Sparingly Solvating Electrolytes for High Energy Density Lithium

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Sparingly Solvating Electrolytes for High Energy Density Lithium-Sulfur Batteries Lei Cheng, Larry A Curtiss, Kevin R. Zavadil, Andrew A. Gewirth, Yuyan Shao, and Kevin G. Gallagher ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00194 • Publication Date (Web): 11 Jul 2016 Downloaded from http://pubs.acs.org on July 14, 2016

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Sparingly Solvating Electrolytes for High Energy Density Lithium-Sulfur Batteries Lei Cheng,1,2 Larry A. Curtiss,1,2 Kevin R. Zavadil,1,4 Andrew A. Gewirth,1,5 Yuyan Shao1,6, Kevin G. Gallagher1,3* 1

Joint Center for Energy Storage Research

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Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, USA

Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, USA 4

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Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

Department of Chemistry, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, USA

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Energy & Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA

AUTHOR INFORMATION Corresponding Author * [email protected], +1-630-252-4473

ACS Paragon Plus Environment

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ABSTRACT Moving to lighter and less expensive battery chemistries compared to contemporary lithium-ion requires the control of energy storage mechanisms based on chemical transformations rather than intercalation. Lithium sulfur (Li/S) has tremendous theoretical specific energy, but contemporary approaches to control this solution-mediated, precipitation-dissolution chemistry require large excesses of electrolyte to fully solubilize the polysulfide intermediates. Achieving reversible electrochemistry under lean electrolyte operation is the most promising path for Li/S to move beyond niche applications to potentially transformational performance. An emerging Li/S research area is the use of sparingly solvating electrolytes and the creation of design rules for discovering new electrolyte systems that fundamentally decouple electrolyte volume from sulfur and polysulfide reaction mechanism. This perspective presents an outlook for sparingly solvating electrolytes as a key path forward for long-lived, high-energy density Li/S batteries including an overview of this promising new concept and some strategies for accomplishing it.

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The commercialization of smaller, lighter and less expensive batteries compared to present-day lithium-ion may provide the critical tipping point to shift the transportation sector away from liquid carbonaceous fuels and to the widespread use of electric vehicles powered by electrochemical energy storage systems1. A step change in battery specific energy requires the control of energy storage mechanisms based on chemical transformations rather than intercalation. Batteries based on the intercalation energy storage mechanism carry the intercalation-host mass and volume penalty in exchange for enhanced stability and reversibility. Chemical transformations harnessed through electrochemical reactions such as metal plating and stripping coupled with the conversion of sulfur to sulfide are an exciting, though challenging pathway to the next generation of transportation batteries. The lithium-sulfur (Li/S) battery has been the subject of intense research and development for the past several decades owing to the low cost of sulfur and high materials-only specific energy and energy density values.2-4 Projections made via techno-economic modeling suggest long-term battery pack performance goals of $100/kWh, 400 Wh/L, and 400 Wh/kg may be reached on a useable energy basis5 through higher utilization of sulfur and significantly lower excesses of electrolyte and lithium metal than used in the vast majority of published works on the topic. Past research efforts have relied upon a large margin for compromise mistakenly judged from the high materials-only specific energy, which is a weak indicator of system level promise6. The ever improving energy density and cost of lithium-ion batteries provides little room for compromise in the Li/S chemistry if mass commercialization is to ever be achieved5, 7. However, the low molecular weight, high abundance, and low cost of sulfur warrant continued investigation as long as the key barriers preventing success are the focus of the efforts. One of the barriers is the development of an electrolyte that enables attainment of useable energy density goals and at the


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same time prevents reactant redistribution. This perspective presents an outlook for sparingly solvating electrolytes as the key path forward for long-lived, high energy density lithium-sulfur batteries including an overview of this promising new concept and some strategies for accomplishing it. The lithium sulfur to sulfide conversion falls within a broader mechanistic class of precipitation-dissolution reactions that undergo a state change during the redox process. The initial reduction of sulfur, S8, to polysulfide species in conventional ether electrolytes with ~1 M lithium salt results in the creation of soluble polysulfides (e.g., Li2Sn where n≥4). Continued reduction of the intermediate polysulfides eventually results in the end member lithium sulfide Li2S. Both S8 and Li2S bulk phases are electronically insulating and exhibit low solubility, though S8 is markedly higher than Li2S (~50 vs < 1 mmolS/L)8-9 in conventional electrolytes. The high solubility of polysulfides in the electrolyte results in the often reported “shuttle effect” between negative and positive electrodes10. Perhaps more importantly, the migration of polysulfides to the lithium metal negative electrode results in capacity loss and impedance rise in the form of precipitated Li2S in the solid electrolyte interphase of lithium metal. Regardless of chosen method to control the sulfur electrode, a viable lithium metal electrode will require protection to eliminate electrolyte consumption. Long-lived, efficient operation under low electrolyte to sulfur (E/S) ratios is a critical but less recognized challenge for Li/S chemistry5, 7, 11. The challenge of lowering the E/S ratio may have been overlooked due to the recent excitement generated by encapsulation strategies for polysulfide control. The pioneering work of Nazar et al., as well as several other excellent examples12-16, has proven that electrode architecture is necessary. The encapsulation schemes have created structures with tortuous paths for polysulfides to travel, but do not alone enable


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operation at low E/S ratios. Many of the architectures themselves have large internal porosities which prevent low E/S operation lest void space remains unfilled with electrolyte. Common E/S values used in the literature are greater than 10 mL/g17-18; conversely, values near 1 mL/g are necessary to compete with lithium-ion technology on an energy density basis (i.e. volume)5. While 1 mL/g may seem challenging, it is equivalent to a cathode design with 50 % volume of electrolyte in the charged state5 (assuming 70:20:10 mass ratio of S:C:binder for loadings greater than 3.5 mgS/cm2). This leaves adequate room within the cathode for the greater than 70% increase in reactant volume during conversion from sulfur to sulfide. State-of-the-art performance has been reported at 4 mL/g10. At E/S ratios less than 4, the electrochemical performance begins to show increased polarization in the second plateau and eventually a dramatic reduction in utilized capacity. The challenge of operating at low E/S ratios is proposed to be related to maximum solubility of polysulfide species ~3.9 mL/g (i.e. 8 molS/L) in conventional electrolytes. Li/S operation under low E/S ratios is further complicated by a high sensitivity to electrolyte consumption from side reactions with lithium and also potential sulfur intermediates that eventually result in cell dry-out and failure19. In the state-of-the-art 1 M Li-bis(trifluoromethane)sulfonamide (LiTFSI) in 1,3dioxolane/1,2-dimethoxyethane (DOL/DME) electrolyte operated at moderate and high E/S ratios (i.e. >4 mL/g), the polysulfides are solution species undergoing electrochemistry and chemical equilibria in the catholyte regime. In addition, the solubilized polysulfide species act as redox mediators to the insoluble and insulating end members. In this catholyte regime, only the primary reactant, S, and product, Li2S, are believed to be precipitated. A further complication of lean electrolyte operation is likely the disproportionation of polysulfides leading to uncontrolled insoluble precipitants and the passivation of electrode surface. Such a disproportionation


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mechanism may require a two- or three-body interaction (i.e. proportional to [Li2Sn]2 or [Li2Sn]3) and thus exacerbated at low E/S ratios where high concentrations of polysulfides are present. A deeper understanding of the limitation of conventional electrolytes under lean operation is missing from the published literature. Long-lived operation of Li/S cells at the target E/S ratio < 1 mL/g may require a fundamental shift in the role of the electrolyte from fully solvating polysulfides to a sparingly solvating regime. Successfully commercialized secondary battery electrodes based on the precipitationdissolution mechanism commonly utilize electrolytes in which the intermediate or product salt is only sparingly soluble20-21. For example, lead acid batteries operate with very low solution concentrations of PbSO4 (2x10-5 M)22-23 and sodium iron-chloride (i.e. ZEBRA) batteries minimize the concentration of FeCl224-25. Ag/AgCl and Cd/Cd(OH)2 are other well-known precipitation-dissolution reactions that successfully operate through limited solubility of active species. The secondary alkaline zinc metal electrode26-27, Zn/ZnO, exhibits limited cycle life, in part, due to the finite solubility of zincate. The meaning of a sparingly soluble reactant in the battery literature is not precisely defined, but perhaps varies from chemistry to chemistry. Generally, the concentrations are on the order of 1 mM or less. The advantages of sparingly soluble reactants are two-fold: i) greater energy density via the intrinsic higher charge density of the precipitated solid phase and ii) control of species migration in the solution phase and subsequent life-limiting mechanisms. Many of these electrodes exhibit high rates of discharge and charge demonstrating that low solubility of a reactant may not limit the rate capability of the battery. In concert with sparing solubility, the electrode architecture plays an important role by controlling product formation and enabling short transport lengths to the electrified interface.


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Electrolytes with low solubility of polysulfides have been the subject of recent review28-29. Only a few key papers are highlighted here with their main scientific contribution. Suo et al. systematically examined increasing LiTFSI concentration in DOL/DME binary solvent solutions resulting in so called “solvent in salt” electrolytes30. The most concentrated mixture of 7 moles of LiTFSI in 1 liter of DOL/DME (i.e. (DOL/DME)1.7-LiTFSI ) exhibited a very low solubility for polysulfides while simultaneously demonstrating comparatively longer cycle life. At a similar time, Dokko et al. examined the polysulfide solubility and corresponding Li/S performance of increasing the LiTFSI concentration in a TEGDME solvent reaching a similar conclusion8. Driving the electrolyte system into a solvate regime where all of the solvent is subject to strong interactions with the LiTFSI results in only small measured solubility of polysulfides (10-100 mMS). Dokko et al.8 proposed using a less viscous and nonsolvating diluent, in this case a hydrofluorinated ether (HFE)31, as a way to maintain or diminish further the polysulfide solubility while increasing performance through lowered electrolyte viscosity. With the addition of 4:1 molar ratio of HFE to TEGDME-LiTFSI solvate, the viscosity decreased from >100 cP to 0.5eV) that most anion receptors in solvents will be occupied by LiDCTA with minimum possibility to bind and dissolve Li2S6.


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BF3 B(C6F5)3

0.0

B(OCH2CF3)3

-0.2

Binding Energy (eV)

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-0.4 -0.6 -0.8 -1.0 -1.2 -1.4

Li2S6

LiTFSI

LiOTF

LiClO4

LiTDI

LiDCTA

Solute

Figure 3. Calculated binding energies of polysulfide Li2S6 and common salts LiTFSI, trifluoromethanesulfonate (LiOTF), LiClO4, 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI) and 4,5-dicyano-1,2,3-triazolate (LiDCTA) with different anion receptors. Calculations were performed using B3LYP/6-31+G* with the PCM solvation model as implemented in Gaussian 09.42 Successful operation of Li/S chemistry under lean electrolyte conditions will require both sparing solubility of the reactants and the protection of lithium metal. An unprotected lithium metal electrode will quickly reduce the electrolyte and thereby limit lifetimes under lean electrolyte operation to only 10’s of cycles19. However a protected lithium electrode alone is not enough for Li/S to meet transportation cycle life goals of 1000 cycles. History teaches that batteries based on precipitation-dissolution chemistries, even those without a reductive metal anode, require sparingly solvating electrolytes as a means to maintain reactants within the electrode and to control reactant/product redistribution preventing shape change over repeated cycling27. In addition, current work in lithium-oxygen electrochemistry is concurrently


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demonstrating the importance of tuning the electrolyte to manipulate the solubility of the intermediates48-49.

Sparingly solvating electrolytes for Li/S batteries have emerged as a powerful new concept that enables the attainment of useable energy density goals through reversible operation at low E/S ratios. The literature contains some demonstration of the success of this new concept, but further research is required to enable the use of sparingly solvating electrolytes in Li/S batteries. Atomic and molecular level understanding is needed of the speciation and mechanistic differences between various sparingly solvating systems. For example, the unique behavior of the solvate system reported by Cuisinier et al3 is not well understood. Due to the very low solubility of polysulfides in these electrolytes, a competition between solution-phase and solidstate reaction pathways may exist. Operando studies using X-ray absorption spectroscopy and Xray diffraction will continue to be invaluable techniques to determine temporal speciation. Moreover, model studies using scanning probe microscopy will enable visualization of any competing solid state and solution mediated processes that may result in particle swelling or redistribution across the electrode. Morphologies of solid phase sulfur, polysulfides, and sulfide species, which are expected to be highly dependent on the charge/discharge rate and cathode architecture, can in turn have profound effects on subsequent reaction kinetics and charge/discharge behaviors as previously demonstrated in both Li/O2 and Li/S batteries.50-52 To meet cycle and calendar life goals, the chemical stability of these electrolyte materials toward sulfide reaction intermediates requires greater understanding and subsequent improvement through the design and testing of new blends of electrolytes.


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A commercialized Li/S battery using a sparingly solvating electrolyte will require an engineered cathode with high sulfur loading to meet energy density goals while simultaneously exhibiting high rates of charge and discharge.53 Demonstrations of sparingly solvating electrodes with areal capacities higher than 3 mAh/cm2 have yet to be reported. The transport properties of the electrolyte will be critical to enabling Li+ access throughout the depth of the porous electrode. Early characterization of solvate based sparingly solvating electrolytes suggest adequate conductivity and potentially higher Li+ transference numbers than dilute or polymer systems.30 In addition to cation access throughout the electrode, electron transfer to and from the solid and solution phase species is paramount. A sparingly solvating system may require heterogeneous surfaces, such as metal oxides mixed with carbons54-55, to always maintain access to electron transfer sites. A balance of sulfur-philic and sulfide-philic nucleation sites in close proximity to one another could potentially enable higher rates of charge and discharge even with low polysulfide solubility. Previously commercialized systems using precipitation/dissolution mechanisms, such as lead-acid, have shown high rate performance; however, the lithium sulfur chemistry has two insulating end members in contrast to the many listed earlier that consist of metal to salt reactions. While having two insulating end members is an additional challenge, reversible and facile operation should be possible if electron transfer sites remain available and in close proximity. To direct local polysulfide transport, functional binders or deposited films on the electrode could create localized zones of tuned solubility or mediation to assist the sparingly solvating electrolyte.56-57 A key design constraint is the greater than 70% volume expansion during the reduction of sulfur. The displaced electrolyte must move to accommodate the swelling and thus binders and films must be designed with this consideration in mind.


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Li/S battery development will be greatly impacted by future reports on the reaction mechanism and reactant spatial distribution in cells operated at E/S ratios approaching 1 mL/g. The utilization of sparingly solvating electrolyte at low E/S ratios is yet to be demonstrated proving the decoupling between electrolyte quantity and reaction mechanism. A protected lithium anode or two-compartment cell, perhaps using a solid ceramic, may be required for such a demonstration. The development of sparingly solvating electrolytes for Li/S batteries is still at an early stage. With better understanding of the requirements of such systems, a large design space is available for exploration of electrolytes that meet all performance requirements for Li/S batteries operating at lean electrolyte condition (low E/S ratio).

ACKNOWLEDGMENT This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We gratefully acknowledge Dr. Junzheng Cheng, Dr. Huilin Pan, Dr. Heng-Liang Wu, Dr. Kimberly A. See, Dr. Kah Chun Lau, Dr. Mahalingam Balasubramanian, Prof. Linda Nazar, Prof. Nitash Balsara, Dr. Zhengcheng Zhang for helpful discussions, and Dr. Venkat Srinivasan for pointing out the connection to historical precipitation-dissolution chemistries.


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(37) Yamada, Y.; Furukawa, K.; Sodeyama, K.; Kikuchi, K.; Yaegashi, M.; Tateyama, Y.; Yamada, A. Unusual Stability of Acetonitrile-Based Superconcentrated Electrolytes for FastCharging Lithium-Ion Batteries. J. Am. Chem. Soc. 2014, 136, 5039-5046. (38) Zhang, C.; Ueno, K.; Yamazaki, A.; Yoshida, K.; Moon, H.; Mandai, T.; Umebayashi, Y.; Dokko, K.; Watanabe, M. Chelate Effects in Glyme/Lithium Bis (Trifluoromethanesulfonyl) Amide Solvate Ionic Liquids. I. Stability of Solvate Cations and Correlation with Electrolyte Properties. J. Phys. Chem. B 2014, 118, 5144-5153. (39) Chen, J.; Han, K. S.; Henderson, W. A.; Lau, K. C.; Vijayakumar, M.; Dzwiniel, T.; Pan, H.; Curtiss, L. A.; Xiao, J.; Mueller, K. T. Restricting the Solubility of Polysulfides in Li‐S Batteries Via Electrolyte Salt Selection. Adv. Energy Mater. 2016, 6, 1600160. (40) Ueno, K.; Tatara, R.; Tsuzuki, S.; Saito, S.; Doi, H.; Yoshida, K.; Mandai, T.; Matsugami, M.; Umebayashi, Y.; Dokko, K. Li+ Solvation in Glyme–Li Salt Solvate Ionic Liquids. Phys. Chem. Chem. Phys. 2015, 17, 8248-8257. (41) Peled, E.; Sternberg, Y.; Gorenshtein, A.; Lavi, Y. Lithium‐Sulfur Battery: Evaluation of Dioxolane‐Based Electrolytes. J. Electrochem. Soc. 1989, 136, 1621-1625. (42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, Revision E.01, Gaussian, Inc.: Wallingford CT, 2009. (43) Lee, H.; Yang, X.; McBreen, J.; Choi, L.; Okamoto, Y. The Synthesis of a New Family of Anion Receptors and the Studies of Their Effect on Ion Pair Dissociation and Conductivity of Lithium Salts in Nonaqueous Solutions. J. Electrochem. Soc. 1996, 143, 3825-3829. (44) McBreen, J.; Lee, H.; Yang, X.; Sun, X. New Approaches to the Design of Polymer and Liquid Electrolytes for Lithium Batteries. J. Power Sources 2000, 89, 163-167. (45) Dietrich, B. Design of Anion Receptors: Applications. Pure Appl. Chem. 1993, 65, 14571464. (46) Schmidtchen, F. P.; Berger, M. Artificial Organic Host Molecules for Anions. Chem. Rev. 1997, 97, 1609-1646. (47) Wong, D. H. C.; Thelen, J. L.; Fu, Y. B.; Devaux, D.; Pandya, A. A.; Battaglia, V. S.; Balsara, N. P.; DeSimone, J. M. Nonflammable Perfluoropolyether-Based Electrolytes for Lithium Batteries. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 3327-3331. (48) Johnson, L.; Li, C.; Liu, Z.; Chen, Y.; Freunberger, S. A.; Ashok, P. C.; Praveen, B. B.; Dholakia, K.; Tarascon, J.-M.; Bruce, P. G. The Role of LiO2 Solubility in O2 Reduction in Aprotic Solvents and Its Consequences for Li–O2 Batteries. Nature Chem. 2014, 6, 1091-1099. (49) Aetukuri, N. B.; McCloskey, B. D.; García, J. M.; Krupp, L. E.; Viswanathan, V.; Luntz, A. C. Solvating Additives Drive Solution-Mediated Electrochemistry and Enhance Toroid Growth in Non-Aqueous Li–O2 Batteries. Nature Chem. 2015, 7, 50-56. (50) Fan, F. Y.; Carter, W. C.; Chiang, Y. M. Mechanism and Kinetics of Li2S Precipitation in Lithium–Sulfur Batteries. Adv. Mater. 2015, 27, 5203-5209. (51) Gallant, B. M.; Kwabi, D. G.; Mitchell, R. R.; Zhou, J.; Thompson, C. V.; Shao-Horn, Y. Influence of Li2O2 Morphology on Oxygen Reduction and Evolution Kinetics in Li–O2 Batteries. Energy Environ. Sci. 2013, 6, 2518-2528. (52) Viswanathan, V.; Thygesen, K. S.; Hummelshøj, J.; Nørskov, J. K.; Girishkumar, G.; McCloskey, B.; Luntz, A. Electrical Conductivity in Li2O2 and Its Role in Determining Capacity Limitations in Non-Aqueous Li-O2 Batteries. J. Chem. Phys. 2011, 135, 214704.


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AUTHOR INFORMATION Corresponding Author * [email protected], +1-630-252-4473 Biographies Lei Cheng is an Assistant Chemist at Materials Science Division at Argonne National Laboratory. Her research interests include first-principles computational studies of energy storage materials and heterogeneous catalysis. Larry A. Curtiss is a Distinguished Fellow at Argonne National Laboratory. His research interests are in the development of new quantum chemical methods and their applications to understanding and discovery of new materials. Kevin R. Zavadil is a Distinguished Member of Technical Staff in the Advanced Materials Laboratory at Sandia National Laboratory. His current research interests include the


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electrodeposition and dissolution of metals, electrochemical energy storage, and the coupling of electrochemical and imaging characterization techniques. Andrew A. Gewirth is a Professor in the Department of Chemistry at University of Illinois at Urbana-Champaign. His research interests are in the study of structure and reactivity of surfaces and interfaces Yuyan Shao is a Senior Research Scientist at the Energy and Environment Directorate at Pacific Northwest National Laboratory. His research interest is in electrochemical materials and technologies for energy storage and generation. Kevin G. Gallagher is a Principal Chemical Engineer in the Chemical Sciences and Engineering Division at Argonne National Laboratory. His research interests span electrochemical characterization and materials-chemistry to the techno-economic modeling of advanced batteries. HIGHLIGHTED QUOTES Long-lived, efficient operation under low electrolyte to sulfur (E/S) ratios is a critical but less recognized challenge for Li/S chemistry. Successfully commercialized secondary battery electrodes based on the precipitationdissolution mechanism commonly utilize electrolytes in which the intermediate or product salt is only sparingly soluble. The design rules for achieving solubility and mobility of the supporting salt (e.g. LiTFSI) and not lithium polysulfides fall within the categories of limited solvent and selective solvent. Li/S battery development will be greatly impacted by future reports on the reaction mechanism and reactant spatial distribution in cells operated at E/S ratios approaching 1 mL/g.


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Sparingly Solvating Electrolytes for High Energy Density Lithium

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