With an aim to improve the 5V capacity and cyclability of the LiMn1.5Ni0.5O4 spinel oxide, three series of Cr substitutions have been pursued with y ≤0.2: LiMn1.5Ni0.5−y Cr y O4, LiMn1.5−0.5y Ni0.5−0.5y Cr y O4, and LiMn1.5−0.33y Li0.33y Ni0.5−y Cr y O4. While the first series involves an increase in the Mn3+ content, the second and third series are designed to maintain charge neutrality (Mn4+, Ni2+, Cr3+, and Li+) without introducing Mn3+ ions. The LiMn1.5Ni0.5−y Cr y O4 series experiences a widening of the 4V plateau and a decrease in the 5V capacity compared to LiMn1.5Ni0.5O4 due to an increase in the Mn3+ content. On the other hand, the LiMn1.5−0.5y Ni0.5−0.5y Cr y O4 series shows a suppression of the 4V plateau and an increase in the 5V capacity due to the elimination of the Mn3+ions. The LiMn1.5−0.33y Li0.33y Ni0.5−y Cr y O4 series shows a suppression of the 4V plateau at low Cr contents, but an increase in the 4V plateau as the Cr content increases above 0.1. Among the various compositions investigated, LiMn1.45Ni0.45Cr0.1O4 exhibits the best combination of high 5V capacity (128mAh/g at 5–4.2V) and excellent capacity retention (98% in 50 cycles) compared to 118mAh/g and 92% for LiMn1.5Ni0.5O4.