Synergistic Effect, Structural and Morphology Evolution, and Doping Mechanism of Spherical Br‐Doped Na3V2(PO4)2F3/C toward Enhanced Sodium Storage

• Published in: Small (Weinheim an der Bergstrasse, Germany) Vol. 18; no. 22
• Main Authors: Hu, Zhuang, Zhang, Ruijie, Fan, Changling, Liu, Xunlin, Gao, Peng, Zhang, Weihua, Liu, Zhixiao, Han, Shaochang, Liu, Jinshui, Liu, Jilei
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.06.2022
• Subjects: Bromine; Br‐doped Na 3V 2(PO 4) 2F 3/C cathodes; Cetyltrimethylammonium bromide; Crystal structure; Density functional theory; Diffusion barriers; Doping; Electrochemical analysis; electrochemical performance; first‐principle calculation; Fluorine; Morphology; Nanotechnology; Polyelectrolytes; Polytetrafluoroethylene; Sodium; sodium‐ion batteries; spherical morphology; Spray drying; Synergistic effect
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.202201719

Na3V2(PO4)2F3 has attracted wide attention due to its high voltage platform, and stable crystal structure. However, its application is limited by the low electronic conductivity and the ease formation of impurity. In this paper, the spherical Br‐doped Na3V2(PO4)2F3/C is successfully obtained by a one‐step spray drying technology. The hard template polytetrafluoroethylene (PTFE) supplements the loss of fluorine, forming porous structure that accelerates the infiltration of electrolyte. The soft template cetyltrimethylammonium bromide (CTAB) enables doping of bromine and can also control the fluorine content, meanwhile, the self‐assembly effect strengthens the structure and refines the size of spherical particles. The loss, compensation, and regulation mechanism of fluorine are investigated. The Br‐doped Na3V2(PO4)2F3/C sphere exhibits superior rate capability with the capacities of 116.1, 105.1, and 95.2 mAh g−1 at 1, 10, and 30 C, and excellent cyclic performance with 98.3% capacity retention after 1000 cycles at 10 C. The density functional theory (DFT) calculation shows weakened charge localization and enhanced conductivity, meanwhile the diffusion energy barrier of sodium ions is reduced with Br doping. This paper proposes a strategy to construct fluorine‐containing polyanions cathode, which enables the precise regulation of structure and morphology, thus leading to superior electrochemical performance. Spherical Br‐doped Na3V2(PO4)2F3/C with superior performance is constructed by one‐step spray drying technology. The pure phase is realized by the supplement of fluorine from polytetrafluoroethylene (PTFE). The synergistic effect of hard template PTFE and soft template cetyltrimethylammonium bromide (CTAB) improves the integrity of the sphere and refines the particles. The first principle calculation shows enhanced conductivity and reduced diffusion energy barrier with Br doping.