- Email: [email protected]
Lithium-Sulfur Batteries: Attaining the Critical Metrics
Please cite this article in press as: Bhargav et al., Lithium-Sulfur Batteries: Attaining the Critical Metrics, Joule (2020), https://doi.org/10.1016/ j.joule.2020.01.001
COMMENTARY
Lithium-Sulfur Batteries: Attaining the Critical Metrics Amruth Bhargav,1 Jiarui He,1 Abhay Gupta,1 and Arumugam Manthiram1,* Amruth Bhargav is a PhD student in the Materials Science and Engineering graduate program at the University of Texas at Austin. He obtained his B.E. in Mechanical Engineering from Visvesvaraya Technological University, India, in 2013 and a Master of Science in Mechanical Engineering from Indiana University-Purdue University Indianapolis in 2016. His research mainly focuses on lithium-sulfur and lithium-organosulfur batteries.
Dr. Jiarui He is a postdoctoral fellow in the Texas Materials Institute at the University of Texas at Austin. He obtained his B.E. (2012) and his PhD (2018) in electronic information materials and devices from the University of Electronic Science and Technology of China. His research interests are in the area of electrochemical conversion and storage materials. He has authored more than 76 publications, with 3,000 citations and an h-index of 33 (Google Scholar).
Abhay Gupta is a PhD student in the Materials Science and Engineering graduate program at the University of Texas at Austin. He received his bachelor’s degree from the Hildebrand Department of Petroleum and Geosystems Engineering at the University of Texas at Austin in 2016. His research mainly focuses on lithium-sulfur batteries with an emphasis on low-temperature performance.
Professor Arumugam Manthiram is the Cockrell Family Regents Chair in engineering and the Director of Texas Materials Institute and the Materials Science and Engineering program at the University of Texas at Austin. His research interests are in the area of materials for rechargeable batteries and fuel cells, including novel synthesis approaches, advanced characterization, and prototype device fabrication. He has authored more than 800 publications, with 60,000 citations and an h-index of 123 (Google Scholar). See https://www.sites.utexas. edu/manthiram for further details.
Introduction Lithium-sulfur (Li-S) batteries represent a potential step-change advance in humanity’s ability to electrochemically store energy, because of the high gravimetric capacity and low cost of sulfur. We are now on the precipice of the next phase of Li-S research, where new developments must palpably contribute to making the Li-S technology commercially relevant. To take this leap, the Li-S community should recognize the shortcomings in the parameters being used in current cell designs and deliberately move toward those that can fully realize the high-energy density promise of Li-S batteries. In this Commentary, we outline key areas for improvement in the Li-S technology. Although this commentary is organized on subsystem by subsystem basis, many of these challenges are interdependent. The following subsections present parameters that are critical to consider in Li-S research and provide a roadmap for overcoming the obstacles that have prevented the technology’s implementation to date. Finally, we introduce the ‘‘five 5s’’—the critical metrics that are essential for meeting the high-energy target of Li-S systems.
Cathode The cathode has received the largest volume of work among all Li-S cell components in academic literature. Here, we seek to highlight the critical challenges remaining in this area and also call attention to the unique interplay between sulfur and the electrolyte. The electrochemistry of the Li-S battery relies on the formation of soluble lithium polysulfide (LiPS) intermediates during charge and discharge. These redox-active species dissolve into the electrolyte and mediate the reaction kinetics of charge-transfer in solution, which in effect regulates the achievable capacity in the cell. The dissolution of the sulfur active material into the electrolyte highlights the need to consider the cathode and electrolyte in conjunction, rather than as separate decoupled entities. For this reason, this section first delves into parameters governing just the sulfur electrode and then proceeds to highlight metrics that best account for the dependencies of the cathode on the electrolyte. Use of Carbon The majority of sulfur cathodes in the literature employ composite frameworks containing significant amounts of conductive carbon (up to 50 wt %).1 This is to overcome the low electronic conductivity of sulfur. However, the addition of large amounts of conductive filler deprecates the cell-level specific energy. Therefore, minimization of carbon content in the cathode is critical to prioritize when optimizing the sulfur cathode. Similar to Li-ion battery cathodes, the carbon content should ideally be reduced to below 5%. Additionally, minimization of carbon content can be highly beneficial to the following three parameters: a. Porosity: Various carbons and their composites used in Li-S cathodes have a high surface area (100–1500 m2 g 1) and low tap density (0.1–0.3 g cm 3).2
Joule 4, 1–6, February 19, 2020 ª 2020 Elsevier Inc.
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Please cite this article in press as: Bhargav et al., Lithium-Sulfur Batteries: Attaining the Critical Metrics, Joule (2020), https://doi.org/10.1016/ j.joule.2020.01.001
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Figure 1. Critical Cathode Parameters (A) A visual comparison of the density of microporous carbon (Ketjenblack EC-600JD), sulfur, and a Li-ion battery cathode, LiCoO 2 . The powders were tapped to ensure efficient packing. (B) Interaction between water and carbon, showing its high hydrophobicity. (C) Interaction of water with a polar host on carbon, showing greatly improved hydrophilicity. (D) A simple polysulfide-absorption test comparing carbon and a polar host. The transparent solution in the case of polar host shows its strong interaction with polysulfides. (E) Analytical model showing the effect of sulfur loading on energy density at various E/S ratios. (F) Model showing the effect of E/C ratio (with subsequent points corresponding to increased sulfur utilization as indicated) has on the specific energy at various E/S ratios.
The density of microporous carbon, sulfur, and an established Li-ion battery material like LiCoO2 is visually compared through the photograph in Figure 1A. The density of carbon is strikingly low, being four and nine times less dense than, respectively, sulfur and LiCoO2. Therefore, the increased use of carbon results in a highly porous cathode. Large amounts of electrolytes are required to sufficiently wet all the surfaces in such a cathode, adding appreciable weight to the cell and diminishing the specific energy. In order to obtain a dense cathode with minimized electrolyte uptake, a sulfur host should possess low surface area (<100 m2 g 1) and high tap density (0.7–1 g cm 3). b. Wettability: The sulfur conversion reaction depends on solution-
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based redox of LiPSs, which in turn relies on facile ion transport. This is encouraged when the liquid electrolyte can completely envelop (i.e., wet) the cathode particles. The hydrophobicity of carbon resists wetting by the polar electrolyte, as seen in Figure 1B. Therefore, high carbon content in the cathode curtails wetting, resulting in poor ion transport and lowering the cell capacity. Hydrophilic hosts or surface treatments can enhance wettability (Figure 1C) and thereby facilitate electrolyte infiltration and promote LiPS conversion. c. Interaction: The sulfur host matrix at the cathode forms the primary electron conduction pathway for the various redox-active species during cycling. Effective electron transfer between the LiPS and conductive host occurs when
they are sufficiently bound. Non-polar carbon interacts weakly with the polar LiPSs in solution (as seen in Figure 1D), resulting in poor binding and electron transfer. Consequently, there is an incomplete conversion of LiPS and generation of an electronically insulating thin film of Li2S, which reduces the attainable capacity.3 A sulfur host should be both conductive and electrocatalytic. Such hosts can simultaneously facilitate LiPS binding (as observed in the case of polar hosts in Figure 1D) while promoting electron transfer, boosting its conversion and attainable capacity. In addition, a thin, lightweight layer of such polar materials could also be incorporated as an interlayer on the separator to simultaneously improve utilization and polysulfide confinement. Sulfur Loading Another critical parameter to consider when designing the sulfur cathode is the areal sulfur loading, in mg cm 2. Higher sulfur loading increases the areal capacity of the cathode (mA h cm 2). This maximizes the specific energy of the cell, because the increased fraction of active material offsets the ‘‘dead weight’’ present from the various inactive components like the current collectors and separator. The dynamic effect of sulfur loading on specific energy is modeled in Figure 1E. In this model, the specific energy of a Li-S battery is plotted as a function of areal sulfur loading, assuming that the cathode contains 70 wt % sulfur and achieves 1,000 mA h g 1 (~60% utilization). Further information about this model is provided in the Supplemental Information. It is evident that irrespective of the electrolyte amount, a sulfur loading of <4 mg cm 2 is insufficient to outbalance the weight of inactive cell components. Future investigations
Please cite this article in press as: Bhargav et al., Lithium-Sulfur Batteries: Attaining the Critical Metrics, Joule (2020), https://doi.org/10.1016/ j.joule.2020.01.001
into the sulfur cathode should strive for sulfur loadings of >5 mg cm 2 in order to obtain practically relevant results. Electrolyte-to-Sulfur (E/S) Ratio The electrolyte constitutes the largest weight fraction of a Li-S cell. Hence, it represents the most important lever in altering the specific energy of the cell. As highlighted above, the electrolyte and the sulfur are coupled owing to the formation of electrolyte-soluble LiPSs. Maximizing specific energy thus necessitates the consideration of the electrolyte volume used in the cell with respect to the amount of sulfur in the cathode. Electrolyte/sulfur (E/S) ratio (expressed in mL mg 1) is a parameter that accurately captures this dependency, as depicted in the model in Figure 1E. This shows that for a given loading, reducing the E/S ratio greatly improves the specific energy. A 50% or higher gain in specific energy can be achieved by lowering the E/S ratio from 5 to 2 mL mg 1. As mentioned above, one way to optimize the E/S ratio is to tune the porosity of a cathode. Although researchers are beginning to recognize the significance of this metric, a large portion of Li-S research output still continues to use E/S ratios >20 mL mg 1 or simply neglects to report E/S ratio altogether.1,4–6 This fails to meet the standards of what is needed in order to realistically evaluate the improvements being reported. Therefore, to fairly gauge the improvements in cathode design and make relevant improvements to specific energy, the community must focus on reducing the E/S ratio below 5 mL mg 1. Electrolyte-to-Capacity (E/C) Ratio Conventional ether-based electrolytes can dissolve up to 8 M sulfur in the form of LiPSs in solution.5 Therefore, at low E/S ratios, the dissolved LiPS concentration can be driven toward the saturation point of the electrolyte. The onset of electrolyte saturation by LiPS presents a barrier to further dissolution of active material, inhibiting the
conversion of remaining sulfur to Li2S. Thus, at limited electrolyte conditions, the sulfur utilization and capacity delivered by the cell tends to be poor. This interplay between electrolyte amount and sulfur utilization can be described by the parameter of electrolyte/capacity (E/C) ratio, defined as the amount of electrolyte used per unit of discharge capacity delivered in a cell (expressed in g [A h] 1 or mL [mA h] 1). E/C ratio is a consequence of sulfur utilization in the cell and thus cannot be directly controlled like the E/S ratio. However, as E/C ratio links the inactive weight of the electrolyte to the capacity derived by the cell, it is an effective determinant of specific energy. This effect at different E/S ratios has been modeled in Figure 1F. As expected, for a given E/S ratio, a higher sulfur utilization maximizes the specific energy, which is reflected by a minimized E/C ratio. This can also be looked at from a specificenergy standpoint. In order to achieve 325 W h kg 1, for example, a cathode operating at a low E/S ratio of 2 mL mg 1 can achieve the target specific energy with only 60% sulfur utilization (corresponding to ~1,000 mA h g 1, or an E/C ratio of 2 mL [mA h] 1). Alternatively, a cathode operating at a high E/S ratio of 5 mL mg 1 can achieve this energy density so long as the sulfur utilization is 90% (~1,500 mA h g 1 or E/C ratio of 3.3 mL [mA h] 1). This example emphasizes that the E/C ratio is a measure of the effectiveness of the electrolyte in enabling utilization for a given cathode architecture. The E/C ratio can be minimized by modulating properties such as wettability and interaction as detailed above. Therefore, beyond just lowering the E/S ratio below 5 mL mg 1, cathodes must be designed to also lower the E/C ratio below 5 mL (mA h) 1 to realistically evaluate new improvements. Electrolyte The electrolyte plays a critical role in the Li-S battery, beyond just ensuring excellent ionic conductivity. As
described previously, the nature of discharge in Li-S batteries relies on the dissolution of redox-active LiPS, which mediates the reaction kinetics of charge transfer. This presents unique challenges and opportunities when developing and optimizing Li-S battery electrolytes. Due to the solution-mediated reaction mechanisms, the Li-S electrolyte must be able to accommodate and solvate a wide array of lithium polysulfide species. However, although the dissolution of lithium polysulfides is essential, this also presents the consequence of parasitic shuttling of active material to the anode. This leads to significant active material loss over the course of cycling. An additional consideration is the long-term stability of the reactive lithium-metal anode in the presence of the chosen electrolyte.7 Therefore, the electrolyte must simultaneously guarantee satisfactory ionic conductivity, favorable solubility of LiPSs, minimization of the LiPS shuttle effect, and long-term stability with the Li-metal anode. The predominantly used ether-based electrolyte represents the currently best-identified compromise among this myriad of considerations. This electrolyte displays quite favorable ionic conductivity and LiPS solubility. Additionally, ethers display relatively high stability with Limetal in comparison to other classes of polar aprotic solvents. The effort to minimize LiPS shuttling and further extend the long-term stability with Limetal presents significant opportunities for liquid electrolytes and will require continued exploration of new electrolyte solvents, salts, and additives. There are also opportunities for exploration of new electrolyte frameworks, such as solid-state electrolytes as well as sparingly solvating electrolytes. In sparingly solvating electrolytes, the solvent is entirely coordinated by the salt in order to prevent dissolution of LiPS. In both systems, the circumvention of
Joule 4, 1–6, February 19, 2020
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Figure 2. Critical Anode and Cell Geometry Parameters (A) Model showing the variation of specific energy as a function of N/P ratio at various E/S ratios. Li anode was retrieved from a cycled (B) coin cell after 300 cycles and (C) pouch cell after 40 cycles, showing vastly different Li deposition morphologies.
LiPS dissolution requires the battery to rely on solid-state reduction from elemental sulfur to Li2S. This can present significant benefits for minimization of LiPS shuttling and long-term stability with Li metal. However, it also presents key difficulties in terms of reaction kinetics, particularly when using high sulfur loadings and minimal carbon content required for practical implementation. Given the diversity of approaches available, it is vital to keep in mind that the implementation of any new electrolyte framework must strive to meet the parameters outlined in this commentary. Through continued development of conventionally solvating, sparingly solvating, and solid-state electrolytes, the stringent design requirements of the Li-S battery could finally be met.
Anode Designing high-energy-density Li-S cells necessitates the use of a Li-metalbased anode, both to offset the low operating voltage (~2.1 V versus Li/ Li+) and match the high capacity of sulfur. However, enabling the stable cycling of the Li-metal anode denotes one of the most formidable challenges and largest areas for improvement in Li-S batteries. In this section, we look at how parameters like excess Li, anode architecture, and electrolyte
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degradation are key for unlocking Li-S performance. Negative-to-Positive (N/P) Capacity Ratio Negative/positive (N/P) ratio is defined as the areal capacity of the Li anode (negative electrode) to that of the sulfur cathode (positive electrode). An ideal Li-S full cell would operate at an N/P ratio of 1.8 However, excess Li is required to offset the Li loss occurring due to electrolyte decomposition and solidelectrolyte interphase (SEI) formation. The excess Li comes at the cost of specific energy. The impact of N/P ratio on cell-level specific energy as a function of different E/S ratios has been modeled in Figure 2A. When the N/P ratio is R20, the excess Li weight penalizes specific energy, limiting it to around 150 W h kg 1. As the N/P ratio approaches unity, the specific energy increases by 100% to 200%, depending on the E/S ratio used. Additionally, the use of low N/P ratios amplifies the gains to specific energy brought about by reducing E/S ratio. For example, at an N/P ratio of 5, the specific energy increases by 56% when the E/S ratio is reduced from 5 to 2 mL mg 1. Typical Li-S cells unfortunately use thick Li-foils (>500 mm or N/P R20) as the anode. Beyond simply lowering specific energy, this excess also compensates for Li loss during cycling, obfuscating the true cycle life. Therefore, in order to
augment the improvements brought about by reducing E/S and E/C ratios, the community should work toward using thin Li-foils (N/P ratio <5). This would vastly improve both gravimetric and volumetric energy density, while providing a realistic view of cycle life. Anode Architecture Li is a relatively soft metal, with a Mohs hardness of 0.6, whereas that of a metal like copper (Cu) is 3. This lack of robustness prevents it from being able to be rolled into thin, mechanically stable foils. For this reason, thin (50 mm) Li foils generally contain a Cu foil backing. Because Cu is very dense (8.9 g cm 3 versus 0.534 g cm 3 for Li), it contributes nearly 75% of the mass of such an anode, dramatically reducing its effective capacity.9 Cu also gets corroded by LiPSs. These issues hinder the use of thin Li foils in this format and hamper the reduction of N/P below 5. Therefore, future Li-S research must focus on employing thin Li anodes wherein the Li is infused into lightweight, robust, and lithiophilic host matrices.10 This will help minimize inactive current collector weight while also lowering the N/P ratio. Electrolyte Decomposition Repeated cycling of Li anode to high capacities involves the stripping and plating of several microns of Li metal. Exposure of the electrolyte to fresh Li surfaces in each cycle leads to their consumption and a continuous growth of insulating SEI. This problem is further exacerbated by the non-uniform deposits of high-surface-area Li metal with mossy and dendritic morphology. Particularly in the limited electrolyte environments that practical Li-S operation requires, this reduces the amount of electrolyte available for stable cathode operation, leading to premature cell failure.11 The instability of the Li-metal anode likely represents the single greatest obstacle to implementation of the Li-S battery. It is thus imperative to understand the Li-anode
Please cite this article in press as: Bhargav et al., Lithium-Sulfur Batteries: Attaining the Critical Metrics, Joule (2020), https://doi.org/10.1016/ j.joule.2020.01.001
Figure 3. Attaining Critical Metrics for Li-S Batteries (A) The five 5s showing the critical metrics required for high-energy-density Li-S batteries. (B) Schematic showing the main issues leading to a low-energy-density configuration and the key changes to overcome these limitations toward a highenergy-density configuration.
degradation behavior in the unique polysulfide-rich environment of Li-S batteries.8 Continued understanding of these unique interfaces could enable the development of additives or artificial SEI layers that could eventually overcome the challenges intrinsic to Li-metal anode.12 Cell Geometry A final overarching consideration is the cell testing geometry. Studies are generally conducted in coin-cell geometries in order to evaluate the fundamental performance of a new material in a high-throughput manner. However, the implementation of battery materials at large size scales brings about new challenges that coin-cell testing fails to highlight. For example, at a coin-cell level, uniform Li stripping and plating can be observed (Figure 2B). In contrast, at the large size scales of pouch-cells, uneven current densities across the anode can cause wide variations in the deposited morphology, which affects cycling behavior (Figure 2C). Another downside of coin cells is the ineffective volume utilization. In a typical 2032-type cell, <2% of the volume is occupied by the cell components. Whereas, in a vacuum-sealed soft-packing pouch cell, >65% of the
volume is utilized. This helps in the optimization of important parameters like E/S ratio and stack pressure. Therefore, testing new materials in large pouchcell geometries can help obtain realistic assessments of new modifications. The evaluation of new materials at size scales relevant to commercial implementation prioritizes the consideration of industry-relevant parameters. This will hopefully discourage the use of materials that cannot be produced at scale and hone the community’s attention to practical solutions.
the transition from a heavy, voluminous low-energy configuration shown in Figure 3B to a light-weight and compact high-energy system as depicted in Figure 3B. It is important to note that the attainment of these metrics does not necessarily signal the culmination of the development path for Li-S batteries. Rather, it points to the minimum metrics that the community must strive to achieve in putting our best foot forward toward making Li-S batteries a reality.
SUPPLEMENTAL INFORMATION Conclusion: The Five 5s for HighEnergy Li-S Cells The history of Li-ion batteries demonstrates that concerted efforts in recognizing and tackling the key technical challenges form the bedrock for their commercial success. Li-S technology can undergo a similar transformation, provided the research community is in unison on maximizing the system-level specific energy. This target can be simplified as the achievement of the following five metrics, which we title the ‘‘five 5s’’ as shown in Figure 3A: sulfur loading >5 mg cm 2, carbon content <5%, E/S ratio <5 mL mg 1, E/C ratio <5 mL (mA h) 1, and N/P ratio <5 in pouch-type cells. This would enable
Supplemental Information can be found online at https://doi.org/10. 1016/j.joule.2020.01.001.
ACKNOWLEDGMENTS The authors would like to thank the support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering under award number DESC0005397.
1. Chung, S.-H., Chang, C.-H., and Manthiram, A. (2018). Progress on the Critical Parameters for Lithium–Sulfur Batteries to be Practically Viable. Adv. Funct. Mater. 28, 1801188. 2. Wild, M., and Offer, G.J. (2019). Lithium– Sulfur Batteries (John Wiley & Sons, Inc.).
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3. Klein, M.J., Veith, G.M., and Manthiram, A. (2017). Rational Design of Lithium-Sulfur Battery Cathodes Based on Experimentally Determined Maximum Active Material Thickness. J. Am. Chem. Soc. 139, 9229– 9237. 4. Hagen, M., Hanselmann, D., Ahlbrecht, K., Mac¸a, R., Gerber, D., and Tu¨bke, J. (2015). Lithium–Sulfur Cells: The Gap between the State-of-the-Art and the Requirements for High Energy Battery Cells. Adv. Energy Mater. 5, 1401986. 5. McCloskey, B.D. (2015). Attainable gravimetric and volumetric energy density of Li-S and li ion battery cells with solid separator-protected Li metal anodes. J. Phys. Chem. Lett. 6, 4581–4588. 6. Zhao, M., Li, B.-Q., Peng, H.-J., Yuan, H., Wei, J.-Y., and Huang, J.-Q. Challenges and Opportunities towards Practical Lithium–
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Sulfur Batteries under Lean Electrolyte Conditions. Angew. Chem. 10.1002/ ange.201909339. 7. Gupta, A., Bhargav, A., and Manthiram, A. (2019). Highly Solvating Electrolytes for Lithium-Sulfur Batteries. Adv. Energy Mater. 9, 1803096. 8. Nanda, S., Gupta, A., and Manthiram, A. (2018). A Lithium–Sulfur Cell Based on Reversible Lithium Deposition from a Li2S Cathode Host onto a Hostless-Anode Substrate. Adv. Energy Mater. 8, 1801556. 9. Heligman, B.T., Kreder, K.J., and Manthiram, A. (2019). Zn-Sn Interdigitated Eutectic Alloy Anodes with High Volumetric Capacity for Lithium-Ion Batteries. Joule 3, 1051–1063. 10. Peng, H.-J., Huang, J.-Q., Cheng, X.-B., and Zhang, Q. (2017). Review on High-Loading
and High-Energy Lithium–Sulfur Batteries. Adv. Energy Mater. 7, 1700260. 11. Cheng, X.-B., Yan, C., Huang, J.-Q., Li, P., Zhu, L., Zhao, L., Zhang, Y., Zhu, W., Yang, S.-T., and Zhang, Q. (2017). The gap between long lifespan Li-S coin and pouch cells: The importance of lithium metal anode protection. Energy Storage Materials 6, 18–25. 12. Yan, C., Zhang, X.-Q., Huang, J.-Q., Liu, Q., and Zhang, Q. (2019). Lithium-Anode Protection in Lithium-Sulfur Batteries. Trends in Chemistry 1, 693–704. 1Materials
Science and Engineering Program and Texas Materials Institute, University of Texas at Austin, Austin, TX 78712, USA *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2020.01.001
COMMENTARY
Lithium-Sulfur Batteries: Attaining the Critical Metrics Amruth Bhargav,1 Jiarui He,1 Abhay Gupta,1 and Arumugam Manthiram1,* Amruth Bhargav is a PhD student in the Materials Science and Engineering graduate program at the University of Texas at Austin. He obtained his B.E. in Mechanical Engineering from Visvesvaraya Technological University, India, in 2013 and a Master of Science in Mechanical Engineering from Indiana University-Purdue University Indianapolis in 2016. His research mainly focuses on lithium-sulfur and lithium-organosulfur batteries.
Dr. Jiarui He is a postdoctoral fellow in the Texas Materials Institute at the University of Texas at Austin. He obtained his B.E. (2012) and his PhD (2018) in electronic information materials and devices from the University of Electronic Science and Technology of China. His research interests are in the area of electrochemical conversion and storage materials. He has authored more than 76 publications, with 3,000 citations and an h-index of 33 (Google Scholar).
Abhay Gupta is a PhD student in the Materials Science and Engineering graduate program at the University of Texas at Austin. He received his bachelor’s degree from the Hildebrand Department of Petroleum and Geosystems Engineering at the University of Texas at Austin in 2016. His research mainly focuses on lithium-sulfur batteries with an emphasis on low-temperature performance.
Professor Arumugam Manthiram is the Cockrell Family Regents Chair in engineering and the Director of Texas Materials Institute and the Materials Science and Engineering program at the University of Texas at Austin. His research interests are in the area of materials for rechargeable batteries and fuel cells, including novel synthesis approaches, advanced characterization, and prototype device fabrication. He has authored more than 800 publications, with 60,000 citations and an h-index of 123 (Google Scholar). See https://www.sites.utexas. edu/manthiram for further details.
Introduction Lithium-sulfur (Li-S) batteries represent a potential step-change advance in humanity’s ability to electrochemically store energy, because of the high gravimetric capacity and low cost of sulfur. We are now on the precipice of the next phase of Li-S research, where new developments must palpably contribute to making the Li-S technology commercially relevant. To take this leap, the Li-S community should recognize the shortcomings in the parameters being used in current cell designs and deliberately move toward those that can fully realize the high-energy density promise of Li-S batteries. In this Commentary, we outline key areas for improvement in the Li-S technology. Although this commentary is organized on subsystem by subsystem basis, many of these challenges are interdependent. The following subsections present parameters that are critical to consider in Li-S research and provide a roadmap for overcoming the obstacles that have prevented the technology’s implementation to date. Finally, we introduce the ‘‘five 5s’’—the critical metrics that are essential for meeting the high-energy target of Li-S systems.
Cathode The cathode has received the largest volume of work among all Li-S cell components in academic literature. Here, we seek to highlight the critical challenges remaining in this area and also call attention to the unique interplay between sulfur and the electrolyte. The electrochemistry of the Li-S battery relies on the formation of soluble lithium polysulfide (LiPS) intermediates during charge and discharge. These redox-active species dissolve into the electrolyte and mediate the reaction kinetics of charge-transfer in solution, which in effect regulates the achievable capacity in the cell. The dissolution of the sulfur active material into the electrolyte highlights the need to consider the cathode and electrolyte in conjunction, rather than as separate decoupled entities. For this reason, this section first delves into parameters governing just the sulfur electrode and then proceeds to highlight metrics that best account for the dependencies of the cathode on the electrolyte. Use of Carbon The majority of sulfur cathodes in the literature employ composite frameworks containing significant amounts of conductive carbon (up to 50 wt %).1 This is to overcome the low electronic conductivity of sulfur. However, the addition of large amounts of conductive filler deprecates the cell-level specific energy. Therefore, minimization of carbon content in the cathode is critical to prioritize when optimizing the sulfur cathode. Similar to Li-ion battery cathodes, the carbon content should ideally be reduced to below 5%. Additionally, minimization of carbon content can be highly beneficial to the following three parameters: a. Porosity: Various carbons and their composites used in Li-S cathodes have a high surface area (100–1500 m2 g 1) and low tap density (0.1–0.3 g cm 3).2
Joule 4, 1–6, February 19, 2020 ª 2020 Elsevier Inc.
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Please cite this article in press as: Bhargav et al., Lithium-Sulfur Batteries: Attaining the Critical Metrics, Joule (2020), https://doi.org/10.1016/ j.joule.2020.01.001
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E/C Ratio ( L [mAh]-1)
Figure 1. Critical Cathode Parameters (A) A visual comparison of the density of microporous carbon (Ketjenblack EC-600JD), sulfur, and a Li-ion battery cathode, LiCoO 2 . The powders were tapped to ensure efficient packing. (B) Interaction between water and carbon, showing its high hydrophobicity. (C) Interaction of water with a polar host on carbon, showing greatly improved hydrophilicity. (D) A simple polysulfide-absorption test comparing carbon and a polar host. The transparent solution in the case of polar host shows its strong interaction with polysulfides. (E) Analytical model showing the effect of sulfur loading on energy density at various E/S ratios. (F) Model showing the effect of E/C ratio (with subsequent points corresponding to increased sulfur utilization as indicated) has on the specific energy at various E/S ratios.
The density of microporous carbon, sulfur, and an established Li-ion battery material like LiCoO2 is visually compared through the photograph in Figure 1A. The density of carbon is strikingly low, being four and nine times less dense than, respectively, sulfur and LiCoO2. Therefore, the increased use of carbon results in a highly porous cathode. Large amounts of electrolytes are required to sufficiently wet all the surfaces in such a cathode, adding appreciable weight to the cell and diminishing the specific energy. In order to obtain a dense cathode with minimized electrolyte uptake, a sulfur host should possess low surface area (<100 m2 g 1) and high tap density (0.7–1 g cm 3). b. Wettability: The sulfur conversion reaction depends on solution-
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based redox of LiPSs, which in turn relies on facile ion transport. This is encouraged when the liquid electrolyte can completely envelop (i.e., wet) the cathode particles. The hydrophobicity of carbon resists wetting by the polar electrolyte, as seen in Figure 1B. Therefore, high carbon content in the cathode curtails wetting, resulting in poor ion transport and lowering the cell capacity. Hydrophilic hosts or surface treatments can enhance wettability (Figure 1C) and thereby facilitate electrolyte infiltration and promote LiPS conversion. c. Interaction: The sulfur host matrix at the cathode forms the primary electron conduction pathway for the various redox-active species during cycling. Effective electron transfer between the LiPS and conductive host occurs when
they are sufficiently bound. Non-polar carbon interacts weakly with the polar LiPSs in solution (as seen in Figure 1D), resulting in poor binding and electron transfer. Consequently, there is an incomplete conversion of LiPS and generation of an electronically insulating thin film of Li2S, which reduces the attainable capacity.3 A sulfur host should be both conductive and electrocatalytic. Such hosts can simultaneously facilitate LiPS binding (as observed in the case of polar hosts in Figure 1D) while promoting electron transfer, boosting its conversion and attainable capacity. In addition, a thin, lightweight layer of such polar materials could also be incorporated as an interlayer on the separator to simultaneously improve utilization and polysulfide confinement. Sulfur Loading Another critical parameter to consider when designing the sulfur cathode is the areal sulfur loading, in mg cm 2. Higher sulfur loading increases the areal capacity of the cathode (mA h cm 2). This maximizes the specific energy of the cell, because the increased fraction of active material offsets the ‘‘dead weight’’ present from the various inactive components like the current collectors and separator. The dynamic effect of sulfur loading on specific energy is modeled in Figure 1E. In this model, the specific energy of a Li-S battery is plotted as a function of areal sulfur loading, assuming that the cathode contains 70 wt % sulfur and achieves 1,000 mA h g 1 (~60% utilization). Further information about this model is provided in the Supplemental Information. It is evident that irrespective of the electrolyte amount, a sulfur loading of <4 mg cm 2 is insufficient to outbalance the weight of inactive cell components. Future investigations
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into the sulfur cathode should strive for sulfur loadings of >5 mg cm 2 in order to obtain practically relevant results. Electrolyte-to-Sulfur (E/S) Ratio The electrolyte constitutes the largest weight fraction of a Li-S cell. Hence, it represents the most important lever in altering the specific energy of the cell. As highlighted above, the electrolyte and the sulfur are coupled owing to the formation of electrolyte-soluble LiPSs. Maximizing specific energy thus necessitates the consideration of the electrolyte volume used in the cell with respect to the amount of sulfur in the cathode. Electrolyte/sulfur (E/S) ratio (expressed in mL mg 1) is a parameter that accurately captures this dependency, as depicted in the model in Figure 1E. This shows that for a given loading, reducing the E/S ratio greatly improves the specific energy. A 50% or higher gain in specific energy can be achieved by lowering the E/S ratio from 5 to 2 mL mg 1. As mentioned above, one way to optimize the E/S ratio is to tune the porosity of a cathode. Although researchers are beginning to recognize the significance of this metric, a large portion of Li-S research output still continues to use E/S ratios >20 mL mg 1 or simply neglects to report E/S ratio altogether.1,4–6 This fails to meet the standards of what is needed in order to realistically evaluate the improvements being reported. Therefore, to fairly gauge the improvements in cathode design and make relevant improvements to specific energy, the community must focus on reducing the E/S ratio below 5 mL mg 1. Electrolyte-to-Capacity (E/C) Ratio Conventional ether-based electrolytes can dissolve up to 8 M sulfur in the form of LiPSs in solution.5 Therefore, at low E/S ratios, the dissolved LiPS concentration can be driven toward the saturation point of the electrolyte. The onset of electrolyte saturation by LiPS presents a barrier to further dissolution of active material, inhibiting the
conversion of remaining sulfur to Li2S. Thus, at limited electrolyte conditions, the sulfur utilization and capacity delivered by the cell tends to be poor. This interplay between electrolyte amount and sulfur utilization can be described by the parameter of electrolyte/capacity (E/C) ratio, defined as the amount of electrolyte used per unit of discharge capacity delivered in a cell (expressed in g [A h] 1 or mL [mA h] 1). E/C ratio is a consequence of sulfur utilization in the cell and thus cannot be directly controlled like the E/S ratio. However, as E/C ratio links the inactive weight of the electrolyte to the capacity derived by the cell, it is an effective determinant of specific energy. This effect at different E/S ratios has been modeled in Figure 1F. As expected, for a given E/S ratio, a higher sulfur utilization maximizes the specific energy, which is reflected by a minimized E/C ratio. This can also be looked at from a specificenergy standpoint. In order to achieve 325 W h kg 1, for example, a cathode operating at a low E/S ratio of 2 mL mg 1 can achieve the target specific energy with only 60% sulfur utilization (corresponding to ~1,000 mA h g 1, or an E/C ratio of 2 mL [mA h] 1). Alternatively, a cathode operating at a high E/S ratio of 5 mL mg 1 can achieve this energy density so long as the sulfur utilization is 90% (~1,500 mA h g 1 or E/C ratio of 3.3 mL [mA h] 1). This example emphasizes that the E/C ratio is a measure of the effectiveness of the electrolyte in enabling utilization for a given cathode architecture. The E/C ratio can be minimized by modulating properties such as wettability and interaction as detailed above. Therefore, beyond just lowering the E/S ratio below 5 mL mg 1, cathodes must be designed to also lower the E/C ratio below 5 mL (mA h) 1 to realistically evaluate new improvements. Electrolyte The electrolyte plays a critical role in the Li-S battery, beyond just ensuring excellent ionic conductivity. As
described previously, the nature of discharge in Li-S batteries relies on the dissolution of redox-active LiPS, which mediates the reaction kinetics of charge transfer. This presents unique challenges and opportunities when developing and optimizing Li-S battery electrolytes. Due to the solution-mediated reaction mechanisms, the Li-S electrolyte must be able to accommodate and solvate a wide array of lithium polysulfide species. However, although the dissolution of lithium polysulfides is essential, this also presents the consequence of parasitic shuttling of active material to the anode. This leads to significant active material loss over the course of cycling. An additional consideration is the long-term stability of the reactive lithium-metal anode in the presence of the chosen electrolyte.7 Therefore, the electrolyte must simultaneously guarantee satisfactory ionic conductivity, favorable solubility of LiPSs, minimization of the LiPS shuttle effect, and long-term stability with the Li-metal anode. The predominantly used ether-based electrolyte represents the currently best-identified compromise among this myriad of considerations. This electrolyte displays quite favorable ionic conductivity and LiPS solubility. Additionally, ethers display relatively high stability with Limetal in comparison to other classes of polar aprotic solvents. The effort to minimize LiPS shuttling and further extend the long-term stability with Limetal presents significant opportunities for liquid electrolytes and will require continued exploration of new electrolyte solvents, salts, and additives. There are also opportunities for exploration of new electrolyte frameworks, such as solid-state electrolytes as well as sparingly solvating electrolytes. In sparingly solvating electrolytes, the solvent is entirely coordinated by the salt in order to prevent dissolution of LiPS. In both systems, the circumvention of
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Specific Energy (Wh kg-1)
A
C
B
600
E/S ratio 2 4
500
L mg-1) 3 5
400 300 200
3 mm
1 cm
100 0
5
10
15
20
N/P ratio
Li anode from coin cell after 300 cycles
Li anode from pouch cell after 40 cycles
Figure 2. Critical Anode and Cell Geometry Parameters (A) Model showing the variation of specific energy as a function of N/P ratio at various E/S ratios. Li anode was retrieved from a cycled (B) coin cell after 300 cycles and (C) pouch cell after 40 cycles, showing vastly different Li deposition morphologies.
LiPS dissolution requires the battery to rely on solid-state reduction from elemental sulfur to Li2S. This can present significant benefits for minimization of LiPS shuttling and long-term stability with Li metal. However, it also presents key difficulties in terms of reaction kinetics, particularly when using high sulfur loadings and minimal carbon content required for practical implementation. Given the diversity of approaches available, it is vital to keep in mind that the implementation of any new electrolyte framework must strive to meet the parameters outlined in this commentary. Through continued development of conventionally solvating, sparingly solvating, and solid-state electrolytes, the stringent design requirements of the Li-S battery could finally be met.
Anode Designing high-energy-density Li-S cells necessitates the use of a Li-metalbased anode, both to offset the low operating voltage (~2.1 V versus Li/ Li+) and match the high capacity of sulfur. However, enabling the stable cycling of the Li-metal anode denotes one of the most formidable challenges and largest areas for improvement in Li-S batteries. In this section, we look at how parameters like excess Li, anode architecture, and electrolyte
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degradation are key for unlocking Li-S performance. Negative-to-Positive (N/P) Capacity Ratio Negative/positive (N/P) ratio is defined as the areal capacity of the Li anode (negative electrode) to that of the sulfur cathode (positive electrode). An ideal Li-S full cell would operate at an N/P ratio of 1.8 However, excess Li is required to offset the Li loss occurring due to electrolyte decomposition and solidelectrolyte interphase (SEI) formation. The excess Li comes at the cost of specific energy. The impact of N/P ratio on cell-level specific energy as a function of different E/S ratios has been modeled in Figure 2A. When the N/P ratio is R20, the excess Li weight penalizes specific energy, limiting it to around 150 W h kg 1. As the N/P ratio approaches unity, the specific energy increases by 100% to 200%, depending on the E/S ratio used. Additionally, the use of low N/P ratios amplifies the gains to specific energy brought about by reducing E/S ratio. For example, at an N/P ratio of 5, the specific energy increases by 56% when the E/S ratio is reduced from 5 to 2 mL mg 1. Typical Li-S cells unfortunately use thick Li-foils (>500 mm or N/P R20) as the anode. Beyond simply lowering specific energy, this excess also compensates for Li loss during cycling, obfuscating the true cycle life. Therefore, in order to
augment the improvements brought about by reducing E/S and E/C ratios, the community should work toward using thin Li-foils (N/P ratio <5). This would vastly improve both gravimetric and volumetric energy density, while providing a realistic view of cycle life. Anode Architecture Li is a relatively soft metal, with a Mohs hardness of 0.6, whereas that of a metal like copper (Cu) is 3. This lack of robustness prevents it from being able to be rolled into thin, mechanically stable foils. For this reason, thin (50 mm) Li foils generally contain a Cu foil backing. Because Cu is very dense (8.9 g cm 3 versus 0.534 g cm 3 for Li), it contributes nearly 75% of the mass of such an anode, dramatically reducing its effective capacity.9 Cu also gets corroded by LiPSs. These issues hinder the use of thin Li foils in this format and hamper the reduction of N/P below 5. Therefore, future Li-S research must focus on employing thin Li anodes wherein the Li is infused into lightweight, robust, and lithiophilic host matrices.10 This will help minimize inactive current collector weight while also lowering the N/P ratio. Electrolyte Decomposition Repeated cycling of Li anode to high capacities involves the stripping and plating of several microns of Li metal. Exposure of the electrolyte to fresh Li surfaces in each cycle leads to their consumption and a continuous growth of insulating SEI. This problem is further exacerbated by the non-uniform deposits of high-surface-area Li metal with mossy and dendritic morphology. Particularly in the limited electrolyte environments that practical Li-S operation requires, this reduces the amount of electrolyte available for stable cathode operation, leading to premature cell failure.11 The instability of the Li-metal anode likely represents the single greatest obstacle to implementation of the Li-S battery. It is thus imperative to understand the Li-anode
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Figure 3. Attaining Critical Metrics for Li-S Batteries (A) The five 5s showing the critical metrics required for high-energy-density Li-S batteries. (B) Schematic showing the main issues leading to a low-energy-density configuration and the key changes to overcome these limitations toward a highenergy-density configuration.
degradation behavior in the unique polysulfide-rich environment of Li-S batteries.8 Continued understanding of these unique interfaces could enable the development of additives or artificial SEI layers that could eventually overcome the challenges intrinsic to Li-metal anode.12 Cell Geometry A final overarching consideration is the cell testing geometry. Studies are generally conducted in coin-cell geometries in order to evaluate the fundamental performance of a new material in a high-throughput manner. However, the implementation of battery materials at large size scales brings about new challenges that coin-cell testing fails to highlight. For example, at a coin-cell level, uniform Li stripping and plating can be observed (Figure 2B). In contrast, at the large size scales of pouch-cells, uneven current densities across the anode can cause wide variations in the deposited morphology, which affects cycling behavior (Figure 2C). Another downside of coin cells is the ineffective volume utilization. In a typical 2032-type cell, <2% of the volume is occupied by the cell components. Whereas, in a vacuum-sealed soft-packing pouch cell, >65% of the
volume is utilized. This helps in the optimization of important parameters like E/S ratio and stack pressure. Therefore, testing new materials in large pouchcell geometries can help obtain realistic assessments of new modifications. The evaluation of new materials at size scales relevant to commercial implementation prioritizes the consideration of industry-relevant parameters. This will hopefully discourage the use of materials that cannot be produced at scale and hone the community’s attention to practical solutions.
the transition from a heavy, voluminous low-energy configuration shown in Figure 3B to a light-weight and compact high-energy system as depicted in Figure 3B. It is important to note that the attainment of these metrics does not necessarily signal the culmination of the development path for Li-S batteries. Rather, it points to the minimum metrics that the community must strive to achieve in putting our best foot forward toward making Li-S batteries a reality.
SUPPLEMENTAL INFORMATION Conclusion: The Five 5s for HighEnergy Li-S Cells The history of Li-ion batteries demonstrates that concerted efforts in recognizing and tackling the key technical challenges form the bedrock for their commercial success. Li-S technology can undergo a similar transformation, provided the research community is in unison on maximizing the system-level specific energy. This target can be simplified as the achievement of the following five metrics, which we title the ‘‘five 5s’’ as shown in Figure 3A: sulfur loading >5 mg cm 2, carbon content <5%, E/S ratio <5 mL mg 1, E/C ratio <5 mL (mA h) 1, and N/P ratio <5 in pouch-type cells. This would enable
Supplemental Information can be found online at https://doi.org/10. 1016/j.joule.2020.01.001.
ACKNOWLEDGMENTS The authors would like to thank the support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering under award number DESC0005397.
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Science and Engineering Program and Texas Materials Institute, University of Texas at Austin, Austin, TX 78712, USA *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2020.01.001