Nowadays, most of commercial Li-ion batteries use organic-based electrolytes, but the organic components are encountering several safety and environmental concerns. For example, ester based solvents are highly flammable and reactive with electrodes. These safety concerns are hindering the development of energy storage devices for high energy applications, such as electric vehicles and grid storage. Aqueous-based electrolytes may alleviate these safety concerns with several additional potential advantages, such as high conductivity and low cost. Nevertheless, aqueous-based electrolyte systems suffer from narrow operation voltage window (1.23 V), as aqueous-based electrolytes due to water-splitting reaction at both the high potential (3.7 V versus Li/Li⁺, oxygen evolution reaction) and low potential (2.5 V versus Li/Li⁺, hydrogen evolution reaction). New approaches based on water-in-salt concept, which involves very high slat concentrations (e.g., 21m LiTFSI_(aq)) to reduce water decomposition reactions, have recently been proposed for widening the voltage window. The extremely high salt concentrations used in these new approaches impose serious cost and new safety concerns. In this work, a new method using electrode surface modification to reduce the salt concentrations needed for widened voltage windows is explored. The surface modification is shown to be effective in reducing water reduction toward the lower end of the voltage window. As a result, a significantly widened operation voltage window can be achieved with salt concentrations that are substantially lower than those typically reported in the water-in-salt methods.

Figure 1: Cyclic voltammetry test of Ti current collectors with or without surface modification in 1m LiTFSI_(aq).