A Na+-storage electrode material free of potential plateaus and its application in electrochemical capacitors

• The charge–discharge curves of this positive material exhibit no plateau, which is a capacitive behavior.
• The capacity of the sodium ion storage device can be 50% larger than AC/AC capacitor.
• The sodium ion storage device can retain 90% of capacity after 1000 charge–discharge cycles.

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 Na0.67[Mn0.75Al0.25]O2 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.