Structural and lithium intercalation studies of Mn(0.5-x)CaxTi2(PO4)3 phases (0≤x≤0.50)

Materials from the Mn(0.5-x)CaxTi2(PO4)(3) (0less than or equal toxless than or equal to0.50) solid solution were obtained by solid-state reaction in air at 1000 degreesC. Selected compositions were investigated by powder X-ray diffraction analysis, P-31 nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R (3) over bar space group. Mn2+/Ca2+ ions occupy only the M1 sites in the Ti-2(PO4)(3) framework. The divalent cations are ordered in one of two M1 sites, except for the Mn0.50Ti2(PO4)(3) phase, where a small departure from the ideal order is observed by XRD and P-31 MAS NMR. The electrochemical behaviour of Mn0.50Ti2(PO4)(3) and Mn(0.5-x)CaxTi2(PO4)(3) phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti-2(PO4)(3) framework. In the 0 less than or equal to y less than or equal to 2 range, the OCV curves of Li//LiyMn0.50Ti2(PO4)(3) cells show two main potential plateaus at 2.90 and 2.50-2.30 V Comparison between the OCV curves of Li//Li(1 + y)Ti2(PO4)(3) and Li//LiyMn0.50Ti2(PO4)(3) shows that the intercalation occurs first in the unoccupied M1 site of Mn0.50Ti2(PO4)(3) at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50-2.30 V voltage range). The effect of calcium substitution in Mn0.50Ti2(PO4)(3) On the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all MI sites. In the case of Mn0.50Ti2(PO4)(3), the stability of Mn2+ either in an octahedral or tetrahedral environment facilitates cationic migration. (C) 2002 Elsevier Science B.V. All rights reserved.