Investigations on some electrochemical aspects of lithium-ion ionic liquid/gel polymer battery systems

Electrochemical and interfacial characteristics of Li-ion battery system based on LiFePO4 cathode and graphite anode with ionic liquid (IL) electrolytes have been investigated, both with and without addition of a small amount of polymer to the electrolyte. The IL electrolyte consisted of bis(fluorosulfonyl)imide (FSI) as anion and 1-ethyl-3-methyleimidazolium (EMI) or N-methyl-N-propylpyrrolidinium (Py13) as cation, and operated at ambient temperature. We reported previously that the SEI formation with IL was stabilized in the graphite anode at 80% coulombic efficiency (CE) in the first cycle, when FSI anion is used. In this work, we extend the study to the LiFePO4 cathode material. Gel polymer with IL is one part of this study. The stepwise impedance spectroscopy was used to characterize the Li/IL-Gel polymer/LiFePO4 at different states of charge. This technique revealed that the interface resistance was stabilized when the cathode is at 70% DoD (Depth of Discharge). The diffusion resistance is higher at the two extremes of discharge when monophase LiFePO4 state (0%DoD and 100%DoD) obtains. When polymer is added to the IL, interface resistance is improved with 1 wt.% but results with IL alone are not improved for the case of 5 wt.% polymer added. Good cycling life stability was obtained with Li/IL-FSI/LiFePO4 cells, with or without polymer. The first evaluation of the Li-ion cell, LiFePO4/IL-FSI-(5 wt.%) gel polymer/graphite, has shown low first CE at 68.4% but it recovers in the third cycle, to 96.5%. Some capacity fade was noticed after 30 cycles. The rate capability of the Li-ion cell shows a stable capacity until 2 C discharge rate.

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