Tailoring Local Sodium Coordination Toward High Initial Coulombic Efficiency Na4Fe3(PO4)2P2O7 Cathodes.

The phosphate cathode Na4Fe3(PO4)2P2O7(NFPP) is promising for large-scale sodium-ion batteries (SIBs) due to its stability and safety. However, its capacity is limited by first-cycle irreversible Na loss, and the fundamental structural origin remains incompletely understood. Here we identify that the extraction of insufficiently occupied five-coordinated Na3 sites induces strong electrostatic repulsion, causing irreversible structural rearrangement and hindering sodium reinsertion, thereby affecting the initial Coulombic efficiency (ICE). Based on this, rapid cooling was employed to reconfigure the initial thermodynamic occupancy of NFPP, achieving the intrinsic trapping of high-energy Na3 sites and reinforcing the local Na─O─P bonding strength. This reinforced initial configuration imposes spatial constraints on the P2O7 polyhedron, enabling reversible elastic twisting during the desodiation process. Theoretical calculations further indicate that this localized sodium coordination regulation lowers the reinsertion energy barrier and stabilizes Na3-related redox processes. Consequently, the optimized sample exhibits high sodium utilization and an ICE of 94.9% (vs. 86.3% for slow cooling), with 86.9% capacity retention after 10 000 cycles at 10 C. Furthermore, the corresponding cylindrical cell demonstrates excellent long-cycle stability and wide temperature adaptability (-40°C to 55°C). These findings provide a strategy for tailoring local sodium coordination to overcome inherent capacity limitations for high-efficiency, long-lifespan SIBs.