Thin alumina coatings on Li-rich nickel cobalt manganese oxide (Li-rich NCM) particles used as cathode material in Li-ion batteries can improve the capacity retention during cycling. However, the underlying mechanisms are still not fully understood. It is crucial to determine the degree of coverage of the particle's coating on various length scales from micrometer to nanometer and to link it to the electrochemical properties. Alumina coatings applied on Li-rich NCM by atomic layer deposition or by chemical solution deposition were examined. The degree of coverage and the morphology of the particle coatings were investigated by time-of-flight secondary-ion mass spectrometry (ToF-SIMS), scanning electron microscopy, elemental analysis using inductively coupled plasma optical emission spectrometry, and scanning/transmission electron microscopy. ToF-SIMS allows investigating the coverage of a coating on large length scales with high lateral resolution and a surface sensitivity of a few nanometers. Regardless of the chosen coating route, analytical investigations revealed that the powder particles were not covered by a fully closed and homogenous alumina film. This study shows that a fully dense coating layer is not necessary to achieve an improvement in capacity retention. The results indicate that rather the coating process itself likely causes the improvement of the capacity retention and increases the initial capacity.
Keywords: Li-ion battery; Li-rich NCM; aluminum oxide cathode coatings; atomic layer deposition (ALD); cathode; chemical solution deposition (CSD) coatings; time-of-flight secondary-ion mass spectrometry (ToF-SIMS).