Si-doped NASICON-type Li1.4Al0.4Ti1.6(PO4)3 solid electrolytes for enhanced stability and performance of Li-CO2 batteries

Abstract

Li-CO₂ batteries (LCBs) have attracted significant research interest owing to their potential as energy storage devices and their contribution to carbon neutrality. In this study, we synthesized a solid electrolyte using Si-doped Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LASTP), by incorporating Si into the NASICON-structured LATP. Through Si doping, P in the tetrahedral PO₄ units within the NASICON framework is substituted with Si, and bridging oxygen bonds are formed after high-temperature heat treatment The LASTP powder synthesized via a solution-based method exhibited uniform particle size and composition, and the resulting pellet achieved high densification and the formation of interconnected structures. The pellet exhibited an ionic conductivity of approximately 8.8 × 10^-4 S/cm at 25℃. The LCB utilizing LASTP demonstrated a maximum discharge capacity of 23,887 mAh/g and successfully operated for 200 cycles at a current density of 100 mA/g with a cut-off capacity of 500 mAh/g. The post-cycling analysis of the cathode confirmed the reversible reactions of the LCB. Additionally, comparative post-cycling XPS analysis of LATP and LASTP revealed that Si doping in LASTP mitigated the reduction of Ti⁴⁺ to Ti³⁺, thereby enhancing the chemical stability of the solid electrolyte. Also, the structural stability of the solid electrolyte was enhanced owing to the formation of new bonds, surpassing the cycle performance and full-depth capacity of LCBs using conventional solid electrolytes. The introduction of structurally and chemically stabilized LASTP enabled the realization of long-lasting, high-capacity LCBs.