Rectangular Tunnel‐Structured Na0.4MnO2 as a Promising Cathode Material Withstanding a High Cutoff Voltage for Na‐Ion Batteries

AbstractThe structural characteristics and electrochemical properties of Na0.4MnO2 (Na2Mn5O10) as a cathode material in sodium‐ion batteries are systematically investigated. Na0.4MnO2 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. Na0.4MnO2 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 Na0.4MnO2 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 Na0.4MnO2 with a (2×3) rectangular tunnel structure is a promising cathode material for sodium‐ion batteries.