Separators For High-performance Lithium-sulfur Batteries Research Articles

Active sites-rich zeolitic imidazolate framework/MXene heterostructure modified separator with improved Li+ transport for high-performance Li-S batteries

The inevitable shuttling of lithium polysulfides (LiPSs) and poor redox kinetics restrict real-world applications of lithium-sulfur (Li-S) batteries, although they have been paid plentiful attention. Herein, a thin and multifunctional heterostructure (ZIF-L/MXene), consisting of leaf-like zeolitic imidazolate framework sheets (ZIF-L) and two-dimensional layered Ti3C2Tx MXene nanosheets, is developed for modification of polyolefin-based separators. A good combination of the merits of the ZIF-L and MXene can hinder the restacking of MXene nanosheets and achieve a large specific surface area, thus exposing plentiful active sites for adsorption and catalytic reaction of LiPSs. Taking these obviously synergistic effects, the ZIF-L/MXene heterostructure modified separators not only alleviate the shuttling of LiPSs but also promote their kinetics conversion. Furthermore, with an improved electrolyte affinity, the ZIF-L/MXene modified separators can accelerate the transport of Li+. Thus, the modified separator endows a Li-S cell with an admirable discharge capacity of 1371.7 mAh g-1 at 0.2 C and favorable cycling stability, with a slow capacity decay ratio of 0.075% per cycle during 500 cycles. Even under a sulfur loading of ~ 4.1 mg cm-2, the Li-S battery can achieve an excellent capacity of 990.6 mAh g-1 at 0.1 C and maintain an excellent cycling performance. The novel ZIF-L/MXene heterostructure modified separator in this work can provide an alternative strategy for designing thin and light separators for high-performance Li-S batteries, via the enhancement of redox kinetics and reduction of shuttling of the LiPSs.

Ti3C2Tx nanosheet@Cu/Fe-MOF separators for high-performance lithium-sulfur batteries: an experimental and density functional theory study.

Lithium-sulfur (Li-S) batteries have attracted much attention due to their superior theoretical specific capacity and high theoretical energy density. However, rapid capacity fading originating from the shuttle effect, insulating the S cathode and the dendrite formation on the Li anode restrict the practical applications of Li-S batteries. Herein, we suggest novel coatings on glass fiber separators to satisfy all high-performance Li-S battery requirements. A conductive Ti3C2Tx (MXene) nanosheet/Fe-MOF or Ti3C2Tx (MXene) nanosheet/Cu-MOF layer was coated on a glass fiber separator to act as a polysulfide trapping layer. The MXene layer with high conductivity and polar surface functional groups could confine polysulfides and accelerate the redox conversions. The porous MOF layer acts as a Li ion sieve, thereby leading to the interception of polysulfides and mitigation of Li dendrite growth. The cells with the Cu-MOF/MXenes and Fe-MOF/MXene separators display superior capacities of 1100 and 1131 mA h g-1 after 300 cycles, respectively, whereas the cell with a pure glass fiber separator delivers a very low capacity of 309 mA h g-1 after 300 cycles. With Fe-MOF/MXene and Cu-MOF/MXene configurations, the discharge capacity, coulombic efficiency, cycling stability, and electrochemical conversion reactions are significantly improved. Our ab initio calculations demonstrate that the MXene layer dissociates lithium polysulfides into adsorbed S and mobile Li ions, which explains the experimental findings.

CoS2@SC modified separator for high-performance lithium-sulfur batteries: suppression of polysulfide shuttling

ABSTRACT Due to their high energy density and cost-effective electrode materials, lithium sulfur batteries (LSBs) have been considered as one promising candidate of energy devices. However, the practical application is impeded by the shuttle effect of lithium polysulfides and slow redox kinetics of electrode. Here, we report a successful synthesis of CoS2@SC composite material via in-situ growth and sulfuration pyrolysis method and prepared CoS2@SC modified separator. The carbon matrix derived from sucrose carbonization with a porous structure promotes the uniform distribution of active adsorption sites. According to Lewis acid–base interaction, CoS2@SC separator exhibits strong chemisorption of lithium polysulfides, reducing the loss of active material during cycling. Furthermore, the enhanced wettability of the CoS2@SC separator with the electrolyte accelerates the ion transfer, improving the reaction kinetics. Compared with the blank separator, the cell with CoS2@SC separator has an initial specific capacity of 1135.5 mAh g−1 and notably, the capacity decay rate of the modified battery is only 0.052% per cycle after 800 long cycles at 0.5 C. This unique attribute ensures enhanced stability and durability of the battery system. Consequently, LSBs with the CoS2@SC separator exhibit an improved electrochemical performance.

Tailoring nickel–cobalt bimetallic alloy as highly effective catalyst in modified separators for high-performance lithium-sulfur batteries

Lithium–sulfur batteries (LSBs) are anticipated as most prospective new generation energy storage systems due to their high theoretical specific capacity, high energy density and low cost. However, their practical applications are seriously hindered by the sluggish redox kinetics of soluble liquid polysulfide intermediates during discharge-charge. Herein, nickel-cobalt bimetallic alloy/bamboo-like carbon nanotubes composite (NiCo/BCNTs) is rationally developed by a one-step calcination method toward this challenge. The results indicate that the Ni and Co nanoparticles not only strongly confine the lithium polysulfides (LiPSs) but also effectively catalyze its conversion reactions by lowering the energy barrier, while the BCNTs enable a large surface area for more active site exposure and provide fast electron/ion pathways. As expected, LSBs with NiCo/BCNTs electrocatalyst modified separators present a reversible specific capacity with 1495.6 mAh g−1 at 0.1 C. More importantly, a high reversible capacity of 544.8 mAh g−1 over 1000 cycles with a low capacity damping of 0.063% per cycle at 2 C is also acquired. This work provides a deeper insight into the regulation of polysulfide redox kinetics in electrocatalysts for LSBs.

Ultra-small TiO2 truncated octahedron nanocrystals with exposed index {1 0 1} facets modified separator for High-Performance Li-S batteries

Lithium sulfur (Li-S) batteries is one of the most attractive candidates due to their high theoretical capacity. However, the effect caused by the dissolution of polysulfides (LiPSs) still prevent the real Li-S batteries commercialization. Herein, we developed the synthesis of ultra-small TiO2 truncated octahedron nanocrystals with exposed index {101} and {001} facets through a one-step hydrothermal method. The as-fabricated samples enable strong adsorption of LiPSs due to their greatly increased number of polar active sites, thus suppressing the shuttle effect. As expected, the TiO2-modified separator improves the rate performance and cycle stability.

Co-Fe phosphide@graphitic carbon nitride nanosheet modified separator for high-performance lithium-sulfur batteries

Lithium-sulfur (Li-S) battery is a promising next-generation energy storage devices due to its high theoretical capacity (1675 mAh g-1), environment-friendliness, low cost, and natural abundance. However, the shuttle effect and sluggish kinetics of lithium polysulfides (LiPs) hinder its commercialization. Herein, we report the synthesis of cobalt iron phosphide@graphitic carbon nitride nanosheet (CFP@CN) modified separator for Li-S batteries. The Li-S cell with CFP@CN separator (CFP@CN-PP) exhibits a high specific capacity of 786.4 mAh g-1 at 1 C. Furthermore, the Li-S cell with CFP@CN-PP can still retain a discharge capacity of 528.3 mAh g-1 and a capacity decay rate of only 0.084% per cycle after 650 cycles. And, the Li-S cell with CFP@CN-PP maintains high coulombic efficiency above 99% after 650 cycles. These results indicate that the CFP@CN with a large specific surface can alleviate the shuttle effect by strong chemical interaction with lithium polysulfides. Furthermore, thanks to high electrical conductivity of CFP@CN, the utilization of sulfur and redox kinetics can be improved. Our study offers useful insights into designing materials for modified separator of lithium-sulfur battery.

Scalable fabrication of Ni(OH)2/carbon/polypropylene separators for high-performance Li-S batteries

Lithium-sulfur (Li-S) batteries are one of the most promising energy storage systems due to their attractive features of high energy density, low cost, and environmental friendliness. However, the shuttle effect of lithium polysulfides stays an obstacle for the further advancement of Li-S batteries. Herein, a novel Ni(OH)2/carbon/ Polypropylene(Ni(OH)2/C/PP) separator was prepared and applied in Li-S batteries to alleviate the shuttle issue. The Ni(OH)2/C/PP separator was constructed through vacuum filtration coating a super thin layer of super P(0.007 mg cm–2) on the Celgard 2325 followed by decorating the carbon layer with Ni(OH)2 nanosheets film formed by a scalable gas-liquid diffusion method. Influence of Ni(OH)2/C on surface hydrophilicity, polysulfide permeability and catalytic effect on polysulfide conversion have been investigated. The results of contact angle, permeation test of Li2S6, XPS S2p spectra, symmetric Li2S6 cells, Li+ ion diffusion coefficient and Tafel slope confirmed that the Ni(OH)2/C/PP greatly improved the wettability for better penetration of electrolyte, lowered polysulfide permeability to retard its diffusion, and enhanced catalytic effect for the polysulfide conversion to boost the electrochemical kinetics. Thus, compared with the bare PP separator, the Ni(OH)2/C/PP separator enabled significantly increased capacity (1153 vs. 703 mAh g−1 at 0.2 C), greatly enhanced rate performance (816 vs. 381 mAh g−1 at 2 C) and satisfactory cyclability ( 0.10% decay rate after 500 cycles). This study provides a feasible separator modification approach for advanced Li-S batteries.

Construction of composite separator with 3D hierarchical spinel structure in Lithium-Sulfur batteries

Lithium-sulfur (Li-S) batteries are considered the most promising candidate for achieving a low-cost high-energy–density system. One of the biggest deterrents during the practical application of Li-S batteries is the shuttle effect which causes fast capacity degradation and poor rate performance. 3D CoNi2S4 with 3D interconnected porous nitrogen doping carbon was prepared by sulfurizing the precursor Prussian blue analogues. The CoNi2S4 nanoparticles are confined in the carbon framework, affording interconnected macro-mesoporous channels. The constructed 3D CoNi2S4 modified separator effectively accommodates enough electrolytes and provides more exposed active sites that suppress the shuttle effect through trapping polysulfides and accelerating the lithiation of S8 to Li2S. Thus, Li-S batteries with 3D CoNi2S4-PP separators exhibit high-rate capacity and remarkable cycle stability. Furthermore, based on the modified separators, a high sulfur loading cathode delivers an areal capacity of 5.62 mAh cm−2. The separator size expands further to assemble the 1Ah Li-S pouch cell, confirming the practicality. This work provides an efficient strategy with a modified separator for high-performance Li-S batteries.

Ten micrometer thick polyethylene separator modified by α-LiAlO2@γ-Al2O3 nanosheets for simultaneous suppression of Li dendrite growth and polysulfide shuttling in Li-S batteries

Lithium-sulfur (Li-S) batteries are of considerable interest for low-cost and high-density energy storage. However, the notorious polysulfide shuttling and the undesired Li dendrite growth impel the development of Li-S batteries. To simultaneously mitigate these problems, a thin (∼10 μm) separator is herein developed by coating α-LiAlO2@γ-Al2O3 (LAO) nanosheets on the dual sides of a commercial polyethylene (PE) separator. Owing to the high anion-anchoring capability and the polysulfide absorbing and catalytic effect of LAO, the LAO modified separator (LAOMS) not only suppresses the polysulfide shuttling, but also homogenizes Li+ flux and reduces concentration polarization. As a result, the Li-Li cell with LAOMS demonstrates an unprecedented stability over 2500 h when cycled at a high current density of 5 mA/cm2. In addition, the Li-S battery with LAOMS exhibits superior cycling lifespan with an ultra-low capacity decay of 0.037% per cycle over 1000 cycles even at 2 C, excellent rate performance, and superior anti-self-discharge capability. Our studies not only shed light on the rational design of functionalized separators for high-performance Li-S batteries, but also provide a thin separator with promising commercial perspective.

Nitrogen doped hollow carbon nanospheres as efficient polysulfide restricted layer on commercial separators for high-performance lithium-sulfur batteries

The polysulfide shuttle limits the development of lithium-sulfur (Li-S) batteries with high energy density and long lifespan. Herein, nitrogen doped hollow carbon nanospheres (NHCS) derived from polymerization of dopamine on SiO2 nanospheres are employed to modify the commercial polypropylene/polyethylene/polypropylene tri-layer separators (PP/PE/PP@NHCS). The abundant nitrogen heteroatoms in NHCS exhibit strong chemical adsorption toward polysulfides, which can effectively suppress the lithium polysulfides shuttle and further enhance the utilization of active sulfur. Lithium-sulfur batteries employing the PP/PE/PP@NHCS deliver an initial discharge capacity of 1355 mAh/g and retain high capacity of 921 mAh/g after 100 cycles at 0.2 C. At a high rate of 2 C, the lithium-sulfur batteries exhibit capacity of 461 mAh/g after 1000 cycles with a capacity fading rate of 0.049% per cycle. This work demonstrates that the NHCS coated PP/PE/PP separator is promising for future commercial applications of lithium-sulfur batteries with improved electrochemical performances.

Plasma and magnetron sputtering constructed dual-functional polysulfides barrier separator for high-performance lithium-sulfur batteries

In order to fundamentally suppress the shuttle effect, N2 Plasma & Al2O3 magnetron sputtered separators (Al2O3@N-PP) are proposed for lithium-sulfur batteries (LSBs). Such a dual-functional polysulfides (LiPSs) barrier separator greatly inhibits the shuttle effect from the perspective of physical and chemical interaction. Physically, the inherently electronegative amorphous Al2O3 first achieves the repulsion of LiPSs to the sulfur cathode through the electrostatic repulsive effect, effectively preventing a large amount of soluble LiPSs from accumulating at the separator. At the same time, the Al2O3 film seals the shuttle channel of LiPSs to a certain extent. Chemically, N2 plasma-doped N heteroatoms form a lithium bond with Li+ in LiPSs to achieve the first step chemical adsorption and anchoring of LiPSs. When the LiPSs reaches the amorphous Al2O3 film, more stable chemical bonds are formed between Al3+ and S2−, Li+ and O2– to achieve more effective adsorption and anchoring of LiPSs. At 1C with a high sulfur loading up to 3–5 mg cm−2 the LSB contributes a specific charge capacity of 717.4 mAh g−1, with high retention rate up to 75.49 % after 450 cycles. The U-shaped electrolytic cell experiment and ultraviolet–visible spectrum experiment confirmed the LiPSs barrier function of the functional separator.

Insight into Lithium–Sulfur Batteries with Novel Modified Separators: Recent Progress and Perspectives

Lithium-sulfur (Li-S) batteries have been found to have sky-scraping theoretical gravimetric energy density and have gained a lot of interest. However, owing to low electrical conductivity, a conducting substance must be coupled with the sulfur cathode. The first important concern in Li-S is diffusion of lithium polysulfides (PSs), resulting in a shuttle linking both the cathode and anode and rapid capacity of degradation. Plating conventional separators is a kind of alleviation of the shuttle issue. To achieve an advanced Li-S battery, understanding of the structure of PSs needs to be considered to design efficient separator coating materials. The main object of this work is to review the most promising recent challenges and address the application of modified coatings on the polypropylene (PP), polyethylene (PE), and glass fiber (GF) separators for high-performance Li-S batteries. The particular focus has been placed on functioning of various functional layers on the surface of PP, PE, and GF, including polymers, carbons, nanomaterials, and their composites. Finally, the polar host materials in the cathode that influence the Li-S performance are summarized. © 2021 American Chemical Society. All rights reserved.

Empowering polypropylene separator with enhanced polysulfide adsorption and reutilization ability for high-performance Li-S batteries

Originating from the severe polysulfide dissolution and notorious polysulfide shuttle, the practical applications of lithium-sulfur (Li-S) batteries confront huge challenges in low active material utilization, rapid capacity degradation, short cycling lifespan, and inferior rate capability. Herein, a rationally modified polypropylene (PP) separator with multi-functional nickel hydroxide@carbon nanotube (NiOOH@CNT) coating layer is proposed to overcome these critical obstacles. The conductive and porous CNT networks ensure the rapid electron/ion transport to achieve high reactivation and reutilization of active materials, as well as physically block the polysulfide migration. Meanwhile, the porous NiOOH nanowalls arrays layer with polar surface demonstrates superior chemical absorption ability for anchoring polysulfides, thereby significantly suppressing the shuttle effect. Such elaborately designed separator endows Li-S batteries with high specific capacity, superior capacity retention and remarkable rate capability. This work sheds new light on the multi-functional separators for high-performance Li-S batteries.

A multifunctional separator for high-performance lithium-sulfur batteries

Lithium-sulfur (Li–S) batteries attract much more attention over the past decades because of its high theoretical specific capacity (1675 mAh g−1). However, the rapid capacity decay caused by the shuttle effect of polysulfides greatly limits its practical applications. In this work, we prepared an acetylene black/γ-Al2O3/polypropylene composite separator in a friendly and simple way to improve the performance of Li–S batteries. The γ-Al2O3 layer can block the polysulfides diffusion and simultaneously improve the thermal stability of the separator. The acetylene black layer can reduce interface impedance, improve the utilization of active materials and also further block the shuttle of the polysulfides. The Li–S batteries equipped with this composite separator show better cycling performance with an initial discharge capacity of 1096 mAh g−1 and a capacity decay as low as 0.08% per cycle for 500 cycles at 0.5 C (1 C = 1675 mA g−1) and rate performance with a reversible discharge capacity of 871 mAh g−1 at 3 C. Our work can be used for coming commercialization of Li–S batteries.

Designing a Multifunctional Separator for High-Performance Li-S Batteries at Elevated Temperature.

The practical applications of lithium-sulfur (Li-S) batteries are seriously limited by the undesirable polysulfide shuttling and lithium dendrite growth. Herein, a multifunctional membrane is designed and prepared by coating a lithiated Nafion (Li@Nafion) layer and an Al2 O3 layer on the two sides of a routine polymer membrane (polypropylene/polyethylene/polypropylene, PEP). The Li@Nafion layer faced to the sulfur cathode builds a "polysulfide-phobic" surface to restrain the shuttle effect via Coulomb repulsion, while the Al2 O3 layer with a uniform porous structure aids in regulating homogeneous Li+ fluxes to achieve stable Li electrodeposition. As a result, the Li//Li symmetric cell with a Li@Nafion/PEP/Al2 O3 (LNPA) separator realizes stable Li plating/striping even after 1000 h at a high current density (5 mA cm-2 ). Moreover, the Li-S batteries incorporating LNPA separators not only can achieve excellent outstanding cyclic stability at an ultrahigh sulfur loading (7.6 mg cm-2 ), but also exhibit impressive electrochemical performance at an elevated temperature (60 °C). The rational design of the LNPA separator presents new insights to develop high-performance Li-S batteries.

Impeding polysulfide shuttling with a three-dimensional conductive carbon nanotubes/MXene framework modified separator for highly efficient lithium-sulfur batteries

The shuttling of polysulfide is still a major issue which greatly hinders the practical application of lithium-sulfur (Li-S) batteries. Here, we report a multifunctional separator for trapping polysulfide and reusing the active sulfur species in Li-S batteries. The multifunctional separator was fabricated by coating the commercial polypropylene (PP) separator with a three-dimensional (3D) interwoven framework structure composed of 1D carbon nanotubes (CNTs) and 2D Ti3C2Tx MXene nanosheets. Taking advantages of the excellent electrical conductivity and strong chemical interaction with polysulfide of Ti3C2Tx MXene, the ability in immobilizing and reusing of polysulfide for the multifunctional separator was significantly enhanced compared with pure CNTs modified separator. At mean time, the presence of CNTs effectively helps avoid the restacking of MXene nanosheets, resulting in an interfacial layer between separator and cathode with an interconnected conductive network, which can facilitate fast Li ion and electron transport and thus improve the rate capabilities and sulfur utilization. Moreover, the effect of MXene content on the structure and electrochemical performance were systematically investigated. At MXene content of 5% and CNTs/MXene mass loading of 0.16 mg cm−2, the cell with CNTs/MXene-PP (CMP) separator delivered an initial capacity of 1415 mA h g-1 at 0.1 C, a capacity retention of 614 mA h g−1 after 600 cycles at 1 C with a low capacity decay of 0.06% per cycle in the 0.8–2.5 mg cm−2 sulfur-loaded cathode. Such 3D carbon nanotubes/MXene conductive framework provides great potential for developing efficient functional separator for high-performance Li-S batteries and can be easily applied in electrode materials for other electrochemical energy storage systems.

Constructing metal-free and cost-effective multifunctional separator for high-performance lithium-sulfur batteries

As numerous efforts have been devoted to the construction of nano-structured sulfur electrodes, the rational design of multi-functional separator is attracting an increasing research enthusiasm recently as another promising approach towards high-performance lithium-sulfur (Li-S) batteries. Herein, red phosphorus nanoparticles are employed, for the first time, to establish a metal-free and cost-effective multifunctional separator, which not only efficiently immobilizes the lithium polysulfides but also facilitates the ion conduction for fast and durable sulfur redox reactions. The computational and experimental results reveal that red phosphorus is capable of chemically confining the polysulfides through a combination of the Lewis acid-base interaction and sulfur-chain catenation, while the highly conductive Li3PO4 is generated during the cell operation and facilitates ion transportation. Attributed to these unique features, the cells based on the as-developed separator exhibit a high sulfur utilization of 1287 mAh g−1 at 0.1 C, superb rate capability of 809 mAh g−1 at up to 2 C, and excellent cycling performance with 82% capacity retention after 500 cycles at 1 C. This facile and low-cost strategy not only offers an effective pathway towards enhancement in battery performance, but could also inspire new thinking on developing metal-free functional agents for improved sulfur electrochemistry.

Carbon-Tungsten Disulfide Composite Bilayer Separator for High-Performance Lithium-Sulfur Batteries.

A bilayer separator based on cost-effective carbon-tungsten disulfide composite and commercial separators is developed in this work. The C-WS2 separator reduces the shuttling effect and increases the cycle life of the battery because of the excellent adsorption capability of polar WS2 to polysulfides. Furthermore, conductive carbon substantially enhances the ionic conductivity of (1 × 10-3 S cm-1). The cell with the C-WS2 separator delivers an excellent discharge capacity of 996 mA h g-1 at 1 C and retained at 416 mA h g-1 after runs over 1000 cycles with a 0.045% capacity decay per cycle. The work provides insights into high-capacity lithium-sulfur batteries with cost-effectiveness.

MXene debris modified eggshell membrane as separator for high-performance lithium-sulfur batteries

A functional separator (MXene/ESM) to suppress the lithium polysulfides shuttling via coating MXene debris on one surface of a biodegradable eggshell membrane (ESM) is designed to enhance the electrochemical performance of Li-S batteries. The excellent electronic conductivity of the porous MXene debris, and the good mechanical strength, superior thermal stability as well as large electrolyte infiltration of ESM make MXene/ESM an ideal separator for high-performance Li-S batteries. The strong chemisorption induced from both Ti-S bond formed between Ti atom in MXene and the lithium polysulfides by the Lewis acid-base interaction and affinity of O and N containing functional groups on ESM to the lithium polysulfides greatly prevents the shuttling effect of the polysulfides. Compared with a commercial polypropylene separator, the Li-S battery with the MXene/ESM separator containing a KJC/S cathode and a Li metal anode displays greatly improved cycling stability with a capacity retention of 74% after 250 cycles at 0.5 C, while the Li-S battery with a polypropylene separator remains only 11%. The rate performance of Li-S battery with the MXene/ESM separator has also been enhanced compared to that with a polypropylene separator. Specifically, the Li-S battery with the MXene/ESM separator has a discharge capacity of 1321 mAh g−1 at 0.1 C, 1112 mAh g−1 at 0.2 C, 1003 mAh g−1 at 0.5 C and 948 mAh g−1 at 1 C.

Novel flower-like hierarchical carbon sphere with multi-scale pores coated on PP separator for high-performance lithium-sulfur batteries

Lithium-sulfur (Li-S) battery is considered as the next-generation of cost-efficient rechargeable battery systems to fulfill clean energy transportation systems due to its remarkable high theoretical energy density. Nevertheless, capacity fading and poor cycling stability mostly caused by polysulfide shuttle and active sulfur materials loss play an obstacle role on large-scale viable commercialization. In this paper, a flower-like hierarchical carbon sphere (FHCS) is prepared and coated on commercialized PP separator for polysulfide trapper. This carbon sphere with multi-scale pores strongly absorbs the polysulfide, further tremendously enhances the cycling performances, and the conducted carbon sphere also acts as current collector for improving the conductivity of active materials. The results show the cell demonstrates an initial discharge specific capacity of up to 1427mAhg−1 at 0.2C, and high reversible capacity of 730mAhg−1 with degradation rate of only 0.061% per cycle after 500 cycles at 0.5C.