In Operando and Ex Situ Investigation of the Formation and Aging Processes of SEI/CEI C–H Chemistry in Liquid Electrolyte Lithium-Ion Batteries

Liquid electrolyte lithium-ion batteries (LIBs) are the fundamental electrochemical technology powering modern electric vehicles. The performance of LIBs is closely related to the quality of the anode solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) layers. For the first time, sum frequency generation (SFG) vibrational spectroscopy was successfully used to study the formation and surface changes of the CEI layer in operando at the solid/liquid interface during the initial formation cycle using a commercially relevant electrode (i.e., NCM622 chemistry and noncontrived geometry). In operando experiments showed the CEI layer formed when the voltage reached 3.23 V and stabilized after 2 h at 3.55 V in the first cycle. Ex situ SFG experiments demonstrated that methylene (CH₂) and methyl (CH₃) groups detected in the CEI and SEI layers after one cycle became less ordered after 10 and 50 cycles.