Fluorosulfate positive electrodes for Li-ion batteries made via a solid-state dry process

Ionothermal synthesis has recently been used to prepare a fluorosulfate (LiFeSO4 F) capable of reversibly intercalating Li at 3.6 V vs Li, making this material a serious contender to LiFePO4 for HEV and electric vehicle applications. Although fluorosulfates are made from low cost and abundant starting materials, their synthesis is costly because of the use of ionic liquids as synthetic medium. Herein, we report a solid-state process by which LiFeSO4 F can be synthesized without the use of ionic liquids but at the expense of both longer reaction time and weakly contaminated samples. Additionally, we show how powerful Mössbauer spectroscopy can be in the optimization of the various stages of electrode preparation as shown through the synthesis of LiFeSO4 F and its implementation into an electrode. The importance of having Fe3+-free hydrated precursors to routinely obtain pure LiFeSO4 F samples is shown together with the need to optimize ballmilling conditions to preserve Fe3+-free LiFeSO 4 F composites. Samples prepared via this low temperature solid-state process show battery performances approaching those of samples prepared using ionic liquids as synthetic medium. Furthermore, this process can be extended to the synthesis of the other members of the fluorosulfates AMSO4 F family with A=Li, Na and M=Fe, Co, and Ni. © 2010 The Electrochemical Society