Preparation method of high-capacity sodium iron phosphate positive electrode material

Abstract

The application relates to the technical field of batteries, and discloses a preparation method of a high-capacity sodium iron phosphate positive electrode material, aiming at the problems of element segregation leading to the generation of impurities, high drying energy consumption and a complex process flow in the existing sodium iron phosphate material preparation process, and providing a preparation method for improving the capacity of a sodium iron phosphate positive electrode material and the sodium iron phosphate positive electrode material, which comprises the following steps: preferentially grinding an iron source into fine particles, then pouring soluble raw materials into stirring to wait for solidification, and then directly sintering the solidified precursor.The method is instantaneously solidified when the slurry is uniformly stirred, and there is no fluidity, so that the influence of element segregation caused by the drying process of conventional fluid slurry is avoided, the prepared precursor has more uniform element distribution, better phase crystal type, higher purity and better electrochemical performance, and the competitiveness of the material in the low-cost market is effectively improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of batteries, in particular to a preparation method of a high-capacity sodium iron phosphate-ferric phosphate positive electrode material. BACKGROUND

[0002] Sodium iron phosphate-ferric phosphate (Na4Fe3(PO4)2P2O7) as a new type of sodium-ion battery positive electrode material has important application prospects in the field of low-cost energy storage due to its advantages of abundant raw materials, stable structure and environmental friendliness. However, its actual reversible capacity is generally lower than the theoretical value (about 128 mAh / g), which seriously restricts the industrialization process. This bottleneck is mainly due to the inherent defects of the traditional preparation process: the conventional wet process usually adopts iron source, sodium source and phosphorus source (partially soluble) to form a flowable slurry. In the subsequent drying process, due to the easy oxidation of iron ions and the difference in migration rate of different ions, serious element segregation inevitably occurs, and finally NaFeO4, Fe2O3 and other impurities are generated in the product, reducing the material purity and electrochemical activity. Although the solid-phase method avoids drying segregation, the mechanical mixing uniformity of the raw materials is poor, and the reaction is not sufficient, which often requires multiple grinding and sintering, resulting in high energy consumption and insufficient product crystal form integrity.

[0003] Although the existing improved technologies such as co-precipitation or sol-gel method can improve the uniformity of element distribution, they have problems such as complex process flow, large amount of organic solvent used, high production cost, etc., which are difficult to meet the needs of large-scale manufacturing. Especially for sodium iron phosphate-ferric phosphate with a multi-anion complex structure, the dispersion stability of elements in the liquid phase directly affects the uniformity of the precursor element distribution, and then determines the phase purity and crystal quality of the final product. Therefore, developing a preparation method with short process, low cost and high performance, which can eliminate element segregation while ensuring phase purity and grain integrity, has become a key path to break through the capacity limitation of the material. SUMMARY

[0004] The application provides a short-process preparation method for improving the capacity and purity of the material, aiming at the problems of element segregation leading to the generation of impurities, high drying energy consumption and complex process flow in the existing preparation process of sodium iron phosphate-ferric phosphate material. The method uses the synergistic effect of iron source preferential nanodispersion and high solid content slurry instantaneous solidification to skip the traditional drying step, block element migration segregation from the source, simplify the process chain and reduce production cost.

[0005] The technical scheme adopted by the application is as follows:

[0006] A preparation method of a high-capacity sodium iron phosphate-ferric phosphate positive electrode material, the preparation method comprising the following steps:

[0007] Step (1), insoluble iron source and dispersant are added into water, and the grinding particle size is controlled by a sand mill to form a high dispersion iron source slurry;

[0008] Step (2), soluble sodium source, phosphorus source and carbon source are added into the high dispersion iron source slurry to quickly dissolve and stir until the slurry is quickly solidified to form a non-fluid precursor block with a solid content of ≥ 35%; the precursor is directly solidified, which avoids the element segregation defect caused by the drying process of the fluid slurry.

[0009] Step (3), the solidified precursor is transferred to a sintering furnace, and sintered at a temperature of 500-600℃ for 8-12h under a nitrogen atmosphere to obtain a high-capacity sodium iron phosphate pyrophosphate positive electrode material.

[0010] Preferably, in step (1), the iron source contains ferrous oxalate, and the molar ratio of iron in the ferrous oxalate to the total iron source is ≥ 20%, at which concentration the ferrous oxalate exhibits high viscosity characteristics.

[0011] Preferably, in step (1), the dispersant is at least one of, but not limited to, citric acid, oxalic acid, polyacrylic acid or polymethacrylic acid.

[0012] Preferably, in step (1), the grinding particle size is controlled to D 50 =0.2-0.5μm.

[0013] Preferably, in step (2), the soluble sodium source and phosphorus source are salts with strong water retention capacity, which are one of, but not limited to, disodium hydrogen phosphate, sodium dihydrogen phosphate, disodium pyrophosphate and sodium pyrophosphate; and the carbon source is a mixture of glucose and sodium carbonate.

[0014] Preferably, in step (3), the temperature for sintering is 500-600℃, and the temperature rising rate is 3℃ / min.

[0015] The present application controls the solid content of the slurry and fully utilizes the characteristics of high water retention sodium, phosphorus source and high viscosity ferrous oxalate to convert their disadvantages in traditional wet process into advantages, which realizes the effect of simultaneously improving the electrochemical performance and reducing the cost.

[0016] Compared with the prior art, the present application has the following advantages:

[0017] (1) Instant solidification innovation: when the ferrous oxalate is ground in air, the Fe 2+Oxidation occurs and triggers severe hydrolysis, resulting in a sharp decrease in the pH value of the slurry system. This process is not simply thickening, but rather a synergistic effect with the water-retaining agent, precisely triggering the transient reversal and failure of the slurry Zeta potential, thereby disrupting the electrostatic stability system between particles, inducing an irreversible phase change from a flowing state to a solid state in the entire slurry, and generating a gel blank with a three-dimensional network structure. This instantaneous solidification effect can perfectly "lock" the uniform dispersion state of active substances, providing a guarantee for obtaining high-performance electrode materials with more uniform microstructure and more excellent electrochemical performance.

[0018] (2) In-situ reduction innovation: The traditional process uses stable iron phosphate (FePO4) as the iron source, and the iron is always +3. The present invention innovatively uses a mixed iron source of ferrous oxalate and iron phosphate. During the subsequent sintering stage in an inert atmosphere, ferrous oxalate will decompose to release reducing gases (CO, CO2, etc.), which form a local "micro-reduction atmosphere field" inside and around the material particles to promote the formation of pure phases.

[0019] (3) Microstructure regulation innovation: The gases produced by the decomposition of ferrous oxalate can act as soft templates, to some extent, inhibiting the excessive growth and hard agglomeration of particles during the sintering process, and helping to form primary particles with more uniform particle size distribution and better crystallinity, providing more optimal transport channels for the rapid migration of sodium ions.

[0020] (4) Segregation elimination mechanism innovation: The present invention first creates a method of completely eliminating element segregation at the mesoscale by constructing a slurry with ultra-high solid content (≥35%) and triggering its transient non-flowing solidification. The core of this mechanism lies in the synergistic effect of high water-retention sodium / phosphorus sources (such as disodium hydrogen phosphate) and high-viscosity iron sources (ferrous oxalate), which greatly bind the free water in the slurry; when all components are added, the system quickly reaches a supersaturated state, and water is firmly "locked" in the salt's crystalline hydrate grid and on the surface of nanoparticles, causing the slurry to lose its flowability in a very short time, and directly changing from a "liquid-solid" mixed state to a "solid-solid" uniform mixed and viscous state without water migration. This process completely skips the long liquid evaporation stage in traditional processes, thereby completely eliminating the long-range migration and segregation of ions (Na + , Fe 2+ / 3+ , PO4 3- ) caused by capillary effects, concentration gradients, and Brownian motion, and achieving the ultimate uniformity of the precursor from the macroscopic to the microscopic.

[0021] (5) Process flow is greatly simplified: The solidified precursor is directly sintered, eliminating the need for processes such as spray drying and crushing, thereby reducing energy consumption.

[0022] (6) Phase purity breakthrough: The nanoscale iron source and the non-segregation precursor work together to reduce the amount of impurities in the sintered product and increase the crystallinity.

[0023] (7) Cost and performance win-win: the process reduces the manufacturing cost while improving the electrochemical performance of the product. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is the XRD comparison chart of sodium iron pyrophosphate phosphate prepared by the present embodiment 1 and the comparative example 3.

[0025] Figure 2 is the 0.1C charge-discharge curve chart of the sodium ion battery prepared in the present embodiment 1.

[0026] Figure 3 is the solid-state precursor prepared in the present embodiment 1. DETAILED DESCRIPTION

[0027] The preparation method of a high-capacity sodium iron pyrophosphate phosphate positive electrode material is further described in detail below in combination with specific embodiments, which are only used for comparison and explanation purposes, and the present application is not limited to these embodiments.

[0028] Performance evaluation is tested by a discharge test, and the test method is as follows:

[0029] The prepared positive electrode material, conductive agent, and binder are mixed in a ratio of 90:5:5, dispersed in an appropriate amount of N-N-dimethylpyrrolidone, fully mixed and stirred to form a positive electrode slurry, uniformly coated on a positive electrode current collector carbon-coated aluminum foil, and a negative electrode is a sodium sheet, a glass fiber separator is used, and a sodium-containing sodium electrolyte is used. Electrolyte. After assembly, charge-discharge test is carried out within the electrochemical window of 2.0-4.0V, and the test rate is 0.1C. Example 1

[0030] 4.2 mol of iron phosphate and 1.8 mol of ferrous oxalate, 0.06 mol of citric acid, and deionized water (total slurry solid content 40%) are weighed and mixed, and after uniform mixing, a sand mill is added at a speed of 1500 rpm for sand milling for 4 hours to obtain a slurry with a particle size of D 50 =0.32 μm; the remaining 3.8 mol of sodium dihydrogen phosphate and 2.1 mol of sodium carbonate and 0.4 mol of glucose are added and dissolved by rapid stirring, and after reducing the speed, the slurry is instantaneously solidified into a solid, and the solid is taken out and transferred to a sintering furnace, and sintered at 3℃ / min to 500℃ under N2 atmosphere for 12 hours, and after air flow crushing, the sodium iron pyrophosphate phosphate positive electrode material is collected.

[0031] The test results of this example are as follows: the discharge test results: 0.1C discharge specific capacity 111.2 mAh / g, and the first efficiency is 94.1%.

[0032] Comparative Example 1

[0033] The preparation method of the positive electrode material in the present comparative example comprises the following steps:

[0034] Take 5.4 mol of iron phosphate and 0.6 mol of ferrous oxalate and 0.06 mol of citric acid and deionized water (total slurry solid content 40%) and mix them evenly, then add them to a sand mill at a speed of 1500 rpm for 4 hours to obtain a slurry with a particle size of D 50 =0.41 μm; the remaining 2.6 mol of sodium dihydrogen phosphate and 2.7 mol of sodium carbonate and 0.4 mol of glucose are added and dissolved under rapid stirring, and after reducing the speed, the slurry becomes unevenly coagulated, the mixture is taken out and transferred to a sintering furnace, and sintered at 3 ℃ / min to 500 ℃ under N2 atmosphere for 12 hours, and then collected after air flow crushing to obtain a sodium iron pyrophosphate phosphate positive electrode material.

[0035] The test results of the present comparative example: the discharge specific capacity of 0.1C is 89.6 mAh / g, and the initial efficiency is 88.5%.

[0036] Comparative Example 2

[0037] The preparation method of the positive electrode material in the present comparative example comprises the following steps:

[0038] Take 4.2 mol of iron phosphate and 1.8 mol of ferrous oxalate and deionized water (total slurry solid content 40%) and mix them evenly, then add them to a sand mill at a speed of 1500 rpm for 4 hours to obtain a slurry with a particle size of D 50 =0.51 μm; the remaining 3.8 mol of sodium dihydrogen phosphate and 2.1 mol of sodium carbonate and 0.4 mol of glucose are added and dissolved under rapid stirring, and after reducing the speed, the slurry is instantaneously coagulated into a solid, the solid is taken out and transferred to a sintering furnace, and sintered at 3 ℃ / min to 500 ℃ under N2 atmosphere for 12 hours, and then collected after air flow crushing to obtain a sodium iron pyrophosphate phosphate positive electrode material.

[0039] The test results of the present comparative example: the discharge specific capacity of 0.1C is 102.0 mAh / g, and the initial efficiency is 92.4%.

[0040] Comparative Example 3

[0041] The preparation method of the positive electrode material in the present comparative example comprises the following steps:

[0042] Take 4.2 mol of iron phosphate and 1.8 mol of ferrous oxalate and deionized water (total slurry solid content 20%) and mix them evenly, then add them to a sand mill at a speed of 1500 rpm for 4 hours to obtain a slurry with a particle size of D 50=0.46 μm slurry; the remaining 3.8 mol of sodium dihydrogen phosphate and 2.1 mol of sodium carbonate and 0.4 mol of glucose were added and dissolved under rapid stirring, and after reducing the stirring speed, the slurry became a thin paste with good fluidity. The mixture was taken out and transferred to a sintering furnace, and sintered at 3 ℃ / min to 500 ℃ under N2 atmosphere for 12 hours. After air flow crushing, the sodium iron phosphate positive electrode material was collected.

[0043] The test results of the present comparative example are as follows: the discharge specific capacity at 0.1 C was 82.6 mAh / g, and the initial efficiency was 90.6%.

[0044] Comparative Example 4

[0045] The preparation method of the positive electrode material in the present comparative example comprises the following steps:

[0046] 6 mol of iron phosphate and deionized water (total slurry solid content 40%) were weighed and mixed, and after being uniformly mixed, they were added to a sand mill and sand-milled at a speed of 1500 rpm for 4 hours to obtain a D 50 =0.47 μm slurry; the remaining 2 mol of sodium dihydrogen phosphate and 3 mol of sodium carbonate and 0.4 mol of glucose were added and dissolved under rapid stirring, and after reducing the stirring speed, the slurry became a thick slurry with partial fluidity. The mixture was taken out and transferred to a sintering furnace, and sintered at 3 ℃ / min to 500 ℃ under N2 atmosphere for 12 hours. After air flow crushing, the positive electrode material was collected.

[0047] The test results of the present comparative example are as follows: the discharge specific capacity at 0.1 C was 80.5 mAh / g, and the initial efficiency was 95.6%.

[0048] Comparative Example 5

[0049] The preparation method of the positive electrode material in the present comparative example comprises the following steps:

[0050] 6 mol of iron phosphate and deionized water (total slurry solid content 40%) were weighed and mixed, and after being uniformly mixed, they were added to a sand mill and sand-milled at a speed of 1500 rpm for 4 hours to obtain a D 50 =0.52 μm slurry; the remaining 2 mol of sodium dihydrogen phosphate and 3 mol of sodium carbonate and 0.4 mol of glucose were added and dissolved under rapid stirring, and after reducing the stirring speed, the slurry became a thick slurry with partial fluidity. The mixture was taken out and transferred to a sintering furnace, and sintered at 3 ℃ / min to 500 ℃ under N2 atmosphere for 12 hours. After air flow crushing, the positive electrode material was collected.

[0051] The test results of the present comparative example are as follows: the discharge specific capacity at 0.1 C was 77.0 mAh / g, and the initial efficiency was 89.6%.

[0052] Comparative Example 6

[0053] The preparation method of the positive electrode material in the present comparative example comprises the following steps:

[0054] Weigh 6 mol of iron phosphate and deionized water (total slurry solid content 30%) and mix them, then add them to a sand mill for sand grinding at a speed of 1500 rpm for 4 hours to obtain a slurry with a particle size of D 50 = 0.47 μm; the remaining 2 mol of sodium dihydrogen phosphate and 3 mol of sodium carbonate and 0.4 mol of glucose are added and dissolved under rapid stirring, and after the speed is reduced, the slurry becomes a dilute mud with good fluidity. The slurry is spray dried (inlet air temperature 240℃ / outlet air temperature 100℃) to obtain a precursor, which is sintered at 500℃ under N2 atmosphere at a rate of 3℃ / min for 12 hours, and then collected after air flow crushing to obtain the positive electrode material.

[0055] The test results of the present comparative example are as follows: the discharge specific capacity at 0.1C is 101.5 mAh / g, and the initial efficiency is 92.7%.

[0056] In summary, from the test results of the above examples and comparative examples, it can be seen that the processing technology provided by the present application can simplify the process, reduce the cost, improve the uniformity of the material and thus improve the electrochemical performance. Specifically, to achieve the effect of solidification, strict process control is required.

[0057] By comparing the results of Example 1 and 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. Ferrous oxalate has the disadvantage of thick slurry during grinding in actual process. In the present application, this feature is fully exploited by adding a proper proportion of ferrous oxalate to increase the viscosity of the slurry, which is a guarantee for obtaining a uniformly solidified slurry. When the content of ferrous oxalate decreases to 10%, even under the condition of high solid content, only a non-uniformly solidified curd-like solid can be obtained, which shows that even under the condition of adding water-retaining salt, the viscosity of the slurry is not enough, and a uniformly solidified slurry cannot be obtained.

[0058] 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 iron phosphate, the use of dispersant can avoid the agglomeration of fine particles in the slurry. The more uniform the slurry, the better the electrochemical performance of the material obtained by sintering.

[0059] 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 decreases to 20%, only a dilute mud-like slurry can be obtained, and the electrochemical performance of the material prepared by sintering is poor, with a discharge specific capacity of 82.6 mAh / g.

[0060] Comparative Example 4, Comparative Example 5 and Comparative Example 6 are the conventional processes for preparing sodium ferric phosphate pyrophosphate. As can be seen from Comparative Example 1, Comparative Example 4 and Comparative Example 6, the slurry cannot be solidified into a solid without adding ferrous oxalate and with a solid content of 30%. In Comparative Example 5, even if the solid content is increased to 40%, the slurry cannot be solidified and only a slightly thickened slurry is obtained. In Comparative Example 6, after the conventional process of spray sintering, the specific discharge capacity is 101.5 mAh / g and the electrochemical performance is at an ordinary level. However, using the process of the present application, the specific discharge capacity is obviously improved and is as high as 111.2 mAh / g. This is mainly due to the fact that the slurry is formed into a solid with uniform dispersion during the preparation process, there is no liquid flow in this state, the segregation of elements is obviously improved, and thus the electrochemical performance is better.

[0061] In addition, it should be noted that various specific technical features described in the foregoing embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, various possible combinations are not described again in the present application.

Claims

1. A method for preparing a high-capacity sodium iron phosphate pyrophosphate cathode material, characterized in that, The preparation method comprises the following steps: Step (1), adding iron source and dispersing agent into water, controlling the grinding particle size by sand mill to form high-dispersed iron source slurry; Step (2), adding soluble sodium source, phosphorus source and carbon source into the high-dispersed iron source slurry to quickly dissolve and stir the slurry to solidify rapidly to form non-fluid precursor block with solid content ≥ 35%; Step (3), transferring the solidified precursor to a sintering furnace, sintering under nitrogen atmosphere for 8-12 h to obtain high-capacity sodium iron phosphate pyrophosphate positive electrode material by sintering; In the step (1), the iron source contains ferrous oxalate, and the molar ratio of iron in the ferrous oxalate accounts for ≥ 20% of the total molar ratio of the iron source; In the step (2), the soluble sodium source and the phosphorus source are salts with strong water-retaining capacity, including but not limited to one of sodium hydrogen phosphate, sodium dihydrogen phosphate, disodium pyrophosphate and sodium pyrophosphate; and the carbon source is a mixture of glucose and sodium carbonate.

2. The method for preparing the high-capacity sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In the step (1), the dispersing agent is at least one of citric acid, oxalic acid, polyacrylic acid or polymethacrylic acid.

3. The method for preparing the high-capacity sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In the step (1), the grinding is controlled to a particle size of D 50 = 0.2 - 0.5 μm.

4. The method for preparing the high-capacity sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In the step (3), the temperature of the temperature-sintering is 500-600℃, and the temperature rising rate is 3℃ / min.

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