The utilization of hydrofluoric acid for the preparation of silver vanadium oxide fluorides has been advanced for the synthesis of Ag₄V₂O₆F₂ and Ag₃VO₂F₄. 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 Ag₂O-V₂O₅ 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-Ag₃VO₄, Ag₄V₂O₇, β-AgVO₃, and Ag₂V₄O₁₁ were synthesized as well as the recently discovered Ag₄V₂O₆F₂. Owing to the high temperature solution synthesis, single crystals of α-Ag₃VO₄ were collected and studied by X-ray diffraction to solve the crystal structure. In α-Ag₃VO₄, 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, Ag₃VO₂F₄. The crystal structure was determined to be cryolite-type with corner sharing Ag⁺- and V⁵⁺ -centered MO₂F₄^n- octahedra and silver in the 10-coordinate A-site. Ag₃VO₂F₄ 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 Ag₄V₂O₆F₂. Ag₂O and V₂O₅ combined in 7% (by wt.) hydrofluoric acid solution at room temperature yield Ag₄V₂O₆F₂ 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.

Last modified

• 09/20/2018

Creator

• Thomas Allen Albrecht

DOI

• https://doi.org/10.21985/N2JM93

Subject

• Chemistry

Keyword

• Chemistry

Date created

• 2008-12-03

Resource type

• Dissertation

Rights statement

• In Copyright