Improved Stability in LiX‐NbCl$_5$ (X = Cl, Br) Glass‐Ceramic Electrolytes Through Anion Mixing for Solid‐State Batteries

The realization of solid-state batteries (SSBs) hinges upon the development of solid electrolytes (SEs) exhibiting superior functional properties. Halide SEs are promising candidates due to their high room-temperature ionic conductivity and favorable (chemo)mechanical properties. However, their electrochemical stability and degradation processes under operating conditions remain largely unexplored. Herein, we present lithium niobium halide SEs, LiX-NbCl$_5$ (X = F$^−$, Cl$^−$, Br$^−$, I$^−$), with emphasis placed on LiNbCl$_6$ and LiNbCl$_5$Br. Structural analysis unveils the materials to be predominantly amorphous, interspersed with nanocrystalline domains, with both LiNbCl$_6$ and LiNbCl$_5$Br exhibiting ionic conductivities above 3.5 mS cm$^{−1}$ at 25°C. Mechanical properties and pressure-dependent ionic conductivities were also examined, revealing good densification behavior and low activation volumes. When used as catholyte in SSBs with layered oxide cathodes, the cells show high initial Coulomb efficiencies (>90%) and deliver specific discharge capacities of over 200 mAh g$^{−1}$. Using differential electrochemical mass spectrometry, we demonstrate that chlorine evolves at the end of charge, which can be mitigated to some extent by introducing bromine, leading to enhanced cyclability. Overall, our study indicates that halide substitution has a positive effect on electrochemical stability without impairing ionic conductivity, and that gas evolution must be considered in halide-based SEs.