Metal-Based Electrocatalysts for High-Performance Lithium-Sulfur Batteries: A Review

: The lithium-sulfur (Li-S) redox battery system is considered to be the most promising next-generation energy storage technology due to its high theoretical specific capacity (1673 mAh g − 1 ), high energy density (2600 Wh kg − 1 ), low cost, and the environmentally friendly nature of sulfur. Though this system is deemed to be the next-generation energy storage device for portable electronics and electric vehicles, its poor cycle life, low coulombic efficiency and low rate capability limit it from practical applications. These performance barriers were linked to several issues like polysulfide (LiPS) shuttle, inherent low conductivity of charge / discharge end products, and poor redox kinetics. Here, we review the recent developments made to alleviate these problems through an electrocatalysis approach, which is considered to be an effective strategy not only to trap the LiPS but also to accelerate their conversion reactions kinetics. Herein, the influence of different chemical interactions between the LiPS and the catalyst surfaces and their effect on the conversion of liquid LiPS to solid end products are reviewed. Finally, we also discussed the challenges and perspectives for designing cathode architectures to enable high sulfur loading along with the capability to rapidly convert the LiPS.


Introduction
The exponential growth in the utilization of portable electronic devices such as laptops, cellular phones, electric vehicles, etc., has surged the demand for high energy storage devices with extended cycle life. Although lithium-ion (Li-ion) battery technologies have been at the forefront for meeting such needs, they cannot sustain the current global trend of increasing energy demand [1][2][3]. In this context, the research efforts have been focused on exploring new mechanisms and chemistries, beyond Li-ion batteries, for developing next-generation energy storage systems [2,4]. In search of alternatives over the past few decades, lithium-sulfur (Li-S) battery was identified as one of the most feasible energy storage technology to satisfy the current and near-future energy needs. Li-S battery offers a unique set of advantages such as high theoretical capacity (1672 mAh g −1 , 10X times higher energy than the conventional Li-ion battery cathodes), high energy density (~2567 Wh kg −1 ), low cost and eco-friendliness of sulfur, non-toxicity, and a wide temperature range of operation [5]. The typical Li-S battery's functioning is governed by several highly complex and non-equilibrium multi-step conversion reactions involving various intermediate sulfur species. More specifically, during discharge upon lithiation, the elemental sulfur (S 8 ) produces several intermediate lithium polysulfides (LiPS, Li 2 S n , n ≤ 8) [6]. The long-chain LiPS (Li 2 S 8 and Li 2 S 6 ) produced during the initial step of discharge undergoes further reduction to generate medium order LiPS in the midway of discharge. Finally, Powered by Pymatgen version 2019. 10.16) [63,64] and VESTA software [65].

Metals-Sulfide Interactions in the Electrocatalysis of LiPS Redox Reaction
Our group has introduced the concept of electrocatalysis for the non-aqueous LiPS reactions [60] by exploring traditional metal-based electrocatalysts such as Pt, gold (Au), and Ni coated on aluminum (Al) or stainless steel as current collectors while eliminating the need for a carbon matrix (Figure 2a). The Ni electrocatalyst displayed an enhanced i0 of 0.071 mA cm −2 compared to carbon (0.049 mA cm −2 ), indicating better redox kinetics on the Ni surface. Besides, higher potentials for cathodic reactions and lower potentials for anodic reactions often reflect in low cell polarization as a  10.16) [63,64] and VESTA software [65].
Based on the function and activity, electrocatalysts designed for Li-S are mainly classified into two types (i) homogenous: catalytic active materials are dispersed in the electrolyte, which in turn produces active species that binds to the LiPS and convert them into solid products. (ii) heterogenous: Catalysts 2020, 10, 1137 4 of 28 cathode surfaces tailored with polarized functional groups that regulate the adsorption and conversion of LiPS into solid ones. Both types of electrocatalysts effectively reduce the charge transfer issues, which leads to the expedited LiPS redox conversion reaction along with less polarization. Additionally, these catalytic materials drastically reduce the internal resistance which arises due to the accumulation of electronically insulating products, by effectively converting them reversibly during charge/discharge. This process is known to vastly improve the Li-S cell performance. However, the homogenous electrocatalyst functions are limited to its electrochemical redox potential and solubility, restricting them from extensive usage. Whereas, being an integral part of the cathode matrix, the robust and cost-effective heterogeneous electrocatalytic surfaces with polar nature has provided widespread opportunity to be explored as cathode hosts for Li-S batteries.
In this approach, the efficiency of the liquid LiPS conversion reaction to solid end products lies in the adsorption mechanism and anchoring effect exhibited by the cathode surface. For instance, the catenation reaction of MnO 2 nanosheets with intermediate LiPS via polythionate complex formation was found to accelerate the liquid to solid conversion in a very efficient manner [33]. Similarly, the incorporation of sulfiphilic CoS 2 promotes the LiPS adsorption onto the surface while significantly manipulating the redox activity of liquid−liquid LiPS conversion, resulting in the prompt deposition of solid Li 2 S [59]. Tuning the electrocatalytic surface to lithiophilic/sulfiphilic nature has also provided a widespread opportunity to increase their anchoring or binding affinity towards LiPS. In addition to this, several other parameters such as nucleophilicity, electrophilic character, Lewis acid-base interaction strength, surface polarity determines the anchoring strength of the electrocatalytic surface towards LiPS and thus improves the performance. Previously, the Li-S electrocatalysts have been subjected to review based on recent developments in metal and non-metal based electrocatalytic surfaces [30]. In this review, we classified the electrocatalytic cathodes for Li-S redox reactions based on their interaction with intermediate LiPS. For this purpose, we will discuss metal-sulfide, metal oxide-sulfide, metal sulfide-sulfide, and metal carbide-sulfide based interactions that influence the LiPS anchoring and subsequent redox reactions in detail, with the help of binding energies, extensive adsorption studies. We have compared electrocatalysts with other control electrodes based on cyclic voltammetry (CV) derived parameters like onset potentials for redox reactions, exchange current densities (i 0 ), etc. Finally, we present our outlook on the development of next-generation electrocatalysts for Li-S batteries.

Metals-Sulfide Interactions in the Electrocatalysis of LiPS Redox Reaction
Our group has introduced the concept of electrocatalysis for the non-aqueous LiPS reactions [60] by exploring traditional metal-based electrocatalysts such as Pt, gold (Au), and Ni coated on aluminum (Al) or stainless steel as current collectors while eliminating the need for a carbon matrix (Figure 2a). The Ni electrocatalyst displayed an enhanced i 0 of 0.071 mA cm −2 compared to carbon (0.049 mA cm −2 ), indicating better redox kinetics on the Ni surface. Besides, higher potentials for cathodic reactions and lower potentials for anodic reactions often reflect in low cell polarization as a result of the efficient electrocatalytic activity. Given this information, Table 1 represents a comparison between the redox peak potentials and the exchange current densities of various electrocatalysts compared to control electrodes. In the initial testing, the proposed Ni and Pt electrocatalysts delivered a discharge capacity of 370 and 395 mAh g −1 at the end of the 50th cycle, respectively. At the same time, Au and Al electrodes exhibited a discharge capacity of only 95 and 25 mAh g −1 , respectively. Given that its surface area strongly influences the electrocatalyst's activity [66], the authors attempted to enhance the LiPS conversion reactions on a 3D Ni foam. Li-S assembled with the microporous and macroporous 3D Ni foams exhibited a discharge capacity of 800 and 900 mAh g −1 respectively for 50 cycles, further confirming that the electrocatalysis surface accessibility is maximum with mesoporous structures compared to microporous ones. The same group has also demonstrated the effect of electrocatalyst's (Ni) particle size on the LIPS conversion reaction [67]. In this work, graphene (Gr) supported Ni nanoparticles when used as sulfur hosts, delivered a significantly enhanced discharge capacity over other electrodes under investigation. Detailed studies on these electrodes revealed that the homogeneous distribution  Figure 2b) while ensuring high sulfur loadings [61]. In this work, the authors have attempted to understand the underlying mechanism behind the enhancement in Li-S battery's performance in the presence of nano-electrocatalysts (Pt, Ni) supported on Gr. For this purpose, ex situ XPS was employed to decipher the interactions between the Pt and LiPS during the charge-discharge processes. The results, on the one hand, revealed the formation of insoluble Li2S and Li2S2 on the Gr surface, both in charged and discharged electrodes, indicating poor reversibility of LiPS. In contrast, XPS results on Pt surface evidenced the limited presence of such products in the charged state along with traces of formation of elemental sulfur. These results demonstrate that unlike the Gr surface, the electrocatalytic Pt binds with the LiPS during the discharge process, which assists in their effective reversibility during the charging process. The catalytic effect on the charge transfer kinetics was demonstrated by the i0 values calculated for Pt/Gr electrodes, which were found to be ~3 times (cathodic process) and ~2 times (anodic process) higher than that of the Gr electrodes. More specifically, the i0 calculated for the pristine and Pt/graphene electrodes were found to be 1.18 and 3.18 mA cm −2 for the cathodic process and 0.17 and 0.29 mA cm −2 for the anodic processes, respectively. Finally, the enhancement in the rate of LiPS conversion reaction due to the electrocatalysis effect [68] was reflected in the excellent specific capacity of the Ni/Gr (740 mAh g −1 ) and Pt/Gr (1100 mAh g −1 ) electrodes, which correspond to 20% and 40% enhancement over pristine Gr electrodes. In a similar effort, Fan et al. have decorated Au nanoparticles (AuNPs) on acetylene black (CB) to be used as a Li-S cathode material (Figure 3a) [69]. Density-functional theory (DFT) calculations were employed to understand the interactions between the Li-S discharge products and the AuNPs. The results demonstrated that the chemical bonding between Au with two Li atoms of Li2S is structurally feasible, and the binding energy between Li2S and Au (1.81 eV) is several times higher than the binding between Li2S and pristine CB (0.37 eV). It was anticipated that the AuNPs could form covalent bonds with sulfur or LiPS, effectively limiting the movement of  [61]. In this work, the authors have attempted to understand the underlying mechanism behind the enhancement in Li-S battery's performance in the presence of nano-electrocatalysts (Pt, Ni) supported on Gr. For this purpose, ex situ XPS was employed to decipher the interactions between the Pt and LiPS during the charge-discharge processes. The results, on the one hand, revealed the formation of insoluble Li 2 S and Li 2 S 2 on the Gr surface, both in charged and discharged electrodes, indicating poor reversibility of LiPS. In contrast, XPS results on Pt surface evidenced the limited presence of such products in the charged state along with traces of formation of elemental sulfur. These results demonstrate that unlike the Gr surface, the electrocatalytic Pt binds with the LiPS during the discharge process, which assists in their effective reversibility during the charging process. The catalytic effect on the charge transfer kinetics was demonstrated by the i 0 values calculated for Pt/Gr electrodes, which were found to be~3 times (cathodic process) and~2 times (anodic process) higher than that of the Gr electrodes. More specifically, the i 0 calculated for the pristine and Pt/graphene electrodes were found to be 1.18 and 3.18 mA cm −2 for the cathodic process and 0.17 and 0.29 mA cm −2 for the anodic processes, respectively. Finally, the enhancement in the rate of LiPS conversion reaction due to the electrocatalysis effect [68] was reflected in the excellent specific capacity of the Ni/Gr (740 mAh g −1 ) and Pt/Gr (1100 mAh g −1 ) electrodes, which correspond to 20% and 40% enhancement over pristine Gr electrodes. In a similar effort, Fan et al. have decorated Au nanoparticles (AuNPs) on acetylene black (CB) to be used as a Li-S cathode material (Figure 3a) [69]. Density-functional theory (DFT) calculations were employed to understand the interactions between the Li-S discharge products and the AuNPs. The results demonstrated that the chemical bonding between Au with two Li atoms of Li 2 S is structurally feasible, and the binding energy between Li 2 S and Au (1.81 eV) is several times higher than the binding between Li 2 S and pristine CB (0.37 eV). It was anticipated that the AuNPs could form covalent bonds with sulfur or LiPS, effectively limiting the movement of discharge products away from the cathode surface, thus inhibiting the shuttle process. Ex situ XPS analysis was also performed to reveal the nature of chemical interactions between AuNPs and LiPS. The results demonstrated the existence of the Au-S bond in both charge and discharge products on the AuNP electrode indicating their effective anchoring on the catalyst surface. The CV results showed the shortening of electron/ion transfer paths, effective restriction for LiPS, and concurrent catalyzation of electrode reactions by the AuNPs. The cell assembled with AuNPs electrode displayed a reversible discharge capacity of 771 mAh g −1 for over 100 cycles, with a capacity retention of 94.8% and capacity decay of 0.05% per cycle; In comparison, the CB-S control electrode delivered a capacity of 297 mAh g −1 only after 100 cycles with 61% retention. Hence, the decoration of the cathode surface with AuNPs had the following effects on the Li-S battery: (i) improved reaction kinetics due to the significant shortening of the electron transfer paths; (ii) controlled nucleation/reduction of LiPS on the AuNP surface, which minimizes the active material dissolution into the electrolyte and associated wastage. Table 2 summarizes the LiPS adsorption energies of various LiPS on different electrocatalytic surfaces, as revealed by first principle DFT calculations.
Towards deploying electrocatalysts even in a polysulfide full cell configuration, Sawas et al. have reported an electrocatalytically active composite cathode against a 3D porous metallic lithium-free anode to stabilize the LiPS and enhance the reaction kinetics while eliminating the use of unsafe Li metal anode [70]. For this purpose, Pt functionalized Gr and porous 3-D silicon electrode were used as cathode and anode materials, respectively ( Figure 3b). The full cell showed a stable specific capacity of 597 mAh g −1 for over 240 cycles with a minimal capacity loss of 0.147% per cycle. On the other hand, Gr electrodes displayed a capacity loss of 2.6% per cycle for the first 10 cycles, followed by 0.54% loss per cycle for over 130 cycles.
More recently, Chen et al. have shown the synthesis and employment of Co-Fe mixed phosphide nanocubes with highly porous interconnected-pore architecture as Li-S cathode electrodes ( Figure 4) [71]. The UV-Vis absorption spectroscopy showed that the Co-Fe-P nanocubes have a robust chemical adsorption capacity towards LiPS, further validated by ex situ XPS studies. Besides, DFT studies revealed that the strong polar-polar interaction behind the Li 2 S 6 and CoP 2 phase leads to the deformation of the LiPS into three Co-S and two Li-P bonds, whose binding energy (−3.926 eV) towards the electrocatalyst was found to be much higher than towards the conventional carbonaceous electrodes. Besides, LiPS were also found to break and form new bonds with the FeP 2 phase of the cathode material whose bindings energies were found to be as high as −7.40 eV. These results indicate the strong chemical trapping ability of nanocubes towards LiPS, while subsequently, these interactions between the cathode material and LiPS dynamically accelerate the kinetics of conversion reactions. As shown in Figure 4, a positive shift in cathodic peaks and a negative shift in anodic peaks for the Co-Fe-P electrode confirms that the LiPS redox kinetics were promoted compared to the Co-Fe electrode. Given these capabilities, the Li-S cells assembled with the nanocubes delivered an initial high specific capacity of 863 mAh g −1 at 1 C. In contrast, after long-term cycling for 500 cycles, the capacity was found to be 678 mAh g −1 , with an average capacity fade of 0.043% per cycle. In effect, while the polar characteristics of the nanocubes contribute to the creation of abundant adsorption sites for confining LiPS, their highly interconnected-pore architecture and inherent metallic conductivity further enhanced the redox kinetics of LiPS conversion reactions.

Metal Oxide-Sulfide Interactions in Electrocatalysis of LiPS Redox Reactions
As alternatives to expensive metal catalysts, metal oxide-based surfaces also show excellent electrocatalytic activity towards LiPS adsorption and their subsequent conversion in Li-S batteries.
The key point in achieving effective LiPS confinement is the strong interaction between polar natured substrate and the LiPS species during the electrochemical reactions. To date, various metal oxide electrocatalysts including MnO2 [79], Ti4O7 [33], MoO [80], WO3−x [81], and CeO2 [74], V2O5 [82], SnO2 [83], TiO2 [84], ZrO2 [85], and La0.6Sr0.4CoO3-δ [86] have been explored. To entrap LiPS and promote the conversion reactions in Li-S batteries, Linda Nazar and her coworkers introduced a highly efficient LiPS mediatorbased on manganese oxide nanosheet [79]. In this chemical approach, the MnO2 nanosheet mediator facilitated the conversion of LiPS to insoluble Li2S2/Li2S species. The idea is that the host MnO2 nanosheets react with the as-formed LiPS to form surface-based intermediates on the MnO2 surface. Further, polythionates formation was anticipated due to the reaction between LiPS and the surface-bound intermediates. The interaction between sulfur and MnO2 nanosheets was studied by XPS which confirmed the formation of polythionates and thiosulfates during the discharge reaction. With this efficient LiPS conversion reaction, the MnO2/S composite delivered high capacity retention of 92% after 200 cycles at a C/5 rate. Additionally, the cathode exhibited a low capacity fade of 0.036% per cycle for 2000 charge-discharge cycles at a 2 C rate. Further, to attain an efficient oxide based catalyst with improved electronic conductivity, the same group introduced a highly conductive titanium oxide-based catalyst for LiPS conversion reactions [33]. A Magnéli phase Ti4O7 based surface-enhanced LiPS redox chemistry was investigated for Li-S batteries. Interestingly, the Ti4O7 has a high surface area and a high electronic conductivity of the order 2 × 10 3 S cm −1 at room temperature. Due to its high electrical conductivity and sulfiphilic nature, this metallic oxide exhibits a high affinity towards LiPS which eventually mitigates the LiPS dissolution by forming an effective interface with Li2S. More specifically, the study elucidated the interactions between Ti4O7 surface with both the terminal and bridging sulfur in the LiPS, which leads to the polarization of electrons away from the sulfur atom and towards titanium and/or oxygen vacancies (which are inherently electropositive). Additionally, the surface-mediated redox between LiPS and the sulfiphilic metal oxide host was extensively studied by XPS and X-ray Absorption Near Edge Structure, evidencing the accelerated conversion of LiPS to Li2S through the interaction between sulfiphilic host and LiPS compared to the carbon surface. With this excellent catalytic activity, the Ti4O7/S composite cathode exhibited a discharge capacity of 1070 mAh g −1 at a moderate rate and delivered a stable cycle life of 500 cycles at 2 C rate.
To improve the oxide catalyst conductivity, Zheng et al. introduced a Fe2O3/Gr hybrid prepared by introducing Fe2O3 nanoparticles onto Gr nanosheets via a one-pot strategy [73]. Due to the strong interaction between LiPS and Fe2O3 nanoparticles, the LiPS shuttle was mitigated. The conversion of  These works demonstrate that the employment of electrocatalysis and, more precisely, the metal catalysts in Li-S batteries can significantly enhance the redox kinetics while curbing the LiPS shuttle. Besides, the inherent high conductivity of the metallic catalysts can also improve contact between the insulating sulfur/discharge end products with the cathode surface, thereby improving the sulfur utilization over prolonged cycling. However, these precious-metal-based electrocatalysts being expensive, low-cost alternative materials are always important from a commercialization point of view.

Metal Oxide-Sulfide Interactions in Electrocatalysis of LiPS Redox Reactions
As alternatives to expensive metal catalysts, metal oxide-based surfaces also show excellent electrocatalytic activity towards LiPS adsorption and their subsequent conversion in Li-S batteries.
The key point in achieving effective LiPS confinement is the strong interaction between polar natured substrate and the LiPS species during the electrochemical reactions. To date, various metal oxide electrocatalysts including MnO 2 [79], Ti 4 O 7 [33], MoO [80], WO 3−x [81], and CeO 2 [74], V 2 O 5 [82], SnO 2 [83], TiO 2 [84], ZrO 2 [85], and La 0.6 Sr 0.4 CoO 3-δ [86] have been explored. To entrap LiPS and promote the conversion reactions in Li-S batteries, Linda Nazar and her coworkers introduced a highly efficient LiPS mediatorbased on manganese oxide nanosheet [79]. In this chemical approach, the MnO 2 nanosheet mediator facilitated the conversion of LiPS to insoluble Li 2 S 2 /Li 2 S species. The idea is that the host MnO 2 nanosheets react with the as-formed LiPS to form surface-based intermediates on the MnO 2 surface. Further, polythionates formation was anticipated due to the reaction between LiPS and the surface-bound intermediates. The interaction between sulfur and MnO 2 nanosheets was studied by XPS which confirmed the formation of polythionates and thiosulfates during the discharge reaction. With this efficient LiPS conversion reaction, the MnO 2 /S composite delivered high capacity retention of 92% after 200 cycles at a C/5 rate. Additionally, the cathode exhibited a low capacity fade of 0.036% per cycle for 2000 charge-discharge cycles at a 2 C rate. Further, to attain an efficient oxide based catalyst with improved electronic conductivity, the same group introduced a highly conductive titanium oxide-based catalyst for LiPS conversion reactions [33]. A Magnéli phase Ti 4 O 7 based surface-enhanced LiPS redox chemistry was investigated for Li-S batteries. Interestingly, the Ti 4 O 7 has a high surface area and a high electronic conductivity of the order 2 × 10 3 S cm −1 at room temperature. Due to its high electrical conductivity and sulfiphilic nature, this metallic oxide exhibits a high affinity towards LiPS which eventually mitigates the LiPS dissolution by forming an effective interface with Li 2 S. More specifically, the study elucidated the interactions between Ti 4 O 7 surface with both the terminal and bridging sulfur in the LiPS, which leads to the polarization of electrons away from the sulfur atom and towards titanium and/or oxygen vacancies (which are inherently electropositive). Additionally, the surface-mediated redox between LiPS and the sulfiphilic metal oxide host was extensively studied by XPS and X-ray Absorption Near Edge Structure, evidencing the accelerated conversion of LiPS to Li 2 S through the interaction between sulfiphilic host and LiPS compared to the carbon surface. With this excellent catalytic activity, the Ti 4 O 7 /S composite cathode exhibited a discharge capacity of 1070 mAh g −1 at a moderate rate and delivered a stable cycle life of 500 cycles at 2 C rate.
To improve the oxide catalyst conductivity, Zheng et al. introduced a Fe 2 O 3 /Gr hybrid prepared by introducing Fe 2 O 3 nanoparticles onto Gr nanosheets via a one-pot strategy [73]. Due to the strong interaction between LiPS and Fe 2 O 3 nanoparticles, the LiPS shuttle was mitigated. The conversion of soluble LiPS to insoluble Li 2 S 2 /Li 2 S was also accelerated, thus improving the sulfur utilization. Detailed DFT calculations to understand the interactions between the LiPS and Fe 2 O 3 revealed that the electrocatalyst showed adsorption energies at least 3 times higher than Gr towards all LiPS, S 8 and Li 2 S. Further, the binding mechanism of LiPS on different surfaces was illustrated as: (i) only weak Van der Waals interaction exists between S clusters and Gr (ii) strong Li-O and Fe-S bonds are responsible for the high binding energy in the case of Fe 2 O 3 surface, which tends to further increase with shortening sulfur chains. The 3D Gr network improved the electronic conductivity and enhanced the electron transfer while providing enough space for volume expansion during charge-discharge reactions. The i 0 calculated from potentiostatic polarization experiments for the Fe 2 O 3 electrode displayed a value of 3.46 and 4.96 mA cm −2 for the cathodic and anodic processes, respectively, which was found to be higher than the porous graphene electrode (2.28 and 4.81 mA cm −2 ). The improved i 0 indicates an enhanced rate of intermediate LiPS conversion into discharge end products during the charge and discharge processes. The sulfur/Fe 2 O 3 hybrid exhibited a high capacity of 565 mAh g −1 with a low capacity decay of 0.049% per cycle for 1000 continuous charge-discharge cycles at 5 C rate. Another exciting strategy of separator modification using MoO 3 nanobelts for electrocatalysis of LiPS was carried out by James A. Anderson and his coworkers [80]. In this different approach, the MoO 3 nanobelt modified separator exhibited a bifunctional catalytic effect for the chemical encapsulation and electrocatalytic conversion of LiPS ( Figure 5a). Further, a strong chemical interaction between the LiPS and MoO 3 successfully blocked the LiPS on the separator and catalyzed the redox reaction of LiPS on the cathode side. The electrocatalytic effect was evidenced from CV curves, where the MoO 3 modified separator showed a significant increment in the current density during CV compared to the pristine separator. In effect, the MoO 3 modified separator delivered a high capacity of 684 mAh g −1 at 0.5 C rate for 200 cycles. discharge reactions. The i0 calculated from potentiostatic polarization experiments for the Fe2O3 electrode displayed a value of 3.46 and 4.96 mA cm −2 for the cathodic and anodic processes, respectively, which was found to be higher than the porous graphene electrode (2.28 and 4.81 mA cm −2 ). The improved i0 indicates an enhanced rate of intermediate LiPS conversion into discharge end products during the charge and discharge processes. The sulfur/Fe2O3 hybrid exhibited a high capacity of 565 mAh g −1 with a low capacity decay of 0.049% per cycle for 1000 continuous chargedischarge cycles at 5 C rate. Another exciting strategy of separator modification using MoO3 nanobelts for electrocatalysis of LiPS was carried out by James A. Anderson and his coworkers [80]. In this different approach, the MoO3 nanobelt modified separator exhibited a bifunctional catalytic effect for the chemical encapsulation and electrocatalytic conversion of LiPS ( Figure 5a). Further, a strong chemical interaction between the LiPS and MoO3 successfully blocked the LiPS on the separator and catalyzed the redox reaction of LiPS on the cathode side. The electrocatalytic effect was evidenced from CV curves, where the MoO3 modified separator showed a significant increment in the current density during CV compared to the pristine separator. In effect, the MoO3 modified separator delivered a high capacity of 684 mAh g −1 at 0.5 C rate for 200 cycles.  It is well known that oxygen deficiency in oxide-based cathodes is known to exhibit excellent electrocatalytic activity towards any electrochemical reactions [87,88]. In this context, to elucidate the cationic activity of oxygen deficiency on the LiPS conversion, oxygen-deficient WO 3 nanoplates were proposed as electrocatalysts for Li-S batteries [81]. In this work, WO 3 was taken as a model compound, and the catalytic effect of oxygen deficiency was elucidated using a self-doped oxide without any heteroatoms. Additionally, the WO 3 is a low-cost material compared to other metallic electrocatalysts, and the oxygen-deficient WO 3 can be synthesized easily (Figure 5b). The beneficial catalytic activity of accelerated LiPS conversion was investigated using UV-Vis spectroscopy, which demonstrated that the oxygen deficiency in WO 3 interacts with the LiPS and significantly mitigates the accumulation of LiPS in the sulfur cathode surface. As a result, the S/WO 3 nanoplate cathode exhibited a capacity decay of only 0.13% per cycle at a 0.5 C rate.
Further, to achieve effective confinement of sulfur, a CeO 2 nanocrystal was implanted homogeneously into a bimodal mesoporous nitrogen-doped carbon sphere (CeO 2 /MMNC), as shown in Figure 6 [74]. The CeO 2 /MMNC showed key characteristics, especially the hybrid material has high porosity, polar adsorption sites, 3D nanochannels, and high electronic conductivity. With the advantage of physical and chemical interactions with LiPS, the polar and conductive nanocrystal catalyst significantly mitigated the LiPS dissolution and facilitated the fast LiPS conversion reactions. In the present work, the strong electrocatalytic effect along with enhanced LiPS redox kinetics on CeO 2 nanocrystals was confirmed by the positive and negative shifts in the reduction and oxidation peaks of CV respectively. By combining the synergetic effect of LiPS adsorption and LiPS fast reaction kinetics, the CeO 2 /MMNC cathode with 1.4 mg cm −2 loading exhibited a reversible capacity of 1064 mAh g −1 at 0.2 C rate after 200 cycles. Interestingly, a highly stable cycle life of 721 mAh g −1 capacity at 2 C rate has achieved 1000 cycles at a low capacity decay of 0.024% per cycle. Moreover, high loading of 3.4 mg cm −2 cathode was also investigated, and stable capacity of 611 mAh g −1 was achieved after 200 cycles.
cationic activity of oxygen deficiency on the LiPS conversion, oxygen-deficient WO3 nanoplates were proposed as electrocatalysts for Li-S batteries [81]. In this work, WO3 was taken as a model compound, and the catalytic effect of oxygen deficiency was elucidated using a self-doped oxide without any heteroatoms. Additionally, the WO3 is a low-cost material compared to other metallic electrocatalysts, and the oxygen-deficient WO3 can be synthesized easily (Figure 5b). The beneficial catalytic activity of accelerated LiPS conversion was investigated using UV-Vis spectroscopy, which demonstrated that the oxygen deficiency in WO3 interacts with the LiPS and significantly mitigates the accumulation of LiPS in the sulfur cathode surface. As a result, the S/WO3 nanoplate cathode exhibited a capacity decay of only 0.13% per cycle at a 0.5 C rate.
Further, to achieve effective confinement of sulfur, a CeO2 nanocrystal was implanted homogeneously into a bimodal mesoporous nitrogen-doped carbon sphere (CeO2/MMNC), as shown in Figure 6 [74]. The CeO2/MMNC showed key characteristics, especially the hybrid material has high porosity, polar adsorption sites, 3D nanochannels, and high electronic conductivity. With the advantage of physical and chemical interactions with LiPS, the polar and conductive nanocrystal catalyst significantly mitigated the LiPS dissolution and facilitated the fast LiPS conversion reactions. In the present work, the strong electrocatalytic effect along with enhanced LiPS redox kinetics on CeO2 nanocrystals was confirmed by the positive and negative shifts in the reduction and oxidation peaks of CV respectively. By combining the synergetic effect of LiPS adsorption and LiPS fast reaction kinetics, the CeO2/MMNC cathode with 1.4 mg cm −2 loading exhibited a reversible capacity of 1064 mAh g −1 at 0.2 C rate after 200 cycles. Interestingly, a highly stable cycle life of 721 mAh g −1 capacity at 2 C rate has achieved 1000 cycles at a low capacity decay of 0.024% per cycle. Moreover, high loading of 3.4 mg cm −2 cathode was also investigated, and stable capacity of 611 mAh g −1 was achieved after 200 cycles.

Metal Sulfides-Sulfide Interactions in Electrocatalysis of LiPS Redox Reactions
Inspired by their application in desulfurization [89], solar cells [90], and hydrogen evolution reaction [91], cost-effective and non-noble metal-based chalcogenide electrocatalysts have attracted immense attention due to their excellent performance toward sulfur chemistries. In general, sulfide-based transition metal electrocatalysts has several advantages: (i) they offer high stability towards sulfur chemistries, (ii) their sulfiphilic nature is highly attractive towards LiPS, (iii) they avoid overlapping with the active voltage region in Li-S batteries due to its low lithiation potentials [30]. Additionally, the sulfide-based electrocatalysts are generally metallic or semi-metallic, which further assists in facile electron transfer during LiPS conversion reactions. Furthermore, the basis behind their electrocatalytic property is that the effective d band structure is obtained by the synergism between metal d orbital and unsaturated S heteroatom, thereby approaching the d band of Pt. In general, the catalytic activity of the metal sulfide materials is correlated to the number of exposed edge sites [92]. To date, numerous sulfide-based electrocatalysts have been explored for LiPS conversion reactions, especially CoS 2 , WS 2 , MoS 2 , SnS 2, and binary sulfides [93]. Table 2. Binding energies (eV) between various electrocatalysts and LiPS, obtained from first principle DFT calculations.

Material Li 2 S (eV) Li 2 S 4 (eV) Li 2 S 6 (eV) Li 2 S 8 (eV) S 8 (eV)
Ref. Our group introduced transition metal sulfides as electrocatalysts for alleviating the shuttling process in Li-S batteries through adsorption of LiPS on edge sites (Figure 7a) [62]. The WS 2 catalyst as a host for LiPS conversion reaction has several advantages (i) since there is no physical encapsulation of LiPS, the Li-ion transport will not be impeded, (ii) the production of non-noble metal-based WS 2 is cost-effective and can also be scaled up further for low-cost Li-S batteries. Due to its intrinsic physicochemical properties, the detrimental LiPS shuttle reaction was significantly stabilized. Further, as evidenced by CV, the onset potential for the reduction reactions was found to be higher (2.24 and 1.78 V) on the WS 2 surface compared to the carbon surface (2.21 and 1.67 V). Additionally, the i 0 was found to be 11.8 µA cm −2 for WS 2 , while it was only 8.5 µA cm −2 on the carbon surface, indicating that catalytic active edge sites of WS 2 enhanced the charge transfer kinetics. The interaction between LiPS and WS 2 edge sites was visualized through systematic scanning electron microscopy (SEM) studies and further evidenced by spectroscopy measurements such as XPS and UV-Vis. The adsorption results demonstrate that unsaturated edge atoms of transition metal dichalcogenide (TMDs) exhibited significant LiPS adsorption due to polarity and inherent catalytic activity. As a result, a specific capacity of 590 mAh g −1 and catalytic driven stable coulombic efficiency of 99% for 350 continuous charge-discharge cycles was realized.  Zhang and his coworkers introduced CoS 2 as an efficient LiPS conversion electrocatalyst in Li-S batteries (Figure 7b) [59]. The CoS 2 is a pyrite mineral and semi-metallic in nature with a conductivity of 6.7 × 10 3 S cm −1 at 300 K, which is relatively high compared to the first-row transition metal sulfides such as Ni 3 S 2 and FeS 2 . Given the sulfiphilic nature and high electron conduction property of CoS 2 , its incorporation could accelerate LiPS redox reactions in Li-S batteries. In addition to the conventional CV for a typical Li-S cell, to investigate the redox kinetics of LiPS, the authors studied the electrocatalyst in a symmetrical cell format containing active material in the form of Li 2 S 6 . From the CV, it is clear that as the CoS 2 weight ratio increases, the current density is increased by order of magnitude, indicating that the CoS 2 -LiPS interactions accelerated the LiPS conversion reactions compared to the Gr surface. More importantly, DFT calculations revealed the Li 2 S 4 binding mechanism to CoS 2 as follows: (i) initially, the Li atoms and terminal sulfur atoms of Li 2 S 4 orients towards sulfur dimers and Co atoms of CoS 2 due to the electrostatic affinities of the donor-acceptor pairs; (ii) simultaneously, S-S bridge of Li 2 S 4 positions far away from the CoS 2 surface, which enables the large charge density atoms to occupy the spaces close to its surface; (iii) these concurrent processes ultimately lead to the generation of sufficient charge transfer and very high binding energy of 1.97 eV between Li 2 S 4 and CoS 2 . In contrast, the absence of polarized charge distribution on the Gr surface is responsible for low atomic interactions and low binding energy of only 0.34 eV with Li 2 S 4 . In short, the thermally reduced CoS 2 /Gr composite employed in the LiPS conversion reaction exhibited accelerated LiPS redox kinetics, low polarization effects, and high energy efficiency under high currents. Overall, the composite promoted Li-S battery delivered a discharge capacity of 60% and an improved energy efficiency of 10%. Further, an improved cycle life was achieved at a high current density of 2 C rate with a low capacity decay rate of only 0.034% per cycle for over 2000 continuous charge-discharge cycles.

Non-metal
To explore more conductive LiPS adsorbents, Pu et al. investigated the hydrothermal synthesized conductive Co 3 S 4 nanotubes for Li-S batteries [98]. In general, a highly conductive LiPS adsorbent with a high aspect ratio is the critical design strategy for highly efficient sulfur cathodes. To achieve a continuous conductive network, nanotube morphology-based Co 3 S 4 with high electronic conductivity was synthesized and investigated as a sulfur host in Li-S batteries. The nanostructured and conductive Co 3 S 4 helped to host sulfur species and catalyzed the sulfur species by relatively large surface area. Further, the authors have employed symmetrical cell studies to elucidate the enhanced electrochemical properties of Co 3 S 4 nanotubes. Chronoamperometric studies on these cells revealed a higher current response upon the addition of Li 2 S 6 in the electrolyte in comparison to its absence, indicating the current response is dominated by the lithiation/delithiation reactions rather than double-layer capacitance. Similarly, CV studies with Li 2 S 6 also presented higher current response in the case of Co 3 S 4 compared to the acetylene black electrode, depicting that the electrocatalyst performs two essential functions: (i) absorb sulfur species (ii) accelerate the LiPS electrochemical conversion reactions. Additionally, the high electronic conductivity of the Co 3 S 4 nanotubes assisted in the acceleration of the sulfur reaction kinetics. As a result, it exhibited a high capacity of 1024 mAh g −1 at 0.05 C while displaying a slow capacity decay of only 0.04% per cycle at a very high rate of 5 C rate for over 1000 cycles.
A metal-organic frameworks (MOF) derived efficient multifunctional Co 9 S 8 was grown directly on a separator to be used as a polar barrier in Li-S batteries, using a liquid reaction-based approach [102]. The in situ grown Co 9 S 8 works as a LiPS polar barrier and improves the LiPS blocking. With this improved strategy, the fabricated pouch cell using Co 9 S 8 grown separator delivers a high capacity of 1200 mAh g −1 for 30 charge-discharge cycles. In another report, a self-template synthesis of metallic and polar Co 9 S 8 /C hollow polyhedra was investigated as a sulfur host in Li-S batteries [103]. The nano polyhedra were synthesized from a facile sulfurization, followed by a melt diffusion process. The hollow polyhedra have numerous advantages: (i) a high sulfur loading can be achieved from highly porous void space, (ii) the nano polyhedral is metallic and highly polar in nature, leading the Co 6 S 8 to strongly bind to LiPS and improve the reaction kinetics, and (iii) the 3D porous Co 6 S 8 /C composite improves the Li-ion transport through conductive outer shells. In effect, the nano polyhedra with high surface area delivered a high capacity of 950 mAh g −1 at 0.1 C rate and a low capacity decay of 0.041% per cycle for 1000 cycles at 2 C rate.
A liquid phase exfoliated, low-cost MoS 2 based electrode was also investigated for high-performance Li-S batteries [104], where a controlled amount of sulfur deficiencies on the MoS 2−x /rGO catalytic surface catalyzed the LiPS conversion reactions (Figure 8a). CV studies on symmetric cells were carried out to understand the catalytic effect of MoS 2−x on the LiPS redox reactions. As shown in Figure 8b, the CVs revealed distinctive highly reversible peaks representing reduction (−0.047 and −0.39 V) and the oxidation process (0.047 and 0.39 V) of Li 2 S 6 . In comparison, MoS 2 /rGO and bare rGO electrodes only displayed indistinct and drawn-out redox peaks. It is noteworthy that the sharp peaks and narrow peak separation in the case of the MoS 2−x electrode represent excellent electrochemical reversibility and facile LiPS conversion reactions. Consequently, the accelerated LiPS conversion controlled the accumulation of discharge products on the cathode surface, indicating that the sulfur utilization was improved by fast LiPS conversion kinetics. With this kinetic effect, sulfur with a small amount of MoS 2 delivered an excellent cycle life of 1600 cycles at a 5 C rate with a 0.083% capacity decay per cycle. The result indicates that the low cost and facile synthesis of MoS 2 based electrode could be an excellent catalyst for LiPS conversion in Li-S batteries. Furthermore, to develop polar LiPS adsorbent with high sulfur loading, Gao et al. developed Co-doped SnS 2 anchored on nitrogen-doped CNT as sulfur host for Li-S batteries [105]. The nanostructured NCNT-Co-SnS 2 was synthesized by a spray pyrolyzed method coupled with a solvothermal reaction. Since Co compounds have an excellent affinity towards LiPS and exhibit an excellent activity towards LiPS conversion, the authors enhanced the LiPS conversion reaction by employing SnS 2 using Co doping and nitrogen-doped conductive carbon nanotube. With the synergetic effect of Co's catalytic activity towards LiPS and SnS 2 LiPS adsorption effects, the sulfur loaded NCNT-Co-SnS 2 exhibited a specific capacity of 1351 mAh g −1 at 13 mA cm −2 current density.  [106].
After the successful exploration of metal sulfides for improving the LiPS reaction kinetics, bimetal sulfide compounds have been studied as electrocatalysts for Li-S batteries. Based on the excellent performance of CoS2 and NiS2 in LiPS conversion, the authors synthesized NiCo2S4 using hydrothermal reaction at low temperatures [107]. The as-synthesized material exhibited excellent LiPS adsorption and suppressed LiPS shuttle efficiently due to the strong affinity of bimetal sulfides towards LiPS. More importantly, with the unique architecture of the cathode, the cell exhibited a capacity of 788 mAh g −1 at 0.5 C and low capacity fade of 0.0489% per cycle at 0.6 C over 1000 cycles. More recently, a flexible binder-free sulfur host composed of FeCo2S4 nanotube arrays grown on Yi Cui and his coworkers have examined various polar transition metal sulfide compounds for LiPS conversion to learn critical parameters in deciphering energy barrier in Li 2 S oxidation and LiPS adsorption chemistry in Li-S batteries (Figure 8c) [106]. This work demonstrates that the intermetallic conductivity, catalyst, LiPS interaction, fast Li-ion transport, controlled Li 2 S precipitation, surface-mediated reaction kinetics, and catalytic conversion capability of metal sulfide catalysts are the critical parameters in designing an efficient LiPS conversion catalyst to reduce the overpotential for Li 2 S decomposition. Further, all of these features are conducive to improve the electrochemical performance of Li-S batteries. Based on a series of investigations by theory combined with experimentation, TiS 2 , VS 2, and CoS 2 based compounds have been reported to show excellent binding energy diffusion barriers due to the strong complete interaction between these metal sulfides and the LiPS (Figure 8d). Eventually, the hybrid materials based on TiS 2 , VS 2, and CoS 2 delivered a high capacity of 1008, 1093, and 1033 mAh g −1 at 2 C rate. From the detailed investigation on metal sulfide electrocatalyst, this work provides a basis behind the LiPS conversion catalyst and idea about the rational design of catalyst for LiPS catalytic reactions.
After the successful exploration of metal sulfides for improving the LiPS reaction kinetics, bimetal sulfide compounds have been studied as electrocatalysts for Li-S batteries. Based on the excellent performance of CoS 2 and NiS 2 in LiPS conversion, the authors synthesized NiCo 2 S 4 using hydrothermal reaction at low temperatures [107]. The as-synthesized material exhibited excellent LiPS adsorption and suppressed LiPS shuttle efficiently due to the strong affinity of bimetal sulfides towards LiPS. More importantly, with the unique architecture of the cathode, the cell exhibited a capacity of 788 mAh g −1 at 0.5 C and low capacity fade of 0.0489% per cycle at 0.6 C over 1000 cycles. More recently, a flexible binder-free sulfur host composed of FeCo 2 S 4 nanotube arrays grown on carbon paper by two-step hydrothermal synthesis was reported [75]. The interconnected carbon skeleton improved the electronic conductivity of the electrode, and the bimetallic sulfides accelerated the electron transfer reaction with reduced LiPS dissolution due to their strong chemical affinity. To evaluate its ability in LiPS conversion, a detailed investigation of CV, charge/discharge, and electrochemical impedance spectroscopy (EIS) studies were carried out and compared with a bare carbon-based electrode. With this improved catalytic effect, the FeCo 2 S 4 nanotube array delivers a high specific capacity of 1384 mAh g −1 at 0.1 C rate, and a stable coulombic efficiency of 98% for 300 cycles was recorded.

Metal Carbide-Sulfide Interactions in Electrocatalysis of LiPS Redox Reactions
While the poor adsorption properties of hydrophobic carbonaceous cathodes render them inactive towards binding with polar natured LiPS, metal carbide, or more specifically, transition metal carbide (TMCs) based hydrophilic hosts chemically bind with LiPS and facilitate multi-step redox processes in Li-S batteries. The unique electrochemical activity of TMCs stems from the 3d electron number of the transition metal atoms and strong interactions between the metals and the electroactive species. Additionally, due to the electron distribution in the carbons, TMCs exhibit unique surface reactivity and catalytic properties close to that of noble metals [76,108,109]. Further, MCs comprises of inherent large interlayer channels that are known to shorten the ionic/electronic transport pathways, which assists in the realization of very high power capabilities. In this context, Salem et al. have reported tungsten carbide (WC) and titanium carbide (TiC) based cathodes for Li-S batteries [76]. Initially, spin-polarized DFT calculations were performed to understand the adsorption state of Li 2 S 8 on MCs. The results revealed that Li 2 S 8 undergoes complete dissociation on MC upon adsorption while remaining intact over the Gr surface. More importantly, WC (3.56 eV/S atom) and TiC (3.68 eV/S atom) exhibited higher binding energies towards Li 2 S 8 due to the polar metal-C bonds compared to Gr (0.11 eV/S atom) and its non-polar C-C bonds. The adsorption capabilities of MCs towards LiPS were further evaluated by UV-Vis spectroscopy, which demonstrated an excellent affinity of W and Ti edge sites towards adsorption of LiPS. Upon performing electrochemical tests, the TiC electrode unveiled a specific capacity of 1156 mAh g −1 and excellent reversibility. Peng et al. have further investigated the role of conductive polar TiC on liquid-liquid LiPS interconversion and liquid-solid (Li 2 S) precipitation in comparison with non-polar carbon and semiconducting titanium dioxide surfaces [110]. The authors have predicted that only a polar conductive material can fulfill the requirements for enhancing the interfacial electrochemical kinetics and anchoring LiPS by strong chemical adsorption. Appropriately, first-principle calculations also revealed that binding energies were very high between Li 2 S 4 (−1.89 eV) and Li 2 S (−2.75 eV) on the TiC surface when compared to the non-polar carbon surface. It was also predicted that improved conductivity can also influence the LiPS interfacial transformations. Further, efforts were directed towards understanding polysulfide redox in liquid phase and Li 2 S precipitation to provide insights into the correlation between kinetics and electrical conductivity. Given the high conductivity of TiC, it was noted that it exhibited the highest activity towards Li 2 S precipitation and promoted its growth in the radial direction while impinging in a perpendicular direction. Improved kinetics towards LiPS conversion on the TiC surface, high conductivity, and interactions between LiPS and TiC has reflected in its excellent capacity of 1032 mAh g −1 and coulombic efficiency of 95% over 100 cycles.
In an attempt to improve the cycling stability and rate capability of the Li-S battery, Zhou et al. reported a low-cost metal carbide nanoparticles (MCNPs) on carbon nanofibers which can provide both catalytic and adsorption sites on the cathode surface, [77]. DFT models created to understand the nature of chemical interactions predicted strong partial bonding between polar W 2 C, Mo 2 C, TiC, and LiPS (Li 2 S 6 ) with binding energies of −2.57 eV, −2.71 eV, and −2.32 eV, respectively. Thus, these polar MCNPs on the CNF were anticipated to act as anchoring sites for long-chain LiPS making these species electrochemically accessible on the electrode's surface, as shown in Figure 9. Additionally, while the redox of LiPS was found to be triggered upon their adsorption onto the MC surface, the electrocatalytic activity of MCNPs was found to lower the passive overpotentials and enhance the redox kinetics. Overall, a typical Li-S battery assembled with W 2 C NPs-CNFs as the cathode material delivered a high specific capacity of 1200 mAh g −1 at 0.2 C rate with a stable performance of up to 200 cycles.
Further, given that the MCs are layered and puckered sheets of covalently bonded mixed atoms, availability of complete surface area is restrained during their restacking after exfoliation, thus effecting the ions accessibility to the surface. In this context, Bao et al. proposed a flexible delamination and reassembly method for MCs by exploring the highly porous MOF system [111]. For this purpose, they have introduced a metal carbide@ mesoporous carbon hybrid architecture loaded with sulfur (Ti 3 C 2 T x @Meso-C/S). The proposed cathode delivered an initial capacity of 1225.8 mAh g −1 with a high reversible capacity of 704.6 mAh g −1 after 300 cycles. Simultaneously, a control meso-C/S cathode delivered an initial capacity of only 1006.8 mAh g −1, which decreased to 470.2 mAh g −1 after 300 cycles. The evident improvement in the electrochemical performance of the MC was ascribed to the synergetic effect of Ti 3 C 2 T x nanosheets and the mesoporous structure. It was reported that, during cycling, the active hydrophilic surfaces of exfoliated MC could effectively trap the LiPS, while the mesoporous matrix can arrest the LiPS diffusion and associated shuttle process through strong physical adsorption. In a similar effort, Pourali et al. have demonstrated Li 2 S/Ti 3 C 2 T x composite as a cathode material for Li-S batteries [112]. As anticipated, the cells showed superior electrochemical performance, which was attributed to the reduction in energy barrier for Li-ion transport though Li 2 S particles by the highly conductive 2D Ti 3 C 2 T x .
Catalysts 2020, 10, x FOR PEER REVIEW 18 of 29 delivered a high specific capacity of 1200 mAh g −1 at 0.2 C rate with a stable performance of up to 200 cycles. Further, given that the MCs are layered and puckered sheets of covalently bonded mixed atoms, availability of complete surface area is restrained during their restacking after exfoliation, thus effecting the ions accessibility to the surface. In this context, Bao et al. proposed a flexible delamination and reassembly method for MCs by exploring the highly porous MOF system [111]. For this purpose, they have introduced a metal carbide@ mesoporous carbon hybrid architecture loaded with sulfur (Ti3C2Tx@Meso-C/S). The proposed cathode delivered an initial capacity of 1225.8 mAh g −1 with a high reversible capacity of 704.6 mAh g −1 after 300 cycles. Simultaneously, a control meso-C/S cathode delivered an initial capacity of only 1006.8 mAh g −1, which decreased to 470.2 mAh g −1 after 300 cycles. The evident improvement in the electrochemical performance of the MC was ascribed to the synergetic effect of Ti3C2Tx nanosheets and the mesoporous structure. It was reported that, during cycling, the active hydrophilic surfaces of exfoliated MC could effectively trap the LiPS, while the mesoporous matrix can arrest the LiPS diffusion and associated shuttle process through strong physical adsorption. In a similar effort, Pourali et al. have demonstrated Li2S/Ti3C2Tx composite as a cathode material for Li-S batteries [112]. As anticipated, the cells showed superior  (Figure 10a) [101]. The sulfiphilic B 4 C nanowires were found to chemically absorb the LiPS, which were effectively confined in the B 4 C and CNF network. Detailed DFT calculations revealed that the B 4 C exhibits the highest binding energy (3.84 to 12.51 eV) towards Li 2 S 4 in comparison with the non-polar graphite surface (1.18 eV). Further, Bader charge analysis performed to understand the electron charge transfer between Li 2 S 4 and B 4 C confirmed the formation of chemical bonds between them, as evidenced by an increase in electron density between S and surface atoms of B 4 C. On the other hand, the inherent catalytic effect of B 4 C was found to be responsible for reducing the overpotentials while facilitating the redox kinetics of LiPS conversion. Additionally, the enhanced LiPS adsorption and subsequent conversion on the free-standing structure can eliminate the need for additional current collector or binders and provide room for increasing the sulfur loading. Based on the CV studies on a Li-S cell, the Li-ion diffusion coefficients were found to be one order higher for the B 4 C@CNF electrodes compared to the conventional Li-S cells, further revealing the accelerated redox kinetics on the MC surface. The improved redox chemistry and excellent polysulfide trapping are also reflected in the enhanced long cycling performance. At 1 C rate, the Li-S cells with B 4 C@CNF and CNF delivered an initial capacity of 1024 and 1021 mAh g −1 , respectively. But, the cell with CNF displayed a rapid capacity fade over cycling, delivering only 447 mAh g −1 after 500 cycles. On the other hand, B 4 C@CNF cell demonstrated and excellent reversibility and cycling stability, delivering 815 mAh g −1 of discharge capacity after 500 cycles.    Another new class of sulfur host materials, the MXenes, has gained much attention in recent times, given their inherently high conductivity and highly active 2D surface with their capability to chemically bond intermediate LiPS through metal-sulfur interactions. Liang et al. have demonstrated an MXene phase Ti 2 C as an effective sulfur host material [113]. As shown in Figure 10b, this material takes advantage of the high 2D electron conductivity of transition metal carbides and the exposed terminal metal sites to bind the LiPS. It was reported that the Ti-OH groups in the MXene Ti 2 C nanosheets are replaced by Ti-S at elevated temperatures during sulfur infusion. The electronegative S atoms decrease the electron density of Ti atoms, leading to higher binding energy between Ti and S compared to the Ti-C bond. Hence, effective anchoring of LiPS was expected on the Ti 2 C surface due to the Lewis acid-base interactions between Ti and S. Such an anchoring effect resulted in the long cycling performance, where an initial discharge capacity of 1090 mAh g −1 was noted at 0.5 C rate, with just 0.05% decay per cycle, the cell delivered a capacity of 723 mAh g −1 even after 650 cycles.
While these studies demonstrate the influence of various electrocatalysts on the enhancement of Li-S battery performance, it is equally important to improve the sulfur loading to realize the true potential of the electrocatalysts for real-world applications. In this context, based on the sulfur loading and current rate, Table 3 provides an overview of some contemporary electrocatalysts (metal, metal-oxide, metal-sulfide and metal-carbide) performance in terms of capacity, cycle life, and Coulombic efficiency. This information indicates that exploring heteroatom or new architecture based electrocatalytic materials can help realize high capacities and enable high sulfur loading capabilities.

Summary and Outlook
Controlling the liquid LiPS dissolution and subsequent shuttling phenomenon while ensuring high rate capability embraces the significance to realize high theoretical energy lithium−sulfur (Li−S) batteries with long cycle life. Towards this goal, it is highly desired to design a cathode architecture that has the simultaneous capability to trap the liquid LiPS while accelerating their conversion reaction kinetics during charge/discharge. This review provides an overview of the contemporary efforts made to capture the liquid LiPS and expedite the subsequent conversion reactions through various chemical anchoring strategies exhibited by several cathode surfaces. Metal nanoparticles dispersed in highly porous architecture and their interactions with LiPS during charge/discharge provides plenty of active sites for confining the LiPS, which helps realize a high sulfur loading. Additionally, the proven metal-sulfide interaction during the LiPS redox reaction not only reduces the LiPS shuttling phenomenon but also improves the reversibility of the electronically insulating end products on the cathode surface along with an enhanced active materials utilization. A combination of electronic conductivity and surface polarity in metal oxide-based host materials provides a bifunctional activity to the sulfur cathodes, which significantly mitigates the LiPS dissolution. Besides, this surface-enhanced redox catalysis of LiPS allows spatially controlled Li 2 S precipitation and forms an effective interface that significantly lowers the polarization potential during the charging process and improves the utilization of the active material. In search of a cost-effective and efficient catalyst for Li-S batteries, these bifunctional oxide type host materials are a promising approach towards high capacity and long cycle life Li-S batteries.
Inspired by the industrial hydrodesulfurization catalytic process with sulfiphilic functional groups, strong LiPS adsorption followed by their conversion is effectively enabled through sulfide-based metallic compounds. In contrast to oxides, sulfiphilic interactions creates a stronger affinity between metal sulfides and LiPS, thus ensuring their redox conversion process at a confined location along with inherent catalytic functions. This approach demonstrates that the mechanism involved in surface interaction holds a significant role in controlling the LiPS dissolution and associated shuttling issues. The advantages of employing this class of catalytic approach in Li−S batteries include (i) retention of the low-cost advantage of Li-S batteries by the economically viable metal sulfides; (ii) the possibility to tune the electrocatalytic activity of metal sulfide surface by adjusting the number of their active sites (iii) feasibility for scaling up the synthesis of such structures, making this approach easily adaptable for practical applications. Further, a synergetic effect between the hydrophilic nature and high electrochemical activity from the 3d electron number in transition metals renders metal carbides a unique capability to adsorb LiPS. More importantly, these metal carbides present themselves as effective and economically viable electrocatalysts for Li-S batteries, given the inherent catalytic activity of metal.
Though existing strategies effectively direct researchers to select appropriate cathode hosts for sulfur, several challenges remain for practical Li-S purposes. Further enhancement of the electrocatalysis concept in Li-S batteries requires consideration of the following factors: (i) As the intermediate LiPS forms with different chain lengths during the reactions and undergoes manifold (electro) chemical transformations, their binding strength varies from surface to surface. In such circumstances, the cathode surfaces no longer can offer a ubiquitous anchoring effect towards all the intermediate LiPS, and subsequently, some tend to undergo dissolution. Exploring the design principle for anchoring of LiPS on the cathode substrates that are capable of adsorbing all the intermediate LiPS while catalyzing the subsequent redox reactions is imperative to completely restrain the PS shuttle. (ii) The catalysts with high surface area and exceptional electronic conductivity need to be developed to promote Li + transportation in the inner parts of cathodes to access the active materials while facilitating the redox conversion reaction. This could provide an opportunity to achieve high sulfur loading with low electrolyte/sulfur ratio to realize high energy density Li-S batteries. Besides, maximizing the sulfur loading without compromising on the electrocatalytic activity holds the key to realize the effective utilization of the electrocatalysts and, ultimately, high energy density Li-S batteries. Additionally, catalytic cathodes should have a rigid structure with enough porosity to enable uniform distribution of active materials and accommodate volume changes during charge/discharge.
(iii) An in-depth understanding of the phenomenon occurring at the electrode/electrolyte interface with theoretical and sophisticated in situ measurements is vital to understand the interactions of LiPS with the catalytic cathodes in real-time. This could reveal essential information such as the nature of such interactions, LiPS reaction pathways on catalyst cathodes during the entire reactions, etc., which are essential to elucidate the accelerated reversible redox pathways of sulfur redox reactions.
Furthermore, the target application of the Li-S batteries, which include pouch-cells with high sulfur loading to deliver high energy density and battery packs for electric vehicles, surges the demand for the metallic lithium (Li) [121]. In this context, the current global Li resource was estimated to be around 39 million tones (Mt) [122]. It is estimated that Li consumption by the battery industry accounts for around 35% of the total Li usage, which is expected to grow to 66% by 2024 [123]. More precisely, demand for Li metal drastically increased over the years, with the requirement being 85 metric tons in 2018 while it was only 39.3 metric tons in 2016. Interestingly, the price of Li (per kWh) has been decreasing over the past few decades, for instance, a 30 kWh lithium-ion battery (LIB) cost over € 7500 in 2007, which reduced to € 800, 200 in 2010 and 2018 respectively, and is expected to drop to € 100 by 2025 [124]. Even with the current rapid rate of development in the battery industry, the global Li resources are estimated to suffice the current and near-future needs, with the highest requirement for the 2010 to 2100 period not exceeding 20 Mt [122]. However, despite their appealing advantages, LIBs are still expensive, and their short lifetime (<3 years) poses several ecological and humanitarian concerns due to the generation of massive wastes [125,126]. Hence, it would be beneficial to accelerate and explore the battery recycling methodologies, to not only reduce the pileup of hazardous battery components but also to support their supply chain and alleviate the depletion of natural resources towards a sustainable future.
In summary, electrocatalysis of LiPS redox reactions in Li-S batteries is a practical approach to mitigate the shuttling phenomenon and electronic conductivity issues during the charge/discharge processes. With the current advancement in the theoretical and experimental understanding and designing strategies, they are expected to guide the development of sophisticated catalytic surfaces and pave new pathways leading to the high performance and long cycle life Li-S batteries for commercial purposes.

Conflicts of Interest:
The authors declare no conflict of interest.