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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 ≈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.