Active material for battery and method of preparing the same

An active material for a battery has a surface-treatment layer including a compound of the formula (1): MXO 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, and a solid-solution compound including surface treating elements M, X and O (oxygen), and Li and M', wherein M' is at least one element selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and a rare-earth element that originate from said lithiated intercalation compound.