Thermal Modulation of Defect Density and Lattice Hydration in Microwave-Hydrothermal Vanadium Pentoxide Cathodes for Aqueous Zinc-Ion Batteries

Abstract

Practical deployment of aqueous zinc-ion batteries (AZIB) is constrained by sluggish Zn2+ transport and cathode structural degradation. Microwave-hydrothermal (MWHT) synthesis leverages dielectric volumetric heating to accelerate nucleation and promote homogeneous crystal growth, yet how synthesis temperature governs structural evolution in V₂O₅·nH₂O cathodes has not been systematically established. This study investigates how a modest 20 °C thermal increment governs the crystalline ordering, defect density, and charge storage kinetics within the hydrated phase framework. V₂O₅ synthesized at 130 °C for 60 minutes yields a disordered, strongly hydrated phase characterized by crumpled nanosheets and a high V⁴⁺ defect concentration (V⁵⁺/V⁴⁺ = 6.5). Increasing the synthesis temperature to 150 °C drives interlayer reorganization and crystallite ripening within the hydrated V₂O₅·nH₂O framework, yielding well-faceted lamellae with improved long-range ordering and a reduced defect density (V⁵⁺/V⁴⁺ = 8.8). The 130 °C cathode exhibits mixed kinetics with a diffusion-dominant contribution (b̄ ≈ 0.70) and elevated charge-transfer resistance (Rct ≈ 400 Ω), whereas the 150 °C cathode shifts toward pseudocapacitive-dominant charge storage (b̄ ≈ 0.85) with 20% lower interfacial resistance (Rct ≈ 320 Ω). The 150 °C cathode delivers 330 mAh g⁻¹ at 0.5 A g⁻¹ and retains 64% capacity after 3000 cycles at 4 A g⁻¹, compared to 52.6% for the 130 °C sample. These findings demonstrate that MWHT synthesis temperature is a readily accessible parameter for simultaneously tuning crystalline ordering, defect density, and long-term cycling stability in hydrated vanadium oxide cathodes.