Method for preparing moisture-resistant sodium ferric sulfate from pyrite

Abstract

The invention relates to the technical field of sodium ion batteries, and particularly discloses a method for preparing moisture-resistant sodium ferric sulfate from pyrite. Aiming at the impurity characteristics of pyrite, a three-step purification process is innovatively adopted: 1) alkali washing to reduce aluminum and remove silicon; 2) deep oxidation with hydrogen peroxide; and 3) adding iron powder for efficient reduction. The purified iron source, the sodium source and the organic carbon source are subjected to spray drying and low-temperature sintering to directly synthesize the sodium ferric sulfate positive electrode material, then the inorganic carbon source is added for coating to enhance the moisture resistance, and finally the multi-element doped high-performance sodium ferric sulfate positive electrode material is obtained. According to the method, pyrite is used for replacing a high-purity reagent, the problems of low phase purity and capacity fading caused by impurities are solved by accurately controlling the alkali concentration and the oxidizing agent, and the impurity removal rate gt is achieved; and activity of 98% of ferrous, trace aluminum, copper, calcium and titanium is retained. The obtained material can be directly used for large-scale energy storage battery positive electrode manufacturing, and the raw material cost is reduced by more than 40%.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium-ion battery cathode material preparation, and particularly relates to a low-cost method for synthesizing a sodium ferrosulfate cathode material by taking pyrite as an iron source. In view of problems such as low material purity and poor electrochemical activity caused by a large amount of silicon and aluminum impurities associated in the pyrite, a synergistic process of "alkali washing and impurity removal-oxidation and reduction refining" is developed to realize efficient removal of silicon impurities and stable retention of ferrous, trace aluminum, copper, calcium and titanium activities. Further, high-performance sodium ferrosulfate cathode materials are synthesized by combining spray drying and high-energy ball milling technologies. The method breaks through the dependence of traditional processes on high-purity chemical reagents and is suitable for low-cost manufacturing of large-scale energy storage battery cathodes. BACKGROUND

[0002] High-energy-density iron-based polyanion cathode materials have attracted much attention in recent years. Sodium ferrosulfate, as an excellent low-cost iron-based cathode material, has a reversible platform as high as 3.8 V and a theoretical specific capacity of 121 mAh•g -1 -1 in polyanion materials. The current mainstream process relies on reagent-grade ferrous sulfate, and its industrialization is subject to high-purity iron sources, with raw material costs accounting for more than 60%. To solve this core contradiction, the abundant pyrite enters the field of view. Not only does its main component FeS2 contain the key elements Fe and S required for sodium ferrosulfate, but also the naturally associated trace elements Al, Cu, Ca and Ti can greatly improve the overall performance of sodium ferrosulfate cathode materials, providing a unique advantage for developing a low-cost, element-source-integrated new iron source process.

[0003] However, the existing doping technology generally uses a single element and has a high doping amount (>1% Fe mole amount), which can improve some performance, but it is difficult to achieve a synergistic improvement in comprehensive performance. Specifically, calcium doping can enhance stability but significantly reduce capacity (CN119764381A); aluminum doping helps improve cycling but sacrifices rate performance (CN120473554A); titanium doping can improve capacity and rate but does not address moisture resistance (CN120581563A); and copper doping can optimize cycling performance but has no effect on capacity (CN116706027A). At the same time, traditional pyrite processing technologies generally have high energy consumption, heavy pollution, or incomplete impurity removal (CN101817563A, CN117865227A), which not only fails to meet the stringent requirements of high-performance cathode materials for iron sources, but also highlights a major gap in existing technologies: the lack of an effective strategy for multi-element trace co-doping or high-entropy doping directly from the composition of mineral impurities. To address the dual challenges of material performance and raw material processing, the present invention proposes a "hydrogen peroxide oxidation and reduction refining method" that efficiently converts pyrite to high-purity ferrous sulfate under mild conditions close to normal temperature and pressure. This method not only significantly reduces energy consumption and is environmentally friendly, but also effectively controls the Fe 3+ generation and deep removal of silicon and other impurities, ultimately cleverly utilizing the naturally occurring trace amounts of Al, Cu, Ca, and Ti elements in the mineral source through multi-element low-dose synergistic regulation to successfully prepare a comprehensive high-performance sodium ferric sulfate cathode material with high capacity, excellent rate performance, high moisture resistance, and long cycle stability, opening up a new path for the high-value utilization of pyrite. SUMMARY

[0004] The technical problem solved by the present invention is the low purity and poor electrochemical activity of sodium ferric sulfate cathode materials due to the large amount of SiO2 and Al2O3 impurities in pyrite. A low-cost synthesis process is provided that achieves efficient aluminum and silicon removal and precise regulation of aluminum, copper, calcium, and titanium elements through a "hydrogen peroxide oxidation refining method".

[0005] The technical solution adopted by the present invention to solve the technical problem is: using pyrite (FeS2 content 88-90%) as raw material, a high-performance sodium ferric sulfate cathode material is prepared by hydrogen peroxide oxidation treatment and spray drying, specifically including the following steps:

[0006] Step 1: Pyrite pretreatment

[0007] ① 30 g of pyrite containing 88-90% FeS2 is crushed and pretreated, then mixed with excess 1 mol•L -1 NaOH solution and sand ground to ≤500 nm to remove silicon impurities and reduce aluminum impurity content; SiO2+2NaOH=Na2SiO3+H2O, Al2O3+2NaOH=2NaAlO2+H2O;

[0008] ②The suspension is filtered by suction;

[0009] ③The filter residue is washed with deionized water, and the obtained sodium hydroxide filtrate is recovered and returned to the first step of alkali washing sand mill;

[0010] Step 2: hydrogen peroxide oxidation refining

[0011] ①An H2O2 solution with a mass fraction of 20% is prepared;

[0012] ②50 ml of the prepared solution in ① is taken in a 1000 ml beaker, and 30 g of the filter residue sample is evenly divided into 30 parts, each of which is 1 g. The next part is added after the previous part completely reacts without exothermic phenomenon and no gas bubbles are generated, until all the reactions are completed;

[0013] ③Fe powder is added to the reacted solution to reduce Fe 3+ to Fe 2+ , and H2SO4 to FeSO4, obtaining an FeSO4 solution;

[0014] Step 3: material synthesis

[0015] ①2.3635 g of citric acid, 2.3635 g of glucose, 8.5224 g of anhydrous Na2SO4, and 0.6 g of carbon nanotubes are added to the above solution;

[0016] ②The mixed solution is stirred at 30°C and 225 rpm for 30 min until the mixed system is uniform and no layering occurs. The mixed solution is heated and concentrated to a volume of 50-80 mL;

[0017] ③The mixed solution is spray dried at 180°C to obtain a precursor powder;

[0018] ④0.05 g of acetylene black is added to the precursor powder in a ball mill and ball milled at a speed of 500 rpm for 20 min;

[0019] ⑤The precursor powder is sintered at a rate of 2°C•min -1 -1 to 350°C for 8 h in a nitrogen atmosphere;

[0020] Compared with the prior art, the present application has the following advantages:

[0021] A pioneering "oxidation-reduction" dual-stage iron valence state regulation mechanism was developed, using excess H2O2 to block sulfur coating. A two-step purification process of alkaline washing and oxidation refining was developed, achieving a SiO2 impurity removal rate of >99% while retaining 1-2% of Al, Cu, Ca, and Ti elements. A synergistic spray drying method was used to ensure uniform phase distribution and obtain a pure-phase precursor. Furthermore, a high-energy ball milling process was introduced to construct a uniform carbon coating layer on the material surface, significantly improving the material's intrinsic moisture resistance. A low-temperature solid-phase synthesis pathway was established, with a critical sintering temperature of 350°C effectively suppressing Fe... 2+ Oxidation phase transition; a dual carbon source synergistic strategy is adopted, using citric acid to construct a conductive network and glucose to generate mesopores to enhance the ion diffusion rate, thereby further optimizing the electrochemical performance of the material. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of preparing sodium ferric sulfate cathode material from pyrite in Embodiment 1 of the present invention; Figure 2 The image shows the XRD pattern of the sodium ferric sulfate cathode material prepared in Example 1 of this invention. Figure 3 This is a SEM-EDS image of the sodium ferric sulfate cathode material prepared in Example 1 of the present invention; Figure 4 This is a Raman curve of the sodium ferric sulfate cathode material prepared in Example 1 of the present invention; Figure 5 This is a cycle performance diagram of the sodium ferric sulfate cathode material prepared in Example 1 of the present invention; Figure 6 This is a rate performance diagram of the sodium ferric sulfate cathode material prepared in Example 1 of the present invention; Figure 7 This is a 25°C first-cycle charge-discharge curve of the sodium ferric sulfate cathode material prepared in Example 1 of the present invention. Figure 8 This is a -10°C first-cycle charge-discharge curve of the sodium ferric sulfate cathode material prepared in Example 1 of the present invention; Figure 9 This is a charge-discharge curve of the sodium ferric sulfate cathode material prepared in Example 1 of the present invention after 7 days of exposure at 25°C and 60%RH. Detailed Implementation

[0023] The present invention will be further described below with reference to embodiments, specifically according to the following implementation methods:

[0024] Example 1: This invention discloses a method for preparing moisture-resistant sodium ferric sulfate using pyrite. The specific procedure is as follows: Take 30g of pyrite (FeS 289.2%), crush it through a 100-mesh sieve, and add 100 mL of 1 mol•L... -1 NaOH solution was mixed and milled until the particle size was ≤500 nm. The mixture was washed with water and filtered. The resulting sodium hydroxide filtrate was recycled and returned to the first step of alkaline washing and milling. The filter residue was washed with deionized water to obtain purified ore. The purified ore was divided into 30 batches, 1 g per batch, and each batch was reacted with 50 ml of 20% H₂O₂ solution for oxidation (the slag was added to the solution). An appropriate amount of iron powder was added to the solution after the reaction to make Fe... 3+ All reduced to Fe 2+ Weigh out 2.3635 g of citric acid, 2.3635 g of glucose, 8.5224 g of anhydrous Na₂SO₄, and 0.6 g of carbon nanotubes, and stir until the mixture is homogeneous and free of stratification. Concentrate the mixture to a volume of 50 ml by heating, and then spray-dry at 180°C to obtain precursor powder. Mix the precursor powder with 0.05 g of acetylene black in a ball mill and ball-mill at 500 rpm for 20 min to uniformly coat the material with carbon. Then sinter at 350°C for 8 h under a nitrogen atmosphere and sieve to obtain the product Na. 2.6 Fe 1.6822 Cu 0.0044 Al 0.0065 Ca 0.0019 Ti 0.0009 (SO4)3@C. The cathode material, acetylene black, and PVDF were mixed at a mass ratio of 8:1:1. PVDF was pre-dissolved in N-methylpyrrolidone at a ratio of 1:24. After thorough mixing, the mixture was coated onto carbon-coated aluminum foil (loading 2 mg•cm). -2 After vacuum drying at 100°C for 8 hours, Φ12 mm electrode sheets were punched out. Using sodium perchlorate as the electrolyte and glass fiber as the separator, CR2032 button cells were assembled. Test conditions: voltage range 2.0~4.5 V. The specific capacity at 1 C charge at room temperature was 105.3 mAh•g. -1 The discharge specific capacity is 97.9 mAh•g. -1 The discharge specific capacity after 7 days of exposure to 60%RH is 83.5 mAh•g. -1 The retention rate after 100 cycles is 94%, and the discharge specific capacity at a rate of 30 C is 76 mAh g. -1 The specific capacity at 1 C discharge at -10°C is 92.6 mAh•g. -1 .

[0025] Example 2: This invention discloses a method for preparing moisture-resistant sodium ferric sulfate using pyrite. The specific procedure is as follows: Take 30g of pyrite (FeS 289.2%), crush it through a 100-mesh sieve, and add 100 mL of 1 mol•L... -1 NaOH solution was mixed and milled until the particle size was ≤500 nm. The mixture was washed with water and filtered. The resulting sodium hydroxide filtrate was recycled and returned to the first step of alkaline washing and milling. The filter residue was washed with deionized water to obtain purified ore. The purified ore was divided into 30 batches, 1 g per batch, and each batch was reacted with 50 ml of 20% H₂O₂ solution for oxidation (the slag was added to the solution). An appropriate amount of iron powder was added to the solution after the reaction to make Fe... 3+ All reduced to Fe 2+ Weigh out 2.5025 g of citric acid, 2.5025 g of glucose, 7.3364 g of anhydrous Na₂SO₄, and 0.6 g of carbon nanotubes, and stir until the mixture is homogeneous and free of stratification. Concentrate the mixture to a volume of 50 ml by heating, and then spray-dry at 180°C to obtain precursor powder. Mix the precursor powder with 0.05 g of acetylene black in a ball mill and ball-mill at 500 rpm for 20 min to uniformly coat the material with carbon. Then sinter at 350°C for 8 h under a nitrogen atmosphere and sieve to obtain the product Na. 2.4 Fe 1.7822 Cu 0.0047 Al 0.0069 Ca 0.0020 Ti 0.0010 (SO4)3@C. The cathode material, acetylene black, and PVDF were mixed at a mass ratio of 8:1:1. PVDF was pre-dissolved in N-methylpyrrolidone at a ratio of 1:24. After thorough mixing, the mixture was coated onto carbon-coated aluminum foil (loading 2 mg•cm). -2 After vacuum drying at 100°C for 8 hours, Φ12 mm electrodes were punched out. Using sodium perchlorate as the electrolyte and glass fiber as the separator, CR2032 button cells were assembled. Test conditions: voltage range 2.0~4.5 V. The specific capacity at 1 C charge at room temperature was 101.3 mAh•g. -1 The discharge specific capacity is 92.3 mAh•g. -1 The discharge specific capacity after 7 days of exposure to 60% RH is 80 mAh•g. -1 The retention rate after 100 cycles is 89%, and the discharge specific capacity at a rate of 30 C is 73.8 mAh g. -1 The specific capacity at 1 C discharge at -10°C is 89.2 mAh•g. -1 .

[0026] Example 3: This invention discloses a method for preparing moisture-resistant sodium ferric sulfate using pyrite. The specific procedure is as follows: Take 30g of pyrite (FeS 289.2%), crush it through a 100-mesh sieve, and add 100 mL of 1 mol•L... -1 NaOH solution was mixed and milled until the particle size was ≤500 nm. The mixture was washed with water and filtered. The resulting sodium hydroxide filtrate was recycled and returned to the first step of alkaline washing and milling. The filter residue was washed with deionized water to obtain purified ore. The purified ore was divided into 30 batches, 1 g per batch, and each batch was reacted with 50 ml of 20% H₂O₂ solution for oxidation (the slag was added to the solution). An appropriate amount of iron powder was added to the solution after the reaction to make Fe... 3+ All reduced to Fe 2+ Weigh out 2.2245 g of citric acid, 2.2245 g of glucose, 9.8540 g of anhydrous Na₂SO₄, and 0.6 g of carbon nanotubes, and stir until the mixture is homogeneous and free of stratification. Concentrate the mixture to 50 ml by heating, and then spray-dry at 180°C to obtain precursor powder. Mix the precursor powder with 0.05 g of acetylene black in a ball mill and ball-mill at 500 rpm for 20 min to uniformly coat the material with carbon. Then sinter at 350°C for 8 h under a nitrogen atmosphere and sieve to obtain the product Na. 2.8 Fe 1.5822 Cu 0.0041 Al 0.0061 Ca 0.0018 Ti 0.0008 (SO4)3@C. The cathode material, acetylene black, and PVDF were mixed at a mass ratio of 8:1:1. PVDF was pre-dissolved in N-methylpyrrolidone at a ratio of 1:24. After thorough mixing, the mixture was coated onto carbon-coated aluminum foil (loading 2 mg•cm). -2 After vacuum drying at 100°C for 8 hours, Φ12 mm electrode sheets were punched out. Using sodium perchlorate as the electrolyte and glass fiber as the separator, CR2032 button cells were assembled. Test conditions: voltage range 2.0~4.5 V. The specific capacity at 1 C charge at room temperature was 98.9 mAh•g. -1 The discharge specific capacity is 88.4 mAh•g. -1 The discharge specific capacity after 7 days of exposure to 60%RH is 77.6 mAh•g. -1 The retention rate after 100 cycles is 83%, and the discharge specific capacity at a rate of 30 C is 69.9 mAh g. -1 The specific capacity at 1 C discharge at -10°C is 83.1 mAh•g. -1 .

[0027] The ICP test results for the ferrous sulfate solution prepared in this embodiment of the invention are shown in the table below:

[0028]

[0029] The results showed that Fe had the highest concentration and was the dominant element in the solution, while small amounts of Cu, Al, Ca, and Ti, naturally occurring dopants, were also present. This elemental concentration distribution helps ensure the purity of the material during the later synthesis of sodium ferric sulfate, and the small amount of dopants can improve the material's performance in battery applications.

Claims

1. A method for preparing moisture-resistant sodium ferric sulfate using pyrite, characterized in that, The method includes the following steps: S1: Crush the pyrite raw material into fine particles of 50-200 mesh; S2: The fine particles after S1 are crushed are mixed with an excess sodium hydroxide solution and then sand-milled to obtain a ferrous disulfide suspension. S3: The suspension in S2 is filtered to obtain ferrous disulfide filter residue and sodium hydroxide filtrate. The filtrate is collected and stored for subsequent recycling. S4: Slowly add the ferrous disulfide filter residue from S3 into the hydrogen peroxide solution to form a mixed solution of ferric sulfate and sulfuric acid. S5: Add reduced iron powder to the solution of S4 to form ferrous sulfate solution; S6: The ferrous sulfate solution in S5 is mixed with anhydrous sodium sulfate, citric acid, glucose, and carbon nanotubes to obtain a suspension, which is then concentrated by heating. S7: Spray-dry the suspension obtained in S6 to obtain sodium ferric sulfate microsphere precursor; S8: Mix the sodium ferric sulfate microsphere precursor described in S7 with an inorganic carbon source and perform high-energy ball milling; S9: The material after high-energy ball milling described in S8 is placed in a tube furnace and sintered in nitrogen to obtain carbon-coated sodium ferric sulfate. The concentration of the sodium hydroxide filtrate collected in S3 is 90-95% of that before collection; The inorganic carbon source in S8 is one or more of graphene, acetylene black, and Ketjen black, and its addition amount is 0.1~0.5% of the mass of the sodium ferric sulfate microsphere precursor.

2. The method according to claim 1, characterized in that: The pyrite raw material described in S1 contains 88-90% ferrous disulfide by mass.

3. The method according to claim 1, characterized in that: The solid-liquid ratio of the fine particles to the sodium hydroxide solution in S2 is 1:3 to 1:4, and the concentration of the sodium hydroxide solution is 1 to 1.5 mol•L. -1 The grinding time is 10-12 hours.

4. The method according to claim 1, characterized in that: The hydrogen peroxide solution in S4 has a mass fraction of 15-20%, and the reaction time is 6-8 h.

5. The method according to claim 1, characterized in that: The amount of citric acid added in S6 is 8-10% of the molar amount of ferrous sulfate, the amount of glucose added is 8-10% of the molar amount of ferrous sulfate, the amount of carbon nanotubes added is 1-1.5% of the mass of ferrous sulfate, the molar mass ratio of anhydrous sodium sulfate to ferrous sulfate is one of several from 1.2:1.8, 1.3:1.7, and 1.4:1.6, the heating and concentration time is 2-4 h, the heating temperature is 60-80°C, and the solid-liquid ratio after concentration is 1:(1-1.5).

6. The method according to claim 1, characterized in that: The spray dryer described in S7 has an inlet temperature of 180~230°C, an outlet temperature of 90~120°C, and a feed rate of 100~300 mL•h. -1 .

7. The method according to claim 1, characterized in that: The sintering temperature described in S9 is 350~380°C, and the sintering time is 6~8 h.

8. The method according to claim 1, characterized in that: The chemical formula of the carbon-coated sodium ferric sulfate according to claim 1 is Na. 2.6 Fe 1.6822 Cu 0.0044 Al 0.0065 Ca 0.0019 Ti 0.0009 (SO4)3@C, Na 2.4 Fe 1.7822 Cu 0.0047 Al 0.0069 Ca 0.0020 Ti 0.0010 (SO4)3@C, Na 2.8 Fe 1.5822 Cu 0.0041 Al 0.0061 Ca 0.0018 Ti 0.0008 Several types of (SO4)3@C.

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