Preparation method and application of gradient phase-control calcined sodium-ion battery positive electrode material

Abstract

The invention discloses a preparation method of a high-purity ferric sodium pyrophosphate (Na4Fe3 (PO3) 2P2O7-Pure, hereinafter referred to as NFPP-P) positive electrode material and an application of the positive electrode material in a sodium ion battery. The invention relates to an innovative sectional gradient calcining process, which is combined with a spray drying precursor synthesis technology to successfully realize effective regulation and control of material phase purity. Experiments show that the process can significantly inhibit generation of a sodium ferric phosphate impure phase by controlling a calcination temperature curve, and the electrochemical performance of the positive electrode material is improved. The method provides a new process route for developing a high-performance sodium ion battery positive electrode material, and has important application value in the field of low-cost and long-cycle-life energy storage devices.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium-ion battery cathode materials, and particularly relates to a method for preparing high-purity sodium iron pyrophosphate (Na4Fe3(PO3)2P2O7-Pure, hereinafter referred to as NFPP-P) cathode material through a multi-section gradient phase control calcination process; and also relates to a method for preparing a sodium-ion battery cathode sheet using the high-purity sodium iron pyrophosphate cathode material and application of the high-purity sodium iron pyrophosphate cathode material in a sodium-ion battery with long cycle life. BACKGROUND

[0002] With the transformation of global energy structure to renewable energy, large-scale energy storage technology has become a key support to achieve the goal. In recent years, market demand has continuously driven the development of energy storage in the direction of electrochemical energy storage. However, traditional lithium-ion battery cathode materials, such as lithium cobaltate, lithium manganate and ternary materials, are limited by the finiteness of lithium resources and uneven geographical distribution, and are not convenient for use in large-scale electrochemical energy storage. Sodium-ion battery cathode material sodium iron pyrophosphate (Na4Fe3(PO3)2P2O7, denoted as NFPP) has the advantages of abundant reserves, wide distribution, low price and stable structure. This feature is exactly in line with the characteristics of large-scale energy storage devices, so sodium-ion batteries are considered to be one of the potential candidates for large-scale energy storage systems. For sodium iron pyrophosphate NFPP, it has a high theoretical specific capacity (~129 mAh / g), a high working voltage (~3.1V, Na + / Na) and a low volume expansion (~4%), and is considered to be the most potential sodium-ion battery cathode material. However, the purity of NFPP prepared by conventional preparation methods is not high, which is not conducive to the reversible specific capacity. SUMMARY

[0003] OBJECTIVE The purpose of the present application is to provide a preparation method of a segmented gradient phase control calcination process for high-purity sodium iron pyrophosphate cathode material. The preparation method uses a segmented gradient solid phase sintering method for the precursor in the material synthesis process, and the recrystallization process can effectively improve the purity of sodium iron pyrophosphate phase in the material and inhibit the generation of irreversible sodium iron phosphate phase, thereby improving the reversible specific capacity of the material.

[0004] TECHNICAL SCHEME In one aspect of the present application, a preparation method of high-purity sodium iron pyrophosphate positive electrode material is provided, which is characterized by comprising the following steps: adding a sodium source, a phosphorus source, an iron source and a carbon source into deionized water to stir uniformly, and then drying to obtain a precursor by using a spray drying technology; and placing the precursor into a vacuum tube furnace to obtain the high-purity sodium iron pyrophosphate positive electrode material by using a segmented gradient phase control sintering process. The segmented gradient phase control sintering process comprises three sintering steps, i.e., pre-sintering, high-temperature sintering and recrystallization sintering. The positive electrode material is a spherical particle with a particle size distribution of 1-6 µm and an average particle size of 3 µm.

[0005] The segmented gradient phase control sintering process comprises the following steps: Step 1: In the pre-sintering, the temperature rising rate is 4-7 ℃ / min, the temperature is 280-320 ℃, the holding time is 5-8 h, the cooling is furnace cooling, the inert reducing atmosphere is argon-hydrogen mixed gas, and the hydrogen volume percentage in the mixed gas is 5%; the precursor obtained by spray drying is subjected to high-temperature calcination in a reducing atmosphere to preliminarily form the sodium iron pyrophosphate positive electrode material; Step 2: In the high-temperature sintering, the temperature is first raised to 280-320 ℃ at a rate of 4-7 ℃ / min, and then raised to 540-580 ℃ at a rate of 1-3 ℃ / min, the holding time is 6-10 h, and then the furnace is cooled to room temperature, the inert reducing atmosphere is argon-hydrogen mixed gas, and the hydrogen volume percentage in the mixed gas is 5%; Step 3: In the recrystallization sintering, the temperature rising rate is 1-3 ℃ / min, the temperature is 480-510 ℃, the holding time is 1-3 h, the cooling rate is 4-7 ℃ / min, the inert reducing atmosphere is argon-hydrogen mixed gas, and the hydrogen volume percentage in the mixed gas is 8%. This step is to eliminate impurities by recrystallization of the sodium iron pyrophosphate positive electrode material obtained in the above steps in a reducing atmosphere to obtain the high-purity sodium iron pyrophosphate positive electrode material.

[0006] Preferably, the segmented gradient phase control sintering process comprises the following steps: Step 1: In the pre-sintering, the temperature rising rate is 5 ℃ / min, the temperature is 300 ℃, the holding time is 6 h, the cooling is furnace cooling, the inert reducing atmosphere is argon-hydrogen mixed gas, and the hydrogen volume percentage in the mixed gas is 5%; Step 2: In the high-temperature sintering, the temperature is first raised to 300 ℃ at a rate of 5 ℃ / min, and then raised to 550 ℃ at a rate of 2 ℃ / min, the holding time is 8 h, and then the furnace is cooled to room temperature, the inert reducing atmosphere is argon-hydrogen mixed gas, and the hydrogen volume percentage in the mixed gas is 5%; In the recrystallization sintering, the temperature rising rate is 2℃ / min, the temperature is 500℃, the temperature holding time is 2h, the temperature falling rate is 5℃ / min, the inert reducing atmosphere is argon-hydrogen mixed gas, and the hydrogen volume percentage is 8%.

[0007] The sodium source, the phosphorus source and the iron source are sodium dihydrogen phosphate, iron nitrate nonahydrate respectively, and the molar ratio is 4:3.

[0008] The carbon source is citric acid, the molar ratio of the carbon source to the sodium source is 3:1, and the concentration of the citric acid solution is 8%-15%, preferably 10%.

[0009] In the preparation of the precursor, the stirring speed is 400-500rpm, and the stirring time is 3-5h; preferably, the stirring speed is 450rpm, and the stirring time is 4h.

[0010] In the spray drying, the feeding rate is 650-750 mL / h, and the nozzle temperature is 240-280℃; preferably, the feeding rate is 700 mL / h, and the nozzle temperature is 260℃.

[0011] In another aspect of the present application, a high-purity sodium iron phosphate pyrophosphate positive electrode material is provided, which is prepared by the above preparation method.

[0012] In another aspect of the present application, a preparation method of a sodium ion battery positive electrode sheet is provided, which comprises the following steps: Step 1: the following components are weighed according to the mass fraction: 70-80 parts of the above high-purity sodium iron phosphate pyrophosphate positive electrode material, 10-20 parts of conductive carbon black, and 5-15 parts of a binder; preferably, 75 parts of the high-purity sodium iron phosphate pyrophosphate positive electrode material, 15 parts of conductive carbon black, and 10 parts of a binder; Step 2: after the high-purity sodium iron phosphate pyrophosphate positive electrode material, the conductive carbon black and the binder weighed in step 1 are dispersed in N-methyl pyrrolidone with a solid content of 15-18%, preferably 16%, they are coated on a treated aluminum foil and dried, and a sodium ion battery positive electrode sheet is obtained.

[0013] The conductive carbon black is Super P, the binder is polyvinylidene fluoride, the coating thickness is 100µm, and the drying temperature is 90℃.

[0014] Advantages The beneficial effects of the present application are: through exploration, it is found that the innovative three-section gradient phase control calcination process is adopted, which is divided into pre-burning, high-temperature sintering and recrystallization sintering, the gradient sintering lies in that the temperature, the heating rate, the holding time and the cooling rate of each sintering process are different; the phase control lies in improving the purity of sodium iron pyrophosphate phase and reducing the content of irreversible sodium iron phosphate phase. Combined with the traditional spray drying process for preparing the precursor, the purity of sodium iron pyrophosphate phase can be controlled, the generation of sodium iron phosphate impurity phase can be significantly inhibited, and finally the high-purity sodium iron pyrophosphate positive electrode material is obtained, thereby the reversible specific capacity of the NFPP positive electrode material is improved, a new synthesis process route for the sodium ion battery positive electrode new material is provided, and a new idea for the development of sodium ion battery technology is provided. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is the SEM diagram of the high-purity sodium iron pyrophosphate sodium ion battery positive electrode material of the present application embodiment 1; Figure 2 is the XRD diagram of the sodium iron pyrophosphate sodium ion battery positive electrode material prepared by the present application embodiment 1, comparative example 2 and comparative example 5; Figure 3 is the charge-discharge curve diagram of the button cell assembled by the sodium iron pyrophosphate positive electrode material prepared by the present application embodiment 1, comparative example 2 and comparative example 5 at 0.1C rate; Figure 4 is the rate performance diagram of the button cell assembled by the sodium iron pyrophosphate positive electrode material prepared by the present application embodiment 1, comparative example 2 and comparative example 5 at different rates; Figure 5 is the cycle performance diagram of the button cell assembled by the sodium iron pyrophosphate positive electrode material prepared by the present application embodiment 1, comparative example 2 and comparative example 5 at 1C rate. DETAILED DESCRIPTION

[0016] The present application will be described in detail below in combination with the drawings and specific embodiments, and the materials used in all embodiments can be obtained through commercial channels.

[0017] Preparation example Sodium dihydrogen phosphate and iron nitrate nonahydrate are weighed and mixed according to the molar ratio of 4:3, 10% citric acid solution is added, 30ml deionized water is added dropwise based on 3.8424g sodium dihydrogen phosphate, and the mixture is stirred at 450rpm for 4 hours to obtain a uniform solution, then spray drying is carried out, the feeding rate is 700mL / h, and the nozzle temperature is 260℃, and the powder precursor is obtained.

[0018] Example 1 The powder precursor prepared in the preparation example was heated to 300°C in a vacuum tube furnace at a heating rate of 5°C / min and held for 6 hours, then cooled to room temperature with the furnace. It was then heated to 300°C again at a heating rate of 5°C / min, followed by heating to 550°C at a heating rate of 2°C / min and holding for 8 hours, then cooled to room temperature with the furnace. All of the above processes were carried out under the protection of an argon-hydrogen mixture with a hydrogen volume content of 5%. The material obtained from the above processes was then subjected to a third-stage sintering: heated to 500°C in a vacuum tube furnace at a heating rate of 2°C / min and held for 2 hours, then cooled to room temperature at a heating rate of 5°C / min. This process was carried out under the protection of an argon-hydrogen mixture with a hydrogen volume content of 8%, thus obtaining the Na4Fe3(PO3)2P2O7 cathode material. Its morphology is as follows... Figure 1 As shown, the cathode material consists of spherical particles with a particle size distribution of 1-6µm and an average particle size of 3µm. Comparative Example 1 The comparative example of sodium iron phosphate pyrophosphate cathode material synthesis process adopts the heat treatment process of Example 1, wherein the difference is that the reducing atmosphere in the third sintering process is an argon-hydrogen mixture with a hydrogen volume content of 5%.

[0019] Comparative Example 2 The synthesis process of the sodium iron phosphate pyrophosphate cathode material in this comparative example adopts the heat treatment process of Example 1, except that the cooling method in the third sintering process is furnace cooling.

[0020] Comparative Example 3 The comparative example of the sodium iron phosphate pyrophosphate cathode material synthesis process adopts the heat treatment process of Example 1, wherein the different parameter is that the temperature reached during the third sintering process is 400°C.

[0021] Comparative Example 4 The comparative example of the sodium iron phosphate pyrophosphate cathode material synthesis process adopts the heat treatment process of Example 1, wherein the different parameter is that the temperature reached during the third sintering process is 600°C.

[0022] Comparative Example 5 The powder precursor prepared in the preparation example was heated to 300°C in a vacuum tube furnace at a heating rate of 5°C / min and held for 6 hours. It was then cooled to room temperature in the furnace and then heated to 300°C again at a heating rate of 5°C / min. After that, it was heated to 550°C at a heating rate of 2°C / min and held for 8 hours. It was then cooled to room temperature in the furnace. The entire process was carried out under the protection of an argon-hydrogen mixture with a hydrogen volume content of 5%, thus obtaining the Na4Fe3(PO3)2P2O7 cathode material.

[0023] Comparative Example 6 The powder precursor prepared in the preparation example was heated to 550°C in a vacuum tube furnace at a heating rate of 5°C / min and kept for 8 hours, followed by furnace cooling to room temperature, and the whole process was carried out under the protection of argon-hydrogen mixed gas with a hydrogen volume content of 5%, to obtain a Na4Fe3(PO3)2P2O7 positive electrode material.

[0024] The assembly process of the button cell: The positive electrode sheet was uniformly mixed by the positive electrode material, conductive carbon black and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 7.5:1.5:1, and the mixture was dispersed in N-methyl pyrrolidone (NMP) with a solid content of 16%, and then adjusted into a slurry and uniformly coated on an aluminum foil, and dried at 90°C for 10 hours. The positive electrode material was prepared by the above-mentioned embodiment 1 or comparative examples 1-6, respectively. The above-mentioned electrode sheet was cut and assembled into a button cell with a sodium sheet in a glove box. The electrolyte is 1 mol / L NaClO4 dissolved in a volume ratio of 1:1 of ethylene carbonate (EC) and ethylene methyl carbonate (EMC) solution, and 5% fluoroethylene carbonate (FEC) additive is additionally added to the electrolyte. - 1 The sodium salt of NaClO4 was dissolved in a volume ratio of 1:1 of ethylene carbonate (EC) and ethylene methyl carbonate (EMC) solution, and 5% fluoroethylene carbonate (FEC) additive was additionally added to the electrolyte.

[0025] Performance test The sodium pyrophosphate iron phosphate positive electrode materials prepared in example 1, comparative example 2 and comparative example 5 were tested by X-ray diffraction (SHIMADZU XRD-7000). The experimental conditions are as follows: copper target (λ = 0.1518 nm), 2θ angle range 5-65°. The measured XRD pattern is shown in Figure 2 .

[0026] From the XRD pattern in Figure 2 , it can be seen that the positive electrode materials of example 1, comparative example 2 and comparative example 5 all have a high crystallinity of sodium pyrophosphate iron phosphate phase crystal structure of Pn21a space group (PDF standard card number: PDF#89-0579). Among them, comparative example 5 and comparative example 2 can be observed to have two sodium iron phosphate diffraction peaks with different intensities (PDF standard card number: PDF#89-0816), while example 1 does not have impurity diffraction peaks. The results show that the recrystallization process in the three-stage gradient phase control sintering and the control of the cooling rate of the third stage sintering can prepare a high-purity and high-crystallinity sodium pyrophosphate iron phosphate positive electrode material.

[0027] The discharge specific capacity of the button cell prepared in example 1, comparative example 2 and comparative example 5 was measured at current densities of 0.1C, 0.2C, 0.5C, 1.0C, 2.0C and 5.0C, respectively, and the cycle stability was measured at a current density of 1C for 500 cycles. The test results are shown in Figure 3 ,Figure 4 and Figure 5 and Table 1.

[0028] Table 1

[0029] in combination with Figure 3 , Figure 4 and Figure 5 and Table 1, it can be seen that, compared with all the comparative examples, Example 1 has the highest capacity release (103.854 mAh / g) and the first charge coulombic efficiency (99.6%) at 0.1C; and also has the highest discharge specific capacity of 99.454 mAh / g at 1C, and the retention rate is as high as 98.7% after 500 cycles.

[0030] In Comparative Example 1, the hydrogen content in the reducing atmosphere in the third stage sintering is 5%, and the specific capacity, the first charge coulombic efficiency, and the specific capacity retention rate of the obtained positive electrode material are all slightly lower than those of Example 1.

[0031] In Comparative Example 2, the cooling in the third stage sintering adopts furnace cooling, and the controlled cooling rate of 5°C / min in Example 1 is not adopted, and from Figure 1 It can be seen that it has two sodium iron phosphate diffraction peaks with different intensities, and the purity is not as good as that of Example 1, and the specific capacity, the first charge coulombic efficiency, and the specific capacity retention rate of the obtained positive electrode material are all lower than those of Example 1.

[0032] In Comparative Example 3, the temperature of the third stage sintering is 400°C, which is lower than that of Example 1, and the specific capacity, the first charge coulombic efficiency, and the specific capacity retention rate of the obtained positive electrode material are all lower than those of Example 1.

[0033] In Comparative Example 4, the temperature of the third stage sintering is 600°C, which is higher than that of Example 1, and the specific capacity, the first charge coulombic efficiency, and the specific capacity retention rate of the obtained positive electrode material are all lower than those of Example 1.

[0034] In Comparative Example 5, the first two stages of sintering are adopted, and the third stage of sintering is not adopted, and from Figure 1 It can be seen that it has two sodium iron phosphate diffraction peaks with different intensities, and the purity is not as good as that of Example 1, and the specific capacity, the first charge coulombic efficiency, and the specific capacity retention rate of the obtained positive electrode material are all much lower than those of Example 1.

[0035] In Comparative Example 6, only the first stage of sintering is adopted, and the second and third stages of sintering are not adopted, and the specific capacity, the first charge coulombic efficiency, and the specific capacity retention rate of the obtained positive electrode material are all far lower than those of Example 1.

[0036] Combined with the XRD chart and the above electrochemical data analysis, it can be concluded that the three-stage gradient phase control sintering method, the specific sintering temperature, the third-stage cooling rate and the hydrogen content in the reducing atmosphere can effectively improve the purity of sodium iron pyrophosphate, and due to the improvement of the phase purity, the content of the opposite irreversible sodium iron phosphate phase is reduced, so that the initial efficiency and the reversible specific capacity of the material are improved. At the same time, due to the recrystallization process, the grain size in the material is uniform and the stress is released, so that the cycle stability of the material is obviously improved.

[0037] Through the analysis and comparison of the above examples and comparative examples, the three-stage gradient phase control sintering method provided by the present application, combined with the simple spray drying preparation of the precursor, can effectively inhibit and eliminate the irreversible sodium iron phosphate phase in the sodium iron pyrophosphate positive electrode material, and then obtain a high-purity sodium iron pyrophosphate positive electrode material. The obtained electrode material has better electrochemical performance and can be used for the positive electrode of a rechargeable sodium ion battery, providing a new process route for developing high-performance sodium ion battery positive electrode materials, and having important application value in the field of sodium ion battery energy storage.

Claims

1. A method for preparing a high-purity sodium iron phosphate pyrophosphate cathode material, characterized in that: The process includes the following steps: adding sodium, phosphorus, iron, and carbon sources to deionized water and stirring until homogeneous, then drying using spray drying technology to obtain a precursor; placing the precursor in a vacuum tube furnace and using a segmented gradient controlled phase sintering process to obtain a high-purity sodium iron phosphate pyrophosphate cathode material. The segmented gradient controlled phase sintering process includes three sintering stages: pre-sintering, high-temperature sintering, and recrystallization sintering. The cathode material consists of spherical particles with a particle size distribution of 1-6µm and an average particle size of 3µm.

2. The method for preparing high-purity sodium iron phosphate pyrophosphate cathode material according to claim 1, wherein the segmented gradient phase-controlled sintering process comprises the following steps: Step 1: During pre-firing, the heating rate is 4-7℃ / min, the temperature is 280-320℃, the holding time is 5-8h, the cooling is carried out with the furnace, and the inert reducing atmosphere is a mixture of argon and hydrogen, with a hydrogen volume percentage of 5% in the mixture. Step 2: During high-temperature sintering, first heat to 280-320℃ at a heating rate of 4-7℃ / min, then heat to 540-580℃ at a heating rate of 1-3℃ / min, hold for 6-10 hours, and then cool to room temperature with the furnace. The inert reducing atmosphere is a mixture of argon and hydrogen, with a hydrogen volume percentage of 5%. Step 3: During recrystallization sintering, the heating rate is 1-3℃ / min, the temperature is 480-510℃, the holding time is 1-3h, the cooling rate is 4-7℃ / min, and the inert reducing atmosphere is a mixture of argon and hydrogen, with a hydrogen volume percentage of 8%.

3. The method for preparing high-purity sodium iron phosphate pyrophosphate cathode material according to claim 2, wherein the segmented gradient phase-controlled sintering process comprises the following steps: Step 1: During pre-firing, the heating rate is 5℃ / min, the temperature is 300℃, the holding time is 6h, the cooling is carried out with the furnace, and the inert reducing atmosphere is a mixture of argon and hydrogen, with a hydrogen volume percentage of 5% in the mixture. Step 2: During high-temperature sintering, first heat to 300℃ at a heating rate of 5℃ / min, then heat to 550℃ at a heating rate of 2℃ / min, hold for 8 hours, and then cool to room temperature with the furnace. The inert reducing atmosphere is a mixture of argon and hydrogen, and the volume percentage of hydrogen in the mixture is 5%. Step 3: During recrystallization sintering, the heating rate is 2℃ / min, the temperature is 500℃, the holding time is 2h, the cooling rate is 5℃ / min, and the inert reducing atmosphere is a mixture of argon and hydrogen, with a hydrogen volume percentage of 8%.

4. The method for preparing high-purity sodium iron phosphate pyrophosphate cathode material according to claim 3, wherein the sodium source, phosphorus source, and iron source are sodium dihydrogen phosphate and ferric nitrate nonahydrate, respectively, and the molar ratio is 4:

3.

5. The method for preparing the high-purity sodium iron phosphate pyrophosphate cathode material according to claim 4, characterized in that, The carbon source is citric acid, the molar ratio of carbon source to sodium source is 3:1, and the concentration of citric acid solution is 8%-15%, preferably 10%.

6. The method for preparing the high-purity sodium iron phosphate pyrophosphate cathode material according to claim 5, characterized in that, During the preparation of the precursor, the stirring speed is 400-500 rpm and the stirring time is 3-5 h. Preferably, the stirring speed is 450 rpm and the stirring time is 4 h.

7. The method for preparing high-purity sodium iron phosphate pyrophosphate cathode material according to claim 6, wherein during spray drying, the feed rate is 650-750 mL / h and the nozzle temperature is 240-280℃; preferably, the feed rate is 700 mL / h and the nozzle temperature is 260℃.

8. A high-purity sodium iron phosphate pyrophosphate cathode material prepared by any one of claims 1-7, wherein the cathode material is spherical particles with a particle size distribution of 1-6µm and an average particle size of 3µm.

9. A method for preparing a positive electrode sheet for a sodium-ion battery, characterized in that, Includes the following steps: Step 1: Weigh the following components according to the following mass percentages: 70-80 parts of the high-purity sodium iron phosphate pyrophosphate cathode material as described in claim 8, 10-20 parts of conductive carbon black, and 5-15 parts of binder; preferably, 75 parts of the high-purity sodium iron phosphate pyrophosphate cathode material as described in claim 8, 15 parts of conductive carbon black, and 10 parts of binder. Step 2: Disperse the high-purity sodium iron pyrophosphate sodium-ion battery positive electrode material, conductive carbon black and binder weighed in Step 1 in N-methylpyrrolidone at a solid content of 15-18%, preferably 16%, and then coat it on the treated aluminum foil and dry it to obtain the sodium-ion battery positive electrode sheet.

10. The method for preparing the positive electrode of a sodium-ion battery according to claim 9, wherein, The conductive carbon black is SuperP, and the binder is polyvinylidene fluoride; the coating thickness is 100µm, and the drying temperature is 90℃.

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