Electrochemical intercalation of Na$^+$ into 1T-HfS$_2$– a comparative study using different electrolyte solvents

Titanium disulfide is a well-known electrode material for batteries and has already been extensively studied for its structural changes during the intercalation of guest ions such as Li or Na. However, the question of how higher homologues, such as HfS$_2$ – which was investigated in this study – react to the intercalation of Na has remained virtually unexplored. Although all group IV transition metal disulfides (TMDSs) are isostructural and undergo a phase transition from 1T-TMS$_2$ to 3R-Na$_x$TMS$_2$ (TM = Ti, Zr, Hf) during intercalation of sodium ions, the reaction mechanism for each is unique and is influenced by the electrolyte used in the case of electrochemical intercalation. For HfS$_2$ co-intercalation of both strong and weakly coordinating electrolyte solvents (namely diglyme and an EC:DEC mixture) was investigated by combining XRPD with IR and TG measurements, and only the use of THF shows no evidence of co-intercalation. Furthermore, the intercalation reaction of Na$^+$ does not occur via explicit intermediate structures (TiS$_2$) or stacking faults (ZrS$_2$). Instead, phases with different d-spacings coexist. The structural studies are complemented with insights in changes of the electronic structure: in situ UV/Vis measurements indicate the transformation from a semiconductor to a conducting material during the intercalation of Na$^+$. ... mehrX-ray absorption near-edge structure spectroscopy shows only a very small shift of the Hf L$_3$-edge energy, and EXAFS shows some expansion of the Hf–S atomic distances in NaHfS$_2$ due to the Na$^+$ intercalation, indicating that sulfur instead of Hf might be involved in the redox process.