Synthesis, structural, and transport properties of novel bihydrated fluorosulphates NaMSO4F 2H2O (M=Fe, Co, and Ni)

3d-metal fluorosulphates that have a tavorite-type structure (e.g., LiFeSO4F) 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, proceedswithwater as the solvent and has led to a new family of fluorosulphatesNaMSO4F3 2H2Othat crystallize in amonoclinic unit cell (SG=P21/m) similar to the uklonskovite-type structure earlier proposed for NaMgSO4F3 2H2O. 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 NaMSO4F.2H2O phases, to prepare NaMSO4F phases adopting a derived tavoritetype 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. © 2010 American Chemical Society