Revealing the enhancement of Li plating/stripping efficiency in TEGDME-based low-concentration electrolytes for anode-free lithium metal batteries

Anode-free lithium metal batteries (AFLMBs) have attracted great attention owing to their higher energy density compared to conventional lithium metal batteries. Unfortunately, AFLMBs still suffer from poor Coulombic efficiency (CE) due to severe dendrite growth and the unstable solid-electrolyte interface (SEI) at the anode. Therefore, we explored the effect of concentration and LiNO3 additive on the SEI layer and on Li plating/stripping efficiency using a low-concentration tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte in an AFLMB anode half-cell configuration. It was found that the formation of protective Li2O-based SEI layers when blocking the oxidative subsequent SEI formation (called "OSS") above 2.2 V vs. Li/Li+ improved Li plating/stripping stability, while the LiNO3 additive suppressed the oxidation of Li2O into Li2O2 and sustained the existence of Li2O without blocking OSS, therefore improving CE performance. Although increasing the concentration (from 0.4 M to 2.0 M) did not have a major effect, the 2.0 M electrolyte with LiNO3 additive shifted the dominant SEI species from Li2O to LiF, further enhancing CE performance. The oxidation of the major SEI component Li2O via the oxidative subsequent SEI formation process is suppressed under the influence of LiNO3 additive, favoring a dendrite-less surface in the dilute LiTFSI/TEGDME electrolyte.