Precisely gradient S-doping to optimize oxysulfide nanowires active centers for high-rate electrochemical energy storage

Abstract

Owing to high theoretical capacitance, nickel cobalt alkaline carbonate (NiCoAC) has attracted wide attention in electrochemical energy storage. However, the high surface ionic resistance and low bulk intrinsic activity result in NiCo-AC being unable to exhibit fast electronic response frequency and charge storage. Herein, via precisely controlling gradient S-doping, NiCo-AC nanowire is in situ converted into core-shell bimetallic oxysulfide (NiCo─O─S), with interior particularly presenting a granular due to the contraction of bulk structure, achieving the synergistic modification of surface and bulk. Specifically, owing to the lower band gap energy of metal sulfides, the derived shell optimizes OH⁻ adsorption center from Ni to Ni─Co─Ni sites with higher binding energy, and the granular core facilitates further ion diffusion with enhanced charge accumulation. Hence, NiCo─O─S reflect higher redox activity than that of nickel-cobalt sulfide (NiCo─S) and NiCo-AC, with an ultrahigh capacitance of 3,298 F g⁻¹ at 1 A g⁻¹. The as-fabricated supercapacitors display an outstanding energy density of 131 Wh kg⁻¹ at 800 W kg⁻¹ and present high capacitance retention of 98.5% and coulomb efficiency of 93.2％ under 12 000 charge-discharge cycles. This study reflects a new insight into activating the intrinsic activity of nanomaterials to further develop high-rate and stable electrodes.