Sulfur Cathodes In Lithium-sulfur Batteries Research Articles

Insights into the aspect ratio effects of ordered mesoporous carbon on the electrochemical performance of sulfur cathode in lithium-sulfur batteries

Tailoring porous host materials, as an effective strategy for storing sulfur and restraining the shuttling of soluble polysulfides in electrolyte, is crucial in the design of high-performance lithium-sulfur (Li-S) batteries. However, for the widely studied conductive hosts such as mesoporous carbon, how the aspect ratio affects the confining ability to polysulfides, ion diffusion as well as the performances of Li-S batteries has been rarely studied. Herein, ordered mesoporous carbon (OMC) is chosen as a proof-of-concept prototype of sulfur host, and its aspect ratio is tuned from over ∼ 2 down to below ∼ 1.2 by using ordered mesoporous silica hard templates with variable length/width scales. The correlation between the aspect ratio of OMCs and the electrochemical performances of the corresponding sulfur-carbon cathodes are systematically studied with combined electrochemical measurements and microscopic characterizations. Moreover, the evolution of sulfur species in OMCs at different discharge states is scrutinized by small-angle X-ray scattering. This study gives insight into the aspect ratio effects of mesoporous host on battery performances of sulfur cathodes, providing guidelines for designing porous host materials for high-energy sulfur cathodes.

Investigating the Influence of Diverse Functionalized Carbon Nanotubes as Conductive Fibers on Paper-Based Sulfur Cathodes in Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries are expected to be one of the next generations of high-energy-density battery systems due to their high theoretical energy density of 2600 Wh kg-1. Embracing the trends toward flexibility, lightweight design, and cost-effectiveness, paper-based electrodes offer a promising alternative to traditional coated cathodes in Li-S batteries. Within paper-based electrodes, conductive fibers such as carbon nanotubes (CNTs) play a crucial role. They help to form a three-dimensional network within the paper matrix to ensure structural integrity over extended cycling while mitigating the shuttle effect by confining sulfur within the cathode. Herein, we explore how variously functionalized CNTs, serving as conductive fibers, impact the physical and electrochemical characteristics of paper-based sulfur cathodes in Li-S batteries. Specifically, graphitized hydroxylated carbon nanotubes (G-CNTs) exhibit remarkable capacity at low currents owing to their excellent conductivity and interaction with lithium polysulfide (LiPS), achieving the highest initial specific capacity of 1033 mAh g-1 at 0.25 C (1.1 mA cm-2). Aminated multi-walled carbon nanotubes (NH2-CNTs) demonstrate an enhanced affinity for LiPS due to the -NH2 groups. However, the uneven distribution of these fibers may induce electrode surface passivation during charge-discharge cycles. Notably, hydroxylated multi-walled carbon nanotubes (OH-CNTs) can establish a uniform and stable 3D network with plant fibers, showcasing superior mechanical properties and helping to mitigate Li2S agglomeration while preserving the electrode porosity. The paper-based electrode integrated with OH-CNTs even retains a specific capacity of approximately 800 mAh g-1 at about 1.25 C (5 mA cm-2), demonstrating good sulfur utilization and rate capacity compared to other CNT variants.

Kinetic Promoters for Sulfur Cathodes in Lithium-Sulfur Batteries.

ConspectusLithium-sulfur (Li-S) batteries have attracted worldwide attention as promising next-generation rechargeable batteries due to their high theoretical energy density of 2600 Wh kg-1. The actual energy density of Li-S batteries at the pouch cell level has significantly exceeded that of state-of-the-art Li-ion batteries. However, the overall performances of Li-S batteries under practical working conditions are limited by the sluggish conversion kinetics of the sulfur cathodes. To overcome the above challenge, various kinetic promotion strategies have been proposed to accelerate the multiphase and multi-electron cathodic redox reactions between sulfur, lithium polysulfides (LiPSs), and lithium sulfide. Nowadays, kinetic promoters have been massively employed in sulfur cathodes to achieve Li-S batteries with high energy densities, high rates, and long lifespans. A comprehensive and timely summary of cutting-edge kinetic promoters for sulfur cathodes is of great essence to afford an in-depth understanding of the unique Li-S electrochemistry.In this Account, we outline the recent efforts on the design of sulfur cathode kinetic promoters for advanced Li-S batteries. The latest progress is discussed in detail regarding heterogeneous, homogeneous, and semi-immobilized kinetic promoters. Heterogeneous promoters, representatively known as electrocatalysts, function mainly by reducing the energy barriers of the interfacial electrochemical reactions. The working mechanism, activity regulation strategies, and reconstitution/deactivation processes of the heterogeneous promoters are reviewed to provide guiding principles for rational design. In comparison, homogeneous promoters are able to fully contact with the reaction interfaces and regulate the electron/ion-inaccessible reactants in working Li-S batteries. Redox mediators and redox comediators are typical homogeneous promoters. The former establishes extra chemical reaction pathways to circumvent the originally sluggish steps and boost the overall kinetics, while the latter fundamentally modifies the LiPS molecules to enhance their redox kinetics. For semi-immobilized promoters, the active units are generally anchored on the cathode substrate through flexible chains with mobile characteristics. Such a design endows the promoter with both heterogeneous and homogeneous characteristics to comprehensively regulate the multiphase sulfur redox reactions involving both mobile and immobile reactants.Overall, this Account summarizes the fundamental electrochemistry, design principles, and practical promotion effects of the various kinetic promoters used for the sulfur cathodes in Li-S batteries. We believe that this Account will provide an in-depth and cutting-edge understanding of the unique sulfur electrochemistry, thereby providing guidance for further development of high-performance Li-S batteries and analogous rechargeable battery systems.

Chitosan binders for sustainable lithium-sulfur batteries: Synergistic effects of mechanical and polysulfide trapping properties

The lithium-sulfur (Li-S) system is a promising candidate for the next generation of lithium batteries; however, for practical applications, there are still problems to solve as the loss of active material due to polysulfides diffusion and migration (shuttle effect), passivation of interfaces of both electrodes by deposition of insulating species. Chitosan, as an abundant and sustainable biopolymer has been previously modified to solve issues related to polysulfides retention through biopolymer functionalization; however, the effect of such modifications on the mechanical properties, related to the structural modifications, has been poorly investigated; moreover, the coupled effects of both phenomena, polysulfide trapping capacity and mechanical properties, and its impact on battery capacity and cyclability has not been discussed. In order to determine those effects, in this work is proposed a new binder based on chitosan modified by Methanesulfonic Acid as a plasticizer in the fabrication of sulfur cathodes for Li-S batteries. The improvement in the overall performance of the system was investigated by C-Rate, deep cycling and tensile test, and the results were compared with those of the sulfur cathode using brittle and water-insoluble pristine chitosan as binder. Our results indicated that the Chitosan – Methanesulfonic Acid, ChMSA25 polymer network, is able to modulate the mechanical strength and binding performance of the system. Due to the aforementioned characteristics, the ChMSA25 based sulfur cathode exhibits a low-capacity fade (0.058% per cycle) at 1.0 C for 500 cycles. Therefore, the ChMSA25 binder is a promising candidate for the development of high-performance, water processable lithium-sulfur batteries.

Tuning Redox Behavior of Sulfur Cathodes Via Ternary-Coordinated Single Fe Atom in Lithium-Sulfur Batteries.

Modulating the coordination configuration of single Fe atom has been an efficient strategy to strengthen the redox dynamics for lithium-sulfur batteries (LSBs) but remains challenging. Herein, the single Fe atom is functioned with nitrogen and carbon atoms in the first shell, and simultaneously, oxidized sulfur (─SOx) in the second shell, which presents a lower antibonding state and well address the redox activity of sulfur cathodes. In the ternary-coordinated single Fe atom catalyst (FeN2C2-SOx-NC), the binary structure of FeN2C2 provides a lower Fe-S bonding strength and d-p orbital hybridization, which obviously optimizes the adsorption and desorption behavior of sulfur species during the reduction and oxidation reaction processes. Simultaneously, the ─SOx redistributes the electron density of the coordinating nitrogen atoms, which possesses high electron-withdrawing ability and develops electrocatalytic activity. As a result, the sulfur cathodes with FeN2C2-SOx-NC present an excellent high-rate cyclic performance, accompanied by a capacity decay rate of 0.08% per cycle for 500 cycles at 4.0 C. This study provides new insights for optimizing the redox dynamics of sulfur cathodes in LSBs at the atomic level.

First-row transition metal carbide nanosheets as high-performance cathode materials for lithium-sulfur batteries.

Despite the prodigious potential of lithium-sulfur (Li-S) batteries as future rechargeable electrochemical systems, their commercial implementation is hindered by several vital issues, including the shuttle effect and sluggish migration of lithium-polysulfides leading to rapid capacity fading. Here, we systematically investigate the potential of first-row two-dimensional transition metal carbides (TMCs) as sulfur cathodes for Li-S batteries. The adsorption strength of lithium-polysulfides on TMCs is induced by the amount of charge transfer from the former to the latter and the proposed periodic relationship between sulfur in Li2S and 3d-transition metals. Our findings show that the VC nanosheet possesses immense anchoring potential and exhibits a comparatively low migration energy barrier for lithium-ion and Li2S molecules. Additionally, we report ab initio molecular dynamics simulations for lithiated polysulfide species anchored on a TMC-based model with a liquid-electrolyte medium. The microscopic reaction mechanism, revealed by the evolution of the reaction voltage during lithiation, demonstrates that the dissolution of high-order lithium-polysulfides in the electrolytes can be prevented due to their robust interaction with TMC-based cathode materials. These appealing features suggest that TMCs present colossal performance improvements for anchoring lithium-polysulfides, stimulating the active design of sulfur cathodes for practical Li-S batteries.

The “dual-layer sulfur cathode” strategy: An In2S3/Bi2S3@rGO heterostructure as an interlayer/modified separator for boosting the areal capacities of lithium-sulfur batteries

The specific energies and energy densities of lithium–sulfur (Li-S) batteries are influenced by various cell parameters, including the sulfur loading, the sulfur weight percentage in the cathode, and the electrolyte/sulfur ratio. An In2S3/Bi2S3@rGO heterostructure was obtained by growing indium sulfide nanoparticles on the surface of bismuth sulfide nanoflowers in a graphene oxide (GO) solution via a one-step solvothermal approach. This structure was introduced as a modified separator/dual-layer sulfur cathode for Li-S batteries. The Bi2S3/In2S3 heterointerfaces act as active sites to speed up interfacial electron transfer, along with the entrapment, diffusion, and transformation of lithium polysulfides. A Li-S cell containing a dual-layer sulfur cathode (thin layer of In2S3/Bi2S3@rGO sandwiched between two thick layers of sulfur) and coupled with an In2S3/Bi2S3@rGO-coated separator suppressed the polysulfide shuttle effect. The cell based on the dual-layer sulfur cathode technology and operated at a current rate of 0.3C achieved a high capacity (7.1 mAh cm−2) after the 200th cycle, giving an electrolyte/sulfur ratio (10 µL mg−1) under a high sulfur loading (11.53 mg cm−2). These results demonstrate the unique nature of the dual-layer sulfur cathode technique, which can yield high energy density Li-S batteries with high sulfur loadings and low electrolyte/sulfur ratios.

The Effect of Carbon Type and Solvent/Binder Ratio on the Structure and Performance of Sulfur Cathodes in Lithium-Sulfur Batteries

Lithium-Sulfur (Li-S) batteries are among the popular candidates for next-generation rechargeable energy storage devices due to their high specific capacity and superior energy storage capabilities. However, commercialization of Li-S batteries has been hindered by several challenges such as the insulating nature of sulfur, and the shuttling of soluble lithium polysulfides and accumulation of insulating deposits on the electrodes – leading to capacity degradation over extended cycling1.Several research efforts have been devoted to mitigating these challenges. Among the different approaches, many studies focus on addressing the poor conductivity of sulfur and lithium polysulfide shuttling simultaneously by developing conductive carbon hosts2,3. While improvements have been achieved in the development of host structures with better electronic conductivity and improved polysulfide trapping capability, many of those structures have advanced architectures that demand complex processing, expensive precursors, and often lower gravimetric sulfur loading (<70 wt%).This leads to expensive-to-produce electrodes that have lower true energy density than desired. What is needed are simple preparation methods that address the issues discussed above without overly complicated processing, and preferably already commercialized materials.In this work, we systematically investigate the effect of different electrode components, including various common carbons and polymeric binders. The effect of carbon type and loading is discussed. The effect of solvent to binder ratio in the electrode slurry preparation is also studied. By tuning the binder composition, types of conductive carbon black, and the amount of solvent, we observed a difference in the structure of the host medium and consequently the sulfur electrode. Specifically, a shell covering surrounding the sulfur particles was observed at low solvent/binder ratio. Increasing the solvent/binder ratio led to the disappearance of the shell coverings and particle agglomeration, which resulted in lower achieved capacity and reduced cycle life. Ketjen black offered the highest specific capacity, while the presence of shell covering achieved at a low solvent/binder ratio was found necessary for cycling stability.This work demonstrates the importance of electrode processing parameters on the structure and electrochemical performance of sulfur cathode in Li-S batteries. The new understandings from this work can provide guidance on electrode designs to achieve Li-S batteries with enhanced capacity and longevity.References G. Li et al., Adv. Mater., 30, 1705590 (2018).D.-W. Wang et al., J. Mater. Chem. A, 1, 9382 (2013).Y. Li and S. Guo, Matter, 4, 1142–1188 (2021).

Single‐atomic tungsten‐doped Co3O4 nanosheets for enhanced electrochemical kinetics in lithium–sulfur batteries

AbstractThe practical application of lithium–sulfur batteries (LSBs) is severely hindered by the undesirable shuttling of lithium polysulfides (LiPSs) and sluggish redox kinetics of sulfur species. Herein, a series of ultrathin single‐atomic tungsten‐doped Co3O4 (Wx‐Co3O4) nanosheets as catalytic additives in the sulfur cathode for LSBs are rationally designed and synthesized. Benefiting from the enhanced catalytic activity and optimized electronic structure by W doping, the Wx‐Co3O4 not only reduces the shuttling of LiPSs but also decreases the energy barrier of sulfur redox reactions of sulfur species, leading to accelerated electrode kinetic. As a result, LSB cathodes with the use of 5.0 wt% W0.02‐Co3O4 as the electrocatalyst show the high reversible capacities of 1217.0 and 558.6 mAh g−1 at 0.2 and 5.0 C, respectively, and maintain a high reversible capacity of 644.6 mAh g−1 at 1.0 C (1.0 C = 1675 mA g−1) after 500 cycles. With a high sulfur loading of 5.5 mg cm−2 and electrolyte–electrode ratio of 8 μLelectrolyte mgsulfur−1, the 5.0 wt% W0.02‐Co3O4‐based sulfur cathode also retains a high reversible areal capacity of 3.86 mAh cm−2 at 0.1 C after 50 cycles with an initial capacity retention of 84.7%.

A COF-coated ordered porous framework as multifunctional polysulfide barrier towards high-performance lithium-sulfur batteries

The practical application of lithium-sulfur batteries (LSBs) is still hindered by the shuttle effect of lithium polysulfides (LiPS) and slow sulfur conversion kinetics. Herein, a LiPS inhibited covalent organic framework (COF)-coated conductive porous metal oxide design strategy is proposed towards the development of efficient and durable sulfur cathode in LSBs. This strategy is demonstrated by coating a TpPa-1 COF layer on cobalt-decorated titanium oxynirtide (TiOxNy) with a three-dimensional ordered microporous framework (3DOM) structure. In this strategy, the oxygen-deficient TiOxNy framework ensures a good conductivity and structural stability of the cathode during the charge/discharge process. The 3DOM macrospores provide a high capacity for sulfur accommodation and exposes active interfaces, whereas the coated TpPa-1 COF featured with ultrafine microspore offer an effective confinement of LiPS within the 3DOM framework, mitigating its shuttling effect. At the same time, the Co embedded in 3DOM TiOxNy servers as efficient catalyst promoting the sulfur electrochemical reaction. Attributed to these structural superiorities, the 3DOM TpPa-1@Co/TiOxNy/S cathode exhibits excellent performance even under high sulfur loading and low electrolyte condition. This work of using microporous COF coating with conductive macroporous metal oxides offers an effective alternative strategy for the design of practical sulfur battery.

Electrocatalytic and Conductive Vanadium Oxide on Carbonized Bacterial Cellulose Aerogel for the Sulfur Cathode in Li-S Batteries

Many transition-metal-oxide-based catalysts have been investigated to chemically bind soluble lithium polysulfides and accelerate their redox kinetics in lithium-sulfur (Li-S) battery chemistry. However, the intrinsic poor electrical conductivities of these oxides restrict their catalytic performance, consequently limiting the sulfur utilization and the rate performance of Li-S batteries. Herein, we report a freestanding electrocatalytic sulfur host consisting of hydrogen-treated VO2 nanoparticles (H-VO2) anchored on nitrogen-doped carbonized bacterial cellulose aerogels (N-CBC). The hydrogen treatment enables the formation and stabilization of the rutile VO2(R) phase with metallic conductivity at room temperature, significantly enhancing its catalytic capability compared to the as-synthesized insulative VO2(M) phase. Several measurements characterize the electrocatalytic performance of this unique H-VO2@N-CBC structure. In particular, the two kinetic barriers between S8, polysulfides, and Li2S are largely reduced by 28.2 and 43.3 kJ/mol, respectively. Accordingly, the Li-S battery performance, in terms of sulfur utilization and charge/discharge rate, is greatly improved. This work suggests an effective strategy to develop conductive catalysts based on a typical transition metal oxide (VO2) for Li-S batteries.

Cycle-Induced Structural Evolution of Sulfur Cathodes in Lithium-Sulfur Batteries

Lithium (Li) ion batteries (LIBs) have been the predominant and fastest growing energy storage technology over the past few decades. A significant amount of LIB research has been carried out and remarkable improvements in the technology have been achieved. As a result, state-of-the-art LIBs offer superior cyclability, high efficiency, and high specific energy relative to competitors [1]. However, the desire for long-range electric vehicles (EVs) and grid-level energy storage and delivery is increasing the demands for batteries with very high gravimetric energy density (e.g. > 500 Wh/kg) [2]. This is simply much higher than what LIB electrode materials can practically offer (~ 260 Wh/kg). Therefore, alternative chemistries are needed at both electrodes.One material that has received significant attention recently as a replacement cathode material in Li-based batteries is sulfur (S). S has 5 times the theoretical specific energy than conventional LIB cathodes and can offer a practical energy density of > 500 Wh/kg when coupled with commercially available lithiated graphite or Li metal anodes [3]. S is also non-toxic, low-cost, and has high natural abundance. These properties make S a promising candidate for next-generation cathodes in Li battery systems. Yet, the path to achieving near theoretical capacity and long cycle life for S cathodes has proven difficult due to numerous unsolved scientific and technical issues including: i) the insulating nature of Sulfur (S8) and its discharged product (Li2S); ii) undesired solubility of the S products in the liquid electrolyte, resulting in the degrading so-called Li polysulfides “shuttling”, and iii) structural change of the S cathode during charge and discharge due to the large volume variation between the fully charged and discharged products [4].Several approaches have been reported to address these challenges and improve the Li-S battery performance and durability. Despite these efforts, the advances have been mostly limited to a small number of cycles, or the need for complex structures and that would lead to expensive synthesis costs at the manufacturing scale. In fact, it is not truly known if such complex structures are even necessary as the literature lacks a truly systematic investigation into i) the influence of the S structure on its behavior; and ii) how the S structure evolves as a result of charging and discharging the cell. It is also likely that complex structures would not be reformed upon deep charging/discharging – making their possible advantages only temporary. Hence, there are a limited number of truly practical S cathodes that can be rationally developed [3, 4]. In this work, new insights are presented regarding the structural evolution of S cathodes throughout cycling. The structural changes experienced by the S cathodes were investigated by scanning electron microscopy (SEM) during charge and discharge (at C/10) over the lifetime of the cell (10’s to 100’s of cycles) for multiple cells. Cycling was done with Li-S coin cells that were made using a Li metal anode and a S cathode. The S cathode was prepared using commercially available S powder, a through low-cost, simple, and scalable electrode recipe and production techniques. Drastic microstructural and compositional transformations were observed in the S cathodes as a consequence of charging and discharging. Results suggest that there was a reversible swelling transfiguration of the support structure (conductive carbon plus binder) during each discharge and charge step. It was also observed that the location and distribution of S was changed, and new structures were formed. These results are expected to cast light on a fairly unknown area in the Li-S battery technology, which can help with future scale-up and manufacturing of these cells.

Catalysis in Lithium-Sulfur Cathodes for Improved Performance and Stability

Among next generation batteries, lithium-sulfur batteries are expected to be first battery to find a commercial route in the next few years. The sulfur cathodes in lithium-sulfur batteries provide both a lower cost and high capacity (1675 mAh/g) in comparison to intercalation cathode materials [1]. Despite these upsides, this conversion reaction type battery suffers from few problems that hinders their adoption. Some of these problems include the sluggish reaction kinetics, low sulfur utilization, rapid capacity loss due to various sulfur loss phenomena, and low Coulombic efficiency [2]. The use of electrocatalysts was shown to significantly boost the polysulfides redox reactions and improves the battery performance [3]. Transition metals, especially platinum group metal (PGM) based catalysts are proven to effectively boost the reaction rate of polysulfides conversion during cycling due to their high electrocatalytic activity [4], [5]. However, catalyst incorporation in conversion reaction batteries systems often would lead to side reactions and strategies on how to incorporate them into the battery system have to be developed.In this work, platinum group metal (PGM) nanocatalysts were implemented in lithium-sulfur cathodes using a process that is tailored to effectively improve catalyst dispersion and to provide controlled catalyst electrolyte contact. The nanocatalysts were loaded in carbon nanotube at variable low contents 0.1 – 5 wt% (Figure 1a) and were used in cathodes with sulfur loading up to 70 wt%. Using standard lithium-sulfur electrolyte based on 1 mol/kg LiTFSI in DOL:DME (v:v = 1:1) with lean electrolyte condition, batteries based on 2032 type coin cells and multilayer pouch cells were studied. The batteries' performance was studied for their impedance growth using electrochemical impedance spectroscopy, the redox performance using cycling voltammetry, and for their sulfur utilization/sulfur loss/Coulombic efficiency using galvanostatic charge-discharge cycling. These cathodes were shown to have improved redox performance in the batteries, improved sulfur utilization, and maintained stable capacity even at high sulfur loadings of 4-5 mg/cm2. Comparison of performance of nanocatalyst-containing batteries versus control batteries show improved first cycle capacity and stabilized capacity retention in the early cycling life of the battery (Figure 1b). Elucidating the underlying phenomena of the stabilization is studied in detail revealing reduced sulfur precipitation and shuttle effects. Higher C-rate performance of up to 1C revealed similar observations of stabilization.

(Invited) From Lithiated Transition Metal Oxide to Silicon and Lithium-Sulfur Systems: An Evolution of Electrochemically Active Materials

Energy storage is vital for driving an energy independent world economy that is currently largely dependent on exploiting natural reserves supplemented by oil and gas exports. The field of Li-ion batteries continues to witness burgeoning progress since the commercialization of the first Li-ion battery (LIB) by Sony in 1991, and is at present, clearly the flagship rechargeable battery system. Correspondingly, enormous progress is seen in cathodes, electrolytes as well as anodes [1]. New materials have continuously evolved. Lithiated transition metal oxides despite advances in various systems, are still the flagship cathodes of choice relying on ubiquitous intercalation chemistries. Newer chemistries exploiting alloying and Zintl phase formation focusing on silicon and tin as alternative anodes have also evolved in the last two decades. Similarly, there is tremendous research in the area of alternative energy storage systems beyond lithium-ion intercalation chemistry. Lithium-sulfur batteries (LSB) and Li metal anodes have putatively emerged at the forefront and are the spotlight of increased research activity in recent years. All these systems are unfortunately, plagued by intransigent inferior electronic conductivity, Li-ion conductivity, voltage-specific phase transition related kinetic limitations accompanied with ensuing chemical, physical, and electrochemical challenges. Nano-engineered approaches aided by concomitant progress in science and technology of synthetic and analytical chemistry of advanced materials appear primed for overcoming these hurdles.We initiated work introducing solution-based chemical synthesis strategies for generating nanostructured lithiated transition metal oxides, including anti-site defect-free LiNiO2[2,3]. This work then transitioned into alternative anodes[4] and presently into the Li-S systems holding much promise, albeit major challenges remain to be overcome. We have thus far, implemented dynamic theoretical and experimental strategies to develop engineered electronic and Li-ion conducting nanomaterials showing considerable promise as supporting components augmenting the performance and overcoming many of the limitations affecting these systems. Additionally, we have developed several approaches utilizing nanoscale droplets, nanoparticles, hollow silicon nanotubes (h-SiNTs), cost-effective template derived nanoscale morphologies, scribable and flexible hetero-structured Si architectures, displaying impressive capacities of ~3000 mAh/g with sustained cyclability and high-rate capability in Si anodes [4]. Electrochemical approaches were also developed creating binder-less Si-based thin film anodes with considerable promise. Similarly, engineering approaches were implemented for generating sulfur cathodes in LSBs exploiting the tailored attributes of inorganic, nanocomposite, tethered, and polymeric lithium ion conductors (LIC) coupled with chemically linked complex framework materials (CFM) based matrices for encapsulating S along with novel fine yarn-like and tethered S architectures yielding 5.5 mAh/cm2 – 12 mAh/cm2 areal capacity and ~1200 mAh/g specific capacity with S loadings as high as 20 mg/cm2 displaying ~250 cycles cycling stability [5, 6]. Single layer pouch cells exhibiting 180-200 Wh/kg were also demonstrated. Developing scalable economic approaches, however, remain a key challenge.Engineering strategies were also developed for identifying new Li metal alloy anodes preventing and eliminating dendrite formation. These systems serve as alternative safe anodes to Li metal. This presentation will thus provide an overview of the above systems. Similarly, insights into the promising future in generating tailored functional engineered systems in the rapidly evolving digitally savvy era of the 21st century will also be presented. Finally, the prospects of these systems offering a pathway to potentially achieving energy independence in the near future will also be outlined. References Wang, P.N. Kumta, et al. ACS Nano (2011).C-C. Chang, P.N. Kumta et al. J. Electrochemical Society 147 (2000).C. Chang, P.N. Kumta et al. J. Power Sources 75 (1998).Gattu, P.N. Kumta et al. Nano Research (2017).M. Shanthi, P.N. Kumta et al. Electrochimica Acta (2017).M. Shanthi, P.N. Kumta et al. Applied Energy Materials (2018).

The Effect of Binder on the Structure and Performance of Sulfur Cathodes in Lithium-Sulfur Batteries

Lithium-ion batteries (LIBs) are a reliable energy storage technology that have been used in various applications such as portable devices and power tools. However, the specific capacities of the electrode materials in the current LIB technology are approaching their theoretical limits which impedes their utilization in a variety of emerging applications such as long-range electric vehicles, next-generation mobile devices, and grid level energy storage and delivery. Therefore, alternative electrode materials with high specific capacity beyond the conventional LIB electrode materials are needed 1.Sulfur has been touted as a promising alternative cathode material in recent years. Sulfur offers superior theoretical capacity, and a high practical energy density when it is paired with a Li metal anode in so called Li-S batteries2. Non-toxicity, low cost and high natural abundance also make Sulfur environmentally and economically appealing. However, achieving the desired high energy density and long cycle life in Li-S batteries have been proven difficult because of the: (1) insulating nature of the two end products of charge and discharge; S8 and Li2S, (2) electrode degradation due to the volumetric change during cycling, and (3) dissolution of the Sulfur discharge products, Li-polysulfides (LiPSs), in the ether-based electrolyte, resulting in the “shuttling effect” that leads to capacity decay over extended cycling 3,4.In this work, new insights are presented on how the binder, its solvent, and dissolution process affect the electrode microstructure and performance. The Sulfur cathodes were prepared using commercially available Sulfur powder, carbon black and various binders and solvents. The cathode structures prepared using different binder and solvent combinations were characterized using scanning electron microscopy (SEM). The cycling performance of the Sulfur cathodes were tested in coin cells. The results showed considerable structural and performance variations between cathodes with similar binders but different solvents, or different treatment conditions with the same solvent. In particular, when binders were minimally dissolved in N-Methylpyrrolidone a porous shell-like structure was observed around the sulfur particles that evolved to a denser sponge-shape structure upon excessive dissolution. The porous shell structure resulted in enhanced performance and cycle life. Using spectroscopic data, it is possible that enhanced cycle life might be attributable to physical trapping of the LiPSs and providing a buffer for the volumetric change during discharge. Thus, a new perspective will be presented that the binder/solvent interaction can impact the performance of sulfur cathodes by manipulating both its structural and chemical behavior. These results are expected to provide a new understanding regarding the effect of binder and its processing on the performance of Li-S batteries and help to write a new narrative regarding electrode chemistry and preparation techniques for future applications.References M. Zhao et al., ACS Cent. Sci., 6, 1095–1104 (2020).A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, and Y.-S. Su, Chem. Rev., 114, 11751–11787 (2014).A. Manthiram, Y. Fu, and Y.-S. Su, Acc. Chem. Res., 46, 1125–1134 (2013).W. Ren, W. Ma, S. Zhang, and B. Tang, Energy Storage Mater., 23, 707–732 (2019).

Biomass-Derived Carbon for High-Performance Lithium-Sulfur Batteries

With the advantages of high conductivity and low cost, porous carbons have been considered as the most attractive materials as hosts of sulfur cathode in lithium-sulfur batteries (LSBs). However, LSBs always suffer short cycle life due to the “shuttle effect” of lithium polysulfide species (polysulfides), which are intermediate products during the charge/discharge processes. The weak interaction between carbon and polysulfides results in the dissolution of polysulfides from the cathodes, loss of active material and eventually fast capacity fading. To overcome these drawbacks, we employed a biomass-derived carbon as the host material in sulfur cathodes. Results from X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen sorption reveals that this biomass-derived product is amorphous carbon and is composed of both large (>10 nm) and small (<5 nm) pores. Using as hosts of cathodes in LSBs, the biomass-derived carbons could deliver a high reversible capacity of > 800 mAh/g and retain >80% of initial capacity after 200 cycles. Especially, the activated carbons exhibited 80% capacity retention after 400 cycles. The promising LSB performance could be ascribed to the unique porous architecture of biomass-derived carbons. The meso/micropores in biomass-derived carbons could provide more sites to anchor sulfur and polysulfides, while macropores provide channels for fast transport of ions. This was corroborated by the results from the electrochemical impedance spectroscopy (EIS), the thermogravimetric analysis (TGA) and absorption measurements.

Low-Cost Processing of Highly Durable (>1000 cycles) Sulfur Cathodes for Li-S Batteries

Lithium-sulfur (Li-S) batteries are one of the promising alternatives to modern Lithium-ion Battery (LIB) technology due to their superior specific energy density, which can satisfy the emerging needs of advanced energy storage applications such as electric vehicles and grid-scale energy storage and delivery. However, achieving this high specific energy density is hampered by several challenges inherent to the properties of sulfur and its discharge products.One major issue is related to the insulating nature of S and its fully discharged product (Li2S), which often leads to low utilization of the active material and poor rate capability. The poor electronic conductivity of these species can be overcome by utilizing conductive hosts, though they are dilutive and decrease the energy density, meaning that their mass ratio to the active material should be as low as possible [1]. Another crucial issue relates to the undesired solubility of certain sulfur discharge products, so-called long-chain Li polysulfides (LiPSs), in the conventional ether-based liquid electrolyte. The solubility of long-chain LiPSs promotes their free back-and-forth transport between the positive and negative electrodes, which results in poor cyclability and capacity decay [2, 3]. Despite the efforts to engineer and control the undesired LiPSs shuttling effect, advances have been mostly limited to a small number of cycles (100-200), or the need for complex and often expensive synthesis that has limited the rational development of new sulfur cathodes.At present, a large majority of the sulfur cathode research has focused on nano-architectured electrodes using 2D and 3D host materials for sulfur, such as carbon nanotubes, graphene, conductive scaffolds, yolk-shell structures, and the like, to increase the conductivity and alleviate the LiPSs shuttling [4]. Although these approaches have helped to increase the achievable capacity, and sometimes the cyclability, their synthesis methods have been highly complex, meaning that their manufacturing cost will be high. Also, in operating cells, it is highly unlikely that these complex structures can be effectively reproduced upon many charge-discharge cycles – meaning that capacity loss is essentially inevitable. Thus, developing novel, yet affordable and scalable, cathode architectures that can enhance the rapid transport of Li-ions to active sites for electrode reactions, accommodate discharge-induced volume expansion, and minimize the shuttling mechanism by sulfur encapsulation are still in great need.In this work, we present a low-cost and scalable processing method for highly durable sulfur cathodes containing commercial sulfur, carbon black, and polyvinylidene fluoride (PVDF). The sulfur cathode slurry was prepared through a simple and scalable recipe where the degree of binder dissolution into the solvent was controlled before electrode deposition. Variables such as the solvent:binder ratio, dissolution time, and agitation will be discussed. The microstructure of the sulfur cathodes was characterized using scanning electron microscopy. Through controlled dissolution of binder, a porous, swollen network of binder was achieved that adhered the sulfur and carbon particles while providing a highly porous structure that can accommodate the sulfur volume expansion during discharge and impede dissolution of the discharge products into the electrolyte by physically trapping them. The cycling performance of the sulfur cathodes prepared through the present novel processing was tested at C/10 and compared with those prepared through the conventional production techniques. The sulfur cathodes prepared with this novel electrode processing offered impressive capacity retention of 80% after 1000 cycles suggesting a considerable improvement in the shuttling effect and active material preservation. These results are expected to help move the production and manufacturing of Li-S batteries forward.References -J. Lee, T.-H. Kang, H.-Y. Lee, J. S. Samdani, Y. Jung, C. Zhang, Z. Yu, G.-L. Xu, L. Cheng, S. Byun et al., Advanced Energy Materials, vol. 10, no. 22, p. 1903934, 2020.Yang, G. Zheng, and Y. Cui, Chemical Society Reviews, vol. 42, no. 7, pp. 3018–3032, 2013.She, Y. Sun, Q. Zhang, and Y. Cui., Chemical society reviews, vol. 45, no. 20, pp. 5605-5634, 2016.Zhou, D. L. Danilov, R.-A. Eichel, and P. H. L. Notten, Advanced Energy Materials, vol. 1, p. 2001304, 2020.

A biomass-based cathode for long-life lithium-sulfur batteries

With the advantages of high conductivity and low cost, porous carbons have been considered as the most attractive host materials of sulfur cathodes in lithium-sulfur batteries (LSBs). However, LSBs always suffer short cycle life due to the “shuttle effect” of lithium polysulfide species (polysulfides), which are intermediate products during the charge/discharge processes. The weak interaction between carbon and polysulfides results in the dissolution of polysulfides from the cathodes, loss of active material of sulfur and eventually fast capacity fading. To overcome these drawbacks, we employed a biomass-derived carbon as the host material in sulfur cathodes. Results from X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen sorption reveals that this biomass-derived product is amorphous carbon and is composed of both large (>10 nm) and small (<5 nm) pores at an appropriate ratio. Using as hosts of cathodes in LSBs, the biomass-derived carbons deliver a high reversible capacity of >800 mAh/g and retain >80% of initial capacity after 200 cycles. Especially, the activated carbons exhibited an unprecedently high durability with 80% capacity retention after 400 cycles. The promising LSB performance can be ascribed to the unique porous architecture of biomass-derived carbons. The small pores in biomass-derived carbons provide more sites to anchor sulfur and polysulfides, while large pores provide channels for fast transport of ions. This is corroborated by the results from the electrochemical impedance spectroscopy (EIS), the thermogravimetric analysis (TGA) and absorption measurements.

Unraveling the morphological evolution mechanism of solid sulfur species in lithium-sulfur batteries with operando light microscopy

Solid-liquid phase conversion between various sulfur species in lithium-sulfur (Li-S) batteries is a fundamental reaction of the sulfur cathode. Disclosing the morphological evolution of solid sulfur species upon cycling is of great significance to achieving high energy densities. However, an in-depth investigation of the internal reaction is still lacking. In this work, the evolution process of solid sulfur species on carbon substrates is systematically studied by using an operando light microscope combined with in situ electrochemical impedance spectra technology. The observation of phenomena such as bulk solid sulfur species can form and dissolve independently of the conductive substrates and the transformation of supercooled liquid sulfur to crystalline sulfur. Based on the phenomena mentioned above, a possible mechanism was proposed in which the dissolution reaction of solid sulfur species is a spatially free reaction that involves isotropic physical dissolution, diffusion of molecules, and finally the electrochemical reaction. Correspondingly, the formation of solid sulfur species tends to be a form of crystallization in a saturated solution rather than electrodeposition, as is commonly believed. Our findings offer new insights into the reaction of sulfur cathodes and provide new opportunities to design advanced sulfur cathodes for Li-S batteries.

Trade-off effect of 3d transition metal doped boron nitride on anchoring polysulfides towards application in lithium-sulfur battery

Sulfur cathodes in lithium-sulfur batteries (LSBs) suffer from the notorious “shuttle effect”, low sulfur use ratio, and tardy transformation of lithium polysulfides (LiPSs), while using two-dimensional (2D) polar anchoring materials combined with single-atom catalysis is one of the promising methods to address these issues. Herein, the 3d transition metal (TM) doped 2D boron nitrides (BN), labeled as TM-BN, are studied for the anchoring and redox kinetics of LiPSs using first principles calculations. From the simulated results, the TM atom and adjacent N atoms are active adsorption sites for binding S atoms in LiPSs/S8 and Li atoms in LiPSs, respectively. A negative d-band center closer to the Fermi level of TM-BN is key for enhancing the binding strength of TM-S and lowering the Li2S decomposition energy barrier, while it deteriorates the activity of adjacent N atoms. Fortunately, the electrolyte environment has little effect on the superiority of the TM-BN for binding polysulfides/S8, guaranteeing the sturdy anchor of polysulfides/S8 in realistic conditions. The trade-off effect on the activities of TM and adjacent N atom sites in TM-BN for binding LiPSs highlights the excellence of Ti/V/Cr-BN as modification materials for LSB.