Silicon–carbon nanocomposites have recently established themselves among the most promising next-generation anodes for lithium-ion batteries. Indeed, they can exist either as a two-host system combining silicon’s high energy density with the good electrochemical response of graphitic carbon or as a single-host system consisting of silicon carbide with a very high mechanical stiffness. However, selecting the composite materials with an optimal electrochemical performance remains particularly challenging because of the poor understanding of the underlying atomistic mechanisms. We address this issue by applying a reactive interatomic potential, for the Li–Si–C system developed using ab initio structure-energy data. Combined with molecular dynamics, our new force field effectively describes chemical processes and properties in various hybrid Si/C structures in a wide range of lithiation, temperature and stress conditions. As an illustration, it validates the high lithiation capacity of silicon carbide because of the formation of amorphous lithium carbide and further demonstrate that this would however lead to a higher volume expansion than in amorphous silicon. Furthermore, it also quantitatively predicts interdiffusion coefficients responsible for (de)lithiation kinetics. On the other hand, our force field demonstrates that lithium (de)insertion in silicon/graphene-like nanocomposites mainly occur via their grain boundaries at a higher voltage due to the prevailing Li interphase stabilized toward graphene. The (de)lithiation-induced change of silicon’s volume and of graphite’s interlayer stacking and spacing are simultaneously depicted. Thus, our newly parametrized Li–Si–C potential energy surface lays a new foundation for modeling, understanding and hence utilizing high-performance silicon-carbon anode materials optimally for upcoming Li-ion battery technologies.