“Water-In-Salt” Electrolyte-Based High-Voltage (2.7 V) Sustainable Symmetric Supercapacitor with Superb Electrochemical Performance—An Analysis of the Role of Electrolytic Ions in Extending the Cell Voltage

The present research on an aqueous electrolyte(s)-based supercapacitor, considered to be the next-generation storage device, is mainly focused on to achieve a wide potential window with high energy density. However, the water decomposition in the aqueous electrolyte has posed a major challenge to be tackled to perceive its future prospect. In this context, the present work has undertaken a systematic investigation to analyze the role of ions in the recently explored "water-in-salt" electrolyte(s) attaining the high cell voltage for the symmetric supercapacitor cell (SSC), constructed using N-doped reduced graphene oxide as the electrode material. The nature of electrolytic ions toward affecting the H-bonding network of the water structure and thereby influencing the cell voltage stability has been investigated by employing different electrolytes: 7 m CH3COONa; 2, 7, 12, and 17 m NaClO4; 2, 5, 7, and 11 m NaNO3; and 7, 17, 22, and 27 m CH3COOK. It exhibited the highest cell voltage of 2.7 V in 17 m NaClO4 with an energy density of 140 W h/kg at 640 W/kg, whereas for 11 m NaNO3, a cell voltage of 2.3 V is observed with an energy density of 72 W h/kg at 545 W/kg. In contrast, for 7 m CH3COONa and 27 m CH3COOK, cell voltages of 1.9 and 2.0 V have been obtained with energy densities of 40 W h/kg at 474 W/kg and 41 W h/kg at 500 W/kg, respectively. Among these electrolytes, the ClO4– anion and NO3– anion are found to be the water structure breakers (chaotrope), but the ClO4– anion exhibited much better chaotropicity as compared to the NO3– anion. In contrast, the CH3COO– anion is found to act as a water structure maker (kosmotrope) anion. To the best of our knowledge, this is among the highest cell voltages (2.7 V) obtained in the aqueous electrolyte-based symmetric supercapacitor with superb energy density and power density. Its potential for energy storage application has been illustrated by lighting 54 white light-emitting diodes (>3 V) for more than 6 min upon charging a single SSC for about 15 s.