Polymer electrolytes with high cation transport number for rechargeable Li-metal batteries: current status and future direction

The development of solid polymer electrolytes for lithium-metal (Li⁰) batteries (LMBs) with high energy density and high safety has been a long-standing goal that attracted intensive efforts for over four decades. The low cation transport number (t₊) of most polymer electrolytes based on polyether linkages often leads to a steep ion concentration gradient near the electrode surfaces that strongly reduces charge/discharge rate and favors Li⁰ dendrite growth. Although theoretically, the single-ion conducting polymer electrolytes (SIPEs) have the intrinsic potential to overcome this limitation, their very low ionic conductivities limit practical applications, especially under high current densities. Only high-cation transport number polymer electrolytes (HTPEs) with simultaneously high t₊ and high ionic conductivity could provide a solution to enable high-performance solid-state batteries (SSBs). Differing from previous reviews focusing on the design and synthesis of SIPEs chemical structure and morphology or their electrochemical performance, this review focuses on various strategies for improving the t₊ of polymer electrolytes. Meanwhile, the mechanical properties, interaction mechanisms and advantages of such cationic conducting polymers have also been explored. By summarizing the existing experimental results, we identify that the synergistic effect of high t₊ and high ionic conductivity enables the tolerance towards high current density and long-cycling stability in SSBs. This review will help to guide the design of new solid-electrolyte systems towards practical next-generation SSBs.