Electrochemical insertion properties of the novel lithium vanadium fluorophosphate, LiVPO4F

The novel fluorophosphate compound, LiVPO4F, P(1) over bar, a = 5.173(8) Angstrom, b = 5.309(6) Angstrom, c = 7.250(3) Angstrom, alpha = 72.479(4)degrees, beta = 107.767(7)degrees, gamma 5 81.375(7)degrees, has been synthesized by a novel two-step reaction method based on a carbothermal reduction (CTR) process. In the initial CTR step, vanadium pentoxide, V2O5, ammonium dihydrogen phosphate, and a high surface area carbon are reacted under an inert atmosphere to yield the trivalent vanadium phosphate, VPO4. 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 V3+ as well as leaving residual carbon, which is useful in the subsequent electrode processing. In the second incorporation step, the CTR VPO4 is reacted with LiF in an argon atmosphere to yield the single phase LiVPO4F product. Preliminary electrochemical evaluation of the LiVPO4F carried out at 23degreesC indicates a reversible specific capacity of around 115 mAh/g, a performance roughly equivalent to cycling of x = 0.74 in Li1-xVPO4F. Elevated temperature testing suggests that the extraction process may yield the novel delithiated phase, VPO4F. 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 LiVPO4F are compared with the other vanadium-based polyanion materials, namely Li3V2(PO4)(3) and VOPO4. The demonstrated performance suggests that the LiVPO4F insertion system may offer some properties favorable for commercial application. (C) 2003 The Electrochemical Society.