Beyond Lithium Lanthanum Titanate: Metal-Stable Hafnium Perovskite Electrolytes for Solid-State Batteries

Perovskite-type electrolytes (PTEs) exhibit high room-temperature ionic conductivity (~1 mS·cm⁻¹) but suffer from poor stability against metal anodes, limiting their application in metal all-solid-state batteries. We investigated alternatives to the prototypical PTE, lithium lanthanum titanate (LLTO), to identify PTE compositions with improved stability against metal-anodes and electrolyte performance. We evaluated composition–stability relationships of PTEs using DFT calculations of thermodynamic and electrochemical stabilities, diffusion barriers, and mechanical properties. Substituting Ti with Zr (LLZrO) and Hf (LLHfO) resulted in promising metastability against metallic Li anodes, although their diffusion barriers (0.188 – 0.576 eV) generally exceed those of LLTO. Unlike LLZrO, which lacks mechanical stability, LLHfO is predicted to be mechanically stable with balanced elasticities, high hardness, good ductility, low anisotropy, and the potential to suppress dendrite formation. These characteristics make LLHfO a promising Li-metal-compatible alternative to LLTO with strong overall performance metrics. Analogous Na-PTEs are stable against Na-metal; however, their high Na+ diffusion barriers (> 0.85 eV) must be lowered to achieve practical ionic conductivity.