Na2Mn3O7: A Suitable Electrode Material for Na-Ion Batteries?

Sodium manganese oxides were studied as a suitable material for Na-ion batteries. Na2Mn3O7 black crystals were prepared hydrothermally from MnO2 and NaOH under 2 kbar of O2 at 595 °C for 7 days. The structure of this compound was solved from single crystals in a triclinic symmetry. The sheets are separated by NaO6 and NaO5 polyhedra. A capacity of 160 mAh/g, i.e., 1.7 lithium was observed through a biphasic process for a cycling between 2.0 and 4.5 V versus Li+/Li with a plateau at 3.0 V. the sodium manganese oxide Na2Mn3O7 by a very simple conventional solid-state method, from stoichiometric amounts of sodium nitrate and manganese carbonate were prepared. The required precursors were mixed together for 15 min and further heated at 600 °C for 4 h in an alumina crucible under oxygen flow. The thermogravimetric study revealed that this phase is stable up to 650 °C under air. The cationic composition was checked by Inductively Coupled Plasma (ICP) analysis and the manganese oxidation state by iodometric titration. It was found that the layered phase Na2Mn3O7 could reversibly insert two sodium through a biphasic process, leading to the formation of a new material with the composition of Na4Mn3O7 showing a capacity of 160 mAh/g at C/20, with a low polarization. The redox potential of 2.1 V vs Na+/Na gives an energy density of 336 Wh/kg.