Insights into tailoring composite solid polymer electrolytes for solid-state lithium batteries

Lithium-ion batteries (LIBs) have widely revolutionised our lifestyle in the Battery of Things era. However, safety issues in LIBs that utilise organic liquid electrolytes have been regarded as a bottleneck to rapidly expanding the application of batteries in electric vehicles and large-scale energy storage systems. Fortunately, solid-state batteries (SSBs) using solid-state electrolytes are emerging as promising alternatives to conventional LIBs since they improve the energy density and stability of the batteries. In this regard, solid electrolytes play a crucial role in developing SSBs. Among various types of solid-state electrolytes, composite solid polymer electrolytes (CSPEs) are of particular interest by harmonizing the advantages of the inorganic solid and solid polymer electrolytes. However, the application of CSPEs is limited by their intrinsic shortcomings. This review discusses the comprehensive and extensive understanding of the state-of-art of CSPEs, mainly focusing on the fundamentals, design strategies, and synthesis methods for CSPEs to improve ionic conductivity and make stable interfaces with electrodes. In addition, future strategies and perspectives are provided with possible directions to accelerate the development of CSPEs.