A non-calcination preparation method of a sulfate-based sodium ion battery positive electrode material

Abstract

The application relates to the technical field of sodium ion battery materials, and discloses a non-calcination preparation method of a sulfate sodium ion battery positive electrode material. + The average voltage of the original Na2Fe(SO4)2 electrode material is 3.5 V (relative to Na / Na 2+ / Fe 3+ redox pair), and the Na2Fe(SO4)2 electrode material has potential application value in sodium ion batteries.

Description

Technical Field

[0001] This invention relates to a non-calcination preparation method for sulfate-based sodium-ion battery cathode materials, belonging to the field of sodium-ion battery technology. Background Technology

[0002] As environmental degradation caused by traditional fossil fuels intensifies, the need to explore and cultivate sustainable, renewable, and clean energy alternatives is becoming increasingly prominent. Given the dispersed and fragmented distribution of these renewable resources, establishing large-scale energy storage systems capable of intelligently storing and distributing energy is crucial. Sodium-ion batteries, with their abundant and uniformly distributed main components, have been found by existing research to offer superior thermal safety and low-temperature electrochemical performance compared to lithium-ion batteries. Therefore, sodium-ion batteries are considered a strong contender in the future large-scale energy storage market. Developing sodium-ion battery electrode materials with long cycle life and high energy density is one of the research focuses in the field of sodium-ion batteries.

[0003] Researchers have conducted extensive studies on various polyanionic sulfate-based sodium-ion battery cathode materials, such as Alluaudite-type Na₂Fe₂(SO₄)₃ and Eldfellite-type NaFe(SO₄)₂. Heating sulfate-based cathode materials at high temperatures causes the sulfate groups in the crystal lattice to decompose violently into SO₄²⁻. x Because of the presence of gases, sulfate materials cannot be coated and modified using traditional high-temperature pyrolysis processes. Researchers' ideal goal is to find a new electrochemically active sulfate cathode material that can be prepared without calcination. Related studies have shown that Alluaudite-type Na₂Fe₂(SO₄)₃ and Eldfellite-type NaFe(SO₄)₂ require calcination at 350–400 °C to obtain a highly crystalline phase. Stanfieldite-type Na₂Fe(SO₄)₂ electrode materials are highly anticipated, but there are currently no reports on their steady-state preparation and electrochemical performance. Specifically, even under stoichiometric preparation conditions, the obtained product does not contain the Na₂Fe(SO₄)₂ phase; instead, it is entirely the Vanthoffite-type Na₆Fe(SO₄)₄ phase. Summary of the Invention

[0004] The purpose of this invention is to provide a calcination-free preparation method for sulfate-based sodium-ion battery cathode materials, which solves the problem of the inability to prepare steady-state Stanfieldite-type Na2Fe(SO4)2 materials, and further confirms their excellent electrochemical activity and potential application value.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A sodium-ion battery prepared from a sulfate-based sodium-ion battery cathode material includes the following steps:

[0007] Step 1: Prepare a mixed aqueous solution of sodium sulfate and ferrous sulfate.

[0008] Sodium sulfate and ferrous sulfate in a 1:1 molar ratio were added to deionized water, and then stirred continuously for 1–2 hours until completely dissolved. During this process, an appropriate amount of antioxidant was added to prevent Fe from being released. 2+ Hydrolysis yields a light green aqueous solution of sodium sulfate and ferrous sulfate with uniformly dispersed solutes.

[0009] Step 2: Two-step continuous vacuum drying of the mixed aqueous solution of sodium sulfate and ferrous sulfate.

[0010] The mixed aqueous solution of sodium sulfate and ferrous sulfate was first dried at low temperature in a vacuum drying oven to obtain a solid powder. After a second high-temperature vacuum drying for 6-12 hours, the obtained material was Stanfieldite type Na2Fe(SO4)2.

[0011] Step 3: Weigh 70-80 parts of Na2Fe(SO4)2 active material, 10-20 parts of conductive carbon black, and 10 parts of binder according to the mass fraction, put them into an agate mortar, dry grind for 10-30 minutes, add NMP and wet grind for 10-30 minutes, and the resulting paste can form a sodium-ion battery positive electrode slurry with good fluidity.

[0012] Step 4: After uniformly coating the slurry onto the aluminum foil, vacuum dry it at 100°C for 12 hours, and then use a cutting machine to cut it into round positive electrode material sheets.

[0013] In Step 2, the initial vacuum drying temperature is 60–100℃ and the drying time is 3–6 hours. The secondary vacuum drying temperature is 150–200℃ and the drying time is 6–12 hours.

[0014] Furthermore, in Step 1, the antioxidant is at least one of ascorbic acid, citric acid, or reduced iron powder.

[0015] Furthermore, in Step 1, the antioxidant: Fe 2+ The molar ratio is 30 to 60:1.

[0016] Furthermore, in Step 2, the two-step continuous vacuum drying is divided into a low-temperature and a high-temperature vacuum drying process.

[0017] Beneficial effects

[0018] This invention discloses a sulfate-based Na2Fe(SO4)2 sodium-ion battery cathode material. The material is prepared using a two-step continuous vacuum drying process, effectively preventing the transformation of Stanfieldite-type Na2Fe(SO4)2 into the Na6Fe(SO4)4 phase. Based on a single-electron reaction, the average voltage of the Na2Fe(SO4)2 cathode material is 3.5V (relative to Na / Na). + Fe 2+ / Fe 3+ (Redox pair), with a discharge specific capacity of approximately 40 mAh / g at 0.1C. In summary, this work not only solves the long-standing problem of the inability to prepare steady-state Na2Fe(SO4)2, but also further reveals the crystal structure and electrochemical activity of Na2Fe(SO4)2 sodium-ion battery cathode materials. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below.

[0020] Figure 1 This is the XRD pattern of the prepared Stanfieldite-type Na2Fe(SO4)2 material.

[0021] Figure 2 This is a SEM image of the prepared Stanfieldite-type Na2Fe(SO4)2 material.

[0022] Figure 3 This is the XRD pattern of the Na6Fe(SO4)4 mixed-phase material prepared in Comparative Example 1.

[0023] Figure 4 This is a SEM image of the Na6Fe(SO4)4 mixed-phase material prepared in Comparative Example 1.

[0024] Figure 5 This refers to the 0.05C constant current charge-discharge performance of the Na2Fe(SO4)2 cathode material in Example 1.

[0025] Figure 6 This is a rate performance test of the Na2Fe(SO4)2 cathode material in Example 1.

[0026] Figure 7 This is the Nyquist plot of the electrochemical impedance spectroscopy of the Na2Fe(SO4)2 cathode material in Example 1. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to specific embodiments.

[0028] Example 1

[0029] Preparation of Stanfieldite-type Na2Fe(SO4)2 materials:

[0030] Step 1: Prepare a mixed aqueous solution of sodium sulfate and ferrous sulfate.

[0031] Weigh sodium sulfate and ferrous sulfate according to a molar ratio of 1:1 and add them to deionized water. Stir continuously for 1-2 hours until completely dissolved. During this process, follow the ascorbic acid:Fe... 2+ An appropriate amount of ascorbic acid was added at a molar ratio of 30:1, resulting in a light green aqueous solution of sodium sulfate and ferrous sulfate with uniform solute dispersion.

[0032] Step 2: Two-step continuous vacuum drying of a mixed aqueous solution of sodium sulfate and ferrous sulfate.

[0033] A mixed aqueous solution of sodium sulfate and ferrous sulfate was first dried in a vacuum drying oven at 100°C for 5 hours to obtain a solid powder. Then, the temperature was immediately raised to 200°C and vacuum dried for another 10 hours. Finally, the dried powder was ground and collected to obtain the Stanfieldite type Na2Fe(SO4)2 material.

[0034] Figure 1 The XRD pattern of the Na2Fe(SO4)2 material prepared in Example 1 shows that the number and position of the main diffraction peaks of the material are consistent with the standard cards of Na2Co(SO4)2 and Na2Ni(SO4)2. At the same time, the material contains only the transition metal element Fe, which further proves that the prepared material is a Stanfieldite type Na2Fe(SO4)2 material.

[0035] Figure 2 The image shows a SEM image of the prepared Stanfieldite-type Na₂Fe(SO₄)₂ material. It can be observed that the material possesses a rich porous structure, which is due to the rapid loss of adsorbed and crystalline water during the two-step continuous vacuum drying process. This porous structure facilitates electrolyte wetting. Simultaneously, a large number of isolated particles with a diameter of approximately 100-200 nm after breakage are visible, indicating that the prepared Stanfieldite-type Na₂Fe(SO₄)₂ material belongs to the micro-nano size category.

[0036] Comparative Example 1

[0037] Step 1: Prepare a mixed aqueous solution of sodium sulfate and ferrous sulfate.

[0038] Weigh sodium sulfate and ferrous sulfate according to a molar ratio of 1:1 and add them to deionized water. Stir continuously for 1-2 hours until completely dissolved. During this process, follow the ascorbic acid:Fe... 2+An appropriate amount of ascorbic acid was added at a molar ratio of 30:1, resulting in a light green aqueous solution of sodium sulfate and ferrous sulfate with uniform solute dispersion.

[0039] Step 2: Air drying of the mixed aqueous solution of sodium sulfate and ferrous sulfate.

[0040] A mixed aqueous solution of sodium sulfate and ferrous sulfate was first dried in air at 80°C for 5 hours to obtain a solid powder. The temperature was then immediately raised to 200°C and dried for another 10 hours. Finally, the dried powder was ground and collected to obtain the mixed phase material.

[0041] Figure 3 The XRD pattern of the material prepared for Comparative Example 1 shows that, based on the number and position of the main diffraction peaks, the material is determined to be a two-phase mixture of Na6Fe(SO4)4 and FeSO4·H2O.

[0042] Figure 4 The image shows the SEM image of the prepared mixed-phase material. It can be observed that the primary particles with a particle size of about 100-500 nm have obviously agglomerated. However, no obvious pore structure was observed on the surface of the primary particles. The adsorbed water and crystal water that are not rapidly lost during the vacuum drying process cannot be achieved during air drying, resulting in the formation of a two-phase mixture.

[0043] Electrochemical activity test experiment

[0044] The Na2Fe(SO4)2 cathode material prepared in Example 1 of this invention was coated onto aluminum foil. After complete drying, it was cut into circular electrode sheets with a diameter of 11 mm using a cutting machine. These were used as the cathode of the sodium-ion battery, with a metallic sodium sheet as the counter electrode, a glass fiber filter membrane as the separator, and 1M NaClO4 as the sodium salt used in the electrolyte. The solvent used in the electrolyte was EC and PC in a 1:1 volume ratio, with 5% FEC added by volume. A coin-type CR2032 battery was assembled in an argon-filled glove box, followed by electrochemical performance tests, including charge and discharge tests. The final electrochemical performance was as follows: Figure 5 and Figure 6 As shown.

[0045] from Figure 5 It can be seen that the sodium-ion battery prepared in Example 1 has an initial discharge specific capacity of approximately 40 mAh / g at a current density of 5 mA / g, and an average operating voltage of approximately 3.5 V (vs. Na / Na). + The almost symmetrical charge-discharge curve shape indicates that the Na2Fe(SO4)2 cathode material can indeed achieve Na… + Reversible embedding.

[0046] from Figure 6It can be seen that the sodium-ion battery prepared in Example 1 has acceptable discharge specific capacity at current densities of 5mA / g, 10mA / g, 20mA / g, and 50mA / g, but poor discharge specific capacity at higher current densities, such as 100mA / g and 200mA / g. The inherent low electronic conductivity of polyanionic materials limits the high-rate capacity.

[0047] from Figure 7 As can be seen, the sodium-ion battery prepared in Example 1 has an ohmic impedance of approximately 8.814 Ω and a charge transfer impedance of approximately 684.2 Ω in the Nyquist spectrum, confirming that its electronic conductivity is indeed poor.

[0048] Combination Figure 5 , Figure 6 and Figure 7 It can be inferred that the original Stanfieldite-type Na2Fe(SO4)2 material cannot fully utilize its discharge specific capacity due to the inherent low electronic conductivity of its polyanionic structure, and requires further carbon modification or particle nano-sizing strategies to further improve its electrochemical performance.

[0049] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions will not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the embodiments and comparative technical solutions of the present invention.

Claims

1. A method for preparing sulfate-based sodium-ion battery cathode material without calcination, characterized in that, The sodium-ion battery cathode material is prepared using the following steps: Step 1: Prepare a mixed aqueous solution of sodium sulfate and ferrous sulfate. Sodium sulfate and ferrous sulfate in a 1:1 molar ratio were added to deionized water, and then stirred continuously for 1-2 hours until completely dissolved. During this process, an appropriate amount of antioxidant was added to prevent Fe from being released. 2+ Hydrolysis yields a light green aqueous solution of sodium sulfate and ferrous sulfate with uniformly dispersed solutes. Step 2: Two-step continuous vacuum drying of the mixed aqueous solution of sodium sulfate and ferrous sulfate. The mixed aqueous solution of sodium sulfate and ferrous sulfate was first dried at low temperature in a vacuum drying oven to obtain a solid powder. After a second high-temperature vacuum drying for 6-12 hours, the obtained material was Stanfieldite type Na2Fe(SO4)2. Step 3: Weigh 70-80 parts of Na2Fe(SO4)2 active material, 10-20 parts of conductive carbon black, and 10 parts of binder according to the mass fraction, and put them into an agate mortar. Dry grind for 10-30 minutes, add NMP and wet grind for 10-30 minutes. The resulting paste can form a sodium-ion battery positive electrode slurry with good fluidity. Step 4: After uniformly coating the slurry onto the aluminum foil, vacuum dry it at 100°C for 12 hours, and then use a cutting machine to cut it into round positive electrode material sheets. In Step 2, the initial vacuum drying temperature is 60-100℃ and the drying time is 3-6 hours. The secondary vacuum drying temperature is 150-200℃ and the drying time is 6-12 hours.

2. The method for preparing a sulfate-based sodium-ion battery cathode material without calcination according to claim 1, characterized in that, The antioxidant mentioned in Step 1 is at least one of ascorbic acid, citric acid, or reduced iron powder.

3. The method for preparing a sulfate-based sodium-ion battery cathode material without calcination according to claim 1, characterized in that, The antioxidant mentioned in Step 1: Fe 2+ The molar ratio is 30-60:1.

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