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_0.44MnO₂ 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 Na_0.66[Mn_0.66Ti_0.34]O₂ is reported. The tunnel-type structure of Na_0.44MnO₂ 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 Na_0.66[Mn_0.66Ti_0.34]O₂ is a promising positive electrode material for rechargeable aqueous sodium-ion batteries. Tunnel-type Na_0.66[Mn_0.66Ti_0.34]O₂ is successfully designed and synthesized as a positive electrode material for aqueous sodium-ion batteries. It shows a high reversible capacity of ≈76 mAh g^-1 at a current rate of 2 C with an average voltage of 1.2 V vs. NaTi₂(PO₄)₃ (NTP). The aqueous full-cell demonstrates excellent cycle performance with small capacity decay after 300 cycles.