Degradation of a lithium cobalt oxide cathode under high voltage operation at an interface with an oxide solid electrolyte†

Lithium (de)intercalation of layered rocksalt-type cathodes in high-voltage regions is of great importance for achieving a high energy density in lithium batteries. The reversible capacity of LiCoO₂ at high voltages is not well known because of oxidative side reactions with the electrolyte species. In this study, a model thin-film battery was fabricated using an epitaxially grown LiCoO₂ cathode and an amorphous Li₃PO₄ solid electrolyte to suppress oxidative degradation. The film battery operated stably at high voltages, ranging up to 4.6 V, without severe side reactions of LiCoO₂ and Li₃PO₄, resulting in a reversible capacity greater than 200 mA h g⁻¹. However, the charge–discharge capacities of the battery decreased with cycling at 4.7 V. In situ synchrotron X-ray diffraction studies revealed an irreversible structural change in LiCoO₂ at 3.0 V after charging at 4.7 V. Structural degradation occurred both in the bulk and surface regions of the LiCoO₂ film, indicating intrinsic irreversibility of the crystal structure changes of highly delithiated LiCoO₂, although the LiCoO₂/electrolyte interface remained stable.