Synthesis and Characterization of a Novel Hydrated Layered Vanadium(III) Phosphate Phase K3V3(PO4)4•H2O: A Functional Cathode Material for Potassium-Ion Batteries

Hydrated vanadium(III) phosphate, K3V3(PO4)(4)center dot H2O, 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 VO6 octahedra connected by corner and edge-sharing PO4 tetrahedra with a hydrated K+ ion interlayer. The unit cell is assigned to the orthorhombic system (space group Pnna) with a = 10.7161(4) angstrom, b = 20.8498(10) angstrom, and c = 6.5316(2) angstrom. Earlier studies of this material report a K3V2(PO4)(3) stoichiometry with a NASICON structure (space group R (3) over barc). Previously reported XRD and electrochemical data on K3V2 (PO4)(3) are critically evaluated and we suggest that they display mixed phase compositions of K3V3(PO4)(4 )+120 and known electrochemically active phases KVP2O7 and K3V(PO4)(2). In the present study, the synthesis conditions, structural parameters, and electrochemical properties (vs K/K+) of K3V3(PO4)4 center dot H2O 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).