Study On The Effect Of Surfactant On Structure And Electrochemical Performance Of LiFePO₄ Material

In this article, LiFePO4 was synthesized by a novel solvothermal method. Surfactant (Oleic acid, using OA for short) was mingled into the solvent system (EG was used as the unique solvent) in order to investigate its effect on structure and electrochemical performances of LiFePO4. Moreover, the micro-structure and morphology of all the samples were characterized by XRD, TEM and SEM method. At the same time, electrochemical performances were tested by galvanostatic charge/discharge technique, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) method. Furthermore, some important electrochemical parameters such as Rct and ΔEp were calculated.XRD test reveals that all the samples which obtained within either different reaction time or different surfactant assisted exhibit a single phase of LiFePO4. Differences only existed in cell parameter, including cell volume and the ratio of the peak intensity between I111 and I131, and so on. For instance, the ratio of I111 I131 of the sample that prepared within 18 h is 0.71, while 0.65,0.70,0.68 for the the sample that prepared within 6 h,10 h,14 h, respectively. Besides, the discharge special capatities are different within the reaction time from 6 hto 18 h, which is 119.7,114.7,121.5 and 133.2 mAh·g-1, respectively.In the experiment, OA acted an essential role on the micro-structure and electrochemical properties of LiFePO4 material. XRD result shows that OA could modify the peak intensity of LiFePO4 material. The ratio of I111/I131 (the ratio of intensity between secondary peak and main peak) is 0.73 of the sample that prepared within OA, while 0.71 for the sample that prepared without OA. The SEM and TEM images show that surfactant indeed affect the structure and size of LiFePO4 material. The size of LiFePO4 material reduced from 200 nm to 50 nm because of the existence of OA. On account of time-dependence experiment, a primary mechanism was proposed:nucleation-adsorption of surfactant-agglomeration-growth-complete crystallization, which can explain this discovery in some degree.Electrochemical performance measurements show that the electrochemical performances of LiFePO4 material which synthesized within OA are better than thoes prepared without OA. It has the maximum discharge special capacity of 161.8 mA·hg-1. However, LiFePO4 material that prepared without OA shares a lower discharge special capacity of 133.2 mAh·g-1, namely, increases 21.5%. And the discharge special capacity at 5 C bumps up from 70.2 to 86.5 mAh·g-1, namely, increases 23.2%. More importantly, the value of Rct decreases from 263.4 Ω to 257.7 Ω. At the same time, the value of ΔEP from CV curves decreases from 0.35 V to 0.34 V, which proves that OA may indeed improve the dynamic performances of LiFePO4.