Study On Synthesis And Performance Of FeF₃and LiTi₂（PO₄）₃as Electrode Materials For Lithium-ion Batteries

FeF₃, which based on conversion reaction, has an attractive theoretical specificcapacity of712mAh/g. But the high ionicity, large bandgap and poor electronicconductivity result in the low actual capacity and fast capacity fading, which holdback the commercial applications. The Nasicon-type LiTi₂（PO₄）₃has an openthree-dimensional framework, in which Li+ions can move easily, thus has high ionconduction. However, the poor rate capability due to its low intrinsic electronicconductivity, preventing it from being widely used in large-scale applications. Thisthesis is focused on improving the electronic conductivity and electrochemicalperformance of FeF₃and LiTi₂（PO₄）₃, especially the rate performance.FeF₃·3H₂O, FeF₃·0.33H₂O and anhydrous FeF₃were synthesized via lowtemperature liquid-phase method followed by heat-treatment at different temperatures.The results showed that FeF₃·0.33H₂O with orthorhombic structure had the bestelectrochemical performance. To improve their electronic conductivity, a ball millingprocess with acetylene black powders has been used to form FeF₃·nH₂O/C （n=3,0.33,0） nanocomposites. Though they have different crystalline structures, they canachieve the identical reversible electrochemical conversion reaction from Fe3+to Fe0in the wide voltage range of1.0⁴.5V. FeF₃·0.33H₂O/C delivered an initial dischargecapacity of177.6and105.1mAh/g at0.1C and5C （1C corresponding to237mA/g）in the voltage range of2.0⁴.5V, and the capacity retentions remain as high as83.8%and83.3%after100cycles.The Fe1-xCoxF₃·0.33H₂O/C （x=0,0.03,0.05,0.07） nanocomposites wereobtained by Co cation doping and ball milling with conductive acetylene blackpowders. It was found that the Co cation had been doped into the Fe site, andCo-doping significantly improved the rate capability and cycle stability.Fe0.95Co0.05F₃·0.33H₂O/C exhibited excellent electrochemical performance with thedischarge capacity of151.7,136.4and127.6mAh/g at rates of1C,2C and5C in thevoltage range of2.0⁴.5V, and its capacity retentions remain as high as92.0%,92.2%and91.7%after100cycles, respectively.LiTi₂（PO₄）₃/C nanocomposite was synthesized by polyethylene glycol （PEG）assisted sol-gel method at a relative low temperature followed by ball milling withacetylene black. The effect of calcination tempreture on crystal structure and properties of LiTi₂（PO₄）₃was discussed in detail. The results showed that LiTi₂（PO₄）₃was synthesized at850°C had the best electrochemical properties, and carbon coatingcould improve the electronic conductivity, decrease electrochemical polarization, thusinhibiting the capacity attenuation. LiTi₂（PO₄）₃/C composites exhibited excellentelectrochemical performance with the maximum discharge capacity of139.9,126.4,115.8,106.0,97.1and91.3mAh/g at rates of0.1C,0.5C,1C,2C,5C and10C （1C corresponding to140mA/g） in the voltage range of1.5³.5V, respectively, and thecapcity retation reached81.5%after100cycles at10C rate.