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

Abstract

Lithium ion batteries are nowadays key devices for energy storage, and a greatresearch effort is under way to develop and apply new materials. Recently, newapproaches have been proposed that rely on the reversible formation of eitherLi2O orLi2O2 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, theirsurfaces and their nanoparticles have been here investigated by density functionaltheory and ab initio thermodynamics. At a pressure of 1 atmosphere of oxygen,Li2O2 is the stable bulk phase below 5 K, where a transition toLi2O takes place. Wulff’s construction predicts an octahedral shape forLi2O nanoparticles and the form of a hexagonal prism forLi2O2. By taking into account the effect of the surfaces, a size-dependent phase diagram iscalculated. 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∼2.5 nm. This size-dependent oxidation behavior of lithium should be taken into account in thedesign of nanostructured oxygen cathodes.