Lawrence Berkeley National Laboratory

Ultra high-resolution is needed to resolve light elements in a heavy-atom matrix, such as Li+ in a transition metal oxide. Recent advances in high-resolution transmission electron microscopes (TEM) and associated software, particularly exit-surface wave (ESW) reconstruction, push the resolution limit to better than 0.1nm, which provides opportunities to examine whether Li+ could be imaged and resolved in a transition metal oxide structure. A layered LiCoO2 compound with space group R-3m was studied here, prepared from a mixture of Li2CO3 and Co3O4 at 900 degrees C in O2 for 24 hrs. It was found from exit-surface wave (ESW) simulations that a microscope resolution of 0.1nm was needed to experimentally resolve individual lithium ions in the layered structure along the [110] zone axis for specimen thickness close to 5nm. Application of neutral Li and charged Li+ scattering factors in the calculations led to no considerable difference in the ESWs. The simulation results were then compared with focus-series reconstructed ESWs along the [110] zone axis. The ESWs were reconstructed from several focus-series of experimental TEM images using the Philips/Brite-Euram software, which were obtained on a modified Philips CM300FEG/UT microscope with a native resolution of 0.17nm and a demonstrated maximum resolution of 0.078nm. It was found consistent with the simulation results that the intensity associated with the Li+ position in the experimental ESWs was the weakest in comparison to those of oxygen and cobalt. Simulations from a model with no Li+ present showed no contrast at the Li+ position. Therefore, both simulation and experimental results support the fact that Li+ was resolved atomically in LiCoO2.