Mn/V Co‐Doping Enables Multielectron Transfer and Above‐Theoretical Capacity in Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ 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 3.5 Fe 2 Mn 0.5 V 0.5 (PO 4 ) 2 P 2 O 7 /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 Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (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 Mn 2+ /Mn 3+ /Mn 4+ and V 3+ /V 4+ redox couples, as well as Fe 2+ /Fe 3+ . 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.