New Amorphous Iron-Based Oxyfluorides as Cathode Materials for High-Capacity Lithium-Ion Batteries

A novel two-step strategy to prepare amorphous oxyfluorides, containing both divalent and trivalent 3d-metals, as cathode materials for lithium batteries is presented. The first step involves the preparation of hydrated fluorides M2+M3+2F8(H2O)2 (M2+ = Mn, Fe, Co, Ni, Cu; M3+ = V, Fe) by microwave-assisted solvothermal synthesis. Besides the MnFe2F8(H2O)2 and CuFe2F8(H2O)2 phases, three new compounds, Fe1.3V1.7F8(H2O)2, CoFe2F8(H2O)2, and NiFe2F8(H2O)2, which are isostructural with Fe3F8(H2O)2, have been unraveled. The second step consists in the decomposition of M2+M3+2F8(H2O)2 into amorphous oxyfluorides M2+M3+2F8–2xOx via suitable thermal treatments.. The amorphous materials show a greater electrochemical activity toward Li than their parent phases, among them being CuFe2F6O, which displays the best performance as a cathode material with a first discharge capacity of 310 mAh·g–1. We show that such a large capacity results from cumulative insertion and displacement reactions.本研究提出了一种新型两步法策略，用于制备兼具二价与三价3d过渡金属的无定形氟氧化物，作为锂离子电池正极材料。第一步通过微波辅助溶剂热合成法制备水合氟化物M²⁺M³⁺₂F₈(H₂O)₂（其中M²⁺为Mn、Fe、Co、Ni、Cu；M³⁺为V、Fe）。除MnFe₂F₈(H₂O)₂与CuFe₂F₈(H₂O)₂物相外，本研究还解析出三种与Fe₃F₈(H₂O)₂同构的新型化合物：Fe₁.₃V₁.₇F₈(H₂O)₂、CoFe₂F₈(H₂O)₂及NiFe₂F₈(H₂O)₂。第二步通过适宜的热处理工艺，将M²⁺M³⁺₂F₈(H₂O)₂分解为无定形氟氧化物M²⁺M³⁺₂F₈₋₂ₓOₓ。相较于其前驱体物相，该类无定形材料展现出更优异的锂电化学活性，其中CuFe₂F₆O的性能最优，作为正极材料的首次放电比容量可达310 mAh·g⁻¹。研究表明，该高比容量源于嵌入反应与置换反应的累积协同作用。