A process for preparing an active material for a battery includes the steps of preparing a coating liquid by adding a compound comprising an element X that is capable of forming a double bond with oxygen, and a compound comprising at least one from the group consisting of an alkali metal, an alkaline earth metal, a group 13 element, a group 14 element, a transition metal, and a rare-earth element, to water, adding a metal source to the coating liquid to surface-treat the metal source material, drying the surface-treated metal source material to prepare an active material precursor; mixing the active material precursor with a lithium source; and heat-treating the resultant mixture to produce an active material with a surface-treatment layer comprising the compound of the formula (1): MXO.sub.k (1) wherein M is at least one selected from the group consisting of 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 that can form a double bond with oxygen; and k is a numerical value in the range of 2 to 4.