Preparation of lithium manganese iron phosphate cathode material by purification of ferrous sulfate and study on its performance influence
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摘要: 采用硫酸法工艺在生产钛白粉的过程中产生大量的副产品硫酸亚铁，以提纯后硫酸亚铁作为原料，采用一步水热法合成经济价值更高的磷酸锰铁锂正极材料，研究原料中部分去除的镁元素对磷酸锰铁锂正极材料的物理和电化学性能的影响。结果显示，原料采用质量分数为6%的氟化铵为化学沉淀剂，得到镁杂质脱除率为98.86%的硫酸亚铁产品，合成的材料为不规则球形形貌正交晶系的磷酸锰铁锂材料，少量镁杂质改变了材料中锂离子的活动空间，使锂离子迁移速率得到提升，合成的磷酸锰铁锂正极材料（LMFP/C-2）放电比容量在0.1C和2C下分别为135.24 mAh/g和86.16 mAh/g，在0.1C下循化100圈后放电比容量保持率可达到97.70%，所得产物稍优于高纯度商业材料的性能。Abstract: The titanium dioxide production process via the sulfuric acid method generates a significant amount of byproduct ferrous sulfate. To fully utilize this resource, ferrous sulfate is purified and then used as a raw material to synthesize value-added cathode material of lithium manganese iron phosphate (LMFP) through a one-step hydrothermal method. In this study the impact of partially removed magnesium from the raw material on the physical and electrochemical properties of LMFP had been investigated. The results show that the using ammonium fluoride with a mass fraction of 6% as a chemical precipitant can obtain ferrous sulfate products with a removal rate of 98.86% magnesium impurities. Consequently the synthesized LMFP features an irregular spherical morphology and an orthorhombic crystal structure. A small amount of magnesium impurities alters the lithium-ion activity space within the material, enhancing lithium-ion migration rates. The discharge specific capacity of the synthesized LMFP cathode material (LMFP/C-2) is 135.24 mAh/g at 0.1C and 86.16 mAh/g at 2C, respectively. After 100 cycles at 0.1C, the discharge specific capacity retention rate reaches 97.70%. The performance of the obtained product slightly surpasses that of high-purity commercial materials.
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图 1 不同氟化铵用量对镁离子脱除的影响
Figure 1. The effect of different ammonium fluoride addition on the removal of magnesium ions
图 2 硫酸亚铁提纯前后XRD分析
Figure 2. XRD spectra of ferrous sulfate before and after purification
图 4 LMFP/C-2正极材料的SEM形貌以及粒径分布
Figure 4. SEM morphology and particle size distribution chart of LMFP/C-2 cathode material
(a)SEM形貌； (b) 粒径分布
图 5 LMFP/C-2正极材料的TEM分析
(a) TEM图像； (b)(c) 高倍TEM图像
Figure 5. TEM images of LMFP/C-2 cathode material
图 6 不同条件下的循环伏安曲线
(a) LMFP/C-2样品前三圈；(b) 1 mV/s下LMFP/C与LMFP/C-2
Figure 6. Cyclic voltammetry curves under different conditions
图 7 不同Mg含量的倍率与循环性能
(a) 不同Mg含量的倍率性能图；(b) LMFP/C材料与LMFP/C-2样品0.1C下的循环性能
Figure 7. Rate capability and cycling performance at different Mg contents
图 8 LMFP/C与LMFP/C-2的EIS对比
(a) LMFP/C与LMFP/C-2样品的Nyquist图和等效电路； (b) Z′与低频区ω−1/2线性拟合
Figure 8. Comparison of EIS between LMFP/C and LMFP/C-2
表 1 七水硫酸亚铁主要化学成分
Table 1. Main chemical composition of ferrous sulfate
% Fe Ti S Mg Ca Al Zn 26 0.03 15 0.76 0. 046 0.002 0.008 注：其余为游离水 
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表 2 七水硫酸亚铁提纯后各元素质量分数
Table 2. Mass fraction of each element in the product after purifying ferrous sulfate heptahydrate
% Fe Ti Mn Mg Ca Al Zn 99.97 0.003 0.004 0.0087 0. 001 0.0015 0.008 注：其余为游离水
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表 3 LMFP/C与LMFP/C-2样品的晶胞参数与晶胞体积
Table 3. Cell parameters and cell volumes of LMFP/C and LMFP/C-2 samples
项目 a/nm b/nm c/nm V/nm3 LMFP/C 1.046 0.604 0.474 29.949 LMFP/C-2 1.041 0.606 0.474 29.863
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