Abstract
Tunnel type Na0.44MnO2 (tt-NMO) is a promising cathode for sodium-ion batteries 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 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 A h g-1 followed by a 115 mA h g-1 discharge capacity within the potential window of 4 ̶1.7V 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 by DFT calculations, that the unique microstructure of this tt-NMO with modulated Na-O bond length and Na-O-Na bond angle resulted in open channels along the c axis in the ab plane providing a wide pathway for ion diffusion. The Na+ migration barriers (Em) along the two pathways of the c-tunnel are calculated to be within the threshold limit of the Na+ migration energy barrier which renders more sites electrochemically active, enabling the high 1st charge capacity. This novel study opens possibilities for using this unique tt-NMO as an efficient SIB cathode by harnessing a modified structure.