Recent Progress in High Donor Electrolytes for Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries continue to be considered promising post‐lithium‐ion batteries owing to their high theoretical energy density. In pursuit of a Li–S cell with long‐term cyclability, most studies thus far have relied on using ether‐based electrolytes. However, their limited ability to dissolve polysulfides requires a high electrolyte‐to‐sulfur ratio, which impairs the achievable specific energy. Recently, the battery community found high donor electrolytes to be a potential solution to this shortcoming because their high solubility toward polysulfides enables a cell to operate under lean electrolyte conditions. Despite the increasing number of promising outcomes with high donor electrolytes, a critical hurdle related to stability of the lithium‐metal counter electrode needs to be overcome. This review provides an overview of recent efforts pertaining to high donor electrolytes in Li–S batteries and is intended to raise interest from within the community. Furthermore, based on analogous efforts in the lithium‐air battery field, strategies for protecting the lithium metal electrode are proposed. It is predicted that high donor electrolytes will be elevated to a higher status in the field of Li–S batteries, with the hope that either existing or upcoming strategies will, to a fair extent, mitigate the degradation of the lithium–metal interface.     

Highly Stable Lithium–Sulfur Batteries Based on Laponite Nanosheet‐Coated Celgard Separators

Lithium–sulfur (Li–S) batteries are of great interest due to their high theoretical energy density. However, one of the key issues hindering their real world applications is polysulfide shuttle, which results in severe capacity decay and self‐discharge. Here, a laponite nanosheets/carbon black coated Celgard (LNS/CB‐Celgard) separator to inhibit polysulfide shuttle and to enhance the Li⁺ conductivity simultaneously is reported. The polysulfide shuttle is efficiently inhibited through strong interactions between the O active sites of the LNS and polysulfides by forming the Li···O and O? S bonds. Moreover, the separator features high Li⁺ conductivity, fast Li⁺ diffusion, excellent electrolyte wettability, and high thermal stability. Consequently, the Li–S batteries with the LNS/CB‐Celgard separator and the pure S cathode show a high initial reversible capacity of 1387 mA h g^?1 at 0.1 C, high rate performance, superior cycling stability (with a capacity decay rate of 0.06% cycle^?1 at 0.2 C and 0.028% cycle^?1 at 1.0 C over 500 cycles), and ultralow self‐discharge. The separator could also enhance the performance of other batteries such as the LiFePO₄/separator/Li battery. This work sheds a new light on the design and preparation of novel separators for highly stable Li–S batteries via a “green” and cost‐effective approach.     

Catalytic Polysulfide Conversion and Physiochemical Confinement for Lithium–Sulfur Batteries

The lithium–sulfur (Li–S) battery is widely regarded as a promising energy storage device due to its low price and the high earth‐abundance of the materials employed. However, the shuttle effect of lithium polysulfides (LiPSs) and sluggish redox conversion result in inefficient sulfur utilization, low power density, and rapid electrode deterioration. Herein, these challenges are addressed with two strategies 1) increasing LiPS conversion kinetics through catalysis, and 2) alleviating the shuttle effect by enhanced trapping and adsorption of LiPSs. These improvements are achieved by constructing double‐shelled hollow nanocages decorated with a cobalt nitride catalyst. The N‐doped hollow inner carbon shell not only serves as a physiochemical absorber for LiPSs, but also improves the electrical conductivity of the electrode; significantly suppressing shuttle effect. Cobalt nitride (Co₄N) nanoparticles, embedded in nitrogen‐doped carbon in the outer shell, catalyze the conversion of LiPSs, leading to decreased polarization and fast kinetics during cycling. Theoretical study of the Li intercalation energetics confirms the improved catalytic activity of the Co₄N compared to metallic Co catalyst. Altogether, the electrode shows large reversible capacity (1242 mAh g^?1 at 0.1 C), robust stability (capacity retention of 658 mAh g^?1 at 5 C after 400 cycles), and superior cycling stability at high sulfur loading (4.5 mg cm^?2).     

A Metal Organic Framework Derived Solid Electrolyte for Lithium–Sulfur Batteries

Lithium–sulfur batteries (LSBs) are currently considered as promising candidates for next‐generation energy storage technologies. However, their practical application is hindered by the critical issue of the polysulfide‐shuttle. Herein, a metal organic framework (MOF)‐derived solid electrolyte is presented to address it. The MOF solid electrolyte is developed based on a Universitetet i Oslo (UIO) structure. By grafting a lithium sulfonate (‐SO₃Li) group to the UIO ligand, both the ionic conductivity and the polysulfide‐suppression capability of the resulting ‐SO₃Li grafted UIO (UIOSLi) solid electrolyte are greatly improved. After integrating a Li‐based ionic liquid (Li‐IL), lithium bis(trifluoromethanesulfonyl)imide in 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide, the resulting Li‐IL/UIOSLi solid electrolyte exhibits an ionic conductivity of 3.3 × 10^?4 S cm^?1 at room temperature. Based on its unique structure, the Li‐IL/UIOSLi solid electrolyte effectively restrains the polysulfide shuttle and suppresses lithium dendritic growth. Lithium–sulfur cells with the Li‐IL/UIOSLi solid electrolyte and a Li₂S₆ catholyte show stable cycling performance that preserves 84% of the initial capacity after 250 cycles with a capacity‐fade rate of 0.06% per cycle.     

Electrochemically Stable Rechargeable Lithium–Sulfur Batteries with a Microporous Carbon Nanofiber Filter for Polysulfide

As a primary component in lithium–sulfur (Li–S) batteries, the separator may require a custom design in order to facilitate electrochemical stability and reversibility. Here, a custom separator with an activated carbon nanofiber (ACNF)‐filter coated onto a polypropylene membrane is presented. The entire configuration is comprised of the ACNF filter arranged adjacent to the sulfur cathode so that it can filter out the freely migrating polysulfides and suppress the severe polysulfide diffusion. Four differently optimized ACNF‐filter‐coated separators have been developed with tunable micropores as an investigation into the electrochemical and engineering design parameters of functionalized separators. The optimized parameters that are verified by electrochemical and microstructural analyses require the coated ACNF filter to possess the following: (i) a porous architecture with abundant micropores, (ii) small micropore sizes, and (iii) high electrical conductivity and effective electrolyte immersion. It is found that the ACNF20‐filter‐coated separator demonstrates an overall superior boost in the electrochemical utilization (discharge capacity: 1270 mA h g^?1) and polysulfide retention (capacity fade rate: 0.13% cycle^?1 after 200 cycles). These results show that the modified thin‐film‐coating technique is a viable approach to designing ultratough ACNF‐filter‐coated separators with outstanding mechanical strength and flexibility as an advanced component in Li–S cells.     

Porphyrin‐Derived Graphene‐Based Nanosheets Enabling Strong Polysulfide Chemisorption and Rapid Kinetics in Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries hold great promise as a next‐generation battery system because of their extremely high theoretical energy density and low cost. However, ready lithium polysulfide (LiPS) diffusion and sluggish redox kinetics hamper their cyclability and rate capability. Herein, porphyrin‐derived graphene‐based nanosheets (PNG) are proposed for Li–S batteries, which are achieved by pyrolyzing a conformal and thin layer of 2D porphyrin organic framework on graphene to form carbon nanosheets with a spatially engineered nitrogen‐dopant‐enriched skin and a highly conductive skeleton. The atomic skin is decorated with fully exposed lithiophilic sites to afford strong chemisorption to LiPSs and improve electrolyte wettability, while graphene substrate provides speedy electron transport to facilitate redox kinetics of sulfur species. The use of PNG as a lightweight interlayer enables efficient operation of Li–S batteries in terms of superb cycle stability (cyclic decay rate of 0.099% during 300 cycles at 0.5 C), good rate capability (988 mAh g^?1 at 2.0 C), and impressive sulfur loading (areal capacity of 8.81 mAh cm^?2 at a sulfur loading of 8.9 mg cm^?2). The distinct interfacial strategy is expected to apply to other conversion reaction batteries relying on dissolution–precipitation mechanisms and requiring interfacial charge‐ and mass‐transport‐mediation concurrently.     

An Effective Lithium Sulfide Encapsulation Strategy for Stable Lithium–Sulfur Batteries

With a high theoretical capacity of 1162 mA h g^?1, Li₂S is a promising cathode that can couple with silicon, tin, or graphite anodes for next‐generation energy storage devices. Unfortunately, Li₂S is highly insulating, exhibits large charge overpotential, and suffers from active‐material loss as soluble polysulfides during battery cycling. To date, low‐cost, scalable synthesis of an electrochemically active Li₂S cathode remains a challenge. This work demonstrates that the low conductivity and material loss issues associated with Li₂S cathodes can be overcome by forming a stable, conductive encapsulation layer at the surface of the Li₂S bulk particles through in situ surface reactions between Li₂S and electrolyte additives containing transition‐metal salts. It is identified that the electronic band structure in the valence band region of the thus‐generated encapsulation layers, consisting largely of transition‐metal sulfides, determines the initial charging resistance of Li₂S. Furthermore, among the transition metals tested, the encapsulation layer formed with an addition of 10 wt% manganese (II) acetylacetonate salt proved to be robust within the cycling window, which is attributed to the chemically generated MnS surface species. This work provides an effective strategy to use micrometer‐sized Li₂S directly as a cathode material and opens up new prospects to tune the surface properties of electrode materials for energy‐storage applications.     

Design Principles for Optimum Performance of Porous Carbons in Lithium–Sulfur Batteries

A series of experiments is presented that establishes for the first time the role of some of the key design parameters of porous carbons including surface area, pore volume, and pore size on battery performance. A series of hierarchical porous carbons is used as a model system with an open, 3D, interconnected porous framework and highly controlled porosity. Specifically, carbons with surface areas ranging from ≈500–2800 m² g^?1, pore volume from ≈0.6–5 cm³ g^?1, and pore size from micropores (≈1 nm) to large mesopores (≈30 nm) are synthesized and tested. At high sulfur loadings (≈80 wt% S), pore volume is more important than surface area with respect to sulfur utilization. Mesopore size, in the range tested, does not affect the sulfur utilization. No relationship between porosity and long‐term cycle life is observed. All systems fail after 200–300 cycles, which is likely due to the consumption of the LiNO₃ additive over cycling. Moreover, cryo‐scanning transmission electron microscopy imaging of these carbon–sulfur composites combined with X‐ray diffraction (XRD) provides further insights into the effect of initial sulfur distribution on sulfur utilization while also revealing the inadequacy of the indirect characterization techniques alone in reliably predicting distribution of sulfur within porous carbon matrices.     

Simultaneous Cobalt and Phosphorous Doping of MoS2 for Improved Catalytic Performance on Polysulfide Conversion in Lithium–Sulfur Batteries

The lithium–sulfur batteries are susceptible to the loss of sulfur as dissolved polysulfides in the electrolyte and their ensuing redox shutting effect. The acceleration of the conversion kinetics of dissolved polysulfides into the insoluble sulfur and lithium sulfide via electrocatalysis has the appeal of being a root‐cause solution. MoS₂ is the most common electrocatalyst used for this purpose. It is demonstrated that how the effectiveness can be improved by simultaneous cobalt and phosphorus doping of MoS₂ nanotubes (P‐Mo_0.9Co_0.1S₂‐2, containing 1.81 at% of P). Cobalt doping induces the transformation of MoS₂ from 2H phase to metallic 1T phase, which improves the electrical conductivity of the MoS₂. The Co–P coordinated sites on the catalyst surface are highly active for the polysulfide conversion reactions. Consequently, a sulfur cathode with P‐Mo_0.9Co_0.1S₂‐2 can decrease the capacity fade rate from 0.28% per cycle before modification (over 150 cycles at 0.5C rate) to 0.046% per cycle after modification (over 600 cycles at 1C rate). P‐Mo_0.9Co_0.1S₂‐2 also enhances the high rate performance from a capacity of 338 to 931 mAh g^?1 at 6C rate. The results of this study provide the first direct evidence of the beneficial effects of heteroatom codoping of polysulfide conversion catalysts.     

A Comprehensive Approach toward Stable Lithium–Sulfur Batteries with High Volumetric Energy Density

A comprehensive approach is reported to construct stable and high volumetric energy density lithium–sulfur batteries, by coupling a multifunctional and hierarchically structured sulfur composite with an in‐situ cross‐linked binder. Through a combination of first‐principles calculations and experimental studies, it is demonstrated that a hybrid sulfur host composed by alternately stacking graphene and layered graphitic carbon nitride embraces high electronic conductivity as well as high polysulfide adsorptivity. It is further shown that the cross‐linked elastomeric binder empowers the hierarchical sulfur composites—multi‐microns in size—with the ability to form crack‐free and compact high‐loading electrodes using traditional slurry processing. Using this approach, electrodes with up to 14.9 mg cm^?2 sulfur loading and an extremely low electrolyte/sulfur ratio as low as 3.5: 1 µL mg^?1 are obtained. This study sheds light on the essential role of multifaceted cathode design and further on the challenges facing lithium metal anodes in building high volumetric energy density lithium–sulfur batteries.     

Tailoring the Pore Size of a Polypropylene Separator with a Polymer Having Intrinsic Nanoporosity for Suppressing the Polysulfide Shuttle in Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are being considered as one of the most promising candidates for the development of next‐generation energy storage technologies. Although much progress has been made over the past decade, the development of Li–S batteries is still held back by a crucial polysulfide‐shuttle problem. To address this critical issue, an approach to reduce the pore size of the separator is presented here, to prevent the penetration of soluble polysulfide species. A polymer with intrinsic nanoporosity (PIN) is developed within the micrometer‐scale pores of a polypropylene separator. The framework of polypropylene acts as a skeleton to sustain reliable mechanical properties with the thin membrane. Upon the formation of PIN in the pores, the polypropylene separator maintains its thickness. With the thin PIN–polypropylene membrane, the Li–S cells can be operated with a relatively high sulfur loading. The PIN allows the transport of Li⁺ ions, but suppresses the penetration of the polysulfide species. The Li–S batteries with the PIN‐modified polypropylene separator exhibit enhanced cycling performance.     

Designing Safe Electrolyte Systems for a High‐Stability Lithium–Sulfur Battery

Safety, nontoxicity, and durability directly determine the applicability of the essential characteristics of the lithium (Li)‐ion battery. Particularly, for the lithium–sulfur battery, due to the low ignition temperature of sulfur, metal lithium as the anode material, and the use of flammable organic electrolytes, addressing security problems is of increased difficulty. In the past few years, two basic electrolyte systems are studied extensively to solve the notorious safety issues. One system is the conventional organic liquid electrolyte, and the other is the inorganic solid‐state or quasi‐solid‐state composite electrolyte. Here, the recent development of engineered liquid electrolytes and design considerations for solid electrolytes in tackling these safety issues are reviewed to ensure the safety of electrolyte systems between sulfur cathode materials and the lithium‐metal anode. Specifically, strategies for designing and modifying liquid electrolytes including introducing gas evolution, flame, aqueous, and dendrite‐free electrolytes are proposed. Moreover, the considerations involving a high‐performance Li⁺ conductor, air‐stable Li⁺ conductors, and stable interface performance between the sulfur cathode and the lithium anode for developing all‐solid‐state electrolytes are discussed. In the end, an outlook for future directions to offer reliable electrolyte systems is presented for the development of commercially viable lithium–sulfur batteries.     

Carbon Quantum Dots–Modified Interfacial Interactions and Ion Conductivity for Enhanced High Current Density Performance in Lithium–Sulfur Batteries

Significant progress has achieved for developing lithium–sulfur (Li–S) batteries with high specific capacities and excellent cyclic stability. However, some critical issues emerge when attempts are made to raise the areal sulfur loading and increase the operation current density to meet the standards for various industrial applications. In this work, polyethylenimine‐functionalized carbon dots (PEI‐CDots) are designed and prepared for enhancing performance of the Li–S batteries with high sulfur loadings and operation under high current density situations. Strong chemical binding effects towards polysulfides and fast ion transport property are achieved in the PEI‐CDots‐modified cathodes. At a high current density of 8 mA cm^?2, the PEI‐CDots‐modified Li–S battery delivers a reversible areal capacity of 3.3 mAh cm^?2 with only 0.07% capacity decay per cycle over 400 cycles at 6.6 mg sulfur loading. Detailed analysis, involving electrochemical impedance spectroscopy, cyclic voltammetry, and density functional theory calculations, is done for the elucidation of the underlying enhancement mechanism by the PEI‐CDots. The strongly localized sulfur species and the promoted Li⁺ ion conductivity at the cathode–electrolyte interface are revealed to enable high‐performance Li–S batteries with high sulfur loading and large operational current.     

Solvent‐Mediated Li2S Electrodeposition: A Critical Manipulator in Lithium–Sulfur Batteries

Controlling electrochemical deposition of lithium sulfide (Li₂S) is a major challenge in lithium–sulfur batteries as premature Li₂S passivation leads to low sulfur utilization and low rate capability. In this work, the solvent's roles in controlling solid Li₂S deposition are revealed, and quantitative solvent‐mediated Li₂S growth models as guides to solvent selection are developed. It is shown that Li₂S electrodeposition is controlled by electrode kinetics, Li₂S solubility, and the diffusion of polysulfide/Li₂S, which is dictated by solvent's donicity, polarity, and viscosity, respectively. These solvent‐controlled properties are essential factors pertaining to the sulfur utilization, energy efficiency and reversibility of lithium–sulfur batteries. It is further demonstrated that the solvent selection criteria developed in this study are effective in guiding the search for new and more effective electrolytes, providing effective screening and design criteria for computational and experimental electrolyte development for lithium–sulfur batteries.     

Facile Synthesis of Crumpled Nitrogen‐Doped MXene Nanosheets as a New Sulfur Host for Lithium–Sulfur Batteries

Crumpled nitrogen‐doped MXene nanosheets with strong physical and chemical coadsorption of polysulfides are synthesized by a novel one‐step approach and then utilized as a new sulfur host for lithium–sulfur batteries. The nitrogen‐doping strategy enables introduction of heteroatoms into MXene nanosheets and simultaneously induces a well‐defined porous structure, high surface area, and large pore volume. The as‐prepared nitrogen‐doped MXene nanosheets have a strong capability of physical and chemical dual‐adsorption for polysulfides and achieve a high areal sulfur loading of 5.1 mg cm^–2. Lithium–sulfur batteries, based on crumpled nitrogen‐doped MXene nanosheets/sulfur composites, demonstrate outstanding electrochemical performances, including a high reversible capacity (1144 mA h g^–1 at 0.2C rate) and an extended cycling stability (610 mA h g^–1 at 2C after 1000 cycles).