Orbital-Enabled Intrinsic Defect Healing via Charge Compensation toward Accelerated Sodium Storage in Mn-Rich Composite Phosphates

Abstract Manganese-rich composite phosphate cathodes, represented by Na4Fe1.25Mn1.75(PO4)2P2O7 (NFMPP), offer high energy density for sodium-ion batteries. However, their limited electrochemical durability and sluggish kinetics hinder practical application. In this work, we first identify abundant intrinsic oxygen vacancies, which are determined to be responsible for the occurrence of Mn/Na antisite defects and deteriorate electrochemical performance of NFMPP. Building on the new insight, we design a trace V-doped Mn-rich composite phosphate cathode, Na4Fe1.25Mn1.65V0.1(PO4)2P2O7 (NFMPP-V). Different from other dopants (Cr3+, Co2+, Ni2+, Nb5+), the unique empty eg*0 orbitals and modest charge compensation (Δz = +1) of V3+ effectively reduce the oxygen-vacancy defects in the NFMPP structure. This reduction leads to a more uniform strain distribution and a lower concentration of in situ formed Mn/Na anti-site defects, ultimately accelerating Na+ kinetics and enabling more complete utilization of the intrinsic low-voltage Fe3+/Fe2+ redox reaction. Consequently, NFMPP-V can deliver an energy density of 421.1 Wh/kg, relating to a reversible capacity of 125.7 mAh g–1 with an enhanced initial Coulombic efficiency of 97.0% (105.8 mAh g–1, 87.0% for the NFMPP), outperforming most Mn-rich composite phosphate cathodes reported to date. Notably, a full pouch cell paired with a hard-carbon anode retains 93.6% of its capacity after 1000 cycles. This work clarifies the decisive role of oxygen-vacancy defects in Mn-rich composite phosphate cathodes and provides a new insight for developing high-performance sodium-ion batteries.