A High-Power Symmetric Na-Ion Pseudocapacitor

Batteries and supercapacitors are critical devices for electrical energy storage with wide applications from portable electronics to transportation and grid. However, rechargeable batteries are typically limited in power density, while supercapacitors suffer low energy density. Here, a novel symmetric Na‐ion pseudocapacitor with a power density exceeding 5.4 kW kg −1 at 11.7 A g −1 , a cycling life retention of 64.5% after 10 000 cycles at 1.17 A g −1 , and an energy density of 26 Wh kg −1 at 0.585 A g −1 is reported. Such a device operates on redox reactions occurring on both electrodes with an identical active material, viz., Na 3 V 2 (PO 4 ) 3 encapsulated inside nanoporous carbon. This device, in a full‐cell scale utilizing highly reversible and high‐rate Na‐ion intercalational pseudocapacitance, can bridge the performance gap between batteries and supercapacitors. The characteristics of the device and the potentially low‐cost production make it attractive for hybrid electric vehicles and low‐maintenance energy storage systems.