Emerging horizons in active filler engineering for next generation solid polymer electrolytes: a mini review

Solid polymer electrolytes (SPEs) are promising alternatives to liquid electrolytes for next-generation lithium batteries, offering improved safety, flexibility, and high-energy-density compatibility. However, low ionic conductivity, insufficient lithium-ion transference number, narrow electrochemical stability window, and poor mechanical strength hinder their practical application. Incorporating active fillers into polymer matrices has emerged as an effective strategy to address these limitations simultaneously. This review discusses the fundamentals of SPEs and the role of active fillers, including garnet, NASICON, perovskite, and sulfide-type ceramics, in enhancing ion transport and structural stability. Ion conduction mechanisms, polymer ceramic interactions, and interfacial transport pathways are systematically analyzed. Key performance metrics, including ionic conductivity, electrochemical stability, mechanical flexibility, and interfacial compatibility, are critically evaluated. Garnet-type fillers offer superior stability and conductivity; NASICON-type fillers provide 3D ion-conduction pathways; perovskite-type fillers enable fast bulk diffusion; and sulfide-type fillers deliver exceptional conductivity with favorable interfacial contact. Finally, remaining challenges and future directions, including interface engineering, hybrid multifunctional fillers, and scalable fabrication, are discussed toward realizing high-performance solid-state lithium batteries.