Ternary NASICON-Type Na_<b>3.25</b>VMn_<b>0.25</b>Fe_<b>0.75</b>(PO_<b>4</b>)_<b>3</b>/NC@CNTs Cathode with Reversible Multielectron Reaction and Long Life for Na-Ion Batteries

Na superionic conductor (NASICON)-structure Na₄MnV(PO₄)₃ (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 Na_3.25VMn_0.25Fe_0.75(PO₄)₃/NC@CNTs (NVMn_0.25Fe_0.75P/NC@CNTs) electrode material to deliver fast Na⁺ diffusion kinetics, high reversible capacity (110.2 mAh g^-1 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 NVMn_0.25Fe_0.75P/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 Fe²⁺/Fe³⁺, Mn²⁺/Mn³⁺, and V³⁺/V⁴⁺ 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 NVMn_0.25Fe_0.75P/NC@CNTs ∥ hard carbon full cell manifests high capacity (101.1 mAh g^-1 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.