Tri-high designed graphene electrodes for long cycle-life supercapacitors with high mass loading

Abstract

Practical supercapacitor electrodes require high mass loading to enhance the energy density of the entire devices, which would limit ion diffusions in thicker electrodes and thus generally result in poor specific capacitance, rate capability and cycle life. Here, we demonstrate a "highly oriented, highly crumpled and highly doped" (3H) design for fabrication of high mass loading yet high performance nitrogen-doped graphene film (NGF) electrodes. NGF exhibits a unique long-range orientating and short-range crumpling structure, ensuring high packing density (up to 1.64 g cm-3) and efficient ion transmission simultaneously. The NGF-based symmetric supercapacitors (NGF-SC) displayed a specific capacitance 413 F cm-3 or 252 F g-1 at areal mass loading 0.32 mg cm-2, and 370 F cm-3 or 226 F g-1 at 11.2 mg cm-2 in aqueous electrolyte. For the case of high mass loading (11.2 mg cm-2), 90.1% retention was achieved after 100,000 cycles. In ionic liquid, the NGF-SC showed a high specific capacitance 352 F cm-3 or 215 F g-1 at 11.2 mg cm-2 in potential window 0–3.5 V, affording an ultrahigh electrode-based energy density 138 W h L-1. Due to the 3H design and high mass loading, the energy density of the whole NGF-SC device attains 65 W h L-1, much higher than those of commercial supercapacitors. Notably, such NGF-SC showed long lifespan up to 50,000 cycles with 84.8% retention, a record cycle-life for high mass loading supercapacitors. The 3H design provides a constructive principle for production of practical supercapacitors with both high energy density and power density.