A sodium ferric pyrophosphate positive electrode material and a preparation method thereof

Abstract

The application discloses a sodium pyrophosphate iron phosphate positive electrode material and a preparation method thereof, and belongs to the technical field of synthesis of sodium ion battery positive electrode materials. 3.6 Fe 2.6 (PO4) 1.6 P2O7; the preparation method of the sodium pyrophosphate iron phosphate positive electrode material comprises the following steps: S1, dissolving a soluble chemical composition containing an iron source, a sodium source, a phosphorus source and a carbon source in deionized water to form a precursor solution, and stirring until uniform; S2, adding the precursor solution into 1-4 times the volume of a strong water-absorbing organic solvent, fully stirring to precipitate the precursor, and then performing reduced-pressure filtration to obtain a precursor with uniform element distribution; and S3, performing high-temperature sintering on the precursor to obtain the sodium pyrophosphate iron phosphate positive electrode material. The application greatly simplifies the preparation process of the sodium pyrophosphate iron phosphate material, and also ensures that the sodium pyrophosphate iron phosphate material has excellent electrochemical performance, and solves the problems of complicated production process, high energy consumption and poor optimization of impurity phases in the prior art.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion battery positive electrode material synthesis, and in particular to a sodium iron pyrophosphate positive electrode material and a preparation method thereof. Background Art

[0002] As lithium-ion batteries face increasing challenges in safety, cost, and resource availability, researchers are working to develop a new generation of low-cost and safe energy storage technologies. Against this backdrop, sodium-ion batteries have garnered widespread attention. While sodium-ion batteries share the same operating principles as lithium-ion batteries, they offer advantages over lithium-ion batteries, including cost advantages, superior rate capability, outstanding safety, and a wide operating temperature range. More importantly, sodium resources are widely available, unrestricted by geographical location, and are inexpensive and stable, making them a promising alternative to lithium-ion batteries.

[0003] At present, the positive electrode materials of sodium-ion batteries are mainly divided into three categories: layered transition metal oxides, polyanionic compounds and Prussian blue compounds. Among them, layered transition metal oxides have developed slowly due to their poor stability in air, and Prussian blue compounds have developed slowly due to the difficulty of water removal process. Polyanionic compounds have shown great energy storage potential due to their outstanding structural stability and long cycle life.

[0004] For polyanionic compounds, taking sodium iron pyrophosphate as an example, it has become the main hot material in current research because of its small volume change during sodium insertion and extraction, environmental friendliness, low cost, moderate operating voltage and high discharge specific capacity.

[0005] In the prior art, the preparation of sodium iron pyrophosphate positive electrode materials mainly has the following two problems:

[0006] First, the preparation methods mainly include ball milling solid-phase synthesis, sand milling-spray drying liquid-phase synthesis, freeze-drying synthesis, and sol-gel synthesis. However, the huge energy consumption, complicated production process, and equipment requirements brought about by these methods in generating precursors have increased the cost of material production, which is undoubtedly contrary to the price advantage of sodium-ion batteries.

[0007] For example, patent publication number: CN116344823A discloses a carbon-coated composite material and its preparation method and application (hereinafter referred to as "prior art 1"), which adopts sand milling-spray drying liquid phase synthesis precursor (that is, wet sand milling is used to produce precursor slurry, and then spray drying is used to evaporate the highly hygroscopic organic solvent to obtain the precursor). Its production cycle is long and the energy consumption is huge.

[0008] Secondly, the sodium iron pyrophosphate cathode material produced by conventional stoichiometric ratio inevitably contains Na2FeP2O7 and m-NaFePO4 impurities, which undoubtedly affects the electrochemical performance of the material. Although the existing technology 1 adopts the method of reducing the iron content in sodium iron pyrophosphate to reduce the NaFePO4 impurity phase in the material, it uses a concept similar to defects, and the entire adjustment process does not meet the charge conservation principle, that is, Na4Fe 3-x (PO4)2P2O7(0 <x<0.15),因此对于杂相的优化较为片面,未考虑到Na4Fe3(PO4)2P2O7和Na3Fe2(PO4)P2O7两个端点的情况。

[0009] In summary, it is necessary for those skilled in the art to improve the existing method for preparing sodium iron pyrophosphate positive electrode materials so that it not only has the characteristics of high synthesis efficiency, low synthesis energy consumption, and simple synthesis equipment, but also can solve the problem of impurities, thereby fully improving the competitive advantage of sodium iron pyrophosphate for sodium ion batteries. Summary of the Invention

[0010] The purpose of the present invention is to provide a sodium iron pyrophosphate cathode material and a preparation method thereof, so as to solve the problems of the prior art such as complicated production process, high energy consumption and poor impurity optimization.

[0011] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0012] One of the purposes of the present invention is to provide a sodium iron pyrophosphate positive electrode material, the molecular formula of the sodium iron pyrophosphate positive electrode material is Na 3.6 Fe 2.6 (PO4) 1.6 P2O7.

[0013] A second object of the present invention is to provide a method for preparing the above-mentioned sodium iron pyrophosphate positive electrode material, comprising the following steps:

[0014] Step S1, dissolving a soluble compound containing an iron source, a sodium source, a phosphorus source, and a carbon source in deionized water to form a precursor solution with a concentration of 0.2-2.4 mol / L, and stirring until uniform; the stoichiometric ratio of the sodium source, the iron source, the phosphorus source, and the carbon source in the soluble compound is 1.33-1.50:1:1.33-1.50:1.33-1.50;

[0015] Step S2, adding the precursor solution to 1-4 times the volume of a strongly water-absorbing organic solvent, stirring thoroughly to precipitate the precursor, and then filtering under reduced pressure to obtain a precursor with uniform element distribution;

[0016] Step S3: sintering the precursor at high temperature to obtain sodium iron pyrophosphate positive electrode material.

[0017] Preferably, the concentration of the precursor solution is 1.2 mol / L.

[0018] Specifically, the soluble compound composition includes the following substances in a molar ratio:

[0019] first soluble compound 2-3;

[0020] second soluble compound 3-4;

[0021] The third soluble compound 3-4.

[0022] Preferably, the first soluble compound is one or more of ferric citrate, ferric acetylacetonate, ferric nitrate, ferrous sulfate, and ferric hydroxide.

[0023] Preferably, the second soluble compound is one of sodium hydroxide, sodium carbonate, sodium acetate, sodium nitrate, sodium nitrite, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphoric acid, diammonium hydrogen phosphate, and sodium pyrophosphate, which contains both a sodium source and a phosphorus source, or a combination of two or more substances containing both a sodium source and a phosphorus source.

[0024] Preferably, the third soluble compound is one or more of soluble starch, citric acid, grape, sucrose and maltose.

[0025] Preferably, the strongly water-absorbing organic solvent is one or more of ethylene glycol, propylene glycol, N-methylpyrrolidone, and dimethylacetamide.

[0026] Preferably, in step S2, the precursor solution is added into 2.5 times the volume of a strongly water-absorbing organic solvent.

[0027] Specifically, step S3 includes the following steps:

[0028] Step S301: pre-calcining the precursor in an inert atmosphere at 300° C.-350° C. for 5-6 hours to obtain an intermediate product;

[0029] Step S302: calcining the intermediate product at 500° C.-550° C. in an inert atmosphere for 15-18 hours to obtain a sodium iron pyrophosphate positive electrode material.

[0030] The design principle of the present invention is:

[0031] First, the iron source, sodium source, phosphorus source, and carbon source are dissolved in deionized water to form a precursor solution. Then, a strongly hygroscopic organic solvent is added. The polar groups such as carbonyl (C=O) and hydroxyl (-OH) in the organic solvent interact with the hydrogen bonds between water molecules to form a hydrogen bond complex between the organic solvent and water, thereby separating the precursor and water. Finally, a precursor with uniform element distribution is obtained. In this way, after high-temperature sintering, the established sodium iron pyrophosphate cathode material, namely, Na 3.6 Fe 2.6 (PO4) 1.6 P2O7.

[0032] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0033] (1) Based on the difference in the preparation system, the organic solvent used in the present invention has a different main function from the existing process, such as the organic solvent used in the scheme recorded in the above-mentioned patent publication number: CN116344823A. The organic solvent in the present invention has the function of absorbing water, forming a hydrogen bond complex, and separating the precursor from the water; while the main function of the organic solvent in CN116344823A is to disperse the precursor as a dispersion medium for sand milling, and there is no change before and after use. Therefore, the present invention, through the selection of raw materials and process design, is more simple in the preparation process of the precursor, with a shorter cycle and lower energy consumption than the existing methods such as ball milling solid phase synthesis, sand milling-spray drying liquid phase synthesis, freeze drying synthesis and sol-gel synthesis, and the elements in the precursor are evenly distributed, and the product quality after high-temperature sintering can also be guaranteed. Experiments have shown that the electrochemical properties of the obtained sodium iron pyrophosphate positive electrode material are improved to a certain extent compared with the electrochemical properties of conventionally produced materials.

[0034] (2) The present invention designs the ratio of sodium source and iron source in the raw materials, which effectively reduces the generation of Na2FeP2O7 impurity phase or m-NaFePO4 impurity phase. The adjustment process always satisfies the conservation of charge and fully considers the two endpoints of Na4Fe3(PO4)2P2O7 and Na3Fe2(PO4)P2O7. Therefore, the optimization of the impurity phase is more comprehensive, which can more effectively improve the electrochemical performance of the sodium iron pyrophosphate positive electrode material and enhance the competitive advantage of the sodium iron pyrophosphate material for sodium ion batteries.

[0035] (3) The various links of the present invention are closely linked, complementary and related to each other. While greatly simplifying the preparation process of sodium iron pyrophosphate material, it also ensures that the sodium iron pyrophosphate material has excellent electrochemical properties. Therefore, the present invention is very suitable for large-scale promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1This is a schematic diagram of the process for preparing sodium iron pyrophosphate material according to Example 1 of the present invention.

[0037] Figure 2 This is the XRD pattern of the sodium iron pyrophosphate material prepared in Examples 1-3 of the present invention.

[0038] Figure 3 This is a charge and discharge curve diagram of the sodium iron pyrophosphate material prepared in Examples 1-3 of the present invention.

[0039] Figure 4 The XRD patterns of the sodium iron pyrophosphate materials prepared in Example 3 and the comparative example of the present invention are shown.

[0040] Figure 5 This is a graph showing the first charge and discharge curves of the sodium iron pyrophosphate material prepared in Example 3 of the present invention and the comparative example at 0.1C.

[0041] Figure 6 This is a magnification diagram of the sodium iron pyrophosphate material prepared in Example 3 of the present invention and the comparative example.

[0042] Figure 7 Schematic diagram of 200 cycles of sodium iron pyrophosphate materials prepared in Example 3 of the present invention and the comparative example at 1C. DETAILED DESCRIPTION

[0043] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with various embodiments.

[0044] Example 1

[0045] This embodiment provides a sodium iron pyrophosphate material with the molecular formula Na 3.6 Fe 2.6 (PO4) 1.6 P2O7 is prepared by precursor preparation + high temperature sintering. The whole preparation process is as follows Figure 1 shown.

[0046] The above process is described in detail below.

[0047] 1. Precursor Preparation

[0048] The precursor preparation is one of the main inventive points of this embodiment, which is characterized by simple process, high efficiency and low energy consumption. The specific process is as follows:

[0049] First, a soluble compound containing an iron source, a sodium source, a phosphorus source, and a carbon source is dissolved in deionized water to form a precursor solution, which is then stirred until homogeneous. The soluble compound in this example consists of 0.2 mol of ferric nitrate, 0.3 mol of sodium dihydrogen phosphate, and 0.3 mol of citric acid. The stoichiometric ratio of the sodium source, iron source, phosphorus source, and carbon source is 1.50:1:1.50:1.50. The resulting precursor solution has a concentration of 1.2 mol / L.

[0050] Next, the precursor solution is added to 3 times the volume of ethylene glycol, stirred for 1-2 hours to allow the precursor to be fully precipitated, and then filtered through a pressure-reducing funnel to obtain a precursor with uniform element distribution.

[0051] 2. High temperature sintering

[0052] The high-temperature sintering of this embodiment is divided into two steps: pre-firing and calcination. The first step is pre-firing: the precursor is pre-firing at 300°C in an inert atmosphere for 5 hours to obtain an intermediate product; then the second step is calcination: the intermediate product is pre-firing at 500°C in an inert atmosphere for 15 hours to obtain the final sodium iron pyrophosphate material.

[0053] Example 2

[0054] The difference from Example 1 is that in this embodiment:

[0055] 1. A soluble compound composition is composed of 0.3 mol of ferric nitrate, 0.4 mol of sodium dihydrogen phosphate and 0.4 mol of citric acid, wherein the stoichiometric ratio of the sodium source, the iron source, the phosphorus source and the carbon source is 1.33:1:1.33:1.33.

[0056] 2. Add the precursor solution to 2.5 times the volume of propylene glycol.

[0057] Example 3

[0058] The difference from Example 1 is that in this embodiment:

[0059] 1. A soluble compound composition is composed of 0.26 mol of ferric nitrate, 0.36 mol of sodium dihydrogen phosphate and 0.36 mol of citric acid, wherein the stoichiometric ratio of the sodium source, the iron source, the phosphorus source and the carbon source is 1.38:1:1.38:1.38.

[0060] 2. Add the precursor solution to 2.5 times the volume of N-methylpyrrolidone.

[0061] Example 4

[0062] The difference from Example 1 is that in this embodiment:

[0063] 1. A soluble compound composition is composed of 0.26 mol of ferric nitrate, 0.36 mol of sodium dihydrogen phosphate, and 0.36 mol of citric acid, wherein the stoichiometric ratio of the sodium source, iron source, phosphorus source, and carbon source is 1.38:1:1.38:1.38. The resulting precursor solution concentration is 0.6 mol / L.

[0064] 2. Add the precursor solution to 2.5 times the volume of dimethylacetamide.

[0065] 3. During high-temperature sintering, the precursor is pre-fired at 300°C in an inert atmosphere for 5 hours to obtain an intermediate product; then calcined: the intermediate product is pre-fired at 550°C in an inert atmosphere for 18 hours to obtain the final sodium iron pyrophosphate material.

[0066] Example 5

[0067] The difference from Example 1 is that in this embodiment:

[0068] 1. A soluble compound composition is composed of 0.26 mol of ferric nitrate, 0.36 mol of sodium dihydrogen phosphate, and 0.36 mol of citric acid, wherein the stoichiometric ratio of the sodium source, iron source, phosphorus source, and carbon source is 1.38:1:1.38:1.38. The resulting precursor solution concentration is 1.8 mol / L.

[0069] 2. Add the precursor solution to a mixed solvent of ethylene glycol and N-methylpyrrolidone with a volume of 2.5 times.

[0070] 3. During high-temperature sintering, the precursor is pre-fired at 350°C in an inert atmosphere for 6 hours to obtain an intermediate product; then calcined: the intermediate product is pre-fired at 500°C in an inert atmosphere for 15 hours to obtain the final sodium iron pyrophosphate material.

[0071] Example 6

[0072] The difference from Example 1 is that in this embodiment:

[0073] 1. A soluble compound composition is composed of 0.26 mol of ferric nitrate, 0.36 mol of sodium dihydrogen phosphate, and 0.36 mol of citric acid, wherein the stoichiometric ratio of the sodium source, iron source, phosphorus source, and carbon source is 1.38:1:1.38:1.38. The resulting precursor solution concentration is 2.4 mol / L.

[0074] 2. Add the precursor solution to a 2.5-fold volume of a mixed solvent of ethylene glycol and propylene glycol.

[0075] 3. In high temperature sintering, the precursor is pre-sintered at 350°C for 6h in inert atmosphere to obtain intermediate product; then calcination: the intermediate product is pre-sintered at 550°C for 18h in inert atmosphere to obtain the final sodium iron pyrophosphate material.

[0076] Comparative Example

[0077] The comparative example uses spray drying method to prepare sodium iron pyrophosphate material (Na 3.6 Fe 2.6 (PO4) 1.6 P2O7), the specific process is as follows:

[0078] First step: 0.26 mol of ferric nitrate, 0.36 mol of sodium dihydrogen phosphate and 0.36 mol of citric acid are weighed and dissolved in deionized water to form a precursor solution with a concentration of 1.2 mol / L, then stirred until uniform, wherein the stoichiometric ratio of sodium source, iron source, phosphorus source and carbon source is 1.38:1:1.38:1.38;

[0079] Second step: the precursor solution is sprayed by a spray dryer to obtain a precursor;

[0080] Third step: the precursor is sintered at 300°C and 500°C in inert atmosphere to obtain sodium iron pyrophosphate positive electrode material, the pre-sintering time is 5h, and the calcination time is 15h.

[0081] The following introduces the experimental conditions and conclusions of the above examples 1-6 and comparative example.

[0082] I. Experimental conditions and conclusions of examples 1-3 and comparative example

[0083] 1. According to Figure 2 It can be seen that in examples 1-3, by properly adjusting the ratio of sodium and iron in the precursor, the impurity phase of the obtained sodium iron pyrophosphate positive electrode material is significantly reduced, among which, example 3 does not exist Na2FeP2O7 impurity peak near 10.7° compared with example 1; does not exist m-NaFePO4 impurity peak near 33° compared with example 2, and the remaining diffraction peaks of example 3 all correspond to the standard card, which shows that the sodium iron pyrophosphate positive electrode material prepared in example 3 has high phase purity.

[0084] 2. According to Figure 3 It can be seen that example 3 has the highest discharge specific capacity and the most ideal charge-discharge curve compared with examples 1 and 2, which shows that the capacity of the material is improved due to the reduction of impurities.

[0085] 3. In Example 3 and the comparative example, there is no difference in crystal structure between the sodium iron pyrophosphate cathode material prepared in Example 3 and the sodium iron pyrophosphate cathode material prepared by conventional spray drying. Figure 4 As shown, each diffraction peak corresponds to the standard card one by one, and there is no impurity peak.

[0086] 4. According to Figure 5 It can be seen that in Example 3 and the control example, the discharge specific capacity of the sodium iron pyrophosphate positive electrode material prepared in Example 3 is slightly higher than that of the sodium iron pyrophosphate positive electrode material prepared by conventional spray drying.

[0087] 5. According to Figure 6 、 Figure 7 It can be seen that in Example 3 and the control example, there is no difference in rate performance and cycle performance between the sodium iron pyrophosphate positive electrode material prepared in Example 3 and the sodium iron pyrophosphate positive electrode material prepared by conventional spray drying.

[0088] II. Experimental Results of Different Precursor Solution Concentrations in Examples 3-6

[0089] In Examples 3-6, the optimal solution concentration was obtained by exploring the concentration of the precursor solution. As shown in Table 1, when the concentration of the precursor solution was below 1.2 mol / L, the capacity of the prepared materials was 112 mAh·g -1 When the concentration of the precursor solution exceeds 1.2 mol / L, the capacity of the prepared material decreases (Examples 5-6).

[0090] Table 1 Na prepared in Examples 3-6 3.6 Fe 2.6 (PO4) 1.6 The first charge and discharge and first coulombic efficiency results of P2O7 cathode material at 0.1C:

[0091]

[0092] This indicates that increasing the precursor solution concentration means an increase in the amount of water absorbed per unit volume of the organic solvent. Excessively high precursor solution concentrations lead to incomplete dehydration of the organic solvent, resulting in uneven distribution of precursor elements and reduced material capacity. Considering both the organic solvent utilization rate and the precursor yield, a precursor solution concentration of 1.2 mol / L is optimal in the present invention.

[0093] The present application scheme is simple in appearance, but is not easy to think. Only through in-depth study of the preparation characteristics of sodium pyrophosphate ferric phosphate material, combining the characteristics of raw materials, and paying creative labor, can high-performance sodium pyrophosphate ferric phosphate material be prepared in a simple and effective way, so that technology and efficiency, quality and cost are more compatible. Therefore, compared with the prior art, the present application has obvious technical progress, and has outstanding substantial characteristics and significant progress.

[0094] The above-described embodiments only express the specific implementation of the present application, which is described in more detail and in more detail, but cannot be understood as a limitation on the protection scope of the present application. It should be pointed out that for ordinary skilled persons in the art, without departing from the technical scheme concept of the present application, a number of modifications and improvements can be made, which belong to the protection scope of the present application.

Claims

1. A method for preparing sodium iron pyrophosphate positive electrode material, comprising the following steps: Step S1, dissolving a soluble compound containing an iron source, a sodium source, a phosphorus source, and a carbon source in deionized water to form a precursor solution with a concentration of 0.2-2.4 mol / L, and stirring until uniform; the stoichiometric ratio of the sodium source, the iron source, the phosphorus source, and the carbon source in the soluble compound is 1.33-1.50:1:1.33-1.50:1.33-1.50; Step S2, adding the precursor solution to 1-4 times the volume of a strongly hygroscopic organic solvent, stirring thoroughly, utilizing the hydrogen bonding interaction between the polar groups in the organic solvent and the water molecules to form a hydrogen bond complex between the organic solvent and water, thereby separating the precursor from the water, precipitating the precursor, and then filtering under reduced pressure to obtain a precursor with uniform element distribution; Step S3: sinter the precursor at high temperature to obtain a sodium iron pyrophosphate cathode material. The molecular formula of the sodium iron pyrophosphate cathode material is Na 3.6 Fe 2.6 (PO4) 1.6 P2O7.

2. The method according to claim 1, characterized in that The concentration of the precursor solution is 1.2 mol / L.

3. The method according to claim 2, characterized in that The iron source is one or more of ferric citrate, ferric acetylacetonate, ferric nitrate, ferrous sulfate and ferric hydroxide.

4. The method according to claim 3, characterized in that The sodium source and phosphorus source are one of sodium hydroxide, sodium carbonate, sodium acetate, sodium nitrate, sodium nitrite, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphoric acid, diammonium hydrogen phosphate, and sodium pyrophosphate, or a combination of two or more substances containing the sodium source and the phosphorus source.

5. The method according to claim 4, characterized in that The carbon source is one or more of soluble starch, citric acid, grape, sucrose and maltose.

6. The method according to any one of claims 1 to 5, characterized in that The strong water-absorbing organic solvent is one or more of ethylene glycol, propylene glycol, N-methylpyrrolidone and dimethylacetamide.

7. The method according to claim 6, characterized in that In the step S2, the precursor solution is added into 2.5 times the volume of a strongly water-absorbing organic solvent.

8. The method according to claim 1 or 2 or 3 or 4 or 5 or 7, characterized in that The step S3 comprises the following steps: Step S301: pre-calcining the precursor in an inert atmosphere at 300° C.-350° C. for 5-6 hours to obtain an intermediate product; Step S302: calcining the intermediate product at 500° C.-550° C. in an inert atmosphere for 15-18 hours to obtain a sodium iron pyrophosphate positive electrode material.

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