Carbon coating may expedite the fracture of carbon-coated silicon core-shell nanoparticles during lithiation

Previous studies on silicon (Si) indicate that lithiation-induced fracture of crystalline Si nanoparticles can be greatly inhibited if their diameter is reduced to below a critical scale of around 150 nm. In this paper, in situ lithiation of individual carbon-coated Si nanoparticles (Si@C NPs) is conducted which shows that Si@C NPs will fracture during lithiation even though their diameter is much smaller than 150 nm, implying a deleterious effect of the carbon coating on the integrity of the Si@C NPs during lithiation. To shed light on this effect, finite element analysis is carried out which reveals that the carbon coating, if fractured during lithiation, will induce cracks terminating at the C/Si interface. Such cracks, upon further lithiation, can immediately propagate into the Si core due to the elevated driving force caused by material inhomogeneity between the coating and core. To prevent the fracture of the carbon coating so as to protect the Si core, a design guideline is proposed by controlling the ratio between the diameter of Si core and the thickness of carbon coating. The results in this paper should be of practical value to the design and application of Si-based core-shell structured anode materials for lithium ion batteries.