Synthesis and Characterization of Silver Vanadium Oxide Fluorides

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The utilization of hydrofluoric acid for the preparation of silver vanadium oxide fluorides has been advanced for the synthesis of Ag<sub>4</sub>V<sub>2</sub>O<sub>6</sub>F<sub>2</sub> and Ag<sub>3</sub>VO<sub>2</sub>F<sub>4</sub>. Silver-rich metal oxide fluorides are important battery cathode materials due to the high reduction potential of silver vs. Li, which is made even higher by the incorporation of the more electronegative fluoride. Methods were invented and developed to synthesize these phases under hydrothermal conditions and at room temperature (a first for any cathode material for primary Li batteries), to grow single crystals, and to optimize electrochemical activity. The Ag<sub>2</sub>O-V<sub>2</sub>O<sub>5</sub> system in hydrofluoric acid was explored under low temperature (T ≤ 200 ºC) and pressure (P < 20 atm) hydrothermal conditions. The well-known silver vanadates α-Ag<sub>3</sub>VO<sub>4</sub>, Ag<sub>4</sub>V<sub>2</sub>O<sub>7</sub>, β-AgVO<sub>3</sub>, and Ag<sub>2</sub>V<sub>4</sub>O<sub>11</sub> were synthesized as well as the recently discovered Ag<sub>4</sub>V<sub>2</sub>O<sub>6</sub>F<sub>2</sub>. Owing to the high temperature solution synthesis, single crystals of α-Ag<sub>3</sub>VO<sub>4</sub> were collected and studied by X-ray diffraction to solve the crystal structure. In α-Ag<sub>3</sub>VO<sub>4</sub>, silver and vanadium have a close-packed arrangement with oxide in tetrahedral positions. While vanadium is also tetrahedrally coordinated, silver adopts see-saw and square-planar orientations. In addition, a novel reaction scheme under hydrothermal conditions led to the discovery of a new silver vanadium oxide fluoride, Ag<sub>3</sub>VO<sub>2</sub>F<sub>4</sub>. The crystal structure was determined to be cryolite-type with corner sharing Ag<sup>+</sup>- and V<sup>5+</sup> -centered MO<sub>2</sub>F<sub>4</sub><sup>n-</sup> octahedra and silver in the 10-coordinate A-site. Ag<sub>3</sub>VO<sub>2</sub>F<sub>4</sub> has a negative temperature coefficient of solubility; therefore, growth of single crystals was achieved by slowly heating the completely dissolved reaction solution from room temperature to 100 ºC. Room temperature synthesis of silver vanadium oxide fluorides arose from hydrolysis studies in the hydrothermal system, literature-inspired solution synthesis of silver vanadates, and a desire to reduce particle size of the electrode material Ag<sub>4</sub>V<sub>2</sub>O<sub>6</sub>F<sub>2</sub>. Ag<sub>2</sub>O and V<sub>2</sub>O<sub>5</sub> combined in 7% (by wt.) hydrofluoric acid solution at room temperature yield Ag<sub>4</sub>V<sub>2</sub>O<sub>6</sub>F<sub>2</sub> particles that are an order of magnitude smaller than manually ground particles from hydrothermal conditions. As a cathode material, the smaller particles exhibited less polarization due to the reduction of silver and therefore provide a higher potential over the course of reaction with the first four equivalents of lithium. This solution synthesis could be further adapted to yield smaller particle morphology via templating.

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  • 09/20/2018
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