From Ab-Initio Calculations to Multiscale Design of Si/C Core-Shell Particles for Li-Ion Anodes

The design of novel Si-enhanced nanocomposite electrodes, that will successfully mitigate mechanical and chemical degradation, is becoming increasingly important for next generation Li-ion batteries. Recently, Si/C hollow core-shell nanoparticles were proposed as a promising anode architecture, which can successfully sustain thousands of cycles with high coulombic efficiency. As the structural integrity and functionality of these heterogeneous Si materials depend on the strength and fracture energy of the active materials, an in-depth understanding of their interface and intrinsic mechanical properties, such as fracture strength and debonding, is critical for the successful design of such composites. Here, we perform ab-initio simulations to calculate these properties for lithiated a-Si/ a-C interface structures, and we combine these results with linear elasticity expressions to model conditions that will avert fracture and debonding in these heterostructures. We calculate that the a-Si/a-C interface retains good adhesion, even at high stages of lithiation. For average lithiated structures, we predict that the strong Si-C bonding averts fracture at the interface; instead, the structure ruptures within lithiated a-Si. From the calculated values and linear elastic fracture mechanics, we construct a continuum level diagram, which outlines the safe regimes of operation in terms of the core and shell thickness and the state of charge. We believe that this multiscale approach can serve as a foundation for developing quantitative failure models, and for subsequent development of flaw-tolerant anode architectures.