Towards elucidating microscopic structural changes in Li-ion conductors Li1+yTi2–yAly[PO4 ]3 and Li1+yTi2–yAly[PO4 ]3–x [MO4 ]x(M=V and Nb) : X-ray and27Al and31P NMR studies

A combination of X-ray powder diffraction (XRD) and nuclear magnetic resonance (NMR) studies has demonstrated that attempted substitutions of Al, V and Nb into the framework of LiTi₂(PO₄)₃ yield several impurity phases in addition to direct substitutions of Al into Ti and V, Nb into P sites. Direct substitutions were confirmed by changes in the unit cell dimensions as indicated by the peak shifts observed in the X-ray diffractographs and by analyses of the ²⁷Al and ³¹P magic angle spinning (MAS) spectra. A major impurity phase was identified as AlPO₄ (found in at least two polymorphs) and the amount present increases with increasing Al additions. The formation of AlPO₄ appeared to be enhanced by further V but suppressed by Nb substitution. These results suggest that the presence of AlPO₄ , together with the non-stoichiometric modified LTP, may be the cause for the observed densification of this material upon sintering and the increased ionic conductivity.