Regulating solvation environment of Li ions via high donor number anions for high-performance Li-metal batteries

Abstract

The evolution of solid electrolyte interphases (SEI) on Li metal anodes strongly depends on the solvation environment of Li ions in the electrolyte. Using solvent molecules with high Gutmann donor number (DN) is a commonly accepted strategy to modulate the solvation environment of Li ions. However, high DN solvent molecules react strongly with Li metal anodes. Herein, a novel strategy is developed to regulate the solvation environment via introducing salt anion with high DN as electrolyte additives, with lithium triflate (LiTf) as a case study. Tf− with high DN can effectively form an anion-dominated solvation sheath, which is confirmed by molecular dynamics (MD) simulations and Raman spectroscopy. Density functional theory (DFT) calculations show that Tf− demonstrates a strong reduction tendency during SEI formation due to its relatively low lowest unoccupied molecular orbital (LUMO) energy. Moreover, Tf− shows weak binding interaction with Li ions in the solvated structure, which is conducive to the desolvation process and preferential decomposition of the anion to form SEI. Consequently, a stable and LiF-enriched SEI is successfully constructed, leading to the effective inhibition of side reactions and achieving dendrite-free Li metal anodes. Cu@Li||Li cells based on LiTf-contained electrolytes exhibit low overpotential and long-term cycle life over 1400 h, and the Cu@Li||LiFePO4 full cells still have higher capacity retention (70 %) after 500 cycles with an overall Coulombic efficiency (99.6 %).