Promoting the Transformation of Li2S2 to Li2S: Significantly Increasing Utilization of Active Materials for High‐Sulfur‐Loading Li–S Batteries

Lithium–sulfur (Li–S) batteries with high sulfur loading are urgently required in order to take advantage of their high theoretical energy density. Ether‐based Li–S batteries involve sophisticated multistep solid–liquid–solid–solid electrochemical reaction mechanisms. Recently, studies on Li–S batteries have widely focused on the initial solid (sulfur)–liquid (soluble polysulfide)–solid (Li₂S₂) conversion reactions, which contribute to the first 50% of the theoretical capacity of the Li–S batteries. Nonetheless, the sluggish kinetics of the solid–solid conversion from solid‐state intermediate product Li₂S₂ to the final discharge product Li₂S (corresponding to the last 50% of the theoretical capacity) leads to the premature end of discharge, resulting in low discharge capacity output and low sulfur utilization. To tackle the aforementioned issue, a catalyst of amorphous cobalt sulfide (CoS₃) is proposed to decrease the dissociation energy of Li₂S₂ and propel the electrochemical transformation of Li₂S₂ to Li₂S. The CoS₃ catalyst plays a critical role in improving the sulfur utilization, especially in high‐loading sulfur cathodes (3–10 mg cm^?2). Accordingly, the Li₂S/Li₂S₂ ratio in the discharge products increased to 5.60/1 from 1/1.63 with CoS₃ catalyst, resulting in a sulfur utilization increase of 20% (335 mAh g^?1) compared to the counterpart sulfur electrode without CoS₃.     

Redox Mediators for Li–O2 Batteries: Status and Perspectives

Li–O₂ batteries have received much attention due to their extremely large theoretical energy density. However, the high overpotentials required for charging Li–O₂ batteries lower their energy efficiency and degrade the electrolytes and carbon electrodes. This problem is one of the main obstacles in developing practical Li–O₂ batteries. To solve this problem, it is important to facilitate the oxidation of Li₂O₂ upon charging by using effective electrocatalysis. Using solid catalysts is not too effective for oxidizing the electronically isolating Li‐peroxide layers. In turn, for soluble catalysts, red‐ox mediators (RMs) are homogeneously dissolved in the electrolyte solutions and can effectively oxidize all of the Li₂O₂ precipitated during discharge. RMs can decompose solid Li₂O₂ species no matter their size, morphology, or thickness and thus dramatically increase energy efficiency. However, some negative side effects, such as the shuttle reactions of RMs and deterioration of the Li‐metal occur. Therefore, it is necessary to study the activity and stability of RMs in Li–O₂ batteries in detail. Herein, recent studies related to redox mediators are reviewed and the mechanisms of redox reactions are illustrated. The development opportunities of RMs for this important battery technology are discussed and future directions are suggested.     

Single Nickel Atoms on Nitrogen‐Doped Graphene Enabling Enhanced Kinetics of Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries have arousing interest because of their high theoretical energy density. However, they often suffer from sluggish conversion of lithium polysulfides (LiPS) during the charge/discharge process. Single nickel (Ni) atoms on nitrogen‐doped graphene (Ni@NG) with Ni–N₄ structure are prepared and introduced to modify the separators of Li–S batteries. The oxidized Ni sites of the Ni–N₄ structure act as polysulfide traps, efficiently accommodating polysulfide ion electrons by forming strong S_x ^2????Ni? N bonding. Additionally, charge transfer between the LiPS and oxidized Ni sites endows the LiPS on Ni@NG with low free energy and decomposition energy barrier in an electrochemical process, accelerating the kinetic conversion of LiPS during the charge/discharge process. Furthermore, the large binding energy of LiPS on Ni@NG also shows its ability to immobilize the LiPS and further suppresses the undesirable shuttle effect. Therefore, a Li–S battery based on a Ni@NG modified separator exhibits excellent rate performance and stable cycling life with only 0.06% capacity decay per cycle. It affords fresh insights for developing single‐atom catalysts to accelerate the kinetic conversion of LiPS for highly stable Li–S batteries.     

Multifunctional Interlayer Based on Molybdenum Diphosphide Catalyst and Carbon Nanotube Film for Lithium–Sulfur Batteries

A multifunctional interlayer, composed of molybdenum diphosphide (MoP₂) nanoparticles and a carbon nanotube (CNT) film, is introduced into a lithium–sulfur (Li–S) battery system to suppress polysulfide migration. Molybdenum diphosphide acts as the catalyst and can capture polysulfides and improve the polysulfide conversion activity during the discharge/charge processes. The CNT film acts as a conductive skeleton to support the MoP₂ nanoparticles and to ensure their uniform distribution. The CNT film physically hinders polysulfide migration, acts as a current collector, and provides abundant electron pathways. The Li–S battery containing the multifunctional MoP₂/CNT interlayer exhibits excellent electrochemical performance. It delivers a reversible specific capacity of 905 mA h g^?1 over 100 cycles at 0.2 C, with a capacity decay of 0.152% per cycle. These results suggest the introduction of the multifunctional CNT/MoP₂ interlayer as an effective and practical method for producing high‐performance Li–S batteries.     

Anode Interface Engineering and Architecture Design for High‐Performance Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are considered as one of the most promising options to realize rechargeable batteries with high energy capacity. Previously, research has mainly focused on solving the polysulfides' shuttle, cathode volume changes, and sulfur conductivity problems. However, the instability of anodes in Li–S batteries has become a bottleneck to achieving high performance. Herein, the main efforts to develop highly stable anodes for Li–S batteries, mainly including lithium metal anodes, carbon‐based anodes, and alloy‐based anodes, are considered. Based on these anodes, their interfacial engineering and structure design are identified as the two most important directions to achieve ideal anodes. Because of high reactivity and large volume change during cycling, Li anodes suffer from severe side reactions and structure collapse. The solid electrolyte interphase formed in situ by modified electrolytes and ex situ artificial coating layers can enhance the interfacial stability of anodes. Replacing common Li foil with rationally designed anodes not only suppresses the formation of dendritic Li but also delays the failure of Li anodes. Manipulating the anode interface engineering and rationally designing anode architecture represents an attractive path to develop high‐performance Li–S batteries.     

Approaching Ultrastable High‐Rate Li–S Batteries through Hierarchically Porous Titanium Nitride Synthesized by Multiscale Phase Separation

Porous architectures are important in determining the performance of lithium–sulfur batteries (LSBs). Among them, multiscale porous architecutures are highly desired to tackle the limitations of single‐sized porous architectures, and to combine the advantages of different pore scales. Although a few carbonaceous materials with multiscale porosity are employed in LSBs, their nonpolar surface properties cause the severe dissolution of lithium polysulfides (LiPSs). In this context, multiscale porous structure design of noncarbonaceous materials is highly required, but has not been exploited in LSBs yet because of the absence of a facile method to control the multiscale porous inorganic materials. Here, a hierarchically porous titanium nitride (h‐TiN) is reported as a multifunctional sulfur host, integrating the advantages of multiscale porous architectures with intrinsic surface properties of TiN to achieve high‐rate and long‐life LSBs. The macropores accommodate the high amount of sulfur, facilitate the electrolyte penetration and transportation of Li⁺ ions, while the mesopores effectively prevent the LiPS dissolution. TiN strongly adsorbs LiPS, mitigates the shuttle effect, and promotes the redox kinetics. Therefore, h‐TiN/S shows a reversible capacity of 557 mA h g^?1 even after 1000 cycles at 5 C rate with only 0.016% of capacity decay per cycle.     

A Bifunctional Perovskite Promoter for Polysulfide Regulation toward Stable Lithium–Sulfur Batteries

Lithium‐sulfur (Li? S) batteries are strongly considered as the next‐generation rechargeable cells. However, both the shuttle of lithium polysulfides (LiPSs) and sluggish kinetics in random deposition of lithium sulfides (Li₂S) significantly degrade the capacity, rate performance, and cycling life of Li? S cells. Herein, bifunctional Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ perovskite nanoparticles (PrNPs) are proposed as a promoter to immobilize LiPSs and guide the deposition of Li₂S in a Li? S cell. The oxygen vacancy in PrNPs increases the metal reactivity to anchor LiPSs, and co‐existence of lithiophilic (O) and sulfiphilic (Sr) sites in PrNP favor the dual‐bonding (Li? O and Sr? S bonds) to anchor LiPSs. The high catalytic nature of PrNP facilitates the kinetics of LiPS redox reaction. The PrNP with intrinsic LiPS affinity serves as nucleation sites for Li₂S deposition and guides its uniform propagation. Therefore, the bifunctional LiPS promoter in Li? S cell yields high rate performance and ultralow capacity decay rate of 0.062% (a quarter of pristine Li? S cells). The proposed strategy to immobilize LiPSs, promotes the conversion of LiPS, and regulates deposition of Li₂S by an emerging perovskite promoter and is also expected to be applied in other energy conversion and storage devices based on multi‐electron redox reactions.     

MoS2/Celgard Separator as Efficient Polysulfide Barrier for Long‐Life Lithium–Sulfur Batteries

A high lithium conductive MoS₂/Celgard composite separator is reported as efficient polysulfides barrier in Li–S batteries. Significantly, thanks to the high density of lithium ions on MoS₂ surface, this composite separator shows high lithium conductivity, fast lithium diffusion, and facile lithium transference. When used in Li–S batteries, the separator is proven to be highly efficient for depressing polysulfides shuttle, leading to high and long cycle stability. With 65% of sulfur loading, the device with MoS₂/Celgard separator delivers an initial capacity of 808 mAh g^?1 and a substantial capacity of 401 mAh g^?1 after 600 cycles, corresponding to only 0.083% of capacity decay per cycle that is comparable to the best reported result so far. In addition, the Coulombic efficiency remains more than 99.5% during all 600 cycles, disclosing an efficient ionic sieve preventing polysulfides migration to the anode while having negligible influence on Li⁺ ions transfer across the separator. The strategy demonstrated in this work will open the door toward developing efficient separators with flexible 2D materials beyond graphene for energy‐storage devices.     

Nb2O5/RGO Nanocomposite Modified Separators with Robust Polysulfide Traps and Catalytic Centers for Boosting Performance of Lithium–Sulfur Batteries

Lithium–sulfur batteries (LSBs) have shown great potential for application in high‐density energy storage systems. However, the performance of LSBs is hindered by the shuttle effect and sluggish reaction kinetics of lithium polysulfides (LiPSs). Herein, heterostructual Nb₂O₅ nanocrystals/reduced graphene oxide (Nb₂O₅/RGO) composites are introduced into LSBs through separator modification for boosting the electrochemical performance. The Nb₂O₅/RGO heterostructures are designed as chemical trappers and conversion accelerators of LiPSs. Originating from the strong chemical interactions between Nb₂O₅ and LiPSs as well as the superior catalytic nature of Nb₂O₅, the Nb₂O₅/RGO nanocomposite possesses high trapping efficiency and efficient electrocatalytic activity to long‐chain LiPSs. The effective regulation of LiPSs conversion enables the LSBs enhanced redox kinetics and suppressed shuttle effect. Moreover, the Nb₂O₅/RGO nanocomposite has abundant sulfophilic sites and defective interfaces, which are beneficial for the nucleation and growth of Li₂S, as evidenced by analysis of the cycled separators. As a result, LSBs with the Nb₂O₅/RGO‐modified separators exhibit excellent rate capability (816 mAh g^?1 at 3 A g^?1) and cyclic performance (628 mAh g^?1 after 500 cycles). Remarkably, high specific capacity and stable cycling performance are demonstrated even at an elevated temperature of 50 °C or with higher sulfur loadings.