Cation-dependent phase formation and electrochemical behavior in Co- and Cu-modified layered vanadium oxide cathodes

Abstract

Cation modification is proposed as a strategy for stabilizing layered mixed-valence vanadium oxide cathodes for aqueous zinc-ion batteries, yet the modifier is rarely considered as a competitor for the reducing agent that generates the layered phase. Here, Co²⁺- and Cu²⁺-containing vanadium oxides are prepared from V₂O₅ and oxalic acid via a single hydrothermal step. XRD, FTIR, XPS, ICP-AES, and electron microscopy reveal that the difference in redox activity between the two cations dictates the final product. Cobalt, which resists reduction and is partly reoxidized to Co³⁺ in situ, leaves most of the oxalate available for vanadium reduction, yielding a two-phase mixture of V₁₀O₂₄·12H₂O and residual V₂O₅ with cobalt homogeneously dispersed throughout the matrix. In contrast, copper is readily reduced to Cu⁺, consuming the reductant and suppressing the layered phase, thereby producing crystalline V₂O₅ with a copper-enriched surface. The unmodified hydrate achieves 281 mAh·g^-1 but retains only 26% after 500 cycles; the cobalt-modified sample peaks at 183.1 mAh·g^-1 but fails within 140 cycles; whereas the copper-modified sample delivers 168.9 mAh·g^-1 and retains 63%, offering the best stability but at lower capacity. We propose that the redox activity of the metal cation rather than ionic radius or the identity alone is a critical but often overlooked synthesis parameter that governs both structure and, consequently, functional properties in these cathode materials.