Tuning the Electron Density of Aromatic Solvent for Stable Solid‐Electrolyte‐Interphase Layer in Carbonate‐Based Lithium Metal Batteries

Lithium metal has been hailed as a key enabler of upcoming rechargeable batteries with high energy densities. Nonetheless, uncontrolled dendritic growth and resulting formation of a nonuniform solid‐electrolyte‐interphase (SEI) layer constitute an ever‐challenging obstacle in long‐term cyclability and safety. So far, these drawbacks have been addressed mainly by using noncarbonate electrolytes based on their relatively mild decomposition under reductive environments. Here, toluene as a co‐solvent of carbonate‐based electrolytes for lithium metal anodes is reported. The electron‐donating nature of the methyl group of toluene shifts the reduction of toluene prior to that of commonly used carbonate solvents, resulting in a more uniform and rigid SEI layer. Moreover, the polymerization process of toluene induces the decomposition of the bis(fluorosulfonyl)imide (FSI) anion in LiFSI salt to yield uniform distribution of lithium fluoride (LiF), and thus lowers an onset salt concentration in realizing the so‐called “high‐concentration effect” to 3 molar concentration, instead of 5 molar concentration as in typical electrolyte cases. This investigation reveals the usefulness of aromatic compounds in improving the stability of the SEI layer in lithium metal anodes, particularly by tuning the electron density of the benzene ring.