Polypyrrole encapsulation-protected porous multishelled CoO hollow microspheres for advanced all-solid-state asymmetric supercapacitors with boosted reaction kinetics and stability.

Transition metal oxides (TMOs) have shown great potential in high-performance supercapacitors (SCs) because of their high theoretical capacities, low cost and simple preparation process. However, considerable challenges still remain in simultaneously improving their electrical conductivity, reaction kinetics and stability. Herein, we deliberately designed a polypyrrole encapsulation-protected porous multishelled CoO hollow microspheres (pMS-CoO/PPy) composite via a modified carbon self-templating method and in situ oxidative polymerization route. The unique porous multishelled structure of the pMS-CoO hollow microspheres assembled by interconnected CoO nanoparticles can provide sufficient active sites, shorted ion diffusion paths and efficiently alleviate the structural strain. Meanwhile, the PPy encapsulation-protected nanolayers significantly improve their electrical conductivity, contribute pseudocapacitance and protect CoO nanoparticles from structural pulverization-chemical dissolution into electrolyte. The prepared pMS-CoO/PPy electrodes exhibited a high specific capacitance (1292.2 F g at 1 A g), excellent rate capability (1205.8 F g at 10 A g) and cycle stability (ultrahigh capacitance retention of 91.5% for 5000 cycles), which has rarely been achieved in previously reported CoO-based electrodes. Furthermore, the assembled all-solid-state asymmetric supercapacitors (pMS-CoO/PPy//AC) delivered a high energy density of 40.2 Wh kg at a power density of 761.7 W kg and superior stability with a capacitance retention of 90.6% for 5000 cycles. This study offers an effective nanostructure design strategy to solve the issues of TMOs and develop high-performance energy storage systems.