Understanding the transport mechanism of lithium ions in solid-electrolyte interphase in lithium metal batteries with liquid electrolytes

Lithium (Li) metal battery is regarded as a high-energy-density battery system beyond Li-ion battery. However, the cycle life of Li metal batteries with liquid electrolytes is severely hindered by the high reactivity and non-uniform plating/stripping behaviors of Li metal anodes. The plating/stripping behaviors of Li metal anodes are mainly dictated by the transport mechanism of Li ions in solid electrolyte interphase (SEI), a nanoscale passivation film between the anode and electrolytes. SEI is composed of various inorganic and organic components and has a diversiform structure, which complicates the transport behaviors of Li ions in it and differentiates the Li-ion transport mechanism in SEI from that in common liquid and solid electrolytes. Therefore, understanding the transport mechanism of Li ions in SEI is imperative for rationally regulating SEI components and structure and enabling uniform Li plating/stripping behaviors. In this review, the recent progress in understanding the Li-ion transport mechanism in SEI in Li metal batteries with liquid electrolytes is summarized, including the detailed transport mechanisms of Li ions in SEI, and the methods to investigate and regulate the Li-ion transport mechanism in SEI. In particular, an insightful outlook is provided to guide future research on investigating the transport mechanism of Li ions in SEI.