Synthesis, Structural, and Transport Properties of Novel Bihydrated Fluorosulphates NaMSO₄F·2H₂O (M = Fe, Co, and Ni)

3 d -metal fluorosulphates that have a tavorite-type structure (e.g., LiFeSO 4 F) were recently reported as attractive positive electrode candidates for future Li-ion batteries aimed at large volume markets. These new fluorosulphates had to be synthesized via ionothermal synthesis owing to both their thermal instability at temperatures greater than 300 °C and their water solubility. In an attempt to depart from ionothermal synthesis, low-temperature solid-state reactions and solvothermal processes were successfully tried. The latter technique, which is reported herein, proceeds with water as the solvent and has led to a new family of fluorosulphates NaMSO 4 F·2H 2 O that crystallize in a monoclinic unit cell (SG = P 2 1 / m) similar to the uklonskovite-type structure earlier proposed for NaMgSO 4 F·2H 2 O. These new phases show no electrochemical activity with either Li or Na metal and have room temperature ionic conductivities on the order of 1 × 10 −9 S cm −1 . Additionally, we have discovered the feasibility, upon controlled dehydration of the NaMSO 4 F.2H 2 O phases, to prepare NaMSO 4 F phases adopting a derived tavorite-type structure and displaying ionic conductivities around 10 −7 S cm −1 . Finally, this finding opens the possibility to achieve Li-based fluorosulphates via two-step synthetic pathways.