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

Here, we combine single-crystal and powder, <i>operando</i> X-ray diffraction, neutron scattering, and <i>ex situ</i> 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. <i>Operando</i> 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.