Lithium (Li)-ion batteries (LIBs) are indispensable for modern energy storage applications. However, Li dendrite formation poses serious challenges to their safety and long-term stability. As a key component of LIBs, separators must simultaneously prevent internal short circuits while enabling efficient Li + transport. Enhancing their electrolyte wettability, thermal stability, and mechanical strength is critical to improving overall battery performance. In this study, we present a functional separator design incorporating polyhedral oligomeric silsesquioxane-modified graphene oxide (GO-POSS) nanofillers to enhance the thermal and electrochemical performance of commercial polyethylene (PE) separators. GO provides abundant oxygen-containing functional groups that improve electrolyte wettability and facilitate Li + transport, whereas POSS forms a thermally stable ceramic char layer that reinforces thermal stability and interfacial adhesion. The resulting GO-POSS-coated separators exhibit improved ionic transport, excellent thermal resilience, enhanced rate capability, and extended cycle life. Notably, Li/NCM622 cells assembled with the GO-POSS-coated separator achieved 82.7% capacity retention after 300 cycles at 0.1 A g −1, compared with 76.5% for cells using unmodified PE separators. Furthermore, the GO-POSS coating suppressed Li dendrite formation, contributing to cycling stability and safety. These findings demonstrate the potential of GO-POSS-based separator designs in advancing high-performance and safe next-generation LIBs.