New Insights into the Reaction Mechanism of Sodium Vanadate for an Aqueous Zn Ion Battery

Sodium vanadate (Na_1+xV₃O₈ or NVO) has recently attracted significant interest as a potential cathode material for an aqueous Zn ion battery for its unique pillared framework facilitating Zn ion migration. Here, we performed a detailed study on reaction mechanisms of hydrated Na₂V₆O₁₆·2H₂O slabs and nonhydrated Na_1.25V₃O₈ nanorods using transmission electron microscopy. Our initial observation reveals that the thin (30-50 nm) Na₂V₆O₁₆·2H₂O system successfully undergoes discharge with Zn ion insertion into the structure while thick (120-170 nm) Na_1.25V₃O₈ allows Zn ion insertion only at the surface, signifying the importance of both the presence of water and the nanostructure thickness in determining the reaction mechanism of NVO. More in-depth analysis of these two systems revealed the irreversible formation of the stable byproduct phase Zn₃Na_x(OH₂)V₂O₇ (ZNVO), which likely evolved through a Zn-ion redox reaction, contributing to overall cell performance. Eventually, the entire discharge/charge process appears to become bifurcated, consisting of primary (Zn redox in NVO) and secondary (Zn redox in ZNVO) reactions, where their relative contribution to overall cell capacity changes with continued cycling. Our study provides a fresh insight into the morphology- and hydration-dependent reaction mechanisms of NVO and their implications on the electrochemistry.