A review on the ionic conductivity and mechanical properties of composite polymer electrolytes (CPEs) for lithium batteries: Insights from the perspective of polymer/filler composites

All-solid-state lithium batteries have become a focal point in both academic and industrial circles. Composite polymer electrolytes (CPEs), amalgamating the benefits of inorganic and polymer electrolytes, offer satisfactory ionic conductivity, robust mechanical properties, and advantageous interfacial interactions with electrodes. Consequently, they have the potential to significantly enhance the electrochemical performance of all-solid-state batteries compared to those relying solely on a polymer or inorganic electrolyte. As a kind of polymer/filler composites, the electrochemical and mechanical properties of CPEs are related to the fundamental characteristics of the inorganic phase, polymer phase and polymer/filler interface. This is the first review on the combined electrochemical and mechanical properties as well as their optimization methods from a polymer/filler composites perspective. Herein, a summary of the fabrication methods of zero-, one- and two-dimensional (i.e., 0D, 1D and 2D) inorganic fillers is presented. Also, the dual mechanical properties and ionic conductivity of some typical inorganic fillers and polymers are highlighted. The key factors (e.g., inorganic fillers - category, concentration, size and shape; polymers - category and molecular weight; and polymer/ filler interface) which influence these dual-functional properties are then discussed. Emphasis is given to the polymer/filler interface optimization methods, which serve as routes to improve both the electrochemical and mechanical properties of CPEs. Finally, future research directions are outlined for the development of high-performance CPEs.