Article

XPS Chemical State Analysis of N-Doped Carbon Surface Coatings for Improved Battery Stability

Surface Analysis Spotlight: XPS

by Sarah Zaccarine XPS Scientist

Lithium-ion batteries have employed nickel manganese cobalt (NCM) cathodes for years, but Co availability and price has driven the need for alternative materials. Referred to as NCM811, nickel-rich layered oxide material LiNi_0.8Co_0.1Mn_0.1O₂ is a promising cathode active material (CAM) due to its lower cobalt content and therefore lower cost, low toxicity, and higher specific capacity. However, Ni-rich NCM811 faces severe transition metal dissolution, corrosion, side reactions, and irreversible phase transitions, hindering battery performance. Significant research efforts have focused on strategies for mitigating degradation, including mono-crystallization, ion doping, and the use of surface coatings. Coating the surface of NCM811 with a layer of stable material to minimize degradation is a promising option since it is a straightforward process in industrial-scale applications.

In a recent study, Wang et al. investigated carbon-coated and nitrogen-doped carbon-coated NCM811 cathode materials. The authors wanted to determine whether the surface coatings would react with the bulk NCM811 material and to discern the impact of introducing N species in addition to the carbon coating. X-ray photoelectron spectroscopy (XPS) was used to answer these questions. XPS is a powerful surface analysis technique capable of resolving not only elemental composition but also specific chemical states of the elements in the surface.

The authors utilized the PHI VersaProbe II spectrometer to understand the specific chemical composition of each CAM in the study. XPS survey scans revealed the expected elements (Li, Ni, Mn, Co, and O), along with increased C content after carbon coating and the appearance of N after nitrogen doping. High-resolution scans in Figure 1 were collected to investigate the chemical states of C, O, N, and Ni in the pristine, uncoated CAM (P-NCM811) and after carbon coating only (NCM811-C0.5) or N-doped carbon coating (NCM811-CN0.5).

The C1s spectra showed the presence of C-C and C-O bonds in all three samples, along with C-N in the N-doped sample (Figure 1a). The relative amount of C-O from lithium carbonate (Li₂CO₃) was lower in the two coated samples, particularly the N-doped sample, due to the higher C-C content from the carbon coating process. It is also possible that the coating was able to partially mitigate the formation of Li₂CO₃ compared to the uncoated NCM. This is supported by the O1s data (Figure 1b), which shows relatively more metal oxides and less carbonate in the coated samples compared to the uncoated NCM. The N1s data collected for the N-doped carbon-coated sample showed the clear presence of multiple N-C chemistries (Figure 1c), including pyrrolic and pyridinic N, which are known to have high Li⁺ adsorption capacity, beneficial for performance.

Figure 1: XPS spectra of (a) C1s, (b) O1s, (c) N1s, and (d) Ni2p of P-NCM811 (bottom row in each spectral window), NCM811-C0.5 (middle row), and NCM811-CN0.5 (top row).

The Ni chemistry of the NCM active material was also affected by the coating processes. In the Ni2p data (Figure 1d), the ratio between Ni³⁺ and Ni²⁺ oxides varied in the three samples with slightly more Ni³⁺ and less Ni²⁺ in the N-doped C-coated sample compared to the uncoated sample while the C-coated sample showed slightly less Ni³⁺/more Ni²⁺ than the uncoated sample. The altered chemistry was attributed to a difference in lattice disorder as demonstrated in XRD results when N species were present.

To understand the impact of the measured chemical changes on performance, various electrochemical tests were performed. Both coated samples outperformed the uncoated P-NCM811, and the N-doped sample had enhanced cyclability, stability, and rate performance compared to the pristine and C-coated NMC811. The mitigation of harmful side reactions is likely due to the ability of the surface coating to create a chemical barrier between the highly reactive cathode material and electrolyte as well as the stabilizing effect of N species in this particular coating.

To learn more about how XPS can be used to understand surface coatings and other properties beneficial for battery performance, see Dr. Sarah Zaccarine’s talk at the upcoming 2023 MRS Fall Meeting in Boston at the end of November.