Probing the Electrode-Electrolyte Interface in Cycled LiNi_0.5Mn_1.5O₄by XPS Using Mg and Synchrotron X-rays

X-ray photoelectron spectroscopy (XPS) was used to investigate the surface chemistry of high voltage spinel, LiNi 0.5 Mn 1.5 O 4 (LNMO) positive electrodes cycled 5 and 10 times in Li-cells with 1 M LiPF 6 in (3:7) EC:DMC. The XPS spectra were collected using conventional Mg X-rays with energy of 1253.6 eV as well as synchrotron X-rays with energies of 2493.6 and 3498.4 eV in order to examine the depth distribution of various surface chemical species induced during cycling. The XPS spectra revealed a 5 – 10 nm surface layer of organic and Li x PF y O z -type species formed as result of electrolyte decomposition, and a comparatively thinner layer composed of transition metal fluorides and LiF. These results suggest that electrolyte decomposition is a major contributor to parasitic reactions in LNMO battery electrochemistry. Limiting electrolyte decomposition with the use of solvents with wide electrochemical stability windows thus comprises a promising strategy for ensuring the practical feasibility of high voltage spinel materials in future Li-ion systems.