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

Abstract

Hydrated vanadium­(III) phosphate, K3V3(PO4)4·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) Å, b = 20.8498(10) Å, and c = 6.5316(2) Å. Earlier studies of this material report a K3V2(PO4)3 stoichiometry with a NASICON structure (space group R3̅c). 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·H2O 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·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).

Validating the Electronic Structure of Vanadium Phosphate Cathode Materials

“…04-015-8358) to their respective PDF 4+ database structures, displaying R wp values of 1.68 and 3.00 with goodness of fit (GoF) values of 1.46 and 2.76, respectively, indicating good agreement with database reference structures and the literature. , The slightly higher GoF for NVP is ascribed to a mismatch of peak intensities due to the preferred orientation of crystallites and grain size effects from the powder sample. The KVP sample was matched to our previously reported powder refinement structure for K 3 V 3 (PO 4 ) 4 ;H 2 O (SG: Pnna ) and showed good agreement with an R wp of 8.52 and a GoF of 1.77. SEM micrographs for LVP, NVP, and KVP are shown in the insets of Figure d–f.…”

“…An expanded view of the 800–1200 cm –1 frequency region is presented for AVPs in Figure S2b. LVP and NVP spectra consist of several weak bands with broad features, consistent with literature spectra. , KVP Raman bands appear sharper than other AVPs and are also consistent with previously reported Raman spectra . No clear evidence of PO 4 distortion/antisymmetric stretching modes is discernible for any of these AVP samples.…”

“…Relatively even intensities between PO 4 and P–O–P peaks in NVP suggest good long-range ordering of PO 4 tetrahedra consistent with the crystal structure, which shows a high proportion of ordered corner-sharing PO 4 tetrahedra. In addition, the reduced intensity of the P–O–P peak compared to the primary peak for KVP also matches well with the crystal structure, which shows a lower proportion of corner-sharing of PO 4 tetrahedra (see Figure c). The P–O–P secondary peak is notably absent in P K-edge TEY spectra for LVP, suggesting that the surface does not possess a high proportion of P–O–P bridging beyond the second P coordination sphere.…”

“…A series of aqueous hydrothermal syntheses were undertaken to optimize the yield and quality of final products. The methods follow that of our previous study, but with modifications to hydrothermal conditions and final calcination time and temperature to successfully crystallize the target phase. Analytical-grade Li 2 CO 3 , Na 2 CO 3 , K 2 CO 3 , V 2 O 5 , C 2 H 2 O 4 ·2H 2 O, and NH 4 H 2 PO 4 obtained from Sigma-Aldrich were used for all experiments.…”

Computational and experimental search for potential polyanionic K-ion cathode materials

“…of the hydrated phase also fits well with a very recent report of orthorhombic K 3 V 3 (PO 4 ) 4 -H 2 O. 54 Similarly, all the K 3 V 3−x Cr x (PO 4 ) 4 (x= 0, 1, 2, 3) compounds exhibit such hydration behavior, as evidenced by the peak change at approximately 30° after 5-h air exposure (Figure 9d). The moisture sensitive nature of K 3 V 3−x Cr x (PO 4 ) 4 (x= 0, 1, 2, 3) requires an inert storage environment of these materials, which might lead to difficulty in practical applications.…”