镁掺杂的非化学计量比磷酸铁锂的制备及性能

摘要

以Li2CO3、FeC2O4·2H2O、MgO、NH4H2PO4为原料,无水葡萄糖为碳源,采用高温固相合成法制备获得4％Mg2+掺杂的LiFe0.96Mg0.04PO4/C,并通过分别减少2％Mg2+和2％Fe2+合成了非化学计量比的LiFe0.96Mg0.02PO4/C和LiFe0.94Mg0.04PO4/C材料.利用X射线衍射(XRD)、X射线光电子能谱(XPS)、红外光谱分析(FTIR)、扫描电镜(SEM)、透射电镜(TEM)表征了Fe2+和Mg2+含量的变化对材料的物质构成、形貌及结构的影响.通过交流阻抗(EIS)及恒流充放电循环测试对材料的电化学性能进行表征.研究结果表明Mg2+均匀地掺入LiFePO4的晶格中,非化学计量比的LiFe0.96Mg0.02PO4/C和LiFe0.94Mg0.04PO4/C材料的晶胞体积减小,导电性杂质Fe2P的含量升高,Li-Fe错位现象减少;其中,LiFe0.94Mg0.04PO4/C表现出最优的充放电容量、倍率性能以及循环性能,0.1C、20C倍率下的放电比容量分别为161.2mAh/g、74.7mAh/g,500次循环后20C的容量保持率达到97.3％.%LiFe0.94Mg0.04PO4/C,LiFe0.96Mg0.02PO4/C and LiFe0.96Mg0.04PO4/C have been synthesized by a solid-state reaction using Li2CO3,FeC2O4-2H2O,MgO,and NH4H2PO4 as starting materials and C6H6O6 as carbon source.X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS),scanning electron microscopy (SEM),transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the effect of Fe2+ or Mg2+ on the morphology and structure of the prepared materials.The electrochemical performances of different samples were characterized by galvanostatic charge/discharge test and electrochemical impedance spectroscopy(EIS).The results indicated that the crystal lattice parameters and the Li-Fe anti-site defects were reduced,whereas the content of conductive Fe2P in non-stoichiometric lithium iron phosphate increased when Mg was doped,and LiFe0.94Mgo.04PO4/C exhibited the highest discharge performance,rate capability and excellent cycle performance,whose initial capacities were 161.2mAh/g at 0.1C,and 74.7mAh/g at 20C and the retention rate was 97.3％ at 20C after 500 cycles.