Single-Atom Cobalt Supported on Nitrogen-Doped Three-Dimensional Carbon Facilitating Polysulfide Conversion in Lithium-Sulfur Batteries

Electron-Deficient Cobalt Centers Realized by Rational p─π Conjugation Regulation for High-Performance Li─S Batteries

Rational design of the coordination environment of single-atom catalysts (SACs) can enhance their catalytic activity, which is of great significance for high-loading and lean-electrolyte lithium-sulfur (Li─S) batteries. Inspired by the Lewis acid-base theory, we design a unique coordination environment for constructing electron-deficient Co SACs on carbon nanotubes (named as CNT@f-CoNC), which function as a Lewis acid, to enhance the chemisorption and catalytic activity towards polysulfides (Lewis base). Compared with porphyrin-like Co SACs, electron-deficient Co SACs (Lewis acid) exhibit much stronger binding affinity towards polysulfides (Lewis base) and a significantly lower energy barrier of the rate-determining step in the sulfur reduction reaction. As expected, even with a high sulfur loading (6.9 mg cm-2) and lean electrolyte to sulfur (E/S) ratio (4.0 µL mg-1), the areal capacity still reaches 7.7 mAh cm-2. Moreover, a 1.6 Ah-class pouch cell is successfully assembled under the harsh conditions and delivers an energy density of 422 Wh kg-1. This work provides novel insights into enhancing the electrochemical performance of Li─S batteries by modulating the local electronic density of metal sites through the rational design of the coordination environment.

Rationally Designed Binder with Polysulfide-Affinitive Moieties and Robust Network Structures for Improved Polysulfide Trapping and Structural Stability of Sulfur Cathode

Lithium-sulfur batteries (LSBs) have emerged as a promising next-generation energy storage application owing to their high specific capacity and energy density. However, inherent insulating property of sulfur, along with its significant volume expansion during cycling, and shuttling behavior of lithium-polysulfides (LiPSs), hinder their practical application. To overcome these issues, a crosslinked cationic waterborne polyurethane (CCWPU) is rationally designed as a binder for LSBs. The mechanical robustness of CCWPU enables it to withstand the high stress derived from volume expansion of sulfur, facilitating charge-transferring through conserved charge-transfer pathway and promoting interconversion of LiPSs. Additionally, polar urethane groups offer favorable interaction sites with LiPSs, mitigating shuttling behavior of LiPSs via polar-polar interaction. Density functional theory investigations further elucidate that the incorporation of cationic moieties enhances LiPSs immobilization by confining Sn x- (x = 1 or 2) in LiPSs, thereby improving sulfur utilization. Benefiting from these, the cell with CCWPU demonstrates reduced polarization, superior LiPSs conversion rates, and stable cycling performance. Moreover, water-processable nature of CCWPU aligns with environmental consciousness. These diverse functionalities of CCWPU provide valuable insights for the development of advanced binder for LSBs, ultimately improving the electrochemical performances of LSBs.

Alloying Strategy Regulating Size and Electronic Structure of Mo0.25Nb0.75Se2 to Achieve High-Performance Lithium-Sulfur Batteries

The utilization of catalysts in lithium--sulfur batteries has proven to be an efficacious avenue for enhancing the kinetics of polysulfide conversion. Specially, the size and electronic structure of catalysts play a pivotal role in harnessing the active sites and intrinsic catalysis activity. Outstanding MoSe2 and NbSe2 are were selected from 16 universal transition metal selenides based on the proposed binary descriptor. Then, an alloying strategy is was devised to prepare Mo0.25Nb0.75Se2 flakelets for further improvement of the intrinsic catalysis. The integration of density functional theory calculations and electrochemical analysis demonstrates that alloying Mo with Nb can regulate the surface energy and indexes of band match and lattice mismatch, thereby enabling Mo0.25Nb0.75Se2 to possess a small size, suitable adsorption energy and low reaction energy barrier. This optimization enhances the catalysis of sulfur reduction/evolution reaction and the reversible deposition/stripping of lithium. Consequently, an assembled Ah-level pouch cell is realized with dramatic cycle stability. With the electrolyte/sulfur ratio of 2.36 μL mg S-1, the cell can deliver a high energy density of up to 505.4 Wh kgtotal -1. This work pioneers a universal strategy for sculpting the geometric configurations and electronic structures of catalysts, to achieve enhanced catalytic activity and precise interpretation of structure-activity relationships.

MOF-derived Carbon-Based Materials for Energy-Related Applications

New carbon-based materials (CMs) are recommended as attractively active materials due to their diverse nanostructures and unique electron transport pathways, demonstrating great potential for highly efficient energy storage applications, electrocatalysis, and beyond. Among these newly reported CMs, metal-organic framework (MOF)-derived CMs have achieved impressive development momentum based on their high specific surface areas, tunable porosity, and flexible structural-functional integration. However, obstacles regarding the integrity of porous structures, the complexity of preparation processes, and the precise control of active components hinder the regulation of precise interface engineering in CMs. In this context, this review systematically summarizes the latest advances in tailored types, processing strategies, and energy-related applications of MOF-derived CMs and focuses on the structure-activity relationship of metal-free carbon, metal-doped carbon, and metallide-doped carbon. Particularly, the intrinsic correlation and evolutionary behavior between the synergistic interaction of micro/nanostructures and active species with electrochemical performances are emphasized. Finally, unique insights and perspectives on the latest relevant research are presented, and the future development prospects and challenges of MOF-derived CMs are discussed, providing valuable guidance to boost high-performance electrochemical electrodes for a broader range of application fields.

Single-Atom Cobalt Catalyst with Boron and Nitrogen Codoped Graphene (Co-BN-G) Enables Adsorption and Catalytic Conversion of Polysulfides for High-Performance Lithium-Sulfur Batteries

In lithium-sulfur batteries, the shuttling effect and sluggish redox conversion of soluble polysulfides lead to unsatisfactory sulfur utilization and capacity retention. In this research, we used a one-pot hydrothermal method to prepare graphene with single-atom Co and B, N codoped as a sulfur host in Li-S batteries, thereby suppressing the shuttling effect and facilitating the redox conversion of lithium polysulfides (LiPSs). A series of characterizations demonstrated that dual doping of B and N introduces more lattice defects and structural deformations in graphene oxide, thus enhancing its adsorption of polysulfides. Simultaneously, single-atom cobalt can also polarize adsorption and accelerate the conversion reaction of LiPSs. The Li-S cell with the as-prepared Co-BN-G sulfur host materials exhibited an excellent capacity of 1034 mAh g-1 at 0.5 C and satisfactory cycle performance (retention of 69% over 500 cycles). Even at a rate of 2 C, a discharge capacity of 851 mAh g-1 is achieved. The results show that the Co-BN-G configuration efficiently captures LiPSs and enhances their rate conversion kinetics in redox reactions, demonstrating significant practical potential.

Embedding cobalt (II, III) oxide nanoparticles into nitrogen-doped carbon nanotubes-grafted hollow polyhedrons as sulfur hosts for ultralong-life lithium-sulfur batteries

The sluggish reaction kinetics and unfavorable shuttling effect are regarded as obstacles to the practical application of lithium-sulfur (Li-S) batteries. To resolve these inherent drawbacks, we synthesized novel multifunctional Co3O4@NHCP/CNT as the cathode materials consisting of carbon nanotubes (CNTs)-grafted N-doped hollow carbon polyhedrons (NHCP) embedded with cobalt (II, III) oxide (Co3O4) nanoparticles. The results indicate that the NHCP and interconnected CNTs could provide favorable channels for electron/ion transport and physically restrict the diffusion of lithium polysulfides (LiPSs). Furthermore, N doping and in-situ Co3O4 embedding could endow the carbon matrix with strong chemisorption and effective electrocatalytic activity toward LiPSs, thus prominently promoting the sulfur redox reaction. Benefiting from these synergistic effects, the Co3O4@NHCP/CNT electrode exhibits a high initial capacity of 1322.1 mAh/g at 0.1 C, and a capacity retention of 710.4 mAh/g after 500 cycles at 1 C. Impressively, even at a relatively high current density of 4 C, the Co3O4@NHCP/CNT electrode achieves a high capacity of 653.4 mAh/g and outstanding long-term cycle stability for 1000 cycles with a low decay rate of 0.035% per cycle. Hence, the design of N-doped CNTs-grafted hollow carbon polyhedrons coupled with transition metal oxides would provide effective promising perspective for developing high-performance Li-S batteries.

Single-atom site catalysis in Li-S batteries

With their high theoretical energy density, Li-S batteries are regarded as the ideal battery system for next generation electrochemical energy storage. In the last 15 years, Li-S batteries have made outstanding academic progress. Recently, research studies have placed more emphasis on their practical application aspects, which puts forward strict requirements for the loading of S cathodes and the amount of electrolytes. To meet the above requirements, electrode catalysis design is of crucial significance. Among all the catalysts, single-atom site catalysts (SASCs) are considered to be ideal catalyst materials for the commercialization of Li-S batteries due to their high activity and highest utilization of catalytic sites. This perspective introduces the kinetic mechanism of S cathodes, the basic concept and synthesis strategy of SASCs, and then systematically summarizes the research progress of SASCs for S cathodes and, the related functional interlayers/separators in recent years. Finally, the opportunities and challenges of SASCs in Li-S batteries are summarized and prospected.

Highly Porous Polypyrrole (PPy) Hydrogel Support for the Design of a Co-N-C Electrocatalyst for Oxygen Reduction Reaction

Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts have emerged as one of the most promising platinum-group metal (PGM)-free cathode catalysts for oxygen reduction reaction (ORR). Among the various approaches to enhance the ORR performance of the catalysts, increasing the density of accessible active sites is of paramount importance. Thus, nitrogen-rich support with abundant porosity can be very propitious. Herein, we report a highly porous polypyrrole (PPy) hydrogel as a versatile support for the facile design of a Co-N-C electrocatalyst for ORR. The resulting Co-N-C catalyst with abundant micro- and mesoporous combinations demonstrates a half-wave potential (E1/2) of 0.825 V vs reversible hydrogen electrode (RHE) in O2-saturated 0.1M KOH with just 2.1 wt % Co content. The ORR performance reduces only 11 mV (E1/2) after 5000 cycles of accelerated durability test (ADT), portraying its excellent stability. The catalyst retains ≈83% of its original current during a short-term durability test at 0.8 V vs RHE for 25 h. Furthermore, the catalyst shows electron transfer approaching ≈4 with low H2O2 yield in the potential range 0.5-0.9 V vs RHE. This work provides a simple design strategy to synthesize M-N-C catalysts with increased accessible active site density and enhanced mass transport for ORR and other electrocatalytic applications.

Enhancing Electron Conductivity and Electron Density of Single Atom Based Core-Shell Nanoboxes for High Redox Activity in Lithium Sulfur Batteries

Herein, an integrated structure of single Fe atom doped core-shell carbon nanoboxes wrapped by self-growing carbon nanotubes (CNTs) is designed. Within the nanoboxes, the single Fe atom doped hollow cores are bonded to the shells via the carbon needles, which act as the highways for the electron transport between cores and shells. Moreover, the single Fe atom doped nanobox shells is further wrapped and connected by self-growing carbon nanotubes. Simultaneously, the needles and carbon nanotubes act as the highways for electron transport, which can improve the overall electron conductivity and electron density within the nanoboxes. Finite element analysis verifies the unique structure including both internal and external connections realize the integration of active sites in nano scale, and results in significant increase in electron transfer and the catalytic performance of Fe-N4 sites in both Li2 Sn lithiation and Li2 S delithiation. The Li-S batteries with the double-shelled single atom catalyst delivered the specific capacity of 702.2 mAh g-1 after 550 cycles at 1.0 C. The regional structure design and evaluation method provide a new strategy for the further development of single atom catalysts for more electrochemical processes.

Enhancement of Organic Oxygen Atoms on Metal Cobalt for Sulfur Adsorption and Catalytic Polysulfide Conversion

Metals and their compounds effectively suppress the polysulfide shuttle effect on the cathodes of a lithium-sulfur (Li-S) battery by chemisorbing polysulfides and catalyzing their conversion. However, S fixation on currently available cathode materials is below the requirements of large-scale practical application of this battery type. In this study, perylenequinone was utilized to improve polysulfide chemisorption and conversion on cobalt (Co)-containing Li-S battery cathodes. According to IGMH analysis, the binding energies of DPD and carbon materials as well as polysulfide adsorption were significantly enhanced in the presence of Co. According to in situ Fourier transform infrared spectroscopy, the hydroxyl and carbonyl groups in perylenequinone are able to form O-Li bonds with Li₂S_n, facilitating chemisorption and catalytic conversion of polysulfides on metallic Co. The newly prepared cathode material demonstrated superior rate and cycling performances in the Li-S battery. It exhibited an initial discharge capacity of 780 mAh g^-1 at 1 C and a minimum capacity decay rate of only 0.041% over 800 cycles. Even with a high S loading, the cathode material maintained an impressive capacity retention rate of 73% after 120 cycles at 0.2 C.

Donor-Acceptor Conjugated Microporous Polymer toward Enhanced Redox Kinetics in Lithium-Sulfur Batteries

Conjugated microporous polymers (CMPs) with porous structure and rich polar units are favorable for high-performance lithium-sulfur (Li-S) batteries. However, understanding the role of building blocks in polysulfide catalytic conversion is still limited. In this work, two triazine-based CMPs are constructed by electron-accepting triazine with electron-donating triphenylbenzene (CMP-B) or electron-accepting triphenyltriazine (CMP-T), which can grow on a conductive carbon nanotube (CNT) to serve as separator modifiers for Li-S batteries. CMP-B@CNT features faster ion transportation than the counterpart of CMP-T@CNT. More importantly, compared with acceptor-acceptor (A-A) CMP-T, donor-acceptor (D-A) CMP-B possesses a higher degree of conjugation and a narrower band gap, which are conducive to the electron transfer along the polymer skeleton, thus accelerating the sulfur redox kinetics. Consequently, the CMP-B@CNT functional separator endows Li-S cells with an outstanding initial capacity of 1371 mAh g^-1 at 0.1 C and favorable cycling stability with a capacity degradation rate of 0.048% per cycle at 1 C for 800 cycles. This work provides insight into the rational design of efficient catalysts for advanced Li-S batteries.

Realizing anti-self-discharged lithium-sulfur batteries by using hierarchical porous carbon nanofibers embedded with Fe/Ni-N catalytic sites

Lithium-sulfur (Li-S) batteries are featured with high gravimetric energy density, yet their commercial application is significantly deteriorated with the severe self-discharging resulted from the polysulfides shuttle and sluggish electrochemical kinetics. Here, a hierarchical porous carbon nanofibers implanted with Fe/Ni-N (denoted as Fe-Ni-HPCNF) catalytic sites are prepared and used as a kinetics booster toward anti-self-discharged Li-S batteries. In this design, the Fe-Ni-HPCNF possesses interconnected porous skeleton and abundant exposed active sites, enabling fast Li-ion conduction, excellent shuttle inhibition and catalytic ability for polysulfides' conversion. Combined with these advantages, this cell with the Fe-Ni-HPCNF equipped separator exhibits an ultralow self-discharged rate of 4.9% after resting for one week. Moreover, the modified batteries deliver a superior rate performance (783.3 mAh g^-1 at 4.0 C) and an outstanding cycling life (over 700 cycles with 0.057% attenuation rate at 1.0 C). This work may guide the advanced design of anti-self-discharged Li-S batteries.

Ultrafine Co-Species Interspersed g-C3N4 Nanosheets and Graphene as an Efficient Polysulfide Barrier to Enable High Performance Li-S Batteries

Lithium-sulfur (Li-S) batteries are regarded as one of the promising advanced energy storage systems due to their ultrahigh capacity and energy density. However, their practical applications are still hindered by the serious shuttle effect and sluggish reaction kinetics of soluble lithium polysulfides. Herein, g-C₃N₄ nanosheets and graphene decorated with an ultrafine Co-species nanodot heterostructure (Co@g-C₃N₄/G) as separator coatings were designed following a facile approach. Such an interlayer can not only enable effective polysulfide affinity through the physical barrier and chemical binding but also simultaneously have a catalytic effect on polysulfide conversion. Because of these superior merits, the Li-S cells assembled with Co@g-C₃N₄/G-PP separators matched with the S/KB composites (up to ~70 wt% sulfur in the final cathode) exhibit excellent rate capability and good cyclic stability. A high specific capacity of ~860 mAh g^-1 at 2.0 C as well as a capacity-fading rate of only ~0.035% per cycle over 350 cycles at 0.5 C can be achieved. This bifunctional separator can even endow a Li-S cell at a low current density to exhibit excellent cycling capability, with a capacity retention rate of ~88.4% at 0.2 C over 250 cycles. Furthermore, a Li-S cell with a Co@g-C₃N₄/G-PP separator possesses a stable specific capacity of 785 mAh g^-1 at 0.2 C after 150 cycles and a superior capacity retention rate of ~84.6% with a high sulfur loading of ~3.0 mg cm^-2. This effective polysulfide-confined separator holds good promise for promoting the further development of high-energy-density Li-S batteries.

Self-Supporting Multicomponent Hierarchical Network Aerogel as Sulfur Anchoring-Catalytic Medium for Highly Stable Lithium-Sulfur Battery

The low utilization rate of active materials, shuttle effect of lithium polysulfides (LiPSs), and slow reaction kinetics lead to the extremely low efficiency and poor high current cycle stability of lithium sulfur batteries (Li-S batteries). In this paper, a self-supporting multicomponent hierarchical network aerogel is proposed as the modified cathode (S/GO@MX@VS₄ ). It consists of graphene (GO) and MXene nanosheets (MX) loaded with VS₄ nanoparticles. The experimental results and first-principles calculations show that the GO@MX@VS₄ aerogel has strong adsorption and reversible conversion effects on LiPSs. It can not only inhibit the shuttle effect and improve the utilization rate of active substances by keeping the chain crystal structure of VS₄ , but also promote the reversibility and kinetics of the reaction by accelerating the liquid-solid transformation in the reduction process and the decomposition of insoluble Li₂ S in the oxidation process. The GO@MX@VS₄ aerogel modified cathode with a multicomponent synergy exhibits the capacity ratios (Q₁ /Q₂ ) at different discharge stages is close to the theoretical value (1:2.8), and the capacity decay per cycle is 0.019% in 1200 cycles at 5C. Also, a high areal capacity of 6.90 mAh cm^-2 is provided even at high sulfur loading (7.39 mg cm^-2 ) and low electrolyte/sulfur ratio (E/S, 8.0 µL mg^-1 ).