Abstract
Topochemical synthesis under ionothermal conditions provides a unique pathway for obtaining the polyanionic electrode material Na3V2(PO4)2FO2 with particles exhibiting a distinctive 2D platelet morphology. This morphology directly originates from the VOPO4·2 H2O precursor through a synthesis process involving water molecules extraction followed by fluorine and sodium insertion in the interlayer space while preserving the overall covalent bond network. This synthesis approach gives the possibility to engineer the active material particles shape through a design of the precursor, while maintaining the same reaction conditions. Herein, three different VOPO4·2 H2O with distinct particle sizes and morphologies were synthetized to evaluate the impact on the resulting Na3V2(PO4)2FO2 electrochemical performance. The optimal material was sintered under argon flow to transform the ionic liquid-based surface layer into a carbon coating and enhance the active material electronic conductivity. The particles morphology optimization using ionic liquid as templating-agent allowed to improve specific capacity from 56 mAh/g to 83 mAh/g at 1 C, while the further carbonization of the ionic liquid surface layer permits to increase active material capacity up to 103 mAh/g, demonstrating the benefits of surface modification.