Identifying a descriptor for d-orbital delocalization in cathodes of Li batteries based on x-ray Compton scattering

We discuss how x-ray Compton scattering spectra can be used for investigating the evolution of electronic states in cathode materials of Li batteries under the lithiation/delithiation process. In particular, our analysis of the Compton spectra taken from polycrystalline Li$_x$CoO$_2$ samples shows that the spectra are dominated by the contribution of the O $2p$ redox orbital. We identify a distinct signature of Co $3d$ orbital delocalization, which is tied directly to the conductivity of the material, providing a descriptor [1] based on Compton spectra for monitoring the lithiation range with improved conductivity and kinetics for electrochemical operation. Our study demonstrates that Compton scattering spectroscopy can provide a window for probing complex electronic mechanisms underlying the charging and discharging processes in Li-battery materials.\\ \\ $\mbox{[1] B. Barbiellini {\em et al.} Applied Physics Letters {\bf 109} , 073102 (2016)}$