Redox Organic Molecules for Battery Applications

Redox organic molecules (ROMs) are one unique family of organic molecules that are electrochemical active. Their unique electrochemical behavior leads to many interesting battery related applications, including electrolyte additives for lithium ion batteries and active species for flow batteries. Redox shuttle additives are electrochemically reversible molecules that are used for overcharge protection for lithium ion batteries. As shown in Figure 1, those molecules have to have an electrochemically reversible potential that is higher than the end-of charge potential of the cathode materials so that they could function as an internal short circuit during the overcharge process. However, finding the stable redox shuttles that could work with various cathode materials is really not an easy task. In this talk, the evolution story of a new family of dimethoxybenzene based redox shuttle molecules will be discussed, which will cover our most progress in this field, including the 2014 winner of R&D 100 award, ANL-RS2.1-3 In the second part, we will introduce an emerging energy storage technology, non-aqueous redox flow batteries (NRFBs). As shown in Figure 1 (right), unlike traditional batteries, the active materials of NRFBs are in mobile phase (solutions). Therefore it is extremely important to find the suitable and stable redox active species that could work well in the solutions. Given ROMs have been proven to work well in electrolyte solutions, many ideas are transferred into the development of the active materials for NRFBs (catholytes and anolytes). Even though many shuttle molecules can be directly used, some difference requirements lead to further evolution of those molecules. For instance, ANL-8, ANL-9 and ANL-10 molecules were developed by incorporation of PEO chains with different lengths on one or both sides of the dimethoxy-di-tert-butyl-benzene based redox structure in order to afford higher solubility. Another category of the molecules, including JH-100 and JH-200, were developed by striping down the non-essential structures, such as tert-butyl groups and ether chains, in order to boost the intrinsic capacities. While many of these molecules can afford electrochemically reversible behavior, some new promising features are discovered. ANL-8 and ANL-9 molecules are found to be liquid at room temperature, which was hoped to function as co-solvent for NRFBs, thus leading to much improved energy density. JH-100 and JH-200 molecules, on the other hand, offer much enhanced intrinsic capacity (nearly two times as big as DBBB), which also is believed to be a key to further improve the energy density and lower the cost of NRFBs.4-6

Figure 1. Simplified schemes of redox shuttle overcharge protection mechanism (left), and non-aqueous redox flow batteries (right)

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