Fluorine-doped Li(Ni0.5Mn0.5)O-2 or Li(Ni0.5+0.5zMn0.5-0.5z)O2-zFz (0 <= z <= 0.2) has been synthesized by a solid-state reaction method. X-ray diffraction patterns showed that the synthesized materials had layered alpha-NaFeO2-type structure (R (3) over barm); the Rietveld analysis revealed that lattice parameters (a and c of hexagonal setting) and degree of cation mixing increased with increasing fluorine content (z). Initial discharge capacity of the cathode materials increased with z, showed maximum at z=0.02, and decreased afterwards. At the same time, impedance of the cathode materials decreased with z, reached minimum at z=0.02, and then increased afterwards. Among the materials prepared and studied in this work, Li(Ni0.51Mn0.49)O1.98F0.02 exhibited the best electrochemical properties in terms of capacity (151 mAh g(-1)), impedance (67 Omega cm(2)) and cycleability (no capacity fading up to 40 cycles). (c) 2005 Elsevier B.V. All rights reserved.