Effects of mixed phases on ionic conductivities for (LaF3)1–x(PbF2)x fast-fluoride-ion-conducting solid electrolytes

Abstract

All-solid-state fluoride-ion batteries (FIBs) with a high theoretical energy density have attracted considerable attention due to their enhanced safety features and superior capacity. The selection of solid electrolyte materials with enhanced fluoride-ion conductivity is crucial for the successful implementation of high fluoride-ion energy storage in realistic scenarios. In this study, heterogeneous solid fluoride-ion electrolytes with different ratios of (LaF₃)_1−x(PbF₂)_x are investigated. In contrast to other research efforts focusing on the development of single-phase, single-lattice-defect solid-state electrolytes, the (LaF₃)_1−x(PbF₂)_x material incorporates two phases with different defects, namely fluoride vacancies and mobile fluoride ions. This unique combination of defects results in a significant enhancement in conductivity while maintaining their coexistence. The crystal gaps are filled by the orthorhombic phase, which is conductive, resulting in a reduction of the contact resistance due to the uniform distribution of the three crystal phases and the heterointerface between the hexagonal phase and the cubic phase. The multi-component electrolyte (LaF₃)_0.4(PbF₂)_0.6 exhibits the best performance among the samples tested. The electrolyte exhibits a high fluoride-ion conductivity, with a value of 2.78 × 10⁻⁴ S cm⁻¹ at 120 °C and 6.37 × 10⁻⁶ S cm⁻¹ at room temperature. The electronic conductivity is negligible with a value of 5.48 × 10⁻⁸ S cm⁻¹. Furthermore, the electrolyte has an electrochemical stability window of 1.85 V vs. Sn/Sn²⁺. Due to its favorable electrochemical properties, this material has considerable potential as a solid-state fluoride-ion electrolyte.