The present invention provides a method of preparing an intercalation compound of the nominal general formula Li.sub.x Mn.sub.2 O.sub.4, where x is greater than 0 and less than 2, which comprises providing a manganese compound and a lithium compound and mixing such compounds in proportion which provides the desired amount of x demonstrated by the nominal general formula. The mixed compounds are then irradiated with electromagnetic radiation, desirably microwave frequency or infrared radiation whereby manganese, lithium, and oxygen combine to form the Li.sub.x Mn.sub.2 O.sub.4 having the desired quantity x of lithium. It is desired that the heating by radiation be conducted in air. Preferably, the radiation is microwave radiation and such radiation is at a frequency which causes atomic bonds to vibrate within at least a portion of the precursor compounds and/or the solvent contained in the mixture.