Swelling, pressure evolution and aging in high-silicon/ graphite composite lithium-ion batteries

The expansion of particles continues to present a challenge to the mechanical integrity of silicon-containing anodes, especially as their elemental silicon content has continued to increase. By applying external mechanical pressure, the anode and cell stack can be stabilized. Therefore, in this study, the silicon materials SiOx and Si-C were investigated in a composite anode with graphite under varying external mechanical resistance, which influences pressure development. By performing a long-term aging study at the pouch cell level with an NMC811 cathode, microstructure degradation, rather than the material used (Si-C or SiOx) or its content, was identified as the main weak point. The two components of the composite anode, namely graphite and silicon, contributed to swelling and degradation behavior in different ways and depending on the bracing condition. This observation was made using a variety of experimental methods, including the use of a powerful, self-developed cell press to study electromechanical effects such as swelling, post-mortem analysis, and differential voltage analysis. In-depth analysis of the correlations identified pressure as the controlling physical quality. These results demonstrate that swelling, pressure, and aging must be considered together in silicon-containing cells, as their correlations impact energy density and lifetime performance.