Development of Non-Aqueous Redox Flow Batteries at Pacific Northwest National Laboratory

Redox flow battery (RFB) is considered as one of the most promising large-scale stationary energy storage technologies and has attracted much attention both academically and industrially. The unique cell architecture renders RFB with attractive advantages including separation of energy and power, exceptional design flexibility, excellent scalability and modularity, long service life, high efficiency, etc.Therefore, RFB is a well-suitable option to stabilize the power grid and tackle the intermittency of the renewable energies. Conventional aqueous RFBs have achieved significant progress such as vanadium-vanadium, iron-vanadium, zinc-bromine, polysulfide-bromine systems. However, aqueous RFBs also are confined by two major intrinsic limitations: narrow voltage window to avoid water electrolysis and relatively low concentrations of the electroactive materials, making them generally low energy density systems.

Non-aqueous electrolytes provide wider electrochemically stable voltage window and examples having cell potential of >2V have been demonstrated recently. The expansion of the operational cell potential has a direct impact on the energy density of the non-aqueous RFB system. Meanwhile, the wider voltage windows make more redox couples available as electroactive material choices and increase the possibility of using multiple electron transfer redox reactions. Therefore, a number of non-aqueous RFB systems have been proposed and investigated [1]. Most of these systems are based on organic active materials that have the advantages of convenient structural tunability and availability of different redox mechanisms.

Here we report the development of non-aqueous RFBs at PNNL with the focus on several lithium-organic RFB systems, each adopting a different organic cathode material [2]. Chemical functionalization to the organic active materials was carried out to increase its solubility targeting high energy density energy storage systems. The flow cells of these systems demonstrated excellent electrochemical performance with impressive cell efficiencies, high operational current density, and remarkable cycling stability that exceeded those of other reported non-aqueous RFBs. Effective protection of the lithium anode employed in these lithium-organic RFB systems will be discussed.

References

1. W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Adv. Funct. Mater., 23, 970 (2013).

2. W. Wang, W. Xu, L. Cosimbescu, D. Choi, L. Li, Z. Yang, Chem. Commun. 48, 6669 (2012).

Figure 1. A lithium – anthraquinone non-aqueous RFB: (a) CV curves of 0.25M structurally modified anthraquinone in 1.0M LiPF6/PC electrolyte with lithium foil as the reference electrode; (b) Cycling energy efficiency and discharge energy density. [2]