Facile topotactic synthesis of tavorite LiFeSO₄F using supercritical methanol

For the first time, tavorite-structured LiFeSO₄F is synthesized using supercritical methanol at 300 °C within a short reaction time of 15 min. The synthesis involves topotactic insertion of LiF in the supercritical solution into szomolnokite-type FeSO₄⋅H₂O, whose structure is similar to that of the tavorite. The introduction of the co-solvent, tetraethylene glycol, solubilizes LiF partially and significantly promotes the incorporation of LiF into the structure, enabling the synthesis of highly uniform granular particles of the tavorite sized approximately 200 nm, in 15 min. The longer reaction time in supercritical methanol at 300 °C results in reduced reversible Li storage capacity and inferior cycling stability. Furthermore, the increase in the synthesis temperature results in a phase transition to triplite LiFeSO₄F, which is a more thermodynamically stable phase in the synthetic temperature ranges considered. This study provides a new insight into the facile synthesis of other related fluorosulfate or fluorophosphate materials using supercritical methanol.