NMR Study of LiCo1-xCrxO2and Li1-xNaxCoO2(x=0and 0.05)

The 7 Li and 6 Li nuclear spin-lattice relaxation rates, ( 7 T 1 ) -1 and ( 6 T 1 ) -1 , have been measured for LiCoO 2 as a function of temperature. Comparison between 7 Li- and 6 Li-rich samples shows that the ratio ( 6 T 1 ) -1 /( 7 T 1 ) -1 =0.06 is considerably larger than the value expected for quadrupolar relaxation processes. Furthermore ( 7 T 1 ) -1 decreases significantly as 7 Li is replaced by 6 Li. These observations indicate that ( 7 T 1 ) -1 is dominated by the nuclear magnetic dipole-dipole interaction between the 7 Li nuclei. The substitution of Cr for Co has no effect on ( 7 T 1 ) -1 in the range below 200 K, whereas the substitution of Na for Li enhances ( 7 T 1 ) -1 . The substitution effect supports the view that the two-dimensional Li ion motion is dominant in LiCoO 2 . The weak dependence of ( 7 T 1 ) -1 upon temperature and frequency suggests the presence of a broad distribution of barrier heights for ionic motion in LiCoO 2 .