X-ray diffraction study on LixCoO2 below ambient temperature

In order to elucidate the structural change of LixCoO2 with temperature (T), powder X-ray diffraction measurements have been carried out using a synchrotron radiation source in the T range between 300 and 90 K for the samples with x=1.02x=1.02, 0.60, 0.56, and 0.53. The samples with x<1.02x<1.02 were prepared by an electrochemical reaction in a non-aqueous lithium cell. The x=1.02x=1.02 and 0.60 samples are in a rhombohedral phase (R3¯m) in the whole T   range measured. On the other hand, the x=0.56x=0.56 and 0.53 samples exhibit a structural transition around 140 K, although the both samples are in a monoclinic phase (C2/mC2/m) down to 90 K. That is, the angle between aMaM- and cMcM-axis (βMβM) increases monotonically down to 150 K, then increases more rapidly with further lowering T  . The values of Δβ and aM/bMaM/bM, which are parameters to characterize a monoclinic distortion from the hexagonal symmetry, are Δβ>+0.6° and aM/bM<1.732aM/bM<1.732 above 140 K, while Δβ<+0.6° and aM/bM≈1.732aM/bM≈1.732 below 140 K. This suggests that the monoclinic distortion below 140 K is mainly caused by a gliding along the basal plane.