Electrochemical Lithium Insertion in two Polymorphs of a Reduced Molybdenum Oxide (gamma and gamma’ Mo4O11)
A study of the electrochemical lithium insertion in two polymorphs of a reduced molybdenum oxide, Mo4O11, is presented in this work. When used as active materials in cells discharged down to 1 V vs. Li+/Li, both forms, the orthorhombic gamma-Mo4O11 and the monoclinic gamma'-Mo4O11, incorporated a similar number of lithium atoms per metal atom (Li/Mo = 2.12 and 2.25, respectively). Step potential electrochemical spectroscopy experiments proved that the insertion reaction proceeds in both cases through different mechanisms. In situ X-ray diffraction studies showed an almost complete loss of crystallinity of both compounds after the first discharge, leading to amorphous materials with different electrochemical behaviour on cycling. When discharged to low potentials (0.5 V vs. Li+/Li), the gamma'-Mo4O11 polymorph showed very good cycling behaviour for at least five lithium atoms per formula unit, corresponding to a specific capacity of 230 Ah/kg after seven complete charge-discharge cycles.