Lydie Bourgeois

Structural and electrochemical studies of a new Tavorite composition: LiVPO4OH

Polyanionic materials attract strong interest in the field of Li-ion battery research thanks to the wide range of compositions, structures and electrochemical properties they offer. Tavorite-type compositions offer a very rich crystal chemistry, among which LiVPO4F has the highest theoretical energy density (i.e. 655 W h kg−1). A new Tavorite-type LiVPO4OH composition was synthesized by a hydrothermal route from three different vanadium-containing precursors. The crystal structure of this new phase was fully determined thanks to synchrotron X-ray and neutron diffraction. 1H, 7Li, and 31P magic angle spinning nuclear magnetic resonance spectroscopy as well as diffuse reflectance infra-red spectroscopy were performed in order to support further the nature of the phases formed. Galvanostatic intermittent titration technique experiments in lithium batteries and ex situ X-ray diffraction analyses revealed that during oxidation the concomitant extraction of Li+ and H+ occurs at the same equilibrium potential (3.95 V vs. Li+/Li) and leads to the formation of the Tavorite phase VPO4O at the end of the charge. LiVPO4OH is also electrochemically active in the low voltage region, upon Li+ insertion. The reversible insertion/extraction of lithium at 1.35 V vs. Li+/Li leads to the formation of Li2VPO4OH at the end of discharge.

Vanadyl-type defects in Tavorite-like NaVPO4F: from the average long range structure to local environments

Tavorite-type compositions offer rich crystal chemistry for positive electrodes in rechargeable batteries, among which LiVIIIPO4F has the highest theoretical energy density (i.e. 655 Wh kg−1). In this article, we report for the first time the synthesis of the related Na-based phase crystallizing in the Tavorite-like structure. Its in-depth structural and electronic characterization was conducted by a combination of several techniques, spanning electron and X-ray powder diffraction as well as infrared and X-ray absorption spectroscopy. The magnetic susceptibility measurement reveals an average oxidation state for vanadium slightly higher than V3+. This slight oxidation is supported by infrared and X-ray absorption spectroscopies which highlight the presence of V4+[double bond, length as m-dash]O vanadyl-type defects leading to an approximated NaVIII0.85(VIVO)0.15(PO4)F0.85 composition. In this material, the profile of the diffraction lines is governed by a strong strain anisotropic broadening arising from the competitive formation between the ionic V3+–F and the covalent V4+[double bond, length as m-dash]O bonds. This material shows a limited extraction of sodium, close to 15% of the theoretical capacity. Indeed, its electrochemical properties are strongly inhibited by the intrinsic low sodium mobility in the Tavorite framework.