What XRD configuration shall one use to analyze battery cathode materials?

Introduction

X-ray diffraction (XRD) is a crucial tool for analyzing cathode materials in lithium-ion batteries. Cathode materials such as lithium iron phosphate (LFP) and nickel manganese cobalt oxide (NMC), commonly used in electric vehicle (EV) batteries, may exhibit defects like cation mixing and grain boundaries, which can affect their performance and durability. XRD is frequently used to investigate these defects, as well as the crystal phases of synthesized cathode materials.

Since XRD is not a fixed-configuration instrument, its optical path must be customized based on the material to ensure high sensitivity and the best data quality. Cathode materials contain transition metals (e.g., Fe, Ni, Co, Mn) that typically exhibit high fluorescence when analyzed with the conventional copper (Cu) anode. This fluorescence leads to a high background in the XRD diffractogram, reducing sensitivity to minor phases. However, selecting specific optics and detector combinations can significantly minimize this background, improving data quality.

Below is a summary of key optical components commonly used in XRD measurements.

Choice of Incident Beam Optics

• Motorized Slit: A widely used optics in XRD. When combined with a beam knife, it provides good low-angle performance. However, a beta filter is necessary to eliminate kβ peaks from the diffractogram, which reduces the kα beam intensity by around 50%.
• BBHD (or iCore) Optics: This multilayer mirror-based monochromator optics offers a resolution of ~450 eV. It filters out beta and white radiation at the source, delivering excellent low-angle performance and a high-intensity diffractogram with low noise.

Choice of Detector

For powder diffraction measurements, either a 0D or 1D detector can be used, with 1D detectors providing faster measurements. 1D detectors come in two categories: low energy resolution (>1500 eV) and high energy resolution (<350 eV). High energy resolution detectors are preferred for samples with high fluorescence as they deliver superior performance.

Case Study: Li-NCM111 Cathode Material

In this case study, we measured the BAM-S014 Li-NCM111 certified reference material using four different XRD configurations, each with a unique combination of incident beam optics and detectors. The configurations are summarized in the table below:

Results and discussion

The diffractograms measured using these four configurations are compared in Figure 1.

Figure 1: Comparison of XRD pattern measured with PDS and BBHD and two different energy resolution settings (1500eV and 340eV) of the 1Der detector.

Zoomed-in views of the low- and high-angle regions of the diffractograms are shown in Figures 2 and 3.

Figure 2: Low angles zoom out of XRD pattern measured with PDS and BBHD and two different energy resolution settings (1500eV and 340eV) of the 1Der detector.

Figure 3: High angles zoom out of XRD pattern measured with PDS and BBHD and two different energy resolution settings (1500eV and 340eV) of the 1Der detector.

The results show that the 340 eV energy resolution provides a distinct advantage over conventional detectors with >1500 eV energy resolution. The signal-to-background (S/B) ratio is significantly better with 340 eV measurements. Furthermore, the use of multilayer mirror optics, such as BBHD or iCore, enhances the signal intensity and reduces background noise even further compared to motorized slits, at a given energy resolution.

The S/B ratios obtained using various XRD configurations are summarized in the table below:

From S/B ratio, it can be concluded that BBHD in combination with 340eV energy resolution detector provides the best quality data for analyzing cathode active materials.

This high-quality data is suitable for Rietveld refinement to extract critical parameters, such as cation mixing and crystallite size. An example refinement in HighScore Plus software is shown in Figure 4, showing a cation mixing of 7% and crystallite size of 92nm in this sample.

Figure 4: Rietveld refinement of XRD data gives 7% cation mixing and 92 nm crystallite size in this sample.

Conclusion

To analyze cathode materials with XRD, a high energy resolution detector (<350 eV) has a distinct advantage in eliminating the fluorescence coming from transition metals like Mn, Fe, Co, and Ni. Use of Co anode instead of Cu can also eliminate the fluorescence from Fe and Co, however, not from Mn. So, a high energy resolution detector is an excellent choice for analyzing a wide variety of cathode materials.

Combining high energy resolution detector with BBHD or iCore optics can further improve the data quality by reducing the background and enhancing the signal, delivering best signal to background in the XRD diffractogram. Such high-quality data can then be used to determine critical parameters like cation mixing and crystallite size, in addition to the crystal phase structure of the cathode material.