New Amorphous Mixed Transition Metal Oxides and Their Li Derivatives:  Synthesis, Characterization, and Electrochemical Behavior

New amorphous MnV2O6+ δ·nH2O (0 < δ < 1) compounds have been prepared by a soft chemistry route including the precipitation of a crystallized precursor, MnV2O6·4H2O, and its ozonation at temperatures below 100 °C. A combination of techniques (TGA, magnetic measurements, IR, XANES, EXAFS, XPS, and EELS) was used for their characterization. At low voltage, very large amounts of Li (e.g., Li12MnV2O6.96) are stored into anhydrous MnV2O6+ δ compounds. After the first Li uptake, subsequent charge/discharge cycles correspond to fully reversible Li uptake between the two compositions: LiαMnV2O6+ δ (α ≥ 2) and Li12MnV2O6+ δ (from 600 to 900 mAh/g). The Li derivatives were characterized at various steps of the discharge/charge process. During the first discharge the VO n polyhedron changes from VO5 to VO6, and reversibly from VO6 to VO4 during subsequent cycles. Meanwhile, the V oxidation state varies reversibly between +3 and +5, respectively. Upon the first discharge Mn4+ cations are reduced to Mn2+. During the subsequent cycles the average Mn oxidation state varies reversibly between +2 and ∼+2.6. The electron transfer from Li to the host matrix is shown to decrease with increasing Li uptake. It is shown that the same soft chemistry route enables the preparation of amorphous CoV2O6+ δ·nH2O (0 < δ < 1) compounds which exhibit similar storage properties.