Steric-hindrance engineering to stabilize structural evolution in biphasic Na4Fe3(PO4)2P2O7Na2FeP2O7 cathode

The low-cost iron-based polyanionic Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) represent a 3D Na⁺ pathways and voltage-advantageous cathode material in sodium-ion batteries. Nevertheless, anisotropic lattice strain and stress generated during sodium (de)intercalation induces prominent local structural changes, deteriorating the long-term stability. Herein, this paper proposes the steric hindrance engineering of Na₂FeP₂O₇ phase (NFPO) to restrict the intramolecular motion and stabilize structural evolution in a biphasic structure Na₄Fe₃(PO₄)₂P₂O₇Na₂FeP₂O₇ (NFPP-4.1). The crystal domains of the NFPO phase are interlaced with the NFPP, and the NFPO with minimal volume change can mitigate local structural changes, thereby ensuring robust structural evolution. In addition, theoretical calculations and experiments corroborate that NFPO has abundant Na⁺ channels and rapid diffusion kinetics. Consequently, NFPP-4.1 exhibits excellent rate performance (91.4 mAh g^-1at 10 C) and prolonged cycle duration (capacity retention of 77.8 % after 8000 cycles). The stable structural evolution is underscored by minimal volume change of only 3.59 % observed in the platform region of the sodium storage. This study provides a new insight into the structural evolution of biphasic materials via steric hindrance engineering which can shed light on the development of long-cycle iron-based cathode materials.