Relationship between Chemical Bonding Nature and Electrochemical Property of LiMn2O4 Spinel Oxides with Various Particle Sizes:  “Electrochemical Grafting” Concept

Systematic Mn 2p XPS and Mn K-edge XAS analyses together with the electrochemical measurement have been carried out for the spinel LiMn2O4 prepared at various sintering temperatures in order to elucidate an origin of the dependence of electrochemical properties on synthetic conditions. From the comparative experiments, it becomes clear that a lowering of synthetic temperature gives rise to an increase of structural disorder and of the average oxidation state of manganese, which is more prominent on the surface than in the bulk. Such results suggest that the modification of surface property induced by a decrease of particle size is closely related to the electrochemical performance. The nanocrystalline LiMn2O4 prepared at 250 °C shows excellent cyclability at the 3 V region compared to that of microcrystalline LiMn2O4 prepared at 700 °C. For the purpose of examining the evolution of the chemical bonding nature of inserted lithium, 7Li MAS NMR studies have been performed for both the spinel compounds before and after Li+ intercalation. While the intercalation of 0.2 mol Li+ does not induce any remarkable spectral change for the microcrystalline LiMn2O4, it leads to a dramatic suppression of the NMR signal for the nanocrystalline LiMn2O4, indicating that the process of grafting Li into the latter phase results in significant modifications of the chemical environment of lithium. On the basis of present experimental findings, it can be concluded that the lowering of synthetic temperature modifies the surface properties, which facilitates the grafting process of Li+ ion and, thereby, enhances the electrochemical properties for the 3 V region corresponding to the Li insertion.