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

The thermodynamic stability and electronic structure of 40 surfaces of lithium peroxide (Li (2) O (2) ) and lithium oxide (Li (2) O) 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 Li (2) O (2) surfaces and 17 unique Li (2) O 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 Li (2) O (2) oxygen-rich 0001 and 1 ̅100 terminations that are more stable than those previously reported. Although oxygen-rich facets predominate in Li (2) O (2), in Li (2) O stoichiometric surfaces are preferred, consistent with prior studies. Surprisingly, surface-state analyses reveal that the stable surfaces of Li (2) O (2) are half-metallic, despite the fact that Li (2) O (2) is a bulk insulator. Surface oxygens in these facets are ferromagnetic with magnetic moments ranging from 0. 2 to 0. 5 μ (B). In contrast, the stable surfaces of Li (2) O are insulating and nonmagnetic. The distinct surface properties of these compounds may explain observations of electrochemical reversibility for systems in which Li (2) O (2) is the discharge product and the irreversibility of systems that discharge to Li (2) O. Moreover, the presence of conductive surface pathways in Li (2) O (2) could offset capacity limitations expected to arise from limited electron transport through the bulk.