The effect of Co substitution for Ni on the structure and electrochemical behavior of T2 and O2 structure Li2/3[COxNi1/3-xMn2/3]O2

The effect of Co substitution for Ni on the structure and the electrochemical behavior of Li-2/3[CoxNi1/3-xMn2/3O2 has been investigated. It is found that Co can substitute for Ni to form the solid solution Na-2/3[CoxNi1/3-xMn2/3]O-2 (0 less than or equal to x less than or equal to 1/3) which adopts the P2 structure. When x less than or equal to 1/24, Li-2/3[CoxNi1/3-xMn2/3]O-2 prepared by ion-exchanging the Na in Na-2/3[CoxNi1/3-xMn2/3]O-2 for Li, adopts the T2 structure. However, when x greater than or equal to 1/12, Li-2/3[CoxNi1/3-xMn2/3]O-2 exists in an O2 structure with stacking faults. This occurs because the Co additions gradually suppress the superlattice ordering of the transition metals in the transition metal layers, which destabilizes the T2 structure. The electrochemical data indicate that a small amount of Co substitution allows the removal of more lithium from the lattice during the first charge process and improves the rate capability of the material. However, preliminary results indicate that the capacity retention with cycle number becomes worse as Co is added. As x in Li-2/3[CoxNi1/3-xMn2/3]O-2 increases, the deintercalation potential moves toward increasingly higher voltage. (C) 2001 The Electrochemical Society. All rights reserved.