Electrode active material, preparation method thereof, and electrode and lithium battery containing the same

An electrode active material includes a core capable of intercalating and deintercalating lithium; and a surface treatment layer disposed on at least a portion of a surface of the core, wherein the surface treatment layer includes a lithium-free oxide having a spinel structure, and an intensity of an X-ray diffraction peak corresponding to impurity phase of the lithium-free oxide, when measured using Cu-K± radiation, is at a noise level of an X-ray diffraction spectrum or less.