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1. (Digital Presentation) Accelerating the Conversion Process of Polysulfides in High Mass Loading Sulfur Cathode for the Longevity Li-S Battery

   https://doi.org/10.1149/MA2022-012383mtgabs  

   Zhang, Yuxuan; Kivevele, Thomas; Song, Han Wook; Lee, Sunghwan (July 2022, ECS Meeting Abstracts)

   Conventional lithium-ion batteries are unable to meet the increasing demands for high-energy storage systems, because of their limited theoretical capacity. 1 In recent years, intensive attention has been paid to enhancing battery energy storage capability to satisfy the increasing energy demand in modern society and reduce the average energy capacity cost. Among the candidates for next generation high energy storage systems, the lithium sulfur battery is especially attractive because of its high theoretical specific energy (around 2600 W h kg-1) and potential cost reduction. In addition, sulfur is a cost effective and environmentally friendly material due to its abundance and low-toxicity. 2 Despite all of these advantages, the practical application of lithium sulfur batteries to date has been hindered by a series of obstacles, including low active material loading, poor cycle life, and sluggish sulfur conversion kinetics. 3 Achieving high mass loading cathode in the traditional 2D planar thick electrode has been challenged. The high distorsion of the traditional planar thick electrodes for ion/electron transfer leads to the limited utilization of active materials and high resistance, which eventually results in restricted energy density and accelerated electrode failure. 4 Furthermore, of the electrolyte to pores in the cathode and utilization ratio of active materials. Catalysts such as MnO 2 and Co dopants were employed to accelerate the sulfur conversion reaction during the charge and discharge process. 5 However, catalysts based on transition metals suffer from poor electronic conductivity. Other catalysts such as transition metal dopants are also limited due to the increased process complexities. . In addition, the severe shuttle effects in Li-S batteries may lead to fast failures of the battery. Constructing a protection layer on the separator for limiting the transmission of soluble polysulfides is considered an effective way to eliminate the shuttle phenomenon. However, the soluble sulfides still can largely dissolve around the cathode side causing the sluggish reaction condition for sulfur conversion. 5 To mitigate the issues above, herein we demonstrate a novel sulfur electrode design strategy enabled by additive manufacturing and oxidative vapor deposition (oCVD). Specifically, the electrode is strategically designed into a hierarchal hollow structure via stereolithography technique to increase sulfur usage. The active material concentration loaded to the battery cathode is controlled precisely during 3D printing by adjusting the number of printed layers. Owing to its freedom in geometry and structure, the suggested design is expected to improve the Li ions and electron transport rate considerably, and hence, the battery power density. The printed cathode is sintered at 700 °C at N 2 atmosphere to achieve carbonization of the cathode during which intrinsic carbon defects (e.g., pentagon carbon) as catalytic defect sites are in-situ generated on the cathode. The intrinsic carbon defects equipped with adequate electronic conductivity. The sintered 3D cathode is then transferred to the oCVD chamber for depositing a thin PEDOT layer as a protection layer to restrict dissolutions of sulfur compounds in the cathode. Density functional theory calculation reveals the electronic state variance between the structures with and without defects, the structure with defects demonstrates the higher kinetic condition for sulfur conversion. To further identify the favorable reaction dynamic process, the in-situ XRD is used to characterize the transformation between soluble and insoluble polysulfides, which is the main barrier in the charge and discharge process of Li-S batteries. The results show the oCVD coated 3D printed sulfur cathode exhibits a much higher kinetic process for sulfur conversion, which benefits from the highly tailored hierarchal hollow structure and the defects engineering on the cathode. Further, the oCVD coated 3D printed sulfur cathode also demonstrates higher stability during long cycling enabled by the oCVD PEDOT protection layer, which is verified by an absorption energy calculation of polysulfides at PEDOT. Such modeling and analysis help to elucidate the fundamental mechanisms that govern cathode performance and degradation in Li-S batteries. The current study also provides design strategies for the sulfur cathode as well as selection approaches to novel battery systems. References: Bhargav, A., (2020). Lithium-Sulfur Batteries: Attaining the Critical Metrics. Joule 4 , 285-291. Chung, S.-H., (2018). Progress on the Critical Parameters for Lithium–Sulfur Batteries to be Practically Viable. Advanced Functional Materials 28 , 1801188. Peng, H.-J.,(2017). Review on High-Loading and High-Energy Lithium–Sulfur Batteries. Advanced Energy Materials 7 , 1700260. Chu, T., (2021). 3D printing‐enabled advanced electrode architecture design. Carbon Energy 3 , 424-439. Shi, Z., (2021). Defect Engineering for Expediting Li–S Chemistry: Strategies, Mechanisms, and Perspectives. Advanced Energy Materials 11 . Figure 1 

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2. Sulfur Cathode Design Strategies Enabled by Stereolithography Technique and Oxidative Chemical Vapor Deposition

   Zhang, Yuxuan; Song, Han Wook; Lee, Sunghwan (May 2022, Materials Research Society)

   It is urgent to enhance battery energy storage capability to satisfy the increasing energy demand in modern society and reduce the average energy capacity cost. Among the candidates for next-generation high energy storage systems, the lithium-sulfur battery is especially attractive because of its high theoretical specific energy (around 2600 W h kg-1) and cost savings potential.1 In addition to the high theoretical capacity of sulfur cathode as high as 1,673 mA h g-1, sulfur is further appealing due to its abundance in nature, low cost, and low toxicity. Despite these advantages, the application of sulfur cathodes to date has been hindered by a number of obstacles, including low active material loading, low electronic conductivity, shuttle effects, and sluggish sulfur conversion kinetics.2 The traditional 2D planer thick electrode is considered as a general approach to enhance the mass loading of the lithium-sulfur (Li-S) battery.3 However, the longer diffusion length of lithium ions required in the thick electrode decrease the wettability of the electrolyte (into the entire cathode) and utilization ratio of active materials.4 Encapsulating active sulfur in carbon hosts is another common method to improve the performance of sulfur cathodes by enhancing the electronic conductivity and restricting shuttle effects. Nevertheless, it is also reported that the encapsulation approach causes unfavorable carbon agglomeration with low dimensional carbons and a low energy density of the battery with high dimensional carbons. Although an effort to induce defects in the cathode was made to promote sulfur conversion kinetic conditions, only one type of defect has demonstrated limited performance due to the strong adsorption of the uncatalyzed clusters to the defects (i.e.: catalyst poisoning). 5 To mitigate the issues listed above, herein we propose a novel sulfur electrode design strategy enabled by additive manufacturing and oxidative chemical vapor deposition (oCVD).6,7 Specifically, the electrode is designed to have a hierarchal hollow structure via a stereolithography technique to increase sulfur usage. Microchannels are constructed on the tailored sulfur cathode to further fortify the wettability of the electrolyte. The as-printed cathode is then sintered at 700 °C in a reducing atmosphere (e.g.: H2) in order to generate a carbon skeleton (i.e.: carbonization of resin) with intrinsic carbon defects. A cathode treatment with benzene sulfonic acid further induces additional defects (non-intrinsic) to enhance the sulfur conversion kinetic. Furthermore, intrinsic defects engineering is expected to synergistically create favorable sulfur conversion conditions and mitigate the catalyst poisoning issue. In this study, the oCVD technique is leveraged to produce a conformal coating layer to eliminate shuttle effects, unfavored in the Li-S battery performance. Identified by SEM and TEM characterizations, the oCVD PEDOT is not only covered on the surface of the cathode but also the inner surface of the microchannels. High resolution x-ray photoelectron spectroscopy analyses (C 1s and S 2p orbitals) between pristine and modified sample demonstrate that the high concentration of the defects have been produced on the sulfur matrix after sintering and posttreatment. In-operando XRD diffractograms show that the Li2S is generated in the oCVD PEDOT-coated sample during the charge and discharge process even with a high current density, confirming an eminent sulfur conversion kinetic condition. In addition, ICP-OES results of lithium metal anode at different states of charge (SoC) verify that the shuttle effects are excellently restricted by oCVD PEDOT. Overall, the high mass loading (> 5 mg cm-2) with elevated sulfur utilization ratio, accelerated reaction kinetics, and stabilized electrochemical process have been achieved on the sulfur cathode by implementing this innovative cathode design strategy. The results of this study demonstrate significant promises of employing pure sulfur powder with high electrochemical performance and suggest a pathway to the higher energy and power density battery. 

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3. Sulfur-deficient MoS ₂ grown inside hollow mesoporous carbon as a functional polysulfide mediator

   https://doi.org/10.1039/c9ta01722d  

   Wang, Hong-En; Li, Xuecheng; Qin, Ning; Zhao, Xu; Cheng, Hua; Cao, Guozhong; Zhang, Wenjun (May 2019, Journal of Materials Chemistry A)

   Lithium–sulfur (Li–S) batteries are regarded as one of the most promising next-generation electrochemical cells. However, shuttling of lithium polysulfide intermediates and sluggish kinetics in random deposition of lithium sulfide (Li 2 S) have significantly degraded their capacity, rate and cycling performance. Herein, few-layered MoS 2 nanosheets enriched with sulfur vacancies are anchored inside hollow mesoporous carbon (MoS 2−x /HMC) via S–C bonding and proposed as a novel functional mediator for Li–S batteries. Ultrathin MoS 2 sheets with abundant sulfur vacancies have strong chemical affinity to polysulfides and in the meantime catalyze their fast redox conversion with enhanced reaction kinetics as proved by experimental observations and first-principles density functional theory (DFT) calculations. At a current density of 1C, the MoS 2−x /HMC-S composite cathode exhibits a high initial capacity of 945 mA h g −1 with a high retained capacity of 526 mA h g −1 and a coulombic efficiency of nearly 100% after 500 cycles. The present work sheds light on the design of novel functional electrodes for next-generation electrochemical cells based on a simple yet effective vacancy engineering strategy. 

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4. Tailored porous framework materials for advancing lithium–sulfur batteries

   https://doi.org/10.1039/d1cc07087h  

   Liu, Bingqian; Thoi, V. Sara (March 2022, Chemical Communications)

   Despite great promise as next-generation high-capacity energy storage devices, lithium–sulfur batteries still face technical challenges in long-term cyclability. With their porous structures and facile synthesis, metal–organic frameworks (MOFs) are tunable platforms for understanding polysulfide redox and can serve as effective sulfur hosts for lithium–sulfur batteries. This feature article describes our design strategies to tailor MOF properties such as polysulfide affinity, ionic conductivity, and porosity for promoting active material utilization and charge transport efficiency. We also present engineering approaches for implementing MOF-based sulfur cathodes for lithium–sulfur batteries with high volumetric density and under low temperature operation. Our studies provide fundamental insights into sulfur–host interactions and polysulfide electrochemistry in the presence of porous matrices, inspiring future designs of advanced batteries. 

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5. Understanding the lithium–sulfur battery redox reactions via operando confocal Raman microscopy

   https://doi.org/10.1038/s41467-022-32139-w  

   Lang, Shuangyan; Yu, Seung-Ho; Feng, Xinran; Krumov, Mihail R.; Abruña, Héctor D. (December 2022, Nature Communications)

   Abstract The complex interplay and only partial understanding of the multi-step phase transitions and reaction kinetics of redox processes in lithium–sulfur batteries are the main stumbling blocks that hinder the advancement and broad deployment of this electrochemical energy storage system. To better understand these aspects, here we report operando confocal Raman microscopy measurements to investigate the reaction kinetics of Li–S redox processes and provide mechanistic insights into polysulfide generation/evolution and sulfur deposition. Operando visualization and quantification of the reactants and intermediates enabled the characterization of potential-dependent rates during Li–S redox and the linking of the electronic conductivity of the sulfur-based electrode and concentrations of polysulfides to the cell performance. We also report the visualization of the interfacial evolution and diffusion processes of different polysulfides that demonstrate stepwise discharge and parallel recharge mechanisms during cell operation. These results provide fundamental insights into the mechanisms and kinetics of Li–S redox reactions. 

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