Heat treatment protocol for modulating ionic conductivity via structural evolution of Li3-xYb1-xMxCl6 (M = Hf4+, Zr4+) new halide superionic conductors for all-solid-state batteries

Owing to their deformability and good (electro)chemical-oxidation stability, halide superionic conductors have emerged as enablers for practical all-solid-state batteries. Herein, we report the dynamic structural evolution of Li3YbCl6, which varies with the heat treatment temperature (400 vs. 500 °C) and aliovalent substitutions with Hf4+ or Zr4+. It is observed that slight differences in Li+ conductivities (0.19 vs. 0.14 mS cm−1 at 30 °C) and activation energies (0.47 vs. 0.53 eV) between unsubstituted Li3YbCl6 heat-treated at 400 and 500 °C diverge upon aliovalent substitution, which results in the evolution of monoclinic and orthorhombic phases, respectively. Enhanced Li+ conductivities reaching 1.5 mS cm−1 with an activation energy of 0.26 eV (Li2.60Yb0.60Hf0.40Cl6 prepared at 400 °C) upon Hf4+- or Zr4+-substitution are ascribed to the optimal concentration of charge carriers of Li+ and vacancies. Importantly, the exclusive comparison of crystal structures affecting Li+ conductivity in halide superionic conductors is enabled for the first time, demonstrating that it is more favorable for the cubic close-packed (ccp) monoclinic structure as compared to the hexagonal close-packed (hcp) orthorhombic structure. Furthermore, the excellent reversibility of single-crystalline LiNi0.88Co0.11Al0.01O2 in all-solid-state cells at 30 °C was achieved by employing ccp monoclinic Li2.60Yb0.60Hf0.40Cl6 prepared at 400 °C with a capacity retention of 83.6% after 1000 cycles.