Kinetics of Li+ reduction in 1-methyl-3-propylpiperidinium bis(trifluoromethylsulfonyl) imide room temperature ionic liquid

Separation of the total Li/LiTFSI + MPPipTFSI/Li cell impedance into single contributions was performed with the use of electrochemical impedance spectroscopy (EIS). The structure of the electrode was observed by scanning electron microscopy (SEM). The charge transfer process occurs at a solid|solid Li+(SEI) → Li(metal) interphase (SEI is the solid electrolyte interphase). SEM images show that the structure of the electrode surface (and of the interphase) is grain-like. The kinetics of Li+ reduction in ionic liquids is interesting as the Li+ desolvation step (which takes place in classical solutions) does not take place which leads to a simpler process. Deconvolution of the EIS spectra indicates three resistances and corresponding three ln R = f(T−1) straight lines of different slopes, associated with the resistance of liquid electrolyte, SEI and the charge transfer process, respectively. Activation energy for the Li+ conduction process in the liquid electrolyte and solid SEI is 23 and 32 kJ mol−1, respectively. Activation energy for the charge transfer process Li+(SEI) → Li is 62 kJ mol−1 and the corresponding exchange current density at T = 298 K is 1.92 × 10−5 A cm−2. As the process occurs at the SEI|Li interphase, the concentration independent rate constant cannot be calculated.