Theory of overlithiation reaction in LiMO2 battery electrodes

Layered compounds with composition LixMO2 and structure R(3) over bar m, in which M is a first-row transition metal or a combination of transition (and possibly nontransition) metals, are attractive materials for lithium-battery cathodes. For most choices of M, lithiation beyond x = 1 results in either decomposition or transformation to a nonlayered structure. In this article, we present first-principles calculations, on the basis of the GGA+U method, to compare addition, decomposition (with metal oxide and lithia as products), and displacement (with metal and lithia as products) reactions when an additional Li is added to LiMO2. We consider M = Mn, Co, and Ni, as well as equiatomic binary mixtures of these elements. Although the displacement reaction is energetically (or thermodynamically) favored, the addition and decomposition reactions are apparently more favored kinetically, as it is the latter that are observed experimentally. The ratio of the open-circuit cell voltage to the magnitude of the reaction energy, V-0/Delta E-r, is higher for systems such as M = Mn0.5Ni0.5, for which an addition reaction occurs, than for those such as M = Co, which undergoes decomposition.