Electrochemical studies of N-Methyl N-Propyl Pyrrolidinium bis(trifluoromethanesulfonyl) imide ionic liquid mixtures with conventional electrolytes in LiFePO₄/Li cells

The electrochemistry of the electrolyte plays a significant role in the performance of lithium ion batteries. In order to further investigate the effect of combining an ionic liquid with a conventional electrolyte, a hybrid electrolyte (HE) based on a mixture of 38% N-methyl-N-propyl-pyrrolidinium bis(trifluromethanesulfonyl) imide in an electrolyte consisting of LiPF₆ in Ethylene Carbonate: Diethyl Carbonate (1:1) was studied and compared with the properties of a standard organic electrolyte in this work. Thermal measurements reveal that the addition of the ionic liquid improves the thermal stability of the hybrid electrolyte. The interfacial behavior was studied in lithium symmetric cells and it was found that the combined effects of the ionic liquid and the conventional electrolyte results in lower interfacial resistance. The charge-discharge behaviour of LiFePO₄ / Li cells was investigated; at the end of 100 cycles, the discharge capacity of the cell with LP30, HE and [C₃mpyr][TFSI] was 141 mAh/g, 148 mAh/g and 45mAh/g, respectively. Analysis of the separator recovered from Li/LiFePO₄ cells that had been stored for 30 days at 20 °C shows iron dissolution from the cathode into the LP30 electrolyte, but not into the hybrid electrolyte. Posthumous studies of the cycled electrodes were carried out using FTIR, XRD, FEG-SEM and HR-TEM analyses, show that the HE forms a permeable interface on the cathode material which prevents metal dissolution from the cathode structure, leading to better cyclic performance.