An active material for a battery has a surface-treatment layer including a compound of the formula (1): MXO.sub.k (1) wherein M is at least one element selected from an alkali metal, an alkaline earth metal, a group 13 element, a group 14 element, a transition metal, and a rare-earth element, X is an element capable of forming a double bond with oxygen, and k is a numerical value in the range of 2 to 4.