LiFePO4 and graphite electrodes with ionic liquids based on bis(fluorosulfonyl)imide (FSI)- for Li-ion batteries

Ambient-temperature ionic liquids (IL) based on bis(fluorosulfonyl)imide (FSI) as anion and 1-ethyl-3-methyleimidazolium (EMI) or N-methyl-N-propylpyrrolidinium (Py13) as cations have been investigated with natural graphite anode and LiFePO4 cathode in lithium cells. The electrochemical performance was compared to the conventional solvent EC/DEC with 1 M LiPF6 or 1 M LiFSI. The ionic liquid showed lower first coulombic efficiency (CE) at 80% compared to EC-DEC at 93%. The impedance spectroscopy measurements showed higher resistance of the diffusion part and it increases in the following order; EC-DEC-LiFSI<EC-DEC-LiPF6<Py13(FSI)-LiFSIE=MI(FSI)-LiFSI. On the cathode side, the lower reversible capacity at 143mAh g(-1) was obtained with Py13(FSI)-LiFSI; however, a comparable reversible capacity was found in EC-DEC and EMI(FSI)-LiFSI. The high viscosity of the ionic liquids suggests that different conditions such as vacuum and 60 degrees C are needed to improve impregnation of IL in the electrodes. With these conditions, the reversible capacity improved to 160 mAh g(-1) at C/24. The high-rate capability of LiFePO4 was evaluated in polymer-IL and compared to the pure IL cells. The reversible capacity at C/10 decreased from 155 to only 126 mAh g(-1) when the polymer was present. (C) 2007 Published by Elsevier B.V.