Ternary NASICON-Type NaVMnFe(PO)/NC@CNTs Cathode with Reversible Multielectron Reaction and Long Life for Na-Ion Batteries.

Na superionic conductor (NASICON)-structure NaMnV(PO) (NVMP) electrode materials reveal highly attractive application prospects due to ultrahigh energy density originating from two-electron reactions. Nevertheless, NVMP also encounters challenges with its poor electronic conductivity, Mn dissolution, and Jahn-Teller distortion. To address this issue, utilizing N-doped carbon layers and carbon nanotubes (CNTs) for dual encapsulation enhances the material's electronic conductivity, creating an effective electron transport network that promotes the rapid diffusion and storage of Na. On this basis, partially substituting Mn in NVMP with Fe, a new sodium superionic conductor (NASICON) structured cathode material has been designed to alleviate Jahn-Teller distortion and prolong the cycling life. The synergistic effect of N-doped double nanocarbon encapsulation and multielectron reactions is employed to promote the optimized NaVMnFe(PO)/NC@CNTs (NVMnFeP/NC@CNTs) electrode material to deliver fast Na diffusion kinetics, high reversible capacity (110.2 mAh g at 0.1 C), and long-term cyclic stability (80.1% of the capacity at 10 C over 2000 cycles). Besides, the electrochemical properties of NVMnFeP/NC@CNTs composites were investigated in detail at high loads and high window voltages to evaluate their potential for practical applications. The reduction/oxidation processes involved in Fe/Fe, Mn/Mn, and V/V redox couples and a solid-solution and biphasic reaction mechanism upon repeated de- and re-intercalation processes are revealed via ex-situ XRD and XPS characterization. Finally, the assembled NVMnFeP/NC@CNTs ∥ hard carbon full cell manifests high capacity (101.1 mAh g at 0.1 C) and good cycling stability (98.2% capacity retention at 1 C after 100 cycles). The rational design with multimetal ion substitution regulation has the potential to open up new possibilities for high-performance sodium-ion batteries.