Disordered Bilayered V2O5 ⋅ nH2O Shells Deposited on Vertically Aligned Carbon Nanofiber Arrays as Stable High‐Capacity Sodium Ion Battery Cathodes ⋅

Abstract Due to the large radius of Na + ions, it has been challenging to find suitable host materials for sustainable sodium ion batteries (SIBs). This study investigates sputter‐coated thin V 2 O 5 shells on vertically aligned carbon nanofiber (VACNF) arrays as a novel three‐dimensional (3D) core‐shell material for SIB cathodes. SEM, TEM, XRD and Raman spectroscopy revealed that the as‐deposited V 2 O 5 shell has a highly disordered bilayered V 2 O 5 ⋅ n H 2 O structure with a large interlayer spacing of 11.0 Å, which can accommodate Na + ions better than orthorhombic α‐V 2 O 5 crystals. This hydrated metastable structure has been systematically characterized for Na + storage. A high initial insertion capacity of 277 mAh g −1 can be achieved at a current density of 250 mA g −1 in the potential window of 4.0–1.0 V (vs Na/Na + ). Using higher charge‐discharge rate or narrower potential windows, the electrode becomes more reversible, able to reach a coulombic efficiency of ∼99 %. Cyclic voltammetry, galvanostatic charge‐discharge and electrochemical impedance spectroscopy measurements indicate that the Na + storage is dominated by a large pseudocapacitive contribution due to fast surface reactions, which facilitates the improved stability and high power density. Such highly disordered bilayered V 2 O 5 ⋅ n H 2 O material in the 3D core‐shell architecture provides new insights for developing future SIB materials.