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Song, Z.; Jiang, W.; Jian, X.; Hu, F. Advanced Nanostructured Materials for Electrocatalysis in Lithium–Sulfur Batteries. Encyclopedia. Available online: https://encyclopedia.pub/entry/41322 (accessed on 21 December 2024).
Song Z, Jiang W, Jian X, Hu F. Advanced Nanostructured Materials for Electrocatalysis in Lithium–Sulfur Batteries. Encyclopedia. Available at: https://encyclopedia.pub/entry/41322. Accessed December 21, 2024.
Song, Zihui, Wanyuan Jiang, Xigao Jian, Fangyuan Hu. "Advanced Nanostructured Materials for Electrocatalysis in Lithium–Sulfur Batteries" Encyclopedia, https://encyclopedia.pub/entry/41322 (accessed December 21, 2024).
Song, Z., Jiang, W., Jian, X., & Hu, F. (2023, February 17). Advanced Nanostructured Materials for Electrocatalysis in Lithium–Sulfur Batteries. In Encyclopedia. https://encyclopedia.pub/entry/41322
Song, Zihui, et al. "Advanced Nanostructured Materials for Electrocatalysis in Lithium–Sulfur Batteries." Encyclopedia. Web. 17 February, 2023.
Advanced Nanostructured Materials for Electrocatalysis in Lithium–Sulfur Batteries
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This entry is adapted from the peer-reviewed paper 10.3390/nano12234341

Lithium–sulfur (Li-S) batteries are considered as among the most promising electrochemical energy storage devices due to their high theoretical energy density and low cost. However, the inherently complex electrochemical mechanism in Li-S batteries leads to problems such as slow internal reaction kinetics and a severe shuttle effect, which seriously affect the practical application of batteries. 

lithium–sulfur batteries nanostructure electrocatalysis

1. Introduction

With the development of society and economy, traditional fossil energy such as coal and oil are continuously consumed, and problems such as environmental pollution and ecological damage have become more serious. Development of green and renewable new energy has become an important task at present [1]. Most common renewable types of energy (solar, wind, hydro and tidal energy, etc.) have uncontrollable shortcomings and cannot be used conveniently. The electrochemical energy storage system can realize the mutual conversion of chemical energy and electrical energy, so as to realize the transformation, storage and utilization of energy, which has become the focus of research [2][3]. Among them, lithium-ion (Li-ion) batteries were first commercialized by Sony in 1991, and are now widely used in electric vehicles, portable electronic devices, energy storage and other fields [4]. However, due to the limitations of the working principle and the materials themselves, the performance of Li-ion batteries has so far been close to the theoretical value.
Lithium–sulfur (Li-S) batteries have attracted extensive attention due to their excellent theoretical capacity (1675 mAh g−1) and energy density (2600 Wh kg−1), as well as the environmental friendliness and cost-effectiveness of elemental sulfur [5][6][7]. A typical Li-S battery that provides high capacity through multi-electron exchange reactions usually uses metallic lithium as the anode and sulfur-containing material as the cathode, and its electrolyte is mostly an organic liquid electrolyte [8][9]. According to the different sulfur forms in the cathode, the reaction mechanism of Li-S batteries is usually divided into two categories: the first is the “(quasi) solid phase” transformation mechanism, which mostly uses atomic/small-molecule sulfur cathodes represented by sulfurized polyacrylonitrile, and the electrolyte are mainly esters [10][11]. Sulfur is confined in the form of atom/small-molecule clusters or fixed by chemical bonds in porous carbon or polymer frameworks [12]. The charge–discharge process is manifested as a solid-phase transition between sulfur and lithium sulfide (Li2S), and the discharge curve presents a single platform [13][14] (Figure 1). The second type of reaction process is the “solid–liquid–solid” transformation mechanism, which is more common [15][16][17]. Most composite materials containing elemental sulfur are used as the cathode electrode, and the ether solvent with good solubility for the intermediate-chain lithium polysulfide product (Li2Sx, x = 4~8) is used as the electrolyte [18][19][20][21]. During the discharge process, the solid elemental sulfur is gradually reduced to Li2Sx, which is dissolved in the electrolyte, and the active material is separated from the conductive substrate. After that, Li2Sx is further reduced to form Li2S with low solubility, which is redeposited on the conductive substrate [22][23][24]. The specific reaction process can be represented by the following chemical equation:
S8 (solid) + 2Li+ + 2e− → Li2S8 (liquid)
3Li2S8 (liquid) + 2Li+ + 2e− → 4Li2S6 (liquid)
Li2S8 (liquid) + 2Li+ + 2e− → 2Li2S4 (liquid)
2Li2S6 (liquid) + 2Li+ + 2e− → 3Li2S4 (liquid)
Li2S4 (liquid) + 2Li+ + 2e− → 2Li2S2 (solid)
Li2S2 (solid) + 2Li+ + 2e− → 2Li2S (solid)
/media/item_content/202302/63ef10423ef76nanomaterials-12-04341-g001.png
Figure 1. (a) The reaction mechanism of Li-S batteries. (b) Electrocatalysis in Li-S batteries.
The process exhibits a typical “solid–liquid–solid” multi-phase transformation mechanism, and the discharge curve presents two plateaus, corresponding to the solid–liquid transition and the liquid–solid transition, respectively [25][26] (Figure 1a). In particular, the first type of mechanism is highly dependent on the dispersion state of atomic/small-molecule sulfur, which greatly limits the sulfur content of the cathode [27]. 
The sluggish redox reaction kinetics causes a large amount of polysulfides dissolved in the electrolyte to fail to react rapidly and to diffuse to the anode under the influence of electric field force and concentration gradient, resulting in irreversible loss of active materials and consumption of a large amount of electrolyte [28][29]. The process, known as the shuttle effect, is the biggest problem that limits the performance improvement of Li-S batteries [30][31][32]. Meanwhile, the drastic change in cathode volume and the infinite growth of anode dendrites during the charge–discharge process further degrade the performance of Li-S batteries in practical applications [33][34]. In order to solve the above problems, strategies such as physical confinement and chemical adsorption have been proposed successively [35][36][37][38]. These strategies mainly include constructing cathodes with novel nanostructures to confine polysulfides [36]; modifying separators to suppress the shuttle effect [33]; introduction of additional interlayer materials with special properties to anchor polysulfides [39][40]; rationally designing novel electrolytes to confine polysulfide shuttles [41][42]; and constructing robust artificial solid electrolyte interfaces on Li anodes to suppress lithium dendrites [43]. Although the above strategies alleviate the existing problems of Li-S batteries to a certain extent through adsorption, the slow redox kinetics and the severe shuttle effect still cannot be completely solved, which inevitably leads to low-rate performance and a short cycle life [44][45][46]. 

2. Heterogeneous Catalysts

In chemical concepts, when the catalyst is in a different phase with the reactants, it is called a heterogeneous catalyst. For Li-S batteries, heterogeneous catalysts generally exist in the reaction system in the form of solid-state catalysts, which are insoluble in the electrolyte during the reaction process. Therefore, the heterogeneous catalysts in Li-S batteries are generally compounded on the cathode, separator, or in the form of interlayers (Figure 2). Heterogeneous catalysts mostly accelerate the chemical reaction rate of Li-S batteries by adsorbing dissolved polysulfides and promoting the transfer of electrons/ions to achieve rapid conversion between sulfur-containing species [47][48].
/media/item_content/202302/63ef1056e5c76nanomaterials-12-04341-g002.png
Figure 2. Schematic diagram of Pt-catalyzed polysulfide conversion.

2.1. Metal Compounds

Metal materials have become among the most used electrocatalysts in industry due to their high electrical conductivity and good coordination. Traditional noble metal materials (Pt, Au, etc.) are widely used as catalysts in various electrochemical processes such as electroreduction and water electrolysis [49][50][51]. Inspired by this, traditional electrocatalyst Pt nanoparticles (NPs) [52] are loaded on graphene as the cathode host of Li-S batteries, and its specific capacity was found to be approximately 40% higher than that of conventional graphene/sulfur composite cathodes. This indicates that rational electrocatalysis can improve the performance of Li-S batteries. The modification of Au NPs on commercial acetylene black/sulfur composite cathodes (CB-S-Au) is also capable of electrocatalysis [53]. Rapid conversion of polysulfides is achieved by regulating the chemistry between Au NPs and polysulfides, thus improving the performance of Li-S batteries. However, noble metal materials are expensive and difficult to use for large-scale production of Li-S batteries [36][54]. Therefore, exploring cost-effective electrocatalysts for Li-S batteries is among the current research hotspots. Metal compounds, which are abundant in yield and relatively low cost, have been widely used in the field of Li-S battery electrocatalysis as substitutes for noble metals.

2.1.1. Metal Oxide

As the most common metal compounds, metal oxides are widely used in the field of Li-S battery electrocatalysis [55][56][57][58][59]. Metal oxides have large electrochemically active surfaces and contain hydrophilic groups. At the same time, the oxygen atom is a good electron acceptor, and it is easy to chemically bond with lithium [60][61]. This can increase the binding energy with polysulfides and decrease the kinetic barrier for Li2S nucleation, which is beneficial to the deposition and transformation of Li2S [62]. In addition, the cost of general metal oxides is relatively low, which is convenient for the large-scale application of catalysts.
Iron-based oxides are among the most common types of metal oxides. Loading α-Fe2O3 NPs on three-dimensional (3D) porous graphene as the cathode of Li-S batteries can achieve electrocatalytic effects [56]. The 3D structure can increase the contact area of the electrocatalyst and improve the reaction kinetics. In addition, there is a strong interaction between Fe2O3 NPs and polysulfides, which can accelerate the rapid conversion of polysulfides to insoluble Li2S and effectively suppress the shuttle effect. In addition to 3D structures, one-dimensional (1D) nanostructures can also promote electrocatalysis. The introduction of Fe3O4-encapsulated NPs into 1D carbon nanotubes (CNTs) as hosts for Li-S batteries can effectively accelerate the electrochemical reactions inside the batteries [62]. 
In addition to iron-based oxides, dioxides such as MnO2 and TiO2 have also been used in the electrocatalysis of Li-S batteries [63][64][65][66]. Two-dimensional (2D) ultrathin MnO2 nanosheets were shown to accelerate the conversion of polysulfides to insoluble Li2S/Li2S2 [64]. Different from the adsorption–catalysis principle, this process is mainly due to the in situ reaction of polysulfides on the 2D ultrathin MnO2 surface to generate thiosulfate. The surface thiosulfate groups form active polythionates complexes in subsequent reactions and acts as anchoring and transfer mediators to inhibit the dissolution of polysulfides into electrolytes. The planar structure of 2D materials is more favorable for the deposition of reactants. Li-S batteries with sulfur/MnO2 nanosheet composites as cathodes exhibit high specific capacity and good cycling stability. At the same time, it is also demonstrated that the mechanism is applicable to graphene oxide, which has broad applicability in many 2D materials. Titanium-deficient anatase TiO2 (TDAT) as a separator modification material can also improve the electrochemical performance of Li-S batteries. [67]. 

2.1.2. Metal Nitride

Metal nitrides have also attracted extensive attention in the field of Li-S battery electrocatalysis due to their unique electronic structures, excellent chemical stability, and outstanding mechanical strength [68]. Unlike metal oxides, metal nitrides tend to have better electrical conductivity, which facilitates the transport of ions/electrons [69]. Meanwhile, abundant nanostructures such as face-centered cubic, hexagonal closed packed, and simple hexagonal also endow metal nitrides with unique physical/chemical properties [70]. Transition metal nitrides, including VN [71], TiN [72], and WN [73], have been widely used in the field of electrocatalysis for Li-S batteries.
VN is a material with high catalytic activity, and its catalytic activity is comparable to that of noble metal materials. The composite of VN nanoribbons with porous graphene and used as cathode materials for Li-S batteries can effectively accelerated the reaction kinetics inside the batteries [71]. VN possesses both high electrical conductivity and catalytic properties, enabling the rapid reaction of polysulfides on its surface. The compatibility of 2D graphene with 2D VN nanoribbons also greatly improves the mechanical strength of the material and enables self-supporting of the cathode. In addition to VN, vanadium nitride oxides (VOxNy) have also received a lot of attention recently [74]. Unlike other electrocatalytic materials, VOxNy exhibit a redox potential window between their oxide counterparts, around which polysulfides are able to form polysulfide complexes and enhance kinetics and polysulfide anchoring through the V–N and V–O interfaces. TiN is a metal nitride with high mechanical properties and is often used as a coating material.

2.1.3. Metal Sulfides

Metal sulfides, as a class of effective catalysts, are widely used in anticorrosion, hydro-desulfurization and solar cells [75][76]. In recent years, metal sulfides have also been widely used as electrocatalysts for Li-S batteries due to their high chemical stability of sulfur [36]. Metal sulfides can form active complexes through chelation with polysulfides, thereby reducing the solvation degree of polysulfides [77]. At the same time, metal sulfides can also promote the redox kinetics inside Li-S batteries. In addition, metal sulfides with nanostructures also have the advantages of low redox potential and high specific capacity [78]. A Co9S8 with interconnected graphene-like nanostructures was synthesized and demonstrated to interact strongly with polysulfides when used as a cathode [79]. Compared with conventional porous carbon, the cyclability of the battery is improved by 10 fold when Co9S8 with a hierarchical nanostructure is used as the cathode. More importantly, after the adsorption of polysulfide by Co9S8, cobalt and sulfur can act as “thiophilic” and “lithiophilic” sites, respectively, to undergo an internally couped charge transfer process, thus achieving higher electrical conductivity than metal oxides. This proves that some metal sulfides with high electrical conductivity can be used as cathode materials for Li-S batteries.
With the increasing abundance of synthetic methods for 2D metal sulfides in recent years, more and more metal sulfides with special nanostructures have been used for electrocatalysis in Li-S batteries. Metal dichalcogenides, as a class of metal sulfides with 2D layered structures, have been used in Li-S battery electrocatalysis due to their highly exposed surface atoms and high electrical conductivity of metalloids. Due to the unique 2D structure, the structural edge and surface of metal dichalcogenides often exhibit different electrocatalytic activities. Therefore, there is a difference in the properties [80] between the unsaturated-coordinated edge metal sites and the saturated-coordinated surface metal sites in MoS2. The adsorption capacity of the edge unsaturated sites was found to be much larger than that of the surface saturated sites, which proved that the adsorption capacity was affected by the chemical environment. 

2.1.4. Metal Phosphides

In the periodic table, almost all transition metals can react with phosphorus to form metal phosphides [81]. Flexible components and structures make metal phosphides often exhibit excellent properties such as magnetism and electrical conductivity. In addition, many metal phosphides have excellent properties similar to those of metal dichalcogenides due to similar bonding methods [82]. This enables the application of metal phosphides in the electrocatalysis of Li–S batteries. However, it is worth noting that the catalytic site of metal phosphide is not in its bulk phase, but at the oxidized compound on its surface [83]. To achieve efficient exposure of active sites to increase electrocatalytic performance, various nanostructures have been developed and utilized. In 2018, Co-O-P was found to be formed in surface-oxidized CoP [84], which enables Co sites to interact strongly with polysulfides and generate Co-S bonds. For the unoxidized CoP, the Co and P sites cannot interact with polysulfides due to the weak polarization of the Co-P bond. Therefore, the oxidized CoP has better redox kinetics than the unoxidized CoP. In addition, the special structure of the carbon nanoarray-coated FeP materials [85] can facilitates the fast conduction of ions/electrons. Meanwhile, the presence of Fe-O and O-P bonds in the XPS spectra indicated that the FeP nanoarrays were oxidized.

2.2. Heterostructures

Although a single type of electrocatalyst shows a certain improvement in electrochemical performance, its properties such as electrical conductivity, specific surface area and nanostructure are relatively fixed, which makes the modification of materials difficult to achieve and adversely affects battery performance. In addition, it is difficult for single-component electrocatalysts to combine the two functions of efficient adsorption and rapid conversion. Therefore, the design of heterostructure electrocatalysts composed of two or more components can overcome the limitation of the function of a single component [86][87][88]. The synergistic effect of multi-component materials to exert different functions can further improve the performance of Li-S batteries [37].

2.2.1. Metal-Based Heterostructures

Metal compounds have received a lot of attention in the electrocatalysis of Li-S batteries due to their diverse types and wide sources. However, single metal compounds often have their limitations, and various properties cannot be optimal at the same time. In response to this problem, the concept of bifunctional heterogeneous structure was proposed [89]. The combination of metal oxides with high adsorption and metal nitrides with high conductivity enables the effective preparation of heterogeneous structured electrocatalysts. The combination of TiO2 for the adsorption of polysulfides and TiN for catalytic polysulfide conversion enables an efficient adsorption–catalysis process. The heterostructure not only improves the poor conductivity of metal oxides, but also improves the adsorption of metal nitrides. Based on this idea, a checkerboard-like heterostructure electrocatalyst [90] was designed by the sulfonation reaction. The electrocatalyst is composed of CoS2 and Co, and the CoS2-Co heterostructure has strong adsorption and catalytic effects on polysulfides.

2.2.2. Carbon-Based Heterostructures

Carbon materials have become among the most used substrates in heterostructures because of their diverse types, rich structures, good conductivity, and large specific surface area [37][91]. Common carbon-based heterostructures refer to hybrid materials that combine carbon materials with other materials (mainly metal compounds). Commonly used carbon materials mainly include CNTs, graphene and activated carbon [92][93]. Compared with metal-based heterostructures, carbon-based heterostructures have more abundant synthesis methods and more diverse structures. In addition, the good processability of carbon materials also provides a broader space for the application of carbon-based heterostructures. Aiming at the problem of good adsorption but poor conductivity of VO2, in situ growth of VO2 on the surface of 2D reduced graphene oxide (rGO) by solvothermal method to construct heterostructures is a good solution [94]. Compared with the single component, the reaction kinetics of VO2@rGO is significantly improved, and the diffusion of Li+ is smoother. This is achieved by VO2 with catalytic properties and 2D rGO with good conductivity, which reflects the superiority of the heterostructure. A heterostructure composed of CNTs and MoP2 NPs was combined and applied to the interlayer of Li-S batteries [95]. Benefit from the synergistic effect of the heterostructure, the cycle performance of batteries is improved.

2.2.3. MOFs/COFs-Based Heterostructures

Metal-organic frameworks (MOFs) are coordination polymers self-assembled by transition metal ions and organic ligands containing oxygen, nitrogen, and other elements [96]. As a new class of porous materials, MOFs have attracted much attention in the field of energy storage due to their tunable nanostructure and functionalization [97][98]. MOFs-based heterostructures are MOFs carbonized in an inert environment to generate MOFs-derived carbon-metal framework structures with high porosity. The framework structure can be customized according to the nanostructure of the precursor, and simultaneously contains heteroatoms and metal ions. This enables MOFs-based heterostructures to exert dual confinement and catalytic effects on polysulfides. Carbonization of MOF precursors is a common method to prepare MOF-based heterostructures.
Carbon materials have become among the most used substrates in heterostructures because of their diverse types, rich structures, good conductivity, and large specific surface area [37][91]. Common carbon-based heterostructures refer to hybrid materials that combine carbon materials with other materials (mainly metal compounds). Commonly used carbon materials mainly include CNTs, graphene and activated carbon [92][93]. Compared with metal-based heterostructures, carbon-based heterostructures have more abundant synthesis methods and more diverse structures. In addition, the good processability of carbon materials also provides a broader space for the application of carbon-based heterostructures. Aiming at the problem of good adsorption but poor conductivity of VO2, in situ growth of VO2 on the surface of 2D reduced graphene oxide (rGO) by solvothermal method to construct heterostructures is a good solution [94]. Compared with the single component, the reaction kinetics of VO2@rGO is significantly improved, and the diffusion of Li+ is smoother. This is achieved by VO2 with catalytic properties and 2D rGO with good conductivity, which reflects the superiority of the heterostructure. A heterostructure composed of CNTs and MoP2 NPs was combined and applied to the interlayer of Li-S batteries [95]. Benefit from the synergistic effect of the heterostructure, the cycle performance of batteries is improved. In addition to binary heterostructures, carbon-based ternary heterostructures also show good properties. The 3D network structure composed of CNTs and rGO exhibits considerable advantages in the fast transport of Li+ and the loading of S. Based on this idea, a 3D porous WS2-rGO-CNTs ternary heterostructure was designed [99], in which rGO is chemically bonded to CNTs and WS2 is grown in situ on CNTs.

2.3. Single Atoms

Defect-rich metal compound electrocatalysts as well as heterostructure electrocatalysts with complementary properties can provide a large number of active sites for enhancing the redox kinetics of Li-S batteries, thus significantly improving the overall electrochemical performance [100][101]. However, the relative content of defects limits the specific catalysis activity of these catalysts, and affects the further improvement of rate performance and energy density. Therefore, making the catalyst more defective while reducing its overall size can form more active sites and increase the specific activity of the catalyst [102]. Based on this theory, single-atom catalysts (SACs) obtained by combining individual metal atoms with catalyst supports can achieve the most efficient utilization of catalyst specific surface area and metal atoms, and their adverse effects on energy density can be minimized due to the light weight of SACs [103][104][105], which together prove that the SACs is a promising research direction in the field of Li-S battery electrocatalysis.
Notably, the uniformly distributed isolated atoms, although having extremely small particle size, have a tendency to aggregate into clusters or particles due to their thermodynamic instability, which can reduce the specific catalytic activity of the catalyst. For this reason, the interaction between isolated atoms and supports was proposed in 1978, through which the movement and aggregation of isolated atoms can be prevented and a uniform and stable distribution of active sites can be constructed, thus positively affecting the selectivity, stability and electrocatalytic activity of the catalysts [106][107][108][109]. Therefore, the selection of coordination configurations with appropriate metal-atom–support interactions is also crucial for the practical application of SACs.

2.3.1. Saturated Coordination Configuration

Typical SACs usually use carbon materials as supports in which they are coordinated with other atoms and form saturated coordination configurations with symmetric electron distribution (most commonly the M-N4 configuration, M stands for metal atoms and N for coordination atoms such as N, O and S atoms) [110][111]. SACs with saturated coordination configurations were first applied to Li-S batteries in 2018, when single-atom Fe (SAFe) coordinated with four pyrrole N on the surface of the supports was used as an electrocatalyst to improve the rate performance and cycling lifetime of the battery, demonstrating the electrocatalytic effect of SAFe on the redox reaction of LiPSs [112]. To investigate the intrinsic mechanism of SACs in the electrochemical reaction of Li-S batteries, a novel theoretical simulation study was proposed, which demonstrated that SAFe can coordinate with Li2S, thus weakening the Li-S bond, promoting the decomposition of Li2S and the growth of LiPSs chains, and that it can regenerate by gaining electrons during the charging phase [113]. A reduction in the reaction energy barrier by SACs can effectively improve the rate performance, cycling performance and sulfur utilization of Li-S battery, thus triggering researchers to further investigate their preparation, catalytic mechanism and structural design. Generally, the catalytic activity of SACs depends on the intrinsic activity of the metal atom, the type of coordination atom, and the coordination configuration, and the precise design of SACs can be achieved by modulating these factors [114][115].
As the center of the active sites provided by SACs, the catalytic activity of the metal atoms themselves and the coordination stability have a direct impact on the electrocatalytic activity of SACs. Therefore, based on the purpose of screening the most suitable metal atoms for the preparation of SACs for catalytic Li-S electrocatalysis, a novel theoretical model verified the effect of SACs constructed with different monometallic atoms in saturated coordination configurations on the electrocatalytic capacity of Li-S batteries [116]. SACs (M-N4 configuration, M represents Fe, Mn, Ru, Zn, Co, Cu, V, Ag) obtained by loading different monometallic atoms on a conductive support were subjected to calculations of the Li2S decomposition potential, Li-S bond parameters and adsorption energy for Li2S6. The results show that, except for the structurally unstable SACu@NG, SAAg@NG, the VN4/NG-SAC group exhibits the lowest Li2S decomposition barrier (1.10 eV) and the highest adsorption energy for Li2S6 (3.38 eV), i.e., the VN4/NG-SAC has theoretically stronger electrocatalytic capacity and inhibition of the shuttle effect, which is fully consistent with the excellent discharge capacity of 1230 mAh g−1 (0.2 C) and the good cycling stability (0.073% capacity decay per cycle after 400 cycles) exhibited by the S@VN4/NG-SAC cathode in electrochemical tests. As the center of the active sites provided by SACs, the catalytic activity of the metal atoms themselves and the coordination stability have a direct impact on the electrocatalytic activity of SACs. Therefore, based on the purpose of screening the most suitable metal atoms for the preparation of SACs for catalytic Li-S electrocatalysis, a novel theoretical model verified the effect of SACs constructed with different monometallic atoms in saturated coordination configurations on the electrocatalytic capacity of Li-S batteries [116]. SACs (M-N4 configuration, M represents Fe, Mn, Ru, Zn, Co, Cu, V, Ag) obtained by loading different monometallic atoms on a conductive support were subjected to calculations of the Li2S decomposition potential, Li-S bond parameters and adsorption energy for Li2S6. The results show that, except for the structurally unstable SACu@NG, SAAg@NG, the VN4/NG-SAC group exhibits the lowest Li2S decomposition barrier (1.10 eV) and the highest adsorption energy for Li2S6 (3.38 eV), i.e., the VN4/NG-SAC has theoretically stronger electrocatalytic capacity and inhibition of the shuttle effect, which is fully consistent with the excellent discharge capacity of 1230 mAh g−1 (0.2 C) and the good cycling stability (0.073% capacity decay per cycle after 400 cycles) exhibited by the S@VN4/NG-SAC cathode in electrochemical tests. 
In addition to supersaturated coordination configurations, unsaturated coordination configurations also have good applications in the design of SACs for Li-S batteries. Single-atom catalysts with Mo-N2-C conformation with good adsorption of LiPSs and Li2S and improved conversion kinetics were applied to obtain Li-S batteries with high reversible capacity (743.9 mAh g−1 at 5 C rate) and long cycling lifetime (after 550 cycles with a capacity decay of only 0.018% per cycle) [117]. Through a combination of supporting structure design and coordination environment modulation, SACs with FeN2 unsaturated coordination structures were introduced onto graphene with a pore-rich structure, achieving uniform Li transport and effective adsorption of LiPSs [118].

3. Effect of Nanostructure on Heterogeneous Electrocatalysts

Nanostructures are small structures with physical dimensions below 100 nm. At the nanoscale, the structure and properties of materials often change in many ways. Therefore, nanostructured materials have broad application prospects in many fields. According to the dimensional classification, nanostructures can be generally classified into 0D, 1D, 2D, and 3D. The 0D nanostructures mainly include quantum dots (QDs), nanoparticles (NPs) and nanoclusters; 1D nanostructures include nanowires, nanorods, nanotubes and nanofibers; 2D nanostructures mainly include nanosheets and nanoribbons; 3D nanostructures are usually composed of the above three dimensions of nanostructures. In the field of electrocatalysis of Li-S batteries, electrocatalysts with different nanostructures usually show different catalytic properties and advantages. Due to the large number of exposed catalytic active sites, the electrocatalysts with 0D nanostructures can improve the catalytic performance while minimizing the amount of catalyst. Such structures are common in the field of single-atom catalysis. Electrocatalysts with 1D and 2D nanostructures have characteristic morphologies and have considerable advantages in ion/electron transfer.
In particular, the electrocatalytic properties of typical cathode materials should be explored in more detail. Here, the sulfur content of the cathode, the electrocatalyst content of the cathode and the specific surface area are listed to facilitate a clearer comparison of the nanostructures of various types of non-homogeneous catalysts in relation to the capacity of the battery. It can be seen that single-atom electrocatalysts usually have a greater advantage in terms of specific surface area and catalyst usage compared to the other two types of electrocatalysts. Because single-atom electrocatalysts expose more active sites, their specific surface area tends to be larger, and their usage in the cathode is lower, enabling them to catalyze electrochemical reactions within Li-S batteries more efficiently. Various types of single-atom electrocatalysts have been designed and reported, and the performance of Li-S batteries has been gradually improved. Certainly, single-atom electrocatalysts also have the disadvantages of complicated synthesis methods and high cost.

4. Homogeneous Electrocatalysts

Although heterogeneous electrocatalysts that are capable of adsorbing polysulfides and reducing the activation energy of bidirectional conversion reactions have shown many advantages in enhancing the practical electrochemical performance of Li-S batteries, they still have some drawbacks due to the limitation of insufficient active sites and the microstructures of materials, which include low practical utilization efficiency and gradual failure in long cycles.
Homogeneous electrocatalysts are in the same phase as polysulfides in Li-S batteries, which allows them to be fully exposed to electronic conductors, ionic conductors and polysulfides, and thus are expected to be free from reliance on active sites and avoid failure by gradual coverage with insulating products. During the electrochemical reaction of Li-S batteries, homogeneous electrocatalysts can achieve faster redox kinetics by constructing additional electronic pathways or combining with active materials to directly change the reaction pathways in the system.
Even though the homogeneous-type electrocatalysts completely dissolved in the electrolyte have impressive merits, the performance loss caused by internal shuttling cannot be ignored, therefore the semi-immobilized-type electrocatalysts with the characteristics of both heterogeneous and homogeneous electrocatalysts have been developed. As another type of homogeneous electrocatalysts, semi-immobilized electrocatalysts can ensure sufficient active sites at sulfur anode while achieving uniform and sufficient electrocatalytic effect, which couples the heterogeneous and homogeneous processes well, so it is a promising research direction in the field of Li-S battery electrocatalysis.

4.1. Homogeneous-Type Electrocatalysts

As elaborated above, the redox reaction of active materials in Li-S system can only occur at the three-phase interface between the ionic conductor, the electronic conductor, and the active materials, while homogeneous-type electrocatalysts in the same phase as the active materials can effectively utilize the active sites and promote both nucleation and growth of Li2S, usually by building additional electronic pathways with faster redox kinetics or directly changing the reaction path. Nickel dimethoxyethane chloride adduct (NiDME) were selected as homogeneous catalysts for accelerating the redox conversion of sulfur as well as for inhibiting the shuttle effect of polysulfides [119]. For one thing, NiDME dissolved in the electrolyte phase can fully bind to the active materials and effectively reduce the activation energy of the multi-stage electrochemical reaction of sulfur, thus greatly accelerating the redox reaction rate. For another, NiCl2 formed by dissociation of NiDME has higher affinity for LiPSs than for electrolytes and lower affinity for elemental sulfur than for electrolytes, thus enabling excellent recycling and effective capture of LiPSs. To verify the catalytic ability of NiDME for electrochemical processes, Tafel curve fitting and calculation of the activation energy of the discharge process were performed from the data of cyclic voltammetry tests, and these two plots demonstrated that the NiDME catalyst greatly enhanced the conversion efficiency of LiPSs through a significant reduction in the activation energy. The high catalytic activity of NiDME was further demonstrated by in situ UV tests in agreement with the theoretical calculations. It is concluded that NiDME is an active and practical electrocatalyst for trapping LiPSs, accelerating their conversion at the homogeneous interface and regulating the homogeneous deposition of Li2S.

4.2. Semi-Immobilized-Type Electrocatalysts

In Li-S batteries, the redox reaction of intrinsic polysulfides is a multi-step reaction involving solid–liquid–solid phase changes with both heterogeneous and homogeneous processes. From this perspective, catalysts capable of regulating both processes simultaneously and coupling heterogeneous electrocatalysis with homogeneous electrocatalysis capabilities, are considered as a promising research direction for accelerating redox kinetics.
A semi-immobilization strategy for the copolymerization of small-molecule imides with medium-chain polyethers to form space-constrained but catalytically active semi-immobilized RMs (PIPEs) was firstly proposed [120]. In PIPE-mediated electrochemical reactions, imides with appropriate redox potentials can achieve sufficient electrocatalytic acceleration of the active species by chain swinging, while polymer macromolecules do not suffer losses caused by shuttle effects due to the semi-constrained nature of polyethers. By injecting PIPE into a fully discharged Li-S battery and further discharging it to 1.7 V, the result that PIPE can release an additional 140 mAh g−1 specific capacity was obtained, which directly demonstrates the excellent electrocatalytic ability of PIPE to enable full electrochemical reduction of unreacted polysulfides.
Based on the semi-immobilization strategy, the semi-immobilized electrocatalyst were further extended to heterogeneous electrocatalysis, which verified the feasibility of coupling homogeneous electrocatalysis with non-homogeneous electrocatalysis [121]. Specifically, G@ppy-por electrocatalysts with semi-fixed active sites were prepared by covalently grafting porphyrin molecules onto graphene collectors and using polypyrrole linkers as active sites. From the LSV curves, it can be seen that the G@ppy-por electrocatalyst-mediated cell exhibited the highest current response in both the kinetic and diffusion control regions during the SRR process, demonstrating the potent trapping and kinetic acceleration of the polysulfide by the porphyrin active site.

5. Effect of Molecular Structure on Homogeneous Electrocatalysts

5.1. Building Fast Electronic Pathway Types

Such electrocatalysts in Li-S batteries are usually able to construct electron exchange paths with accelerated kinetics between the active material and the current collector and induce three-dimensional deposition of Li2S, and such catalytic mechanisms are usually determined by the following molecular structures: imide structures [120][122], anthraquinone structures [123][124], porphyrin structures [121], transition metal metallocenes, etc. [125][126].
Take PIPE, an electrocatalyst with an imide structure, as an example: in the electrochemical reaction, PIPE exhibits a two-electron reaction mechanism similar to that of pyromellitic dimide due to the intrinsic reactivity of carbonyl groups in the imide segments. Combining the electrochemical reaction mechanism with its CV curves shows that the reduction peaks at 2.46, 2.33 V and 2.01, 1.9 V vs. Li/Li+, corresponding to PIPE from neutral to monoanion (r1PIPE) and from monoanion to dianion (r2PIPE), respectively, appear, which corresponds to the entire charge–discharge potential window of sulfur in the electrolyte (usually at 2.3–2.4 V and 2.0–2.1 V) properly.

5.2. Change the Electrochemical Reaction Mechanism Types

In Li-S batteries, such electrocatalysts usually combine with active substances to form specific intermediates that directly modify the inferior redox kinetics of intrinsic polysulfides, and such catalytic mechanisms are usually determined by the following molecular structures: disulfide structures [127], phenylselenides [128], ethyl viologen structure [129], etc.
Take diphenyl diselenide DPDSe as an example: in electrochemical reactions, DPDSe reacts spontaneously with lithium polysulfide to form soluble lithium phenyl selenide polysulfide LiPhSePSs [128]. For the redox reactions of long-chain polysulfides, DPDSe exhibits excellent redox mediating ability due to the significant advantage of LiPhSePSs formed under Se–S interactions over intrinsic polysulfides in terms of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels.

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Subjects: Electrochemistry
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Zihui Song
,
Wanyuan Jiang
,
Xigao Jian
,
Fangyuan Hu
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Update Date: 20 Feb 2023
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