Synthesis and characterization of high-temperature hexagonal P2-Na0.6MnO2 and its electrochemical behaviour as cathode in sodium cells

A layered sodium manganese oxide, Na0.6MnO2, stable at temperatures above 800 degreesC, was synthesized by using a sol-gel method that employs Mn(acac)(3) (ac = acetylacetonate), Na2CO3 and propionic acid to form the resin framework. This layered bronze possesses a hexagonal, P2-type structure, in which the distortion associated with Mn3+ is hardly perceptible. It reacts slowly, though reversibly, with atmospheric moisture, which causes the interlayer spacing in the structure to increase by ca. 2.5 Angstrom, through intercalation of water molecules into the interlayer gap occupied by Na+ ions. The anhydrous material was tested as a cathode in sodium cells. Although the electrochemical intercalation of Na+ occurs in two steps, the host retains its main structural features, with a slight tendency in the interlayer spacing to contract as the sodium content increases. The similarity between the discharge and charge profiles of the first cycles reveals a quasi-reversible nature in the intercalation process and that the cell can deliver a constant specific capacity of ca. 140 A h kg(-1) at 0.1 mA cm(-2) when cycled in a voltage window of 3.8-2.0 V. However, the continuous strains and distortions resulting from the insertion and extraction of Na+ ions cause the host structure to gradually collapse and yield an amorphous material after the first eight cycles. This leads to a progressive reduction of the cell capacity, irrespective of the specific voltage window used.