(Keynote) Non-Destructive Analysis of Electrochemical Systems By Electrochemical Impedance Spectroscopy

Abstract

The importance of electrochemical impedance spectroscopy (EIS) has grown in accordance with its expanding fields of application. A number of important papers that contributed greatly to the development of this field were published, and the timeline of the refinement of EIS has been well summarized by Orazem and Tribollet.[1] Since we applied to an impedance measurement using FFT on electrochemical systems in 1982,[2] we have proposed many approaches in order to facilitate status evaluation of electrochemical devices on the basis of reaction models of the components and interfaces. In situ observation of electrodeposited lithium was evaluated by EIS and optical microscopy. Microscopic observation indicated that lithium deposited more uniformly in mixture propylene carbonate and 1,2-dimethoxyethane than in sole propylene carbonate solution. The results of morphological evaluation of the electrodeposited lithium using optical microscopy corresponded to the variations in the value of the charge transfer resistance obtained from EIS measurements, showing that the behavior of the charge transfer resistance accurately represented the morphology.[3] Subsequently, we have strenuously investigated lithium ion battery (LIB). The impedance response of the LIB cell is composed of the responses of the two electrodes and the electrolyte with the separator. Elemental processes that affect the impedance response can be assumed as follows; electron migrations in a conductive additive and at interface of active material/current collector, charge transfer processes, a solid state diffusion consists of Warburg. It is difficult to separate the contributions of both electrodes and, if these responses are not separated, the information obtained via the EIS is the internal resistance of the cell only. We have three approaches to overcoming this problem, installation of a reference electrode, separation using a symmetric or temperature-controlled cell.[4] EIS has been utilized to characterize each elemental process of electrochemical devices, because it enables us to analyze dynamics of each elemental process sensitively and separately without destruction of the cell. The equivalent circuit to express each elemental process in a commercial LIB by EIS has been carefully investigated. An equivalent circuit was designed for the analysis of lithium ion batteries with the contributions of a variety of diffusion parameters resulting from the various particle sizes for the cathode and the solid-electrolyte interphase formed on the anode particles, as well as electrochemical reactions and inductive components. The electrochemical impedance of the electrodes in commercial LIB was analyzed to evaluate the proposed circuit at various states of charge[6] and capacity fading were analyzed with continuous charge-discharge cycling.[7] Besides, we have evaluated catalyst layers in direct methanol fuel cell (DMFC)[7] and polymer electrolyte fuel cell (PEFC)[8] using an equivalent circuit composed of transmission line model (TLM) in which primary and secondary pores were considered. As these results, we achieved the detection of the state of the devices from the EIS analysis. The conventional EIS using FRA – potentiostat systems is not easy to measure the impedance of the large-scale LIB because of its low internal resistance. Moreover, FRA – potentiostat system for conventional EIS measurement could not be mounted on the vehicle. Thus, impedance measurement system is needed without using FRA – potentiostat systems. In our study, application of square wave potential for input signals of EIS was investigated in simple electrochemical reaction to verify a new technique called “Square-potential/current electrochemical impedance spectroscopy (SP-EIS, SC-EIS)” which is a method for EIS without using the FRA systems. Introduction of SC-EIS to diagnosis technology of laminated LIBs and the LIB module was discussed.[9] We applied SC-EIS to evaluate a state of a commercial stationary storage battery system with LIB (Fig. 1). This work was partly supported by “Research & Development Initiative for Scientific Innovation of New Generation Batteries” and “Development of Safety and Cost Competitive Energy Storage System for Renewable Energy” from NEDO, Japan.