Phase Stability and Charge Capacity of Cubic Spinel in Li[sub 1+δ]Mn[sub 2−δ]O[sub 4] and Consequence of Magnesium Doping

The X-ray diffraction analysis of Li 1 + δ Mn 2 − δ O 4 having δ = 0.00 , 0.03, 0.06, 0.12, 0.18, 0.24, 0.33, 0.42, and 0.50 powders, synthesized by a sol–gel process, shows that a single-phase cubic spinel structure is stable only when δ ≤ 0.24 . When lithium content increases to δ 0.24 , the percentage of monoclinic Li 2 MnO 3 increases until δ = 0.42 and saturates thereafter. On magnesium doping, the range of stability of cubic spinel Li 1 + δ Mn 2 − δ − 0.1 Mg 0.1 O 4 decreases to δ ≤ 0.18 , and monoclinic Li 2 MnO 3 starts to appear when δ 0.18 . In magnesium-doped Li 1 + δ Mn 1.9 − δ − 0.1 Mg 0.1 O 4 , the ion Mg 2 + goes to octahedral sites and decreases the population of Mn 3 + in the 16d octahedral sites, and so the lattice parameter of the magnesium-doped cubic spinel for the same δ is lower than that for undoped Li 1 + δ Mn 2 − δ O 4 . However, at higher δ, the population of Mn 3 + may get totally exhausted and the charge balance may be maintained by the creation of defects. The voltage step appearing in the discharge curve has been attributed to a transition from two cubic phases to one cubic phase. With increasing δ, the size of the step decreases and becomes broader. However, this voltage step does not appear to be affected by magnesium doping.