Tailoring Atomistic Interactions in Li-S Battery Via a Computational Multi-Scale-Data-Driven Approach

Introduction As the demand for energy is harshly increasing nowadays, great effort has been devoted to developing various advanced energy-storage technologies with high energy density, long-term cyclability, and inexpensive costs. The lithium-ion battery technology has been the mainstream option for the portable energy supply for the past decades. However, the commercial lithium-ion battery has limited cathode capacity and energy density of about 170 mAh g-1 and 600 Wh kg-1, as well as the high costs of $10,000 per ton. To further increase the cathode capacity and lower the fabrication costs, sulfur, with a high theoretical capacity of 1,675 mAh g-1, a high theoretical energy density of 2,600 Wh kg-1 in lithium-sulfur batteries, and a cheap price of $150 per ton, has been chosen to become new cathode materials. Thus, lithium–sulfur battery, as an alternative branch of lithium-ion batteries, is a potential technology to solve the problems in portable energy supply.In lithium–sulfur system, multiple redox reactions occur during different steps of charge and discharge reactions. In the discharge process, the solid-state sulfur (S8) will be reduced to form liquid-state lithium polysulfides (Li2Sx, 4 ≤ x ≤ 8) and solid-state lithium sulfides (Li2S2/Li2S). The lithium sulfides will be charged back to solid-state sulfur. These reactions in charge and discharge processes repeat during cycling. Three key factors, the sulfur loading, the sulfur content, and the electrolyte-to-sulfur ratio, have been thoroughly researched for developing high-performance lithium–sulfur batteries in the past two decades. To commercialize the lithium–sulfur battery technology, limits on these three factors have been set, which are a sulfur loading of 5 mg cm-2 and above, a sulfur content of 65 wt% and above, and an electrolyte-to-sulfur ratio of 10 μL mg-1 and below. However, due to the complex reaction steps in lithium–sulfur system, these three factors are conflicted or cannot be achieved simultaneously in most of the research cases. Results and Discussion Here, in our presentation, we report a high-loading polysulfide cathode, achieving both high sulfur content and low electrolyte-to-sulfur ratio at the same time. A non-nanoporous carbon nanotube/nanofiber (CNT/CNF) is adopted as the electrode substrate to accommodate the active material, liquid polysulfide catholyte, forming a polysulfide cathode with a high sulfur loading of 8.64 mg cm-2, a high sulfur content of 68 wt%, and low electrolyte-to-sulfur ratios of 4–7 μL mg-1. With such a cathode design, the lithium-sulfur cell attain high discharge capacities of 630–870 mAh g-1 and 580–631 mAh g-1 at C/10 and C/5 cycling rates, which corresponds to the high energy densities of 10.5–11.5 and 11.4–15.7 mWh cm-2. In addition to the high electrochemical utilization, our lean-electrolyte lithium-sulfur cells demonstrate a long-term cyclability of 200 cycles.As a result, we further investigate the possible failure mechanism for a high-loading polysulfide cathode operating in a lean-electrolyte cell under a semi-dry electrolyte condition. The extended cycling test is performed to explore the discharge and charge efficiency as well as the lithiation and delithiation mechanisms. Two cycling rates, C/10 and C/5, are applied for comparison. It reveals that due to the semi-dry electrolyte condition, the decomposition of the polysulfide catholyte occurs during extended cycles. The overcharge issue is thus occurred as a result of the rapid consumption of electrolyte, and leads to slow ion diffusion, decreased Coulombic efficiency, and irreversible capacity loss. Additionally, the decayed cycling performances are more easily observed at the slow C/10 cycling rate than at the fast C/5 cycling rate. Conclusion In summary, according to the excellent cycling performances of our cell design with a high-loading cathode, high sulfur content, and lean electrolyte addition, we might be very close to overcome the three limiting factors for commercialization of lithium–sulfur battery technology. Moreover, the failure analysis of our lean-electrolyte lithium-sulfur cell demonstrates the main challenges in developing a lean-electrolyte lithium–sulfur cell. The attainment of a high and stable Coulombic efficiency as well as the fast electrolyte consumption during regular cycling rate would challenge the development of lean-electrolyte lithium-sulfur cell with more practical cell design parameters.

Energy storage devices with high energy density and low cost are highly demanding since the existing state-of-the-art Li-ion battery systems have intrinsic limitations in energy and price. Thus, lithium-sulfur (Li-S) batteries have attracted a great attention because sulfur is abundant and it has high theoretical capacity (1672 mAh g-1) and energy density (2600 Wh kg-1) when coupled with metallic lithium anode [1]. However, it is still challenging to device Li-S batteries offering a long-term cycling stability at a high level of sulfur loading due to the dissolution of polysulfides (Li2Sx, 4<x<8) into the electrolyte and their shuttle effect causing poor capacity retention and low coulombic efficiency [2]. Recently, many important technological breakthroughs have been reported to suppress the polysulfides shuttle effects, which potentially leads to long cycle life of Li-S batteries [3]. Nonetheless, the loading level of sulfur has to be much increased for the Li-S batteries to go beyond current LIBs in energy density. In this aspect, a rational design of sulfur cathode is still regarded as one of the most important measures in mitigating the polysulfides shuttle problems. In this study, we employed mesoporous carbons (MCs) and N-doped mesoporous carbon (N-MC) as the sulfur host materials. The MCs were prepared by a sol-gel process in an acidic aqueous mixture of commercial colloidal silica (20nm in diameter) as the template and sucrose as the carbon source [4]. The ratio of colloidal silica/sucrose was varied to obtain MCs with different textual properties. The N-MC was prepared by aniline polymerization in MC dispersion with ammonium persulfate as the initiator followed by carbonization of resulting composite product. The sulfur loading level of S/MCs and S/N-MC was varied in the range of 35-80 wt%, which corresponds to the sulfur loading density of 0.6~4 mg/cm2. We also studied the effects of sulfur loading methods (melt-impregnation in a flow or a closed system) and the use of a conductive separator (CS) on the electrochemical performances of Li-S batteries. The CS was prepared by direct coating of a flake-type graphite on a polypropylene (PP) membrane (Cellgard 2400). The electrochemical tests were conducted using CR203 coin-type cells with Li foil as the counter electrode. The working electrode was prepared by casting a paste that consisted of the active material, conductive additive and PVdF binder with a mass ratio of 7:1:2 or 8:1:1 onto a Al foil current collector. 1.0 M LiTFSI in 1,2-dioxolane(DOL)/1,2-dimethoxyethane(DME) containing 0.1 or 0.4M LiNO3 was used as the electrolyte. The cells were cycled in a cut-off voltage range of 1.5-3.0V Li+/Li. The MC-1(SBET = 1213 m2/g, Vpore = 2.75 cm3/g) is highly porous with pore size distribution in the range of 10-30 nm as shown in the TEM image in Fig. 1a. As compared in Fig. 1b and 1c, flake-type graphite are evenly coated onto the surface of porous PP membrane. The reversible capacities of various Li-S batteries at 0.5C are compared in Fig. 1d as a function of sulfur content. In overall, the reversible capacity was decreased with sulfur content. However, the reversible capacity was improved with strategic approaches depicted in Fig. 1d; (1) S/MC-1 showed much higher capacity than S/MC-2 (SBET = 887 m2/g, Vpore = 1.55 cm3/g ), (2) S/N-MC-1 showed much enhanced capacity compared to S/MC-1 at high sulfur content, and (3) the use of CS exhibited further improvement in the reversible capacity. As shown in Fig. 1e, an electrode of S/MC-1 (S = 65.7wt%) assembled with CS exhibited an excellent cycling stability at 0.5C up to 300 cycles. [1] P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J.-M. Tarascon, Nat. Mater., 11, 19–29 (2012). [2] Y. Diao, K. Xie, S. Xiong, X. Hong, J. Power Sources, 235, 181 –186. (2013). [3] A. Rosenman, E. Markevich, G. Salitra, D. Aurbach, A. Garsuch, and F. F. Chesneau, Adv. Energy Mater., DOI: 10.1002/aenm.201500212 (2015). [4] H. I. Lee, G. D. Stucky, J. H. Kim, C. Pak, H. Chang, and J. M. Kim, Adv. Mater., 23, 2357-2361 (2011). Fig.1. (a) TEM image of MC-1 (Inset shows pore size distribution of MCs), SEM images of (b) PP membrane and (c) conductive separator, (d) reversible capacities (after 50 cycles at 0.5C) of various samples as a function of sulfur content, and (e) cycling performance of S/MC-1 assembled with CS. Figure 1

As a secondary Li-ion battery with high energy density, lithium-sulfur (Li-S) batteries possess high potential development prospects. One of the important ingredients to improve the safety and energy density in Li-S batteries is the solid-state electrolyte. However, the poor ionic conductivity largely limits its application for the commercial market. At present, the gel electrolyte prepared by combining the electrolyte or ionic liquid with the all-solid electrolyte is an effective method to solve the low ion conductivity of the solid electrolyte. We present a cross-linked gel polymer electrolyte with fluoroethylene carbonate (FEC) as a solid electrolyte interface (SEI) film formed for Li-S quasi-solid-state batteries, which can be simply synthesized without initiators. This gel polymer electrolyte with FEC as an additive (GPE@FEC) possesses high ionic conductivity (0.830 × 10-3 S/cm at 25 °C and 1.577 × 10-3 S/cm at 85 °C) and extremely high Li-ion transference number (tLi+ = 0.674). In addition, the strong ability toward anchoring polysulfides resulting in the high electrochemical performance of Li-S batteries was confirmed in GPE@FEC by the diffusion experiment, X-ray photoelectron spectroscopy analysis (XPS), and scanning electron microscopy (SEM) mapping of the S element. Such a high ion conductivity (IC) gel polymer electrolyte enables a competitive specific capacity of 940 mAh/g at 0.2C and supreme cycling performance for 180 cycles at 0.5C, which is far beyond that of conventional poly(ethylene oxide)-based quasi-solid-state Li-S batteries.

A lithium-sulfur (Li-S) battery has a high theoretical specific capacity (1675 mAh g-1 of elemental sulfur) and a high nominal energy density (2500 Wh kg-1 of cell weight), which offers the prospect of a significant energy density improvement compared to the mainstream lithium-ion batteries (150 Wh kg-1). However, the development of Li-S battery technology has been plagued by problems arising from the highly insulating nature of sulfur (5×10-30 S cm-1 at 25 °C), the high solubility of lithium polysulfides in the electrolyte and volumetric expansion of sulfur during lithiation. Here, we demonstrate the design and preparation of few-layer graphene (FLG) foam scaffold for sulfur loading to fabricate cathode structures for lithium-sulfur batteries. In this work, a free-standing FLG monolithic network foam was formed as a negative of a Ni metallic foam template by CVD followed by etching away of Ni. Then, a three-dimensional (3-D) cathode system for lithium sulfur (Li-S) batteries has been synthesised by loading sulfur on to this interconnected network of few-layered graphene (FLG) via a sulfur solution infiltration method. This cathode system did not require any additional binding agents, conductive additives or a separate metallic current collector thus decreasing the weight of the cathode by typically ~20-30 wt%. A Li-S battery with the sulfur/FLG foam cathode shows good electrochemical stability and high rate discharge capacity retention for up to 400 discharge/charge cycles at a high current density of 3200 mA g-1, which due to the FLG foam offers excellent electrical conductivity, an appropriate pore structure for containing the electro-active sulfur and facilitates rapid electron/ion transport. Even after 400 cycles the capacity decay is only ~0.064% per cycle relative to the early (e.g. the 5th cycle) discharge capacity, while yielding an average columbic efficiency of ~96.2%. To further understand the sulfur loading ability of FLG foam, the composites with high content (up to 72 wt %) were also investigated. The composite with high content of sulfur still presented good initial discharge capacity and good cycle stability after 200 cycles. Our results indicate the potential suitability of graphene foam for efficient and high-performance batteries.

Because of its high energy density and promising potential to revolutionize the future electric energy storage systems, the technology of Li-S batteries has attracted significant interests in recent years. With the significant progresses made on the development of cathode materials in lithium-sulfur (Li-S) batteries, the stability of anode in Li-S batteries has become one of the more urgent challenges in order to reach the long-term stability of Li-S batteries. Lithium metal is the preferential option for the anode material in Li-S batteries because of its high specific capacity. However, lithium polysulfides formed during discharge and charge processes may migrate from cathode to anode side to react with the lithium metal, so called shuttle problem, leading to limited cycle life and poor Coulombic efficiency of Li-S batteries. A passivation layer is easily formed on the metallic Li anode surface because of the presence of polysulfides and electrolyte additives, which is supposed to corrode Li anode and result in failure of the Li-S battery. Various strategies used to minimize the corrosion of Li anode and to reduce its impedance increase have been developed, including electrolyte additives, polymer electrolytes and interlayers. Recently, concentrated electrolytes were demonstrated to offer lower solubility of polysulfides and could lead to more stable cycling stability in Li-S batteries. Concentrated electrolytes tend to reduce the solubility of lithium polysulfides due to the lack of available solvent to partially solvate the Li+ cations. Coordinating anions also affect the solubility of lithium polysulfides through interacting with the Li+ cations with different solvation strength. In this presentation, the failure mechanism and the stability of anode/electrolyte interface in Li-S batteries with highly concentrated electrolyte will be reported.

Lithium-sulfur (Li-S) batteries have a high theoretical energy density and are regarded to be an ideal choice for the next generation of electrochemical energy storage systems. However, their practical application is hindered by several bottlenecks, including the insulating nature of sulfur and its discharge products (Li2S2/Li2S), the shuttling behavior of intermediate polysulfides, and slow redox reactions. Herein, we propose a highly efficient bimetallic selenide electrocatalyst featuring a hollow porous core-shell spherical structure, which serves as both a cathode host and a modified separator coated on a commercially available polypropylene separator to address the above issues. The bimetallic selenide enhances cathode conductivity, and its unique hollow porous core-shell spherical structure provides rapid ion transport channels, along with ample spatial confinement for lithium polysulfides. Additionally, the abundant reactive sites on the bimetallic selenides exhibit high intrinsic electrocatalytic activity, accelerating polysulfide conversion and improving redox kinetics. Density functional theory calculations indicate that bimetallic selenides interact more strongly with polysulfides and present lower reaction barriers compared to those of their sulfide counterparts. Consequently, these bimetallic selenide materials demonstrate superior rate performance and cycling stability in Li-S batteries, achieving an impressive lifespan of 1400 cycles with a minimal decay rate of 0.030% per cycle at 1.0 C. This work provides unique insights into enhancing the performance of transition metal compounds in Li-S batteries.

Practical applications of lithium-sulfur (Li-S) batteries have been severely hindered by their low capacity, poor rate performance, and fast capacity degradation, which mainly originate from the notorious polysulfide shuttle effect. Herein, with density functional theory calculations, we show that the alloying of Fe into carbon-coated Co not only provides moderate binding interactions with the polysulfides to hinder their diffusion but also serves as an active catalyst in the spontaneous and successive lithiation of S8 to Li2S. Based on the fast migration of Li ions and the spontaneous lithiation of Li2S2 on the carbon-coated Fe-Co alloy, the entrapping-conversion processes of polysulfides are both thermodynamically and kinetically promoted in redox cycling. Experimentally, rationally designed Co7Fe3@porous graphite carbon-carbon nanotubes (Co7Fe3@PGC-CNT) electrocatalysts are introduced into Li-S batteries through separator functionalization. Consistent with theoretical predictions, Li-S batteries with Co7Fe3@PGC-CNT modified separators exhibit a dramatically enhanced rate capacity (788 and 631 mAh g-1 at 10 and 15 C rates, respectively) and cycling stability (a slow capacity decay of 0.05% per cycle over 1000 cycles at 2.0 C), which are superior to those of most reported Li-S batteries coupled with state-of-the-art separators. Furthermore, it is shown that the excellent hindering of the shuttle effects enables a high areal capacity of 4.7 mAh cm-2 after 90 cycles at a high sulfur loading of 6.7 mg cm-2. Our work provides a feasible method for developing high-energy and long-life Li-S batteries, which might drive the commercialization of Li-S batteries.

Rechargeable lithium-sulfur (Li-S) batteries have emerged as the most promising electrochemical energy storage systems due to their high energy density, wide range of temperature operation, cost effectiveness, and eco-friendly in nature. Nevertheless, the drawbacks associated with the Li-S batteries such as insulating nature of sulfur species, corrosive nature of predeposited polysulfides at electrodes during discharge-charge processes, and formation of thick SEI layer still need to be circumvented. One of the promising ways to alleviate these problems is to use highly porous, conductive, and active species which can improve the kinetics of polysulfide conversion reactions and diffusion processes. In this context, we have investigated electronically conductive species incorporated graphene, specifically Metal/graphene hybrids (metal = Pt, Ni, and Co) as cathode in Li-S batteries and performed systematic studies to understand the electrochemical behavior of polysulfides during discharge-charge processes. Metal/graphene hybrids were prepared through wet chemical routes and employed them as electrodes for the conversion of polysulfides in Li-S batteries. Structural and morphological techniques depicted phase purity of the materials and well-dispersed metallic nanocrystallites with an average size of 5 nm on the graphene sheets. Electrochemical studies were carried out by assembling Li/sulfur cells inside glove box using a porous metal/graphene as electrodes cathode, 0.6M Li2S8-1M LiTFSI in TEGDME as electrolyte, and metallic lithium as anode. Galvanostatic discharge-charge tests indicated a two-fold increment in the Li-S battery performance with metal/graphene compared to pure graphene. A high discharge capacity of 750 mAh/g at C-rates of C/10 has been observed with Pt/graphene. The studies on metal/graphene based Li-S battery revealed pivotal role of metallic nanoparticles in enhancing the polysulfide conversion processes. # - Corresponding Author - leela.arava@wayne.edu

The lithium-sulfur (Li-S) battery has emerged as a next-generation rechargeable battery, owing to the involvement of multiple electrons during the redox reactions between Li and S, which provides high theoretical energy density (2,600 Wh kg-1) and specific capacity (1,675 mAh g-1). In addition, due to abundance of elemental sulfur and its environmental compatibility, Li-S battery has been considered the much more attractive power sources.1,2 The present study demonstrates how to improve the electrochemical performance and energy density of lithium-sulfur batteries based on controlling the wettability between the freestanding electrode and the electrolyte. First, to improve the volumetric energy density, we reduced the thickness of freestanding electrode by the applying pressure. Second, in order to improve electrolyte conductivity and wettability for pressed freestanding electrode, we reduced the viscosity of electrolyte solution by reducing the LiTFSI salt concentration.3,4 Through these controls, the Li-S cell is constructed from a pressed freestanding carbon nanotube-sulfur cathode, 0.5 M LiTFSI in DME/DOL with 0.4 M LiNO3 as a low-viscosity electrolyte (0.72 cP), and Li-metal foil as an anode. As a result, we can obtain a high discharge capacity of 1,400 mAh g-1 at 0.1 C and excellent cycle retention of 95% after 80 cycles at 0.5 C in coin-type cell. In addition, scale-up (3 by 5 cm) pouch-type Li-S cell delivers a high discharge capacity of 500 mAh at 0.1 C with high gravimetric and volumetric energy densities.5 References B. Scrosati, J. Hassoun, Y.-K. Sun, Energy Environ. Sci. 2011, 4, 3287 J.-Y. Hwang, H. M. Kim, S. K. Lee, J. H. Lee, A. Abouimrane, M. A. Khaleel, I. Belharouak, A. Manthiram, Y.-K. Sun, Adv. Energy Mater. 2016, 6, 1501480 J.J. Hu, G. K. Long, S. Liu, G. R. Li, X. P. Gao, Chem. Commun., 2014, 50, 14647 H.-S. Kang, Y.-K. Sun, Adv. Funct. Mater., 2016, 26, 1225 H.-S. Kang, E. Park, J.-Y. Hwang, H. Kim, D. Aurbach, A. Rosenman, Y.-K. Sun, Adv. Mater. Technol. 2016, 1600052

Lithium-sulfur (Li-S) batteries are potential alternatives to lithium-ion batteries, which are known to have certain drawbacks. This has drawn special technological attention. Sulfur’s low electrical conductivity, shuttling of the intermediate polysulfides, the slow conversion reaction kinetics of various lithium polysulfides (LiPS), and the major volume change of sulfur in the lithiation reaction are some of the main issues that still need to be resolved before Li-S batteries can be used in practical applications. Additionally, it is likely because there are not enough prognostic models determining energy densities at the cell and pack level based on materials and cell design, most of the articles do not report system-level metrics, specific energy (Wh kg-1), and energy density (Wh L-1).To overcome these problems, atomic vanadium (V) and cobalt (Co) modified ketjen black-sulfur composite (VCKBS) was prepared and used as a cathode in Li-S batteries. The synthesized composite was characterized by various techniques, e.g., XRD, Raman, SEM, HR-TEM, HAADF-STEM, and XPS spectra. The polysulfide adsorption test showed that V had increased the adsorption ability, and cyclic voltammetry displayed the increased catalytic activity resulting from Co, offering a large number of interfacial active sites and ensuring smooth electron transfer. Cycling performance tests showed an initial specific capacity of 1329 mAh g−1, maintained at 1249 mAh g−1 after 100 cycles. Under a higher sulfur loading (2.4 mg cm−2), at 0.2 C, Li-S cells demonstrated an initial specific capacity of 1201 mAh g−1 and retained 985 mAh g−1 after 300 cycles with a low fading rate of 0.059% per cycle. The rate performance tests showed that the prior capacity is nearly recovered, going from 1C back to 0.1C, demonstrating the superiority of the VCKBS material.In addition to the experimental characterization of the Li-S cell performance, energy densities were also predicted by a proposed system-level performance model. The 1st and 100th discharge capacities were used to forecast the system-level specific energies and energy densities. The system-level specific energies and energy densities based on the 100th discharge capacity of VCKBS cathodes had improvements of 1342 and 568%, respectively. This suggests that an easy route to construct the highly stable cathode material may pave the way for the development of highly efficient Li-S batteries. Acknowledgments Hira Fazal acknowledges support from the Türkiye Scholarship/Research Fellowship Program sponsored by the Prime Ministry of Turkey Presidency for Turks Abroad and Related Communities (21PK070917). Damla Eroglu acknowledges support from the Istanbul Development Agency (Award No: TR10/21/YEP/0001). We also thank the National Natural Science Foundation of China (22171180) and the Science and Technology Commission of Shanghai Municipality (20520710400).

Lithium-sulfur (Li-S) batteries constitute promising next-generation energy storage devices due to the ultrahigh theoretical energy density of 2600 Wh kg-1. However, the multiphase sulfur redox reactions with sophisticated homogeneous and heterogeneous electrochemical processes are sluggish in kinetics, thus requiring targeted and high-efficient electrocatalysts. Herein, a semi-immobilized molecular electrocatalyst is designed to tailor the characters of the sulfur redox reactions in working Li-S batteries. Specifically, porphyrin active sites are covalently grafted onto conductive and flexible polypyrrole linkers on graphene current collectors. The electrocatalyst with the semi-immobilized active sites exhibits homogeneous and heterogeneous functions simultaneously, performing enhanced redox kinetics and a regulated phase transition mode. The efficiency of the semi-immobilizing strategy is further verified in practical Li-S batteries that realize superior rate performances and long lifespan as well as a 343 Wh kg-1 high-energy-density Li-S pouch cell. This contribution not only proposes an efficient semi-immobilizing electrocatalyst design strategy to promote the Li-S battery performances but also inspires electrocatalyst development facing analogous multiphase electrochemical energy processes.

Two-dimensional porous transition metal organic framework materials with strongly anchoring ability as lithium-sulfur cathode

Lithium-sulfur (Li-S) batteries are considered as one of the most potential next-generation rechargeable batteries due to their high theoretical energy density. However, some critical issues, such as low capacity, poor cycling stability, and safety concerns, must be solved before Li-S batteries can be used practically. During the past decade, tremendous efforts have been devoted to the design and synthesis of electrode materials. Benefiting from their tunable structural parameters, hollow porous carbon materials (HPCM) remarkably enhance the performances of both sulfur cathodes and lithium anodes, promoting the development of high-performance Li-S batteries. Here, together with the templated synthesis of HPCM, recent progresses of Li-S batteries based on HPCM are reviewed. Several important issues in Li-S batteries, including sulfur loading, polysulfide entrapping, and Li metal protection, are discussed, followed by a summary on recent research on HPCM-based sulfur cathodes, modified separators, and lithium anodes. After the discussion on emerging technical obstacles toward high-energy Li-S batteries, prospects for the future directions of HPCM research in the field of Li-S batteries are also proposed.

Comparative life cycle assessment of Li-Sulphur and Li-ion batteries for electric vehicles

Flame-retardant ammonium polyphosphate/MXene decorated carbon foam materials as polysulfide traps for fire-safe and stable lithium-sulfur batteries