Mechanisms of Morphological Evolution of Li2O2 Particles during Electrochemical Growth

Li-O-2 batteries, wherein solid Li2O2 is formed at the porous air cathode during discharge, are candidates for high gravimetric energy (3212 Wh/kg(Li2O2)) storage for electric vehicles. Understanding and controlling the nucleation and morphological evolution of Li2O2 particles upon discharge is key to achieving high volumetric energy densities. Scanning and transmission electron microscopy were used to characterize the discharge product formed in Li-O-2 batteries on electrodes composed of carpets of aligned carbon nanotubes. At low discharge rates, Li2O2 particles form first as stacked thin plates, similar to 10 nm in thickness, which spontaneously splay so that secondary nucleation of new plates eventually leads to the development of a particle with a toroidal shape. Li2O2 crystallites have large (001) crystal faces consistent with the theoretical Wulff shape and appear to grow by a layer-by-layer mechanism. In contrast, at high discharge rates, copious nucleation of equiaxed Li2O2 particles precedes growth of discs and toroids.