Silicon has the highest known theoretical capacity (∼4140 mAhg−1) to store lithium. Among different forms of Si, Si nanowires (SiNWs) are the most promising candidates for the next-generation lithium-ion batteries. Lithiation of SiNWs is a complex process, which is not very well understood. Here, we present density functional theory calculations on Li incorporation in SiNWs using surface and interface geometries. Our results show that, initially, Li intercalation proceeds through Si(110) facets, leading to the formation of an amorphous Li2Si shell. For interfaces between the lithiated (amorphous Li2Si) shell and unlithiated (pristine c-Si) core region we find that the Li intercalation barriers are independent of the actual interface orientation, while interface energies show an orientation dependence similar to surface energies. In particular, a-Li2Si/c-Si(111) is most favorable while a-Li2Si/c-Si(110) is least favorable. Since high-energy interfaces typically show a higher mobility than low-energy interfaces, the experimentally observed anisotropic swelling of SiNWs can be understood on the basis of interface energetics.