Enhanced electrochemical properties of LiFe_(1-x)Mn_xPO_4/C composites synthesized from FePO_4·2H_2O nanocrystallites

摘要

The high discharge potential of LiMnPO_4,4.1 V vs. Li/Li~+, and its theoretical capacity of 170 mAh g~(-1) make it a promising candidate as a cathode material in lithium-ion batteries. But extremely low electronics conductivity, slow lithium diffusion kinetics, and the Jahn-Teller effect of Mn~(3+) limit the electrochemical performances of LiMnPO_4. In this work, the pre-synthesized and defined FePO_4·2H_2O nanocrystallites are used as one of the raw materials to synthesize LiFe_(1-x)Mn_xPO_4/C (x = 0.85, 0.75, 0.65) composites via solid-state reactions. The synthesized LiFe_(1-x)Mn_xPO_4 samples show well-crystallized structures and have enhanced electrochemical properties. There exist two plateaus around 3.5 and 4.1 V on both of their charge and discharge curves. Among the samples, the Fe_(0.25)Mn_(0.7)PO_4 one exhibits the longest highvoltage charge/discharge plateau at 4.10 V/4.05 V, and has an average discharge voltage of ～3.78 V vs. Li/Li~+ and a discharge capacity of ～130 mAh g~(-1) at 0.05 C rate. For the Fe_(0.25)Mn_(0.75)PO_4 sample, the noticeable improvement of its electrochemical performances is mainly attributed to iron substitution, the appropriate Mn/Fe ratio, and the well-ordered crystal structure forming by using FePO_4·2H_2O nanocrystallites as one of the raw materials.