Revealing lithium distribution at the microscale with Cipher

Results and discussion

Assessing the applicability of Li-CDM to compound materials

The qBSE signal as a function of mean atomic number, $$\bar{Z}$$, ​​is plotted in Figure 2 with $$\bar{Z}$$ calculated​ using the modified electron approach of equation (1) (after [5]):

$$\bar{Z}=\frac{\sum_{i=1}^{n} a_{i} Z_{i}^{\,1+x}}{\sum_{i=1}^{n} a_{i} Z_{i}^{\,x}}$$​

where a_i represents the atomic fraction of element i and x ≃ 0.7. In line with other publications (e.g., [6]), the qBSE signal was fitted to the function:

$$f(\bar{Z})=C\left(1 - e^{-\bar{Z}/q}\right)$$

​

Equation 2.

where C and q are constants related to the SEM and detector settings.

For compounds with $\bar{Z}$ < 40, an excellent fit of the experimental data to the exponential function of equation 2 was observed with few—if any—outliers. However, for materials of $\bar{Z}$ > 40, although the experimental data continues to follow the general trendline, the increased scatter of the experimental data indicates that a large uncertainty would be expected in the Li-CDM calculation. Notwithstanding, it was confirmed that the Li-CDM is suitable for a wide range of metallic and non-metallic samples of $\bar{Z}$ < 40.

Evaluating the lithium content of lithium aluminate

Cipher was used to determine the lithium content in a lithium aluminate sample. The compositional analysis results are summarized in Table 1. The lithium content was determined to be 22.6 ± 3.5 at.% (9.5 ± 1.7 wt.%); within 2.4 at.% and only 0.9 wt.% of the nominal composition 25.0 at.% (10.5 wt.%).

Table 1. Elemental quantification results of LiAlO₂ sample.

Evaluating the lithium content of Li-ion battery cathode materials

The NMC 811 powder analyzed consisted of approximately spherical 'secondary' particles of 5 – 30 µm in diameter. These secondary particles are agglomerates of smaller 'primary' particles that can be observed in a crystal orientation map collected by electron backscatter diffraction (EBSD) (Figure 3). The orientation map captured by the EDAX Clarity EBSD Detector reveals primary particles 200 – 2,000 nm in size with random crystal orientations and low/no gaps between primary particles.

Quantitative EDS analysis at select locations within NMC particles was performed and revealed O, Ni, Mn, and Co with little-to-no variation within or between particles that were analyzed (Table 2). No other elements were found to be present above the minimum detection level. The Ni:Mn:Co ratio of 8.07:1.00:1.01 was determined experimentally and was found to be consistent with the nominal 8:1:1 ratio.

Table 2. Quantitative EDS analysis of six NMC particles. Analysis positions as shown in Figure 4.

The lithium content from six different NMC particles was determined using Cipher; analysis locations are shown in Figure 4. The mean lithium concentration was determined to be 22.5 at.% (5.7 wt.%) and within ~1.5 wt.% of the nominal composition value of 7.3 ± 0.3 wt.%.

Introduction

Lithium-containing compounds and alloys are critical to many key technologies of the twenty-first century, from Li-ion batteries used to power mobile electronic devices and cars to lightweight structural alloys. Progress in these fields has been remarkable, given the lack of a method to determine lithium content at the microscale. Commonly, energy dispersive x-ray spectroscopy (EDS) in the scanning electron microscope (SEM) is employed for microanalysis. However, this has not been possible for elements with atomic number (Z) <4 as the characteristic x-rays emitted (e.g., Li K at ~55 eV) are easily attenuated by the sample or the presence of an oxide layer or contamination and require the use of highly specialized detectors. Even so, a limit of detection of ~20 wt.% Li and the inability to perform quantitative measurements due to the dependence of the fluorescence yield on the Li bonding state present significant issues [1]. More recently, it has been suggested that elemental Li maps captured with these detectors are unreliable [2].

However, quantification of Li in the SEM was demonstrated recently by researchers at LKR/AIT using a composition by difference method using EDS and quantitative backscattered electron imaging (qBEI) [3, 4]. EDS analysis was used to quantify elements Z = 4 – 94, while qBEI was used to determine the mean atomic mass (the qBEI signal being a function of atomic number for Z = 1 – 94). The fraction of light elements (Z = 1 – 3) was calculated and assumed to be Li, given the MgLi alloy analyzed. Using this lithium by composition by difference method (Li-CDM), detection of <5 wt% Li was demonstrated with acceptable accuracy (~1 wt.%).

Gatan announced the Cipher^® System that integrates the EDAX^® Octane Elite or Elect Super EDS Detectors with the OnPoint^® backscatter electron detector and a composition-by-difference module for DigitalMicrograph^® software to quantitatively measure the lithium composition of a sample. In this article, we assess the accuracy of Li-CDM in a range of non-metallic materials and describe the latest results using Cipher to analyze the lithium content in a stoichiometric lithium aluminate and a lithium nickel manganese cobalt oxide powder commonly employed in cathode materials of Li-ion batteries.

Materials and methods

Quantitative backscattered electron measurements were recorded from 55 samples (Micro-Analysis Consultants Ltd) using Cipher. The samples included elemental, mineral, semiconductor, and alloy materials and ranged in atomic number from 4 – 83. The samples were mechanically polished and coated with a 2.0 nm thick carbon layer to avoid charging in the SEM using a PECS™ II system.

Quantitative lithium analysis using Cipher was applied to two samples that are available commercially—a high purity (>99.99%) LiAlO₂ (100) crystal substrate and a powder form of Lithium Nickel Manganese Cobalt Oxide (NMC) with nominal Ni:Mn:Co ratio 8:1:1 and 5.7 wt.% Li (approximately 25 at.%).

The samples were prepared by broad beam argon milling using an Ilion^® II or PECS^™ II polisher and coated with a 2.0 nm thick carbon layer to avoid charging. Before sample preparation, the NMC 811 powder sample was embedded in epoxy to form a solid block. A field emission SEM was used to collect EDS and qBEI data at 20 and 25 kV, respectively, selected to ensure that all x-ray lines were excited efficiently while also providing comparable sampling volumes of the signals. Electron backscatter diffraction (EBSD) using the EDAX Clarity^™ detector was also performed on a commercial NMC sample to reveal the crystal structure.

Figure 3. An example secondary electron image and b) orientation map of a lithium nickel manganese oxide powder sample.

Figure 4. a) A quantitative backscattered electron image where the grayscale intensity value can be related directly to the mean atomic number;​ locations used for quantitative EDS and Li-CDM analysis are indicated. b) A graphical representation of the lithium content of the six NMC particles analyzed using Cipher.

This is a significant step forward in the analysis of battery materials as, for the first time, the charge state of a cathode material was determined in a conventional SEM. Here the ~25 at.% Li corresponds to the uncharged battery state.

Summary

The lithium by composition by difference method was demonstrated in stoichiometric compound samples. The lithium content of a high-purity lithium aluminate crystal substrate was determined to be 9.5 wt.% within ~ 1 wt.% of the stoichiometric value, an accuracy similar to previous reports for metallic samples [2].

For the first time, the lithium content of an NMC cathode material was determined quantitatively in a conventional scanning electron microscope. A mean lithium content of 22.5 at.% was measured experimentally, corresponding to the uncharged battery state in this material. These results validate Li-CDM for a wider range of materials, opening exciting characterization possibilities in lithiated battery materials using Cipher.

References

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2. R. Gauvin and N. Brodusch, Microsc. Microanal. 28 (Suppl 1), 2022. doi:10.1017/S1431927622002847
3. J.A. Österreicher et al., Scripta Materialia 194 (2021) 113664
4. Austrian Patent Application A 50783/2020
5. J. Donovan et al., (2003) Microsc. Microanal. 9, p. 202. DOI: 10.1017/S143192760030137
6. A. Garitagoitia Cid et al., Ultramicroscopy 195 (2018) 47

7. D. Kaczmarek, Scanning Microscopy 12 (1) (1998) p. 161-169