Facet-dependent electrochemical performance and electronic structure of LiCoO 2 polyhedral particles revealed by microscopic resonant X-ray photoelectron spectroscopy

The morphology of active material particles has a significant impact on the charge–discharge cycle performances of lithium-ion batteries because each crystal surface constructed by different elemental arrangements indicates various surface energies. The surface properties of each facet affect the stability of the interface between the facets of particles and electrolytes. In this study, we applied microscopic resonant photoelectron spectroscopy with a spatial resolution of 100 nm (3DnanoESCA system) to reveal the electronic structure of each facet of prototypical layered LiCoO 2 cathode particles, where the characteristic facets are (001), (104), and (012). We detected the difference in an electronic structure near the valence-band maximum (around 1–3 eV) on different LiCoO 2 facets, where the dominant Co 3d bands at the valence band of the (001), (104) and (012) facets showed binding energies of 2.48, 2.25 and 2.02 eV, respectively. The closer Co 3d band of the (012) facet to the Fermi level makes it easier to lose electrons than the other facets, suggesting its more reactive property than the other facets. This technique, which provides the electronic structure of each crystal facet, is useful for designing active materials with excellent charge–discharge cycle performances.