Synthesis And Electrochemical Properties Of LiFe_1-xV_xPO₄F?0?x?1?

The structure of fluorophosphate cathode materials based on the polyanion[PO₄F]can remain stable upon repeated Li⁺-extraction/insertion.Additionally,redox energies of cations can be tuned through the inductive effect introduced by cation substitution.LiFePO₄F and LiVPO₄F are homotypic with tavorite structre and have a working platform of².8/⁴.2 V,respectively.Cathode materials with high voltage is one of the key determinants to enhance energy densities for Li-ion batteries.Tavorite triclinic-structured LiFe_1–xV_xPO₄F?x=0,0.1,0.3,0.5,0.7,0.9&1?solid-solution powders were prepared from hydrothermal-synthesized and commercial raw materials.The samples have been systematically investigated by X-ray powder diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,scanning electron microscopy?SEM?/energy dispersive spectrometer?EDS?,charge-discharge,cyclic voltammetry?CV?measurement and galvanostatic intermittent titration technique?GITT?.Results confirm the formation of LiFe_1–xV_xPO₄F solid solution.The broad shape of sloping charge/discharge profiles,cyclic voltammetry?CV?peaks and sloping open-circuit voltage?OCV?profiles were observed in LiFe_1–xV_xPO₄F solid-solution cathodes while 0<x<1.This imply a single-phase behavior which is different from the two-phase behavior in the end-members?x=0 or 1?.Occurrence of the Li_0.67VPO₄F intermediate phase at⁴.3 V made the the end member of LiVPO₄F undergo a complex electrochemical oxidation reaction.We investigated for the first time the formattion of intermediate phases and the electrochemical mechanism in the LiFe_1–xV_xPO₄F?0<x<1?solid solution.Cyclic voltammetry curves reveal a systematic shift in the redox potential of Fe^2+/3+couple and,surprisingly,no shift for V^3+/4+couple in the LiFe_1–xV_xPO₄F solid solution compared to their pristine end members.The electronegativity of M(M=Fe_1–xV_x)plays a dominant role compared to the bond length of M–O₄F₂ for the redox potential of Fe^2+/3+couple.As for the V^3+/4+couple,electronegativity and the bond length of M(M=Fe_1–xV_x)play a collectively dominant role and thereby no shift in the redox potential of Fe^2+/3+couple.The obtained Li-ion diffusion coffients of LiVPO₄F by CV measurement are the same order of magnitude for that of LiFePO₄F,and the diffusion coffients of LiFe_0.3V_0.7PO₄F and LiFe_0.5V_0.5PO₄F are lower than that of LiVPO₄F and LiFePO₄F.The reason is that 1–x Li,uniformly distributing around Fe,in LiFe1–xVxPO4F solid-solution can not be extacted and thereby hinders Li⁺extaction/insertion.The obtained Li-ion diffusion coefficients of LiVPO₄F by GITT measurement are 1–2 orders of magnitude than that of LiFePO₄F,and the diffusion coefficients of LiFe_0.3V_0.7PO₄F and LiFe_0.5V_0.5PO₄F are lower than that of LiVPO₄F and higher than that of LiFePO4F.