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.
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