Binder-Free Electrode from Hemp-Derived Porous Carbon for Li-Sulfur Battery

Lithium–sulfur batteries (LSBs) are considered to be one of the most promising next-generation electrochemical power sources to replace commercial lithium-ion batteries because of their high energy density. However, practical application of LSBs is hindered by two critical drawbacks: “redox shuttle reactions” of dissolved polysulfides at the cathode side and Li dendrites at the Li anode side. Therefore, various approaches have been proposed to break down technical barriers in LSB systems. The overall device performance of LSBs depends on not only the development of host materials but also the superior architecture design of electrodes. Among these architectures, binder-free electrodes are verified to be one of the most effective structural designs for high-performance LSBs. Therefore, it is urgent to review recent advances in binder-free electrodes for promoting the fundamental and technical advancements of LSBs. Herein, recently emergent studies using various binder-free architectures in sulfur cathodes and lithium metal anodes are reviewed. These binder-free electrodes, with well-interconnected structures and abundant structural space, can provide a continuous pathway for fast/uniform electron transport/distribution, load sufficient active materials for ensuring high energy density, and afford large electrochemically active surface areas where electrons and Li ions can come into contact with the active materials for fast conversion reactions, thus leading to suitable applications for LSBs. Subsequently, the advantages and challenges of binder-free architectures are discussed from several recently emergent studies using binder-free structured sulfur cathodes or Li metal anodes. The future prospects of LSBs with binder-free electrode structure designs are also discussed.

6 - Nanocomposites for binder-free Li-S electrodes

The success of the rechargeable Li-S cell is limited in part by the dissolution of lithium-polysulfide in the electrolyte. Remarkably, it is found that removal of the conventional membrane separator in a Li-S cell improves sulfur utilization and cycling performance, whether the sulfur is initially contained in the cathode or electrolyte. An optimized cell design yields discharge capacities as high as 980 mA h g-1 after 100 cycles.

Electrode-electrolyte interfacial properties play a vital role in the cycling performance of lithium-sulfur (Li-S) batteries. The issues at an electrode-electrolyte interface include electrochemical and chemical reactions occurring at the interface, formation mechanism of interfacial layers, compositional/structural characteristics of the interfacial layers, ionic transport across the interface, and thermodynamic and kinetic behaviors at the interface. Understanding the above critical issues is paramount for the development of strategies to enhance the overall performance of Li-S batteries. Liquid electrolytes commonly used in Li-S batteries bear resemblance to those employed in traditional lithium-ion batteries, which are generally composed of a lithium salt dissolved in a solvent matrix. However, due to a series of unique features associated with sulfur or polysulfides, ether-based solvents are generally employed in Li-S batteries rather than simply adopting the carbonate-type solvents that are generally used in the traditional Li+-ion batteries. In addition, the electrolytes of Li-S batteries usually comprise an important additive, LiNO3. The unique electrolyte components of Li-S batteries do not allow us to directly take the interfacial theories of the traditional Li+-ion batteries and apply them to Li-S batteries. On the other hand, during charging/discharging a Li-S battery, the dissolved polysulfide species migrate through the battery separator and react with the Li anode, which magnifies the complexity of the interfacial problems of Li-S batteries. However, current Li-S battery development paths have primarily been energized by advances in sulfur cathodes. Insight into the electrode-electrolyte interfacial behaviors has relatively been overshadowed. In this Account, we first examine the state-of-the-art contributions in understanding the solid-electrolyte interphase (SEI) formed on the Li-metal anode and sulfur cathode in conventional liquid-electrolyte Li-S batteries and how the resulting chemical and physical properties of the SEI affect the overall battery performance. A few strategies recently proposed for improving the stability of SEI are briefly summarized. Solid Li+-ion conductive electrolytes have been attempted for the development of Li-S batteries to eliminate the polysulfide shuttle issues. One approach is based on a concept of "all-solid-state Li-S battery," in which all the cell components are in the solid state. Another approach is based on a "hybrid-electrolyte Li-S battery" concept, in which the solid electrolyte plays roles both as a Li+-ion conductor for the electrochemical reaction and as a separator to prevent polysulfide shuttle. However, these endeavors with the solid electrolyte are not able to provide an overall satisfactory cell performance. In addition to the low ionic conductivity of solid-state electrolytes, a critical issue lies in the poor interfacial properties between the electrode and the solid electrolyte. This Account provides a survey of the relevant research progress in understanding and manipulating the interfaces of electrode and solid electrolytes in both the "all-solid-state Li-S batteries" and the "hybrid-electrolyte Li-S batteries". A recently proposed "semi-solid-state Li-S battery" concept is also briefly discussed. Finally, future research and development directions in all the above areas are suggested.

Electrochemical stability of energy storage devices is one of their major concerns. Polymeric binders are generally used to enhance the stability of the electrode, but the electrochemical performance of the device is compromised due to the poor conductivity of the binders. Herein, 3D binder-free electrode based on nickel oxide deposited on graphene (G-NiO) was fabricated by a simple two-step method. First, graphene was deposited on nickel foam via atmospheric pressure chemical vapour deposition followed by electrodeposition of NiO. The structural and morphological analyses of the fabricated G-NiO electrode were conducted through Raman spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS). XRD and Raman results confirmed the successful growth of high-quality graphene on nickel foam. FESEM images revealed the sheet and urchin-like morphology of the graphene and NiO, respectively. The electrochemical performance of the fabricated electrode was evaluated through cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) in aqueous solution at room temperature. The G-NiO binder-free electrode exhibited a specific capacity of ≈ 243 C g−1 at 3 mV s−1 in a three-electrode cell. A two-electrode configuration of G-NiO//activated charcoal was fabricated to form a hybrid device (supercapattery) that operated in a stable potential window of 1.4 V. The energy density and power density of the asymmetric device measured at a current density of 0.2 A g−1 were estimated to be 47.3 W h kg−1 and 140 W kg−1, respectively. Additionally, the fabricated supercapattery showed high cyclic stability with 98.7% retention of specific capacity after 5,000 cycles. Thus, the proposed fabrication technique is highly suitable for large scale production of highly stable and binder-free electrodes for electrochemical energy storage devices.

In the scientific mission for sustainable and efficient energy storage solutions, the importance of developing high performance energy storage systems along with environmentally friendly manufacturing technologies has become important. Among various energy storage systems, Lithium-Sulfur (Li/S) batteries are particularly noteworthy for their high theoretical specific energy (2,600 Wh/kg) and cost-effectiveness, traits largely attributable to abundance and favorable properties of sulfur. This positions Li/S batteries as a compelling alternative to the prevalent lithium-ion batteries, setting a new horizon in the pursuit of advanced energy storage solutions. A critical aspect of Li/S battery manufacturing is the fabrication process of sulfur electrodes, traditionally reliant on the slurry casting method with polymer binders like N-Methyl-2-pyrrolidone (NMP), which poses environmental and economic challenges due to the energy-intensive slurry drying and subsequent NMP vapor recovery processes. Although alternatives such as aqueous binders have been explored, they offer limited benefits in reducing the overall environmental impact. Moreover, the use of binders generally increases the resistance of electrodes due to their insulating nature, affecting the electrochemical performance of battery electrodes. Our research marks a significant departure from conventional practices by introducing a novel binder-free and solvent-free approach to fabricate sulfur-carbon composite electrodes. This pioneering method represents a paradigm shift towards green energy technology, addressing both the environmental and economic concerns associated with traditional electrode fabrication. This advancement contributes significantly to reducing the environmental footprint of battery technologies, aligning with the global imperative for cleaner energy solutions. Our comprehensive study focuses on the structural, compositional, and electrochemical characteristics of the binder-free sulfur-carbon electrodes produced through this method. Utilizing advanced spectroscopic and imaging techniques, we examine the mechanisms that underlie the enhanced performance of these electrodes. The investigation reveals critical insights into the morphological and crystalline attributes that promote electrochemical activity, as well as the synergistic interactions between sulfur and carbon in the absence of traditional polymer binder.

Development of flexible and freestanding electrode is fascinating for lithium-sulfur (Li-S) batteries to enable higher gravimetric energy density and wider potential applications. However, the lithium polysulfides shuttle effect remains challenging and leads to a severe capacity fading. Here, we study the sulfur cathode prepared via phase inversion method with polyethersulfone (PES) polymeric binder and the coupling strategy of the electrode with polysulfide-modified electrolyte to enhance the cells performance. The flexible/freestanding electrode was successfully prepared by the efficient and straight forward method, even it is possible for massive production of electrodes with various size. The resultant polymeric networks in the electrode can effectively trap the soluble polysulfides owing to the higher hydrophilicity and stronger affinity of PES than that with the routine polyvinylidene fluoride (PVDF) binder. Furthermore, we show a two-fold enhancement of the cell performance by coupling the electrode with polysulfide-modified electrolyte, giving an improved capacity of 1141 mAh g-1, lower polarization of 192 mV and more stable capacity retention with 100% Coulombic efficiency over 100 cycles at 0.25C. The advantages of favored strategy are further demonstrated in lithium-ion sulfur full battery with lithiated graphite anode, which demonstrates much improved performance than those previously reported. This work successfully introduces an efficient strategy for flexible/freestanding electrodes and enlightens the importance of coupling strategy to enhance the performances of Li-S batteries. Figure 1. (a) Digital photographs of the as-prepared composite via phase inversion method. (b) Cycle performance of half-cells using 2-layer PES-CNT-S and PVDF-CNT-S cathodes with polysulfide(PS)-free electrolyte and polysulfide(PS)-modified electrolyte at 0.25C. REFERENCES W. Wahyudi, Z. Cao, P. Kumar, M. Li, Y. Wu, M. N. Hedhili, T. D. Anthopoulos, L. Cavallo, L. J. Li, J. Ming, Adv. Funct. Mater., 2018, 28 (34), 1802244.J. Ming, Z. Cao, W. Wahyudi, M. Li, P. Kumar, Y. Wu, J.-Y. Hwang, M. N. Hedhili, L. Cavallo, Y.-K. Sun, L. J. Li, ACS Energy Lett., 2018, 3 (2), 335-340.W. Wang, Z. Cao, G. A. Elia, Y. Wu, W. Wahyudi, E. Abou-Hamad, A. H. Emwas, L. Cavallo, L. J. Li, J. Ming, ACS Energy Lett., 2018, 3 (12), 2899-2907. Figure 1

Development of flexible and freestanding electrode is fascinating for lithium-sulfur (Li-S) batteries to enable higher gravimetric energy density and wider potential applications. However, the lithium polysulfides shuttle effect remains challenging and leads to a severe capacity fading. Here, we study the sulfur cathode prepared via phase inversion method with polyethersulfone (PES) polymeric binder and the coupling strategy of the electrode with polysulfide-modified electrolyte to enhance the cells performance. The flexible/freestanding electrode was successfully prepared by the efficient and straight forward method, even it is possible for massive production of electrodes with various size. The resultant polymeric networks in the electrode can effectively trap the soluble polysulfides owing to the higher hydrophilicity and stronger affinity of PES than that with the routine polyvinylidene fluoride (PVDF) binder. Furthermore, we show a two-fold enhancement of the cell performance by coupling the electrode with polysulfide-modified electrolyte, giving an improved capacity of 1141 mAh g-1, lower polarization of 192 mV and more stable capacity retention with 100% Coulombic efficiency over 100 cycles at 0.25C. The advantages of favored strategy are further demonstrated in lithium-ion sulfur full battery with lithiated graphite anode, which demonstrates much improved performance than those previously reported. This work successfully introduces an efficient strategy for flexible/freestanding electrodes and enlightens the importance of coupling strategy to enhance the performances of Li-S batteries. Figure 1. (a) Digital photographs of the as-prepared composite via phase inversion method. (b) Cycle performance of half-cells using 2-layer PES-CNT-S and PVDF-CNT-S cathodes with polysulfide(PS)-free electrolyte and polysulfide(PS)-modified electrolyte at 0.25C. REFERENCES W. Wahyudi, Z. Cao, P. Kumar, M. Li, Y. Wu, M. N. Hedhili, T. D. Anthopoulos, L. Cavallo, L. J. Li, J. Ming, Adv. Funct. Mater., 2018, 28 (34), 1802244.Ming, Z. Cao, W. Wahyudi, M. Li, P. Kumar, Y. Wu, J. Y. Hwang, M. N. Hedhili, L. Cavallo, Y. K. Sun, L. J. Li, ACS Energy Lett., 2018, 3 (2), 335-340.Wang, Z. Cao, G. A. Elia, Y. Wu, W. Wahyudi, E. Abou-Hamad, A. H. Emwas, L. Cavallo, L. J. Li, J. Ming, ACS Energy Lett., 2018, 3 (12), 2899-2907. Figure 1

Lithium-sulfur (Li-S) batteries has recently attracted worldwide attention as sulfur possesses an extremely high theoretical capacity of 1675 mAh g-1 in addition to the advantages of natural abundance, low cost and environmental friendless, making it as one of the most promising cathode materials for the next generation of high energy batteries.1, 2 However, it’s widely reported that Li-S batteries suffer from low utilization of sulfur, poor reaction kinetics and inferior charge/discharge efficiency owing to its poor electronic and ionic conductivity, intrinsic polysulfides shuttling and Li corrosion. Therefore, construction of nanostructured sulfur cathode with fast diffusion paths for lithium ions and electrons as well as good trapping of polysulfides are the key factors to enhance its electrochemical performance.3 Nanostructured porous carbon materials such as ordered mesoporous carbon, porous carbon spheres, porous graphene, porous carbon nanofibers and etc have been widely used in Li-S batteries.4 Thanks to their high electronic conductivity, unique pore structure, high specific surface area and good affinity for sulfur active materials, significant improvement on the cycle life and rate capability of sulfur cathode has been achieved. However, these carbon materials are struggling to the complicated synthetic process such as difficult template removal, long synthesis period, high cost fabrication method, low yield and so on, leading to the need of further exploration for porous carbon materials with low cost, high scale-up and superior electrochemical properties. In the current work, a leaf-derived microporous carbon (LMC) with high surface area was prepared through one-pot carbonization followed by KOH activation. The sulfur/carbon composite with sulfur loading around 60wt. % (S/LMC) was then synthesized through a melt-diffusion strategy. The electrochemical test results shown that it can keep a stable capacity around 450 mAh g-1 in 200 cycles without obvious capacity fading at a charge/discharge rate of 0.5 C (1C=1675 mA g-1), showing higher reversible capacity and better cycle stability than commercial multi-walled carbon nanotubes with even lower sulfur loading of 50wt. % (S/MWCNTs). Figure 1a and the inset show the SEM image of LMC, in which micro-sized carbon aggregate with existence of numerous pores was obvious observed. The porous structure and the pore size were further confirmed by the N2 adsorption-desorption curve in the Figure 1b and the inset, showing a surface area of 1412.0 m2 g-1 and a pore volume of 0.759 cm3 g-1, and main pore size around 2-4 nm. Figure 1c is the TGA curves of S/LMC and S/MWCNTs from room temperature to 600 °C at argon atmosphere. A significant mass loss owing to the evaporation of sulfur in the sulfur/carbon composite was observed in the both two TGA curves. However, it can be seen that sulfur has been already evaporated below 350 °C for S/WCNTs; while in the case of S/LMC, most of the sulfur was removed at a temperature up to 500 °C. This finding may indicate a stronger adsorption of between sulfur and LMC than that of sulfur and MWCNTs. Figure 1d gives the comparison of the cycle life of S/MWCNTs and S/LMC at a rate of 0.5C. It was shown that S/LMC can deliver a stable capacity ca. 450 mAh g-1 in 200 cycles without any obvious capacity loss. On the contrary, S/MWCNTs exhibited a continuous capacity fading. Such difference in their electrochemical performance may originate from the high specific surface area and unique microporous structure of LMC, leading to better adsorption of polysulfides during charge/discharge. In summary, a low cost leaf-derived microporous carbon with high specific surface area was harvested from dead leaf and can demonstrate high performance in Li-S batteries. Acknowledgements: Research at the Argonne National Laboratory was funded by U.S. Department of Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357.

The editorial summarizes the contents of the special issue for energy storage in Advanced Functional Materials.

Due to their high theoretical specific energies, lithium-sulfur (Li-S) batteries have drawn significant attention as one of the promising chemistries to replace commercially used Li-ion batteries. However, in Li-S batteries attaining high capacities and long cycle lives are severely hindered by major challenges. Insulating nature of sulfur is one of the main problems that results in low sulfur utilization and requirement of electronically conductive materials in the cathode. In addition, escaping polysulfides from the cathode, in other words the polysulfide shuttle mechanism, result in sulfur loss and lithium corrosion. Hence, efficient material selection and cell design are crucial for attaining high performance Li-S batteries. Even though the battery performance depends on these materials and cell design factors significantly, the critical link between these variables and the performance is not clear. To identify this link, machine learning is a highly effective tool, especially for such complex systems with many variables and large dataset.1 In this study, we use machine learning, association rule mining (ARM) specifically, to determine the critical materials and cell design factors in Li-S batteries providing high performances at prolonged cycle lives. First, a comprehensive literature search is performed and the experimental articles are collected randomly. 353 papers accounting for almost 10 % of the literature, published between 01.01.2020 and 18.07.2018(search day), are used to collect 1660 experimental data. As performance indicators, peak discharge capacity (PDC) and cycle number at which 80% of PDC retained are chosen. 19 important factors, such as anode, encapsulation and conductive materials and their wt.% are determined and extracted from papers. This dataset is analyzed via ARM, which provides single factor associations by presenting three parameters as results: support, confidence and lift. Lift is the ratio of the fraction of a specific factor with high PDC to the fraction of that specific factor in the total data. In our analysis, lift value is chosen as the performance metric to compare the results since it provides both positive and negative correlations of factors with the output (lift values greater and lower than 1 show positive and negative correlations, respectively). The change of lift values with increasing PDCs and cycle numbers are presented as bubble graphs, where bubble size shows the number of datapoints obeying that rule. In addition, other analyses are performed to investigate the rules for high energy density cells by individually restricting the minimum sulfur loading and the maximum electrolyte-to-sulfur (E/S) ratio to 5 mg cm-2 and 5 mL g-1, respectively, for PDCs higher than or equal to 1000 mAh g-1.Figure 1 shows the change of lift values for the encapsulation material type with increasing PDC and cycle number limits. Figure 1a clearly shows the significance of the encapsulation strategy since no encapsulation case has lift values around 0.5. Porous carbons, collection of infrequently used materials such as polyacrylonitrile defined as others, and CNT with additives show promise for PDCs higher than 1400 mAh g-1, since promising factors for high performance should have increasing lift trends with increasing capacity or cycle number limits. Moreover, it is found that the encapsulation materials wt. %’s should be higher than 40% for improved cell performances, which may be due to enhanced sulfur utilization and suppressed polysulfide shuttle mechanism. According to the rest of the ARM results (not shown here), ethylene carbonate:diethyl carbonate (EC:DEC) or tetraethylene glycol dimethyl ether (TEGDME) and LiPF6 salts are proposed to be very efficient as electrolyte materials especially for lean electrolyte conditions. Furthermore, polytetrafluoroethylene (PTFE) and polymer n-lauryl acrylate (LA) are shown to be efficient binders for high PDCs. Similar results are found for the maximization of the cycling performance of the batteries. To sum up, in this study, it was concluded that novel electrode and electrolyte materials are essential for reaching high capacities at prolonged cycle lives.Figure 1. Lift vs. PDC (a) and cycle number graphs (b) for encapsulation type. References A. Kilic, Ç. Odabaşı, R. Yildirim, and D. Eroglu, Chem. Eng. J., 390, 124117 (2020) Figure 1

Lithium-sulfur (Li–S) batteries attract great attention of researchers due to their high theoretical energy density (2600 W∙h∙kg-1) and economic efficiency compared to state-of-art lithium-ion batteries [1]. The depth of the electrochemical reduction of sulfur and the cycle life of lithium-sulfur batteries are determined by the amount of electrolyte retained in the positive electrode [2]. In turn, the amount of electrolyte in the positive electrode is determined by its porosity and the ability of binders to swell in the electrolyte. Therefore, polymer binders used in the positive electrode can significantly affect the characteristics of lithium-sulfur batteries. The aim of the work was to study the effect of the swelling of polymer binders of the positive electrode in the electrolyte solution on the characteristics of lithium-sulfur batteries. The sulfur electrodes, with the composition of 70 wt.% Sulfur (99.5%, Russia), 10 wt.% multi-walled carbon nanotubes (Sigma Aldrich) and 20 wt% binder was the object of study. Polymeric binders were PEO (ММ 4∙106), PVDF-HFP (ММ 3∙105), LA-132. Lithium-sulfur cells were assembled as described in [3]. Electrolyte was1M LiCF3SO3 in sulfolane. Sulfolane is promising solvent for lithium–sulfur batteries. Electrolyte solutions based on sulfolane have high chemical and electrochemical stability [4], and moderate conductivity [5]. Sulfolane has a high flash point (166°C) [6]. Lithium trifluoromethanesulfonate was chosen as a support salt since electrolytes based on it have good scattering ability and provide lithium–sulfur cells with the smallest capacity depletion compared to electrolyte systems based on other salts [4]. Studies have shown that all investigated sulfur electrodes had good elasticity, mechanical strength and adhesion. Swelling of the electrode layer of sulfur electrodes was estimated by weight method at room temperature (23-25 °C) as follows. Pre-weighted sulfur electrodes were placed into the electrolyte solution for 48 hours. Then the sulfur electrodes were removed from the electrolyte, the excess of electrolyte was removed from the surface of the electrodes by a filter paper, and weighed. The mass of the electrolyte sorbed by the sulfur electrode was calculated as the difference in the masses of the sulfur electrodes after and before their storage into the electrolyte solution. It is established that the composition of the polymer binder has a significant effect on the swelling of the positive electrode in the electrolyte (Table). Sulfur electrodes containing PEO as a polymer binder can soak up to 2.8 µL/mAh(S). Sulfur electrodes containing PVDF-HFP and LA-132 can soak 1.5 and 1.1 µL/mAh(S), respectively. The discharge voltage profiles of lithium-sulfur cells with sulfur electrodes containing various polymeric binders differ significantly (Figure). The shape of the discharge voltage profile of lithium-sulfur cells, with the polymer binder PEO, is traditional: there are two voltage plateau: high voltage and low voltage. And in the case of polymer binders LA132 and PVDF-HFP, the discharge capacity corresponding to the end of the high-voltage stage is significantly lower than for cells with PEO binder, and the low-voltage plateau is almost indistinguishable. The difference in the swelling of sulfur electrodes containing various polymeric binders in the electrolyte solution explains the differences in the characteristics of the lithium-sulfur cells. This work was performed as part of a Government Order to Ufa Institute of Chemistry of the Russian Academy of Sciences by the Ministry of Science and Higher Education of the Russian Federation (Theme No. AAAA-A17-117011910031-7) Russia and was also financially supported by the Russian Science Foundation (project No 17-73-20115).

The mesopores and macropores within porous carbon materials help increasethe surface for the depostion of solid-state products, reducethe Li2S film thickness, enhanceelectron and mass transport, and accelerate the reaction kinetics. However, an excessive amount of mesopores and macropores can lead to increased electrolyte consumption, particularly at high sulfur loadings, where excessive electrolyte usage hampers the enhancement of practical energy density in lithium-sulfur (Li-S) batteries. A rational pore structure can minimize the amount of electrolyte to fill the pores, thereby reducing electrolyte consumption while achieving rapid reaction kinetics and a high gravimetric energy density. In this work, the pore structure of carbon nanosheet-based electrocatalysts is precisely controlled by adjusting the content of a water-soluble potassium chloride template, allowing for in-depth investigation of the relationship between pore structure, electrolyte usage, and electrochemical performance in Li-S batteries. The molybdenum carbide-embedded carbon nanosheet (MoC-CNS) electrocatalyst, with an optimized pore structure, facilitates exceptional electrochemical performance under high sulfur loading and lean electrolyte conditions. Ultimately, the MoC-CNS-3-based Li-S battery achieved stable operation over 50 cycles under high sulfur loading (12mgcm-2) and a low electrolyte-to-sulfur (E/S) ratio of 4uLmg-1, delivering a high gravimetric energy density of 354.5Whkg-1. This work provides a viable strategy for developing high-performance Li-S batteries.

Currently, lithium sulfur (Li-S) battery with high theoretical energy density has attracted great research interest. However, the diffusion and loss process of intermediate lithium polysulfide during charge-discharge hindered the application of the Li-S battery in modern life. To overcome this issue, metal organic frameworks (MOFs) and their composites have been regarded as effective additions to restrain the LiPS diffusion process for Li-S battery. Benefiting from the unique structure with rich active sites to adsorb LiPS and accelerate the LiPS redox, the Li-S batteries with MOFs modified exhibit superior electrochemical performance. Considering the rapid development of MOFs in Li-S battery, this review summarizes the recent studies of MOFs and their composites as the sulfur host materials, functional interlayer, separator coating layer, and separator/solid electrolyte for Li-S batteries in detail. In addition, the promising design strategies of functional MOF materials are proposed to improve the electrochemical performance of Li-S battery.

Lithium sulfur (Li-S) batteries have been intensively concerned in recent years as next generation Li batteries1. Compared with state-of-the-art Li-ion batteries, Li-S batteries own ultrahigh theoretical capacity and energy density, which are considered as most promising candidates applied to electric vehicles and hybrid electric vehicles. However, insulation of sulfur and dissolution of polysulfides are two main issues in sulfur cathodes which hinder the cycle life and application safety of Li-S batteries. To overcome these issues, developing conductive hosts and coating materials are prevailing approaches to improve the performance of Li-S batteries. Herein, we report molecular layer deposited (MLD) alucone coating and nanoscale metal organic framework derived carbon (MOF-C) confined sulfur cathodes in Li-S batteries. Surface coating with carbon or metal oxides has been proven to be a promising approach towards mitigating the shuttle effect in Li-S batteries. An ideal coating material should both cover on based material completely to prevent sulfur migration and also allow Li-ions and electrons transferring through smoothly. Based on these requirements, atomic and molecular layer deposition (ALD and MLD) are ideal technique to synthesize ultrathin and conformal coatings due to the self-limiting nature2 , 3. For the first time, we demonstrate that an MLD alucone coating directly on sulfur electrodes can dramatically improve the cycling stability and capability of Li-S cells. Furthermore, the alucone coated sulfur cathode delivers a discharge capacity of 710 mAh g-1, which is over two times higher than the bare sulfur cathode after 100 cycles4. The alucone coating demonstrated long durability during cell cycling, which explores a new direction in the protection of sulfur cathodes. Porous structured materials are prevailing in sulfur cathode to confine sulfur molecule in host materials. Herein, we developed a new carbon family, metal organic framework derived carbon materials (MOF-Cs) with tunable porous structure via in-situ ammonia treatment5. The ammonia treated MOF-C as carbon host shows an impressive improvement on sulfur cathodes, which performed twice higher discharge capacity retention than that of the pristine MOF-C. This research sheds light to design MOF-C materials with controlled nanostructure not only for Li–S batteries, but also for expanded applications in different energy storage systems. References 1 Bruce, P. G., Freunberger, S. A., Hardwick, L. J. & Tarascon, J. M. Li-O2 and Li-S batteries with high energy storage. Nature materials11, 19-29, (2012). 2 Meng, X., Yang, X. Q. & Sun, X. Emerging applications of atomic layer deposition for lithium-ion battery studies. Adv Mater24, 3589-3615, (2012). 3 Li, X. et al. Atomic layer deposition of solid-state electrolyte coated cathode materials with superior high-voltage cycling behavior for lithium ion battery application. Energy & Environmental Science7, 768, (2014). 4 Li, X., Lushington, A., Liu, J., Li, R. & Sun, X. Superior stable sulfur cathodes of Li-S batteries enabled by molecular layer deposition. Chem Commun (Camb)50, 9757-9760, (2014). 5 Li, X. et al. Tunable porous structure of metal organic framework derived carbon and the application in lithium–sulfur batteries. J Power Sources 302, 174-179, (2016). Figure 1