Cobalt(III) effect on 27Al NMR chemical shifts in LiAlxCo1-xO2

A preparative route based on citrate precursors is used to obtain LiAlxCo1-xO2 solid solutions over the whole composition range. From XRD, IR, and NMR the compounds have been shown to be a substitution of Co by Al in the octahedral sites of the layer. A detailed Al-27, Co-59, and Li-7 MAS NMR study is carried out on these products by using a high spinning speed in a high magnetic field (11.7 T). Aluminum is surrounded by six second-neighbor metals. Co/Al substitution leads to an almost regular chemical shift increment of +7 ppm, thus giving a maximum chemical shift of 62.5 ppm for the aluminum surrounded by six cobalt atoms. This value is in the classical chemical shift range of tetrahedral environments but is, however, characteristic of an octahedral environment. Despite this unusual aspect of the Al-27 NMR chemical shift, all the other NMR, XRD, and IR observations converge to a consistent description of the phase as being a continuous solid state solution of octahedral sites randomly occupied by Co or Al. In the composition range from 0 to 0.20 in Al it seems that the phase is a modified LiCoO2 type and beyond 0.2 in Al looks like a ideal mixture. It is only beyond 0.9 that the phase more resembles LiAlO2.