High-energy Sodium-Ion Hybrid Capacitors Through Nanograin-Boundary-induced Pseudocapacitance of Co3O4 Nanorods

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 similar to 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 (similar to 100%).(C) 2022 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.