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

We report a novel delithiation process for epitaxial thin films of LiCoO2(001) cathodes using only physical methods, based on ion sputtering and annealing cycles. Preferential Li sputtering followed by annealing produces a surface layer with a Li molar fraction in the range 0.5 < x < 1, characterized by good crystalline quality. This delithiation procedure allows the unambiguous identification of the effects of Li extraction without chemical byproducts and experimental complications caused by electrolyte interaction with the LiCoO2 surface. An analysis by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides a detailed description of the delithiation process and the role of O and Co atoms in charge compensation. We observe the simultaneous formation of Co4+ ions and of holes localized near O atoms upon Li removal, while the surface shows a (2 × 1) reconstruction. The delithiation method described here can be applied to other crystalline battery elements and provide information on their properties that is otherwise difficult to obtainThis work was supported by the Spanish MICINN (grant nos. PID2021-123295NB-I00 and PID2020-117024GB-C43), MAT2017-83722-R, “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M), within the framework of UE M-ERA.NET 2018 program under StressLIC Project (grant no. PCI2019-103594) and by the Comunidad Autónoma de Madrid (contract no. PEJD-2019- PRE/IND-15769 and S2108-NMT4321). The authors acknowledge Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities. They thank Ignacio Carabias from the Diffraction Unit CAI UCM for his experimental support and helpful comments. The research leading to this result has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. M.J., P.M., I.P., and F.B. acknowledge funding from EUROFEL (RoadMap Esfri). The work at the University of Maryland was supported by ONR MURI (Award No. N00014-17-1-2661). The work at Sandia National Laboratories was supported by the Laboratory-Directed Research and Development (LDRD) Program and the DOE Basic Energy Sciences Award number DE-SC0021070. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the US Department of Energy’s National Nuclear Security Administration under contract DE-NA 000352

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

We report a novel delithiation process for epitaxial thin films of LiCoO2(001) cathodes using only physical methods, based on ion sputtering and annealing cycles. Preferential Li sputtering followed by annealing produces a surface layer with a Li molar fraction in the range 0.5 x < 1, characterized by good crystalline quality. This delithiation procedure allows the unambiguous identification of the effects of Li extraction without chemical byproducts and experimental complications caused by electrolyte interaction with the LiCoO2 surface. An analysis by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides a detailed description of the delithiation process and the role of O and Co atoms in charge compensation. We observe the simultaneous formation of Co4+ ions and of holes localized near O atoms upon Li removal, while the surface shows a (2 × 1) reconstruction. The delithiation method described here can be applied to other crystalline battery elements and provide information on their properties that is otherwise difficult to obtain