Mille-Crêpe-like Metal Phosphide Nanocrystals/Carbon Nanotube Film Composites as High-Capacitance Negative Electrodes in Asymmetric Supercapacitors

Transition metal phosphides (TMPs) are emerging as high-performance and promising electrode materials for use in asymmetric supercapacitors (ASCs). Herein, we demonstrate that cobalt phosphide (CoP) nanocrystals supported on carbon nanofibers (CoP/CNF) can serve as negative electrode materials in ASCs and show outstanding specific capacitance 748 F g^-1 at 2 A g^-1 and superior long-term cycle stability, outperforming conventional carbon-based and nearly all TMP-based negative electrode materials reported previously. A binder-free mille-crêpe-like negative electrode is fabricated by loading CoP/CNF onto an ultrathin, interconnected carbon nanotube (CNT) film current collector, followed by folding the film multiple times, which exhibits a high total mass (active materials and current collector) based specific capacitance of 200.0 F g^-1 at 1 A g^-1, due to the lightweight of the CNT film collector. Furthermore, we fabricate an all-phosphide ASC (APASC) using CoP/CNF/CNT as the negative electrode and NiP/CNF/CNT as the positive electrode, which delivers a specific capacitance of 163.8 F g^-1 and shows excellent long-cycle stability up to 50000 cycles. The APASC's capacitance can be further improved to 209 F g^-1 if a bimetallic CoNiP/CNF/CNT positive electrode is used. The APASC holds substantial promise for use in the next generation energy storage systems.