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Improving EBSD Characterization of Li-Ion Battery Cathode Materials using Clarity

Introduction

There is increasing demand for high-performance lithium-ion batteries for energy storage, electric vehicles, and electronic devices. Typically, these batteries use layered cathode materials, with one of the most common of these being LiNixMnyCozO2 (NMC). The NMC material is used as polycrystalline particles, with the orientation, grain size, and grain boundary structure within each particle influencing the charging-discharging properties and degradation behavior of the battery cell. Electron Backscatter Diffraction (EBSD) is the ideal microanalysis technique to measure the crystallographic microstructure of these materials within a Scanning Electron Microscope (SEM). However, due to the grain size of the NMC crystals and stringent sample preparation requirements, it can be challenging to satisfactorily resolve the grains at typical EBSD analytical conditions. In this work, the Clarity™ Direct Detector was used to collect data at lower beam energies and currents for improved EBSD spatial resolutions.

Results and Discussion

A cross-section of an NMC cathode layer was prepared using a low-energy broad ion beam source. This approach produces a pristine surface with high-quality EBSD patterns. Ideally, a lithium-ion battery sample prepared this way would immediately be transferred into the SEM using a vacuum exchange system, as is optionally available with the PECS™ II System from Gatan. In this case, however, the sample was shipped for analysis, and exposed to the atmosphere for some time before EBSD analysis, causing degradation of the sample surface.

EBSD data was collected using the EDAX Clarity EBSD Analysis System with APEX™ 2.0 Software for EBSD. This system captures the diffracted electrons directly onto a pixelated sensor, removing the need for a phosphor screen and optical coupling resulting in higher sensitivity and better performance at lower beam energies and currents. Figure 1a shows an EBSD Inverse Pole Figure (IPF) orientation map collected at 20 kV acceleration voltage and 1.6 nA beam current with an acquisition rate of approximately 45 patterns per second. The colors correspond to the crystallographic orientations aligned with the sample normal direction, using the colored IPF triangle as a key. The black areas in this map correspond to points with a low confidence index, indicating that the acquired EBSD patterns can not be reliably analyzed.

Most of these points correspond to the binding material used in the cathode layer. However, for the large NMC polycrystalline particle in the center of this image, there are also significant black areas between measured grains within the particle. This is due to the size of the interaction volume at this acceleration voltage spreading across multiple grains when acquiring data near grain boundaries, coupled with the degradation of EBSD pattern quality due to exposure. Note that the index of the single-crystal particle is much more reliable. Figure 1b shows an IPF map collected at the same rate from a different particle while reducing the beam dose to 10 kV energy and 400 pA current. The reduction in beam dose improves the spatial resolution within the polycrystalline NMC particle as well as the microstructural characterization, correlating grain boundary structure with degradation behavior. In addition, it achieves this enhanced spatial resolution without having to create an electron beam transparent sample for Transmission Kikuchi Diffraction (TKD). The lower beam doses also reduce charging effects, which is important for these samples with non-conductive binder material.

EBSD IPF orientation maps of NMC cathode particles used in lithium-ion batteries obtained using the Clarity Direct Detector. For Figure 1a, the data was collected at 20 kV acceleration voltage and 1.6 nA beam current. The black points represent measurements that could not be analyzed with high confidence. For Figure 1b, the data was collected at 10 kV acceleration voltage and 400 pA beam current. The characterization of the grains within the primary particle is much better. No data cleanup routines were used for these figures.
Figure 1. EBSD IPF orientation maps of NMC cathode particles used in lithium-ion batteries were obtained using the Clarity Direct Detector. For Figure 1a, the data was collected at 20 kV acceleration voltage and 1.6 nA beam current. The black points represent measurements that could not be analyzed with high confidence. For Figure 1b, the data was collected at 10 kV acceleration voltage and 400 pA beam current. The characterization of the grains within the primary particle is much better. No data cleanup routines were used for these figures.

 

In these examples, no data clean-up routines were used, and only the IPF coloring is shown. The indexing performance at grain boundaries can be camouflaged by combining the IPF map with an EBSD image or pattern quality greyscale map. The image quality (IQ) value near grain boundaries decreases due to overlapping patterns, and thus the darker shading of the IQ value will hide the lower confidence indexing. It is important to understand how these combined contrasts affect data interpretation, especially when evaluating indexing performance.

Conclusion

Using the Clarity Direct Detector at lower electron beam energies and currents reduces the beam interaction volume, resulting in improved EBSD spatial resolution. This leads to improved characterization of the NMC cathode materials used in lithium-ion batteries and assists in understanding and improving the performance of these materials.

References

1. Quinn et al. (2020) Electron Backscatter Diffraction for Investigating Lithium-Ion Electrode Particle Architectures. Cell Reports Physical Science 1,100137.