Defect-Driven Reconstruction of Na-Ion Diffusion Channels Enabling High-Performance Co-Doped TiO2 Anodes for Na-Ion Hybrid Capacitors

Na-ion hybrid capacitors (NICs) are known for their potential to integrate high power and energy density along with superior lifespan into a single energy storage device. However, the practical implementation of NICs is delayed due to their inadequate energy densities (<100 Wh kg −1 ), which is a result of the lack of anodes with rapid Na-ion diffusion kinetics to match the cathodes. To accelerate Na-ion diffusion kinetics, cobalt-doped TiO 2 (Co x Ti 1−x O y ) nanosheet anodes with reconstructed low-energy barrier channels for Na-ion transfer are designed. Crystal defects, including nanointerfaces, Ti interstitials, and oxygen vacancies, are intentionally introduced to the Co x Ti 1−x O y structure to improve its conductivity and induce pseudocapacitive-type Na-ion storage. Moreover, these crystal defects subtly alter the Na-ion transfer pathways in the bulk Co x Ti 1−x O y and reduce the energy barrier, as confirmed by density functional theory (DFT) simulations. Rapid Na-ion diffusion kinetics can minimize the kinetics discrepancy between anodes and cathodes, presenting great potential for achieving high-performance anodes for NIC applications. When integrated with activated carbon/reduced graphene oxide composite (AC/rGO) cathodes, the fabricated NICs demonstrate remarkable energy density (164 Wh kg −1 at 31 W kg −1 ), power density (8307 W kg −1 at 56 Wh kg −1 ), and an ultralong lifespan (83% capacity retention after 15000 cycles).