N/P-dual-doped carbon coated Na3V2(PO4)2O2F microspheres as high-performance cathode material for sodium-ion batteries.

Na3V2(PO4)2O2F (NVPOF) is attracting great interest due to its large capacity and high working voltage. However, the poor electronic conductivity limits the electrochemical performance of NVPOF. Herein, we fabricate N/P-dual-doped carbon coated NVPOF microspheres (labeled as NVPOF@P/N/C) via a hydrothermal process followed by heat treatment. This microsphere-structured NVPOF@P/N/C composite has a relatively high tap density of 1.22 g cm-3. TEM and XPS results reveal that the dual-doped carbon layer is tightly coated on the NVPOF surface due to the bridging effect of P and has a good protective effect on NVPOF. Density functional theory (DFT) calculations confirm that N/P-dual-doped carbon layer is advantageous to achieve higher electronic conductivity and lower migration activation energy than the undoped and single N- or P-doped carbon layer. As cathode material for sodium-ion battery (SIB), the NVPOF@P/N/C exhibits high capacity (128 mAh g-1 at 0.5 C and 122 mAh g-1 at 2 C) and ultra-long cycle performance (only 0.037% capacity fading rate per cycle in 500 cycles at 2 C). We believe that the NVPOF@P/N/C composite is appealing for high-performance SIBs with large energy density.