Nb-doped LiMn1.5Ni0.5O4 materials have been synthesized through a solid-state reaction, and Nb doping achieves some encouraging results. Both crystal domain size and electronic conductivity are influenced by this kind of doping. The lattice parameter of the Nb-doped LiMn1.5Ni0.5O4 samples are slightly larger than that of pure LiMn1.5Ni0.5O4 samples, and Nb doping does not change the basic spinel structure. Even though the material has a particle size of 1–2 μm, the capacity retention is improved remarkably compared to that of the undoped one when charge-discharged at high rates. The LiNi0.525Mn1.425Nb0.05O4 has a discharge capacity of 102.7 mAh g−1 at 1 C charge–discharge rate after 100 cycles. Though all samples exhibit similar initial discharge capacities at various high C rates, the Nb-doped LiMn1.5Ni0.5O4 samples display remarkable cyclabilities. Capacity retention of Nb-doped LiMn1.5Ni0.5O4 is excellent without a significant capacity loss at various high C rates. This is ascribed to a smaller crystallite, a higher conductivity, and a higher lithium diffusion coefficient (D Li) observed in this material. As a result, our microscale Nb-doped LiMn1.5Ni0.5O4 can be used for battery applications that require high power and long life, including HEVs and energy storage devices for renewable energy systems.