A Novel High Capacity Positive Electrode Material with Tunnel‐Type Structure for Aqueous Sodium‐Ion Batteries

Aqueous sodium‐ion batteries have shown desired properties of high safety characteristics and low‐cost for large‐scale energy storage applications such as smart grid, because of the abundant sodium resources as well as the inherently safer aqueous electrolytes. Among various Na insertion electrode materials, tunnel‐type Na 0.44 MnO 2 has been widely investigated as a positive electrode for aqueous sodium‐ion batteries. However, the low achievable capacity hinders its practical applications. Here, a novel sodium rich tunnel‐type positive material with a nominal composition of Na 0.66 [Mn 0.66 Ti 0.34 ]O 2 is reported. The tunnel‐type structure of Na 0.44 MnO 2 obtained for this compound is confirmed by X‐ray diffraction and atomic‐scale spherical aberration‐corrected scanning transmission electron microscopy/electron energy‐loss spectrum. When cycled as positive electrode in full cells using NaTi 2 (PO 4 ) 3 /C as negative electrode in 1 m Na 2 SO 4 aqueous electrolyte, this material shows the highest capacity of 76 mAh g −1 among the Na insertion oxides with an average operating voltage of 1.2 V at a current rate of 2 C. These results demonstrate that Na 0.66 [Mn 0.66 Ti 0.34 ]O 2 is a promising positive electrode material for rechargeable aqueous sodium‐ion batteries.