| Nanostructured LiMn1.5+deltaNi0.5-deltaO 4 spinel powders were synthesized by a solution based chemistry method called modified Pechini. The impacts of processing parameters such as synthesis temperature, oxygen-partial-pressure and mole ratio of ethylene glycol to citric acid on the morphology, structure and properties of spinel materials have been studied thoroughly via various in-situ and ex-situ characterization techniques. Later, these parameters were tied with the electrochemical properties of spinel electrodes.; After optimization of processing steps and glycol/acid ratio, a unique mesoporous morphology with interconnected nanoparticles were successfully obtained. Such morphology was found to be very conducive to achieve high power density lithium-ion battery spinel cathodes. This was attributed to (i) large number of mesoporosities that favor electrolyte penetration, thereby enabling better wetting of spinel cathodes and faster lithium ion transfer at electrolyte/cathode interface and (ii) particle interconnectivity that allows continuous electron transport, which becomes highly critical especially at high current rates.; The synthesis temperature and oxygen-partial-pressure were found to affect the structure significantly. Depending on the ordering/disordering of transition metal ions on octahedral sites, spinels were assigned to either ordered P4 332 or disordered Fd3m space groups. The spinel of the two symmetry groups differed significantly in fast discharge rate capability. In an effort to identify the origin of this electrochemical disparity, intensive characterizations of both structures were undertaken (in-situ: XRD, Impedance spectroscopy, Raman; ex-situ: XRD, FTIR, TGA, electronic conductivity and lithium diffusivity and more). The poor performance of the ordered phase was attributed to its intrinsic properties, namely lower electronic conductivity and lithium diffusion coefficient (DLi). Regarding the former, the mechanism of electron conduction in LiMn1.5+deltaNi 0.5-deltaO4 was demonstrated to be electron hopping from Mn3+ (non existent in P4332 when delta=0.5) to Mn4+ sites. The difference between highest electronic conductivity spinel (Fd3m phase) and lowest conductivity spinel (P4332 phase) was measured to be as much as 2.5 orders-of-magnitude. As for the latter, the disordered spinel was found to have an order-of-magnitude higher D Li than its ordered counterpart for the same composition.; Finally, elevated temperature performance of spinel electrodes was studied. A plausible scenario for the failure mechanism of battery cells was suggested. |
