Application of vanadium phosphate in aqueous zinc-ion batteries

With the increasing shortage of petroleum resources and serious environmental pollution, the demand for green technology development is growing stronger. Electrical energy storage is an excellent way to store intermittent clean energy and transport clean energy from one place to another. The lithium-ion battery (LIB) is broadly recognized as the first choice for electrical energy storage due to its high energy density, especially in smart electronics and electric cars. Nevertheless, the application of LIB in large-scale energy storage has been limited by various factors, including the limited and uneven distribution of lithium resources, safety issues and toxic organic electrolytes. The aqueous zinc-ion battery (AZIB) has been regarded as a potential substitute for LIB in large-scale energy storage devices because of the competitive theoretical volumetric capacity (5855 mA·h·cm⁻³) and gravimetric capacity (820 mA·h·g⁻¹) of the Zn anode, the low electrochemical potential of Zn²⁺ (−0.76 V vs SHE), and the high ionic conductivity of the aqueous electrolyte, the ease of manufacturing (e.g., manufacture in an open-air environment), and the merits of rich resources, low cost and high safety. Finding a cathode material with high energy density and power density is proposed as a strategy to accelerate the progress of AZIB because the cathode material largely dominates the electrochemical properties and the cost of the battery. However, the strong electrostatic interaction between Zn²⁺ and the host material results in sluggish reaction kinetics, leading to inferior cycling performance and rate property. Some cathode materials are dissolved in aqueous electrolytes, which restrict the development of AZIB. In comparison to the reported AZIB cathodes, including vanadium-based materials, manganese-based materials, Prussian blue analogs, and organic materials, vanadium phosphates have received a lot of attention as cathodes due to their stable structures, high voltage plateaus, and high power densities. This review presents an overview of various vanadium phosphates such as Li₃V₂(PO₄)_3, Na₃V₂(PO₄)_3, VOPO₄, Na₃V₂(PO₄)₂F₃, NaVPO₄F and their derivatives that are applied as cathodes for AZIB. The summary includes their phase structures, synthetic methods, electrochemical performance, electrochemical Zn²⁺ storage mechanisms and existing problems. The two major challenges in using vanadium phosphates as cathode materials for AZIB are low electronic conductivity and material dissolution problems, both of which result in inferior cycling performance and rate capacity. The resolution strategies for the mentioned challenges include designing the nanostructure, adjusting the electronic structure, coating with conductive materials, and regulating electrolytes to enhance electrochemical properties. Experimental techniques for studying electrochemical mechanisms are also proposed. Finally, the prospects for the future development of these cathodes in AZIB are advanced. It can be expected that this review has some significance for the development of new vanadium phosphates as cathode materials.