InSitu DRIFTS Analysis of Solid-Electrolyte Interphase Formation on Li-Rich Li1.2Ni0.2Mn0.6O2 and LiCoO2 Cathodes during Oxidative Electrolyte Decomposition

Insitu diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) investigations have been made to examine solid-electrolyte interphase (SEI) formation on lithium-rich Li1.2Ni0.2Mn0.6O2 (LLNMO) and LiCoO2 cathodes during first- and second-cycle charging and discharging. This DRIFTS technique allows us to clarify SEI formation with different charging voltages. Both cathodes revealed the formation of the same surface species during first-cycle charging, initially including ethylene carbonate (EC) adsorption, and SEI species, for example, ROCOF, RCOOR, Li2CO3, ROCO2Li, and PFx, are formed above the onset potential, namely 4.0 and 4.5V for LiCoO2 and LLNMO, respectively. The onset potentials correspond to the upper limit of the reversible redox potential range for transition-metal couples (e.g. Co3+/Co4+ in LiCoO2 and Ni2+/Ni4+ in LLNMO), which account for the intrinsic instability of these cathode materials. Such results suggest the participation of intermediate reactive oxygen species in SEI formation. SEI species continue to form during the discharge process when the potential is scanned cathodically below 3.6 and 4.0V for LiCoO2 and LLNMO, respectively. Similar SEI species are also observed during the second cycle charge-discharge over LLNMO, where additional oxidized species such as carboxylate (-COO-) and CO2 are also found during charging. With the exception of PFx, all of the observed SEI species can be attributed to the oxidative decomposition of the organic solvent, EC. Finally, possible reaction mechanisms related to the oxidative decomposition of EC are discussed.