Review on Multi-Functional Separator for Li-S Batteries

The design and manufacture of a Li-S separator from scratch is often accompanied by complex preparation process, high cost and harsh technological conditions. The synthesis of specific polymer structures makes it expensive to prepare a composite separator, which is not conducive to large-scale commercial application. It is a simpler and more practical way to make functional surface modification directly on the mature PE and PP separator. Currently, a variety of materials such as carbon-based materials (graphene (GE), carbon nanotubes (CNT), carbon black (CB), ketjen black (KB)and etc), metal oxides, metal sulfides, metal nitrides and MOFs have been widely used to modify commercial PP/PE separators to gain excellent performance. Carbon-based materials are the earliest used to modify the Li-S separator. In 2012, the method of inserting a free-standing multiwalled carbon nanotube (MWCNT) paper between the cathode and the separator was firstly reported by Yu-Sheng Su and Arumugam Manthiram [28]. In 2020, Zhu et al. [29] reported a N-doping carbon aerogel interlayer in coral-like structure (NHCA) to modify the separator. Besides, they also explored influences of specific surface area of interlayer. Rana et al. [30] proposed a Nafion/KB (KBN) composite coating which possessed abundant SO₃⁻ groups and electronically conductive porous framework. Since there have already been many summaries about carbon-based materials modified separator, this paper will mainly focus on other materials. *3.1. Metal Oxides* Metal oxides have been widely used in separator modification because of their lower price and stronger polarity which have been proved to effectively adsorb LPS in many studies. In Cheng et al.’s work [31], spontaneously polarized bismuth ferrite BiFeO₃ (BFO) particles were firstly used in separator modification of Li-S battery. Combining with graphene oxide (GO) and acetylene black (AB), they fabricated a functional BFO/GO/AB@PP separator. The spontaneous polarization of BiFeO₃ is regarded as the key factor which achieves effective LPS anchoring. In 2020, Wang et al. [32] proposed to use FeOOH and C₃N₄/KB to build a self-assembled bifunctional separator. Different from the physical adhesion method used in previous studies, they were the first to chemically synthesize a layer of FeOOH to capture LPS by floating commercial PP separator onto acidic K₂FeO₄ solution, and then coated a layer of C₃N₄/KB on this basis to improve the conductivity and promote Li⁺ diffusion. Benefiting from this bifunctional separator, the battery displays rate capacities of 1000, 901, and 802 mAh g⁻¹ at 0.5, 1, and 2 C as well as long cycle discharge capacity of 444 mAh g⁻¹ after 900 cycles at 1 C. In addition, MoO₂ was also used in construction of Li-S separator. Choi et al. [33] reported a MoO₂@CNT nanocomposite and applied it to the surface modification of separator. Such MoO₂@CNT nanocomposite was prepared through a customized electrical wire-explosion process conducted at room temperature using Mo wires and CNTs. The CNTs scaffold served as a physical barrier for lithium polysulfides and a current collector for Li⁺ transport as well as lithium polysulfides conversion. At the same time, MoO₂ was supposed to chemically anchor the polysulfides. Benefited from a number of voids and mesopores, the as-prepared MoO₂@CNT nanocomposite can also alleviate the volume change of cathode and provide additional active sites. The Li-S batteries introduced with MoO₂@CNT-modified separator exhibited a significant cycle performance of 1067 mAh g⁻¹ at 0.2 C and achieved a coulombic efficiency of more than 96% after the first ten cycles as well as quite low-capacity decay of only 0.066% per cycle. As displayed in Figure 5, Zuo et al. [34] used acrylic cloth carbon (ACC) decorated with MnO₂ to modify the separator. On the one hand, MnO₂ has been proved to be effective in capturing LPS. On the other hand, acrylic cloth carbon (ACC) was reported to be more reactive and conductive than graphite nitrogen, rGO, dopamine and CNT because of sufficient pyridine and pyrrolic nitrogen generated during the carbonation. The ACC/MnO₂ hybrids coating significantly improved the stability of the obtained Li-S battery.Wan et al. [35] developed a Titanium-based nanorods/Ketjen Black (KB) modified separator named as KTO-2/PP, and applied it to a typical Li-S battery which showed wonderful electrochemical capability. The KTO-2 was fabricated by a simple one-pot hydrothermal method using Ketjen black as well as tetrabutyl titanate and then vacuum-filtered through Celgard PP separators to prepare KTO-2/PP. The results of A.C. impedance test showed that the separator coated with KTO-2 presented a lower charge transfer resistance (25.7 Ω) than that of either KB/PP (39.9 Ω) or PP (102.3 Ω). In other words, the KTO-2 can really improve the conductivity of the coating and its lithium-ion transport capacity. Besides, the great physical and chemical LPS adsorption properties owned respectively by Ketjen black and Titanium-based nanorods are also proved which both relieve the shuttle effect of such Li-S battery. After 200 cycles, the battery equipped with KTO-2/PP displays a reversible capacity of 754 mAh g⁻¹ under the rate of 1 C. Huang et al. [36] developed a Sodium-containing Titanium oxides nanowires/nanosheets (denoted as STO-W/S) composite coating by one-pot method. Instead of simply physically mixing, the reaction condition controlling is more helpful for structure fabrication of STO-W and STO-S. The STO-W/S is able to enhance Li⁺ conductivity as well as suppressing shuttle effect. The battery assembled with this functional separator can achieve an initial capacity of up to 813 mAh g⁻¹ at 1 C and remain the capacity of 541 mAh g⁻¹ after 500 cycles. Recently, a Ni/SiO₂/Graphene coated separator was reported by Chen et al. [37] in order to make an improvement in such typical Li-S battery. As shown in Figure 6, the coating consists of Ni/SiO₂ hollow sphere with mesoporous structure as well as graphene and a great number of silica nanosheets and Ni nanoparticles are evenly distributed on the Ni/SiO₂ hollow sphere. The function of silica nanosheets and Ni nanoparticles is to anchor polysulfides and catalyze to quicken their transformation. Meanwhile, the introduction of graphene aims to fabricate conductive network as well as hampering polysulfides to shuttle from cathode to anode. Moreover, the rapid Li⁺ transportation is achieved by outstanding electrolyte infiltration and retention benefited from mesopores structure. Because of these advantages mentioned above, the battery equipped with Ni/SiO₂/Graphene exhibits a significant cycle performance of 922 mAh g⁻¹ after 100 cycles at 0.2 C as well as 772 mAh g⁻¹ after 300 cycles at 1 C and rate capability of 1463 mAh g⁻¹ at 0.1 C and 782 mAh g⁻¹ at 2 C. Li et al. [38] proposed a CNTs-COOH@Fe₃O₄ coating made up of Fe₃O₄ nanoparticles along with carboxylate carbon nanotubes (CNTs) and put it on a commercial separator so as to form a functional Li-S separator. The introduction of CNTs-COOH is to fabricate a physical barrier and network structure, effectively catching polysulfides. At the same time, ultrasmall Fe₃O₄ nanoparticles help trap the polysulfides. Furthermore, the coating with CNTs-COOH and Fe₃O₄ possesses a superior conductivity so that it can be considered as the second current collector to promote the conversion and reutilization of insulated polysulfides. As a result, the Li-S battery equipped with CNTs-COOH@Fe₃O₄ achieved a prominent cycle stability of 0.023% capacity decay per cycle at 1 C over 2000 cycles. Wu et al. [39] developed a carbon black/LiMn2O4-modified separator in which the LiMn₂O₄ (LMO) nanoparticles could chemically adsorb LPS and serve as a fast channel for Li⁺ transportation. Wang et al. [40] firstly used binary transition metal oxide manganese ferrite MnFe₂O₄ prepared by coprecipitation as well as conductive acetylene black to modify the separator. Zhu et al. [41] used magnetic mesoporous Fe₃O₄ mixed with Ketjen Black to modify the glass fiber separator. Angulakshmi et al. [42] reported a bi-functional permselective separator prepared by using Li₇La₃Zr₂O₁₂ as well as activated carbon. They proved that Li₇La₃Zr₂O₁₂ is helpful for Li⁺ transport and hindering LPS shuttling. Cho et al. [43] developed a composite separator which consisted of the poly(vinylidene fluoride-co-hexafluoropropylene) and anodic aluminum oxide. The above works prove that metal oxides have great application potential as the separator modification materials. *3.2. Metal Sulfides* Metal sulfides and metal oxides have similar properties in many respects, but metal sulfides have better chemical adsorption of LPS. For example, Yan et al. [44] used TiS₂ sheets prepared by fast solid-phase synthesis method to modify the separator. TiS₂ has strong chemisorption and electrocatalysis towards LPS, which can capture LPS and promote its catalytic conversion and recycling. The authors also proved that vacuum filtration for separator modification is a more advanced process than slurry coating due to the formation of the more densely packed film in a fast solvent removal process. Mao et al. [45] developed a ZENs/G/Celgard separator in which the ZENs refers to ZnS(en)_0.5 prepared by microwave-assist exfoliation method and G refers to graphene. The ZENs mixed with graphene can facilitate the Li⁺ diffusion as well as hindering the polysulfides shuttling so that the battery induced with such separator displayed as low as 0.036% capacity decay per cycle during 1000 cycles at 0.5 C. Li et al. [46] proposed to embed ZnS into a nitrogen-doped carbon nanosheet skeleton and then use it to modify the separator, thereby fabricating a dual-function ZnS-modified separator. SnS was researched to modify the Li-S separator by Li et al. for the first time [47]. The specific preparation process is shown in the Figure 7. They used NaCl and Na₂SiO₃ as double templates in order to prepare extremely thin porous carbon nanosheet skeletons (PCNS). The hexadecyl trimethyl ammonium bromide was employed as the surfactant and then the SnS₂ was loaded onto PCNS by the hydrothermal method. After heated to 600 °C, the SnS was synthesized from SnS₂ decomposement and therefore the as-prepared material was named as SnS/PCNS. The function of SnS/PCNS is to facilitate the adsorption and reduction of polysulfides as well as enhancing the rapid nucleation and uniform deposition of Li₂S, effectively inhibiting the shuttle effect. The electrochemical performance was tested to be significant with an initial capacity of 1270 mAh g⁻¹ at 0.5 C and a low capacity decay of 0.039% per cycle at 1 C. NiCo₂S₄@C composites was used to fabricate interlayer by Li et al. [48] to improve the performance of Li-S battery. The NiCo₂S₄ was derived from MOF, so it has significant three-dimensional structure with nanopores. The utilization of carbon materials is to offer electron transport pathway. NiCo₂S₄ is helpful for Li⁺ transport and LPS intercepting.*3.3. Metal Carbides/Nitrides* In addition to metal oxides and metal sulfides, some metal carbon/nitrides with good catalytic effects are also used to modify Li-S separators. Zhang et al. [49] modified the separator with bimetallic carbide electrocatalyst Co₃Mo₃C in 2020. The test results showed that the electrochemical performance was significantly improved due to the addition of Co₃Mo₃C. In their report, effects of other bimetallic carbides with similar catalytic effects on Li-S batteries were also demonstrated, which reflected the potential of this material family for further application. Shen et al. [50] reported a TiN/Nitrogen-doped Graphene (TiN/NG) nanocomposite synthesized by one-step carbonization method using g-C₃N₄ generated from the decomposition of dicyandiamide as a self-sacrificed template and nitrogen source. The mixture of dicyandiamide, glucose and titanium potassium oxalate were heated to 900 °C under nitrogen (N₂) atmosphere and then the TiN/NG was made after cooling. The obtained slurry was coated on a commercial glass microfiber separator. Graphene nanosheets can help TiN Nanoparticles avoid aggregating and tiny TiN grain with crystal defects is beneficial to the adsorption of polysulfides. The TiN/NG has total pore volume of 2.7 cm³ g⁻¹ and BET surface area of 328.1 m² g⁻¹ which can effectively enhance the adsorption of polysulfides. Besides, the high electronic conductivity of TiN/NG can also induce the polysulfides to be transformed. Such separator modified with TiN/NG applied in Li-S battery shows an initial discharge capacity of 1005 mAh g⁻¹ at 1C and retains 718 mAh g⁻¹ after 300 cycles. *3.4. Framework Materials* A large specific surface area is a key factor in achieving LPS blocking or capture. Various kinds of frameworks materials like metal organic framworks (MOFs) with abundant polar active sites become ideal materials for separator functional modification. The adjustable pore size of MOF is beneficial for the suppression of LPS shuttle effect. Qi et al. [51] developed a multi-functional separator by coating homogeneous MIL-125(Ti) layer on a conventional PP/PE membrane. Because of the pore size of ~1.5 nm, the separator can not only regulate Li⁺ transportation but also hinder the shuttle effect by Lewis base–acid interaction between MIL-125(Ti) and Li₂Sn. Wang et al. [52] proposed an ion-permselective LiX@Celgard separator composed of LiX zeolite membrane and commercial Celgard separator. The NaX zeolite was firstly synthesized over commercial Celgard separator and then it was immersed into a 0.3 M LiCl aqueous solution and heated to reflux at 95 °C for 2 h to form LiX zeolite membrane. The thickness of LiX zeolite layer is found to be about 6 μm and the morphology of LiX zeolite layer is pretty continuous and compact through SEM images. As shown in Figure 8, the porous structure of LiX zeolite can physically obstruct the shuttle effect of polysulfides and effectively improve the electrolyte wettability of Celgard separator. Benefited from above excellent properties of LiX@Celgard separator, the electrochemical test showed a specific capacity of 1083 mAh g⁻¹ and a capacity retainment of 70% after 400 cycles at 0.2 C.As reported in Wang’s works [53], cobalt-based zeolitic imidazolate framework-67 (ZIF-67), a crucial member of MOFs, has also been applied in modification of Li-S battery separator. The ZIF-67 was synthesized by mixing 2-methylimidazole and CoCl₂·6H₂O in deionized water and then the prepared ZIF-67 was mixed with polyvinylidene difluoride (PVDF) and NMP to form slurry, which needed to be uniformly coated on the surface of the pristine polypropylene film. Figure 9a showed that the ZIF-67 which was synthesized by such route was relatively pure through x-rays diffraction (XRD) and Figure 9b nicely showed the cubic structure morphology with a dimeter of 80–100 nm of ZIF-67 through SEM. Benefited from strong chemical adsorption for polysulfides, the ZIF-67 could effectively inhibit the shuttle effect. Furthermore, electrochemical tests exhibited that the Li-S battery equipped with ZIF-67/PP separator showed a high specific discharging capacity of 1365 mAh g⁻¹ at 0.1 C and 816 mAh g⁻¹ after 300 cycles at 2 C.*3.5. Other Materials* In addition to the above chemicals, a few natural clay minerals were also researched, such as illite and smectite [32]. Wang and co-workers proposed an ISC/C@Celgard separator which was fabricated by coating natural clay mineral (illite/smectite, ISC) and carbon black (C) onto a commercial Celgard separator. The ISC/C@Celgard separator can nicely infiltrate electrolyte and keep its structure stable during the cycling process. It is worth mentioning that the polysulfides anchoring function of such separator is realized by formation of Li-O and Li-S bonds between LPS and OH groups as well as S element which are uniformly distributed on the separator surface. The ISC/C layer also has nanopores which are beneficial for physically inhibiting the shuttle effect. The Li-S battery equipped with ISC/C@Celgard separator displayed an initial capacity of 1322 mAh g⁻¹ at 0.1 C and wonderful cycle stability with low-capacity decay of 0.054% per cycle during 500 cycles at 1.0 C. In recent years, transition metal compounds have caught researchers’ attention. As one of the representative transition metal selenides, MoSe₂ has been widely used in many fields such as catalysis, energy storage and optoelectronics due to the crystal structure and thermodynamic properties. It has been found that MoSe₂ has a great improvement in inhibiting the shuttling effect of lithium polysulfide [54]. You et al. developed a MoSe₂@rGO coating which is constructed by two-dimensional MoSe₂ and reduced graphene oxide (rGO), and placed it on the cathode side. The coating is made up of MoSe₂ and rGO in a ratio of nearly 1 to 1. The polar MoSe₂ provides the capacity for chemically absorbing polysulfides and catalyzing the reduction reaction and the rGO serves as a conductive network for its wonderful electric conductivity, capable flexibility, and enormous specific surface area. Therefore, the above-mentioned features of MoSe₂@rGO contribute to as low as 0.03% capacity decay per cycle at 1 C after 1000 cycles. Ferrocene (Fc), an organic transition metal compound with aromatic properties, was also studied by Zeng et al. [55]. In their works, the Fc was mixed with graphene oxide (GO) under the operation of ultrasonication. And then the GO/Fc-modified separator (PP–GO/Fc) was constructed through vacuum filtration. The Fc and GO do have a synergistic effect to suppress the polysulfides shuttling and help them to deposit and carry out catalytic conversion so that the battery exhibit a high reversible capacity of 409 mAh g⁻¹ after 500 cycles at 0.2 C. Additionally, cobalt is also widely developed due to its unique physical and chemical properties, which can effectively block polysulfides shuttling when applied in Li-S batteries. Lu et al. [56] developed a functional separator prepared via in-situ growth of hollow CoSO₄ hydrate arrays on pristine polypropylene membrane. Liu et al. [57] utilized waste LiCoO₂ to modify the separator of Li-S batteries so as to effectively inhibit the shuttle effect and facilitate the conversion reactions of LPS and environmentally friendly recycle of LiCoO₂. Besides, intermetallic nickel–iron with prospective electrocatalysis of effective polysulfide conversion was also utilized in separator modification in Li-S batteries. Zhang et al. [58] synthesized the above-mentioned nickel–iron intermetallics by urea-assisted precipitation method and the combustion in nitrogen atmosphere. Figure 10 shows the illustration of operation principle of batteries assembled with different separator. Because of the insert of extraneous iron atoms into the nickel phase, the resulting distorted lattice of Ni₃Fe intermetallic became prominently catalytically active for accelerating the redox kinetics of LPS. Therefore, outstanding rate capability with a capacity of 598 mAh g⁻¹ at 4 C and long cycle life of 1000 cycles at 1 C with a capacity decay rate of down to 0.034% per cycle were achieved.Studies have shown that polymers can also play a role in inhibiting the shuttle effect. Recently, Ma et al. [59] proposed a dual-engineered separator by coating ammonium alcohol polyvinyl phosphate/carbon black (AAPP/CB) and Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) on two different surfaces of a commercial PP using blade coating method, which is denoted as AAPP/CB@PP@LAGP. Because of conductive CB frameworks and plentiful hydroxyl groups/phosphate radical in AAPP, the AAPP/CB layer performs as a “polysulfides stockroom” as well as a physical barrier to guarantee the instantaneous polysulfides absorption and conversion. At the same time, the LAGP layer filled with three-dimensional Li⁺ migration pathways exhibits great Li⁺ transmission ability. Thus, the Li-S batteries equipped with AAPP/CB@PP@LAGP separator achieves superior rate performance of 758 mAh g⁻¹ at 4 C, excellent cycle stability of capacity fade rate of 0.065% per cycle at 1 C over 500 cycles and high reversible capacities of 976.5 and 611.5 mAh g⁻¹ after 150 cycles. It is because of good inhibition for the shuttle effect of polysulfides and the growth of lithium dendrites. In addition, Polyetherimide was used to modify the PE separator by Liu et al. [60] and the battery with such separator showed an initial specific capacity of 1265 mAh g⁻¹ at 0.2 C. Mathew et al. [61] developed a permselective separator by coating triazine-phenylenediamine-porous organic polymer (TP-POP) onto a commercial Celgard (2320) membrane. The secondary amine groups of POP are beneficial for the separator to facilitate Li⁺ diffusion as well as confining LPS shuttling both chemically and physically. Specially, Zhou et al. [62] proposed a functionalized separator prepared by single-sided chemical tailoring in the oxidative solution and grafting in the organosiloxane vapor. The functional groups on separator formed by organosiloxane fumigation grafting can well suppress LPS diffusion and the channel structure formed by chemical tailoring can well retain the dissolved LPS. Attapulgite, a kind of natural multi-metal oxide nanorods consisting of Al, Mg, Fe, Si and O ions, was applied as multifunctional ionic sieve to capture LPS between separator and cathode by Sun et al. [63] Furthermore, they find that Fe ions in attapulgite can catalyze LPS converting, faster and relative amount of polysulfide adsorbent and the conductive agent plays an important role in the performance of Li–S batteries. The key parameters of more relevant studies are summarized in Table 2. As remarked above, because of the simple preparation process and abundant alternative modification materials, method of Li-S separator modification has shown great potential for inhibiting the shuttle effect of LPS, reducing capacity attenuation of both cathode and anode and improving stability of lithium metal anode.