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.