PEO/LiTFSI-Based Composite Solid Polymer Electrolytes with Lithium Phosphate Fillers for Solid-State Lithium Batteries

PEO-based solid polymer electrolytes have been widely investigated as promising electrolyte materials for solid-state lithium batteries owing to their flexibility, processability, lightweight characteristics, and good compatibility with lithium salts. PEO enables Li-ion transport through coordination between ether oxygen groups and Li ions. However, its practical application remains limited by insufficient ionic conductivity at room temperature. This limitation is mainly associated with the high crystallinity of PEO and restricted segmental motion of polymer chains, which hinder efficient Li-ion migration within the polymer matrix. In this study, lithium phosphate-incorporated composite solid polymer electrolytes are prepared using a PEO/LiTFSI matrix to improve the ion conduction environment. The lithium phosphate fillers are introduced to suppress PEO crystallization, increase the amorphous region of the polymer matrix, and promote segmental motion, which is closely related to Li-ion transport. In addition, the fillers are expected to provide additional Li-ion migration pathways and contribute to improved interfacial characteristics between the polymer electrolyte and electrode. The influence of filler incorporation is investigated through thermal, structural, and electrochemical analyses, including crystallinity, thermal behavior, ionic conductivity, and interfacial stability. Based on these analyses, this study aims to clarify the relationship between filler-induced structural modification and Li-ion conduction behavior in PEO/LiTFSI-based composite electrolytes. This work provides useful insight into the design of polymer–ceramic composite solid electrolytes and their potential application in solid-state lithium batteries.