Redox Paradox of Vanadium in Tavorite LiVPO4F1–yOy

Abstract

Vanadyl-type defects in vanadium oxy-fluoride phosphates confer interesting properties to these materials as positive electrodes in Li-ion or Na-ion batteries. The influence of defect concentration on the phase diagram and redox mechanisms in LiVPO4F1–yOy tavorite phases has been investigated by combining operando synchrotron X-ray powder diffraction (SXRPD) and X-ray absorption spectroscopy (XAS). Operando X-ray absorption near edge structure (XANES) reveals the activation of the V4+═O/V5+═O redox couple during the first step of the charge, and then upon further Li+ extraction the activation of the V3+/V4+ redox couple in fluorine-rich environments. This result, although surprising, is in full agreement with the operating voltages of the end-member phases, as LixVIII,IVPO4F operates at a higher voltage than LixVIV,VPO4O. The unexpected succession of these redox couples is observed for the first time in the same material. The small volume changes undergone by the materials upon electrochemical operation were proposed to be at the origin of the better performance observed for the mixed valence samples: “freezing” of the framework is observed all along the activation of the V4+═O/V5+═O redox couple in a solid-solution-type domain.