Aluminum–silicon alloys are an important class of commercial casting materials having wide applications in automotive and aerospace industries. Etching the Al–Si eutectic leads to selective dissolution of Al, resulting in novel morphology – macroporous Si spheres with a three-dimensional nano network. Up to 5% Al is dissolved in Si, leading to an expansion of the crystal lattice. The resulting porous Si is electrochemically active with lithium and thus can be used as a high capacity anode for lithium-ion batteries. The etching of Al–Si provides a simple and low-cost method of producing nano-structured Si materials.