Emerging Plasma FIB-SEM Techniques Advancing Battery Innovation

In recent years, focused ion beam scanning electron microscope (FIB-SEM) has been widely adopted in battery research, from interfacial analysis via 2D cross-sectioning to 3D tomography for quantitative characterization and modeling. Traditional gallium (Ga) FIB-SEM is an effective tool for Li-ion battery research, but its capabilities are limited when evaluating certain advanced battery systems. For example, novel batteries may need large-volume analysis that is impractical for Ga-FIB-SEM. This article introduces an emerging plasma FIB-SEM (PFIB-SEM) technology and describes how it is facilitating the investigation of new battery technology.

Thick electrodes have emerged as a practical way to meet the demand for increased battery energy density, but their characterization requires large representative volumes, which are difficult to access with Ga FIB-SEM. Recent studies have shown that 2D and 3D PFIB-SEM can be used to explore the microstructural characteristics of battery electrodes and explain how they relate to battery performance.

Figure 1. Combined use of 3D PFIB-SEM imaging and 3D simulation for the degradation analysis of thick NMC811 cathodes. Image reproduced from Reference 3. (Image: Thermo Fisher Scientific)

In particular, 3D volume analysis of thick NMC811 cathodes, paired with TEM analysis and 3D simulations, has been used to understand their degradation mechanisms. The 3D PFIB-SEM technique provides quantitative analysis of cracks generation and contact loss, which correlate to cell degradation. TEM and 3D modeling further revealed how the homogeneity of the 3D carbon and binder network impacts cell degradation. This knowledge was subsequently applied to dry-processed LNMO cathodes for performance optimization. Structural uniformity was also validated with large-area 2D cross-sections created with PFIB-SEM (Figure 1).

Overall, PFIB-SEM is offering exciting new analytical capabilities for materials research, including novel opportunities for advanced battery development. PFIB-SEM will serve as an indispensable technique, supporting battery innovation in both academic and industrial environments.

Electrode Characterization

The widespread popularity of portable electronics can be attributed, at least in part, to the Lithium-ion (Li-ion) batteries that power them. The same can be said for electric vehicles, which also predominantly use Li-ion batteries, and have grown in popularity as an environmentally friendly alternative to gas-powered automobiles. However, current commercially available batteries are approaching limits in terms of performance and their ability to meet demand, as they require scarce and expensive materials such as cobalt to produce. Ongoing efforts in battery research are investigating more sustainable and scalable alternatives, not only by reducing the usage of rare materials, but also by improving energy density and efficiency.

Thick electrodes are being explored as a practical solution that can effectively improve energy density, but the increased thickness also results in mass transport limitation and increased electron impedance. Current optimization and production of these electrodes are hampered by a lack of high-resolution means to survey the electrode structure in detail, including statistically relevant 3D electrode volumes that would provide an in-depth understanding of how microstructure correlates to cell performance. 1

PFIB-SEM is an ideally suited technique for the characterization of advanced battery materials, as it provides nanometer-scale imaging of 3D electrodes across large (hundred micrometer scale) volumes. In this article, recent applications of PFIB-SEM for battery electrode analysis are highlighted, showcasing the critical details this method can reveal to guide the next generation of Li-ion batteries. 2, 3

Battery Analysis and Imaging

Typical approaches for electrode characterization utilize some combination of multi-scale imaging and elemental analysis techniques. For instance, X-ray tomography is capable of generating 3D reconstructions of whole electrode volumes, but its limited resolution makes it difficult to resolve porous electrode structures and other phases within the electrode, such as binder and carbon. However, as a nondestructive method, it can identify regions of interest in the large electrode volume for further analysis with other higher-resolution techniques. Scanning electron microscopy (SEM), for example, provides high-resolution 2D images of electrodes, and can be paired with focused ion beam (FIB) serial sectioning to produce a series of sequential 2D cross-section images that are recombined into a 3D representation of the electrode.

Figure 2. Visualization of phase distribution in LNMO electrodes, comparing dry and slurry-based methods for electrode manufacturing. PFIB cross-sections show (A) Dry-LNMO along with slurry-based LNMO produced with (D) Super C65 carbon or (G) VGCF. Corresponding color segmentation and phase.

Traditionally, this serial sectioning is accomplished by a gallium FIB. With such liquid-metal ion sources (LMIS), field emission at a fine tungsten tip is used to produce a beam of ionized liquid metal gallium. The impact of the high-energy gallium ions then mills away surface atoms from the sample.

Unfortunately, Ga-FIB produces a relative low collision energy, and it would take an impractical amount of time to section the large volumes needed to fully analyze thick electrodes. Additionally, gallium ions can interact adversely with a number of materials (i.e., lithium metal, forming Li-Ga alloys), 4 potentially distorting results.

Plasma FIB, meanwhile, utilizes a range of gas sources to form a high-energy beam of plasma ions. The higher collision energy of these ions compared to Ga+ results in more efficient material removal. Additionally, the flexibility to choose from several gas species (Xe+, Ar+, N+, and O+) allows the milling conditions to be tailored to the material, ensuring the surface created by the PFIB is optimized for subsequent SEM imaging.

Recent Applications of PFIB-SEM Analysis in Batteries

PFIB-SEM analysis of novel battery electrodes was recently demonstrated in a collaboration between Thermo Fisher Scientific and Professor Shirley Meng’s group at the University of Chicago. Large-area 2D imaging was paired with 3D tomography to reveal critical microstructural details for thick Li-ion electrodes across their lifetime, particularly for different LNMO electrode manufacturing methods. 2, 3

Processing Methods

There are two main approaches currently being used for electrode manufacturing: wet and dry processing. The conventional slurry-based wet-processing approach applies the electrode materials in a liquid suspension that is subsequently dried to form the electrode layer. Scalability and cost continue to challenge industrial-scale slurry deposition, however, as the solvents necessary (i.e., NMP, N-methyl-2-pyrrolidone) are toxic and must be carefully recycled, this results in a lengthy and expensive process. 5 In dry coating, the electrode materials are instead bound by binder material to conductive carbon fibers, removing the necessity for these prohibitive solvents. However, this method is comparatively new, and there is still ongoing work to optimize manufacturing conditions to meet energy density and longevity demands.

Multiscale Analysis of Thick Li-ion Electrodes

In their initial experiment, 2 Prof. Meng’s group tested the utility of multiscale analysis on a thick Li-ion electrode with low cobalt content, NMC811 (LiNi0.8Mn0.1Co0.1O2). This material was analyzed at multiple stages as it was cycled in order to track structural changes and how they relate to performance at different electrochemical cycling conditions. Multiscale imaging was essential for this characterization as it allowed them to observe changes at varying length scales (i.e., from micrometer to atomic scale). For instance, qualitative PFIB-SEM 3D analysis revealed an increase in large intergranular cracks along with contact loss between the carbon binder domain (CBD) and the active particles.

Additional transmission electron microscopy (TEM) imaging provided finer details at the sub-100-nm level, such as electrolyte interface thickness in the cathode. Finally, high-angle annular dark-field (HAADF) STEM imaging offered atomic-scale information on phase transformations at the grain surface, further clarifying degradation mechanisms.

A critical benefit of 3D PFIB-SEM analysis is that the measured electrode structures can serve as a template for electrochemical modeling, providing an even greater understanding of electrode degradation and optimization. For example, the models developed by Prof. Meng’s group revealed the critical role of the CBD network in NMC811 performance. To validate this approach, they concluded by investigating LNMO, 6 another environmentally friendly electrode material with high energy density.

In their second study, 3 Professor Meng’s group continued to analyze and compare the performance of LNMO electrodes. Measurements were taken before and after cycling in order to understand how the microstructure evolves in each battery type over time, and how much they degraded after the same number of cycles. PFIB-SEM provided high-quality, large-area 2D cross-sections, which were used to evaluate the uniformity of the CBD depending on the process method (wet or dry) ( Figure 2).

The slurry-based electrodes showed phase segregation during the solvent drying process, leading to an uneven distribution of carbon, binder, and LNMO. The dry method, meanwhile, maintained consistent adhesion and resulted in a more homogeneous distribution of electrode materials. These differences explained the superior performance of dry-process LNMO electrodes and corroborated the findings of the first study (Figure 3).

Figure 3. Schematic of dry and slurry-based cathode fabrication processes. Image reproduced from Reference 2 under CC BY-NC 3.0. (Image: Thermo Fisher Scientific)

The Future of Li-ion Batteries

Improved battery performance and energy density are becoming increasingly critical as the global need for alternative energy sources continues to grow. Thick Li-ion electrodes have proven to be a promising material for higher energy-density cells, but are facing challenges in terms of scalability, performance, and cost. Optimization of these materials necessitates multiscale, high-quality imaging and analysis, as these allow battery performance (i.e., degradation) to be correlated from microscopic observations down to the atomic scale. Additionally, changes that impact performance could originate deep within the electrode, and the full story can only be revealed with large-scale 3D information.

PFIB-SEM is a valuable technique that neatly complements other analytical and imaging methods, providing high-resolution 2D and 3D information for larger areas, as compared to traditional gallium FIB-SEM. The volumetric information produced by PFIB-SEM can completely capture cracks, faults, and various inhomogeneities, pointing to regions of interest for further elemental or atomic-scale analysis. It is expected to remain an indispensable technique, supporting battery innovation in both academic and industrial environments.

This article is written by Alex llitchev, Scientific Editor, and Zhao Liu, Senior Market Development Manager, both at Thermo Fisher Scientific (Waltham, MA). For more information, visit here .

References

  1. Kuang, Y, et al. Thick Electrode Batteries: Principles, Opportunities, and Challenges. Advanced Energy Materials 9:33 (2019). doi:10.1002/aenm.201901457.
  2. Yao, W, et al. A 5V-class Cobalt-free Battery Cathode with High Loading Enabled by Dry Coating. Energy Environ. Sci. 16, 1620-1630 (2023). doi:10.1039/D2EE03840D.
  3. Zhang, M, et al. Coupling of multiscale imaging analysis and computational modeling for understanding thick cathode degradation mechanisms. Joule 7:1 (2023). doi:10.1016/j. joule.2022.12.001.
  4. Lee, JZ, et al. Cryogenic Focused Ion Beam Characterization of Lithium Metal Anodes. ACS Energy Lett. 4:2 (2019). doi:10.1021/acsenergylett.8b02381.
  5. Wood, DL, et al. Technical and Economic Analysis of Solvent-Based Lithium-ion Electrode Drying with Water and NMP. Drying Technology 36:2 (2017). doi:10.1080/07373937.2017.1319855.
  6. Li, W, et al. Enabling High Areal Capacity for Co-free High Voltage Spinel Materials in Next-Generation Li-ion Batteries. Journal of Power Sources 473 (2020). doi:10.1016/j.jpowsour.2020.228579.


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This article first appeared in the September, 2023 issue of Battery & Electrification Technology Magazine.

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