into the vanadium-doped lithium iron phosphate precursor to obtain vanadium-sulfur-doped lithium iron phosphate precursor;

[0008] (3) carbon nanotubes are used for carbon coating on the vanadium-sulfur-doped lithium iron phosphate precursor obtained in step (2) to obtain vanadium-sulfur-doped lithium iron phosphate positive electrode material.

[0009] The preparation method of the lithium iron phosphate precursor is that iron phosphate dihydrate, lithium hydroxide monohydrate and phosphoric acid are put into deionized water, and the lithium iron phosphate precursor is obtained through hydrothermal reaction at 200-220 ℃ for 12-48 h, and the dosage ratio of the iron phosphate dihydrate, lithium hydroxide monohydrate, phosphoric acid and deionized water is 1-3 mol: 1-3 mol: 1-20 mL: 500-1500 mL.

[0010] Further, in step (1), V2O5 powder and lithium iron phosphate precursor are mixed and ball milled to obtain a mixture, and then the mixture and glucose are calcined to obtain the vanadium-doped lithium iron phosphate precursor, and the mass ratio of the mixture and glucose is 50: 1-10.

[0011] Further, in step (1), anhydrous ethanol is used as a ball milling medium and zirconium oxide is used as a grinding ball in the ball milling process, and the ball milling is carried out under the condition that the vacuum degree is less than 1.5 × 10 -2 Pa for 2-6 h, and the mixture is obtained by drying; the dosage ratio of V2O5 powder, lithium iron phosphate precursor, anhydrous ethanol and zirconium oxide is 3-5 g: 20-50 g: 20-100 mL: 100-200 g; the calcination is carried out in a tube furnace under a nitrogen atmosphere, the first stage is calcined at 300-400 ℃ for 20-60 min, the second stage is calcined at 500-600 ℃ for 20-40 min, and the vanadium-doped lithium iron phosphate precursor is obtained after cooling to room temperature.

[0012] Further, in step (2), thiourea and the vanadium-doped lithium iron phosphate precursor are dispersed in deionized water, citric acid is added as a complexing agent, heated to 80-90 ℃ and kept for 10-30 min to form a wet gel; then the wet gel is pre-fired in an argon atmosphere, and then high-temperature calcination is carried out to obtain the vanadium-sulfur-doped lithium iron phosphate precursor.

[0013] Further, the dosage ratio of the thiourea, vanadium-doped lithium iron phosphate precursor, citric acid and deionized water is 1-5 g: 20-50 g: 10-25 mL: 300-500 mL.

[0014] The pre-sintering temperature of the wet gel is 400-500 DEG C in an argon atmosphere, and the pre-sintering time is 1-3 h; the high-temperature calcination temperature is 750-800 DEG C, and the high-temperature calcination time is 3-9 h.

[0015] Further, in the step (3), the carbon coating is to disperse 20-50 nm carbon nanotube powder and the vanadium-sulfur doped lithium iron phosphate precursor obtained in the step (2) in anhydrous ethanol, stir uniformly, and then stand for 6-12 h; after solid-liquid separation, vacuum drying is carried out at 60 DEG C; under a nitrogen atmosphere, calcination is carried out at 500-600 DEG C for 6-12 h, and cooling is carried out to obtain a vanadium-sulfur doped lithium iron phosphate positive electrode material.

[0016] The amount ratio of the carbon nanotube powder, the vanadium-sulfur doped lithium iron phosphate precursor, and the anhydrous ethanol is 1-3 g: 10-50 g: 50-500 mL.

[0017] Further, in the step (3), the carbon nanotube powder is a sulfonic acid / carboxyl bifunctionalized carbon nanotube powder.

[0018] The carbon nanotube powder is first subjected to carboxyl and sulfonic acid bifunctionalization treatment, and carboxyl and sulfonic acid groups are modified on the surface of the carbon nanotube, which reduces the agglomeration of the carbon nanotube particles after dispersion, so that the carbon nanotube uniformly and stably coated on the surface of the doped precursor after the carboxyl and sulfonic acid bifunctionalization treatment forms a uniform conductive layer; after heat treatment, the carbon nanotube is tightly combined with the doped precursor, thereby improving the conductivity of the lithium iron phosphate positive electrode material; the vanadium-sulfur doping and the carbon nanotube coating cooperatively reduce the charge transfer impedance of the lithium iron phosphate positive electrode material, improve the lithium ion diffusion coefficient of the lithium iron phosphate positive electrode material, and inhibit the growth of lithium dendrites, thereby improving the rate performance and capacity retention rate of the lithium iron phosphate positive electrode material; at the same time, the carbon nanotube tightly connects the lithium iron phosphate particles by physical adsorption and bonding to form a tight three-dimensional conductive network, thereby further improving the electrical performance of the lithium iron phosphate positive electrode material.

[0019] Further, in the step (3), the preparation method of the sulfonic acid / carboxyl bifunctionalized carbon nanotube powder is as follows:

[0020] (a) adding 20-50 nm carbon nanotube powder into a mixed solution of concentrated nitric acid and concentrated sulfuric acid, stirring at a stirring speed of 300-600 rpm to heat to 70-90 DEG C, keeping for 4-8 h, cooling to room temperature, then diluting with deionized water, repeatedly filtering until the solution is neutral, separating the solid from the solution, and drying to obtain carboxylated carbon nanotube powder; the volume ratio of concentrated nitric acid to concentrated sulfuric acid in the mixed solution of concentrated nitric acid and concentrated sulfuric acid is 1:4, and the use amount ratio of carbon nanotube powder to the mixed solution of concentrated nitric acid and concentrated sulfuric acid is 1-30 g:400-600 mL; the mass fraction of the concentrated nitric acid is 60-70 wt%, and the mass fraction of the concentrated sulfuric acid is 68-75 wt%;

[0021] (b) adding the carboxylated carbon nanotube powder into a 5-10 wt% sodium dodecylbenzenesulfonate solution, stirring at 70-90 DEG C for 3-6 h, centrifuging, washing with ethanol, and vacuum drying at 30-60 DEG C to obtain sulfonic acid / carboxyl bifunctionalized carbon nanotube powder; the mass ratio of the carboxylated carbon nanotube powder to sodium dodecylbenzenesulfonate is 5-20:1-3.

[0022] A vanadium-sulfur doped lithium iron phosphate positive electrode material, which is prepared by the preparation method of any one of the above.

[0023] The application of the above vanadium-sulfur doped lithium iron phosphate positive electrode material in preparing a lithium ion battery.

[0024] The preparation method of the vanadium-sulfur doped lithium iron phosphate positive electrode material of the present application, by first doping vanadium in a lithium iron phosphate precursor, then doping sulfur, and then coating with carbon nanotubes, a vanadium / lithium iron phosphate positive electrode material coated with double layers of sulfur and carbon nanotubes is prepared, which significantly improves the capacity retention rate and the specific discharge capacity of the lithium iron phosphate positive electrode material, the 5C specific discharge capacity is up to 147 mAh / g, the capacity retention rate is up to 95.5% after 1000 cycles of 5C rate charge and discharge, and the charge transfer impedance is minimum 55.4 Ω. DETAILED DESCRIPTION

[0025] The present application is further described below in conjunction with examples.

[0026] Example 1

[0027] The present embodiment provides a preparation method of a vanadium-sulfur doped lithium iron phosphate positive electrode material, comprising the following steps:

[0028] S1, preparing a lithium iron phosphate precursor:

[0029] Put 182.8g FePO4·2H2O, 41.9g LiOH·H2O, 20.0mL phosphoric acid into 1000.0mL deionized water, and hydrothermally react at 200℃ for 12h to obtain a lithium iron phosphate precursor;

[0030] S2, doping vanadium:

[0031] Mix 3g V2O5 powder with 47g lithium iron phosphate precursor, and ball mill in 50mL anhydrous ethanol as a ball mill medium with 200g zirconium oxide as a grinding ball under a vacuum degree less than 1.5×10-3Pa for 6h, and take out and dry to obtain a mixture; -2 Pa condition for 6h, take out and dry to obtain a mixture;

[0032] Mix 50g of the mixture with 3g glucose, and place in a tube furnace, and calcine at 300℃ for 30min in the first stage and at 500℃ for 30min in the second stage under a nitrogen atmosphere, and cool to room temperature to obtain a vanadium-doped lithium iron phosphate precursor;

[0033] S3, doping sulfur:

[0034] Disperse 5g thiourea and the vanadium-doped lithium iron phosphate precursor obtained in S2 in 300mL deionized water, add 25mL citric acid as a complexing agent, heat to 80℃ and keep for 30min to form a wet gel;

[0035] Pre-burn the wet gel at 400℃ for 1h in an argon atmosphere, and then high-temperature calcine at 750℃ for 3h to obtain a vanadium-sulfur-doped lithium iron phosphate precursor;

[0036] S4, carbon coating:

[0037] Disperse 1g carbon nanotube powder with a particle size distribution of 20-50nm and 50g vanadium-sulfur-doped lithium iron phosphate precursor in 500mL anhydrous ethanol, uniformly stir, and then stand for 12h, and after solid-liquid separation, vacuum dry at 60℃, and then calcine at 600℃ for 6h under a nitrogen atmosphere, and cool to obtain a vanadium-sulfur-doped lithium iron phosphate positive electrode material.

[0038] Example 2:

[0039] The embodiment provides a preparation method of a vanadium-sulfur-doped lithium iron phosphate positive electrode material, and comprises the following steps:

[0040] S1, preparing a lithium iron phosphate precursor:

[0041] Put 182.8g FePO4·2H2O, 41.9g LiOH·H2O, 20.0mL phosphoric acid into 1000.0mL deionized water, and hydrothermally react at 220℃ for 12h to obtain a lithium iron phosphate precursor;

[0042] S2, doping vanadium:

[0043] Mix 3g V2O5 powder with 45g lithium iron phosphate precursor, ball mill in 50mL anhydrous ethanol as ball mill medium, 200g zirconium oxide as grinding ball, under vacuum degree less than 1.5x10 -2 Pa for 5h, take out and dry to obtain the mixture;

[0044] Mix 50g mixture with 5g glucose, place in a tube furnace, under nitrogen atmosphere, first stage 300℃ calcine for 30min, second stage 600℃ calcine for 20min, cool to room temperature to obtain vanadium doped lithium iron phosphate precursor;

[0045] S3, doping sulfur:

[0046] Disperse 5g thiourea and vanadium doped lithium iron phosphate precursor obtained in S2 in 400mL deionized water, add 25mL citric acid as complexing agent, heat to 80℃ for 30min to form wet gel;

[0047] Wet gel is pre-fired at 400℃ for 1h under argon atmosphere, then high temperature calcined at 750℃ for 3h, vanadium and sulfur doped lithium iron phosphate precursor;

[0048] S4, carbon coating:

[0049] Disperse 2g carbon nanotube powder with particle size distribution of 20-50nm and 50g vanadium and sulfur doped lithium iron phosphate precursor in 500mL anhydrous ethanol, stir uniformly, then stand for 12h, solid-liquid separation, vacuum drying at 60℃, heat to 600℃ under nitrogen atmosphere for 6h, cool to obtain vanadium and sulfur doped lithium iron phosphate anode material.

[0050] Example 3:

[0051] The embodiment provides a preparation method of vanadium and sulfur doped lithium iron phosphate anode material, comprising the following steps:

[0052] S1, preparation of lithium iron phosphate precursor:

[0053] Add 182.8g FePO4·2H2O, 41.9g LiOH·H2O and 20.0mL phosphoric acid into 1000.0mL deionized water, hydrothermal reaction at 220℃ for 12h to obtain lithium iron phosphate precursor;

[0054] S2, doping vanadium:

[0055] Mix 5g V2O5 powder with 45g lithium iron phosphate precursor, ball mill in 50mL anhydrous ethanol as ball mill medium, 200g zirconium oxide as grinding ball, under vacuum degree less than 1.5x10 -2 Pa for 6h, take out and dry to obtain the mixture;

[0056] Mix 50g of the mixture with 4g of glucose, place it in a tube furnace, and calcine it at 400℃ for 20min in the first stage and at 600℃ for 20min in the second stage under a nitrogen atmosphere. After cooling to room temperature, a vanadium-doped lithium iron phosphate precursor is obtained;

[0057] S3, doping sulfur:

[0058] Disperse 5g of thiourea and the vanadium-doped lithium iron phosphate precursor obtained in S2 in 300mL of deionized water, add 25mL of citric acid as a complexing agent, heat to 80℃ for 20min to form a wet gel;

[0059] Pre-burn the wet gel at 400℃ for 1h under an argon atmosphere, and then high-temperature calcine it at 750℃ for 3h to obtain a vanadium-sulfur-doped lithium iron phosphate precursor;

[0060] S4, carbon coating:

[0061] Disperse 3g of carbon nanotube powder with a particle size distribution of 20-50nm and 50g of the vanadium-sulfur-doped lithium iron phosphate precursor in 500mL of anhydrous ethanol, stir uniformly, and then stand for 12h. After solid-liquid separation, vacuum dry at 60℃, and then calcine at 600℃ for 12h under a nitrogen atmosphere. After cooling, a vanadium-sulfur-doped lithium iron phosphate positive electrode material is obtained.

[0062] Example 4:

[0063] The embodiment provides a preparation method of a vanadium-sulfur-doped lithium iron phosphate positive electrode material, which comprises the following steps:

[0064] S1, preparation of a lithium iron phosphate precursor:

[0065] Add 182.8g of FePO4·2H2O, 41.9g of LiOH·H2O, and 20.0mL of phosphoric acid to 1000.0mL of deionized water, and hydrothermally react at 200℃ for 12h to obtain a lithium iron phosphate precursor;

[0066] S2, doping vanadium:

[0067] Mix 3g of V2O5 powder with 47g of the lithium iron phosphate precursor, and ball mill in 50mL of anhydrous ethanol as a ball milling medium and 200g of zirconia as a grinding ball under a vacuum degree of less than 1.5×10 -2 Pa for 6h, and then take out and dry to obtain a mixture;

[0068] Mix 50g of the mixture with 3g of glucose, place it in a tube furnace, and calcine it at 300℃ for 30min in the first stage and at 500℃ for 30min in the second stage under a nitrogen atmosphere. After cooling to room temperature, a vanadium-doped lithium iron phosphate precursor is obtained;

[0069] S3, doping sulfur:

[0070] 5g thiourea and the vanadium-doped lithium iron phosphate precursor obtained in S2 were dispersed in 300mL deionized water, 25mL citric acid was added as a complexing agent, heated to 80℃ for 30min to form a wet gel;

[0071] The wet gel was pre-fired at 400℃ for 1h under argon atmosphere, and then calcined at 750℃ for 3h to obtain the vanadium-sulfur-doped lithium iron phosphate precursor;

[0072] S4, carbon coating:

[0073] 20g carbon nanotube powder with a particle size of 20-50nm was added to a mixture of 100mL concentrated nitric acid and 400mL concentrated sulfuric acid, stirred at 300rpm, and heated to 80℃ for 6h. After cooling to room temperature, the solution was diluted with deionized water, and repeatedly filtered until the pH of the solution was neutral. The carboxylated carbon nanotube powder was obtained by drying after solid-liquid separation. The mass fraction of the concentrated nitric acid was 65wt%, and the mass fraction of the concentrated sulfuric acid was 72wt%;

[0074] 20g carboxylated carbon nanotube powder was added to 40g 5wt% sodium dodecylbenzenesulfonate solution, and stirred at 80℃ for 5h. After centrifugal separation, the powder was washed with ethanol and vacuum dried at 60℃ to obtain a sulfonic / carboxyl bifunctionalized carbon nanotube powder;

[0075] 1g sulfonic / carboxyl bifunctionalized carbon nanotube powder and 50g vanadium-sulfur-doped lithium iron phosphate precursor were dispersed in 500mL anhydrous ethanol, stirred uniformly, and then left to stand for 12h. After solid-liquid separation, the mixture was vacuum dried at 60℃, and then calcined at 600℃ for 6h under nitrogen atmosphere. After cooling, a vanadium-sulfur-doped lithium iron phosphate positive electrode material was obtained.

[0076] Example 5:

[0077] The present embodiment provides a preparation method of a vanadium-sulfur-doped lithium iron phosphate positive electrode material, comprising the following steps:

[0078] S1, preparation of lithium iron phosphate precursor:

[0079] 182.8g FePO4·2H2O, 41.9g LiOH·H2O, and 20.0mL phosphoric acid were added to 1000.0mL deionized water, and hydrothermally reacted at 200℃ for 12h to obtain a lithium iron phosphate precursor;

[0080] S2, doping vanadium:

[0081] 5g V2O5 powder is mixed with 45g lithium iron phosphate precursor and ball milled, 50mL anhydrous ethanol is used as the ball milling medium, 200g zirconium oxide is used as the grinding ball, the vacuum degree is less than 1.5x10 -2 The mixture is taken out and dried after ball milling for 6h under the condition of 50Pa;

[0082] 50g of the mixture is mixed with 5g of glucose, and placed in a tube furnace, calcined at 300℃ for 30min in the first stage and at 600℃ for 20min in the second stage under nitrogen atmosphere, and the vanadium-doped lithium iron phosphate precursor is obtained after cooling to room temperature;

[0083] S3, doping sulfur:

[0084] 5g thiourea and the vanadium-doped lithium iron phosphate precursor obtained in S2 are dispersed in 400mL deionized water, 25mL citric acid is added as a complexing agent, heated to 80℃ for 30min to form a wet gel;

[0085] The wet gel is pre-fired at 400℃ for 1h and then calcined at 750℃ for 3h in an argon atmosphere, and the vanadium-sulfur-doped lithium iron phosphate precursor is obtained;

[0086] S4, carbon coating:

[0087] 30g of carbon nanotube powder with a particle size of 20-50nm is added to a mixture of 100mL concentrated nitric acid and 400mL concentrated sulfuric acid, stirred at 300rpm, heated to 80℃, and kept for 6h, then cooled to room temperature, diluted with deionized water, repeatedly filtered until the solution pH is neutral, and dried after solid-liquid separation to obtain carboxylated carbon nanotube powder; the mass fraction of the concentrated nitric acid is 65wt%, and the mass fraction of the concentrated sulfuric acid is 72wt%;

[0088] 20g of carboxylated carbon nanotube powder is added to 60g of 5wt% sodium dodecylbenzenesulfonate solution, magnetically stirred at 90℃ for 3h, centrifuged, washed with ethanol, and vacuum dried at 60℃ to obtain sulfonic acid / carboxyl bifunctionalized carbon nanotube powder;

[0089] 2g of sulfonic acid / carboxyl bifunctionalized carbon nanotube powder and 50g of vanadium-sulfur-doped lithium iron phosphate precursor are dispersed in 500mL anhydrous ethanol, stirred uniformly, and then left to stand for 12h, and after solid-liquid separation, vacuum dried at 60℃, calcined at 600℃ for 6h under nitrogen atmosphere, and cooled to obtain vanadium-sulfur-doped lithium iron phosphate positive electrode material.

[0090] Example 6:

[0091] The embodiment provides a preparation method of vanadium-sulfur-doped lithium iron phosphate positive electrode material, comprising the following steps:

[0092] S1, preparing lithium iron phosphate precursor:

[0093] Add 182.8 g FePO4·2H2O, 41.9 g LiOH·H2O, 20.0 mL phosphoric acid into 1000.0 mL deionized water, hydrothermal reaction at 200 ℃ for 12 h to obtain a lithium iron phosphate precursor;

[0094] S2, vanadium doping:

[0095] Mix 5 g V2O5 powder with 45 g lithium iron phosphate precursor and ball mill, 50 mL anhydrous ethanol as ball milling medium, 200 g zirconium oxide as grinding ball, ball mill under vacuum degree less than 1.5 × 10 -2 Pa for 6 h, take out and dry to obtain a mixture;

[0096] Mix 50 g of the mixture with 4 g of glucose, place it in a tube furnace, and calcine at 400 ℃ for 20 min in the first stage and at 600 ℃ for 20 min in the second stage under a nitrogen atmosphere to obtain a vanadium-doped lithium iron phosphate precursor;

[0097] S3, sulfur doping:

[0098] Disperse 5 g of thiourea and the vanadium-doped lithium iron phosphate precursor obtained in S2 in 300 mL of deionized water, add 25 mL of citric acid as a complexing agent, heat to 80 ℃ for 30 min to form a wet gel;

[0099] Pre-burn the wet gel at 400 ℃ for 1 h in an argon atmosphere, and then high-temperature calcine at 750 ℃ for 3 h to obtain a vanadium-sulfur-doped lithium iron phosphate precursor;

[0100] S4, carbon coating:

[0101] Add 30 g of carbon nanotube powder with a particle size of 20-50 nm to a mixture of 100 mL of concentrated nitric acid and 400 mL of concentrated sulfuric acid, stir at 300 rpm, and heat to 80 ℃ for 6 h. After cooling to room temperature, dilute with deionized water, repeatedly filter until the solution pH is neutral, and dry after solid-liquid separation to obtain carboxylated carbon nanotube powder. The mass fraction of the concentrated nitric acid is 65 wt%, and the mass fraction of the concentrated sulfuric acid is 72 wt%;

[0102] Add 20 g of carboxylated carbon nanotube powder to 20 g of 5 wt% sodium dodecylbenzenesulfonate solution, magnetically stir at 90 ℃ for 3 h, centrifuge, wash with ethanol, and vacuum dry at 60 ℃ to obtain sulfonic acid / carboxyl bifunctionalized carbon nanotube powder;

[0103] 3g sulfonic / carboxyl bifunctionalized carbon nanotube powder and 50g vanadium and sulfur doped lithium iron phosphate precursor were dispersed in 500mL anhydrous ethanol, after stirring uniformly, standing for 12h, after solid-liquid separation, vacuum drying at 60℃, calcining under nitrogen atmosphere, heating to 600℃ for 6h, after cooling, vanadium and sulfur doped lithium iron phosphate positive electrode material was obtained.

[0104] Comparative Example 1

[0105] The embodiment provides a preparation method of vanadium and sulfur doped lithium iron phosphate positive electrode material, comprising the following steps:

[0106] S1, preparing lithium iron phosphate precursor:

[0107] 182.8g FePO4·2H2O, 41.9g LiOH·H2O, 20.0mL phosphoric acid were added into 1000.0mL deionized water, hydrothermal reaction at 200℃ for 12h, and then lithium iron phosphate precursor was obtained;

[0108] S2, doping vanadium and sulfur:

[0109] 3g V2O5 powder was mixed with 47g lithium iron phosphate precursor, 3g glucose and 5g thiourea, 50mL anhydrous ethanol was used as a ball milling medium, 200g zirconium oxide was used as a grinding ball, and the ball milling was carried out under a vacuum degree less than 1.5×10 -2 Pa for 6h, and then the mixture was taken out and dried to obtain a mixed material;

[0110] The mixed material was placed in a tube furnace, calcined at 300℃ for 30min in the first stage and calcined at 500℃ for 30min in the second stage under a nitrogen atmosphere, and then the vanadium and sulfur doped lithium iron phosphate precursor was obtained after cooling to room temperature;

[0111] S3, carbon coating:

[0112] 20g carbon nanotube powder with a particle size of 20-50nm was added into a mixed solution of 100mL concentrated nitric acid and 400mL concentrated sulfuric acid, stirring was carried out at 300rpm, and then the temperature was increased to 80℃ and kept for 6h, after cooling to room temperature, deionized water was used for dilution, repeated suction filtration was carried out until the pH of the solution was neutral, and then the carboxylated carbon nanotube powder was obtained after drying after solid-liquid separation; the mass fraction of the concentrated nitric acid is 65wt%, and the mass fraction of the concentrated sulfuric acid is 72wt%.

[0113] 20g carboxylated carbon nanotube powder was added into 40g 5wt% sodium dodecylbenzenesulfonate solution, magnetic stirring was carried out at 80℃ for 5h, centrifugal separation was carried out, ethanol was used for washing, and then vacuum drying was carried out at 60℃ to obtain sulfonic / carboxyl bifunctionalized carbon nanotube powder;

[0114] 1g sulfonic / carboxyl bifunctionalized carbon nanotube powder and 50g vanadium and sulfur doped lithium iron phosphate precursor were dispersed in 500mL anhydrous ethanol, after stirring uniformly, standing for 12h, after solid-liquid separation, vacuum drying at 60℃, calcining under nitrogen atmosphere, heating to 600℃ for 6h, after cooling, vanadium and sulfur doped lithium iron phosphate positive electrode material was obtained.

[0115] Comparative Example 2:

[0116] The embodiment provides a preparation method of vanadium and sulfur doped lithium iron phosphate positive electrode material, comprising the following steps:

[0117] S1, preparing lithium iron phosphate precursor:

[0118] 182.8g FePO4·2H2O, 41.9g LiOH·H2O, 20.0mL phosphoric acid were added into 1000.0mL deionized water, hydrothermal reaction at 200℃ for 12h, and then lithium iron phosphate precursor was obtained;

[0119] S2, doping vanadium and sulfur:

[0120] 5g V2O5 powder, 45g lithium iron phosphate precursor, 5g glucose and 5g thiourea were mixed and ball milled, 50mL anhydrous ethanol was used as ball milling medium, 200g zirconium oxide was used as grinding ball, and the ball milling was carried out under a vacuum degree less than 1.5×10 -2 Pa for 6h, and then the mixed material was obtained by drying;

[0121] The mixed material was placed in a tube furnace, and calcining was carried out under a nitrogen atmosphere, the first stage was 300℃ for 30min, and the second stage was 600℃ for 20min, and then vanadium and sulfur doped lithium iron phosphate precursor was obtained after cooling to room temperature;

[0122] S3, carbon coating:

[0123] 30g carbon nanotube powder with a particle size of 20-50nm was added into a mixed solution of 100mL concentrated nitric acid and 400mL concentrated sulfuric acid, stirring was carried out at 300rpm, and heating was carried out to 80℃, and then the temperature was kept for 6h, after cooling to room temperature, deionized water was used for dilution, repeated suction filtration was carried out until the pH of the solution was neutral, and then the carboxylated carbon nanotube powder was obtained by drying after solid-liquid separation; the mass fraction of the concentrated nitric acid is 65wt%, and the mass fraction of the concentrated sulfuric acid is 72wt%.

[0124] 20g carboxylated carbon nanotube powder was added into 60g 5wt% sodium dodecylbenzenesulfonate solution, magnetic stirring was carried out at 80℃ for 3h, centrifugal separation was carried out, ethanol was used for washing, and then vacuum drying was carried out at 60℃, and sulfonic / carboxyl bifunctionalized carbon nanotube powder was obtained.

[0125] 2g of sulfonic acid / carboxyl bifunctionalized carbon nanotube powder and 50g of vanadium-sulfur-doped lithium iron phosphate precursor were dispersed in 500mL of anhydrous ethanol. After stirring evenly, the mixture was allowed to stand for 12h. After solid-liquid separation, the mixture was vacuum dried at 60℃ and calcined at 600℃ for 6h under a nitrogen atmosphere. After cooling, vanadium-sulfur-doped lithium iron phosphate cathode material was obtained.

[0126] Comparative Example 3:

[0127] This embodiment provides a method for preparing vanadium-sulfur-doped lithium iron phosphate cathode material, including the following steps:

[0128] S1. Preparation of lithium iron phosphate precursor:

[0129] 182.8g FePO4·2H2O, 41.9g LiOH·H2O, and 20.0mL phosphoric acid were added to 1000.0mL deionized water and the mixture was hydrothermally reacted at 200℃ for 12h to obtain lithium iron phosphate precursor.

[0130] S2, Vanadium doping:

[0131] 3g of V2O5 powder and 47g of lithium iron phosphate precursor were mixed and ball-milled. 50mL of anhydrous ethanol was used as the ball milling medium and 200g of zirconium oxide was used as the grinding balls. The mixture was ball-milled for 6 hours under a vacuum of less than 1.5×10-2Pa. The mixture was then removed and dried to obtain the final product.

[0132] Mix 50g of the mixture with 3g of glucose, place it in a tube furnace, and calcine at 300℃ for 30min in the first stage and at 500℃ for 30min in the second stage under a nitrogen atmosphere. After cooling to room temperature, a vanadium-doped lithium iron phosphate precursor is obtained.

[0133] S3, sulfur doping:

[0134] Disperse 5g of thiourea and the vanadium-doped lithium iron phosphate precursor obtained from S2 in 300mL of deionized water, add 25mL of citric acid as a complexing agent, heat to 80℃ and keep warm for 30min to form a wet gel.

[0135] The wet gel was pre-calcined at 400°C for 1 hour in an argon atmosphere, and then calcined at 750°C for 3 hours to obtain a lithium iron phosphate precursor doped with vanadium and sulfur.

[0136] S4, Carbon Coating:

[0137] 20g carbon nanotube powder with particle size of 20-50nm was added into a mixture of 100mL concentrated nitric acid and 400mL concentrated sulfuric acid, and stirred at 300rpm to raise the temperature to 80℃, and kept for 6h. After cooling to room temperature, the solution was diluted with deionized water, and repeatedly filtered until the pH of the solution was neutral. After solid-liquid separation and drying, carboxylated carbon nanotube powder was obtained; the mass fraction of the concentrated nitric acid was 65wt%, and the mass fraction of the concentrated sulfuric acid was 72wt%;

[0138] 1g carboxyl-functionalized carbon nanotube powder and 50g vanadium-sulfur-doped lithium iron phosphate precursor were dispersed in 500mL anhydrous ethanol, stirred uniformly, and then left to stand for 12h. After solid-liquid separation, vacuum drying was performed at 60℃, and then calcination was performed at 600℃ for 6h under a nitrogen atmosphere. After cooling, a vanadium-sulfur-doped lithium iron phosphate positive electrode material was obtained.

[0139] The vanadium-sulfur-doped lithium iron phosphate positive electrode materials prepared in Examples 1-6 and Comparative Examples 1-3 were respectively prepared into positive electrode sheets, and then the positive electrode sheets were assembled into lithium iron phosphate batteries. The specific preparation method was as follows:

[0140] 0.5g of vanadium-sulfur-doped lithium iron phosphate positive electrode material, acetylene black, and polyvinylidene fluoride (PVDF) with a mass ratio of 85:10:5 were mixed with an appropriate amount of N-methyl pyrrolidone to form a uniform slurry, which was coated on an aluminum foil and dried to prepare a positive electrode sheet.

[0141] A lithium metal sheet was used as a negative electrode sheet, a separator was a polypropylene microporous film (Celgard 2400), and an electrolyte was an equal amount of a mixture of 1mol / L LiPF6, ethylene carbonate (EC), and dimethyl carbonate (DMC). A button cell was assembled in an argon-filled glove box, and the 1C, 2C, and 5C discharge specific capacities of each sample were measured, with the results shown in Table 1:

[0142] Table 1

[0143]

[0144]

[0145] As can be seen from Table 1, the 5C discharge specific capacities of Examples 1-6 were significantly higher than those of Comparative Examples 1-3, and in particular, the discharge specific capacities of Examples 4-6 at a 5C rate were higher than 142mAh / g.

[0146] The capacity retention rates of the batteries were measured by performing 5C rate charge-discharge tests for 1000 cycles at 25℃ for each sample, with the results shown in Table 2:

[0147] Table 2

[0148]

[0149] As shown in Table 2, the capacity retention rate of the batteries prepared from the positive electrode materials of Examples 1-6 is greater than 88.5% after 1000 cycles, and the highest is 95.5%, while the capacity retention rate of the batteries prepared from the positive electrode materials of Comparative Examples 1-3 is not higher than 74.6%.

[0150] The charge transfer impedance of the button cells assembled from the vanadium and sulfur doped lithium iron phosphate positive electrode materials prepared from Examples 1-6 and Comparative Examples 1-3 was measured on an electrochemical workstation (Shanghai Chenhua 760E, the test frequency range is 0.01 Hz-100 kHz), and the results are shown in Table 3.

[0151] Table 3

[0152]

[0153] As shown in Table 3, the positive electrode sheets prepared from the lithium iron phosphate positive electrode materials of Examples 1-6 have lower charge transfer impedance after being assembled into button cells, and thus have more excellent electrical performance. The lithium iron phosphate positive electrode materials of Examples 4-6 are coated with the carbon nanotubes with sulfonic acid / carboxyl bifunctional groups, and thus have charge transfer impedance not higher than 56.7 Ω, and thus have more excellent electrical performance. In Comparative Examples 1 and 2, the simultaneous doping method is used, and thus the charge transfer impedance is not less than 158.9 Ω. In Comparative Example 3, the carbon nanotubes with carboxyl functional groups are used for coating instead of the carbon nanotubes with sulfonic acid / carboxyl bifunctional groups used in Examples 4-6, and thus the charge transfer impedance is increased by more than 50% relative to Example 1.

Claims

1. A method for preparing a vanadium-sulfur-doped lithium iron phosphate cathode material, characterized in that, The preparation method includes the following steps: (1) Vanadium is doped into lithium iron phosphate precursor to obtain vanadium-doped lithium iron phosphate precursor. (2) Sulfur is added to the vanadium-doped lithium iron phosphate precursor to obtain vanadium-sulfur-doped lithium iron phosphate precursor. (3) Carbon nanotubes are used to carbon-coat the vanadium-sulfur-doped lithium iron phosphate precursor obtained in step (2) to obtain vanadium-sulfur-doped lithium iron phosphate cathode material.

2. The method for preparing a vanadium-sulfur-doped lithium iron phosphate cathode material according to claim 1, characterized in that: In step (1), V2O5 powder and lithium iron phosphate precursor are mixed and ball-milled to obtain a mixture. Then, the mixture is calcined with glucose to obtain vanadium-doped lithium iron phosphate precursor. The mass ratio of the mixture to glucose is 50:1 to 10.

3. The method for preparing a vanadium-sulfur-doped lithium iron phosphate cathode material according to claim 2, characterized in that: In step (1), anhydrous ethanol is used as the milling medium and zirconium oxide is used as the milling ball in the ball milling process, under a vacuum degree of less than 1.5 × 10⁻⁶. - 2 Ball milling for 2–6 hours under Pa conditions, then drying to obtain a mixture; the ratio of V2O5 powder, lithium iron phosphate precursor, anhydrous ethanol, and zirconium oxide is 3–5 g: 20–50 g: 20–100 mL: 100–200 g; the calcination is carried out in a tube furnace under a nitrogen atmosphere, with the first stage calcination at 300–400 °C for 20–60 min, the second stage calcination at 500–600 °C for 20–40 min, and after cooling to room temperature, vanadium-doped lithium iron phosphate precursor is obtained.

4. The method for preparing a vanadium-sulfur-doped lithium iron phosphate cathode material according to claim 1, characterized in that: In step (2), thiourea and vanadium-doped lithium iron phosphate precursor are dispersed in deionized water, citric acid is added as a complexing agent, and the mixture is heated to 80-90°C and kept at that temperature for 10-30 minutes to form a wet gel. The wet gel is then pre-calcined in an argon atmosphere and then calcined at high temperature to form the vanadium-doped lithium iron phosphate precursor.

5. The method for preparing a vanadium-sulfur-doped lithium iron phosphate cathode material according to claim 4, characterized in that: The amounts of thiourea, vanadium-doped lithium iron phosphate precursor, citric acid, and deionized water are in the ratio of 1-5g: 20-50g: 10-25mL: 300-500mL. The wet gel is pre-calcined at 400–500°C in an argon atmosphere for 1–3 hours; and calcined at 750–800°C for 3–9 hours.

6. The method for preparing a vanadium-sulfur-doped lithium iron phosphate cathode material according to claim 1, characterized in that: In step (3), carbon coating is achieved by dispersing 20-50 nm carbon nanotube powder and the vanadium-sulfur-doped lithium iron phosphate precursor obtained in step (2) in anhydrous ethanol, stirring evenly, letting stand for 6-12 h, separating the solid and liquid, drying under vacuum at 60 °C, calcining at 500-600 °C for 6-12 h under a nitrogen atmosphere, and cooling to obtain vanadium-sulfur-doped lithium iron phosphate cathode material. The ratio of carbon nanotube powder, vanadium-sulfur-doped lithium iron phosphate precursor, and anhydrous ethanol is 1–3 g: 10–50 g: 50–500 mL.

7. The method for preparing a vanadium-sulfur-doped lithium iron phosphate cathode material according to claim 6, characterized in that: In step (3), the carbon nanotube powder is a sulfonic acid / carboxyl bifunctionalized carbon nanotube powder.

8. The method for preparing a vanadium-sulfur-doped lithium iron phosphate cathode material according to claim 7, characterized in that, The preparation method of the sulfonic acid / carboxyl bifunctionalized carbon nanotube powder in step (3) is as follows: (a) 20-50 nm carbon nanotube powder is added to a mixture of concentrated nitric acid and concentrated sulfuric acid. The mixture is stirred at 300-600 rpm and heated to 70-90 °C. The temperature is maintained for 4-8 h. After cooling to room temperature, the mixture is diluted with deionized water and repeatedly filtered until the pH of the solution is neutral. After solid-liquid separation, the solution is dried to obtain carboxylated carbon nanotube powder. The volume ratio of concentrated nitric acid to concentrated sulfuric acid in the mixture is 1:4, and the volume ratio of carbon nanotube powder to the mixture is 1-30 g: 400-600 mL. The mass fraction of the concentrated nitric acid is 60-70 wt%, and the mass fraction of the concentrated sulfuric acid is 68-75 wt%. (b) The carboxylated carbon nanotube powder was added to a 5-10 wt% sodium dodecylbenzenesulfonate solution, magnetically stirred at 70-90°C for 3-6 h, centrifuged, washed with ethanol, and vacuum dried at 30-60°C to obtain sulfonic acid / carboxyl bifunctionalized carbon nanotube powder; the mass ratio of the carboxylated carbon nanotube powder to sodium dodecylbenzenesulfonate was 5-20:1-3.

9. A vanadium-sulfur-doped lithium iron phosphate cathode material, characterized in that: The vanadium-sulfur-doped lithium iron phosphate cathode material is prepared by the preparation method described in any one of claims 1 to 8.

10. The application of the vanadium-sulfur-doped lithium iron phosphate cathode material as described in claim 9 in the preparation of lithium-ion batteries.

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