The N-doped carbon coated Na3V2(PO4)3 with different N sources as cathode material for sodium-ion batteries: Experimental and theoretical study

Abstract

NASICON (super ion conductor)-type Na3V2(PO4)3 (NVP) is considered to be one of the most promising cathode materials for sodium-ion batteries (SIBs) due to its good structural stability, fast Na+ diffusion coefficient, and excellent electrochemical properties. However, the poor intrinsic conductivity and electronic conductivity and severe volume shrinkage of NVP, resulting in serious limitations in the practical application of NVP materials. Carbon, especially N-doped carbon modification, is the most effective method and strategy to improve the conductivity of NVP. The study of N source and N content in carbon plays a very important role in improving the electronic properties. Herein, N-doped carbon coated NVP composites was synthesized via the classical sol-gel method using common, green and inexpensive urea and polyvinylpyrrolidone (PVP) as N sources, respectively. According to EIS tests and GITT tests, N-doped carbon layers with urea as the N source significantly improved the conductivity of the carbon layers and the Na+ diffusion kinetics. The effects of N-doped carbon layers on the electrode kinetics and electrochemical properties of NVP materials were investigated in detail. The optimized U-NC15@NVP composite exhibited a maximum discharge capacity of 114.2 mAh g−1 at 0.2C and 92.1% capacity retention after 400 cycles at 1C. When the optimal U-NC15@NVP electrode was selected to assemble a symmetric full cell, the reversible discharge capacity was 75.0 mAh g−1 after 100 cycles at 1C. The density functional theory (DFT) calculations establish that N-doped carbon can generate lots of active sites and external defects, which is beneficial to reduce the energy bandwidth of the carbon layer and thus improve the conductivity of the carbon layer. Additionally, it can also reduce the Na+ diffusion barrier and improve the adsorption energy for Na+. Experimental tests and theoretical calculations show that N-doped carbon can significantly increase the conductivity of the carbon layer and improve the electrochemical properties of NVP.