Redox Flow Lithium-Ion Batteries for Large-Scale Energy Storage

Based on the discovery of reversible redox targeting reactions of battery materials¹, we have recently reported a conceptually new device — redox flow lithium battery (RFLB)² for large-scale energy storage. In RFLB, the active materials are kept in separate containers without attaching to the current collectors. Upon operation, the exchange of electrons between the materials and current collectors is mediated by the flow of redox shuttle molecules dissolved in the electrolyte fluids via redox targeting reactions. With the cathodic and anodic lithium-ion battery materials LiFePO₄² and TiO₂³ as an example, we have demonstrated that both materials could be chemically delithiated and lithiated reversibly in the presence of suitable redox mediators. Since the concentrations of Li⁺ in both materials are as high as 22 M, around 10 times tank energy density could be achieved compared with the conventional vanadium redox-flow battery. 

In our previous study, we have demonstrated RFLB half-cells. Here we report our latest progress in the development of RFLB full systems. This comprises the development of robust redox mediators for both anode and cathode, Li⁺-conducting membranes, as well as mechanistic studies on the Li⁺-coupled heterogeneous charge transfer — the essential process for the operation of RFLB. With the above efforts, we have successfully fabricated the first RFLB full cell with LiFePO₄ and TiO₂ as the cathodic and anodic Li-storage materials, respectively. The system functions with power generated in the cell via the redox reactions of shuttle molecules dissolved in the catholyte and anolyte, while energy stored in the two Li-storage materials, which are kept in two separate energy tanks. RFLB combines the great system flexibility (decoupled power and energy units) of redox flow battery with the high energy density (the same Li-storage materials while no conducting additives and binder required) of lithium-ion batteries, revealing significant potential to its application for large-scale stationary and automotive energy storage.⁴

References:

1. Q. Wang, et al., Redox Targeting of Insulating Electrode Materials: A New Approach to High Energy Density Batteries.
Angew. Chem. Int. Ed., 45 (2006), 8197-8200.
2. Q. Huang, et al., Reversible Chemical Delithiation/ Lithiation of LiFePO₄: Towards A Redox Flow Lithium-ion Battery. 
Phys. Chem. Chem. Phys., 15 (2013), 1793-1797.
3. F. Pan, et al., Redox Targeting of Anatase TiO₂ for Redox Flow Lithium-ion Battery, Adv. Energy Mater., 4 (2014), 1400567.
4. Q. Huang and Q. Wang, Next-Generation, High-Energy-Density Redox Flow Batteries. ChemPlusChem, (2014), in press. (doi: 10.1002/cplu.201402099)