High rate capabilities and remarkably cycle-stable flexible pseudocapacitors based on nano-coralloid arrays with sulfide vacancies enhanced Ni-Co-S nanoparticle covering.

Both poor electron conductivity and low ion diffusion of electrode materials are two main issues limiting the rate performance of pseudocapacitors. The present work reports the design and fabrication of hierarchically nano-architectured electrode consisting of sulfide vacancies enhanced Ni-Co-S nanoparticle covering bent nickel nano-forest (BNNF). We propose new insight into largely increased ion-accessible active sites and fast charge storage/delivery enhanced the reaction kinetics. The Ni-Co-S@BNNF electrode exhibits extremely high rate performance with 91% capacity retention from 1 to 20 A g-1, and even still remains 83.6% capacity at 40 A g-1, much superior to reported NiCo2S4-based electrodes. This is attributed to the unique nano-architecture providing increased ion availability of electrochemical active sites and high conductivity for fast electron transport. Especailly the electrode achieves remarkable long-term cycle stability with more than 100% initial capacity value after 5,000 cycles at 5 A g-1, and exhibits excellent cycle reversibility even at 20 A g-1. This should be attributed to the sulfide vacancies in Ni-Co-S nanobranches and the electrode architecture sustaining structural strain during fast redox reactions. An asymmetric pseudocapacitor applying such electrode achieves a high energy density of 99.9 W h kg-1 and exhibit superior cycling stability at a high current density of 20 A g-1. This study underscores the potential importance of developing nanoarrays covered with highly redox-active materials with increasing ions/charge kinetics for energy storage.