  The study focuses on NMC811, a promising material for high-capacity
batteries, and investigates the challenges associated with its use,
specifically the formation of the Cathode Electrolyte Interphase (CEI) layer
due to chemical reactions. This layer is a consequence of the position of the
LUMO energy level of NMC811 that is close to the HOMO level of liquid
electrolyte, resulting in electrolyte oxidation and cathode surface alterations
during charging. A stable CEI layer can mitigate further degradation by
reducing the interaction between the reactive cathode material and the
electrolyte. Our research analyzed the CEI layer on NMC811 using advanced
techniques such as 4D-STEM ACOM and STEM- EDX, focusing on the effects of
different charging voltages (4.3 V and 4.5 V). The findings revealed varying
degrees of degradation and the formation of a fluorine-rich layer on the
secondary particles. Detailed analysis showed the composition of this layer
differed based on the voltage: only LiF at 4.5 V and a combination of LiF and
LiOH at 4.3 V. Despite LiF's known stability as a CEI protective layer, our
observations indicate it does not effectively prevent degradation in NMC811.
The study concludes that impurities and unwanted chemical reactions leading to
suboptimal CEI formation are inevitable. Therefore, future efforts should focus
on developing protective strategies for NMC811, such as the use of specific
additives or coatings.
