Polymer electrolytes have attracted great interest for next-generation lithium (Li)-based batteries in terms of high energy density and safety. In this review, we summarize the ion-transport mechanisms, fundamental properties, and preparation techniques of various classes of polymer electrolytes, such as solvent-free polymer electrolytes (SPEs), gel polymer electrolytes (GPEs), and composite polymer electrolytes (CPEs). We also introduce the recent advances of non-aqueous Li-based battery systems, in which their performances can be intrinsically enhanced by polymer electrolytes. Those include high-voltage Li-ion batteries, flexible Li-ion batteries, Li-metal batteries, lithium-sulfur (Li-S) batteries, lithium-oxygen (Li-O2) batteries, and smart Li-ion batteries. Especially, the advantages of polymer electrolytes beyond safety improvement are highlighted. Finally, the remaining challenges and future perspectives are outlined to provide strategies to develop novel polymer electrolytes for high-performance Li-based batteries. The progress of lithium (Li)-based batteries has been greatly hindered by the safety issues originating from traditional non-aqueous liquid electrolytes. As alternatives of liquid electrolytes, polymer electrolytes have attracted great attention because of their merits such as low flammability, flexible processability, and more tolerance to vibration, shock, and mechanical deformation. Recently, the applications of polymer electrolytes in fields such as high-voltage Li-ion batteries, flexible Li-ion batteries, Li-metal batteries, Li-sulfur batteries, Li-oxygen batteries, and smart Li-ion batteries have inspired new research enthusiasm in both electrochemistry and material science communities. This review presents a survey of emerging polymer electrolytes, including solvent-free polymer electrolytes, gel polymer electrolytes, and composite polymer electrolytes, and highlights their recent developments in Li-based battery applications.