Polymer-mediated growth of highly crystalline nano- and micro-sized LiNi0.5Mn1.5O4 spinels

The use of polymers containing oxygen-based functional groups [polyethylene glycol (PEG), poly(methyl methacrylate) (PMMA)] has been studied to synthesize highly crystalline nanometric LiNi0.5Mn1.5O4 spinel. Polyvinylidene fluoride (PVDF), which was also tested, failed in the spinet synthesis. Mechanical activation of hydrated salts in the presence of oxalic acid and the polymer, followed by heating at 800 degrees C for a few minutes, is sufficient to obtain pseudopolyhedral particles ranging from 60-80 nm in size. X-ray broadening analysis and the electron micrographs of the microstructure reveal that the polymer has an improved particle crystallinity. Calcining at 400 degrees C tailors the particle shape towards a nanorod-like morphology due to the nonionic surfactant properties of PEG. The ability of the functional groups of these polymers to bind to metal ions brings them closer and therefore shortens the diffusion paths followed to adopt the spinel structure. At high temperature, the particles adopt a well-defined pseudopolyhedral morphology with a smooth texture and a lower microstrain content than that obtained in the absence of polymer. The results of the electrochemical tests show that the polymer-assisted nanoparticles exhibit improved reversible capacity and better cycling properties as electrode materials in lithium cells. The improved crystallinity of the particles is the key factor in this respect, particularly when the cells are operated at low rates. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).