Rapid mechanochemical synthesis of high-performance Na4Fe2.94Al0.04(PO4)2(P2O7)/C cathode material for sodium-ion storage

Abstract

Na₄Fe₃(PO₄)₂(P₂O₇) is regarded as a promising cathode material for sodium-ion batteries due to its affordability, non-toxic nature, and excellent structural stability. However, its electrochemical performance is hampered by its poor electronic conductivity. Meanwhile, most of the previous studies utilized spray-drying and sol-gel methods to synthesize Na₄Fe₃(PO₄)₂(P₂O₇), and the large-scale synthesis of the cathode material is still challenging. This study presents a composite cathode material, Na₄Fe_2.94Al_0.04(PO₄)₂(P₂O₇)/C, prepared via a straightforward ball-milling technique. By substituting Al³⁺ minimally into the Fe²⁺ site of NFPP, Fe defects are introduced into the structure, hindering the formation of NaFePO₄ and thereby enhancing Na-ion diffusion kinetics and conductivity. Additionally, the average length of AlO bonds (2.18 Å) is slightly smaller than that of FeO bonds (2.19 Å), contributing to the superior structural stability. The smaller ionic radii of Al³⁺ induce lattice contraction, further enhancing the structural stability. Moreover, the surface of material particles is coated with a thin layer of carbon, ensuring excellent electrical conductivity and outstanding structure stability. As a result, the Na₄Fe_2.94Al_0.04(PO₄)₂(P₂O₇)/C cathode exhibits excellent electrochemical performance, leading to high discharge capacity (128.1 mAh g^-1 at 0.2 C), outstanding rate performance (98.1 mAh g^-1 at 10 C), and long cycle stability (83.7 % capacity retention after 3000 cycles at 10 C). This study demonstrates a low-cost, ultra-stable, and high-rate cathode material prepared by simple mechanical activation for sodium-ion batteries which has application prospects for large-scale production.

Keywords: Al-doping; Cathode materials; Na(4)Fe(3)(PO(4))(2)(P(2)O(7)); Na-ion batteries; Solid-state method.