Influence of the preparation methods on the electrochemical properties and structural changes of alpha-sodium iron oxide as a positive electrode material for rechargeable sodium batteries

摘要

A potential positive electrode material, alpha-NaFeO2, can be viewed one of the base material for rechargeable Na batteries. In this study, we found that its structural evolution during charge process differentiates depending on its synthesis route. The alpha-NaFeO2 is prepared by three methods: one is by the hydrothermal technique (HT), and the other two by solid-state reactions, SSn and SS mu, in which nano-sized Fe3O4 and micro-sized Fe3O4 are used as the Fe source, respectively. The synchrotron radiation X-ray diffraction profiles of the initial state suggest that all three samples are apparently identified as the target product, alpha-NaFeO2, with rhombohedral structures. Though all three samples remain stable for more than 50 cycles under the cycling conditions of a constant amount of charging (Na extraction) to 70 mAh g(-1), their structural evolution differs during the sodium extraction. The SS mu transitions from the rhombohedral structure to a spinel-like cubic structure, while the HT transforms into a monoclinic structure. On the other hand, both the monoclinic and cubic phases co-exist in SSn. What produces these differences among the seemingly identical materials is discussed. (C) 2015 Elsevier Ltd. All rights reserved.