Lithium Vanadium Polyanionic Composite Multielectron Intercalation Cathode Derived from Thermodynamically Unstable Li2VP2O7/Li2VP2O7F

LiVP2O7 is a well-explored cathode that supports reversible single electron extraction with a theoretical capacity of 115 mA h g–1. Hypothetical Li2VP2O7F or Li2VP2O7 compounds, with an additional Li and F in the structure or Li with vanadium in the lower oxidation state V2+, are expected to exchange two moles of lithium from the cathode. Li2VP2O7F synthesis is attempted through a sol–gel process to explore the possibility of such multielectron cathode material. Several syntheses with final annealing at 800 °C in the Ar or N2/H2 (95:5) resulted in compounds exhibiting identical X-ray diffraction (XRD) patterns. However, none of these compositions has any significant fluorine content in them. Surprisingly, sol–gel synthesis of the nominal composition, i.e., Li2VP2O7, without any F addition in the initial precursor also resulted in a similar XRD pattern. These XRD patterns are comparable to the ones reported in the literature for Li2VP2O7 composition. Interestingly, we find that neither the Li2VP2O7F nor Li2VP2O7 compound is thermodynamically stable. Rather, it forms roughly an equimolar structural mixture of three different phases, viz., Li9V3(P2O7)3(PO4)2, Li3V2(PO4)3, and LiVP2O7, as discerned from the detailed structural studies performed using X-ray diffraction, Rietveld refinement, and chemical composition analysis performed using proton-induced γ-ray emission and proton elastic backscattering spectrometry. The electrochemical studies, interestingly, exhibit two reversible redox processes with nearly flat potential and an average voltage of 3.86 and 1.72 V. A very high first cycle capacity of ∼340 mA h g–1 is reported for the first time. The capacity related to second Li fades upon cycling, providing a net capacity of ∼200 mA h g–1. The nominal composition of Li2VP2O7F synthesized in N2/H2 (95:5) gas flow exhibited the best electrochemical properties for a single lithium intercalation reaction.