Electrochemical Intercalation Reaction of Sodium into the Layered Transition Metal Dichalcogenide ZrS$_2$─Influence of the Electrolyte Solvent

While TiS$_2$ has been extensively studied for its ability to intercalate alkali metals like Li or Na, the higher homologue ZrS$_2$ was studied only sparsely. Furthermore, an influence of different coordinating and noncoordinating electrolyte solvents on cyclability as well as the structural changes of the host structures had been observed for different active materials. In this study, we therefore investigated the intercalation mechanism of Na$^+$ ions into layered 1T-ZrS$_2$ using electrolytes with solvents of different coordination strengths toward Na+, namely sodium trifluoromethanesulfonimide in ethylene carbonate and diethyl carbonate (1:1, EC/DEC) and sodium triflate in bis(2-methoxyethyl) ether (diglyme). At low intercalation degrees, coordinating solvents (e.g., diglyme) lead to a cointercalation in combination with a large expansion of the interlayer distance. After deintercalation, a turbostratically disordered material was obtained. In contrast, for weakly coordinating solvents (EC/DEC) no cointercalation was observed which enabled us to observe the reversible phase transitions from 1T-ZrS$_2$ to 3R-NaZrS$_2$ upon (de)intercalation. ... mehrThis transition proceeds via stacking faults and was analyzed in detail. Further, the intermediates of the electrochemical intercalation were analyzed by solid-state NMR and cycling stability tests were carried out. The long-term stability of cells prepared from ZrS$_2$ is comparable, independent of the electrolyte solvent.