M2Mn3O8 (M=Ca2+, Cu2+) compounds were synthesized and characterized in lithium cells. The M2+ cations, which reside in the van der Waals gaps between adjacent sheets of Mn3O84- may be replaced chemically (by ion-exchange) or electrochemically with Li. More than 7 Li+/Cu2Mn3O8 may be inserted electrochemically, with concomitant reduction of Cu2+ to Cu metal, but less Li can be inserted into Ca2Mn3O8. In the case of Cu2+, this process is partially reversible when the cell is charged above 3.5 V vs. Li, but intercalation of Cu+ rather than Cu2+ and Li+/Cu+ exchange occurs during the subsequent discharge. If the cell potential is kept below 3AV, the Li in excess of 4 Li+/Cu2Mn3O8 can be cycled reversibly. The unusual mobility of +2 cations in a layered structure has important implications both for the design of cathodes for Li batteries and for new systems that could be based on M2+ intercalation compounds. (c) 2006 Elsevier B.V. All rights reserved.