Utilization of (oxalato)borate-based organic electrolytes in activated carbon/graphite capacitors

The electrolyte salts composed of tetramethylammonium (TMA+) cation and difluoro(oxalato)borate (DFOB−) or bis(oxalato)borate (BOB−) anions have been proposed for the application in activated carbon (AC)/graphite capacitors. The electrochemical performance of AC/graphite capacitors has been studied using these electrolyte salts dissolved in propylene carbonate (PC). The intercalation behaviors of anions (BF4−, DFOB−, and BOB−) at the graphite positive electrodes have been investigated by in situ XRD measurements. The bigger the anion is, the higher the cell voltage is where the intercalation happens. Accordingly, the bigger the anion is, the smaller discharge capacity delivered by an AC/graphite capacitor. The charge mechanism of TMA+ at the AC negative side has also been addressed. Compared with other bigger quaternary alkyl ammonium cations, the specific capacitance of the AC negative electrode towards TMA+ adsorption is somehow smaller as estimated.

► (oxalato)borate-based electrolyte salts for activated carbon/graphite capacitors.
► Bigger anions are difficult to be intercalated into graphite electrodes smoothly.
► PC-solvated tetramethylammonium cation is adsorbed at activated carbon electrodes.