Synthesis and characterisation of sulphided lithium manganese spinels LiMn2O4−ySy prepared by sol-gel method

The series of LiMn2O4−ySy spinels (0.02 ≤ y ≤ 0.50) were obtained by the sol-gel method followed by the calcination at 300 °C. All the samples showed the XRD patterns typical for defected Li1−xMn2−2xO4 spinels. The TGA measurements with simultaneous EGA showed evolution of oxygen in the same temperature range as in the case of Li1−xMn2−2xO4 samples obtained and calcined in the same way that may suggest the elimination of the cationic defects and the improvement of the crystal structure. Additional calcination in the air at 800 °C was required to receive well crystallized spinels. For such samples single phase materials were obtained up to composition of y = 0.25 mol of sulphur. For the higher concentration of sulphur additional phase of bixbyite (Mn2O3) was observed on the XRD patterns. Calcination at 800 °C improves the electrical conductivity of the spinels. The sulphided spinels showed the phase transition at about room temperature, but the effect is strongly suppressed with the increase of substitution of sulphur for oxygen in the spinel lattice. This substitution may be used to stabilize the Li-Mn spinels without change in Mn3+/Mn4+ ratio, which should be close to 1, to provide high cathode capacity. Above 920 °C the sulphided spinels decomposed with evolution of sulphur oxides.