A new study by a team at MIT led by Dr. Yang Shao-Horn and Dr. Carl Thompson sheds more light on the morphological evolution of Li2O2 particles in Lithium-air batteries. An understanding of the interior structure of these particles and their growth processes is key to engineering air cathode electrode structures that can provide high energy and power densities and lower charging voltages for high round-trip efficiency, they note in their paper published in the ACS Journal of Physical Chemistry Letters.
Lithium-air (Li−O2) batteries have attracted considerable interest in the past few years because of their intrinsically high gravimetric energy densities compared with Li-ion batteries, which make them promising for electric vehicle applications. During discharge, electrons electrochemically reduce O2 on the surface of an air cathode combining with Li+ ions to form Li2O2, in a fundamentally different energy-storage mechanism than the intercalation reactions of Li-ion batteries. The nucleation, growth, and morphological evolution of Li2O2 particles have not been thoroughly investigated to date.
In previous studies, it has been demonstrated that the first discharge in Li−O2 cells containing a relatively stable electrolyte (e.g., 1,2-dimethoxyethane (DME)) and carbon-based air cathode can lead to a range of discharge product morphologies, including conformal films or disc- and toroid- shaped particles. However, few studies have suggested a mechanism for growth or probed the interior structure of the Li2O2 particles. In this study, we investigated the structure of Li2O2 discharge product as a function of rate and capacity using scanning and transmission electron microscopy (SEM and TEM).—Mitchell et al.
They found that at low discharge rates, electrochemically grown Li2O2 particles form first as stacked thin plates (∼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.
They also observed that at high rates growth of small equiaxed particles precedes growth of discs, which form only at high capacities.
Robert R. Mitchell, Betar M. Gallant, Yang Shao-Horn, and Carl V. Thompson (2013) Mechanisms of Morphological Evolution of Li2O2 Particles during Electrochemical Growth. The Journal of Physical Chemistry Letters. doi: 10.1021/jz4003586