Layered monodiphosphate Li9V3(P2O7)3(PO4)2: A novel cathode material for lithium-ion batteries

Single phase Li₉V₃(P₂O₇)₃(PO₄)₂ is synthesized at 750 °C via solid-state reaction method for the first time. The Rietveld refinement results show that the trigonal system (space group: ) with the lattice parameters a = 0.9724 nm, c = 1.3596 nm are obtained. Its intrinsic electrical conductivity of 1.43 × 10⁻⁸ S cm⁻¹ is higher than that of LiFePO₄ and as the same order of Li₃V₂(PO₃)₄. The electrochemical measurement results show that there are two plateaus (3.77 V and 4.51 V) and three plateaus (3.77 V, 4.51 V and 4.75 V) in the potential ranges of 2.0–4.6 V and 2.0–4.8 V, respectively. In the range of 2.0–4.6 V, two discharge plateaus (4.46 V and 3.74 V) can be observed and 110 mAh g⁻¹ of discharge capacity is achieved. The Rietveld refinement result of the X-ray diffraction (XRD) data at the end of discharge after the first cycle suggests that the structural reversibility can be retained during electrochemical reactions in Li₉V₃(P₂O₇)₃(PO₄)_2. In the range of 2.0–4.8 V, almost six lithium ions are extracted and the trigonal structure is still recovered after 30 cycles. Therefore, this novel layered vanadium monodiphosphate offers a promising candidate as cathode material for lithium-ion batteries.