Design of a pseudocapacitive cathode based on polypyrrole-derived carbon tube supported anthraquinone for lithium-ion hybrid capacitors

The research on lithium-ion hybrid capacitors (LIHCs) has become one of the hottest areas due to larger specific energy than supercapacitors along with higher specific power than lithium-ion batteries (LIBs). LIHCs' cathode composed of electric double layer capacitive material has significantly lower specific capacity than their anode based on the LIB-type material leading to the capacity imbalance between the cathode and anode. Therefore, in the LIHC field, improving the specific capacity of cathode is one of the key issues that needs to be solved urgently. Herein, to improve the specific capacity of LIHCs' cathode, anthraquinone (AQ) which can undergo a redox reaction with the Li+ ions in the electrolyte to generate pseudocapacitance is introduced. Polypyrrolederived carbon tube (CT) with an ultrahigh surface area is used as the supporter of AQ to form a pseudocapacitive carbon-based composite cathode with a high initial discharge specific capacity (125.1 mAh g-1@0.5 A g-1) together with 75.8% capacity retention after 1000 cycles under the optimal AQ/CT mass ratio. A CT//AQ/CT0.25 LIHC device was assembled with the optimized AQ/CT-0.25 cathode and a CT anode (mass ratio of 1:1). The constructed device delivers a remarkable specific energy (121.3 Wh kg-1@225.0 W kg-1 specific power) as well as an exceptional cyclic stability, as indicated by high capacity retention rate (81.5%) after 5000 constant charge/discharge cycles@1 A g-1.