Optimizing strategies for high Li+ transference number in solid state electrolytes for lithium batteries: A review

Abstract

Lithium-ion batteries (LIBs) have revolutionized in the field of energy storage. However, commercial LIBs contain highly volatile flammable and organic electrolytes, which makes LIBs posing significant safety hazard. Solid polymer electrolytes have the potential to address these safety problems and are expected to become the next generation of high safety all-solid LIBs electrolyte materials. Nonetheless, the strong solvation effect between lithium ions (Li⁺) and solvent molecules in common electrolytes limits the mobility of Li⁺ ions. As a result, anions dominate charge conduction in electrolytes, and in most cases, the Li⁺ transference number (t_Li⁺) is between 0.2 and 0.4. A low t_Li⁺ exacerbates concentration polarization during the charging and discharging process, especially at high rates. This not only increases the overpotential but also intensifies side reactions, leading to uneven deposition of lithium (Li) and the growth of lithium dendrites when lithium metal is used as anode. This review briefly discussed the lithium-ion conduction mechanism of solid polymer electrolytes and the research progress in improving the comprehensive electrochemical performance of solid polymer electrolytes. With this in mind, we classify and summarize approaches of enhancing t_Li⁺ from three perspectives: lithium salts, modification of polymers, and addition of fillers to polymer electrolytes. We believe this review will provide a systematic understanding and summary of t_Li⁺ and point out some feasible strategies to enhance battery performance by enhancing t_Li⁺.