Influence of doping on the cation ordering and charge–discharge behavior of LiMn1.5Ni0.5−xMxO4 (M = Cr, Fe, Co, and Ga) spinels between 5.0 and 2.0 V

The charge–discharge behaviors of doped high-voltage spinel oxides LiMn1.5Ni0.5−xMxO4 (M = Cr, Fe, Co, and Ga) have been investigated between 5.0 and 2.0 V before and after post-annealing at 700 °C. It is found that the discharge profiles of LiMn1.5Ni0.5−xMxO4 below 3 V are influenced by the dopant ions due to the variations in lattice strain during lithium ion insertion into 16c octahedral sites caused by the differences in the degree of cation ordering in the 16d sites of the spinel lattice. The differences in the degree of cation ordering can be attributed to the size differences between the Mn4+ and M3+ (M = Cr, Fe, Co, and Ga) ions. Furthermore, after post-annealing at 700 °C, the degree of cation ordering in the 16d sites of the undoped and Fe-doped samples increases, as supported by the discharge profile below 3 V, decrease in lattice parameter, and a reduction in microstrain. In contrast, the highest degree of cation disorder in the 16d sites is found with Co doping due to the uniform distribution of the Co dopant ions in the bulk and the lower concentration of Ni2+ ions in the bulk compared to the other doped samples that encounter a segregation of the dopant ions to the surface. The study demonstrates that the charge–discharge behavior below 3 V is an effective way to assess precisely the degree of cation ordering in the 5 V spinels.