ABSTRACT
Developing battery-supercapacitor hybrid devices (BSHs) is viewed as an efficient route to shorten the gap between supercapacitors and batteries. In this study, a composite hydrogel consisting of perylene tetracarboxylic diimide (PTCDI) and reduced graphene oxide (rGO) is tested as the anode for BSHs in the electrolyte of ammonium acetate (NH4Ac) with a record concentration of 32 molality (m). This water-in-salt electrolyte exhibits a wide electrochemical stability window of 2.13 V and high conductivity of 23.3 mS cm-1 even at -12 °C. Molecular dynamics calculations and spectroscopic measurements reveal that a favorable water-acetate interaction occurs in a high concentration NH4Ac electrolyte. On the other hand, the study of electrode kinetics in 32 m NH4Ac demonstrates a high capacitive contribution to charge storage in PTCDI-rGO although an electrode redox reaction involves reversible enolization of carbonyl groups in PTCDI. This result suggests fast NH4+-ion intercalation kinetics in charge-discharge processes. Furthermore, the electrode performance is improved by optimizing the loading amount of rGO in composites. The best-performing composite electrode delivers the maximum capacity of 165 mAh g-1 at 0.5 A g-1 and sustains a great capacity retention of 66% at 8 A g-1. Finally, an all-organic BSH device is tested in a broad temperature window from -20 to 50 °C and is well operated at 1.9 V regardless of operating temperatures. Due to the synergetic effect of splendid electrolyte properties and high anode capacities, BSH devices possess the maximum energy density of 12.9 Wh kg-1 at the power density of 827 W kg-1 and retain 74 % of the initial capacity after 3000 cycles at 1 A g-1. Our study paves a novel route towards designing inexpensive and environmentally friendly BSH devices with high performances.
