Unravelling Electro-Chemo-Mechanical Processes in Graphite/µ-Si Composite Electrodes for Evidence-Based Design of Advanced Microstructures

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Silicon (Si) is a promising negative electrode material for high-energy automotive batteries, but its significant volume changes during cycling cause rapid degradation, limiting its loading to just 10 wt.% in commercial graphite/Si composite negative electrodes as a compromise between energy density and cycle life. Overcoming this threshold requires evidence-based design of advanced electrodes. Here we combine operando optical microscopy, synchrotron X-ray CT 4D imaging, digital image/volume correlation and machine learning-assisted image processing techniques, to elucidate the multiscale electro-chemo-mechanical processes in graphite/µ-Si composite negative electrode. Presented with multimodal high-resolution videos, here we show the expansion of porous µ-Si particles strongly depend on the morphology of the intra-particle porosity. One dimensional tubular porosity is conducive to suppressed volume expansion and cracking compared to planar porosity, which incurs highly anisotropic particle strain and crack. Moreover, the encapsulation and loss of active Si particles result in excessive charging current being directed to the graphite particles, thereby increasing the risk of premature lithium plating—an overlooked safety concern. Surprisingly, the electrode expansion is not necessarily governed by Si; rather, its influence only becomes pronounced at high SOCs during the first lithiation cycle but is dominated by graphite in the subsequent cycles. Severe thickness expansion (20%) and reduction in nano-pores (from 43% to 21%) are observed in the CBD, undermining accessible capacity and fast charging capability. Finally, in response to the five identified major challenges in graphite/µ-Si composite electrodes, we develop a double-layered graphite/µ-Si composite negative electrode, which demonstrates significantly lower polarization and mitigated capacity decay compared to its homogeneous counterparts. Overall, this study provides a comprehensive framework for advancing Si-based negative electrodes through hierarchical engineering, from particle level to the 3D architecture of the electrode. Figure 1

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