Unveiling the structural integrity of tunnel-type Na_0.44MnO₂ cathode for sodium ion battery

Tunnel-type Na_0.44MnO₂ (tt-NMO) is a promising cathode for sodium ion battery having excellent structural stability, diffusion kinetics, and low cost. However, this cathode is reported to suffer from low initial charge capacity (e.g., ≤60 mA h g⁻¹) due to the limited accessibility of sodium ion extraction (0.22-0.24 Na⁺ per formula unit) from the structure, which hinders the practical viability of this material in a full battery cell. In this study, we report a tailored tt-NMO structure, synthesized using a two-step facile and scalable process, with >95% yield. Our tt-NMO demonstrated a 1st charge capacity of 110 mA h g⁻¹, followed by a discharge capacity of 115 mA h g⁻¹ within the potential window of 4-1.7 V versus Na/Na⁺. The long-term cycling performance at 0.5C rate and 1C rate (1C = 120 mA h g⁻¹) shows excellent structural integrity for over 400 cycles with >75% capacity retention. We show experimentally and support it with DFT (density functional theory) calculations that the unique microstructure of this tt-NMO, with modulated Na-O bond length and Na-O-Na bond angle, results in open channels along the c-axis in the ab plane, providing a wide pathway for ion diffusion. The Na⁺ migration barriers (E_m) along the two pathways of the c-tunnel are calculated to be within the threshold limit of Na⁺ migration energy barrier, which renders more sites electrochemically active, enabling the high 1st charge capacity. This novel study opens possibilities to use this unique tt-NMO as an efficient SIB (sodium ion battery) cathode by harnessing the modified structure.