In-situ constructing pearl necklace-shaped heterostructure: Zn2+ substituted Na3V2(PO4)3 attached on carbon nano fibers with high performance for half and full Na ion cells

Abstract

Na3V2(PO4)3 (NVP) emerges as prospective cathode for sodium ion batteries. However, the poor electronic and ionic conductivity hinders its development. Traditional synthesis methods only allow ex-situ bonding between the NVP grains and carbon-based substrate, leading to unstable combination. Herein, a simultaneous modification strategy to optimize the morphological features and crystal construction of NVP system is proposed. The in-situ synthesized framework consisting of NVP and high conductive carbon nano fibers (CNFs) can efficiently elevate the electrochemical performance. A distinctive pearl necklace-shaped heterostructure is successfully constructed by electrospinning and carbon-thermal reduction routes. The necklace substrate is derived from the CNFs, possessing the unique unidimensional morphology and bridging well with each other to build a high conductive network. The Zn2+-substituted NVP grains with nano size are grown on the surface of the substrate. The shortened size provides short pathway for Na+ migration, resulting in the improved kinetic characteristics. Furthermore, the substitution of Zn2+ generates p-type doping to introduce favorable hole carriers, enhancing the ionic conductivity. The reduced band gap and migration energy barrier of Na+ for Zn2+ doped NVP is demonstrated by DFT calculations. Accordingly, the modified Zn0.07-ES-800 sample releases a high capacity of 117.5 mA h g−1 at 0.1C. It delivers a capacity of 92.3 mA h g−1 at 100C and maintains 72.7 mA h g−1 after 1000 cycles. Moreover, the Zn0.07-ES-800//Zn0.07-ES-800 full cell shows a high capacity of 94.4 mA h g−1 at 0.1C and keeps 80 mA h g−1 at 1C with a high retention of 95% after 70 cycles.