Interfacial Degradation Analysis via the Combined In Situ Method of Gas Monitoring and Electrochemical Impedance Spectroscopy upon Fast-Aging Cycling at 45 °C

In situ analysis methods have been widely developed to investigate the degradation of high-energy density lithium-ion batteries with NMC811 as a positive electrode. The in situ/operando monitoring method is based on sensors, and impedance is an effective strategy for detecting electrolyte-related gas and interface variations upon cycling. However, the correlation between the liquid and solid interface variation with gas evolution and battery degradation during operation remains ambiguous. To understand such a relationship, we developed the combined in situ method of gas monitoring and electrochemical impedance spectra (EIS) to study the NMC811 materials coated with Al₂O₃ and borides. The operando gas evolution analysis indicates that CO₂ and CO related to electrolyte decomposition show similar evolution behavior during cycling, which is opposite to that of H₂ and CH₄. The increase of resistance via in situ EIS analysis implies that interface degradation leads to changes in the ratios of CO₂ to CO and CO₂ to H₂. The compositional variation of the interfaces via X-ray photoelectron spectroscopy (XPS) depth profiling suggests that aluminum shuttled to the negative electrode within the Li-ion batteries (LIBs) assembled with U-Al/B-NMC and prevents the interface from degradation, forming stable inorganic species, i.e., LiF and Li₂O. This work provides a novel in situ/operando method to analyze the relationship between interface variation, gas evolution, and battery degradation.