Cation substituted spinel LiMn1.5-yNi0.5-zMy+zO4 cathodes exhibit superior capacity retention and rate capability in the 5 V region compared to the unsubstituted LiMn1.5Ni0.5O4 cathode. For example, LiMn1.42Ni0.42Fe0.16O4, LiMn1.42Ni0.42Co0.16O4, and LiMn1.5Ni0.42Zn0.08O4 exhibit capacity fades of < 3% in 50 cycles with 115-125 mAh/g at C/5 rate compared to 8% fade and 131 mAh/g for LiMn1.5Ni0.5O4. Also, LiMn1.42Ni0.42Co0.16O4 retains 92% of its capacity on going from C/10 to the 4C rate compared to 82% for LiMn1.5Ni0.5O4. The superior performances of these cathodes are attributed to the smaller lattice parameter differences among the three cubic phases formed during the charge-discharge process. (c) 2005 The Electrochemical Society.