Transition metal sulfide nanostructures: synthesis and application in metal-air batteries

Owing to great energy density, eco-friendliness, safety and security, and cost-effectiveness, rechargeable metal–air batteries (MABs) have engrossed substantial devotion. The MABs signify one of the most feasible forthcoming alternatives to powering electric vehicles (EVs) and smart-grid energy storage. The progress of MABs has offered a solution benefitting from its much higher theoretical energy density than that of lithium-ion batteries (LIB). However, certain technical difficulties allied with metal–air batteries include sluggish electrochemical oxygen reaction kinetics that has yet to be fixed. The transition single metal and mixed metals sulfides (TMS) nanostructures have validated an advanced electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance, due to their higher electronic conductivity and fast-charge transfer kinetics. The bifunctional electrocatalytic act of the TMSs can be enhanced by altering the electronic configuration, double layer structure and interface, valence state, and vacancies. In this minireview, the preparation, properties, and testing of electrode components of transition metal sulfides (TMS) nanomaterials towards different types of metal–air batteries (aqueous and non-aqueous), the fundamentals, configuration of battery, choice of electrode materials, electrolyte, and separator, current challenges as well as perspectives of the design of high-performance MABs are also discussed based on the existing execution.


Introduction
The design of high-performance electrochemical energy storage systems are highly vital component in the energy sector to safeguard impulsive energy production and provisions through renewable sources [1][2][3].Metal-air batteries (MABs) include Zn−air [4][5][6], Li−air [7][8][9], Mg−air [10,11], Al−air [12], etc have grown revitalized attentiveness among numerous existing candidates because of their high energy density, eco-friendly, protection and security, and cost-effective (table 1) [13].In metal-air batteries, metals, including Zn, Al, Mg, and Fe are thermodynamically unsteady in the aqueous solution, however, metal surfaces can be easily passivated through the formation of oxides or hydroxides in definite conditions (figure 1) [14][15][16].Thus, these metals are well-suited with aqueous electrolytes to a certain degree and have been examined for aqueous MABs [17][18][19][20].Metals are oxidized at the surface of the anode and oxygen molecules (O 2 ) from the environmental air are reduced on the surface of the catalysts at the gas-diffusion cathode during the process of discharge (figure 2) via the following equations [21][22][23][24]: + + « redox reaction between metal and oxygen (O 2 ) in the air under an appropriate electrolyte [25][26][27][28].Two basic electrochemical reactions such as oxygen reduction reaction (ORR) and OER occur in discharge and charge processes, respectively in rechargeable metal-air batteries [29][30][31][32].Besides, the reactions and the inclusive yield may vary from each other amongst several metal-air battery types, subject to the explicit metal, the electrolyte, and the catalysts employed.In recent years, numerous research efforts have been dedicated to MABs research under aqueous and non-aqueous for designing next-generation energy storage systems [33][34][35].Literature studies show that excessive contests quite occur in designing oxygen electrocatalysts for MABs [16,36,37].
In metal-air batteries, the overpotentials of both OER (η charge) and ORR (η discharge) majorly reduce the power output and round-trip efficacy.Thus, the strategy of economic and extremely effectual oxygen electrocatalysts (ORR and OER) is key to improving the power density, cycling ability, and energy conversion effectiveness of metal-air batteries [31,38].Over recent years, great research struggles have been prepared in the advance of oxygen electrocatalysts for both primary and secondary MABs [37,39].Until now, precious metal- Voltage (V) Specific energy density Volumetric energy density (Wh dm −3 ) derived composites have been employed as effectual electrocatalysts.However, their expensive cost and poor stability are constrained by their tremendous catalytic activity [40].Therefore, over the recent years, numerous non-noble metal-based catalysts, including carbon-based materials [41,42], transition metal sulfides (TMSs) [43,44], phosphides [45][46][47][48], oxides [49,50], etc often used as the catalysts for MABs.In particular, nanostructured transition single metal and mixed metals sulfides have validated an advanced electrocatalytic ORR/OER performance, due to their higher electronic conductivity and fast-charge transfer kinetics.In recent years, reports have shown that the electrocatalytic act of the TMSs can be enhanced by altering the electronic configuration, double layer structure and interface, valence state, and vacancies [51][52][53].
The rational design for the interfacial structure may lead to local charge redistribution and improve the electronic system's stability and conductivity [54][55][56].Additionally, the heterostructured composites with varied valence and electronic states optimize intermediates' adsorption and desorption energies.It creates a synergistic impact between the metal ions throughout the electrocatalysis process [48,[57][58][59].However, the actual performance of batteries in real-world environmental conditions should also be assessed to practically develop the TMSs as effective bifunctional oxygen catalysts in metal-air batteries [60][61][62].Owing to their better ORR/ OER catalytic activity, and tunable surface valence and electronic structure, transition metal sulfides-based nanostructures are frequently utilized as catalysts [63][64][65].
Extensive research has been done on TMSs and their composites to progress competitive ORR and OER catalysts for high-performance MABs, that do not contain precious metals [71][72][73][74].Due to the rapid development of MABs, the evaluation of TMSs is urgently necessary to create forthcoming bifunctional ORR/ OER catalysts.The introduction of TMSs precisely as bifunctional oxygen catalysts in Li-air, Zn-air batteries, and other metal-air batteries has not yet received much attention, even though bifunctional oxygen catalysts in metal-air batteries have been discussed in multiple notable studies [61,[75][76][77].Therefore, the objective of this mini-review is the preparation, properties and electrochemical testing of the bifunctional TMSs catalysts for next-generation MABs.The employment of oxygen electrocatalysts (ORR and OER) based on transition single metal and mixed metal sulfides nanostructures in MABs applications and encapsulate the progress of MABs based on the outlook of materials chemistry is also described.

Synthesis and characteristics
Owing to outstanding optical, catalytic, electronic, and conductive characteristics, transition metal sulfide (TMS) nanostructures have fascinated great devotion in various electrochemical energy conversion systems.The TMS nanomaterials are a main collection of minerals that offer a route to investigate crystals in numerous aspects because of their different crystal structural types and high abundance in Earth as Ni 3 S 2 , FeS 2 , Cu 2 S, etc [78].A variety of nanostructured TMS has been prepared using various physical and chemical strategies, including the solution-phase method, hydrothermal method, sol-gel method, electrochemical approach [79], physical vapour deposition (PVD), chemical vapour deposition (CVD), etc.
It has been observed that the chemical process for the dimension-and morphology-controlled preparation of TMS nanostructures through a green approach is a vital challenge.The choice of environmentally friendly solvent molecules and biomolecules has the potential to build special nanostructures and assembly functions of nanomaterials.Recently, we have methodically explored the relationship among surface morphologycontrolled preparation approaches for FeS nanomaterials [80].A variety of nanostructured FeS materials include nanoparticles-, flowersand rice grains-like morphological structures using chemical, solvothermal, and electrochemical strategies, respectively.As depicted in figure 3, the FeS nanoparticles exhibited hexagonal crystalline nature whereas flowerand rice grain-structured FeS nanomaterials presented orthorhombic and cubic crystallinity.The FeS ricegrain nanomaterials possessed a huge quantity of oxygen moieties (∼47%), edgecontrolled active spots, a huge capacity of electrochemically active surface area, and active defect sites, facilitating OER performance.
Shankar et al have recently reported a one-step electrochemical preparation of bimetallic FeCo nanoclusters embedded in three-dimensional nanosheets on nickel foam (NF) substrate with improved OER catalytic capabilities under alkaline electrolyte [81].The as-developed FeCoS-based electrode materials exhibited nanosheet-like surface morphology with orthorhombic and cubic crystal structures, as shown in figure 4.An exclusive and identical formation of 3D-nanosheet-like FeCoS heterostructures caused no difficulty of OH - access via the great size of active sites.The FeCoS nanomaterials showed good corrosion acceptance under oxidizing conditions are all features of the electrode fabrication strategy.It also avoids high power utilization and does not release any toxic gases.The MnCo-based metal-organic framework was prepared by He and coworkers [82].The heterogeneous MnS-CoS nanocrystals were anchored to free-standing porous N-doped carbon fibers (PNCFs) using in situ growth and vulcanization.The MnS-CoS|PNCFs displayed a twodimensional surface morphology with a cubic crystal structure (figure 5).The physico-chemical properties suggested that there are many defects presented with high electrical conductivity, superior BET surface area, and large pore volume.These defects may be beneficial for enhancing the OER/ORR catalytic activity.To compare the ORR and OER activity of the MnS-CoS|PNCFs, the ORR and OER activity of CoS|NC, CoS|PNCFs, and MnS-CoS|NC were fabricated under similar experimental conditions.
Opallo and co-workers prepared different Ag-based metallic, intermetallic, and metal sulfide nanostructures to improve the interface of solid-liquid and liquid-liquid towards ORR [83].The different combinations of elements were used to vary the types of samples, such as combining silver with additional metal to form the intermetallic nature (Ag 3 Sb), with a non-metal to create the binary chalcogenide (Ag 2 S), and with both of these elements (metal and non-metal) to establish the ternary chalcogenide of silver (AgSbS 2 ).Based on these findings, the properties of the metal-organic precursors may not disturb the reaction, notwithstanding their dissimilar steadiness and decomposition mechanism.The antimony xanthate complex and silver antimony sulfide nanoparticles displayed orthorhombic stibnite and cubic phase crystalline nature.When the morphology of intermetallic nanoparticles was investigated using TEM analysis, larger, irregularly shaped particles than silver nanoparticles were made comparably visible (figure 6).
Sandhyarani Co-workers prepared layered rhenium disulphide (ReS 2 ) nanomaterials via chemical synthesis approach [84].The WS 2 , NiS, and CoS 2 were also prepared through similar experimental conditions to compare the ORR activity with the ReS 2 nanosheets.The oxygen species rather binds on the surface of Re atoms with adsorption energies in the range of −0.14 to −0.15 eV, suggesting the sturdiest locations for adsorption.The adsorbed oxygen species is vertical against the ReS 2 nanosheet.Zhang et al have described an easy and effective way to make molybdenum disulfide nanosheets with gold nanoparticles attached to them (MoS 2 |AuNP) using a   hydrothermal approach [34].The intention is to use these nanosheets in rechargeable Li-O 2 batteries (figure 7).The produced MoS 2 exhibits a consistent morphology, forming nanoflowers with a diameter ranging from 100 to 150 nm folded characteristic is evident, as corroborated by the accompanying TEM.When HAuCl 4 precursors were added to the reaction solution, the MoS 2 nanostructures got smaller and changed shape from nanoflowers to nanosheets, which looked like a hexagonal structure with a lattice face.The attachment of compact and diminutive AuNPs may account for the increased surface area of MoS 2 |AuNP nanohybrids.We also found that the pore sizes of Super P and MoS 2 /AuNP nanohybrids are about the same, which could make the processes of lithiation and delithiation better.Recent research has revealed that MoS 2 -based nanomaterials perform exceptionally well in energy-related applications.Given this fact, they conducted more research on the efficiency of Li-O 2 batteries made in a non-aqueous environment using MoS 2 /AuNP nanohybrids as cathodes.
Numerous transition metal sulfides nanomaterials with tailor-made geometries, dimensions, and active sites are ultimate systems for potential metal-air battery applications.The chemical interactions among metal and S atoms, improving the electronic structures, electrochemical active sites, and intrinsic catalytic OER/ORR activity are key and are often reliant on the selection of reagents, precursors, and preparation approaches.

Metal-air battery applications
In MABs, the positive electrodes, including Li, Na, Mg, Al, Zn, and Fe are not necessarily stored in any reactants and retain tremendously high energy densities.The MABs are generally inhale oxygen from the environment.The positive electrode (anode) often controls the distinctive features of MABs.Specifically, the cell voltage, energy density, and recharging ability of the electrochemical cell certainly depend on the metal electrode and oxygen electrocatalysts [85].Owing to high theoretical density and high-reactive characteristics, Li-air batteries are attractive among other types of MABs.Other positive electrodes including Zn, Fe and, Al are usually abundant and less reactive, increasing the feasibility of MABs.Zinc has been extensively employed as an anode material among MABs due to its high earth abundance, inexpensive, environmentally friendly, high capacity, and long-term durability in aqueous electrolytes.As depicted in table 1, the chemical reaction of numerous aqueous metal-air batteries is given as follows [85][86][87]:

+ +  -
In recent years, an advance development in single active oxygen electrocatalysts for OER/ORR to construct a rechargeable air electrode.At present, a huge volume of research studies on oxygen electrocatalysts based on TMS nanomaterials is performed under alkaline electrolytes because of the least corrosion, fast electrode kinetics, low cost, environment-friendly, and safe operation.We have developed numerous nanostructured FeS, CoS, NiS, and CuS using a single-step electrochemical approach for improved OER in 1.0 M KOH.Among the developed TMS nanostructures, the FeS nanomaterials delivered the best OER activity when compared to other Reproduced from [34] with permission from the Royal Society of Chemistry.nanostructures because of three-dimensional (3D) sheet-like surface morphology, a large volume of active sites, and small polarization resistance (figure 8).Following this study, we have synthesized various FeS nanomaterials include nanoparticles-, flowersand rice grains-like morphological structures towards OER [80].The rice grains structured FeS demonstrated the best OER catalytic activity under alkaline electrolytes among other FeS nanostructures due to the direct growth approach and large amount of active edges of rice grain structures which offered high OER performance.
Continuing this research, Maduraiveeran and co-workers have established morphologically controlled bimetallic FeCo nanoclusters embedded in three-dimensional nanosheets (3D-FeCoS NS) towards OER.In comparison to other reported FeCoS nanostructures, the 3D-FeCoS-B (1:1) showed the best OER catalytic activity in an alkaline medium (figure 9) [81].The advantages of this study include its self-activation and selfsupported fabrication (binder-free) method, low power consumption, avoidance of toxic gas release, lack of the use of hazardous reducing agents and organic solvents, the distinctive surface morphology of FeCoS nanoclusters/nanosheet-based heterostructures, more stable FeCoS nanoclusters under alkaline conditions during electrochemical OER processes, and the format-facilitating effects of Fe and CoS.Various other TMSsbased nanocrystals (CoS/NC, CoS/PNCFs, MnS-CoS/NC, and MnS-CoS/PNCFs) were developed using a situ growth and vulcanization method for improved ORR/OER in 1.0 M KOH [82].Among the developed TMS nanocrystals, the MnS-CoS/PNCFs nanomaterials delivered the greatest ORR/OER activity, when compared to other electrodes (figure 10).Following this work, they prepared various TMS nanocrystal phases, and the nanostructures of the materials, revealed MnS-CoS/PNCF electrode designed via MnS-CoS heterostructure.It was anticipated that the resulting electrode materials may considerably reduce the energy barriers of the oxygen reactions (ORR/OER) through the possession of improved electron-occupied states at the Fermi level, the smallest charge transfer resistance, a larger electrochemical double layer capacitance, more active sites, good long-term stability, and outstanding electrochemical performance as a consequence.
A foundational understanding of the appropriate pairings of Ag with other elements for further studies can be gained from the organized learning.They were developed utilizing metal-organic precursors and the hot injection process [70].A shift in the Ag-oxidation state in the lattice structure is not possible due to the strength of the Ag-S bond.Compared to an oxide ion, which is a hard base, the sulfide ion forms a considerably stronger bond with silver, which is a soft metal.On metal and intermetallic alloy-modified electrodes, the set of peaks corresponding to the oxidation/reduction of silver can also be detected, and the presence of oxygen increases the size of the peak currents.The number of oxygen binding sites changes when the crystal structure changes from face-centered cubic for Ag to orthorhombic (Ag 3 Sb) and monoclinic (Ag 2 S), as well as the occupancy of various atoms (Sb or S).The bonds between the metal and the chalcogenide atoms in these precursors have already been established for favouring ORR catalytic properties [83].
Recently   OER [87].The design of highly active electrocatalysts for ORR is crucial for MABs.Efforts have been made to fabricate various non-precious TMS catalysts toward ORR because of less price and long-term durability than that of metal oxides.Recent progressions in the development of electrocatalysts have prepared a potential assembly of MAB electrodes with a single active and bi-functional electrode material that catalyzes both OER and ORR effectively.Literature shows that the development of bi-functional electrocatalysts for OER/ORR is performed in a much greater portion under alkaline electrolytes which are highly relevant to rechargeable MABs.
In general, bi-functional catalysts may be categorized into three wide collections: (1) non-noble metal-based materials (oxides, sulfides, phosphides, nitrides, etc), (2) carbon-derived materials (carbon nanotubes (CNTs) and graphene), and (3) hybrids/composites materials (mostly both carbon and metal oxide/sulfide/phosphide).At present, the advancement in the development of bi-functional oxygen electrocatalysts is majorly motivated by employing acquaintance attained from highly catalytically active non-precious TMS established earlier or preparing them via a single-pot process to acquire encouraging edges for a synergistic bi-functional electrocatalytic consequence.Shinde et al recently developed zinc-air pouch cells using of copper phosphosulfide employed as a cathode, chitosan-biocellulosics acted as electrolytes, and zinc utilized as the anode (figure 11).These MABs exhibited high energy density of ∼460 W h kg cell −1 and ∼1389 W h L −1 , good rate ability of 5-200 mA cm −2 , possessed a durable cycle life of ∼6000 cycles @ 25 mA cm −2 , and broad range of operating temperature, starting from ∼20 to ∼80 °C.Schmidt and co-workers presented the initial discharge-charge patterns of the Li-O 2 battery using MoS 2 |AuNP nanohybrids [34].3)).On the other hand, the cell containing MoS 2 |AuNP nanohybrids shows discharge and charge overpotentials of 0.21 and 1.28 V, respectively.This results in the highest round-trip efficiency, with a specific capacity of approximately 4336 mAh.g −1 .The cell with MoS 2 |AuNP nanohybrids had a discharge voltage that was 0.09 and 0.05 V higher than the cell with pure Super P and MoS 2 nanoflowers.This showed that MoS 2 |AuNP nanohybrids were better at speeding up ORR than super P and MoS 2 nanoflowers. .The cells exhibited satisfactory performance for up to 50 cycles without any noticeable decrease in voltage.The decrease in capacity may be due to the deterioration of the electrolyte, which may be vulnerable to attack by oxygen-containing intermediates.

Conclusion and outlook
Metal-air batteries have been well-thought-out as next-generation battery devices.Low-cost and enhanced energy-density batteries are highly desired in energy storage devices in the universal urban environs.The MABs possess outstanding scenes due to their merits, such as high energy density, low price, and security.However, numerous tasks must be overlooked for industrial-scale usage.It is highly essential to progress air cathodes and electrocatalysts majorly based on TMS nanostructures that may be employed in several situations (aqueous/ non-aqueous electrolyte, voltage, and operating temperature).Table 2 summarizes the synthetic methods and performance of the MABs based on various transition metal sulfide nanostructures in various metal-air batteries.The notable development of MABs marks the advancement in this field, primarily due to the highly active kinetics of oxygen reactions at the cathode.It is also suggested that further enhancements in this field through strategies like designing electrode materials, modifying electrolytes, carefully selecting separators, and gaining a comprehensive understanding of the reaction mechanism.
An efficient system requires a reversible metal electrode (anode) that has multiple active sites, high recharge efficacy, and long-term solidity.Modifying the structure and composition of the electrode through novel synthetic techniques and adding the appropriate amount of additives or chemical doping can achieve this.The MABs are a safe and reliable choice because they work for a long time.Consequently, the following prospects are  required to be considered to build high-performance MABs: (i) An in-depth understanding of bi-functional mechanisms and oxygen intermediates reaction; (ii) Operation settings, onset potential, current density and temperature to be considered; (iii) Interaction of OER/ORR catalyst with other components of MABs (separator, metal anode, and electrolyte); and (iv) Optimization of cell structure.We strongly believe that the significant progress in this field, together with ongoing devotion and research, will lead to the implementation of TMS materials in a wide range of industrial applications in the future.

Figure 1 .
Figure 1.Pictorial representation of rechargeable MABs.Reproduced from [25] with permission from the Royal Society of Chemistry.

Figure 2 .
Figure 2. Schematic illustration of a typical metal-air battery.

Figure 8 .
Figure 8. LSV curves (a), the plot of E onset versus electrodes (b), the plot of overpotential versus electrodes (c), and Tafel plots (d) of the various electrodes.Reproduced from [86] with permission from the Royal Society of Chemistry.

Figure 12
compares these patterns to those using super P and MoS 2 nanoflowers at identical current densities.This comparison lets us figure out how the MoS 2 |AuNP nanohybrids change the speed of the ORR and the OER in the battery (equations (1)-(

Figure 11 .Figure 12
Figure 11.Method for high-energy zinc-air pouch cell and an evaluation with commercial energy storage cells with appropriate electrochemical results.Reproduced from[88], with permission from Springer Nature.

Figure 12 .
Figure 12.Discharge/charge curves of the Li-O 2 battery at a current density of 70 mA.g−1 (a); The first discharge/charge curves of the Li-O 2 battery (b); The MoS 2 |AuNP electrodes at current densities of 200 and 500 mA•g −1 (c); Curtailing capacity at a current density of 300 mA.g −1 and terminal discharge voltage (d).Reproduced from[34] with permission from the Royal Society of Chemistry.

Table 1 .
List of significant properties of typical metal-air batteries.

Table 2 .
List of preparation methods, and performance of different transition metal sulfide nanostructures in various metal-air batteries.