Ab initio thermodynamics of lithium oxides: from bulk phases to nanoparticles

Lithium ion batteries are nowadays key devices for energy storage, and a great research effort is under way to develop and apply new materials. Recently, new approaches have been proposed that rely on the reversible formation of either Li2O or Li2O2 at the electrodes. The details of their formation and dissolution are, however, still unclear. As a first step towards the understanding of these processes, bulk lithium oxides, their surfaces and their nanoparticles have been here investigated by density functional theory and ab initio thermodynamics. At a pressure of 1 atmosphere of oxygen, Li2O2 is the stable bulk phase below 5 K, where a transition to Li2O takes place. Wulff's construction predicts an octahedral shape for Li2O nanoparticles and the form of a hexagonal prism for Li2O2. By taking into account the effect of the surfaces, a size-dependent phase diagram is calculated. At an oxygen pressure of 1 atmosphere and a temperature of 300 K, Li2O is the stable phase for particles with a diameter larger than approximately 2.5 nm. This size-dependent oxidation behavior of lithium should be taken into account in the design of nanostructured oxygen cathodes.