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Lithium Peroxide Surfaces Are Metallic, While Lithium Oxide Surfaces Are Not

Radin, Maxwell D., Rodriguez, Jill F., Tian, Feng, Siegel, Donald J.
Journal of the American Chemical Society 2012 v.134 no.2 pp. 1093-1103
batteries, crystallites, electrochemistry, electron transfer, lithium, methodology, oxygen, thermodynamics
The thermodynamic stability and electronic structure of 40 surfaces of lithium peroxide (Li₂O₂) and lithium oxide (Li₂O) were characterized using first-principles calculations. As these compounds constitute potential discharge products in Li–oxygen batteries, their surface properties are expected to play a key role in understanding electrochemical behavior in these systems. Stable surfaces were identified by comparing 23 distinct Li₂O₂ surfaces and 17 unique Li₂O surfaces; crystallite areal fractions were determined through application of the Wulff construction. Accounting for the oxygen overbinding error in density functional theory results in the identification of several new Li₂O₂ oxygen-rich {0001} and {11̅00} terminations that are more stable than those previously reported. Although oxygen-rich facets predominate in Li₂O₂, in Li₂O stoichiometric surfaces are preferred, consistent with prior studies. Surprisingly, surface-state analyses reveal that the stable surfaces of Li₂O₂ are half-metallic, despite the fact that Li₂O₂ is a bulk insulator. Surface oxygens in these facets are ferromagnetic with magnetic moments ranging from 0.2 to 0.5 μB. In contrast, the stable surfaces of Li₂O are insulating and nonmagnetic. The distinct surface properties of these compounds may explain observations of electrochemical reversibility for systems in which Li₂O₂ is the discharge product and the irreversibility of systems that discharge to Li₂O. Moreover, the presence of conductive surface pathways in Li₂O₂ could offset capacity limitations expected to arise from limited electron transport through the bulk.