Synergistic effect of electrolyte and intercalation engineering: comprehensive enhancement of electrochemical performance in a mixed proton–electron conducting layered vanadate cathode for aqueous zinc-ion batteries

Abstract

This study synthesized a layered vanadate cathode, (H₃N(CH₂)₄NH₃)[V₆O₁₄] (C₄N₂-VO), for aqueous zinc-ion batteries (AZIBs) through an intercalation strategy using 1,4-butanediamine cations. These cations effectively expand the interlayer spacing and form a dense hydrogen–bonding network with the vanadium oxide layers. At a relative humidity of 98%, C₄N₂-VO exhibits mixed proton and electron conductivity, with maximum proton and electron conductivities of 1.18×10^-3 S cm^-1 and 7.36×10^-5 S cm^-1 at 358 K, respectively. Owing to its unique structural and conductive properties, the C₄N₂-VO cathode exhibits excellent capacity performance in an aqueous Zn(CF₃SO₃)₂ electrolyte. Nevertheless, the structural collapse of the C₄N₂-VO cathode during charge–discharge processes degrades cyclic stability. To address this issue, this study introduced polyethylene glycol (PEG) additive into the original aqueous electrolyte. PEG effectively suppresses structural collapse of the cathode; however, it concurrently increases resistance to ionic migration, thereby causing a decline in capacity performance. When the PEG volume fraction in the electrolyte is optimised to 20%, the C₄N₂-VO cathode exhibits a significant improvement in cyclic stability while maintaining a high specific capacity. Moreover, the distinct lattice planes of the C₄N₂-VO cathode exhibit opposing expansions (positive vs. negative expansion) during charge–discharge processes, effectively minimising volume changes. This research not only confirms that a layered vanadate incorporating organic amine cations can serve as a potential mixed proton–electron conductor, but also provides an innovative strategy to comprehensively enhance the electrochemical performance of layered vanadate cathodes for AZIBs.