Unraveling the Electrochemical Energy Storage Mechanism of Vanadium Hexacyanoferrate-Based Metal-Organic Frameworks As a Cathode for Aqueous Rechargeable Batteries

Considering the environmental pollution crisis caused by fossil fuel and the limited supply of those, developing alternative power stations is strongly urged. Accompanying the development of alternative power stations, the introduction of large-scale stationary energy storage systems (ESSs) connected with the electrical power grid is vital for enhanced accessibility of electricity and leveling-off the intermittent spikes in the generated power. To date, Li ion batteries (LIBs) have been successfully demonstrated as feasible power sources for portable devices. Nonetheless, the application of LIBs into ESSs faces obstacles due to the limited source and continuously increasing cost of raw lithium and high flammability and the high cost of the organic electrolytes. Consequently, aqueous rechargeable batteries (ARBs) with readily accessible electroactive guest ions of H 3 O + , Na + , or Mg 2+ , are the most suitable energy devices for ESSs. Very recently, Prussian blue analogues (PBAs) holding an important category of metal-organic frameworks have been highlighted as a promising candidate of electrode materials to be used in rechargeable batteries due to their ideal crystal structure and desirable electrochemical characteristics. Typically, PBAs have a typical cubic structure constructed with two transition metal ions (or clusters) linked via cyanide (−CN−) groups. However, the specific capacity of most PBAs obtained with aqueous electrolytes is limited to less than ≈60 mA h g −1 . It indicates that the optimal combination of transition metal ions is the most crucial factor in improving their electrochemical performances, because the performance of PBA is mostly governed by the electrochemical process of the metal ions upon insertion/extraction of guest ions. Herein, new kind of PBAs constituted with vanadium and iron ions (V/Fe PBAs) were fabricated using a simple co-precipitation method, and their electrochemical performance as well as related mechanisms of energy storage was investigated under aqueous electrolytes. The V/Fe PBAs provided an improved capacity of ~ 100 mAh -1 under a C-rate of ~ 0.55 C, taking advantage of the multiple-electron redox reactions of V and Fe ions. In addition, the mechanism of energy storage and the cycling behavior were elucidated by monitoring the ex-situ X-ray absorption near-edge spectra, microstructural changes, dissolution of the transition metal ions, and changes in the resistance of charge transfer and surface layer during cycling. V/Fe PBA is the most attractive candidate cathode material for ARBs because of their high cost efficiency, simple synthesis, and the promising electrochemical performance, which is sufficient to meet the requirements of ARBs for large-scale ESSs. Figure 1