Insight into Nickel‐Cobalt Oxysulfide Nanowires as Advanced Anode for Sodium‐Ion Capacitors

Transition metal oxides have a great potential in sodium‐ion capacitors (SICs) due to their pronouncedly higher capacity and low cost. However, their poor conductivity and fragile structure hinder their development. Herein, core‐shell‐like nickel‐cobalt oxysulfide (NCOS) nanowires are synthesized and demonstrated as an advanced SICs anode. The bimetallic oxysulfide with multiple cation valence can promote the sodium ion adsorption and redox reaction, massive defects enable accommodation of the volume change in the sodiation/desodiation process, meanwhile the core‐shell‐like structure provides abundant channels for fast transfer of sodium ions, thereby synergistically making the NCOS electrode exhibit a high reversible sodium ion storage capacity (1468.5 mAh g−1 at 0.1 A g−1) and an excellent cyclability (90.5% capacity retention after 1000 cycles). The in‐situ X‐ray diffraction analysis unravels the insertion and conversion mechanism for sodium storage in NCOS, and the enhanced capability of NCOS is further verified by the kinetic analysis and theoretical calculations. Finally, SICs consisting of the NCOS anode and a boron‐nitrogen co‐doped carbon nanotubes cathode deliver an energy density of 205.7 Wh kg−1, a power density of 22.5 kW kg−1, and an outstanding cycling lifespan. These results indicate an efficient strategy in designing a high‐performance anode for sodium storage based on bimetallic dianion compounds. Core‐shell‐like nickel‐cobalt oxysulfide nanowires (NCOS) as an advanced anode for sodium‐ion capacitors are synthesized via a feasible anion‐exchange strategy. Benefiting from the multiple cation valence and massive defects of the oxysulfide and the tunnel structure of the NCOS, the anode exhibits a superior sodium storage capability. In‐situ X‐ray diffraction and density function theory calculations reveal the mechanism for the improvement in sodium storage performance.