LiNixCu0.5-xMn1.5O4 (0 less than or equal to x less than or equal to 0.5) spinel materials that are of interest as electrodes for lithium-ion batteries have been studied by crystallographic and electrochemical methods. Electrochemical results show that the inclusion of Ni into the Cu-modified spinel increases the overall reversible capacity from 72 mAh/g with LiCu0.5Mn1.5O4 (x = 0) at 4.95 V to 120 mAh/g with LiNi0.5Mn1.5O4 (x = 0.5) at 4.6 V. The Cu-rich spinels in the LiNixCu0.5-xMn1.5O4 system show enhanced stability upon cycling. These spinel products are difficult to prepare in single-phase form. Neutron diffraction data show that the end members LiNi0.5Mn1.5O4 and LiCu0.5Mn1.5O4 contain small amounts of lithium-nickel-oxide and lithium-copper-oxide impurity phases, respectively; these impurities affect the composition of the spinel component. The composition of the spinel component in LiNi0.5Mn1.5O4 was determined from a combined X-ray and neutron diffraction profile refinement to be Li0.97Ni0.42Mn1.61O4.