Unraveling the Mechanism of Enhanced Lithium Salt Solubility in Water-in-Salt Electrolytes Containing Ionic Liquids

Abstract

The narrow electrochemical stability window of aqueous electrolytes of ~1.5 V can be extended to >2.5 V employing high salt concentration.1-3 At a molality of, e.g., 21 moles of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) per kilogram of water, the water:lithium ratio equals 2.65, causing strong cation-water interactions and the alignment of TFSI anions in a water-blocking electrochemical double-layer at the cathode, resulting in particularly high oxidative stability.1,2 As these electrolytes are typically saturated solutions at room temperature, they tend to crystallize below room temperature. We recently showed that employing salts with asymmetric anions such as lithium (pentafluoroethanesulfonyl)(trifluoromethanesulfonyl)imide (LiPTFSI) is a very effective strategy to suppress crystallization in such water-in-salt electrolytes to temperatures below −10 °C.4,5 LiPTFSI has further been combined with LiTFSI to form a solution with an overall molality of 56 mol kg−1, lowering the water:lithium ratio to 1.6 Reducing the water:lithium ratio is a key challenge to enhance the reductive stability of water-in-salt electrolytes. However, increasing the salt concentration to such high levels leads to a low ionic conductivity of only 0.1 mS cm−1 at 30 °C.6 Recently it was demonstrated that the addition of quaternary ammonium-based ionic liquids to a water-in-salt electrolyte increases LiTFSI solubility, while maintaining high ionic conductivity.7 Yet it is unclear, where the enhanced LiTFSI solubility originates from.In this contribution, we unravel the mechanism enabling very high lithium salt solubility in the presence of ionic liquids in water-in-salt electrolytes via nuclear magnetic resonance and vibrational spectroscopy.8 We investigate hybrid electrolytes with a water:lithium ratio of only 0.93 and a LiTFSI molality of up to 60 mol kg−1, almost three times as high as in water. Generally, these ternary water-ionic liquid-lithium salt electrolytes have much better transport properties at high salt concentration reaching 1.4 mS cm−1 at 25 °C at a molality of 40 mol kg−1 LiTFSI and 20 mol kg−1 ionic liquid. In addition, the enhanced reductive stability enables stable cycling of a Li4Ti5O12 anode in combination with a nickel-rich lithium nickel manganese cobalt oxide (NMC811) cathode with high Coulombic efficiency demonstrating the promising potential of water-in-salt electrolytes.