Structure Deformation and Electronic Spin States Modification in the Surfaces of Charged and Discharged Lithium-Ion Battery Cathodes

Abstract

Structures and electronic spin states on active-surfaces for Li-ion (de) intercalation of Lithium Cobalt Oxides (LCO) are investigated by first principle calculation and molecular orbital analysis focusing on differences between charged and discharged states. Instability of the cathodes prevents realizing higher voltage charge of Li-ion Batteries (LIB). Especially, the degradation of the edge surfaces where Li-ions propagate in the charging and discharging processes decreases the capacity of the batteries severely. Despite of the long history of research and development of LIB, the degradation of surfaces of the cathodes has not been known well from electronic states point of view. In this study, by using electronic state calculations and orbitals analyses, we approach the mechanism of the surface deformation, focusing especially on the most probable edge surface of LCO. First, we established the edge surface model in the discharged states and performed the structure optimization with evaluation of the surface formation energy. Second, we deleted three quarters of Li-ions from the surface model and optimized the structure again. Comparison of the both structures shows that Li-ion de-insertion gives the flexibility of the coordination structures of the surface cobalt and promotes the surface structure distortion. Third, the electronic states of the both charged and discharged states were examined. We developed an analysis method of Kohn-sham orbitals which enable us to know the symmetry of the orbitals for valence band minimum and other bands, indicating that we can figure out the surface conditions from the ligand field theory for the surface cobalt and oxygen atoms. Our analyses show that the d-electrons of the surface cobalt atoms in the charged state occupy more unstable orbitals than those in the discharged states. Finally, we concluded that lithium de-intercalation causes surface deformation with unstable cobalt spin states and considered that the deformation brings about further structure degradations. We believe that the results can be applied to the other cathode materials containing transition-metal atoms.