Fluorophosphate-Based Polyanion Cathode Materials for Rechargeable Magnesium Batteries

Abstract

Rechargeable magnesium-ion batteries are promising candidates to compete with the state-of-the-art lithium-ion batteries for a plethora of large-scale energy storage devices. The terrestrial reserves of magnesium are plentiful and magnesium metal is safe to handle under ambient atmosphere containing water and oxygen than metallic lithium. In addition, the multivalent nature of magnesium metal itself makes rechargeable magnesium-ion battery very attractive not only due to the inherent low cost and safety, but also as a high-energy density battery system. However, in spite of the aforementioned advantages, the energy density of rechargeable magnesium batteries reported was low. The low energy density stems from the low voltage and capacity which is mainly constrained by the cathode material. A number of feasible cathode materials that can reversibly (de)insert magnesium ion have been reported in recent years, typical examples being the molybdenum chalcogenides[1] and transition metal oxides such as V2O5 [2]and MnO2 [3]; nonetheless, the energy density attained still remains unsatisfactory. Therefore, high energy density cathode frameworks are highly desirable for rechargeable magnesium battery system to be at par or even outperform the current lithium-ion battery technology. In pursuit for alternative cathode materials, this study explores a promising broad class of poly(oxy)anion inorganic frameworks based on fluorophosphates[4] that exhibit multi-redox reactions (high theoretical capacity) and high redox voltage. The synthesis, crystal structure and electrochemical magnesium (de)insertion behaviour in representative fluorophosphates cathode materials based on environmentally-benign constituents such as MgFePO4F will be highlighted. Their analogues in the mineral world are hydroxyfluorophosphates, including triplite, panasqueiraite and magniotriplite [(Mg, Fe)2(PO4)(F, OH)], where F- can partially replace an OH- group. Complete substitution of F- is necessary to enhance electrochemical properties because the hydroxyl tends to undergo irreversible redox reactivity. Furthermore, the replacement of PO4 3- with PO4F4-in principle alters the charge balance and the dimensionality of the structure.