Fig. 3a shows the nitrogen adsorption isotherms of untreated Si@C and activated Si@C samples. The isotherm of pristine Si was flat, indicating near zero pore volume and a calculated surface area of 4 m2 g−1. However, after activation, the isotherms for all Si@C samples were type I according to IUPAC classification. The Si@C samples exhibited a dramatic increase in the quantity of N2 adsorbed below the relative pressure (P/Po) 0.01, which then increased slightly at higher relative pressures, indicating the development of micropores and some mesopores in the carbon layer due to activation.