Identifying
and understanding charge storage mechanisms is important
for advancing energy storage. Well-separated peaks in cyclic voltammograms
(CVs) are considered key indicators of diffusion-controlled electrochemical
processes with distinct Faradaic charge transfer. Herein, we report
on an electrochemical system with separated CV peaks, accompanied
by surface-controlled partial charge transfer, in 2D Ti3C2Tx MXene in water-in-salt
electrolytes. The process involves the insertion/desertion of desolvation-free
cations, leading to an abrupt change of the interlayer spacing between
MXene sheets. This unusual behavior increases charge storage at positive
potentials, thereby increasing the amount of energy stored. This also
demonstrates opportunities for the development of high-rate aqueous
energy storage devices and electrochemical actuators using safe and
inexpensive aqueous electrolytes.