Nanostructured lithium nickel manganese oxides were investigated as advanced positive electrode materials for lithium-ion batteries designated to power plug-in hybrid electric vehicles and all-electric vehicles. The investigation included material characterization and electrochemical testing. In cell tests, the Li1.375Ni0.25Mn0.75O2.4375 composition achieved high capacity (210 mAh g(-1)) at an elevated rate (230 mA g(-1)), which makes this material a promising candidate for high energy density Li-ion batteries, as does its being cobalt-free and uncoated. The material has spherical morphology with nanoprimary particles embedded in micrometer-sized secondary particles, possesses a multiphase character (spinel and layered), and exhibits a high packing density (over 2 g cm(-3)) that is essential for the design of high energy density positive electrodes. When combined with the Li4Ti5O12 stable anode, the cell showed a capacity of 225 mAh g(-1) at the C/3 rate (73 mA g(-1)) with no capacity fading for 200 cycles. Other chemical compositions, Li(1+x)Ni0.25Mn0.75O(2.25+x/2) (0.32 < x < 0.65), were also studied, and the relationships among their structural, morphological, and electrochemical properties are reported.