Revealing the regulation mechanism of Na3V2(PO4)2O2F crystal growth in sodium alginate solution for high-performance sodium ion batteries

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).