Rectangular tunnel-structured Na_0.4MnO₂ as a promising cathode material withstanding a high cutoff voltage for na-ion batteries

The structural characteristics and electrochemical properties of Na_0.4MnO₂ (Na₂Mn₅O₁₀) as a cathode material in sodium-ion batteries are systematically investigated. Na_0.4MnO₂ 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. Na_0.4MnO₂ 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 mAhg^–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_0.4MnO₂ nanorod cathode shows a much better electrochemical performance when adjusting the high cutoff voltage to 4.5 V. The discharge capacity of 98.3 mAhg^–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 mAhg^–1 can be obtained. These data strongly suggest that Na_0.4MnO₂ with a (2×3) rectangular tunnel structure is a promising cathode material for sodium-ion batteries.