摘要
采用溶胶-凝胶法合成Li3V2-2/3xMnx(PO4)3(0≤x≤0.12)。采用XRD、SEM、XPS、恒流充放电和电化学阻抗谱(EIS)研究Mn掺杂对Li3V2(PO4)3/C结构和电化学性能的影响。XRD研究表明:掺杂少量的Mn2+不会影响材料的结构,所有样品均具有单一相态的单斜结构(P21/n空间群)。XPS分析表明:在Li3V1.94Mn0.09(PO4)3/C中,V和Mn的化合价分别为+3和+2,原料中的柠檬酸在煅烧过程中分解成C而残留在Li3V1.94Mn0.09(PO4)3/C中。电化学测试表明:掺杂Mn改善了电极材料的循环性能和倍率性能,正极材料Li3V1.94Mn0.09(PO4)3/C表现出最好的循环稳定性和倍率性能。在40mA/g的放电电流密度下,循环100次后,Li3V1.94Mn0.09(PO4)3/C的放电容量从158.8mA·h/g衰减到120.5mA·h/g,容量保持率为75.9%,而未掺杂样品的放电容量从164.2mA·h/g衰减到72.6mA·h/g,容量保持率为44.2%。当放电电流密度增加到1C时,Li3V1.94Mn0.09(PO4)3/C的初始放电容量仍能达到146.4mA·h/g,循环100次后,放电容量保持为107.5mA·h/g。EIS测试表明,掺杂适量的Mn2+减小了电荷转移阻抗,这有利于Li+的脱嵌。
Li3V2-2/3xMnx（PO4）3（0≤x≤0.12） powders were synthesized by sol-gel method. The effect of Mn2＋-doping on the structure and electrochemical performances of Li3V2（PO4）3/C was characterized by XRD, SEM, XPS, galvanostatic charge /discharge and electrochemical impedance spectroscopy（EIS）. The XRD study shows that a small amount of Mn2＋-doped does not alter the structure of Li3V2（PO4）3/C materials, and all Mn2＋-doped samples are of pure single phase with a monoclinic structure （space group P21/n）. The XPS analysis indicates that valences state of V and Mn are ＋3 and ＋2 in Li3V1.94Mn0.09（PO4）3/C, respectively, and the citric acid in raw materials was decomposed into carbon during calcination, and residual carbon exists in Li3V1.94Mn0.09（PO4）/C. The results of electrochemical measurements show that Mn2＋-doping can improve the cyclic stability and rate performance of these cathode materials. The Li3V1.94Mn0.09（PO4）3/C cathode material shows the best cyclic stability and rate performance. For example, at the discharge current density of 40 mA/g, after 100 cycles, the discharge capacity of Li3V1.94Mn0.09（PO4）3/C declines from initial 158.8 mA·h/g to 120.5 mA·h/g with a capacity retention of 75.9%; however, that of the Mn-undoed sample declines from 164.2 mA·h/g to 72.6 mA·h/g with a capacity retention of 44.2%. When the discharge current is increased up to 1C, the intial discharge capacity of Li3V1.94Mn0.09（PO4）3/C still reaches 146.4 mA·h/g, and the discharge capacity maintains at 107.5 mA·h/g after 100 cycles. The EIS measurement indicates that Mn2＋-doping with a appropriate amount of Mn2＋ decreases the charge transfer resistance, which is favorable for the insertion/extraction of Li＋.
基金
Project (20771100) supported by the National Natural Science Foundation of China
