Controlled design of metal oxide-based (Mn2+/Nb5+) anodes for superior sodium-ion hybrid supercapacitors: Synergistic mechanisms of hybrid ion storage

Abstract

Due to the different energy storage mechanisms, anode electrodes have been challenging to match cathode ones in sodium-ion hybrid supercapacitors (Na–HSCs), resulting in low energy and power densities of the device. As a result, engineering anode materials with excellent electrochemical performance has aroused a great deal of interest. In this work, metal oxides (Mn2+/Nb5+) embedded into interconnected hollow carbon nanoboxes (MnO@HCNb and NaNbO3@HCNb) are fabricated by a simple template-assisted CVD method. Such a unique structure provides shortened solid-state transportation pathway and sufficient adsorption sites for Na+, with differing energy storage mechanisms from traditional Faradic reaction. Benefiting from it, the MnO@HCNb electrode exhibits an outstanding rate performance and an ultra-long lifespan of more than 10,000 cycles with 88.6% of capacity retention, which is rarely reported for oxide-based anodes. Moreover, energy storage mechanism of the composite and the important role of Mn/Na2O interface generated inside the core (MnO) - shell (C) structure during conversion reaction have been revealed. Hybrid ion storage mechanisms (adsorption, Faradic reaction and intercalation) are proposed with desirable compatibility, exhibiting uniform capacitive electrochemical behavior. Coupling with activated carbon cathode, a full cell of Na-HSC demonstrates a high energy density of 116 Wh kg−1 at 99 W kg−1 and 56.4 Wh kg−1 at 1.8 kW kg−1. Indeed, this work is inspiring to other researchers to further improve the electrochemical performance of anode materials by adjusting their energy storage mechanisms through structural reconstruction.