Zirconium Modification Induced Small-Polaron Breakdown in Na4Fe3(PO4)2P2O7 as Superior Cathode in Sodium-Ion Battery

Abstract

Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is a promising cathode material for sodium-ion batteries (SIBs) due to its low cost, facile synthesis, environmental compatibility, high structural stability, and suitable operating voltage. However, its practical application is hindered by poor cycling stability, limited rate capability, and low electronic/ionic conductivity. Herein, these challenges are addressed by strategically incorporating Zr⁴⁺ ions at the Fe1 site of NFPP (denoted as NZFPP-X, where X represents the Zr/Fe ratio). The optimized NZFPP-0.05 exhibits significantly enhanced thermodynamic stability and electrochemical performance. Zr substitution induces depolarization effects, which promote electron mobility, thereby improving conductivity, cycling stability, and rate performance. Specifically, NZFPP-0.05 delivers an exceptional capacity retention of 86.6% after 6000 cycles at 10 C and a remarkable rate capability of 58.5 mAh g⁻¹ at 50 C. These advancements are attributed to the reduced energy barrier and accelerated kinetics of NZFPP-0.05 compared to pristine NFPP. This work presents a novel Zr-substitution strategy to enhance the performance of NFPP cathodes and introduces NZFPP-0.05 as a cost-effective, ultra-stable, and high-rate cathode material for SIBs.