Structural Aspects, Ionic Conductivity, and Electrochemical Properties of New Bromine-Substituted Alkali-Based Crystalline Phases MTa(Nb)X _{6– y} Br _y (M = Li, Na, K; X = Cl, F)

Solid electrolytes, the key to safe solid-state batteries, have been recently revitalized with the discovery of superionic, ductile, and redox-active Ta and Nb halides. We investigate anion substitution in crystalline LiTaCl 6 and related MTaCl 6 (M = Na, K) halides and study effects on ionic conductivity and Li reactivity. We found that Br-substituted MTaCl 6– y Br y compounds form solid solution series for M = Li, Na (0 ≤ y ≤ 6) and for M = K ( y ≥ 2), with a structure transition at y = 1 for KTaCl 6– y Br y, and exhibit low ionic conductivities from 10 –6 to 10 –3 mS/cm, peaking at KTaCl 5 Br. Aggressive ball-milling increases the ionic conductivity by nearly 1800 times for LiTaBr 6 but remains insufficient for solid electrolytes. Nevertheless, we showed reversible stepwise Li insertion in hcp-LiTaCl 6– y Br y (0 ≤ y ≤ 6), unlike a single-step conversion reaction in ccp-LiTaF 6, highlighting structural effects. Our findings reveal anion substitution in Ta/Nb halides as a route to develop redox-active catholytes with tunable structural and transport properties.