Optical Impedance Spectroscopy - a Technique for Characterizing the Lithium Intercalation Process in a Porous Graphite Electrode

Graphite is the most commonly used anode material in Lithium-ion batteries. It allows high energy and power densities (depending on material parameters). Lithium intercalation losses can be reduced by fine-tuning anode design. In previous literature, several electrochemical models (1-4) were proposed to better understand and simulate this process.

The challenge is to parameterize and unambiguously validate these models. This would require electrochemical investigations e.g. electrochemical impedance spectroscopy (EIS) in half cell setups under a variation of boundary conditions, namely state of charge (SOC) or temperature (1,5). However, it can be difficult to get sufficient information for parameterization. Some parameters, e.g. electrical and ionic conductivity, show similar dependencies in their physical equations.

Lithium intercalation decreases the electrode potential and changes the diffusive reflectance. This is due to SOC dependent photo-molecular interactions, e.g. intraband transitions (6). In this work we present a new optical cell setup with a Lithium ring reference electrode. This innovation allows simultaneous in-situ optical and electrical measurements. We can now observe the dynamic behavior of the macroscopic anode surface reflectance. We demonstrate the excitation by using a modulated voltage signal to measure the reflectance as a response signal. With this new measurement method, we mapped an optical impedance spectrum (OIS), revealing further information about the Lithium intercalation process.

Conventional electrochemical models usually only contain electrical measurement figures. The most appropriate model assumption for porous graphite electrodes is based on a transmission line (1,7). In this study, the transmission line model is extended so that it reproduces both the EIS and OIS measurements. It is thereby possible to investigate different model assumptions using a combined electro-optical parameterization (Fig. 1). It becomes apparent that combined electro-optical parameterization can reduce ambiguities in the electrochemical model. It will be demonstrated that the OIS is highly sensitive to changes in ionic and electronic conductivities; ergo, these parameters can be reliably determined.