An investigation of spinel NiCo2O4 as anode for Na-ion capacitors

• Spinel NiCo2O4 as anode for hybrid NiCo2O4/AC Na-ion capacitors was investigated.
• The designed 3.0-V-class capacitor exhibited superior performance to the theoretical 4.3-V-class capacitor.
• The designed 3.0 V-class capacitor was mass-optimized.
• The Na-ion insertion/extraction reactions in the NiCo2O4 anode were controlled.

Sodium (Na)-ion-based energy storage devices have been regarded as a promising alternative to the traditional lithium (Li)-ion-based energy systems because of the more abundant Na reserves on the earth. This work, we have made an investigation about spinel nickel cobaltite (NiCo2O4) as an anode material for Na-ion capacitors (NICs). The NiCo2O4 materials have been fabricated via a facile and scalable chemical deposition route. The obtained NiCo2O4 materials display a typical agglomerate porous morphology with large specific surface area (190.1 m2 g−1) and high mesopore volume (0.943 cm3 g−1). The NiCo2O4 electrode was found to show non-preferable Na-ion insertion/extraction behavior under the full charge/discharge condition. The designed 3.0-V class NiCo2O4/activated carbon (AC) hybrid capacitor exhibits a much superior performance to the theoretical 4.3-V class capacitor, owing to the effective control of Na-ion insertion/extraction reactions in the NiCo2O4 electrode. The mass-optimized (1:1) 3.0-V class capacitor exhibits desirable energy and power performances (13.8 Wh kg−1, 308 W kg−1 at 0.4 A g−1), good cycling stability (61.2% energy retention after 2000 cycles at 0.15 A g−1) and excellent Coulombic efficiency (almost at 100%), indicating a potential application of spinel NiCo2O4 for NICs.