The Lithium Oxygen Battery

 

The carbon electrode in nonaqueous Li–O2 cells

The carbon electrode in nonaqueous Li–O<sub>2</sub> cells.

Muhammed M. Ottakam Thotiyl , Stefan A. Freunberger , Zhangquan Peng and Peter G. Bruce, "The Carbon Electrode in Nonaqueous Li–O2 Cells", J. Am. Chem. Soc., 2013, 135 (1), pp 494–500.

Carbon has been used widely as the basis of porous cathodes for nonaqueous Li–O2 cells. However, the stability of carbon and the effect of carbon on electrolyte decomposition in such cells are complex and depend on the hydrophobicity/hydrophilicity of the carbon surface. Analysing carbon cathodes, cycled in Li–O2 cells between 2 and 4 V, using acid treatment and Fenton’s reagent, and combined with differential electrochemical mass spectrometry and FTIR, demonstrates the following: Carbon is relatively stable below 3.5 V (vs. Li/Li+) on discharge or charge, especially so for hydrophobic carbon, but is unstable on charging above 3.5 V (in the presence of Li2O2), oxidatively decomposing to form Li2CO3. The results emphasize that stable cycling of Li2O2 at the cathode in a Li–O2 cell depends on the synergy between electrolyte and electrode; the stability of the electrode and the electrolyte cannot be considered in isolation.

 

A reversible and higher-rate Li-O2 battery

Z. Peng, S. A. Freunberger, Y. Chen and P. G. Bruce, "A Reversible and Higher-Rate Li-O2 Battery", Science, 2012, 337, 563-566.

Operation of the rechargeable Li-O2 battery depends critically on repeated and highly reversible formation/decomposition of lithium peroxide (Li2O2) at the cathode upon cycling. Recent studies by others and us have shown that this process is plagued by side-reactions typically involving decomposition of the electrolyte. However, we have shown stable cycling is possible with the use of a dimethyl sulfoxide (DMSO) electrolyte and a nanoporous gold (NPG) electrode. Using this system we observed 95 % capacity retention from cycles 1 to 100, whereas with other electrolyte/cathode combinations only partial Li2O2 formation/decomposition and limited cycling occurs.

Stable cycling using a dimethyl sulfoxide (DMSO) electrolyte and a nanoporous gold (NPG) electrode.

Using IR spectroscopy we have confirmed that at the NPG/DMSO interface few by-products form and that the dominant product is Li2O2, which is completely oxidised during charging; both of these are critical requirements of a reversible Li-O2 battery!

IR spectroscopy confirms that few by-products are forms during the cycling.

 

Charging a Li–O2 battery using a redox mediator

Yuhui Chen, Stefan A. Freunberger, Zhangquan Peng, Olivier Fontaine & Peter G. Bruce, "Charging a Li–O2 battery using a redox mediator", Nature Chemistry, 2013, 5, 489–494.

Charging a Li-O2 battery depends on oxidizing solid Li2O2, which is formed on discharge within the porous cathode. However, significant voltage polarisation occurs on charging even at a modest rate. This is because transporting charge between Li2O2 particles and the solid electrode surface is very difficult due to its poor conduction, both electronic and ionic conductivity. Therefore, we have introduced a redox mediator, i.e. tetrathiafulvalene (TTF), to transfer the charge between electrode surface and solid Li2O2, enabling high rate charging. On charge, the redox mediator Mred is oxidised to Mox and Mox diffuses to Li2O2 surface to oxidize Li2O2, forming O2 and Li+. After oxidising the Li2O2, Mox is reduced to Mred and is then regenerated by oxidation at the electrode surface.

A redox mediator enables high rate charging when conductivity between Li<sub>2</sub>O<sub>2</sub> particles and the solid electrode surface is poor.