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

Abstract

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, proceeds with water as the solvent and has led to a new family of fluorosulphates NaMSO4F·2H2O that crystallize in a monoclinic unit cell (SG =P21/m) similar to the uklonskovite-type structure earlier proposed for NaMgSO4F·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 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.