A post-annealing strategy at 600 °C was used to modify the oxygen deficiency during synthesis of a spinel LiMn1.45Cr0.1Ni0.45O4 cathode for lithium-ion batteries. Structural analyses revealed that post-annealing is an effective way to eliminate the impurity phase without changing the F d 3 ¯ m space group. The substitution of a small amount of Cr leads to better rate performance along with cyclability at room temperature, compared to the commercial LiMn1.5Ni0.5O4. LiMn1.45Cr0.1Ni0.45O4 delivered a reversible capacity of ∼115, 104, 95 and 40 mAh g−1 at 0.2C, 1C, 2C and 5C, respectively. While commercial LiMn1.5Ni0.5O4 offered a lower reversible capacity of ∼110, 98, 85 and 20 mAh g−1 at the same C rates. After 125 cycles, about 99% of reversible capacity was retained for the LiMn1.45Cr0.1Ni0.45O4, while about 6% of capacity loss was obtained after 125 cycles for the commercial LiMn1.5Ni0.5O4. Electrochemical impedance spectroscopy measurements revealed that the LiMn1.45Cr0.1Ni0.45O4 had a smaller surface resistance, which may be due to the segregation of Ni from the surface to the bulk.