Synthesis and electrochemical properties of pure LiFeSO₄F in the triplite structure

Recently in the Li-ion battery community there has been an intense amount of attention focusing on developing positive electrodes which operate on the Fe²⁺/Fe³⁺ redox potential given that the environmental and economic advantages of Fe-based compounds compared to other transition metals are tremendous. Here we report that we have succeeded in preparing for the first time LiFeSO₄F in the triplite structure which shows an open circuit voltage of 3.9 V versus Li with a reversible capacity of approximately 85 mA h g^{- 1}, still well below the theoretical value of 148 mA h g ^{- 1} due to poor electrode kinetics. Nevertheless, we additionally found that the volume change on removal of Li is 0.6%. Once optimized, this triplite phase of LiFeSO₄F could stand as a promising contender to LiFePO₄.