A Novel Reference Electrode for EIS Measurements in Sodium-Ion Batteries
Authors: Linsenmann Fabian , Pritzl Daniel , Fellinger Tim Patrick , Gasteiger Hubert A. 
Published: February 2020

Electrochemical impedance spectroscopy (EIS) is a powerful and non-invasive technique to gain valuable insights into the lithium or sodium intercalation kinetics via monitoring of the associated resistances.[1,2,3] To date, the measurement of symmetrical cells is the most commonly used approach to differentiate between the contributions of anode and cathode to the full-cell impedance. However, this method requires the reassembly of two identically treated anodes and cathodes from (aged) full-cells into a symmetric cell. Therefore, disassembly and reassembly of numerous cells is needed to perform state-of-charge (SOC) and/or state-of-life dependent EIS analyses. On the other hand, simultaneous in-situ measurement of the anode and cathode impedance can be achieved via incorporation of a micro-reference electrode (µ-RE) within the cell setup. Solchenbach et al.
[4] and others [5] have shown such a µ-RE has to fulfill the following fundamental requirements: (a) its potential has to be stable within the measuring time of the impedance spectrum, (b) it has to be located centrally between anode and cathode, and (c) its cross-sectional dimensions have to be small compared to the distance between the electrodes.
In this talk, we will introduce a novel micro-reference electrode for sodium-ion batteries based on an insulated tin wire with a diameter of ≈75 mm, further on referred to as Tin Wire Reference Electrode (TWRE), which is electrochemically alloyed with sodium after cell assembly from either the working or the counter electrode in order to receive a stable reference potential. This TWRE enables measuring the impedance response of sodium-based active materials in-situ during cycling. Since hard carbons (HCs) currently are the most promising candidates as anode materials for emerging sodium ion batteries (SIBs),[3,6] we will present data on the impedance evolution of a commercial HC anode as a function of SOC during sodiation and desodiation (see Figure 1).
By comparing in-situ EIS data with impedance measurements performed using the symmetrical cell approach, we will prove that reliable impedance responses can be obtained in-situ. Figure 1a exemplary shows two in-situ impedance spectra of a HC anode in a Nyquist plot representation, one measured directly after cell assembly and sodiation of the tin wire (0% SOC, black) and the other measured after sodiation of the HC anode to 300 mAh/g (ca. 100% SOC, green). The former displays a transmission-line like behavior with a 45° line at high frequencies and a largely capacitive behavior at low frequencies. From the impedance response collected at close to 100% SOC, the HC anode charge transfer resistance (RCT) can be quantified by fitting the semi-circle in the Nyquist plot with an R/Q element. Figure 1b depicts the RCT-values of the HC anode measured at various state-of-charge levels during sodiation and desodiation, showing continuously decreasing RCT values upon Na intercalation. These findings will be compared to the magnitude and the evolution of the cell resistances for the lithiation of the same HC, and implications on the rate capability originating therefrom will be discussed.

References

[1] D. Pritzl, J. Landesfeind, S. Solchenbach and H. A. Gasteiger., J. Electrochem.
Soc. 165 (10), A2145-A2153, 2018.

[2] R. Petibon, C. P. Aiken, N. N. Sinha, J. C. Burns, H. Ye, C. M. VanElzen, G. Jain, S. Trussler, and J. R. Dahn et al., J. Electrochem. Soc. 160 (1), A117-A124, 2013.

[3] C. Bommier, W. Luo, W.Y. Gao, A. Greaney, S. Ma, X. Ji, Carbon 76, 165–174, 2014.

[4] P. Abraham, S. D. Poppen, A. N. Jansen, J. Liu, and D. W. Dees, Electrochim.
Acta. 49, 4763-4775, 2004.

[5] S. Solchenbach, D. Pritzl, E. Kong, J. Landesfeind, and H. A. Gasteiger, J. Electrochem.
Soc. 163 (10), A2265-A2272, 2016.

[6] P. Bai, Y. He, X. Zou, X. Zhao, P. Xiong, Y. Xu, Adv. Energy Mater. 1703217, 2018.

Figure 1