Lithium‐ion Mobility in Li6B18(Li3N) and Li Vacancy Tuning in the Solid Solution Li6B18(Li3N)1−x(Li2O)x.

All‐solid‐state batteries are promising candidates for safe energy‐storage systems due to non‐flammable solid electrolytes and the possibility to use metallic lithium as an anode. Thus, there is a challenge to design new solid electrolytes and to understand the principles of ion conduction on an atomic scale. We report on a new concept for compounds with high lithium ion mobility based on a rigid open‐framework boron structure. The host–guest structure Li6B18(Li3N) comprises large hexagonal pores filled with ∞1[ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ Li7N] strands that represent a perfect cutout from the structure of α‐Li3N. Variable‐temperature 7Li NMR spectroscopy reveals a very high Li mobility in the template phase with a remarkably low activation energy below 19 kJ mol−1 and thus much lower than pristine Li3N. The formation of the solid solution of Li6B18(Li3N) and Li6B18(Li2O) over the complete compositional range allows the tuning of lithium defects in the template structure that is not possible for pristine Li3N and Li2O. [ABSTRACT FROM AUTHOR]