The present disclosure provides a silicon clathrate electrode active material having low expansion during charging and a solid-state lithium-ion battery comprising such a silicon clathrate electrode active material. The silicon clathrate electrode active material of the present disclosure comprises a sodium compound. In the silicon clathrate electrode active material of the present disclosure, a ratio of a mass of the sodium compound relative to a total mass of the silicon clathrate electrode active material and the sodium compound is 0.1% by mass or greater and 5.0% by mass or less. The solid-state lithium-ion battery of the present disclosure comprises the negative electrode active material layer of the present disclosure. The negative electrode active material layer of the present disclosure contains the negative electrode mixture of the present disclosure. The negative electrode mixture of the present disclosure comprises a silicon clathrate electrode active material and a solid electrolyte.