Hydrogen-Bond Restructuring of Water-in-Salt Electrolyte Confined in Ti 3 C 2 T x 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 Ti 3 C 2 T x 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.