The Hydrothermal Synthesis and Electrochemistry of Oxyfluorides and Fluoro-bronzes for Lithium and Multivalent Battery Cathodes.

摘要

The next major breakthrough in multivalent batteries will come in the form of a cathode. To that end, the low temperature, phase-pure synthesis of both cubic and orthorhombic molybdenum fluoro-bronzes is presented. A study of temperature and fluoride concentration on reaction products proves to be critical, enabling control over formation of product phases. The orthorhombic fluoro-bronze appears as a solid solution with the formula MoO3-xF x and plausible end members (0.20≤x≤0.25). Microscopy of the cubic and orthorhombic molybdenum fluoro-bronzes reveal that the size and morphology of the products can largely be predicted from physical characteristics of alpha-MoO 3 reagent, suggesting the low temperature reaction may not follow a "traditional" hydrothermal mechanism. The cubic WO3-xF x can also be made at low temperatures; however, the low solubility of WO3 at these temperatures both enables and requires a different approach.;Cathodes composed of orthorhombic molybdenum fluoro-bronze are found to reversibly intercalate magnesium. Fluoro-bronze cathodes have a higher capacity than isostructural alpha-MoO3 by an order of magnitude. Structural, spectroscopic, and modeling techniques indicate that the fluoro-bronze can accommodate the Mg2+ ion, without co-intercalation of anions, solvent imbibition, or the formation of conversion products such as MgO or MgF2. Fluorine's preference for bridging anion sites may account for the absence of fluoride abstraction.;In lithium cells, electrochemical techniques indicate that cubic molybdenum fluoro-bronze discharges via a different mechanism than the orthorhombic fluoro-bronze. In addition, cubic fluoro-bronze has an irreversible two-phase plateau on its first discharge only, suggesting it undergoes a distinct break-in step compared to the orthorhombic fluoro-bronze. Both molybdenum fluoro-bronzes prove to have similar measured energy densities, approximately 350 Wh/Kg, though with distinct battery properties.;In experiments on the electrochemistry of organic materials, a new phenomenon in contact electrification is described wherein insulators charge with one polarity before becoming uncharged then finally charging with the opposite polarity. The likelihood of contact charging insulators to undergo polarity reversal is linked to the difference in the materials' Young's moduli. X-ray photoelectron spectroscopy, atomic force microscopy, and Kelvin force microscopy surface studies reveal that exchanged materials act as charge carriers in these systems.