Physical Delithiation of Epitaxial LiCoO₂ Battery Cathodes as a Platform for Surface Electronic Structure Investigation

We report a novel delithiation process for epitaxialthin filmsof LiCoO2(001) cathodes using only physical methods, basedon ion sputtering and annealing cycles. Preferential Li sputteringfollowed by annealing produces a surface layer with a Li molar fractionin the range 0.5 < x < 1, characterized bygood crystalline quality. This delithiation procedure allows the unambiguousidentification of the effects of Li extraction without chemical byproductsand experimental complications caused by electrolyte interaction withthe LiCoO2 surface. An analysis by X-ray photoelectronspectroscopy (XPS) and X-ray absorption spectroscopy (XAS) providesa detailed description of the delithiation process and the role ofO and Co atoms in charge compensation. We observe the simultaneousformation of Co4+ ions and of holes localized near O atomsupon Li removal, while the surface shows a (2 x 1) reconstruction.The delithiation method described here can be applied to other crystallinebattery elements and provide information on their properties thatis otherwise difficult to obtain.