Heat treatment protocol for modulating ionic conductivity via structural evolution of Li_3-xYb_1-xM_xCl₆ (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 (Li_2.60Yb_0.60Hf_0.40Cl₆ 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 LiNi_0.88Co_0.11Al_0.01O₂ in all-solid-state cells at 30 °C was achieved by employing ccp monoclinic Li_2.60Yb_0.60Hf_0.40Cl₆ prepared at 400 °C with a capacity retention of 83.6% after 1000 cycles.