Decoration of Silica Nanoparticles on Polypropylene Separator for Lithium-Sulfur Batteries

Research and application of polypropylene: a review

Polypropylene (PP) is a versatile polymer with numerous applications that has undergone substantial changes in recent years, focusing on the demand for next-generation polymers. This article provides a comprehensive review of recent research in PP and its advanced functional applications. The chronological development and fundamentals of PP are mentioned. Notably, the incorporation of nanomaterial like graphene, MXene, nano-clay, borophane, silver nanoparticles, etc., with PP for advanced applications has been tabulated with their key features and challenges. The article also conducts a detailed analysis of advancements and research gaps within three key forms of PP: fiber, membrane, and matrix. The versatile applications of PP across sectors like biomedical, automotive, aerospace, and air/water filtration are highlighted. However, challenges such as limited UV resistance, bonding issues, and flammability are noted. The study emphasizes the promising potential of PP while addressing unresolved concerns, with the goal of guiding future research and promoting innovation in polymer applications.

Single-atom site catalysis in Li-S batteries

With their high theoretical energy density, Li-S batteries are regarded as the ideal battery system for next generation electrochemical energy storage. In the last 15 years, Li-S batteries have made outstanding academic progress. Recently, research studies have placed more emphasis on their practical application aspects, which puts forward strict requirements for the loading of S cathodes and the amount of electrolytes. To meet the above requirements, electrode catalysis design is of crucial significance. Among all the catalysts, single-atom site catalysts (SASCs) are considered to be ideal catalyst materials for the commercialization of Li-S batteries due to their high activity and highest utilization of catalytic sites. This perspective introduces the kinetic mechanism of S cathodes, the basic concept and synthesis strategy of SASCs, and then systematically summarizes the research progress of SASCs for S cathodes and, the related functional interlayers/separators in recent years. Finally, the opportunities and challenges of SASCs in Li-S batteries are summarized and prospected.

Effect of Heterostructure-Modified Separator in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries with high energy density and low cost are the most promising competitor in the next generation of new energy reserve devices. However, there are still many problems that hinder its commercialization, mainly including shuttle of soluble polysulfides, slow reaction kinetics, and growth of Li dendrites. In order to solve above issues, various explorations have been carried out for various configurations, such as electrodes, separators, and electrolytes. Among them, the separator in contact with both anode and cathode is in a particularly special position. Reasonable design-modified material of separator can solve above key problems. Heterostructure engineering as a promising modification method can combine characteristics of different materials to generate synergistic effect at heterogeneous interface that is conducive to Li-S electrochemical behavior. This review not only elaborates the role of heterostructure-modified separators in dealing with above problems, but also analyzes the improvement of wettability and thermal stability of separators by modification of heterostructure materials, systematically clarifies its advantages, and summarizes some related progress in recent years. Finally, future development direction of heterostructure-based separator in Li-S batteries is given.

Ultrafast self-assembly of supramolecular hydrogels toward novel flame-retardant separator for safe lithium ion battery

Traditional polyolefin separators for lithium-ion batteries (LIBs) often experience limited thermal stability and intrinsic flammability, resulting in great safety risks during their usage. Therefore, it is highly important to develop novel flame-retardant separators for safe LIBs with high performance. In this work, we report a flame-retardant separator derived from boron nitride (BN) aerogel with a high BET surface area of 1127.3 m² g^-1. The aerogel was pyrolyzed from a melamine-boric acid (MBA) supramolecular hydrogel, which was self-assembled at an ultrafast speed. The in-situ evolution details of the nucleation-growth process of the supramolecules could be observed in real-time using a polarizing microscope under ambient conditions. The BN aerogel was further composited with bacterial cellulose (BC) to form a BN/BC composite aerogel with excellent flame-retardant performance, electrolyte-wetting ability and high mechanical property. By using the BN/BC composite aerogel as the separator, the developed LIBs exhibited high specific discharge capacity of 146.5 mAh g^-1 and excellent cyclic performance, maintaining 500 cycles with a capacity degradation of only 0.012% per cycle. The high-performance flame-retardant BN/BC composite aerogel represents a promising candidate for separators not only in LIBs but also in other flexible electronics.

Hierarchical Porous and Three-Dimensional MXene/SiO2 Hybrid Aerogel through a Sol-Gel Approach for Lithium–Sulfur Batteries

A unique porous material, namely, MXene/SiO₂ hybrid aerogel, with a high surface area, was prepared via sol-gel and freeze-drying methods. The hierarchical porous hybrid aerogel possesses a three-dimensional integrated network structure of SiO₂ cross-link with two-dimensional MXene; it is used not only as a scaffold to prepare sulfur-based cathode material, but also as an efficient functional separator to block the polysulfides shuttle. MXene/SiO₂ hybrid aerogel as sulfur carrier exhibits good electrochemical performance, such as high discharge capacities (1007 mAh g^–1 at 0.1 C) and stable cycling performance (823 mA h g^–1 over 200 cycles at 0.5 C). Furthermore, the battery assembled with hybrid aerogel-modified separator remains at 623 mA h g^–1 over 200 cycles at 0.5 C based on the conductive porous framework and abundant functional groups in hybrid aerogel. This work might provide further impetus to explore other applications of MXene-based composite aerogel.

P-Doped Cotton Stalk Carbon for High-Performance Lithium-Ion Batteries and Lithium-Sulfur Batteries

Biomass as a carbon material source is the characteristic of green chemistry. Herein, a series of hierarchical P-doped cotton stalk carbon materials (HPCSCMs) were prepared from cheap and abundant biowaste cotton stalk. These materials possess a surface area of 3463.14 m² g^-1 and hierarchical pores. As lithium-ion battery (LIB) anodes, the samples exhibit 1100 mAh g^-1 at 0.1 A g^-1 after 100 cycles and hold 419 mAh g^-1 at 1 A g^-1 after 1000 cycles, with nearly 100% capacity retention. After HPCSCMs are loaded with sulfur (S/HPCSCMs), the samples (S/HPCSCMs-2) deliver a discharge capacity of 413 mAh g^-1 at 0.1 A g^-1 after 100 cycles as lithium-sulfur (Li-S) battery cathodes. This excellent electrochemical performance can be attributed to P in carbon networks, which not only provides more active sites, but also improves electrical conductivity.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Energy-Saving Synthesis of Functional CoS2/rGO Interlayer With Enhanced Conversion Kinetics for High-Performance Lithium-Sulfur Batteries

Lithium sulfur (Li-S) battery has exhibited great application potential in next-generation high-density secondary battery systems due to their excellent energy density and high specific capacity. However, the practical industrialization of Li-S battery is still affected by the low conductivity of sulfur and its discharge product (Li2S2/Li2S), the shuttle effect of lithium polysulfide (Li2Sn, 4 ≤ n ≤ 8) during charging/discharging process and so on. Here, cobalt disulfide/reduced graphene oxide (CoS2/rGO) composites were easily and efficiently prepared through an energy-saving microwave-assisted hydrothermal method and employed as functional interlayer on commercial polypropylene separator to enhance the electrochemical performance of Li-S battery. As a physical barrier and second current collector, the porous conductive rGO can relieve the shuttle effect of polysulfides and ensure fast electron/ion transfer. Polar CoS2 nanoparticles uniformly distributed on rGO provide strong chemical adsorption to capture polysulfides. Benefitting from the synergy of physical and chemical constraints on polysulfides, the Li-S battery with CoS2/rGO functional separator exhibits enhanced conversion kinetics and excellent electrochemical performance with a high cycling initial capacity of 1,122.3 mAh g-1 at 0.2 C, good rate capabilities with 583.9 mAh g-1 at 2 C, and long-term cycle stability (decay rate of 0.08% per cycle at 0.5 C). This work provides an efficient and energy/time-saving microwave hydrothermal method for the synthesis of functional materials in stable Li-S battery.

Hierarchically Porous Silica Membrane as Separator for High-Performance Lithium-Ion Batteries

Commercial polymeric separators in lithium-ion batteries (LIBs) typically suffer from limited porosity, low electrolyte wettability, and poor thermal and mechanical stability, which can degrade the battery performance especially at high current densities. Here, the design of hierarchically porous, ultralight silica membranes as separator for high-performance LIBs is reported through the assembly of hollow mesoporous silica (HMS) particles on the cathode surface. The rich mesopores and large cavity of individual HMS particles provide low-tortuosity pathways for ionic transport, while simultaneously serving as electrolyte reservoir to further boost the electrochemical kinetics. Moreover, benefiting from their inorganic and hierarchically porous nature, such HMS separators display better electrolyte affinity, thermal stability, and mechanical strength than commercial polypropylene (PP) separators. As a demonstration, LIBs with a LiFePO₄ cathode coated with HMS separators exhibit exceptional rate capability and cycling stability, outperforming LIBs with PP and Al₂ O₃ -modified PP separators as well as separators made of solid silica particles.

Efficient capture and conversion of polysulfides by zinc protoporphyrin framework-embedded triple-layer nanofiber separator for advanced Li-S batteries

The practical application of Lithium-sulfur (Li-S) batteries is significantly inhibited by (i) the notable 'shuttle effect' of lithium polysulfides (LiPS), (ii) the corrosion of the lithium interface, and (iii) the sluggish redox reaction kinetics. The functional separator in the Li-S battery has the potential to provide the perfect solution to these problems. Herein a triple-layer multifunctional PVDF-based nanofiber separator, which contains GoTiN/PVDF layer on the top and bottom and ZnTPP/PVDF layer on the middle, is designed. The polarity and porous structure of this multifunctional separator can greatly improve the wettability of electrolytes and enhance the transportation of Li⁺. With the zinc-based porphyrin framework (ZnTPP) structure, this separator has a strong chemisorption and LiPS conversion ability, which greatly prevent the 'shuttle effect'. Consequently, the designed multilayer separator showed excellent electrochemical performance. As a result, the cell with GoTiN@ZnTPP@GoTiN nanofiber membrane displayed an initial discharge capacity of 1180 mAh/g with a benign capacity retention of 65.9% at 0.5C and high coulombic efficiency of more than 98.5% after 100 cycles. Even at 2C, it can still release a capacity of 798 mAh/g. Moreover, the remarkable capacity of 591 mAh/g could be achieved with a high sulfur load of 5.76 mg/cm² under a current density of 0.1C. Based on these merits, this novel and scalable multifunctional separator is a promising candidate to replace the conventional PP separator for advanced Li-S batteries to deal with various challenges.

Robust and High-Temperature-Resistant Nanofiber Membrane Separators for Li-Metal, Li-Sulfur, and Aqueous Li-Ion Batteries

Mechanically strong separators with good electrolyte wettability and low-shrinkage properties are desirable for highly efficient and safe lithium batteries. In this study, multifunctional nanofiber membranes are fabricated by electrospinning a homogeneous solution containing amphiphilic poly(ethylene glycol)diacrylate-grafted siloxane and polyacrylonitrile. After the chemical cross-linking of siloxane, the prepared nanofiber membranes are found to exhibit good mechanical properties, high thermostability, and superior electrolyte-philicity with aqueous and nonaqueous electrolytes. Li-metal cells with the fabricated membrane separator exhibit high cycling stability (Coulombic efficiency of 99.8% after 1000 cycles). Moreover, improved cycling stability of Li-sulfur batteries can be achieved using these membrane separators. These membrane separators can be further used in flexible aqueous lithium-ion batteries and exhibit steady electrochemistry performance. This work opens up a potential route for designing multifunctional universal separators for rechargeable batteries.

Novel Bifunctional Separator with a Self-Assembled FeOOH/Coated g-C3N4/KB Bilayer in Lithium-Sulfur Batteries

Separator modification with metal oxide and carbon composite recently has become a potential and competitive way to confine polysulfide diffusion and mitigate the shuttling effect. However, other modification methods also have an impact on the stability of the modified layer and the enhancement of electrochemical performance. Herein, we first design a novel bifunctional separator combined with one self-assembled FeOOH layer via a chemical way and one conductive g-C₃N₄/KB layer by physical coating. Different from directly coating the metal oxide and carbon composite on the separator, the self-assembled FeOOH layer is firmly formed on the PP separator, which enables the chemical capture of the soluble polysulfide and prohibit the shuttling effect. Then, the coated g-C₃N₄/KB layer is further introduced to greatly enhance the transportation of lithium ions and physically confine the migration of intermediates. As a result, the battery with this bifunctional separator (G-SFO) achieves outstanding rate capacities (1000, 901, and 802 mA h/g at 0.5, 1, and 2 C). After 900 cycles at 1 C, it also shows excellent long cycle performance, with relatively low fading (0.055%). This original fabrication will present a new and feasible strategy for fabricating a bifunctional separator with metal oxide and carbon material.

Enhancement of the Surface Properties on Polypropylene Film Using Side-Chain Crystalline Block Copolymers

The consumption of polypropylene (PP) has significantly increased over that of other materials because of its light weight, easy molding, and high mechanical strength. However, the applications of PP are limited, owing to the lack of surface properties, especially with respect to adhesive properties and hydrophilicity. In this study, we developed a surface modification method for enhancing the adhesive properties and hydrophilicity on the PP surface using a side-chain crystalline block copolymer (SCCBC). This method was simple and involved the dipping of a PP film in a diluted SCCBC solution. The optimized modification conditions for enhancing the adhesive properties of PP were investigated. The results revealed that the adhesion strength of PP modified with the SCCBC of behenyl acrylate and 2-(tert-butylamino)ethyl methacrylate was enhanced to 2.00 N/mm (T-peel test) and 1.05 N/mm2 (tensile shear test). In addition, the hydrophilicity of PP modified with the SCCBC of behenyl acrylate and di(ethylene glycol)ethyl ether acrylate was enhanced to a water contact angle of 69 ± 4°. Surface analysis was also performed to elucidate a plausible mechanism for PP modification by the SCCBCs. This surface modification method is facile and enhances desirable properties for the wide application of PP.

Semihollow Core-Shell Nanoparticles with Porous SiO2 Shells Encapsulating Elemental Sulfur for Lithium-Sulfur Batteries

Lithium-sulfur batteries have shown great promise as next-generation high energy density power sources, but their commercial applications are hindered by short battery cycle life arising from the dissolution and shuttling of polysulfides. To address this shortcoming, we prepared two types of semihollow core-shell nanoparticles in which (1) elemental sulfur is encapsulated within a porous silica shell (S@SiO₂) and (2) elemental sulfur is encapsulated within a porous silica shell where the inner surface of the shell is decorated with small Au nanoparticles (S@Au@SiO₂). These core-shell nanoparticles, both ∼300 nm in diameter, were generated from analogous zinc sulfide-based core-shell nanoparticles (ZnS@SiO₂ and ZnS@Au@SiO₂, respectively) by converting the ZnS cores to elemental sulfur upon treatment with Fe(NO₃)₃. With a high surface area and strong host-polysulfide interaction, the SiO₂ shells effectively trap the polysulfides; moreover, the internal void space of these nanostructures accommodates the volume expansion of the sulfur core upon lithiation. By decorating ∼5-7 nm Au nanoparticles evenly on the inner surface of the porous SiO₂ shells (i.e., S@Au@SiO₂), electron transport is enhanced, with consequently enhanced sulfur conversion kinetics at high current rates. Studies of battery performance showed that the S@SiO₂ cathode can deliver an initial capacity of 1153 mA h g^-1 under 0.2 C and retain 816 mA h g^-1 after 100 cycles. More importantly, the Au-decorated S@Au@SiO₂ cathode can deliver a high capacity of 500 mA h g^-1 under 5 C.

A Review of Functional Separators for Lithium Metal Battery Applications

Lithium metal batteries are considered “rough diamonds” in electrochemical energy storage systems. Li-metal anodes have the versatile advantages of high theoretical capacity, low density, and low reaction potential, making them feasible candidates for next-generation battery applications. However, unsolved problems, such as dendritic growths, high reactivity of Li-metal, low Coulombic efficiency, and safety hazards, still exist and hamper the improvement of cell performance and reliability. The use of functional separators is one of the technologies that can contribute to solving these problems. Recently, functional separators have been actively studied and developed. In this paper, we summarize trends in the research on separators and predict future prospects.

Toward Commercially Viable Li-S Batteries: Overall Performance Improvements Enabled by a Multipurpose Interlayer of Hyperbranched Polymer-Grafted Carbon Nanotubes

Shuttle effect and the low utilization of dissolved lithium polysulfides (LiPSs) are two prevailing concerns in Li-S battery (LSB) research. Energy efficiency on the other hand is often overlooked but vital to the commercial deployment of battery technology. In this work, a composite of hyperbranched poly(amidoamine)-modified multiwalled carbon nanotubes (PAMAM-CNTs) is successfully prepared by chemical grafting and employed as an interlayer material in LSBs. The high content and highly dispersed polar functional groups of PAMAM can efficiently adsorb and enhance the redox reaction of LiPSs. The CNTs function as a scaffold and current collector that reduces the internal polarization. The assembled LSB displays a high energy efficiency of 86% and a low capacity fading rate of 0.037% per cycle over 1200 cycles at 2 C. The cell also shows excellent cycle performance, high sulfur utilization, and improved stability at a high areal capacity of 9 mAh cm^-2 (achieved at a sulfur loading of 8.7 mg cm^-2) and low electrolyte/sulfur ratio of 6.1 mL g^-1. This thin (12 μm) and lightweight (0.34 mg cm^-2) interlayer has a negligible impact on the overall cell energy density.

Covalent Organic Frameworks: Design, Synthesis, and Functions

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Enhanced performance of lithium–sulfur batteries based on single-sided chemical tailoring, and organosiloxane grafted PP separator

Even after a decade of research and rapid development of lithium–sulfur (Li–S) batteries, the infamous shuttle effect of lithium polysulfide is still the major challenge hindering the commercialization of Li–S batteries. In order to further address this issue, a functionalized PP separator is obtained through selective single-sided chemical tailoring, and then organosiloxane fumigation grafting. During the charge–discharge process, the grafted functional groups can effectively block the transportation of the dissolved polysulfides through strong chemical anchoring, inhibit the shuttle effect and greatly enhance the cycle stability of the Li–S battery. Interestingly, the specially designed single-sided enlarged channel structure formed by chemical tailoring can well accommodate the deposition with intermediate polysulfides on the separator surface toward the cathode chamber, resulting in enhanced initial discharge capacity and rate performance. Compared to the battery assembled with PP, the Li–S battery employing the separator grafted with a 3-ureidopropyltrimethoxysilane (PP–O^x−–U) displays better electrochemical performance. Even at 2C, it can still deliver a high capacity of 786 mA h g⁻¹, and retain a capacity of 410 mA h g⁻¹ with a low capacity fading of 0.095% per cycle over 500 cycles. This work provides a very promising and feasible strategy for the development of a special functionalization PP separator for Li–S batteries with high electrochemical performance.

Even after a decade of research and rapid development of lithium–sulfur (Li–S) batteries, the infamous shuttle effect of lithium polysulfide is still the major challenge hindering the commercialization of Li–S batteries.

Metal Coated Polypropylene Separator with Enhanced Surface Wettability for High Capacity Lithium Metal Batteries

Lithium metal batteries are among the strong contenders to meet the increasing energy demands of the modern world. Metallic lithium (Li) is light in weight, possesses very low standard negative electrochemical potential and offers an enhanced theoretical capacity (3860 mA h g-1). As a negative electrode Li paves way to explore variety of elements including oxygen, sulfur and various other complex oxides as potential positive electrodes with a promise of much higher energy densities than that of conventional positive electrodes. However, there are technical challenges in utilizing metallic lithium due to its higher reactivity towards liquid electrolytes and higher affinity to form Li dendrites, leading to serious safety concerns. Here, we report on preparation of niobium (Nb) metal-coated binder-free and highly hydrophilic polypropylene separator prepared via radio frequency (RF) magnetron sputtering. Thin layer of niobium metal (Nb) particles were deposited onto the polypropylene (PP) sheet for various time periods to achieve desired coating thickness. The as-prepared separator revealed excellent hydrophilic behaviour due to enhanced surface wettability. Symmetric cells display reduced interface resistance and uniform voltage profiles for 1000 cycles with reduced polarization at higher current densities suggesting smooth stripping and plating of Li and homogeneous current distribution at electrode/electrolyte interface under room temperature conditions. Nb nanolayer protected separator with LiNi0.33M0.33Co0.33O2 (LNMC) and composite sulfur cathodes revealed an enhanced cycling stability.

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.

Suppressing the Shuttle Effect in Lithium-Sulfur Batteries by a UiO-66-Modified Polypropylene Separator

The lithium-sulfur battery is one of the most promising battery technologies with high energy density that exceeds the presently commercialized ones. The shuttle effect caused by the migration of soluble polysulfides to the lithium anode is known as one of the crucial issues that prevent the Li-S batteries from practical application. Modification of the separator is regarded as a convenient yet efficient strategy to alleviate the shuttle effect. In this report, we use a thermally stable and chemically robust metal-organic framework (MOF), UiO-66, as a physical and chemical barrier for soluble polysulfides to functionalize the commercial polypropylene separator. The Li-S cell assembled with such a separator shows a significantly improved cycling stability with an average specific capacity of ca. 720 mA h g^-1 at a current rate of 0.5 C for 500 cycles. Experimental and theoretical investigations indicate that the cell performance enhancement results from the physical restriction of the MOF barrier layer and strong chemical interaction between UiO-66 and polysulfides. The excellent thermal stability and chemical robustness (in acid/alkali solutions, conventional organic solvents, and polysulfide electrolytes) of UiO-66 make it highly competitive among various materials developed for separator modification in Li-S batteries.

A SuperLEphilic/Superhydrophobic and Thermostable Separator Based on Silicone Nanofilaments for Li Metal Batteries

Conventional polyolefin separators suffer from poor wettability to liquid electrolytes (LEs). Although some modified separators exhibit improved wettability, they are hydrophilic, causing inevitable moisture uptake. Trace water could result in poor performance and safety hazard of Li metal batteries. Here, we report a design idea of superLEphilic/superhydrophobic and thermostable separators by modifying the Celgard separator using silicone nanofilaments. The separator features low moisture uptake (∼0%), fast LE diffusion (454 ms), and high LE uptake (287.8%), LE retention rate, and Li⁺ conductivity. Consequently, the Li/LiFePO₄ cells show high cycling stability (96.05% after 350 cycles), good rate performance (125 mA h g⁻¹ at 5.0 C), low resistance, and stable open circuit voltage at 160°C. Moreover, the separator could improve performance of the other Li metal batteries with high-voltage cathodes and the LiFePO₄/graphite pouch cells. This work provides an avenue for designing advanced separators by using bioinspired superwetting surfaces.

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A superLEphilic/superhydrophobic separator is first reported for Li metal batteries

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The separator has low moisture uptake and could improve performance of Li/Li cells

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The separator features fast LE diffusion, high LE uptake, and Li⁺ conductivity

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The separator could enhance performance of high-voltage Li metal batteries

The adsorption effect of freestanding SiOx-decorated stabilized polyacrylonitrile interlayers in lithium-sulfur batteries

Lithium-sulfur batteries are well-known for their high theoretical specific capacity and high energy density. However, they undergo rapid capacity fading after the initial cycles due to the dissolution of polysulfides which further results in the shuttle effect. To address this issue and to protect the Li anode surface, silicon suboxide decorated stabilized polyacrylonitrile (sPAN-SiOx) fibermats are used as a freestanding interlayer on the cathode side. Polysulfides are easily captured at the cathode side with the help of the complementary adsorption effect of oxygen-containing functional groups, SiOx and the pyridinic-N structure of sPAN-SiOx resulting in better electrochemical cell performance. The adsorption effect of those functional groups and SiOx is confirmed by X-Ray Photoelectron Spectroscopy (XPS) analysis as obvious shifts in the binding energies and reductions of the peak intensities in the presence of polysulfides. The battery cell with the sPAN-SiOx interlayer shows a discharge capacity of 646 mA h g-1 after 100 cycles of charge-discharge at C/5 current density which is a significant increase compared to the cells with a stabilized polyacrylonitrile (sPAN) interlayer or the cells without an interlayer.

A lithium passivated MoO3 nanobelt decorated polypropylene separator for fast-charging long-life Li-S batteries

Dissolution of lithium polysulfide (LiPS) into the electrolyte during discharging, causing shuttling of LiPS from the cathode to the lithium (Li) metal, is mainly responsible for the capacity decay and short battery life of lithium-sulfur batteries (LSBs). Herein, we designed a separator comprising polypropylene (PP) coated with MoO3 nanobelts (MNBs), prepared through facile grinding of commercial MoO3 powder. The formation of Li2Sn-MoO3 during discharging inhibited the polysulfide shuttling; during charging, Li passivated LixMoO3 facilitated ionic transfer during the redox reaction by decreasing the charge transfer resistance. This dual-interaction mechanism of LiPS-with both Mo and the formation of LixMoO3-resulted in a substantially high initial discharge capacity at a very high current density of 5C, with 29.4% of the capacity retained after 5000 cycles. The simple fabrication approach and extraordinary cycle life observed when using this MNB-coated separator suggest a scalable solution for future commercialization of LSBs.

In Situ Armoring: A Robust, High-Wettability, and Fire-Resistant Hybrid Separator for Advanced and Safe Batteries

Development of nonflammable separators with excellent properties is in urgent need by next-generation advanced and safe energy storage devices. However, it has been extremely challenging to simultaneously achieve fire resistance, high mechanical strength, good thermomechanical stability, and low ion-transport resistance for polymeric separators. Herein, to address all these needs, we report an in situ formed silica@silica-imbedded polyimide (in situ SiO₂@(PI/SiO₂)) nanofabric as a new high-performance inorganic-organic hybrid separator. Different from conventional ceramics-modified separators, this in situ SiO₂@(PI/SiO₂) hybrid separator is realized for the first time via an inverse in situ hydrolysis process. Benefiting from the in situ formed silica nanoshell, the in situ SiO₂@(PI/SiO₂) hybrid separator shows the highest tensile strength of 42 MPa among all reported nanofiber-based separators, excellent wettability to the electrolyte, good thermomechanical stability at 300 °C, and fire resistance. The LiFePO₄ half-cell assembled with this hybrid separator showed a high capacity of 139 mAh·g^-1@5C, which is much higher than that of the battery with the pristine PI separator (126.2 mAh·g^-1@5C) and Celgard-2400 separator (95.1 mAh·g^-1@5C). More importantly, the battery showed excellent cycling stability with no capacity decay over 100 cycles at the high temperature of 120 °C. This study provides a novel method for the fabrication of high-performance and nonflammable polymeric-inorganic hybrid battery separators.

Inhibiting polysulfide shuttling using dual-functional nanowire/nanotube modified layers for highly stable lithium–sulfur batteries

Dual-functional MnO₂ nanowire/CNT modified layers were prepared to inhibit the polysulfide shuttle effect utilizing their strong adsorption capability and high conductivity.

Polyoxometalates/Active Carbon Thin Separator for Improving Cycle Performance of Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have great potential for the next generation of energy-storage devices owing to their high theoretical energy density. However, the polysulfides' shuttling effect seriously degraded the cycle stability and capacity and hindered their commercial applications. Here, we design and fabricate a bifunctional composite separator including a polypropylene (PP) matrix layer and Keggin polyoxometalate [PW₁₂O₄₀]^3-/Super P composite retarding layer by utilizing the Coulombic repulsion between polyanion and polysulfides. Such a binary composite separator shows the effects in enhancing the Coulombic efficiency and cycling stability. Compared with the polypropylene (PP) matrix separator, the capacity is improved by 41% after 120 cycles when using the PW₁₂/Super P separator. It is the first time that the polyoxometalate (POM) matrix is used as a bifunctional separator for lithium-sulfur batteries, demonstrating the promise of POM-based separators in reducing the shuttling effect of Li-S battery.

Lightweight Reduced Graphene Oxide@MoS2 Interlayer as Polysulfide Barrier for High-Performance Lithium-Sulfur Batteries

The further development of lithium-sulfur (Li-S) batteries is limited by the fact that the soluble polysulfide leads to the shuttle effect, thereby reducing the cycle stability and cycle life of the batteries. To address this issue, here a thin and lightweight (8 μm and 0.24 mg cm^-2) reduced graphene oxide@MoS₂ (rGO@MoS₂) interlayer between the cathode and the commercial separator is developed as a polysulfide barrier. The rGO plays the roles of both a polysulfide physical barrier and an additional current collector, while MoS₂ has a high chemical adsorption for polysulfides. The experiments demonstrate that the Li-S cell constructed with an rGO@MoS₂-coated separator shows a high reversible capacity of 1122 mAh g^-1 at 0.2 C, a low capacity fading rate of 0.116% for 500 cycles at 1 C, and an outstanding rate performance (615 mAh g^-1 at 2 C). Such an interlayer is expected to be ideal for lithium-sulfur battery applications because of its excellent electrochemical performance and simple synthesis process.