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

Abstract

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 Na0.44MnO2 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 Na0.66[Mn0.66Ti0.34]O2 is reported. The tunnel‐type structure of Na0.44MnO2 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 NaTi2(PO4)3/C as negative electrode in 1 m Na2SO4 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 Na0.66[Mn0.66Ti0.34]O2 is a promising positive electrode material for rechargeable aqueous sodium‐ion batteries.