Lithium insertion in the two crystallographic forms of the binary phase Mo(15)Se(19).

Compounds which can undergo topotactic insertion of lithium are of potential technological importance in secondary lithium batteries. We recently discovered such a new electrode material, Mo(6)Se(6) (molybdenum selenide), which exhibits the desirable structural and electrochemical properties useful for secondary batteries. In our continuing study of the chemistry of such materials and our search for new cathode materials, we investigated the recently synthesized (Mo(15)Se(19) phase which we have shown to be structurally related to Mo(6)Se(6). The crystal structure results for this material, as well as its ability to reversibly insert lithium, are the subject of this manuscript. We showed that Mo(15)Se(19) can reversibly insert 8 lithiums at an average potential of 2 volts. The Li/Mo(15)Se(19) cells maintain their capacity over several cycles. However, because of the large molecular weight, the theoretical energy density of Li/Mo(15)Se(19) cells is only one third that of Li/Mo(6)Se (6) cells. We also show that the evolution of changes in electrode material crystal structure with the state of charge using a recently designed in situ x-ray cell.