LiFeyMn2-yO4 (0 less than or equal to y < 1) is prepared and examined by electrochemical, X-ray diffractional (XRD), and Moss- bauer-spectral methods. Li[FeyMn2-y]O-4(Fd3m) having a normal spinel-framework structure can be prepared with 0 less than or equal to y < 0.6 and an intermediate between a normal and inverse spinel for 0.6 y. Li[FeyMn2-y]O-4 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]O-4 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 square [Fe0.5Mn1.5]O-4(cubic) to Li-2[Fe0.5Mn1.5]O-4(tetragonal) via Li[Fe0.5Mn1.5]O-4(cubic). Mossbauer 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.