Abstract
Nanostructured LixMn1.5Ni0.5O4 (x = 0.95,1.0,1.05) spinel powders were synthesized by a modified Pechini method. The powders were annealed at different temperatures between 500 and 800 C for 15 h. Depending on the ordering/disordering of transition metal ions on octahedral sites, spinels were assigned to either ordered P4(3)32 (P) or disordered Fd3m (F) space groups. The spinels of the two symmetry groups differed significantly in fast discharge rate capability. Extensive characterization was employed to identify the source of the difference. Vibrational spectroscopy techniques (FTIR and Raman), in situ and ex situ XRD and impedance spectroscopy did not reveal any sign of structural degradation for electrochemically inferior P4(3)32 spinels even after rigorous cycling. The poor performance was assigned to an intrinsic property, the lower electrical conductivity of the cation ordered samples. Arrhenius plots of sintered pellets revealed that the ordered spinels were shown to have two orders of magnitude lower electronic conductivity than disordered samples. The difference in electronic conductivity was assigned to the presence of a small amount of Mn(3+) in disordered samples.