xLi[Li1/3Mn2/3]O2-(1-x)LiNi5/12Mn5/12Co2/12O2(0≤x≤0.8)的结构与电化学性能

采用喷雾干燥法制备了xLi[Li_1/3Mn_2/3]O₂-(1-x)LiNi_5/12Mn_5/12Co_2/12O₂(0≤x≤0.8)系列富锂层状固溶体正极材料, 并通过X射线衍射(XRD)、高分辨透射电子显微镜(HRTEM)、X射线光电子能谱(XPS)、电化学阻抗测试(EIS)以及充放电测试等多种手段研究了样品组分中Li₂MnO₃ 含量变化对材料结构及电化学性能的影响.研究发现, 材料的微观结构随着Li₂MnO₃含量的增加而逐渐发生转变.当x≤0.2时, 样品的微观结构与其母体材料LiNi_5/12Mn_5/12Co_2/12O₂相似; 而当x≥0.4时, 样品的微观结构与Li₂MnO₃有很高的相似性.当x=0.3时, 材料表现出两相共存的特征.HRTEM结果显示, 随着Li₂MnO₃含量的增加, 样品中过渡金属原子的排列逐渐由长程有序转变为长程无序而短程有序, 并且在高Li₂MnO₃含量的样品中观察到了金属阳离子混排的现象.充放电测试结果表明, 当x≤0.6时, 材料的放电比容量随着x的增加而增加; 当x>0.6时, 其放电比容量则随着x的增加而下降; 当x=0.6时, 放电比容量最高, 室温及高温(50℃)下分别为260 和304 mA·h/g.EIS研究结果表明, 这种微观结构上由有序向无序的转变会导致材料电荷转移阻抗的增加, 进而影响材料的电化学性能.

Structure and Electrochemical Properties of xLi[Li1/3Mn2/3] O2-(1-x)LiNi5/12Mn5/12Co2/12O2(0≤x≤0.8)

Li-rich layered cathode materials xLi[Li_1/3Mn_2/3]O₂-(1-x)LiNi_5/12Mn_5/12Co_2/12O₂(0≤x≤0.8)were prepared via spray-dry method. X-ray diffraction(XRD), high resolution transmission electron microscopy(HRTEM), X-ray photoelectron spectroscopy(XPS), electrochemical impedance spectroscopy(EIS) and charge-discharge tests were carried out to investigate the influence of the amount of Li₂MnO₃ component on the structure and electrochemical properties of the materials. It is found that the micro-structure changes with the increase of Li₂MnO₃ content in composition. In the case x≤0.2, the crystal structure of material is similar to parent material LiNi_5/12Mn_5/12Co_2/12O₂. When x≥0.4, the materials show Li₂MnO₃-like structure. And the two structures co-exist in sample of x=0.3. HRTEM observations reveal that the arrangement of transition mental ions in the internal structure transfer from long-range order to long-range disorder and short-range order. As the x value increases from 0.1 to 0.8, the discharge capacity of material increases gradually when x≤0.6, and decreases gradually when x>0.6. It is reasonable considering the increase of charge transfer resistence detected by EIS. As a result, when x=0.6, the material exhibts the highest discharge capacity, which is about 260 mA·h/g at room tempreature and 304 mA·h/g at high tempreature(50℃). The EIS research shows that the micro-structural transformation from order to disorder increases the charge transfer resistance of material, thus yield poor electrochemical performances.