【作者】 易惠华；

【导师】 戴永年； 方海升；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2012， 博士

【摘要】 新型正极材料的研究和开发一直是锂离子电池研发的主要方向之一。橄榄石型LiMnPO4具有电压平台高(4.1V)、能量密度高(697Wh·kg-1)以及价格便宜等优点,是一种非常有应用前景的锂离子电池正极材料。然而LiMnPO4材料本征电导率很低,严重影响其倍率性能,阻碍了它的商业化应用进程。因此,如何提高LiMnPO4材料的倍率性能是当前研究的重点和热点。围绕这一问题,本论文对磷酸锰锂的制备、Mn位阳离子替换、结构及电化学性能进行了较系统的研究。利用XRD、SEM和EDS等技术对产物的微观结构和形貌进行了分析,并采用恒流充放电、循环伏安(CV)和电化学阻抗谱(EIS)技术测试其电化学性能。采用固相法合成了碳复合LiMnPO4材料,原料中加入草酸后,合成的碳复合LiMnPO4粒径小,具有更好的电化学性能,0.1C放电比容量为42.54mAh·g-1,是未使用草酸样品放电比容量的两倍以上,而且550℃退火1h能进一步提高样品的放电比容量。以蔗糖为碳源合成的原位碳包覆LiMnPO4比以Super P为碳源合成的样品表现出更好的电化学性能。在制备工艺研究的基础上,本文研究了Mn位Fe、Zn和Mg替换LiMn1-xMxPO4(M=Fe、Zn和Mg)材料。研究表明,三种阳离子替换都可以提高LiMnPO4材料的导电率,减小充放电过程中的电化学极化,改善材料的倍率性能和循环稳定性。Mn位Fe替换样品的晶粒会随着铁替换量增加和合成温度升高而增大,并且颗粒均一性变差,从而导致材料性能下降,650℃反应10h合成的C-LiMn0.9Fe0.1PO4具有最好的性能,0.1和2C的比容量分别为130和90mAh·g-1。Mn位Zn替换研究表明,少量的锌替换可以改善LiMnPO4材料的性能,这不同于前期文献中报道的结果。与单纯碳包覆LiMnPO4材料的性能相比较,LiMnPO4具有更高的比容量和更好的倍率性能,700℃反应10h合成的C-LiMn1-xZn0.05PO4表现出最好的电化学性能,0.1和2C的放电比容量分别为144和120mAh·g-1。采用固相法和共沉淀法合成了系列C-LiMn1-xMgxPO4材料,固相法合成的C-LiMn1-xMgxPO4材料颗粒更小,具有更好的电化学活性,但两种方法合成的样品都是当x=0.03时表现出最好的倍率性能。在LiMnPO4材料Mn位阳离子单一替换研究的基础上,本论文提出了Mn位Fe-Zn、Mg-Zn和Fe-Mg共替换。实验结果表明阳离子共替换可以产生协同效应,比阳离子单一替换具有更好的改性效果,而在这些样品中,Fe-Mg共替换材料具有最好的性能。因此对C-LiMn0.9Fe1-xMgxPO4材料的性能进行了优化,x=0.01时,800℃反应10h合成的材料具有最好的性能,0.1、1和10C的放电比容量分别为154、135和63mAh·g-1。最后,本论文采用真空方法合成了原位碳包覆LiMn0.9Fe0.09Mg0.01PO4材料并对其制备过程中各工艺参数的影响进行研究。真空法制备的LiMn0.9Fe0.09Mg0.01PO4材料具有多孔特性,700℃反应10h合成的材料极化最小,显示出最好的倍率性能。更多还原

【Abstract】 Investigation and improvement of novel cathode material has been one of the main directions of lithium-ion battery. Olivine structured lithium manganese phosphate (LiMnPO4) has the advantage of high operating voltage (4.1V), high energy density (697Wh·kg-1) and low cost etc.. However, LiMnP04has very low conductivity which limits its electrochemical properties at rapid charge/discharge and hinders its commercialization. Therefore, how to improve rate performance of LiMnP04becomes an important and hot point of current research. According to this object, present dissertation gives a systematic research on preparation of LiMnP04, the substitution with cation, the structure and the electrochemical properties. The micro-structures and morphologies of these composites were investigated by XRD SEM and EDS. The electrochemical performance has been evaluated by galvanostatic charge/discharge, cyclic votammetry (CV) and electrochemical impedance spectra (EIS).LiMnPO4/C composites were synthesized by solid state reaction which show significantly enhanced electrochemical performance due to smaller particle size after adding oxalic acid in the raw materials. The sample delivers a discharge capacity of42.54mAh·g-1at0.1C, as well as above twice higher than the capacity of the sample without using oxalic acid. The discharge capacity could be further enhanced when both of the samples were post annealed at550℃for1h. Then in situ carbon coating LiMnPO4composites were synthesized using sucrose as conductive additive and they exhibit better electrochemical performance compared with the sample using Super P as carbon resource.Based on the process research of LiMnPO4/C, Fe, Zn and Mg substituted LiMn1-xxMxPO4(M=Fe、Zn and Mg)solid solutions were synthesized and studied. The results show that the conductivity was increased and the charge-discharge property was improved with obviously reduced electrochemical polarization and the cycle performance was also enhanced after substituting with three cations. The crystallite grains of Fe substituted LiMnPO4grew and became less uniform with an increase Fe contents and synthesis temperatures resulting in a fall in the discharge capacity. Among of them, the sample in situ carbon coating LiMno.9Fe0.1PO4synthesized at650℃for10h shows the optimized performance, delivering the capacity of～130mAh·g-1at0.1C and～90mAhg-1at2C. For Zn substituted LiMnPO4, the results reveal that the Zn substitution is highly beneficial for the performance of LiMnPO4, which is different from those previously reported in the former documents. Compared with the pure C-LiMnPO4, the C-LiMn1-xZnxPO4has higher capacity and better rate capability. C-LiMn0.95Zn0.05PO4synthesized at700℃for10h exhibits the best performance providing capacity of144mAh℉g-1at0.1C and102mAh·g-1at2C. Two different processes, solid-state reaction and co-precipitation, were carried out to prepare Mg substituted LiMnPO4. The samples of C-LiMn1-xMgxPO4synthesized by solid-state reaction possess better electrochemical activity but the samples via two methods shows the highest discharge specific capacity when x=0.03.On the basis of the study on single substitution of LiMnPO4, Fe-Zn, Mg-Zn and Fe-Mg co-substitutions were proposed and a synergistic effect of the cation co-doping or co-substitution has been evidenced by experimental research. Co-substitutions of Mn sites have better modification effect than single substitution and Fe-Mg co-substituted LiMnPO4delivers the highest capacity in all samples. Therefore, C-LiMno.9Fe1-xMgxPO4composites were further optimized showing more higher capacity at0.1,1and10C are154,135and63mAh·g-1, respectively, when*=0.01.Finally, C-LiMn0.9Fe0.09Mg0.01PO4composites were synthesized under vacuum condition. Technological conditions of this synthesis method were investigated systematically and optimized which have a porous characteristic. The sample synthesized at700℃for5h shows optimum electrochemical performance with the smallest polarization and the best rate performance.更多还原