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 configura-tion. 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 sta-bility (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 speci-fic 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%).

Key words: Sodium-ion hybrid capacitor, Cobalt oxide nanorod, Nanograin-boundary, Pseudocapacitance