Showing papers by "Dany Carlier published in 2023"
TL;DR: In this article , the authors used V K-edge X-ray absorption spectroscopy to unveil the redox mechanism of KVPO4F1-yOy (y = 0, 0.5, 1), performing data analysis via a chemometric approach.
Abstract: M-ion batteries (M = Li, Na, K ...) positive electrode materials most often operate through the reversible oxidation of transition-metal ions. In complex materials involving many transition metals or many redox centers, understanding the sequence in which they participate to the reaction is not trivial but is often necessary to explain the electrochemical properties. Mixed anion vanadium phosphates, such as KVPO4F0.5O0.5, are known to contain two different redox entities that are V3+O4F2 and V3+O5F “ionic” entities on the one hand and {V4+═O}O5 and {V4+═O}O4F “covalent” vanadyl-type units on the other hand. However, their participation to the redox mechanism occurring during the charge of this material has never been studied. Here, we use V K-edge X-ray absorption spectroscopy to unveil the redox mechanism of KVPO4F1–yOy (y = 0, 0.5, 1), performing data analysis via a chemometric approach. With XAS being very sensitive to the oxidation state and bond length, it was found that the ionic V3+–F units oxidize at a lower potential than the covalent {V4+═O} ones, which is surprising considering the high electronegativity of fluoride anions but is consistent with the redox potential observed for KVPO4F and KVOPO4. Further, ab initio calculations and ex situ X-ray diffraction analyses allowed an atomistic description of the redox mechanism with the sequential oxidation of the cis V site before the trans V site in KVPO4F upon charge. Finally, the complete atomically resolved redox mechanism of KVPO4F0.5O0.5 is proposed.
TL;DR: In this article , the thermal stability and decomposition mechanisms of carbon-coated Na3V2(PO4)2F3 and two fluorine-rich phases belonging to the solid-solution NaxV3+2-2-yV4+y(PO 4)-naxV2-PO4-yOy (y = 0, 0.07 and 0.12), that family of compounds being often considered among the most promising positive electrode materials for Na-ion batteries.
Abstract: The Na-ion battery technology appears as a reliable, sustainable, and environmentally friendly alternative to the Li-ion one, especially for stationary energy storage. As for the Li-ion technology, the safety aspect is of high importance to ensure large-scale development. In this work, we studied the thermal stability and decomposition mechanisms of carbon-coated Na3V2(PO4)2F3 and two fluorine-rich phases belonging to the solid-solution Na3V3+2–yV4+y(PO4)2F3–yOy (y = 0.07 and y = 0.12), that family of compounds being often considered among the most promising positive electrode materials for Na-ion batteries. This study shows the good thermal stability of these polyanionic materials and reveals that a low O2– for F– substitution has a very limited effect on the thermal stability of fully reintercalated materials recovered in the discharged state of the battery, whereas it has a beneficial impact for highly deintercalated ones, obtained by in-depth charges. Furthermore, whatever the state of charge and the oxygen content in NaxV2(PO4)2F3–yOy (1 ≤ x ≤ 3 and y = 0, 0.07 and 0.12), the thermal degradation leads, quite unexpectedly, to the formation of crystalline Na3V3+2(PO4)2F3 in addition to an amorphous phase. The fluorination of the partial oxygen for fluorine substituted material was clearly demonstrated by X-ray diffraction (XRD) and solid state nuclear magnetic resonance spectroscopy (NMR) on materials recovered after differential scanning calorimetry (DSC) analyses. The formation of a fully sodiated crystalline phase from the thermal degradation of the material obtained in charged states of the battery, with or without the presence of electrolyte, was never reported before.
TL;DR: In this article , the authors used synchrotron-based operando techniques to determine the origins of the rapid structural decay of Na4MnV(PO4)3 positive electrode materials when the third Na+ is extracted.
Abstract: As the importance of developing low-cost and high-capacity materials is emerging, Mn-based Na4MnV(PO4)3 positive electrode materials that allow multiredox reactions are in the spotlight for Na-ion batteries. The structure that gives highly reversible electrochemical reactions, when two Na+ (out of four) are de-inserted, deteriorates rapidly when the third Na+ is extracted at high voltage, resulting in poor cyclability. In this work, using synchrotron-based operando techniques, we perform long-range and local structural analyses to determine the origins of the rapid structural decay of Na4MnV(PO4)3 when the third Na+ is extracted. Operando XRD shows a significant change in the crystal structure (c parameter increases rapidly) as the occupancy of the Na (1) site decreases at high voltage. The local environments of Mn and V, monitored by operando XAS, remain rather symmetrical up to the extraction of two Na+, while both Mn and V show drastic local distortions when the third Na+ is extracted. These structural degradations are found to further progress when cycling to high voltage. This study presents important aspects of how local and long-range structure modifications can affect the electrochemical performance in multiredox NASICON materials.