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

Abstract

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 LiHf<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf>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 × 10–5S 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 Li<inf>1.5</inf>Al<inf>0.5</inf>Hf<inf>1.5</inf>(PO<inf>4</inf>)<inf>3</inf>electrolyte demonstrated stable lithium plating/stripping in a Li

Li cell for over 170 h. Furthermore, when employed in a solid-state Li

LiFePO<inf>4</inf>cell, it exhibited high Coulombic efficiency and decent cycling stability, demonstrating its potential for use in high-temperature solid-state batteries. © 2025 Elsevier B.V., All rights reserved.