High-Voltage Fe-Based Tunnel-Type Na_0.66[Mn_0.33Fe_0.33Ti_0.3Sn_0.04]O₂ Cathode for Aqueous Rechargeable Sodium-Ion...

Tunnel-type-structure Na_0.44MnO₂ has been extensively researched for cathode material in aqueous rechargeable sodium-ion battery owing to its high specific capacity (120 mA h g^-1), large channels facilitating Na extraction/insertion, chemical and electrochemical stability in aqueous electrolytes, and low cost. However, the low average working potential (0.1 V versus standard hydrogen electrode, SHE) and no more than half of its available theoretical capacity within full batteries limit the practical application. Herein, we develop an Fe-based tunnel-type Na_0.66[Mn_0.33Fe_0.33Ti_0.3Sn_0.04]O₂ cathode, delivering a high reversible specific capacity (95 mA h g^-1) under a high working voltage (0.75 V versus SHE). A full battery, assembled with a NaTi₂(PO₄)₃@C anode, exhibits a high energy density of 80 W h kg^-1 (total mass of cathode and anode active materials) and a long cycle life with 84% capacity retention after 1000 cycles at 1 C.