Synthesis and Characterization of a Novel Hydrated Layered Vanadium(III) Phosphate Phase K(3)V(3)(PO(4))(4)·H(2)O: A Functional Cathode Material for Potassium-Ion Batteries

[Image: see text] Hydrated vanadium(III) phosphate, K(3)V(3)(PO(4))(4)·H(2)O, has been synthesized by a facile aqueous hydrothermal reaction. The crystal structure of the compound is determined using X-ray diffraction (XRD) analysis aided by density functional theory (DFT) computational investigation. The structure contains layers of corner-sharing VO(6) octahedra connected by corner and edge-sharing PO(4) tetrahedra with a hydrated K(+) ion interlayer. The unit cell is assigned to the orthorhombic system (space group Pnna) with a = 10.7161(4) Å, b = 20.8498(10) Å, and c = 6.5316(2) Å. Earlier studies of this material report a K(3)V(2)(PO(4))(3) stoichiometry with a NASICON structure (space group R3®c). Previously reported XRD and electrochemical data on K(3)V(2)(PO(4))(3) are critically evaluated and we suggest that they display mixed phase compositions of K(3)V(3)(PO(4))(4)·H(2)O and known electrochemically active phases KVP(2)O(7) and K(3)V(PO(4))(2). In the present study, the synthesis conditions, structural parameters, and electrochemical properties (vs K/K(+)) of K(3)V(3)(PO(4))(4)·H(2)O are clarified along with further physical characterization by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray fluorescence (XRF), Raman spectroscopy, Fourier transform infrared (FT-IR), and thermogravimetric analysis (TGA).