“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.