LiFe y Mn2−y O4 (0≤y<1) is prepared and examined by electrochemical, X-ray diffractional (XRD), and Mössbauer-spectral methods. Li[Fe y Mn2−y ]O4(Fd 3 ̄ m) having a normal spinel-framework structure can be prepared with 0≤y<0.6 and an intermediate between a normal and inverse spinel for 0.6<y. Li[Fe y Mn2−y ]O4 shows discrete operating voltages at 3, 4, and 5 V versus Li. When the iron concentration y is increased from 0 to 0.5, the capacity at 5 V increases while the capacity at 4 V decreases. The operating voltage at 3 V is not affected so much by the iron concentration. The XRD examinations for the oxidation and reduction of Li[Fe0.5Mn1.5]O4 show that the reaction proceeds in a topotactic manner over an entire range, i.e. the cubic phase at voltages above 3 V, the cubic/tetragonal phases in the range 2–3 V, and the tetragonal phase below 2 V. Reversible capacity observed below 2 V is not explained in terms of a topotactic reaction from □[Fe0.5Mn1.5]O4(cubic) to Li2[Fe0.5Mn1.5]O4(tetragonal) via Li[Fe0.5Mn1.5]O4(cubic). Mössbauer spectra indicate that Fe4+ is formed above 4.5 V and Fe3+ (high spin state) below 4.5 V. From these results the reaction mechanism is discussed in terms of the solid-state redox reaction.