Preparation method of high-capacity ferric sodium pyrophosphate positive electrode material

Abstract

The invention relates to the technical field of batteries, discloses a preparation method of a high-capacity ferric sodium phosphate pyrophosphate positive electrode material, and aims to solve the problems of impurity phase generation, high drying energy consumption, complex process flow and the like caused by element segregation in an existing ferric sodium phosphate pyrophosphate material preparation process. The invention provides a preparation method for improving the capacity of a sodium ferric phosphate pyrophosphate positive electrode material and the sodium ferric phosphate pyrophosphate positive electrode material, and the preparation method comprises the following steps: preferentially grinding an iron source into fine particles, then pouring soluble raw materials into the fine particles, stirring and waiting for solidification, and then directly sintering a solidified precursor. According to the method, the slurry is instantly solidified when being uniformly stirred, fluidity does not exist, the element segregation influence caused by conventional fluidity slurry in the drying process is avoided, the prepared precursor element distribution is more uniform, the phase crystal form is better, the purity is higher, the electrochemical performance is better, and the competitiveness of the material in the low-cost market is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a method for preparing a high-capacity sodium iron phosphate pyrophosphate positive electrode material. Background Art

[0002] Sodium ferric pyrophosphate (Na4Fe3(PO4)2P2O7), a novel cathode material for sodium-ion batteries, holds significant promise for low-cost energy storage applications due to its abundant raw materials, stable structure, and environmentally friendly properties. However, its actual reversible capacity is generally lower than its theoretical value (approximately 128 mAh / g), severely hindering its industrialization. This bottleneck stems primarily from inherent flaws in traditional preparation methods. Conventional wet processes typically blend an iron source with a sodium source and a partially soluble phosphorus source to form a fluid slurry. During the subsequent drying process, the readily oxidizable iron ions and the varying migration rates of different ions inevitably lead to severe elemental segregation. This leads to the formation of impurities such as NaFeO4 and Fe2O3 in the final product, reducing its purity and electrochemical activity. While solid-phase methods avoid drying segregation, they suffer from poor mechanical mixing of the raw materials, resulting in incomplete reactions and the need for multiple grinding and sintering steps. This results in high energy consumption and insufficient product crystal integrity.

[0003] Although existing improved technologies such as co-precipitation or sol-gel methods can improve the uniformity of element distribution, they have problems such as complex process flow, large amount of organic solvents used, and high production costs, making it difficult to meet the needs of large-scale manufacturing. Especially for multi-anion composite structures such as sodium iron phosphate pyrophosphate, the dispersion stability of elements in the liquid phase directly affects the uniformity of the precursor element distribution, which in turn determines the phase purity and crystal quality of the final product. Therefore, developing a preparation method that combines short process, low cost and high performance, while eliminating element segregation and ensuring phase purity and grain integrity, has become a key path to breaking through the capacity limitations of this material. Summary of the Invention

[0004] This invention addresses the challenges of existing sodium ferric pyrophosphate (SFP) material preparation processes, including elemental segregation leading to impurity formation, high drying energy consumption, and complex process flows. By providing a shortened process to improve material capacity and purity, this method utilizes the synergistic effect of preferential nano-dispersion of the iron source and the instantaneous solidification of a high-solids slurry, bypassing the traditional drying step and preventing elemental migration and segregation at the source. This method also simplifies the process chain and reduces production costs.

[0005] The technical solution adopted in the present invention is as follows: A method for preparing a high-capacity sodium iron pyrophosphate positive electrode material, the preparation method comprising the following steps: Step (1), adding an insoluble iron source and a dispersant into water, and controlling the grinding particle size by a sand mill to form a highly dispersed iron source slurry; Step (2): adding a soluble sodium source, a phosphorus source, and a carbon source to the highly dispersed iron source slurry, rapidly dissolving and stirring until the slurry rapidly solidifies to form a non-flowable precursor block with a solid content of ≥35%; the direct solidification of the precursor eliminates the conventional spray drying step, thus avoiding the disadvantage of element segregation caused by the drying process of the flowable slurry.

[0006] Step (3): transferring the solidified precursor to a sintering furnace and sintering it at elevated temperature for 8-12 hours under a nitrogen atmosphere to obtain a high-capacity sodium iron pyrophosphate positive electrode material.

[0007] Preferably, in step (1), the iron source contains ferrous oxalate, and the molar ratio of iron in the ferrous oxalate in the iron source accounts for ≥20% of the molar ratio of the total iron source. At this concentration, ferrous oxalate exhibits the characteristic of high viscosity.

[0008] Preferably, in step (1), the dispersant is at least one of, but not limited to, citric acid, oxalic acid, polyacrylic acid or polymethacrylic acid.

[0009] Preferably, in the step (1), during grinding, the grinding particle size is controlled to D 50 =0.2-0.5μm.

[0010] Preferably, in step (2), the soluble sodium source and phosphorus source are salts with strong water-retaining capacity, including but not limited to one of disodium hydrogen phosphate, sodium dihydrogen phosphate, disodium dihydrogen pyrophosphate, and sodium pyrophosphate; and the carbon source is a mixture of glucose and sodium carbonate.

[0011] Preferably, in step (3), the sintering temperature is 500-600°C, and the heating rate is 3°C / min.

[0012] The present invention controls the solid content of the slurry and fully utilizes the properties of high water retention sodium, phosphorus source and high viscosity ferrous oxalate, turning their disadvantages in traditional wet processes into advantages, taking a different approach and achieving the effects of simultaneously improving electrochemical performance and reducing costs in the process.

[0013] Compared with the prior art, the present invention has the following significant advantages: (1) Instantaneous solidification innovation: When ferrous oxalate is ground in air, its Fe 2+ Oxidation occurs and triggers violent hydrolysis, causing a sharp drop in the pH value of the slurry system. This process is not a simple thickening process, but rather a synergistic effect with the water-retaining agent, precisely triggering the instantaneous reversal and failure of the slurry's Zeta potential, thereby disrupting the electrostatic stabilization system between particles and inducing an irreversible phase transition from a fluid state to a solid state throughout the slurry, forming a gel body with a three-dimensional network structure. This instantaneous solidification effect perfectly "locks" the uniform dispersion of the active material, ensuring the production of high-performance electrode materials with more uniform microstructures and superior electrochemical properties.

[0014] (2) In-situ reduction innovation: The traditional process uses stable iron phosphate (FePO4) as the iron source, and the iron is always +3 valent. The present invention innovatively uses a mixed iron source of ferrous oxalate and iron phosphate. During the subsequent inert atmosphere sintering stage, the ferrous oxalate decomposes to release reducing gases (CO, CO2, etc.). These gases form a local "micro-reducing atmosphere field" inside and around the material particles, promoting the formation of pure phases.

[0015] (3) Innovation in microstructure regulation: The gas produced by the decomposition of ferrous oxalate can serve as a soft template, which can inhibit the excessive growth and hard agglomeration of particles during the sintering process to a certain extent, and help form primary particles with more uniform particle size distribution and better crystallinity, providing a better transmission channel for the rapid migration of sodium ions.

[0016] (4) Innovation in segregation elimination mechanism: The present invention pioneered a method to completely eliminate element segregation at the mesoscopic scale by constructing an ultra-high solid content (≥35%) slurry and triggering its instantaneous non-fluid solidification. The core of this mechanism is: utilizing the synergistic effect of a high water-retaining sodium / phosphorus source (such as disodium hydrogen phosphate) and a high-viscosity iron source (ferrous oxalate) to greatly constrain the free water in the slurry; when all components are added, the system quickly reaches an oversaturated state, and the water is firmly "locked" in the salt crystal hydrate grid and the surface of the nanoparticles, causing the slurry to lose fluidity in a very short time, and directly transform from a "liquid-solid" mixed state to a "solid-solid" uniform mixed viscous state where water does not migrate. This process completely skips the long liquid phase evaporation stage in the traditional process, thereby eliminating the ion (Na) caused by capillary effect, concentration gradient and Brownian motion from the root. + ,Fe 2+ / 3+ ,PO4 3- ) long-range migration and segregation, achieving the ultimate uniformity of the precursor from macro to micro.

[0017] (5) The process flow is extremely simplified: the solidified precursor is directly sintered, eliminating the spray drying, crushing and other processes, and reducing energy consumption.

[0018] (6) Breakthrough in phase purity: The nano-iron source and the non-segregated precursor work together to reduce the impurities in the sintered product and increase the crystallinity.

[0019] (7) Win-win situation of cost and performance: This process reduces manufacturing costs while improving the electrochemical performance of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 3 is an XRD comparison diagram of sodium ferric pyrophosphate prepared in Example 1 of the present invention and Comparative Example 3.

[0021] Figure 2This is a 0.1C charge and discharge curve of the sodium ion battery prepared in Example 1 of the present invention.

[0022] Figure 3 It is the solid precursor prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0023] The following is a further detailed description of a method for preparing a high-capacity sodium iron pyrophosphate positive electrode material in conjunction with specific examples. These examples are only for comparison and explanation purposes, and the present invention is not limited to these examples.

[0024] The performance evaluation is carried out through the power-off test, and the test method is as follows: The prepared positive electrode material, conductive agent, and binder were mixed in a 90:5:5 ratio and dispersed in an appropriate amount of NN-dimethylpyrrolidone. The mixture was thoroughly stirred to form a positive electrode slurry, which was then evenly coated onto a carbon-coated aluminum foil for the positive electrode current collector. The negative electrode was a sodium sheet, the separator was a glass fiber separator, and the electrolyte was a sodium electrolyte containing sodium hexafluorophosphate. After assembly, charge and discharge tests were performed within the electrochemical window of 2.0-4.0V at a test rate of 0.1C. Example 1

[0025] Weigh 4.2 mol of ferric phosphate and 1.8 mol of ferrous oxalate, mix them with 0.06 mol of citric acid and deionized water (total slurry solid content 40%), mix them evenly, put them into a sand mill and grind them at 1500 rpm for 4 hours to obtain D 50 =0.32 μm slurry; the remaining 3.8 mol of sodium dihydrogen phosphate, 2.1 mol of sodium carbonate and 0.4 mol of glucose were added and dissolved by rapid stirring. After reducing the rotation speed, the slurry instantly solidified into a solid state. The solid was taken out and transferred to a sintering furnace. The temperature was increased to 500 ° C at 3 ° C / min under N2 atmosphere and sintered for 12 hours. After air flow crushing, it was collected to obtain sodium iron pyrophosphate positive electrode material.

[0026] Test results of this embodiment: Buckle test results: 0.1C discharge specific capacity 111.2 mAh / g, first efficiency 94.1%.

[0027] Comparative Example 1 The preparation method of the positive electrode material in this comparative example includes the following steps: Weigh 5.4 mol of ferric phosphate and 0.6 mol of ferrous oxalate, mix them with 0.06 mol of citric acid and deionized water (total slurry solid content 40%), mix them evenly, put them into a sand mill and grind them at 1500 rpm for 4 hours to obtain D 50=0.41 μm slurry; the remaining 2.6 mol of sodium dihydrogen phosphate, 2.7 mol of sodium carbonate and 0.4 mol of glucose were added and dissolved by rapid stirring. After the rotation speed was reduced, the slurry became an uneven curd. The mixture was taken out and transferred to a sintering furnace. The temperature was raised to 500 ° C at 3 ° C / min in a N2 atmosphere and sintered for 12 hours. After air flow crushing, it was collected to obtain sodium iron pyrophosphate positive electrode material.

[0028] The test results of this comparative example are as follows: the battery-withdrawal test results: 0.1C discharge capacity is 89.6 mAh / g, and the first efficiency is 88.5%.

[0029] Comparative Example 2 The preparation method of the positive electrode material in this comparative example includes the following steps: Weigh 4.2 mol of ferric phosphate, 1.8 mol of ferrous oxalate and deionized water (total slurry solid content 40%), mix them evenly, add them into a sand mill and sand grind them at 1500 rpm for 4 hours to obtain D 50 =0.51 μm slurry; the remaining 3.8 mol of sodium dihydrogen phosphate, 2.1 mol of sodium carbonate and 0.4 mol of glucose were added and dissolved by rapid stirring. After reducing the rotation speed, the slurry instantly solidified into a solid state. The solid was taken out and transferred to a sintering furnace. The temperature was raised to 500°C at 3°C / min under N2 atmosphere and sintered for 12 hours. After being crushed by air flow, it was collected to obtain sodium iron pyrophosphate positive electrode material.

[0030] The test results of this comparative example: The power-down test results: 0.1C discharge specific capacity is 102.0 mAh / g, and the first efficiency is 92.4%.

[0031] Comparative Example 3 The preparation method of the positive electrode material in this comparative example includes the following steps: Weigh 4.2 mol of ferric phosphate, 1.8 mol of ferrous oxalate and deionized water (total slurry solid content 20%), mix them evenly, add them into a sand mill and grind them at 1500 rpm for 4 hours to obtain D 50 =0.46 μm slurry; the remaining 3.8 mol of sodium dihydrogen phosphate, 2.1 mol of sodium carbonate and 0.4 mol of glucose were added and dissolved by rapid stirring. After reducing the rotation speed, the slurry became a thin mud with good fluidity. The mixture was taken out and transferred to a sintering furnace. The temperature was increased to 500 ° C at 3 ° C / min under N2 atmosphere and sintered for 12 hours. After air flow crushing, it was collected to obtain sodium iron pyrophosphate positive electrode material.

[0032] The test results of this comparative example: The power-off test results: 0.1C discharge capacity is 82.6mAh / g, and the first efficiency is 90.6%.

[0033] Comparative Example 4 The preparation method of the positive electrode material in this comparative example includes the following steps: Weigh 6 mol of ferric phosphate and deionized water (total slurry solid content 30%), mix well, add to a sand mill and sand grind at 1500 rpm for 4 hours to obtain D 50 =0.47μm slurry; the remaining 2mol of sodium dihydrogen phosphate, 3mol of sodium carbonate and 0.4mol of glucose were added and quickly stirred to dissolve. After reducing the rotation speed, the slurry became a thin mud with good fluidity. The mixture was taken out and transferred to a sintering furnace. The temperature was raised to 500℃ at 3℃ / min under N2 atmosphere and sintered for 12 hours. After airflow crushing, it was collected to obtain the positive electrode material.

[0034] The test results of this comparative example: The power-down test results: 0.1C discharge capacity is 80.5mAh / g, and the first efficiency is 95.6%.

[0035] Comparative Example 5 The preparation method of the positive electrode material in this comparative example includes the following steps: Weigh 6 mol of ferric phosphate and deionized water (total slurry solid content 40%), mix well, add to a sand mill and grind at 1500 rpm for 4 hours to obtain D 50 =0.52μm slurry; the remaining 2mol of sodium dihydrogen phosphate, 3mol of sodium carbonate, and 0.4mol of glucose were added and rapidly stirred to dissolve. The rotation speed was reduced to a thick slurry with some fluidity. The mixture was removed and transferred to a sintering furnace. The temperature was increased at 3°C / min to 500°C under an N2 atmosphere and sintered for 12 hours. The positive electrode material was then collected after airflow pulverization.

[0036] The test results of this comparative example: The power-off test results: 0.1C discharge specific capacity is 77.0mAh / g, and the first efficiency is 89.6%.

[0037] Comparative Example 6 The preparation method of the positive electrode material in this comparative example includes the following steps: Weigh 6 mol of ferric phosphate and deionized water (total slurry solid content 30%), mix well, add to a sand mill and sand grind at 1500 rpm for 4 hours to obtain D 50 =0.47μm slurry; the remaining 2mol of sodium dihydrogen phosphate, 3mol of sodium carbonate and 0.4mol of glucose were added and dissolved by rapid stirring. After reducing the rotation speed, the slurry became a thin mud with good fluidity. The slurry was spray-dried (inlet air temperature 240℃ / outlet air temperature 100℃) to obtain a precursor, which was heated to 500℃ at 3℃ / min in an N2 atmosphere and sintered for 12 hours. After being pulverized by air flow, it was collected to obtain the positive electrode material.

[0038] The test results of this comparative example are as follows: the battery charge test results: 0.1C discharge capacity is 101.5mAh / g, and the first efficiency is 92.7%.

[0039] In summary, the test results of the above examples and comparative examples show that the processing technology provided by the present invention can simplify the process and reduce costs while improving the uniformity of the material and thus improving the electrochemical performance. Specifically, to achieve the coagulation effect of this processing technology, strict process control is required.

[0040] By comparing the results of Example 1 with those of Comparative Example 1, it can be seen that the proportion of ferrous oxalate in the slurry is one of the conditions for obtaining a uniformly solidified slurry. In actual processes, ferrous oxalate has the disadvantage of making the slurry viscous during the grinding process. This feature is fully exploited in the present invention. Adding an appropriate proportion of ferrous oxalate to increase the viscosity of the slurry is a guarantee for obtaining a uniformly solidified slurry. When the content of ferrous oxalate drops to 10%, even under high solid content conditions, only uneven curd-like solids can be obtained, indicating that even with the addition of water-retaining salt, the slurry is not viscous enough and a uniformly solidified slurry cannot be obtained.

[0041] By comparing the results of Example 1 and Comparative Example 2, it can be seen that the dispersant is a necessary factor for obtaining a uniform material. During the grinding of ferric phosphate, the use of the dispersant can prevent the agglomeration of fine particles in the slurry. The more uniform the slurry is, the better the electrochemical properties of the sintered material will be.

[0042] By comparing the results of Example 1, Comparative Example 2 and Comparative Example 3, it can be seen that the solid content of the slurry is one of the conditions for obtaining a uniformly solidified slurry. When the solid content of the slurry is reduced to 20%, only a thin muddy slurry can be obtained, and the electrochemical performance of the sintered material is poor, with a discharge specific capacity of 82.6 mAh / g.

[0043] Comparative Example 4, Comparative Example 5 and Comparative Example 6 are conventional processes for preparing sodium ferric phosphate pyrophosphate. It can be seen from Comparative Example 1, Comparative Example 4 and Comparative Example 6 that, with 30% solid content and without the addition of ferrous oxalate, the slurry cannot be solidified into a solid. In Comparative Example 5, even if the solid content is continued to be increased to 40%, it cannot be solidified, and only a slightly thick fluidity slurry is obtained. In Comparative Example 6, after conventional process spray sintering, the discharge specific capacity is 101.5mAh / g, and the electrochemical performance reaches an ordinary level. However, using the process of the present invention, the discharge specific capacity is significantly improved, reaching a maximum of 111.2mAh / g. This is mainly due to the fact that the slurry forms a uniformly dispersed solid during the preparation process, and there is no flowing liquid in this state, and the segregation problem of the element is significantly improved, so that the electrochemical performance is better.

[0044] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. To avoid unnecessary repetition, the present invention will not further describe various possible combinations.

Claims

1. A method for preparing a high-capacity sodium iron pyrophosphate positive electrode material, characterized in that: The preparation method comprises the following steps: Step (1), adding an iron source and a dispersant into water, and controlling the grinding particle size by a sand mill to form a highly dispersed iron source slurry; Step (2), adding a soluble sodium source, a phosphorus source, and a carbon source to the highly dispersed iron source slurry, rapidly dissolving and stirring until the slurry rapidly solidifies to form a non-flowable precursor block with a solid content of ≥35%; Step (3): transferring the solidified precursor to a sintering furnace and sintering it at elevated temperature for 8-12 hours under a nitrogen atmosphere to obtain a high-capacity sodium iron pyrophosphate positive electrode material.

2. The method for preparing a high-capacity sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: In the step (1), the iron source contains ferrous oxalate, and the molar ratio of iron in the ferrous oxalate in the iron source accounts for ≥20% of the molar ratio of the total iron source.

3. The method for preparing a high-capacity sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: In the step (1), the dispersant includes but is not limited to at least one of citric acid, oxalic acid, polyacrylic acid or polymethacrylic acid.

4. The method for preparing a high-capacity sodium iron pyrophosphate positive electrode material according to claim 1, wherein: In the step (1), during grinding, the grinding particle size is controlled to D 50 =0.2-0.5μm.

5. The method for preparing a high-capacity sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: In step (2), the soluble sodium source and phosphorus source are salts with strong water-retaining capacity, including but not limited to one of disodium hydrogen phosphate, sodium dihydrogen phosphate, disodium dihydrogen pyrophosphate, and sodium pyrophosphate; and the carbon source is a mixture of glucose and sodium carbonate.

6. The method for preparing a high-capacity sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: In the step (3), the sintering temperature is 500-600°C, and the heating rate is 3°C / min.

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