Influence of Na+/Li+ Co-doping on the structure, thermal behavior, and ionic conductivity of fluorapatite-based materials

Abstract

Fluorapatite-based materials were synthesized in three compositions: pure fluorapatite (FAp), and two Na + /Li + co-doped derivatives with substitution levels of x = 1 and x = 2(LNFAp x = 1, LNFAp x = 2). These materials were developed for potential use in energy storage systems, particularly as solid electrolytes in batteries. Structural, morphological, thermal, and electrochemical characterizations were performed using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), BET surface area analysis, thermogravimetric/differential thermal analysis (TGA/DTG/DSC), and electrochemical impedance spectroscopy (EIS). Introducing Na + and Li + into the fluorapatite lattice led to variations in crystallinity, microstructure, and surface area, with the doped samples exhibiting reduced surface area and porosity, likely due to enhanced grain growth and densification. A notable decrease in the calcium-to-phosphorus (Ca/P) atomic ratio was observed with increasing dopant concentration. This shift in the Ca/P ratio may favor the formation of vacancies or mobile charge carriers, thereby contributing to the observed enhancement in ionic conductivity, especially in the sample with x = 2. These results position Na/Li co-doped fluorapatites as promising candidates for sustainable solid-state battery components. This study aligns with the United Nations Sustainable Development Goals, particularly SDG 7 (Affordable and Clean Energy), SDG 9 (Industry, Innovation and Infrastructure), and SDG 12 (Responsible Consumption and Production), by promoting the development of bio-inspired, resource-efficient energy materials.