Composite polymer electrolytes: progress, challenges, and future outlook for sodium-ion batteries

Sodium-ion battery (SIB) arises as propitious energy sources complementing the energy supply demands amidst of proliferating energy crises and environmental trauma due to fossil fuel consumption. Higher earth abundance, similar electrochemistry as lithium, and cost-effectiveness have driven the research focused on building better SIBs. Solid inorganic and polymer electrolytes (PEs) are prevailing electrolyte candidates for SIBs. The bottleneck of both the electrolytes, such as low ionic conductivity, poor mechanical and thermal stability, and high interfacial charge resistance, has retarded the rate of their commercial acceptance for futuristic energy devices. To tackle these burning issues, strategies to couple inorganic and polymer electrolytes as composite polymer electrolytes (CPEs) are drawing immense interest in academia and industry. The present review discusses the state-of-the-art composite polymer electrolytes for SIBs. It comprises three parts. The first part briefs about the introduction and performance index of CPEs to assess the importance of CPEs over existing electrolytes. In the second part, various synthesis methods for CPEs preparations are encapsulated. The third part is focused on the role of extrinsic fillers (active and passive) and the corresponding mechanism involved in ionic transport in CPEs by recently reported works. The role of filler engineering in addressing the remedies of CPEs is also intensely scrutinized. Finally, this review is concluded with the perspective of CPEs toward the future of SIB development. This review is aiming to understand the insight of fillers within CPEs and their impact on the performance of SIBs.