Abstract: Aqueous zinc-vanadium oxide (Zn-V₂O₅) batteries exhibit considerable potential for large-scale energy storage systems due to their safety and high theoretical capacity. However, the narrow interlayer spacing and poor structural stability of V₂O₅ cathodes result in unsatisfactory practical specific capacity and rate performance. The corrosion reactions and parasitic hydrogen evolution reaction (HER) on the Zn anode side hinder the overall practical implementation. In this work, we designed a viologen-based bifunctional regulator to modulate the interfacial chemistry at both the cathode and anode, enhancing the overall battery performance and stability. At the cathode, viologen molecules act as structural pillars when intercalated into the V₂O₅·nH₂O interlayers to suppress the strong polarization effect of Zn²⁺ and enhance structural stability. X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM) results reveal that the interlayer distance of the intercalated products increases with the length of the alkyl chains. By modulating the structure of viologen molecules, the spacing of V₂O₅·nH₂O interlayer can be finely adjusted in the range of 0.57 and 1.37 nm. Cyclic voltammetry (CV) measurements and galvanostatic intermittent titration technique (GITT) further confirm a positive correlation between interlayer spacing and Zn²⁺ diffusion kinetics, highlighting improved ion transport and electrochemical activity. Furthermore, density functional theory (DFT) calculations indicate that viologen intercalation weakens the interaction between Zn²⁺ and the V₂O₅·nH₂O host, facilitating faster ion migration. On the anode side, the use of a Zn(OTf)₂-viologen hybrid electrolyte promotes the in-situ formation of a gradient solid-electrolyte interphase (SEI) during cycling. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) results demonstrate that the SEI possesses a composite gradient structure, characterized by an organic-rich outer layer and an inorganic-rich inner layer primarily composed of ZnF₂, ZnS, and Zn₃N₂. Potentiodynamic polarization (Tafel) tests revealed a lower corrosion current density (6.48 mA cm-2) and a higher corrosion potential (-1.013 V vs. SCE) for the system with the viologen additive. Consistent with these findings, Zn||Zn symmetric cells employing the hybrid electrolyte demonstrated significantly extended cycle life compared to those using the blank electrolyte. This combined electrochemical evidence confirms that the in-situ formed SEI effectively protects the zinc electrode and suppresses the HER. Owing to the synergistic dual-electrode interface engineering, the optimal Zn-V₂O₅ battery achieves an enhanced energy density of 297 mAh g^-1 at a current density of 5 A g^-1 and maintains 60% of its capacity after 10000 cycles, demonstrating outstanding rate capability and long-term cycling stability. The systematic design concept will inspire novel paradigms for the development of high-performance rechargeable batteries.

Key words: Vanadium oxide, Viologen, Interlayer spacing, Gradient interphase, Aqueous zinc-vanadium oxide battery