Todorokite-type manganese oxide nanowires as an intercalation cathode for Li-ion and Na-ion batteries

Abstract

Extended hydrothermal treatment at an elevated temperature of 220 °C allowed high yield synthesis of manganese oxide nanowires with a todorokite crystal structure suitable for ions intercalation. The flexible, high aspect ratio nanowires are 50–100 nm in diameter and up to several microns long, with 3 × 3 structural tunnels running parallel to the nanowire longitudinal axis. The tunnels are occupied by magnesium ions and water molecules, with the chemical composition found to be Mg_0.2MnO₂·0.5H₂O. The todorokite nanowires were, for the first time, electrochemically tested in both Li-ion and Na-ion cells. A first discharge capacity of 158 mA h g⁻¹ was achieved in a Na-ion system, which was found to be greater than the first discharge capacity in a Li-ion system (133 mA h g⁻¹). Despite large structural tunnel dimensions, todorokite showed a significant first cycle capacity loss in a Na-ion battery. After 20 cycles, the capacity was found to stabilize around 50 mA h g⁻¹ and remained at this level for 100 cycles. In a Li-ion system, todorokite nanowires showed significantly better capacity retention with 78% of its initial capacity remaining after 100 cycles. Rate capability tests also showed superior performance of todorokite nanowires in Li-ion cells compared to Na-ion cells at higher current rates. These results highlight the difference in electrochemical cycling behavior of Li-ion and Na-ion batteries for a host material with spacious 3 × 3 tunnels tailored for large Na⁺ ion intercalation.