A high-performance capacitor battery AC/λ-MnO2 based on Na-ion insertion-deinsertion into λ-MnO2 in 1 mol dm−3 Na2SO4 solution is successfully demonstrated. A simple acid leaching technique is used to produce λ-MnO2 by treating LiMn2O4 with H2SO4. X-ray power diffraction and inductively coupled plasma analysis provide supporting evidences for the extraction of Li+ from the LiMn2O4, and the particle morphology of λ-MnO2 is studied by scanning and transmission electron microscopy. A high specific capacity of 390.7 mAh g−1 is obtained for the λ-MnO2 electrode at a current density of 13.6 mA g−1 with a nearly 100% efficiency. The capacitor battery AC/λ-MnO2 can operate at a cell voltage as high as 2.2 V, and it exhibits a specific energy of 19.7 Wh Kg−1 at a high power density of 3500 W Kg−1 and a high energy density of 71.7 Wh Kg−1 at a low power density of 403 W Kg−1. It has potentials to be used as an energy storage device for the integration of solar and wind power into the electrical power grid.