A comparative study of lithium and sodium insertion into HfMo2O8 and two polymorphs of ZrMo2O8

Lithium and sodium insertion into RMo2O8 (R = Zr,Hf) has been studied by galvanostatic chronopotentiometry, cyclic voltammetry (CV) and quantitative XRD phase analysis as well as by flame photometry after treatment with n-butyllithium or sodium naphtalide solutions. Low-temperature modification of ZrMo2O8 with monoclinic framework structure accommodates two lithiums, in agreement with the topological analysis based on the Voronoi tessellation, at a constant open-circuit voltage of 2.4 V and discharge voltage of ca. 2.1 V. According to CV, the process is partially reversible, but lithium extraction is kinetically hindered, presumably due to the low electronic conductivity of the oxidized phase. Further lithium insertion results in complete amorphization at 4Li per formula unit. This value is limiting also for the isostructural trigonal layered phases, HfMo2O8 and high-temperature ZrMo2O8; in those cases, however, amorphization starts from the beginning of reduction. Sodium insertion (both electrochemical and chemical) in the three phases is sterically hindered and could only be detected by CV.