Na₂FePO₄F/C composite synthesized via a simple solid state route for lithium-ion batteries

Abstract

Using low-cost FePO₄·2H₂O as iron source, Na₂FePO₄F/C composite is prepared by alcohol-assisted ball milling and solid-state reaction method. The XRD pattern of Na₂FePO₄F/C composite demonstrates sharp peaks, indicating high crystalline and phase purity. The SEM and TEM images reveal that diameter of the spherical-like Na₂FePO₄F/C particles ranges from 50 to 300 nm, and HRTEM image shows that the surface of Na₂FePO₄F/C composite is uniformly coated by carbon layer with a average thickness of about 3.6 nm. The carbon coating constrains the growth of the particles and effectively reduces the agglomeration of nanoparticles. Using lithium metal as anode, the composite delivers a discharge capacities of 102.8, 96.4 and 90.3 mA·h/g at rates of 0.5C, 1C and 2C, respectively. After 100 cycles at 0.5C, a discharge capacity of 98.9 mA·h/g is maintained with capacity retention of 96.2%. The Li⁺ diffusion coefficient (D) of Na₂FePO₄F/C composite is calculated as 1.71×10⁻⁹ cm²/s. This study reveals that the simple solid state reaction could be a practical and effective synthetic route for the industrial production of Na₂FePO₄F/C material.

摘要

选用二水合磷酸铁为铁源, 经乙醇辅助球磨和固相反应制备了氟磷酸亚铁钠/碳复合材料。 X- 射线衍射证实产品有高的结晶度和相纯度。 扫描电镜和透射电镜照片显示, 球形氟磷酸亚铁钠粒子的 粒径分布在 50~300 nm 之间; 从高分辨透射电镜图可以看出, 在氟磷酸亚铁钠/碳复合材料的表面包 覆了一层厚度为 3.6 nm 的碳层。碳层的包覆能有效地遏制氟磷酸亚铁钠粒子的长大及粒子的团聚。 以 锂片作为负极组装半电池, 在 0.5C,1C,2C 倍率下,复合材料的放电比容量分别为 102.8,96.4, 90.3 mA·h/g。0.5C 循环 100 次后电池的放电比容量为 98·9 mA·h/g,容量保持率为 96.2%。 从循环伏 安曲线计算得到氟磷酸亚铁钠/碳复合材料的锂离子扩散系数为 1.71×10⁻⁹ cm²/s。 显然, 固相法是制备 锂离子电池正极材料用氟磷酸亚铁钠/C 复合材料的有效方法。