Understanding Graphite Impedance: Determining Solid Electrolyte Interphase, Charge Transfer, and Pore Resistance
Authors: Morasch Robert , Suthar Bharatkumar , Keilhofer Josef , Gasteiger Hubert A. 
Published: May 2020

Electrochemical Impedance Spectroscopy (EIS) is a powerful tool, as it is a non-invasive technique that allows to understand and deconvolute internal resistances of electrochemical systems. However, great caution has to be exercised when choosing a proper equivalent circuit for graphite anodes in Li-ion batteries and assigning resistances to specific mechanisms such as solid-electrolyte-interphase (SEI, at the electrolyte-SEI interface) or charge transfer resistance (at the SEI-graphite interface). The ionic resistance in the electrolyte of the porous electrode is hereby often overlooked or assumed negligibly small. Such assumptions lead to improper equivalent circuits and the desired analysis of a specific resistance is questionable.

In this study we present a rigorous impedance analysis where the equivalent circuit assigned to the electrode is validated through measurements in multiple configurations. First, we show how the impedance response of a graphite electrode at standard loading (~3mAh/cm2) changes depending on its state of charge (SOC, see Fig. 1) and assign a preliminary equivalent circuit based on the transmission line model, similar to Refs. 1 and 2. We then validate the initial assumptions by changing the graphite loading - a simple but impactful procedure - as all resistances associated with the active material surface area are expected to directly correlate to the change in loading. Both SEI and charge transfer resistance are expected to increase with decreasing surface area, while the pore resistance that depends on electrode thickness, porosity, and tortuosity is expected to decrease. Eventually we will show a full impedance model for the graphite electrode and show how the SEI resistance is dominant compared to the charge transfer resistance, and that pore resistances are a major contribution to the overall resistance for typical battery graphite electrodes. Lastly, we will show the influence of additives on the SEI resistance in low-loaded graphite electrodes and how they affect the impedance spectra for graphite electrodes with a standard loading.

Figure 1 Half-cell impedance of a graphite electrode (~3mAh/cm2) at varying state of charge (SOC) measured via a gold-wire µ-reference electrode before and after three formation cycles in 1M LiPF6 in EC:EMC 3:7. Measured at open circuit voltage with a voltage amplitude of 10 mV between 30 kHz and 0.1 Hz.

Acknowledgements: This work was supported by the BMBF (Federal Ministry of Education and Research, Germany) under the auspices of the ExZellTUM II project (grant number 03XP0081) and by the BMWI (Federal Ministry for Economic Affairs and Energy) under the auspices of the SurfaLib project (grant number 03ET6103F).

References:

(1) Landesfeind, J.; Pritzl, D.; Gasteiger, H. A. An Analysis Protocol for Three-Electrode Li-Ion Battery Impedance Spectra: Part I. Analysis of a High-Voltage Positive Electrode. J. Electrochem. Soc.
2017, 164 (7), A1773–A1783.

(2) Pritzl, D.; Landesfeind, J.; Solchenbach, S.; Gasteiger, H. A. An Analysis Protocol for Three-Electrode Li-Ion Battery Impedance Spectra: Part II. Analysis of a Graphite Anode Cycled vs. LNMO. J. Electrochem. Soc.
2018, 165 (10), A2145–A2153.

Figure 1