Real-space measurement of orbital electron populations for LiCoO.

The operation of lithium-ion batteries involves electron removal from and filling into the redox orbitals of cathode materials, experimentally probing the orbital electron population thus is highly desirable to resolve the redox processes and charge compensation mechanism. Here, we combine quantitative convergent-beam electron diffraction with high-energy synchrotron powder X-ray diffraction to quantify the orbital populations of Co and O in the archetypal cathode material LiCoO. The results indicate that removing Li ions from LiCoO decreases Co t orbital population, and the intensified covalency of Co-O bond upon delithiation enables charge transfer from O 2p orbital to Co e orbital, leading to increased Co e orbital population and oxygen oxidation. Theoretical calculations verify these experimental findings, which not only provide an intuitive picture of the redox reaction process in real space, but also offer a guidance for designing high-capacity electrodes by mediating the covalency of the TM-O interactions.