Non-aqueous Li-air (O2) batteries promise a significantly higher theoretical energy-storage density than conventional lithium-ion batteries. However, major increases in the lithium-oxygen round-trip charging/discharging efficiency and cycle life are needed before lithium-air batteries become viable.
When a Li-O2 battery discharges, electrons, lithium ions, and oxygen gas react to form nanoparticles of Li2O2, and the opposite may be expected when the battery charges. However, transporting charge between the Li2O2 particles and the solid electrode surface is very difficult, and in practice, most of the Li2O2 polarizes, preventing the battery from completely recharging. Reporting online May 12 in Nature Chemistry (DOI: 10.1038/NCHEM.1646,) Y. Chen, S.A. Freunberger, Z. Peng, O. Fontaine, and P.G. Bruce from the University of St. Andrews, Graz University of Technology, Chinese Academy of Sciences, and Université Montpellier may have found a solution to the polarization puzzle in the form of a redox mediator, tetrathiafulvalene (TTF).
In a discharging Li-O2 battery, oxygen accepts electrons (reduces) when it forms lithium peroxide in the porous electrode bathed in non-aqueous electrolyte. When the battery charges, only the surface of the nanoparticles in direct contact with the electrode readily gives up electrons (oxidizes). Realizing that they could prevent polarization and increase charging if they could find a way to oxidize all of the Li2O2 particles, the researchers identified TTF as a highly promising mediator.
They surmised that upon charging, TTF adjacent to the electrode and in the electrolyte would first be oxidized to TTF+. The TTF+ would then in turn oxidize Li2O2 across its entire surface, while simultaneously transforming the mediator back to TTF. The mediator would create more efficient electron transfer.
In identical Li-O2 test cells with and without the mediator, the TTF cells maintained low, steady voltages on cycling across a range of charging rates. The porous TTF test cell electrode had the equivalent capacity of a carbon electrode with 3000 mAh/g carbon. Up to the highest charging rate, the cells with TTF allowed stable cycling for 100 cycles while the cells without TTF were impossible to charge. Using infrared spectroscopy, surface-enhanced Raman spectroscopy, and differential electrochemical mass spectrometry, the researchers showed that the electrochemical results came from the formation and oxidation of Li2O2 and not just TTF.
Explaining that TTF is just one of a whole range of potential mediators, the researchers state that “[this study] demonstrates the feasibility of using a redox mediator to oxidize Li2O2 in a Li-O2 cell and thereby overcome the difficulty of an inherently insulating active material.”
