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The electroactive vanadium fluorophosphate compounds, and have been successfully synthesized using a range of preparative strategies, including solid state (ceramic), hydrothermal, and ion exchange methods. In all preparative approaches a carbothermally synthesized vanadium(III) phosphate, was used as a suitable intermediate phase. The is found to crystallize with a tetragonal symmetry (space group and is structurally related to the previously reported sodium aluminum fluorophosphate phase, Low rate testing of the solid state and hydrothermally prepared samples reveals a reversible specific capacity in the range 97-110 mAh/g, and this performance is coupled to exceptional insertion stability at both 23 and 60°C. The voltage profile response during extraction/insertion for the system is consistent with the alkali ions occupying two energetically nonequivalent crystallographic sites within the fluorophosphate framework. The phase crystallizes with a triclinic structure (space group and is found to be isostructural with the known mineral, Tavorite, High-resolution electrochemical measurements reveal a structured voltage response for the lithium extraction process consistent with the lithium ions occupying two crystallographic positions. For the solid-state synthesized sample, the discharge process suggests a two-phase insertion mechanism coupled to phase nucleation behavior. Elevated temperature testing indicates that the lithium extraction process may yield the novel delithiated phase, The reversible specific capacities for the solid-state and ion-exchanged materials are found to approach 120 mAh/g and when coupled to an average discharge voltage in the range 4.10-4.20 V vs. Li, we believe this insertion system may offer some favorable characteristics for commercial application. © 2004 The Electrochemical Society. All rights reserved.
Barker et al. (Thu,) studied this question.