Electrochemical Insertion Properties of the Novel Lithium Vanadium Fluorophosphate, LiVPO4 F

The novel fluorophosphate compound, LiVPO 4 F, PI, a = 5.173(8) A, b = 5.309(6) A, c = 7.250(3) A, a = 72.479(4)°, β = 107.767(7)°, γ = 81.375(7)°, has been synthesized by a novel two-step reaction method based on a carbothermal reduction (CTR) 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 3+ 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 = 0.74 in Li 1-x 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.

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