Review—Carbon Electrodes in Magnesium Sulphur Batteries: Performance Comparison of Electrodes and Future Directions

Rechargeable magnesium sulfur batteries have been chosen as an electrochemical power accumulator device because Mg has a high capacity (3,832 mAh cm⁻³), significantly less reduction potential, naturally abundant, operational safety, ¹ and Mg anodes are highly reversible. The Mg anode material is considered the safe electrode in liquid electrolytes because it does not form dendrites on the electrode surface. Mg favors the expansion of smooth surfaces because of its highly coordinated configuration and lower diffusion barriers. As compare to other anode metals, Mg is economical, and compounds of Mg are non-toxic. Sulfur theoretical capacity is also high (3, 459 mAh cm⁻³ or 1,671 mAh g⁻¹ ). ² So together, the Mg–S battery has high discharge capacity,coulombic efficiency, and cyclic reversibility. Figure 1 shows the working of an Mg–S battery, the charging/discharging processes, and its components.

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An Mg–S battery binder should be non-toxic, inexpensive, have good bond strength, have less electrolyte absorption, and be highly soluble in solvents. Electrodes are commonly the product of carbon host fabric, so this desires to have a robust chemical anchoring of sulfur, excessive electric conductivity, a robotically solid framework, tiny pores without large openings, easy access to a liquid electrolyte, etc. The major disadvantage of Mg is its more positive reduction potential, sulfur's electrophilic solid nature, and the electrical insulating property. These create problems in the advancement of the Mg–S battery.

Mg–S battery work at the electrochemical transformation of sulfur (S₈) to magnesium sulfide (MgS). These reactions occur during the discharging process. Opposite of these reactions occur in the process of charging. ³

At negative electrode:

$$\begin{eqnarray*}&&{\rm{Mg}}\to {{\rm{Mg}}}^{2+}+2{\rm{e}}\end{eqnarray*}$$

At positive electrode:

$$\begin{eqnarray*}&&{\rm{S}}8+8{{\rm{Mg}}}^{2+}+{{\rm{16e}}}^{-}\to 8{\rm{MgS}}\mathop{\mathrm{lim}}\limits_{x\to \infty }\end{eqnarray*}$$

But along with these reactions some side reactions also takes place which forms polysulphides:

Step 1. (solid-liquid two-phase reaction): S8(solid) + 4e⁻ + 2Mg²⁺ → 2MgS4(liquid)

Step 2. (liquid-solid two-phase reaction): MgS4(liquid) + 2e⁻ + Mg²⁺ → 2MgS2(solid)

Step 3. (solid-state reaction): MgS_2(solid) + 2e⁻ + Mg²⁺ → 2MgS_(solid) Some of the problems occur in the Mg–S battery:

In the discharge process, low-order polysulfide forms and precipitates at the sulfur cathode. In the solid phase, magnetization slows down the conversion of long-chain polysulfides to ^MgS. The nature of polysulfides like Mg₃S₈, MgS₂, or MgS is insulating, alongside the lower diffusivity of Mg²⁺ within solid phase, leading to massive overpotential on the last step of discharge likewise final products re-conversion to long-chain polysulfide in charging process is lacking. ³ Dissolution of polysulfides in the electrolyte is another problem. Passivation of magnesium metal anode occurs in most Mg–S batteries; therefore, the localized or inhomogeneity increase of Mg deposits, especially at excessive current densities, is the issue associated with the Mg anode passivation. After considering all these issues, the design of the Mg–S battery is done. Figure 2 depicts the various essential components and some additional components in the design and successful operation of the Mg–S battery.

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The high theoretical energy density and natural abundance of sulphur in Mg–S batteries are considered to be a great substitute for high-energy rechargeable energy storage devices. However, the issues including insulation of sulphur, formation of polysulfides, their dissolution which results in short cycle lifespan have prevented Mg–S battery from being used in practical applications. Here we have categorised various physical and chemical methods to improve the effectiveness and performance of the battery. Through surface coating the shuttle effect in Mg–S batteries can be suppressed. Carbon has a high electrical conductivity, elastic nature and dense structure which makes it a remarkable coating material in enhancing the electrochemical performances of various electrode materials that are used in battery. A solvent system containing of a mixture of tetraethylene glycol dimethyl ether (TEGDME)/diethylene glycol dimethyl ether (DEGDME) with commonly used additives such as LiNO₃ when added to Mg−S batteries can increase their cycling stability and reduce the polysulfide shuttle. An artificial SEI between the Magnesium anode and separator which reduces the number of side reactions on the metal surface is an appropriate strategy to mitigate the self-discharge and various negative effects of the polysulfide shuttle in the battery. The barriers for polysulfide physical separation i.e. use of separators such as carbonaceous materials. Many porous carbon materials, such as microporous and mesoporous carbon have the ability of adsorbing polysulfides into their large surface area and abundant pores structure. Many other separators are also mention while discussing the cathode material preparation in this study. Sulphur loading is another process through which sulphur can be introduced into the fabricated matrixes containing enough pores for the accommodation of sulphur. Melt diffusion gives a closer contact between sulphur and matrixes, vapour deposition is another approach to obtain cathodes that can easily accommodate sulphur molecules. Both these methods are highly efficient in preparing composite cathodes with carbon present in them though carbon and sulphur bonding is strong but it also facilitates the sulphur particles into the porous voids while the matrix consists of oxygen-rich carbon. Many carbon structures have been described further in the report that are good matrix to host sulphur such as graphene, porous carbon, CNTs, mesoporous carbon and graphene, nanofibers, a hollow carbon fibers foam which consists of multiwalled carbon nanotube and carbon black. The host exhibited higher absorbability and fixation of polysulfides and also showed improved electrochemical performance keeping good cycling stability with sulphur loadings. Doping carbon with various elements like boron, nitrogen, phosphorus, or sulphur can be done to enhance the chemical adsorption towards polysulfides without affecting the conductivity. Apart from this, many nano-sized metal oxides structures can be used to fabricate feasible cathodes. Materials containing porous and hollow structures can provide physical confinement and enhance adsorption of polysulfides. Many sulfurized polymers were also design to improve the physical adsorption of sulphur particles. Thus composite electrodes with various chemical and physical confinement ways can be a promising approach to reduce the shuttle effect of polysulfides. The cathode design can ideally consist of higher sulphur loading capacity, conductivity of material, larger specific surface area and high adsorption sites such as porous or hollow structures. ⁴

A battery consists of mainly three components: cathode, anode, and electrolyte. Different types and forms of anode, cathode, and electrolyte can be used according to the need. Other additional materials are added to these three components in order to enhance the battery performance. These materials affect the performance of the battery directly or indirectly. Various components help in improving distinct properties of electrodes and electrolytes. An anode is made up of magnesium material for an Mg–S battery, and a cathode is made up of porous carbonaceous sulfur-doped material. The main reason for capacity fading in reversible magnesium batteries is the dissolution of sulfur in electrolytes. This can be prevented by using a non-nucleophilic electrolyte.

Cathode

Current collector

Binders

Anode

Electrolytes

Electrolyte is a crucial part of a battery that acts as a medium for the transfer of Mg ions between the S cathode and the Mg anode in Mg–S batteries. Many attempts have been made to outline electrolytes that can be suitable for magnesium-sulfur batteries. It should be noted that ordinarily nucleophilic electrolytes are not compatible with S due to their electrophilic nature, which causes battery failure. This new study on nucleophilic electrolytes was further discussed along with the preparation of non-nucleophilic electrolytes. Types of electrolytes used in Mg–S batteries:

• 1)  
  Electrolyte with a high concentration of Mg/Al is compounded by TiCl₄ reaction. Electrolyte synthesis reaction was preceded by room temperature.
• 2)  
  MgCl2, AlCl₃, anthracene, and (CF3SO₃)₂, dissolved in THF and tetraglyme (TG) under pressure at room temperature. Solutions obtained without additional purification were used as electrolytes.
• 3)  
  Mg(TFSI)2 - This electrolyte contains magnesium bis (trifluoromethane sulfonyl)imide (Mg(TFSI)2) in dimethoxyethane, tetrahydrofuran, triethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetra-ethylene glycol dimethyl ether. The most typical electrolyte used for electrochemical experiments of Mg/CMK3 and Mg/PTMA cells S was 0.3 M magnesium bis (trifluoromethane sulfonyl) along with (Mg (TFSI)2) dissolved in the chemical ether chemical glyme/diglyme. Outcomes of several solvents, such as THF, acetonitrile, glyme, triglyme, tetraglyme, and diglyme in Mg stripping from the Mg electrode, were scrutinized.
• 4)  
  (Mg2(m-Cl)3·6THF) (HMDSnAlCl4−n) (n = 1, 2)—The addition of AlCl3 upgraded the performance of HMDSMgCl. Active material crystallization can be an important process in synthesizing a non-nucleophilic electrolyte with desirable properties like cathode compatibility. This step can augment coulombic efficiency, potential stability, and much more.
• 5)  
  Ether-based electrolytes—An essential requirement for the electrolyte applied in sulfur batteries are chemical stability, and Ethers are preferred for their ability to enhance chemical stability. The more the number of oxygen atoms, the more is the solvation ability. A cell with the PEGDME-based electrolyte evinced a capacity close to 1100 mAhg⁻¹ of sulfur material at 0.1 C.
• 6)  
  Mg(Tf)2- MgAl2O4-PVDF-HFP, a ceramic polymer composite electrolyte, was selected for a battery design, with a maximum performance of 4 mS cm⁻¹, a potential window of about 3.3 V, and a transmission number of 0.66.
• 7)  
  A rechargeable Mg–S battery using an electrolyte consisting of complex active electrochemical in situ using HMDSMgCl and AlCl3 salt. Subsequently, bisamide salt Mg (HMDS)2 was widespread using Mg (TFSI)₂ salt. However, in both of the scenarios, the forced availability of chloride additives (MgCl₂, AlCl₃) increased the degradation of the electrolyte and compelled the use of current metal collectors that are unfit for large-scale production.
• 8)  
  Addition of many THFPB activities for the preparation of an electrolyte based on magnesium borohydride. The primary purpose of THFPB is to significantly enhance the softness of Magnesium Borohydride in diglyme solvent due to the strong bond linking the THFPB molecule and BH4⁻ anion. Taking the help of the strong electron-acceptor power of THFPB infused, high concentration (0.5 M, approximately fifty times more than the DGM electrolyte/Pristine Mg (BH4)2) of Mg (BH4)2 was attained. Another function of the THFPB supplement is to modify the anodic stiffness of the Mg(BH4)2 electrolyte by generating new anion and cation types as indicated by the NMR, Raman, and MS spectra. The potential power window has been increased from 1.8 V to 2.8 V compared to Mg, which makes electrolytes have the ability to test high voltages. The next activity of the THFPB supplement may result in the nucleophilicity of the newly formed anion and cation, hence making the electrolytes of Mg(BH4)2/THFPB suitable with the sulfur cathode. Therefore, we explain the first use of Mg (BH4) 2/THFPB electrolyte in magnesium-sulfur (Mg–S) batteries. These studies present a practical and explicit approach to getting rid of hurdles faced by Mg(BH4)2 based electrolytes and bring out the significant impact of active additives on unsatisfactory Mg-ion electrolyte structures.
• 9)  
  In Mg-cells, the [(HMDS)₂Mg] compounds containing MgCl₂ and AlCl₃ in the TEG and DEG mixture were exploited. It was shown that the (HMDS)₂Mg based electrolyte comprised of [HMDSAlCl₃]+ [Mg₂Cl₃] as active electrochemical species existed as a balance in solution and no free Cl-anions. As indicated in MACC (magnesium aluminum chloride complex), an electrolytic correction process is not required for this electrolyte. The presence of MgCl₂ on the anode surface after initial release, initial activation, and secondary release has been detected by EDX but may be due to contamination of Mg-based electrons, which may not be thoroughly washed. If a chlorine shuttle facing the anode were to occur, the highest value would be considered at 199. 7 eV, possessed by MgCl₂, would be considered, which was not the case, and vanadium would be reduced, which was also unprecedented. Therefore, we do not exclude that Cl-shuttle occurs in the VOCl/Mg system. Fig. 3 describe the difference between battery and supercapcitor on the basis of energy density, power density, life cycle, and applications.

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In this section, we mention some important and ground-breaking developments reported in recent years regarding Mg–S batteries. Figure 4 represents some of the technical advancements in recent years with respect to the electrolyte used in the Mg–S battery.

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In one of the earliest developments in the electrolyte, Zhao-Karger, Z. et al. (2014) reported a novel method of synthesizing nucleophilic magnesium electrolyte using a single pot two steps reaction. The reaction used magnesium bis(hexamethyldisilazane) [(HMDS) ₂ Mg] and AlCl₃ as reactants in various ethers. This was synthesized in different ethers or Glymes CH₃O- (CH₂CH₂O)n-CH₃ and ionic liquids. They used (HMDS) ₂ Mg-based Diglyme(DEG), Tetraglyme(TEG), and their mixtures in ionic liquids represented as DEGIL and TEGIL electrolytes in their Mg–S batteries. These reactants used by them produced magnesium complexes that were non-nucleophilic. They used sulfur-carbon composite S/CMK400PEG and S/CMK160PEG as a cathode. The performance of batteries was improved as they reported for the first time a discharge potential of 1.65 V, a value close to the thermodynamic value. ¹²

Further, Robba A. et al. (2017) investigated the operation of an Mg–S battery using electrolytes composed of simple salts in binary mixtures of glymes. MgCl₂ and Mg(TFSI)₂ were dissolved in a ratio of 1:1 in a mixture of TEGDME: DOL. The study resulted in the conversion of 80% of sulfur into MgS during the first discharge. Reduction gave two plateaus while oxidation proceeded through a single plateau. However, the capacity was fading fast. ¹³ Manjuladevi R. et al. (2018) described a new development in the electrolyte system. They studied a magnesium ions conduction Blend Polymer electrolyte, a type of solid polymer electrolyte(SPE). These BPE films consisted of Mg(NO₃)₂ salts to boost conductivity and 92.5PVA:7.5PAN blend. PVA(polyvinyl alcohol) was used as a host matrix. They used PAN(polyacrylonitrile), a special conjugated polymer, to create a blend to enhance conductivity. The experiments were carried out in different concentrations of Mg(NO₃)₂ during synthesis to determine the effect of salt concentration on conductivity. 92.5PVA:7.5PAN:0.3 m.m.% Mg(NO₃)₂ composition resulted in highest electrical conductivity. ¹⁴ Supriyono., 2018 evaluated a completely new electrolyte for the Mg–S battery. This battery used 3.5%wt NaCl solution as electrolyte. He also did modeling using a simulation to obtain the optimum conditions for the usage of the battery. The discharge mechanism of the battery was studied during main and side reactions. The model was used to evaluate the maximum and minimum state of charge (SOC). ¹⁵ To further enhance the cyclicity of magnesium sulfur batteries, Wang et al.,2018 introduced a new electrolyte (PhMgCl)₂−AlCl₃/tetrahydrofuran-based, a nucleophilic electrolyte. The cathode was sulfur impregnated in mesoporous carbon matrix type (S@MC). The sulfur in the matrix existed as many sulfur species like S₂₋₄ molecules and huge ringlike S₈. However, the surface consisted of S₈ species. The carbon matrix was chosen to improve the reaction kinetics and for adsorbing polysulphide species. Earlier, copper was used as a current collector in Mg–S batteries. ¹⁶ In the same year, Liu R. et al. (2018) demonstrated various electrolytes for Mg- S batteries. They have shown that Magnesium mono carborane (MMC) Mg(CB₁₁H₁₂)₂ in glyme results in high anodic stability (approx 3.8 V) and an elevated Magnesium cyclicity. Magnesium hexafluoroisopropanol aluminate Mg[Al(HFIP)₄]₂ in ethereal solvents, the deposition of Mg is reversible with elevated conductivity (greater than 6 m Scm⁻¹) and oxidative stability (greater than 3.5 V vs Mg/Mg²⁺). The thermal stability of Mg[B(HFIP)₄]₂·3DME is up to 150 °C, and it is also hydrolytically and air-stable, Mg[B(HFIP)₄]₂ is, in fact, adaptable with any cathode and anode materials which makes it an all-inclusive electrolyte for Mg batteries. The weakly cooperating nature of the [B(HFIP)₄]⁻ anion allows the effortless segregation of the salt, assists suitable Magnesium dissolution and deposition in the electrolyte. Thus the MgBhfip electrolyte can be used for cathode materials with high voltage. As stated in the common ion effect in Magnesium-Sulfur batteries with Mg[B(HFIP)₄]₂ electrolyte, the plateau of potential during discharge faintly rises after the first complete cycle, the voltage gap between the charging and discharging decreases. ¹⁰ Later, Wang W. et al. (2019) introduced an electrolyte of Mg(CF₃SO₃)₂−AlCl₃ dissolved in tetrahydrofuran and tetraglyme mixed solvent. DME is a potential solvent for Mg(TFSI)₂ + MgCl₂ electrolyte because it makes MgCl₂ more soluble in the solution. Therefore MgTFSI₂ + MgCl₂/DME solutions result in enhanced reversible deposition of Mg. Using THF + TG mixture (in 1:1 volumetric ratio) in the form of solvent, a reduced overpotential of 0.1 V for Mg deposition and dissolution was found. The Coulombic efficiency here was found to be 89.6% at the end of the first cycle, and there is 95% stability after the third cycle, and further, the performance continues to be stable for 200 cycles. ⁶

Figure 5 shows the development of carbon electrodes in the field concerning the dopping material in the current years to increase the sulfur loading of electrodes. While discussing the developments of electrodes in Mg–S batteries, an advanced class of cathode was presented by NuLi, Y. et al. (2007). These novel cathodes used organo-sulfur compounds with S–S bonds for rechargeable magnesium sulfur batteries. The battery's capacity during charge and discharge was due to the cleavage and recombination of these S–S compounds. They studied three organo-sulfur materials, namely 2,5-dimercapto-1,3,4-thiadiazole (DMcT), poly-2,20 dithiodianiline (PDTDA), and a conductive sulfur-containing material (CMS). ¹⁷ More progress occurred when Vinayan B. P. et al. (2015) developed a magnesium battery with Reduced Graphene oxide (r-GO) and Sulphur composite as cathode material. The electrolyte used consisted of a non-nucleophilic base of magnesium-bis (hexamethyldisilazide) [(HMDS)2Mg] metallated with AlCl3 dissolved in solvents of various ethers. The anode consisted of a magnesium-carbon composite. The r-GO material in the cathode was used to exploit its excellent properties like colossal surface area, elevated electronic and thermal conductivity, and increased charge mobility. ¹ They reported that this kind of battery delivered a high capacity, high cyclicity, and rate capability. From the usual developments in cathode material, Sievert, B. et al. (2017) have developed and taken an incredible stride in anode material development. They reported a design of magnesium powder anode. Their studies showed an improved behavior of magnesium powder anode batteries over magnesium foil anode batteries. ¹⁸ Later, Vinayan, B.P. et al. (2019) developed a new cathode material for magnesium sulfur battery using different sulfur loadings ranging from 3–0.5 Mg–Sulfur per cm². This cathode used nitrogen-doped hybrid nanocomposite of multiwall carbon nanotubes (MWCNTs) and graphene as a host material. They studied the generation of various polysulphide species during the charge and discharge of this battery using operando Raman spectroscopy and XPS. They synthesized Graphite oxide from Graphite, and MWCNTs were synthesized by the method of chemical vapor deposition. These materials were further mixed and treated to produce the cathodic material. ¹⁹ In further works, He et al., 2019 proposed a novel cathode using Cu on carbon nanofibers as an additive to the cathode. These composites were then mixed with sulfur to make a Cu-S cathode. They reported excellent cyclicity up to 100 cycles. Mg–S battery operations are hindered because of the powerful electrostatic attraction between magnesium cations and cathodic material. ²⁰ To improvise the reversibility of the Mg–S battery, Sheha et al., 2019 (cathode) have proposed a new design for cathode material with V2O5(vanadium oxide) as host matrix. This can enhance the reversibility of Mg ion intercalation and extraction. ²¹ Since separator material in Mg–S batteries was not explored that much, therefore, Zhou et al., 2019 (separator) have demonstrated the use of novel separators for Mg–S batteries. Also, since most of the magnesium sulfur poses serious safety hazards caused due to the shortcomings of separators which can be flammable, and of shrinking nature at high temperatures, this could lead to undesirable polysulphide dissolution. Therefore, they have developed a multifunctional Janus separator of polyimide nonwovens with a copper nanowire-graphene nanosheet functional layer and an inflexible lithium lanthanum zirconium oxide-polyethylene oxide matrix. ²²

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The reaction products formed during discharge were also able to reverse back. Moreover, the most common problem encountered in the magnesium sulfur battery operation is the polysulfide shuttling, corrosion of magnesium anode, and low cathode stability. To alleviate this issue, Sun, J. et al. (2020) introduced a design of cathode material having impregnated sulfur in carbon-confined Cobalt in a mesoporous matrix (MesoCo@C). The CoSx species played a crucial role in trapping these polysulphide species. Also, this design had the dual advantage of having Cobalt and carbon as good electron conductors. ²³ Zhang et al., 2020 studied the effects of different types of binder in cathode performance. They have used sulfurized pyrolyzed polyacrylonitrile (S@pPAN) composite as cathode material. Binders studied were poly(ethylene oxide) (PEO),carboxymethyl cellulose (SCMC), sodium polyacrylate (PAAS), guar gum (GG),locust bean gum (LBG), poly(vinylidene fluoride) (PVDF), and β-cyclodextrin (β-CD). They reported that the PAAS binder showed better cyclicity and electrochemical performance compared to the standard PVDF binder. ¹⁶ To counter the sluggish kinetics of redox reactions in magnesium sulfur batteries involving Sulphur and its low electronic conductivity, Zhao et al., 2020 developed a new host for sulfur, i.e., Co₃S₄@MXene heterostructure. This host proved to be an effective solution as it catalyzed the conversions of polysulphide intermediates and was thus successful in improving the kinetics. Moreover, the MX_ene matrix allowed the diffusion of magnesium ions. ²⁴ A new composite material as a cathodic material in Mg–S batteries was investigated by Wang P. et al. (2020). Sulfurized poly(acrylonitrile) ("SPAN") was used as a composite in their work. They have used a fluorinated electrolyte inTetraglyme solvent. A high concentration of electrolytes was preferred to reduce the dissolution of magnesium polysulfides. They studied the electrochemical results of Mg–S batteries based upon SPAN and compared it with two different types of anodes. Mg foil anode and Mg-powder anode were contrasted. Their studies showed that Mg-powder anode with SPAN gave better results as compared to Mg-foil anode. ¹¹ Oxygen vacancies to spherical NaV₆O₁₅ cross-linked with carbon nanotubes (CNTs) (SNVO_X -CNT) as a cathode material were introduced by Yang Y. et al. (2020) to achieve an excellent long-time cycle life of rechargeable magnesium batteries (RMBs). They reported that SNVO_X-CNT showed a higher discharge and charge-specific capacity than CNVO and SNVO. SNVO_X -CNT was able to sustain 120 mAh·g⁻¹ after 180 cycles at 50 mA·g⁻¹. The initial Coulombic efficiency of SNVO_X—CNT reached 98%, indicating excellent reversibility. ²⁵ The establishment of oxygen vacancies improved the diffusion kinetics of Mg²⁺. ²⁶

In Fig. 6, different reversible batteries compatible with the Mg–S battery and good charge—discharge capacities are mentioned. Lithium-ion battery (LIB) has attained great success and is currently an excellent device for energy storage for portable electronics, powering electric vehicles considering its high power and energy density and eco-friendly nature. Li has meager oxidation potential (−3.04 V) among metals, hence considered as an appropriate anode to provide high energy density for a fuel cell system. In these batteries, electrolyte typically reduces on an anode to form an (SEI)solid electrolyte interface layer that opposes more electrolyte decomposition while enabling lithium-ion movement. Also, the Earth's crust has an uneven and lesser distribution of lithium metal resources. However, currently used lithium-ion batteries cannot meet the increasing requirements for higher energy density, low cost, and safety. The need for Li-ion batteries is expected to increase in the future, and hence it raises concerns about its demand fulfillment due to lack of element sources. Lithium-sulfur batteries have higher potential, seeing as it is cheap and less toxic nature of sulfur, and significantly higher theoretical capacities of the sulfur cathode (3459 mAh ml⁻¹, 1672 mAh g⁻¹) and lithium anode (2046 mAh ml⁻¹, 3861 mAh g⁻¹). ²² However, dendrite formation and weakened coulombic efficiency during the repeated deposition-dissolution limit the growth of these batteries. In addition to this, its price and lack of resource availability of Li metal reduced its Large scale applications. In Li-S batteries, current collector Cu and powdered Cu were used to increase the electrochemical property of sulfur cathodes in lithium-sulfur batteries by converting sulfur to electrochemically active Cu₂S. In these batteries, there is no such complex multielectron solid-liquid-solid conversion as in the Mg–S battery. Usage of separators in both Li-S batteries and Mg–S batteries improves battery-safe operation and electrochemical properties of batteries. When the polysulfides are generated, the shuttling phenomena occur, resulting in loss of capacity, corrosive nature of anode, and coulombic efficiency decay. Researchers have also focussed on several new technologies which have emerged, such as magnesium-ion batteries (MIBs), aluminum-ion batteries (AIBs), potassium-ion batteries (KIBs), and sulfur-ion batteries (SIBs). ²⁷ Benefitting from sulfur's low redox potential, it can be easily combined with metals such as Na, K, and more, increasing the cell voltage of the particular cells. The energy density gets maximized in this case. The formation of sulfides from sulfur leads to significant volume change in the cells. There is an expansion in the volume in the range of 80% (S to Li2S) to 300% (S to K2S), indicating more space is required, and also solid electrodes can crack. There is more dendrite growth as well as electrolyte decomposition in the cells. Table I given below shows how advantageous the various properties of Mg are compared to different metallic anodes. The Na-S battery is the currently developed high-temperature battery that operates at near 300 C, which is also based on liquid electrodes such as molten sulfur or polysulfides and molten sodium, which are set apart throughout the solid electrolyte. Many other membranes are used to prevent the polysulfides shuttle phenomena from the electrodes. Polymer-based membrane material Nafion suppresses the shuttle effect in Li-S and Na-S electrochemistry. Recent observations declared that there is no complete suppression. Finer overall cell performance and cycle stability have been displayed when kept at room temperature in Na-S batteries. On the contrary to Mg electrolyte, potassium electrolyte solutions are prepared with traditional salt in solvent methods. Potassium metal electrodes provide 685.5 mAhg, which is the lowest among the metals. The theoretical voltage of K–S cell (1.88 V) is close to Na-S cell but is not very high to Li or Na and fulfills the requirements. Although, the lower melting point of K for liquid electrode understanding can be beneficial. ²⁸

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Rechargeable Mg-ion batteries have an excellent capability to supersede Li-ion batteries because of the properties like the absence of formation of dendrites during cycling, having excellent energy density, and being less expensive. Table II gives insights into the high performance and operation when pure sulfur is used as a cathode for lithium and Magnesium sulfur batteries. Magnesium metal has shown more superficial processing features, lesser reactivity toward moisture or air, and lesser dendrite formation than lithium gaining from the strongly bonded magnesium(Mg) atoms along with better ion mobility. These are significant for the current developing technology because lower costs, high energy density, and safety make metallic Mg a potential anode in practical batteries. Also, the large size of ion and slow diffusion capacity of bi-valent Mg ion(Mg2+) slows the kinetics of the reaction to advance cathode reaction kinetics to gain fast Mg conduction and diffusion. However, the large size also leads to high polarisation, which results in the diffusion of Mg2+ ions in the host materials.

In Table III, a comparison based on various parameters is drawn between the RMBs and other metal sulfur batteries. It demonstrates the high performance of RMBs than the metal-sulfur battery as a promising safe and better design with the durable operation of the battery for future generations.

We know that Mg ion batteries have become very popular due to their ability of storing large amount of energy in energy storage materials, which are used in huge mobile and immobile appliances. Recently vanadium oxychloride has been utilised as cathode material in Mg based battery. But unfortunately, the battery delivered a very capacity of 45 mA h g⁻¹ in the first cycle. For improving the results, the interlayer of VOCl has to be expanded for the easy insertion of Mg ions into the structure of electrode material. Therefore, the VOCl cathode was cycled with Li. As a result of which, the spacing between the layers of cathode increased and thus higher capacities were obtained. 170 mA h g⁻¹ capacity was obtained after the successful treatment. ³⁴ Vanadium oxides have always been an important option as electrode materials. This may be due to their cheap cost and abundance. Furthermore, H2V3O8 nanowires has been evaluated as cathode material. But here, the spacing between the layers is greater in V3O7 structure as they are connected by H-bonds. The battery displayed a very high specific capacity of 305. 4 mAh g⁻¹. The battery retained 261.2 m Ah g⁻¹ even at the end of 20 cycles. ³⁵ Fig. 7 represents various other types of cathode materials have been studied for rechargeable Mg based batteries. Another type of cathode material is the Chevral phase, they consist of Mo₆S₈, Mo₆Se₈, Mo₆Te₈. They also work as intercalation materials accommodating Mg ions. These materials are crystalline in nature, with S8 anions occupy the corners and Mo6 atoms occupy the face side. However, a battery using this cathode material gives low capacity on discharge. And for maximum it has to be operated at a higher temperature. ³⁶ Polymer based materials have also been evaluated as cathode materials. One of them is PAQS(poly(antraquinoyl) sulfide). It was developed to increase the cycling efficiency of battery. This battery gave an initial discharge capacity of 100 mAh g⁻¹. The cycling efficiency was also found to be moderate, as after 80 cycles the capacity reduced to 60 mAh g⁻¹ from 100 mAh g⁻¹. ³⁷

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Herein, we report the development of different electrodes that can be used in Mg–S batteries. Cheng Y. and his coworkers in 2012 reported a Li+ intercalation cathode (Mo₆S₈) and Mg anode. This battery delivered an excellent performance rate (about 83% retention of capacity at 15 C). These hybrid batteries show outstanding rate capability, reliability, and superior safety with suitable discharge voltages. This battery showed low capacity fade (∼5%) after 3000 cycles, and the Mg metal surface was also clean and smooth with no dendritic structure. These Chevrel phase compounds display good Mg²⁺ ions intercalation characteristics. However, the diffusion of Mg²⁺ ions is relatively slow, and the rate capability is also constrained at room temperature. ³⁸

Shunsuke Yagi et al. 2013 used a glassy carbon electrode and coating the same on Ti and stainless steel electrodes with a Grignard reagent-based electrolyte. ³⁹

Huiea et al., 2014 presented that magnesiated Prussian blue or Platinum is used as a counter electrode and water considerably plays a crucial role in the electrolyte and cathode crystal to help determine the characteristics of aqueous Prussian blue Mg-based battery. As observed in hydrated V₂O₅, water present in interstitial sites within Prussian blue structure helps disturb Mg²⁺ ions as it is very well known that the use of water in Mg batteries causes the magnesium oxides to accumulate on the surface of the anode because of the reaction between Mg and water and it restricts the usage of water in Mg batteries. Magnesium oxide formed on the surface leads to power and current losses as they are unstable and inhibits ion diffusion. Various cathodes materials for Mg and Mg-ion batteries are molybdenum oxide, molybdenum sulfide, Chevrel phases, vanadium oxide, manganese oxide, Prussian blue, and transition metal silicates. Mg ion transfer properties are enhanced by using layered vanadium oxides, which exhibit high capacity, high surface area, and enhance operating voltage and formation of tiny crystallites with acceptable water content. Manganese oxides provide a range of structural forms with varying electrochemistry. The layered spinel structured materials (birnessite) presented better cycling than (hollandite), with a tunneled structure. Irreversible structure changes are induced by insertion and removal of divalent charges through charge trapping exhibited by orthorhombic structured molybdenum oxides, which thin films could address. One of the most studied cathode materials for Mg batteries, Molybdenum sulfides exhibit less structural degradation upon cycling because of less structural rigidity than oxides. Although slow diffusion rate and magnesium ion charge trapping are still an issue, high-temperature cycling for improving diffusion rate and nanomaterials' usage can better perform. Molten salt and sol-gel synthesis methods are used to prepare silica with iron, cobalt, or manganese, which is a key to generating viable electrochemical performance in Mg batteries. The advantage of involving molecularly based aqueous systems and active cathode materials having a structure analogous to Prussian blue (nickel and copper hexacyanoferrate) is that it paves pathways for Mg2+ ions mobility. ⁴⁰

Shuojian Su et al. 2015 introduced an Mg–S battery with a titanium dioxide cathode containing Mg anode coupled with magnesium borohydride/tetraglyme electrolyte. Commercially available TiO₂ was used without further treatment and investigation of TiO₂ to be used in batteries. The capacities at various current rates from 0.1 to 2 C were measured for evaluating the rate performance of TiO₂. It was noted that TiO₂ exhibited almost the same theoretical capacity and initial discharge capacity. The Mg–S batteries showed enhanced rate capability and capacity. ⁴¹ NaWu et al., 2015 exhibited an improved electrochemical performance of Li₄Ti₅O₁₂ electrode in an Mg battery system. This was done by and Li⁺ and Mg²⁺ co-insertion into large-sized LTO electrodes. It showed excellent energy storage capability of different large-sized electrode materials, effective in rechargeable Mg batteries. ⁴² Bucur et al. 2015 mentioned that some cathode substances which showed better performance in Li-ion batteries did not respond the same way in Mg batteries showing poor reversibility; this suggested that the variety of compatible materials can be entirely different for Magnesium batteries than in Lithium batteries. The effect of water on the cathode's overall performance can be used to enhance it if the electrolyte stability and Mg electrode reactions can be minimized. Electron delocalization is preferred over transition metal oxidation/reduction to improve charge stability in insertion cathodes. An electrolyte that can be catalyzed through highly electropositive cathodes can form blockading films through their oxidation, which is considered one of the most significant issues. Ether solvents exhibit good reductive stability, making them suitable for most successful Mg electrolytes, whereas their oxidative stability is not as good as that of carbonate solvents used in Li-ion battery electrolytes. ⁴³ Itaoka et al.,2015 showed that sulfur-containing composite electrodes showed a faster electrochemical reaction rate when compared between triglyme electrolyte and acetonitrile electrolyte. The cyclic ether unit has a better cycling behavior of the cell with the sulfur-containing composite electrode than the linear ether unit. The reason behind that may be the formation of the paths for ion conduction in the cathode by the cyclic ether units and Mg ions. An increase in the internal resistance can be prevented by these paths. ⁴⁴

Bonnick P. et al. (2016) suggested layered TiS₂ as a favorable intercalation material for positive electrodes. TiS₂ electrode particles are micrometer-sized. This layered TiS₂ is prepared by grounding titanium powder with 10% excess sulfur. For the Mg'sentire cell, this provides a 115 mAh g⁻¹ capacity. By cycling at high temperatures, the excessive migration barrier to magnesium diffusion can be reduced. With the partially magnesia TiS₂ formed on the first cycle, reversible Mg²⁺ de/intercalation can be attained. ⁵ Vinayan et al.,2016 presented the Mg–S mobile in which sulfur diffused reduced graphene oxide because cathode gives excellent cyclability and a reasonable charge functionality because of reduced graphene oxide's unique purposeful and morphological properties. An Mg/S battery with exact cyclability the usage of a graphene-based sulfur composite electrode and a non-nucleophilic electrolyte. Reduced graphene oxide represents a buffer layer, houses the quantity modifications on top of electrochemical cycling among sulfur and magnesium sulfide, and gives accurate electronic conductivity and an excessive surface area for the dispersal of active material. ⁴⁵

Sievert, B. et al. (2017) have reported a novel Mg–S battery design using Mg powder anode. They synthesized the metal anode using magnesium powder(99.5%) and graphite powder. They prepared two different powder anodes, namely PALP(powder anode at low pressure) and PAHP(powder anode at high pressure), depending on the pressure they applied to this powder while pressing it in a hydraulic press. They compared these electrodes with magnesium foil anodes. Their research has utilized the 50 S and 70 S cathodes, where 50 and 70 represent the mass fraction of sulfur in the cathode. They reported that Mg-PALP cathode in 50 S and 70 S delivers a very high capacity of about 600 Ah/Kg_sulfur in the first cycle. The capacity fades quickly. Moreover, an insufficient capacity was obtained after ten cycles.

Their experiments reported that the PALP magnesium anode absorbs the electrolyte while the PAHP anode does not. PAHP anode is more similar to foil anode due to its dense structure. PALP anode showed more clearer voltage plateaus in both charge and discharge as compared to PAHP anode. PAHP anode produced a single plateau, and the capacity retention was not good at least up to ten cycles. The charge/discharge profiles of PAHP anode and foil anode were similar. To conclude, powder anodes show better results than foil anodes. ¹⁸ Tian, H. et al. (2017) studied the use of Iodine as a cathode material. Here, I2 can be reduced to I₃ ⁻. Its reversible redox reaction involved I₃ ⁻ to I⁻. The battery could deliver a capacity of 180 mAh g⁻¹, a voltage of 2 V. The capacity retention was about 95% at 0.5 C rate even after 120 cycles. ⁴⁶ Minella et al., 2017 showed that the original, non-increased VOCl confirmed no magnesium intercalation/deintercalation. After the primary discharge at 298 K, the specific potential was enhanced by an aspect of 4 to 170 mAhg⁻¹ because of Li pre-treatment. During the first recharge in Mg-cell, the Li present in the VOCl structure goes back and forth from cathode to electrolyte, following the concept of hybrid batteries. Intercalation of magnesium is caused after this, and it exists over the Li intercalation, which is confirmed by battery testing. Easy and quick in situ methods have been presented to customize layered nanostructures in non-aqueous electrolytes and the adaptability of VOCl for electrode applications. ⁴⁷ Karger et al., 2017 showed that nucleophilic, organomagnesium compounds could damage the electrophilic cathodes, which include sulfur; otherwise, they are, ironically, pivotal in synthetic organic chemistry. The nucleophilic attack at the sulfur cathode was mitigated utilizing crystallizing the electrochemically energetic species from a 3:1 mixture of HMDSMgCl: AlCl₃. The wider voltage window and improved coulombic performance, and electrochemical blessings are carried out through crystallization. XPS analysis supplied us with a method to reveal the electrochemical conversion of the cathode while it was charged and discharged. ²⁴

Du et al., 2017 effectively synthesized a singular SGDY cathode for Li–S and Mg–S batteries, displaying first-rate electrochemical performances including advanced rate capability, potential solid retention, high coulombic efficiency, and massive ability, except for the first cycle. Electrolytes like the commonly used Grignardreagents-based electrolyte and commercially available carbonate electrolytes are all stable with this cathode. The richness and flexibility of the graphyne family confirmed the high and strong ability of assembled Mg–S battery as much as 36 cycles, which is advanced to previous reviews, this is because of its promising ability to enhance the content material of sulfur through tuning graphyne structure, thus presenting a considerable potential towards high-overall performance and low-cost sulfur-based batteries. ⁴⁸

Manjuladevi R. et al. (2018) have reported a Blend polymer electrolyte film capable of conducting magnesium ions. This Mg–S battery has used Mg anode, BPE film as an electrolyte, and MnO₂ and Graphite composite cathode. Graphite was used in this to make the cathode electronic conductor. Following are the reactions occurring in the battery-

$$\begin{eqnarray*}&&{\rm{Mg}}+\left({{\rm{2OH}}}^{-}\right)\to {\rm{Mg}}{\left({\rm{OH}}\right)}_{2}+2{\rm{e}}\left(\,({\rm{oxidationatanode}}\right)\end{eqnarray*}$$

$$\begin{eqnarray*}&&2{{\rm{MnO}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}+2{\rm{e}}\to {{\rm{Mn}}}_{2}{{\rm{O}}}_{3}+2{{\rm{OH}}}^{-}\left(\,({\rm{reduction}}\,{\rm{atcathode}}\right)\end{eqnarray*}$$

$$\begin{eqnarray*}&&{\rm{Overallreaction}}:{\rm{Mg}}+2{{\rm{MnO}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{Mn}}}_{2}{{\rm{O}}}_{3}+{\rm{Mg}}{\left({\rm{OH}}\right)}_{2}\end{eqnarray*}$$

The source of hydroxyl ions is from the structure of PVA, which is the host in BPE films. Also, due to the porous nature of the film, there is some moisture present called occluded moisture. ¹⁴ Kong et al., 2018 proposed that a cogent design of S hosts as the cathode is favored to accomplish the high overall performance of Mg–S batteries. Consequently, for the durability of Mg–S batteries, a unique design of conductive hosts is required to undergo vigorous chemical interactions with Mg-PS that were based entirely on interfacial phenomena instead of spatial confinement to diminish the back and forth effect and enhance the S utilization through its high electric conductivity. Stable cyclic performance in Mg–S batteries is achieved by using porous nanostructured materials, which provide enough interfaces and the uncovered surfaces provide an opportunity to enhance to anchor Mg-PS. Practical techniques to shield the metal surface are using high capacity alloy anode substances and a suitable protecting and conductive synthetic interphase on Mg anode. Crucial mechanistic understandings of running Mg–S batteries also need attention other than the requirement of substantial breakthroughs in the sulfur cathode, magnesium anode, and electrolyte. ⁴⁹ Wu et al., 2018 reported that, for Mg secondary batteries, CuS nanoparticles could be used because of their extreme performance as a cathode. The cathode shows incredible cycling stability (over 350 cycles), high ability (one hundred seventy-five ma hg⁻¹), and excellent rate capability. Small-sized particles favorable for the conversion response are formed due to the higher conductivity of Copper Sulfide and slow stimulation of the electrode. A decrease in sulfur content was observed for CuS while investigating the mechanism of transformation of MgS to CuS, and reversible conversion response paramountly came about between Cu and Cu₂S. ⁵⁰

Peng He et al., 2019 works have suggested that using Cu current collector at the cathode improves the cyclability. Here the cathode was made with both h-Cu@CNF and Cu@CNF and magnesium disk anode. The sulfur loading of these cathodes was in the range of 0.06−0.12 mg cm⁻². ²⁰ Raphael Richter et al., 2020 used sulfur composite electrodes were prepared by the widely used melt infiltration method and a magnesium metal foil with 18 mm diameter as an anode. The kinetics of the side reactions on the anode surface has a high impact and can be responsible for the rapid self-discharge in Mg–S cells. ²⁹ Meng et al., 2019 proposed that Mg₃Bi₂ was turned in as an alloy-type negative electrode due to how bismuth behaved in Mg batteries. However, using Mg₂Sn or Mg₂Pb anodes, higher theoretical electricity densities could be expected. ⁵¹

Shuxin Zhang et al., 2020 reported using S@pPAN cathodes with Sodium Polyacrylate as a great aqueous binder for Magnesium−Sulfur batteries. The PAAS-based S@pPAN electrode exhibits good high rate capability and cycling stability due to its amorphous structure and an overlapping 3D network formed after the cross-linking upon cycling. It showed improved electrochemical performance of electrochemical batteries. ⁵²

Divyamahalakshmi Muthuraj et al., 2021 introduced a magnesium polysulfide (MgSx) catholyte as a liquid-phase active material and Mg anode to conquer the sluggish redox kinetics of Mg/S batteries. ⁵³ Talian et al., 2021 showed that the impedance spectroscopy outcomes on porous carbon electrodes also offer data on the bottle-neck procedures in the course of their operation, that is, the diffusion of polysulfide species from the majority of the electrolyte in the separator in the direction of the electrode surface.^{5 l} et al., 2020 reported that KB/S is amalgamated by the melt diffusion method. The cathode electrodes were formulated by casting KB/S on the current collector, which was beneficial to boost conversion kinetics of the sulfur cathode. The stoichiometric Ketjenblack carbon (KB) and sulfur were soaked in CS₂ and agitated for 12 hr to volatilize CS₂. The KB/S composites were acquired after the mixtures were pulverized and heated at 155 °C under Argon atmosphere for 12 h. ⁵⁴

Carbon is known as a highly conductive agent as it widely helps improve the material's conductivity. Carbon has proven to be a simplified and efficient way to ensure adequate access to electrons. MWCNTs are also good conductivity agents to ensure mechanical integrity and smooth mobility of the Mg²⁺ ions and electrons. Graphene, an allotrope of carbon, is another conductive agent because of its excellent thermal, electrical, and mechanical properties, improving the low electronic conductivity of materials. Carbon is used as a host matrix in most Mg–S batteries because there are always some large overpotentials between charging and discharging processes in most Mg–S batteries. Capacity fading is fast. There is also a loss of active material. These problems are enhanced when sulfur loading in cathodes is high for more than >1 mgsulfur cm⁻². These problems correspond to the formation and shuttling of soluble polysulphide species between cathode and anode during battery operation. Sometimes, the inferior performance is also due to the slow reaction kinetics of magnesium ions. These polysulphide species at the anode increase anodic impedance and causes a considerable lag between charge and discharge values. Hence, to avoid the problems mentioned above, the trapping or confinement of polysulphide species is essential. Therefore, cathode materials with a polar conductive host matrix with a large surface and high affinity for sulfur species can help bind them by adsorbing them. ¹⁹ Also, they enable the easy transfer of electrons and ions through them during reduction and oxidation. From Fig. 8, we can look at the various developments in the carbon electrode as discussed further in the section and different carbon electrodes used in various studies and research work.

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In Table IV, we gathered different materials doped while preparing the carbon electrode and the latest developments in the fabrication of carbon-based electrode composites to improve the overall electrochemical performance of the battery. Wang W. et al. 2020 proposed a carbon-encapsulated cuprous sulfide(Cu₂S@C) cathode of this Metal-Organic Framework- derived cuprous sulfide prepared by a melt-diffusion method. In this, through ball-milling, sublimed sulfur and Cu-MOF were mixed after that carbonization process was done. The significant amount of carbon in composite posses good elasticity. The surface area of Cu₂S@C composite carbon enhances the contact of electrode and electrolyte by this polarization decrease, and capacity improves. Extra ions stored at the Cu₂S@C surface also increase the capacity. This cathode shows better cycle stability as compare to Cu₂S cathode due to MOF-derived carbon frameworks. A slower current decay, more negligible voltage polarization, and l larger peak currents can be seen in Cu₂S@C∣Mg cell. ²³ Wang P. et al. 2020, proposed a Sulfurised poly(acrylonitrile)(SPAN)composite.SPAN is synthesized by reacting PAN, and excess S8 in a nitrogen atmosphere and electrodes are prepared by coating poly(vinylidene difluoride):SPAN: carbon black in N -methyl-2-pyrrolidone (NMP) on an Al/C foil. By pressing Mg powder with a hydraulic press, the anode is made. In this cathode, sulfur is chemically bound to the polymeric backbone. Poly(sulfide) shuttle-free cycling discharge/charge rates of 8 C for more than 1100 cycles with less than 10% loss is obtained. In the 30th cycle, a discharge capacity of ca. 300 mAh/g_sulfur is reached. Mg–SPAN cells are one the best Mg–S cells if the electrolyte contains magnesium conductive salt. ⁵⁵ Manthiram A. et al. 2016 studied a sulfur material filled inside preactivated carbon nanofiber (CNF) electrodes, and the separator is also coated with CNF; this separator acts like a polysulfide trapper. After cycling, homogeneous distribution of sulfur is achieved. CNF coating is porous so that it can absorb diffused active cathode materials. Contrarily, CNF coating is electronically conductive, due to which it will act as a current collector because of this cyclability, and sulfur utilization is improved. A discharge capacity of ∼1200 mAhg⁻¹ is obtained and lasts up to 20 cycles. ⁵⁶

Figure 9 explains the cathode structure when carbon microparticles surround sulfur particles as binder material providing stability to the S-C composite. Nuli Y. et al. 2019 prepared Sulfur@Microporous Carbon (S@MC) Composite as a cathode. A composite containing 55 wt % sulfur content was prepared by the melt diffusion method by ball milling a mixture of sublimed sulfur and microporous carbon in a 1:4 ratio. Coin cell was constructed using S@MC as cathode,Mg ribbon anode and Mg(CF₃SO₃)₂ + AlCl₃ + MgCl₂ + anthracene/THF + TG as electrolyte. During 55 cycles, discharge capacity keeps a stable value of 420 mAhg⁻¹.During 200 cycles, cell performance was stable, and after the third cycle, coulombic efficiency was 95%. ⁶ Zhao-Karger, Z.et al. (2014) have demonstrated a magnesium sulfur battery using [(HMDS)₂ Mg] based non-nucleophilic electrolyte and CMK-3 based cathodes. CMK-3 material is a mesoporous carbon material. This material has a huge pore volume which provides a conductive framework for electron and ion transfer. The cathode is fabricated by filling liquid sulfur into these pore spaces. When the heating is done at 160- degree Celsius, the cathode is called S/CMK160, and when at 400 degrees celsius, S/CMK400. They used PEG as a coating material for these cathodes. Moreover, two binders, PVDF and CMC, were compared for their application in these cathodes. The cells using the CMC binder displayed a relatively lower initial discharge capacity. However, the charge- discharge profiles were similar. Cells using PVDF binder in TEGIL electrolyte delivered the highest capacity of 800 mAh g⁻¹ in the first cycle. However, the value dropped steeply to 350 mAH g⁻¹ in the second cycle. However, cells using PVD or CMC binder in the same electrolyte solutions could maintain about 260 mAh g⁻¹ capacity even after 20 cycles. ¹² Wang W et al. (2018) prepared a sulfur@ microporous carbon (S@MC) composite with an excellent S loading (64.7 weight %). This provides stabilization of S₂₋₄ small molecules in microporous carbon, and chemical bonding of copper to large molecules such as S₈ with non-nucleophilic electrolytes also provides a productive approach to advance the electrochemical functioning of Mg−S batteries. ⁵⁸

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Activated carbon cloth (ACC) is an anchor material in magnesium-sulfur batteries with permeable carbon materials. Here also, the S-ACC composite is infused with sulfur to make cathodes. This cathode can also accommodate a higher amount of sulfur for high-energy- density batteries. Gao, T. et al. (2018), in their study of the kinetics and thermodynamics of this cathode, showed that as the amount of S per carbon increases, the capacity decreases. During the first stage of reduction involving S₈ to MgS₈ and further reduction to MgS₂, the kinetics is independent of the S/C ratio, but during the last step(MgS₂ to MgS), there is a termination of discharge due to large overpotentials, and this can be more pronounced at high S/C ratio. They reported that the cell had an initial cut-off voltage of 1.4 V and coulombic efficiency of 100%. When the cut-off voltage is 0.5 V, the coulombic efficiency is 82%. ⁵⁹ Dai W. et al.,2018 used activated carbon cloth (ACC) based sulfur composites. This was produced by an improved melt-diffusion method with round about 1 mgcm⁻² sulfur loading. Swagelok-type cells with MgBhfip/DME electrolyte, Mg foil anode, and ACCS cathode give a first discharge capacity of 1000 mAh g⁻¹, and at the end of 20 cycles 660 mAhg⁻¹. ¹⁰

Sun, J. et al. (2020) introduced an in situ sulfurization of carbon confined Cobalt in a mesoporous matrix (MesoCo@C), which could alleviate the Mg anode corrosion. This in situ sulfurization system takes place by inserting melted sulfur inside the mesoporous carbon- confined cobalt matrix. Due to the vast amount of surface energy of nanosized Co particles, their surfaces get oxidized by the hot sulfur melt forming CoSx bonds at the surface. The CoSx species can control the polysulfide shuttle. In-situ sulfurization technique to supply carbon-confined sulfurized Cobalt in a mesoporous matrix (MesoCo@C−S).MesoCo@C−S cathode has a very smooth surface after cycling. Mesoporous carbon structure acts as a substrate material for the confinement of the sulfur, and Cobalt in bulk has conductive properties to allow transfer of electrons; The MesoCo@C−S cathode has 40% sulfur loading and has good electrochemical performance with a specific capacity of 830 mAh g⁻¹ at 0.1 C and maintains 280 mAh g⁻¹ capacity after 400 cycles. The battery uses Mg-foil anode and MMAC-DME as electrolytes. The coulombic efficiency was also up to 70%-80% during the first ten cycles. The charge(2.2−2.5 and 2.5−3 V) and discharge(1.8−1.5 and 1.5−1.2 V) profiles showed two plateaus. Compared to the CB-S cathode, the MesoCo@C−S cathode gave 780 and 380 mAh g⁻¹ at 0.2 and 0.4 C, while the CB-S cathode could give 200 and 80 mAh g⁻¹ of capacity. ²³

Chen et al. prepared the multiwalled carbon nanotubes(MWNT/C/Mg_1.03Mn_0.97SiO₄ nanocomposite) by the one-step CVD method. The hierarchical structure of the MWNT and coated carbon layer provided a larger contact surface of electrolyte or electrode. When used against Mg anode, it exhibits a higher capacity of 300 mAhg⁻¹, better rate capability, and cycling stability than the pure Mg_1.03Mn_0.97SiO₄ nanoparticles and carbon-coated composite. ⁶⁰

Zhuang Y. et al. (2021) developed oxygen vacancies to spherical NaV6O15 cross-linked with carbon nanotubes (CNTs) (SNVOX -CNT) as a cathode material. The storage capacity of Mg2+was enhanced due to the introduction of metal ions, i.e., Na + in layered transition metal oxide, i.e., V2O5. Insertion of Mg2 + inside the cathode is responsible for the electrochemical performance of the cathode. By the Oxygen vacancies, diffusion can be improved. SNVOX - CNT maintains 120 mAh·g−1 after 180 cycles at 50 mA·g−1.SNVOX-CNT shows 98% initial Coulombic efficiency. This electrode can maintain its morphology after 50 cyclings. Therefore, Vinayan, B.P. et al. (2019) developed a new cathode design with a host matrix consisting of nitrogen-doped hybrid nanocomposite of multiwall carbon nanotubes (MWCNTs) graphene. The cathode used different sulfur loadings ranging from 3 to 0.5 mgsulfur cm⁻². For 3 mgsulfur cm⁻² loading in the cathode, the cell displayed a capacity of 431 mAh g⁻¹ during 1st, 406 mAh g⁻¹ during 2nd, and 228 mAh g⁻¹ during 50th cycles. The capacity retention is about 94.2% and 53% during the second and 50th cycles. The capacity fade may be attributed to the high sulfur loading, which can augment polysulfide dissolution, and also, there is less utilization of sulfur in the cathode. When loading is decreased to 0.5 mgsulfur cm⁻², the capacity during the first cycle increases to 1384 mAhg⁻¹. ¹⁹

Nuli, Y. et al. (2007) have proposed a new type of cathodes using organo-sulfur compounds. These electrodes consist of S–S bonds, and the capacity generated during charge and discharge is because of the breaking and recombination of these bonds. They have suggested that the redox reaction between (-SH) thiol and (S–S) disulfide compounds can store a large amount of energy. The theoretical energy of these compounds is more than many polymer composites and intercalation compounds. They have used PDTDA(Poly-2,20 -dithiodianiline ), DMcT( 2,5-dimercapto-1,3,4-thiadiazole) and PAn(polyaniline ) composite and CMS(conductive sulfur-containing material). For DMcT and PAn composite, the capacity of first discharge was only 16.8 mAh g⁻¹. This value increases up to 29.2 mAh g⁻¹ during the tenth discharge cycle. They also observed two voltage plateaus at 1.4, and 0.9 V. For PDTDA cathode, the battery discharge capacity was reported to be 78 mAh g⁻¹ in the second cycle. This cell exhibited low degradation of capacity even after 30 cycles. The reason may be the structure of PDTDA which enables intramolecular catalysis. CMS contains the main chain to provide electric conductivity, and the sidechain contains disulfide bonds. Since the performance of only CSM cathode was inferior, they have used CSM-PAn composite. It gave a discharge capacity of 117.3 mAh g⁻¹ during the first, 93.0 during the second, and 72.7 mAh g⁻¹ during the twenty-second cycle. ¹⁷

Diemant T. et al. 2016 has used a sulfur-reduced graphene oxide cathode made by the graphite oxide (GO) thermal exfoliation at 400 °C in an argon gas atmosphere. Mg-carbon composite is used as an anode prepared by together ball milling of conductive carbon black and Mg powder in 20:80 ratio. Graphene (G) has high charge mobility, high mechanical strength, huge surface area, good chemical stability, and outstanding thermal and electronic conductivity. Due to these properties, G is preferred for energy storage applications. The presence of an oxygen functional group over the rGO surface helps increase the bond formation with sulfur, which helps improve the capacity. This cell offers a good current rate capability and reversible discharge capacity of 448 mAh g⁻¹ and after the 50th cycle 236 mAh g⁻¹. Wang P. et al.2019 prepared the graphene-wrapped V2O5 microparticles by the solvothermal reaction. While used as cathode in Mg(AlCl2BuEt)2/THF electrolyte against the Mg foil counter and electrodes, the assembled composite can provide a high capacity of 178 mAh g⁻¹ in the initial discharge. V2O5 cathode with the presence of graphene demonstrates excellent, more extraordinary performance as a complete Mg cell cathode. ⁵⁵

Several types of electrode materials are used in Mg–S batteries, as discussed above, because of their different microstructure. The material's microstructure highly influences the physical properties like conductivity, hardness porosity, absorption, corrosion resistance, etc. An electrode mainly consists of a binder, current collector, active material, and doping material. From Fig. 10, we understand the microstructural arrangement inside an electrode and a current collector and binder, strengthening the bond between carbon and active material. The microstructure of these components has a high impact on the ionic conductivity, Large specific capacity, cyclic stability, and coulombic efficiency because the material structure decides the electrode's reactivity with electrolyte and microstructures vary considerably with the processing or synthesis conditions and methods. So understanding microstructure is very important to find the cell performance. The crystal structure is an inherent property of a material, but the crystal parameters can be slightly modified by introducing dopants. The introduction of Na⁺ to V₂O₅ resulted in enhanced cyclic stability ²⁶ and enhanced the storage capacity of Mg²⁺. Scientists performed different diffraction and scanning techniques for getting information about the electrode materials' microstructure. In Mg–S batteries, the anode is mainly made up of magnesium metal only. In order to improve its physical properties, different forms of this material like Mg foil, Mg powder, or pressed magnesium powder electrodes are used commonly. Liu R. et al. 2018 by SEM showed no morphology change in Mg foil after and before soaking in electrolyte and because of adsorption of solvent, a layer form on Mg electrode which is good for stability. ¹⁰ Sievert B. et al. 2020 showed that pressed Mg powder anode has more surface area than Mg foil, so the possibility of Mg deposition is high, resulting in better charging behavior and higher cycling performance. This anode has a highly porous structure, and magnesium powder's average particle size is 44 μm, so the surface area of this anode is 3000 times more than the Mg foil anode.

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Nanostructured materials are commonly used because these materials enhance the reversible Mg insertion and extraction reaction and reduce the diffusion of Mg ions. Therefore cathode is made up of carbon and its derivatives or non-carbonaceous compounds like V₂O₅, Mo₆S₈, Ti₂O, etc. Lighter elements are mainly used as electrode materials due to their higher specific capacities. Variable valence states of transition metal oxide facilitate more electron-storing sites due to which these oxides are generally used as cathode materials, and the electronegativity helps find the types of bonds between transition metal ions and ligands. More electronegativity differences form a more ionic bond. As a result, a dense structure. The electrode's electrochemical potential depends on the valence state, electronegativity, ionic radius, and the local environment of the cations in compounds. Thus small particle-sized transition metal electrodes provide a high surface area which results in high specific energy.

Carbon electrodes are constructed by different arrangements of carbon, such as they form mesopores or micropores structures. Graphene oxide, graphite, or CNTs are also used as electrodes because of the physical properties of materials. As discussed in this section, Fig. 11 gives us a complete microstructural visualization of the sulfur-carbon composite cathode. Zhou J. et al. 2018, presented sulfur@ microporous carbon (S@MC) composite the micropores, provide extensive sulfur loading approximately 64.7 wt % with more active sites for immobilizing sulfur species. In the micropores of carbon, small chainlike S₂₋₄ molecules get highly dispersed inside and ringlike S₈ molecules on the outside surface. Microporous carbon has ultra-micropores with median pore width (<0.4 nm) and narrow pore diameters (<2 nm). Better confinement of S₂₋₄ is achieved in this composite; therefore, this cathode exhibits enhanced rate performance and cycling stability in the nucleophilic electrolytes. ¹⁰

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Carbon nanotubes electrodes also give a conductivity; Kong W. et al. 2015 suggested a polymer encapsulated nano sulfur/super-aligned carbon nanotube (PVP@S-SACNT) composite electrode because of the porous and high conductive structure of the SACNT network and the PVP surface modification, PVP formed a protective layer around SACNTs, and this will suppress the diffusion of polysulphides. As a result, this cathode provides excellent cyclic stability and 100% coulombic efficiency. Thus Mg–S battery performance is the highly dependent microstructure of the electrode.

Now let us see the microstructural changes taking place during a particular battery's operation. First of all, let us talk about the working of an Mg–S battery having a MesoCo@C cathode reported by Sun, J.et al. (2020). The CoSx species plays a significant role in alleviating the shuttle effect by binding with polysulphide species. This CoSx species confined inside the carbon matrix enable surface-catalyzed redox reactions like S/S2 since they have a high binding affinity for polysulphides and high electronic conductivity during the battery's work. The post-discharge XPS analysis of the cathode. The XPS spectra show a splitting of Co 2p spectra into two peaks and a slight shift of Co²⁺ peak at 781.5 eV and a new peak at 778.6 eV. This current peak of Co(0) was due to the reduction process, and the shifted peak displayed the adsorption of polysulphide. Also, the XPS spectra of S 2p showed a peak at 162.2 eV, which was due to the formation of MgS_x species formation during the working of the battery. ²³ Further, in Mg–S batteries using organo-sulfur electrodes, the changes happening in microstructure are the breaking of disulfide bonds in the cathode during discharge and the formation of disulfide bonds during the charging process. In DMcT and PAn composite, the redox reactions are fast due to the close contact between these two materials at the molecular level. The PDTDA cathode was better than DMcT and PAn composite because the disulfide bonds were enclosed between polyaniline chains. So it exhibited an intramolecular catalytic effect. The breaking and formation of bonds were easy in this case because the leading polymer chains remained unchanged. The CSM-PAn composite cathode has a coplanar structure with pi-conjugated bonds. The intramolecular reaction of S–S bonds is fast due to planar structure, and there is no change in the leading polymer chains. ¹⁷ In Mg–S battery using nitrogen-doped graphene-MWCNT hybrid nanocomposite cathode as reported by Vinayan, B.P. et al. (2019), studied the microstructural changes, especially the formation of polysulphide species during operation. They have used Operando Raman Spectroscopy during the charging and discharging processes. The nitrogen doping in graphene created three functional groups: graphitic, pyridinic, and pyrrolic N. Studies have proven that doping nitrogen creates polarity in the matrix and thus helps bind polysulphides. The bands corresponding to bulk sulfur were observed at 150, 219, and 470 cm⁻¹. During the discharge, these bands decreased and, in the end, wholly disappeared. This behavior might be attributed to the reduction of bulk sulfur to polysulphide chains. The bands for ${{{\rm{S}}}_{4}}^{2-}$ were maximum between 1.4 to 1.2 V and started decreasing due to further reduction to S²⁻. This shows the formation of short-chain polysulphides. These results were further validated using XPS spectra. When XPS analysis was done of the new cathode, it displayed two peaks attributed to S₈. Further, when the discharged cathode was analyzed, it displayed several low-intensity peaks attributed to other reduced sulfur species. Then all the polysulphide-related peaks in the Raman spectra disappeared, and broad peaks were observed between 180 to 350 cm⁻¹. These peaks confirmed the conversion of the polysulphides to MgS during battery operation. They also concluded that the structure of the final MgS product was of zinc blend phase. ¹⁹

After understanding the microstructural changes happening during the battery's operation, we explain to you how these changes can sometimes hinder the working of a battery. Wang et al. (2019) presented that by the elemental sulfur reduction, the high-order magnesium polysulfides (MgSx, 3 < x < 8) formed are certainly soluble in ether-based solvents that makes them electrochemically transportable. However, they can relocate concurrently from the cathode to the anode, which results in capacity fading and active material loss. ⁵⁵ Yang et al. (2019) manifested that the reaction between intermediary polysulfides formed through the discharge and charge process with the Mg anode. Mg−S batteries suffer from the shuttle effect mainly. The decomposition of polysulfides results in never-ending active material loss restricts the cycling stability and decreases the specific capacity of Mg−S batteries. Here, low Coulombic performance suggests that excessive polysulfide shuttling is not the principal reason for the apparent decrease of the capability. ²⁵ Vinayan et al. (2016) conveyed that the increase of charge capacity over the discharge capacity after some initial cycles acts as a parasitic side reaction primarily because of the Mg polysulfide shuttle effect. The voltage figures represent that both the capacity and discharge voltage decrease slowly with an increasing rate, which can be responsible for high kinetic, ohmic overvoltages at high current rates and inactive kinetics of Magnesium with Sulfur. Muthuraj et al., 2019 presented a Dual- doped carbon cloth that affects the intermediate higher-order polysulfides, influences their rapid dissolution, and reduces the shuttle effect. ⁴⁵ Du et al., 2013 proposed that overcharge behavior and low charge-discharge efficiency of a battery is mainly because of polysulfides shuttle behavior. The main reason behind this might be the electrolyte decomposition and corrosion of the current collector, primarily because of charging cells to above 2.5 V. Mg/S battery set forth varied voltage figures while applying various Mg-ion electrolytes, which shows the crucial roles of electrolytes in the working of Mg–S battery. ⁴⁸ Du et al., 2017 revealed that fast capacity fading and poor cell reversibility result from the dissolution of greater-order polysulfides that are moderately soluble in carbonate-based solvents. High- order polysulfides are easily dissolved in Glyme-based and dioxolane solvents, still accompanying the greater-order polysulfides drifting between the anode and cathode, leading to the "shuttle effect," which results in the and low Coulombic efficiency and active materials loss. ⁴⁸ Herein, Fig. 12 explains the development of cathode materials and their microstructure's involvement in improving battery efficiency.

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The below-discussed process parameters are instructions to guide the researchers in the field to obtain an efficient result using the outcomes we have discussed in the literature survey. The key findings of the literature are provided in "Discussions," which explains why these parameters are optimum.

• 1.  
  The presence of a binder on an electrode highly influences the battery's performance because the physical properties of electrodes are remarkably affected. Various polymeric binders that are organic or water-soluble are commonly used, but the sodium polyacrylate(PAAS)binder with sulfurized pyrolyzed polyacrylonitrile (S@pPAN) composite cathode show significantly high capacity retention and cyclic stability due to its outstanding cohesion and adhesion property.
• 2.  
  The electrolyte's compatibility with the sulfur and Mg electrodes is a foremost concern while constructing a reversible magnesium battery. Most of the electrolytes contain AlCl3, which is corrosive to the electrodes. Boron centered magnesium electrolyte in DME solvent show excellent performance as it is non-corrosive and shows non- nucleophilic characteristics hence highly compatible with the electrophilic sulfur. Research shows that for more than 100 cycles, organic magnesium borate-based electrolyte offers a stable discharge capacity above 1000 mAhg⁻¹, and this electrolyte is synthesized efficiently compared to another non-nucleophilic electrolyte.
• 3.  
  The doping of ions can improve the Mg–S battery electrochemical performance, nanostructure construction, fabrication of the prepared composites, change in the crystal structure, electrode-electrolyte contact enhancement makes Mg-deposition and stripping more straightforward on Mg anode and also are proved to be efficient modification methods.
• 4.  
  The non-nucleophilic electrolytes are found to be more compatible, non-corrosive when electrolyte additives are added to boost the functioning of the battery. Also, improved cycle life with sulfur-based cathodes than the nucleophilic electrolytes.
• 5.  
  Using a current collector can give a revamped performance for an Mg–S battery, but it depends on the selection of metal as a current collector. In the same electrolyte, cathode, and anode, Aluminium as current collector shows inferior electrochemical performance than copper, with large overpotentials and low capacities. Therefore, copper as a current collector can result better by promoting the segmentation of polysulphide chains due to copper-polysulphide reactions.
• 6.  
  A combination of Mg(TFSI)2 and MgCl2 salts in 1:1 ratio in dioxolane and tetraglyme as an electrolyte gives the highest first discharge capacity(1320 mAh g⁻¹) compared to other non-nucleophilic electrolytes previously reported like Mg-HMDS based electrolytes in THF and glymes and BCM electrolyte. Potential during first discharge is low due to the passivating film present on the anode surface, but it increases in further cycles after activation of the anode surface.
• 7.  
  The mobility of Mg2 + ions is enhanced, and charging-discharging results are also improved by using electrolytes consisting of small chain ethers with low boiling points and blowing-up temperatures.
• 8.  
  To achieve high energy density in Mg batteries, the most promising positive electrode is Sulfur based electrode. Sulfurized Carbon-containing cobalt (MesoCo@C) prevents the shuttling of polysulphides when binded with it. ^69–90