Operando Fourier Transform Infrared Investigation of Cathode Electrolyte Interphase Dynamic Reversible Evolution on Li_1.2Ni_0.2Mn_0.6O₂

One of the keys to the cycling stability of electrode materials is the formation of a stable interface film on cathode materials, which is called a cathode electrolyte interphase (CEI). For a Li/Mn-rich cathode, especially, the high working voltage will cause an extremely unstable electrolyte environment, becoming a challenge for the stable interface film formation. In this work, an operando-attenuated total reflection–Fourier transform infrared (ATR–FTIR) technique is developed to monitor in real time the dynamic mechanism of CEI formation in a carbonate-based electrolyte with or without the moderate additive tris(trimethylsilyl)borate (TMSB), which could promote the formation and stability of high-quality CEI films when it charges to 4.8 V. It is interesting that the components of CEI are basically generated in the first cycle owing to ethylene carbonate (EC) priority decomposition. Besides, the presence of TMSB can suppress the decomposition of EC in part and modify the stability of the CEI film. This is because TMSB containing an electron-deficient boron atom can easily combine with an electron-rich F– and PF6– forming a polyanion group initially, which will weaken the electrostatic force between the anionic groups and EC to reduce the concentration of EC on the cathode surface and prevent the continuous decomposition of EC at a high voltage. X-ray photoelectron spectroscopy also verifies the presence of polyanion groups and their further participation in CEI formation. This work highlights the dynamical stability of CEI modified by moderate TMSB and the formation mechanism of this dynamical change during cycling characterized by the operando ATR–FTIR technique, which paves the way for a better understanding of the complex and hard-characterized cathode interface reactions.