A Progress Report on Metal–Sulfur Batteries

Nonaqueous conversion‐reaction sulfur chemistry has been attracting increasing attention over the past decade for the development of next‐generation lithium‐based batteries. Li–S batteries are currently approaching a nexus stage from lab‐scale experiments to possible pragmatic applications. Inspired by the success of Li–S chemistry, other metal–sulfur batteries with a variety of metallic anodes, such as sodium, potassium, magnesium, calcium, and aluminum, have also started to attract attention. In comparison to lithium, Na, Mg, Al, K, and Ca are naturally more abundant and affordable. The Na‐S, Mg‐S, Al‐S, K‐S, and Ca‐S battery systems provide a great potential for improving the volumetric energy density of sulfur‐based batteries. The multivalent metal‐sulfur systems, Mg‐S, Al‐S, and Ca‐S, offer better safety features as well. However, the research and development on Na‐S, Mg‐S, Al‐S, K‐S, and Ca‐S batteries is far behind the Li–S system due to many critical challenges. In this progress report, the fundamental principles of various metal–sulfur chemistries are first presented and compared. Then, the historical progress, recent advances, and key challenges of the Li–S, Na‐S, Mg‐S, Al‐S, K‐S, and Ca‐S systems are summarized and discussed. Finally, future efforts and directions for both the fundamental and practical research are prospected.     

Activation of Oxygen‐Stabilized Sulfur for Li and Na Batteries

The sulfur‐based cathode materials suffer severely from poor cycling stability and low utilization, incurred by their stepwise reaction mechanism that generates polysulfide intermediates and the subsequent irreversible losses. In this work, those issues are significantly relieved by entrapping sulfur species in carbon host rich in oxygen functionalities. Sulfur species in such C/S composite are highly stabilized by their interaction with oxygen, and can deliver a reversible capacity of 508 mAh/(g of S) for 2000 cycles when coupled with Li, representing the best cycling stability up to date. More interestingly, extra capacity can be accessed by simply prelithiating the oxygen‐stabilized C/S composites down to 0.6 V for a few cycles, which enables a high capacity of 1621 mAh/(g of S) that eventually stabilizes at 820 mAh/(g of S) for 600 cycles. The mechanism for this electrochemical activation process is investigated with both spectroscopic and electrochemical techniques, which reveal that the inactive sulfur bonded to oxygen is liberated in the initial deep lithiation precycles and becomes electrochemically active. The oxygen‐stabilized sulfur can also be coupled with Na anode to form Na/S cell, confirming that the formation of S?O interaction in C/S composite generates promising sulfur‐based cathode materials for Li–S and Na–S batteries.     

Lithium–Magnesium Alloy as a Stable Anode for Lithium–Sulfur Battery

Lithium–sulfur (Li–S) batteries are regarded as the promising next‐generation energy storage device due to the high theoretical energy density and low cost. However, the practical application of Li–S batteries is still limited owing to the cycle stability of both the sulfur cathode and lithium anode. In particular, the instability in the bulk and at the surface of the lithium anode during cycling becomes a huge obstacle for the practical application of Li–S battery. Herein, a Li‐rich lithium–magnesium (Li–Mg) alloy is investigated as an anode for Li–S batteries, based on the consideration of improving the stability in the bulk and at the surface of the lithium anode. Our experimental results reveal that the robust passivation layer is formed on the surface of the Li–Mg alloy anode, which is helpful to reduce side reactions, and enable the smooth surface morphology of anode during cycling. Meanwhile, the mixed electron and Li‐ion conducting matrix of the Li‐poor Li–Mg alloy as a porous skeleton structure can also be formed after delithiation, which can guarantee the structural integrity of the anode in the bulk during Li stripping/plating process. Therefore, the Li‐rich Li–Mg alloy is demonstrated to be a very promising anode material for Li–S battery.     

Multifunctional Chitosan–rGO Network Binder for Enhancing the Cycle Stability of Li–S Batteries

Although Li–S batteries are currently receiving great interest, due to their high energy density and the low cost of sulfur, practical applications are still inhibited by capacity fading that is caused by various undesirable processes. In this study, a new multifunctional network binder composed of chitosan and reduced graphene oxide (rGO) is introduced to enhance the capacitive performance of Li–S batteries. Chitosan is reacted with graphene oxide in aqueous solution to produce a homogenous network, which effectively enhances the redox system by entrapping lithium polysulfides, reinforces the mechanical properties, and allows electrical conductivity through the binder system. Collaborative relationship‐based chitosan–rGO network binder allows noteworthy improvement in the capacity decay of 0.016% per cycle at 1 C for 1000 cycles.     

Interface Design and Development of Coating Materials in Lithium–Sulfur Batteries

High‐energy Li‐S batteries have received extensive attention and are considered to be the most promising next‐generation electric energy storage devices beyond Li‐ion batteries. Interface design is an important direction to address challenges in the development of Li–S batteries. This review summarizes recently developed coatings and interlayer materials at various interfaces of Li–S batteries. In particular, advanced nanostructures and novel fabrication methods of coating and interlayer materials applied to Li–S batteries are highlighted. Furthermore, underlying mechanisms at the interfaces and electrochemical performance of the developed Li–S batteries are also discussed. Finally, existing challenges and the future development of interface design in high‐energy Li–S batteries are summarized and prospected.     

Reversible Crosslinked Polymer Binder for Recyclable Lithium Sulfur Batteries with High Performance

Owing to the negative impact of the extensive utilization of batteries on the environment, sustainability of the cells needs to be included in the systemic research of batteries. Herein, a dissolvable ionic crosslinked polymer (DICP) is exploited as a binder for lithium–sulfur batteries by crosslinking the polyacrylic acid and polyethyleneimine through carboxy‐amino ionic interaction. This interaction is pH‐controlled, and therefore, the crosslinked binder network can be readily dissociated under basic conditions, providing a facile strategy enabling valuable components recycled through a convenient washing method. The sulfur cathode prepared using the recycled carbon–sulfur composite can deliver comparable capacity as that of fresh electrode. In addition, evidence from cell performance and characterizations, such as in situ X‐ray absorption spectroscopy, in situ UV–visible spectroscopy, X‐ray photoelectron spectroscopy, and density functional theory calculation, confirms that DICP is a more effective binder than its commercial counterpart on suppressing polysulfide dissolution in the electrolyte. Exploiting reversible crosslinked polymer binder for recyclable Li–S batteries with ameliorated electrochemical performance, this study illuminates sustainable development for large‐scale energy storage systems.     

Yolk–Shell Structured Assembly of Bamboo‐Like Nitrogen‐Doped Carbon Nanotubes Embedded with Co Nanocrystals and Their Application as Cathode Material for Li–S Batteries

Despite their high theoretical specific capacity (1675 mA h g^?1), the practical application of Li–S batteries remains limited because the capacity rapidly degrades through severe dissolution of lithium polysulfide and the rate capability is low because of the low electronic conductivity of sulfur. This paper describes novel hierarchical yolk–shell microspheres comprising 1D bamboo‐like N‐doped carbon nanotubes (CNTs) encapsulating Co nanoparticles (Co@BNCNTs YS microspheres) as efficient cathode hosts for Li–S batteries. The microspheres are produced via a two‐step process that involves generation of the microsphere followed by N‐doped CNTs growth. The hierarchical yolk–shell structure enables efficient sulfur loading and mitigates the dissolution of lithium polysulfides, and metallic Co and N doping improves the chemical affinity of the microspheres with sulfur species. Accordingly, a Co@BNCNTs YS microsphere‐based cathode containing 64 wt% sulfur exhibits a high discharge capacity of 700.2 mA h g^?1 after 400 cycles at a current density of 1 C (based on the mass of sulfur); this corresponds to a good capacity retention of 76% and capacity fading rate of 0.06% per cycle with an excellent rate performance (752 mA h g^?1 at 2.0 C) when applied as cathode hosts for Li–S batteries.     

In Situ Synthesis of Bipyramidal Sulfur with 3D Carbon Nanotube Framework for Lithium–Sulfur Batteries

A one‐pot synthesis of three‐dimensional carbon nanotube frameworks with bipyramidal sulfur particles and the application of these materials for a cathode in lithium–sulfur (Li–S) battery are reported. By simple mixing of multi‐walled carbon nanotubes (MWCNTs), sulfur powder, and capping agents in water/tetrahydrofuran, micrometer bipyramidal sulfur particles enclosed with MWCNTs are synthesized. The MWCNTs spontaneously form a 3D conducting network inside and outside the sulfur particle. Along the edge of MWCNT framework, a sulfur particle‐free region is present, which comprises ≈35 vol% based on the total volume. These sulfur‐MWCNT bipyramidal particles are mixed with conductive carbon additive to prepare binder‐free cathode for Li–S cells. The Li–S cells deliver a specific discharge capacity of ≈1600 mAh g^?1 at 0.05 C on the first cycle. In particular, these Li–S cells show high rate stability and Coulombic efficiency with deep discharge and charge (1.0–3.0 V vs Li/Li⁺). This resultant performance can arise from 1) homogeneous distribution of the conducting MWCNT framework and the carbon additive coating layer on the sulfur particle, which allow rapid Li⁺ ion/electrolyte diffusion and mitigation of polysulfide shuttle, respectively, and 2) the sulfur‐free buffer space accommodating volume expansion. It is expected that this new cathode design with the simple synthetic process can reduce the number of preparation steps, thus allowing the construction of a low‐cost Li–S battery.     

Phase Inversion Strategy to Flexible Freestanding Electrode: Critical Coupling of Binders and Electrolytes for High Performance Li–S Battery

Development of flexible and freestanding electrode is attracting great attention in lithium–sulfur (Li–S) batteries, but the severe capacity fading caused by the lithium polysulfides (PSs) shuttle effect remains challenging. Herein, a completely new polymeric binder of polyethersulfone is introduced. Not only it enables massive production of flexible/current‐free electrode by a novel concept of “phase‐inversion” approach but also the resultant polymeric networks can effectively trap the soluble polysulfides within the electrode, owing to the higher hydrophilicity and stronger affinity properties than the routine polyvinylidene fluoride. Coupling with polysulfide‐based electrolyte, the Li–S cell shows a higher capacity of 1141 mAh g^?1, a lower polarization of 192 mV, and a more stable capacity retention with 100% Coulombic efficiency over 100 cycles at 0.25C. The advantages of favored binder and electrolyte are further demonstrated in lithium‐ion sulfur full battery with lithiated graphite anode, which demonstrates much improved performance than those previously reported. This work not only introduces a novel strategy for flexible freestanding electrodes but also enlightens the importance of coupling electrodes and electrolytes to higher performances for Li–S battery.     

Constructing a High‐Strength Solid Electrolyte Layer by In Vivo Alloying with Aluminum for an Ultrahigh‐Rate Lithium Metal Anode

The serious safety issues caused by uncontrollable lithium (Li) dendrite growth, especially at high current densities, seriously hamper the rapid charging of Li metal‐based batteries. Here, the construction of Al–Li alloy/LiCl‐based Li anode (ALA/Li anode) is reported by displacement and alloying reaction between an AlCl₃‐ionic liquid and a Li foil. This layer not only has high ion‐conductivity and good electron resistivity but also much improved mechanical strength (776 MPa) as well as good flexibility compared to a common solid electrolyte interphase layer (585 MPa). The high mechanical strength of the Al–Li alloy interlayer effectively eliminates volume expansion and dendrite growth in Li metal batteries, so that the ALA/Li anode achieves superior cycling for 1600 h (2.0 mA cm^?2) and 1000 cycles at an ultrahigh current density (20 mA cm^?2) without dendrite formation in symmetric batteries. In lithium–sulfur batteries, the dense alloy layer prevents direct contact between polysulfides and Li metal, inhibiting the shuttle effect and electrolyte decomposition. Long cycling performance is achieved even at a high current density (4 C) and a low electrolyte/sulfur (6.0 µL mg^?1). This easy fabrication process provides a strategy to realize reliable safety during the rapid charging of Li‐metal batteries.     

3D Carbonaceous Current Collectors: The Origin of Enhanced Cycling Stability for High‐Sulfur‐Loading Lithium–Sulfur Batteries

The cycling stability of high‐sulfur‐loading lithium–sulfur (Li–S) batteries remains a great challenge owing to the exaggerated shuttle problem and interface instability. Despite enormous efforts on design of advanced electrodes and electrolytes, the stability issue raised from current collectors has been rarely concerned. This study demonstrates that rationally designing a 3D carbonaceous macroporous current collector is an efficient and effective “two‐in‐one” strategy to improve the cycling stability of high‐sulfur‐loading Li–S batteries, which is highly versatile to enable various composite cathodes with sulfur loading >3.7 mAh cm^?2. The best cycling performance can be achieved upon 950 cycles with a very low decay rate of 0.029%. Moreover, the origin of such a huge enhancement in cycling stability is ascribed to (1) the inhibition of electrochemical corrosion, which severely occurs on the typical Al foil and disables its long‐term sustainability for charge transfer, and (2) the passivation of cathode surface. The role of the chemical resistivity against corrosion and favorable macroscopic porous structure is highlighted for exploiting novel current collectors toward exceptional cycling stability of high‐sulfur‐loading Li–S batteries.     

Functional Mesoporous Carbon‐Coated Separator for Long‐Life,High‐Energy Lithium–Sulfur Batteries

The lithium–sulfur (Li–S) battery is regarded as the most promising rechargeable energy storage technology for the increasing applications of clean energy transportation systems due to its remarkable high theoretical energy density of 2.6 kWh kg^?1, considerably outperforming today's lithium‐ion batteries. Additionally, the use of sulfur as active cathode material has the advantages of being inexpensive, environmentally benign, and naturally abundant. However, the insulating nature of sulfur, the fast capacity fading, and the short lifespan of Li–S batteries have been hampered their commercialization. In this paper, a functional mesoporous carbon‐coated separator is presented for improving the overall performance of Li–S batteries. A straightforward coating modification of the commercial polypropylene separator allows the integration of a conductive mesoporous carbon layer which offers a physical place to localize dissolved polysulfide intermediates and retain them as active material within the cathode side. Despite the use of a simple sulfur–carbon black mixture as cathode, the Li–S cell with a mesoporous carbon‐coated separator offers outstanding performance with an initial capacity of 1378 mAh g^?1 at 0.2 C, and high reversible capacity of 723 mAh g^?1, and degradation rate of only 0.081% per cycle, after 500 cycles at 0.5 C.     

Cobalt‐Doped SnS2 with Dual Active Centers of Synergistic Absorption‐Catalysis Effect for High‐S Loading Li‐S Batteries

The application of Li‐S batteries is hindered by low sulfur utilization and rapid capacity decay originating from slow electrochemical kinetics of polysulfide transformation to Li₂S at the second discharge plateau around 2.1 V and harsh shuttling effects for high‐S‐loading cathodes. Herein, a cobalt‐doped SnS₂ anchored on N‐doped carbon nanotube (NCNT@Co‐SnS₂) substrate is rationally designed as both a polysulfide shield to mitigate the shuttling effects and an electrocatalyst to improve the interconversion kinetics from polysulfides to Li₂S. As a result, high‐S‐loading cathodes are demonstrated to achieve good cycling stability with high sulfur utilization. It is shown that Co‐doping plays an important role in realizing high initial capacity and good capacity retention for Li‐S batteries. The S/NCNT@Co‐SnS₂ cell (3 mg cm^?2 sulfur loading) delivers a high initial specific capacity of 1337.1 mA h g^?1 (excluding the Co‐SnS₂ capacity contribution) and 1004.3 mA h g^?1 after 100 cycles at a current density of 1.3 mA cm^?2, while the counterpart cell (S/NCNT@SnS₂) only shows an initial capacity of 1074.7 and 843 mA h g^?1 at the 100th cycle. The synergy effect of polysulfide confinement and catalyzed polysulfide conversion provides an effective strategy in improving the electrochemical performance for high‐sulfur‐loading Li‐S batteries.     

Hierarchical Defective Fe3‐xC@C Hollow Microsphere Enables Fast and Long‐Lasting Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries present one of the most promising energy storage systems owing to their high energy density and low cost. However, the commercialization of Li–S batteries is still hindered by several technical issues; the notorious polysulfide shuttling and sluggish sulfur conversion kinetics. In this work, unique hierarchical Fe_3‐xC@C hollow microspheres as an advanced sulfur immobilizer and promoter for enabling high‐efficiency Li–S batteries is developed. The porous hollow architecture not only accommodates the volume variation upon the lithiation–delithiation processes, but also exposes vast active interfaces for facilitated sulfur redox reactions. Meanwhile, the mesoporous carbon coating establishes a highly conductive network for fast electron transportation. More importantly, the defective Fe_3‐xC nanosized subunits impose strong LiPS adsorption and catalyzation, enabling fast and durable sulfur electrochemistry. Attributed to these structural superiorities, the obtained sulfur electrodes exhibit excellent electrochemical performance, i.e., high areal capacity of 5.6 mAh cm^?2, rate capability up to 5 C, and stable cycling over 1000 cycles with a low capacity fading rate of 0.04% per cycle at 1 C, demonstrating great promise in the development of practical Li–S batteries.     

Effect of the Anion Activity on the Stability of Li Metal Anodes in Lithium‐Sulfur Batteries

With the significant progress made in the development of cathodes in lithium‐sulfur (Li‐S) batteries, the stability of Li metal anodes becomes a more urgent challenge in these batteries. Here the systematic investigation of the stability of the anode/electrolyte interface in Li‐S batteries with concentrated electrolytes containing various lithium salts is reported. It is found that Li‐S batteries using LiTFSI‐based electrolytes are more stable than those using LiFSI‐based electrolytes. The decreased stability is because the N–S bond in the FSI^? anion is fairly weak and the scission of this bond leads to the formation of lithium sulfate (LiSO_x) in the presence of polysulfide species. In contrast, in the LiTFSI‐based electrolyte, the lithium metal anode tends to react with polysulfide to form lithium sulfide (LiS_x), which is more reversible than LiSO_x formed in the LiFSI‐based electrolyte. This fundamental difference in the bond strength of the salt anions in the presence of polysulfide species leads to a large difference in the stability of the anode‐electrolyte interface and performance of the Li‐S batteries with electrolytes composed of these salts. Therefore, anion selection is one of the key parameters in the search for new electrolytes for stable operation of Li‐S batteries.     

Elastic Carbon Nanotube Aerogel Meets Tellurium Nanowires: A Binder‐ and Collector‐Free Electrode for Li‐Te Batteries

Rechargeable Li batteries based on group VIA element cathodes, such as tellurium, are emerging due to their capability to provide equivalent theoretical volumetric capacity density to O and S, as well as an improved activity to react with Li. Herein, bifunctional and elastic carbon nanotube (CNT) aerogel is fabricated to combine with Te nanowires, yielding two types of binder/collector‐free Te cathodes to assemble Li‐Te batteries. The CNTs with high electronic conductivity and hollow porous structure enable stable electric contact and fast transportation of Li⁺, while trapping Te and Li₂Te in its network, triggering fast and stable Li‐Te electrochemistry. Both cathodes are also provided with fine compressibility, helping to buffer their volume changes during lithiation/delithiation and improving electrode integrity. Both cathodes deliver high specific capacity, fine cycling stability, and favorable high‐rate capability, proving their competence in building high‐energy rechargeable Li‐ion batteries.     

The Fusion of Imidazolium‐Based Ionic Polymer and Carbon Nanotubes: One Type of New Heteroatom‐Doped Carbon Precursors for High‐Performance Lithium–Sulfur Batteries

Rational design of sulfur host materials with high electrical conductivity and strong polysulfides (PS) confinement is indispensable for high‐performance lithium–sulfur (Li–S) batteries. This study presents one type of new polymer material based on main‐chain imidazolium‐based ionic polymer (ImIP) and carbon nanotubes (CNTs); the polymer composites can serve as a precursor of CNT/NPC‐300, in which close coverage and seamless junction of CNTs by N‐doped porous carbon (NPC) form a 3D conductive network. CNT/NPC‐300 inherits and strengthens the advantages of both high electrical conductivity from CNTs and strong PS entrapping ability from NPC. Benefiting from the improved attributes, the CNT/NPC‐300‐57S electrode shows much higher reversible capacity, rate capability, and cycling stability than NPC‐57S and CNTs‐56S. The initial discharge capacity of 1065 mA h g^?1 is achieved at 0.5 C with the capacity retention of 817 mA h g^?1 over 300 cycles. Importantly, when counter bromide anion in the composite of CNTs and ImIP is metathesized to bis(trifluoromethane sulfonimide), heteroatom sulfur is cooperatively incorporated into the carbon hosts, and the surface area is increased with the promotion of micropore formation, thus further improving electrochemical performance. This provides a new method for optimizing porous properties and dopant components of the cathode materials in Li–S batteries.     

Efficient Ni2Co4P3 Nanowires Catalysts Enhance Ultrahigh‐Loading Lithium–Sulfur Conversion in a Microreactor‐Like Battery

High‐loading lithium–sulfur (Li–S) batteries suffer from poor electrochemical properties. Electrocatalysts can accelerate polysulfides conversion and suppress their migration to improve battery cyclability. However, catalysts for Li–S batteries usually lack a rational design. A d‐band tuning strategy is reported by alloying cobalt to metal sites of Ni₂P to enhance the interaction between polysulfides and catalysts. A molecular or atomic level analysis reveals that Ni₂Co₄P₃ is able to weaken the S? S bonds and lower the activation energy of polysulfides conversion, which is confirmed with temperature‐dependent experiments. Ni₂Co₄P₃ nanowires are further fabricated on a porous nickel scaffold to unfold the catalytic activity by its large surface area. Using a simple ion‐selective filtration shell, a microreactor‐like S cathode (MLSC) is constructed to realize ultrahigh S loading (25 mg cm^?2). As such, a microreactor design integrates reaction and separation in one cell and can effectively address the polysulfide issues, the MLSC cell demonstrates excellent properties of cyclability and high capacity (1223 mAh g^?1 at 0.1 C). More importantly, the catalyst's designs and microreactor strategies provide new approaches for addressing the complicated issues of Li–S batteries.     

Quasi‐Stable Electroless Ni–P Deposition: A Pivotal Strategy to Create Flexible Li–S Pouch Batteries with Bench Mark Cycle Stability and Specific Capacity

Development of flexible Li–S batteries brings along the flourishing prospective for energy‐hungry wearable devices. However, it is still seriously restricted due to lack of facile methods to solve its inherent problems and flexible device‐related current collection issues. Herein, quasi‐stable electroless deposition method is firstly proposed to solve these problems by fabricating 3D tunable Ni–P networks in the C/S free‐standing electrode. The ultrathin Ni–P layers which are highly conductive and strongly adhesive with electrode substrates improve the electronic conductivity by two orders of magnitude and rise initial specific capacity from 1200 to 1600 mAh g^?1. The harmful shuttle effect of polysulfide is also effectively alleviated due to the chemical adsorption and physical sieving properties of the 3D networks. The flexible pouch Li–S batteries assembled with commercially applicable structure also show high flexibility and as high as 1420 mAh g^?1 output capacity at 0.1C in cycling test. This method can definitely be extended to other flexible devices such as Li‐ion batteries, Li–O₂ batteries, and supercapacitors.     

Biomimetic Molecule Catalysts to Promote the Conversion of Polysulfides for Advanced Lithium–Sulfur Batteries

To overcome the shuttle effect in Li–S batteries, novel biomimetic molecule catalysts are synthesized by grafting hemin molecules to three functionalized carbon nanotube systems (CNTs–COOH, CNTs–OH, and CNTs–NH₂). The Li–S battery using the CNTs–COOH@hemin cathode exhibits the optimal initial specific capacity (1637.8 mAh g^?1) and cycle durability (up to 1800 cycles). Various in situ characterization techniques, such as Raman spectroscopy, Fourier‐transform infrared reflection absorption spectroscopy, and UV–vis spectroscopy, combined with density functional theory computations are used to investigate the structure–reactivity correlation and the working mechanism in the Li–S system. It is demonstrated that the unique structure of the CNTs‐COOH@hemin composite with good conductivity and adequate active sites resulting from molecule catalyst as well as the strong absorption to polysulfides entrapped by the coordinated Fe(III) complex with Fe? O bond enables the homogeneous dispersion of S, facilitates the catalysis and conversion of polysulfides, and improves the battery's performance.