Preparation and characterization of LiMn2O4 spinel nanoparticles as cathode materials in secondary Li batteries

Nanoparticles of LiMn2O4 spinels have been synthesized through a sol-gel method followed by annealing at various temperatures between 350 and 550degreesC. The particle sizes depend on annealing temperatures. At a low annealing temperature (350degreesC), the nanoparticles may have a size of 10 nm, estimated from transmission electron microscopy photographs. The annealing temperatures higher than 550degreesC give rise to particles larger than nanoscale. Some large porous Li1.4Mn2O4 particles with 1 mum 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 Li+ 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. (C) 2004 The Electrochemical Society.