A simple acid leaching technique is demonstrated to produce λ-MnO2 by treating LiMn2O4 with H2SO4. λ-MnO2 shows high intercalation capacities as the positive electrode material for rechargeable aqueous batteries. The structure and morphology of the nano-particles are characterized by X-ray power diffraction, inductively coupled plasma, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy. The electrochemical performance of λ-MnO2 is investigated by cyclic voltammetry, constant current charge-discharge tests and electrochemical impedance spectroscopy. λ-MnO2 has ideal battery behaviors for polyvalent cations (Mg2+, Zn2+) to intercalate/deintercalate in the aqueous electrolyte. The λ-MnO2 electrode shows a high initial discharge capacity of 545.6 mAh g−1 at 13.6mAg−1 in 0.5mol dm−3 MgCl2 with a columbic efficiency of nearly 100%. In addition, it shows good cycling stability and maintains a capacity of 155.6 mAh g−1 after 300 cycles in 1mol dm−3 MgCl2 electrolyte.