XPS Studies of Surface Chemistry Changes of LiNi_0.5Mn_0.5O₂Electrodes during High-Voltage Cycling

We have examined the electrochemical performance and the surface chemistry of quenched and annealed LiNi 0.5 Mn 0.5 O 2 electrodes as a function of high voltage cutoff via X-ray photoelectron spectroscopy (XPS). After cycling for 20 cycles, the surface regions of quenched and annealed electrodes are significantly enriched in fluorine and lithium relative to pristine electrodes, with the degree of enrichment being greater for the annealed electrodes. Mn and Ni fluorides are observed on the surface of the cycled annealed electrodes while both Mn and Ni oxides and fluorides are observed on the surface of the cycled quenched electrodes. The XPS data indicate that the surface films formed on the annealed electrodes as a result of cycling are thicker than the films formed on the quenched electrodes. Significantly, the results indicate that Mn and Ni dissolution are suppressed by annealing, further supporting previous findings that showed an increase in the relative concentration of Mn 4+ to Mn 3+ and a decrease in the Ni/Li interlayer mixing for the annealed material relative to the quenched material. The formation of Mn and Ni fluorides and the suppression of highly resistive LiF upon cycling appear to play an important role toward the improved capacity retention of annealed LiN 0.5 Mn 0.5 O 2 .