Surface Layer Formation and Stripping Process on LiMn2O4 and LiNi1/2Mn3/2O4 Thin Film Electrodes

Surface layer formation and stripping processes on LiMn2O4 and LiNi1/2Mn3/2O4 thin film electrodes were analyzed by using in situ Fourier transform infrared (FTIR) spectroscopy. For LiMn2O4, the peak intensity of in situ FTIR gradually increased up to 4.2 V vs Li/Li+ during anodic polarization and decreased more anodic electrode potential. In situ FTIR spectra of LiNi1/2Mn3/2O4 showed a strong peak intensity at a two electrode potential, 4.2 V vs Li/Li+ and 4.8 V vs Li/Li+. These observations indicated that a surface layer formation process is accelerated by a redox couple of transition metals (Mn3+/Mn4+ or Ni2+/Ni4+) in active materials. In addition, in situ FTIR spectra for a 3 V plateau area of LiMn2O4 were also investigated to confirm the surface layer formation and stripping by redox couple of transition metals at low electrode potential. The surface layer stripping was observed at 2.9 V vs Li/Li+, and the lower electrode potential and formation process was also observed from 2.9 to 3.2 V vs Li/Li+. It can be explained that the surface layer was formed by immersing the cathode materials into the electrolyte solution.