Redox Flow Lithium Oxygen Batteries

Lithium-oxygen battery, because of its extremely high theoretical specific energy, has been intensively pursued as a potential solution for next generation energy storage. Despite numerous studies to date, it suffers from unbearable overpotential loss and poor cycling stability. To address the issues, research has been focused on the development of new cathode and catalysts, while limited success was made so far. The inevitable surface passivation and pore clogging of the cathode by insoluble Li₂O₂ precipitation severely aggravate the cell performance especially upon relatively deep discharge. Inspired by the development of redox flow lithium-ion batteries^1,2, here we introduce a catalyst-free redox flow lithium-oxygen battery (RFLOB) for large-scale energy storage. The new device enables the formation of Li₂O₂ via redox targeting reactions³ in a gas diffusion tank (GDT), spatially separated from the electrode. With ethyl viologen and iodide as the redox mediators, an integrated RFLOB was demonstrated which presented excellent performance even in the absence of catalysts on the cathode.

Upon discharging, O₂ is reduced by dissolved viologen in the electrolyte, with which Li₂O₂ is formed in the GDT tank in the presence of Li⁺. Upon charging, Li₂O₂ in the GDT tank is oxidized by I₃^-, which releases O₂. As such, the discharging product Li₂O₂ could be remotely formed in the GDT tank and reversibly oxidized in the charging process without depositing onto the cathode inside the cell. Such decoupled reactions of Li₂O₂ provide great flexibility to circumvent the issues confronted by the conventional Li-O₂ batteries. The surface passivation and pore clogging of the cathode resulted from Li₂O₂ precipitations, which is inevitable in conventional cells, are essentially avoided under the new operation mode. In addition, the capacity of the cathode could be expanded by simply enlarging the size of GDT tank, which is however constrained by the pore volume of cathode and catalysts deposited on it in the conventional Li-O₂ batteries. Moreover, as the reaction of O₂ in GDT is far apart from the electrodes, the tolerance of the cell towards air would be enhanced as well.

References:

1. 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.
2. F. Pan, et al., Redox Targeting of Anatase TiO₂ for Redox Flow Lithium-ion Battery, Adv. Energy Mater., 4 (2014), 1400567.
3. 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.