Water/Ionic Liquid/Succinonitrile Hybrid Electrolytes

Abstract

The water-in-salt concept has significantly improved the electrochemical stability of aqueous electrolytes, and the hybridization with organic solvents or ionic liquids has further enhanced their reductive stability.[1] Here, we open a large design space by introducing succinonitrile as a cosolvent in water/ionic liquid/succinonitrile hybrid electrolytes. Via addition of the nitrile, electrolyte performance metrics such as electrochemical stability, conductivity, or cost can be tuned, and salt solubility limits can be fully circumvented. We elucidate the solution structure of two select hybrid electrolytes and highlight the impact of each electrolyte component on the final formulation, showing that excess ionic liquid fractions decrease the lithium transport number, while excess nitrile addition reduces electrochemical stability and yields flammable electrolytes. If component ratios are tuned appropriately, high electrochemical stability is achieved and aqueous Li4Ti5O12 - LiNi0.8Mn0.1Co0.1O2 full cells show excellent cycling stability with a maximum energy density of ca. 140 Wh/kg of active material, and Coulombic efficiencies of close to 99.5% at 1C. Furthermore, strong rate performance over a wide temperature range, facilitated by the fast conformational dynamics of succinonitrile, with a capacity retention of 53% at 10C relative to 1C is observed.[2]