LiPF6 and lithium oxalyldifluoroborate blend salts electrolyte for LiFePO4/artificial graphite lithium-ion cells

The electrochemical behaviors of LiPF6 and lithium oxalyldifluoroborate (LiODFB) blend salts in ethylene carbonate + propylene carbonate + dimethyl carbonate (EC + PC + DMC, 1:1:3, v/v/v) for LiFePO4/artificial graphite (AG) lithium-ion cells have been investigated in this work. It is demonstrated by conductivity test that LiPF6 and LiODFB blend salts electrolytes have superior conductivity to pure LiODFB-based electrolyte. The results show that the performances of LiFePO4/Li half cells with LiFF6 and LiODFB blend salts electrolytes are inferior to pure LiPF6-based electrolyte, the capacity and cycling efficiency of Li/AG half cells are distinctly improved by blend salts electrolytes, and the optimum LiODFB/LiPF6 molar ratio is around 4:1. A reduction peak is observed around 1.5V in LiODFB containing electrolyte systems by means of CV tests for Li/AG cells. Excellent capacity and cycling performance are obtained on LiFePO4/AG 063048-type cells tests with blend salts electrolytes. A plateau near 1.7-2.0 V is shown in electrolytes containing LiODFB salt, and extends with increasing LiODFB concentration in charge curve of LiFePO4/AG cells. At 1C discharge current rate, the initial discharge capacity of 063048-type cell with the optimum electrolyte is 376.0 mAh, and the capacity retention is 90.8% after 100 cycles at 25 degrees C. When at 65 degrees C, the capacity and capacity retention after 100 cycles are 351.3 mAh and 88.7%, respectively. The performances of LiFePO4/AG cells are remarkably improved by blending LiODFB and LiPF6 salts compared to those of pure LiPF6-based electrolyte system, especially at elevated temperature to 65 degrees C. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.