A Novel High-Energy-Density Storage and Operation Concept for Redox Flow Batteries
Authors: Nguyen Trung , Li Yuanchao 
Published: July 2021

The environmental impact of the use of fossil fuels for energy can be greatly reduced if electricity, which represents one-third of all energy uses, can be generated totally from renewable/sustainable sources such as wind and solar. However, this is only possible if cost-effective long-duration (>5 days) storage technologies are available to allow the highly variable and unpredictable wind and solar energy sources to become reliable baseline energy sources like coal or natural gases. Redox flow battery (RFB) energy storage systems are highly suitable for this large-scale, long-duration storage application because while their power output scales with the size of the battery, an expensive component of the storage system, their energy content resides in the amount of active materials that are stored in external tanks and can be easily scaled up for longer duration.1 A stationary battery system like the lithium-ion battery does not have this separation of power and energy characteristic.
Therefore, an approach that can significantly increase the operational duration of the RFB systems at a minimal cost is of great interest. The conventional redox flow batteries store electrical energy in the form of some aqueous or non-aqueous soluble ions or compounds in the electrolyte solution. Because of the low solubility (~1M-2M) of most ions and compounds in aqueous and non-aqueous solvents, these redox flow battery systems have low energy density (as compared to solid reactants like lithium in lithium batteries).2–4 For example, the commercialized all-vanadium RFB system has an average energy density of 20 Wh/kg while that of the lithium-ion battery system is 100-265 Wh/kg.5 To store enough energy for 3-5 days in these RFBs requires a very large volume of solution in a large number of tanks, making these RFB systems expensive due to the cost of tanks and the fluid distribution system and floor space.
This presentation will discuss a concept that increases the amount of active materials stored in a given storage volume while still enabling the flow battery concept. The novel storage design is to i) store these reactants in a solution of soluble ions and an additional amount of these ions in their undissolved solid form and ii) to circulate only the liquids through the batteries. In their solid form, their energy densities will be much higher than that of the aqueous solution. For example, the concentration of vanadyl sulfate (VOSO4), an active compound used in the all-vanadium RFB system, is around 1.5M. At 1.5M, one has 1.5 moles of vanadium ions per liter or 40.2 Ah/L. In its 6-ligand-water solid form (VOSO4·6H2O) which has a density of 1920 g/L,6 a liter of this solid contains 7.1 moles of vanadium ions or 189 Ah energy. This is a tremendous increase and will put the energy storage density of the RFB systems within those of solid systems like lithium-ion batteries. An existing 6-hr RFB system can become a >24-hr system with minimal modifications. This concept and the processes used to demonstrate it will be discussed in this presentation. A demonstration of this aqueous/solid storage concept in a hydrogen-vanadium flow battery will be presented in another presentation from our group.
Acknowledgments
This work was also supported by the National Science Foundation under grant number CBET-2024378.
References
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