Invited Presentation: Quantifying and Controlling Microstructure in Lithium Ion Battery Porous Electrodes

It is becoming increasing clear that the microstructure of lithium ion battery (LIB) electrodes strongly influences key performance metrics, such as charge and discharge rate, cell life, and safety, and that future, high performance LIBs will require not only improved active materials but also optimized microstructures.

To study microstructure on statistically significant porous electrode volumes, we image porous electrodes using synchrotron radiation x-ray tomographic microscopy (SRXTM). We implement a segmentation algorithm that allows identification of individual particles and validate it by showing that the calculated particle size distribution (PSD) is in agreement with experimentally determined PSD obtained with laser diffraction.[1]

We then systematically quantify and determine the origin of tortuosity anisotropy in porous electrodes.[2] LIB electrode manufacturing orients non-spherical particles in the plane parallel to the current collector. This increases tortuosity in the direction perpendicular to the current collector, which can limit battery performance (Figure 1a).

We demonstrate that based on a simple assessment of the particle shape, obtained for example from scanning electron microscope (SEM) images of the active particles, and estimation of particle alignment, it is possible to use the Differential Effective Medium (DEM) approximation to predict the in-plane (i.e., parallel to the current collector) and through–plane (i.e., perpendicular to the current collector) tortuosities that result from standard porous electrode preparation. We conclude that while anisotropic tortuosity could be mitigated by the use of spherical particles, when non-spherical particles are aligned in the direction perpendicular to the current collectors, low tortuosities can be achieved in this direction.

Based on these findings, we present a technique to enable alignment of anisotropic particles within an electrode with a low cost technique that is compatible with current LIB manufacturing. To prove the feasibility of the alignment process, we fabricate vertically-aligned graphite electrodes and use x-ray tomography to quantify the degree of alignment (Figure 1b and c). We further show improved rate capability in thick electrodes using the alignment procedure in agreement with expectations based on simulation. We discuss the engineering challenges associated with this technique as well as its potential.

[1] M. Ebner, F. Geldmacher, F. Marone, M. Stampanoni, and V. Wood. “X-ray Tomography of Porous, Transition Metal Oxide Based Lithium Ion Battery Electrodes” Advanced Energy Materials 3 (2013).

[2] M. Ebner, D.-W. Chung, R. E. García, and V. Wood. “Tortuosity Anisotropy in Lithium-Ion Battery Electrodes”  Advanced Energy Materials (2013). DOI: 10.1002/aenm.201301278

Figure 1: a) In-plane and through-plane tortuosities of spherical Li(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC) and platelet-shaped graphite-based electrodes with varying porosities. b, c) in-plane (top) and through-plane(bottom) tomography cross-sections showing graphite porous electrodes with high- and low-tortuosity in the direction perpendicular to the current collector. The microstructure b) is obtained with the lithium ion battery manufacturing techniques today, while the microstructure c) is desirable for high performance lithium ion batteries.