Improved Li⁺ Transport and Interfacial Stability in PEO-LiTFSI Solid Electrolytes via Al₂O₃ Ceramic Fillers at 20 °C

Incorporation of ceramic nanofillers (Al₂O₃, SiO₂, TiO₂) into PEO-based solid electrolytes is investigated to compare their effects on Li⁺ transport and address the low room-temperature conductivity of PEO by reducing polymer crystallinity and facilitating Li⁺ migration through the amorphous phase. Among the tested fillers, 5 wt % Al₂O₃ reduces membrane crystallinity by ≈ 11% and increases ionic conductivity by ≈1.8-fold. Comparative analysis suggests that Al₂O₃ stands out due to possible interactions with PEO ether oxygens, as supported by structural and spectroscopic analyses. These interactions generate interconnected Li⁺ transport pathways along polymer-filler interfaces, improving ionic conduction. Structural (XRD, FTIR, Raman) and thermal (TGA/DSC) analyses confirm an amorphous and thermally stable network. Electrochemical studies demonstrate that the Al₂O₃-containing membrane provides extended oxidation stability up to 5.22 V, with Li⁺ transference number and diffusion coefficient 2.58 and 8 times higher than those of the filler-free PEO-LiTFSI system, respectively. This membrane displays reduced overpotential and an almost 100% recovery ratio after current-step cycling, emphasizing superior interfacial compatibility. Stable Li plating/stripping for ≈780 h at 50 μA cm^-2 further confirms suppression of lithium dendrite growth, achieving a cycling lifetime 5.5 times longer. These findings highlight the role of Al₂O₃ in promoting efficient low-temperature interfacial Li⁺ conduction, offering promising insights for the design of solid polymer electrolytes.