High-energy sodium-ion hybrid capacitors through nanograin-boundary-induced pseudocapacitance of Co3O4 nanorods

Abstract

Sodium-ion hybrid capacitors (SICs) have been proposed to bridge performance gaps between batteries and supercapacitors, and thus realize both high energy density and power density in a single configuration. Nevertheless, applications of SICs are severely restricted by their insufficient energy densities (<100 Wh/kg) resulted from the kinetics imbalance between cathodes and anodes. Herein, we report a nanograin-boundary-rich hierarchical Co3O4 nanorod anode composed of ∼20 nm nanocrystallites. Extreme pseudocapacitance (up to 72%@1.0 mV/s) is achieved through nanograin-boundary-induced pseudocapacitive-type Na+ storage process. Co3O4 nanorod anode delivers in this case highly reversible capacity (810 mAh/g@0.025 A/g), excellent rate capability (335 mAh/g@5.0 A/g), and improved cycle stability (100 cycles@1.0 A/g with negligible capacity degradation). The outstanding performance can be credited to the hierarchical morphology of Co3O4 nanorods and the well-designed nanograin-boundaries between nanocrystallites that avoid particle agglomeration, induce pseudocapacitive-type Na+ storage, and accommodate volume variation during sodiation-desodiation processes. Nitrogen-doping of the Co3O4 nanorods not only generates defects for extra surficial Na+ storage but also increases the electronic conductivity for efficient charge separation and lowers energy barrier for Na+ intercalation. Synergy of conventional reaction mechanism and pseudocapacitive-type Na+ storage enables high specific capacity, rapid Na+ diffusion, and improved structural stability of the Co3O4 nanorod electrode. The SIC integrating this highly pseudocapacitive anode and activated carbon cathode delivers exceptional energy density (175 Wh/kg@40 W/kg), power density (6632 W/kg@37 Wh/kg), cycle life (6000 cycles@1.0 A/g with a capacity retention of 81%), and coulombic efficiency (∼100%).