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.

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.

Revisiting the Electrochemical Impedance Spectroscopy of Porous Electrodes in Li-ion Batteries by Employing Reference Electrode.

Electrochemical impedance spectroscopy (EIS), characterized by its non-destructive and in-situ nature, plays a crucial role in comprehending the thermodynamic and kinetic processes occurring with Li-ion batteries. However, there is a lack of consistent and coherent physical interpretations for the EIS of porous electrodes. Therefore, it is imperative to conduct thorough investigations into the underlying physical mechanisms of EIS. Herein, by employing reference electrode in batteries, we revisit the associated physical interpretation of EIS at different frequency. Combining different battery configurations, temperature-dependent experiments, and elaborated distribution of relaxation time analysis, we find that the ion transport in porous electrode channels and pseudo-capacitance behavior dominate the high-frequency and mid-frequency impedance arcs, respectively. This work offers a perspective for the physical interpretation of EIS and also sheds light on the understanding of EIS characteristics in other advanced energy storage systems.

Improving Rate Performance of Encapsulating Lithium-Polysulfide Electrolytes for Practical Lithium-Sulfur Batteries.

The cycle life of high-energy-density lithium-sulfur (Li-S) batteries is severely plagued by the incessant parasitic reactions between Li metal anodes and reactive Li polysulfides (LiPSs). Encapsulating Li-polysulfide electrolyte (EPSE) emerges as an effective electrolyte design to mitigate the parasitic reactions kinetically. Nevertheless, the rate performance of Li-S batteries with EPSE is synchronously suppressed. Herein, the sacrifice in rate performance by EPSE is circumvented while mitigating parasitic reactions by employing hexyl methyl ether (HME) as a co-solvent. The specific capacity of Li-S batteries with HME-based EPSE is nearly not decreased at 0.1 C compared with conventional ether electrolytes. With an ultrathin Li metal anode (50 µm) and a high-areal-loading sulfur cathode (4.4 mgS cm-2 ), a longer cycle life of 113 cycles was achieved in HME-based EPSE compared with that of 65 cycles in conventional ether electrolytes at 0.1 C. Furthermore, both high energy density of 387 Wh kg-1 and stable cycle life of 27 cycles were achieved in a Li-S pouch cell (2.7 Ah). This work inspires the feasibility of regulating the solvation structure of LiPSs in EPSE for Li-S batteries with balanced performance.

Cathode Kinetics Evaluation in Lean-Electrolyte Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries afford great promise on achieving practical high energy density beyond lithium-ion batteries. Lean-electrolyte conditions constitute the prerequisite for achieving high-energy-density Li-S batteries but inevitably deteriorates battery performances, especially the sulfur cathode kinetics. Herein, the polarizations of the sulfur cathode are systematically decoupled to identify the key kinetic limiting factor in lean-electrolyte Li-S batteries. Concretely, an electrochemical impedance spectroscopy combined galvanostatic intermittent titration technique method is developed to decouple the cathodic polarizations into activation, concentration, and ohmic parts. Therein, activation polarization during lithium sulfide nucleation emerges as the dominant polarization as the electrolyte-to-sulfur ratio (E/S ratio) decreases, and the sluggish interfacial charge transfer kinetics is identified as the main reason for degraded cell performances under lean-electrolyte conditions. Accordingly, a lithium bis(fluorosulfonyl)imide electrolyte is proposed to decrease activation polarization, and Li-S batteries adopting this electrolyte provide a discharge capacity of 985 mAh g-1 under a low E/S ratio of 4 µL mg-1 at 0.2 C. This work identifies the key kinetic limiting factor of lean-electrolyte Li-S batteries and provides guidance on designing rational promotion strategies to achieve advanced Li-S batteries.

Reforming the Uniformity of Solid Electrolyte Interphase by Nanoscale Structure Regulation for Stable Lithium Metal Batteries.

The stability of high-energy-density lithium metal batteries depends on the uniformity of solid electrolyte interphase (SEI) on lithium metal anodes. Rationally improving SEI uniformity is hindered by poorly understanding the effect of structure and components of SEI on its uniformity. Herein, a bilayer structure of SEI formed by isosorbide dinitrate (ISDN) additives in localized high-concentration electrolytes was demonstrated to improve SEI uniformity. In the bilayer SEI, LiNx Oy generated by ISDN occupies top layer and LiF dominates bottom layer next to anode. The uniformity of lithium deposition is remarkably improved with the bilayer SEI, mitigating the consumption rate of active lithium and electrolytes. The cycle life of lithium metal batteries with bilayer SEI is three times as that with common anion-derived SEI under practical conditions. A prototype lithium metal pouch cell of 430 Wh kg-1 undergoes 173 cycles. This work demonstrates the effect of a reasonable structure of SEI on reforming SEI uniformity.

Electrolyte Design for Improving Mechanical Stability of Solid Electrolyte Interphase in Lithium-Sulfur Batteries.

Practical lithium-sulfur (Li-S) batteries are severely plagued by the instability of solid electrolyte interphase (SEI) formed in routine ether electrolytes. Herein, an electrolyte with 1,3,5-trioxane (TO) and 1,2-dimethoxyethane (DME) as co-solvents is proposed to construct a high-mechanical-stability SEI by enriching organic components in Li-S batteries. The high-mechanical-stability SEI works compatibly in Li-S batteries. TO with high polymerization capability can preferentially decompose and form organic-rich SEI, strengthening mechanical stability of SEI, which mitigates crack and regeneration of SEI and reduces the consumption rate of active Li, Li polysulfides, and electrolytes. Meanwhile, DME ensures high specific capacity of S cathodes. Accordingly, the lifespan of Li-S batteries increases from 75 cycles in routine ether electrolyte to 216 cycles in TO-based electrolyte. Furthermore, a 417 Wh kg-1 Li-S pouch cell undergoes 20 cycles. This work provides an emerging electrolyte design for practical Li-S batteries.

An Organodiselenide Comediator to Facilitate Sulfur Redox Kinetics in Lithium-Sulfur Batteries with Encapsulating Lithium Polysulfide Electrolyte.

Lithium-sulfur (Li-S) batteries are regarded as promising high-energy-density energy storage devices. However, the cycling stability of Li-S batteries is restricted by the parasitic reactions between Li metal anodes and soluble lithium polysulfides (LiPSs). Encapsulating LiPS electrolyte (EPSE) can efficiently suppress the parasitic reactions but inevitably sacrifices the cathode sulfur redox kinetics. To address the above dilemma, a redox comediation strategy for EPSE is proposed to realize high-energy-density and long-cycling Li-S batteries. Concretely, dimethyl diselenide (DMDSe) is employed as an efficient redox comediator to facilitate the sulfur redox kinetics in Li-S batteries with EPSE. DMDSe enhances the liquid-liquid and liquid-solid conversion kinetics of LiPS in EPSE while maintains the ability to alleviate the anode parasitic reactions from LiPSs. Consequently, a Li-S pouch cell with a high energy density of 359 Wh kg-1 at cell level and stable 37 cycles is realized. This work provides an effective redox comediation strategy for EPSE to simultaneously achieve high energy density and long cycling stability in Li-S batteries and inspires rational integration of multi-strategies for practical working batteries.

Regulating Lithium Salt to Inhibit Surface Gelation on an Electrocatalyst for High-Energy-Density Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries have great potential as high-energy-density energy storage devices. Electrocatalysts are widely adopted to accelerate the cathodic sulfur redox kinetics. The interactions among the electrocatalysts, solvents, and lithium salts significantly determine the actual performance of working Li-S batteries. Herein, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), a commonly used lithium salt, is identified to aggravate surface gelation on the MoS2 electrocatalyst. In detail, the trifluoromethanesulfonyl group in LiTFSI interacts with the Lewis acidic sites on the MoS2 electrocatalyst to generate an electron-deficient center. The electron-deficient center with high Lewis acidity triggers cationic polymerization of the 1,3-dioxolane solvent and generates a surface gel layer that reduces the electrocatalytic activity. To address the above issue, Lewis basic salt lithium iodide (LiI) is introduced to block the interaction between LiTFSI and MoS2 and inhibit the surface gelation. Consequently, the Li-S batteries with the MoS2 electrocatalyst and the LiI additive realize an ultrahigh actual energy density of 416 W h kg-1 at the pouch cell level. This work affords an effective lithium salt to boost the electrocatalytic activity in practical working Li-S batteries and deepens the fundamental understanding of the interactions among electrocatalysts, solvents, and salts in energy storage systems.

Carrier Free O2 -Economizer for Photodynamic Therapy Against Hypoxic Tumor by Inhibiting Cell Respiration.

Abnormal tumor metabolism causes the hypoxic microenvironment, which greatly limits the efficacy of photodynamic therapy (PDT). In this work, a strategy of metabolic reprogramming is proposed to economize O2 for enhanced PDT against hypoxic tumors. The carrier-free O2 -economizer (designated as LonCe) is prepared based on the metabolic antitumor drug of Lonidamine (Lon) and the photosensitizer of chlorin e6 (Ce6). By virtue of intermolecular interactions, Lon and Ce6 self-assemble into nanosized LonCe with favorable stability and high drug contents. Compared with Ce6, LonCe exhibits an improved cellular uptake and photodynamic property for tumor treatment. Moreover, LonCe is capable of inhibiting cell metabolism and mitochondrial respiration to remit the tumor hypoxia, which would promote reactive oxygen species (ROS) production and elevate the PDT efficacy on tumor suppression. In vivo experiments indicate that intravenously injected LonCe prefers to accumulate at the tumor site for highly efficient PDT regardless of the hypoxic environment. Besides, the self-delivery LonCe is fabricated without any carriers, which avoids the excipients induced system toxicity and immunogenicity in vivo. This carrier-free nanomedicine with cell respiratory inhibition mechanism would expedite the development and clinical translation of photodynamic nanoplatforms in tumor treatment.

Curcumin regulates the homeostasis of Th17/Treg and improves the composition of gut microbiota in type 2 diabetic mice with colitis.

Diabetes mellitus (DM) is one of the most common complications in patients with ulcerative colitis (UC). Curcumin has a wide range of bioactive and pharmacological properties and is commonly used as an adjunct to the treatment of UC and DM. However, the role of curcumin in UC complicated by DM has not been elucidated. Therefore, this study was conducted to construct a model of UC complicating diabetes by inducing UC in DB mice (spontaneously diabetic) with dextran sodium sulfate. In this study, curcumin (100 mg/kg/day) significantly improved the symptoms of diabetes complicated by UC, with a lower insulin level, heavier weight, longer and lighter colons, fewer mucosal ulcers and less inflammatory cell infiltration. Moreover, compared to untreated DB mice with colitis, curcumin-treated mice showed weaker Th17 responses and stronger Treg responses. In addition, curcumin regulated the diversity and relative abundance of intestinal microbiota in mice with UC complicated by DM at the phylum, class, order, family and genus levels. Collectively, curcumin effectively alleviated colitis in mice with type 2 diabetes mellitus by restoring the homeostasis of Th17/Treg and improving the composition of the intestinal microbiota.

Untangling Degradation Chemistries of Lithium-Sulfur Batteries Through Interpretable Hybrid Machine Learning.

The development of emerging rechargeable batteries is often hindered by limited chemical understanding composing of entangled patterns in an enormous space. Herein, we propose an interpretable hybrid machine learning framework to untangle intractable degradation chemistries of conversion-type batteries. Rather than being a black box, this framework not only demonstrates an ability to accurately forecast lithium-sulfur batteries (with a test mean absolute error of 8.9 % for the end-of-life prediction) but also generate useful physical understandings that illuminate future battery design and optimization. The framework also enables the discovery of a previously unknown performance indicator, the ratio of electrolyte amount to high-voltage-region capacity at the first discharge, for lithium-sulfur batteries complying practical merits. The present data-driven approach is readily applicable to other energy storage systems due to its versatility and flexibility in modules and inputs.

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.

Quantification of the Dynamic Interface Evolution in High-Efficiency Working Li-Metal Batteries.

Lithium (Li) metal has been considered a promising anode for next-generation high-energy-density batteries. However, the low reversibility and intricate Li loss hinder the widespread implementation of Li metal batteries. Herein, we quantitatively differentiate the dynamic evolution of inactive Li, and decipher the fundamental interplay among dynamic Li loss, electrolyte chemistry, and the structure of the solid electrolyte interphase (SEI). The actual dominant form in inactive Li loss is practically determined by the relative growth rates of dead Li0 and SEI Li+ because of the persistent evolution of the Li metal interface during cycling. Distinct inactive Li evolution scenarios are disclosed by ingeniously tuning the inorganic anion-derived SEI chemistry with a low amount of film-forming additive. An optimal polymeric film enabler of 1,3-dioxolane is demonstrated to derive a highly uniform multilayer SEI and decreased SEI Li+ /dead Li0 growth rates, thus achieving enhanced Li cycling reversibility.

Fluorinating the Solid Electrolyte Interphase by Rational Molecular Design for Practical Lithium-Metal Batteries.

The lifespan of practical lithium (Li)-metal batteries is severely hindered by the instability of Li-metal anodes. Fluorinated solid electrolyte interphase (SEI) emerges as a promising strategy to improve the stability of Li-metal anodes. The rational design of fluorinated molecules is pivotal to construct fluorinated SEI. Herein, design principles of fluorinated molecules are proposed. Fluoroalkyl (-CF2 CF2 -) is selected as an enriched F reservoir and the defluorination of the C-F bond is driven by leaving groups on ß-sites. An activated fluoroalkyl molecule (AFA), 2,2,3,3-tetrafluorobutane-1,4-diol dinitrate is unprecedentedly proposed to render fast and complete defluorination and generate uniform fluorinated SEI on Li-metal anodes. In Li-sulfur (Li-S) batteries under practical conditions, the fluorinated SEI constructed by AFA undergoes 183 cycles, which is three times the SEI formed by LiNO3 . Furthermore, a Li-S pouch cell of 360 Wh kg-1 delivers 25 cycles with AFA. This work demonstrates rational molecular design principles of fluorinated molecules to construct fluorinated SEI for practical Li-metal batteries.

Thermally Stable Polymer-Rich Solid Electrolyte Interphase for Safe Lithium Metal Pouch Cells.

Serious safety risks caused by the high reactivity of lithium metal against electrolytes severely hamper the practicability of lithium metal batteries. By introducing unique polymerization site and more fluoride substitution, we built an in situ formed polymer-rich solid electrolyte interphase upon lithium anode to improve battery safety. The fluorine-rich and hydrogen-free polymer exhibits high thermal stability, which effectively reduces the continuous exothermic reaction between electrolyte and anode/cathode. As a result, the critical temperature for thermal safety of 1.0 Ah lithium-LiNi0.5 Co0.2 Mn0.3 O2 pouch cell can be increased from 143.2 °C to 174.2 °C. The more dangerous "ignition" point of lithium metal batteries, the starting temperature of battery thermal runaway, has been dramatically raised from 240.0 °C to 338.0 °C. This work affords novel strategies upon electrolyte design, aiming to pave the way for high-energy-density and thermally safe lithium metal batteries.

Operando Quantified Lithium Plating Determination Enabled by Dynamic Capacitance Measurement in Working Li-Ion Batteries.

The access to full performance of state-of-the-art Li-ion batteries (LIBs) is hindered by the mysterious lithium plating behavior. A rapid quantified lithium plating determination method compatible with actual working conditions is an urgent necessity for safe working LIBs. In this contribution, the relationship between electrical double layer (EDL) capacitance and electrochemical active surface area (ECSA) of graphite anodes during the Li-ion intercalation and Li plating processes is unveiled. We propose an operando lithium plating determination method based on the dynamic capacitance measurement (DCM) test. Reasonable selection of alternating current (AC) frequency protects the anodic responses from the interference of cathodic responses, which allows DCM to be applied in practical LIBs. The onset of lithium plating can be quantitatively traced, demonstrating the promise for real-time operando determination for lithium plating in a working battery.

Semi-Immobilized Molecular Electrocatalysts for High-Performance Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries constitute promising next-generation energy storage devices due to the ultrahigh theoretical energy density of 2600 Wh kg-1. However, the multiphase sulfur redox reactions with sophisticated homogeneous and heterogeneous electrochemical processes are sluggish in kinetics, thus requiring targeted and high-efficient electrocatalysts. Herein, a semi-immobilized molecular electrocatalyst is designed to tailor the characters of the sulfur redox reactions in working Li-S batteries. Specifically, porphyrin active sites are covalently grafted onto conductive and flexible polypyrrole linkers on graphene current collectors. The electrocatalyst with the semi-immobilized active sites exhibits homogeneous and heterogeneous functions simultaneously, performing enhanced redox kinetics and a regulated phase transition mode. The efficiency of the semi-immobilizing strategy is further verified in practical Li-S batteries that realize superior rate performances and long lifespan as well as a 343 Wh kg-1 high-energy-density Li-S pouch cell. This contribution not only proposes an efficient semi-immobilizing electrocatalyst design strategy to promote the Li-S battery performances but also inspires electrocatalyst development facing analogous multiphase electrochemical energy processes.

A review on energy chemistry of fast-charging anodes.

With the impetus to accelerate worldwide market adoption of electrical vehicles and afford consumer electronics with better user experience, advancing fast-charging technology is an inevitable trend. However, current high-energy lithium-ion batteries are unable to support ultrafast power input without any adverse consequences, with the capacity fade and safety concerns of the mainstream graphite-based anodes being the key technological barrier. The aim of this review is to summarise the fundamentals, challenges, and solutions to enable graphite anodes that are capable of high-rate charging. First, we explore the complicated yet intriguing graphite-electrolyte interface during intercalation based on existing theories. Second, we analyse the key dilemmas facing fast-charging graphite anodes. Finally, some promising strategies proposed during the past few years are highlighted so as to outline current trends and future perspectives in this field.