 LiF–NixMn1−xO composites are prepared by the mechanical milling of LiF and NixMn1−xO (x ≤ 1) solid solution for 72 h. The obtained composites are examined by XRD, charge–discharge measurements, CV, and XPS. X-ray diffraction peaks of LiF–NixMn1−xO are broadened by milling. The diffraction peaks of NixMn1−xO in x < 0.5 are shifted to the higher angle by milling together with LiF. The discharge capacity and voltage of the LiF–NixMn1−xO composites changes with the molar ratio of NiO and MnO. The composite containing a large amount of Mn showed a lower discharge voltage and a larger discharge capacity. The composites containing a large amount of Ni show higher discharge voltage. The maximum discharge capacity among LiF–NixMn1−xO composites is obtained at x = 0.2, and its value is 279 mAh g−1 for cut-off voltages of 2.0–4.8 V at 0.1 C. The XPS measurement shows that the Mn2+ and Ni2+ ions are oxidized to Mn3+ and Ni3+ions in charging process and are reduced to Mn2+ and Ni2+ ions in the discharging. The oxidation and reduction process between Mn2+ and Mn3+ greatly contribute in the charge and discharge process of LiF–Ni0.5Mn0.5O composite.