Synergistic Effect, Structural and Morphology Evolution, and Doping Mechanism of Spherical Br-Doped Na 3 V 2 (PO 4 ) 2 F 3 /C toward Enhanced Sodium Storage.

Na 3 V 2 (PO 4 ) 2 F 3 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 Na 3 V 2 (PO 4 ) 2 F 3 /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 Na 3 V 2 (PO 4 ) 2 F 3 /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.