Triggering reversible V4+/V5+ redox couples in Na3V2(PO4)3 porous flower-like cathodes by Mg2+ substitution for high-performance Na-ion batteries

Na3V2(PO4)3 has received a lot of attention as a prospective cathode for sodium-ion batteries (SIBs) owing to its strong 3D wide-open architecture for Na+ transport. However, the practical application is still hampered by its poor electronic conductivity and weak Na+ diffusion kinetics. In this paper, a novel Mg2+-doped Na3V2(PO4)3 flower-like cathode (Na3+xV2-xMgx(PO4)3/C, x = 0, 0.02, 0.04, 0.06) with abundant mesoporous has been synthesized by a convenient template-assisted sol-gel method. Mg2+ doping successfully triggers V4+/V5+ reversible redox with improved voltage plateau and reversible capacity. The hierarchical mesoporous structure combined with carbon layers is beneficial to the fast transport of Na+ and e−, which guarantee improved kinetic properties and enhanced conductivity. Attributed to these competitive advantages, significantly improved electrochemical properties of Na3.04V1.96Mg0.04(PO4)3/C (NVMP-4) are acquired, including extremely high specific capacity (123.8 mAh g−1 at 1C), excellent rate performance (73.8 mAh·g−1 at 100C) and exceptionally stable energy output with 89.1 % capacity retention after 3000 cycles at 20C. Additionally, when assembled with a hard carbon anode (HC), NVMP-4||HC provides an exceptional energy density of 216.9 Wh kg−1 at 0.2C. By enhancing nanostructure and rational surface engineering, this project offers fresh perspectives for creating superior electrodes, increasing the energy density of SIBs.