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 (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 [e.g. Na, K, Mg, Ca, Sr, Ni, Co, Si, Ti, B, Al, Sn, Mn, Cr, Fe, V, Zr], X is an element capable of forming a double bond with oxygen, e.g. P,S or W, and k is a numerical value in the range of 2 to 4.