Contents
2 The principle and problems
3 Catalytic mechanism and materials
3.1 Adsorption⁃transformation mechanism
3.2 Redox⁃mediated mechanism
3.3 Other catalytic mechanisms
4 Characterization technology and research method of catalytic mechanism
4.1 Electrochemical characterization
4.2 Characterization of materials and structures
4.3 Theoretical simulation method
1 Introduction
In today's society, the problems of environmental pollution and resource shortage are gradually aggravated. With the enhancement of environmental awareness and crisis awareness, the energy structure dominated by fossil energy is gradually changing to renewable clean energy such as solar energy, wind energy and biomass energy. However, due to the contradiction between energy supply and demand in time and space, energy storage devices have become an indispensable part of the new energy system. As a portable energy storage component, secondary batteries provide great convenience for energy storage and utilization. Lithium-ion batteries (LIB) have attracted much attention due to their high energy density and long cycle life since they were developed in 1991. Over the past three decades, lithium-ion batteries based on graphite negative electrodes have achieved great commercial success in powering portable electronic devices
[1]. Since 2017, new energy vehicles based on lithium-ion batteries have developed rapidly. For this reason, many countries have planned development goals for the range (> 500 km), charging time (10 ~ 15 min) and energy storage cost (<$80 kWh
-1) of LIB-powered electric vehicles, and accordingly require the battery energy density to reach the 400 Wh·kg
-1[1][2]. However, the energy density of LIB is limited to about 300 Wh·kg
-1 due to the large relative molecular mass of transition metal oxide cathode and the limited lithium storage capacity of graphite anode
[3].
In order to achieve the above goals, lithium metal batteries have been widely studied. Lithium-sulfur battery (LSB) is a typical Lithium metal battery, which is similar to the traditional lithium-ion battery in structure
[4]. The LSB uses elemental sulfur and metal lithium as the positive and negative electrodes respectively, and the theoretical specific capacity of elemental sulfur can reach 1675 mAh·g
-1, and the theoretical energy density is as high as 2600 Wh·kg
-1. Because elemental sulfur has the advantages of abundant reserves, low price and environmental friendliness, LSB can effectively reduce the cost of energy storage and avoid environmental pollution, and realize energy storage and utilization with large capacity, high performance and low cost
[5].
2 Working principle and problems
The lithium-sulfur battery mainly comprises a sulfur positive electrode, a separator, a lithium negative electrode and an electrolyte (fig. 1), wherein the electrolyte generally uses a 1,3-dioxolane (DOL)/dimethylether (DME) -based lithium bis (trifluoromethanesulfonate) imide (LiTFSI) solution.
The electrochemical reaction equation of the lithium-sulfur battery is as follows:
Total reaction :S8+16Li 8Li2S
Anode :S8+16Li++16e-8Li2S
Elemental sulfur usually exists in nature as a cyclic S
8 formed by eight sulfur atoms. The discharge process is a reduction process in which the sulfur-containing material is lithiated, and the S
8 undergoes a two-platform electrochemical reaction, passes through the Lithium polysulfides (LiPSs) intermediate state, and is finally reduced to Li
2S. According to the typical charge-discharge curve of the lithium-sulfur battery (Fig. 2), the discharge process of the lithium-sulfur battery can be divided into three stages: the first stage is a solid-liquid phase transition process, in which the solid S
8 gains electrons, the ring structure opens, and a long-chain Li
2S
8 soluble in the electrolyte is generated, which corresponds to a discharge plateau at about 2.4 V
[5,7]. The second stage is the liquid phase transformation process, in which the Li
2S
8 undergoes a series of complex reduction reactions and rapidly transforms into Li
2S
4 through liquid intermediate States such as S
62- and S
52-. As the LiPSs changed from long chain to short chain, the voltage decreased sharply. The third stage is the liquid-solid phase change process, in which the dissolved Li
2S
4 is transformed into insoluble Li
2S
2 and Li
2S. This process contributes about 3/4 of the capacity of the whole discharge process, showing a long discharge plateau at about 2. 1 V. At the end of the discharge plateau, the Li
2S
2 is converted to a Li
2S, and the reaction kinetics is slow because both are insulating solids.
The charging process is the oxidation process of lithium removal. The first step is the solid-liquid phase transition from solid Li2S to short-chain LiPSs. Because the Li2S is an insulating solid, it needs to overcome a large phase transition energy barrier and has a large polarization voltage. With the increase of LiPSs concentration on the cathode side, the concentration polarization increases, which leads to the increase of voltage. Finally, the long-chain LiPSs undergo a liquid-solid phase transition and are oxidized into S8, the electrochemical polarization increases rapidly, and the voltage increases significantly.
At present, the practical application of lithium-sulfur batteries is still hindered by a series of problems: (1) The "shuttle effect" of polysulfides is the most serious problem
[8]. LiPSs dissolved in the electrolyte on the positive electrode side will gradually migrate to the negative electrode under the combined action of concentration gradient and electric field, while the pore size of commercial polyolefin separator is much larger than that of LiPSs, so LiPSs can easily penetrate through the separator to reach the surface of the negative electrode. During the discharge process, LiPSs will directly react with the negative electrode to form an insulating passivation layer, which will reduce the coulombic efficiency and the capacity of the battery. In the process of standing, the self-discharge phenomenon of the battery is serious. The poor conductivity of (2)S
8 and Li
2S/Li
2S
2 seriously hinders the electron transfer in the reaction process, and a large energy barrier needs to be overcome in the initial activation. (3) The growth of lithium dendrites and the volume expansion of electrodes will cause the loss of active materials and even lead to safety problems.
To suppress the "shuttle effect", physical confinement, chemisorption, and catalytic conversion strategies (Figure 3) have been extensively studied for cathode or membrane modification
[9]. (1) when that physical limitation strategy is apply to the membrane, the selective permeation of the Li
+ is realized by use a functional intermediate layer based on the difference of the size, charge and other properties of the Li
+ and the S
n2-[3]; When applied to the cathode, the LiPSs are confined to the cathode side by the Van der Waals interaction between the porous carbon material and the LiPSs or the physical confinement effect of the hollow structure
[10⇓~12]. (2) The chemisorption strategy is to anchor the LiPSs on the functional surface through the strong interaction between the polar material and the LiPSs, thus reducing the dissolution of LiPSs in the electrolyte and weakening the "shuttle effect"
[13]. (3) The strategy of catalytic conversion is to accelerate the kinetics of oxidation-reduction reaction through catalysis, so as to accelerate the mutual conversion between sulfur-containing substances
[14⇓~16]. Different from physical confinement and chemisorption strategies, which inhibit the "shuttle effect" by "blocking" the diffusion of LiPSs, catalytic conversion can reduce the concentration and residence time of LiPSs in the electrolyte, and play a "dredging" role for LiPSs, thus reducing the diffusion time and diffusion driving force of LiPS, and achieving the purpose of inhibiting the "shuttle effect". At the same time, the catalytic conversion strategy can also induce the uniform deposition of charge-discharge products, reduce the internal polarization and activation energy barrier of the battery
[17][18].
Catalytic materials in lithium-sulfur batteries have been reviewed and reported, most of which are classified according to the composition or state of the catalytic materials
[19⇓⇓⇓⇓~24]. In this paper, the catalysis in lithium-sulfur batteries is divided into adsorption-conversion mechanism and redox mediated mechanism according to whether there is a clear intermediate state. The catalytic mechanisms and related materials involved are summarized. Finally, the common characterization techniques and research methods of catalytic mechanisms are introduced.
3 Catalytic mechanism and material
3.1 Adsorption-transformation mechanism
In the study of chemisorption materials, researchers have found that the Li — S bond is weakened due to the strong adsorption of LiPSs on the surface of some polar materials. At the same time, due to the high conductivity of the adsorption material, the kinetics of the redox reaction is enhanced. In addition, the enrichment of reactants on the active surface can also promote the redox reaction. Unlike the chemisorption strategy, which only reduces the dissolution of LiPSs, the adsorption-conversion mechanism is based on chemisorption and catalyzes the rapid conversion between sulfur-containing species through a surface reaction pathway. The performance of the adsorption-conversion process is influenced by properties such as adsorption strength, electronic/ionic conductivity, and specific surface area.
High adsorption strength is the basis of the adsorption-conversion process. Cui et al. Calculated the adsorption energy of metal oxides, sulfides, chlorides and graphene on different LiPSs, as well as the charge transfer and bond length changes before and after adsorption by density functional theory (DFT)
[25]. When the adsorption energy is too low, LiPSs can not be well immobilized on the surface of the material; When the adsorption energy is too high, the Li — S bond will be broken, and the polysulfide ions will be separated from the surface of the material and can not be effectively utilized. Therefore, the adsorption strength needs to be controlled within a suitable range (0.8 to 2.0 eV). Conductivity directly affects the reaction rate of lithiation/delithiation of LiPSs. For some catalytic materials with low intrinsic conductivity, LiPSs need to diffuse to the conductive surface after adsorption to achieve conversion, so the adsorption strength needs to match the diffusivity of LiPSs
[26].
The surface area of the catalytic interface has a direct impact on the catalytic effect. The specific surface area of the material can be effectively increased by nanocrystallization or low-dimensional methods, which can increase the exposure of surface active sites and thus increase the catalytic interface
[27,28].
Because the reduction process of sulfur-containing substances is an electron-gaining lithiation process, while the oxidation process is an electron-losing delithiation process, the performance requirements for catalytic materials are also different.At the same time, considering the shortcomings of single material in some aspects, this part will introduce the catalytic mechanism and materials in the process of reduction and oxidation, as well as the design method of bifunctional catalytic mechanism and materials.
3.1.1 Catalytic mechanism in the reduction proces
In the reduction process, the catalytic material has two main functions: (1) the reduction process of LiPSs is an electron-gaining lithiation process, and the reaction kinetics is controlled by the charge transfer rate and the interfacial impedance, and the use of catalytic materials with high conductivity can promote the charge transfer
[29]; (2) The Li
2S generated by the chemical disproportionation of free LiPSs can not be recycled during the charging process, and the use of catalytic materials with high adsorption strength can induce the uniform deposition of Li
2S on the conductive surface
[17].
(1) Metal materials are commonly used industrial catalysts, which have catalytic activity due to the coordination of their d orbitals with reactants and their high electrical conductivity. Arava et al. loaded traditional electrocatalyst Pt nanoparticles on graphene (Fig. 4A) as the cathode host, and found that its specific capacity was about 40% higher than that of graphene/sulfur composite cathode
[30]. Zhang et al. Induced the controlled deposition of LiPSs on the cathode through the chemical interaction between Au nanoparticles and LiPSs, which can effectively improve the cycle capacity and stability of the battery with a very small addition of 3 wt%
[31].
In addition to noble metals, other transition metal materials such as Ni and Co have also been shown to have catalytic effects
[32,33][34]. Sun et al. Supported trace (0.05 mg·cm
-2) Mo metal clusters on carbon nanofiber (CNF) membranes, and achieved catalytic conversion of LiPSs and regular deposition of Li
2S on the basis of physical confinement and chemisorption of LiPSs (Fig. 4B)
[35]. For cathodes with sulfur loadings up to 7.64 mg·cm
-2, a reversible area-specific capacity of 4.75 mAh·cm
-2 at 0.2 C can be achieved.
(2) Metal oxides have been widely studied due to the strong chemisorption between LiPSs and metal oxides
[36,37]. Cai et al. Loaded La
2O
3 nanoparticles on nitrogen-rich mesoporous carbon (NMC), and found that the initial potential of sulfur reduction increased significantly, and the reaction kinetics was accelerated
[38]. However, due to the insulation of the La
2O
3 itself, the performance decreases when the addition is more than 15 wt%. Nazar et al. Used Ti
4O
7 with high conductivity and strong "thiophilicity" as the cathode host, and the Lewis acidic metal center promoted the reduction of LiPSs while anchoring them, and induced the uniform deposition of Li
2S on the cathode surface (Fig. 5A)
[17].
In order to solve the problem of low conductivity of metal oxides, Li et al. Used ultra-thin TiO
2-x nanosheets and carbon nanotubes (CNT) to form a defect-rich heterojunction catalyst to achieve the area specific capacity of 5.4 mAh·cm
-2 at 5. 0 C
[39]. The conductivity of the material is obviously improved due to the introduction of carbon doping and oxygen vacancies; By combining with carbon materials, the d-band center of the TiO
2 is closer to the Fermi level, indicating that electrons are transferred from carbon to titanium atoms, and the change of interface electron distribution and energy band structure improves the catalytic activity of the heterojunction (Fig. 5B). Metal oxides such as α-Fe
2O
3, VO
2 and V
2O
3 have also been shown to have catalytic activity
[40][41][42]. In addition, some metal oxides can react with LiPSs to form active intermediate polysulfates, which will be described in detail later
[43].
Metal hydroxides and metal oxides have similar chemical composition and structure, which can catalyze the reduction of LiPSs and the nucleation of Li
2S through their "thiophilic" metal sites
[44].
(3) Metal sulfides can reduce the solvation of LiPSs by chelating with LiPSs to form active complexes
[45]. Nazar et al. Found that after LiPSs were adsorbed on the Co
9S
8, the cobalt and sulfur in the LiPSs acted as "thiophilic" and "lipophilic" sites, respectively, and internal coupling charge transfer processes (i.e. Sn
2-→Co
δ+ and Li
+→S
δ-) occurred, resulting in higher conductivity than metal oxides
[46].
Transition metal disulfides are a class of metal sulfides with two-dimensional layered structure, which have been applied in the cathode of lithium-sulfur batteries due to their highly exposed surface atoms and high conductivity of metalloids
[47,48]. Because of its unique two-dimensional structure, the edge and surface show different electrocatalytic activities. Cui et al. Studied the performance difference between the edge metal sites with unsaturated coordination and the surface metal sites with saturated coordination in the MoS
2, and found that the adsorption performance was independent of the size and shape of the lamella, but only related to the chemical environment, that is, the adsorption strength of the edge unsaturated sites of the lamella was much higher than that of the surface sites
[49]. Zhang et al. Found that CoS
2 can form chemical bonds with LiPSs and provide electrons to them. The addition of CoS
2 increases the transfer rate of electrons from the current collector to LiPSs, so that the reduction process is no longer controlled by the charge transfer rate (Fig. 6a)
[29]. At 2.0 C, the capacity decay rate per cycle is only 0.034%, while the initial capacity at 0.5 C can reach 1368 mAh·g
-1. Arava et al. And Goodenough et al. Further found that the dangling bonds at the edge of two-dimensional nanosheets had strong adsorption effect on LiPSs, and could promote charge transfer and improve the discharge capacity of high voltage platform
[50][51]. In addition to metal disulfides, ZnS, Co
3S
4 and other materials can also catalyze redox reactions through the process of adsorption-diffusion-transformation
[52][53].
In addition to adsorption-conversion, some metal sulfides can catalyze the formation of redox intermediates, such as the chelation of Mo
6S
8 with LiPSs to form Li
xMo
6S
8[54]. The Li
xMo
6S
8 has multiple redox couples in the range of 1.7 to 2.8 V, the Gibbs free energy of the reaction is much smaller than that of the interconversion between LiPSs, and can stably and reversibly intercalate lithium ions (Fig. 6B). During the discharge process, the reduction of LiPSs is promoted by the rapid conversion of Li
3Mo
6S
8/LiMo
6S
8 and Li
4Mo
6S
8/Li
3Mo
6S
8. During the charging process, the Li
xMo
6S
8 rapidly removes lithium and oxidizes the LiPSs to S
8. Mo
6S
8 not only provides a real catalytic mechanism for sulfur conversion, but also realizes a quasi-solid sulfur conversion pathway, which fundamentally solves the "shuttle effect". In the case of a high sulfur load (4~10 mg·cm
-2) and a low E/S ratio (4.2μL·mg
-1), an initial discharge capacity of 1057 mAh·g
-1 at room temperature can be achieved. The strategy of catalyzing the conversion of the sulfur-containing species by the redox couple will be described in detail later.
(4) Metal nitrides, phosphides and carbides are favorable for the electrochemical conversion of LiPSs due to their strong chemisorption and high conductivity.
Kim et al., based on theoretical calculation and XPS characterization results, found that the adsorption energy of S
8 molecules on the initial TiN surface was much higher than that of oxides or sulfides such as TiO
2 and Ti
4O
7, because the ring structure of S
8 was destroyed and all sulfur atoms were bonded with titanium atoms (Fig. 7A, B)
[55]. After the TiN surface is passivated by sulfur, it shows moderate bond strength to polysulfides. When the Li
2S
4 is adsorbed on the surface of TiN, the reduction reaction changes from an endothermic process to an exothermic process, and the energy barrier of the chemical disproportionation process is also significantly reduced. Therefore, under the same sulfur content, the discharge capacity of the cathode with TiN additive can be increased by 4. 5 times compared with that of the cathode without additive. The conductivity of VN is as high as 1.17×10
4S·cm
-1,Li at room temperature. A self-supporting three-dimensional network structure composed of VN and graphene is used as the cathode host, which provides a high specific capacity of 1461 mAh·g
-1 at 0.2 C and a coulombic efficiency of nearly 100%
[56]. Salley et al systematically studied the cell performance when WN, VN, Mo
2N and TiN were used as cathodes, and found that WN and TiN could achieve rapid electron transfer and reduce the passivation of the cathode surface due to the formation of S-W-N bonds or S-Ti-N bonds on the surface
[57,58].
Wang et al. First reported the use of MoP as a cathode host to enhance the adsorption and conversion of LiPSs
[59]. Wang et al. Proposed that the oxide layer on the surface of CoP nanoparticles plays an important role in enhancing adsorption and catalytic performance
[60]. The surface of CoP is easy to be oxidized to form Co — O — P bonds, and the high electronegativity of oxygen promotes the formation of high-valence Co sites, which form Co — S bonds with short-chain LiPSs to promote charge transfer. After reduction, however, the Co — P bond on the surface is weakly polarized and has no strong binding force for either Li or S (fig. 7C). In order to explain the influence of non-metal anions on the catalytic performance, Qian et al systematically studied the differences in structure and catalytic performance of Co
3O
4, CoS
2, Co
4N and CoP anchored on reduced graphene oxide
[61]. The experimental results show that CoP has the lowest electrochemical energy barrier, so it shows the best rate performance, and it can still provide 417.3 mAh·g
-1 specific capacity at 40. 0 C ultra-high rate. By analyzing the results of DFT calculation, it is found that the adsorption ability of CoP to Li
2S
6 is moderate, the p-band center of P in CoP moves up, and the d-band center of Co has a smaller band gap, which strengthens the bonding state energy and produces higher electron energy, and promotes the reaction kinetics at the interface by promoting electron exchange.
Metal carbides, such as NbC and TiC, have highly exposed polar surfaces and high conductivity of metalloids, which can promote the rapid conversion of LiPSs on the basis of adsorption
[62][63]. However, because the electronegativity of carbon is lower than that of sulfur and oxygen, the polarity of metal carbides is weak, and the adsorption capacity of metal carbides for LiPSs is moderate.
3.1.2 Catalytic mechanism in oxidation proces
In contrast to the reduction process, the oxidation process is a delithiation process that loses electrons. The two main processes involved are the conversion of insoluble insulating Li2S to short-chain LiPSs and the conversion of long-chain LiPSs to elemental sulfur, which usually need to overcome higher energy barriers due to the phase transition involved.
Cui et al studied a series of transition metal sulfides by DFT simulation to determine the mechanism of catalytic conversion of Li
2S
[14]. The results show that the intrinsic metal-like conductivity, the strong interaction with LiPSs, the promotion of lithium ion transfer, the ability to induce controlled deposition of Li
2S, and the rapid surface redox reaction all play a significant role in reducing the reaction energy barrier and improving the battery performance. The decomposition process of Li
2S is related to the interaction between the separated lithium ions and sulfur in sulfides, so some strong adsorption materials can induce lower activation energy barriers. Some non-metallic materials, such as phosphorus materials, heteroatom-doped carbon materials, carbon and nitrogen compounds, as well as the metal-based materials mentioned in the previous section, have catalytic effects on the oxidation process.
(1) phosphorus simple substance material. Phosphorus is a typical nonmetallic material with many allotropes. Black phosphorus is one of the stable ones, which has a wrinkled honeycomb two-dimensional lamellar structure similar to graphite.
Koratkar et al. mixed a few layers of black phosphorus with CNF to prepare a composite sulfur cathode body
[27]. Black phosphorus can effectively adsorb LiPSs due to its large specific surface area and high adsorption strength; The conductivity of black phosphorus is about 4.5 S·cm
-1, which is higher than that of metal nitrides, so it can reduce polarization, accelerate redox reaction and improve sulfur utilization. Lau et al. Used zero-dimensional black phosphorus quantum dots as catalysts, which have two advantages compared with metal-based catalysts: the density of black phosphorus is much lower than that of metal-based materials, which has less impact on the overall energy density of batteries
[28]; Compared with the two-dimensional materials, the zero-dimensional materials can expose more edge active sites; The black phosphorus quantum dots have a higher conductivity than the few-layer black phosphorus. The capacity retention of the cathode was 89% after 200 cycles at 0.1 C under high sulfur loading of 8 mg·cm
-2, and the experimental results showed that there was no "shuttle effect" in the cycling process.
(2) heteroatom-doped carbon material. Carbon materials are usually non-polar materials, although they have high conductivity, but there is only Van der Waals interaction between them and LiPSs, which is generally used only for physical confinement strategies. By doping some heteroatoms into the carbon material, the polarity can be enhanced, the conductivity can be improved, and the effect of catalytic conversion can be achieved
[64⇓⇓~67].
Dong et al. Found by in situ Raman characterization and DFT calculation that during the charging process, the Li
2S
8 is easier to gain electrons than the Li
2S
6 and S
8, so it is unstable in thermodynamics and kinetics
[68]. However, the weak adsorption of LiPSs on the surface of carbon materials makes it difficult to be reduced to S
8. Therefore, the charging process usually ends at the Li
2S
6, resulting in a low utilization of the active material. After the conductive carbon black super P is doped with nitrogen, it has strong Lewis basicity due to the existence of surface lone pair electrons, and the adsorption energy of LiPSs is increased by about 30 kcal·mol
-1, thus promoting the oxidation process of Li
2S
6→Li
2S
8→S
8 (Figure 8). Tao et al. compared the adsorption performance and Li
2S dissociation performance of LiPSs on the surface of nitrogen atoms in different chemical environments (graphite nitrogen N
G, pyridine nitrogen N
PL and pyrrole nitrogen N
PD). The results showed that pyridine nitrogen not only had the strongest adsorption capacity for Li
2S, but also provided the lowest activation energy to decompose it
[69]. In addition to adjusting the chemical environment, the construction of nanostructures with high surface area, such as nanoflowers, can also enhance performance
[70,71]. In addition, the structure doped with N/P, B/O, N/S and other heteroatoms can combine the characteristics of different heteroatoms to provide more adsorption sites and stronger catalytic performance
[72][73][74].
(3) Carbon and nitrogen compounds. Carbon and nitrogen can form a range of complex compounds, including some framework materials and polymer materials.
Inspired by the coordinated electron transfer between porphyrin and sulfur clusters in cytochrome C reductase, Zhang et al. Connected porphyrin to a two-dimensional framework to prepare a framework porphyrin (POF) electrocatalyst, and compounded it with graphene to further increase conductivity (Fig. 9a)
[75]. POF has a highly polar and conjugated structure with delocalized electron and polarization ability, which significantly enhances the interaction between POF and LiPSs through electron transfer and intermolecular polarization. The results of the density of States analysis show that there is an obvious overlap state between the C and N 2p orbitals in POF and the S 3p orbitals of LiPSs, and the delocalization of POF provides a wide state adjacent to the S 3p orbitals, which is conducive to the rapid transfer of electrons and promotes the mutual transformation between LiPSs (Fig. 9B, C).
Graphite-like carbon nitride (g-C
3N
4) is an inorganic polymer semiconductor with a large number of active adsorption sites (pyridine nitrogen) on its surface, which can induce the controlled deposition of Li
2S and elemental sulfur. Zhang et al. developed a one-step free-standing solid-state pyrolysis method for the preparation of g-C
3N
4 and oxidized carbon nitride (OCN)
[76]. The OCN is composed of two-dimensional corrugated sheets of micron scale, and the sheet layer is a micro-mesoporous structure doped with oxygen atoms. Due to the combined effect of pyridine nitrogen and active oxygen, it can achieve a higher initial discharge capacity, but the specific surface area is low and the capacity decay is fast. Nazar et al. Synthesized g-C
3N
4 by hard template method. Because of the low reaction temperature, only the edge of the lamella was oxidized, and the nitrogen content was four times that of OCN
[77]. High concentration of pyridine nitrogen promoted the adsorption and conversion of LiPSs, and the capacity decay rate was only 0. 04% per cycle after 1500 cycles at 0. 5 C. In addition, the polymerization of carbon nitride (p-C
3N
4) can also reduce the kinetic energy barrier of the redox of LiPSs
[78]. It was found by DFT calculation that the configuration of LiPSs changed after adsorption due to electrostatic interaction (Fig. 9d), which made the delithiation process easy to occur. At the same time, the binding energy between nitrogen atoms and lithium ions is stronger than that of the electrolyte solvent, so lithium ions in the electrolyte will be enriched on the surface of the p-C
3N
4, which is conducive to the lithiation process.
3.1.3 Bifunctional catalytic mechanism
(1) Most materials are difficult to combine the characteristics of high adsorption strength, high conductivity and high specific surface area, for example, metal oxides have strong adsorption performance but low conductivity, which is not conducive to the conversion of LiPSs and may lead to more serious side effects; The adsorption of highly conductive materials such as metal nitrides is generally poor. Therefore, the characteristics and advantages of different materials can be combined to improve the overall performance by building heterostructures.
Yang et al. First proposed the concept of a bifunctional heterostructure by designing a twinned TiO
2-TiN heterostructure, which combines the high adsorption performance of TiO
2 and the high conductivity of TiN to achieve a smooth adsorption-diffusion-conversion process of LiPSs on its surface (Fig. 10a)
[79]. The heterostructure is loaded on graphene as a membrane modification layer, which can be used as a physical barrier to inhibit the diffusion of LiPSs. Based on a similar idea, Zhao et al. Constructed Co
9S
8/CoO heterojunction, the two functions not only achieve synergy, but also promote and strengthen each other
[80]. The high adsorption strength of CoO can make LiPSs diffuse to the surface of the Co
9S
8 more quickly to realize the conversion; The high catalytic efficiency of the Co
9S
8 allows the active sites of CoO to be released quickly, thereby adsorbing more LiPSs. Sun et al. Show that the presence of the SnS
2/SnO
2 heterointerface increases the electron transfer rate and the lithium ion diffusion rate, thereby accelerating the redox reaction (Fig. 10B)
[81].
Heterostructures can not only coordinate the relationship between adsorption strength and conductivity and enhance ion diffusion, but also make up for the lack of specific surface area and conductivity of some materials
[82,83][84]. MXene, as a typical two-dimensional transition metal carbide, has a two-dimensional lamellar structure, high conductivity and high catalytic activity surface, but its advantages are difficult to play due to the problems of lamellar repacking and end group inertia. Yang et al. partially converted the MXene(Ti
3C
2T
x) into TiO
2 by controlled oxidation (Fig. 10c),TiO
2, which can serve as the adsorption center of LiPSs and achieve rapid electron/ion diffusion through the heterointerface
[85]. At the same time, the presence of TiO
2 nanoparticles inhibits the restacking of MXene and maintains the two-dimensional structure with high specific surface area. Yu et al. Constructed a C/Fe/Fe
3C heterostructure by the pyrolytic reduction of iron ions, and the in-situ formation of Fe promoted the graphitization of carbon, thus improving the conductivity
[86].
(2) Due to the different properties and sites of action, some catalytic materials can only catalyze a specific process, such as the strong interaction between metal atoms and sulfur atoms, which is beneficial to the reduction process; Nitrogen and oxygen promote the oxidation process by interacting with lithium atoms. Therefore, the use of co-doping strategy can make the catalytic materials have two different types of sites, and show catalytic activity for both oxidation and reduction processes
[87⇓⇓~90].
Dong et al. prepared cobalt and nitrogen Co-doped graphite carbon (Co-N-GC) using ZIF-67 as precursor, and studied the synergistic catalytic effect of Co-N
[91]. By testing the performance of the battery after removing Co or N, it was found that cobalt atoms were beneficial to improve the capacity, while nitrogen atoms could promote the oxidation process of Li
2S
6→Li
2S
8→S
8 to improve the cycle performance, thus improving the utilization of active materials. Qiu et al. explained the synergistic effect of cobalt and nitrogen by DFT calculations
[92]. Doped nitrogen atoms act as electron donors to increase the charge density of carbon, and the charge in cobalt atoms is transferred to nearby carbon, further increasing the charge density. In addition, the adsorption energy of the Li-S cluster decreases from-0. 46 eV to-0. 78 eV after nitrogen doping, and further decreases to-1. 57 eV after cobalt doping, which indicates the synergistic effect of cobalt and nitrogen on the adsorption performance.
3.2 Redox mediated mechanism
Adsorption-conversion mechanism-related materials mostly adsorb dissolved LiPSs and promote electron/ion transfer to achieve rapid conversion between sulfur-containing substances. These materials are usually solid, and the catalytic effect is achieved by surface reaction, so the catalytic effect of non-conductive solid sulfur and lithium sulfide will be limited. In addition, the non-uniformity of the distribution in space can also lead to the local deposition of insulating substances, resulting in the deactivation of catalytic materials.
Recent studies have found that some redox couples/redox intermediates with suitable redox potentials can react chemically with LiPSs through electrochemical reactions on the surface of conductive materials, thus achieving rapid conversion between sulfur-containing substances, which are called redox mediators (RM). RM is mostly liquid in the lithium-sulfur battery system, so it can achieve rapid conversion and uniform distribution of sulfur-containing substances through solution reaction.
With the development of in-situ characterization technology, the redox mechanism in lithium-sulfur batteries has been studied more deeply
[93,94]. The results show that the reduction process involves complex electrochemical reactions and disproportionation reactions, and a series of polysulfide anions and free radicals are produced, which undoubtedly promote the reaction. LiPSs, an intermediate product of lithium-sulfur batteries, is a natural RM
[18,95].
Cui et al. Found that when using a Li
2S as the cathode, the first cycle charge has an energy barrier of about 1.0 V, so a higher cut-off voltage needs to be applied (Fig. 11A)
[18]. After the first cycle of charging, the formation of LiPSs in the electrolyte significantly accelerates the charge and ion migration process, and the charging energy barrier is no longer obvious in the subsequent cycles. Zhang et al. achieved uniform deposition and reoxidation of the discharge products by pre-adding LiPSs to the electrolyte, which mediated liquid-solid phase transfer (Fig. 11B)
[96]. When the sulfur loading was up to 4.2 mg·cm
-2 and the current density was 1.5 A·g
-1, the initial capacity decreased slightly, but the capacity recovered and exceeded the initial value after 1000 cycles, reflecting the "self-healing" ability.
3.2.1 Thiosulfate/polysulfate mediated
For example, Liang et al. Prepared sulfur-rich lithium polythiophosphate by mixing S
8 with Li
3PS
4, and found that it had high lithium ion conductivity (25℃,3.0×10
-5S·cm
-1) and good cycle performance
[97]. It is speculated that thiosulfates and polysulfates may have similar properties.
The adsorption and catalytic properties of transition metal oxides (TMOs) have been confirmed by a large number of studies, which accelerate the charge transfer and promote the conversion of LiPSs through the strong interaction between TMOs and LiPSs. Nazar et al. Found that in addition to adsorption, MnO
2 can oxidize LiPSs to insoluble thiosulfate S
2O
32- and fix them on the surface when using MnO
2 nanosheets as the cathode host
[41]. The long-chain LiPSs can be connected into the polysulfates through S-S single bonds, and can generate solid Li
2S
2 or Li
2S through an internal disproportionation reaction (Wackenroder reaction). However, this mechanism is not applicable to all TMOs. Nazar et al. Further studied and found that the formation of initial S
2O
32- is closely related to the redox potential of TMO, and proposed the "Goldilocks" principle: as shown in Figure 12A, the electrochemical oxidation window of LiPSs is 2.1 ~ 2.4 V. When the redox potential E of TMO is ≤ 2.4 V, only adsorption occurs and LiPSs cannot be oxidized
[98][94]; When 2.4 V < E ≤ 3.05 V, the LiPSs adsorbed by TMO can be oxidized to S
2O
32-. When E > 3.05 V, the LiPSs are oxidized to SO
42- with no electrochemical activity, resulting in the loss of active species and contamination of the active surface.
Based on the "Goldilocks" principle, Sun et al. Used VO
2 nanoribbons and graphene composites as the main body of the cathode, and the initial discharge capacity reached 1405 mAh·g
-1 at 0.2 C
[99]. The VO
2 nanoribbons showed ultrafast adsorption and catalytic properties, significantly better than V
2O
5 with similar specific surface areas (fig. 12 B). However, VO
2 has polycrystalline phase (VO
2(A), VO
2(B), VO
2(C)) and mineral phase VO
2(P), etc.Among them, VO
2(B)) has suitable redox potential, but it is easy to transform into other crystal forms at high temperature, and its inherent conductivity is not high. Sun et al. Used high-temperature stable subferrovanadite VO
2(P) as the main cathode, and formed polysulfate through Wackenroder reaction, which still maintained high stability under high sulfur loading of 9.6 mg·cm
-2[100].
Transition metal oxides generally have the problem of high adsorption capacity and low conductivity. Long et al. Prepared MnO
2 nanospheres with rich oxygen vacancies on the surface, which improved the conductivity and surface reactivity of MnO
2, adsorbed more LiPSs and accelerated the formation of polysulfates
[101]. Wang et al modified ultra-thin hafnium oxide (HfO
2) on the surface of CNT by atomic layer deposition, and the resulting material as an intermediate layer can effectively block the shuttle of LiPSs
[102]. Among them, the HfO
2 acts as the active material for LiPSs capture and catalytic conversion, while the CNT acts as a physical barrier to inhibit LiPSs diffusion and accelerate charge transfer.
Some highly conductive metal oxides, such as RuO
2, have excellent catalytic properties, which can effectively improve the utilization of sulfur and rate capability. Jiang et al. Proposed to use RuO
2 nanodots (NDs) to anchor LiPSs and trigger their surface-mediated reduction to promote the formation of Li
2S
2/Li
2S through catalysis
[103]. The conductivity of the RuO
2NDs is as high as 10
5S·cm
-1, and the size is only 2 nm, which provides enough contact area for the conversion of LiPSs and promotes the rapid transfer of charge. The results of natural bond analysis, density functional theory calculation and XPS show that LiPSs undergo catalytic reduction at the interface, and the reduction process undergoes a polysulfate intermediate state. Subsequently, Shao et al. Embedded RuO
2 nanoparticles into CNT to prepare a self-supporting 3D CNT@10%RuO
2 cathode, which had a high initial discharge capacity of 1325 and 750 mAh·g
-1 at 0. 2 C and 2. 0 C, respectively, and a decay rate of only 0. 06% per cycle at 0. 5 C
[104].
In addition to transition metal oxides, some other materials have shown similar properties. Chen et al. Prepared two-dimensional porous carbon nanosheets PCN by self-assembly of sulfonated pitch and triblock polymer F127 followed by carbonization. The oxygen-containing functional groups on the surface of PCN can trigger the formation of thiosulfate
[105]. The monatomic iron catalyst (SAFe) can promote the delithiation process of the Li
2S and extend the polysulfide chain continuously through the strong nucleophilic interaction with the sulfur atom, and can charge the Li
2S based battery at an ultra-high rate of 12. 0 C
[106].
3.2.2 Sulfur radical mediated
The reactions in lithium-sulfur batteries are complex, and elemental sulfur does not exist as a single polysulfide in the electrolyte, but as a dynamic equilibrium of multiple intermediates. Alloin et al. studied the electrolyte composition under different discharge conditions by means of liquid chromatography and UV-visible absorption spectroscopy, and found that after the coexistence of electrochemical reaction and disproportionation reaction in the reduction process, the intermediate state of sulfur radical (
) would be experienced when the S
62- was converted into S
42-[93]. The reaction mechanism of lithium-sulfur battery proposed by Xiao et al. Based on the in-situ observation during charge and discharge is shown in Fig. 13A. The dynamic balance between S
62- and S
3. always exists during charge and discharge. The S
62- generally occurs in the high potential region (> 2.1 V), while the S
3. tends to occur in the low potential region (≤ 2.1 V)
[107]. Higher sulfur utilization is easier to achieve due to the higher reactivity of the sulfur radicals. The proposed reaction mechanism provides a new idea for the study of lithium-sulfur batteries.
The formation of sulfur radicals is closely related to the properties of electrolyte solvents, and solvents with high donor number and high dielectric constant are beneficial to the formation of sulfur radicals
[108,109]. Nazar et al. Found that electron pair donor can exist stably in electron pair donor (EPD) solvents, and took dimethylacetamide as an example to study the mechanism
[110]. EPD is beneficial to the dissociation of LiPSs due to its high donor number, thus promoting the formation of
. EPD can strengthen the electrochemical equilibrium between the sulfur species. Under ideal conditions, the
can reoxidize the Li
2S to prevent the passivation of the positive electrode until the sulfur species are completely reduced. During the charging process, EPD can partially solvate the Li
2S, alleviating the problems caused by low conductivity and reducing the dependence on the three-phase interface (Fig. 13B). In addition, the electrochemical reaction is assisted by the
through a chemical reaction path.
The introduction of defects/vacancies in metal-based catalytic materials can provide more active sites for adsorption and catalysis, and improve the conductivity of the materials
[111, 112, 113]. Studies have shown that oxygen vacancies can induce the generation of superoxide radicals in the electrochemical process, so it can be speculated that vacancies also have a positive effect on the generation of sulfur radicals
[114]. Lin et al. Found that when WO
3-x containing oxygen vacancies are used as the host of the electrode, the charge and discharge processes go through the free radical pathway, thus catalyzing both oxidation and reduction processes
[115]. The generation of
is difficult to characterize by electrochemical testing, but its presence can be directly demonstrated using UV-Vis spectroscopy (Fig. 14 a).
is usually produced by the decomposition of
, whereas direct conversion between
and
can be achieved using commercially available titanium silicalite-1 (TS-1)
[116]. In-situ UV-Vis spectroscopy analysis showed that the utilization of 14b),S
8 was significantly improved by the direct disproportionation of
to
and
/
(Fig. 2) when discharged to 2. 5-2. 3 V.
3.2.3 Redox couple mediated
In addition to the internally generated sulfur-containing intermediates, redox mediators in lithium-sulfur batteries can also introduce additional redox couples with redox activity in the Li-S voltage range from the outside, and their selection is mainly based on redox potential (relative to Li/Li
+). Taking the charging process as an example (Figure 15A), the redox potential of RM is required to be higher than the equilibrium potential of the Li
2S. RM chemically oxidizes the Li
2S on the surface in contact with the electrolyte, and the reduced RM diffuses to the current collector for electrochemical reoxidation, thus promoting the oxidation process of the Li
2S
[117]. Accordingly, if the redox potential of RM is lower than the formation potential of the Li
2S, the LiPSs reduction process can be promoted (fig. 15B).
Transition metal ions with multiple valences are the most common redox couples. Aurbach et al. study three types of RM in term of redox potential (E),A E
RM<
(dicyclopentadienyl cobalt), a E
RM≈
(diphenylchromium),E
RM>
(decamethylferrocene, lithium iodide, ferrocene)
[117]. It was found that the Li
2S could be fully oxidized at a lower potential when only the
was E
RM> (Fig. 16A), and the initial charging voltage of the Li
2S could be reduced from 4.0 V to 3.2 V by adding decamethylferrocene. The redox potential of cobaltocene is about 2. 0 V, which is lower than the second reduction peak of sulfur, so it can be used as the RM of LiPSs reduction
[118,119]. This strategy has the potential to be applied in the state of poor electrolyte, because cobaltocene can convert LiPSs dissolved in the electrolyte into Li
2S through chemical reaction, and the discharge capacity of the low voltage plateau is significantly improved (Fig. 16 B).
In addition to metallocene, some compounds containing double bonds (such as imides, quinonimines, etc.) Can also be reversibly oxidized and reduced, and their redox potentials can be precisely controlled by the design of adjacent functional groups. Helms et al. First proposed the use of benzo [GHI] perylene imides (BPI) containing imide groups as redox mediators for lithium-sulfur batteries
[120]. Without the addition of BPI, the Li
2S is deposited only on the conductive surface of the current collector; After the addition of BPI, the deposition of Li
2S is no longer dependent on electrochemical reaction, so it can continue to deposit on the surface of insulating Li
2S, and eventually form a three-dimensional structure. In addition to being used as an electrolyte additive, electrode binders with similar functions can be obtained by grafting BPI into polymer chains
[121]. Mecerreyes et al. studied three polyimide-polyethylene oxide (PI-PEO) block copolymers with different structures and found that the closer the reduction potential of the imide is to the reduction potential of sulfur, the stronger the mediation
[122].
When RM is used in the charging process, its redox potential is the charging voltage, so it is ideal to be slightly higher than the equilibrium potential of the Li
2S. However, when metallocene is used as RM, the charging voltage is still as high as 3. 0 V. How to further reduce the charging voltage is a realistic problem. In addition, the solubility and stability of RM also have a direct impact on the mediation performance. Anthraquinone-2-carboxylic acid (AQC) has a suitable redox potential, but has poor electrochemical stability and limited solubility (20 mM) under working conditions. Bao et al. designed 1,5-bis (2- (2- (2- (2-methoxyethoxy) ethoxy) ethoxy) anthracene 9,10-quinone (AQT) with similar structure to AQC by adjusting the molecular structure
[123]. AQT has a redox potential similar to that of AQC due to structural similarity, but its stability and solubility (> 500 mM) are greatly improved due to its more polar substituents. When 160 mM AQT was added to the electrolyte, the initial charge voltage of the Li
2S cathode at 0. 5 C was reduced to less than 2. 5 V, and the first cycle discharge capacity was up to 1261 mAh·g
-1. Zhang et al. And Sun et al. Have studied the reaction mechanism of quinoid imine (Fig. 17A) and imide (Fig. 17b) as RM, respectively
[124][125].
3.3 Other catalytic mechanisms
3.3.1 Monatomic catalysis
The monoatomic catalyst (SAC) refers to the monodisperse atoms dispersed on the solid substrate, whose theoretical atom utilization rate is 100%, which has the characteristics of heterogeneous and homogeneous catalysis
[126]. SAC has several advantages over conventional catalysts: the unsaturated coordination environment of the metal atom reduces its adsorption and reaction energy barriers
[127]; It has a unique electronic structure due to the influence of quantum size effect
[128]. The bonding between the metal atom and the carrier promotes charge transfer
[129]. SAC has a similar catalytic mechanism to metal-based materials, but it can achieve excellent performance at very low loadings. Monatomic Fe, Co and Ni have been proved to be effective catalysts for lithium-sulfur batteries.
Because of its high polarity, SAC has a strong interaction with polysulfides, and as a membrane modification layer, it can have both chemisorption and catalytic conversion functions. Xie et al. Prepared nitrogen-doped graphene (M
1/NG,M=Fe, Co, Ni) supported by different SACs by liquid impregnation method, and supported on polyolefin separator after mixing with conductive agent and binder (Fig. 18 a)
[130]. Compared with other metal-containing catalytic materials, the metal loading of SAC in the whole cell is very low, only 2 μg. It was found that the lithium-sulfur battery with the Fe
1/NG modified separator performed best, showing a high discharge capacity of 673 mAh·g
-1 at a high sulfur loading of ~4.5 mg·cm
-2 and a high current density of 5.0 C, which is directly related to the strongest adsorption performance of iron atoms (Fig. 18 B). Niu et al. Prepared Ni mono atoms embedded in nitrogen-doped graphene (Ni @ NG) by pyrolysis
[131]. Among them, the Ni-N
4structure provides active Ni centers and high conductivity, and can reversibly form
… Ni — N bonds with LiPSs. LiPSs have a lower Gibbs free energy at the Ni @ NG surface than NG (Fig. 18c), so the kinetic conversion is faster. In addition, the binding energy between LiPSs and Ni @ NG is large, so adsorption and conversion can occur rapidly (Fig. 18d).
In addition to membrane modification, SAC can also be used as the main body of sulfur cathode
[132,133]. Ji et al. Found that monatomic cobalt embedded in nitrogen-doped graphene (Co-N/G) has the ability to catalyze both oxidation and reduction reactions
[134]. The conversion of Li
2S
2 to Li
2S is an endothermic rate-limiting step in the reduction process, and Co-N/G can effectively reduce its Gibbs free energy, thus promoting the reduction of sulfur-containing substances (Fig. 19A). During the oxidation process, the decomposition of the Li
2S solid has a high energy barrier, and the doping of a single Co atom can reduce the decomposition energy barrier from 2.29 V to 1.43 V, which significantly accelerates the phase transition of the Li
2S (Fig. 19b). Zhang et al. Found that monatomic iron supported on nitrogen-doped carbon (NC: SAFe) can promote the delithiation reaction of Li
2S: SAFe first coordinates with Li
2S, weakening the strength of Li — S bond, and Li
+ is easy to dissociate from the intermediate [N—Fe…S—Li
2]
[106]; The partially delithiated intermediate is coupled with another Li
2S and forms a polysulfide chain through repeated delithiation processes; This process is repeated until the polysulfide chain is detached from the SAFe at a certain potential. The released SAFe sites reoccur the above reaction. Due to the unsaturated coordination environment of Fe, it can coordinate with multiple Li
2S, thus catalyzing multiple reaction processes simultaneously.
3.3.2 Chemistry of organic sulfur and mercaptans
In order to give full play to the role of LiPSs as RM, a large amount of electrolyte is usually needed to dissolve them, and the electrolyte system is generally electrochemically inert, which may lead to a decrease in the overall energy density of the battery. Based on the study of thiol chemistry, Wang et al. Used liquid organic sulfur compound dimethyl disulfide (DMDS,CH
3SSCH
3) as a cosolvent. DMDS can produce soluble dimethyl polysulfide (DMPS) in the reduction process, and further reduce to organic lithium sulfide and lithium sulfide, which provides a new path for the reduction process of sulfur
[135]. In addition, disulfide bonds in DMDS can be reversibly broken/connected during charge and discharge, providing a theoretical capacity of 570 mAh·g
-1, making the battery capacity about twice that of conventional lithium-sulfur batteries (Figure 20 a). Fu et al. Used a similar dimethyl trisulfide (DMTS,CH
3SSSCH
3) as the cathode active material, where the disulfide bond dissociation energy is about 45 kcal·mol
-1, lower than the DMDS(~70 kcal·mol
-1), and therefore has faster reaction kinetics
[136]. DMTS undergoes a highly reactive free radical process during reduction (Figure 20 B), which inhibits the formation of long-chain polysulfides and elemental sulfur.
In addition to organic sulfur, thiol reductants, such as Dithiothreitol (DTT) and Tris (2-carboxyethyl) phosphine (TCEP), are commonly used in biochemistry to break disulfide bonds in proteins, and can also promote the reduction of LiPSs when applied to lithium-sulfur batteries. Huang et al. Used lightweight graphene/dithiothreitol (Gra/DTT) as an intermediate layer for lithium-sulfur batteries, in which DTT can cut disulfide bonds (Fig. 20c) and promote the reduction of long-chain LiPSs, thus reducing their accumulation in the electrolyte
[137]. Chen et al. Used TCEP as a LiPSs cleavage reagent to truncate long-chain LiPSs into two short-chain LiPSs, which were themselves oxidized
[105]; At the same time, the oxidation state oxidizes the LiPSs to thiosulfate, the role of which is described in Section 4.1. Excellent rate capability (503 mAh·g
-1 at 7. 5 C) and cycling performance (700 cycles at 1. 0 C, capacity decay rate of 0. 0356% per cycle) can be obtained by using TCEP as the host of sulfur cathode.
4 Characterization Techniques and Research Methods of Catalytic Mechanism
In this part, the electrochemical measurement techniques, material characterization techniques and theoretical simulation methods commonly used in the study of catalytic mechanism are briefly introduced.
4.1 Lectrochemical test
(1) Constant current charge-discharge method is often used to characterize the charge-discharge capacity of batteries and the change of potential with time/capacity.
The improvement of battery capacity after adding catalyst is the most direct manifestation of catalytic performance. The performance of electrocatalyst is positively correlated with temperature and surface area, so the increase of capacity with the increase of temperature and surface area can prove the existence of catalysis
[30].
From the charge-discharge curve, the contribution of each discharge platform to the capacity of the battery, the degree of polarization and the change of overpotential can be analyzed. As shown in fig. 16 (B), the residual Li
2S
4 in the electrolyte is fully reduced through the mediation of cobaltocene, thereby extending the low voltage discharge plateau
[118]. The most serious problem of Li
2S cathode is the high overpotential in the first cycle of charging. Taking the mediation of LiPSs as an example, the activation energy barrier of ~ 3.5 V needs to be overcome in the first cycle of charging. In the following cycles, the overpotential decreased significantly due to the formation of LiPSs. The overpotential can be calculated from the voltage difference between the first charge plateau and the second discharge plateau, and a decrease in the overpotential can indicate an enhancement in the reaction kinetics
[38].
(2) Cyclic Voltammetry (CV) is often used to characterize the redox potential, reaction kinetics and reversibility of materials.
The conventional lithium-sulfur battery has two reduction characteristic peaks and one oxidation characteristic peak, and the corresponding voltage is the oxidation/reduction potential. The shift of oxidation characteristic peak to low voltage region, the shift of reduction characteristic peak to high voltage region, and the narrowing and sharpening of peak shape can explain the enhancement of reaction kinetics
[38].
RM requires good redox reversibility and suitable redox potential, so CV test is very important in the selection of RM. For example, 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) has a reversible redox couple, and the redox potential of sulfur is between its redox potential, so PTCDI can effectively mediate the redox of LiPSs
[125].
(3) Electrochemical impedance spectroscopy (EIS) can reflect the electronic/ionic conductivity of the material, and the increase of conductivity can indicate the existence of catalysis to some extent.
(4) Tafel chart reflects the relationship between current density and overpotential, from which the exchange current density of different materials can be calculated. The higher the exchange current density is, the higher the conversion rate is.
(5) Galvanostatic Iintermittent titration technique (GITT) is often used to measure the diffusion coefficient of lithium ions. The voltage is rapidly increased by applying a positive current pulse; Then the relaxation process is entered, and the potential gradually decreases until equilibrium due to the diffusion of lithium ions. The lithium ion diffusion coefficient can be calculated from the voltage change during the pulse and relaxation processes.
4.2 Material and structure characterization
In addition to electrochemical testing, some of the catalytically active intermediates mentioned above need to be characterized by conventional or in situ structural and compositional analysis.
Thiosulfates and polysulfates are generally characterized by XPS
[98]. In the XPS S 2p spectrum, the characteristic peaks at 167.2 eV and 168.2 eV prove the presence of thiosulfate and polysulfate, respectively (Figure 21 a), and the presence of the characteristic peak of sulfate (~ 170.0 eV) indicates that the redox potential of the metal oxide is high (Figure 21 B). The formation of thiosulfate and polysulfate is usually accompanied by the change of the valence state of metal elements, so the mechanism analysis is generally combined with the XPS characterization of metal elements.
The sulfur radical can be characterized by some optical methods, such as the UV-vis characteristic peak of the
is 615 nm (Figure 21c), and the Raman spectrum characteristic peak is 535 cm
-1 (Figure 21d)
[109,138]. With the development of in situ characterization technology in recent years, researchers have directly proved the existence of sulfur radicals by in situ X-ray diffraction, near-edge absorption spectroscopy and in situ electron paramagnetic resonance, and further studied the important role of sulfur radicals in battery reaction
[107,138,139]4.3 Theoretical simulation method
Electrochemical and chemical characterization mostly focus on phenomena, and combined with theoretical simulation, the actual reaction process and catalytic mechanism of lithium-sulfur batteries can be studied more deeply, which can guide the selection and design of materials.
The adsorption strength and conductivity of LiPSs on the catalytic material have a direct impact on the catalytic performance, and these parameters can be quantified by theoretical simulation
[14]. The conductivity of the material can be analyzed by electronic band structure simulation (Fig. 22 a), and the adsorption energy of LiPSs on the surface of the material can be calculated by first-principles calculation (Fig. 22 B). In addition, the diffusion barrier of the Li
+ and the decomposition barrier of the Li
2S can be calculated by the transition state search method (Fig. 22c, d).
5 Conclusion and prospect
The "shuttle effect" of LiPSs in lithium-sulfur batteries is a key problem that limits its development and application, and researchers have proposed a series of solutions from different perspectives. In the early stage of research, LiPSs were mainly blocked at the cathode side by physical confinement or chemisorption, but the local enrichment of LiPSs led to severe polarization and slow redox reaction. At the same time, considering the conditions of high sulfur loading and poor electrolyte in practical applications, how to inhibit the "shuttle effect" and enhance the reaction process becomes the key problem. Based on the findings in the study of adsorption materials, researchers have proposed a strategy to channel LiPSs by catalytic conversion. In this paper, two main types of catalytic mechanisms and related materials are reviewed: one is the adsorption-conversion mechanism, which promotes the conversion of LiPSs by adsorbing them on the active surface with high ionic/electronic conductivity through strong interactions with LiPSs; The other is redox mediated mechanism, which is achieved by the chemical reaction between LiPSs and redox intermediates added in advance or generated in situ.
Although the research of catalytic materials has been widely involved in metals and their compounds, carbon materials, polymer materials and so on, the catalytic mechanism of some materials is not yet in-depth, especially the adsorption-conversion mechanism of related materials. Therefore, it is necessary to further explore the actual reaction pathway and catalytic mechanism in the working process of the battery by combining in-situ characterization technology and theoretical simulation. Several factors need to be considered in the selection and design of catalytic materials: (1) Lithium-sulfur batteries use lithium metal as the anode, so safety is of vital importance. The catalytic material not only needs to be stable in the working voltage window, but also its influence on the anode can not be ignored. (2) Catalytic materials are usually used under the condition of high sulfur loading and poor electrolyte, and the actual reaction pathway of lithium-sulfur batteries under this condition is obviously different from that of button batteries in the laboratory, so further exploration is needed; (3) As an additional additive, low density and low cost are the inevitable requirements for the commercialization of lithium-sulfur batteries, and the restrictive relationship between low addition and high catalytic performance will promote the development of new materials and new structures.
At present, physical confinement and chemical adsorption strategies have been widely used because of their simplicity and feasibility. However, these strategies are still insufficient to solve the problems of high initial activation energy and slow redox reaction of Li2S. The introduction of catalytic conversion strategy into the research of lithium-sulfur batteries and the simultaneous progress of theory and application will open up a new path to promote the industrialization and practical process of lithium-sulfur batteries.
State Key Laboratory of Catalytic Materials and Reaction Engineering, State Key Laboratory of Catalytic Materials and Reaction Engineering(中国石油化工股份有限公司石油化工科学研究院)
the National Natural Science Foundation of China(21878216)
the National Natural Science Foundation of China(22005215)
Tianjin Science and Technology Project(19YFSLQY0070)
the National Key Research and Development Program of China(2019YFE0118800)
and Hebei Province Innovation Ability Promotion Project(20312201D)
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