Structure and reversible lithium intercalation in a new P′3-phase: Na2/3Mn1−yFeyO2 (y = 0, 1/3, 2/3)

Abstract

In this contribution, new data on the reversible Li+ intercalation in iron substituted sodium manganates are provided. Novel Na2/3Mn1−yFeyO2 (y = 0, 1/3 and 2/3) compounds with a P′3-type structure are prepared from freeze-dried citrate precursors at 500 °C. A new structural element is the development of three-layer oxygen stacking contrary to the well-known P2-type Na2/3MnO2 with a two-layer sequence. The effect of Fe additives on the structure of Na2/3MnO2 was examined by XRD powder diffraction and TEM analysis. The oxidation state and the distribution of transition metal ions in Na2/3Mn1−yFeyO2 were analysed using electron paramagnetic resonance spectroscopy. The lithium intercalation in Na2/3Mn1−yFeyO2 was investigated in two-electrode lithium cells of the type Li|LiPF6 (EC:DMC)|Na2/3Mn1−yFeyO2. The stability of the layered phases during lithium intercalation was studied by ex situ Raman spectroscopy. It was found that the intermediate Na2/3Mn2/3Fe1/3O2 composition is able to intercalate Li+ reversibly in high amounts. Details of the structure and its stability during the Li+ intercalation are discussed.