Lithium-sulfur (Li-S) batteries have been viewed as one of the promising electric energy storage technologies due to the high theoretical energy density and the abundance of sulfur [1]. However, the utilization of sulfur is typically only 60-70% given the commonly reported reversible capacity of ~1000 mAh/g compared with the theoretical capacity of 1672 mAh/g [2]. The complicated reaction mechanism at the positive electrode is one of the causes for this discrepancy [3]. Though the charged (elemental sulfur, S) and discharged (lithium sulfide, Li₂S) states are solid, the reaction intermediates (Li₂S_x, x=4-8) are soluble in ether-based electrolytes, e.g. 1,3-dioxolane (DOL) [3]. Therefore, the active material goes through repeated dissolution and precipitation during cell operation. Furthermore, both S and Li₂S are insulating, so a conductive porous carbon matrix is required as a host structure [1]. The precipitation of the insulating S and Li₂S particles can decrease the electronically-conductive surface area or even block the pores in the carbon matrix, which leads to incomplete reaction and thus low utilization of active material [3]. In this work, the correlation between the evolution of the solid species and the electrochemical response of the porous electrode is investigated by operando X-ray diffraction (XRD) coupled with the Intermittent Current Interruption (ICI) method [4].

The ICI method is a facile way to follow the real-time internal resistance of a battery cycling at a constant current. Periodic transient current pauses are applied to obtain the potential response of the system [4]. From the potential-time relationship, both time-independent and time-dependent parts of the resistance can be derived. The time-independent resistance has been shown to correspond to the sum of the resistive elements in an equivalent circuit model of the electrochemical impedance spectroscopic (EIS) response [4]. In terms of the electrochemical process, the time-independent resistance includes the electronic, ionic and charge transfer resistances. In our recent work [5], the time-dependent part of the resistance from ICI measurements is analyzed by comparison with the existing porous electrode model. This enables us to characterize the mass transport in the positive electrode during the evolution of the solid species as captured by operando XRD.

Along with the resistance profile, this work will present the results from in-house operando XRD of a Li-S cell with our cell design, which is dedicated to preserve the reproducible electrochemical properties while offering a satisfactory signal-to-noise ratio in the XRD data. With the information of the mass transport in porous electrode and evolution of crystalline species, the concurrent analyses of both techniques will illustrate the dynamic interaction between the solid sulfur species and the carbon matrix at the positive electrode of Li-S batteries.

Figure: Operando XRD results with the diffraction pattern as the heatmap at the top, followed by the cell voltage (E), time-independent resistance (R) and time-dependent resistance (k). The legend for the colors used in the heatmap and following curves are labeled at the top and bottom of the graph.

References

[1] S. S. Zhang, “Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions,” J. Power Sources, vol. 231, pp. 153–162, 2013.
[2] M. Hagen et al., “Sulfur Cathodes with Carbon Current Collector for Li-S cells,” J. Electrochem. Soc., vol. 160, no. 6, pp. A996–A1002, 2013.
[3] M. Wild et al., “Lithium sulfur batteries, a mechanistic review,” Energy Environ. Sci., vol. 8, no. 12, pp. 3477–3494, 2015.
[4] M. J. Lacey, “Influence of the Electrolyte on the Internal Resistance of Lithium−Sulfur Batteries Studied with an Intermittent Current Interruption Method,” ChemElectroChem, vol. 4, no. 8, pp. 1997–2004, 2017.
[5] In Manuscript.