Composite solid electrolytes for all-solid-state lithium batteries

Compared to currently used liquid-electrolyte lithium batteries, all-solid-state lithium batteries are safer and possess longer cycle life and have less requirements on packaging and state-of-charge monitoring circuits. Among various types of solid electrolytes, composite solid electrolytes, which are composed of active or passive inorganic fillers and polymer matrices, have been considered as promising electrolyte candidates for all-solid-state lithium batteries. Incorporation of inorganic fillers into the polymer matrices has been demonstrated as an effective method to achieve high ionic conductivity and excellent interfacial contact with the electrodes. In this review article, we first summarize the historical development of composite solid electrolytes. Contribution of both inert inorganic fillers and active Li-ion conductors to the ionic conductivity, electrochemical stability, and mechanical properties of the composite solid electrolytes are elaborated. Possible mechanisms of conductivity enhancement by inorganic fillers are broadly discussed. Examples of different composite solid electrolyte design concepts, such as inorganic nanoparticle/polymer, inorganic nanofiber/polymer, and other inorganic/polymer composite solid electrolytes, are introduced and their advantages and disadvantages are discussed. Inorganic filler/polymer composite solid electrolytes studied for use in various Li battery systems including Li-ion, Li-sulfur, and Li-metal batteries are evaluated. Promising designs of composite solid electrolytes and cathode materials used in all-solid-state Li batteries are also introduced. Finally, future perspectives on current requirements of composite solid electrolyte technologies are highlighted.