Three-Phase 3D Reconstruction of Li-Ion Batteries Electrodes Via FIB-SEM Tomography

Three-dimensional (3D) reconstructions of lithium-ion battery electrodes provide quantitative microstructural characteristics and input for 3D computational modeling of electrode lithiation and de-lithiation (1-3).  In much of the work reported to date, it was not possible to identify all three electrode phases – oxide particles, carbon, and electrolyte – because there was little or no image contrast between the carbon-based epoxy used to fill the pore regions of the electrode (after electrolyte removal) and the carbon phase (4).  Recently, it was shown that filling with a silicone resin allows good electrolyte/carbon contrast, although the high viscosity of the resins may make effective pore filling difficult (5).  

In this work, three-phase 3D reconstructions of a commercial LiCoO₂ cathode and lab-made LiFePO₄ cathodes (6) are demonstrated using a low viscosity silicone resin as a filling material.  As illustrated in Figure 1, contrast among oxide, carbonaceous materials (conducting carbon and binders, CB) and resin-filled porosity (electrolyte space) is obtained using focused ion beam-scanning electron microscopy tomography. Structural parameters including, phase volume fraction, surface area, particle size distribution, pore connectivity and tortuosity are extracted for electrode microstructure-performance correlation analyses. For the commercial LiCoO₂ cathode, the electrolyte tortuosity within the electrode is found to be inhomogeneous. For LiFePO_­4 cathodes, the microstructure characteristics are compared with conventional electrodes and those with additional large-scale pores added using a sacrificial templating technique. The improved high rate performance found in the templated LiFePO₄ electrode can be explained by 3D microstructural characteristics created by the sacrificial template.  The present study demonstrates the importance of obtaining all three phases for accurate analysis of microstructure-performance correlations.

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