Room Temperature, Hybrid Sodium-Based Flow Batteries with Multi-Electron Transfer Redox Reactions

The development of renewable energy has become a high priority for environmental sustainability and energy independence. Some renewable energies (e.g., solar and wind), however, are intermittent, and require integration with energy storage systems to provide predictable and dispatchable supply of electricity. Redox flow batteries (RFBs) are excellent candidates for such grid-scale energy storage of electricity. However, the state-of-the-art RFBs have low volumetric and gravimetric energy densities (20 – 33 Wh/L and 15 – 25 Wh/kg) and their costs are high for many applications. To address these issues, we have been working on a completely new type of flow batteries, termed as hybrid Na-based flow batteries (HNFBs). These HNFBs offer multiple unmatched advantages over RFBs. In particular, HNFBs can operate at ambient temperature, allow catholytes to have multiple electron transfer redox reactions per active ion, offer wide selection of catholyte chemistries with multiple active ions to couple with the highly negative Na alloy anode, and enable the use of both aqueous and non-aqueous catholytes. Further, the molten Na alloy anode permits the decoupled design of power and energy since a large volume of the molten Na alloy can be used with a limited ion-exchange membrane size. In this talk, examples of aqueous and non-aqueous catholytes will be discussed and the feasibility of multi-electron transfer redox reactions per active ion will, for the first time, be demonstrated. The critical barriers to mature these new HNFBs will also be discussed. If successfully developed, these HNFBs with ultrahigh energy densities can be used for all levels of electric energy storage, including electricity generation site storage, electricity transmission-substation storage, community storage, and end user storage.

Acknowledgments: Financial support from the U.S. DOE Office of Electricity Delivery and Energy Reliability (OE) Energy Storage Program is greatly appreciated.