Na-Ion Interstitial Site Filling and Structural Evolution in a Tunnel-Structured Vanadium Oxide Insertion Host

Abstract The tunnel-structured metastable polymorph ζ-V2O5 has emerged as a promising insertion host for alkali and multivalent ions, yet the mechanisms governing Na-ion storage remain poorly understood. Here, we combine single-crystal and powder, operando X-ray diffraction, neutron scattering, and ex situ X-ray absorption spectroscopy techniques with electrochemical measurements to elucidate Na-ion insertion and structural evolution in ζ-V2O5. Topochemical sodium insertion is limited to approximately 0.27 Na per formula unit, with Na ions initially occupying split seven-coordinate β sites while preserving the tunnel framework. Electrochemical sodiation accesses higher Na-ion stoichiometries and reveals a sequential filling mechanism involving β, β′, and C sites concomitant with Na-ion redistribution and reversible structural distortions. Despite the larger size of Na ions in comparison to Li ions, the tunnel framework remains intact throughout cycling and is recovered upon desodiation. Operando diffraction and X-ray absorption spectroscopy reveal that approximately 0.3 Na per formula unit remains trapped after the first discharge process, thereby pillaring and stabilizing the framework. The preinserted pillared phase enables subsequent reversible Na-ion storage. The ability to reversibly insert both Li and Na ions in ζ-V2O5 provides the rare opportunity to contrast ion insertion and migration mechanisms. Comparison of lithiation and sodiation mechanisms in ζ-V2O5 demonstrates distinct differences in the thermodynamics and kinetics of ion insertion despite similar insertion sites and ion mobility. These findings provide mechanistic understanding of sodium storage in metastable ζ-V2O5 and establish design principles for robust tunnel-type cathodes for Na-ion and dual-ion batteries.