JP7812000B2 - Method for producing sodium iron phosphate material and sodium iron phosphate material - Google Patents

Description

The present invention relates to the field of electrode materials, and specifically to a method for producing sodium iron phosphate material used as an electrode material in secondary batteries of alkali metals such as lithium, sodium, and potassium, and the sodium iron phosphate material.

Polyanionic compounds are excellent electrode materials for alkali metal secondary batteries due to their excellent ion transfer efficiency and favorable structural and thermal stability. Vanadium-iron sodium phosphate, a typical polyanionic compound, has attracted widespread attention due to its structural stability and high safety. Sodium iron phosphate, compared to sodium vanadium phosphate, has a wider range of raw material sources and lower costs, making it highly competitive in practical applications. Currently, conventional methods for producing sodium iron phosphate materials are limited to high-temperature solid-state synthesis, sol-gel methods, and electrochemical synthesis. These processes often involve harsh conditions, such as high vacuums, specific atmospheres, and high temperatures, resulting in relatively complicated operations and high production costs. The resulting sodium iron phosphate material has a relatively large microparticle size, typically on the order of μm. Because sodium iron phosphate has relatively low electronic conductivity, the large particle size limits the sodium storage performance of the resulting sodium iron phosphate material. To address issues such as the relatively low conductivity of polyanionic compounds such as sodium vanadium phosphate, members of this project team proposed the idea of nano-synthesis in Chinese patent applications (202210162593.6, 202210249477.8). Research demonstrated that small-particle polyanionic compounds contribute to improving the electron/ion transport performance of materials. It is anticipated that designing and manufacturing nano-sized sodium iron phosphate will improve the electron/ion transport efficiency of materials and result in sodium iron phosphate products with superior performance. However, to date, little research has been reported on technologies for efficiently manufacturing nano-sized sodium iron phosphate.

The present invention aims to provide a method for producing sodium iron phosphate material and the sodium iron phosphate material. It proposes a novel method for efficiently producing nano-sodium iron phosphate using a two-stage particle size control method under low-temperature conditions. Due to the relatively small Ksp of iron hydroxide and the relatively large reaction rate constant of the precipitation reaction, iron ions are rapidly precipitated under basic conditions to form an iron hydroxide precursor with a controllable particle size. Subsequently, nano-sodium iron phosphate material is obtained by utilizing the ion exchange reaction between anions such as PO ₄ ³⁻ , F ⁻ , and Cl ⁻ and OH ⁻ . The present invention utilizes the rapid nucleation by the precipitation reaction and the in-situ conversion by anion exchange to achieve the rapid and efficient production of nano-sodium iron phosphate products at room temperature.

In order to achieve the above object, the present invention employs the following technical means.
The nano-sodium iron phosphate material is prepared by a two-stage particle size control method under room temperature conditions, specifically comprising the following steps:

1) Solution A is prepared by dissolving a divalent or trivalent iron source in deionized water. The divalent or trivalent iron source is one or more of the following inorganic or organic divalent or trivalent iron salts: ferrous sulfate, ferric sulfate, ferrous chloride, ferric nitrate, and ferrous oxalate. The concentration of the iron source in Solution A is 0.1 to 5.0 mol/L.

2) Solution B is prepared by dissolving a basic substance in deionized water. The basic substance is one or more of the following substances whose aqueous solutions are basic: sodium hydroxide, potassium hydroxide, ammonia water, sodium carbonate, sodium bicarbonate, etc. The concentration of the basic substance in Solution B is 0.1 to 5.0 mol/L. Solution B is added dropwise to Solution A at a predetermined rate, and stirring is continued until the solution reaches a pH of 4 to 8. The solution is then centrifuged and filtered to obtain a slurry.

3) Prepare solution C by dissolving chloride and/or fluoride and phosphate in deionized water. The chloride is one or more of sodium chloride, potassium chloride, and ammonium chloride; the fluoride is one or more of sodium fluoride, potassium fluoride, and ammonium fluoride; and the phosphate is one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate. The slurry obtained in step 2) is dispersed in solution C, and stirring is continued for 0.5 to 24 hours. The mixture is then filtered and dried to obtain the desired product.

The concentration ratio of chloride and/or fluoride to phosphate in solution C is 0 to 1:10, and the molar concentration ratio of the divalent or trivalent iron source to phosphate is 1:1 to 1:3.

A sodium iron phosphate material manufactured by a manufacturing method for a sodium iron phosphate material, having a chemical formula of Na _x FeO _y PO ₄ F _a Cl _b (where 0<x≦3, 0≦y≦1, 0≦a+b≦1) and a nanoparticulate microtopography.

The particle size distribution is relatively concentrated. The manufacturing method of the present invention is simple and efficient, and can achieve efficient adjustment and control of the particle size of sodium iron phosphate particles under room temperature conditions, with the average particle size being concentrated between 50 and 200 nm. The sodium iron phosphate material of the present invention has good electrochemical performance and can be used as an electrode material for alkaline metal secondary batteries such as lithium, sodium, and potassium.

A key feature of this invention is the production of nano-sodium iron phosphate material using a two-stage particle size control method. Taking advantage of the relatively small Ksp of iron hydroxide and the relatively large reaction rate constant of the precipitation reaction, iron ions are rapidly precipitated to form iron hydroxide, thereby achieving the goal of controlling the particle size of the precursor. Furthermore, anion exchange is used to perform in situ ion exchange of anions such as PO ₄ ^3- , F ^- , and Cl ^- with OH ^- to achieve the tuning and control effect of polyanions. This dual control of the particle size of the target product results in a nano-sodium iron phosphate product with small particle sizes and uniform particle distribution. The rapid nucleation rate of the iron hydroxide precipitation reaction limits the growth of iron hydroxide crystal nuclei, thereby controlling the particle size of the precursor. The ion exchange process not only does not contribute to further nucleation, but also achieves in situ exfoliation of key components of the precursor through an in situ conversion mechanism, further reducing the particle size of the product. In other words, this two-stage particle size control process allows for the rapid and efficient production of nano-sodium iron phosphate.

The present invention has the following advantageous effects compared to the prior art.
Using a two-step particle size control method—iron precipitation under basic conditions and anion exchange—we were able to produce nano-sized sodium iron phosphate materials with small particle sizes and uniform particle size distributions. The rapid nucleation process of the precipitation reaction rapidly formed iron hydroxide precursors, effectively controlling the precursor particle size. The anion exchange process in situ converted anions such as _PO4 ^3- , ^F- , and ^Cl- into ^OH- , allowing the polyanions to adjust and control the structure (microstructure, electronic structure) and performance, resulting in the efficient production of nano-sized sodium iron phosphate. This production method effectively controlled the particle size and uniformity of the sodium iron phosphate particles, achieving a nanoscale particle size distribution. The introduction of anions such as ^F- and ^Cl- improved the ionic and electronic conductivity of the sodium iron phosphate material, enhancing its electrochemical performance. Furthermore, because the precipitation and ion exchange reactions are efficient, have short reaction times, and can both occur at room temperature, the preparation method of the present invention can be completed efficiently and quickly at room temperature and is easily scaled up for production. Due to its technical advantages and the structural and performance advantages of the produced products, this method is expected to overcome the limitations of the application of sodium iron phosphate-based materials in the energy storage field and may be widely used in research on electrode materials for alkaline metal secondary batteries such as lithium, sodium, and potassium. The method of the present invention has the advantages of simple operation, high efficiency, and amenable to industrial scale-up, facilitating research and application of sodium iron phosphate materials in the field of electrochemical energy storage.

1 is an XRD graph of sodium iron phosphate produced by the method of Example 1. 1 is an SEM characterization of the sodium iron phosphate material produced by the method of Example 1. 1 is a characterization of the sodium iron phosphate material produced by the method of Example 1 by EDS elemental analysis. 1 is a particle size distribution characterization of the sodium iron phosphate material produced by the method of Example 2. 1 is a BET characterization of the sodium iron phosphate material produced by the method of Example 2.

The following provides a clear and complete description of the technical means in the embodiments of the present invention with reference to examples of the present invention, but it is clear that the described embodiments are merely examples and do not limit the present invention.

Example 1
1) 0.01 mol of ferrous sulfate was weighed and dissolved in 50 mL of deionized water to prepare solution A.
2) 0.1 mol of sodium hydroxide was weighed out and dissolved in 50 mL of deionized water to prepare solution B. Solution B was added dropwise to solution A at a rate of 1 drop/second, and stirring was continued until the solution reached a pH of 5. The mixture was then centrifuged and filtered to obtain a slurry.
3) 0.01 mol of sodium fluoride and 0.03 mol of sodium dihydrogen phosphate were weighed and dissolved in 50 mL of deionized water to prepare solution C. The slurry obtained in step 2) was dispersed in solution C, and stirring was continued for 24 hours, followed by filtration and drying to obtain the target product.

As is clear from Figure 1, characteristic diffraction peaks of _Na3Fe (III) _OPO4F appear at 11.2°, 13.2°, 18.2°, 19.5°, and 23.1° on the graph, and these diffraction peaks have high intensity. This indicates that the method of Example 1 was successful in producing sodium iron phosphate, and that the obtained sodium iron phosphate had high purity and no impurities were generated.

As is clear from Figure 2, the sodium iron phosphate produced in Example 1 has a nanoparticulate microtopography, with a uniform particle size distribution that is primarily concentrated around 150 nm.

As is clear from Figure 3, the sodium iron phosphate produced in Example 1 contains elements such as Na, Fe, P, O, and F, with the ratio of each element being Na:Fe:P:O:F = 25:7:10:36:8, which is consistent with the stoichiometric ratio of the constituent elements in _Na3Fe (III) _OPO4F . This further demonstrates that the produced sodium iron phosphate sample is _Na3Fe (III) _OPO4F .

Example 2
1) 0.01 mol of ferrous chloride was weighed and dissolved in 50 mL of deionized water to prepare solution A.
2) 0.1 mol of sodium hydroxide was weighed out and dissolved in 50 mL of deionized water to prepare solution B. Solution B was added dropwise to solution A at a rate of 1 drop/second, and stirring was continued until the pH of the solution reached 7, followed by centrifugation and filtration to obtain a slurry.
3) 0.01 mol of potassium chloride and 0.02 mol of disodium hydrogen phosphate were weighed and dissolved in 50 mL of deionized water to prepare solution C. The slurry obtained in step 2) was dispersed in solution C, and stirring was continued for 6 hours. The mixture was filtered and dried to obtain the target product.

As is clear from Figure 4, the sodium iron phosphate produced in Example 2 has a uniform particle size distribution, with the particle sizes concentrated mainly around 150 nm.

As is clear from FIG. 5, the sodium iron phosphate produced in Example 2 has a relatively high specific surface area (31.8 m ² /g).

Example 3
1) 0.01 mol of ferrous oxalate was weighed and dissolved in 50 mL of deionized water to prepare solution A.
2) 0.1 mol of sodium bicarbonate was weighed out and dissolved in 50 mL of deionized water to prepare solution B. Solution B was added dropwise to solution A at a rate of 1 drop/second, and stirring was continued until the solution reached a pH of 6. The mixture was then centrifuged and filtered to obtain a slurry.
3) 0.02 mol of sodium phosphate was weighed and dissolved in 50 mL of deionized water to prepare solution C. The slurry obtained in step 2) was dispersed in solution C, and stirring was continued for 16 hours. The mixture was filtered and dried to obtain the target product.

Comparative Example 1
As a control, the sodium iron phosphate was prepared by a solid phase method with appropriate modifications based on the method for preparing sodium iron phosphate described in Chinese Patent Application No. 202210454752.X. The specific procedure is as follows:

1) Raw material mixing using the dry ball mill method: 150.82 g of iron phosphate, 103.00 g of sodium bicarbonate, and 22.62 g of glucose were weighed out and placed in a ball mill tank where the raw materials were mixed using the dry ball mill method. The rotation speed was set to 300 rpm/min, and the mixing time using the ball mill was set to 1 hour.

2) Raw material mixing using a wet ball mill: 300 g of deionized water was added to 1) and mixed using a high-energy wet ball mill. The ball mill rotation speed was set to 500 rpm/min, and the mixing time using the ball mill was set to 5 hours. The mixture was then dried to obtain a precursor powder material.

3) The precursor powder material was placed in a tubular furnace and subjected to high-temperature firing under an Ar atmosphere. Specifically, the temperature was raised to 350°C at 2°C/min and maintained at that temperature for 4 hours, then raised to 700°C at 5°C/min and maintained at that temperature for 12 hours. After cooling, the material was sieved and pulverized to obtain sodium iron phosphate positive electrode material.

Table 1 compares the specific discharge capacity of the sodium iron phosphate material produced by the method of Example 3 and the sodium iron phosphate material produced by Comparative Example 1.

As can be seen from Table 1, under conditions of a charge/discharge current density of 50 mA/g, the specific capacity of the sodium iron phosphate produced in Example 3 was 98 mAh/g, which was higher than the specific capacity (91 mAh/g) of the sodium iron phosphate sample produced by the method of Comparative Example 1, and the impedances of the corresponding button batteries were 160 Ω and 250 Ω, respectively. The nanosized sodium iron phosphate produced was found to have lower impedance and higher electronic/ionic conductivity, thereby exhibiting better sodium storage performance.

The above-described examples are intended to illustrate, not limit, the technical means of the present invention. While the present invention has been described in detail based on the above examples, those skilled in the art should understand that modifications and equivalent substitutions can be made to the present invention, and that all modifications and partial substitutions made without departing from the spirit and scope of the present invention are included within the scope of the claims of the present invention.

Claims (10)

A method for producing nano sodium iron phosphate material with the chemical formula Na _x FeO _y PO ₄ F _a Cl _b (where 0<x≦3, 0≦y≦1, 0≦a+b≦1) is used under room temperature conditions using a two-stage particle size control method, and specifically includes the following steps:
1) preparing solution A by dissolving a divalent or trivalent iron source in deionized water;
2) dissolving a basic substance in deionized water to prepare solution B, adding solution B dropwise to solution A, stirring continuously until the pH of the solution reaches 4 to 8, and then centrifuging and filtering to obtain a slurry; and
3) Dissolve chloride and/or fluoride and phosphate in deionized water to prepare solution C, disperse the slurry obtained in step 2) in solution C, continue stirring for 0.5 to 24 hours, filter, and dry to obtain the target product. The method for producing sodium iron phosphate material according to claim 1, characterized in that the divalent or trivalent iron source is one or more of ferrous sulfate, ferric sulfate, ferrous chloride, ferric nitrate, and ferrous oxalate. The method for producing sodium iron phosphate material described in claim 1, characterized in that the concentration of the iron source in solution A is 0.1 to 5.0 mol/L. The method for producing sodium iron phosphate material according to claim 1, characterized in that the basic substance is one or more of sodium hydroxide, potassium hydroxide, aqueous ammonia, sodium carbonate, and sodium bicarbonate. The method for producing sodium iron phosphate material described in claim 1, characterized in that the concentration of the basic substance in solution B is 0.1 to 5.0 mol/L. The method for producing sodium iron phosphate material according to claim 1, characterized in that the chloride is one or more of sodium chloride, potassium chloride, and ammonium chloride, the fluoride is one or more of sodium fluoride, potassium fluoride, and ammonium fluoride, and the phosphate is one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate. The method for producing sodium iron phosphate material described in claim 1, characterized in that in step 3), the concentration ratio of chloride and/or fluoride to phosphate in solution C is 0 to 1:10, and the molar concentration ratio of the divalent or trivalent iron source to phosphate is 1:1 to 1:3. A sodium iron phosphate material having the chemical formula Na _x FeO _y PO _{4 Fa} Cl _b (where 0<x≦3, 0<y≦1 , 0≦a+b≦1) and a nanoparticulate microtopography. The chemical formula is Na _x FeO _y P.O. ₄ F _a Cl _b (where x=3, 0≦y≦1, 0≦a+b≦1), and characterized in that the microtopography is nanoparticulate. The sodium iron phosphate material according to claim 8, characterized in that the average particle size of the sodium iron phosphate material is 50 to 200 nm.