Synthesis and structural characterisation of transition metal fluoride sulfates

Li5VF4(SO4)2: A Prototype High-Voltage Li-Ion Cathode

A Li-rich polyanionic compound based on V³⁺ with a previously unknown structure, Li₅VF₄(SO₄)₂, has been developed as a high-voltage cathode material for Li-ion batteries. The solvothermal preparation of this material, crystal structure solution, and initial electrochemical characterization are presented. An analysis based on density functional theory electronic structure calculations suggests that a high voltage close to 5 V is required to extract two Li ions and to reach the oxidation state of V⁵⁺. However, the use of conventional carbonate-based electrolytes, which exhibit increasing degradation above a potential of 4.3 V, does not permit the full capacity of this compound to be achieved at this time.

Two halide-containing cesium manganese vanadates: synthesis, characterization, and magnetic properties

Two new halide-containing cesium manganese vanadates have been synthesized by a high-temperature (580 °C) hydrothermal synthetic method from aqueous brine solutions.

Multiple Fluorine-Substituted Phosphate Germanium Fluorides and Their Thermal Stabilities

Anhydrous compounds are crucially important for many technological applications, such as achieving high performance in lithium/sodium cells, but are often challenging to synthesize under hydrothermal conditions. Herein we report that a modified solvo-/hydro-fluorothermal method with fluoride-rich and water-deficient condition is highly effective for synthesizing anhydrous compounds by the replacement of hydroxyl groups and water molecules with fluorine. Two anhydrous phosphate germanium fluorides, namely, Na₃[GeF₄(PO₄)] and K₄[Ge₂F₉(PO₄)], with chainlike structures involving multiple fluorine substitutions, were synthesized using the modified solvo-/hydro-fluorothermal method. The crystal structure of Na₃[GeF₄(PO₄)] is constructed by the common single chains _∞¹{[GeF₄(PO₄)]^3-} built from alternating GeO₂F₄ octahedra and PO₄ tetrahedra. For K₄[Ge₂F₉(PO₄)], it takes the same single chain in Na₃[GeF₄(PO₄)] as the backbone but has additional flanking GeOF₅ octahedra via an O-corner of the PO₄ groups, resulting in a dendrite zigzag single chain _∞¹{[Ge₂F₉(PO₄)]^4-}. The multiple fluorine substitutions in these compounds not only force them to adopt the low-dimensional structures because of the "tailor effect" but also improve their thermal stabilities. The thermal behavior of Na₃[GeF₄(PO₄)] was investigated by an in situ powder X-ray diffraction experiment from room temperature to 700 °C. The modified solvo-/hydro-fluorothermal method is also shown to be effective in producing the most germanium-rich compounds in the germanophosphate system.