High-Voltage Fe-Based\nTunnel-Type Na<sub>0.66</sub>[Mn<sub>0.33</sub>Fe<sub>0.33</sub>Ti<sub>0.3</sub>Sn<sub>0.04</sub>]O<sub>2</sub> Cathode for Aqueous Rechargeable\nSodium-Io

Tunnel-type-structure Na<sub>0.44</sub>MnO<sub>2</sub> has been\nextensively researched for cathode material in aqueous rechargeable\nsodium-ion battery owing to its high specific capacity (120 mA h g<sup>–1</sup>), large channels facilitating Na extraction/insertion,\nchemical and electrochemical stability in aqueous electrolytes, and\nlow cost. However, the low average working potential (0.1 V versus\nstandard hydrogen electrode, SHE) and no more than half of its available\ntheoretical capacity within full batteries limit the practical application.\nHerein, we develop an Fe-based tunnel-type Na<sub>0.66</sub>[Mn<sub>0.33</sub>Fe<sub>0.33</sub>Ti<sub>0.3</sub>Sn<sub>0.04</sub>]O<sub>2</sub> cathode, delivering a high reversible specific capacity (95\nmA h g<sup>–1</sup>) under a high working voltage (0.75 V versus\nSHE). A full battery, assembled with a NaTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>@C anode, exhibits a high energy density of 80 W h kg<sup>–1</sup> (total mass of cathode and anode active materials)\nand a long cycle life with 84% capacity retention after 1000 cycles\nat 1 C.