About four years ago my team and I started at MCL, Austria with the image-based characterization of silicon (Si)-based lithium (Li)-ion cells using micro-X-ray computed tomography (XCT). These initial studies were triggered by a granted EU-H2020 project entitled “Silicon based materials and new processing technologies for improved lithium-ion batteries” or "Sintbat" (Proj. No. 685716).  Si-based Li-ion batteries are among the most promising candidates for decentralized storage systems in the area of renewable energy, e-mobility, or mobile electronic devices¹. A main advantage to add Si into Li-ion anode materials is that the theoretical specific capacity of Si (Li₁₅Si₄ with 3578 mAh g^-1) is about ten times that of graphite (LiC₆ with 372 mAh g^-1)².

However, Si undergoes high volumetric expansions upon lithiation (up to 300%)³ leading to insufficient lifetime expectancy due to degradation and high capacity fade. The volumetric expansion and contraction during cycling result in mechanically induced changes in the microstructure of the anode material such as fracture, peeling off or delamination⁴. In addition, it leads to the unfavorable continuous solid electrolyte interface (SEI) formation limiting the migration of the Li-ions between the electrolyte solvent and the active material, which finally causes irreversible capacity loss²³. 

Advanced material design strategies for the anode material utilizing for instance dual phase alloy systems have shown potential to decrease the capacity degrading and improve the cycling capability⁵. Their characterization at different scales and throughout their ageing is however challenging.

The motivation of our work was to gain a deep understanding of the structure-property relationship and ageing on all relevant length scales, since it is critical to yield design guidelines to improve the different cell component materials beyond the state of the art.

We started our scientific journey on the 3D characterization of industrial relevant Si/FeSi₂ nano-composite anode materials by using XCT. We recognized soon that XCT was only appropriate to perform characterization on cell-level (micrometer and above) but not on the anode level (micrometer and below) due to the lack of resolution and contrast necessary to understand the rather complex multi-scale structure.

In particular, a multi-scale characterization approach as well as modeling to collect quantified structural information at all relevant length scales, and gain insight how the evolution of the Si/FeSi₂ compound particles during cycling affects Li-ion diffusion within the proximate pore network, was required. Therefore, a multidisciplinary team spread across Europe including mainly CEA (France), University of Warwick (GB), VARTA Micro Innovation GmbH (Austria) and my team at MCL (Austria) gathered to reach the goal.

Advanced industrial relevant anode samples were fabricated in close collaboration with VARTA Micro Innovation GmbH.  In particular we utilized (1) scattering experiments performed and analyzed by CEA mainly to understand the (de-) lithiation and volumetric irreversible structural changes of the Si/FeSi₂ nanoscale phase, combined with (2) nano-FIB-SEM tomography experiments also performed by CEA as well as Synchrotron tomography executed at ESRF by MCL and supported by L. Helfen, to quantify the morphology evolution of the Si/FeSi₂ compound particles, as well as their impact on the proximate pore network with cycling numbers of up to 300 cycles. (3) Finally, the experimental characterization was complemented by modeling the local concentration of Li-ions in the anode material, performed at the University of Warwick, using the image analyzed collected morphology data as an input.  

Definitely, the quantification of the morphology, based on the collected image data from both the nano-FIB-SEM as well as the Synchrotron tomography data was crucial but also very tough due to the complex multi-phase structure and generated artefacts during the measurements. Here, the development of an appropriate image analysis workflow was necessary in order to quantify the data as accurate as possible. At MCL within my team in particular T. Vorauer (PhD student) and F.F. Chamasemani (postdoc) took great effort on this endeavor. We could finally find a way to succeed and gain the necessary structural information. It was a first step towards more advanced image analysis algorithm.

We think that the work provides novel insights and possibilities with respect to the characterization of advanced anode materials. Further, an important message of our work is that the expanded perception considering the whole morphology of the anode on different length scales and not only the active material, is highly crucial to yield future design guidelines and to improve the anode material beyond the state of the art. 

A link to the manuscript can be found here: https://www.nature.com/articles/s42004-020-00386-x

References:

1          Tarascon, J.-M. & Armand, M. Issues and challenges facing rechargeable lithium batteries. in Materials for sustainable energy: a collection of peer-reviewed research and review articles from Nature Publishing Group 171–179 (World Scientific, 2011).

2         McDowell, M. T., Lee, S. W., Nix, W. D. & Cui, Y. 25th anniversary article: understanding the lithiation of silicon and other alloying anodes for lithium‐ion batteries. Adv. Mater. 25, 4966–4985 (2013).

3          Beaulieu, L. Y., Eberman, K. W., Turner, R. L., Krause, L. J. & Dahn, J. R. Colossal reversible volume changes in lithium alloys. Electrochem. Solid-State Lett. 4, A137–A140 (2001).

4          Müller, S. et al. Quantification and modeling of mechanical degradation in lithium-ion batteries based on nanoscale imaging. Nat. Commun. 9, 1–8 (2018).

5          Ko, M. et al. Scalable synthesis of silicon-nanolayer-embedded graphite for high-energy lithium-ion batteries. Nat. Energy 1, 16113 (2016).