Cork-Derived Carbon Sheets for High-Performance Na-Ion Capacitors

Abstract

S-doped carbon sheets have been easily prepared by deconstructing the 3D cellular structure of a fully sustainable and renewable biomass material such as cork through a mild ball-milling process. S-doping of the material (>14 wt % S) has been achieved by using sulfur as an earth-abundant, cost-effective, and environmentally benign S-dopant. Such synthesized materials provide large Na storage capacities in the range of 300-550 mAh g-1 at 0.1 A g-1 and can handle large current densities of 10 A g-1, providing 55-140 mAh g-1. Their increased packing density compared to the 3D pristine structure allows them to also provide good volumetric capacities in the range of 285-522 mAh cm-3 at 0.1 A g-1 and 53-133 mAh cm-3 at 10 A g-1. In addition, highly porous carbon sheets (SBET > 2700 m2 g-1) have been produced from the same carbon precursor by rationally designing the chemical activation approach. These materials are able to provide good anion storage capacities/capacitances of up to 100-114 mAh g-1/163-196 F g-1. A sodium-ion capacitor assembled with the optimized S-doped carbon sheets and the highly porous carbon sheets with mass matching ratios provided the best energy/power characteristics (90 Wh kg-1 at 29 kW kg-1) in combination with robust cycling stability over 10,000 cycles, with a capacity fade of only 0.0018% per cycle.