Healing High-Loading Sulfur Electrodes with Unprecedented Long Cycling Life: Spatial Heterogeneity Control

Kinetic Evaluation on Lithium Polysulfide in Weakly Solvating Electrolyte toward Practical Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are highly considered as next-generation energy storage techniques. Weakly solvating electrolyte with low lithium polysulfide (LiPS) solvating power promises Li anode protection and improved cycling stability. However, the cathodic LiPS kinetics is inevitably deteriorated, resulting in severe cathodic polarization and limited energy density. Herein, the LiPS kinetic degradation mechanism in weakly solvating electrolytes is disclosed to construct high-energy-density Li-S batteries. Activation polarization instead of concentration or ohmic polarization is identified as the dominant kinetic limitation, which originates from higher charge-transfer activation energy and a changed rate-determining step. To solve the kinetic issue, a titanium nitride (TiN) electrocatalyst is introduced and corresponding Li-S batteries exhibit reduced polarization, prolonged cycling lifespan, and high actual energy density of 381 Wh kg-1 in 2.5 Ah-level pouch cells. This work clarifies the LiPS reaction mechanism in protective weakly solvating electrolytes and highlights the electrocatalytic regulation strategy toward high-energy-density and long-cycling Li-S batteries.

Development of Electrolytes under Lean Condition in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries stand out as one of the promising candidates for next-generation electrochemical energy storage technologies. A key requirement to realize high-specific-energy Li-S batteries is to implement low amount of electrolyte, often characterized by the electrolyte/sulfur (E/S) ratio. Low E/S ratio aggravates the known challenges for Li-S batteries and introduces new ones originated from the high concentration of polysulfides in limited electrolyte reservoir. In this review, the connections between the fundamental properties of electrolytes and the electrochemical/chemical reactions in Li-S batteries under lean electrolyte condition are elucidated. The emphasis is on how the solvating properties of the electrolyte affect the fate of polysulfides. Built upon the mechanistic analysis, different strategies to design lean electrolytes to improve the overall process of Li-S reactions and Li anode protection are discussed.

New Emerging Fast Charging Microscale Electrode Materials

Fast charging lithium (Li)-ion batteries are intensively pursued for next-generation energy storage devices, whose electrochemical performance is largely determined by their constituent electrode materials. While nanosizing of electrode materials enhances high-rate capability in academic research, it presents practical limitations like volumetric packing density and high synthetic cost. As an alternative to nanosizing, microscale electrode materials cannot only effectively overcome the limitations of the nanosizing strategy but also satisfy the requirement of fast-charging batteries. Therefore, this review summarizes the new emerging microscale electrode materials for fast charging from the commercialization perspective. First, the fundamental theory of electronic/ionic motion in both individual active particles and the whole electrode is proposed. Then, based on these theories, the corresponding optimization strategies are summarized toward fast-charging microscale electrode materials. In addition, advanced functional design to tackle the mechanical degradation problems related to next generation high capacity alloy- and conversion-type electrode materials (Li, S, Si et al.) for achieving fast charging and stable cycling batteries. Finally, general conclusions and the future perspective on the potential research directions of microscale electrode materials are proposed. It is anticipated that this review will provide the basic guidelines for both fundamental research and practical applications of fast-charging batteries.

Bio-inspired carbon electrodes for metal-ion batteries

Carbon has been widely used as an electrode material in commercial metal-ion batteries (MIBs) because of its desirable electrical, mechanical, and physical properties. Still, traditional carbon electrodes suffer from limited mechanical stability and electrochemical performance in MIBs. Drawing inspiration from biological species, the carbon allotropes, such as fullerenes, carbon nanotubes, and graphene, can be engineered into mechanically robust, highly conductive frameworks with enhanced ion storage and transport capabilities for MIBs. Here, we present an assortment of bio-inspired carbon electrodes that have enhanced the cycling stability, capacity retention, and overall performance of MIBs. In addition, mimicking the structure and functionality of biological systems has led to the development of flexible MIBs whose performance does not degrade even when stretched, bent, or twisted. Finite element analysis (FEA) is a useful guide in identifying such bio-inspired carbon frameworks because it can simulate and analyze potential failure scenarios, such as stress build-up or structural collapse in MIBs. This review highlights through several examples that there is much scope for improving carbon-based electrode materials through bio-inspired designs for practical high-performance MIBs.

Layered S-Bridged Covalent Triazine Frameworks via a Bifunctional Template-Catalytic Strategy Enabling High-Performance Zinc-Ion Hybrid Supercapacitors

Exploring covalent triazine frameworks (CTFs) with high capacitative activity is highly desirable and challenging. Herein, the S-rich CTFs cathode is pioneeringly introduced in Zn-ion hybrid supercapacitors (ZSC), achieving outstanding capacity and energy density, and satisfactory anti-freezing flexibility. Specifically, the S-bridged CTFs are synthesized by a bifunctional template-catalytic strategy, where ZnCl2 serves as both the catalyst/solvent and in situ template to construct triazine frameworks with interconnected pores and layered gaps. The resultant CTFs (CTFS-750) are employed as a reasonable pattern-like system to more deeply scrutinize the synergistic effect of S-bridged triazine and layered porous architecture for polymer-based cathodes in Zn-ion storage. The experimental results indicate that the adsorption barriers of Zn-ions on CTFS-750 are effectively weakened, and accessible Zn2+ -absorption sites provided by the C─S─C and C═N bonds have been confirmed via DFT calculations. Consequently, the CTFS-750 cathode-assembled ZSC displays an ultra-high capacity of 211.6 mAh g-1 at 1.0 A g-1 , an outstanding energy density of 202.7 Wh kg-1 , and attractive cycling performance. Moreover, the resulting flexible ZSC device shows superior capacity, good adaptability, and satisfactory anti-freezing behavior. This approach sheds new light on constructing advanced polymer-based cathodes at the atom level and paves the way for fabricating high-performance ZSC and beyond.

Kinetic Promoters for Sulfur Cathodes in Lithium-Sulfur Batteries

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

Metal-organic-framework derived NiS2/C hollow structures for enhanced polysulfide redox kinetics in lithium-sulfur batteries

To cope with the shuttling of soluble lithium polysulfides in lithium-sulfur batteries, confinement tactics, such as trapping of sulfur within porous carbon structures, have been extensively studied. Although performance has improved a bit, the slow polysulfide conversion inducing fast capacity decay remains a big challenge. Herein, a NiS2/carbon (NiS2/C) composite with NiS2 nanoparticles embedded in a thin layer of carbon over the surface of micro-sized hollow structures has been prepared from Ni-metal-organic frameworks. These unique structures can physically entrap sulfur species and also influence their redox conversion kinetics. By improving the reaction kinetics of polysulfides, the NiS2/carbon@sulfur (NiS2/C@S) composite cathode with a suppressed shuttle effect shows a high columbic efficiency and decent rate performance. An initial capacity of 900 mAh g-1 at the rate of 1 C (1 C = 1675 mA g-1) and a low-capacity decline rate of 0.132% per cycle after 500 cycles are obtained, suggesting that this work provides a rational design of a sulfur cathode.

Boosting Lean Electrolyte Lithium-Sulfur Battery Performance with Transition Metals: A Comprehensive Review

Lithium-sulfur (Li-S) batteries have received widespread attention, and lean electrolyte Li-S batteries have attracted additional interest because of their higher energy densities. This review systematically analyzes the effect of the electrolyte-to-sulfur (E/S) ratios on battery energy density and the challenges for sulfur reduction reactions (SRR) under lean electrolyte conditions. Accordingly, we review the use of various polar transition metal sulfur hosts as corresponding solutions to facilitate SRR kinetics at low E/S ratios (< 10 µL mg-1), and the strengths and limitations of different transition metal compounds are presented and discussed from a fundamental perspective. Subsequently, three promising strategies for sulfur hosts that act as anchors and catalysts are proposed to boost lean electrolyte Li-S battery performance. Finally, an outlook is provided to guide future research on high energy density Li-S batteries.

Application of Inorganic Quantum Dots in Advanced Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have emerged as one of the most attractive alternatives for post-lithium-ion battery energy storage systems, owing to their ultrahigh theoretical energy density. However, the large-scale application of Li-S batteries remains enormously problematic because of the poor cycling life and safety problems, induced by the low conductivity , severe shuttling effect, poor reaction kinetics, and lithium dendrite formation. In recent studies, catalytic techniques are reported to promote the commercial application of Li-S batteries. Compared with the conventional catalytic sites on host materials, quantum dots (QDs) with ultrafine particle size (<10 nm) can provide large accessible surface area and strong polarity to restrict the shuttling effect, excellent catalytic effect to enhance the kinetics of redox reactions, as well as abundant lithiophilic nucleation sites to regulate Li deposition. In this review, the intrinsic hurdles of S conversion and Li stripping/plating reactions are first summarized. More importantly, a comprehensive overview is provided of inorganic QDs, in improving the efficiency and stability of Li-S batteries, with the strategies including composition optimization, defect and morphological engineering, design of heterostructures, and so forth. Finally, the prospects and challenges of QDs in Li-S batteries are discussed.

Intrinsic Self-Healing Chemistry for Next-Generation Flexible Energy Storage Devices

The booming wearable/portable electronic devices industry has stimulated the progress of supporting flexible energy storage devices. Excellent performance of flexible devices not only requires the component units of each device to maintain the original performance under external forces, but also demands the overall device to be flexible in response to external fields. However, flexible energy storage devices inevitably occur mechanical damages (extrusion, impact, vibration)/electrical damages (overcharge, over-discharge, external short circuit) during long-term complex deformation conditions, causing serious performance degradation and safety risks. Inspired by the healing phenomenon of nature, endowing energy storage devices with self-healing capability has become a promising strategy to effectively improve the durability and functionality of devices. Herein, this review systematically summarizes the latest progress in intrinsic self-healing chemistry for energy storage devices. Firstly, the main intrinsic self-healing mechanism is introduced. Then, the research situation of electrodes, electrolytes, artificial interface layers and integrated devices based on intrinsic self-healing and advanced characterization technology is reviewed. Finally, the current challenges and perspective are provided. We believe this critical review will contribute to the development of intrinsic self-healing chemistry in the flexible energy storage field.

Electric Eel Biomimetics for Energy Storage and Conversion

The electric eel is known as the most powerful creature to generate electricity with a discharge voltage up to 860 V and peak current up to 1 A. These surprising properties are the results of billions of years of evolution on the electrical biological structure and bulk, and now have triggered great research interest in electric eel biomimetics for designing innovated configurations and components of energy storage and conversion devices. In this review, first, the bioelectrical behavior of electric eels is surveyed, followed by the physiological structure to reveal the discharge characteristics and principles of electric organs and electrocytes. Additionally, underlying electrochemical mechanisms and models for calculating the potential and current of electrocytes are presented. Central to this review is the recent progress of electric-eel-inspired innovations and applications for energy storage and conversion, particularly including novel power sources, triboelectric nanogenerators, and nanochannel ion-selective membranes for salinity gradient energy harvesting. Finally, insights on the challenges at the moment and the perspectives on the future research prospects are critically compiled. It is suggested that energy-related electric eel biomimetics will greatly boost the development of next-generation high performance, green, and functional electronics.

Interface Engineering Toward Expedited Li2 S Deposition in Lithium-Sulfur Batteries: A Critical Review

Lithium-sulfur batteries (LSBs) with superior energy density are among the most promising candidates of next-generation energy storage techniques. As the key step contributing to 75% of the overall capacity, Li₂ S deposition remains a formidable challenge for LSBs applications because of its sluggish kinetics. The severe kinetic issue originates from the huge interfacial impedances, indicative of the interface-dominated nature of Li₂ S deposition. Accordingly, increasing efforts have been devoted to interface engineering for efficient Li₂ S deposition, which has attained inspiring success to date. However, a systematic overview and in-depth understanding of this critical field are still absent. In this review, the principles of interface-controlled Li₂ S precipitation are presented, clarifying the pivotal roles of electrolyte-substrate and electrolyte-Li₂ S interfaces in regulating Li₂ S depositing behavior. For the optimization of the electrolyte-substrate interface, efforts on the design of substrates including metal compounds, functionalized carbons, and organic compounds are systematically summarized. Regarding the regulation of electrolyte-Li₂ S interface, the progress of applying polysulfides catholytes, redox mediators, and high-donicity/polarity electrolytes is overviewed in detail. Finally, the challenges and possible solutions aiming at optimizing Li₂ S deposition are given for further development of practical LSBs. This review would inspire more insightful works and, more importantly, may enlighten other electrochemical areas concerning heterogeneous deposition processes.

Toward Practical High-Energy-Density Lithium-Sulfur Pouch Cells: A Review

Lithium-sulfur (Li-S) batteries promise great potential as high-energy-density energy-storage devices due to their ultrahigh theoretical energy density of 2600 Wh kg^-1 . Evaluation and analysis on practical Li-S pouch cells are essential for achieving actual high energy density under working conditions and affording developing directions for practical applications. This review aims to afford a comprehensive overview of high-energy-density Li-S pouch cells regarding 7 years of development and to point out further research directions. Key design parameters to achieve actual high energy density are addressed first, to define the research boundaries distinguished from coin-cell-level evaluation. Systematic analysis of the published literature and cutting-edge performances is then conducted to demonstrate the achieved progress and the gap toward practical applications. Following that, failure analysis as well as promotion strategies at the pouch cell level are, respectively, discussed to reveal the unique working and failure mechanism that shall be accordingly addressed. Finally, perspectives toward high-performance Li-S pouch cells are presented regarding the challenges and opportunities of this field.

TiH2 Nanodots Exfoliated via Facile Sonication as Bifunctional Electrocatalysts for Li-S Batteries

Mediating the redox kinetics of polysulfides is a promising strategy to mitigate the shuttling and sluggish conversion of polysulfides for practical application of lithium-sulfur (Li-S) batteries. Herein, novel TiH₂ nanodots (THNDs) fabricated by sonication-assisted liquid-phase exfoliation are used as bifunctional electrocatalysts for Li-S batteries. Both experimental and theoretical results reveal that THNDs can not only provide a strong chemical affinity to polysulfides but also bidirectionally promote the precipitation/decomposition of Li₂S from/to polysulfides during the discharge/charge process, thus effectively suppressing the shuttle effect and improving the redox kinetics of polysulfides. Owing to these advantages accompanied by the abundant catalytically active sites of THNDs, the assembled Li-S batteries deliver a low capacity fading rate of 0.055% per cycle over 1000 cycles at 1C and a high areal capacity of 5.38 mAh cm^-2 after 50 cycles with a high sulfur loading of 8.5 mg cm^-2. This work demonstrates the great potential of utilizing functional metal hydrides as effective electrocatalysts for Li-S batteries, which will incite more investigation into the specific selection of metal compounds to boost the redox kinetics of polysulfides.

Bioinspired Materials for Energy Storage

Nature offers a variety of interesting structures and intriguing functions for researchers to be learnt for advanced materials innovations. Recently, bioinspired materials have received intensive attention in energy storage applications. Inspired by various natural species, many new configurations and components of energy storage devices, such as rechargeable batteries and supercapacitors, have been designed and innovated. The bioinspired designs on energy devices, such as electrodes and electrolytes, have brought about excellent physical, chemical, and mechanical properties compared to the counterparts at their conventional forms. In this review, the design principles for bioinspired materials ranging from structures, synthesis, and functionalization to multi-scale ordering and device integration are first discussed, and then a brief summary is given on the recent progress on bioinspired materials for energy storage systems, particularly the widely studied rechargeable batteries and supercapacitors. Finally, a critical review on the current challenges and brief perspective on the future research focuses are proposed. It is expected that this review can offer some insights into the smart energy storage system design by learning from nature.

Ni3FeN functionalized carbon nanofibers boosting polysulfide conversion for Li–S chemistry

Limiting the shuttle effect of polysulfides is an important means to realizing high energy density lithium–sulfur batteries (Li–S).

Surface Gelation on Disulfide Electrocatalysts in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are deemed as future energy storage devices due to ultrahigh theoretical energy density. Cathodic polysulfide electrocatalysts have been widely investigated to promote sluggish sulfur redox kinetics. Probing the surface structure of electrocatalysts is vital to understanding the mechanism of polysulfide electrocatalysis. In this work, we for the first time identify surface gelation on disulfide electrocatalysts. Concretely, the Lewis acid sites on disulfides trigger the ring-opening polymerization of the dioxolane solvent to generate a surface gel layer, covering disulfides and reducing the electrocatalytic activity. Accordingly, a Lewis base triethylamine (TEA) is introduced as a competitive inhibitor. Consequently, Li-S batteries with disulfide electrocatalysts and TEA afford high specific capacity and improved rate responses. This work affords new insights on the actual surface structure of electrocatalysts in Li-S batteries.

Revamping Lithium-Sulfur Batteries for High Cell-Level Energy Density by Synergistic Utilization of Polysulfide Additives and Artificial Solid-Electrolyte Interphase Layers

Despite the high theoretical capacity of lithium-sulfur (Li-S) batteries, a high cell-level energy density and a long cycling life are barely achieved, mainly due to the large electrolyte-to-sulfur ratio, polysulfide (PS) shuttle causing the loss of active sulfur, and the formation of passivation layers on the Li anode. To raise the energy density, holding PS in the cathode has been the most popular approach. Still, it has failed, particularly, when the sulfur loading is high enough to have energy densities similar to those of commercial Li-ion batteries. Here, a practical approach of achieving high "cell-level" energy densities is attempted using lithium PS (LPS)-containing electrolytes instead of a pure electrolyte, reducing the electrolyte-to-sulfur ratio and PS diffusion out of the cathode due to concentration differences. Meanwhile, the persistent problems including PS passivation and Li dendrites are suppressed using Li₂ S-phobic artificial solid-electrolyte interphase (A-SEI) layers on Li metal. The synergistic effects from the LPS additives and A-SEI result in a superior cell-level volumetric energy density of 650 Wh L^-1 as well as large cumulative energy densities considering cycling life. This approach provides an important stepping stone to realize commercial Li-S batteries rivaling the current Li-ion batteries.

Efficient Polysulfide Trapping and Conversion on N-Doped CoTe2 via Enhanced Dual-Anchoring Effect

Polysulfide shuttling and sluggish sulfur redox kinetics hinder the cyclability and rate capability of lithium-sulfur (Li-S) batteries. The intrinsic redox kinetics of sulfur cathodes strongly depends on the interaction between catalysts and sulfur species. Herein, N-doped CoTe₂ is proposed as an effective dual-anchoring electrocatalyst, which can simultaneously bind Li and S atoms in lithium polysulfides via ionic Te-Li/N-Li bonding and coordinate covalent Co-S bonding. The incorporated N not only serves as enhanced lithiophilic site, but also an agent to improve the sulfiphilicity of the Co site as revealed by a series of experimental and computational results. Benefiting from these superiorities, the use of N-doped CoTe₂ as a catalytic interlayer enables efficient operation of Li-S batteries in terms of impressive rate capability of 758 mAh g^-1 at 4 C and very low capacity decay of 0.021% per cycle over 1000 cycles. The material and strategy demonstrated in this work may open the door toward developing more advanced Li-S electrocatalysts.

Electrode Architecture Design to Promote Charge-Transport Kinetics in High-Loading and High-Energy Lithium-Based Batteries

Rechargeable lithium-ion batteries have built much of our modern society. Developing high-loading and high-energy batteries have become an inevitable trend to satisfy the ever-growing demand of energy consumption. However, issues related to mechanical instability and electrochemical polarization have become more prominent accompanying the increase of electrode thickness. How to establish a robust and rapid charge transport network within the electrode architecture plays a vital role for the mechanical property and the reaction dynamics of thick electrodes. In this review, principles of charge transport mechanism and challenges of thick electrode development are elaborated. Next, recent progress on advanced electrode architecture design focused on structural engineering is summarized. Finally, a transmission line model is proposed as an effective tool to guide the engineering of thick electrodes.

Ultralight Electrolyte for High-Energy Lithium-Sulfur Pouch Cells

The high weight fraction of the electrolyte in lithium-sulfur (Li-S) full cell is the primary reason its specific energy is much below expectations. Thus far, it is still a challenge to reduce the electrolyte volume of Li-S batteries owing to their high cathode porosity and electrolyte depletion from the Li metal anode. Herein, we propose an ultralight electrolyte (0.83 g mL^-1 ) by introducing a weakly-coordinating and Li-compatible monoether, which greatly reduces the weight fraction of electrolyte within the whole cell and also enables Li-S pouch cell functionality under lean-electrolyte conditions. Compared to Li-S batteries using conventional counterparts (≈1.2 g mL^-1 ), the Li-S pouch cells equipped with our ultralight electrolyte could achieve an ultralow electrolyte weight/capacity ratio (E/C) of 2.2 g Ah^-1 and realize a 19.2 % improvement in specific energy (from 329.9 to 393.4 Wh kg^-1 ) under E/S=3.0 μL mg^-1 . Moreover, more than 20 % improvement in specific energy could be achieved using our ultralight electrolyte at various E/S ratios.

An Organodiselenide Comediator to Facilitate Sulfur Redox Kinetics in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are considered as promising next-generation energy storage devices due to their ultrahigh theoretical energy density, where soluble lithium polysulfides are crucial in the Li-S electrochemistry as intrinsic redox mediators. However, the poor mediation capability of the intrinsic polysulfide mediators leads to sluggish redox kinetics, further rendering limited rate performances, low discharge capacity, and rapid capacity decay. Here, an organodiselenide, diphenyl diselenide (DPDSe), is proposed to accelerate the sulfur redox kinetics as a redox comediator. DPDSe spontaneously reacts with lithium polysulfides to generate lithium phenylseleno polysulfides (LiPhSePSs) with improved redox mediation capability. The as-generated LiPhSePSs afford faster sulfur redox kinetics and increase the deposition dimension of lithium sulfide. Consequently, the DPDSe comediator endows Li-S batteries with superb rate performance of 817 mAh g^-1 at 2 C and remarkable cycling stability with limited anode excess. Moreover, Li-S pouch cells with the DPDSe comediator achieve an actual initial energy density of 301 Wh kg^-1 and 30 stable cycles. This work demonstrates a novel redox comediation strategy with an effective organodiselenide comediator to facilitate the sulfur redox kinetics under pouch cell conditions and inspires further exploration in mediating Li-S kinetics for practical high-energy-density batteries.

Cationic covalent-organic framework for sulfur storage with high-performance in lithium-sulfur batteries

Covalent organic frameworks (COFs) with pre-designed structure and customized properties have been employed as sulfur storage materials for lithium-sulfur (Li-S) batteries. In this work, a cationic mesoporous COF (COF-NI) was synthesized by grafting a quaternary ammonium salt group onto the pore channel of COFs via a one-pot three components tandem reaction strategy. The post-functionalized COFs were utilized as the matrix framework to successfully construct the Li-S battery with high-speed capacity and long-term stability. The experimental results showed that, after loading active material sulfur, cationic COF-NI effectively suppressed the shuttle effect of the intermediate lithium polysulfide species in Li-S batteries, and exhibited better cycle stability than the as-obtained neutral COF (COF-Bu). For example, compared with COF-Bu based sulfur cathode (521 mA h g^-1), the cationic COF-NI based sulfur cathode maintained a discharge capacity of 758 mA h g^-1 after 100 cycles. These results clearly showed that appropriate pore environment of COFs can be prepared by rational design, which can reduce the shuttle effect of lithium polysulfide species and improve the performance of Li-S battery.

ZnSe/N-Doped Carbon Nanoreactor with Multiple Adsorption Sites for Stable Lithium-Sulfur Batteries

To commercially realize the enormous potential of lithium-sulfur batteries (LSBs) several challenges remain to be overcome. At the cathode, the lithium polysulfide (LiPS) shuttle effect must be inhibited and the redox reaction kinetics need to be substantially promoted. In this direction, this work proposes a cathode material based on a transition-metal selenide (TMSe) as both adsorber and catalyst and a hollow nanoreactor architecture: ZnSe/N-doped hollow carbon (ZnSe/NHC). It is here demonstrated both experimentally and by means of density functional theory that this composite provides three key benefits to the LSBs cathode: (i) A highly effective trapping of LiPS due to the combination of sulfiphilic sites of ZnSe, lithiophilic sites of NHC, and the confinement effect of the cage-based structure; (ii) a redox kinetic improvement in part associated with the multiple adsorption sites that facilitate the Li⁺ diffusion; and (iii) an easier accommodation of the volume expansion preventing the cathode damage due to the hollow design. As a result, LSB cathodes based on S@ZnSe/NHC are characterized by high initial capacities, superior rate capability, and an excellent stability. Overall, this work not only demonstrates the large potential of TMSe as cathode materials in LSBs but also probes the nanoreactor design to be a highly suitable architecture to enhance cycle stability.

Anomalous Photoinduced Reconstructing and Dark Self-Healing Processes on Bi2O2S Nanoplates

We report an anomalous photoinduced reconstructing and dark self-healing process on Bi₂O₂S nanoplates by monitoring the time profile of open-circuit potential (OCP). When the light was switched on and off on the nanoplates, we observed pronounced and repeatable decrement-recovery cycles of the OCP signal, which are inexplicable by a rapid electron-hole separation-recombination process only as in a conventional semiconductor. It is proposed that upon irradiation, accumulation of photogenerated holes at the electrode surface caused oxidation of the S layers of Bi₂O₂S nanoplates into certain intermediates, which, when the light was turned off, were then reduced back to the original state by the electron back flow. Raman scattering spectroscopy provided te S-S vibrational signature of the intermediate, evidencing the hole oxidative dimerization of the S^2- species and the inverse reductive S-S dissociation process. The photophysics and photochemistry of semiconductor nanoplates reported here may inspire the development of energy devices, switches, and memristors.

Spatial and Kinetic Regulation of Sulfur Electrochemistry on Semi-Immobilized Redox Mediators in Working Batteries

Use of redox mediators (RMs) is an effective strategy to enhance reaction kinetics of multi-electron sulfur electrochemistry. However, the soluble small-molecule RMs usually aggravate the internal shuttle and thus further reduce the battery efficiency and cyclability. A semi-immobilization strategy is now proposed for RM design to effectively regulate the sulfur electrochemistry while circumvent the inherent shuttle issue in a working battery. Small imide molecules as the model RMs were co-polymerized with moderate-chained polyether, rendering a semi-immobilized RM (PIPE) that is spatially restrained yet kinetically active. A small amount of PIPE (5 % in cathode) extended the cyclability of sulfur cathode from 37 to 190 cycles with 80 % capacity retention at 0.5 C. The semi-immobilization strategy helps to understand RM-assisted sulfur electrochemistry in alkali metal batteries and enlightens the chemical design of active additives for advanced electrochemical energy storage devices.

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.

Nanostructured metal chalcogenides confined in hollow structures for promoting energy storage

The engineering of progressive nanostructures with subtle construction and abundant active sites is a key factor for the advance of highly efficient energy storage devices. Nanostructured metal chalcogenides confined in hollow structures possess abundant electroactive sites, more ions and electron pathways, and high local conductivity, as well as large interior free space in a quasi-closed structure, thus showing promising prospects for boosting energy-related applications. This review focuses on the most recent progress in the creation of diverse confined hollow metal chalcogenides (CHMCs), and their electrochemical applications. Particularly, by highlighting certain typical examples from these studies, a deep understanding of the formation mechanism of confined hollow structures and the decisive role of microstructure engineering in related performances are discussed and analyzed, aiming at prompting the nanoscale engineering and conceptual design of some advanced confined metal chalcogenide nanostructures. This will appeal to not only the chemistry-, energy-, and materials-related fields, but also environmental protection and nanotechnology, thus opening up new opportunities for applications of CHMCs in various fields, such as catalysis, adsorption and separation, and energy conversion and storage.

Polysulfide Regulation by the Zwitterionic Barrier toward Durable Lithium-Sulfur Batteries

Rational regulation on polysulfide behaviors is of great significance in pursuit of reliable solution-based lithium-sulfur (Li-S) battery chemistry. Herein, we develop a unique polymeric zwitterion (PZI) to establish a smart polysulfide regulation in Li-S batteries. The zwitterionic nature of PZI integrates sulfophilicity and lithiophilicity in the matrix, fostering an ionic environment for selective ion transfer through the chemical interactions with lithium polysulfides (LiPS). When implemented as a functional interlayer in the cell configuration, PZI empowers strong obstruction against polysulfide permeation but simultaneously allows fast Li⁺ conduction, thus contributing to significant shuttle inhibition as well as the resultant facile and stable sulfur electrochemistry. The PZI-based cells realize excellent cyclability over 1000 cycles with a minimum capacity fading rate of 0.012% per cycle and favorable rate capability up to 5 C. Moreover, a high areal capacity retention of 5.3 mAh cm^-2 after 300 cycles can be also obtained under raised sulfur loading and limited electrolyte, demonstrating great promise in developing high-efficiency and long-lasting Li-S batteries.

Enhanced Chemical Immobilization and Catalytic Conversion of Polysulfide Intermediates Using Metallic Mo Nanoclusters for High-Performance Li-S Batteries

Rechargeable lithium-sulfur batteries have attracted tremendous scientific attention owing to their high energy density. However, their practical application is greatly hindered by the notorious shuttling of soluble lithium polysulfide (LPS) intermediates with sluggish redox reactions and uncontrolled precipitation behavior. Herein, we report a semiliquid cathode composed of an active LPS solution/carbon nanofiber (CNF) composite layer, capped with a carbon nanotube (CNT) thin film decorated with metallic Mo nanoclusters that regulate the electrochemical redox reactions of LPS. The trace amount (0.05 mg cm^-2) of metallic Mo on the CNT film provides sufficient capturing centers for the chemical immobilization of LPS. Together with physical blocking of LPS by the compact CNT film, free diffusion of LPS is significantly restrained and the self-discharge behavior of the Li-S cell is thus effectively suppressed. Importantly, the metallic Mo nanoclusters enable fast catalytic conversion of LPS and regular deposition of lithium sulfide. As a result, the engineered cathode exhibits a high active sulfur utilization (1401 mAh g^-1 at 0.1 C), stable cycling (500 cycles at 1 C with 0.06% decay per cycle), high rate performance (694 mAh g^-1 at 5 C), and low self-discharge rate (3% after 72 h of rest). Moreover, a high reversible areal capacity of 4.75 mAh cm^-2 is maintained after 100 cycles at 0.2 C for a cathode with a high sulfur loading of 7.64 mg cm^-2. This work provides significant insight into the structural and materials design of an advanced sulfur-based cathode that effectively regulates the electrochemical reactions of sulfur species in high-energy Li-S batteries.