The advancement of lithium-based batteries has spurred anticipation for enhanced energy density, extended cycle life and reduced capacity degradation. However, these benefits are accompanied by potential risks, such as thermal runaway and explosions due to higher energy density. Currently, liquid organic electrolytes are the predominant choice for lithium batteries, despite their limitations in terms of mechanical strength and vulnerability to leakage. The development of polymer electrolytes, with their high Young’s modulus and enhanced safety features, offers a potential solution to the drawbacks of traditional liquid electrolytes. Despite these advantages, polymer electrolytes are still susceptible to burning and decomposition. To address this issue, researchers have conducted extensive studies to improve their flame-retardant properties from various perspectives. This review provides a concise overview of the thermal runaway mechanisms, flame-retardant mechanisms and electrochemical performance of polymer electrolytes. It also outlines the advancements in flame-retardant polymer electrolytes through the incorporation of various additives and the selection of inherently flame-retardant matrix. This review aims to offer a comprehensive understanding of flame-retardant polymer electrolytes and serve as a guide for future research in this field.

Growing market demand from portable electronics to electric automobiles boosts the development of lithium-ion batteries (LIBs) with high energy density and rate performance. However, strong solvation effect between lithium ions (Li⁺) and solvent molecules in common electrolytes limits the mobility of Li⁺ ions in electrolytes. Consequently, anions dominate the charge conduction in electrolytes, and in most cases, the value of Li⁺ transference number (T₊) is between 0.2 and 0.4. A low T₊ will aggravate concentration polarization in the process of charging and discharging, especially at high rate, which not only increases the overpotential but also intensifies side reactions, along with uneven deposition of lithium (Li) and the growth of lithium dendrites when lithium metal is used as anode. In this review, promising strategies to improve T₊ in liquid electrolytes would be summarized. The migration of Li⁺ ions is affected directly by the types and concentration of lithium salts, solvents, and additives in bulk electrolytes. Besides, Li⁺ ions will pass through the separator and solid electrolyte interphase (SEI) when transferring between anodes and cathodes. With this in mind, we will classify and summarize threads of enhancing T₊ from five aspects: lithium salts, solvents, additives, separators, and SEI based on different mechanisms, including covalently bonding, desolvation effect, Lewis acid-base interaction, electrostatic interaction, pore sieving, and supramolecular interaction. We believe this review will present a systematic understanding and summary on T₊ and point out some feasible threads to enhance battery performance by enhancing T₊.