Among the many challenges for Li-ion batteries (LIBs) to achieve long cycle life, formation of high quality solid electrolyte interphase (SEI) is of critical importance. SEI forms on LIB anode during the initial cycles, and its properties directly impact battery life and power performance. In-depth understanding of SEI growth mechanism can provide critical parameter input for modeling and simulation, set foundation for the SEI properties and performance correlation, and lead to the ultimate effective artificial SEI or coating design to achieve satisfactory cell cycle efficiency. However, due to the nm thickness and air/moisture sensitivity of SEI, designing and performing in-situ diagnostics are technically challenging. To date, most studies on SEI properties are performed on electrodes harvested after SEI formation, and the time-dependent growth mechanism of SEI during its formation remains to be elucidated.

In this talk, we will present an approximation of in-situ diagnostic method, by utilizing isotope-assisted time-of-flight secondary ion mass spectrometry (TOF-SIMS). This method is capable of obtaining time-dependent information during SEI growth, even after the growth process has completed. The model battery design includes a Cu electrode, ⁶Li labeled electrolyte (1M ⁶LiBF₄ in EC:DEC 1:2) and a ⁷Li metal counter electrode. As SEI grows on the Cu electrode, the ⁶Li:⁷Li isotope ratio in the electrolyte decreases due to ⁶LiBF₄ consumption. The time scale of SEI growth thus can be represented by the ⁶Li:⁷Li ratio in the SEI, and each ⁶Li:⁷Li ratio corresponds to a snapshot during SEI growth. The isotope ratio are measured with TOF-SIMS after harvesting the SEI. With this method, we have found that the SEI portion formed first (higher ⁶Li:⁷Li ratio) presents at the electrolyte/SEI interface, and the SEI formed last (lower ⁶Li:⁷Li ratio) exists at the SEI/electrode interface. These results also indicate that new SEI forms at the SEI/electrode interface. Further chemistry analyses with TOF-SIMS and XPS show that the portion formed first are mostly organic compounds while the portion formed last are mostly inorganic compounds. This method could also be applied in other fields where growth dynamics are of interest, such as investigating Li dendrite growth.