Polysulfide-Breathing/Dual-Conductive, Heterolayered Battery Separator Membranes Based on 0D/1D Mingled Nanomaterial Composite Mats

Versatile Separators Toward Advanced Lithium-Sulfur Batteries: Status, Recent Progress, Challenges and Perspective

Lithium-sulfur batteries (LSBs) have recently gained extensive attention due to their high energy density, low cost, and environmental friendliness. However, serious shuttle effect and uncontrolled growth of lithium dendrites restrict them from further commercial applications. As "the third electrode", functional separators are of equal significance as both anodes and cathodes in LSBs. The challenges mentioned above are effectively addressed with rational design and optimization in separators, thereby enhancing their reversible capacities and cycle stability. The review discusses the status/operation mechanism of functional separators, then primarily focuses on recent research progress in versatile separators with purposeful modifications for LSBs, and summarizes the methods and characteristics of separator modification, including heterojunction engineering, single atoms, quantum dots, and defect engineering. From the perspective of the anodes, distinct methods to inhibit the growth of lithium dendrites by modifying the separator are discussed. Modifying the separators with flame retardant materials or choosing a solid electrolyte is expected to improve the safety of LSBs. Besides, in-situ techniques and theoretical simulation calculations are proposed to advance LSBs. Finally, future challenges and prospects of separator modifications for next-generation LSBs are highlighted. We believe that the review will be enormously essential to the practical development of advanced LSBs.

Zeolitic imidazolate framework as humidity-resistant solid state-chemiresistive gas sensors: A review

With significant technological advances, solid-state gas sensors have been extensively applied to detect toxic gases and volatile organic compounds (VOCs) in confined areas such as indoor environments and industries and to identify gas leakage. Semiconductor metal oxides are the primary sensing materials, although their major drawbacks include a lack of sensitivity, poor performance at high humidity, and operating at high temperatures ranging between 140 and 400 °C. Recently, the use of zeolitic imidazolate frameworks (ZIFs) in gas sensors has received considerable attention as a promising material to overcome the drawbacks possessed by semiconductor metal oxide-based gas sensors. Because of their unique properties, including size tunability, high surface area, and stability in humidity, ZIF becomes a preferred candidate for sensing materials. The use of ZIF materials in gas sensors is limited because of their high-temperature operation and low gas responses. This review outlines the strategies and developments in the utilization of ZIF-based materials in gas sensing. The significant influence of the addition of carbon additives in ZIF materials for temperature operation sensors is discussed. Finally, ZIF-carbon additives and SMO@ZIFs/carbon additives are the proposed materials to be studied for future prospects for the detection of VOCs at low temperatures and exhibiting good selectivity towards the gas of interest.

Bifunctional Separator with Ultra-Lightweight MnO2 Coating for Highly Stable Lithium-Sulfur Batteries

The severe shuttling behavior in the discharging-charging process largely hampers the commercialization of lithium-sulfur (Li-S) batteries. Herein, we design a bifunctional separator with an ultra-lightweight MnO₂ coating to establish strong chemical adsorption barriers for shuttling effect alleviation. The double-sided polar MnO₂ layers not only trap the lithium polysulfides through extraordinary chemical bonding but also ensure the uniform Li⁺ flux on the lithium anode and inhibit the side reaction, resulting in homogeneous plating and stripping to avoid corrosion of the Li anode. Consequently, the assembled Li-S battery with the MnO₂-modified separator retains a capacity of 665 mA h g^-1 at 1 C after 1000 cycles at the areal sulfur loading of 2.5 mg cm^-2, corresponding to only 0.028% capacity decay per cycle. Notably, the areal loading of ultra-lightweight MnO₂ coating is as low as 0.007 mg cm^-2, facilitating the achievement of a high energy density of Li-S batteries. This work reveals that the polar metal oxide-modified separator can effectively inhibit the shuttle effect and protect the Li anode for high-performance Li-S batteries.

pH-Regulated Single and Double Charge Inversions on PEI-Coated Surfaces

The pH-regulated charge inversions on polyethylenimine (PEI)-coated surfaces are indispensable to their applications in biomaterials and nanomaterials. Various PEI-coated surfaces, where single charge inversion happens, have been extensively investigated, while the surfaces where double charge inversion appears are less reported. Here, using a molecular theory, we systematically study the pH-regulated charge density of PEI-coated surfaces. The results suggest whether single or double charge inversion happens depends on PEI affinity to the surface and the bare surface charge density. The region of double charge inversion is much smaller than that of single charge inversion, revealing the reason why double charge inversion is less observed in experiments. Besides, the charge inversions are significantly influenced by the solution condition. The present work provides a useful guideline to the selection of the coated materials and the parameters of PEI solution in the design of PEI-coated surfaces aiming to promote their applications in multifunctional nanomaterials.

Facile Separator Modification Strategy for Trapping Soluble Polyphosphides and Enhancing the Electrochemical Performance of Phosphorus Anode

Phosphorus anode is one of the most promising candidates for high-energy-density lithium-ion batteries. Recent studies found the lithiation process of phosphorus is accompanied by the soluble intermediates of lithium polyphosphides. The trans-separator diffusion of polyphosphides is responsible for the capacity decay. Herein, a facile separator modification strategy is proposed for improving the performance of phosphorus anode. The lightweight CNT-modified layer that has a continuous conductive skeleton, a dense structure, and a strong interaction with the soluble lithium polyphosphides can trap, stabilize, and reactivate the active material. Without sophisticated electrode structure design, the cyclability and high-rate performance of the phosphorus anode has been significantly improved, leading to a higher specific capacity of 1505 mAh/g at 250 mA/g (200th cycle) and 1312 mAh/g at 2 A/g. With the advantages of simplicity and low cost, the separator modification strategy provides a new feasible way for further improvement of the phosphorus-based anode.

A novel battery separator coated by a europium oxide/carbon nanocomposite enhances the performance of lithium sulfur batteries

Lithium sulfur (Li-S) batteries represent one of the most promising future power batteries due to their remarkable advantages of low cost and ultrahigh theoretical energy density. However, the commercial applications of Li-S batteries have long been plagued by the shuttling effect of polysulfides and sluggish redox kinetics of these species. Herein, we designed a novel battery separator coated by a europium oxide-doped porous Ketjen Black (Eu₂O₃/KB) and tested its performance for the Li-S batteries for the first time. Experimental results and theoretical calculations reveal that the improved electrochemical performance can be attributed to the presence of Eu₂O₃. The strong binding effect between Eu₂O₃ and polysulfides is demonstrated in two aspects: (1) there exist strong interactions between Eu₂O₃ as a Lewis acid and polysulfides of strong Lewis basicity; (2) Eu₂O₃ with oxygen-vacancy defects provides active sites for catalyzing polysulfide conversion and polysulfide trapping. Thus, a Li-S battery with the Eu₂O₃/KB modified separator delivers highly stable cycling performance and excellent rate capability, with the capacity decay ratio of merely 0.05% per cycle under 1 C rate during 500 cycles, and high specific capacity of 563 mAh g^-1 at 3 C rate. This work offers a meaningful exploration of the application of rare earth oxides for the modification of the separator towards high performance Li-S batteries.

Engineering Oxygen Vacancies in a Polysulfide-Blocking Layer with Enhanced Catalytic Ability

The practical application of the lithium-sulfur (Li-S) battery is seriously restricted by its shuttle effect, low conductivity, and low sulfur loading. Herein, first-principles calculations are conducted to verify that the introduction of oxygen vacancies in TiO₂ not only enhances polysulfide adsorption but also greatly improves the catalytic ability and both the ion and electron conductivities. A commercial polypropylene (PP) separator decorated with TiO₂ nanosheets with oxygen vacancies (OVs-TiO₂ @PP) is fabricated as a strong polysulfide barrier for the Li-S battery. The thickness of the OVs-TiO₂ modification layer is only 500 nm with a low areal mass of around 0.12 mg cm^-2 , which enhances the fast lithium-ion penetration and the high energy density of the whole cell. As a result, the cell with the OVs-TiO₂ @PP separator exhibits a stable electrochemical behavior at 2.0 C over 500 cycles, even under a high sulfur loading of 7.1 mg cm^-2 , and an areal capacity of 5.83 mAh cm^-2 remains after 100 cycles. The proposed strategy of engineering oxygen vacancies is expected to have wide applications in Li-S batteries.

Bifunctional separator with sandwich structure for high-performance lithium-sulfur batteries

Severe "Shuttle effect" and uncontrollable lithium-dendrite growth are ongoing challenges that hinder the practical application of Lithium-sulfur (Li-S) batteries. Herein, a bifunctional separator was modified by Al₂O₃ and carbon nanotubes (CNTs) via a facile method. Li-S battery assembled with the modified separator shows excellent cycling stability (760.4 mA h g^-1 at 0.2 C after 100 cycles) and promising rate performance. The reason is ascribed to synergistic effect of CNTs and Al₂O₃ double coating layers, the strong physicochemical interaction between Al₂O₃ and the polysulfides could alleviate the shuttle effect, and the high conductivity of CNTs can facilitate the reaction kinetics of sulfur and its corresponding discharge products, respectively, which can improve the utilization ratio of sulfur. In addition, the double protection layers improve the hardness of the separator, as well as regulate Li⁺ ion deposition, which can effectively prevent the formation of lithium dendrites, thus the safety of the batteries are significant improved.

Graphdiyne-Modified Polyimide Separator: A Polysulfide-Immobilizing Net Hinders the Shuttling of Polysulfides in Lithium-Sulfur Battery

Graphdiyne (GDY), a new type of carbon material with an electron-rich conjugated structure, has been investigated as a separator coating layer to enhance the electrochemical performance of lithium-sulfur (Li-S) battery. Acetylenic bond (-C≡C-C≡C-) and benzene ring in the GDY coating layer are experimentally verified to reversibly attract the soluble lithium polysulfides by chemical adsorption during cycling. Meanwhile, the shuttle effect of soluble polysulfides is further physically restricted by the GDY coating layer due to the evenly distributed pores (5.42 Å) and a consistent interlayer spacing (3.65 Å) of GDY. Moreover, GDY is a conducting carbon skeleton with high Li⁺ mobility that can improve the rate performance. Hence, Li-S battery with an as-prepared GDY coating layer shows excellent electrochemical performances including superior specific capacity, excellent rate performance, and low capacity attenuation rate. The high initial discharge capacity of 1648.5 mA h g^-1 at 0.1C and 819.5 mA h g^-1 even at a high rate of 2C is achieved by this novel separator. The initial capacity of 1112.9 mA h g^-1 at 0.5C is retained to 816.7 mA h g^-1 after 200 cycles with a low attenuation rate of 0.13% per cycle. Compared with other coated separators, these results show that the GDY coating layer endows the separator with superior electrochemical performances for Li-S battery.

Revisiting the Role of Polysulfides in Lithium-Sulfur Batteries

Intermediate polysulfides (S_n , where n = 2-8) play a critical role in both mechanistic understanding and performance improvement of lithium-sulfur batteries. The rational management of polysulfides is of profound significance for high-efficiency sulfur electrochemistry. Here, the key roles of polysulfides are discussed, with regard to their status, behavior, and their correspondingimpact on the lithium-sulfur system. Two schools of thoughts for polysulfide management are proposed, their advantages and disadvantages are compared, and future developments are discussed.

Chemical Adsorption and Physical Confinement of Polysulfides with the Janus-faced Interlayer for High-performance Lithium-Sulfur Batteries

We design the Janus-like interlayer with two different functional faces for suppressing the shuttle of soluble lithium polysulfides (LPSs) in lithium-sulfur batteries (LSBs). At the front face, the conductive functionalized carbon fiber paper (f-CFP) having oxygen-containing groups i.e., -OH and -COOH on its surface was placed face to face with the sulfur cathode serving as the first barrier accommodating the volume expansion during cycling process and the oxygen-containing groups can also adsorb the soluble LPSs via lithium bonds. At the back face, a crystalline coordination network of [Zn(H₂PO₄)₂(TzH)₂]_n (ZnPTz) was coated on the back side of f-CFP serving as the second barrier retarding the left LPSs passing through the front face via both physical confinement and chemical adsorption (i.e. Li bonding). The LSB using the Janus-like interlayer exhibits a high reversible discharge capacity of 1,416 mAh g⁻¹ at 0.1C with a low capacity fading of 0.05% per cycle, 92% capacity retention after 200 cycles and ca. 100% coulombic efficiency. The fully charged LSB cell can practically supply electricity to a spinning motor with a nominal voltage of 3.0 V for 28 min demonstrating many potential applications.