Polyaniline lamellated Na3V2(PO4)2O2F with fast kinetics toward high-performance sodium-ion batteries

Abstract

The polyanion-type fluorophosphate Na₃V₂(PO₄)₂O₂F (NVPOF) is a prospective cathode candidate for high-energy sodium-ion batteries (SIBs) because of its high voltage plateau, high theoretical specific capacity and three-dimensional sodium super-ionic conductor (NASICON) framework. However, the inherently low electronic conductivity and poor thermal stability of NVPOF pose challenges to its electrochemical properties and synthesis in SIBs. For the first time, using phenylamine-intercalated VOPO₄·2H₂O (AI-VOP) nanosheets as precursors, polyaniline lamellated Na₃V₂(PO₄)₂O₂F particles (PA-NVPOF) are synthesized in a solvent mixture of ethylene glycol and deionized water (v/v = 1/1) as cathodes for high-performance SIBs, enabling an excellent reversible capacity of 129.5 mAh g⁻¹ at 0.1C and a high-energy density of 478 Wh kg⁻¹. Specifically, the crystalline H₂O in VOPO₄·2H₂O (VOP) is extracted from the interlayer space by the intercalation of phenylamine molecules. After an in-situ polymerization of phenylamine between layers, PA-NVPOF with markedly enhanced electronic conductivity is produced at a relatively low temperature (180 °C). Ex-situ X-ray diffraction and electrochemical kinetics investigations reveal the reversible structural stability and fast Na⁺/electron transport rate of PA-NVPOF. This route, based on the phenylamine interaction between layers and an in-situ polymerization at a low temperature, provides valuable insight into the design of thermosensitive polyanionic cathodes for high-performance SIBs.