Cross-linked Na2VTi(PO4)3@C hierarchical nanofibers as high-performance bi-functional electrodes for symmetric aqueous rechargeable sodium batteries

Abstract

Significant interest has been devoted to the design and fabrication of flexible electronic devices owing to their immense potential in modern society. Constructing freestanding electrodes with superior electrochemical performance and excellent mechanical durability is the key to the development of high-performance energy storage devices. In this study, we introduce a new cross-linked Na₂VTi(PO₄)₃/porous carbon nanofiber, which is employed as a self-supported bi-functional electrode in aqueous sodium ion batteries. The sodium intercalation mechanism of the Na₂VTi(PO₄)₃/C electrode in an aqueous electrolyte is investigated. The reversible phase transformations and the valence state change of Na₂VTi(PO₄)₃ in both high- and low-potential ranges (i.e. 0–0.6 V vs. Ag/AgCl; −1–0 V vs. Ag/AgCl) demonstrate its reliability as both the anode and the cathode in the aqueous electrolyte. Favored by the highly porous architecture and aligned cross-linked arrangement, the Na₂VTi(PO₄)₃/C nanofiber achieves fast kinetics and good mechanical characteristics. Both result in its superior high-rate properties, good cycling durability and high structural stability. Moreover, a symmetric aqueous rechargeable sodium battery is prepared by assembling two cross-arranged Na₂VTi(PO₄)₃/C nanofiber electrodes. The cell exhibits a flat potential plateau of ∼1.2 V and is capable of high-rate long-term cycling. It retains 83% of the capacity after six hundred cycles at alternate 40 and 4C. Therefore, this work not only introduces a novel symmetric aqueous sodium ion system, but also provides a highly efficient architecture for flexible high-performance electrodes. More importantly, it provides a new clue to construct energy storage devices with a simple configuration, low-cost and superior properties.