LiVPO₄F_1–yO_y Tavorite-Type Compositions: Influence of the Concentration of Vanadyl-Type Defects on the Structure and Electrochemical Performance

Abstract

Mixed-valence LiVPO4F1–y O y materials were obtained for the first time over a large composition range (here 0.35 ≤ y ≤ 0.75) through a single-step solid-state synthesis. Interestingly, the competition between the ionic character of the V3+–F bond and the strong covalency of the V4+O vanadyl bond originates complex crystal chemistry at the local scale, which allows stabilization of a solid solution between LiVPO4F and LiVPO4O despite a significant deviation from Vegard’s law for the cell parameters. A combined study using IR, Raman, and X-ray absorption spectroscopies highlights the effect of the vanadyl environment on the electronic structure of the vanadium orbitals and a fortiori the electrochemical behavior. Our results underline that the electrochemical performance of LiVPO4F1–y O y -type materials can be controlled by tuning the concentration of vanadyl-type defects, i.e., by playing on the competition between the ionic V3+–F bond and the covalent V4+O bond.

Stability in water and electrochemical properties of the Na3V2(PO4)2F3 – Na3(VO)2(PO4)2F solid solution

“…The forbidden 1s  3d quadrupolar transitions may become allowed thanks to 3d-p orbital hybridization and lead to contributions in the pre-edge region. Their position and intensity are linked to the electronic structure and local environment (coordination number and the nature of the surrounding ligands) of vanadium [25][26][27][28][29]. The more distorted the vanadium site, the more intense is the pre-edge signal, versus that observed for highly symmetric V(O,F)6 environment.…”

Local atomic and electronic structure in the LiVPO4(F,O) tavorite‐type materials from solid‐state NMR combined with DFT calculations

“…The mixed valence (i.e., V III /V IV ) LiVPO 4 F 1‐y O y phases were obtained through one‐step solid‐state syntheses as described in Boivin et al [ 8 ] from V 2 O 5 , LiF, and H 3 PO 4 precursors. Three mixed valence compositions were prepared: LiVPO 4 F 0.65 O 0.35 , LiVPO 4 F 0.45 O 0.55 , and LiVPO 4 F 0.25 O 0.75 .…”

“…The LiVPO 4 F and LiVPO 4 O structures are not described in the same model, due to the presence of vanadyl bonds (i.e., short V IV O bonds) along the chains in LiVPO 4 O that requires a unit cell twice larger than the one used for LiVPO 4 F (Figure 1). As described in Boivin et al [ 8 ] , all the intermediate compositions LiVPO 4 F 1‐y O y (y < 0.9) can be described considering a LiVPO 4 F‐type model. No superstructure peaks were observed suggesting thus a disordered sequence of V III and V IV (involved in vanadyl V IV O bond) along the chains.…”

“…[5] In a recent study, we synthesized several LiVPO 4 F 1-y O y phases with control of the substitution ratio of oxygen for fluorine in order to study the influence of vanadyl-type defects on the average structure as well as on the local environment around vanadium. [8] The concentration of vanadyl-type defects was shown to have a strong influence on the electrochemical behavior in terms of redox processes involved, energy density, cyclability, and rate capability. [8,9] A better understanding of the local structural arrangement is of prime importance for further material optimization.…”

“…[8] The concentration of vanadyl-type defects was shown to have a strong influence on the electrochemical behavior in terms of redox processes involved, energy density, cyclability, and rate capability. [8,9] A better understanding of the local structural arrangement is of prime importance for further material optimization.…”