Amorphous iron vanadate positive electrode enabling fast pseudocapacitive sodium-ion storage

Abstract

Amorphous iron vanadate (a-FVO) is a promising positive electrode for sodium-ion hybrid capacitors owing to its high-rate capability and abundant raw material. However, the correlation between elemental composition of a-FVO and its electrochemical properties remains poorly understood. Herein, we investigate the electrochemical performance of a-FVO with various Fe/V ratios for sodium-ion storage. It is found that a decrease in the Fe/V ratio enhances the reversible capacity of the electrode, while increasing the charge-transfer energy barrier and hindering Na⁺ diffusion. Ex situ structural characterization reveals that the increased V content strengthens the interactions between metal–oxygen bonds and Na⁺ ions. Importantly, a-FVO with an Fe/V molar ratio of 1 : 2 features an optimal configuration of active sites and bridging bonds, achieving balanced specific capacity and rate capability along with good cycling stability. Kinetic analysis reveals a pseudocapacitive-dominated process (capacitive contribution of ∼90%) of Na⁺ storage in it, accompanied by reversible local bond vibrations. This work provides insights into the important role of the local bonding environments in pseudocapacitive charge storage, which is promising for high-rate electrochemical capacitors.