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
. 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
. 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 VO
on the surface of 2D reduced graphene oxide (rGO) by solvothermal method to construct heterostructures is a good solution
. Compared with the single component, the reaction kinetics of VO
is smoother. This is achieved by VO
with catalytic properties and 2D rGO with good conductivity, which reflects the superiority of the heterostructure. A heterostructure composed of CNTs and MoP
. 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 WS
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-N
4 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 Li
2S, thus weakening the Li-S bond, promoting the decomposition of Li
2S 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-N
4 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 Li
2S decomposition potential, Li-S bond parameters and adsorption energy for Li
2S
6. The results show that, except for the structurally unstable SACu@NG, SAAg@NG, the VN
4/NG-SAC group exhibits the lowest Li
2S decomposition barrier (1.10 eV) and the highest adsorption energy for Li
2S
6 (3.38 eV), i.e., the VN
4/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-N
4 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 Li
2S decomposition potential, Li-S bond parameters and adsorption energy for Li
2S
6. The results show that, except for the structurally unstable SACu@NG, SAAg@NG, the VN
4/NG-SAC group exhibits the lowest Li
2S decomposition barrier (1.10 eV) and the highest adsorption energy for Li
2S
6 (3.38 eV), i.e., the VN
4/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-N
2-C conformation with good adsorption of LiPSs and Li
2S 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 FeN
2 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 Li
2S, 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, NiCl
2 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 Li
2S.
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 Li
2S, 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.