Crystallinity control of a nanostructured LiNi0.5Mn1.5O4 spinet via polymer-assisted synthesis:: A method for improving its rate capability and performance in 5 V lithium batteries

Li-Ni-Mn spinets of nominal composition LiNi0.5Mn1.5O4, which are functional materials for electrodes in high-voltage lithium batteries, are prepared by thermal decomposition of mixed nanocrystalline oxalates obtained by grinding hydrated salts and oxalic acid in the presence of polyethyleneglycol 400. Their structure, microstructure, and texture are established from combined X-ray photoelectron spectroscopy (XPS), X-ray diffraction, transmission electron microscopy (TEM), IR spectroscopy, and N-2 absorption measurements. The polymer tailors the shape of particles, which adopt a nanorodlike morphology at low temperatures (400 degrees C). In fact, the nanorods consist of highly distorted oriented nanocrystals connected by a polymer-based film as inferred from IR and XPS spectra. The electrochemical properties of spinels in this peculiar form are quite poor, mainly as a result of the high microstrain content of their nanocrystals. Raising the temperature up to 800 degrees C partially destroys the nanorods, which become highly crystalline nanoparticles approximately 80 nm in size. At this temperature, the polymer facilitates crystal growth; this leads to highly crystalline polyhedral nanoparticles as revealed from TEM images and microstrain data. Following functionalization as a cathode in lithium cells, this material exhibits a very good rate capability, coulombic efficiency, and capacity retention even upon cycling at voltages as high as 5 V. Moreover, it withstands fast-charge-slow-discharge processes, which is an important cycle-life-related property for commercial batteries.