A Comparative Investigation of the Li Insertion Properties of the Novel Fluorophosphate Phases, NaVPO[sub 4]F and LiVPO[sub 4]F

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Abstract
The electroactive vanadium fluorophosphate compounds, NaVPO4FNaVPO4F and LiVPO4F,LiVPO4F, 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, VPO4,VPO4, was used as a suitable intermediate phase. The NaVPO4FNaVPO4F is found to crystallize with a tetragonal symmetry (space group I4/mmm)I4/mmm) and is structurally related to the previously reported sodium aluminum fluorophosphate phase, Na3Al2(PO4)3F2.Na3Al2(PO4)3F2. Low rate testing of the solid state and hydrothermally prepared NaVPO4FNaVPO4F 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 NaVPO4FNaVPO4F system is consistent with the alkali ions occupying two energetically nonequivalent crystallographic sites within the fluorophosphate framework. The LiVPO4FLiVPO4F phase crystallizes with a triclinic structure (space group P1¯),P1¯), and is found to be isostructural with the known mineral, Tavorite, LiFePO4⋅OH.LiFePO4⋅OH. 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 LiVPO4FLiVPO4F 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, VPO4F.VPO4F. The reversible specific capacities for the solid-state and ion-exchanged LiVPO4FLiVPO4F 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.