Stabilization of V4+ in VOPO4 via Organophosphonate-Modified Electrolyte for High-Capacity Zinc-Ion Batteries

Abstract Vanadium oxyphosphate (VOPO₄) is a promising cathode material for zinc-ion batteries (ZIBs), but its capacity is critically limited by the irreversible oxidation of electrochemically active V⁴⁺ to inactive V⁵⁺ during cycling. To address this challenge, we introduce 2-hydroxyphosphonoacetic acid (HPAA) as a multifunctional additive for polyacrylamide (PAM) gel electrolyte. Leveraging its reducing properties, HPAA effectively preserves a higher proportion of V⁴⁺ with redox activity during charge/discharge process. X-ray photoelectron spectroscopy (XPS) analysis confirms this interfacial modulation by HPAA, revealing increased concentration of V⁴⁺ and decreased concentration of V⁵⁺ on cycled cathodes, indicating a more reactive surface. Furthermore, HPAA lowers the reaction energy barrier for the V⁴⁺/V⁵⁺ redox couple and mitigates vanadium dissolution, collectively optimizing the reaction kinetics. Consequently, the HPAA-modified PAM gel electrolyte enables VOPO₄ cathode delivers a significantly enhanced discharge capacity of 387 mAh·g⁻ ¹ at 0.1 A·g⁻ ¹ and retains 163 mAh·g⁻ ¹ after 100 cycles. This work unveils a novel strategy utilizing organophosphonic reductants to regulate vanadium valence states in cathode interface, providing crucial insights for designing high-capacity ZIBs.