Li2xAl1+xP1-xCl8: A Halide Lithium-Ion Conductor Family Derived from the AlPCl8 Framework

Halide solid electrolytes are promising candidates for high-voltage all-solid-state batteries due to their high anodic stability. Here, we report the synthesis and characterization of lithium-containing halide solid electrolytes, Li2xAl1+xP1-xCl8, based on the recently identified orthorhombic Pbcm structure of AlPCl8. Among the nominal compositions characterized by PXRD (x = 0.15, 0.2, 0.333, and 0.5), the x = 0.15 and 0.20 phases retain the AlPCl8-derived framework as single-phase products. The materials were prepared via stoichiometric mechanochemical synthesis followed by low-temperature annealing. Structural analysis using joint Rietveld refinements of X-ray and neutron diffraction data confirmed distorted tetrahedral interstitial lithium sites that interconnect AlCl4 and (P/Al)Cl4 polyhedra. Bond-valence site energy calculations reveal crystallographically accessible Li+ migration pathways with low local migration barriers of ∼0.3 eV. The x = 0.20 composition exhibits an ionic conductivity of 6.3 × 10-7 S cm-1 at room temperature, with a negligible electronic conductivity of 1.9 × 10-10 S cm-1 and an apparent activation energy of ∼1.6 eV. Despite the modest ionic transport, linear sweep voltammetry indicates a high oxidation onset at ∼7.8 V vs In/In-Li, demonstrating high anodic stability among halide electrolytes. The large discrepancy between the calculated local barriers and the experimentally measured activation energy suggests that macroscopic Li+ transport is governed by factors beyond the intrinsic local hopping barrier. These results establish the AlPCl8-derived framework as a useful structural platform for exploring chloride-based Li+ conductors with accessible migration pathways and high oxidative stability.