The need of large-scale energy storage calls for the application of environmentally benign and economic sodium storage electrode materials. At present, most of the existing positive electrode materials for Na+-storage demonstrate many small plateaus, which are a disadvantage for the practical application. We found that some inert elements like Al can straighten the charge-discharge curves and stable the structure of the positive electrode materials. Here we synthesized positive electrode material of Na-0.67[Mn0.75Al0.25]O-2 by the spray drying method. This positive electrode material was matched with the negative electrode material of activated carbon to assemble a Na+-capacitor. This capacitor can retain 90% of capacity after 1000 charge-discharge cycles. The advantage of the Na+-capacitor in properly assessing the cycle performance of the positive electrode material has been specially pointed out. Moreover, the Na+-capacitor possesses higher volumetric energy and power densities than electric double-layer capacitors. (C) 2016 Elsevier B.V. All rights reserved.