Al₂O₃ coated Na_0.44MnO₂ as high-voltage cathode for sodium ion batteries

Na_0.44MnO₂ is expected to be a cathode material with an excellent electrochemical performance for sodium ion batteries due to its unique 3-dimensional (3D) S-type tunnel structure. However, the high cut-off working potential of Na_0.44MnO₂ is usually restricted below 4.0 V (vs. Na/Na⁺) to inhibit fast capacity fading. Although pristine Na_0.44MnO₂ submicron rods deliver a high initial discharge capacity of 108.0 mAh g⁻¹ at 0.4C (1C = 121 mAh g⁻¹) in the wide voltage range of 2.0–4.5 V however the discharge capacity reduces to 80.5 mAh g⁻¹ after 200 cycles. For improving the electrochemical properties under high voltage, Al₂O₃ coated Na_0.44MnO₂ is prepared by a wet-coating process and the coating amount is optimized. The as-prepared 2 wt% Al₂O₃-coated Na_0.44MnO₂ submicron rods possess the best electrochemical performance between 2.0 V–4.5 V, which deliver an initial discharge capacity of 109.8 mAh g⁻¹ at 0.4C and maintain capacity retention of 93.2% after 200 cycles. Moreover, in long-term cycle performance at high current density (4C) between 2.0 and 4.5 V, 2 wt% Al₂O₃-coated Na_0.44MnO₂ can retain 79% capacity after 500 cycles. The mechanism of elevated electrochemical performance for Al₂O₃-coated Na_0.44MnO₂ submicron rods in high voltage is systematically investigated.