Versatile Redox-Active Organic Materials for Rechargeable Energy Storage

ConspectusWith the ever-increasing demand on energy storage systems and subsequent mass production, there is an urgent need for the development of batteries with not only improved electrochemical performance but also better sustainability-related features such as environmental friendliness and low production cost. To date, transition metals that are sparse have been centrally employed in energy storage devices ranging from portable lithium ion batteries (e.g., cobalt and nickel) to large-scale redox flow batteries (e.g., vanadium). Toward the sustainable battery chemistry, there are ongoing efforts to replace the transition metal-based electrode materials in these systems to redox-active organic materials (ROMs). Most ROMs are composed of the earth abundant elements (e.g., carbon, nitrogen, oxygen, sulfur), thus are less restrained by the resource, and their production does not require high-energy consuming processes. Furthermore, the structural diversity and chemical tunability of organic compounds make them more attractive for the versatile design of future energy storage systems. Accordingly, the timely development of high-performance ROM-based electrodes would expedite the shift from the current resource-limited battery chemistry to more sustainable energy solutions.In this Account, we provide an overview of the endeavors to employ and develop ROMs as high-performance active materials for various battery systems. Diverse approaches will be introduced starting from the new ROM design mimicking the energy carrying molecules in biological metabolism to the chemical modifications to tailor the properties for specific battery systems. The molecular redesign of ROM, for example, can be carried out by substituting heteroatoms in the redox center, which leads to the enhancement of the redox potential by the inductive effect. Or, tailoring the ROM molecule by removing redox-inactive functionals results in a reduced molecular weight, thereby an increased specific capacity. The intrinsic limitations of ROMs, such as the low electrical conductivity and the dissolving nature, have been under extensive scrutiny; however, they can be partly addressed through efforts including intermolecular fusion and/or nanoscale hybridization with a conducting scaffold. On the other hand, this problematic dissolving nature of ROMs makes them appealing for some new battery configurations such as redox flow batteries that employ the liquid-state active materials. The high solubility and the stability of the ROM were found to be beneficial in attaining the enhanced energy density and the cycle stability of flow batteries, which could be further optimized by the chemical modifications of ROMs. Besides the role of active materials, the redox activity of ROMs has also enabled their use as catalysts to promote the electrode reaction in metal–air batteries. The redox capability of the ROM was often proven to be effective in the solution-based redox mediation that facilitates both the charging and discharging reaction in metal-air batteries. Finally, we conclude this account by proposing the future research directions regarding the fundamental electrochemistry and the further practical development of ROMs for the sustainable rechargeable energy storage.