Stabilizing fluorine to achieve high-voltage and ultra-stable Na3V2(PO4)2F3 cathode for sodium ion batteries. (April 2021)

• Record Type: Journal Article
• Authors: Deng, Liang; Yu, Fu-Da; Xia, Yang; Jiang, Yun-Shan; Sui, Xu-Lei; Zhao, Lei; Meng, Xiang-Hui; Que, Lan-Fang; Wang, Zhen-Bo
• Is Part Of: Nano energy. Volume 82(2021)
• Publication Date: 2021-04
• Subjects: Sodium ion batteries -- Cathode -- Na3V2(PO4)2F3 -- Stabilizing fluorine -- High working voltage; Nanoscience -- Periodicals; Nanotechnology -- Periodicals; Nanostructured materials -- Periodicals; Power resources -- Technological innovations -- Periodicals; Nanoscience; Nanostructured materials; Nanotechnology; Power resources -- Technological innovations; Periodicals; 621.042
• DOI: 10.1016/j.nanoen.2020.105659
• Languages: English
• ISSNs: 2211-2855

Abstract

Abstract: Na3 V2 (PO4 )2 F3 is a promising cathode candidate for sodium ion batteries due to its high working voltage. However, an "unknown" low-voltage plateau around ~3.3 V lowers its operating voltage and puzzles researchers. Herein, the reasons for its appearance are explored. It is found that the formation of the low-voltage plateau originates from the extra-generated [VO6 ] induced by the fluorine loss during the synthesis. To enable better performance, a feasible strategy of stabilizing fluorine is proposed to eliminate the low-voltage plateau. It is demonstrated that this strategy can effectively guarantee the existence of fluorine and modulate the local electronic structure of V to maintain [VO4 F2 ] in Na3 V2 (PO4 )2 F3 phase, thus eliminating the undesirable plateau and achieving an operating-voltage increase of ~100 mV. Moreover, the enhanced structural stability and kinetics of the optimized Na3 V2 (PO4 )2 F3 cathode are confirmed by in-situ XRD. As expected, the Na3 V2 (PO4 )2 F3 cathode delivers higher energy density (446.4 Wh kg −1 ), better rate capability and longer cycling performance (89.2%@30 C after 1000 cycles). Furthermore, its excellent adaptability to wide temperature range (−25 °C–55 °C) and strong competence in Na3 V2 (PO4 )2 F3 ||hard carbon full-cell are verified. These excellent properties further qualify Na3 V2 (PO4 )2 F3 as a competitive cathode for sodium ion batteries. Graphical Abstract: The strategy of stabilizing fluorine guarantees fluorine to maintain [VO4 F2 ] in Na3 V2 (PO4 )2 F3 cathode and suppress the formation of [VO6 ], thus modulating the local electronic structure of V and eliminating the undesirable low-voltage plateaus (circled in purple dashed lines). ga1 Highlights: Stabilizing fluorine to regulate local electronic structure around vanadium. Eliminating low-voltage plateau to increase operating voltage of ~100 mV. Optimizing structure to improve long-term durability and cyclic stability.