A New Technique for Measuring the Diffusion Coefficient of Electrolytes for Lithium-Ion Batteries

Abstract

The electrolyte is a crucial component in modern Lithium-Ion batteries. It is responsible for the transport Lithium-Ions not only through the separator but also within the porous structure of the electrodes. A small diffusion coefficient of the electrolyte leads to an increased voltage drop due to depletion of Lithium-Ions and thus limits the performance of a battery for high currents. While the diffusion coefficient is mostly measured by potentiostatic or galvanostatic intermittent titration techniques (PITT or GITT), also an evaluation of the diffusion impedance of the electrolyte should yield according values. However, standard impedance measurement techniques like Electrochemical Impedance Spectroscopy (EIS) suffer severe disadvantages: Due to the sinusoidal excitation a current is permanently flowing and, in case of beneficially used metallic Lithium electrodes, Lithium is continuously being deposited and stripped. This causes a continuously changing surface area during the measurement and hence a time-variant and thus invalid impedance spectrum. Moreover, the diffusion impedance needs to be measured down to very low frequencies, which results in very long measurement times for regular Electrochemical Impedance Spectroscopy. As an adequate solution for both issues we suggest computing the diffusion impedance from time-domain measurements [1]: By choosing an appropriate excitation signal, changes within the system during measurement can be prevented and, as all frequencies are excited simultaneously, measurement time can be minimized. By experimentally shifting the diffusion process to lower frequencies we are further able to clearly separate it from other, surface reaction related processes and thus to isolate the diffusion impedance. For an example see Figure 1, where repeated measurements of the diffusion impedance of LiPF6 in EC/DMC (1:1) are shown. From the time constant of the diffusion impedance, the diffusion coefficient of the electrolyte is finally obtained. Caption Figure 1: Repeated measurement of the diffusion impedance of LiPF6 in EC/DMC (1:1), as it is commonly used in Lithium-Ion batteries (real-parts shifted to origin).