Elucidating Li-ion adsorption and diffusion behavior on the surface of Cu0.7Co2.3O4 and improvement of performance as flexible full solid-state supercapacitor

Flexible lightweight supercapacitors are now actively investigated as one of the most promising power supplies for the portable and wearable electronics. Herein, radial nanowire arrays of Cu0.7Co2.3O4 are grown on the carbon fiber for a flexible electrode by hydrothermal process and subsequent thermal annealing. The Cu0.7Co2.3O4 electrode, with large exposure of low adsorption energy sites for Li ions due to the nanowires assembled by nanoparticles, displays superior electrochemical performance in 1 M LiCl aqueous solution and demonstrates areal capacitance of 340 mF cm−2 and 685Fg-1 at scan rate of 5 mV s−1. The contribution of Faraday process and capacitive process are analyzed in detail. The density functional theory is employed to calculate the Li+ ion adsorption energy and diffusion barrier, and the results indicate that Li ion can adsorb on 001 facet of the Co2.3Cu0.7O4 easily and the Li ion diffusion barrier along 001 plane is as low as 0.16 eV. Furthermore, a flexible full solid-state supercapacitor is obtained by assembling two pieces of Co2.3Cu0.7O4 electrode face to face, which presents good performance (112.5 mF cm−2 at 5 mV s−1), stability (74% after 5000 cycles), flexibility and wide operating voltage range. Three charged solid-state supercapacitors in series can light 48 green light emitting diodes for 150 s. The Cu0.7Co2.3O4 electrode is expected to be a potential candidate for actual applications.