Structural, Transport, and Electrochemical Investigation of Novel AMSO4F (A = Na, Li; M = Fe, Co, Ni, Mn) Metal Fluorosulphates Prepared Using Low Temperature Synthesis Routes

We have recently reported a promising 3.6 V metal fluorosulphate (LiFeSO4F) electrode, capable of high capacity, rate capability, and cycling stability. In the current work, we extend the fluorosulphate chemistry from lithium to sodium-based systems. In this venture, we have reported the synthesis and crystal structure of NaMSO4F candidates for the first time. As opposed to the triclinic-based LiMSO4F phases, the NaMSO4F phases adopt a monoclinic structure. We further report the degree and possibility of forming Na(Fe1-xMx)SO4F and (Na1-xLix)MSO4F (M = Fe, Co, Ni) solid-solution phases for the first time. Relying on the underlying topochemical reaction, we have successfully synthesized the NaMSO4F, Na(Fe1-xMx)SO4F, and (Na1-xLix)MSO4F products at a low temperature of 300 degrees C using both ionothermal and solid-state syntheses. The crystal structure, thermal stability, ionic conductivity, and reactivity of these new phases toward Li and Na have been investigated. Among them, NaFeSO4F is the only one to present some redox activity (Fe2+/Fe3+) toward Li at 3.6 V. Additionally, this phase shows a pressed-pellet ionic conductivity of 10(-7) S.cm(-1). These findings further illustrate the richness of the fluorosulphate crystal chemistry, which has just been recently unveiled.