Synergistic effect of electrolyte and intercalation engineering: comprehensive enhancement of the electrochemical performance of 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 its cycling stability. To address this issue, this study introduced a polyethylene glycol (PEG) additive into the original aqueous electrolyte. PEG effectively suppresses the structural collapse of the cathode; however, it concurrently increases resistance to ion 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 cycling 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.