Two birds with one stone: Mn2+/Mg2+ dual-substitution in Na3V2(PO4)3 for robust Zn-ion storage performance
NASICON-type cathodes as promising candidates have attracted more and more attention, and Na 3 V 2 (PO 4 ) 3 is one of the outstanding representatives. In particular, it can tolerate complex interface reaction processes during the Zn 2+ insertion/desertion in the aqueous environment. However, existing three-dimensional channels in the intrinsic Na 3 V 2 (PO 4 ) 3 always hardly play to their advantages as narrow pathways and some other defects. This work has considered both the conductivity and capacity of this vanadium-based NASICON cathode at the same time, where a sodium-rich cathode material Na 3.2 V 1.8 Mn 0.15 Mg 0.05 (PO 4 ) 3 (NVMMP) with Mn 2+ and Mg 2+ dual-substituents has been designed considering the low cost and environmentally friendly characteristics. The synergistic effects introduced by Mn 2+ and Mg 2+ substitution led to a significant increase in the capacity and cycle life of the optimized NVMMP cathode material (with a capacity retention rate of 90.6 % after 2500 cycles at 2 A g −1 ). Furthermore, the reversible storage mechanism of Zn 2+ in NVMMP cathode materials and the theoretical study of such synergistic substitution on intrinsic electronic conductivity have been revealed by different in/ex-situ characterization techniques and theoretical calculations, where redox processes generated by Mn 3+ /Mn 4+ , enhanced ionic/electron pathway, and the pinning effect caused by injecting inert MgO 6 octahedra into the NASICON framework are key points to high capacity and long-life span.
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