Synthesis and characterization of lithium manganese oxide cathode materials for rechargeable lithium batteries

Lithium manganese oxide (LiMn₂O₄) is recognized as the most promising cathode material for the next-generation of lithium batteries. It has been the subject of a great deal of synthetic effort. In this thesis, a synthesis technique used by ceramists to produce ultrafine powders of zirconium oxide (ZrO₂) is applied to produce LiMn ₂O₄ in nanoparticle form. The object is to study the effect of particle size on the performance of a lithium battery. The synthesis technique is based on the reaction of manganese sulfate monohydrate (MnSO₄.H ₂O) with various lithiated nitrate baths. Several nitrate salts and manganese salts were used in order to influence the particle size and morphology. However, the size of the LiMn₂O₄ grains was only 100 nm compared to 5 rum for ZrO₂. The morphology changed when potassium nitrate (KNO₃) was added to the melt due to formation of K + stabilized alfa manganese oxide (α-MnO₂). The one dimensional character of α-MnO₂ induced a needle shape morphology. The reaction mechanism was investigated using thermogravimetry, X-ray diffraction and atomic absorption spectroscopy. The reaction was found to proceed in three steps: dehydration, oxidation, and lithiation. The reaction could be explained using the Lux-Flood formalism. Powders with different composition and grain morphology were tested as cathode materials in a Li battery. This research did not show evidence of improvement in performance for the cathode with small grain size; indeed the small Particle size actually seems to be slightly detrimental to the capacity retention of the cell. These results were interpreted in terms of dissolution kinetic of the cathode in the electrolyte. It was confirmed that Li_1+xMn_2-xO₄ with small excess of lithium exhibit improved capacity retention. Li_1+xMn_2-xO ₄ also improves the high discharge current performance and this is interpreted in term of the effect of the excess lithium on the Jahn-Teller distortion. The drop in operating voltage at high discharge current is interpreted in terms of mixed lithium insertion into tetrahedral and octahedral sites. It was demonstrated that this synthesis technique can be used to produce lithium manganate doped with certain transition metals. Finally, it was found that oxygen loss at high annealing temperature induces a catastrophic capacity loss.