Effect of Ca Doping on Structure and Electrochemical Properties of Li$_{3/8}$Sr$_{7/16}$Hf$_{1/4}$Nb$_{3/4}$O$_3$ Lithium‐Ion Conductors

Perovskite-structured solid electrolytes are promising candidates for next-generation solid-state lithium batteries due to their high ionic conductivity and structural stability. In this work, Li$_{3/8}$Sr$_{7/16−x}$Ca$_x$Hf$_{1/4}$Nb$_{3/4}$O$_3$ solid electrolytes with various calcium contents (x = 0, 0.02, 0.05, 0.08, and 0.44) were synthesized via a sol–gel method, followed by high-temperature treatment. Comprehensive structural and electrochemical characterizations were conducted to elucidate the effect of Ca$1 {2+}$ substitution on material performance. Increasing Ca$^{2+}$ concentration induced a structural transformation from cubic perovskite to an unidentified phase, suggesting a compositional limit for maintaining structural integrity. Among the investigated compositions, Li$_{3/8}$Sr$_{6.2/16}$Ca$_{0.05}$Hf$_{1/4}$Nb$_{3/4}$O$_3$ (LSCaHN) exhibited the highest ionic conductivity of 1.4 × 10$^{−4}$ S cm$^{−1}$ at room temperature, with negligible electronic contribution. Cyclic voltammetry revealed limited electrochemical stability against lithium metal, attributed to interfacial reactions that lead to electrolyte degradation. ... mehrThese findings highlight the importance of precise compositional design to balance structural stability and ionic transport in perovskite-type solid electrolytes for solid-state battery applications.