K0.25Mn2O4 microclusters assembled from single-crystalline nanofibers were synthesized via a hydrothermal process at different temperatures. The possibility of using these materials as cathode material for lithium ion batteries was studied for the first time. The charge/discharge results showed that the K0.25Mn2O4 nanofiber microclusters synthesized at 120 degrees C exhibit excellent lithium storage properties, with a high reversible capability (360 mA h g-1 at current density of 100 mA g-1) and stable lithium-ion insertion/de-insertion reversibility. The charge/discharge mechanism in lithium ion batteries was studied and proposed for the first time. During the charge process, K+ was extracted from the electrode, which made active vacated sites suitable for lithium ion intercalation and was beneficial for increasing capacity.