Lithium Polysulfides Research Articles

Recent Progress and Challenge in Metal–Organic Frameworks for Lithium–Sulfur Battery Separators

AbstractThe separators used in lithium‐sulfur (Li–S) batteries play a crucial role in their cycling performance and safety. Current commercial separators lack the ability to efficiently regulate polysulfide shuttling and are prone to thermal runaway at high temperatures. Recent studies have shown that multifunctional separators can boost the electrochemical performance and safety of Li–S batteries. Metal–organic frameworks (MOFs) have emerged as promising materials for modifying separators due to their large specific surface areas and highly ordered tunable nanopores. Herein, this review focuses on the advancements in developing MOFs, their derivatives, and MOFs‐based composites as separator coatings to address the shortcomings of Li–S batteries. The mechanisms behind these modified separators, designed to inhibit lithium polysulfide shuttling and lithium dendrite growth, are discussed, emphasizing the relationship between the structure and electrochemical performance. The impact of these modified separator coatings on battery safety is also explored, aiming to design suitable composite films for high‐safety Li–S batteries. Furthermore, future perspectives are outlined to guide practical applications and overcome the remaining challenges associated with MOFs‐modified separators.

Synergistic engineering of structural and electronic regulation in necklace-like UiO-66/ACNT structure toward lithium-sulfur batteries with fast polysulfide redox

Although UiO-66-based metal-organic framework (MOF) composites have been widely applied in the entrapment and catalytic effect toward lithium polysulfides (LiPSs), the synergistic engineering of structural and electronic regulation strategy has not been carefully considered. Herein, we optimize the internal cluster structure, rational regulate the size of UiO-66, and design the necklace-like structure composed of UiO-66 nanoparticles and acid-treated carbon nanotube (UiO-66/ACNT) through simple hydrothermal reaction and thermal treatment strategy. According to experiments and density universal function theory calculations, the improved UiO-66/ACNT architecture effectively provides more polarity to capture polysulfides via exposing more active sites and effectively delivers electrocatalytic activity of polysulfide conversion. Furthermore, systematic electrocatalytic kinetics, in situ Raman spectroscopy and in-situ impedance analysis indicate the functional separator can inhibit the shuttling effect, and catalyze the conversion of lithium polysulfide. As a result, Li-S batteries with functional separators exhibit excellent electrochemical performance (low fading rate over 1000 cycles of 0.06 % per cycle at 1C, high areal capacity of 4.01 mAh cm−2 at 0.2C). This work offers insight into the rational design of UiO-66-based metal-organic frameworks (MOFs) through synergistic engineering for high-performance Li-S batteries.

Entropy‐Driven Highly Chaotic MXene‐Based Heterostructures as an Efficient Sulfur Redox Electrocatalysts for Li‐S Battery

AbstractBoth the sluggish sulfur redox reaction (SRR) kinetics and lithium polysulfides (LiPSs) shuttle effect limit the practical application of Li‐S batteries. Designing heterostructure sulfur hosts has emerged as an effective way to address these two issues with one material. However, the principles of heterostructures reinforced Li‐S batteries remain inadequately understood. Here, it is demonstrated for the first time that increasing the entropy of heterostructure can promote its SRR catalytic activity and alleviate the LiPSs shuttling. By a simple solution‐based strategy, a highly chaotic MXene‐based heterostructure (HCMH, TiS2/TiN/TiO2/Ti3C2Tx) is fabricated. The smart integration of “high entropy”, heterostructure, and MXene endow the HCMH catalyst with significantly improved performance, demonstrated by a much smaller Tafel slope of 62.9 mV dec−1 and a higher electron transfer number of 7.10, compared with the moderately chaotic MXene‐based heterostructure (MCMH, TiO2/TiN/Ti3C2Tx) and MXene. DFT theoretical calculations reveal that introducing new phases lowers the Gibbs energy barriers of both rate‐limiting Li2S2/Li2S reduction and Li2S decomposition. Upon the addition of only 5 wt.% HCMH to the sulfur cathode, both the reversible capacity and rate capability of Li‐S cells are greatly improved, which further highlights the importance of the high entropy “cocktail effect” in the design of SRR electrocatalysts in the future.

Screening Conductive MXenes for Lithium Polysulfide Adsorption

AbstractMXenes are promising passive components that enable lithium‐sulfur batteries (LSBs) by effectively trapping lithium polysulfides (LiPSs) and facilitating surface‐mediated redox reactions. Despite numerous studies highlighting the potential of MXenes in LSBs, there are no systematic studies of MXenes’ composition influence on polysulfide adsorption, which is foundational to their applications in LSB. Here, a comprehensive investigation of LiPS adsorption on seven MXenes with varying chemistries (Ti2CTx, Ti3C2Tx, Ti3CNTx, Mo2TiC2Tx, V2CTx, Nb2CTx, and Nb4C3Tx), utilizing optical and analytical spectroscopic methods is performed. This work reports on the influence of polysulfide concentration, interaction time, and MXenes’ chemistry (transition metal layer, carbide and carbonitride inner layer, surface terminations and structure) on the amount of adsorbed LiPSs and the adsorption mechanism. These findings reveal the formation of insoluble thiosulfate and polythionate complex species on the surfaces of all tested MXenes. Furthermore, the selective adsorption of lithium and sulfur, and the extent of conversion of the adsorbed species on MXenes varied based on their chemistry. For instance, Ti2CTx exhibits a strong tendency to adsorb lithium ions, while Mo2TiC2Tx is effective in trapping sulfur by forming long‐chain polythionates. The latter demonstrates a significant conversion of intermediate polysulfides into low‐order species. This study offers valuable guidance for the informed selection of MXenes in various functional components benefiting the future development of high‐performance LSBs.

The Interactions between Ionic Liquids and Lithium Polysulfides in Lithium-Sulfur Batteries: A Systematic Density Functional Theory Study.

Ionic liquids (ILs) based on hybrid anions have recently garnered attention as beguiling alternative electrolytes for energy storage devices. This attention stems from the potential of these asymmetric anions to reduce the melting point of ILs and impede the crystallization of ILs. Furthermore, they uphold the advantages associated with their more conventional symmetric counterparts. In this study, we employed dispersion-corrected density functional theory (DFT-D) calculations to scrutinize the interplay between two hybrid anions found in ionic liquids [FTFSA]- and [MCTFSA]- and the [C4mpyr]+ cation, as well as in lithium polysulfides in lithium-sulfur batteries. For comparison, we also examined the corresponding ILs containing symmetric anions, [TFSA]- and [FSA]-. We found that the hybrid anion [MCTFSA]- and its ionic liquid exhibited exceptional stability and interaction strength. Additionally, our investigation unveiled a remarkably consistent interaction between ionic liquids (ILs) and anions with lithium polysulfides (and S8) during the transition from octathiocane (S8) to the liquid long-chain Li2Sn (4 ≤ n ≤ 8). This contrasts with the gradual alignment observed between cations and lithium polysulfides during the intermediate state from Li2S4 to the solid short-chain Li2S2 and Li2S1. We thoroughly analyzed the interaction mechanism of ionic liquids composed of different symmetry anions and their interactions with lithium polysulfides.

Melamine Polymerization Promotes Compact Phosphorus/Carbon Composite for High-Performance and Safe Lithium Storage.

Phosphorus is regarded as a promising material for high-performance lithium-ion batteries (LIBs) due to its high theoretical capacity, appropriate lithiation potential, and low lithium-ion diffusion barrier. Phosphorus/carbon composites (PC) are engineered to serve as high-capacity high-rate anodes; the interaction between phosphorus and carbon, long-term capacity retention, and safety problems are important issues that must be well addressed simultaneously. Herein, an in situ polymerization approach to fabricate a poly-melamine-hybridized (pMA) phosphorus/carbon composite (pMA-PC) is employed. The pMA hybridization enhances the density and electrical conductivity of the PC, improves the structural integrity, and facilitates stable electron transfer within the pMA-PC composite. Moreover, the pMA-PC composite exhibits efficient adsorption of lithium polysulfides, enabling stable transport of Li+ ions. Therefore, the pMA-PC anode demonstrates a high specific charging capacity of 1,381mAhg-1 at 10Ag-1, and a great capacity retention of 86.7% at 1Ag-1 over 500 cycles. The synergistic effect of phosphorus and nitrogen further confers excellent flame retardant properties to the pMA-PC anode, including self-extinguishing in 2.5s, and a much lower combustion temperature than PC. The enhanced capacity and safety performance of pMA-PC show potential in future high-capacity and high-rate LIBs.

Ingenious separator architecture: Revealing the versatile 3D heterostructured MXene-hydrogen titanate electrocatalysts for advanced lithium-sulfur battery

Lithium-sulfur batteries (LSBs) with high theoretical energy density and cost-effectiveness have great promise as alternative energy storage systems to traditional lithium-ion batteries. Nevertheless, their practical application is severely hindered due to a myriad of shortcomings such as “shuttling effect” of lithium polysulfides (LiPSs), sluggish conversion kinetics of sulfur (S) species, low electrical conductivity of S and discharged products, and intractable lithium dendrite growth. Herein, a novel 3D-heterostructured electrocatalyst consisting of the in-situ grown H2Ti3O7 (HTO) nano-whiskers on a Ti3C2Tx (MXene) substrate (M-HTO-0.5) is designed using a MXene-derived hydrothermal process combined with following ion exchange between Na+ and H+, wherein the hydrothermal reaction time regulation (0.5 h) mainly induces the balanced co-existence of HTO and MXene in the microstructure of M-HTO-0.5. Benefiting from the synergistic effect between M-HTO-0.5 (strong LiPS chemisorption, outstanding catalytic activity with abundant active sites, and boosted Li+ ion migration) and carbon nanotubes (CNTs) (construction of ion/electron transport network), the ingenious design of multi-functional separators (M-HTO-0.5-C) effectively restricts LiPS shuttling and dendritic Li growth simultaneously. Consequently, LSBs with M-HTO-0.5-C separators achieve remarkable rate capability, and stable long-term cyclability even at 5.0 C. This study opens an innovative avenue for developing high-performance LSB configurations.

Tuning the D-Band Center of Bi2S3─MoS2 Heterostructure Towards Superior Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries are one of the most promising energy storage devices due to their environmental friendliness, low cost, and high specific capacity. However, the slow electrochemical kinetics and the "shuttle effect" have seriously hindered their commercialization. Herein, the nanoflower Bi2S3─MoS2 (BMS) heterostructure is synthesized by a two-step hydrothermal method, and then the Bi2S3─MoS2-Polypropylene (BMS-PP) interlayer is constructed. The heterostructure is rich in active sites, in which BMS has strong adsorption to lithium polysulfides (LiPSs) and can effectively anchor LiPSs while catalyzing LiPSs and promote the redox of Li2S at the same time, which can improve the utilization of active substances. More importantly, the d-band center can be tuned by the formation of Bi2S3─MoS2 heterostructure. Thus, Li-S batteries containing the BMS-PP interlayer show excellent rate performance (841.6 mAh g-1 at 5 C) and cycling performance (70.3% capacity retention after 500 cycles at 3 C). This work provides a new route for high-performance lithium-sulfur batteries.

High-Entropy Alloy Electrocatalysts Bidirectionally Promote Lithium Polysulfide Conversions for Long-Cycle-Life Lithium-Sulfur Batteries.

High-entropy alloys (HEAs) have attracted considerable attention, owing to their exceptional characteristics and high configurational entropy. Recent findings demonstrated that incorporating HEAs into sulfur cathodes can alleviate the shuttling effect of lithium polysulfides (LiPSs) and accelerate their redox reactions. Herein, we synthesized nano Pt0.25Cu0.25Fe0.15Co0.15Ni0.2 HEAs on hollow carbons (HCs; denoted as HEA/HC) by a facile pyrolysis strategy. The HEA/HC nanostructures were further integrated into hypha carbon nanobelts (HCNBs). The solid-solution phase formed by the uniform mixture of the five metal elements, i.e., Pt0.25Cu0.25Fe0.15Co0.15Ni0.2 HEAs, gave rise to a strong interaction between neighboring atoms in different metals, resulting in their adsorption energy transformation across a wide, multipeak, and nearly continuous spectrum. Meanwhile, the HEAs exhibited numerous active sites on their surface, which is beneficial to catalyzing the cascade conversion of LiPSs. Combining density functional theory (DFT) calculations with detailed experimental investigations, the prepared HEAs bidirectionally catalyze the cascade reactions of LiPSs and boost their conversion reaction rates. S/HEA@HC/HCNB cathodes achieved a low 0.034% decay rate for 2000 cycles at 1.0 C. Notably, the S/HEA@HC/HCNB cathode delivered a high initial areal capacity of 10.2 mAh cm-2 with a sulfur loading of 9 mg cm-2 at 0.1 C. The assembled pouch cell exhibited a capacity of 1077.9 mAh g-1 at the first discharge at 0.1 C. The capacity declined to 71.3% after 43 cycles at 0.1 C. In this work, we propose to utilize HEAs as catalysts not only to improve the cycling stability of lithium-sulfur batteries, but also to promote HEAs in energy storage applications.

Recent progress in MXenes-based lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) are considered as the potent candidates for next-generation energy storage systems due to their high theoretical energy density. However, some inherent problems, including sulfur insulation, shuttle effect caused by lithium polysulfides, and lithium dendrites, hinder their practical application. Various materials have been studied to address the aforementioned issues. A class of two-dimensional inorganic compounds (MXenes), such as transition metal carbides, nitrides, and carbon nitrides, have recently emerged. In this review, we summarize the characteristics and commonly used preparation methods of MXenes and outline the latest development of MXenes and their composites in LSBs. When utilized as sulfur carriers, modified layers of separators, hosts for lithium metal anodes, and electrolyte additives in LSBs, the diversity of structure, excellent conductivity, and high mechanical strength of MXenes and their composites highlight the competitive advantages. This review provides some ideas for the future development of MXenes in LSBs.

Recent advances in carbon-based sulfur host materials for lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) have been brought into focus as the development direction of the next-generation power battery system due to their high energy density, eco-friendliness, and low cost, which has a broad application prospect in the field of energy storage. However, some problems are still unresolved in the sulfur cathode, e.g., poor electric conductivity, serious volume expansion of sulfur, shuttle effect caused by easy dissolution of lithium polysulfides in the electrolyte, and slow redox reaction kinetics of sulfur species. These issues lead to poor cycle stability and rate performance, making it hard to meet the requirement for LSBs in practical applications. Since the inherent nature of sulfur is the root cause of the above problems, reasonable design of functional sulfur hosts will be an effective way to break through the current bottlenecks of LSBs. The review covers the latest research progress on carbon-based sulfur host materials of LSBs, including structural design and functional optimization strategies, aiming to prepare multifunctional sulfur host materials by integrating physical confinement, chemical adsorption, and catalytic effect towards lithium polysulfides. The obstacles and future prospects of carbon-based sulfur hosts have also been brought forward, which provides in-depth guidance for practical application of LSBs.

Electrochemical active interlayer with porous architecture for reliable lithium–sulfur batteries

Although lithium–sulfur batteries have excellent theoretical specific capacity (1675 mAh g−1) and high energy density (2600 wh kg−1), their practical application is hampered by the severe “shuttle effect” of polysulfides in ether-based electrolytes. Soluble polysulfides can migrate across separator and react with lithium metal, resulting in a rapid capacity decay. In this study, we proposed an electrochemical active porous architecture (EPA) interlayer to address this tricky issue. Firstly, the porous architecture guarantees efficient electrolyte diffusion. Additionally, the catalytic vanadium nitride particles within the conductive skeleton can capture soluble polysulfides, accelerate their redox reaction, and suppress the “shuttle effect”, thereby further restraining the parasitic reaction between lithium metal anode and polysulfides. Therefore, integrating the EPA interlayer in the battery enables an initial capability of 1465.8 mAh g−1 at 0.1C and delivers a good rate performance at 2.0C (668.0 mAh g−1). The battery achieves an average capacity of 796.7 mAh g−1 within the first 500 cycles under 0.5C, with a high average Coulombic efficiency of 99.8 %, indicating the practical potential of EPA interlayer design for application in reliable lithium–sulfur batteries.

Insight into the effect of N-doping level in N-doped carbons modified separator on improving the electrochemical performance of Li-S batteries

Carbon materials are widely applied in lithium-sulfur (Li-S) batteries due to their large specific surface area, high conductivity, and tunable properties. An appropriate heteroatom doping can enhance the adsorption ability and electrochemical activity toward lithium polysulfides, thus improving the overall performance of Li-S batteries. Herein, three nitrogen-doped carbons (NCs) with different nitrogen doping levels are firstly prepared and then used as the model materials to coat the common polypropylene (PP) separator, aiming to elaborate the effect of adsorption behavior from various N-doping level carbons surface toward lithium polysulfides on the subsequent catalytic conversion kinetics. The experimental data demonstrate that the NC800 with appropriate N-doped level shows the moderate adsorption of polysulfides as well as exhibits the best reaction kinetics on polysulfides conversion. In comparison, an assembled Li-S battery with NC800-PP separator delivers high capacity and good cycling stability, achieving an initial specific capacity of ∼ 1302.2 mAh g−1 at 0.2 C and a capacity fading rate of ∼ 0.061 % per cycle after 500 cycles at 1.0 C. Moreover, a Li-S battery using NC800-PP separator still delivers an initial specific capacity of ∼ 1147.6 mAh g−1 at 0.1 C and maintains a stable specific capacity of ∼ 833.5 mAh g−1 even with a sulfur loading of ∼ 3.9 mg cm−2. This study is believed to give better insight to reveal the internal correlation of adsorption and catalytic effect on polysulfides, as well as provide a guidance for rationally designing heteroatom doping carbon materials in Li-S batteries.

Suppressing Shuttle Effect via Cobalt Phthalocyanine Mediated Dissociation of Lithium Polysulfides for Enhanced Li‐S Battery Performance

AbstractCationic lithium polysulfides (LiPSs) within ether‐based electrolytes are essential intermediates pivotal in instigating the shuttle effect. Suppressing the generation of cationic LiPSs is crucial for enhancing the performance of high‐energy‐density Li‐S batteries in practical applications. Herein, for the first time, it is demonstrated that cobalt phthalocyanine (CoPc) can significantly enhance the dissociation of LiPSs, leading to a decrease in cationic LiPSs species within the electrolyte. Employing NMR and in‐situ Raman spectroscopy, it is observed that CoPc interacts with sulfur anions within LiPSs, modifying the dissociation equilibrium of LiPSs in 1,2‐dimethoxyethane (DME)/1,3‐dioxolane (DOL) electrolyte. This CoPc‐mediated shift toward dissociation equilibrium reduces the concentration of cationic LiPSs, effectively suppressing the electromigration of LiPSs intermediates toward the Li anode during the charging process of Li‐S battery. As a result, the Li‐S battery enabled a high reversible areal capacity of 4.2 mAh cm−2 at a high sulfur loading of 5.0 mg cm−2 and a low electrolyte/sulfur (E/S) ratio of 3 µL mgs−1. This work affords a promising strategy for tackling the fundamental challenges regarding Li‐S batteries.

High-Entropy Metal Oxide-Coated Carbon Cloth as Catalysts for Long-Life Li-S Batteries.

Lithium-sulfur (Li-S) batteries with high specific energy density, low cost, and environmental friendliness of sulfur have been regarded as a competitive alternative to replace lithium-ion batteries. However, the shuttle effect and the sluggish conversion rate of lithium polysulfides (LiPSs) have seriously limited the practical application of Li-S batteries. Herein, high-entropy oxides grown on the carbon cloth (CC/HEO) are synthesized by a simple and ultrafast solution combustion method for the sulfur cathode. The as-prepared composites possess abundant HEO active sites for strong interaction with LiPSs, which can significantly promote redox kinetics. Besides, the carbon fiber substrate not only ensures high electrical conductivity but also accommodates large volume change, leading to a stable sulfur electrochemistry. Benefiting from the rational design, the Li-S batteries with CC/HEO as cathode skeleton exhibits good cyclability with a capacity decay rate of 0.057% per cycle after 1000 cycles at 2 C. More importantly, the Li-S batteries with 4.3 mg cm-2 high sulfur loading can still retain a high capacity retention of 78.2% after 100 cycles.

Iodine-doped carbon nanotubes boosting the adsorption effect and conversion kinetics of lithium-sulfur batteries

Compared with lithium-ion batteries (LIBs), lithium-sulfur batteries (LSBs), based on electrochemical reactions involving multi-step 16-electron transformations provide higher specific capacity (1672 mAh g−1) and specific energy (2600 Wh kg−1), exhibiting great potential in the field of energy storage. However, the inherent insulation of sulfur, slow electrochemical reaction kinetics and detrimental shuttle-effect of lithium polysulfides (LiPSs) restrict the development of LSBs in practical applications. Herein, the iodine-doped carbon nanotubes (I-CNTs) is firstly reported as sulfur host material to the enhance the adsorption-conversion kinetics of LSBs. Iodine doping can significantly improve the polarity of I-CNTs. Iodine atoms with lone pair electrons (Lewis base) in iodine-doped CNTs can interact with lithium cations (Lewis acidic) in LiPSs, thereby anchoring polysulfides and suppressing subsequent shuttling behavior. Moreover, the charge transfer between iodine species (electron acceptor) and CNTs (electron donor) decreases the gap band and subsequently improves the conductivity of I-CNTs. The enhanced adsorption effect and conductivity are beneficial for accelerating reaction kinetics and enhancing electrocatalytic activity. The in-situ Raman spectroscopy, quasi in-situ electrochemical impedance spectroscopy (EIS) and Li2S potentiostatic deposition current–time (i-t) curves were conducted to verify mechanism of complex sulfur reduction reaction (SRR). Owing to above advantages, the I-CNTs@S composite cathode exhibits an ultrahigh initial capacity of 1326 mAh g−1 as well as outstanding cyclicability and rate performance. Our research results provide inspirations for the design of multifunctional host material for sulfur/carbon composite cathodes in LSBs.

Oxygen‐Doped Porous Carbon Nanotubes Encapsulated Bismuth Oxide to Enhance the Performance of Lithium‐Sulfur Battery

AbstractLithium‐sulfur batteries (LSBs) are greatly promising next‐generation energy storage systems due to their high theoretical specific capacities and energy density. However, the dissolution of lithium polysulfides (LIPs) causees low capacity and short lifespan, impeding the practical application of LSBs. To address this challenge, the composites of oxygen‐doped porous carbon nanotubes (CNTs) wrapped in Bi2O3 were synthesized by the calcinating method (CNTs@Bi2O3). The CNTs@Bi2O3 composites as absorbent immobilize LIPs via the physical limitations of porous CNTs and the chemical adsorption of Bi2O3 and carbon‐oxygen polar bond. After loading sulfur, the CNTs@Bi2O3/S composites with the encapsulated structure can provide a large space to withstand the significant volume expansion during the electrochemical reaction process, ensuring the stability of the electrode structure. Thus, the batteries with CNTs@Bi2O3/S exhibit a high capacity (1232 mA h g−1 at 0.1 C) and good cycle stability (639.1 mA h g−1 after 150 cycles at 0.5 C). This work presents an innovative design suitable for the encapsulated structure of Bi2O3, serving as a valuable reference for the development of Bi2O3 in LSBs.

High-efficiency Lithium Metal Stabilization and Polysulfide Suppression in Li-S Battery Enabled by Weakly Solvating Solvent.

Lithium-sulfur batteries (LSBs) are considered a highly promising next-generation energy storage technology due to their exceptional energy density and cost-effectiveness. However, the practical use of current LSBs is hindered primarily by issues related to the "shuttle effect" of lithium polysulfide (LiPS) intermediates and the growth of lithium dendrites. In strongly solvating electrolytes, the solvent-derived solid electrolyte interphase (SEI) lacks mechanical strength due to organic components, leading to ineffective lithium dendrite suppression and severe LiPS dissolution and shuttling. In contrast, the weakly solvating electrolyte (WSE) can create an anion-derived SEI layer which can enhance compatibility with lithium metal anode, and restricting LiPS solubility. Herein, a WSE consisting of 0.4м LiTFSI in the mixture of 1,4-dioxane (DX):dimethoxymethane (DMM) is designed to overcome the issues associated with LSB. Surface analyses confirmed the formation of a beneficial SEI layer rich in LiF, enabling homogeneous lithium deposition with an average Coulombic efficiency CE exceeding 99% over 100 cycles. Implementing the low-concentration WSE in Li||SPAN cells yielded an impressive initial specific capacity of 671mAhg-1. This research highlights the advantages of WSE and offers the pathway for cost-effective electrolyte development, enabling the realization of high-performance LSBs.

Influence of the Nitrogen Precursor in the Development of N-Functionalities in a Mesoporous Carbon Material and Its Effect on the Li–S Cells’ Electrochemistry

Li–S batteries are positioned as a strong alternative for efficient energy storage due to their high theoretical energy density and their theoretical specific capacity (1675 mA h g−1) compared to current Li-ion batteries; however, their commercialization is affected by the rapid decay of the specific capacity as a consequence of the different species of lithium polysulfides that are generated during the charge–discharge processes. The use of nitrogen-doped mesoporous carbon materials has been shown to have the ability to confer electronic conductivity to sulfur and retain the lithium polysulfide species. However, there are not enough studies to help understand how the type of nitrogen precursor influences the development of specific nitrogen functionalities to favor the retention of lithium polysulfide species. This work seeks to determine the effect of the use of different nitrogen precursors on the structural changes of the mesoporous carbon materials prepared, and thus evaluate the electrochemical behavior of Li–S cells correlating the type of nitrogen functionality generated when the precursor is variated with the charge/discharge capacity developed during the cell operation. For this study, different carbon materials were prepared by the variation of the nitrogen source (melamine, ethylenediamine, and hexadecylamine) to obtain a N-doped mesoporous carbon with different distributions of nitrogen functionalities in its structure. The use of the primary amine ethylenediamine as a nitrogen precursor in the formation of structured carbon materials favored elemental sulfur infiltration into its pores, resulting in the maximum sulfur content within the pores and interacting with the carbonaceous matrix (78.8 wt.%). The carbon material prepared with this precursor resulted in a higher content of N-pyridinic functionality, which, combined with the high content of N-pyrrolic, resulted in the highest specific discharge capacity at 0.1 C after 100 cycles when compared to cells assembled with materials derived from the use of melamine and hexadecylamine precursors. The cell assembled with the electrode formed from ethylenediamine as a nitrogen precursor presented an initial discharge capacity of 918 mA h g−1 with a Coulombic efficiency of ~83.4% at 0.1 C after 100 cycles.

Cellulose Fiber based self-supporting paper cathode with multi-scale network structure for high performance lithium-sulfur battery

Lithium-sulfur batteries (LSBs) are gaining attention as a potential energy storage option due to their impressive theoretical specific capacity. However, the main challenges hindering its implementation include the expansion in volume of sulfur and lithium sulfide during the conversion process of charge-discharge, the limited conductivity of the active material, and specifically, the phenomenon known as shuttle effect caused by lithium polysulfides (LiPSs). The development of high-performance LSBs have led to the need for self-supporting cathode materials that can efficiently accommodate volume expansion, build up efficient conductive pathways, and suppress shuttle effects. The poly(3,4-ethylenedioxythiophene) (PEDOT) was synthesized in situ on cellulose nanofibers (CNFs), resulting in the formation of CNF@PEDOT. Subsequently, CNF@PEDOT was combined with plant fibers (PFs) and multi-wall carbon nanotubes (MWCNTs) to fabricate a self-supporting cathode featuring a multi-scale network structure denoted as CNF@PEDOT/MWCNT/PF/S. The PFs serve as the primary backbone network, CNF@PEDOT to form the secondary conductive network, and MWCNTs act as tertiary conducting networks. In fact, the design of multi-level conductive network effectively improves the conductivity of the material, and PEDOT can further adsorb and catalyze LiPSs. The incorporation of PFs significantly enhanced the mechanical robustness of the cathode and augmented its electrolyte permeability. The self-supporting CNF@PEDOT/MWCNT/PF/S cathode exhibits an initial capacity of 1204.6 mA h g−1 when loaded with 2.4 mg cm−2 of sulfur at a rate of 0.2 C. Notably, it maintains an initial discharge specific capacity of 712.9 mA h g−1 even when the sulfur load is increased to 4.3 mg cm−2. This study presents a comprehensive framework for the development of self-supporting cathodes in LSBs.