High-Voltage Superionic Halide Solid Electrolytes for All-Solid-State Li-Ion Batteries

All-solid-state Li-ion batteries (ASSBs), considered to be potential next-generation energy storage devices, require solid electrolytes (SEs). Thiophosphate-based materials are popular, but these sulfides exhibit poor anodic stability and require specialty coatings on lithium metal oxide cathodes. Moreover, electrode designs aimed at high energy density are limited by their narrow electrochemical stability window. Here, we report new mixed-metal halide Li_3–xM_1–xZr_xCl₆ (M = Y, Er) SEs with high ionic conductivity—up to 1.4 mS cm^–1 at 25 °C—that are stable to high voltage. Substitution of M (M = Y, Er) by Zr is accompanied by a trigonal-to-orthorhombic phase transition, and structure solution using combined neutron and single-crystal X-ray diffraction methods reveal a new framework. The employment of >4 V-class cathode materials without any protective coating is enabled by the high electrochemical oxidation stability of these halides. An ASSB showcasing their electrolyte properties exhibits very promising cycling stability up to 4.5 V at room temperature.