Lithium–Sulfur Batteries under Lean Electrolyte Conditions: Challenges and Opportunities

The development of energy‐storage devices has received increasing attention as a transformative technology to realize a low‐carbon economy and sustainable energy supply. Lithium–sulfur (Li–S) batteries are considered to be one of the most promising next‐generation energy‐storage devices due to their ultrahigh energy density. Despite the extraordinary progress in the last few years, the actual energy density of Li–S batteries is still far from satisfactory to meet the demand for practical applications. Considering the sulfur electrochemistry is highly dependent on solid‐liquid‐solid multi‐phase conversion, the electrolyte amount plays a primary role in the practical performances of Li–S cells. Therefore, a lean electrolyte volume with low electrolyte/sulfur ratio is essential for practical Li–S batteries, yet under these conditions it is highly challenging to achieve acceptable electrochemical performances regarding sulfur kinetics, discharge capacity, Coulombic efficiency, and cycling stability especially for high‐sulfur‐loading cathodes. In this Review, the impact of the electrolyte/sulfur ratio on the actual energy density and the economic cost of Li–S batteries is addressed. Challenges and recent progress are presented in terms of the sulfur electrochemical processes: the dissolution–precipitation conversion and the solid–solid multi‐phasic transition. Finally, prospects of future lean‐electrolyte Li–S battery design and engineering are discussed.     

Electrolyte Regulation towards Stable Lithium‐Metal Anodes in Lithium–Sulfur Batteries with Sulfurized Polyacrylonitrile Cathodes

Lithium–sulfur (Li–S) batteries are highly regarded as the next‐generation energy‐storage devices because of their ultrahigh theoretical energy density of 2600 Wh kg^?1. Sulfurized polyacrylonitrile (SPAN) is considered a promising sulfur cathode to substitute carbon/sulfur (C/S) composites to afford higher Coulombic efficiency, improved cycling stability, and potential high‐energy‐density Li–SPAN batteries. However, the instability of the Li‐metal anode threatens the performances of Li–SPAN batteries bringing limited lifespan and safety hazards. Li‐metal can react with most kinds of electrolyte to generate a protective solid electrolyte interphase (SEI), electrolyte regulation is a widely accepted strategy to protect Li‐metal anodes in rechargeable batteries. Herein, the basic principles and current challenges of Li–SPAN batteries are addressed. Recent advances on electrolyte regulation towards stable Li‐metal anodes in Li–SPAN batteries are summarized to suggest design strategies of solvents, lithium salts, additives, and gel electrolyte. Finally, prospects for future electrolyte design and Li anode protection in Li–SPAN batteries are discussed.     

A Stable Quasi‐Solid‐State Sodium–Sulfur Battery

Ambient‐temperature sodium–sulfur (Na–S) batteries are considered a promising energy storage system due to their high theoretical energy density and low costs. However, great challenges remain in achieving a high rechargeable capacity and long cycle life. Herein we report a stable quasi‐solid‐state Na‐S battery enabled by a poly(S‐pentaerythritol tetraacrylate (PETEA))‐based cathode and a (PETEA‐tris[2‐(acryloyloxy)ethyl] isocyanurate (THEICTA))‐based gel polymer electrolyte. The polymeric sulfur electrode strongly anchors sulfur through chemical binding and inhibits the shuttle effect. Meanwhile, the in situ formed polymer electrolyte with high ionic conductivity and enhanced safety successfully stabilizes the Na anode/electrolyte interface, and simultaneously immobilizes soluble Na polysulfides. The as‐developed quasi‐solid‐state Na‐S cells exhibit a high reversible capacity of 877 mA h g^?1 at 0.1 C and an extended cycling stability.     

An Intrinsic Flame‐Retardant Organic Electrolyte for Safe Lithium‐Sulfur Batteries

Safety concerns pose a significant challenge for the large‐scale employment of lithium–sulfur batteries. Extremely flammable conventional electrolytes and dendritic lithium deposition cause severe safety issues. Now, an intrinsic flame‐retardant (IFR) electrolyte is presented consisting of 1.1 m lithium bis(fluorosulfonyl)imide in a solvent mixture of flame‐retardant triethyl phosphate and high flashpoint solvent 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropyl (1:3, v/v) for safe lithium–sulfur (Li?S) batteries. This electrolyte exhibits favorable flame‐retardant properties and high reversibility of the lithium metal anode (Coulombic efficiency >99 %). This IFR electrolyte enables stable lithium plating/stripping behavior with micro‐sized and dense‐packing lithium deposition at high temperatures. When coupled with a sulfurized pyrolyzed poly(acrylonitrile) cathode, Li?S batteries deliver a high composite capacity (840.1 mAh g^?1) and high sulfur utilization of 95.6 %.     

A Selection Rule for Hydrofluoroether Electrolyte Cosolvent: Establishing a Linear Free‐Energy Relationship in Lithium–Sulfur Batteries

Hydrofluoroethers (HFEs) have been adopted widely as electrolyte cosolvents for battery systems because of their unique low solvating behavior. The electrolyte is currently utilized in lithium‐ion, lithium–sulfur, lithium–air, and sodium‐ion batteries. By evaluating the relative solvating power of different HFEs with distinct structural features, and considering the shuttle factor displayed by electrolytes that employ HFE cosolvents, we have established the quantitative structure–activity relationship between the organic structure and the electrochemical performance of the HFEs. Moreover, we have established the linear free‐energy relationship between the structural properties of the electrolyte cosolvents and the polysulfide shuttle effect in lithium–sulfur batteries. These findings provide valuable mechanistic insight into the polysulfide shuttle effect in lithium–sulfur batteries, and are instructive when it comes to selecting the most suitable HFE electrolyte cosolvent for different battery systems.     

Full Dissolution of the Whole Lithium Sulfide Family (Li2S8 to Li2S) in a Safe Eutectic Solvent for Rechargeable Lithium–Sulfur Batteries

The lithium–sulfur battery is an attractive option for next‐generation energy storage owing to its much higher theoretical energy density than state‐of‐the‐art lithium‐ion batteries. However, the massive volume changes of the sulfur cathode and the uncontrollable deposition of Li₂S₂/Li₂S significantly deteriorate cycling life and increase voltage polarization. To address these challenges, we develop an ?‐caprolactam/acetamide based eutectic‐solvent electrolyte, which can dissolve all lithium polysulfides and lithium sulfide (Li₂S₈–Li₂S). With this new electrolyte, high specific capacity (1360 mAh g^?1) and reasonable cycling stability are achieved. Moreover, in contrast to conventional ether electrolyte with a low flash point (ca. 2 °C), such low‐cost eutectic‐solvent‐based electrolyte is difficult to ignite, and thus can dramatically enhance battery safety. This research provides a new approach to improving lithium–sulfur batteries in aspects of both safety and performance.     

X‐ray Absorption Near‐Edge Structure and Nuclear Magnetic Resonance Study of the Lithium–Sulfur Battery and its Components

Understanding the mechanism(s) of polysulfide formation and knowledge about the interactions of sulfur and polysulfides with a host matrix and electrolyte are essential for the development of long‐cycle‐life lithium–sulfur (Li–S) batteries. To achieve this goal, new analytical tools need to be developed. Herein, sulfur K‐edge X‐ray absorption near‐edge structure (XANES) and ^6,7Li magic‐angle spinning (MAS) NMR studies on a Li–S battery and its sulfur components are reported. The characterization of different stoichiometric mixtures of sulfur and lithium compounds (polysulfides), synthesized through a chemical route with all‐sulfur‐based components in the Li–S battery (sulfur and electrolyte), enables the understanding of changes in the batteries measured in postmortem mode and in operando mode. A detailed XANES analysis is performed on different battery components (cathode composite and separator). The relative amounts of each sulfur compound in the cathode and separator are determined precisely, according to the linear combination fit of the XANES spectra, by using reference compounds. Complementary information about the lithium species within the cathode are obtained by using ⁷Li MAS NMR spectroscopy. The setup for the in operando XANES measurements can be viewed as a valuable analytical tool that can aid the understanding of the sulfur environment in Li–S batteries.     

Functional Organosulfide Electrolyte Promotes an Alternate Reaction Pathway to Achieve High Performance in Lithium–Sulfur Batteries

Lithium–sulfur (Li‐S) batteries have recently received great attention because they promise to provide energy density far beyond current lithium ion batteries. Typically, Li‐S batteries operate by conversion of sulfur to reversibly form different soluble lithium polysulfide intermediates and insoluble lithium sulfides through multistep redox reactions. Herein, we report a functional electrolyte system incorporating dimethyl disulfide as a co‐solvent that enables a new electrochemical reduction pathway for sulfur cathodes. This pathway uses soluble dimethyl polysulfides and lithium organosulfides as intermediates and products, which can boost cell capacity and lead to improved discharge–charge reversibility and cycling performance of sulfur cathodes. This electrolyte system can potentially enable Li‐S batteries to achieve high energy density.     

Sulfur Spillover on Carbon Materials and Possible Impacts on Metal–Sulfur Batteries

There is currently intense research on sulfur/carbon composite materials as positive electrodes for rechargeable batteries. Such composites are commonly prepared by ball milling or (melt/solution) impregnation to achieve intimate contact between both elements with the hope to improve battery performance. Herein, we report that sulfur shows an unexpected “spillover” effect when in contact with porous carbon materials under ambient conditions. When sulfur and porous carbon are gently mixed in a 1:1 mass ratio, complete surface coverage takes place within just a few days along with the loss of the sulfur bulk properties (crystallinity, melting point, Raman signals). Sulfur spillover also occurs in the presence of a liquid phase. Consequences of this phenomenon are discussed by considering a sodium–sulfur cell with a solid electrolyte membrane.     

A Rechargeable Al/S Battery with an Ionic‐Liquid Electrolyte

Aluminum metal is a promising anode material for next generation rechargeable batteries owing to its abundance, potentially dendrite‐free deposition, and high capacity. The rechargeable aluminum/sulfur (Al/S) battery is of great interest owing to its high energy density (1340 Wh kg^?1) and low cost. However, Al/S chemistry suffers poor reversibility owing to the difficulty of oxidizing AlS_x. Herein, we demonstrate the first reversible Al/S battery in ionic‐liquid electrolyte with an activated carbon cloth/sulfur composite cathode. Electrochemical, spectroscopic, and microscopic results suggest that sulfur undergoes a solid‐state conversion reaction in the electrolyte. Kinetics analysis identifies that the slow solid‐state sulfur conversion reaction causes large voltage hysteresis and limits the energy efficiency of the system.     

Elemental‐Sulfur‐Mediated Facile Synthesis of a Covalent Triazine Framework for High‐Performance Lithium–Sulfur Batteries

A covalent triazine framework (CTF) with embedded polymeric sulfur and a high sulfur content of 62 wt % was synthesized under catalyst‐ and solvent‐free reaction conditions from 1,4‐dicyanobenzene and elemental sulfur. Our synthetic approach introduces a new way of preparing CTFs under environmentally benign conditions by the direct utilization of elemental sulfur. The homogeneous sulfur distribution is due to the in situ formation of the framework structure, and chemical sulfur impregnation within the micropores of CTF effectively suppresses the dissolution of polysulfides into the electrolyte. Furthermore, the triazine framework facilitates electron and ion transport, which leads to a high‐performance lithium–sulfur battery.     

Radially Inwardly Aligned Hierarchical Porous Carbon for Ultra‐Long‐Life Lithium–Sulfur Batteries

Rational design of hollow micro‐ and/or nano‐structured cathodes as sulfur hosts has potential for high‐performance lithium‐sulfur batteries. However, their further commercial application is hindered because infusing sulfur into hollow hosts is hard to control and the interactions between high loading sulfur and electrolyte are poor. Herein, we designed hierarchical porous hollow carbon nanospheres with radially inwardly aligned supporting ribs to mitigate these problems. Such a structure could aid the sulfur infusion and maximize sulfur utilization owing to the well‐ordered pore channels. This highly organized internal carbon skeleton can also enhance the electronic conductivity. The hollow carbon nanospheres with further nitrogen‐doping as the sulfur host material exhibit good capacity and excellent cycling performance (0.044 % capacity degradation per each cycle for 1000 cycles).     

Towards a Safe Lithium–Sulfur Battery with a Flame‐Inhibiting Electrolyte and a Sulfur‐Based Composite Cathode

Of the various beyond‐lithium‐ion batteries, lithium–sulfur (Li‐S) batteries were recently reported as possibly being the closest to market. However, its theoretically high energy density makes it potentially hazardous under conditions of abuse. Therefore, addressing the safety issues of Li‐S cells is necessary before they can be used in practical applications. Here, we report a concept to build a safe and highly efficient Li‐S battery with a flame‐inhibiting electrolyte and a sulfur‐based composite cathode. The flame retardant not only makes the carbonates nonflammable but also dramatically enhances the electrochemical performance of the sulfur‐based composite cathode, without an apparent capacity decline over 750 cycles, and with a capacity greater than 800 mA h^?1 g^?1(sulfur) at a rate of 10 C.     

Encapsulating Sulfur into Hierarchically Ordered Porous Carbon as a High‐Performance Cathode for Lithium–Sulfur Batteries

A three‐dimensional (3D) hierarchical carbon–sulfur nanocomposite that is useful as a high‐performance cathode for rechargeable lithium–sulfur batteries is reported. The 3D hierarchically ordered porous carbon (HOPC) with mesoporous walls and interconnected macropores was prepared by in situ self‐assembly of colloidal polymer and silica spheres with sucrose as the carbon source. The obtained porous carbon possesses a large specific surface area and pore volume with narrow mesopore size distribution, and acts as a host and conducting framework to contain highly dispersed elemental sulfur. Electrochemical tests reveal that the HOPC/S nanocomposite with well‐defined nanostructure delivers a high initial specific capacity up to 1193 mAh g^?1 and a stable capacity of 884 mAh g^?1 after 50 cycles at 0.1 C. In addition, the HOPC/S nanocomposite exhibits high reversible capacity at high rates. The excellent electrochemical performance is attributed exclusively to the beneficial integration of the mesopores for the electrochemical reaction and macropores for ion transport. The mesoporous walls of the HOPC act as solvent‐restricted reactors for the redox reaction of sulfur and aid in suppressing the diffusion of polysulfide species into the electrolyte. The “open” ordered interconnected macropores and windows facilitate transportation of electrolyte and solvated lithium ions during the charge/discharge process. These results show that nanostructured carbon with hierarchical pore distribution could be a promising scaffold for encapsulating sulfur to approach high specific capacity and energy density with long cycling performance.     

The Relationship between the Relative Solvating Power of Electrolytes and Shuttling Effect of Lithium Polysulfides in Lithium–Sulfur Batteries

Relative solvating power, that is, the ratio of the coordination ratios between a solvent and the reference solvent, was used to probe the quantitative structure–activity relationship of electrolyte solvents and the lithium polysulfide (LiPS) dissolution in lithium–sulfur batteries. Internally referenced diffusion‐ordered nuclear magnetic resonance spectroscopy (IR‐DOSY) was used to determine the diffusion coefficient and coordination ratio, from which the relative solvating power can be easily measured. The higher the relative solvating power of an ethereal solvent, the more severe will be the LiPS dissolution and the lower the coulombic efficiency of the lithium–sulfur battery. A linear relationship was established between the logarithm of relative solvating power of a solvent and the degree of LiPS dissolution, rendering relative solvating power an important parameter in choosing the electrolyte solvent for lithium–sulfur batteries.     

Prospect of Sulfurized Pyrolyzed Poly(acrylonitrile) (S@pPAN) Cathode Materials for Rechargeable Lithium Batteries

Lithium–sulfur (Li–S) batteries are one of the most promising next‐generation batteries owing to their ultra‐high theoretical energy density and that sulfur is an abundant resource. During the past 20 years, various sulfur materials have been reported. As a molecular‐scale sulfur‐composite cathode, sulfurized pyrolyzed poly(acrylonitrile) (S@pPAN) exhibits several competitive advantages in terms of its electrochemical behavior. Although it was first reported in 2002 S@pPAN is currently attracting increasing attention. In this Minireview, we summarize its molecular model and explore the correlation between its structure and its exceptional electrochemical performance. We classify the modification strategies into three types, including material improvement, binder, and electrolyte screening. Several research and development directions are also suggested.     

Carbon Materials for Lithium Sulfur Batteries—Ten Critical Questions

Lithium–sulfur batteries are among the most promising electrochemical energy storage devices of the near future. Especially the low price and abundant availability of sulfur as the cathode material and the high theoretical capacity in comparison to state‐of‐the art lithium‐ion technologies are attractive features. Despite significant research achievements that have been made over the last years, fundamental (electro‐) chemical questions still remain unanswered. This review addresses ten crucial questions associated with lithium–sulfur batteries and critically evaluates current research with respect to them. The sulfur–carbon composite cathode is a particular focus, but its complex interplay with other hardware components in the cell, such as the electrolyte and the anode, necessitates a critical discussion of other cell components. Modern in situ characterisation methods are ideally suited to illuminate the role of each component. This article does not pretend to summarise all recently published data, but instead is a critical overview over lithium–sulfur batteries based on recent research findings.     

Developing A “Polysulfide‐Phobic” Strategy to Restrain Shuttle Effect in Lithium–Sulfur Batteries

Inspired by hydrophobic interface, a novel design of “polysulfide‐phobic” interface was proposed and developed to restrain shuttle effect in lithium–sulfur batteries. Two‐dimensional VOPO₄ sheets with adequate active sites were employed to immobilize the polysulfides through the formation of a V?S bond. Moreover, owing to the intrinsic Coulomb repulsion between polysulfide anions, the surface anchored with polysulfides can be further evolved into a “polysulfide‐phobic” interface, which was demonstrated by the advanced time/space‐resolved operando Raman evidences. In particular, by introducing the “polysulfide‐phobic” surface design into separator fabrication, the lithium–sulfur battery performed a superior long‐term cycling stability. This work expands a novel strategy to build a “polysulfide‐phobic” surface by “self‐defense” mechanism for suppressing polysulfides shuttle, which provides new insights and opportunities to develop advanced lithium–sulfur batteries.     

Iron monosulfide (FeS) oxidation by dissolved oxygen: characteristics of the product layer

Samples of FeS were oxidized by oxygen‐bearing acidic solutions at 25 °C and different initial pH values (2.75 ≤ pH ≤ 3.45). The reacted FeS samples were investigated by scanning electron microscopy (SEM), nitrogen adsorption/desorption isotherms measurements, and FT‐Raman spectroscopy. A sulfur rich phase (layer) is inferred to be form on reacting FeS surface based on sequential extraction with acidic chromium(II) chloride solutions. The sulfur‐rich layer (SRL) formed on oxidized FeS samples is traversed by various mesopores with average pore size < 450Å. It is reasonable to assume that the mesopores facilitate the migration of iron and sulfur from FeS structure into solution across SRL. The FT‐Raman spectra of oxidized FeS samples show an intense peak at 162 cm^?1, which can be attributed to SRL developed on FeS surface. A model for SRL development on FeS during its oxidation by dissolved oxygen has been proposed from the present results and previous studies. Copyright © 2009 John Wiley & Sons, Ltd.     

Polyolefin‐Based Janus Separator for Rechargeable Sodium Batteries

Rechargeable sodium batteries are a promising technology for low‐cost energy storage. However, the undesirable drawbacks originating from the use of glass fiber membrane separators have long been overlooked. A versatile grafting–filtering strategy was developed to controllably tune commercial polyolefin separators for sodium batteries. The as‐developed Janus separators contain a single–ion‐conducting polymer‐grafted side and a functional low‐dimensional material coated side. When employed in room‐temperature sodium–sulfur batteries, the poly(1‐[3‐(methacryloyloxy)propylsulfonyl]‐1‐(trifluoromethanesulfonyl)imide sodium)‐grafted side effectively enhances the electrolyte wettability, and inhibits polysulfide diffusion and sodium dendrite growth. Moreover, a titanium‐deficient nitrogen‐containing MXene‐coated side electrocatalytically improved the polysulfide conversion kinetics. The as‐developed batteries demonstrate high capacity and extended cycling life with lean electrolyte loading.