Adding salt to expand voltage window of humid ionic liquids

Humid hydrophobic ionic liquids-widely used as electrolytes-have narrowed electrochemical windows due to the involvement of water, absorbed on the electrode surface, in electrolysis. In this work, we performed molecular dynamics simulations to explore effects of adding Li salt in humid ionic liquids on the water adsorbed on the electrode surface. Results reveal that most of the water molecules are pushed away from both cathode and anode, by adding salt. The water remaining on the electrode is almost bound with Li+, having significantly lowered activity. The Li+-bonding and re-arrangement of the surface-adsorbed water both facilitate the inhibition of water electrolysis, and thus prevent the reduction of electrochemical windows of humid hydrophobic ionic liquids. This finding is testified by cyclic voltammetry measurements where salt-in-humid ionic liquids exhibit enlarged electrochemical windows. Our work provides the underlying mechanism and a simple but practical approach for protection of humid ionic liquids from electrochemical performance degradation. Water adsorbed on the electrode surface of electrochemical energy storage devices narrows the electrochemical windows. Combining atomistic simulations and cyclic voltammetry experiments the authors demonstrate an approach to expand the voltage window of humid ionic liquids based on the addition of lithium salt.