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

Abstract 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 LiFePO 4 cathode in lithium cells. The electrochemical performance was compared to the conventional solvent EC/DEC with 1 M LiPF 6 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  6 −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 °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 LiFePO 4 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.