Structural Aspects, Ionic Conductivity, and Electrochemical Properties of New Bromine-Substituted Alkali-Based Crystalline Phases MTa(Nb)X _{6– <i>y</i>} Br _<i>y</i> (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 LiTaCl6 and related MTaCl6 (M = Na, K) halides and study effects on ionic conductivity and Li reactivity. We found that Br-substituted MTaCl6–yBry 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 KTaCl6–yBry, and exhibit low ionic conductivities from 10–6 to 10–3 mS/cm, peaking at KTaCl5Br. Aggressive ball-milling increases the ionic conductivity by nearly 1800 times for LiTaBr6 but remains insufficient for solid electrolytes. Nevertheless, we showed reversible stepwise Li insertion in hcp-LiTaCl6–yBry (0 ≤ y ≤ 6), unlike a single-step conversion reaction in ccp-LiTaF6, 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.