In a Li-air battery, the discharge reaction at the cathode involves the reduction of O₂ and the formation of solid Li₂O₂; the process is reversed on charging, i.e., 2Li⁺ + O₂ + 2^e− ↔ Li₂O₂. The solid Li₂O₂ that forms on discharge needs to be stored in a porous conducting matrix, with high conductivity and low cost and ease of fabrication; this makes carbon attractive, the team noted in their paper.

Given the role of carbon as a possible porous positive electrode for nonaqueous Li−O₂ cells and the different observations reported in the literature, we undertook further investigation of the carbon electrode.

—Thotiyl et al.

For their study, they cycled carbon cathodes in Li−O₂ cells between 2 and 4 V; their analysis determined that:

• 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. Direct chemical reaction with Li₂O₂ accounts for only a small proportion of the total carbon decomposition on cycling.

• Carbon promotes electrolyte decomposition during discharge and charge in a Li−O₂ cell, giving rise to Li₂CO₃ and Li carboxylates (DMSO and tetraglyme electrolytes). The Li₂CO₃ and Li carboxylates present at the end of discharge and those that form on charge result in polarization on the subsequent charge.

• Li₂CO₃ (derived from carbon and from the electrolyte) as well as the Li carboxylates (derived from the electrolyte) decompose and form on charging.

• Oxidation of Li₂CO₃ on charging to ∼4 V is incomplete; Li₂CO₃ accumulates on cycling resulting in electrode passivation and capacity fading.

• Hydrophilic carbon is less stable and more catalytically active toward electrolyte decomposition than carbon with a hydrophobic surface. If the Li−O₂ cell could be charged at or below 3.5 V, then carbon may be relatively stable, however, its ability to promote electrolyte decomposition, presenting problems for its use in a practical Li−O2 battery.

The behavior of the carbon electrode in a nonaqueous Li−O₂ cell is complex. It depends on the hydrophobicity/hydrophilicity of the carbon surface and involves potential dependent carbon decomposition and electrolyte decomposition promoted by the carbon surface...The net effect is that the formation of Li₂CO₃ from electrolyte and electrode exceeds Li₂CO₃ decomposition, and hence Li₂CO₃ accumulates in the electrode, leading to rapid polarization on charging as well as electrode passivation and capacity fading on cycling, as described previously.

The results emphasize that stable cycling of Li₂O₂ at the cathode in a Li−O₂ cell depends on the interplay between the electrode and the electrolyte rather than each in isolation.

—Thotiyl et al.

Resources

• Muhammed M. Ottakam Thotiyl, Stefan A. Freunberger, Zhangquan Peng, and Peter G. Bruce (2012) The Carbon Electrode in Nonaqueous Li–O₂ Cells. Journal of the American Chemical Society doi: 10.1021/ja310258x