Changing Microstructure of Lithium Metal

Battery materials scientists focus on developing new materials and improving existing materials to overcome cell performance challenges such as capacity degradation, thermal stability, and cycle life. This work requires researchers to investigate the behavior of lithium and the resulting dendrite formation when batteries age.

Image Credit: Juan Roballo/Shutterstock.com

The Importance of Lithium Metal in Battery Functionality

Many consider lithium metal to be the most important material for the negative electrodes in lithium-based batteries. This is because it has an extremely high capacity for storing energy: 3,860 mAh g⁻¹This is much higher than the graphite currently used in batteries. Lithium is also the most electropositive metal.

When paired with high-capacity cathodes (whether using lithium insertion materials, conversion chemistries, or implemented in solid-state design), lithium metal can significantly increase the energy density of the battery.^1,2,3,4,5

Microstructure Formation During Battery Cell Cycling

One of the difficulties in using lithium metal directly as an anode lies in the formation of mossy and dendritic structures during battery cycling. This leads to low efficiency, constant electrolyte consumption due to undesirable side reactions, growth of the solid electrolyte interphase (SEI) and irreversible loss of capacity.^6,7,8 These problems occur both during charging (when lithium is plated) and during discharge (when lithium is stripped), resulting in an unstable electrode structure.

When the battery is charged, lithium accumulates irregularly, forming sparse and dendritic structures. Rapid growth of these lithium structures can cause internal short circuits and even cause the battery to overheat.^9,10

During discharge, unstable structures formed during deposition can collapse, creating isolated lithium domains encapsulated by SEI. These isolated areas, known as “dead lithium”, are disconnected from the main electrode and no longer participate in the electrochemical processes of the battery.¹¹ Therefore, understanding how lithium microstructures form and evolve during charging and discharging is important to guide the use of lithium metal anodes in next-generation batteries.

Various strategies have been developed to control the microstructure of lithium during cycling and reduce the formation of inactive lithium. However, the formation of an ideal lithium microstructure that can achieve coulombic efficiency above 99.9%, a basic standard for the long-term cycle of lithium metal batteries, has not yet been realized.

3D Reconstruction SEvolution of Lithium Microstructures

Researchers from Chalmers University of Technology in Sweden used X-ray tomographic microscopy (XTM) technology to capture lithium microstructures.¹² They later used Thermo Scientific Dispatch boat Software Analyzing all collected tomograms. Segmenting the lithium phases in the reconstructed tomograms allowed the researchers to monitor changes in individual microstructures during plating and stripping.

These advantages helped researchers identify lithium, quantitatively track the spatial distribution of accumulated lithium over time, and observe the formation of dead lithium during plating and stripping processes.

3D images of segmented lithium provided insight into the structural connectivity and growth mechanisms of lithium microstructures (Figure 1). After coating, tomograms showed the formation of several distinct regions at low current density.

At high current density, lithium was almost completely bound, except for a few inactive structures. This indicates improved coupling at higher current density. Inactive lithium structures persisted after two cycles. The segmented lithium volume shows the evolution of microstructures; There is an initial increase in surface area during coating due to rapid growth of the surface, followed by a decrease in surface area, possibly due to the integration of lithium microstructures.

Figure 1: 3D images of segmented lithium. a, b Voltage profiles for galvanostatic coating and stripping during Operando XTM are shown at 0.5 mA/cm^-2 (a) and 1.0 mA/cm for the first cycle^-2 (b) for the second loop. Circles mark the points where X-ray tomograms were captured. Orange indicates the coating process and green indicates the stripping process. c, d 3D renderings display segmented lithium and identify isolated regions in different colors. Green arrows highlight inactive lithium sites. Different colors such as dark blue, light blue, dark green, light green, and purple represent various separated lithium sites. e The volume and surface area of ​​lithium deposited during the plating and stripping cycles are shown. White spaces between colored areas indicate points where tomograms were taken after coating or stripping. Image Credit: Nature Communications (Nat Commun) ISSN 2041-1723 (online)

Reconstructed 3D tomograms were also used to examine the integration of mossy and needle-like structures (Figure 2).

Figure 2: Formation of lithium dendrites and mossy structures. Images were obtained from tomograms taken at 1.0 mA/cm during coating.^-2 For 10, 20, 30, 40, 50 and 60 minutes. Lithium is shown in blue; purple represents the SEI, or high atomic number components covering the lithium, and the bottom purple indicates the copper substrate. Image Credit: Nature Communications (Nat Commun) ISSN 2041-1723 (online)

Specifically, during lithium plating at a current density of 1.0 mA/cm^-2A single lithium cluster grew rapidly, but the longer it got longer, the more likely it was to collapse. At the same time, algae-like lithium spread on the surface and thickened. After 40 minutes, the isolated lithium cluster bonded to the mossy layer, forming a stronger and more stable structure.

This work allowed researchers to monitor the real-time evolution of internal lithium structures. This breakthrough is very important; It underlines the need to avoid mossy and needle-like lithium formations in batteries due to significant safety hazards.

Looking ahead, this innovative workflow has the potential to revolutionize the study of microstructure formation and progression in other metal anodes, including sodium (Na), zinc (Zn), and magnesium (Mg) batteries. By analyzing the geometry, connectivity and electrochemical activity of lithium structures under various conditions, we pave the way for the development of safer, high-performance lithium metal batteries.

Advanced Imaging and Analysis with Avizo Software from Thermo Fisher Scientific

Advanced XTM technology produces significant amounts of diverse data. Image processing software is required to effectively interpret and understand this information. It provides a more comprehensive perspective and numerical measurement of the studied features.

Avizo Software¹³ It offers a range of visualization and imaging data analysis tools to create a comprehensive picture of the material being analyzed. Avizo Software It is designed to simplify and automate many complex steps in image processing and analysis.

In the study discussed here, researchers used Avizo Software’s segmentation tools to create 3D volumetric visualizations and its quantitative tools to calculate the surface and volume of lithium microstructures.

Learn more: How to automate battery quality monitoring with image interpretation?

References and Further Reading

1. Tarascon, J.M. & Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 414, 359–367 (2001).
2. Xue, W. et al. Ultrahigh-voltage, Ni-rich layered cathodes in practical Li metal batteries activated by a sulfonamide-based electrolyte. Nat. Energy 6, 495–505 (2021).
3. Lee, Y.-G. et al. High-energy, long-cycle, all-solid-state lithium metal batteries powered by silver carbon composite anodes. Nat. Energy 5, 299–308 (2020).
4. Varzi, A., Raccichini, R., Passerini, S. & Scrosati, B. Challenges and prospects of the role of solid electrolytes in the revitalization of lithium metal batteries. J. Mater. Chemistry A 4, 17251–17259 (2016).
5. Xu, X. et al. Electro-chemo-mechanical breakdown of solid electrolytes caused by the growth of internal lithium filaments. Hunting. Mother. 34, 2207232 (2022).
6. Yoshimatsu, I., Hirai, T. & Yamaki, J. I. Lithium electrode morphology during cycling in lithium cells. J. Electrochemistry. SOS. 135, 2422–2427 (1988).
7. Liu, J. et al. Pathways to practical, high-energy, long-cycle lithium metal batteries. Nat. Energy 4, 180–186 (2019).
8. Wood, K.N., Noked, M. & Dasgupta, N.P. Lithium metal anodes: towards a better understanding of combined morphological, electrochemical and mechanical behavior. ACS Energy Let. 2, 664–672 (2017).
9. Cheng, X.B., Zhang, R., Zhao, C.Z. & Zhang, Q. Towards safe lithium metal anode in rechargeable batteries: a review. Chemistry Rev. 117, 10403–10473 (2017).
10. Xu, W. et al. Lithium metal anodes for rechargeable batteries. Energy Environment. Science. 7, 513–537 (2014).
11. Fang, C.C. et al. Determination of the amount of inactive lithium in lithium metal batteries. Nature 572, 511–515
12. sadd et al. Investigation of microstructural evolution of lithium metal during plating and stripping by operando X-ray tomographic microscopy. Nature Communication 14, 854 (2023)
13. Avizo Software for battery and energy materials characterization

This information was obtained, reviewed and adapted from materials provided by Thermo Fisher Scientific – Software.

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