The development of low-cost electrode with high capacity and long cycle-life is an essential issue for energy storage devices. Herein a unique nanostructured tetragonal LiMn2O4 (TLMO) electrode exhibiting mixed pseudocapacitor and battery behaviors in Li ion-containing aqueous electrolytes is derived via a novel in-situ low-temperature ion-exchange method from nanocrystalline ZnMn2O4. The complementary actions between the pseudocapacitance and battery properties enable TLMO to substantially outperform, in terms of the combination of high capacity, long cycle stability and suppressed self-discharge, its counterparts having only either one of the electrochemical behaviors, along with many other notable pseudocapacitive materials. The phase transformation mechanism during the ion-exchange process and charge-storage mechanisms are revealed using operando/in situ synchrotron X-ray analyses. This study may open up a new design concept in charge-storage electrode materials.