Whole Landscape of the Origin and Evolution of Gassing in Supercapacitors at a High Voltage.

Although supercapacitors with acetonitrile-based electrolytes (AN-based SCs) have realized high-voltage (3.0 V) applications by manufacturers, gas generation at high voltages is a critical issue. Also, the exact origins and evolution mechanisms of gas generation during SC aging at 3.0 V still lack a whole landscape. In this work, floating tests under realistic working conditions are conducted by 22450-type cylindrical cells with an AN-based commercial electrolyte. Comprehensive insights into the origins and evolution mechanisms of gas species at 2.7 and 3.0 V are acquired, which involves multiple side reactions related to the electrode, current collector, and electrolyte. Both experimental evidence and density functional theory calculations demonstrate that the primary reasons for gas generation are residual water and oxygen-containing functional groups, especially hydroxyl and carboxyl. More importantly, additional types of gas (such as CO 2 , NH 3 , and alkenes) can only be detected at a higher voltage of 3.0 V rather than 2.7 V after failure, suggesting that these gas species can be regarded as the failure signatures at 3.0 V. This breakthrough analysis will provide fundamental guidance for failure evaluation and designing AN-based SCs with extended lifetime at 3.0 V.