Effects of Catalysis and Separator Functionalization on High-Energy Lithium–Sulfur Batteries: A Complete Review
催化和隔膜功能化对高能锂硫电池的影响：完整综述

4.1 Inhibition of Polysulfide Diffusion by Physical Barrier

It has been proved that physical confinement is an effective measure to limit the lithium polysulfide shuttle, including electrostatic shielding, electrostatic adsorption, and ion screening.^{[63, 64]} On the one hand, the long-chain polysulfide (, 2 < n ≤ 8) dissolved in electrolyte is a kind of negatively charged ion, and the most direct way is to adopt the principle of “same-charges repulsion, opposite-charges attraction.” In addition, because the high-order polysulfide ion is a long-chain form, and its size is much larger than that of lithium ion, a specific size channel can be constructed, so that small-size lithium ion can pass through, while large-size polysulfide ion cannot pass through, to achieve the role of “ion sieve.”

Yim et al.^[65] studied the material BaTiO₃ (BTO) polarizes by applying an electric field to produce a permanent dipole. The negative electric field formed on the positive side of the separator can prevent the polysulfide ions from penetrating the separator. In addition, PE separator will contract irreversibly at 120 °C, which will lead to short circuit inside the battery and eventually cause fire. As a ceramic material, BTO has the characteristics of high temperature resistance, so it can improve the electrochemical performance and safety performance of the battery at the same time. The separator modified with polarized BTO nanoparticles retained 82.8% of capacity after 50 cycles with the high sulfur loading of 3 mg cm⁻² and 100 μL electrolyte per coin cell. Moreover, compared with the ordinary PE separator and the separator modified by unpolarized BTO nanoparticles, the separator also showed excellent migration retardation of lithium polysulfide (Figure 3a,b). On the contrary, Raja et al.^[66] reported that polysulfide ions can also be well bound by electrostatic adsorption. MgAl₂O₄ coated on the negative side of the Celgard 2320 separator inhibits the shuttle of polysulfides by electrostatic attraction, while MWCNTs on the positive side provides electronic conductivity. This synergistic effect improves the ionic conductivity and thermal stability of the separator. Multilayer modified layer was prepared on one side of PP separator. The results show that the modified separator has excellent rate performance and long cycle performance. After 120 cycles, the initial specific capacity of 84.5% can still be maintained at the current density of 9 A g⁻¹ under the high sulfur loading of 2.6 mg cm⁻² and 250 μL of electrolyte. Another new physical confinement method is in the positive side of separator. The ion screening layer was constructed on the side of the cathode, so that the soluble long-chain lithium polysulfide could not pass through the separator. Metal organic frameworks (MOFs) are a kind of materials with small pore size. Zhou et al.^[40] creatively grown Cu₃(BTC)₂ with a pore size of only 0.9 nm on graphene oxide (GO) sheets and prepared MOF@GO separator (Figure 3c). The results show that the lithium ion penetrate through the modified separator smoothly, but the large size polysulfides cannot. The diffusion experiment of lithium polysulfide showed that there is no brown lithium polysulfide in the electrolyte on the other side of the separator (MOF@GO) after 48 h (Figure 3d). The separator can well block the shuttle of polysulfide ions. Electrochemical tests show that the cell is still stable at 800 mAh g⁻¹ after 500 cycles at 0.2C rate with the sulfur contents of 70 wt%. After 1600 cycles at high rate 1C, it still keeps at an ultra-high capacity of 900 mAh g⁻¹.

The study of modification has great inspiration. Chen et al.^[67] used conductive MOF Ni₃(HITP)₂ was used to modify the separator, and the synergistic effect of micropores and abundant active sites of MOF was used to improve the rate performance and cycle stability at 0.1C. Tian et al.^[68] synthesized Cu₂(CuTCPP) crystal MOF with only a few molecular layers by one-step solvent method. Ultra-thin nanosheets were prepared by vacuum filtration method, and the highly oriented and flexible ultra-light films were used as the modify separator, which effectively inhibited the shuttle of polysulfides in the lithium–sulfur battery and improved the cycle stability still at high temperature. The uniform pore size of MOF particles promotes the homogenization of Li flux, which can fundamentally limit the growth of Li dendrites. MOF-based modified separator also has obvious effect on lithium anode protection.^[69]

Huang et al.^[70] first reported the research work of Nafion ultrathin functional layer loaded on commercial separator in 2014 and realized the design of ion selective multifunctional lithium–sulfur battery separator. In this ion selective separator, the sulfonate ions modified in the pore channel repel polysulfide ions through coulomb interaction and provide lithium-ion transition sites to allow lithium-ion free transport. By introducing the ion selective separator, the capacity of the assembled Li-S battery only decays by 0.08% in the initial 500 cycles, which shows excellent cycle performance. Xu et al.^[71] modified the separator with exfoliated two-dimensional flakes. This kind of inorganic nanoflake cannot only inhibit the diffusion of polysulfide through space physical barrier and electrostatic effect, but also effectively prevent the penetration of lithium dendrite by its high mechanical strength and Young's modulus.

Kong et al.^[72] studied the high sulfur loading at 4.5 mg cm⁻². The effect of different area of positive electrode combined with carbon nanotube modified separator lithium metal negative electrode on the performance of Li-S battery was investigated. The results show that the smaller area (= 5 mm) of the positive plate can reduce the concentration of LiPSs and inhibit the damage of lithium metal negative electrode, so as to obtain larger positive plate (Φ = 13 mm), higher reaction kinetics, and better cycle rate performance.

Zhou et al.^[73] proposed the flexible integrated structure design of sulfur and graphene modified PP separator. Different from the previous self-supporting metal and graphene collectors, this sulfur/graphene modified separator integrated structure gives the electrode excellent flexibility and has been successfully assembled into a flexible soft pack battery. When the current density is 0.75 A g⁻¹, the initial specific capacity of the flexible cell is 985 mAh g⁻¹. After 30 cycles, it can maintain 722 mAh g⁻¹ specific capacity and 98% Coulombic efficiency. In this study, PP separator is used as the carrier of sulfur and graphene, which provides a new design idea for flexible energy storage devices.

Rana et al.^[74] modified commercial celgard PP separator with Ketjen black/Nafion. Ketjen black has enough specific surface area and pore structure to adsorb polysulfides, and its high conductivity is helpful to improve the initial capacity of Li-S battery. The sulfonate ion group of Nafion can limit the migration of polysulfides through electrostatic interaction, which effectively promotes the stability of the battery. The capacity degradation of Nafion is only 0.06% after 150 cycles. When the sulfur loading is 7.8 mg cm⁻², the initial area capacity is 6.70 mAh cm⁻², which is higher than that of the commercial lithium-ion battery (4.0 mAh cm⁻²).

Wen et al.^[75] prepared a sulfonated reduced graphene oxide (SRGO) is used for separator functionalization: On the one hand, graphene sheet physically limits polysulfide migration, which is conducive to electron transfer; on the other hand, the presence of sulfonic acid group can limit the migration of polysulfide ions and promote the transmission of lithium ions. It can double inhibit the shuttle of polysulfide ions and promote the conduction of electrons and lithium ions, which is helpful to improve the electrochemical activity of Li-S battery. Using 56% sulfur cathode, the battery showed an initial discharge specific capacity of more than 1300 mAh g⁻¹ at 0.5C under the mass load of 1.2–1.5 mg cm⁻². After 250 cycles, it still maintains the reversible capacity of 802 mAh g⁻¹ and has excellent rate performance (Figure 4a). Through the cross-sectional SEM and EDS tests of lithium negative electrodes after cycles, it is found that the SRGO functionalized separator shows the lowest sulfur signal, further proving that the shuttle effect is effectively suppressed. Furthermore, the black phosphorus modified separator was introduced by Sun et al.^[76] (Figure 4b), and it not damaged even after folding and rubbing (Figure 4c,d). At the same time, the strong interaction between black phosphorus and S atoms can well bind polysulfide ions (Figure 4e).

4.2 Inhibition of Polysulfide Diffusion by Chemical Adsorption

Polar inorganic compounds have excellent absorption of polysulfide. Therefore, introducing polar materials into the conductive modified layer can more effectively adsorb dissolved polysulfide and achieve high-efficiency sulfur fixation.^[95] In the literature, SiO₂,^[96] Al₂O₃,^{[97, 98]} MnO₂,^[99] SnO₂,^[100] V₂O₅,^[101] Mg_0.6Ni_0.4O^[102] and hydroxyapatite,^[103] are used to enhance the sulfur adsorption and fixation performance of the conductive layer. The sulfur fixation effect of the modified layer can be greatly improved by adding polar inorganic compounds.

However, the conductivity of most inorganic compounds is very poor. Increasing the doping amount of polar materials is bound to reduce the conductivity of the modified layer and sacrifice the electrochemical activity of the modified layer. Therefore, researchers try to functionalize polar materials or modify the separator with polar adsorption materials with excellent conductivity to improve the comprehensive performance of Li-S battery. For example, Huang et al.^[104] used crab shell as the raw material of polar adsorption material, and used the tubular calcium carbonate with large specific surface area in crab shell and a small amount of organic matter wrapped on the surface to pyrolyze into the structure of carbon coated polar inorganic compound nanotube to prepare conductive tubular CaO, which was hydrolyzed into conductive Ca(OH)₂ modified membrane in aqueous binder gelatin (Figure 7a–c). A conductive modification layer with excellent sulfur fixation performance is obtained. The electrochemical performance test of the sulfur positive electrode with content of 63% and load of 1.2–1.5 mg cm⁻² shows that the initial discharge specific capacity of 1215 mAh g⁻¹ is displayed at the rate of 0.5C, and the reversible specific capacity of 873.5 mAh g⁻¹ is still maintained after 250 cycles (Figure 7e). By comparing and analyzing the surface morphology of the lithium negative electrode before and after the cycle, the electrode with conductive Ca(OH)₂ modified separator has a smooth surface structure. It fully shows that the functional separator can effectively inhibit the shuttle effect and protect the structure of the negative electrode (Figure 7d).

In addition to polar inorganic materials, functional carbon materials, such as -OH functionalized carbon nanotubes, were used to modify the separator. Kim et al.^[105] designed a functional modified separator using hydroxyl functionalized carbon nanotubes (CNT-OH) (Figure 8a). The hydroxyl groups of CNT-OH capture lithium polysulfide on the cathode side through strong polar interaction. Therefore, its initial discharge capacity of the battery is up to 1056 mAh g⁻¹ at 0.5C and the capacity attenuation rate is 0.11%. Similarly, it also shows good rate capabilities (Figure 8b,c). Titanium oxides are attractive materials for the modification of separators in Li-S battery. TiO₂ has become a popular choice because of its low cost, non-toxic and easy to manufacture. Huang et al.^[106] applied nano-TiO₂ modified carbon layer to modify the separator of Li-S battery. The carbon layer physically captures polysulfides, while nano-TiO₂ chemically captures polysulfides. Using this modified separator, the battery performance is significantly improved. The high specific capacity of 883 mAh g⁻¹ at 0.1C after 180 cycles can be obtained. In addition, the excellent reversibility at different rates shows that the modified separator can effectively immobilize polysulfide even at higher rates. However, the poor conductivity of TiO₂ has an adverse effect on the utilization of active materials, which is not suitable for LSB, especially in the case of high sulfur loading. Therefore, Wang et al.^[107] further designed a TiO/MWCNT coated separator with high conductivity (Figure 8d). After replacing TiO₂ with TiO, the utilization rate of sulfur increased from 78% to 91%. Density functional theory (DFT) calculation shows that the adsorption effect of TiO on polysulfide is better than that of TiO₂. The battery with TiO/MWCNT coated separator showed a relatively high initial discharge capacity of 1527.2 mAh g⁻¹ and a high reversible capacity of 1033.8 mAh g⁻¹ after 200 cycles at 0.5C. In the same case, these values are higher than those of batteries with TiO₂/MWCNT coated separator. When the sulfur utilization rate reaches 61.72%, the discharge capacity remains at 620.7 mAh g⁻¹ at 0.5C after 1000 cycles, and the attenuation rate of each cycle is only 0.057%, showing good cycle performance. Also, the average self-discharge value of the battery is low, which is 8.7%. After standing for 48 hours, the open circuit voltage (OCV) decreases by only 0.09 V. This excellent self-discharge resistance shows that there is less polysulfide shuttle and reaction with lithium anode.

ZnO is a polar metal oxide, which has also attracted extensive attention. Xiong et al.^[108] successfully prepared a functional separator using a mixture of two-dimensional graphene and ZnO of different sizes (Figure 8e,f). Polysulfides can be effectively binds by polar bonds through the pores of one-dimensional ZnO and the cavities in the hollow structure. Therefore, the battery with one-dimensional ZnO/two-dimensional graphene functional separator has excellent cycle performance of 927 mAh g⁻¹ at 1C after 200 cycles and high rate capacity of 754 mAh g⁻¹ at 6C. The MoS₂^[109] and WS₂^[110] material with two-dimensional structure can selectively adsorb long-chain polysulfides at the boundary of the lamella and convert them into short-chain Li₂S (Figure 9). These properties are used to accelerate the dynamic process of polysulfide conversion in the Li-S battery and improve the energy density, magnification performance and cycle performance.^[111] Tang et al.^[109] modified the separator with MoS₂ (Figure 9a–d). The functionalized separator showed high lithium-ion conductivity and moderate lithium-ion transference number. When used in Li-S battery, this kind of separator can well inhibit the shuttle of polysulfides and improve its cycle performance. After 600 cycles, it still has a Coulombic efficiency of higher than 99.5%. Researchers also tried to use two-dimensional black phosphorus,^[76] C₃N₄,^[112] and other materials to prepare functional membranes, which also made the battery show excellent sulfur fixation effect. A large number of reports fully show that two-dimensional material modified separator are very effective strategy to improve the electrochemical performance of Li-S battery. Recently, Xiao et al.^[113] introduced a new strategy, they constructed heterostructured ZnS-SnS@NC Cubic nano material on commercial polypropylene (PP) membrane, without affecting its original pore structure and lithium-ion conduction, prepared a modified functional membrane (ZnS-SnS@NC separator) with the dual characteristics of “sulfur affinity” and “lithium affinity.” The membrane has high interfacial conductivity and unique cubic nanostructure. It can not only effectively inhibit the shuttle effect of polysulfide and the rapid migration of ions/electrons but also help to realize the uniform nucleation of lithium ions on the surface of the negative electrode in the process of intercalation/de-lithium (Figure 10a). The heterostructure skillfully combines SnS with strong chemical adsorption and high conductivity with ZnS with strong catalytic activity and realizes a rapid fixation diffusion conversion reaction for polysulfides. Through DFT simulation, ex situ XANES and in situ Raman spectroscopy, it is confirmed that the membrane modified layer can effectively inhibit the shuttle of polysulfides and improve the reaction kinetics of sulfur. Therefore, when ZnS-SnS@NC modified membrane layer is applied to Li-S battery, it shows good electrochemical performance. After 300 cycles at 0.2 C, it has a high reversible capacity of 1149, 661 mAh g⁻¹ at high rate of 10 C, and 2000 cycles at 4 C current density, and the average capacity attenuation rate per cycle is only 0.0126%. Under the conditions of high sulfur load (10.3 mg cm⁻²) and low electrolyte/sulfur ratio (4 mL mg⁻¹), it has high area specific capacity (9.46 mAh cm⁻²) and good cycle stability.

In addition to all the above strategies, Li et al.^[114] introduced the in situ curing strategy into lithium–sulfur batteries, adding 2,5-dichloro-1,4-benzoquinone (DCBQ) to the electrolyte, so that polysulfide ions generated in the electrochemical reaction of lithium–sulfur batteries can react with DCBQ in nucleophilic substitution, and in situ generate solid organic sulfur polymers that are not easily soluble in ether electrolyte and catalytically converted to small chain polysulfides (Figure 10b), so as to achieve the purpose of inhibiting the shuttle effect. Through the combination of experimental characterization and theoretical calculation, it is found that the polysulfide in the organic sulfur polymer can be limited by covalent bond cooperation. The solid organic sulfur polymer can prevent the subsequent migration of polysulfide, keep the active material in the positive electrode, and increase the cycle stability and utilization rate of active material.

4.3 Inhibition of Polysulfide Diffusion by Catalysis

In order to solve the bottleneck problems of LSBs, researchers from the perspective of electrode structure design, electrolyte modification, and the addition of polar polysulfide adsorbent to inhibit polysulfide shuttle and improve the conductivity of sulfur cathode.^[115] The catalysis of LiPSs electrochemical reaction is also a research hotspot in recent years. Zhang's research group has carried out a series of studies around polysulfide adsorption and catalyst design. By developing polar Co₃S₄ nanotubes with network structure^[116] and “bionic” Biomimetic bipolar microcapsules derived from staphylococcus aureus,^[117] it can effectively inhibit polysulfide shuttle and improve the performance of Li-S battery. On the basis of previous work and theoretical calculation, researchers designed transition metal doped catalysts, regulated the interaction between metal d orbital and sulfur 3p orbital, studied the mechanism of catalytic process, and prepared highly active polysulfide conversion catalysts.^[118] LSBs system is a multi-electron redox reaction process: An S molecule needs 16 electrons to be completely reduced to Li₂S. A variety of polysulfides and free radicals are involved in the redox process. Moreover, the very complex conversion reaction relationship between polysulfide ions and free radicals makes the internal electrochemical reaction of lithium–sulfur battery system more complex. Previous studies have shown that the addition of transition metal sulfide additives can balance the adsorption of additives on LiPSs and reduce the diffusion of LiPSs to electrolyte, so as to catalyze the internal electrochemical redox reaction of Li-S battery, effectively improve the reaction kinetics and improve the battery performance, including higher initial discharge specific capacity, lower polarization and higher storage capacity, Coulombic efficiency, excellent cycle stability, and rate performance.^[119]

The kinetics of electrochemical reaction is mainly determined by two factors: First, sufficient binding affinity allows adsorption with sufficient surface coverage of active substances; Second, the effective charge transfer on the liquid–solid boundary promotes the electron transport of the adsorbate in the redox process (Figure 11a,b)^{[120, 121]} in the electrochemical reaction process, the surface reaction and subsequent deposition of solid products can be carried out on relatively abundant surface sites, and speeds up the electron and ion transport in the solid–liquid conversion process. This is the fundamental reason why polar transition metal sulfides have electrochemical kinetic catalysis as LSBs cathode.

The static adsorption of LiPSs also includes the electrochemical reaction of dynamically accelerating the conversion of LiPSs. Comparing Figure 11c,d, the addition of CoS₂ accelerates the reversible conversion of LiPSs on its surface from two aspects.^[122] The catalytic effect on electrochemical reaction is mainly reflected in the binding affinity of polar CoS₂ to LiPSs, which greatly coordinates the adsorption and diffusion of LiPSs, and provides more active sites to ensure that enough active substances participate in electrochemical chemical reaction. In addition, the introduction of Co²⁺ further enhances the effective charge transfer and electron transport. The interaction promoted the transformation from high-order polysulfide to low-order polysulfide (Li₂S₈ → Li₂S₆ → Li₂S₄). Therefore, the energy efficiency of graphene sulfur cathode added with CoS₂ is increased by 10% compared with that without CoS₂. At 2C, the capacity attenuation rate is only 0.034%/cycle, and at 0.5C, the initial discharge specific capacity is up to 1368 mAh g⁻¹ with 0.4 mg cm⁻² sulfur loading and 20 μL mg⁻¹ electrolyte/sulfur ratio.

High adsorption strength is the basis for realizing the adsorption conversion process, Cui et al.^[123] calculated the adsorption energy of metal oxides, sulfides, chlorides, and graphene for different LiPSs, as well as the charge transfer and bond length changes before and after adsorption by density functional theory (DFT). When the adsorption energy is too low, LiPSs cannot be well fixed on the material surface; when the adsorption energy is too high, the Li-S bond will break, and the polysulfides will be separated from the material surface and cannot be used effectively. Therefore, the adsorption strength needs to be controlled within an appropriate range (0.8–2.0 eV). The conductivity directly affects the conversion reaction rate of LiPSs and lithiation/delithiation. For some catalytic materials with low intrinsic conductivity, LiPSs needs to diffuse to the conductive surface to realize conversion after being adsorbed. Based on polar interaction and Lewis acid–base interaction, researchers have done a lot of research on heteroatom doped carbon materials, metal oxides, metal sulfides and organic metal frameworks (MOFs). The results show that polysulfide bonding and confinement are very important for the design of electrode materials, which also provides important theoretical guidance for further improving the performance of Li-S battery. Zhou et al.^[124] published a research work for the catalytic oxidation of Li₂S on the surface of metal sulfides for Li-S batteries (Figure 12a). The catalytic oxidation mechanism of discharge product Li₂S on the surface of metal sulfides in lithium–sulfur (Li-S) battery was revealed, and a series of metal sulfides were deeply studied. By combining density functional theory (DFT) simulation and experimental test, they found that when metal sulfide is used as the main material, its catalytic oxidation/reduction ability is very important for the transportation of lithium ion and the adsorption of polysulfides (LiPSs). The inherent metal conductivity of metal sulfide and the strong interaction between Li₂S/Li₂S_x can reduce the energy barrier, promote the transportation of lithium ions, control the surface precipitation of Li₂S, and accelerate the surface mediated redox process, so as to improve the overall performance of Li-S battery. Through the systematic research and analysis of a series of metal sulfides, it is found that when they are polar main materials, due to the inherent metal properties of these materials and the strong interaction between polysulfides, they promote the transmission of lithium ions and accelerate the surface mediated redox reaction. It is very important to reduce the energy barrier of lithium-ion transport and improve the performance of Li-S battery. Therefore, compared with pure carbon materials, Ni₃S₂-, SnS₂-, and FeS-based composite electrodes, and VS₂-, TiS₂-, and CoS₂-based materials have higher specific capacity, lower overpotential, and good cycle stability. By combining DFT simulation and experimental test, they described the catalytic oxidation mechanism of metal sulfide on Li₂S, which provided a practical and feasible guiding scheme for designing new electrode materials and improving the performance of Li-S battery. As same as this work, Pan et al. use Co₃S₄ nanotubes as polar and catalytic sulfur host for Li-S batteries (Figure 12b). This host material also exhibits good electrochemical performance due to high electron conductivity, polarity, and catalytic activity. It displays 1267 mAh g⁻¹ specific capacity at 0.5C, and more interestingly, it displays 305 mAh g⁻¹ after 1000 cycles with 0.041% decay rate even at 5C with the sulfur loading of 2–4 mg cm⁻² and 40 μL electrolyte per coin cell. Beside polar materials, metal materials are also commonly used industrial catalysts. Because of the coordination between its d orbital and reactants and its high conductivity, it has catalytic activity. Arava et al.^[58] loaded the traditional electrocatalyst Pt nanoparticles on graphene (Figure 13a) as the cathode. It was found that its specific capacity is about 40% higher than that of graphene/sulfur composite cathode (Figure 13b). Zhang et al.^[125] the controllable deposition of LiPSs on the cathode is induced by the chemical interaction between Au nanoparticles and LiPSs. The cycle capacity and stability of the battery can be effectively improved at a very small addition of 3 wt%.

Chen et al.^[126] designed and prepared a perovskite type fast ion conductor La_0.56Li_0.33TiO₃ (LLTO), and used it as an adsorbent for polysulfide and a catalyst for polysulfide conversion for the first time (Figure 13c). It has been confirmed that LLTO has high ionic conductivity (2.94 × 10⁻⁴ S cm⁻¹) and can be used as an efficient electrocatalyst for polysulfide conversion under the condition of poor electrolyte. The coexistence of sulfur (O) and sulfur (Ti) sites in LLTO can form a good interfacial affinity with polysulfides. Therefore, the lithium–sulfur battery containing LLTO can effectively reduce the proportion of electrolyte/active material, accelerate the reaction kinetics, inhibit the shuttle of polysulfides, significantly reduce the overpotential in the charge and discharge process, and improve the rate performance and cycle performance of lithium–sulfur battery. In addition to coordinating the relationship between adsorption strength and conductivity^{[127, 128]} and enhancing ion diffusion,^[129] heterostructures can also make up for the shortcomings of specific surface area and conductivity of some materials. As a typical two-dimensional transition metal carbide, MXene has a two-dimensional lamellar structure. It has high conductivity and high catalytic activity, but its advantages are difficult to play due to the problems of lamellar re-stacking. Jiao et al.^[130] through controlled oxidation Mxene (Ti₃C₂Tx) was partially converted to TiO₂ (Figure 14a). TiO₂ can be used as the adsorption center of LiPSs and realize rapid electron/ion diffusion through the heterogeneous interface (Figure 14b). At the same time, the existence of TiO₂ nanoparticles inhibits the lamellar re-stacking of MXene and maintains the two-dimensional structure with high specific surface area.

Wang et al.^[131] developed a cathode material with fine ordered multistage structure and multiple reactive active sites. Based on the previous research on oxygen defect materials, sulfur atoms with highly polarized electron clouds are selected as the defect body in order to achieve stronger adsorption and catalysis on lithium polysulfide. ZnS_1-x nanotube arrays rich in sulfur defects were prepared on the surface of carbon cloth by sacrificial template method (Figure 14c,d). The accurately ordered ZnS_1-x nanotube structure provides rich active sites for the redox reaction of lithium–sulfur battery and can effectively alleviate the volume change of sulfur in the process of charge discharge reaction. In addition, the density functional theory calculation results show that the introduction of sulfur defects can optimize the electronic structure of Zn atom, enhance the bonding between Zn atom and lithium polysulfide (Figure 14e), accelerate the migration rate of lithium ion on its surface, reduce the decomposition energy barrier of Li₂S on its surface, and improve the utilization rate of sulfur. Based on these advantages, the assembled lithium–sulfur battery shows excellent rate performance (up to 5C) and excellent cycle stability (the capacity attenuation rate per cycle is only 0.04%) (Figure 14f), and still has high surface capacity after 100 cycles under high sulfur load (5 mg cm⁻²) and 15 mL g⁻¹ E/S ratio. In addition, in this work, the electronic structure regulation mechanism and lithium polysulfide adsorption catalytic reaction mechanism are fully explained, which provides a reliable experimental and theoretical reference for the subsequent research and structure design of lithium–sulfur battery. The combination of defect engineering and morphology design promotes the development of cathode electrocatalyst for lithium–sulfur battery and is expected to play an important role in multifunctional electrocatalyst. Recently, Yang et al.^[132] designed an In-based catalyst with selective catalysis. Experimental research and numerical simulation show that the catalyst reduces the conversion rate of sulfur to soluble Li₂S_n during the discharge of Li-S battery; more importantly, the LiInS₂ intermediate produced by the reaction can improve the conversion rate of soluble Li₂S_n to insoluble discharge product with 1.0 mg cm⁻² sulfur loading and 15 μL mg⁻¹ E/S ratio. This selective catalytic strategy effectively reduces the accumulation of Li₂S_n in the electrolyte, and the “quasi source” inhibits the shuttle effect (Figure 15a–c). Through the information of dynamic surface components of the catalyst, the mechanism of selective catalysis is revealed, which provides a new strategy for inhibiting the shuttle effect of Li-S battery and improving the cycle stability of the battery. Wang et al.^[133] reported a novel 3D hierarchical material NiMoO₄ nanosheets anchored on nitrogen sulfur Co doped carbon cloth (NiMoO₄@NSCC), as an independent host of lithium–sulfur battery. Theoretical calculations show that NiMoO₄ has strong metal properties due to the Mo embedded atoms, which can effectively adsorb polysulfides, catalyze conversion reaction, and slow down the shuttle effect (Figure 15d). NiMoO₄ significantly improves the critical step (Li₂S₂ to Li₂S) of Li-S battery and reduces the decomposition barrier of Li₂S. Further atomic-level simulation shows that NiMoO₄ prolongs the S-S bond length of Li₂S₄ and li-s bond length of Li₂S, and then promotes the oxidation and reduction of sulfur species in the process of charge and discharge. Open interconnected three-dimensional layered conductivity with mechanically flexible space and abundant active sites NiMoO₄@NSCC. It can not only accommodate sulfur species but also promote the rapid transfer of electrons/ions and accelerate the oxidation and reduction process. So, NiMoO₄@NSCC/S electrode has good long cycle stability with 15 μL mg⁻¹ E/S ratio. Transition metal sulfides are also good catalyst for the conversion of long chain polysulfides to small-chain polysulfides; recently, Zhang et al.^[134] identified the surface gelation of two sulfide electrocatalysts (MoS₂) in Li-S batteries. Specifically, the gel layer will simultaneously cover two sulfide electrocatalysts and change the surface structure during the cycle (Figure 15e). According to the experimental and theoretical results, the Lewis acid sites of disulfide trigger the ring opening cationic polymerization of DOL solvent to form the surface gel layer. The surface gel layer will cover the active sites of the disulfide electrocatalysts and further hamper their catalytic activity in promoting sulfur redox kinetics. In order to solve these problems, Lewis base three ethylamine (TEA) was used as a competitive inhibitor for Li-S battery to inhibit surface gelation. Specifically, TEA molecules compete with DOL on Lewis acid sites of disulfide to prevent subsequent gelation. Therefore, the Li-S battery using disulfide electrocatalyst and TEA inhibitor can prolong the cycle life (250 stable cycles at 3.0 C), improve the rate response of 4.0 C, and increase the discharge capacity of high sulfur loaded cathode by up to 4 times with 1.2 mg cm⁻² sulfur loading and 15.7 μL mg⁻¹ E/S ratio. The excellent electrochemical performance proved the effectiveness of TEA inhibitors in inhibiting surface gelation, maintaining the surface structure of disulfide electrocatalysts and restoring the electrocatalytic activity of Li-S batteries. In addition, Arumugam et al.^[135] loaded Li₂S nanoparticles onto a carbon host decorated by Co₉S₈ and Co (Li₂S-Co₉S₈/Co) through a simple carbothermal reaction. Co₉S₈/Co electrocatalyst is used as the nucleation site to ensure the uniform distribution of Li₂S in the composite (Figure 16a). The mixing of Li₂S and electrocatalyst at the molecular level enhances the redox activity of Li-S, improves the utilization rate of Li₂S, alleviates the shuttle effect, and prevents the massive accumulation of Li₂S on the lithium negative electrode, so as to ensure the effective utilization of lithium in the battery. The anode free battery based on Li₂S-Co₉S₈/Co cathode realizes a high capacity of 969 mAh g⁻¹. This work lays a foundation for the development of low-cost negative electrode free lithium batteries. Wang et al.^[136] designed and synthesized a heterostructure ZnSe-CoSe₂ embedded egg yolk shell nitrogen doped carbon skeleton material (ZnSe-CoSe₂@NC), as a “two in one” carrier for sulfur positive electrode and lithium negative electrode protection (Figure 16b). As a negative carrier, the Li₂Se phase formed in situ contributes to the transfer of Li. Co and Zn guide the uniform growth of Li in the 3D framework, so as to effectively inhibit the growth of Li dendrites. At the same time, compared with single metal selenides, ZnSe-CoSe with heterostructure gives the cathode carrier excellent electronic conductivity, strong polysulfide chemical adsorption, and efficient polysulfide redox catalytic activity. Combined with these advantages, this Li-S full battery shows an ultra-long life of more than 1000 cycles at 2C. The areal sulfur loading is about 1.2 mg cm⁻² with the E/S ratio of 20 μL mg⁻¹. Even under the condition of high sulfur load and lean electrolyte, the full battery can still achieve an area capacity of 4.16 mAh cm⁻² after 100 cycles at 0.2 C.

4.3.1 Inhibition of Polysulfide Diffusion Using SAC Electrode

Another effective strategy is to add transition metal or precious metal atoms to the electrode materials to accelerate the redox reaction and achieve higher sulfur utilization. Various SACs have been prepared for cathodes and separators for Li-S batteries, such as carbon materials containing single atoms of Co, Fe, Ni, and Zn^[137-139] by promoting polysulfide conversion, SACs can effectively reduce polarization and inhibit shuttle effect, so as to improve rate and cycle performance. In order to improve the electrochemical performance of Li-S battery, SACs carbon materials as sulfur host were synthesized. For example, a single Co atom in nitrogen doped graphene (Co-N/G) is obtained by heat treating a mixture of graphene oxide and cobalt chloride in NH3 and Ar at 750 °C. These atoms have been used as cathode materials S@Co-N/G for Li-S batteries.^[137] As shown in the HAADF-STEM image in Figure 17a, it is confirmed that the Co atoms are evenly dispersed on the graphene support. As shown in Figure 17b–d, X-ray absorption spectroscopy (XAS) reveals the formation of Co-N-C coordination part. In order to understand the catalytic effect of Co-N-C center in graphene support, the CV curve (as shown in Figure 17e) S@Co-N/G material shows obvious cathodic and anodic peaks. The discharge and charge curves in Figure 17f,g show the lowest overpotential, which reveals the high catalytic activity of Co-N/G material on the conversion of lithium polysulfides. The further theoretical calculation in Figure 17h is used to study the improvement of reaction kinetics during discharge and charging, which shows that the reaction activation energy is greatly reduced, and the reduction of sulfur on Co-N/G carrier is more favorable in thermodynamics. When used as the cathode material of Li-S battery, the prepared cathode containing 90 wt% sulfur load maintains a discharge capacity of 681 mAh g⁻¹, the Coulomb efficiency is 99.6%, and the average capacity attenuation rate is 0.053% (Figure 17i) with sulfur loading is about 2 mg cm⁻² with the E/S ratio of 20 μL mg⁻¹. In further work, Zhang et al.^[138] Also designed and synthesized single Fe atom catalysts supported on cathode materials containing L₂S and porous nitrogen doped carbon materials (Li₂S@NC:SAFe) by calcining the mixture, including polyaniline, lithium sulfide, and iron acetate precursors, in an argon atmosphere at 700 °C. Theoretical calculation shows that the use of highly active single Fe atom catalyst can reduce the potential barrier of Li₂S de-lithium, so as to promote the transmission of Li ions. The HAADF-STEM shows the uniform distribution of single Fe atoms, which is also confirmed by the EXAFS results. According to the experimental results and theoretical calculation, the mechanism of using single Fe catalyst in Li-S battery is proposed (Figure 17j). First, through the coordination between SAC and Li₂S, the length of Li-S bond will increase. During the charging process, Li ions can be easily separated from the intermediate [N-F….S-Li₂], which can combine with adjacent Li₂S molecules to form polysulfides through repeated Li removal process. When tested as a Li-S battery, the cathode material shows high rate performance (588 mAh g⁻¹ at 12C) and excellent cycle capacity, and the capacity attenuation is low (0.06% per cycle of 1000 cycles at 5C). In addition, SAC can also be used in the separator of Li-S battery. Xie et al.^[139] prepared a separator for Li-S batteries by coating the commercial polypropylene with foam based graphene foam. The synthesis of catalyst (Fe1/NG) is achieved by heat treatment of GO foam containing FeCl₃ precursor under Ar/NH₃ atmosphere at 750 °C. As shown in Figure 17k, Fe1/NG catalyst shows strong adsorption capacity for polysulfides. After the Fe1/NG catalyst was coated on the commercial separator, it was observed that after 48 h, no polysulfide diffused through the Fe1/NG modified separator (Figure 17l), while the ordinary commercial separator had obvious leakage. This shows that Fe1/NG catalyst can fix lithium polysulfide through the strong electrostatic ability between metal and non-metal atoms, so as to minimize the shuttle effect and improve the battery performance under lean conditions of electrolyte (sulfur loading is about 4.5 mg cm⁻² with the E/S ratio of 10 μL mg⁻¹) as a whole.