Due to the capricious nature of renewable energy resources, such as wind and solar, large-scale energy storage devices are increasingly required to make the best use of the renewable power. The redox flow battery is considered suitable for large-scale applications due to its modular design, good scalability and flexible operation. The biggest challenge of the redox flow battery is the low energy density. The redox active species is the most important component in redox flow batteries, and the redox potential and solubility of redox species dictate the system energy density. This review is focused on the recent development of redox species. Different categories of redox species, including simple inorganic ions, metal complexes, metal-free organic compounds, polysulfide/sulfur and lithium storage active materials, are reviewed. The future development of redox species towards higher energy density is also suggested.

Redox flow batteries (RFBs) are considered one of the most promising large-scale energy storage technologies. However, conventional RFBs suffer from low energy density due to the low solubility of the active materials in electrolyte. On the basis of the redox targeting reactions of battery materials, the redox flow lithium battery (RFLB) demonstrated in this report presents a disruptive approach to drastically enhancing the energy density of flow batteries. With LiFePO₄ and TiO₂ as the cathodic and anodic Li storage materials, respectively, the tank energy density of RFLB could reach ~500 watt-hours per liter (50% porosity), which is 10 times higher than that of a vanadium redox flow battery. The cell exhibits good electrochemical performance under a prolonged cycling test. Our prototype RFLB full cell paves the way toward the development of a new generation of flow batteries for large-scale energy storage.

With the retirement of a massive amount of end-of-life lithium ion batteries (LIBs), proper disposal of the hazardous wastes and cost-effective valorization of useful materials have become increasingly pressing and have attracted extensive attention worldwide. The state-of-the-art recycling technologies, which are generally based on chemical leaching methods, have critical issues like enormous consumption of chemicals and secondary pollution, and generally involve tedious procedures. Here, we report an innovative approach in light of a redox targeting-based process and demonstrate the operation for the recycling of spent LiFePO₄. With 0.20 M [Fe(CN)₆]³⁻ solution as a selective and regenerative redox mediator, LiFePO₄ is readily broken down into FePO₄ and Li⁺via the redox-targeting reaction between the redox mediator and the material, with a recycling efficiency for lithium removal up to 99.8% at room temperature. The reacted redox species are instantaneously regenerated on the electrode for subsequent rounds of reaction while Li⁺ is separated from the counter electrode compartment. Consequently, the consumption of chemicals is drastically minimized and secondary pollutants are eliminated. By virtue of the new process, high purity LiOH (99.90%) and FePO₄ (99.97%) have been obtained. We envisage that this cost-effective and environment-friendly approach can be extended to other materials and promote the sustainability of LIB technologies.

Reversible chemical delithiation/lithiation of LiFePO₄ was successfully demonstrated using ferrocene derivatives, based on which a novel energy storage system — the redox flow lithium-ion battery (RFLB), was devised by integrating the operation flexibility of a redox flow battery and high energy density of a lithium-ion battery. Distinct from the recent semi-solid lithium rechargeable flow battery, the energy storage materials of RFLB stored in separate energy tanks remain stationary upon operation, giving us a fresh perspective on building large-scale energy storage systems with higher energy density and improved safety.