The novel fluorophosphate compound, LiVPO 4 F, P1 ¯, a ϭ 5.173(8) Å, b ϭ 5.309(6) Å, c ϭ 7.250(3) Å, ␣ ϭ 72.479(4)°,  ϭ 107.767(7)°, ␥ ϭ 81.375(7)°, has been synthesized by a novel two-step reaction method based on a carbothermal reduction ͑CTR͒ process. In the initial CTR step, vanadium pentoxide, V 2 O 5 , ammonium dihydrogen phosphate, and a high surface area carbon are reacted under an inert atmosphere to yield the trivalent vanadium phosphate, VPO 4 . The transition-metal reduction is facilitated by the high temperature carbothermal reaction based on the C → CO transition. These CTR conditions favor stabilization of the vanadium as V 3ϩ as well as leaving residual carbon, which is useful in the subsequent electrode processing. In the second incorporation step, the CTR VPO 4 is reacted with LiF in an argon atmosphere to yield the single phase LiVPO 4 F product. Preliminary electrochemical evaluation of the LiVPO 4 F carried out at 23°C indicates a reversible specific capacity of around 115 mAh/g, a performance roughly equivalent to cycling of x ϭ 0.74 in Li 1Ϫx VPO 4 F. Elevated temperature testing suggests that the extraction process may yield the novel delithiated phase, VPO 4 F. High resolution measurements reveal a structured voltage response for the lithium extraction process characterized by two well-defined peaks in the differential capacity data. The corresponding discharge process, centered at around 4.19 V vs. Li, indicates a two-phase reaction mechanism coupled to phase nucleation behavior. The insertion properties of the LiVPO 4 F are compared with the other vanadium-based polyanion materials, namely Li 3 V 2 (PO 4 ) 3 and VOPO 4 . The demonstrated performance suggests that the LiVPO 4 F insertion system may offer some properties favorable for commercial application.
A novel preparative method based on a carbothermal reduction ͑CTR͒ process is described for the synthesis of the representative electroactive materials, ␥-LiV 2 O 5 and Li 3 V 2 (PO 4 ) 3 . In the CTR procedure a high surface area carbon is mixed intimately with appropriate precursor compounds and the mixture heated in an inert atmosphere. Use is then made of the two carbon oxidation reactions, namely, C → CO 2 and C → CO which facilitate controlled transition metal reduction while also allowing lithium ion incorporation. Electrochemical performance evaluation indicates that the CTR ␥-LiV 2 O 5 is capable of cycling at a material utilization of 130 mAh/g, a figure that compares favorably with the theoretical specific capacity of 142 mAh/g. The insertion behavior of the lithium vanadium phosphate shows a specific capacity equivalent to the reversible cycling of two lithium ions per Li 3 V 2 (PO 4 ) 3 formula unit. In summary, we believe the CTR method to be an energy-efficient, economical, and convenient process to produce a wide range of electroactive compounds. It appears ideally suited to preparation of active materials for use in lithium ion applications.
This paper presents a combined computational and experimental study of the structural and electrochemical properties of monoclinic and rhombohedral Li x M 2 (PO 4 ) 3 (with a focus on M ) V). The preferred sites for dilute Li occupation and stable Li ordered phases are identified. Features of the voltage curve are understood as emerging from site energetics, Li ordering, and redox couples. These features are found to be largely independent of alloying and a simple additive model is proposed to analyze the voltage curve for any cation substitution in the monoclinic structure. The model is shown to be very useful for understanding experimental results for a number of substituted compounds. Voltages for most important cations are calculated from first principles and can be combined with the simple model to predict voltage curves for new alloyed monoclinic systems.
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