Synthesis, Structure, and Electrochemistry of Crystallized Layered Chlorides, LiMCl₆ (M = Ta/Nb)

Abstract

A suitable solid electrolyte can turn the long‐standing dream of a safe commercial all‐solid‐statebattery into reality in the same way as appropriate liquid electrolytes kickstarted the first commercial lithium‐ion battery. Presently, halide‐based electrolytes despite their reactivity with lithium metal are extensively studied as they offer better processability, malleability, oxidative stability, and safety over sulfide‐based electrolytes. Particularly, LiMCl6 (M = Ta/Nb) chloride‐based amorphous electrolytes are generating widespread interest with their high conductivities, comparable to liquid electrolytes. In this context, with synchrotron X‐ray and neutron powder diffraction techniques, well‐crystallized triclinic layered LiMCl6 (M = Ta/Nb) chlorides with an hcp AB‐type stacking of chloride ions is reported. The hexagonally ordered NbCl6 octahedra share edges with LiCl6 octahedra forming a honeycomb pattern, whereas Li+ is intermixed with M5+ for M = Ta. Furthermore, it is found that crystalline LiTaCl6 has an ionic conductivity of ≈10−5 mS cm−1, six orders of magnitude lower than that reported for superionic amorphous LiTaCl6. Nevertheless, crystalline LiTaCl6 is utilized as an intercalation compound in an all‐solid‐state‐battery, uncovering a solid‐solution/two‐phase/conversion pathway for lithium‐insertion during the three distinct redox plateaus below 3 V versus Li+/Li°. Broadly, the findings give insights into the structure, ionic conduction, and intercalation in a new family of layered halides.