Effect of K/Zr co-doping on the elevated electrochemical performance of Na 3 V 2 (PO 4 ) 3 /C cathode material for sodium ion batteries

The Na3V2(PO4)3 (NVP) and its binary-doped Na2.96K0.04V2-xZr(3/4)x(PO4)3/C are prepared by a facile solid-phase method. The crystal structure, morphological characteristics, and electrochemical properties are analyzed by XRD, XPS, SEM, and electrochemical tests. The results reveal that K+ and Zr4+ have been successfully doped into NVP system without damaging the original structure. The co-doping strategy can broaden the channels of Na+ migration to facilitate the ionic conductivities. Meanwhile, it is beneficial to stabilizing the crystal structure effectively by introducing the K+ and Zr4+ with larger ionic radius. All the electrochemical properties of co-doped system are better than that of NVP, resulting from the lager channel for Na+ diffusion and enhanced intrinsic electrical conductivities by co-doping. Notably, Na2.96K0.04V1.93Zr0.0525(PO4)3/C exhibits the best electrochemical performance. It delivers a high discharge capacity of 107.3 mAh g−1 at 0.1 C; it remains 92.3 mAh g−1 after 400 cycles at 2 C, corresponding to the capacity retention of 92.02%; it still maintains 100.0 mAh g−1 even at 10 C rate.