Layered Li-ion batteries: interpretation of oxygen K-edge core loss spectra

The cathode and elements reacting to the DFT and a plot of the excitation energy plotted against intensity

Core loss spectroscopies can provide powerful element-specific insight into the redox processes occurring in Li-ion battery cathodes, but this requires an accurate interpretation of the spectral features.

In the paper 'Atomistic interpretation of the oxygen K-edge X-ray absorption spectra of layered Li-ion battery cathode materials', published Chemistry of Materials, the authors* systematically interpret oxygen K-edge core loss spectra of layered lithium transition-metal (TM) oxides (LiM02, where M =  Co, Ni, Mn) from first principles using density-functional theory (DFT).  Spectra were simulated using three exchange-correlated functionals, comprised of the generalised gradient approximation (GGA) functional PBE, the DFT-PBE + Hubbard U method, and the meta-GGA functional rSCAN. 

Generally, the rSCAN provided a better match to experimentally observed excitation energies of spectral features when compared to both PBE and PBE + U, especially at energies close to the main edge.  Projected density of states at core-hole calculations show that the O orbitals are better described by rSCAN.  Hybridisation, structural distortions, chemical composition and magnetism significantly influenced the spectra.  The O K-edge spectrum of LiNi02 obtained which included a Jahn-Teller distortion) showed that the DFT-calculated pre-edge feature contained information about not only chemical species but also geometric distortion.  Core loss spectra derived from DFT can also differentiate between materials with the same structure and magnetic configuration​​ but comprising different TMs; these differences were comparable with those observed in experimental XAS from the same materials.  ​​​​​

This foundational work helps establish the extent to which DFT can be used to bridge the interpretation gap between experimental spectroscopic signatures and ab initio methods describing complex battery materials, such as lithium nickel manganese cobalt oxides.

 

*Led by this department: Oxford Materials, University of Cambridge (Chemistry), University of Dundee (Science and Engineering), The Faraday Institution, University of York (Physics) and the University of Birmingham (Metallurgy and Materials).