Characterisation of 3-D Microstructural Evolution within Electrode Materials during Lithium-Ion Battery Operation
Authors: Taiwo Oluwadamilola O. , Paz-García Juan M. , Mokso Rajmund , Eastwood David , Lee Peter , Brett Daniel J. , Hall Stephen , Shearing Paul R. 
Published: February 2020

Electrochemical reactions that occur in lithium-ion batteries are supported by electrode materials which possess complex microstructures and play a significant role in dictating the battery performance and lifetime. Understanding the relationship between electrode microstructure and battery performance will help in optimizing electrode design and manufacture and in elucidating electrode behaviour during battery operation and failure: for instance, during cell discharge, graphite electrodes have been observed to expand when intercalated with lithium ions, with about 10% change in volume, while alloy-type electrodes such as tin and silicon undergo larger volume changes of up to 400%1,2. Investigations involving in situ and operando characterization are necessary to visualize battery electrode operation and failure.

Microscopic imaging techniques such as SEM, TEM and AFM (in combination with other complementary techniques) have been widely used to characterize microstructural evolution in lithium-ion battery electrodes3. Although these surface techniques reveal a wealth of qualitative information such as particle shape and arrangement, they are unable to completely capture the three-dimensional morphological changes the electrodes undergo during operation, degradation and failure. X-ray tomographic imaging, however, provides a non-destructive platform for visualizing electrode microstructure and microstructural evolution processes in situ and in operando and in three-dimensions at multiple length scales4,5.

We have developed capabilities for imaging lithium-ion battery electrode materials in situ and in operando using laboratory and synchrotron X-ray sources. With the aid of a custom-built X-ray transparent and functional Li-ion battery cell (Fig. 1a), we are able to simultaneously perform tomographic imaging and electrochemical testing of a graphite electrode and a silicon-based electrode. Initial results from the silicon electrode study show expansion and displacement of individual active silicon particles and the bulk electrode volume during the first charge-discharge cycle. We also visualize crack initiation and propagation within larger silicon particles during the first lithiation stage, as well as decrease in the X-ray attenuation of the silicon particles which can be attributed to lithiation-induced phase change. With the aid of these techniques, we aim to examine further changes in microstructure over extended battery cycling and different battery operating conditions.

Besides qualitative assessment of battery electrode microstructures, quantitative characterisation of their morphology also plays a crucial role in improving battery design and performance. We compare the use of stereological prediction and direct three-dimensional analysis techniques for measuring key geometric parameters that characterise lithium-ion battery electrode microstructures such as porosity, tortuosity and pore size distribution. We also use advanced quantification techniques such as digital volume correlation6 to track three-dimensional volume strain in an electrode sample during battery operation from displacement measurements.

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(6)         Bay, B. K.; Smith, T. S.; Fyhrie, D. P.; Saad, M. Experimental Mechanics
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Figure 1