Vanadium redox flow batteries: A review of electrolyte production methodologies, the critical role of electrolyte impurities, and future research directions

The push to integrate more renewable power into the electricity grid has highlighted efficient energy storage systems, such as vanadium redox flow batteries (VRFBs). VRFBs offer an electrochemical energy storage solution with low self-discharge rates, fast response times, prolonged cycle longevity, and eco-friendliness. However, the widespread adoption is hindered by inherent limitations such as lower energy density, more restricted operating temperature range and higher upfront capital costs compared to other energy storage technologies like lithium-ion batteries. These limitations are partly due to costly and stringent purity requirements for vanadium electrolytes and expensive ion exchange membranes. Consequently, in efforts to enhance the properties of the electrolyte and reduce costs by utilising lower-grade vanadium materials, the reduction of impurities is being investigated. These impurities can originate from multiple sources, including the vanadium raw material, the extraction and processing of vanadium from minerals, and failing components within the VRFB. Such impurities affect electrolyte stability, electrochemical reaction kinetics, and cell performance. Furthermore, literature reveals a lack of established industry standards for electrolyte specifications concerning impurity effects in VRFB electrolytes. As a result, the industry relies predominantly on high-purity electrolytes to avoid potential adverse effects from impurities. This approach, however, leads to a significant increase in electrolyte costs. While a few studies have reported that certain impurities present in the VRFB electrolytes can either affect or enhance electrolyte performance, these findings remain preliminary and scarce. This review provides an overview of the effects of impurities in VRFB electrolytes, identifying their sources and impacts on performance. • Impurities originate from vanadium extraction processing and battery component corrosion. • Impurity effects are linked to electrolyte stability and electrode kinetics. • Cell performance and electrochemical kinetics study define impurity thresholds. • Undefined impurity tolerance levels for vanadium electrolyte increase cost. • An alternative method for producing battery-grade electrolyte is to omit the precipitation stage.