Encapsulation of NiCo2O4 in nitrogen-doped reduced graphene oxide for sodium ion capacitors

Abstract

Sodium-ion capacitors are considered as promising energy storage devices for medium/large-scale energy storage applications including electric vehicles (EVs) and smart grid technologies because of their high energy/power densities and long cycle life. However, finding a high-performance anode material has been one of the great challenges in developing this sustainable electrochemical energy storage technology. Nickel cobaltite (NiCo₂O₄) with rich electroactive sites is a promising anode material for electrochemical capacitors and hybrid ion capacitors. However, this material is unstable due to large volume changes during repeated cycles. Here, we report an approach to improve the stability of NiCo₂O₄ against cycling by using a nitrogen-doped graphene framework to encapsulate NiCo₂O₄ particles. The graphene framework guarantees good electronic conductivity and serves as a buffer to alleviate the volume changes of NiCo₂O₄. In a sodium half cell, the composite electrode displayed a reversible capacity of about 450 mA h g⁻¹ with a current rate of 0.1 A g⁻¹ at the 100^th cycle. A full-cell sodium ion capacitor configured with the composite as the anode and a commercial activated carbon as the cathode delivered an energy density of 48.8 W h kg⁻¹ at a power density of 9750 W kg⁻¹ with a stable cycle life. The good electrochemical performance of the electrode material indicates that using nitrogen-doped graphene sheets to stabilise NiCo₂O₄ particles is a feasible approach towards developing high-performance anode materials for sodium ion capacitors.