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16 May 2023

Recent developments in STEM-based characterization of energy materials

Overview

Scanning transmission electron microscopy (STEM) provides both sub-Angstrom spatial resolution for sample visualization, and the ability to obtain information about the elemental composition of the sample at the atomic level – capabilities that are proving particularly relevant to analyzing energy materials such as battery anodes and cathodes. In this article, the focus will be on recent developments in STEM-based characterization techniques that provide enhanced efficiency and applicability, with particular attention paid to detection methods, real-time dose control and chemical spectroscopy, and their application to the study of energy materials.


Introduction

In spite of the widespread applications of rechargeable lithium-ion batteries (LIBs), they still face various materials and analytical challenges. Problems are exacerbated further due to the composition of battery anodes, cathodes, and electrolytes which cannot be easily investigated under the required sealed working environment. Using analytical imaging techniques, researchers can obtain the critical structural and chemical information they need to build better batteries.

Scanning transmission electron microscopy (STEM) is an increasingly useful analytical technique in the field of materials science. It involves scanning a focused beam of electrons across a sample (as in scanning electron microscopy [SEM]) but generating an image using the transmitted electrons (as in transmission electron microscopy [TEM]).

The STEM approach is best advantaged when used with TEM instruments. In that configuration, it provides very high resolution for the internal structure of the sample, while also being compatible with the use of supplementary techniques for elemental identification, namely energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS). STEM is generally robust to thickness and defocus changes, making it valuable for tasks such as defect analysis and structural determination. Recent advances in STEM technology show how it can be used to investigate energy materials such as battery cathodes and anodes at the atomic scale.

 

Detection methods

Detection using STEM can be carried out in a variety of ways, high-angle annular dark-field (HAADF) detection is likely the most well-established. This involves collecting only those electrons that have been scattered at a high angle, resulting in an image that is considerably more sensitive to heavy elements than light elements (the cross-section depends upon Za). As a result, although suitable for many materials, HAADF is of limited value for lithium-ion batteries and other energy materials containing light elements.

An alternative to HAADF, bright-field (BF) detection, resolves this issue by providing high sensitivity for light elements, but has been less favored for STEM imaging because it is very sensitive to sample thickness and the degree of probe defocus, rendering it generally less directly interpretable than HAADF.

Over the last ten years, annular bright-field (ABF) detection has been developed to resolve many of these issues, by collecting a bright-field signal but blocking out the very center part of the beam. It is therefore more immune to sample thickness and defocus, and yields images that are more directly interpretable. On its own it does not provide the well-known ~Z2 type contrast for heavier elements, but by collecting both ABF and HAADF it is possible to obtain high-sensitivity, high-resolution images across the elemental range. This is illustrated in Figure 1 for a LiMn2O4 cathode material.

Fig. 1: (A) STEM configuration for HAADF and ABF detection. (B) STEM-HAADF image of LiMn2O4, with Z-contrast imaging showing only the Mn atoms. (C) STEM-ABF image of LiMn2O4, with overlay showing detection of lithium (red), oxygen (blue) and manganese (green).


Sample preparation considerations

Lithium-containing materials, because of their air-sensitive nature, require careful sample preparation in order not to compromise sample integrity. This presents a challenge for STEM imaging, because of the frequent need to use ion-beam cross-section polishing to obtain a uniformly flat sample without damaging the specimen. Inert-gas specimen exchange chambers enable air-sensitive lithium-containing specimens to be transferred from the ion mill to the electron microscope without exposing them to the atmosphere.

The use of such exchange chambers can also be integrated into a focused ion beam (FIB)/SEM, by using them for site-specific sample preparation of a TEM lamella for STEM analysis, as shown in Figure 2.

Fig. 2: (A,B) SEM images of a lithium-ion battery material, used in conjunction with (C) EDS mapping to identify a region of interest. (D) Image of the site-specific lamella subsequently created by the FIB/SEM for study by (S)TEM analysis. Transfer of the air-sensitive samples throughout the process is aided by the use of inert-gas specimen exchange chambers.

 

Application to energy materials

STEM has the capability to provide a well-rounded picture of energy materials at the atomic scale, and one that is sensitive to the full variety of elements that may be present and their electronic structure or oxidation state. This opens up possibilities for studying both pristine and cycled material, with the effects on structural ordering (Figure 3), transition-metal replacement (Figure 4) or intercalation (Figure 5) being of particular interest.

Fig. 3: STEM-ABF images of pristine Li2MnO3 in two orientations (a) and (b), showing imaging of oxygen (red), pure columns of lithium (green) and manganese (blue), and mixed columns with manganese (yellow). Scale bars are 0.5 nm. Data from ref. 1

 

Fig. 4: STEM-DF images of cycled Li2MnO3: (a) Contrast can be seen around the edge of the particle; (b) A zoomed-in section shows replacement of lithium columns with a heavier element; (c) Closer inspection of a region containing both pristine and modified structures shows at top right (red) well-defined manganese ‘dumbbells’ (with lithium between), whereas in the middle of the image (blue) the site contrast fades, and it can be inferred that transition metals are replacing the lithium. Data from ref. 1

 

Fig. 5: (a) STEM-ABF image of a spinel, with overlay indicating the position of an intercalated multivalent magnesium. (b) Corresponding line scans showing the location of the magnesium at the positions expected. Data from ref. 2

 

 

Reducing electron-beam damage

Despite the above-mentioned advantages of STEM, its use for imaging energy materials has a major drawback that, because the materials are so sensitive, the incident electron beam can induce damage that is almost identical to that caused by electrochemical cycling. This is a problem if a material that has been cycled is the subject of study, as it would be very difficult to know how the damage has arisen.

Recent advances in STEM imaging and dose modulation methodologies have uncovered ways in which such effects can be mitigated, and two such methods are discussed below.

The first method is known as segmented STEM. This involves using a detector that is divided into distinct regions, which can be independently read, and the responses mathematically manipulated, either live or during post-processing. One way of using segmented STEM is optimized bright-field (OBF)[3], which provides maximum signal-to-noise under extremely low doses. This approach is demonstrated in Figure 6 and has particular value for imaging beam-sensitive materials such as zeolites. These low-scattering materials do not respond well to conventional dark-field imaging (Figure 7), or while under conditions of small convergence angles such as in uncorrected or Lorentz modes (Figure 8).

Fig. 6: STEM imaging of SrTiO3 along [001] using electron energies, using ABF (top) and OBF (bottom), showing that a reduction in electron energy from 0.8 pA (left) to 0.1 pA (right) still results in good signal-to-noise with OBF.

 

Fig. 7: STEM-OBF imaging of a low-scattering LiCoO2 cathode-type material, showing clear visualization of columns of lithium (yellow), oxygen (white) and cobalt (orange).

 

Fig. 8: STEM-OBF imaging of a layered semiconductor material, showing excellent contrast between layers of light-element components, using a convergence semi-angle of just 2 mrad.

 

A second approach to avoiding electron-beam damage of sensitive materials is to use electrostatic dose modulation (EDM) with technology from IDES, Inc. This development uses an electrostatic shutter to provide ‘beam-blanking’ with a speed of about 50 ns, representing a 105-fold increase in speed compared to a magnetic blanker. This allows modifications of the duty ratio (and therefore the dose) without affecting spatial resolution (Figure 9).

Software accompanying this technology can be used to apply a dose for every pixel in a predefined ‘mask’, enabling temporal dose structuring with regions of interest to be precisely targeted for higher/lower doses.

Fig. 9: STEM-HAADF image of Si[110] showing pairs of silicon atoms separated by 136 pm, and no loss in resolution on reducing the duty cycle to 30% or below. For every pixel in this scan, the probe is turned on and off ten times.

 

Conclusions

Battery materials are dominated by components containing light elements and/or with beam-sensitive structures. This has provided a considerable challenge to electron microscopy and has proven to be an important application for new developments in instrument technologies.

Following these developments, STEM is emerging as the method of choice for the imaging of thin sections of energy materials. Imaging techniques with sensitivity for the full range of elements, spectroscopic options for element identification, inert techniques for sample transfer, and methods for reducing damage from the electron beam are all valuable tools for obtaining the best possible results from an analysis.

As shown by the examples, these tools can be applied very successfully to real-life applications, and it is hoped that by using them, materials scientists will continue to be able to improve their understanding of the structure and composition of energy materials at the atomic scale.

For more information regarding STEM imaging in energy materials, please visit: https://www.jeolusa.com/APPLICATIONS/Application-List/Energy


Acknowledgements

With contributions from JEOL USA and JEOL Ltd.

 

Contact
Patrick Phillips
Assistant TEM Product Manager
JEOL USA
[email protected]

About the authors

  • Patrick Phillips profile image
    Patrick Phillips
    Assistant TEM Product Manager, JEOL USA, USA

    Dr. Phillips received his PhD in Materials Science and Engineering in 2012 from Ohio State University. Following an appointment as a Research Assistant Professor at the University of Illinois - Chicago, he joined JEOL USA in 2016. Previous research projects and current interests include Cs-Corrected STEM/EELS/EDS of metals, battery materials, oxides, and 2D structures.

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