Can all the lithium be removed from T2-Li2/3[Ni1/3Mn2/3]O2?

In this paper, it is demonstrated that all the lithium cannot he removed electrochemically from Li-2/3[Ni1/3Mn2/3]O-2 at room temperature below 5.4 V vs. Li/Li+. On the contrary, all the Na atoms can he extracted and reinserted reversibly from P2-Na-2/3[Ni1/3Mn2/3]O-2 electrochemically to form Na-x[Ni1/3Mn2/3]O-2 with 0 < x 0.67. Both of these results have been confirmed by X-ray diffraction measurements. Furthermore, when all the Na is removed, [Ni1/3Mn2/3]O-2 adopts the O2 structure with random stacking faults. The diffraction pattern can be well matched by simulations of these stacking faults using the DIFFaX program. Lithium can be inserted into square [Ni1/3Mn2/3]O-2, however. only about one-half of the lithium which is inserted can be removed. X-ray diffraction shows that when the Li is inserted into square [Ni1/3Mn2/3]O-2, the structure transforms from O2 with stacking faults to O2 without stacking faults. We speculate upon the reasons for the difference between the behavior of Li and Na in this host system. (C) 2001 The Electrochemical Society.