Hollandite-Type VO1.75(OH)0.5: Effective Sodium Storage for High-Performance Sodium-Ion Batteries

Nanosized hollandite-type VO1.75(OH)(0.5) is introduced as a novel cathode material for Na-ion batteries. Structural investigation based on X-ray diffraction and Rietveld refinement suggests the presence of numerous vacant sites for Na+ intercalation in the VO1.75(OH)(0.5) structure. All of the possible Na+ sites and tunnel-type Na+ diffusion pathways along the c-axis are confirmed by bond-valence-sum analyses. The nanosized hollandite-type VO1.75(OH)(0.5) delivers an unexpectedly high specific capacity of approximate to 351 mAh g(-1) at 15.5 mA g(-1) in the voltage range of 1.0-3.7 V (vs Na+/Na), which agrees well with the results predicted by first-principles calculations. In addition, combined studies using first-principles calculations and several experimental techniques including in situ operando X-ray diffraction and ex situ X-ray absorption spectroscopy confirm that the nanosized hollandite-type VO1.75(OH)(0.5) undergoes a single-phase reaction with a capacity retention of 71% over 200 cycles. Furthermore, the open structure and nanosized particles of hollandite-type VO1.75(OH)(0.5) contribute to its excellent power capability with 56% of the capacity measured at 0.05 C being delivered at 7 C.