Preparation method of titanium-doped and carbon-coated synergistically modified sodium ferric fluorophosphate material for sodium-ion battery

Abstract

The invention discloses a preparation method of a titanium-doped and carbon-coated synergistically modified sodium ferric fluorophosphate material for a sodium ion battery, which specifically comprises the following steps: 1, adding ferrous oxalate, ammonium dihydrogen phosphate, a fluorine source, sodium bicarbonate and a titanium source into a ball milling tank for mixing, adding a solvent for ball milling for 3-8 hours, and drying in a drying oven to obtain a precursor material; and 2, putting the precursor material into a tubular furnace filled with inert gas, sintering at 300-500 DEG C for 3-6 hours, and cooling to room temperature. And 3, taking out the sintered material, adding the sintered material into a ball-milling tank, adding a carbon source and a solvent, carrying out secondary ball-milling mixing for 2-5 hours, drying in a drying box, putting into a tubular furnace, sintering for 4-10 hours in an inert atmosphere at 550-700 DEG C, and cooling to obtain the titanium-doped and carbon-coated synergistically modified sodium ferric fluorophosphate positive electrode material.

Description

Technical Field

[0001] The invention relates to a method for preparing a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for a sodium ion battery. Background Art

[0002] Unlike lithium, sodium is one of the elements with a high content in the earth's crust. It is also one of the commonly used elements in sodium-ion batteries. The reserves of iron, manganese, and aluminum in the earth's crust are relatively high. These characteristics help to reduce the material cost of sodium-ion batteries. Among them, sodium iron fluorophosphate (Na2FePO4F, NFPF) positive electrode material has attracted widespread attention due to its green and non-toxic characteristics, abundant raw materials, and low cost. However, the inherent poor electronic conductivity and slow sodium ion transmission of NFPF seriously affect its capacity release and cycle stability. Summary of the invention The purpose of the present invention is to overcome the problems in the prior art that the sodium ferric fluorophosphate positive electrode material has poor conductivity and cycle stability and is difficult to improve electrochemical performance, and to provide a method for preparing a titanium-doped and carbon-coated synergistically modified sodium ferric fluorophosphate material for sodium ion batteries.

[0003] In order to achieve the above objectives, the present invention mainly reduces the band gap of the material by doping Ti at the iron position, thereby improving the intrinsic electronic conductivity of the material. At the same time, the carbon layer obtained by in-situ pyrolysis of the added carbon source at high temperature improves the surface / interface electronic conductivity of the material and inhibits the side reaction between the material and the electrolyte.

[0004] The technical solution of the present invention is a method for preparing a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for sodium ion batteries, comprising the following steps: S1. Sodium bicarbonate, ferrous oxalate, titanium source, ammonium dihydrogen phosphate and fluorine source are added into a ball mill in a ratio of Na: (Fe + Ti): P: F = 2:1:1:1 and mixed. Solvent is added and ball milled until the particle size D50 is less than 5 μm. The precursor material is dried in a drying oven to obtain the precursor material.

[0005] S2. Place the precursor material in a tubular furnace filled with inert gas at 300-500 o C for 3-6 hours, and then cool to room temperature.

[0006] S3. Take out the sintered material and add it into a ball mill, add appropriate amount of carbon source and solvent, and mix them by ball milling for the second time until the particle size D50 is less than 1 μm. Put it into a drying oven for drying, place it in a tubular furnace, and sinter it at 550-700°C in an inert atmosphere for 4-10 hours. After cooling, the titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate positive electrode material is obtained.

[0007] Furthermore, the titanium source described in S1 includes at least one of titanium dioxide, titanium tetrachloride, and tetrabutyl titanate, and the doping amount of the titanium source is 1-10%. The fluorine source includes at least one of sodium fluoride, polyvinylidene fluoride, ammonium fluoride, and polytetrafluoroethylene.

[0008] Furthermore, the inert atmosphere is nitrogen or argon.

[0009] Furthermore, the carbon source comprises at least one of fructose, glucose, sucrose, polyethylene glycol, polyvinyl pyrrolidone, and N,N-di(L-alanine)perylene diimide, and the added amount of the carbon source is 10-30% of the mass of the precursor material.

[0010] Furthermore, the ball milling solvent comprises at least one of ethanol, acetone and water.

[0011] Furthermore, in step S3, the sintering conditions are 550-700 o C for 4-10h.

[0012] Compared with the prior art, the present invention has at least the following beneficial effects: 1. The present invention provides a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for sodium ion batteries and a preparation method thereof. On the one hand, the band gap of the material is greatly reduced by doping Ti at the iron position, thereby improving the intrinsic electronic conductivity of the material. On the other hand, a carbon source is introduced, and the carbon layer obtained by in-situ pyrolysis at high temperature can improve the surface / interface electronic conductivity of the material and inhibit the side reaction between the material and the electrolyte.

[0013] 2. The present invention adopts a solid phase method, the preparation process is simple, and it is suitable for large-scale production. The method has a simple synthesis process and is environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS Attached Figure 1 This is a SEM image of sodium iron fluorophosphate prepared in Example 1 of the present invention; Attached Figure 2 This is a charge and discharge curve diagram of the sodium iron fluorophosphate positive electrode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0014] The embodiments of the present invention will be described in detail below with reference to examples, but those skilled in the art will appreciate that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention.

[0015] Example 1 Sodium bicarbonate, ferrous oxalate, titanium dioxide, ammonium dihydrogen phosphate and sodium fluoride were added to a ball mill according to the molar ratio of Na: (Fe + Ti): P: F = 2: 1 (Fe: Ti = 0.95: 0.05): 1: 1, mixed, acetone was added and ball milled until the particle size D50 was less than 5 μm, and then placed in a drying oven for drying to obtain a precursor material. The precursor material was placed in a tubular furnace with nitrogen and heated at 350 o C for 3 hours, and then cooled to room temperature, the material was taken out and added to a ball mill, and 15% polyvinyl pyrrolidone and acetone were added for a second ball milling to mix until the particle size D50 was less than 1 μm, and then placed in a drying oven for drying, and then placed in a tubular furnace, sintered at 600°C in a nitrogen atmosphere for 8 hours, and after cooling, the titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate positive electrode material was obtained.

[0016] Example 2 Sodium bicarbonate, ferrous oxalate, tetrabutyl titanate, ammonium dihydrogen phosphate and sodium fluoride were added into a ball mill according to the ratio of Na: (Fe + Ti): P: F = 2: 1 (Fe: Ti = 0.95: 0.05): 1: 1, and mixed. Acetone was added and ball milled until the particle size D50 was less than 5 μm. The precursor material was dried in a drying oven to obtain the precursor material. The precursor material was placed in a tubular furnace with nitrogen and heated at 500 °C. o C for 3 hours, and then cooled to room temperature, the material was taken out and added to a ball mill, and 15% polyvinyl pyrrolidone and acetone were added for a second ball milling to mix until the particle size D50 was less than 1 μm, and then placed in a drying oven for drying, and then placed in a tubular furnace, and sintered at 700°C in a nitrogen atmosphere for 4 hours. After cooling, the titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate positive electrode material was obtained.

[0017] Example 3 Sodium bicarbonate, ferrous oxalate, titanium dioxide, ammonium dihydrogen phosphate and ammonium fluoride were added into a ball mill according to the ratio of Na: (Fe + Ti): P: F = 2: 1 (Fe: Ti = 0.95: 0.05): 1: 1, mixed, ethanol was added and ball milled until the particle size D50 was less than 5 μm, and then placed in a drying oven for drying to obtain a precursor material. The precursor material was placed in a tubular furnace with nitrogen and heated at 300 °C. o C for 6 hours, and then cooled to room temperature, the material was taken out and added to a ball mill, and 20% glucose and ethanol were added for a second ball milling to mix until the particle size D50 was less than 1 μm, and then placed in a drying oven for drying, and then placed in a tubular furnace, and sintered at 550°C in a nitrogen atmosphere for 10 hours. After cooling, the titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate positive electrode material was obtained.

[0018] Example 4 Based on Example 1, 15% of polyvinyl pyrrolidone was replaced by N,N-di(L-alanyl)perylene diimide, and the rest was the same as Example 1. The precursor material was placed in a tube furnace filled with nitrogen and heated at 350 o C for 3 h, and then cooled to room temperature, the material was taken out and added to a ball mill, and 15% N, N-di(L-alanine)perylene diimide was added and ball-milled for the second time to mix until the particle size D50 was less than 1 μm, and then placed in a drying oven for drying, and then placed in a tubular furnace and sintered at 600°C in a nitrogen atmosphere for 8 h. After cooling, the titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate positive electrode material was obtained. Comparative Example 1 Sodium bicarbonate, ferrous oxalate, ammonium dihydrogen phosphate and sodium fluoride were added into a ball mill according to the ratio of Na:Fe:P:F =2:1:1:1, and mixed. Acetone was added and ball milled until the particle size D50 was less than 5 μm. The precursor material was dried in a drying oven to obtain the precursor material. The precursor material was placed in a tubular furnace with nitrogen and heated at 350 o C for 3 h, and then cooled to room temperature, the material was taken out and added to a ball mill, and 15% polyvinyl pyrrolidone and acetone were added to the ball mill for the second time to mix until the particle size D50 was less than 1 μm, and then placed in a drying oven for drying, and then placed in a tube furnace and heated at 600 °C in a nitrogen atmosphere. o C was sintered for 8 h, and then the carbon-coated modified sodium iron fluorophosphate positive electrode material was obtained after cooling.

[0019] Comparative Example 2 Sodium bicarbonate, ferrous oxalate, titanium dioxide, ammonium dihydrogen phosphate and sodium fluoride were added to a ball mill according to the ratio of Na: (Fe + Ti): P: F = 2: 1: 1: 1, and mixed. Ethanol was added and ball milled until the particle size D50 was less than 5 μm. The precursor material was dried in a drying oven to obtain the precursor material. The precursor material was placed in a tubular furnace with nitrogen and heated at 350 o C for 3 h, and then cooled to room temperature, the material was taken out and added to a ball mill, and 5% glucose and ethanol were added to the ball mill for the second time until the particle size D50 was less than 1 μm. After drying in a drying oven, it was placed in a tube furnace and heated at 600 °C in a nitrogen atmosphere. o C was sintered for 8 h, and then cooled to obtain the titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate positive electrode material.

[0020] Comparative Example 3 Sodium bicarbonate, ferrous oxalate, titanium dioxide, ammonium dihydrogen phosphate and sodium fluoride were added into a ball mill according to the ratio of Na: (Fe + Ti): P: F = 2: 1: 1: 1, and mixed. Acetone was added and ball milled until the particle size D50 was less than 5 μm. The precursor material was dried in a drying oven to obtain the precursor material. The precursor material was placed in a tubular furnace with nitrogen and heated at 350 oC for 3 hours, and then cooled to room temperature, the material was taken out and added to a ball mill, and acetone was added for a second ball milling to mix until the particle size D50 was less than 1 μm, and then placed in a drying oven for drying, and then placed in a tubular furnace, sintered at 600°C in a nitrogen atmosphere for 8 hours, and after cooling, titanium-doped modified sodium iron fluorophosphate positive electrode material was obtained.

[0021] The electrochemical performance of the sodium iron fluorophosphate positive electrode material prepared in the above examples and comparative examples was tested. The mass ratio of sodium iron fluorophosphate: acetylene black: PVDF = 8:1:1 was mixed, and the organic solvent N-methylpyrrolidone (NMP) was added. After stirring, it was coated on aluminum foil and dried to make a positive electrode sheet. The negative electrode used a metal sodium sheet; the diaphragm used a polypropylene porous membrane; and the 2023 button half-cell was assembled in a glove box. After standing for 10 hours at room temperature 25°C and standing for 2 hours in a -10°C constant temperature box, the battery was tested for charge and discharge cycle performance. Within the cut-off voltage range of 2 to 4.2V, the 0.1C and 1C discharge specific capacities were tested; the test electrochemical performance results are shown in Table 1. It can be seen from the data that the titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material has a higher discharge specific capacity, and Example 4 can still maintain more than 90% of the capacity at room temperature at low temperatures, and has excellent low temperature performance.

[0022] Table 1

[0023] The above-mentioned embodiments are only preferred technical solutions of the present invention and should not be regarded as limitations of the present invention. The embodiments and features in the embodiments of the present application can be arbitrarily combined with each other without conflict. The protection scope of the present invention shall be the technical solutions recorded in the claims, including the equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A method for preparing a titanium-doped and carbon-coated modified sodium iron fluorophosphate material for sodium ion batteries, characterized in that: The method comprises the following steps: S1. Sodium bicarbonate, ferrous oxalate, titanium source, ammonium dihydrogen phosphate and fluorine source are mixed in a ball mill, a solvent is added and ball milled for the first time for 3-8 hours, and then the precursor material is obtained by drying in a drying oven; S2, placing the precursor material into a tubular furnace filled with inert gas, sintering it for the first time, and then cooling it to room temperature; S3. Take out the sintered material and add it into a ball mill, add carbon source and solvent for a second ball milling and mix for 2-5 hours, put it into a drying oven for drying, place it in a tubular furnace, and sinter it for a second time in an inert atmosphere. After cooling, the titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate positive electrode material is obtained.

2. The method for preparing a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for sodium ion batteries according to claim 1, characterized in that: In step S1, the titanium source comprises at least one of titanium dioxide, titanium tetrachloride, and tetrabutyl titanate, and the doping amount of the titanium source is 1-10%; in step S1, the fluorine source comprises at least one of sodium fluoride, polyvinylidene fluoride, ammonium fluoride, and polytetrafluoroethylene.

3. The method for preparing a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for sodium ion batteries according to claim 1, characterized in that: In step S1, the molar ratio of Na:(Fe+Ti):P:F is (1.5-2.5):(0.8-1.2):(0.8-1.2):(0.8-1.2); preferably, the molar ratio of Na:(Fe+Ti):P:F is 2:1:1:

1.

4. The method for preparing a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for sodium ion batteries according to claim 1, characterized in that: In step S1, the first ball milling is performed for 3-8 hours until the particle size D50 is less than 5 μm; in step S3, a carbon source and a solvent are added and the mixture is ball milled for a second time for 2-5 hours until the particle size D50 is less than 1 μm.

5. The method for preparing a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for sodium ion batteries according to claim 1, characterized in that: In steps S2 and S3, the inert atmosphere is nitrogen or argon.

6. The method for preparing a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for sodium ion batteries according to claim 1, characterized in that: In step S3, the carbon source comprises one or more of fructose, glucose, sucrose, polyethylene glycol, polyvinyl pyrrolidone, and N,N-di(L-alanine)perylene diimide, and the added amount of the carbon source is 10-30% of the mass of the precursor material.

7. The method for preparing a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for sodium ion batteries according to claim 1, characterized in that: In steps S1 and S3, the ball milling solvent includes at least one of ethanol, acetone and water.

8. The method for preparing a titanium-doped and carbon-coated synergistically modified sodium iron fluorophosphate material for sodium ion batteries according to claim 1, characterized in that: In step S2, the first sintering condition is 300-500 o C sintering 3-6h; In step S3, the second sintering condition is 550~700 o C sintering 4-10h.

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