In Situ Reduction in Carbon Disorder during Electrochemical Cycling of Silicon-Carbon Composite Electrodes

Si’s enormous volume change (> 300%) causes pulverization and rapid capacity deterioration in lithium-ion batteries with Si electrodes. One strategy that has been explored previously for improving capacity retention and cycling stability is embedding Si into carbon matrices with optimized porosity and conductivity. We present a viable two-step emulsion polymerization and carbonization technique for transforming biomass into porous amorphous carbon cloud with commercial Si nanoparticles embedded within it. Henceforth, we refer to it as Si@CC. We developed and synthesized two different types of Si@CC (both with similar Si content). While, Si@CC1 was synthesized using Kraft lignin as the carbon source, Si@CC2 used a mixture of Kraft lignin and pre-carbonized soyhulls as the carbon source. Through a careful electrochemical analysis of Si@CC2 we identified anomalous electrochemical behavior - a constantly decreasing internal resistance and a downward shift in the charging plateau with cycling. This led us to speculate about novel in situ disorder reduction in the amorphous carbon cloud, which was later confirmed via Raman spectroscopy and X-ray diffraction studies. A higher proportion of mesopores (2 nm < pore sizes < 50 nm) in Si@CC2 led to a greater disorder reduction. Notably, we also propose a mechanism for the in situ disorder reduction (first report for LIBs) based on the interplay of Si’s volume fluctuations, the low compressibility of the liquid electrolyte, and the load transfer between the liquid electrolyte, Si nanoparticles, and the carbon cloud. The in situ disorder reduction leads to superior capacity retention for Si@CC2 (81% after 500 cycles at 0.42 A g-1) compared to pristine Si and Si@CC1. Hence, encapsulating Si in carbon cloud and the in situ disorder reduction in amorphous carbon cloud during cycling holds promise for the development of long-lasting, and energy-dense Si-anode LIBs. Acknowledgements: This work was partially supported by the United Soybean Board (USB) to develop the synthesis process of Si@CC materials described in this study. The authors also acknowledge financial support through Clemson University's Virtual Prototyping of Autonomy Enabled Ground Systems (VIPR-GS), under Cooperative Agreement W56HZV-21-2-0001 with the US Army DEVCOM Ground Vehicle Systems Center (GVSC), to develop and test batteries described in this study. The battery data presented herein was obtained at the Clemson Nanomaterials Institute, which Clemson University operates. The authors would like to thank Dr. Yanying Lu, Clemson University, for performing particle size analysis and tap density measurements. The authors also acknowledge informative discussions with Dr. Shailendra Chiluwal, Clemson University, on material synthesis and battery testing procedures. DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited. OPSEC9603. Figure 1