Structural study of two layered phases in the NaxMnyO2 system. Electrochemical behavior of their lithium substituted derivatives

Two manganese oxide phases, alpha-Na0.51Mn0.93O2 and beta-Na0.52MnO2, have been synthesized and their structures determined by Rietveld analysis: alph a-Na0.51Mn0.93O2 (isostructural to NaCo2O4) space group P6(3)/mmc, a = 2.86 61(4) Angstrom, c = 11.165(4) Angstrom and beta-Na0.52MnO2 (isostructural t o alpha-K0.51Mn0.93O2), space group Cmcm, a = 2.834(6) Angstrom, b = 5.216( 1) Angstrom and c = 11.280(3) Angstrom. Both phases are layered, with alter nate sheets comprised of (MnO6) octahedra and (NaO6) trigonal prisms. Ion e xchange of Na for Li in a sample having the mean composition Na0.56Mn0.86O2 that represents a 1 : 3 mixture of the alpha- and beta-phases, was effecte d by immersion in an LiCl/LiNO3 eutectic to obtain the lithiated compositio n Li0.54Na0.02Mn0.86O2. The latter material displayed excellent electrochem ical behavior when employed as a cathode in a lithium battery. A high and r elatively stable capacity of 160 mA g(-1) was obtained in the 2.5-4.3 V vol tage window for a C/15 regime. No transition to a spinel structure occurred , on cycling: the discharge/charge curves showed only a monotonous potentia l variation even after 50 cycles.