Soft X-ray Absorption Spectroscopic and Raman Studies on Li1.2Ni0.2Mn0.6O2 for Lithium-Ion Batteries

Soft X-ray absorption spectroscopy and Raman spectroscopy studies were conducted on Li1.2Ni0.2Mn0.6O2 in order to understand the electronic structures of metal and oxygen ions and the local structural changes that take place during electrochemical cycling. The O K-edge under fluorescent yield mode shows that the spectral ratio significantly drops during the oxygen plateau, while the Ni and Mn LII,III-edges under total electron yield mode show a change in the electronic condition of the surface transition-metal ions during the oxygen plateau as evidenced by a sharp increase in the spectral features. This supports the transition-metal densification model where the surface transition-metal ions migrate to the bulk while oxygen leaves the lattice. Raman spectra show that Li migration is substantially suppressed during the plateau at 4.55 V due to shrinkage along the axial plane. Finally, both Raman spectroscopy and the Mn LII,III-edges show the surface of the material changes from layered to spinel and grows continuously with subsequent cycles, while Mn is reduced from 4+ after the first cycle and is directly involved in the charge compensation mechanism.