Polyoxometalate Modified Separator for Performance Enhancement of Magnesium–Sulfur Batteries

The magnesium–sulfur (Mg-S) battery has attracted considerable attention as a candidate of post-lithium battery systems owing to its high volumetric energy density, safety, and cost effectiveness. However, the known shuttle effect of the soluble polysulfides during charge and discharge leads to a rapid capacity fade and hinders the realization of sulfur-based battery technology. Along with the approaches for cathode design and electrolyte formulation, functionalization of separators can be employed to suppress the polysulfide shuttle. In this study, a glass fiber separator coated with decavanadate-based polyoxometalate (POM) clusters/carbon composite is fabricated by electrospinning technique and its impacts on battery performance and suppression of polysulfide shuttling are investigated. Mg–S batteries with such coated separators and non-corrosive Mg[B(hfip)₄]₂ electrolyte show significantly enhanced reversible capacity and cycling stability. Functional modification of separator provides a promising approach for improving metal–sulfur batteries.     

Review of Separator Modification Strategies: Targeting Undesired Anion Transport in Room Temperature Sodium–Sulfur/Selenium/Iodine Batteries

Rechargeable sodium–sulfur/selenium/iodine (Na–S/Se/I₂) batteries are regarded as promising candidates for large-scale energy storage systems, with the advantages of high energy density, low cost, and environmental friendliness. However, the electrochemical performances of Na–S/Se/I₂ batteries are still restricted by several inherent issues, including the “shuttle effect” of polysulfides/polyselenides/polyiodides (PSs/PSes/PIs), sluggish kinetics of the conversion reactions at the cathodes, and Na dendrite growth at the anodes. Among these challenges, uncontrolled “shuttle effect” of PSs/PSes/PIs is a major contributing factor for the irreversible loss of active cathode materials and severe side reactions on Na metal anodes, leading to rapid failure of the batteries. Separator modification has been demonstrated to be an effective strategy to suppress the shuttling of PSs/PSes/PIs. Herein, the latest achievement in modifying separators for high-performance Na–S/Se/I₂ batteries is comprehensively reviewed. The reaction mechanisms of each battery system are first discussed. Then, strategies of separator modification based on the different functions for regulating the transportation of PSs/PSes/PIs are summarized, including applying electrostatic repulsive interaction, introducing conductive layers, improving sieving effects, enhancing chemisorption capability, and adding efficient electrocatalysts. Finally, future perspectives on the practical application of modified separators in high-energy rechargeable batteries are provided.     

In Situ Reconstruction of Electrocatalysts for Lithium–Sulfur Batteries: Progress and Prospects

The current research of Li–S batteries primarily focuses on increasing the catalytic activity of electrocatalysts to inhibit the polysulfide shuttling and enhance the redox kinetics. However, the stability of electrocatalysts is largely neglected, given the premise that they are stable over extended cycles. Notably, the reconstruction of electrocatalysts during the electrochemical reaction process has recently been proposed. Such in situ reconstruction process inevitably leads to varied electrocatalytic behaviors, such as catalytic sites, selectivity, activity, and amounts of catalytic sites. Therefore, a crucial prerequisite for the design of highly effective electrocatalysts for Li–S batteries is an in-depth understanding of the variation of active sites and the influence factors for the in situ reconstruction behaviors, which has not achieved a fundamental understanding and summary. This review comprehensively summarizes the recent advances in understanding the reconstruction behaviors of different electrocatalysts for Li–S batteries during the electrochemical reaction process, mainly including metal nitrides, metal oxides, metal selenides, metal fluorides, metals/alloys, and metal sulfides. Moreover, the unexplored issues and major challenges of understanding the reconstruction chemistry are summarized and prospected. Based on this review, new perspectives are offered into the reconstruction and true active sites of electrocatalysts for Li–S batteries.     

Wide Working Temperature Range Rechargeable Lithium–Sulfur Batteries: A Critical Review

At the technological forefront of energy storage, there is still a continuous upsurge in demand for high energy and power density batteries that can operate at a wide range of temperature. Rechargeable lithium sulfur batteries stand out among other advanced cell concepts owing to their ultrahigh theoretical gravimetric energy density characteristic as well as merits of low cost and environmental friendliness. Although achieving good operability of ambient lithium sulfur batteries, extending their workability to both higher and lower temperatures is also of paramount importance especially for future task-specific applications. As a first attempt, this review presents a comprehensive understanding on the advances, challenges, and future research directions on lithium sulfur batteries operating at both low and high temperature extremes. From a material perspective, the workability of sulfur-containing cathode materials, advanced electrolytes (from conventional liquid to quasi- and all-solid-state electrolytes), lithium metal anodes and the electrochemically inert components (separators and interlayer materials) at extreme temperatures are thoroughly analyzed. The insurmountable challenges and mechanistic understandings caused by thermal changes are critically reviewed. Finally, potential future research directions and prospects for lithium sulfur batteries operated at a wide range of temperature are also proposed.     

Boosting Catalytic Activity by Seeding Nanocatalysts onto Interlayers to Inhibit Polysulfide Shuttling in Li–S Batteries

The shuttling of soluble lithium polysulfides (LiPSs) is one of the main bottlenecks to the practical use of Li–S batteries. It is reported that in situ synthesized ultrasmall vanadium nitride nanoparticles dispersed on porous nitrogen-doped graphene (denoted VN@NG) as a catalytic interlayer solves this problem. The ultrasmall size of VN particles provide ample triple-phase interfaces (the reactive interfaces among VN nanocatalyst, NG conductive substrate, and electrolyte) for accelerating LiPS conversion and Li₂S deposition, which greatly reduces the accumulation of LiPSs in the electrolyte and therefore inhibits the shuttle effect. Their high catalytic activity is confirmed by a reduced activation energy of the Li₂S₄ conversion step based on temperature-dependent cyclic voltammetric (CV) measurements and the reduced shuttle effect is detected by in situ Raman spectra. With the VN nanocatalyst, Li–S batteries have an outstanding cycling performance with a low capacity decay rate of 0.075% per cycle over 500 cycles at 2 C. A high capacity retention of 84.5% over 200 cycles at 0.2 C is achieved with a high sulfur loading of 7.3 mg cm⁻².     

Anchoring Polysulfides and Accelerating Redox Reaction Enabled by Fe-Based Compounds in Lithium–Sulfur Batteries

The synergetic mechanism of chemisorption and catalysis play an important role in developing high-performance lithium–sulfur (Li–S) batteries. Herein, a 3D lather-like porous carbon framework containing Fe-based compounds (including Fe₃C, Fe₃O₄, and Fe₂O₃), named FeCFeOC, is designed as the sulfur host and the interlayer on separator. Due to the strong chemisorption and catalytic ability of FeCFeOC composite, the soluble lithium polysulfides (LiPSs) are first adsorbed and anchored on the surface of the FeCFeOC composite and then are catalyzed to accelerate their conversion reaction. In addition, the Fe_xO_y in Fe-based compounds can spontaneously react with LiPSs to form magnetic FeS_x species with a larger size, further blocking the penetration of LiPSs cross the separator. As a result, the assembled Li–S cells show excellent long-term stability (748 mAh g⁻¹ over 500 cycles at 1.0 C, and ≈0.036% decay per cycle for 1000 cycles at 3.0 C), a superb rate capability with 659 mAh g⁻¹ at 5.0 C, and lower electrochemical polarization. This work introduces a feasible strategy to anchor and accelerate the conversion of LiPSs by designing the multifunctional Fe-based compounds with high chemisorption and catalytic activity, which advances the large-scale application of high-performance Li–S batteries.     

Accelerated Sulfur Evolution Reactions by TiS2/TiO2@MXene Host for High-Volumetric-Energy-Density Lithium–Sulfur Batteries (Adv. Funct. Mater. 35/2023)

The poor cycling stability and low volumetric energy density of lithium–sulfur batteries compared with lithium-ion batteries are hindering their practical applications. Here, it is demonstrated that a dense sulfur electrode containing heavy TiS₂/TiO₂@MXene heterostructures can tackle these issues. It is observed that the TiO₂ part functionally anchors the lithium polysulfides through the strong chemical affinity, and the TiS₂ part serves as an efficient electrocatalyst to enhance the kinetics of sulfur evolution reactions. Benefitting from these synergistic effects, the TiS₂/TiO₂@MXene heterostructures effectively suppress the shuttle effects, leading to superior cyclability of the sulfur cathode with a low capacity decay of 0.038% per cycle for 500 cycles at a current rate of 1 C. More encouragingly, a highly dense S/TiS₂/TiO₂@MXene cathode exhibits a high volumetric energy density of 2476 Wh L⁻¹ (based on the volume of the composite) at a high sulfur mass loading of 7.5 mg cm⁻² and lean electrolyte of 5 µL mg⁻¹. The electrochemical performance is comparable to or even superior to the lithium-ion and lithium–sulfur batteries reported in the literature. This study provides an effective strategy to design stable and high-volumetric-energy-density lithium–sulfur batteries for practical energy storage applications.