Highly-pure triplite 3.9V-LiFeSO4F synthesized by a single-step solid-state process and its high electrochemical performance

Abstract In this study, triplite 3.9V-LiFeSO4F in single step solid-state reaction with various precursors was synthesized by understanding the reaction pathway that it forms. The loss of fluorine during a heat treatment can be a main problem because it can cause the formation of non-fluorine compounds such as Li2Fe(SO4)2 phase, which are undesirable impurity phases. An additional fluorine source was used to compensate the loss of fluorine and led to the formation of highly-pure triplite LiFeSO4F. Furthermore, additional supplementation with sucrose as a carbon source produced highly-pure LiFeSO4F nano-particles, coated with a carbon layer. This resulting material can achieve superior rate capability up to 20C rate (3 min discharge), ∼73 mAhg−1 and have excellent capacity retention for 350 cycles at 1C charge/1C discharge rate even though the structure of triplite LiFeSO4F does not have an apparent long Li diffusion pathway due to a disordered Li/Fe distribution. These results demonstrate that triplite LiFeSO4F is a promising cathode material with respect to energy density and scalable synthesis capability. Furthermore, this solid-state reaction with additional fluorine source will open novel ways to further engineer the material properties of LiFeSO4F or other fluorine containing compounds.