Neutron and X-ray powder diffraction and Rietveld refinements have been carried out for the well-crystallized samples with a homogeneous composition Li1+xMn2-xO4 (0 less than or equal to x less than or equal to 0.125). The lattice parameter of these spinels changes with both the composition and the synthesis temperature. SEM and TEM micrographs show that the crystallites appear as single crystals with a size of 0.1-2 mu m. The phase transition from cubic Fd (3) over bar m to orthorhombic Fddd around 280 K of samples Li1+xMn2-xO4 (x less than or equal to 0.04) was observed using low temperature XRD and DSC. The orthorhombic phase further converts to a single tetragonal phase I4(1)/amd around 65 K. The charge/discharge capacity, and cyclability of these samples were examined in both the 3 V and the 4 V regions. We found that the discharge capacity as well as the cyclability of the cell Li/Li1+xMn2-xO4 change largely with not only the value of x but also the crystal structure which depends on the synthesis conditions.