On the room temperature synthesis of monoclinic Li3FeF6: A new cathode material for rechargeable lithium batteries

The α-polymorph of Li3FeF6 has been obtained at room temperature by a precipitation reaction from aqueous solution. This easy procedure that does not require any further annealing at high temperature yields a white powder with particles sizes ranging from 250 to 400 nm. After proper processing by mechanical milling, particle sizes clearly below 100 nm are achieved and this fluoride reacts reversibly with lithium at an average voltage of 3.1 V, which is expected for the Fe3+/Fe2+ redox couple. After 12 h of ball milling observed average particle size is ca. 50 nm and a reversible capacity of 100 mA h g−1 is obtained. This corresponds to 70% of the theoretical capacity expected for the one electron reduction of Fe3+. The mentioned capacity is fairly kept upon cycling making the new material useful as the positive electrode in lithium batteries. This is the very first time that a material for cathode application in lithium batteries is prepared at room temperature and from aqueous solution, a procedure which is easily scalable and hence of high industrial interest.