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 and ²⁷Al and ³¹P 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 LiTi 2 (PO 4 ) 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 difiractographs and by analyses of the 27 Al and 31 P magic angle spinning (MAS) spectra. A major impurity phase was identified as AlPO 4 (found in at least two polymorphs) and the amount present increases with increasing Al additions. The formation of AlPO 4 appeared to be enhanced by further V but suppressed by Nb substitution. These results suggest that the presence of AlPO 4 , 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.