Preparation method of olivine-type sodium iron phosphate positive electrode material of sodium ion battery

Abstract

The invention relates to a preparation method of an olivine-type sodium iron phosphate positive electrode material of a sodium ion battery. The preparation method of the sodium iron phosphate positive electrode material comprises the following steps: firstly, weighing iron phosphate, sodium salt and phosphate according to a molar ratio of 1: (1.1-2): (0.1-1.3); the preparation method comprises the following steps: uniformly mixing the raw materials, adding a carbon source accounting for 5-10% of the total mass of the raw materials, carrying out ball milling treatment, heating and calcining in an inert atmosphere, and cooling to room temperature to obtain the olivine-type sodium ferric phosphate positive electrode material. When the material is used for a sodium-ion battery positive electrode, high specific discharge capacity and excellent cycle performance are shown. The addition of a proper amount of phosphate is beneficial to construction of a molten salt preparation system, the reaction efficiency is improved, and the sodium iron phosphate positive electrode material with perfect crystal form and excellent sodium storage performance is prepared. The sodium iron phosphate positive electrode material is simple to prepare, high in practical operability, favorable for batch production, excellent in sodium storage performance and wide in application prospect.

Description

Technical Field

[0001] The invention relates to the field of sodium ion battery positive electrode materials, and in particular to a method for preparing an olivine-type sodium iron phosphate positive electrode material. Background Art

[0002] Sodium iron phosphate (NaFePO 4 ) positive electrode materials have the advantages of good structural stability, high safety and environmental friendliness, making them an important candidate for positive electrode materials for sodium ion batteries.

[0003] There are two main types of sodium iron phosphate positive electrode materials: jellyfish type and olivine type. Sodium iron phosphate prepared by high-temperature solid phase method is usually thermodynamically stable jellyfish type sodium iron phosphate, which has poor electrochemical activity and is difficult to be directly used as a positive electrode material for sodium ion batteries. It needs to be improved through nano-processing. However, the nano-processing preparation process is not only energy-intensive and costly, but also has problems such as particle agglomeration during the nano-processing process, which seriously restricts the large-scale preparation and industrial application of jellyfish type sodium iron phosphate.

[0004] Olivine-type sodium iron phosphate has good electrochemical properties and does not require nano-processing. It is an ideal positive electrode material for sodium ion batteries, but it is difficult to prepare by high-temperature solid-phase method. At present, the main preparation method of olivine-type sodium iron phosphate positive electrode material is to adopt ion exchange method, and commonly used ion exchange methods include electrochemical ion exchange method, solution ion exchange method, etc. For example, CN105047913A and CN117660986A prepared olivine-type sodium iron phosphate positive electrode material by electrochemical ion exchange method. This method first electrochemically removes lithium from lithium iron phosphate in a lithium-containing solution to obtain iron phosphate, and the iron phosphate is electrochemically embedded in sodium in a sodium-containing solution to obtain olivine-type sodium iron phosphate. However, this electrochemical ion exchange preparation method consumes a lot of power when applied on a large scale, and has high requirements for raw material purity. The subsequent treatment of the electrolyte is also relatively complicated, and it is difficult to apply on a large scale. CN114572956A uses a solution ion exchange method to prepare olivine-type sodium iron phosphate, firstly removing lithium from lithium iron phosphate under argon protection to obtain iron phosphate, and then inserting sodium in a sodium iodide solution to prepare olivine-type sodium iron phosphate. This method has the problems of complex pre-treatment and high cost of sodium salt reagent.

[0005] In summary, the preparation process of olivine-type sodium iron phosphate by ion exchange method is cumbersome, resulting in high cost, and there are problems such as poor compatibility with organic sodium salts and low sodium salt utilization. Therefore, it is of great significance to construct a simple and efficient method for preparing high-performance olivine-type sodium iron phosphate positive electrode materials. Summary of the invention

[0006] In view of the above problems, the present invention provides a molten salt-assisted solid-phase preparation method for olivine-type sodium iron phosphate positive electrode material. Using iron phosphate, sodium salt, phosphate and carbon source as raw materials, phosphate as molten salt reaction medium, combined with ball milling activation, a high-temperature solid-phase method is used to prepare an olivine-type sodium iron phosphate positive electrode material with excellent energy storage performance. Among them, the phosphate provides a molten salt reaction medium, which is beneficial to improve the reaction efficiency, make the crystallinity of the olivine-type sodium iron phosphate more perfect, and form a more complete ion migration channel, effectively improving the reversible discharge specific capacity and cycle stability of the olivine-type sodium iron phosphate positive electrode material during the charge and discharge process.

[0007] A method for preparing an olivine-type sodium iron phosphate positive electrode material, characterized in that it comprises the following steps:

[0008] The iron phosphate, sodium salt, phosphate and carbon source are mixed in a dispersant and ball milled;

[0009] The ball-milled mixture is sintered at elevated temperature in an inert gas atmosphere;

[0010] The sintered product is cooled to room temperature to obtain an olivine-type sodium iron phosphate positive electrode material.

[0011] Preferably, the molar ratio of the iron phosphate, the sodium salt, and the phosphate is 1:1.1-2:0.1-1.3. Further, the molar ratio of the iron phosphate, the sodium salt, and the phosphate is 1:1.3:1.3.

[0012] Preferably, the ratio of the phosphate is positively correlated with the electrochemical performance of the olivine-type sodium iron phosphate positive electrode material.

[0013] Phosphate can be used as a phosphorus source and also provide a molten salt medium during the calcination process, which is beneficial to the melting of sodium salts, promotes the embedding of sodium ions into the iron phosphate lattice, and improves the crystallinity of the product.

[0014] Preferably, the crystal form of the iron phosphate is isophosphite-type. Compared with other crystal forms of iron phosphate, isophosphite-type iron phosphate has an olivine structure, which has a higher stability and is conducive to the embedding of sodium ions into its crystal lattice in the molten salt state to form NaFePO with an olivine structure. 4 .

[0015] Preferably, the phosphate includes one or more mixtures of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate. Further, the phosphate is ammonium dihydrogen phosphate. Due to the low melting point of ammonium dihydrogen phosphate, the contact area with the sodium salt is increased, and the subsequent sintering can fully molten salt to promote the dissolution of sodium carbonate. The low melting point phosphate is the key to being compatible with high melting point sodium salt during calcination. The molten phosphate can provide a liquid environment and increase the contact area with the sodium salt. The molten phosphate can penetrate between the particles of the high melting point sodium salt, and the sodium salt is gradually decomposed and melted through physical erosion and diffusion. The melting points of diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate are relatively high, although they are not conducive to improving the compatibility of the calcination system with the high melting point sodium salt, they still have use significance in actual production.

[0016] Preferably, the sodium salt includes any one or more of low melting point sodium salts such as sodium acetate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium hydroxide and high melting point sodium salts such as sodium carbonate. In the present invention, since low melting point phosphates are used, the difference in melting points will lead to a eutectic effect. During the calcination process, the low melting point phosphates will melt first. The molten phosphates can improve the heat transfer and mass transfer conditions during the calcination process, making it easier for heat to be transferred to the interior of the high melting point sodium salt particles, thereby improving the reaction efficiency and compatibility with the high melting point sodium salts.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. The prior art (CN105047913A, CN117660986A) mainly utilizes electrochemical ion exchange method to prepare olivine sodium iron phosphate, which has complicated process and high energy consumption when applied on a large scale. The present invention prepares olivine sodium iron phosphate by molten salt assisted solid phase method, which has simple process and complete olivine sodium iron phosphate crystal form. In addition, the electrochemical ion exchange method needs to separate the sample into solid and liquid in the later stage of preparation, and the process is cumbersome. The sodium iron phosphate prepared by the present invention can be directly applied to the positive electrode material of sodium ion battery, and the operation is simple.

[0019] 2. The present invention proposes a solid phase method for preparing olivine sodium iron phosphate using a molten salt phosphate-assisted strategy. The addition of low melting point phosphate helps to enhance the reaction activity of sodium salt and iron phosphate, improve the crystallinity of olivine sodium iron phosphate, and enhance the structural stability of olivine sodium iron phosphate, thereby improving its cycle stability.

[0020] 3. The prior art (CN117660986A) mainly uses sodium iodide, (CN117583600A) hot-melt metallic sodium, (CN115472825A) organic sodium salt or (CN114368736A) sodium nitrate low melting point sodium salt, the sodium source cost is relatively high, and the preparation method thereof is not compatible with high melting point sodium salt (sodium carbonate, etc.). The present invention can use sodium carbonate for synthesis, which has low cost and is convenient for large-scale application.

[0021] 4. The prior art (CN114368736A) uses excessive sodium salt (the mass ratio of sodium salt to lithium iron phosphate is 20 to 10:1) during the preparation process. The excessive amount of sodium salt used is high in cost and has caused waste. The subsequent removal of excess sodium salt in the product and the purification process of sodium salt are relatively cumbersome. The amount of sodium salt used in the present invention is Na 2 CO 3 :FePO 4 =0.35:1 (molar ratio of Na:Fe=1:1) to synthesize NaFePO 4 The utilization rate of sodium salt is high, the cost of raw materials can be significantly reduced, and no additional treatment is required, and the process is simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0024] In the attached figure:

[0025] Figure 1 This is the X-ray diffraction pattern of the olive-type sodium iron phosphate prepared in Example 1.

[0026] Figure 2 This is the X-ray diffraction pattern of the olive-type sodium iron phosphate prepared in Example 2.

[0027] Figure 3 This is the X-ray diffraction pattern of the olive-type sodium iron phosphate prepared in Example 3.

[0028] Figure 4 This is the X-ray diffraction pattern of the olive-type sodium iron phosphate prepared in Example 4.

[0029] Figure 5 This is the X-ray diffraction pattern of the olive-type sodium iron phosphate prepared in Example 5.

[0030] Figure 6 This is the X-ray diffraction pattern of the sodium iron phosphate prepared in Comparative Example 1.

[0031] in, Figure 1-Figure 6 Intensity is the diffraction intensity, and degree is the diffraction angle.

[0032] Figure 7 This is the charge and discharge curve of the olive-type sodium iron phosphate prepared in Example 1 at 0.1C.

[0033] Figure 8 This is the charge and discharge curve of the olive-type sodium iron phosphate prepared in Example 2 at 0.1C.

[0034] Fig. 9 This is the charge and discharge curve of the olive-type sodium iron phosphate prepared in Example 3 at 0.1C.

[0035] Fig.10 This is the charge and discharge curve of the olive-type sodium iron phosphate prepared in Example 4 at 0.1C.

[0036] Fig.11 This is the charge and discharge curve of the olive-type sodium iron phosphate prepared in Example 5 at 0.1C.

[0037] Fig.12 This is the charge and discharge curve of sodium iron phosphate prepared in Comparative Example 1 at 0.1C.

[0038] in, Figure 7-Figure 12 Potential is potential, and Specific capacity is specific capacity. DETAILED DESCRIPTION

[0039] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention. Without departing from the spirit and essence of the present invention, modifications or substitutions made to the methods, steps or conditions of the present invention all belong to the scope of protection of the present invention. Unless otherwise specified, the experimental materials, reagents, instruments, etc. used in the examples of the present invention are all commercially available; unless otherwise specified, all technical means in the examples of the present invention are conventional means well known to those skilled in the art.

[0040] A method for preparing an olivine-type sodium iron phosphate positive electrode material comprises the following steps:

[0041] Weigh the corresponding iron phosphate, sodium salt, and phosphate in a molar ratio of 1:1.1-2:0.1-1.3, weigh the carbon source, which is 5-10% of the total mass of the iron phosphate, sodium salt, and phosphate, mix the four in ethanol, and ball mill at 100-500 rpm for 0.5-12 hours;

[0042] The ball-milled mixture is heated and sintered in nitrogen or argon, the heating rate is 1-20°C / min, the sintering temperature is 190-410°C, and the sintering time is 2-24 hours.

[0043] The sintered product is cooled to room temperature to obtain an olivine-type sodium iron phosphate positive electrode material.

[0044] Among them, iron phosphate includes isophosphite-type iron phosphate, a recycled product of waste lithium iron phosphate; phosphates include diammonium phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate; sodium salts include sodium acetate, sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, and sodium hydroxide; carbon sources include glucose, sucrose, chitosan, citric acid, and tartaric acid.

[0045] Example 1

[0046] Weigh 15.0810 g of iron phosphate, a recovered product of waste lithium iron phosphate, 7.0979 g of disodium hydrogen phosphate as sodium salt, 11.9269 g of ammonium phosphate as phosphate, and 3.1883 g of glucose as a carbon source, all of which are placed in a ball mill, and ethanol is added and mixed, and ball milled at 100 rpm for 12 hours;

[0047] The ball-milled mixture was sintered in argon at a heating rate of 5°C / min, a sintering temperature of 190°C, and a sintering time of 24 hours.

[0048] The sintered product is cooled to room temperature to obtain an olivine-type sodium iron phosphate positive electrode material. The diffraction pattern of the material is as follows Figure 1 As shown, there are multiple diffraction angles in the range of 10° to 20° that show outstanding diffraction intensity, indicating that the prepared material corresponds to the diffraction peak of olivine-type sodium iron phosphate.

[0049] Example 2

[0050] Weigh 15.0810 g of iron phosphate, a recovered product of waste lithium iron phosphate, 8.4994 g of sodium nitrate as sodium salt, 6.9014 g of diammonium phosphate as phosphate, and 2.7282 g of glucose as a carbon source, all of which are placed in a ball mill, and ethanol is added and mixed, and ball milled at 200 rpm for 8 hours;

[0051] The ball-milled mixture was sintered in argon at a heating rate of 5°C / min, a sintering temperature of 260°C, and a sintering time of 18 hours.

[0052] The sintered product is cooled to room temperature to obtain an olivine-type sodium iron phosphate positive electrode material. The diffraction pattern of the material is as follows Figure 2As shown, there are multiple diffraction angles in the range of 10° to 20° that show outstanding diffraction intensity, indicating that the prepared material corresponds to the diffraction peak of olivine-type sodium iron phosphate.

[0053] Example 3

[0054] Weigh 15.0810 g of iron phosphate, a recovered product of waste lithium iron phosphate, 8.2030 g of sodium acetate as sodium salt, 9.2439 g of ammonium dihydrogen phosphate as phosphate, and 3.1207 g of glucose as a carbon source, all of which are placed in a ball mill, and ethanol is added and mixed, and ball milled at 300 rpm for 4 hours;

[0055] The ball-milled mixture was sintered in argon at a heating rate of 5°C / min, a sintering temperature of 330°C, and a sintering time of 12 hours.

[0056] The sintered product is cooled to room temperature to obtain an olivine-type sodium iron phosphate positive electrode material. The diffraction pattern of the material is as follows Figure 3 As shown, there are multiple diffraction angles in the range of 10° to 20° that show outstanding diffraction intensity, indicating that the prepared material corresponds to the diffraction peak of olivine-type sodium iron phosphate.

[0057] Example 4

[0058] Weigh 15.0810 g of iron phosphate, a recovered product of waste lithium iron phosphate, 11.9976 g of sodium dihydrogen phosphate as sodium salt and phosphate, and 2.7079 g of glucose as a carbon source, all of which are placed in a ball mill, and ethanol is added and mixed, and ball milled at 400 rpm for 4 hours;

[0059] The ball-milled mixture was sintered in argon at a heating rate of 5°C / min, a sintering temperature of 400°C, and a sintering time of 8 hours.

[0060] The sintered product is cooled to room temperature to obtain an olivine-type sodium iron phosphate positive electrode material. The diffraction pattern of the material is as follows Figure 4 As shown, there are multiple diffraction angles in the range of 10° to 20° that show outstanding diffraction intensity, indicating that the prepared material corresponds to the diffraction peak of olivine-type sodium iron phosphate.

[0061] Example 5

[0062] Weigh 15.0810 g of iron phosphate, a recovered product of waste lithium iron phosphate, 5.2994 g of sodium carbonate as sodium salt, 5.7512 g of ammonium dihydrogen phosphate as phosphate, and 2.7282 g of glucose as a carbon source, all of which are placed in a ball mill, and ethanol is added and mixed, and ball milled at 500 rpm for 2 hours;

[0063] The ball-milled mixture was sintered in argon gas at a heating rate of 5°C / min, a sintering temperature of 410°C, and a sintering time of 5 hours.

[0064] The sintered product is cooled to room temperature to obtain an olivine-type sodium iron phosphate positive electrode material. The diffraction pattern of the material is as follows Figure 5 As shown, there are multiple diffraction angles in the range of 10° to 20° that show outstanding diffraction intensity, indicating that the prepared material corresponds to the diffraction peak of olivine-type sodium iron phosphate.

[0065] Comparative Example

[0066] Weigh 15.0810 g of iron phosphate, a recovered product of waste lithium iron phosphate, 5.2994 g of sodium carbonate as sodium salt, and 2.0380 g of glucose as a carbon source, all of which are placed in a ball mill, and ethanol is added and mixed, and ball milled at 500 rpm for 2 hours;

[0067] The ball-milled mixture was sintered in argon gas at a heating rate of 5°C / min, a sintering temperature of 410°C, and a sintering time of 5 hours.

[0068] The sintered product is cooled to room temperature to obtain an olivine-type sodium iron phosphate positive electrode material. The diffraction pattern of the material is as follows Figure 6 As shown, the diffraction intensity corresponding to the diffraction angle in the range of 10° to 20° is basically consistent, indicating that the lack of phosphate addition has a significant effect on the diffraction peak intensity of sodium iron phosphate.

[0069] Example 6

[0070] The olivine-type sodium iron phosphate positive electrode materials obtained in the above Examples 1-5 and the comparative example are used as positive electrode materials for sodium ion batteries. The positive electrode material, acetylene black, and a binder are mixed in a weight ratio of 8:1:1, and a solvent (containing 5% by weight of fluoroethylene carbonate, ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1) is added and mixed evenly to prepare a positive electrode slurry. The positive electrode slurry is evenly coated on an aluminum foil, and the positive electrode plate is obtained after drying and slicing. A metal sodium plate is used as a negative electrode, and a glass fiber diaphragm is used as a diaphragm; the positive electrode plate, the negative electrode, and the diaphragm are placed in a button battery shell, and an electrolyte (the electrolyte of the electrolyte is NaPF 6 The concentration is 1 mol / L), and the sodium ion button half-cell is obtained after the battery shell is pressed. After being placed for 12 hours, it can be used for testing.

[0071] Three sodium ion button half-cells made of the same material were tested, including material specific capacity test and cycle performance test. The test results were the average of the three sample batteries (test results with a difference of more than 5% were considered abnormal values ​​and were discarded). The test results are shown in Table 1 below.

[0072] From Table 1 and Fig.12 It can be seen that due to the lack of phosphate in the comparative example, the obtained sodium iron phosphate has a discharge capacity of less than 100 mAh g at 0.1C. -1 , the capacity retention rate after 100 1C charge-discharge cycles is less than 90%. However, due to the excessive addition of phosphate, the olivine-type sodium iron phosphate obtained in Examples 1-5 has a discharge capacity of more than 100 mAh·g at 0.1C. -1 , the capacity retention rate after 100 1C charge-discharge cycles is higher than 90%. In Example 3, sodium acetate, a sodium salt with a low melting point, is added, and the sodium salt melts more fully during the calcination process, so the product has a better charge-discharge performance capacity retention rate at 1C. The reaction systems of Examples 1, 2, 4, and 5 are all compatible with high-melting-point sodium salts, which significantly improve the electrochemical performance of sodium iron phosphate prepared by using high-melting-point sodium salts as reaction raw materials compared with the comparative example.

[0073] Table 1 Electrochemical performance test results

[0074]

[0075] Based on the above experimental results, using iron phosphate, sodium salt, phosphate and carbon source as raw materials and increasing the proportion of phosphate in the preparation process can make the olivine sodium iron phosphate crystallize more completely during the sintering process, and the diffusion channel of sodium ions is more regular and continuous, which is conducive to the rapid diffusion of sodium ions inside the material, thereby improving the reversible specific capacity.

[0076] It can be understood that the above embodiments only express the preferred implementation modes of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the patent scope of the present invention. It should be pointed out that, for ordinary technicians in this field, the above technical features can be freely combined without departing from the concept of the present invention, and several deformations and improvements can be made, which all belong to the protection scope of the present invention. Therefore, all equivalent changes and modifications made to the scope of the claims of the present invention should belong to the coverage of the claims of the present invention.

Claims

1. A method for preparing an olivine-type sodium iron phosphate positive electrode material for a sodium ion battery, characterized in that: The following steps are involved: The iron phosphate, sodium salt, phosphate and carbon source are mixed in a dispersant and ball milled; The ball-milled mixture is sintered at elevated temperature in an inert gas atmosphere; The sintered product is cooled to room temperature to obtain an olivine-type sodium iron phosphate positive electrode material.

2. The preparation method according to claim 1, characterized in that: The molar ratio of the iron phosphate, the sodium salt and the phosphate is 1:1.1-2:0.1-1.

3.

3. The preparation method according to claim 2, characterized in that: The molar ratio of the phosphate is positively correlated with the electrochemical performance of the olivine-type sodium iron phosphate positive electrode material.

4. The preparation method according to claim 1, characterized in that: The mass ratio of the carbon source to the total mass of the iron phosphate, the sodium salt and the phosphate is 0.05 to 0.1:

1.

5. The preparation method according to claim 1, characterized in that: The crystal form of the iron phosphate is isophosphite type.

6. The preparation method according to claim 1, characterized in that: The phosphate includes any one of diammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, or a mixture of two or more thereof.

7. The preparation method according to claim 1, characterized in that: The sodium salt includes one or more of low melting point sodium salts such as sodium acetate, sodium bicarbonate, sodium dihydrogen phosphate, and high melting point sodium salts such as sodium carbonate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, and sodium hydroxide.

8. The preparation method according to claim 1, characterized in that: The carbon source includes any one of glucose, sucrose, chitosan, citric acid, and tartaric acid, or a mixture of two or more thereof.

9. The preparation method according to claim 1, characterized in that: The ball mill has a rotation speed of 100 to 500 rpm and a time of 0.5 to 12 hours.

10. The preparation method according to claim 1, characterized in that: The heating rate is 1-20°C / min, the sintering temperature is 190-410°C, and the sintering time is 2-24 hours.

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