In Situ Polysulfide Detection in Lithium Sulfur Cells

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Energy Conversion and Storage; Plasmonics and Optoelectronics

In Situ Polysulfide Detection in Lithium Sulfur Cells John-Paul Jones, Simon Christopher Jones, Frederick C. Krause, Jasmina Pasalic, and Ratnakumar V. Bugga J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01400 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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The Journal of Physical Chemistry Letters

In Situ Polysulfide Detection in Lithium Sulfur Cells John-Paul Jones,1,* Simon C. Jones,1 Frederick C. Krause,1 Jasmina Pasalic1 and Ratnakumar Bugga1 1

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 *

email: [email protected]

Abstract Lithium sulfur batteries promise significant improvements in specific energy compared to Li-ion, but are limited by capacity fade upon cycling. Efforts to improve durability focused on suppressing the solubility of intermediate polysulfides in the electrolyte. Here we describe an in situ electrochemical polysulfide detection method based on the cyclic volatmmetric response. The voltammetric peaks correlate with increased discharge, consistent with increased polysulfide species in the electrolyte as demonstrated by prior literature measurements using spectroscopic methods. We verified that adding metal sulfide species to the sulfur cathode and ceramiccoatings on the polyolefin separator result in reduced polysulfide concentration, consistent with improved cycle life reported earlier. Further, the use of highly concentrated electrolytes produces no detectable dissolved polysulfide species. Future advances in Li/S technology could utilize this method to determine the polysulfide contents in the electrolyte, and thus quantify the efficacy of the sulfur-sequestering strategies.

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Manuscript: Future energy storage needs demand higher energy density than is currently achievable using lithium-ion (Li-ion) technology. Among the forerunners for the next generation battery technologies is the Li sulfur (Li/S) system that can potentially offer 2-3 times improvement in specific energy, and is also attractive due to the low cost and abundance of S.1 These favorable attributes have prompted the intense development of Li/S technology for over three decades, but some of its inherent challenges remain unresolved as described in detail in earlier reviews.2–4 Polysulfide intermediates generated at the cathode (S82-, S62−, S42−, S32−) are soluble in the organic electrolytes and form a redox shuttle,5 causing impedance growth at the anode,6 loss of active materials at the cathode, low coulombic efficiency during cycling, and eventually rapid capacity fade.7–9 Equations 1 and 210 respectively represent the high and low plateaus observed experimentally during discharge of a sulfur cathode.


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3S8 + 8Li+ + 8e- ⇋ 4Li2S6

[1]

Li2S6 + xLi+ + xe- ⇋ (x - 4)Li2S + (5 - 0.5x)Li2S2 (8 ≤ x ≤ 10)

[2]

Several interesting strategies have been developed to sequester S and its reduction products within the cathode by modifying either the cathode structure, the separator and/or the electrolyte.2–4 Most Li/S-specific issues trace their roots back to the presence of Li polysulfide species dissolved in the electrolyte and yet the existence of these polysulfides is essential to extract meaningful capacity from Li/S cells.11 Dissolved polysulfide species are reactive and readily rearrange and oxidize,12 making their ex situ determination challenging. Instead, several researchers have investigated polysulfides in situ using X-ray diffraction (XRD),13,14 X-ray absorption spectroscopy (XAS),15,16 Raman spectroscopy,17,18 and nuclear magnetic resonance spectroscopy (NMR).19 While powerful, these techniques are difficult to implement and generally require specially designed cells which are often quite different from the conventional cell configurations. We sought to develop a straightforward in situ method to detect the presence of polysulfide species and assess their relative changes in concentration in the electrolyte of a Li/S cell, in a relatively simple configuration using potentiodynamic measurements (cyclic voltammetry, CV). We introduced a platinum (Pt) wire electrode into an otherwise standard three-electrode prismatic cell with a S cathode, Li anode, and Li reference electrode. CV experiments were carried out on the Pt electrode as the working electrode against the Li (anode) counter electrode and the Li reference electrode. Because of the redox activity of the higher order polysulfides (S82-, S62−, S42−, S32−), they can be detected from their voltage-dependent current responses at the Pt electrode. Since Pt is inert, it should neither contaminate the electrolytes with any extraneous ions nor affect the Li/S chemistry in any manner. This approach can also be



extended to pouch cell formats, that are typically employed in prototype cells. Electrolyte quantity, which is a critical factor in the design of high-energy Li/S cells,20 needs to be adequate for maintaining ionic contact with the Pt working electrode. CV has been employed previously to study the mechanism of Li/S cells,21,22 as well as in situ in a modified Swagelok cell using a Ni working electrode and Pt counter electrode.23 Although Dominko et al.’s 2011 study employed CV in a Li/S cell, our approach is significantly different because it employs a Pt working electrode (eliminating the possibility for contamination from foreign ions) and utilizes large electrodes in a prismatic glass cell which allows for other in situ (IR, UV/Vis, etc.) or ex situ (LC, NMR, etc.) measurements to be coupled with the CV experiments reported here. Two cathode variations were included in the study, namely “S” (S + carbon black + PDVF, 55:40:5 by mass) and “S-MoS2” (S + carbon black + MoS2 + PVDF, 65:15:15:5 by mass), which has been shown in our earlier studies to provide improved cycle life even with higher sulfur loadings. The two electrolytes tested were the standard 1.0 M LiTFSI/0.20 M LiNO3 in DME+DOL (95:5 vol.) and the concentrated “7M” LiTFSI in DME+DOL (1:1 vol.). Preparation of these cathodes and electrolytes has been described previously.24,25 Four-electrode cells were fabricated using 2.5×2.5 cm cathodes and anodes consisting of lithium foil (Foote Mineral Company) pressed onto copper foil, a 0.5×1.0 cm reference electrode consisting of lithium foil pressed onto a nickel mesh, and a 0.8 cm2 working electrode consisting of a Pt wire with heat shrink tubing wrapped around the upper part to isolate a uniform area within the electrolyte. The battery electrodes were either sealed individually with (Tonen-Setela 20 µm thick polyethylene “Tonen” separator) or separated using the Z-fold technique (Asahi CS Al2O3coated “Asahi” separator) because the Asahi separator could not be sealed to itself reliably. Tonen separator was also used with a spray-coated AlF3 layer as described previously.25 The


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separated anode, reference and cathode electrodes were placed on a 6.35 mm (¼”) thick polyethylene shim and inserted into the prismatic glass cell. The Pt working electrode was inserted through the side port in the glass cell so it was in contact with the electrolyte but not with the cell itself. Figure S1 shows a cartoon and a photograph of the cells used here. Charge-discharge measurements were performed using an Arbin battery cycler at 0.75 mA/cm2 (4.7 mA) with a 1.8 V discharge voltage cutoff and a 3.2 V charge cutoff. 1st cycle discharges were paused every 2.23 mAh (approximately 10% of the theoretical capacity based on 3.6 mg/cm2 S cathode, assuming a theoretical capacity of 1000 mAh/g) to measure CV at different states of charge (SOC). CV measurements were performed on a Princeton Applied Research 273A potentiostat using CorrWare software for control and referenced to Li/Li+ in all cases. The Pt working electrode was swept at 10 mV/s from OCV to 3.5 V, then down to 1.3 V, and back to OCV, with two cycles at each condition investigated. The initial scans were compared across cells to verify that the Pt working electrode maintained the same activity after use and cleaning. CV results from the baseline Li/S cell with a S cathode and Tonen separator are reproduced in Figure 1A. There is a low reductive peak around 1.4 V that is present in the initial CV in all cells in approximately equal magnitude and may be related to LiNO3 reduction.21 Oxidative current starts out very low with no discernable peaks in the as-assembled state (Fig. 1A, CV1, light blue), suggesting that there are no dissolved polysulfides in the electrolyte prior to discharge. As the discharge is continued or the state of charge decreased, there is a gradual increase in both the reductive and oxidative peaks, implying increasing amounts of polysulfides in the electrodes at lower states of charge. The voltammetric response increases by ~25-fold at the end of the discharge (Fig. 1A, CV7, purple). During this time, the electrolyte goes from



colorless to yellow to dark brown. By contrast, Figure 1B presents the results from an identical cell with a “7M” electrolyte instead of the standard 1.0 M electrolyte. In this case, there is no appreciable oxidative peak present during the CV regardless of state of discharge and the solution remains colorless during the discharge. The “7M” electrolyte contains no LiNO3, and reduction currents are also much weaker than in the standard electrolyte.

Figure 1. CV of Pt wire immersed in electrolyte of Li/S cell at different stages during the 1st discharge for standard 1.0 M LiTFSI/0.20 M LiNO3 in DME+DOL (95:5 vol.) (A) and high concentration “7M” LiTFSI in DME+DOL (1:1 vol.) (B) electrolytes Given the identity of the electrolyte and electrode components, the process occurring on the anodic sweep was identified as the oxidation of dissolved polysulfide species in the electrolyte. While the reduction peak (~1.6 V) also increases in intensity during the discharge, the change is not as drastic as the oxidation peak, nor is there a consistent trend vs. SOC. This suggests that the oxidized S species are not as easily reduced, therefore oxidation was taken as the measure of S concentration. It is also possible that after each oxidation, a portion of the soluble polysulfide reduced at the anode may be chemically reduced further against Li and thus not available for subsequent reduction (e.g., through formation of Li2S and its subsequent precipitation). Although there should be several dissolved polysulfide species in solution,4,17,18,26–


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28

only one significant oxidation peak was observed on the Pt working electrode, possibly

because the redox potentials are very similar (as expected from the discharge curve). Slower scan rates were used to see if multiple peaks could be resolved (Figure S2), however only one oxidative peak remained even at 1 mV/s. The peak position shifts from 2.8 to 3.4 V over the course of the discharge, indicating that the concentrations of various dissolved species may be changing. The full peak cannot be resolved without increasing the potential beyond the oxidation potential of the electrolyte, so integration of the full peak is challenging. Instead, the maximum oxidative current was taken as the measure to compare between cells with different cathodes, separators and electrolytes. Correlating oxidative current to concentration can be achieved by making a series of calibration standards. Previous work in our group focused on cathode24 and separator25 development, hence the following results where the polysulfide detection method was applied focus on variations in these materials. It can be useful to overlay the maximum oxidative current obtained during the CV with the discharge profile to visualize how the polysulfide concentration in the electrolyte varies during the 1st discharge (see Figure 2). The yellow traces in Figure 2 represent the cell potential during the 1st discharge (right axis) while the blue bars represent the maximum current observed during the CV taken at the discharge steps (left axis). Figure 2 compares four different cells, with varying cathode composition (baseline S or S-MoS2) and separator (Tonen and Asahi). The common feature that all graphs share is that the 1st voltage plateau around 2.3 V produces virtually no detectable polysulfide species in solution. Polysulfides begin to appear in the electrolyte beginning with the transition to the lower voltage plateau around 2.0 V. While polysulfides generally appear to increase approximately linearly with discharge capacity, when



an S-MoS2 cathode is employed with Tonen (Fig. 2C) the concentration of polysulfide species appears to stabilize during the second discharge plateau.

Figure 2. Discharge profile (yellow trace) and maximum oxidative peak height (blue bars) for S cathode with Tonen separator (A), S cathode with Asahi separator (B), S-MoS2 cathode with Tonen separator (C) and S-MoS2 cathode with Asahi separator (D). Leaving aside the details of each individual cell’s voltage profile, we can focus more on the change in polysulfide species during discharge, as in Figure 3. Although all the cells depicted in Fig. 3 are the same physical size, the cells with S-MoS2 cathodes have approximately double the theoretical capacity due to the higher loading. The electrolyte volume (3 mL) is the same in all cases, so the traces are effectively normalized to the geometric electrode area and electrolyte volume. Dissolved polysulfide concentration appears to decrease from the S/Tonen cell (Fig. 3,


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green stars) to the S/Asahi cell (Fig. 3, grey triangles) to the S/AlF3-coated Tonen cell (Fig. 3, orange squares). These results mirror earlier findings by Gewirth et al.17 and strengthen our earlier finding that ceramic-coated separators aid in cycle life, specific capacity and efficiency.25 Additionally, we find that S-MoS2 composite cathodes (Fig. 3 blue diamonds and purple circles) show lower concentrations of polysulfides in the electrolyte than the baseline S cells, in spite of the higher loading of the S-MoS2 cathode. Finally, a high concentration “7M” electrolyte was investigated in an S/Asahi cell (Fig. 3 yellow crosses) and was determined to have virtually no polysulfide content in the electrolyte even at full discharge at 40 °C. High concentration electrolytes have been shown previously to exhibit exceptional efficiency,25,29 albeit at reduced specific capacity, especially at room temperature.

Figure 3. Maximum oxidative peak height (A) and S-specific maximum oxidative peak height (B) as a function of 1st discharge capacity for Li/S cells with S or S-MoS2 cathodes and various separators This study provides a confirmatory explanation for the improved performance of hybrid S-MoS2 cathodes or ceramic-coated separators we reported earlier, which we attributed to some ‘polysulfide trapping’ within or near the sulfur cathode. It has been shown computationally30,31 that polysulfides interact more strongly with certain materials than others, therefore it seems plausible that some chemical binding between either the MoS2 in the cathode or Al2O3/AlF3 on



the separator leads to a decreased availability of polysulfides in the electrolyte. Continued cycling with the S-MoS2 cathode revealed that additional polysulfide species leach out into the electrolyte after 10 cycles (Figure S3). For the coated separators, one must also consider the physical aspect, whereby the additional ceramic coating may increase the tortuosity of the separator and thereby limit the migration of polysulfide species away from the cathode. These results strengthen our earlier findings25 that ceramic coated separators likely block or slow the passage of polysulfide species, but do not suppress their dissolution completely. Highly concentrated electrolytes severely limit the solubility of polysulfides, effectively constraining them to the cathode. Accessing the full cell capacity is difficult to achieve, however, as only the first plateau is readily available for discharge in these viscous electrolytes. In summary, we describe an in situ electrochemical method to measure the concentration of polysulfides in solution during cycling of the sulfur cathode, which is straightforward to implement and can provide an assessment of various strategies being reported in the literature to mitigate the ‘polysulfide shuttle effects’ – a key aspect in the development of long-life Li/S cells. However, much work remains to be done, particularly in understanding how polysulfide concentrations change during cell cycling and over time, to fully ascertain the role that polysulfides play and how best to mitigate their negative effects while utilizing the capacity that they offer. Acknowledgements This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The authors acknowledge the funding support of the US Army (CERDEC). Copyright 2018 California Institute of Technology. U.S. Government sponsorship acknowledged.


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In Situ Polysulfide Detection in Lithium Sulfur Cells

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