Sulfur-Bridged Dual Fe-N4 Sites to Boost Lithium-Sulfur Battery Performance.

The development of high-performance lithium-sulfur (Li-S) batteries is hindered by the complex interplay of lithium polysulfides (LiPSs) shuttle effects and uncontrolled lithium dendrite growth. Herein, we introduce a dual-functional-phosphorus-doped iron single-atom catalysts on reduced graphene oxide (Fe-NPC@rGO)-to address both issues. Density functional theory (DFT) and experiments reveal that Fe-NPC@rGO enhances sulfur redox kinetics and regulates lithium deposition. The Fe-NPC high charge density and enhanced electron transfer (vs. Fe-N4) enable Fe-NPC@rGO to trap polysulfides (LiPSs) and boost their conversion, reducing shuttle effects. Simultaneously, its lithiophilic properties enable uniform Li plating, inhibiting dendrites. Li-S cells with Fe-NPC@GO modified separators deliver a high discharge capacity of 1156 mAh g-1 at 1 C, with an exceptionally low-capacity decay of 0.032% per cycle over 1000 cycles. Moreover, full Li-S battery configuration (Fe-NPC@rGO-Li||Fe-NPC@rGO-PP||ROCNT-S) achieves high areal capacity of 4.9 mAh cm-2 at 5 mg cm-2 sulfur loading, low electrolyte to sulfur (E/S) ratio of 6 µL mg-1, and an ultralow negative to positive (N/P) ratio of 1.2. These findings provide valuable insights into the structural optimization of electrocatalysts and underscore the significant potential of Fe-NPC@rGO in advancing the electrochemical performance of next-generation Li-S batteries.

Although single-atom catalysts (SACs) have been largely explored in lithium-sulfur (Li-S) batteries, the commonly reported nonpolar transition metal-N4 coordinations only demonstrate inferior adsorption and catalytic activity toward shuttled lithium polysulfides (LiPSs). Herein, single Fe atoms with asymmetric coordination configurations of Fe-N3C2-C were precisely designed and synthesized as efficient immobilizer and catalyst for LiPSs. The experimental and theoretical results elucidate that the asymmetrically coordinated Fe-N3C2-C moieties not only enhance the LiPSs anchoring capability by the formation of extra π-bonds originating from S p orbital and Fe dx2-y2/dxy orbital hybridization but also boost the redox kinetics of LiPSs with reduced Li2S precipitation/decomposition barrier, leading to suppressed shuttle effect. Consequently, the Li-S batteries assembled with Fe-N3C2-C exhibit high areal capacity and cycling stability even under high sulfur loading and lean electrolyte conditions. This work highlights the important role of coordination symmetry of SACs for promoting the practical application of Li-S batteries.

Lithium-sulfur (Li-S) batteries possess high theoretical specific energy but suffer from lithium polysulfide (LiPS) shuttling and sluggish reaction kinetics. Catalysts in Li-S batteries are deemed as a cornerstone for improving the sluggish kinetics and simultaneously mitigating the LiPS shuttling. Herein, a cost-effective hexagonal close-packed (hcp)-phase Fe-Ni alloy is shown to serve as an efficient electrocatalyst to promote the LiPS conversion reaction in Li-S batteries. Importantly, the electrocatalysis mechanisms of Fe-Ni toward LiPS conversion is thoroughly revealed by coupling electrochemical results and post mortem transmission electron microscopy, X-ray photoelectron spectroscopy, and in situ X-ray diffraction characterization. Benefiting from the good catalytic property, the Fe-Ni alloy enables a long lifespan (over 800 cycles) and high areal capacity (6.1 mA h cm-2) Li-S batteries under lean electrolyte conditions with a high sulfur loading of 6.4 mg cm-2. Impressively, pouch cells fabricated with the Fe-Ni/S cathodes achieve stable cycling performance under practically necessary conditions with a low electrolyte/sulfur (E/S) ratio of 4.5 μL mg-1. This work is expected to design highly efficient, cost-effective electrocatalysts for high-performance Li-S batteries.

Lithium-sulfur (Li-S) batteries feature a high theoretical capacity of 1675 mAh/g and hence is considered as a promising alternative to lithium-ion batteries. However, deployment of Li-S batteries has been hindered by the low practical energy and limited cycle life.1, 2 Reducing cathode porosity is essential to balancing the electrolyte distribution in Li-S cell, conserving more pore-filling electrolyte to extend cell cycle life.3-5 However, low-porosity electrodes built with nanosized sulfur/carbon (S/C) materials suffer from high tortuosity that significantly deteriorates electrode wetting and hence sulfur utilization. Enabling operation of high-loading sulfur electrodes under both low-porosity and lean-electrolyte conditions is still a challenge and is seldom discussed. In this study,6 we demonstrated a novel and facile strategy for constructing low-tortuosity through-pores across both vertical and planar directions of electrodes by casting large particles into single-particle-layer electrodes. Through multi-scale characterizations and simulations, correlations between material/electrode structures, electrolyte permeability, polysulfide migration, and sulfur reactions were elucidated. The high-loading and dense sulfur cathode fabricated by this method delivers a high specific capacity (>1000 mAh g-1) at a very low electrolyte/sulfur (E/S) ratio of 4 μL mg-1. This study provides a novel approach to reducing the tortuosity of dense sulfur electrodes by manipulating the porosity distribution, which would be also applicable to improving the rate capability of other high-energy electrodes. More details of the progress will be discussed at the meeting.Reference. Dörfler, S.; Althues, H.; Härtel, P.; Abendroth, T.; Schumm, B.; Kaskel, S., Challenges and Key Parameters of Lithium-Sulfur Batteries on Pouch Cell Level. Joule 2020, 4 (3), 539-554.Xue, W.; Miao, L.; Qie, L.; Wang, C.; Li, S.; Wang, J.; Li, J., Gravimetric and volumetric energy densities of lithium-sulfur batteries. Current Opinion in Electrochemistry 2017, 6 (1), 92-99.Lu, D.; Li, Q.; Liu, J.; Zheng, J.; Wang, Y.; Ferrara, S.; Xiao, J.; Zhang, J. G.; Liu, J., Enabling High-Energy-Density Cathode for Lithium-Sulfur Batteries. ACS Appl Mater Interfaces 2018, 10 (27), 23094-23102.Kang, N.; Lin, Y.; Yang, L.; Lu, D.; Xiao, J.; Qi, Y.; Cai, M., Cathode porosity is a missing key parameter to optimize lithium-sulfur battery energy density. Nat Commun 2019, 10 (1), 4597.Feng, S.; Liu, J.; Zhang, X.; Shi, L.; Anderson, C.; Lin, Y.; Song, M.-K.; Liu, J.; Xiao, J.; Lu, D., Rationalizing nitrogen-doped secondary carbon particles for practical lithium-sulfur batteries. Nano Energy 2022, 103.Feng, S.; Singh, R. K.; Fu, Y.; Li, Z.; Wang, Y.; Bao, J.; Xu, Z.; Li, G.; Anderson, C.; Shi, L.; Lin, Y.; Khalifah, P. G.; Wang, W.; Liu, J.; Xiao, J.; Lu, D., Low-tortuous and dense single-particle-layer electrode for high-energy lithium-sulfur batteries. Energy & Environ. Sci. 2022.

The use of single-atom catalysts (SACs) with abundant electrocatalytic centers has been identified as the most desirable strategy to inhibit the shuttle effect in lithium-sulfur batteries. However, the co-contribution from SAC and its support via their interactions for accelerating the sulfur reduction reactions (SRR) has so far received little attention, since the underlying mechanism remains elusive. Herein, guided by density functional theory calculations, Cobalt-SACs supported on a graphitic carbon nitride substrate (Co-GCN), are selected to elucidate the co-catalytic role in enhancing the SRR. The inherent high charge polarity of GCN, combined with its unique tri-s-triazine structure, offers multiple binding sites for lithium polysulfides (LiPSs) through Li-N bonds, as well as N/C-coordinated frameworks for anchoring Co-SACs. This structural configuration further amplifies the interaction with LiPSs via Co-S bonds. Consequently, both Co-SACs and GCN actively participate in sulfur reduction electrocatalysis by binding LiPS intermediates, lowering the conversion energy barrier of SRR. Benefitting from such unique synergy, the battery demonstrates outstanding rate performance (718.9 mAh g-1 at 5.0 C) and yields a high areal capacity of up to 13.8 mAh cm-2 (1584.3 mAh g-1) under a high areal sulfur loading of 8.7 mg cm-2 but a low electrolyte/sulfur ratio of 5.0 μL mg-1.

Lithium-sulfur batteries (LSBs) are considered promising next-generation batteries due to their high energy density (>500W h kg-1). However, LSBs exhibit an unsatisfactory energy density (<400W h kg-1) and cycle life (<300 cycles) because of the shuttle effect caused by soluble lithium polysulfide (LiPS) intermediates and the sluggish conversion reaction kinetics caused by insulating sulfur (S8) and lithium sulfide (Li2S). Although various types of catalysts, including metal-based compounds to single-atom catalysts, have been reported to address these issues, most catalysts exhibited limited catalytic activity under practical lean electrolyte conditions (<5µL mg-1). A comprehensive understanding of the synthetic strategy and catalytic mechanism of catalysts is essential for their design, but understanding the electronic effects of the catalysts and LiPS is more important. Furthermore, the electronic design of these catalysts is not well understood. In this review, we introduce the catalytic mechanisms in LSBs and discuss catalyst design strategies in terms of electronic effects on the interactions between reactants and catalysts, with a primary focus on heterogeneous catalytic systems. We additionally consider how the electronic property of homogeneous systems, particularly redox mediators, affects catalytic behavior under lean electrolyte conditions and propose future research directions for catalyst development in LSBs.

Lithium-sulfur (Li-S) batteries hold great promise as a next-generation energy storage system due to their high theoretical energy density (2600 W h kg-1), surpassing conventional lithium-ion batteries. However, their performance is often limited by the intrinsic transformation of soluble lithium polysulfides (LiPSs) into short-chain insoluble sulfur species (Li2S2/Li2S), which induces significant cell polarization, particularly under lean-electrolyte conditions. Through a galvanic replacement reaction (GRR), enabling precise tailoring of interfacial properties, AgCl-PVP nanocubes (NCs) were synthesized and utilized as sulfur host materials. These materials demonstrated effective entrapment of LiPSs, as confirmed by in situ electrochemical visualization. Furthermore, the AgCl-PVP NCs significantly reduced whole-cell polarization, particularly during the Li2S nucleation step, as validated by galvanostatic intermittent titration technique across the depth of discharge. Under lean-electrolyte conditions (5.6 μL mg-1), the AgCl-PVP NCs cathode exhibited high specific capacity (563.62 mA h g-1 at 0.2 C) with a low-capacity decay rate (1.81% per cycle). These results demonstrate the potential of GRR-engineered nanostructures as sulfur host material for enhancing the electrochemical performance and practical applicability of lean-electrolyte Li-S batteries.

Single-atom catalysts have been paid more attention to improving sluggish reaction kinetics and anchoring polysulfide for lithium-sulfur (Li-S) batteries. It has been demonstrated that d-block single-atom elements in the fourth period can chemically interact with the local environment, leading to effective adsorption and catalytic activity toward lithium polysulfides. Enlightened by theoretical screening, for the first time, we design novel single-atom Nb catalysts toward improved sulfur immobilization and catalyzation. Calculations reveal that Nb-N4 active moiety possesses abundant unfilled antibonding orbitals, which promotes d-p hybridization and enhances anchoring capability toward lithium polysulfides via a "trapping-coupling-conversion" mechanism. The Nb-SAs@NC cell exhibits a high capacity retention of over 85% after 1000 cycles, a superior rate performance of 740 mA h g-1 at 7 C, and a competitive areal capacity of 5.2 mAh cm-2 (5.6 mg cm-2). Our work provides a new perspective to extend cathodes enabling high-energy-density Li-S batteries.

Lithium-sulfur (Li-S) batteries are increasingly recognized for their exceptional energy storage capabilities, offering an energy density of approximately 2600 Wh kg-1. This is largely attributed to the use of sulfur in the cathode—a material both abundantly available and cost-effective. However, the widespread adoption of Li-S batteries faces significant challenges, including the low electrical conductivity of sulfur and its discharge byproducts (e.g., Li2S2, Li2S), the migration of polysulfides within the electrolyte, and electrode volume expansion, which collectively impede their practical application.In the realm of recent advancements, transition metal (TM) sulfides have attracted considerable interest for their superior electrical conductivity and the presence of effective binding sites for lithium polysulfides (LiPSs), arising from a high density of valence electrons. This is a consequence of the soft basic nature of S2-/S2 2- anions, as opposed to the hard basic O2- ions. Particularly, TM sulfides with pyrite-type cubic crystalline structures, including NiS2 and CoS2, have proven to be potent electrocatalysts. They facilitate enhanced polysulfide redox reactions owing to their conductivity surpassing that of TM oxides. Furthermore, the integration of multi-cationic species into a single composite electrocatalyst has emerged as a strategic approach to refine the electronic configuration and augment the catalytic synergy of TM sulfides. Despite these developments, the discrete roles and synergistic contributions of cations within bimetallic TM sulfides remain largely unexplored, especially in comparison to the unveiled catalytic mechanisms of TM oxides.This study introduces a novel, highly active, and durable sulfur catalyst system composed of NixCo1-xS2 nanocrystals dispersed on N-doped porous carbon nanotubes (NixCo1-xS2@NPCTs). This system serves as an efficient cathode electrocatalyst for rechargeable Li-S batteries. The carbonized, porous architecture offers extensive surface area and buffering capacity for cyclic redox processes, alongside numerous chemical binding sites on the CNT framework. The ingeniously designed dual-active TM sulfides, characterized by NiOh 2+−S−CoOh 2+ bonds within the octahedral TMS6 structure, effectively catalyze sulfur cathode reactions. They fulfill multifaceted roles; specifically, CoOh 2+ sites with vacancies robustly interact with LiPSs to mitigate the shuttle effect. Concurrently, the NiOh 2+ species, with an optimal doping concentration, precisely modulate the chemical interaction with LiPSs to facilitate a sequential redox reaction, thereby promoting sustained cooperative catalysis and reducing Li2S passivation on the catalyst surface. The Ni0.261Co0.739S2 catalyst, supported by NPCT carbon, excels by achieving excellent discharge capacity and exhibiting remarkable long-term electrochemical stability. Even under harsh conditions with a low electrolyte-to-sulfur ratio (E/S = 10.0 μL mg-1), this catalyst system demonstrates superior cycling performance and durability. Notably, the Ni0.261Co0.739S2@NPCTs catalyst system showcases exceptional cyclic endurance, maintaining a capacity of 511 mAh g-1 with a mere 0.055% decay per cycle at a 5.0 C rate over 1000 cycles, alongside a significant areal capacity of 2.20 mAh cm-2 under a sulfur loading of 4.61 mg cm-2 after 200 cycles at 0.2 C. Complementary theoretical DFT calculations corroborate the experimental results, enhancing our understanding of the efficacy of the catalyst system, particularly in terms of weakend sulfur bonding strength and optimzed binding energy which are pivotal for the catalytic performance of NixCo1-xS2@NPCTs in interactions with TM sulfides and lithium polysulfides. Our findings could highlight the importance of leveraging the synergistic potential of advanced carbon hosts and optimized bimetallic sulfide electrocatalysts, while also illuminating the intricate atomic configurations crucial for augmenting the redox activity of Li-S batteries in practical applications. Figure 1

Single-atom catalysts (SACs) have been increasingly explored in lithium-sulfur (Li-S) batteries to address the issues of severe polysulfide shuttle effects and sluggish redox kinetics. However, the structure-activity relationship between single-atom coordination structures and the performance of Li-Sbatteries remain unclear. In this study, a P, S co-coordination asymmetric configuration of single atoms is designed to enhance the catalytic activity of Co central atoms and promote d-p orbital hybridization between Co and S atoms, thereby limiting polysulfides and accelerating the bidirectional redox process of sulfur. The well-designed SACs enable Li-S batteries to demonstrate an ultralow capacity fading rate of 0.027% per cycle after 2000 cycles at a high rate of 5 C. Furthermore, they display excellent rate performance with a capacity of 619 mAh g-1 at an ultrahigh rate of 10 C due to the efficient catalysis of CoSA-N3PS. Importantly, the assembled pouch cell still retains a high discharge capacity of 660 mAh g-1 after 100 cycles at 0.2 C and provides a high areal capacity of 4.4 mAh cm-2 even with a high sulfur loading of 6mg cm-2. This work demonstrates that regulating the coordination environment of SACs is of great significance for achieving state-of-the-art Li-S batteries.

Catalytic conversion of polysulfides emerges as a promising approach to improve the kinetics and mitigate polysulfide shuttling in lithium-sulfur (Li-S) batteries, especially under conditions of high sulfur loading and lean electrolyte. Herein, we present a separator architecture that incorporates double-terminal binding (DTB) sites within a nitrogen-doped carbon framework, consisting of polar Co0.85Se and Co clusters (Co/Co0.85Se@NC), to enhance the durability of Li-S batteries. The uniformly dispersed clusters of polar Co0.85Se and Co offer abundant active sites for lithium polysulfides (LiPSs), enabling efficient LiPS conversion while also serving as anchors through a combination of chemical interactions. Density functional theory calculations, along with in situ Raman and X-ray diffraction characterizations, reveal that the DTB effect strengthens the binding energy to polysulfides and lowers the energy barriers of polysulfide redox reactions. Li-S batteries utilizing the Co/Co0.85Se@NC-modified separator demonstrate exceptional cycling stability (0.042% per cycle over 1000 cycles at 2 C) and rate capability (849 mAh g-1 at 3 C), as well as deliver an impressive areal capacity of 10.0 mAh cm-2 even in challenging conditions with a high sulfur loading (10.7 mg cm-2) and lean electrolyte environments (5.8 μL mg-1). The DTB site strategy offers valuable insights into the development of high-performance Li-S batteries.

Lithium-sulfur (Li-S) batteries with high energy density and low cost are promising for next-generation energy storage. However, their cycling stability is plagued by the high solubility of lithium polysulfide (LiPS) intermediates, causing fast capacity decay and severe self-discharge. Exploring electrolytes with low LiPS solubility has shown promising results toward addressing these challenges. However, here, we report that electrolytes with moderate LiPS solubility are more effective for simultaneously limiting the shuttling effect and achieving good Li-S reaction kinetics. We explored a range of solubility from 37 to 1,100 mM (based on S atom, [S]) and found that a moderate solubility from 50 to 200 mM [S] performed the best. Using a series of electrolyte solvents with various degrees of fluorination, we formulated the Single-Solvent, Single-Salt, Standard Salt concentration with Moderate LiPSs solubility Electrolytes (termed S6MILE) for Li-S batteries. Among the designed electrolytes, Li-S cells using fluorinated-1,2-diethoxyethane S6MILE (F4DEE-S6MILE) showed the highest capacity of 1,160 mAh g-1 at 0.05 C at room temperature. At 60 °C, fluorinated-1,4-dimethoxybutane S6MILE (F4DMB-S6MILE) gave the highest capacity of 1,526 mAh g-1 at 0.05 C and an average CE of 99.89% for 150 cycles at 0.2 C under lean electrolyte conditions. This is a fivefold increase in cycle life compared with other conventional ether-based electrolytes. Moreover, we observed a long calendar aging life, with a capacity increase/recovery of 4.3% after resting for 30 d using F4DMB-S6MILE. Furthermore, the correlation between LiPS solubility, degree of fluorination of the electrolyte solvent, and battery performance was systematically investigated.

Lithium-sulfur (Li-S) batteries can theoretically deliver high energy densities exceeding 2500 Wh kg-1. However, high sulfur loading and lean electrolyte conditions are two major requirements to enhance the actual energy density of the Li-S batteries. Herein, the use of carbon-dispersed highly concentrated electrolyte (HCE) gels with sparingly solvating characteristics as sulfur hosts in Li-S batteries is proposed as a unique approach to construct continuous electron-transport and ion-conduction paths in sulfur cathodes as well as achieve high energy density under lean-electrolyte conditions. The sol-gel behavior of carbon-dispersed sulfolane-based HCEs was investigated using phase diagrams. The sol-to-gel transition was mainly dependent on the amount of the carbonaceous material and the Li salt content. The gelation was caused by the carbonaceous-material-induced formation of an integrated network. Density functional theory (DFT) calculations revealed that the strong cation-π interactions between Li+ and the induced dipole of graphitic carbon were responsible for facilitating the dispersion of the carbonaceous material into the HCEs, thereby permitting gel formation at high Li-salt concentrations. The as-prepared carbon-dispersed sulfolane-based composite gels were employed as efficient sulfur hosts in Li-S batteries. The use of gel-type sulfur hosts eliminates the requirement for excess electrolytes and thus facilitates the practical realization of Li-S batteries under lean-electrolyte conditions. A Li-S pouch cell that achieved a high cell-energy density (up to 253 Wh kg-1) at a high sulfur loading (4.1 mg cm-2) and low electrolyte/sulfur ratio (4.2 μL mg-1) was developed. Furthermore, a Li-S polymer battery was fabricated by combining the composite gel cathode and a polymer gel electrolyte.

Lithium-sulfur (Li-S) batteries under low-temperature and lean electrolyte conditions for practical application are hindered by a sluggish conversion reaction, low sulfur utilization, and cycling stability. Herein, we designed a high-entropy (HE) electrolyte by mixing three Li salts. The HE electrolyte simultaneously improves lithium sulfide (Li2S) conversion reaction kinetics, sulfur utilization, and cyclability due to the anticlustering effect on lithium polysulfides, three-dimensional Li2S growth, and robust anion-derived solid electrolyte interphase layer formation, respectively. Consequently, the HE electrolyte exhibits a high initial reversible capacity (1159.9 mAh g-1) and cycling stability for 40 cycles under a low electrolyte-to-sulfur ratio (3.5 μL mg-1) at the pouch cell level. In addition, the Li-S cell with HE electrolyte exhibits high cycling stability with a capacity decay of 0.01% per cycle during 200 cycles at -15 °C.

Developing lithium-sulfur cells with a high-loading cathode at a lean-electrolyte condition is the key to bringing the lithium-sulfur technology into the energy-storage market. However, it has proven to be extremely challenging to develop a cell that simultaneously satisfies the abovementioned metrics while also displaying high electrochemical efficiency and stability. Here, we present a concept of constructing a conductive cathode substrate with a low surface area and optimized nanoporosity (i.e., limited micropores in the porous matrix) that enables achieving a high sulfur loading of 13 mg cm-2 and a high sulfur content of 75 wt % with an extremely low electrolyte/sulfur ratio of just 4.0 μL mg-1. The high-loading nanocomposite cathodes demonstrate high-areal capacities of 9.3 mA h cm-2, high energy densities of 18.6 mW h cm-2, and superior cyclability with excellent capacity retention of 85% after 200 cycles. These values are higher than the benchmarks set up for developing future commercial lithium-sulfur cells (i.e., areal capacity of >2-4 mA h cm-2, energy density of >8-13 mW h cm-2, and a long cycle life of 200 cycles with a capacity retention of 80%). The cathode design further exhibits high-rate capability from C/20 to 1 C rates and great potential to attain ultrahigh sulfur loading and a content of 17 mg cm-2 and 80 wt %. The key nanostructural feature that enables realizing fast-charge transport is the low surface area and limited microporosity that avoid the fast consumption of the electrolyte during cell cycling.