Hybrid Activated Carbon/Na 0.44 MnO 2 Positive Electrodes for High‐Rate and High‐Volumetric Capacitive Sodium‐Ion Storage

Sodium‐ion capacitors (SICs) combine high energy and power densities, promising for meeting the frequency regulation demands of large‐scale green grids. However, activated carbon (AC) positive electrodes suffer from low specific capacity and compaction density. Herein, we design a hybrid positive electrode composed of AC and Na 0.44 MnO 2 (NMO) with optimized comprehensive electrochemical performance, including high‐rate capability, cycling stability, and gravimetric/volumetric capacity. The hybrid AC/NMO electrode delivers enhanced high‐rate capacities through integrating the electric double‐layer capacitance of AC with the fast Faradaic intercalation of NMO. Compared with the AC electrode, the incorporation of NMO increases the compaction density and the volumetric capacity of the hybrid AC/NMO electrode. The optimized weight ratio of the hybrid AC/NMO is 60/40, which delivers ~1.5 times the gravimetric capacity and ~4 times the volumetric capacity compared with the AC electrode. Specifically, this optimized AC/NMO electrode delivers a high volumetric capacity of 82 mAh cm −3 at 12 mA cm −2 , far surpassing that of 20 mAh cm −3 for the AC electrode. This work highlights the potential of the hybrid AC/NMO strategy in achieving superior comprehensive performance for fast sodium‐ion storage.