The electrochemical properties of LiMn2O4 and LiMyMn2-yO4 (M = Ti, Ge, Fe, Zn, or Ni) were studied for different conditions of sample preparation and different degrees of cation substitution (y). In the voltage range 3.5-4.5 V, cells of either spinel LiMn2O4 or lambda-MnO2 (made by leaching the Li from LiMn2O4) reversibly insert 0.4 Li per Mn at an average voltage of 4.1 V, leading to an energy density of 480 Wh/kg of cathode. Cells cycled 50 times lost less than 10% of their initial capacity, suggesting that this material could be used instead of LiCoO2 or LiNiO2 as the cathode in the new generation of "rocking chair batteries." Replacing Mn with cations of valence 2 (Ni, Zn) or 3 (Fe) reduces the amount of Mn+3 and correspondingly reduces the capacity of the cells at 4.1 V, but does not affect their cycling performance