NEGATIVE ELECTRODE ACTIVE MATERIAL CONTAINING SILICON CLATHRATE II

Provided is a negative electrode active material that contains silicon clathrate II and that is suitable for a negative electrode of a lithium ion secondary battery. The negative electrode active material includes a silicon material in which silicon clathrate II represented by composition formula NaSi(≤x≤) is contained and a volume of a pore having a diameter of not greater than nm is not less than cm/g.