Hydrogen-Bond Restructuring of Water-in-Salt Electrolyte Confined in TiCT MXene Monitored by Operando Infrared Spectroscopy.

Highly concentrated water-in-salt aqueous electrolytes exhibit a wider potential window compared to conventional, dilute aqueous electrolytes. Coupled with MXenes, a family of two-dimensional transition metal carbides and nitrides with impressive charge storage capabilities, water-in-salt electrolytes present a potential candidate to replace flammable and toxic organic solvents in electrochemical energy storage devices. A new charge storage mechanism was recently discovered during electrochemical cycling of TiCT MXene electrodes in lithium-based water-in-salt electrolytes, attributed to intercalation and deintercalation of solvated Li ions at anodic potentials. Nevertheless, direct evidence of the state of Li solvation during cycling is still missing. Here, we investigate the hydrogen bonding of water intercalated between MXene layers during electrochemical cycling in a water-in-salt electrolyte with operando infrared spectroscopy. The hydrogen-bonding state of the confined water was found to change significantly as a function of potential and the concentration of Li ions in the interlayer space. This study provides fundamentally new insights into the electrolyte structural changes while intercalating Li in the MXene interlayer space.