The effect of Co substitution for Ni on the structure and electrochemical behavior of T2 and O2 structure Li-2/3[COxNi1/3-xMn2/3]O-2

The effect of Co substitution for Ni on the structure and the electrochemic al 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-xM n2/3]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 L i, 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 or dering of the transition metals in the transition metal layers, which desta bilizes the T2 structure. The electrochemical data indicate that a small am ount of Co substitution allows the removal of more lithium from the lattice during the first charge process and improves the rate capability of the ma terial. However, preliminary results indicate that the capacity retention w ith 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 h igher voltage. (C) 2001 The Electrochemical Society. All rights reserved.