Tuning the Ionic Conductivity and Structural Stability of LiHf₂(PO4)₃ Solid Electrolytes through Al Substitution

Rechargeable Li-metal batteries with solid electrolytes offer enhanced safety and higher specific capacity than conventional liquid-electrolyte-based Li-ion batteries. However, low Li-ion conductivity and high interfacial resistance between the electrolyte and electrode often hinder their performance. In this study, Al-substituted LiHf2(PO4)3 inorganic electrolytes are fabricated via the conventional solid-state reaction method. The Rietveld refinement of room-temperature X-ray diffraction data confirms a rhombohedral phase. The highest total ionic conductivity was observed for 25% Al substitution at the Hf site (7.09 × 10–5 S cm–1). The activation energy for total ionic conductivity decreased from 0.50 to 0.29 eV with increasing aluminum substitution until x = 0.5. The Li-ion transference number was ∼0.99, indicating that lithium ion dominates the charge transport in the material. Electrochemical stability tests using linear sweep voltammetry revealed the ceramic electrolyte’s stability up to approximately 4.61 V. The Li1.5Al0.5Hf1.5(PO4)3 electrolyte demonstrated stable lithium plating/stripping in a Li||Li cell for over 170 h. Furthermore, when employed in a solid-state Li||LiFePO4 cell, it exhibited high Coulombic efficiency and decent cycling stability, demonstrating its potential for use in high-temperature solid-state batteries.