A Comparative Insight of Potassium Vanadates as Positive Electrode Materials for Li Batteries: Influence of the Long-Range and Local Structure

Potassium vanadates with ratio K/V = 1:3, 1:4, and 1:8, prepared by a fast and facile synthesis route, were investigated as positive electrode materials in lithium batteries. KV₃O₈ and K_0.5V₂O₅ have layered structures, while K_0.25V₂O₅ exhibits a tunnel framework isomorphic to that of β-Na_0.33V₂O₅. The Raman spectra of KV₃O₈, K_0.5V₂O₅, and K_0.25V₂O₅ compounds are reported here for the first time, and a detailed comparative analysis distinguishes spectral patterns specific to each structural arrangement. The electrochemical performances of these potassium vanadates toward lithium insertion were investigated. The potassium-richer compound KV₃O₈ shows a good rechargeability in spite of a low discharge capacity of 70 mAh g^–1, while the potassium-poorer bronze K_0.25V₂O₅ exhibits the highest specific capacity of 230 mAh g^–1 but a slow and continuous capacity fade with cycling. We demonstrate that the K_0.5V₂O₅ compound, with its double-sheet V₂O₅ layered framework characterized by a large interlayer spacing of 7.7 Å, is the best candidate as positive electrode for lithium battery among the potassium–vanadium bronzes and oxides. A remarkable specific capacity of 210 mAh g^–1, combined with excellent capacity retention, is achieved

Unraveling the Structure–Raman Spectra Relationships in V₂O₅ Polymorphs via a Comprehensive Experimental and DFT Study

Vanadium pentoxide polymorphs (α-, β-, γ′-, and ε′-V₂O₅) have been studied using the Raman spectroscopy and quantum-chemical calculations based on density functional theory. All crystal structures have been optimized by minimizing the total energy with respect to the lattice parameters and the positions of atoms in the unit cell. The structural optimization has been followed by the analysis of the phonon states in the Γ-point of the Brillouin zone, and the analysis has been completed by the computation of the Raman scattering intensities of the vibrational modes of the structures. The optimized structural characteristics compare well with the experimental data, and the calculated Raman spectra match the experimental ones remarkably well. With the good agreement between the spectra, a reliable assignment of the observed Raman peaks to the vibrations of specific structurals units in the V₂O₅ lattices is proposed. The obtained results support the viewpoint on the layered structure of vanadium pentoxide polymorphs as an ensemble of V₂O₅ chains held together by weaker interchain and interlayer interactions. Similarities and distinctions in the Raman spectra of the polymorphs have been highlighted, and the analysis of the experimental and computational data allows us, for the first time, to put forward spectrum–structure correlations for the four V₂O₅ structures. These findings are of the utmost importance for an efficient use of Raman spectroscopy to probe the changes at the atomic scale in the V₂O₅-based materials under electrochemical operation