Artificial Solid Electrolyte Interphases for Lithium Metal Electrodes by Wet Processing: The Role of Metal Salt Concentration and Solvent Choice

In this study, the artificial solid electrolyte interphase (SEI) formed on lithium metal when treated in ZnCl₂ solutions is thoroughly investigated. The artificial SEI on lithium metal electrodes substantially decreases the interfacial resistance by ca. 80% and improves cycling stability in comparison to untreated lithium. The presence of a native SEI negatively affects the morphology and interfacial resistance of the artificial SEI. Increasing the ZnCl₂ concentration in tetrahydrofuran (THF) (precursor solution) results in higher homogeneity of the surface morphology. Independent of the ZnCl₂ concentrations, the artificial SEI is composed of C_<i>x</i>, CO, LiCl, Li₂CO₃, ZnCl₂, and Li_<i>x</i>Zn_<i>y</i> alloys. ZnCl₂ (1 M) produces the most homogenous surface and additional surface species with carbonyl side groups. Nonetheless, the ZnCl₂ concentration only has a small effect on the interfacial resistance or cycling stability. Using ethyl methyl carbonate (EMC) as the solvent significantly reduces the interfacial resistance to 7 Ω cm², in comparison to 25 Ω cm² for THF. The composition of the artificial SEIs varies depending on the solvent. Either way, the SEI consists of C_<i>x</i> Li_<i>x</i>C, LiCl, Li₂CO₃, ZnCl₂, and LiZn alloys. The THF-based SEI additionally features ether and carbonyl groups, LiZnO, and Zn metal. For the artificial SEI formed with both solvents, the atomic percentage of the LiZn alloy increases close to the Li surface.