Preparation process of high-stability sodium ion battery positive electrode material and sodium ion battery

Abstract

The present invention discloses a process for preparing a high-stability positive electrode material for sodium-ion batteries. The process comprises the following steps: first, using lithium iron phosphate as the positive electrode, an electrolyte solution made of lithium salt, and a carbon material as the negative electrode; connecting the solution to a power source for constant current charging; allowing lithium ions to migrate out of the positive electrode in the liquid phase solution; and after charging, exfoliating the positive electrode powder, washing, and drying it to obtain an olivine-structured iron phosphate. Second, the iron phosphate is uniformly mixed with an organic sodium salt, ball-milled, and calcined in a protective atmosphere at 200-400°C to allow sodium ions in the organic sodium salt to migrate into the iron phosphate. After calcination, the solution is first cooled to 80-120°C at a controlled rate, and then cooled to room temperature at a controlled rate, wherein the first rate is less than the second rate, to obtain the sodium-ion battery positive electrode material. This sodium-ion battery positive electrode material has high purity and a stable structure. Furthermore, a sodium-ion battery with excellent cycle performance and high capacity is provided.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy materials, and in particular to the preparation of a positive electrode material for a sodium ion battery and a sodium ion battery using the positive electrode material. Background Art

[0002] With economic development and progress, society's demands for environmental protection are becoming increasingly stringent, necessitating the development of clean, sustainable energy sources to replace fossil fuels such as coal and oil. Electricity, as a renewable energy source, can be widely used in various applications. Currently, lithium-ion batteries are primarily used to store electrical energy. However, lithium resources are scarce, lithium ore grades are low, and mining costs are high. To meet society's demand for large-scale electricity storage, the development of energy storage materials with high resource availability and low production costs is urgently needed.

[0003] Sodium is abundant on Earth and its raw material costs are low. The development of sodium-ion batteries will meet the demand for large-scale electricity storage. Sodium iron phosphate (NaFePO4) offers advantages such as structural stability, a high voltage platform, excellent thermal stability, and high specific capacity, making it one of the most promising cathode materials for sodium-ion batteries.

[0004] At present, the main processes for synthesizing NaFePO4 are solid-phase high-temperature calcination and ion exchange (solution ion exchange, high-temperature molten salt ion exchange). The high-temperature calcination process is simple, but the crystal form of sodium iron phosphate changes after high-temperature calcination, and its electrochemical performance remains to be studied; the product obtained by the solution ion exchange method has high purity and the crystal form does not change, but it involves the preparation and disassembly of the battery, the process is complex, and the production cost is high; the high-temperature molten salt ion exchange process is simple, but there is incomplete ion exchange, and there are impurity phases in the obtained positive electrode material. At the same time, the size of the ion migration channel in the obtained material is inherited from lithium iron phosphate, and the radius of sodium ions is larger than that of lithium ions. During the cycle, the removal and embedding of sodium ions will squeeze the crystal structure, affecting the stability and cycle performance of the material. Therefore, the development of a NaFePO4 preparation method with simple process, stable crystal structure and high specific capacity is of great significance for the application and promotion of sodium ion batteries. Summary of the Invention

[0005] The object of the present invention is to provide a preparation process for a high-stability sodium ion battery positive electrode material to achieve the preparation of high-purity olivine-type iron phosphate and sodium iron phosphate, the process comprising the following steps:

[0006] In the first step, lithium iron phosphate is used as the positive electrode, an electrolyte solution is made of lithium salt, and a carbon material is used as the negative electrode. A power supply is connected for constant current charging. Lithium ions migrate out of the positive electrode in the liquid solution. After charging is completed, the powder of the positive electrode is peeled off, washed, and dried to obtain olivine-structured iron phosphate.

[0007] In the second step, the ferric phosphate and the organic sodium salt are evenly mixed, ball-milled, placed in a protective atmosphere, and calcined at 200-400° C. to allow the sodium ions in the organic sodium salt to migrate into the ferric phosphate. After the calcination is completed, the temperature is first controlled to be cooled to 80-120° C. at a first cooling rate, and then cooled to room temperature at a second cooling rate, wherein the first cooling rate is less than the second cooling rate, to prepare a sodium ion battery positive electrode material.

[0008] Preferably, in the first step, the lithium salt is stirred and dissolved in pure water to prepare the electrolyte solution, and the concentration of lithium ions in the electrolyte solution is 1 to 2 mol / L.

[0009] Preferably, in the first step, the carbon material is a composite of one or more of hard carbon and soft carbon.

[0010] Preferably, in the first step, the drying is performed in a vacuum drying oven at 60-100° C. for 8-15 hours.

[0011] Preferably, the organic sodium salt in the second step is one or a mixture of sodium formate, sodium acetate, sodium oxalate, sodium citrate, sodium edetate, and sodium ethoxide.

[0012] Preferably, in the second step, the molar ratio of the organic sodium salt to the sodium element and the iron element in the ferric phosphate is (1.0-1.2):1.

[0013] Preferably, in the second step, the ball milling is performed at a rotation speed of 300 rpm for 3 to 8 hours.

[0014] Preferably, in the second step, the calcination temperature is 330-380° C., and the calcination time is 4-10 hours.

[0015] Preferably, in the second step, the first cooling rate is 5-10° C. / min, and the second cooling rate is 1200-2400° C. / min, and the second cooling rate is faster than the first cooling rate.

[0016] Also provided is a sodium ion battery, comprising a sodium ion battery positive electrode material prepared by the above process, wherein the sodium ion battery has a discharge capacity of ≥100 mAh / g at room temperature 0.2C, and a battery capacity retention rate of ≥79% after 3000 charge and discharge cycles at room temperature 10C.

[0017] The present invention has the following beneficial effects:

[0018] 1. The first step is to migrate lithium ions from the lithium iron phosphate cathode in the liquid phase, maintaining the crystal structure of the iron phosphate olivine and achieving high purity of the resulting iron phosphate. The second step is calcination to migrate sodium ions into the iron phosphate, and then a staged cooling process is used to obtain the sodium-ion battery cathode material. The staged cooling process reduces the internal stress in the crystal caused by the compression of the crystal structure by the insertion of sodium ions after the crystal is de-lithiated and sodium is inserted. Since the radius of sodium ions is larger than that of lithium ions, this process improves the structural stability and ultimately enhances the cycle performance and specific capacity of the sodium battery cathode material.

[0019] 2. A two-step heterogeneous ion migration method is used. In the first step, while preparing high-purity sodium cathode materials, the lithium resources in lithium iron phosphate can also be recovered and the iron phosphate can be reused.

[0020] 3. Using organic sodium salt as sodium source increases the migration rate of sodium ions, reduces the calcination temperature and saves energy.

[0021] 4. The raw materials used in the process are common and easily available, non-toxic and harmless, the process is mature and simple, easy to operate, and highly feasible. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is further described below through specific implementation methods. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making any creative work are within the scope of protection of this application.

[0023] The present invention will be further described below through specific embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of this application.

[0024] The object of the present invention is to provide a preparation process for a high-stability sodium ion battery positive electrode material to achieve the preparation of high-purity olivine-type iron phosphate and high-stability sodium iron phosphate, the process comprising the following steps:

[0025] In the first step, lithium iron phosphate is used as the positive electrode, an electrolyte solution is made of lithium salt, and a carbon material is used as the negative electrode. A power supply is connected for constant current charging. Lithium ions migrate out of the positive electrode in the liquid solution. After charging is completed, the powder of the positive electrode is peeled off, washed, and dried to obtain olivine-structured iron phosphate.

[0026] In the second step, the ferric phosphate and the organic sodium salt are evenly mixed, ball-milled, and calcined at 200-400° C. in a protective atmosphere to allow the sodium ions in the organic sodium salt to migrate into the ferric phosphate. After the calcination is completed, the temperature is first controlled to be cooled to 80-120° C. at a first cooling rate, and then cooled to room temperature at a second cooling rate, wherein the first cooling rate is less than the second cooling rate, to prepare a sodium ion battery positive electrode material, i.e., a sodium iron phosphate positive electrode material.

[0027] In the first step, the lithium salt is stirred and dissolved in pure water to form the electrolyte solution. The lithium ion concentration in the electrolyte solution is 1 to 2 mol / L. The carbon material is a composite of one or more of hard carbon and soft carbon. The solution is dried in a vacuum drying oven at 60 to 100°C for 8 to 15 hours.

[0028] In the second step, the organic sodium salt is one or a mixture of sodium formate, sodium acetate, sodium oxalate, sodium citrate, sodium ethylenediaminetetraacetic acid, and sodium ethoxide. The molar ratio of the organic sodium salt to the sodium element and the iron element in the ferric phosphate is (1.0-1.2):1. The calcination temperature is preferably 330-380°C, and the calcination time is 4-10 hours. The ball milling is performed at a ball mill speed of 300 rpm for 3-8 hours. In the second step, the first cooling rate is 5-10°C / min, and the second cooling rate is 1200-2400°C / min.

[0029] The sodium ion battery made using the above-mentioned sodium iron phosphate positive electrode material has a discharge capacity of ≥100mAh / g at room temperature and 0.2C, and a battery capacity retention rate of ≥79% after 3000 charge and discharge cycles at room temperature and 10C.

[0030] In order to further understand the present invention, preferred embodiments of the present invention are described below with reference to examples.

[0031] Example 1

[0032] The first step is to weigh 27.49g of anhydrous lithium sulfate, add it to 500mL of pure water, and stir to dissolve it to obtain the lithium ion concentration [Li + ] is a 1 mol / L electrolyte. A used lithium iron phosphate electrode and a graphite electrode were placed in an electrolytic cell containing the aforementioned electrolyte. The lithium iron phosphate electrode was connected to the working electrode of a potentiostat, and the graphite electrode was connected to the counter electrode. Constant current charging was performed using a silver / silver chloride electrode as the reference electrode. After charging, the lithium iron phosphate was stripped, washed with pure water, and dried in a vacuum drying oven at 80°C for 12 hours to obtain olivine-type iron phosphate.

[0033] In the second step, 100g of dried ferric phosphate and 64.92g of sodium citrate dihydrate (the molar ratio of iron to sodium is 1:1) were weighed and mixed evenly in a beaker, and then transferred to a ball mill for ball milling at 300rpm for 5h. The mixed materials were calcined at 350℃ for 6h. After the calcination, the cooling rate was controlled at 5℃ / min, and the temperature was lowered to 100℃, and then the rate was controlled to be reduced to 1200℃ / min and quickly cooled to room temperature to obtain the sodium positive electrode material.

[0034] Example 2

[0035] The first step is to weigh 27.49g of anhydrous lithium sulfate, add it to 500mL of pure water, and stir to dissolve it to obtain the lithium ion concentration [Li + ] is a 1 mol / L electrolyte. A used lithium iron phosphate electrode and a graphite electrode were placed in an electrolytic cell containing the aforementioned electrolyte. The lithium iron phosphate electrode was connected to the working electrode of a potentiostat, and the graphite electrode was connected to the counter electrode. Constant current charging was performed using a silver / silver chloride electrode as the reference electrode. After charging, the lithium iron phosphate was stripped, washed with pure water, and dried in a vacuum drying oven at 80°C for 12 hours to obtain olivine-type iron phosphate.

[0036] In the second step, 100g of dried ferric phosphate and 64.92g of sodium citrate dihydrate (the molar ratio of iron to sodium is 1:1) were weighed and mixed evenly in a beaker, and then transferred to a ball mill for ball milling at 300rpm for 5h. The mixed materials were calcined at 350℃ for 6h. After the calcination, the cooling rate was controlled at 5℃ / min, and the temperature was lowered to 100℃. Then, the cooling rate was controlled at 2400℃ / min and the sodium positive electrode material was obtained after rapid cooling to room temperature.

[0037] Example 3

[0038] The first step is to weigh 27.49g of anhydrous lithium sulfate, add it to 500mL of pure water, and stir to dissolve it to obtain the lithium ion concentration [Li + ] is a 1 mol / L electrolyte. A used lithium iron phosphate electrode and a graphite electrode were placed in an electrolytic cell containing the aforementioned electrolyte. The lithium iron phosphate electrode was connected to the working electrode of a potentiostat, and the graphite electrode was connected to the counter electrode. Constant current charging was performed using a silver / silver chloride electrode as the reference electrode. After charging, the lithium iron phosphate was stripped, washed with pure water, and dried in a vacuum drying oven at 80°C for 12 hours to obtain olivine-type iron phosphate.

[0039] In the second step, 100g of dried ferric phosphate and 71.41g of sodium citrate dihydrate (the molar ratio of iron to sodium is 1:1.1) were weighed and mixed evenly in a beaker, and then transferred to a ball mill for ball milling at 300rpm for 5h. The mixed materials were calcined at 350℃ for 6h. After the calcination, the cooling rate was controlled to 5℃ / min, and the temperature was lowered to 100℃, and then the rate was controlled to be reduced to 1200℃ / min and quickly cooled to room temperature to obtain the sodium positive electrode material.

[0040] Example 4

[0041] The first step is to weigh 27.49g of anhydrous lithium sulfate, add it to 500mL of pure water, and stir to dissolve it to obtain the lithium ion concentration [Li + ] is a 1 mol / L electrolyte. A used lithium iron phosphate electrode and a graphite electrode were placed in an electrolytic cell containing the aforementioned electrolyte. The lithium iron phosphate electrode was connected to the working electrode of a potentiostat, and the graphite electrode was connected to the counter electrode. Constant current charging was performed using a silver / silver chloride electrode as the reference electrode. After charging, the lithium iron phosphate was stripped, washed with pure water, and dried in a vacuum drying oven at 80°C for 12 hours to obtain olivine-type iron phosphate.

[0042] In the second step, 100g of dried ferric phosphate and 71.41g of sodium citrate dihydrate (the molar ratio of iron to sodium is 1:1.1) were weighed and mixed evenly in a beaker, and then transferred to a ball mill for ball milling at 300rpm for 5h. The mixed materials were calcined at 400℃ for 6h. After the calcination, the cooling rate was controlled to 5℃ / min, and the temperature was lowered to 100℃, and then the rate was controlled to be reduced to 1200℃ / min and quickly cooled to room temperature to obtain the sodium positive electrode material.

[0043] Example 5

[0044] The first step is to weigh 54.97g of anhydrous lithium sulfate, add it to 500mL of pure water, and stir to dissolve it to obtain the lithium ion concentration [Li + ] is a 2 mol / L electrolyte. A used lithium iron phosphate electrode and a graphite electrode were placed in an electrolytic cell containing the aforementioned electrolyte. The lithium iron phosphate electrode was connected to the working electrode of a potentiostat, and the graphite electrode was connected to the counter electrode. Constant current charging was performed using a silver / silver chloride electrode as the reference electrode. After charging, the lithium iron phosphate was stripped, washed with pure water, and dried in a vacuum drying oven at 80°C for 12 hours to obtain olivine-type iron phosphate.

[0045] In the second step, 100g of dried ferric phosphate and 71.41g of sodium citrate dihydrate (the molar ratio of iron to sodium is 1:1.1) were weighed and mixed evenly in a beaker, and then transferred to a ball mill for ball milling at 300rpm for 5h. The mixed materials were calcined at 400℃ for 6h. After the calcination, the cooling rate was controlled to 5℃ / min, and the temperature was lowered to 100℃, and then the rate was controlled to be reduced to 1200℃ / min and quickly cooled to room temperature to obtain the sodium positive electrode material.

[0046] Example 6

[0047] The first step is to weigh 27.49g of anhydrous lithium sulfate, add it to 500mL of pure water, and stir to dissolve it to obtain the lithium ion concentration [Li + ] is a 1 mol / L electrolyte. A used lithium iron phosphate electrode and a graphite electrode were placed in an electrolytic cell containing the aforementioned electrolyte. The lithium iron phosphate electrode was connected to the working electrode of a potentiostat, and the graphite electrode was connected to the counter electrode. Constant current charging was performed using a silver / silver chloride electrode as the reference electrode. After charging, the lithium iron phosphate was stripped, washed with pure water, and dried in a vacuum drying oven at 80°C for 12 hours to obtain olivine-type iron phosphate.

[0048] In the second step, 100g of dried ferric phosphate and 90.12g of sodium acetate trihydrate (the molar ratio of iron to sodium is 1:1) were weighed and mixed evenly in a beaker, and then transferred to a ball mill for ball milling at 300rpm for 5h. The mixed materials were calcined at 380℃ for 6h. After the calcination, the cooling rate was controlled to 5℃ / min, and the temperature was lowered to 100℃, and then the rate was controlled to be reduced to 1200℃ / min and quickly cooled to room temperature to obtain the sodium positive electrode material.

[0049] Example 7

[0050] The first step is to weigh 27.49g of anhydrous lithium sulfate, add it to 500mL of pure water, and stir to dissolve it to obtain the lithium ion concentration [Li + ] is a 1 mol / L electrolyte. A used lithium iron phosphate electrode and a graphite electrode were placed in an electrolytic cell containing the aforementioned electrolyte. The lithium iron phosphate electrode was connected to the working electrode of a potentiostat, and the graphite electrode was connected to the counter electrode. Constant current charging was performed using a silver / silver chloride electrode as the reference electrode. After charging, the lithium iron phosphate was stripped, washed with pure water, and dried in a vacuum drying oven at 80°C for 12 hours to obtain olivine-type iron phosphate.

[0051] In the second step, 100g of dried iron phosphate and 88.74g of sodium oxalate (the molar ratio of iron to sodium is 1:1) were weighed and mixed evenly in a beaker, and then transferred to a ball mill for ball milling at 300rpm for 5h. The mixed materials were calcined at 330℃ for 6h. After the calcination, the cooling rate was controlled at 5℃ / min, and the temperature was lowered to 100℃, and then the rate was controlled to be reduced to 1200℃ / min and quickly cooled to room temperature to obtain the sodium positive electrode material.

[0052] Comparative Example 1

[0053] The first step is to weigh 54.97g of anhydrous lithium sulfate, add it to 500mL of pure water, and stir to dissolve it to obtain the lithium ion concentration [Li + ] is a 1 mol / L Li2SO4 electrolyte. A used lithium iron phosphate electrode and a graphite electrode were placed in a tank containing the Li2SO4 electrolyte. The lithium iron phosphate electrode was connected to the working electrode of a potentiostat, and the graphite electrode was connected to the counter electrode. Constant current charging was performed using a silver / silver chloride electrode as the reference electrode. After charging, the lithium iron phosphate was stripped, washed with pure water, and dried in a vacuum drying oven at 80°C for 12 hours to obtain olivine-type iron phosphate.

[0054] In the second step, 100g of dried ferric phosphate and 64.92g of sodium citrate dihydrate (the molar ratio of iron to sodium is 1:1) were weighed and mixed evenly in a beaker, and then transferred to a ball mill for ball milling at 300rpm for 5h. The mixed materials were calcined at 350℃ for 6h. After calcination, the cooling rate was controlled at 5℃ / min. After cooling to room temperature, the sodium positive electrode material was obtained.

[0055] Comparative Example 2

[0056] Weigh 100 g of raw material iron phosphate and 64.92 g of sodium citrate dihydrate (the molar ratio of iron element to sodium element is 1:1) in a beaker, mix them evenly, and then transfer them to a ball mill and ball mill them at 300 rpm for 5 h. The mixed material is calcined at 350 ° C for 6 h. After calcination, it is naturally cooled to room temperature to obtain a sodium positive electrode material.

[0057] test

[0058] The above-mentioned sodium cathode material, acetylene black, and binder were mixed with a solvent in a weight ratio of 7:2:1 to prepare a positive electrode slurry. This slurry was then evenly coated onto aluminum foil, dried, roll-pressed, and sliced ​​to produce a positive electrode sheet. A sodium metal sheet was then used as the negative electrode, and a polypropylene film was used as the separator. The positive electrode sheet, negative electrode, and separator were placed into a button-type battery case, and an electrolyte solution (1M NaPF6) was injected. The solvents were EC, DEC, and FEC (volume = 9:9:2). After pressing, a sodium ion button half-cell was obtained. After 12 hours of storage, it was ready for testing. Three sodium ion button half-cells made with the same materials were tested simultaneously, including material gram capacity and cycling performance. The test results were averaged across the three samples (results with a difference greater than 5% were considered outliers and discarded). The results are shown in Table 1.

[0059] Table 1

[0060]

[0061] The sodium iron phosphate cathode material samples prepared in the examples of the present invention and the comparative examples were made into batteries and then subjected to performance testing, and the results are shown in Table 1. As can be seen from Table 1, the sodium iron phosphate cathode material obtained in the present invention has a room temperature 0.2C discharge capacity of ≥100 mAh / g for sodium ion batteries, a room temperature 10C charge and discharge cycle rate of ≥79% after 3000 cycles, and a rate performance of ≥90%, which are significantly improved compared to comparative examples 1 and 2.

[0062] The applicant states that the present invention is intended to illustrate the detailed methods of the present invention through the above-described embodiments, but the present invention is not limited to the above-described detailed methods, that is, it does not mean that the present invention must rely on the above-described detailed methods in order to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for various raw materials in the products of the present invention, addition of auxiliary ingredients, and selection of specific methods, etc., are all within the scope of protection and disclosure of the present invention.

Claims

1. A process for preparing a high-stability sodium ion battery cathode material, characterized in that: The steps include: In the first step, lithium iron phosphate is used as the positive electrode, an electrolyte solution made of lithium salt, and a carbon material is used as the negative electrode. A power supply is connected for constant current charging. Lithium ions migrate out of the positive electrode in the liquid phase solution. After charging is completed, the positive electrode powder is peeled off, washed, and dried to obtain olivine-structured iron phosphate. In the second step, the ferric phosphate and the organic sodium salt are evenly mixed, ball-milled, placed in a protective atmosphere, and calcined at 200-400° C. to allow the sodium ions in the organic sodium salt to migrate into the ferric phosphate. After the calcination is completed, the temperature is first controlled to be cooled to 80-120° C. at a first cooling rate of 5-10° C. / min, and then cooled to room temperature at a second cooling rate, the first cooling rate is less than the second cooling rate, and the second cooling rate is 1200-2400° C. / min to prepare a sodium ferric phosphate positive electrode material.

2. The process for preparing a high-stability sodium ion battery positive electrode material according to claim 1, wherein: In the first step, the lithium salt is stirred and dissolved in pure water to prepare the electrolyte solution, wherein the concentration of lithium ions in the electrolyte solution is 1 to 2 mol / L.

3. The process for preparing a high-stability sodium ion battery positive electrode material according to claim 1, wherein: In the first step, the carbon material is a composite of one or more of hard carbon and soft carbon.

4. The process for preparing a high-stability sodium ion battery cathode material according to claim 1, wherein: In the first step, the drying is carried out in a vacuum drying oven at 60-100° C. for 8-15 hours.

5. The process for preparing a high-stability sodium ion battery cathode material according to claim 1, wherein: The organic sodium salt in the second step is one or a mixture of sodium formate, sodium acetate, sodium oxalate, sodium citrate, sodium edetate, and sodium ethoxide.

6. The process for preparing a high-stability sodium ion battery cathode material according to claim 1, wherein: In the second step, the molar ratio of the organic sodium salt to the sodium element and the iron element in the ferric phosphate is (1.0-1.2):

1.

7. The process for preparing a high-stability sodium ion battery cathode material according to claim 1, wherein: In the second step, the calcination temperature is 330-380° C., and the calcination time is 4-10 hours.

8. The process for preparing a high-stability sodium ion battery cathode material according to claim 1, wherein: In the second step, the ball milling is performed at a rotation speed of 300 rpm for 3 to 8 hours.

9. A sodium ion battery, characterized in that: The invention comprises a sodium iron phosphate positive electrode material prepared by the process according to any one of claims 1 to 8, wherein the sodium ion battery has a discharge capacity of ≥100 mAh / g at room temperature 0.2C, and a battery capacity retention rate of ≥79% after 3000 charge and discharge cycles at room temperature 10C.

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