| Polyanion-type cathode materials of lithium ion batteries have gained great attention due to the stable structure, cheap raw materials and environmental friendly characters. Among the iron-based polyanion-type compounds, Li2FeSiO4has showed the highest theoretical capacity (332mAh g-1, corresponding to two Li+storage per molecule). In addition, iron and silicon are the most abundant and cheap elements on earth, making Li2FeSiO4viable for large scale production. However, Li2FeSiO4is plagued by the poor electronic conductivity and poor ion transmittability, which greatly limit its practival elelctrochemical performance. Furthermore, the reversible insertion/extraction of two lithium ions is hard to realize due to the hard realization of redox reaction of Fe3+/Fe4+. Additionly, in the aspect of synthesizing, hydrothermal method is an attractive synthetic method, as itoffers high temperature and high pressure synthesis conditions. Thus, this method generates crystalline materials with unique morphologies thus has excellent properties. Up to now, the hydrothermal method is not widly used for the synthesis of Li2FeSiO4due to the long reacting time, exhaust of more energy and the combination of impurity phases. To overcome these problems, a great effort has been paid. The main points are summarized as follows:(1) Spindle-like Li2FeSi04and Li2FeSiO4/C composites have been synthesized by a facile hydrothermal method using the reactants of silica (SiO2),ammonium ferrous sulfate ((NH4)2Fe(SO4)2·6H2O) and lithium hydroxide (LiOH·H2O), and the solvent of deionized water. The possible formation mechanism of spindle-like architecture is discussed in detail. The experimental conditions such as concentration and temperature have been optimized. The synthesis condition is optimized to be190℃with the reaction time of24h and the Fe concentration of0.1M.After carbon coating, the Li2FeSiO4/C composites showed greatly improved electrochemical performance.In particular,Li2FeSiO4/Cwith carbon content of7.21wt.%showed the best electrochemical performance, withthe discharge capacitiy of160.9mAh g-1at room temperature and213mAh g-1(about1.23Li+per molecular) at45℃(0.1C).(2) Intergrown (1-x)Li2FeSiO4·xLiFePO4/C composite and mixed phase C composite were synthesized by LiFePO4modified Li2FeSi04. The existence state of LiFePO4in (1-x)Li2FeSi04·xLiFeP04/C and (1-xmix)Li2FeSiO4-xmixLiFePO4/C composite has been studied. The effect of LiFePO4content on the electrochemical performance of Li2FeSiO4has also been studied. The results showed that LiFePO4and Li2FeSiO4intergrowize with each other in the intergrown (1-x)Li2FeSiO4·xLiFePO4/C composite. However, they grew independently in the mixed phase of (1-xmix)Li2FeSiO4·xmixLiFePO4/C composite. It is noted that in the intergrown (1-x)Li2FeSiO4·xLiFePO4/C composite, increased electronic conductivity and decreased activation emergies have been achieved after LiFePO4modification. This is benefit to the insertion/extraction of the second Li+. For the mixed phase (1-xmix)Li2FeSiO4·xmixLiFePO4/C composite, the apperarnce of LiFePO4can increase the stability of Li2FeSiO4during the charge-discharge process. Sample with x=0.04in the (1-x)Li2FeSiO4·xLiFePO4/C composite shows the best electrochemical performance with a high discharge capacity of210.4mAh g-1(about1.27Li+per molecular) and at15℃and276.3mAh g-1(about1.26Li+per molecular) at45℃(at0.2C).(3) Al3+doped Li2-xFe1-xAlxSiO4/C composites have been synthesized by a facile sol-gel method. The doping positions of Al3+and the effect of Al3+on the structure of Li2FeSi04have been studied in detail. By modifying the content of Al3+, we found that when xvalue smaller than0.05, pure phase Li2FeSiO4can be obtained. The sample with x=0.03shows the best elelctochemcial performance with209.3mAh g-1at0.2C, which corresponds to a release of1.26Li+per molecule. |
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