Untangling the Structure and Dynamics of Lithium-Rich Anti-Perovskites Envisaged as Solid Electrolytes for Batteries

Lithium-rich anti-perovskites (LiRAPs) have attracted a great deal of attention as they have been praised as another superior group of solid electrolytes that can be used to realize all-solid-state batteries free of flammable liquids. Despite several studies that have reported on the properties of LiRAPs, many questions remain unanswered. In particular, these include fundamental ones concerning the structure, stability, and Li-ion conductivity and diffusivity. Moreover, it is not clear whether some of the previously reported compounds do really exist. To untangle the current picture of LiRAPs, we synthesized “Li₃OCl” and Li₂OHCl polymorphs and applied a wide spectrum of methods, such as powder X-ray diffraction (PXRD), powder neutron diffraction (PND), nuclear magnetic resonance spectroscopy, and impedance spectroscopy to carefully shed some light on LiRAPs. Here we self-critically conclude that the cubic polymorph of the two compounds cannot be easily distinguished by PXRD alone as the lattice metrics and the lattice parameters are very similar. Furthermore, PXRD suffers from the difficulty of detecting H and Li. Even Rietveld refinement of our PND data turned out to be complicated and not easily interpreted in a straightforward way. Nevertheless, here we report the first structural models for the cubic and a new orthorhombic polymorph containing also structural information about the H atoms. In situ PXRD of “Li₃OCl”, intentionally exposed to air, revealed rapid degradation into Li₂CO₃ and amorphous LiCl·xH₂O. Most likely, the instability of “Li₃OCl” explains earlier findings about the unusually high ion conductivities as the decomposition product LiCl·xH₂O offers an electrical conductivity that is good enough for some applications, excluding, of course, those that need aprotic conditions or electrolytes free of any moisture. Considering “H-free Li₃OCl” as well as Li₅(OH)₃Cl₂, Li₅(OH)₂Cl₃, Li₃(OH)₂Cl, and Li₃(OH)­Cl₂, we are confident that Li₄(OH)₃Cl and variants of Li_3–x(OH_x)­Cl, where x > 0, are, from a practical point of view, so far the only stable lithium-rich anti-perovskites.