Understanding the Anomalous Capacity of Li/Li[Ni[sub x]Li[sub (1/3−2x/3)]Mn[sub (2/3−x/3)]]O[sub 2] Cells Using In Situ X-Ray Diffraction and Electrochemical Studies

The electrochemical behavior of cells for 1/4, 1/3, 5/12, and 1/2 is reported. is derived from or by substitution of and by while maintaining all the remaining Mn atoms in the oxidation state. Conventional wisdom suggests that lithium can be removed from these materials only until both the Ni and Mn oxidation states reach giving a charge capacity of We show that cells give smooth reversible voltage profiles reaching about 4.45 V when Li atoms per formula unit are removed, as expected. If the cells are charged to higher voltages, surprisingly, they exhibit a long plateau of length approximately equal to in the range between 4.5 and 4.7 V. Subsequent to this plateau, the materials can reversibly cycle over 225 mAh/g (almost one Li atom per formula unit) between 2.0 and 4.8 V. In situ X-ray diffraction and differential capacity measurements are used to infer that irreversible loss of oxygen from the compounds with occurs during the first charge to 4.8 V. This results in oxygen deficient layered materials with stoichiometry approximately equal to at 4.8 V. These oxygen deficient materials then react reversibly with lithium. © 2002 The Electrochemical Society. All rights reserved.