St. Andrews team elucidates behavior of carbon cathodes in Li-air batteries; the importance of the synergy between electrode and electrolyte
Carbon is seen as an attractive potential cathode material for aprotic (non-aqueous) Lithium-air batteries, which are themselves of great interest for applications such as in electric vehicles because of the cells’ high theoretical specific energy.
However, the stability of carbon and the effect of carbon on electrolyte decomposition in such cells are not deeply understood at this point. A team at the University of St. Andrews (Scotland) led by Prof. Peter Bruce has further investigated the behavior of carbon as a possible porous cathode for aprotic Li-air cells; a paper on their work is published in the Journal of the American Chemical Society.
|2012 AkzoNobel UK Science Award|
|Professor Bruce, a Fellow of the Royal Society and the Royal Society of Edinburgh, received the inaugural UK Science Award from global coatings company AkzoNobel earlier this year.|
|Professor Bruce was selected by an independent panel convened by the Royal Society of Chemistry (RSC) for...|
|“...his outstanding contributions in the fields of solid state chemistry and electrochemistry, particularly in the area of materials chemistry related to energy storage. He is noted for his groundbreaking research into nanostructured intercalation electrodes and polymer electrolytes that underpin rechargeable lithium ion batteries and for his seminal studies of the lithium-air battery. The former technology has revolutionized the portable electronics industry and latter possesses the theoretical energy density that could transform electric and hybrid electric motor vehicles.”|
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 + 2e− ↔ 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.
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