Dual Electrolyte Additives Enabling Bilayer SEI to Suppress Hydrogen Evolution Reaction in Aqueous Li-Ion Batteries

Abstract

Aqueous Li-ion batteries (LIBs) feature safe operation, low cost, and environmental friendliness, but suffer from low energy density due to narrow electrochemical stability window (ESW) of aqueous electrolytes. While exploiting high-salt-concentration strategy of water-in-salt electrolytes (WiSEs) is effective in improving the electrochemical stability, the improvement is still insufficient to ensure practical feasibility of aqueous LIBs. In particular, the reduction stability of WiSEs needs further improvement to enable stable operation of low-voltage anode materials including Li4Ti5O12.This talk reports that the co-use of two electrolyte additives can effectively expand the cathodic limit of various WiSE systems. The synergistic effect between the two additives is attributed to the formation of unique solid-electrolyte interphase composed of organic and inorganic bilayers. The inner inorganic layer suppresses hydrogen evolution reaction, while the outer hydrophobic organic polymer mitigates the dissolution of inner SEI. LiMn2O4/Li4Ti5O12 cells employing 21 m LiTFSI WISE with the dual additives exhibit excellent long-term cycling (>70% retention after 400 cycles at 25 oC) and good rate capability (110 mAh/g at 6 mA/cm2). The efficacy of the dual-additive approach is also demonstrated for other WiSE solutions including water-in-bisalt, hydrate melt, and aqueous/organic hybrid electrolytes, suggesting the general applicability of the dual-additive strategy. Figure 1