NMR study of LiCo1-xCrxO2 and Li1-xNaxCoO2 (x = 0 and 0.05)

The Li-7 and Li-6 nuclear spin-lattice relaxation rates, (T-7(1))(-1) and (T-6(1))(-1), have been measured for LiCoO2 as a function of temperature. Comparison between Li-7- and Li-6- rich samples shows that the ratio (T-6(1))(-1)/(T-7(1))(-1) = 0.06 is considerably larger than the value expected for quadrupolar relaxation processes. Furthermore (T-7(1))(-1) decreases significantly as 7Li is replaced by Li-6. These observations indicate that (T-7(1)) is dominated by the nuclear magnetic dipole-dipole interaction between the Li-7 nuclei. The substitution of Cr for Co has no effect on (T-7(1))(-1) in the range below 200 K, whereas the substitution of Na for Li enhances (T-7(1))(-1). The substitution effect supports the view that the two-dimensional Li ion motion is dominant in LiCoO2. The weak dependence of (T-7(1))-1 upon temperature and frequency suggests the presence of a broad distribution of barrier heights for ionic motion in LiCoO2.