Preparation and Characterization of LiMn[sub 2]O[sub 4] Spinel Nanoparticles as Cathode Materials in Secondary Li Batteries

Nanoparticles of spinels have been synthesized through a sol-gel method followed by annealing at various temperatures between 350 and 550°C. The particle sizes depend on annealing temperatures. At a low annealing temperature (350°C), the nanoparticles may have a size of 10 nm, estimated from transmission electron microscopy photographs. The annealing temperatures higher than 550°C give rise to particles larger than nanoscale. Some large porous particles with 1 μm size were prepared for comparison through an emission process. The electrochemical tests indicate that those nanoparticles reduce capacities in the 4 V discharge region but improve rechargeability of the electrode materials. On the other hand, the nanoparticles significantly improve both capacities and cycling performance in the 3 V discharge region. The electrochemical properties of these materials are further investigated using cyclic voltammetry and ac impedance spectroscopy. The results reveal that the homogeneous-phase transition occurring in the 3 V region and at the first discharge plateau of the 4 V discharge region increases the difficulty of the insertion/extraction process and affects rechargeability of the electrode materials. However, the nanoparticles and porous materials reduce charge-transfer resistances between the composite electrodes and electrolyte interface and increase the effectiveness of lithium intercalation into the active materials in comparison with those large but nonporous materials.