Tris(pentafluorophenyl)borane as an Electrolyte Additive to Improve the High Temperature Cycling Performance of LiFePO(4) Cathode

The capacity fading of LiFePO(4) batteries using 1.2 mol dm(-3) LiPF(6) ethylene carbonate/dimethyl carbonate electrolyte with tris (pentafluorophenyl) borane (TPFPB) additive at 60 degrees C is investigated by cyclic voltammetry, cyclability, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). The cyclability results show that the LiFePO(4)-Li cell with the TPFPB additive electrolytes exhibits high discharge capacity and good cycling performance at 60 degrees C. The LiFePO(4) electrode cycled at 60 degrees C without the TPFPB additive shows a significant increase in charge-transfer resistance by EIS analysis. The TPFPB additive in the electrolyte reduces the difference in the charge-transfer resistance before and after 100 cycles at elevated temperatures. SEM results show that after 100 cycles at 60 degrees C in an electrolyte without TPFPB additive, the LiFePO(4) electrode forms a visible material on the electrode surface. The observed material is presumably related with capacity fading in the LiFePO(4)-Li cell. The results indicate that TPFPB in the electrolyte prevents the formation of the observed material on the electrode, reduces charge-transfer resistance, and enhances cycling performance of the LiFePO(4)-Li cells at 60 degrees C. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3205861] All rights reserved.