New Insight on Open-Structured Sodium Vanadium Oxide as High-Capacity and Long Life Cathode for Zn–Ion Storage: Structure, Electrochemistry, and First-Principles Calculation

Herein, the promising properties of open-structured NaV₃O₈ as a cathode material for Zn-ion batteries (ZIBs) are investigated. First-principles calculations predict the insertion of Zn²⁺ (0.74 Å) in NaV₃O₈ with an interlayer distance of ≈7 Å, enabling delivery of a high discharge capacity of 353 mAh g⁻¹ at 70 mA g⁻¹ (0.2 C) for 300 cycles in the operating window of 0.3−1.5 V in 1 m Zn(CF₃SO₃)₂ aqueous solution. Operando synchrotron X-ray diffraction, X-ray absorption near edge structure spectroscopy, and first-principles calculations validate the insertion of Zn²⁺ into the NaV₃O₈ structure within the operation range. Moreover, operando synchrotron X-ray diffraction and operando Raman spectroscopy reveal the formation of layered zinc hydroxytriflate (Zn₅(OH)₈(CF₃SO₃)₂∙xH₂O) as a side reaction below 0.8 V on discharge (reduction) and its dissolution into the electrolyte above 0.8 V on charge (oxidation). The formation of the Zn hydroxytriflate interfacial layer increases the charge-transfer activation energy from 15.5 to 48 kJ mol⁻¹, leading to kinetics fade below 0.8 V. The findings reveal the charge-storage mechanism for NaV₃O₈, which may also be applicable to other vanadate cathodes, providing new insights for the investigation and design of ZIBs.