Degradation Responses of Activated-Carbon-Based EDLCs for Higher Voltage Operation and Their Factors

To investigate the degradation mechanisms of electric double-layer capacitor (EDLC) components using 1.0 M triethylmethylammonium (TEMA) tetrafluoroborate (BF4) in propylene carbonate (PC), the failure-mode processes of positive and negative electrodes were characterized as a function of the applied voltage (2.5-4.0 V). When the cell voltage ranges below 3.0 V, no impedance spectra or surface morphology changes were observed, indicating that no side reactions occur in this case. In the voltage range from 3.0 to 3.7 V, the exfoliation of graphene layers in activated carbon (AC) and the formation of cracks were observed in the positive electrode over 4.9 V vs Li/Li+ possibly due to the gasification of surface functional groups with adsorbed water. On the negative electrode, the adsorbed water is electrochemically reduced to H-2 gas and OH-. The generated OH-induces the Hoffman elimination of TEMA(+) and activates the hydrolysis of PC. These water-induced side reactions could be the most critical factors for higher voltage operation. In the higher voltage range (over 3.7 V), the accumulation of solid electrolyte interface films by electrochemical oxidation and the reduction of PC were observed for both electrodes, indicating that the electrochemical oxidation and the reduction of PC on the AC surfaces occur above 5.2 V and below 1.5 V vs Li/Li+, respectively. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3126423] All rights reserved.