Electrochemical impedance spectroscopy analysis using blocking symmetric cell for lithium ion batteries

Abstract

Electrochemical impedance spectroscopy (EIS) can observe various resistance component inside the cell by applying an alternating electric signal (AC) with a small amplitude. Applying AC is advantageous with distinguishing electrochemical reaction by frequency, whereas all resistances appear at the same time in direct electric signal (DC). When performing an EIS test on a general lithium-ion battery (LIB), the results of cathode and anode are mixed at a specific frequency. Therefore, the effect of a specific electrode of can be confirmed by testing symmetric cell which is a cell configuration assembled with same two electrodes. When AC with small amplitude (~10 mV) is applied to symmetric cell, Li-ions are adsorbed along the electrode surface rather than lithation/delithation reactions. Therefore, following reaction can be observed for each frequency in the symmetric cell: (1) Electrical structure can be investigated in the high frequency range; (2) Diffusion and adsorption of Li-ions are occur in the middle frequency range; (3) adsorbed Li-ions along the electrode surface cause side reactions with the electrode in low frequency range. Using the diffusion and adsorption of Li-ions at middle frequency range, microstructure of electrode can be quantified. The quantified information form EIS helps to understand the electrochemical reaction of LIB. This is because the electrode information obtained from mercury of nitrogen adsorption, which is commonly used, does not provide the Li-ion reaction information. In this thesis, physical electrode information related to Li-ions is quantified using EIS, and the electrochemical mechanism of LIBs are investigated based on the quantified information. At low frequency, leakage current appears due to the inherent parasitic reaction of the electrode-electrolyte. This is related to the solid electrolyte layer (SEI), which is the result of the decomposition of the electrolyte. The SEI layer is an important component affecting cycle performance of LIB. Analyzing the AC leakage current revealed in the symmetric cell give an insight of electrode-electrolyte reactivity by excluding the faradaic reaction. In this thesis, the change of the cathode material and the generation of the SEI layer during AC leakage current is observed in real time by using in-situ X-ray diffraction (XRD). Evaluating symmetric cell using EIS helps to fundamentally understand LIBs. Using this analysis, it will help to design electrode and cell configuration by understanding mechanisms of Li-ion diffusion, adsorption and reactivity.