Conversion Reaction of CoO Polycrystalline Thin Films Exposed to Atomic Lithium

We have studied the reaction of 5 nm thick polycrystalline CoO films with atomic lithium as a model for the discharge of lithium-ion conversion battery electrodes. The electronic structure has been investigated with X-ray photoemission, ultraviolet photoemission, and inverse photoemission, while the morphology, crystal structure, and unoccupied states of the films have been examined with transmission electron microscopy. It is found that exposure to atomic lithium leads, at room temperature, to partial conversion with formation of Co and Li2O, but also of a Li2O2 or LiOH overlayer at the surface of the sample. As full conversion was obtained at 150 degrees C, a comparison with room-temperature measurements enables the understanding of the kinetic limitations during lithiation.