Study on Synthesis and Performance of FeF3and LiTi2(PO4)3as Electrode Materials for Lithium-ion Batteries
|Course||Physical and chemical|
|Keywords||Iron fluoride cobalt doping LiTi2（PO4）3 composite Lithium-ionbatteries|
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.