Hydrated vanadium pentoxide - reduced graphene oxide hybrid for enhanced performance in aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIB) are emerging as a safer and more cost-effective alternative to Li-ion batteries for large-scale electrical energy storage due to lower-cost raw materials, greater safety, and the higher ionic conductivity of water as a solvent compared with flammable organic solvents. Despite the high gravimetric specific capacity of zinc (Zn) (820 mAh g⁻¹). The performance of AZIBs is limited by the capacity and stability of the cathode material. Vanadium pentoxide (V₂O₅) is one of the best candidates for cathode material in aqueous zinc-ion batteries. However, the performance of commercially available crystalline [alpha]-V₂O₅ is limited by low Zn²⁺ diffusion rate, low electrical conductivity, and structural instability of V₂O₅ nanoparticles due to the tendency to aggregate. These limitations of low diffusion rate can be surpassed by using a hydrated V₂O₅ xerogel structure having a larger interlayer spacing compared to a crystalline [alpha]-V₂O₅, due to the presence of intercalated water molecules in between V₂O₅ layers, and low conductivity can be addressed by making a hybrid with conductive carbon material such as reduced graphene oxide (rGO). We have developed a quick and efficient method to synthesize a hybrid material consisting of hydrated V₂O₅ xerogel and reduced graphene oxide (rGO) via microwave irradiation of vanadium (V) triisopropoxideoxide (VTIP) and graphene oxide (GO) at 80°C and 14 bar, which benefits from the increased interlayer spacing of hydrated V₂O₅ and the high electrical conductivity of reduced graphene oxide. In this synthesis method, reaction is completed in less than 10 minutes, offering an advantage over traditional ambient temperature vanadium alkoxide hydrolysis reactions requiring multiple days for the aging process, hydrothermal reaction requiring >10 hours, or melt quenching of solid V₂O₅ requiring heating the V₂O₅ solid to above 700°C. The as-synthesized product (named as-synthesized V₂O₅-rGO) was then annealed at 220 °C and 300 °C for two hours in air atmosphere (named 220 °C annealed V₂O₅-rGO and 300 °C annealed V₂O₅-rGO respectively) to study the effect of annealing temperature on structure and electrochemical performance. The products were characterized by Raman spectroscopy, X-ray diffraction (XRD), and thermogravimetric analysis (TGA), which revealed that a hybrid material containing hydrated V₂O₅ and rGO was successfully synthesized and revealed the structural and compositional evolution with increasing annealing temperature from hydrated bilayer V₂O₅ to crystalline [alpha]-V₂O₅, accompanied by loss of interlayer water molecules. Electrochemical tests (galvanostatic charge-discharge and cyclic voltammetry) using the three products as cathode material coated on Ti discs (along with additive conductive carbon Super P, and polyvinylidene fluoride binder) in coin cells with Zn metal anode give promising results with reversible capacity (calculated with respect to mass of V₂O₅ in hybrid material) of 513, 468 and 304 mAh g⁻¹ at 0.4 A g⁻¹ for the as-synthesized, 220 °C annealed and 300 °C annealed samples respectively. Despite high specific gravimetric capacity and respectable rate performance, the materials suffer from low cyclic stability and capacity retention, which was not improved on annealing. The as-synthesized material showed the best performance and is a promising candidate for aqueous zinc-ion batteries, and future focus should be on improving cycling stability by optimizing electrolyte, optimizing potential window for charge and discharge, and looking for a deeper understanding of structural degradation on cycling.