Electronic Structure of Oxygen in Delitiated LiTMO₂ Studied by Electron Energy-Loss Spectrometry

Lithium transition-metal oxides (LiTMO₂) are currently the preferred cathode materials for secondary Li ion batteries. Several of these materials have a layered structure suitable for intercalation compounds owing to their good kinetics and ability to withstand internal strains of Li intercalation. Despite extensive research on this system, it is still not understood how the electronic charge is redistributed during lithiation. Traditionally it has been accepted that the charge on the intercalated Li is accommodated by the transition metal, allowing the O valence to remain 2-. However, recent computational studies suggest that there is substantial charge transfer from the ionic Li to the anion [1,2]. Empirical data from x-ray photoemission spectroscopy (XPS) and x-ray-absorption spectroscopy (XAS) show that LiTMO₂ is strongly covalent with available O 2p holes lying in the TM 3d band [3]. These techniques also show a decrease in O 2p holes during Li intercalation indicating an increase in electron density around the O ion [3,4]