Abstract

Two manganese oxide phases, α-Na_0.51Mn_0.93O₂ and β-Na_0.52MnO₂, have been synthesized and their structures determined by Rietveld analysis: α-Na_0.51Mn_0.93O₂ (isostructural to NaCo₂O₄) space group P6₃/mmc, a = 2.8661(4) Å, c = 11.165(4) Å and β-Na_0.52MnO₂ (isostructural to α-K_0.51Mn_0.93O₂), space group Cmcm, a = 2.834(6) Å, b = 5.216(1) Å and c = 11.280(3) Å. Both phases are layered, with alternate sheets comprised of (MnO₆) octahedra and (NaO₆) trigonal prisms. Ion exchange of Na for Li in a sample having the mean composition Na_0.56Mn_0.86O₂ that represents a 1∶3 mixture of the α- and β-phases, was effected by immersion in an LiCl/LiNO₃ eutectic to obtain the lithiated composition Li_0.54Na_0.02Mn_0.86O₂. The latter material displayed excellent electrochemical behavior when employed as a cathode in a lithium battery. A high and relatively stable capacity of 160 mA g⁻¹ was obtained in the 2.5–4.3 V voltage window for a C/15 regime. No transition to a spinel structure occurred, on cycling: the discharge/charge curves showed only a monotonous potential variation even after 50 cycles.