Lithium-Sulfur (Li-S) batteries are an emerging energy storage technology, which has started recently to enter the market. This battery chemistry is appealing because of its high theoretical specific capacity (1675 Ah/g) and high theoretical energy density (2600 Wh/kg). [1]. On the other hand, the developed Li-S batteries are characterized by fast capacity fading, high self-discharge, and poor coulombic efficiency [2].

Therefore, to analyze and assess the feasibility of using this new battery chemistry for various applications and for the improvement of the chemistry, a comprehensive understanding of the battery static and dynamic behavior is required. Thus, in the present work, we have performed an in-depth characterization and investigation of the impedance of a Li-S pouch cell by means of the electrochemical impedance spectroscopy (EIS) technique. Few similar studies are available in the literature, but the vast majority were performed on Li-S coin cells [1], [3].

Experimental

For this study a pre-commercial Li-S pouch cell manufactured by Oxis Energy was used (Fig. 1) The main electrical parameters of this Li-S battery cell are summarized in Table I.

In order to characterize and analyze the impedance behavior of the considered 3.4Ah Li-S pouch cell, EIS measurements were carried out considering a broad range of state of charges (SOCs), temperatures, and load currents. All the EIS measurements were performed in galvanostatic mode for a frequency sweep between 6.5 kHz and 10 mHz, considering 48 frequency points. For exemplification, the influence on the impedance spectra of the SOC and temperature are presented in Fig. 2 and Fig 3, respectively.

Results

To further analyze the data obtained from the EIS measurements, the impedance spectra were curve fitted with the equivalent electrical circuit (EEC) presented in Fig. 4. Thus, it was possible to assign different segments of the impedance spectra to different chemical/ physical processes and evaluate kinetic parameters (e.g., ionic diffusion, charge-transfer etc.).

A comparison between the Li-S battery cell’s measured and estimated impedance spectra is presented in Fig. 5. The results are showing that the selected EEC is able to fit very accurately the measured Nyquist curves.

For exemplification of the curve fitting process, the obatined values of R_s, R₁, R₂, and R₃ for different SOC levels and T=25°C are presented in Fig. 6.

References

[1] Deng Z. set al.. (2013). Electrochemical Impedance Spectroscopy Study of a Lithium/Sulfur Battery: Modeling and Analysis of Capacity Fading. J. Electrochem. Soc. 160(4), A553–A558.

[2] Bresser, D., Passerini, S., & Scrosati, B. (2013). Recent progress and remaining challenges in sulfur-based lithium secondary batteries - a review. Chem. Commun., 49(90), 10545–10562.

[3] Canas N.A. et al., (2013) Investigations of lithium-sulfur batteries using electrochemical impedance spectroscopy. Electrochemica Acta 97, 42-51.

Tables and Figures

Table I. Main electrical parameters of the pouch cell under test

Fig. 1. Li-S pouch cell produced by Oxis Energy

Fig. 2. Influence of SOC on the impedance spectra of the Li-S battery cell (T=25°C).

Fig. 3. Influence of temperature on the impedance spectra of the Li-S battery cell (SOC=0%).

Fig. 4. EEC used to fit the measured impedance spectra of the Li-S pouch cell.

Fig. 5. Measured and estimated impedance spectra.

Fig. 6. Variation of R_s (top-left), R₁ (top-right), R₂ (bottom-left), and R₃ (bottom-right) with the Li-S cell SOC.