Two-Dimensional Materials Modified Separator Strategies of Suppressing the Shuttle Effect in Lithium-Sulfur Batteries

Linli Guo; Xin Zhang; Min Xiao; Shuanjin Wang; Dongmei Han; Yuezhong Meng

doi:10.7536/PC200721

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (7) : 1212-1220 DOI: 10.7536/PC200721

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Two-Dimensional Materials Modified Separator Strategies of Suppressing the Shuttle Effect in Lithium-Sulfur Batteries

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Abstract

Lithium-sulfur batteries have a theoretical discharge specific capacity (1675 mAh /g) and energy density (2600 Wh /kg) that are much higher than those of lithium-ion batteries. Lithium-sulfur batteries are considered as very promising battery systems, thus they have been widely concerned and researched. However, the poor conductivity of sulfur, low utilization, and the shuttle effect of polysulfides make the cycling performance of lithium-sulfur batteries unstable. In order to overcome the impact of the shuttle effect, a variety of new separator designs and preparation methods have been developed in recent years to improve the cycle stability of the battery. This article reviews the latest research progress from the perspective of 2D material modification of the separator, and uses high-quality separators to suppress the shuttle effect, which will better achieve high stability of Li-S batteries. Besides, future developments are prospected.

Contents

1 Introduction

1.1 The structure and working principle of lithium-sulfur batteries

1.2 The challenges of lithium-sulfur batteries

2 Two-dimensional material modified separator

2.1 Graphene

2.2 MXenes

2.3 Two-dimensional transition metal dichalcogenideso

2.4 Other two-dimensional materials

3 Conclusion and outlook

Graphical abstract

Keywords

2D materials / separator / shuttle effect / Li-S batteries

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Linli Guo, Xin Zhang, Min Xiao, Shuanjin Wang, Dongmei Han, Yuezhong Meng. Two-Dimensional Materials Modified Separator Strategies of Suppressing the Shuttle Effect in Lithium-Sulfur Batteries. Progress in Chemistry, 2021, 33(7): 1212-1220 DOI:10.7536/PC200721

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1 Introduction

Nowadays, the energy and environmental problems are becoming more and more urgent. With the development of human society, our demand for energy is also increasing. It is urgent to study and develop clean energy^[1]. In 2016, the new energy automobile industry was listed as the "13th Five-Year" national strategic emerging industry development plan, and the development of power lithium-ion batteries ushered in new opportunities^[2⇓~4]. At present, the consumer electronics industry is developing rapidly, but the specific capacity of secondary batteries often becomes a restriction on the use experience of equipment. The development of electric vehicle industry also requires the energy density of secondary batteries to be further improved. As the electrochemical active material of the cathode of lithium-sulfur batteries, sulfur has a high capacity of 1675 mAh/G, low cost and abundant resources. Therefore, lithium-sulfur batteries are considered to be the next generation battery system with potential for development, and are expected to be used in power batteries for new energy vehicles and large-scale energy storage power stations^[5,6]. Lithium-sulfur battery is a new type of energy storage device with low cost, high energy storage density and environmental protection, which is an important tool for future battery systems. However, the shuttle of polysulfide (PS), the insulating property of sulfur, and the volume change lead to capacity fading and cycle instability, which limit the future application of lithium-sulfur batteries. Two-dimensional materials, such as graphene, carbides, nitrides, sulfides, oxides, and aggregates thereof, can provide a higher surface area to improve sulfur utilization and cycling performance^[7].

1.1 Structure and working principle of lithium-sulfur battery

Lithium-sulfur batteries generally use sulfur or sulfur-containing compounds as the positive electrode, while lithium metal or lithium storage materials are used as the negative electrode. The structure of lithium-sulfur battery is similar to that of current lithium-ion battery, which is mainly composed of lithium metal cathode, diaphragm, electrolyte, sulfur-based composite anode, current collector and shell. But its principle is very different from that of lithium-ion batteries. Lithium-sulfur batteries rely on the breaking and generation of S-S bonds to convert electrical energy and chemical energy, that is to say, lithium-sulfur batteries rely on the redox reaction between S and Li to charge and discharge. The electrochemical reaction equation when it is fully charged and discharged is as follows:

(1)

S 8 + 16 L i + + 16 e - → 8 L i 2 S

Specifically, during charging, the Li₂S is electrolyzed to generate long-chain lithium polysulfide (Li₂S_n),Li⁺, which migrates to the negative electrode and is deposited as metal lithium; During discharge, the Li⁺ migrates from the negative electrode to the positive electrode, the electrons reach the positive electrode through the external circuit, and the S — S bond of the long-chain Li₂S_n is broken to form lithium sulfide^[8].

Although the reaction principle of lithium-sulfur batteries is simple, the actual electrochemical process is complex and multi-step, accompanied by the formation of soluble high-order polysulfides and insoluble low-order sulfides. The specific discharge process can be divided into three stages and two discharge platforms (Figure 1):

In the first stage, the cyclic S₈ is reduced to a long-chain soluble high-order polysulfide

S 6 2 -

in two steps, and a flat discharge plateau is observed at about 2. 4 V. The specific reaction process is as follows:

(2)

S 8 + 2 e - → S 8 2 -

(3)

3 S 8 2 - + 2 e - → 4 S 6 2 -

In the second stage, the

S 6 2 -

is further reduced to the

S 4 2 -

, and the voltage drops sharply from 2.4 V to 2.1 V. The specific reaction process is as follows:

(4)

2 S 6 2 - + 2 e - → 3 S 4 2 -

In the third stage, the long-chain high-order polysulfides are further reduced to insoluble low-order sulfides, and the reaction kinetics is slow due to the solid-phase reaction involved in this stage, which shows a flat plateau at 2. 1 V and thus contributes most of the discharge capacity. The specific reaction process is as follows:

(5)

S 4 2 - + 2 e - + 4 L i + → 2 L i 2 S 2

(6)

L i 2 S 2 + 2 e - + 2 L i + → 2 L i 2 S

The charging process, in fact, is the reverse reaction of the discharging process. The lithium ions in the negative electrode are reduced to lithium metal by electrons, and the positive electrode is the oxidation process of sulfide. At about 2. 2 V, there is a long and smooth voltage curve, and this process is the transformation from insoluble low-order sulfide to long-chain soluble high-order polysulfide, while at 2. 5 V, it is the transformation from long-chain soluble high-order polysulfide to elemental sulfur.

1.2 Challenges for lithium-sulfur batteries

Although lithium-sulfur batteries have high theoretical energy density (2600 Wh/kg) and theoretical capacity (1675 mAh/G), there are still some problems and challenges that hinder their commercialization. During the charge and discharge of lithium-sulfur batteries, elemental sulfur can be converted into soluble and insoluble polysulfides. During the discharge process of the lithium-sulfur battery, the Li₂S can be converted into Li₂S_n(4≤n≤8) which can be dissolved in the electrolyte at the high voltage discharge platform (2.4 ~ 2.1 V).At the low voltage discharge plateau, it will be further reduced to low-valent polysulfide ions (

S n 2 -

, 3 ≤ n ≤ 4) which are soluble in the electrolyte, and Li₂S₂ and Li₂S which are insoluble in the electrolyte. Under the action of concentration gradient, soluble Li₂S_n will diffuse and pass through the diaphragm to the surface of lithium anode, and react with lithium metal to produce short-chain lithium disulfide (Li₂S₂) and lithium sulfide (Li₂S).Eventually, the irreversible loss of active substances in the battery, the attenuation of battery life and the low coulombic efficiency are caused. Under the action of electric field and concentration difference, soluble polysulfides will diffuse back to the positive electrode through the electrolyte and be further oxidized. This phenomenon of cyclic migration of polysulfides between the positive and negative electrodes through the separator is called "Shuttle effect", or Shuttle effect^[10,11]. The formation of the polysulfide shuttle effect is divided into five steps: (I) the formation of long-chain polysulfides, (ii) the separation of polysulfides from the sulfur host; (iii) the polysulfide is dissolved in the electrolyte; (iv) Migration of polysulphides; (V) Reaction between polysulfide and lithium negative electrode^[12]. At the same time, the conductivity of sulfur is very poor (5×10^-30S/cm at room temperature), the utilization rate is low, and the large volume change of sulfur during charge and discharge leads to the pulverization of sulfur cathode, which leads to its poor cycle performance. In addition, lithium polysulfide (Li₂S_n,1≤n≤8), the intermediate product of lithiation, has very poor conductivity, which restricts the transmission of electrons in the sulfur cathode and the utilization of active sulfur, making deep discharge difficult, resulting in poor cycle stability and low specific capacity of the electrode. Finally, as the electrode of the battery, metal lithium usually has dendrite formation in the organic electrolyte, which is prone to internal short circuit and other safety problems.

One of the most serious problems in Li-S batteries is the shuttle effect between polysulfide intermediates (Li₂S_n,4≤n≤8) dissolved in the electrolyte^[13,14]. In order to inhibit the shuttle effect of polysulfide intermediates, advanced sulfur matrix materials have been designed as physical barriers to inhibit polysulfide shuttle by physical adsorption or chemical adsorption^{[15⇓⇓~18]}. At the same time, the performance of Li-S battery has been significantly improved due to the continuous development of research and development of lithium-sulfur battery separator. The separator is an integral part of the battery and plays a vital role in preventing internal short circuits and maintaining the diffusion path of ions. The porous polymer separator is suitable and effective for meeting the requirements of conventional lithium ion batteries. However, multi-electron electrochemical reactions can produce polysulfides in Li-S batteries. This reduces the performance of the battery due to the irreversible decomposition of the polysulfide due to its susceptibility to reaction with the negative lithium metal or due to its metastability and formation of "dead" sulfur-containing species^[19]. In effect, the separator is a complete screen between the positive and negative electrodes. The separator is a perfect retrofit platform to introduce novel battery configurations for Li-S batteries. Recent advances in advanced separators are aimed at high-stability Li-S batteries by (I) shielding the diffusion of polysulfides to suppress the shuttle effect; (ii) reactivating the deposited dead sulfur-containing material; (iii) inhibition of dendrite growth on the lithium metal surface. Various functional layers, including polymers, carbon, oxides, and their composites, have been applied to controllably alter the separator to achieve better battery performance. Similarly, the novel Li-S battery configuration with the intermediate layer is also particularly suitable for Li-S batteries with high capacity, high stability, and high rate capability. Manthiram et al. First proposed this important "intermediate layer" concept in 2012, introducing a porous carbon paper as an intermediate layer between the positive electrode and the separator. Subsequently, extensive research on interlayer materials, including various carbon-based interlayers and polymer-based interlayers, has greatly improved the electrochemical performance of Li-S batteries, and to some extent, can inhibit the diffusion of lithium polysulfide^[20]. In this paper, we focus on the application of functional separators in lithium-sulfur batteries. Functional separators are easy to prepare and act as both an effective barrier to polysulfide shuttling and a reservoir for various sulfur species^[21]. The primary function of the functional separator is to chemically or physically trap the migrating PS, and the secondary function is to ensure a uniform charge density distribution across the electrode, thereby keeping the entire electrode volume active during the electrochemical process^[22]. Currently, a variety of modified materials have been developed to limit the movement of polysulfide intermediates to the positive electrode. These modified materials include MgBO₂(OH)/CNT and MoS₂/CNT, and the use of modified 2D materials in functional separators has aroused great enthusiasm among researchers^[23]^[24]. One-dimensional nanomaterials are considered to be very efficient and stable battery electrode materials because of their directional electron and ion conduction direction, strong stress tolerance and short axial electron and ion transport path. Due to the inherent advantages of two-dimensional materials, i.e., large integrated coverage, mechanical robustness, and abundance of surface chemical groups, they can be ideal modifiers for functional separator batteries^[25]. A technical challenge inherent in lithium-sulfur (Li-S) batteries arises from the inherent redox electrochemistry that occurs on the Li and S electrodes, which can severely reduce S utilization and service life. Two-dimensional (2D) nanomaterials, which are composed of atomic or near-atomic thickness and have infinite lateral dimensions, have been shown to be promising materials for energy storage applications due to their unique structures, excellent physical properties, and tunable surfaces. We believe that surface functionalization or heteroatom doping of 2D materials, hybridization with other nanostructured materials (0 D quantum dots, 1 D nanotubes/nanowires, and other 2D nanosheets), and eventual construction into 3D structures and 2D dominant building blocks could open up new opportunities for lithium batteries^[26].

2 Two-dimensional material diaphragm modification

Two-dimensional materials are easy to stack into thin film materials, and have good conductivity, stronger mechanical properties and extremely high specific surface area, so they can be combined with ordinary membranes to inhibit the shuttle of polysulfides. In the past decade, two-dimensional materials, defined as materials with a thickness of a few atomic layers and extremely high aspect ratios, have attracted great interest from researchers in many applications. Due to their two-dimensional nature, nanosheet materials have extremely high specific surface areas and exhibit excellent physical and chemical properties due to quantum confinement effects in one major direction. Among them, graphene, which is composed of thin layers of carbon atoms bonded by sp², has attracted extensive research due to its excellent physical, chemical, optical and mechanical properties^[27]. Since 2010, other 2D materials have also attracted a lot of new attention, including hexagonal boron nitride, transition metal sulfides (TMDCs), metal oxides, P, silylene, germanylene, and MXenes^[28]^[29]^[30]^[31]^[32]^[33]^[34].

2.1 Graphene

Graphene is a two-dimensional layered sheet composed of carbon atoms and sp² orbital hybrids. Due to its special layered stacking structure and high specific surface area, it can not only provide enough storage space for active sulfur, but also has a good inhibition effect on the shuttle of polysulfides during the reaction. Li et al., Shenyang National Laboratory of Materials Science, achieved graphene coating on the surface of the diaphragm (Fig. 2)^[35]. On the one hand, graphene can effectively act as an internal "current collector" to effectively improve the transmission rate of electrons, on the other hand, the graphene layer also has a certain inhibitory effect on the diffusion of polysulfides. With this separator, the energy density of Li-S batteries can be greatly improved. In order to obtain a convenient and scalable green separator manufacturing method, Zhang et al. Used fluidized bed CVD method to prepare porous graphene (PG)^[36]. In the method, commercial magnesium oxide particles are used as a template for graphene deposition by a water boiling method to obtain a porous magnesium oxide nanosheet, and then the obtained MgO @ PG is added with 5.0 mol·L^-1 HCl at 70 deg C to react for 4.0 H to remove the magnesium oxide template. To prepare the PG membrane, PG was first dispersed in a planetary ball mill to form a homogeneous slurry with PVP and water. The mass ratio of PG to PVP was 8:2. Finally, the aqueous dispersion was coated on a continuously rolled PP film with a width of 0.10 m, and the loading of the composite film was controlled at 0.54 mg·cm^-2. The combination of PG and amphiphilic polymer binder makes the functional layer have higher conductivity and surface area, and stronger chemisorption of polysulfide. Therefore, when the sulfur load of the lithium-sulfur battery is 1.8~2.0 mg·cm^-2,PG, the sulfur utilization rate of the separator is very high at 86.5% (compared with 55.6% for a conventional separator). At 0.05 C, the self-discharge rate is very low, remaining 90% (65% for normal separator), and the rate capability is enhanced. Chen et al. Suggested that Li-S cells with ODC/rGO coated separators exhibit excellent rate performance and good long-term cycle stability (fig. 3) due to the synergistic interaction of physical adsorption and chemical action to immobilize soluble polysulfides^[37].

2.2 MXenes

MXenes are a class of two-dimensional inorganic compounds composed of transition metal carbides, nitrides, or carbonitrides several atomic layers thick. It was first reported in 2011 that MXene materials have the metallic conductivity of transition metal carbides due to hydroxyl or terminal oxygen on their surface^[38]. MXene prepared by HF etching has an accordion-like morphology and is a multilayer MXene (ML-MXene), which is referred to as a thin layer MXene (FL-MXene) when there are less than 5 layers. Since functional groups can be attached to the surface of MXene,

M n + 1

X_nT_x (where T is a functional group, O, F, OH) can be named in a common way. MXene materials are widely used due to their high conductivity and high activity^{[39⇓⇓~42]}. At the same time, the highly active two-dimensional surface of MXenes can effectively fix soluble polysulfides through metal-sulfur interactions^[43,44]. In addition to exhibiting high electron conductivity, MXenes shows unique advantages in improving the electrochemical performance of Li-S batteries.

Wang et al. Found that MXene materials can be used to modify the separator to improve the stability and rate performance of Li-S batteries (Figure 4)^[45]. The membrane is coated with Ti₃C₂T_xMXene nanosheets to trap polysulfides, which can suppress the shuttle effect. The experimental results show that the lithium-sulfur battery with MXene functionalized separator has better performance, including high specific capacity and high cycle stability. Aiming at the problem of easy stacking of 2D sheets, Lai et al., in order to avoid the limitation of effective sites caused by electrolyte ions entering the gap, used a simple electrostatic self-assembly method.The assembly of two-dimensional (2D) MXene and positively charged 1D CNT-polyethyleneimine with the membrane controllably solves the key problem of slow ion migration, and can effectively improve the adsorption capacity and interfacial reaction of lithium polysulfide (LiPSs)^[46]. The separator prepared by the method contains well-organized nanochannels, has better antifouling performance and high cycle rate, and shows a new strategy for enhancing the sulfur loading and rate capability of the Li-S battery. Similarly, Han et al. prepared a functional Ti₃C₂T_x/CNTs-PP separator with a mass loading of 0.016 mg·cm^-2 and a CNTs content of 10%, which retained a discharge capacity of 640 mAh·g^-1 after a capacity fade of only 0.079% per cycle^[47]. The coulombic efficiency remained above 99% for long cycles. This ultra-lightweight separator not only expands the application scope of MXene materials, but also may provide an effective method for high-performance Li-S batteries. Li et al. prepared a novel MXene/MOF derivative 2D hybrid (N-Ti₃C₂/C) for commercial separators for lithium-sulfur batteries by modifying nitrogen-doped MXene nanosheets with porous carbon generated by in-situ nucleation and ZIF-67 conversion on exfoliated MXene^[48]. Uniformly decorated ZIF-67 can isolate MXene nanosheets, prevent them from re-stacking, and leave exposed MXene with sufficient Lewis acid adsorption surface. In addition, it can also provide a nitrogen source to dope nitrogen into MXene and improve its surface properties. PP separator (N-Ti₃C₂/C@PP) coated with N-Ti₃C₂/C can not only effectively prevent the shuttle of polysulfides, but also successfully inhibit the dendrite growth of lithium anode. The results showed that the Li-S battery with N-Ti₃C₂/C@PP separator exhibited a high specific capacity of 1332 mAh·g^-1 at 0. 1 C, and the capacity decay rate per cycle was stable at 0. 07% in 500 cycles at 0. 5 C under the sulfur surface loading of 3.4 mg·cm^-2. These Li-S batteries can stably provide an areal specific capacity of 6.3 mAh·cm^-2 even at higher sulfur loadings exceeding the 10 mg·cm^-2.

2.3 Metal sulfides

Two-dimensional transition metal sulfides (TMDC) are Shi -type semiconductors where M is a transition metal atom (e.g., Mo or W) and X is a chalcogen atom (e.g., S, Se, or Te). In recent years, transition metal disulfides with layered structures (MoS₂,WS₂, etc.) Have attracted great interest^[49,50]. Due to the highly covalent nature of the bonds between transition metals and sulfur, TMDC is perfectly conductive, and the benefit of using multiple layers of TMDC in Li-S batteries is that a large number of valence electrons can be utilized to interact with LiPSs and help absorb and trap them^[51]. MoS₂, in particular, has been shown to exhibit strong interactions with LiPSs due to the large size of its own S^2- anion and its ease of binding to LiPSs. Because of its excellent physical properties, MoS₂ is the most studied material in this family, and the separator coated with MoS₂ can be used as an effective barrier to inhibit the shuttle effect of lithium batteries. The excellent flexibility and unique optoelectronic properties of MoS₂ make them widely used in many scientific fields, such as thin film transistors, photovoltaic devices, supercapacitors and lithium-ion batteries^[52]^[53]. Due to its 2D flexibility, a layer of MoS₂ can be easily deposited on a conventional Celgard membrane by a simple method to produce a flexible MoS₂/Celgard composite membrane. MoS₂ is one of the most typical materials in the transition metal dihalide (TMDC) family. The monolayer of the MoS₂ has three atomic planes, while the middle Mo atomic plane separates the S atoms in the two hexagonal planes, and the adjacent layers are bonded by weak van der Waals forces^[54].

In the work of Liu et al., MoS₂ modified separators were prepared and used as an effective barrier to inhibit the shuttle effect of Li-S batteries^[55]. As the prepared MoS₂ coating modified separator shows excellent wettability, the whole battery can realize rapid lithium ion diffusion. In addition, the metal sulfide MoS₂ provides chemisorption of polysulfides. The Li-S battery by using the separator coated with the MoS₂ shows higher specific capacity and more excellent cycle stability than the original separator. Similarly, Li et al. creatively reported a simple strategy of using a highly lithium conductive MoS₂ film as a new PS barrier layer in Li-S batteries^[56]. When used in Li-S batteries, the MoS₂/Celgard composite separator can effectively reduce PS shuttling, thereby improving coulombic efficiency and long-cycle stability. The strategies demonstrated in this work will open the door to the development of flexible 2D materials other than graphene for efficient energy storage for energy storage devices.

Experiments by Wang et al. (Fig. 5) showed that a Li-S cell with a separator modified with a rGO@MoS₂ coating showed a high reversible capacity of 1122 mAh·g^-1 at 0.2 C, a low capacity decay rate of 0.116% after 500 cycles at 1 C, and excellent rate performance (615 mAh·g^-1 at 2 C)^[57].

2.4 Other two-dimensional material

Notably, significant advances in two-dimensional materials offer new opportunities for long-life and high energy density Li-S batteries. Two-dimensional materials, such as MXene, reduced graphene oxide, and MoS₂, have been extensively investigated for use in lithium-sulfur batteries^[58]^[59]^[55]. In addition, other 2D materials, such as metal-organic frameworks (MOF), molybdenum nitride nanosheets, Sb₂S₃ nanosheets, covalent organic frameworks (COF), porphyrin frameworks (POF), two-dimensional exfoliated diamond sheets, two-dimensional siloxane nanosheets and so on, have attracted increasing research interest due to their strong ability to capture polysulfides^[60]^[61]^[62]^[63]^[64,65]^[66]^[67].

MOF-based materials with large surface areas and highly ordered pores with tunable porosity are suitable as ion sieves to mitigate these shuttling polysulfide ions. In addition, the natural insulating properties of MOF-based materials can also meet the electrical requirements when used as separators for batteries. Fang et al. Assembled ultrathin MOF nanosheets by a simple vacuum filtration method, and obtained highly oriented and flexible MOF films with good mechanical properties for the first time^[68]. The excellent properties make the prepared MOF nanosheets very suitable for manufacturing lightweight interlayer modified materials for inhibiting polysulfide shuttling separators of Li-S batteries. Li-S batteries made from commercial carbon materials using MOF membrane modified separators show significantly enhanced cycle stability, which opens up new opportunities for the synthesis and application of MOF materials. Zhou et al. designed and prepared a microporous MOF @ GO separator for lithium-sulfur batteries^[60]. In the electrochemical process, the MOF @ GO separator can act as an effective ion sieve for polysulfides migrating to the negative electrode, and the lithium-sulfur battery equipped with the MOF @ GO separator has excellent cycle stability and low capacity decay rate. The stable performance is mainly attributed to the size effect of the homogeneous porous framework of the separator. Zheng et al. Also pointed out that MOF can modify the electrochemical performance of the battery (Fig. 6), so that the MOF-modified separator has a microporous structure, hydrophilicity and conductivity to easily capture PS and enhance the electrochemical kinetic reaction, which is conducive to reducing the shuttle effect^[69].

2D nanosheets with high specific surface area also provide abundant active sites, and the layered structure may be an inherent physical barrier to sulfur species^[70⇓~72]. For example, the Li-S battery with MoN_x/Celgard separator prepared by Wang et al. Using ultra-thin and lightweight 2D molybdenum nitride nanosheets to modify the separator has high capacity reversibility, effective control of self-discharge performance and excellent cyclability^[61]. After 500 cycles, the decay rate is only 0.063% per cycle at 0.5 C. The method solves the fatal defect of the Li-S battery and improves the commercial feasibility thereof by applying two-dimensional molybdenum nitride nanosheets. Chen et al. reported a multifunctional membrane (fig. 7) made by modifying a commercial Celgard membrane with functionalized boron nitride nanosheets and negatively charged groups^[73]. Due to the strong ionic repulsion of negatively charged boron nitride nanosheets to negatively charged polysulfides, the boron nitride membrane effectively prevents polysulfides from migrating through the membrane. Lithium-sulfur batteries with boron nitride separators have excellent long-term cycle stability (up to 1000 cycles) and high capacity of 718 mAh·g^-1 at very high 7 C(1.18 A·g^-1) currents. Kim et al. Integrated two-dimensional Sb₂S₃ nanosheets (SSNSs) into the Li-S separator, and applied the new 2D coupling material to the battery for the first time^[62]. The specific capacity of the cell with a well-designed separator coated with an SSNS/carbon nanotube (CNT) coupling layer was significantly increased with a decay rate of 0.05%. The above research provides a new direction for the discovery of high-performance Li-S batteries and a new idea for the application of 2D materials in energy storage systems. In addition, Huang et al used a multi-metal oxide nanorod called attapulgite as a multi-functional ion sieve to assist the modification of the separator, which can effectively inhibit the "shuttle effect" and further guide the management of LiPSs in Li-S batteries^[74]. Attapulgite, which is composed of aluminum, magnesium, iron, silicon and oxygen ions, has more polar sites and can immobilize LiPSs compared with monometallic oxides. In addition, the catalytic properties of palygorskite (iron ions) utilize the LiPSs conversion reaction. In addition, thanks to the synergistic effect of the multimetal oxide and the conductive carbon, the Li-S battery with the modified separator can provide excellent discharge capacities of 1059.4 mAh·g^-1 and 792.5 mAh·g^-1 in the first and 200th cycles at 0.5 C. This work provides a feasible strategy for rationally designing a high-performance Li-S battery by constructing a composite layer.

3 Conclusion and prospect

Li-S batteries are seen as a potential candidate to meet the growing demand for energy due to their high energy density and environmental friendliness. The shuttle effect is one of the key problems to realize the practical application of Li-S battery. This paper summarizes the methods to suppress the shuttle effect from the point of view of the diaphragm. The modification and functionalization of the membrane is one of the effective strategies to prevent the shuttle effect. The modification of the separator by the two-dimensional material is beneficial to inhibiting the shuttle effect and realizing the high cycle stability of the Li-S battery. In this paper, the strategies for improving the electrochemical performance of lithium-sulfur batteries by functionalized composite separators coated with 2D materials are summarized by evaluating the shuttle mechanism and the interaction with soluble polysulfides.

The 2D material has a large specific surface area, and the 2D material with a high specific surface area also provides abundant active sites, and the layered structure may become an inherent physical barrier to sulfur species, in addition, the 2D material has a microporous membrane structure or pores and defects, and is easy to physically adsorb polysulfides. At the same time, the high exposure of surface atoms of 2D materials provides conditions for the regulation of material properties by surface modification/functionalization and element doping, which also provides a chemisorption strategy for the anchoring of polysulfides. In a word, two-dimensional materials can interact with polysulfides through physical adsorption and chemical adsorption to achieve efficient adsorption of polysulfides.Dissolved polysulfides are confined in the positive electrode area, the flux of soluble polysulfides diffusing to the negative electrode is reduced, and the stable utilization of active sulfur materials is realized, thereby improving the stability of the lithium-sulfur battery. 2D materials have become one of the most popular research topics in the field of storage due to their unique structural characteristics, excellent properties and promising applications. With the further study of multi-functional separators, there is still a broad prospect for the performance and large-scale application of 2D materials in lithium-sulfur batteries.

The intrinsic properties and controllable surface chemistry of 2D nanomaterials make them useful as active materials, hosts, protective layers, conductive additives, and reaction promoters. However, there is still a long way to go to develop practical Li-S batteries that meet the performance and safety requirements. In the lithium-sulfur battery, in order to improve the energy density of the battery and reduce the cost of manufacturing the battery, it is required to have a low electrolyte/sulfur (E/S) ratio at a practical level, and a low E/S ratio requires a 2D material to have a highly conductive skeleton, an abundant reaction region, a stable electrode structure, an effective ion transport channel, and an effective electrolyte permeability. In addition, the high micropore volume requires a large amount of electrolyte to achieve a high E/S ratio, so it is expected that a 2D carbon body with no micropores and very thin and low density TMDC (for extended life cycle, lower E/S ratio) can be applied to the Li-S battery separator design of carbon nanotubes to achieve the target energy density.

For the practical application of lithium-sulfur batteries, in addition to inhibiting the shuttle effect, other problems should be solved, such as low sulfur loading, volume expansion of sulfur electrode and lithium dendrite of negative electrode, which are important problems affecting the practical process of lithium-sulfur batteries. In the future application research of lithium-sulfur batteries, how to increase the loading of active material sulfur, limit the loss of active material, reduce the shuttle effect and inhibit the lithium dendrite in lithium metal anode need to be considered comprehensively.

Lithium-sulfur battery, as a new type of battery with great development potential, has attracted great attention from academia. However, there are still some unsolved problems in lithium-sulfur batteries, such as short cycle life, low discharge power and low Coulomb efficiency. Through the modification of electrode, diaphragm and electrolyte, many problems have been improved, but there are still shortcomings. After these problems are solved in the future, it is believed that lithium-sulfur batteries will become the next generation of secondary batteries with low cost and good performance, and will be applied in the market on a large scale.

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