Tailoring the Properties of Al-doped LiCoO₂ Surfaces

Undesired reactions at electrode/electrolyte interfaces impose challenges in the durability of Li-ion battery. Core−shell heterostructures at the nanocrystal level offer opportunities for improving the functionality of the electrode by precise control of the interfacial structures and electronic properties. This ability is demonstrated with the compositional and structural tailoring of passivating layers based on Al³⁺, grown conformally onto LiCoO₂ nanoplates, using thermal treatments. They result in the heterostructures from core−shell LiCoO₂/Al₂O₃ to LiCo_1-xAl_xO₂ gradient structures composed by an Al-rich outer layer on a Co-rich core. Scanning transmission electron microscopy and X-ray adsorption spectroscopy are performed to characterize the Al-doped LiCoO₂ surface structures. First-principles density functional theory calculations are further performed to investigate the Co 3d spin states and O 2p states at different LiCo_1-xAl_xO₂ surfaces. The presence of the surface Co high spin states, combined with the structural epitaxy and the Al/Co ratio at the surfaces, is found to be critical to obtain the best electrode properties and electrode/electrolyte interface stabilization.