A series of Ti-doped Na 3 V 2 (PO 4 ) 2 O 2 F materials were synthesized through a controlled coprecipitation method with precise Ti content regulation. Experimental results indicate that a 15 wt% Ti doping level (NVTOPF-0.15) optimally enhances electrochemical performance, yielding an initial discharge capacity of 119.3 mAh g −1 at 0.2C and retaining 84.3 % of its capacity after 2000 cycles at 5C. Titanium doping exerted a dual-functional effect: streamlining the synthesis pathway through synergistic coprecipitation with controllable particle size and enhancing the lattice substitution of Ti 4+ for V in VO 6 octahedra. This substitution refined the crystal structure and enhanced Na + diffusion kinetics. Titanium doping-induced structural reconstruction led to a reduction in particle size, thereby facilitating the electrochemical reaction kinetics. Additionally, using a combination of the Randles-Sevcik equation and theoretical calculations, it was found that titanium doping significantly increased the Na + diffusion coefficient and decreased the band gap. This study presents a dual-functional titanium-doping approach for high-rate and long-life sodium-ion cathode materials. The synthesis of NVTOPF via the co-precipitation method will provide new guidance for the development and application of positive electrode materials in sodium-ion batteries.
Kaidi Gao,Qiao Hu,Guangming Han,Yu Xia,Jiaying Liao,Jianfeng Yao
The polyanion-type fluorophosphate Na 3 V 2 (PO 4 ) 2 O 2 F (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 VOPO 4 ·2H 2 O (AI-VOP) nanosheets as precursors, polyaniline lamellated Na 3 V 2 (PO 4 ) 2 O 2 F 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 H 2 O in VOPO 4 ·2H 2 O (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.
Na 3 V 2 (PO 4 ) 2 O 2 F (NVPFO) attracts tremendous attention as a cathode material for sodium-ion batteries (SIBs) because of its higher capacity and structural stability . However, its inherent poor conductivity affects rate performance and long cycle performance. Herein, a special lamellar NVPFO are successfully synthesized by sodium alginate (SA) assisted hydrothermal method . SA dissolves in water to form hydrogel, due to the specific cross-linking of SA and V 4+ , which restricts the crystal growth of nanoparticles . Meanwhile, a conductive carbon layer for sodium ion transport channels is constructed after carbonization . Based on multidimensional experiments and density functional theory (DFT) calculations, a mechanism for formation of special lamellar NVPFO in SA solution is proposed for the first time. The results show that special lamellar morphology shortens Na + migration path and conductive coating facilitates electronic transmission. The optimized NVPFO@C-2 has better electrochemical performance with outstanding high rate capacity (106.8 mAh g −1 at 15 C) and long-term cyclability (99.8 mAh g −1 capacity retention of 87.1 % even after 1000 cycles at 5 C).
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