Spatiotemporal Resolution of Phase Formation in Thick Porous Sodium Vanadium Oxide (NaV₃O₈) Electrodes via Operando Energy Dispersive X-ray Diffraction

Herein, zinc vanadium oxide (ZVO) and zinc hydroxy-sulfate (ZHS) formation as discharge products in sodium vanadium oxide (NVO) cathode materials of two distinct morphologies, NVO(300) and NVO(500), is studied with ex situ and operando X-ray diffraction methods. ZHS formation upon discharge is shown to be favored at higher current densities and reversible upon charge, while ZVO formation is found to be favored at lower current densities but persists throughout cycling. Operando synchrotron-based energy dispersive X-ray diffraction (EDXRD) reveals reversible expansion of the NVO lattice due to Zn²⁺ during discharge, spontaneous ZVO formation following cell assembly, and ZHS formation concomitant with H⁺ insertion at potentials less than ∼0.8 V vs Zn/Zn²⁺. With spatially resolved EDXRD, ZVO formation is show to occur near the separator region first, eventually moving to the current collector region as discharge depth increases. ZHS formation, however, is found to originate from the current collector side of the positive electrode and then propagate through the porous electrode network. This study highlights the special benefits of the EDXRD method to gain mechanistic insight into structural evolution within the electrode and at its interface.