Compositional flexibility in irreducible antifluorite electrolytes for next-generation battery anodes

Solid-state batteries currently receive ample attention due to their potential to outperform lithium-ion batteries in terms of energy density when featuring next-generation anodes such as lithium metal or silicon. One key remaining challenge is identifying solid electrolytes that combine high ionic conductivity with stability in contact with the highly reducing potentials of next-generation anodes. Fully reduced electrolytes, based on irreducible anions, offer a promising solution by avoiding electrolyte decomposition altogether. In this study, we demonstrate the compositional flexibility of the disordered antifluorite framework accessible by mechanochemical synthesis and leverage it to discover irreducible electrolytes with high ionic conductivities. We show that the recently investigated Li₉N₂Cl₃ and Li₅NCl₂ phases are part of the same solid solution of Li-deficient antifluorite phases existing on the LiCl-Li₃N tie line with a general chemical formula of Li₁₊₂ Cl_1- N (0.33 < x < 0.5). Using density functional theory calculations, we identify the origin of the 5-order-of-magnitude conductivity increase of the Li₁₊₂ Cl_1- N phases compared to the structurally related rock-salt LiCl phase. Finally, we demonstrate that S_Cl- and Br_Cl-substituted analogues of the Li₁₊₂ Cl_1- N phases may be synthesized, enabling significant conductivity improvements by a factor of 10, reaching 0.2 mS cm^-1 for Li_2.31S_0.41Br_0.14N_0.45. This investigation demonstrates for the first time that irreducible antifluorite-like phases are compositionally highly modifiable; this finding lays the ground for discovery of new compositions of irreducible antifluorite-like phases with even further increased conductivities, which could help eliminate solid-electrolyte decomposition and decomposition-induced Li losses on the anode side in high-performance next-generation batteries.