Li-air batteries (LAB) can theoretically enable energy storage with 5-fold increased specific energy in comparison to that of Li-ion systems. A number of fundamental problems prevent LAB from being reversibly rechargeable however. Precipitation of the discharge products, e.g. lithium peroxide (Li₂O₂), inside a positive electrode is one of the key processes that control battery performance [1], thus detailed understanding of product deposition mechanisms is required to achieve deeper discharge and complete reversibility.

Product formation is a result of multistep chemical process ending up with crystallization. Depending on the discharge conditions and, especially, electrolyte solvent, different growth mechanisms occur giving rise to quite different morphologies. Recently, it has been reported that solvent’s donor number play a key role in lithium peroxide formation processes [2]. Here we test the proposed theory in a wider range of solvents, including DMA and sulfolane.

Carbon paper was used as an electrode material with controlled porosity. Solvent list included DMSO, MeCN, DMA, DME and sulfolane and was demonstrated to be relatively stable to ORR products and intermediates. Composition, structure and morphology of the discharge products formed during potentiostatic discharge were analysed.

It was shown that oxygen reduction in the presence of lithium ions in the electrolyte proceeds with the formation of plate-like particles Li₂O₂ that form a film or agglomerate complex morphology. The formation of Li₂O₂ crystalline plates proceeds via superoxide disproportionation in solution [3].  

[1] Christensen J, Albertus P, Sanchez-Carrera RS et al. A Critical Review of Li-Air Batteries// J. Electrochem. Soc. 2012, 159:R1.

[2] Johnson L, Li C, Liu Z et al. The role of LiO₂ solubility in O₂ reduction in aprotic solvents and its consequences for Li–O₂batteries// Nature Chem. 2014, 6:1091–1099.

[3] T.K. Zakharchenko, A. Y. Kozmenkova, D.M. Itkis et al. Lithium peroxide crystal clusters as a natural growth feature of discharge product in Li-O₂ cell// Beilstein J. Nanotechnol., 2013:4.