Method for preparing carbon-coated titanium-doped composite sodium iron phosphate positive electrode material by supercritical method

Abstract

The application provides a method for preparing a carbon-coated titanium-doped composite sodium iron phosphate positive electrode material by a supercritical method, and comprises the following steps: 1) dissolving iron nitrate in preheated deionized water, slowly adding oxalic acid, and maintaining the temperature of the solution for a certain period of time; 2) placing the solution obtained in the step 1, a titanium source, a carbon source, a sodium source and a phosphorus source into a high-temperature and high-pressure reaction kettle, adding anhydrous ethanol, heating and pressurizing the reaction kettle to a certain temperature and pressure to make the ethanol form a supercritical state to perform a reaction; 3) performing spray drying treatment on the reaction liquid obtained in the step 2 to obtain a precursor powder; and 4) performing high-temperature sintering on the precursor in an inert atmosphere, and then crushing to obtain the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material. By the supercritical method, nanocrystallization, carbon coating and titanium doping are simultaneously realized, the stability of the crystal structure and good reversible performance are improved, and the electronic conductivity and ion migration rate of the material are greatly improved, so that the electrochemical performance such as the cycle performance and the rate performance of the material is improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion battery positive electrode materials, in particular to a method for preparing a carbon-coated titanium-doped composite sodium iron phosphate positive electrode material by a supercritical method. BACKGROUND

[0002] In recent years, with the rapid development of the new energy vehicle market, the lithium ion battery industry has continued to grow rapidly, while the domestic lithium resources are scarce, and the supply is in a state of tension. Lithium ion batteries urgently need to find a high cost-effective replacement solution. Sodium ion batteries are currently the best replacement solution for lithium ion batteries due to their overall cost-effective advantage.

[0003] Polyanion-type materials have the characteristics of stable crystal structure, high thermal stability, adjustable voltage, and adjustable chemical composition, and are one of the most widely studied sodium ion battery positive electrode materials. PO4 3- and P2O7 4- A composite sodium iron phosphate positive electrode material (Na4Fe3(PO4)2(P2O7), referred to as NFPP) coexisting with polyanions has the advantages of low cost, good safety, simple preparation process, and strong cycle stability, and has attracted widespread attention from researchers. For Na4Fe3(PO4)2(P2O7), it can be regarded as a combination of NaFePO4 and Na2FeP2O7, that is, Na4Fe3(PO4)2(P2O7) = 2"NaFePO4" + "Na2FeP2O7". As a three-electron transfer positive electrode material, the composite sodium iron phosphate has a higher theoretical capacity (129 mAh / g) and an average working voltage (3.2 V vs. Na + / Na), and becomes the most promising polyanion-type positive electrode material for sodium ion batteries.

[0004] However, the electronic conductivity of NEPP is low, which greatly affects the electrochemical performance of the material, causing poor high-rate performance and other problems, which limits its application in practice. Therefore, in order to industrialize and popularize the NFPP material, modification research is needed, and the current main modification methods are: nanocrystallization, carbon coating, ion doping, etc. Nanocrystallization can effectively reduce the size of the material and reduce the distance of sodium ion transmission. At the same time, the specific surface area of nanoparticles is larger, the electrolyte is more easily infiltrated, the charging and discharging process is faster, thereby optimizing the electrochemical performance of the material. Carbon coating is an effective means of modifying the positive electrode material. Carbon materials have good electrical conductivity and can coat the surface of the material to enhance the electrical conductivity of the material, provide a channel for electron diffusion, and help improve the electrical conductivity of the material. Doping can reduce the band gap of the material system, reduce the sodium ion diffusion barrier, widen the sodium ion diffusion channel, improve the electronic conductivity, and improve the rate performance.

[0005] The preparation method of the currently used NFPP positive electrode material is mainly high-temperature solid-phase method and conventional liquid-phase method (sol-gel method / hydrothermal method, etc.), wherein the material synthesized by the high-temperature solid-phase method has the characteristics of wide particle size distribution range and uneven particle distribution; and the conventional liquid-phase method has the disadvantages of long reaction process time and low reaction efficiency. Therefore, aiming at the above problems, it is urgent to establish an effective preparation method to simultaneously realize the nanocrystallization, carbon coating and ion doping composite modification of the positive electrode material, solve the problems of low electronic conductivity of the NFPP, and improve the key electrochemical performance indexes such as the rate performance and cycle performance of the material, which are the problems to be solved in the application research of Na4Fe3(PO4)2(P2O7) material. SUMMARY

[0006] To solve the problems in the background art, the application provides a method for preparing carbon-coated titanium-doped composite sodium iron phosphate positive electrode material by a supercritical method, which simultaneously realizes uniform nanocrystallization, carbon coating and ion doping, and has high preparation efficiency.

[0007] The technical scheme for solving the above technical problems is as follows:

[0008] A method for preparing carbon-coated titanium-doped composite sodium iron phosphate positive electrode material by a supercritical method, comprising the following steps:

[0009] 1) Dissolve ferric nitrate in preheated deionized water, slowly add oxalic acid under stirring, and then maintain the solution temperature to react for a certain time to form a ferric trivalent complex;

[0010] 2) Put the solution obtained in step 1), a titanium source, a carbon source, a sodium source and a phosphorus source into a high-temperature and high-pressure reaction kettle, add anhydrous ethanol, heat and pressurize the reaction kettle to a certain temperature and pressure to make the ethanol form a supercritical state for reaction under stirring;

[0011] 3) Spray dry the reaction liquid obtained in step 2) to obtain a precursor powder;

[0012] 4) Sinter the precursor in an inert atmosphere, and then crush to obtain the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material.

[0013] Further, the preheated deionized water in step 1) has a temperature of 80-90℃, the oxalic acid is added dropwise for 20-30 min, and the reaction time is 30 min-1 h.

[0014] Further, the molar ratio of ferric nitrate to oxalic acid is 1:1-1.5, and the content of ferric nitrate in the ferric nitrate solution is 50-60wt%.

[0015] Further, the titanium source is tetrabutyl titanate, the sodium source is sodium acetate, the phosphorus source is ammonium dihydrogen phosphate, and the carbon source is at least one of sucrose and glucose.

[0016] Further, in step 2), the molar ratio of the sodium source, the phosphorus source, the titanium source and the ferric nitrate is (1.3-1.4):(1.3-1.4):X:1-X, X is between 0.05 and 0.1.

[0017] Further, in step 2), the amount of the carbon source added is 4-6% of the total mass of the ferric nitrate, the sodium source and the phosphorus source.

[0018] Further, in step 2), the temperature in the reaction kettle is 260-280℃, the pressure in the kettle is adjusted to 7-9 MPa, and the reaction time is 20-30 min.

[0019] In some preferred embodiments, the ethanol accounts for 50%-70% of the volume of the reaction kettle.

[0020] Further, the inlet air temperature of the spray drying is 250-270℃, and the outlet air temperature is 80-100℃.

[0021] Further, in step 4), the inert atmosphere is nitrogen, the sintering temperature is 500-650℃, and the sintering time is 9-12 h.

[0022] The present application has the following beneficial effects:

[0023] 1) The present application uses ethanol as a solvent, and the reactants are more fully dissolved and contacted in the solvent ethanol under supercritical conditions, the reactants are uniformly mixed and distributed in the solution, the reaction, heat transfer and mass transfer rates are improved, and the Na4Fe3(PO4)2(P2O7) material is simultaneously realized by three means of uniform nanocrystallization, carbon coating and ion doping for composite modification;

[0024] 2) The organic solvent ethanol under supercritical conditions can effectively inhibit the growth of precursor particles, so that the prepared particles have small particle size and uniform particle size distribution, and then the ethanol, oxalic acid, sucrose or glucose, etc. are carbonized by a high-temperature calcination method to convert them into a carbon layer, which is uniformly coated on the surface of the material, can effectively improve the electrical conductivity of the positive electrode material, and at the same time provides effective buffer for the volume change of the material during charging and discharging process, and improves the cycle performance of the material;

[0025] 3) The titanium source is more easily dissolved and dispersed under supercritical conditions of the ethanol solvent, the titanium source is in a free state and fully contacts with each reactant, is uniformly distributed in the interior and surface of the precursor material after spray drying, and ensures the effectiveness and beneficial effects of the subsequent titanium ion doping reaction, the titanium doping can improve the stability of the crystal structure and good reversible performance, and at the same time, speeds up the ion diffusion and transmission, the synergistic effect of uniform nanocrystallization, carbon coating and titanium doping greatly improves the electronic conductivity and ion mobility of the positive electrode material, and improves the electrochemical performance such as cycle performance and rate performance of the material;

[0026] 4) The preparation method provided by the application, ammonium dihydrogen phosphate, oxalate, and acetate in the material reaction system decompose to generate water vapor, carbon dioxide, and ammonia gas and other gases in the reaction process of the composite sodium iron phosphate, a large number of pores are generated in the overflow process of the gases, meanwhile, the growth and agglomeration of the positive electrode material particles are hindered, the particles are uniformly distributed, and the electronic conductivity of the material is improved, and the diffusion path of sodium ions in the material into the electrolyte is shortened. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 A scanning electron microscope image of the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material prepared in Embodiment 1 of the application;

[0028] Figure 2 A rate performance graph of the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material prepared in Embodiment 1 of the application;

[0029] Figure 3 A cycle performance graph of the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material prepared in Embodiment 1 of the application. DETAILED DESCRIPTION

[0030] The principles and characteristics of the application are described below in combination with the drawings and specific embodiments, and the examples are used only to explain the application and are not used to limit the scope of the application.

[0031] Embodiment 1

[0032] This embodiment discloses a method for preparing a carbon-coated titanium-doped composite sodium iron phosphate positive electrode material by a supercritical method, comprising the following steps:

[0033] (1) 6.05 g of iron nitrate is weighed and dissolved in deionized water at 80°C, stirring is started, and then 2.5 g of oxalic acid is added dropwise into the deionized water containing the iron nitrate, the dropping speed of the oxalic acid is controlled, the dropping is completed within 25 min, the temperature of the mixed solution is maintained at 80°C, and the reaction is continued for 30 min;

[0034] (2) The solution obtained in step (1) is moved into a high-temperature and high-pressure reaction kettle, 2.76 g of sodium acetate, 3.86 g of ammonium dihydrogen phosphate, 0.94 g of tetrabutyl titanate, and 0.6 g of sucrose are weighed and placed in the high-temperature and high-pressure reaction kettle, anhydrous ethanol is added, the volume of the ethanol accounts for 60% of the volume of the reaction kettle, then stirring is started, the reaction kettle is heated to 270°C, the pressure in the kettle is adjusted to 8 MPa, and the reaction time is maintained for 25 min.

[0035] (3) The reaction solution obtained in step (2) is subjected to spray drying at an inlet temperature of 260°C and an outlet temperature of 90°C, and a precursor powder is obtained.

[0036] (4) The powder is placed in a high-temperature furnace filled with a nitrogen atmosphere, sintered at 550℃ for 10h to obtain the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material.

[0037] Example 2

[0038] The present embodiment discloses a method for preparing a carbon-coated titanium-doped composite sodium iron phosphate positive electrode material by a supercritical method, comprising the following steps:

[0039] (1) 6.53g of iron nitrate is weighed and dissolved in deionized water at 85℃, stirring is started, then 2.43g of oxalic acid is added dropwise into the deionized water containing iron nitrate, the dropping speed of the oxalic acid is controlled, the temperature of the mixed solution is maintained at 85℃ after the oxalic acid is completely added within 25min, and the reaction is continued for 50min;

[0040] (2) The solution obtained in step (1) is moved into a high-temperature and high-pressure reaction kettle, then 3.20g of sodium acetate, 4.48g of ammonium dihydrogen phosphate, 1.02g of tetrabutyl titanate and 0.57g of glucose are weighed and placed in the high-temperature and high-pressure reaction kettle, anhydrous ethanol is added, the volume of the ethanol accounts for 60% of the volume of the reaction kettle, then stirring is started, the reaction kettle is heated to 280℃, the pressure in the kettle is adjusted to 7MPa, and the reaction time is maintained for 20min.

[0041] (3) The reaction solution obtained in step (2) is subjected to spray drying at an inlet temperature of 270℃ and an outlet temperature of 80℃ to obtain a precursor powder.

[0042] (4) The powder is placed in a high-temperature furnace filled with a nitrogen atmosphere, sintered at 650℃ for 9h to obtain the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material.

[0043] Example 3

[0044] The present embodiment discloses a method for preparing a carbon-coated titanium-doped composite sodium iron phosphate positive electrode material by a supercritical method, comprising the following steps:

[0045] (1) 6.89g of iron nitrate is weighed and dissolved in deionized water at 90℃, stirring is started, then 2.56g of oxalic acid is added dropwise into the deionized water containing iron nitrate, the dropping speed of the oxalic acid is controlled, the temperature of the mixed solution is maintained at 90℃ after the oxalic acid is completely added within 25min, and the reaction is continued for 30min;

[0046] (2) The solution obtained in step (1) is moved into a high-temperature and high-pressure reaction kettle, then 3.53g of sodium acetate, 4.83g of ammonium dihydrogen phosphate, 0.51g of tetrabutyl titanate and 0.9g of sucrose are weighed and placed in the high-temperature and high-pressure reaction kettle, anhydrous ethanol is added, the volume of the ethanol accounts for 60% of the volume of the reaction kettle, then stirring is started, the reaction kettle is heated to 260℃, the pressure in the kettle is adjusted to 9MPa, and the reaction time is maintained for 30min.

[0047] (3) The reaction solution obtained in step (2) is subjected to spray drying at an inlet temperature of 270°C and an outlet temperature of 80°C to obtain a precursor powder.

[0048] (4) The powder is placed in a high-temperature furnace filled with a nitrogen atmosphere and sintered at 500°C for 12h to obtain a composite sodium iron phosphate positive electrode material.

[0049] Figure 1 A scanning electron microscope image of the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material prepared in Example 1 shows that the composite sodium iron phosphate positive electrode material is a spherical nanostructured material. Figure 1 The water vapor, carbon dioxide and ammonia gas produced during the precursor sintering and decomposition process overflow and create a large number of pores, which hinder the growth and agglomeration of the positive electrode material particles, resulting in a particle size range of 50-100nm, which is beneficial to improving the electronic conductivity of the material and shortening the diffusion path of sodium ions into the electrolyte.

[0050] Figure 2 The rate performance graph of the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material prepared in Example 1 shows that the discharge capacity of the composite sodium iron phosphate positive electrode material at 0.5C, 1C, 2C, 5C, 10C, 20C and 30C is 110.67, 110.35, 109.13, 106.9, 104.07, 98.47 and 95.8mAh·g -1 respectively. When the current density returns to 0.5C, the discharge capacity is 109.39mAh·g -1 , which is basically the same as the initial capacity. This indicates that the material structure remains intact after rapid sodium ion extraction, demonstrating high structural stability and good reversibility, resulting in excellent rate performance and cycle performance.

[0051] Figure 3 The cycle performance graph of the carbon-coated titanium-doped composite sodium iron phosphate positive electrode material prepared in Example 1 shows that the discharge capacity of the composite sodium iron phosphate positive electrode material at a current density of 1C is 110.69mAh·g -1 , and after 100 cycles, the capacity is 109.25mAh·g -1 , with a capacity retention rate of 99.3%. It can be clearly seen that the material has excellent cycle performance with almost no loss after 100 cycles.

[0052] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a carbon-coated titanium-doped composite sodium iron phosphate cathode material by a supercritical method, characterized in that, Comprising the following steps: 1) dissolving ferric nitrate in preheated deionized water, slowly adding oxalic acid under stirring, then maintaining the solution temperature for a certain time to form a ferric complex; 2) placing the solution obtained in step 1), a titanium source, a carbon source, a sodium source and a phosphorus source in a high-temperature and high-pressure reaction kettle, adding anhydrous ethanol, heating and pressurizing the reaction kettle to a certain temperature and pressure to form a supercritical state of ethanol under stirring to react; 3) obtaining a precursor powder by spray drying the reaction liquid obtained in step 2); 4) high-temperature sintering of the precursor in an inert atmosphere, then crushing to obtain a carbon-coated titanium-doped composite sodium iron phosphate positive electrode material; The titanium source in step 2) is tetrabutyl titanate, the sodium source is sodium acetate, the phosphorus source is ammonium dihydrogen phosphate, and the carbon source is at least one of sucrose and glucose; the temperature in the reaction kettle is 260-280℃, the pressure in the kettle is adjusted to 7-9MPa, and the reaction time is 20-30min.

2. The method for preparing carbon-coated titanium-doped composite sodium iron phosphate cathode material by supercritical method according to claim 1, characterized in that, The preheated deionized water in step 1) has a temperature of 80-90℃, the oxalic acid is added dropwise within 20-30min, and the reaction time is 30min-1h.

3. The method for preparing carbon-coated titanium-doped composite sodium iron phosphate cathode material by supercritical method according to claim 1, characterized in that, The molar ratio of ferric nitrate to oxalic acid is 1:1-1.5, and the content of ferric nitrate in the ferric nitrate solution is 50-60wt%.

4. The method for preparing carbon-coated titanium-doped composite sodium iron phosphate cathode material by supercritical method according to claim 1, characterized in that, In step 2), the molar ratio of the sodium source, the phosphorus source, the titanium source to ferric nitrate is (1.3-1.4):(1.3-1.4):X:1-X, and X is between 0.05 and 0.

1.

5. The method for preparing carbon-coated titanium-doped composite sodium iron phosphate cathode material by supercritical method according to claim 4, characterized in that, The amount of carbon source added in step 2) is 4-6% of the total mass of ferric nitrate, sodium source and phosphorus source.

6. The method of claim 1, wherein the supercritical process for preparing a carbon-coated titanium-doped composite sodium iron phosphate cathode material is characterized by, The inlet air temperature of the spray drying is 250-270℃, and the outlet air temperature is 80-100℃.

7. The method for preparing carbon-coated titanium-doped composite sodium iron phosphate cathode material by supercritical method according to any one of claims 1-6, characterized in that, The sintering temperature in step 4) is 500-650℃, and the sintering time is 9-12h.

8. The method for preparing carbon-coated titanium-doped composite sodium iron phosphate cathode material by supercritical method according to claim 7, characterized in that, The inert atmosphere in step 4) is nitrogen.

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