Si/C composite is one of the most promising candidate materials for next-generation lithium-ion battery anodes. Herein, we demonstrate the novel structure of carbon cages encapsulating porous Si synthesized by the reaction between magnesium silicide (Mg2Si) and carbon dioxide (CO2) and subsequent acid washing. Benefitting from the in situ deposition through magnesiothermic reduction of CO2, the carbon cage seals the inner Si completely and shows higher graphitization than that obtained from the decomposition of acetylene. After removing MgO, pores are created, which can accommodate the volume change of the Si anode during the charge/discharge process. As the anode material for lithium-ion batteries, the porous Si/C electrode shows a charge capacity of ∼1124 mA h g-1 after 100 cycles with 86.4% capacity retention at the current density of 0.4 A g-1. When the current density increases to 1.6 and 3.2 A g-1, the capacity can still be maintained at ∼860 and ∼460 mA h g-1, respectively. The prominent cycling and rate performance is contributed by the built-in space for Si expansion, static carbon cages that prevent penetration of electrolyte and stabilize the solid electrolyte interface (SEI) outside, and fast charge transport by the novel structure.