Crystalline Li2Ta2OCl10 elucidates the origin of high ionic conductivity in oxychloride solid electrolytes

Great hopes are being placed in all solid-state batteries (ASSBs) to clear the challenging hurdles of energy density and safety found in current Li-ion technology. However, several unresolved issues hinder their commercialization, such as interfacial instability, loss of mechanical integrity during cycling and limited ionic conductivity of solid electrolytes, with the latter factor driving research towards ever-improved ionic conductors and sparking interest in novel Li-based oxyhalides. We herein report a new crystalline Li2Ta2OCl10 phase, prepared via a single step, low-temperature process. This phase crystallizes in a triclinic structure (space group P-1) consisting of [Ta2OCl10]2- dimers and demonstrates an ionic conductivity of 0.002 mS.cm-1 at room temperature with an activation energy (Ea) of 0.46 eV. This activation energy decreases further to 0.27 eV after only 1 to 3 hours of high-energy ball milling while RT increases commensurately by a factor of 5000 (reaching 11 mS.cm-1). To grasp its structure and origins of its performance, we utilized a combination of X-ray diffraction (XRD), pair distribution function (PDF) analysis, Raman spectroscopy, infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) measurements, density functional theory (DFT), and molecular dynamics (MD) calculations to demonstrate that the ball milling step introduces a "liquid-like" sublattice of Li+ cations, similar to the Ag+ sublattice found in highly conducting AgI, and is responsible for the ionic conductivity increase. ASSBs were assembled using Li2Ta2OCl10 as a solid electrolyte with an electrochemical stability window of 1.84-3.89 V relative to LixIn, exhibiting low polarization and long-term capacity retention (300 cycles at 1C with net capacity loss of less than 8%). Built from a distinct framework of dimer units, crystalline Li2Ta2OCl10 paves the way for new high-performance ionic conductors thanks to a wide assortment of structures afforded by the compositional flexibility of the Li-M-O-Cl family.