A joint research team, led by Prof. WU Zhongshuai and Prof. FU Qiang from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), recently proposed a strategy for boosting the capacitance of graphene-based planar micro-supercapacitors (MSCs) using highly concentrated water-in-salt ambipolar redox electrolyte (ZnI2 + ZnCl2).
Using redox-active electrolytes to boost graphene electrodes is a highly-efficient strategy to increase the capacitive performance of MSCs. However, previously reported redox mediators could only offer a certain capacitance for a single electrode, leading to limited energy density due to the unmatched capacitances of two electrodes.
These two species allowed two electrons to be oxidized at the positive electrode and to be reduced at the negative electrode synchronously and individually, thus offering a large pseudocapacitive contribution for EG-MSCs.
They have realized high volumetric capacity of 106 mAh/cm3, energy density of 111 mWh/cm3, and long-term cycling stability with 92.1% retention after 5,300 cycles.
In situ characterizations confirmed that the good performance was attributed to the frustrated self-discharge by suppressing the formation and diffusion of polyiodide ions of I3- and I5-.
Moreover, EG-MSCs showed stable cycling performance at -20oC owing to the reduced freezing point of water by strong interactions between water molecules and zinc ions.
"This work opens a new avenue of introducing ambipolar redox mediators into highly concentrated electrolytes for high-performance MSCs," said Prof. WU.