National Synchrotron Light Source Seminar

Supercapacitors (SCs) are energy storage devices with much higher specific power, much longer cycle-life, much greater charge/discharge rate, but lower specific energy than rechargeable batteries. It is desired to develop all-solid-state supercapacitors that are compact, light-weighted, flexible, environmentally benign, and suitable for use in a wide temperature range with a comparable specific energy to rechargeable batteries. However, conventional all-solid-state supercapacitors suffer from both very low specific power and energy mainly due to very limited access of ions to active sites and low conductivity of polymer electrolytes. This presentation describes a new type of supercapacitors, mediator supercapacitors. Introducing and dispersing mediator molecules into polymer electrolytes enable the ultimate accessibility of the ions in polymer electrolytes to the active sites of the active material, mediators, and greatly enhance charge transport in polymer electrolytes. Two series of all-solid-state mediator supercapacitors have been developed: one is based on Nafion separator and the other one is based on polyvinylidene fluoride (PVDF)/LiCF3SO3 separator. Both provide much greater specific power and energy than those for other solid-state electrolyte supercapacitors and greater than those for conventional liquid electrolyte supercapacitors. SEM/EDS, FTIR, XRD, and in situ XAS measurements using the BNL facility were conducted to characterize the physicochemical states of mediators in the electrodes and to elucidate the mechanisms of charge/discharge, ionic conduction, and cycle stability. In situ XAS demonstrate that the charge/discharge cycle correlate with the oxidation/reduction of the mediators and the majority of the charge capacity is provided by the mediators as the receivers or donors of electrons. As the sideline of this presentation, new developments and findings of atomistic simulation of polymer electrolyte/electrode interfaces and in situ electrochem