Development and Validation of a ReaxFF Reactive Force Field for Modeling Silicon-Carbon Composite Anode Materials in Lithium-Ion Batteries

Silicon-carbon (Si-C) nanocomposites have re-cently established themselves among the most promising next-generation anode materials for lithium (Li)-ion batteries. Indeed, combined Si and graphitic (sp2-C) phases exhibit high energy densities with reasonable electrochemical responses, while covalently bonded silicon carbide (SiC) compounds offer high mechanical stiffness to withstand structural degradation. However, to form and utilize a Si-C composite structure that effectively satisfies specific battery energy, power, and lifetime requirements, it is necessary to understand the underlying atomistic mechanisms at work within its individual phases and at their interfaces. Here, we develop and validate a reactive interatomic potential (ReaxFF) for the Li-Si-C system, trained and tested against a large, diverse collection of ab initio data. In conjunction with molecular dynamics and Monte Carlo simulations, our new force field links macroscale phenomena to the nanostructures of various hybrid Si-C systems in a wide range of lithiation, temperature, and stress conditions. As an illustration, it demonstrates that the high capacity of SiC (5882 mA h g-1) is caused by amorphous lithium carbides (e.g., a-Li4.4C), which soften the overall a-Li4.4(SiC)0.5 system while swelling it volumetrically up to 668%. Furthermore, our force field accurately depicts step-wise lithiation mechanisms and volume changes in Si-sp2-C composites throughout the lithiation domain of each host subsystem. It reveals the formation of a Li-rich interphase at their grain boundary, which favors their adhesion and increases the local Li (de)insertion voltage up to 1 V. These examples demonstrate the ability of the new Li-Si-C ReaxFF potential to provide atomistic-scale insights required for designing and optimizing a wide range of Si-C-based anode candidates for upcoming Li-ion battery technologies.