High voltage cathodes for Na-ion batteries: Na3V2O2x(PO4)2F3-2x system

Abstract

Batteries are one of the most promising energy storage devices. Among them, sodium-ion batteries are presented as the best alternative to lithium-ion batteries especially in the field of stationary energy storage due to the more abundance and lower cost of sodium. Thus, it is necessary to search and optimize new electrode and electrolyte materials in order to better understand the behaviour of these sodium based devices. One of the most promising cathodic materials for sodium-ion batteries are the sodium vanadium fluorophosphates. They show high voltage performance and long-term stability so they could lead to high energy density materials. In this work the relationship between V3+ Na3V2(PO4)2F3 and V4+ Na3V2O2(PO4)2F phases has been established as end members of Na3V2O2x(PO4)2F3-2x family of compounds. Different compounds belonging to this family have been hydrothermally synthesized by variating the type and amount of in-situ carbon in the final product. Among them, Na3V2O2x(PO4)2F3-2x (x = 0.8) and Na3V2O2(PO4)2F samples have been deeply studied. Several characterization methods, such as X-ray diffraction, infrared spectroscopy, electronic microscopy, magnetic measurements and electron and neutron magnetic resonance (EPR and NMR, respectively) have been used to determine the most important features of each material. The influence of the different compositions and materials coatings on their electrochemical performance has also been evaluated. Finally, the electrochemical mechanism of Na3V2O2x(PO4)2F3-2x (x = 0.8) and Na3V2O2(PO4)2F phases has been studied by both, ex-situ and in-situ analysis and the most remarkable differences and similarities between them have been discussed.