High-Electrochemical-Activity Composite Cathode Enabled by Fast Segmental Relaxation for Solid-State Lithium-Sulfur Batteries

Solid-state lithium-sulfur batteries (SSLSBs) have attracted a great deal of attention because of their high theoretical energy density and intrinsic safety. However, their practical applications are severely impeded by slow redox kinetics and poor cycling stability. Herein, we revealed the detrimental effect of aggregation of lithium polysulfides (LiPSs) on the redox kinetics and reversibility of SSLSBs. As a paradigm, we introduced a multifunctional hyperbranched ionic conducting (HIC) polymer serving as a solid polymer electrolyte (SPE) and cathode binder for constructing SSLSBs featuring high electrochemical activity and high cycling stability. It is demonstrated that the unique structure of the HIC polymer with numerous flexible ether oxygen dangling chains and fast segmental relaxation enables the dissociation of LiPS clusters, facilitates the conversion kinetics of LiPSs, and improves the battery's performance. A Li|HIC SPE|HIC-S battery, in which the HIC polymer acts as an SPE and cathode binder, exhibits an initial capacity of 910.1 mA h gS-1 at 0.1C and 40 °C, a capacity retention of 73.7% at the end of 200 cycles, and an average Coulombic efficiency of approximately 99.0%, demonstrating high potential for application in SSLSBs. This work provides insights into the electrochemistry performance of SSLSBs and provides a guideline for SPE design for SSLSBs with high specific energy and high safety.

Structure and Physicochemical Properties of Solutions of Lithium Polysulfides in Tetrasolvate of Lithium Perchlorate with Sulfolane Molecular Dynamics Modeling

Lithium polysulfides are intermediate products formed during the discharge and charge of lithium-sulfur batteries and have good solubility in electrolyte solutions. Therefore, the properties and structure of solutions of lithium polysulfides in electrolyte solutions affect the energy characteristics of the lithium-sulfur battery. In this work, the structure and physicochemical properties (density, ionic conductivity, and self-diffusion coefficients) of solutions of lithium disulfide, tetrasulfide, and octasulfide in lithium perchlorate tetrasolvate with sulfolane in a wide range of concentrations (0.2-12 M) were studied using the molecular dynamics method. With increasing concentrations of lithium polysulfides, the proportion of sulfur atoms in the coordination sphere of the lithium cation increases and the proportion of sulfolane molecules decreases. It has been established that the ability of the polysulfide anion to act as a bridging ligand leads to the formation of clusters, including lithium perchlorate, lithium polysulfides, and sulfolane. It has been shown that the tendency to form clusters increases with an increasing number of sulfur atoms in the polysulfide anion. At high concentrations of lithium polysulfides, regardless of their size, the electrolyte system becomes a single cluster. With an increase in the concentration of lithium polysulfides in the electrolyte system, its density increases and the ionic conductivity and diffusion coefficients of the system components decrease.

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.

Rechargeable Metal-Sulfur Batteries: Key Materials to Mechanisms

Rechargeable metal-sulfur batteries are considered promising candidates for energy storage due to their high energy density along with high natural abundance and low cost of raw materials. However, they could not yet be practically implemented due to several key challenges: (i) poor conductivity of sulfur and the discharge product metal sulfide, causing sluggish redox kinetics, (ii) polysulfide shuttling, and (iii) parasitic side reactions between the electrolyte and the metal anode. To overcome these obstacles, numerous strategies have been explored, including modifications to the cathode, anode, electrolyte, and binder. In this review, the fundamental principles and challenges of metal-sulfur batteries are first discussed. Second, the latest research on metal-sulfur batteries is presented and discussed, covering their material design, synthesis methods, and electrochemical performances. Third, emerging advanced characterization techniques that reveal the working mechanisms of metal-sulfur batteries are highlighted. Finally, the possible future research directions for the practical applications of metal-sulfur batteries are discussed. This comprehensive review aims to provide experimental strategies and theoretical guidance for designing and understanding the intricacies of metal-sulfur batteries; thus, it can illuminate promising pathways for progressing high-energy-density metal-sulfur battery systems.

Conformational Preference of Lithium Polysulfide Clusters Li2Sx (x = 4-8) in Lithium-Sulfur Batteries

Structures are of fundamental importance for diverse studies of lithium polysulfide clusters, which govern the performance of lithium-sulfur batteries. The ring-like geometries were regarded as the most stable structures, but their physical origin remains elusive. In this work, we systematically explored the minimal structures of Li2Sx (x = 4-8) clusters to uncover the driving force for their conformational preferences. All low-lying isomers were generated by performing global searches using the ABCluster program, and the ionic nature of the Li···S interactions was evidenced with the energy decomposition analysis based on the block-localized wave function (BLW-ED) approach and further confirmed with the quantum theory of atoms in molecule (QTAIM). By analysis of the contributions of various energy components to the relative stability with the references of the lowest-lying isomers, the controlling factor for isomer preferences was found to be the polarization interaction. Notably, although the electrostatic interaction dominates the binding energies, it contributes favorably to the relative stabilities of most isomers. The Li+···Li+ distance is identified as the key geometrical parameter that correlates with the strength of the polarization of the Sx2- fragment imposed by the Li+ cations. Further BLW-ED analyses reveal that the cooperativity of the Li+ cations primarily determines the relative strength of the polarization.

Polysulfides in Magnesium-Sulfur Batteries

Mg-S batteries hold great promise as a potential alternative to Li-based technologies. Their further development hinges on solving a few key challenges, including the lower capacity and poorer cycling performance when compared to Li counterparts. At the heart of the issues is the lack of knowledge on polysulfide chemical behaviors in the Mg-S battery environment. In this Review, a comprehensive overview of the current understanding of polysulfide behaviors in Mg-S batteries is provided. First, a systematic summary of experimental and computational techniques for polysulfide characterization is provided. Next, conversion pathways for Mg polysulfide species within the battery environment are discussed, highlighting the important role of polysulfide solubility in determining reaction kinetics and overall battery performance. The focus then shifts to the negative effects of polysulfide shuttling on Mg-S batteries. The authors outline various strategies for achieving an optimal balance between polysulfide solubility and shuttling, including the use of electrolyte additives, polysulfide-trapping materials, and dual-functional catalysts. Based on the current understanding, the directions for further advancing knowledge of Mg polysulfide chemistry are identified, emphasizing the integration of experiment with computation as a powerful approach to accelerate the development of Mg-S battery technology.

Size Effect for Inhibiting Polysulfides Shuttle in Lithium-Sulfur Batteries

It is undeniable that the dissolution of polysulfides is beneficial in speeding up the conversion rate of sulfur in electrochemical reactions. But it also brings the bothersome "shuttle effect". Therefore, if polysulfides can be retained on the cathode side, the efficient utilization of the polysulfides can be guaranteed to achieve the excellent performance of lithium-sulfur batteries. Based on this idea, considerable methods have been developed to inhibit the shuttling of polysulfides. It is necessary to emphasize that no matter which method is used, the solvation mechanism, and existence forms of polysulfides are essential to analyze. Especially, it is important to clarify the sizes of different forms of polysulfides when using the size effect to inhibit the shuttling of polysulfides. In this review, a comprehensive summary and in-depth discussion of the solvation mechanism, the existing forms of polysulfides, and the influencing factors affecting polysulfides species are presented. Meanwhile, the size of diverse polysulfide species is sorted out for the first time. Depending on the size of polysulfides, tactics of using size effect in cathode, separator, and interlayer parts are elaborated. Finally, a design idea of materials pore size is proposed to satisfy the use of size effect to inhibit polysulfides shuttle.

Accelerating Sulfur Redox Chemistry by Atomically Dispersed Zn-N4 Sites Coupled with Pyridine-N Defects on Porous Carbon Sheets

Single-atom catalysts (SACs) with specific N-coordinated configurations immobilized on the carbon substrates have recently been verified to effectively alleviate the shuttle effect of lithium polysulfides (LiPSs) in lithium-sulfur (Li─S) batteries. Herein, a versatile molten salt (KCl/ZnCl2 )-mediated pyrolysis strategy is demonstrated to fabricate Zn SACs composed of well-defined Zn-N4 sites embedded into porous carbon sheets with rich pyridine-N defects (Zn─N/CS). The electrochemical kinetic analysis and theoretical calculations reveal the critical roles of Zn-N4 active sites and surrounding pyridine-N defects in enhancing adsorption toward LiPS intermediates and catalyzing their liquid-solid conversion. It is confirmed by reducing the overpotential of the rate-determining step of Li2 S2 to Li2 S and the energy barrier for Li2 S decomposition, thus the Zn─N/CS guarantees fast redox kinetics between LiPSs and Li2 S products. As a proof of concept demonstration, the assembled Li─S batteries with the Zn─N/CS-based sulfur cathode deliver a high specific capacity of 1132 mAh g-1 at 0.1 C and remarkable capacity retention of 72.2% over 800 cycles at 2 C. Furthermore, a considerable areal capacity of 6.14 mAh cm-2 at 0.2 C can still be released with a high sulfur loading of 7.0 mg cm-2 , highlighting the practical applications of the as-obtained Zn─N/CS cathode in Li─S batteries.

Manipulating Li2S2/Li2S mixed discharge products of all-solid-state lithium sulfur batteries for improved cycle life

All-solid-state lithium-sulfur batteries offer a compelling opportunity for next-generation energy storage, due to their high theoretical energy density, low cost, and improved safety. However, their widespread adoption is hindered by an inadequate understanding of their discharge products. Using X-ray absorption spectroscopy and time-of-flight secondary ion mass spectrometry, we reveal that the discharge product of all-solid-state lithium-sulfur batteries is not solely composed of Li2S, but rather consists of a mixture of Li2S and Li2S2. Employing this insight, we propose an integrated strategy that: (1) manipulates the lower cutoff potential to promote a Li2S2-dominant discharge product and (2) incorporates a trace amount of solid-state catalyst (LiI) into the S composite electrode. This approach leads to all-solid-state cells with a Li-In alloy negative electrode that deliver a reversible capacity of 979.6 mAh g-1 for 1500 cycles at 2.0 A g-1 at 25 °C. Our findings provide crucial insights into the discharge products of all-solid-state lithium-sulfur batteries and may offer a feasible approach to enhance their overall performance.

Guiding maps of solvents for lithium-sulfur batteries via a computational data-driven approach

Practical realization of lithium-sulfur batteries requires designing optimal electrolytes with controlled dissolution of polysulfides, high ionic conductivity, and low viscosity. Computational chemistry techniques enable tuning atomistic interactions to discover electrolytes with targeted properties. Here, we introduce ComBat (Computational Database for Lithium-Sulfur Batteries), a public database of ∼2,000 quantum-chemical and molecular dynamics properties for lithium-sulfur electrolytes composed of solvents spanning 16 chemical classes. We discuss the microscopic origins of polysulfide clustering and the diffusion mechanism of electrolyte components. Our findings reveal that polysulfide solubility cannot be determined by a single solvent property like dielectric constant. Rather, observed trends result from the synergistic effect of multiple factors, including solvent C/O ratio, fluorination degree, and steric hindrance effects. We propose binding energy as a proxy for Li+ dissociation, which is a property that impacts the ionic conductivity. The insights obtained in this work can serve as guiding maps to design optimal lithium-sulfur electrolyte compositions.

Effect of Polysulfide Speciation on Mg Anode Passivation in Mg-S Batteries

Mg-S batteries are a promising next-generation system for beyond conventional Li-ion chemistry. The Mg-S architecture pairs a Mg metal anode with an inexpensive, high-capacity S8 cathode. However, S8-based cathodes exhibit the "polysulfide shuttle" effect, wherein soluble partially reduced Sx2- species generated at the cathode diffuse to and react with the anode. While dissolved polysulfides may undergo reactions to form Li+-permeable layers in Li-S systems, the interfaces on Mg anodes are passivating. In this work, we probe the reactivity of various Mg polysulfide solutions at the Mg anode interface. Mg polysulfide solutions are prepared without any chelating agents to closely mimic conditions in a Mg-S cell. The polysulfides are synthesized by reacting Mg metal and S8 in electrolyte, and the speciation is controlled by varying the Mg:S precursor ratio. S-poor precursor ratios produce magnesium polysulfide solutions with a higher proportion of short-chain polysulfides that react at the Mg anode faster than the longer-chain analogues. Anode passivation can be slowed by shifting the polysulfide equilibria toward longer-chain polysulfides through addition of S8.

In Situ Separator Modification with an N-Rich Conjugated Microporous Polymer for the Effective Suppression of Polysulfide Shuttle and Li Dendrite Growth

Lithium-sulfur (Li-S) batteries are very promising high-energy-density electrochemical energy storage devices, but suffer from serious Li polysulfide (LiPS) shuttle and uncontrollable Li dendrite growth. Here, we show in situ polyolefin separator modification with an N-rich conjugated microporous polymer (NCMP) for advanced Li-S battery. In situ polymerization generates an ultrathin NCMP coating on the whole external surface and the internal surface of the separator, which is substantially different from the conventional approaches with thick coatings only on the external surface. The NCMP coating with abundant N-containing groups (-NH₂ and -N═), uniform nanopores (12.294 Å), and π-conjugated structure can simultaneously inhibit LiPS shuttle and regulate uniform nucleation and growth of Li dendrites. Consequently, the NCMP-based separator endows the Li-S battery with significantly enhanced cycling stability at high S loading (5.4 mg cm^-2), lean electrolyte (E/S = 6.3 μL mg^-1), and limited Li excess (50 μm).

Qualitative Determination of Polysulfide Species in a Lithium-Sulfur Battery by HR-LC-APCI-MSn with One-Step Derivatization

Lithium-Sulfur (Li-S) batteries are one of the most promising next-generation batteries due to their ultrahigh energy density up to 500 W h kg^-1. However, despite the steady progress during the last several decades, there have been significant challenges for practical applications and commercialization. One of the major issues is controlling the lithium polysulfide (LiPS) shuttling process, which causes premature cell failure. To better understand the mechanism of the LiPS shuttling chemistry, a qualitative and quantitative analysis on polysulfide species in Li-S cell has profound significance for realizing highly efficient sulfur electrochemistry. Here we report a qualitative determination of the derivatized polysulfides in the electrolyte of a custom-made Li-S pouch cell with a high-resolution liquid chromatography-atmospheric pressure chemical ionization tandem mass spectrometry method for the first time. The ionization efficiency of the methylated polysulfides was affected by the tune parameters such as the corona discharge current, the vaporizer temperature, and the source capillary temperature. It was found that the source capillary temperature was the dominant parameter to increase the peak intensity of CH₃S₇^- ion, which was the smallest peak in the spectrum. An unusual and unique ionization pattern for methylated polysulfides detected in atmospheric pressure chemical ionization negative mode was elucidated by using first-principles calculations.

Computational approaches for XANES, VtC-XES, and RIXS using linear-response time-dependent density functional theory based methods

The emergence of state-of-the-art X-ray light sources has paved the way for novel spectroscopies that take advantage of their atomic specificity to shed light on fundamental physical, chemical, and biological processes both in the static and time domains. The success of these experiments hinges on the ability to interpret and predict core-level spectra, which has opened avenues for theory to play a key role. Over the last two decades, linear-response time-dependent density functional theory (LR-TDDFT), despite various theoretical challenges, has become a computationally attractive and versatile framework to study excited-state spectra including X-ray spectroscopies. In this context, we focus our discussion on LR-TDDFT approaches for the computation of X-ray Near-Edge Structure (XANES), Valence-to-Core X-ray Emission (VtC-XES), and Resonant Inelastic X-ray Scattering (RIXS) spectroscopies in molecular systems with an emphasis on Gaussian basis set implementations. We illustrate these approaches with applications and provide a brief outlook of possible new directions.

Garnet solid-state electrolyte with benzenedithiolate catholyte for rechargeable lithium batteries

A solid-state electrolyte, Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO), is used to block the crossover of lithium 1,2-benzenedithiolate (LBDT) in the catholyte to the lithium metal anode in a rechargeable lithium battery. The cell maintains a relatively stable cycle life (65.6% capacity retention after 100 cycles). The density functional theory simulation confirms that LBDT has strong interaction with LLZTO, which is helpful for reducing the interfacial resistance.

Electrocatalytic Activity of Polyaniline in Magnesium-Sulfur Batteries

Rechargeable magnesium-sulfur (Mg-S) batteries offer the potential for inexpensive energy storage alternatives to other metal-ion batteries for the grid scale and household applications. Despite all economic and resource advantages, Mg-S battery chemistry suffers from a complicated reaction mechanism and extremely slow reaction kinetics. To improve the kinetics, we improvise a new electrode architecture where a conductive polymer is used along with a carbon network. This report will bring an important insight of electrocatalytic activity of polyaniline, on the basis of free-radical coupling and is a completely new concept in Mg-S battery chemistry. By the combined electron spin resonance spectroscopy, X-ray photoelectron spectroscopy, and fluorescence lifetime measurements, we perceived that the polyaniline anchors the S₃^•- species from the electrolyte/catholyte through a free-radical-coupling process and thus promotes the reduction to end-discharged products, via a chemical adduct. The concept of free-radical catalysis in Mg/S batteries will open a new knowledge to enhance the active material utilization in the Mg-S batteries.

Ion-Selective Covalent Organic Framework Membranes as a Catalytic Polysulfide Trap to Arrest the Redox Shuttle Effect in Lithium-Sulfur Batteries

In the wake of shaping the energy future through materials innovation, lithium-sulfur batteries (LSBs) are top-of-the-line energy storage system attributed to their high theoretical energy density and specific capacity inclusive of low material costs. Despite their strengths, LSBs suffer from the cross-over of soluble polysulfide redox species to the anode, entailing fast capacity fading and inferior cycling stability. Adding to the concern, the insulating character of polysulfides lends to sluggish reaction kinetics. To address these challenges, we construct optimized polysulfide blockers-cum-conversion catalysts by accommodating the battery separator with covalent organic framework@Graphene (COF@G) composites. We settle on a crystalline TAPP-ETTB COF in the interest of its nitrogen-enriched scaffold with a regular pore geometry, providing ample lithiophilic sites for strong chemisorption and catalytic effect to polysulfides. On another front, graphene enables high electron mobility, boosting the sulfur redox kinetics. Consequently, a lithium-sulfur battery with a TAPP-ETTB COF@G-based separator demonstrates a high reversible capacity of 1489.8 mA h g^-1 at 0.2 A g^-1 after the first cycle and good cyclic performance (920 mA h g^-1 after 400 cycles) together with excellent rate performance (827.7 mA h g^-1 at 2 A g^-1). The scope and opportunities to harness the designability and synthetic structural control in crystalline organic materials is a promising domain at the interface of sustainable materials, energy storage, and Li-S chemistry.

Multifactorial engineering of biomimetic membranes for batteries with multiple high-performance parameters

Lithium–sulfur (Li–S) batteries have a high specific capacity, but lithium polysulfide (LPS) diffusion and lithium dendrite growth drastically reduce their cycle life. High discharge rates also necessitate their resilience to high temperature. Here we show that biomimetic self-assembled membranes from aramid nanofibers (ANFs) address these challenges. Replicating the fibrous structure of cartilage, multifactorial engineering of ion-selective mechanical, and thermal properties becomes possible. LPS adsorption on ANF surface creates a layer of negative charge on nanoscale pores blocking LPS transport. The batteries using cartilage-like bioinspired ANF membranes exhibited a close-to-theoretical-maximum capacity of 1268 mAh g⁻¹, up to 3500+ cycle life, and up to 3C discharge rates. Essential for safety, the high thermal resilience of ANFs enables operation at temperatures up to 80 °C. The simplicity of synthesis and recyclability of ANFs open the door for engineering high-performance materials for numerous energy technologies.

Lithium–sulfur batteries have a high specific capacity, but lithium polysulfide diffusion (LPS) and dendrite growth reduce their cycle life. Here, the authors show a biomimetic aramid nanofiber membrane for effectively suppressing LPS diffusion as well as lithium dendrites while allowing lithium ions to be transported. The membranes resists performance degradation at high temperatures and can be produced at scale by Kevlar recycling.

Role of Polysulfide Anions in Solid-Electrolyte Interphase Formation at the Lithium Metal Surface in Li-S Batteries

Delineating intricate interactions between highly reactive Li-metal electrodes and the diverse constituents of battery electrolytes has been a long-standing scientific challenge in materials design for advanced energy storage devices. Here, we isolated lithium polysulfide anions (LiS₄^-) from an electrolyte solution based on their mass-to-charge ratio and deposited them on Li-metal electrodes under clean vacuum conditions using ion soft landing (ISL), a highly controlled interface preparation technique. The molecular level precision in the construction of these model interfaces with ISL, coupled with in situ X-ray photoelectron spectroscopy and ab initio theoretical calculations, allowed us to obtain unprecedented insight into the parasitic reactions of well-defined polysulfides on Li-metal electrodes. Our study revealed that the oxide-rich surface layer, which is amenable to direct electron exchange, drives multielectron sulfur oxidation (S⁰ → S⁶⁺) processes. Our results have substantial implications for the rational design of future Li-S batteries with improved efficiency and durability.

Mo2C/C Hierarchical Double-Shelled Hollow Spheres as Sulfur Host for Advanced Li-S Batteries

One of the major challenges in the sulfur cathode of the Li-S batteries is to achieve high sulfur loading, fast Li ions transfer, and lithium polysulfides (LiPSs) shuttling suppressing simultaneously. This issue can be well solved by the development of molybdenum carbide decorated N-doped carbon hierarchical double-shelled hollow spheres (Mo2C/C HDS-HSs). The mesoporous thick inner shell and the central void of the HDS-HSs achieve the high sulfur loading, facilitate the ion/electrolyte penetration, and accelerate the charge transfer. The microporous thin outer shell suppresses the LiPSs shuttling and reduces the charge/mass diffusion distance. The double-shelled hollow structure accommodates the volume expansion during lithiation. Furthermore, Mo2C/C composition renders the HDS-HSs cathode with improved conductivity, enhanced affinity to LiPSs, and accelerated kinetics of LiPSs conversion. The structural and compositional advantages render the Mo2C/C/S HDS-HSs electrode with the high specific capacity, excellent rate capability, and ultra-long cycling stability in the composed Li-S batteries.

Triazine-based porous organic polymers with enhanced electronegativity as multifunctional separator coatings in lithium-sulfur batteries

The commercialization of lithium-sulfur batteries is seriously affected by the shuttle behavior and slow conversion kinetics of polysulfides. Herein, a new porous organic polymer (POP) is synthesized and grown on reduced graphene oxide (rGO) in situ to improve battery performance, which serves as an efficient polysulfide adsorber and catalytic promoter for polysulfide conversion. The polar POP shows strong chemisorption to polysulfides, which is confirmed by a series of calculations and experimental results. As a popular conductive substrate, rGO offers an electron transport channel for sulfur and polysulfide conversion. Due to the synergistic functions of composite materials, the batteries with POP@rGO modified separators retain a high specific capacity of 697.3 mA h g-1 and a minimum capacity fading rate of 0.04% per cycle at 1C over 500 cycles. Besides, even at a high sulfur loading of 5 mg cm-2, a high area capacity of 4.27 mA h cm-2 can also be achieved, which shows that it has great potential in promoting the commercialization of lithium-sulfur batteries.

Concentration-Dependent Solvation Structure and Dynamics of Aqueous Sulfuric Acid Using Multinuclear NMR and DFT

Sulfuric acid is a ubiquitous compound for industrial processes, and aqueous sulfate solutions also play a critical role as electrolytes for many prominent battery chemistries. While the thermodynamic literature on it is quite well-developed, comprehensive studies of the solvation structure, particularly molecular-scale dynamical and transport properties, are less available. This study applies a multinuclear nuclear magnetic resonance (NMR) approach to the elucidation of the solvation structure and dynamics over wide temperature (-10 to 50 °C) and concentration (0-18 M) ranges, combining the ¹⁷O shift, line width, and T₁ relaxation measurements, ³³S shift and line width measurements, and ¹H pulsed-field gradient NMR measurements of proton self-diffusivity. In conjunction, these results indicate a crossover between two regimes of solvation structure and dynamics, occurring above the concentration associated with the deep eutectic point (∼4.5 M), with the high-concentration regime dominated by a strong water-sulfate correlation. This description was borne out in detail by the activation energy trends with increasing concentration derived from the relaxation of both the H₂O/H₃O⁺ and H₂SO₄/HSO₄^-/SO₄^{2- 17}O resonances and the ¹H self-diffusivity. However, the ¹⁷O chemical shift difference between the H₂O/H₃O⁺ and H₂SO₄/HSO₄^-/SO₄^2- resonances across the entire temperature range is nevertheless strikingly linear. A computational approach coupling molecular dynamics simulations and density functional theory NMR shift calculations to reproduce this trend is presented, which will be the subject of further development. This combination of multinuclear, dynamical NMR, and computational methods, and the results furnished by this study, will provide a platform for future studies on battery electrolytes where aqueous sulfate chemistry plays a central role in the solution structure.

Tailoring Lithium Polysulfide Coordination and Clustering Behavior through Cationic Electrostatic Competition

The materials chemistry underlying lithium-sulfur (Li-S) batteries is uniquely dependent on the behavior of soluble lithium polysulfide intermediates, which form during operation and mediate the charge-transfer process in solution. The manner by which lithium polysulfides are solvated by surrounding solvent and salt compounds is a critical factor with regards to electrochemical utilization and reversibility of the sulfur active material. Particularly at low-temperature and lean electrolyte conditions, lithium polysulfides tend to coordinate with other polysulfide units in solution, forming large, aggregated clusters that stymie the electrochemical conversion process. However, the tendency to cluster is known to be influenced by the presence of strongly binding anionic species in solution, which present electrostatic competing interactions with Li+. The heightened electrostatic competition in turn can dissuade the formation of clustered Li+-Sx 2- bond networks. Here, we extend that understanding to the influence of distinct cationic species in solution, which can present analogous competing interactions with Sx 2- dianions to stymie polysulfide cluster formation. We find that introducing NH4 + cations into solution through an ammonium trifluoroacetate additive positively tailors the polysulfide coordination shell. This improves the electrochemical conversion kinetics at challenging lean electrolyte and subzero low-temperature conditions, and provides a more holistic understanding of polysulfide coordination behavior.

Flexible Quasi-Solid-State Composite Electrolyte of Poly (Propylene Glycol)-co-Pentaerythritol Triacry-Late/Li1.5Al0.5Ge1.5(PO4)3 for High-Performance Lithium-Sulfur Battery

With a higher theoretical specific capacity (1675 mAh g^-1) and energy density (2600 Wh kg^-1), the lithium-sulfur (Li-S) battery is considered as a promising candidate for a next-generation energy storage device. However, the shuttle effect of polysulfides as well as the large interfacial impedance between brittle solid electrolyte and electrodes lead to the capacity of the Li-S battery decaying rapidly, which limits the practical commercial applications of the Li-S battery. Herein, we reported a facile in situ ultraviolet (UV) curing method to prepare a flexible quasi-solid-state composite electrolyte (QSSCE) of poly(propylene glycol)-co-pentaerythritol triacrylate/Li_1.5Al_0.5Ge_1.5(PO₄)₃ (PPG-co-PETA/LAGP). By combining the high Li-ion conductivity and mechanical strength of inorganic NASICON-structure LAGP and good flexibility of the crosslinked PPG-co-PETA with nanopore structure, the flexible QSSCE with 66.85 wt% LAGP exhibited high Li-ion conductivity of 5.95 × 10^-3 S cm^-1 at 25 °C, Li-ion transference number of 0.83 and wide electrochemical window of ~5.0 V (vs. Li/Li⁺). In addition, the application of QSSCE in the Li-S battery could suppress the shuttle effect of polysulfides effectively, thus the Li-S battery possessed the excellent electrochemical cyclic performance, showing the first-cycle discharge-specific capacity of 1508.1 mAh g^-1, the capacity retention of 73.6% after 200 cycles with 0.25 C at 25 °C and good rate performance.

Thermodynamic aspect of sulfur, polysulfide anion and lithium polysulfide: plausible reaction path during discharge of lithium-sulfur battery

The elucidation of elemental redox reactions of sulfur is important for improving the performance of lithium-sulfur batteries. The energies of stable structures of S_n, S_n˙^-, S_n^2-, [LiS_n]^- and Li₂S_n (n = 1-8) were calculated at the CCSD(T)/cc-pVTZ//MP3/cc-pVDZ level. The heats of reduction reactions of S₈ and Li₂S_n with Li in the solid phase were estimated from the calculated energies and sublimation energies. The estimated heats of the redox reactions show that there are several redox reactions with nearly identical heats of reaction, suggesting that several reactions can proceed simultaneously at the same discharge voltage, although the discharging process was often explained by stepwise reduction reactions. The reduction reaction for the formation of Li₂S_n (n = 2-6 and 8) from S₈ normalized as a one electron reaction is more exothermic than that for the formation of Li₂S directly from S₈, while the reduction reactions for the formation of Li₂S from Li₂S_n are slightly less exothermic than that for the formation of Li₂S directly from S₈. If the reduction reactions with large exotherm occur first, these results suggest that the reduction reactions forming Li₂S_n (n = 2-6 and 8) from S₈ occur first, then Li₂S is formed, and therefore, a two-step discharge-curve is observed.

Understanding Structural and Transport Properties of Dissolved Li2 S8 in Ionic Liquid Electrolytes through Molecular Dynamics Simulations

Lithium-sulfur batteries with high energy density are considered as one of the most promising future energy storage devices. However, the parasitic lithium polysulfides shuttle phenomenon severely hinders the commercialization of such batteries. Ionic liquids have been found to suppress the lithium polysulfides solubility, diminishing the shuttle effect effectively. Herein, we performed classical molecular dynamics simulations to explore the microscopic mechanism and transport behaviors of typical Li₂ S₈ species in ionic liquids and ionic liquid-based electrolyte systems. We found that the trifluoromethanesulfonate anions ([OTf]^- ) exhibit higher coordination strength with lithium ions compared with bis(trifluoromethanesulfonyl)imide anions ([TFSI]^- ) in static microstructures. However, the dynamical characteristics indicate that the presence of the [OTf]^- anions in ionic liquid electrolytes bring faster Li⁺ exchange rate and easier dissociation of Li⁺ solvation structures. Our simulation models offer a significant guidance to future studies on designing ionic liquid electrolytes for lithium-sulfur batteries.

Recent Progress and Emerging Application Areas for Lithium-Sulfur Battery Technology

Electrification is progressing significantly within the present and future vehicle sectors such as large commercial vehicles (e.g., trucks and buses), high-altitude long endurance (HALE), high-altitude pseudosatellites (HAPS), and electric vertical take-off and landing (eVTOL). The battery systems' performance requirements differ across these applications in terms of power, cycle life, system cost, etc. However, the need for high gravimetric energy density, 400 Wh kg-1 and beyond, is common across them all, as it enables vehicles to achieve extended range, a longer mission duration, lighter weight, or increased payload. The system-level requirements of these emerging applications are broken down into the component-level developments required to integrate Li-S technology as the power system of choice. To adapt batteries' properties, such as energy and power density, to the respective application, the academic research community has a key role to play in component-level development. However, materials and component research must be conducted within the context of a viable Li-S cell system. Herein, the key performance benefits, limitations, modeling, and recent progress of the Li-S battery technology and its adaption toward real-world application are discussed.

In Situ Imaging Polysulfides Electrochemistry of Li-S Batteries in a Hollow Carbon Nanotubule Wet Electrochemical Cell

Understanding polysulfide electrochemistry is critical for mitigation of the polysulfide shuttle effect in Li-S batteries. However, in situ imaging polysulfides evolution in Li-S batteries has not been possible. Herein, we constructed a hollow carbon nanotubule (CNT) wet electrochemical cell that permits real-time imaging of polysulfide evolutions in Li-S batteries in a Cs-corrected environmental transmission electron microscope. Upon discharge, sulfur was electrochemically reduced to long-chain polysulfides, which dissolved into the electrolyte instantly and were stabilized by Py₁₄⁺ cations solvation. Metastable polysulfides prove to be problematic for Li-S batteries, therefore, destabilizing the Py₁₄⁺-solvated polysulfides by adding low polarized solvents into the electrolyte to weaken the interaction between Py₁₄⁺ cation and long-chain polysulfides renders a rapid polysulfides-to-Li₂S transition, thus efficiently mitigating polysulfide formation and improving the performance of Li-S batteries dramatically. Moreover, the CNT wet electrochemical cell proves to be a universal platform for in situ probing electrochemistry of various batteries.

Janus Conductive/Insulating Microporous Ion-Sieving Membranes for Stable Li-S Batteries

Lithium-sulfur batteries are one of the most promising next-generation high-density energy storage systems. Despite progress, the poor electrical conductivity and cycling stability of sulfur cathodes still hinder their practical implementation. Here, we developed a facile approach for the engineering of Janus double-sided conductive/insulating microporous ion-sieving membranes that significantly enhance recharge efficiency and long-term stability of Li-S batteries. Our membrane consists of an insulating Li-anode side and an electrically conductive S-cathode side. The insulating side consists of a standard polypropylene separator, while the conductive side is made of closely packed multilayers of high-aspect-ratio MOF/graphene nanosheets having a thickness of few nanometers and a specific surface area of 996 m² g^-1 (MOF, metal-organic framework). Our models and experiments reveal that this electrically conductive microporous nanosheet architecture enables the reuse of polysulfide trapped in the membrane and decreases the polysulfide flux and concentration on the anode side by a factor of 250× over recent microporous membranes made of granular MOFs and standard battery separators. Notably, Li-S batteries using our Janus microporous membranes achieve an outstanding rate capability and long-term stability with 75.3% capacity retention over 1700 cycles. We demonstrate the broad applicability of our high-aspect-ratio MOF/graphene nanosheet preparation strategy by the synthesis of a diverse range of MOFs, including ZIF-67, ZIF-8, HKUST-1, NiFe-BTC, and Ni-NDC, providing a flexible approach for the design of Janus microporous membranes and electrically conductive microporous building blocks for energy storage and various other electrochemical applications.

Operando soft X-ray absorption spectroscopic study on microporous carbon-supported sulfur cathodes

Sulfur is a promising material for next-generation cathodes, owing to its high energy and low cost. However, sulfur cathodes have the disadvantage of serious cyclability issues due to the dissolution of polysulfides that form as intermediate products during discharge/charge cycling. Filling sulfur into the micropores of porous carbon is an effective method to suppress its dissolution. Although microporous carbon-supported sulfur cathodes show an electrochemical behavior different from that of the conventional sulfur ones, the corresponding reaction mechanism is not clearly understood. In this study, we focused on clarifying the reaction mechanism of microporous carbon-supported sulfur cathodes by operando soft X-ray absorption spectroscopy. In the microporous carbon support, sulfur was present as smaller fragments compared to conventional sulfur. During the first discharge process, the sulfur species in the microporous carbon were initially reduced to S₆²⁻ and S₂²⁻ and then to Li₂S. The S₆²⁻ and S₂²⁻ species were observed first, with S₂²⁻ being the main polysulfide species during the discharge process, while Li₂S was produced in the final discharge process. The narrow pores of microporous carbon prevent the dissolution of polysulfides and influence the reaction mechanism of sulfur cathodes.

The reaction mechanism of the sulfur cathode in the microporous carbon during discharge was observed by operando XAS.

Dual redox mediators accelerate the electrochemical kinetics of lithium-sulfur batteries

The sluggish electrochemical kinetics of sulfur species has impeded the wide adoption of lithium-sulfur battery, which is one of the most promising candidates for next-generation energy storage system. Here, we present the electronic and geometric structures of all possible sulfur species and construct an electronic energy diagram to unveil their reaction pathways in batteries, as well as the molecular origin of their sluggish kinetics. By decoupling the contradictory requirements of accelerating charging and discharging processes, we select two pseudocapacitive oxides as electron-ion source and drain to enable the efficient transport of electron/Li⁺ to and from sulfur intermediates respectively. After incorporating dual oxides, the electrochemical kinetics of sulfur cathode is significantly accelerated. This strategy, which couples a fast-electrochemical reaction with a spontaneous chemical reaction to bypass a slow-electrochemical reaction pathway, offers a solution to accelerate an electrochemical reaction, providing new perspectives for the development of high-energy battery systems.

The sluggish electrochemical kinetics of sulfur species remains a major hurdle for the broad adoption of lithium-sulfur batteries. Here, the authors construct an energy diagram of sulfur species to unveil their reaction pathways and propose a general strategy to accelerate electrochemical reactions.

Dynamic single-site polysulfide immobilization in long-range disorder Cu-MOFs

The structural transformation of MOFs in a polysulfide electrode process is poorly understood. We report the electrochemical amorphization of Cu3(BTC)2 MOFs in polysulfide electrolyte. We unveil the dynamic single-site polysulfide immobilization at the interconvertible Cu2+/Cu+ cation centres upon polysulfide adsorption and desorption, along with the reversible distortion of the Cu-O square planar unit.

A DFT Investigation on the Origins of Solvent-Dependent Polysulfide Reduction Mechanism in Rechargeable Li-S Batteries

The lithium-sulfur (Li-S) battery is one of the promising energy storage alternatives because of its high theoretical capacity and energy density. Factors governing the stability of polysulfide intermediates in Li-S batteries are complex and are strongly affected by the solvent used. Herein, the polysulfide reduction and the bond cleavage reactions are calculated in different solvent environments by the density functional theory (DFT) methods. We investigate the relationship between the donor numbers (DN) as well as the dielectric constants (ε) of the solvent system and the relative stability of different polysulfide intermediates. Our results show that the polysulfide reduction mechanism is dominated by its tendency to form the ion-pair with Li+ in different organic solvents.

Probing sulphur clusters in a microfluidic electrochemical cell with synchrotron-based photoionization mass spectrometry

We present synchrotron-based mass spectrometry to probe products formed in a lithium sulphide electrolyte. In operando analysis was carried out at two different potentials in a vacuum compatible microfluidic electrochemical cell. Mass spectral observations show that the charged electrolyte formed sulphur clusters under dynamic conditions, demonstrating electrolyte electron shuttling.

A Micelle Electrolyte Enabled by Fluorinated Ether Additives for Polysulfide Suppression and Li Metal Stabilization in Li-S Battery

The Li-S battery is a promising next-generation technology due to its high theoretical energy density (2600 Wh kg⁻¹) and low active material cost. However, poor cycling stability and coulombic efficiency caused by polysulfide dissolution have proven to be major obstacles for a practical Li-S battery implementation. In this work, we develop a novel strategy to suppress polysulfide dissolution using hydrofluoroethers (HFEs) with bi-functional, amphiphlic surfactant-like design: a polar lithiophilic “head” attached to a fluorinated lithiophobic “tail.” A unique solvation mechanism is proposed for these solvents whereby dissociated lithium ions are readily coordinated with lithiophilic “head” to induce self-assembly into micelle-like complex structures. Complex formation is verified experimentally by changing the additive structure and concentration using small angle X-ray scattering (SAXS). These HFE-based electrolytes are found to prevent polysulfide dissolution and to have excellent chemical compatibility with lithium metal: Li||Cu stripping/plating tests reveal high coulombic efficiency (>99.5%), modest polarization, and smooth surface morphology of the uniformly deposited lithium. Li-S cells are demonstrated with 1395 mAh g⁻¹ initial capacity and 71.9% retention over 100 cycles at >99.5% efficiency—evidence that the micelle structure of the amphiphilic additives in HFEs can prohibit polysulfide dissolution while enabling facile Li⁺ transport and anode passivation.

Probing Sulfur Chemical and Electronic Structure with Experimental Observation and Quantitative Theoretical Prediction of Kα and Valence-to-Core Kβ X-ray Emission Spectroscopy

An extensive experimental and theoretical study of the Kα and Kβ high-resolution X-ray emission spectroscopy (XES) of sulfur-bearing systems is presented. This study encompasses a wide range of organic and inorganic compounds, including numerous experimental spectra from both prior published work and new measurements. Employing a linear-response time-dependent density functional theory (LR-TDDFT) approach, strong quantitative agreement is found in the calculation of energy shifts of the core-to-core Kα as well as the full range of spectral features in the valence-to-core Kβ spectrum. The ability to accurately calculate the sulfur Kα energy shift supports the use of sulfur Kα XES as a bulk-sensitive tool for assessing sulfur speciation. The fine structure of the sulfur Kβ spectrum, in conjunction with the theoretical results, is shown to be sensitive to the local electronic structure including effects of symmetry, ligand type and number, and, in the case of organosulfur compounds, to the nature of the bonded organic moiety. This agreement between theory and experiment, augmented by the potential for high-access XES measurements with the latest generation of laboratory-based spectrometers, demonstrates the possibility of broad analytical use of XES for sulfur and nearby third-row elements. The effective solution of the forward problem, i.e., successful prediction of detailed spectra from known molecular structure, also suggests future use of supervised machine learning approaches to experimental inference, as has seen recent interest for interpretation of X-ray absorption near-edge structure (XANES).

Atomically Engineered Transition Metal Dichalcogenides for Liquid Polysulfide Adsorption and Their Effective Conversion in Li-S Batteries

Curtailing the polysulfide shuttle by anchoring the intermediate lithium polysulfides (LiPS) within the electrode structure is essential to impede the rapid capacity fade in lithium-sulfur (Li-S) batteries. While most of the contemporary Li-S cathode surfaces are capable of entrapping certain LiPS, developing a unique electrode material that can adsorb all the intermediates of sulfur redox is imperative. Herein, we report doping of the MoS₂ atomic structure with nickel (Ni@1TMoS₂) to modulate its absorption capability toward all LiPS and function as an electrocatalyst for Li-S redox. Detailed in situ and ex situ spectroscopic analysis revealed that both Ni and Mo sites chemically anchor all the intermediate of LiPS. Electrochemical studies and detailed kinetics analysis suggested that the conversion of liquid LiPS to solid end products are facilitated on the Ni@1TMoS₂ electrocatalytic surface. Further, the employment of the Ni@1TMoS₂ electrocatalyst enhances the Li⁺ diffusion coefficient, thus contributing to the realization of a high capacity of 1107 mA h g^-1 at 0.2C with a very limited capacity fade of 0.19% per cycle for over 100 cycles. In addition, this cathode demonstrated an excellent high rate and long cycling performance for over 300 cycles at a 1C rate.

Influence of Lithium Polysulfide Clustering on the Kinetics of Electrochemical Conversion in Lithium-Sulfur Batteries

The electrochemistry of lithium-sulfur (Li-S) batteries is heavily reliant on the structure and dynamics of lithium polysulfides, which dissolve into the liquid electrolyte and mediate the electrochemical conversion process during operation. This behavior is considerably distinct from the widely used lithium-ion batteries, necessitating new mechanistic insights to fully understand the electrochemical phenomena. Testing at low-temperature conditions presents a unique opportunity to glean new insights into the chemistry in kinetically constrained environments. Under such conditions, despite the low freezing point and favorable ionic conductivity of the glyme-based electrolyte, Li-S batteries exhibit counterintuitively poor performance. Here, we show that beyond just existing in single-molecule conformations, lithium polysulfides tend to cluster and aggregate in solution, particularly at low-temperature conditions, which subsequently constrains the kinetics of electrochemical conversion. Energetics and coordination implications of this behavior are extended towards a new framework for understanding the solution-coordination dynamics of dissolved lithium species. Based off this framework, a favorable strongly-bound lithium salt is introduced in the Li-S electrolyte to disrupt polysulfide clustered networks, enabling substantially enhanced low-temperature electrochemical performance. More broadly, this mechanistic insight heightens our understanding of polysulfide chemistry irrespective of temperature, confirming the link between the solution conformation of active material and electrochemical behavior.

Li2S growth on graphene: Impact on the electrochemical performance of Li-S batteries

Lithium-sulfur batteries show remarkable potential for energy storage applications due to their high-specific capacity and the low cost of active materials, especially sulfur. However, whereas there is a consensus about the use of lithium metal as the negative electrode, there is not a clear and widely accepted architectural design for the positive electrode of sulfur batteries. The difficulties arise when trying to find a balance between high-surface-area architectures and practical utilization of the sulfur content. Intensive understanding of the interfacial mechanisms becomes then crucial to design optimized carbon-hosted sulfur architectures with enhanced electrochemical performance. In this work, we use density functional theory (DFT)-based first principles calculations to describe and characterize the growing mechanisms of Li₂S active material on graphene, taken as an example of a nonencapsulated carbon host for the positive electrode of Li-S batteries. We first unravel the two growing mechanisms of Li₂S supported nanostructures, which explain recent experimental findings on real-time monitoring of interfacial deposition of lithium sulfides during discharge, obtained by means of in situ atomic force microscopy. Then, using a combination of mathematical tools and DFT calculations, we obtain the first cycle voltage plot, explaining the three different regions observed that ultimately lead to the formation of high-order polysulfides upon charge. Finally, we show how the different Li₂S supported nanostructures can be characterized in X-ray photoelectron spectroscopy measurements. Altogether, this work provides useful insights for the rational design of new carbon-hosted sulfur architectures with optimized characteristics for the positive electrode of lithium-sulfur batteries.