Mn/V Co-Doping Enables Multielectron Transfer and Above-Theoretical Capacity in Na₄Fe₃(PO₄)₂P₂O₇ Cathode

The practical deployment of polyanionic cathodes in sodium-ion batteries is severely restricted by their limited theoretical capacity and poor electron/ion kinetics. Herein, we overcome these inherent obstacles through the construction of a high-performance Na_3.5Fe₂Mn_0.5V_0.5(PO₄)₂P₂O₇/C@CNT (NFMVPP) composite via a synergistic strategy, which integrates Mn/V co-doping with a dual-carbon modification involving in situ carbon coating and carbon nanotube networking. The NFMVPP cathode achieves an unprecedented initial discharge capacity of 149.11 mAh g^-1 at 0.05 C, successfully breaking through the theoretical capacity limit of 129 mAh g^-1 of pristine Na₄Fe₃(PO₄)₂P₂O₇ (NFPP). Correspondingly, it achieves a remarkable energy density as a cathode material of 398.44 Wh kg^-1 at 0.05 C, significantly surpassing the unmodified NFPP. Mechanistic studies utilizing ex situ X-ray photoelectron spectroscopy (XPS) and in situ X-ray diffraction (XRD) reveal that this breakthrough originates from the reversible multielectron transfer process triggered by the activation of high-voltage Mn²⁺/Mn³⁺/Mn⁴⁺ and V³⁺/V⁴⁺ redox couples, as well as Fe²⁺/Fe³⁺. Furthermore, benefiting from the hierarchical conductive network, the cathode exhibits excellent rate capability of 86.02 mAh g^-1 at 20 C and robust long-term cycling stability. This work provides a new paradigm for transforming the latent capacity of polyanionic materials to construct high-energy-density sodium-ion batteries.