Article
XPS Insights into Chemical Degradation Mitigation of Protective Coatings on NCM Cathodes
Surface Analysis Spotlight: XPS
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by Sarah Zaccarine XPS Scientist |
Nickel manganese cobalt (NCM) oxides are a popular cathode material for lithium-ion batteries (LIBs) due to their high capacity, stability, and rate capability as well as improved cost compared to cobalt oxides. However, these battery systems are susceptible to side reactions at the electrode/electrolyte interfaces, particularly the cathode-electrolyte interphase (CEI). One approach to mitigate degradation is to coat the cathode with a stabilizing layer that restricts contact between the cathode and electrolyte. Many materials have been explored as coatings, but reactions in the stabilizing layer and its interface with the cathode are not yet well-understood.
A recent study by Hemmelmann et al. explores a novel approach to create model NCM thin film cathodes that enable more accurate chemical analysis of the cathode/protective coating interface. The group studied Al2O3 coatings before and after cycling to understand reactions occurring between the cathode and the protective coating under device-relevant conditions. X-ray photoelectron spectroscopy (XPS) was used to analyze the formation and composition of the CEI formed on pristine and coated NCM cathodes, and complementary physico- and electrochemical characterization analyzed structural changes and the ultimate impact of all properties on performance.
XPS was first used to verify the formation of the Al2O3 coating on the NCM substrate (Figure 1). A pristine sample was compared to two thicknesses of Al2O3 and the samples were analyzed at the surface (top spectrum in each plot) as well as deeper into the sample with successive sputter etching steps. The pristine sample was comprised of a clear Ni signal as expected (Figure 1a) at a binding energy (BE) position in good agreement with NCM in the literature. A small amount of Al was also present in the pristine sample from contamination (Figure 1d). Both coated samples showed a strong Al signal at the surface (Figure 1e-f), but the samples had differences in the Ni2p3/2 spectra. The 2 nm Al2O3 had a more visible Ni 2 p3/2 peak due to its thinner surface layer on the NCM film (Figure 1b), whereas the 5 nm Al2O3 sample did not have a clear Ni signal until the sample had been sputtered (Figure 1c). The BE positions of the Al 2p peaks were in good agreement with literature values for Al2O3.
Figure 1: XPS analysis of a,d) the pristine thin film and the thin films coated with b,e) 2 nm and c,f ) 5 nm alumina.
The absence of Ni 2p3/2 signal in XPS analysis of the 5 nm coated sample also suggests uniform, complete coverage of the NCM surface since the NCM species were not detected. This was confirmed by complementary electron and atomic force microscopy characterization which displayed a uniform and homogeneous distribution of the Al2O3 layer on the NCM thin film surface. Structural analysis by Raman and X-ray diffraction also showed a loss of structural order after cycling in the pristine sample, while the coated samples retained some of their order.
XPS was also used to analyze the sample surface chemistry after cycling (Figure 2). Since these model systems did not contain polymer additives or binders in the cathode, the F 1s spectrum can be interpreted strictly as an illustration of decomposition between the electrolyte and the CAM, without interference that could lead to erroneous peak assignments. The two coated samples showed the presence of Li and Al species that reacted with F in the electrolyte (Figure 2b-c), supported by the Li 1s data (inset). These species suggest the reaction of the electrolyte at the coating interface with mobile Li ions and Al from the coating. The formation of AlF3 species indicates that fluorination of Al2O3 is favorable; it has been shown in the literature that alumina can scavenge corrosive F species and thereby protect the underlying CAM. No fluorinated NCM species were detected, supporting the protective nature of the Al2O3 layer. By comparison, the XPS results on the uncoated sample showed significant degradation of the NCM material itself (Figure 2a). The Li 1s data for the uncoated sample (Figure 2a inset) established that there was no Li detected at the surface, indicating the species at ~685 eV (Figure 2a) was actually Ni from the cathode that reacted with F species in the electrolyte. Similarly, MnF2 species were detected, along with species from the LiPF6 electrolyte additive.
Figure 2: Post-mortem XPS analysis of cycled NCM thin film cathodes a) without coating and films coated with b) 2 nm and c) 5 nm alumina. For better visualization, the spectrum of the uncoated sample was 3x magnified.
Electrochemical testing discovered that there was an additional electrochemical process occurring on the uncoated sample that did not occur on either coated sample, which could be indicative of mitigated electrolyte decomposition at the cathode surface, as observed in XPS analysis. Extended cycling studies also confirmed superior performance stability of the two coated samples, which overall experienced much lower performance losses with extended operation. This could be related to the preservation of CAM species in the coated samples, whereas the uncoated sample showed degradation of NCM components that reacted strongly with the electrolyte during cycling.
In summary, the thin Al2O3 protective coating on NCM thin film cathodes was able to mitigate harmful interfacial degradation reactions and leads to improved performance lifetime. XPS analysis highlighted chemical degradation in the uncoated sample compared to LiF and AlF species in the coated samples that confirm protective reactions occurring in the Al2O3 coating. These chemical changes measured by XPS also hindered structural degradation of the cathode which led to cell capacity decline in the uncoated NCM.
To see how other coating chemistries can affect CEI composition and cell performance, please attend Dr. Sarah Zaccarine’s presentation “Surface Analysis of Engineered Particles for Improved Battery Performance and Stability” at ECASIA 2024 in June in Gothenburg, Sweden.