Rectangular Tunnel‐Structured Na 0.4 MnO 2 as a Promising Cathode Material Withstanding a High Cutoff Voltage for Na‐Ion Batteries

The structural characteristics and electrochemical properties of Na 0.4 MnO 2 (Na 2 Mn 5 O 10 ) as a cathode material in sodium‐ion batteries are systematically investigated. Na 0.4 MnO 2 possesses a unique (2×3) rectangular tunnel structure and the barrier for Na diffusion along the rectangular channel is only 18 meV, based on density functional theory calculations. Na 0.4 MnO 2 nanorods show good rate capability and cycle performance with coulombic efficiencies near 100 % in the voltage range of 2.0–4.0 V as well as 2.0–4.5 V. It delivers an initial discharge capacity of 83.7 mAh g −1 at 0.1 C and maintains 84.7 % after 50 cycles and 74.5 % after 100 cycles with a voltage range of 2.0–4.0 V. It is interesting that the Na 0.4 MnO 2 nanorod cathode shows a much better electrochemical performance when adjusting the high cutoff voltage to 4.5 V. The discharge capacity of 98.3 mAh g −1 can be obtained after 50 cycles with a capacity retention as high as 89.1 % at 0.1 C and a capacity retention of 86.6 % can be maintained after 100 cycles in a voltage range of 2.0–4.5 V. Even at 4  C, a high discharge capacity of 79.3 mAh g −1 can be obtained. These data strongly suggest that Na 0.4 MnO 2 with a (2×3) rectangular tunnel structure is a promising cathode material for sodium‐ion batteries.