Fluorine-doped Li(Ni0.5Mn0.5)O2 or Li(Ni0.5+0.5z Mn0.5−0.5z )O2−z F z (0≤ z ≤0.2) has been synthesized by a solid-state reaction method. X-ray diffraction patterns showed that the synthesized materials had layered α-NaFeO2-type structure ( R 3 ¯ m ) ; 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 (151mAhg−1), impedance (67Ωcm2) and cycleability (no capacity fading up to 40 cycles).