Antifluorite-type materials, Li5+x Fe1−x Co x O4, were prepared and studied as a cathode for use in a lithium secondary battery. During the first charge process, the structure undergoes a phase change as the removal of lithium progresses. The original antifluorite structure is almost completely lost when 1.5 equivalents of lithium are deintercalated from the structure, as in the case of Li5FeO4. This tendency was less obvious in cobalt-doped samples, and 2.1 equivalents of lithium could be removed before the original lattice disappeared. Cobalt doping helps to maintain the original lattice configuration and enhances cycling stability. The Mössbauer spectra suggest that the electrons of oxygen atoms play a role in addition to those of iron. A 1.3 equivalents of lithium could be reversibly deintercalated from Li5.6Fe0.4Co0.6O4, which corresponds to 220mAhg−1.