Modulation of a Dual-Nitrile Electrolyte for High-Voltage and High-Reliability Carbon-Based Supercapacitors

Acetonitrile (ACN) is a common solvent in organic electrolytes for carbon-based supercapacitors due to its low viscosity and proper dielectric constant, but its high volatility and limited stability hinder high-voltage applications. This study develops a dual-nitrile electrolyte comprising ACN and adiponitrile (ADN) in varying volume ratios combined with tetraethylammonium tetrafluoroborate salt. The influence of ACN/ADN ratios on electrolyte properties, such as ionic conductivity, viscosity, and flammability, is evaluated. The electrochemical properties of porous carbon electrodes in various electrolytes, including voltage windows, rate capability, cycle life, self-discharge, leakage current, and gas evolution, are systematically examined. Among the recipes, the ACN/ADN-50/50 electrolyte emerges as the optimal composition. Then, we compare the conventional ACN electrolyte with the optimal ACN/ADN-50/50 electrolyte at 3.0 V, which is a harsh condition but favorable for achieving high energy/power densities. At 3.0 V, the proposed electrolyte shows promising performance and outperforms the ACN electrolyte in terms of rate capability and cycle life. Three-electrode cells are employed to monitor the capacitance and resistance variations of positive and negative electrodes simultaneously during cycling. In addition, post-mortem analyses are conducted on both electrodes to study electrode degradation mechanisms. These findings highlight the potential of this dual-nitrile electrolyte for durable and reliable high-voltage supercapacitors.