Na2+2x Fe2-x(SO4)3鈉離子電池正極材料的 合成與電化學性能研究

Abstract

本研究以硫酸鈉與七水硫酸亞鐵為主要前驅物，採用簡易固相法合成高電壓鐵基正極材料 Na2+2x Fe2-x(SO4)3。透過優化球磨參數，在提升混合效率的同時降低對研磨槽的損傷。最佳條件為使用直徑 5 mm 之研磨球，在 400 rpm 下運轉 30 分鐘，共進行四個循環，並結合手動研磨與混合。原位高解析度 X 光繞射（XRD）分析顯示，400 ⁰C 加熱 1.5 小時為最佳退火條件。所得材料之初級粒徑約為 86.95 nm，次級粒徑介於 8 至 10 µm 之間。XRD 分析證實產物為相純材料，且與參考圖譜一致。 所合成材料對濕氣高度敏感，在暴露於空氣中時會由 alluaudite 相轉變為含水的 bloedite 相。此相變可透過熱處理逆轉，使 bloedite 回復為無水的 alluaudite 相。以銅取代部分鐵元素可提升材料在空氣中的穩定性，並減緩其劣化速率。在電化學測試方面，製備了乾式與濕式電極並進行活化測試。乾式電極之容量為 78.11 mAh g−1，低於理論值；濕式電極則在改善部分厚度限制後，展現相近之容量（77.84 mAh g−1）。容量降低主要歸因於合成前驅物中 Na:Fe 比例不足，其數值低於 1，而最佳比例為 1.33。總體而言，本研究建立了 Na2+2x Fe2-x(SO4)3的實用合成與製程條件，並指出影響其結構穩定性與電化學性能的關鍵因素。

This study synthesized Na2+2x Fe2-x(SO4)3 as a high-voltage iron-based cathode using a straightforward solid-state process with sodium sulfate and ferrous sulfate heptahydrate as primary precursors. Ball milling parameters were optimized to enhance mixing efficiency while minimizing damage to the grinding bowl. Optimal performance was achieved using 5 mm balls at 400 rpm for 30 minutes over four cycles, combined with manual grinding and hand mixing. In situ high-resolution X-ray diffraction (XRD) identified 400 ⁰C for 1.5 hours as the optimal annealing condition. The resulting material exhibited primary particles of approximately 86.95 nm and secondary particles ranging from 8 to 10 µm. XRD analysis confirmed the formation of a phase-pure product consistent with the reference pattern. The synthesized material exhibited high sensitivity to moisture, undergoing a transformation from the alluaudite phase to hydrated bloedite upon air exposure. This phase transition was reversible through heat treatment, which restored the anhydrous alluaudite phase from bloedite. Substitution of copper for iron improved air stability by slowing degradation, as copper exhibits lower hydrophilicity and has a lower coordination tendency compared to Fe, which reduces its interaction with water molecules and enhances moisture stability. In this work, 2% atomic Cu was substituted for Fe in the iron-based sulfate framework. This partial Cu substitution slows down the leftward shift of the diffraction peaks and allows the main peaks to remain intensity even after ten days of air exposure, in clear contrast to the pristine sample, whose main peaks completely disappear over seven days. For electrochemical evaluation, both dry- and wet-processed electrodes were fabricated and subjected to formation tests. The dry-processed electrode achieved a capacity of 78.11 mAh g−1, which was lower than the theoretical value. The wet-processed electrode, which addressed some thickness-related limitations, demonstrated a comparable capacity of 77.84 mAh g−1. The reduced capacity was primarily attributed to an insufficient Na:Fe ratio in the synthesized precursors, with values below 1 compared to the optimal ratio of 1.33. In summary, this study establishes practical synthesis and processing conditions for Na2+2x Fe2-x(SO4)3 and identifies key factors influencing its structural stability and electrochemical performance.