Muhmmd Rshd, Muhmmd Asif, Iftikhr Ahmed, Zhen He, Li Yin,Zhou Xio Wei, Yuxin Wng

a School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang, 212003 Jiangsu, China

b Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China

c Faculty of Science, Jiangsu University, Zhenjiang, 212013 Jiangsu, China

Abstract Rechargeable magnesium ion batteries are potential candidates to replace the lithium ion batteries due to their high volumetric energy density, dendrite free cycling, and low costs.In present work, we have critically reviewed the recent advances made in the fiel of cathode materials development to achieve the high reversible capacities and working potentials.In firs part, carbon-based cathodes such as fluorine doped graphene nanosheets and graphite fluorid (CF0.8)are discussed in terms of compatibilities of positive electrode materials and electrolyte solutions for rechargeable magnesium-ion batteries.Whereas, the second part of this review focuses on crystal structure of vanadium oxide and its capability to accommodate the Mg2+ ions.Likewise,electrochemical performance of selected vanadium oxide based cathodes including VO2 (B), FeVO4.0.9H2O, Mo2.5+yVO9+δ, RFC/V2O5 and V2O5/Graphene composite, are discussed at different temperatures.To support the future research on magnesium ion batteries, particularly positive electrode material developments, several innovative research directions are proposed.

Keywords: Magnesium-ion battery; Energy storage; Cathode materials; Electrolytes; Vanadium oxide; Electrochemical properties.

1.Introduction

Owing to ever-increasing burdens of energy storage devices and environmental concerns from burning of nonrenewable fossils, researchers have focused on renewable energy strategies to fulfil these requirements [1-5].Among energy storage systems, super-capacitors and rechargeable metal-ion batteries (lithium-ion batteries, metal-air batteries,sodium-ion batteries etc.) are most appropriate technologies to get rid of energy crises issue [6,7].During last two decades, lithium-ion batteries (LIBs) have progressed a lot,and have been extensively used in portable electronic devices and auto vehicles [8,9].Although lithium-ion technology is commercialized and dominating the current market, however this technology is restricted due to several reasons such as safety concerns, dendrite growth on lithium anode and expensive.Several new technologies such as magnesium-ion batteries (MIBs) [10], aluminum-ion batteries (AIBs) [11,12], and potassium-ion batteries (KIBs) [13-15] are emerged as postlithium ion batteries.Compared to AIBs or KIBs,MIBs offers several advantages such as cost effective,safe nature,environmental friendly, dendrite free magnesiation, high theoretical energy density and so on[16].Despite these advantages, MIB technology is restricted due to large ionic radius of Mg2+ion,which is around 0.86 °A.The large ionic size and divalent nature of Mg2+ion leads to high polarization, which ultimately results in sluggish diffusion of Mg2+ion inside the host materials [17].Another dispute of MIB technology is formation of non-permeable solid electrolyte interface (SEI) layer on metal magnesium anode, which restrict the reversibility of Mg2+ion [18].These problems prevented the development of MIBs technology during 19thcentury.

Inspired from the advantages of MIBs, Aurbach et al.[19] and his coworkers made a breakthrough in the history of Mg-ion technology.They made a successful attempt to synthesize magnesium ion electrolytes comprising of organohaloaluminate salt of magnesium (i.e.Mg(AlCl3X)2and Mg(AlCl2XY)2, X &Y= Alkyl groups) in glyme family (i.e.tetrahydrofuran (THF)).The fina electrolyte solution Mg(AlCl2BuEt)2-THF was employed in MIB containing metal Mg anode and MgxMo3S4cathode.Final MIB system was cycled between 0.2-2.0V vs.Mg2+/Mg at room temperature.The battery exhibited stable voltage plateau at about 1.1V and delivered the energy density of 60Wh.Kg?1.Furthermore, battery exhibited excellent capacity retentions even after 600 cycles, which proved the fast reversibility of Mg2+ions during striping and plating.After pioneering work of Aurbach, different kind of molybdenum sulfide were extensively used as positive electrode materials in MIBs.For example, Cheng et al.[20] used Chevrel phase virgin Mo6S8(nano and micro particles) for MIBs which delivered the maximum reversible capacity of around 100mAhg?1at 0.1C.Similarly in another work,role of Cu metal inside Chevrel phase Mo6S8was explored thoroughly which confirme that Cu doped Mo6S8cathode revealed high rate performance [21].Following these works, Liang et al.[22] used the firs principle simulation to apply atomic level lattice engineering strategy for activating the host material MoS2for Mg2+ion accommodations.Theoretical calculations revealed that Mg2+ion could diffuse and eject into/from expanded layers of host material.Furthermore, interlayer expansion inside MoS2layer was carried out experimentally by inserting the sufficien amount of poly(ethylene oxide).Experimental results confirme that interlayer distance was increased from 0.62 to 1.45nm, which boosted the diffusion of Mg2+ions twice.Similarly, graphene like was also employed from Mg2+ion storage which delivered the highest capacity around 170mAhg?1at the current density of 200mAg?1[23].

Unlike molybdenum sulfides transition metal oxides based host materials exhibit large polarization during intercalations of Mg2+ions [24,25].Several strategies including electrolyte compositions, organic or inorganic additives, doping host materials and so on have been employed to reduce these polarizations [26,27].In present work, we will critically review and discuss some selected cathode materials, which can reversibly accommodate the Mg2+ions.Firstly, carbon based cathode materials will be summarized and their electrochemical properties will be debated.Secondly, we will discuss how structure of vanadium oxide base cathodes influenc the electrochemistry of MIBs.Different classes of vanadium oxides and their composites will be summarized in terms of their capabilities to accommodate Mg2+ions.The electrochemical properties including rate performance, cycle stability, cut-off voltage, working potentials and measuring temperatures will be explored in details.(Fig.1)

Fig.1.Carbon and vanadium oxide based cathodes for rechargeable magnesium-ion batteries.

2.Carbon based cathode materials

Graphene,a 2D carbon allotrope,is an ideal material which can be used as high energy electrode material owing to its excellent mechanical, electrical, and thermal properties [28].Due to its intrinsic properties,graphene have been extensively used as anode materials for lithium ion batteries [29].Similarly, several attempts were made to use the graphene and its derivatives for Mg ion batteries.One such attempt was made by Xie et al.[30] where they used the graphene nanosheets doped with fluorin (FGS) as electrode material for MIBs.It was found that FGS electrode revealed the specifi capacity of 100mAhg?1at the current density of 10 mAg?1when cycled in MgClO4-DMSO electrolyte from 0.5 to 2.75V verses Mg anode.The large size complex cations are easily accessible to redox sites of FGS, thus lead to very low voltage polarizations.The sluggish diffusion of complex cations into cathode lattices was bypassed by redox processes with functional groups attached to the surface of graphene nanosheets.The sketch mechanism of redox reaction is depicted in Fig.2.

Another attempt was made by Mesallam et al.[31] when they used the multilayer graphene (graphene nanoplatelets;GNPs) as cathode material for Mg-ion batteries.The GNP and H-GNP-Mg cathodes exhibited the reversible capacity of 387/170, and 179/165 mAhg?1for firs two cycles.The plateau at around 1.6V was observed however; large overpotential (about 1.4V) restricted the electrochemical performance to only 8 cycles.Furthermore,graphite fluorid (CF0.8)was also used as cathode materials for Mg ion batteries by Miao et al.[32].CF0.8exhibited the maximum specifi capacity of about 813mAhg?1at the current density of 20 mAg?1when using all-phenyl-complex (APC) magnesium electrolytes and it was higher than that of Li ion battery (737mAhg?1).Also, Mg-Li hybrid electrolytes such as APC-LiCl in tetrahydrofuran (THF) and Mg(BH4)2-LiBH4in Tetraglyme (TG) were used and their gravimetric energy densities as depicted in Fig.3.However, main disadvantage of this kind of cathode is its poor reversibility during second cycles, which prohibits its use on commercial scales.

Fig.2.Scheme of electrochemical reaction mechanisms of a Mg/FGS battery involving multiple (complex) cation or anion (de)intercalations during the early cycles.1) The firs charging is an initial activation process characterized by the insertion of ClO4?related anions, a large fraction of which are converted to Cl?based groups at the surface of carbon framework.2) The following second discharging is achieved by the insertion of Mg2+-containing solvate cations(e.g., Mg(DMSO)nClO4+) rather than anionic desorption.The solvent molecules of DMSO are removed at the electrode-electrolyte interface before bonding of charge carriers with the carbon framework occurs, leading to the formation of potential interactions of Mg-F, Mg-ClO4, and Mg-Cl.3) The consequent second charging and deeper cycling are driven by the shuttle of cations likely consisting of Mg(DMSO)nClO4+, MgCl+, and Mg2Cl3+.During the charging,DMSO molecules participate in the solvation process to form mobilizable Mg(DMSO)nClO4+again [30].

3.Vanadium oxide base cathode materials

Among vanadium based cathode materials, vanadium penta-oxide (V2O5) is most common materials that have been extensively used in lithium ion batteries (LIBs) [33].The layered structure of V2O5is supported by weak van der Waals forces and it belongs to space group Pmmn.It crystalizes as orthorhombic structures as shown in Fig.3, where square pyramids are connected via edges and corners to make a V2O5structure[33,34].The oxygen atoms are attached to vanadium via weak interactions in c-directions.As shown in Fig.4,three oxygen centers referred as O1, O2, and O3, are present in a single-layer slab.The single coordinated vanadyl oxygen atom, O (1) of 1.54 °A bond lengths connected with O2 oxygen bridges two V atoms (through corner-shared VO5square pyramids and have V-O bond length of 1.77 °A).On the other hand, triple coordinated oxygen atom O (3), bridges three V atoms through edge-shared VO5square pyramids.The corresponding bond lengths of V-O are 1.88, 1.88, and 2.02 °A[33].Furthermore, addition of one more oxygen atom at base will change square pyramids into distorted octahedral.This polyhedron is unique because V-O bond, which is oriented in [001] direction, remains constants.Whereas, the extra Oatoms form V-O bond via Van der Waals forces, in opposite direction and corresponding bond length is about 2.81 °A.

Unlike Li+ion, diffusion of Mg2+ion inside V2O5is very sluggish owing of crystal surface modification as well as low electrical conductivity of V2O5[35].To achieve the fast interactions of Mg2+ion inside V2O5several strategies have been employed which are summarized in review articles published in 2014 [16,17].Thus, this review will only focus on recent advancements made after 2014 in magnesium ion batteries.Water intercalated vanadium oxide V2O5·nH2O xerogels are beneficia for fast ion kinetics and consist of two V2O5layers separated by H2O as depicted in Fig.5 [36,37].The average distance between them is 11.5 °A, which can be altered during insertion and extractions of guest species.On the other hand,average distance between vanadium oxide bilayer slabs is 2.90 °A.The crystal water can be removed by heating xerogels at 320°C or above and orthorhombic V2O5can be obtained[38].However, crystal water plays a key role to alleviate the thin bilayers of vanadium oxide as confirme by Kristoffersen et al.[39] and his co-workers through XRD analysis.

Fig.3.Discharge profile of Mg-CF0.8 cell with APC/THF electrolyte and Li-CF0.8 cell with LiPF6/EC-DMC-EMC electrolyte at a current density of 20mAg?1(a); Discharge profile (b) and the corresponding gravimetric energy densities (c) of Mg-CF0.8 cells with the different electrolytes of 0.4M APC/THF, 0.4M APC/1M LiCl/THF and 0.5M Mg(BH4)2/1.5MLiBH4/TG at 20mAg?1 [32].

Fig.4.Crystal structure of orthorhombic V2O5.Views from (a) ac plane and (b) ab plane.(c) The coordination environment around a single V atom.The boxed region indicates the unit cell and the dotted line shows the sixth, electrostatically much weaker, V…O bond.V atoms are in yellow and O atoms are in blue [33].

Fig.5.Schematic of the crystal structure of V2O5·nH2O xerogel [33].

Recently, hydrated vanadium oxide decorated on graphene sheets were used as cathode materials in (Mg(TFSI)2)-Acetonitrile electrolyte solutions and reversible capacity of 330 mAhg?1at 1Ag?1were achieved [40].Furthermore, this cathode can accommodate Mg2+ion even at temperatures ranging from ?30 to 55°C (Fig.6).The enhanced Mg2+ion intercalation is attributed to charge shielding induced by crystal water present between the lattices of V2O5.The removal of crystal water reduces the reversible capacity, which is due to reduction in interlayer spacing that makes Mg2+ion diffusion very difficult

Fig.6.(a) Rate performance and (b) charge-discharge profile at various current density, (c) Discharge capacity at various temperatures from ?30 to 55°C at 1.0Ag?1.(d) GITT curves, (e) GITT potential response curve with time.The experiment was carried out at constant current pulse of 20mAg?1 for 10min followed by a relaxation period of 30min.(f) Diffusivity versus state of discharge [40].

Although, hydrated vanadium oxide cathodes exhibit high capabilities to facilitate the Mg2+ion through shielding effect between water molecules and magnesium ions [41], but water will react with metal magnesium and results in poor capacity retentions [42].Another strategy to improve the intercalation of Mg2+ion into host V2O5material is to develop nano-sized cathodes instead of bulk crystalline materials.For example V2O5nano-clusters that work on “delocalizing the electron for efficien attainment of local electroneutrality” mechanism have been successfully used for MIBs [43].The composite of V2O5and porous carbon was prepared using ambient hydrolysis deposition (AHD) technique [44] and then applied as cathode material for MIBs in [Mg2(μ-Cl)2(DME)4][AlCl4]2in DME electrolytes [43].As shown in Fig.7 (a-c), the STEM analysis of composite revealed that V2O5nanoclusters are dispersed as atomically dispersed catalysts [45].Thus, highly reversible storage of Mg2+ion into host V2O5nanoclusters was achieved as confirme by electrochemical characterizations.Fig.7 (d-f), shows the rate capability and cycle performance of V2O5/RFC composite which revealed that composite exhibits excellent performance.For example,reversible capacities of 225 and 100mAhg?1were obtained at the current densities of 40 and 640mAg?1, respectively.Furthermore, it was seen that the nanoclusters based composite have high reversible capacity and working voltage as compared with previously reported Chevrel phase Mo6S8[20] and RFC.The capacity retention of about 70% was obtained after 100 cycles at the current density of 320mA g?1.Thus, such approaches and cathode materials may hold great promises for high voltage as well as high capacity for MIBs.

Generally, metal oxides possess dense crystal structures,which inhibit the fast insertion/extraction of Mg2+ions inside host materials.To get rid of this problem, softly-bonded materials have also been used to control the polarizations where material’s lattices fluctuat to accommodate the guest Mg2+ions.For example, Tepavcevic et al.[46] synthesized bilayered V2O5structure for MIBs.The layered V2O5was feasible for adjusting interlayer spacing; employing defects, integrate the hydroxyl and water groups and these entire boosts up the reaction kinetics of Mg2+ions.Experimental results revealed that Mg2+ions can be inserted/extracted into interlayer spacing of V2O5when using Mg(ClO4)2-Acetonitrile based electrolytes and reversible capacity of 240mAhg?1were achieved.The high reversibility of Mg2+ions is attributed to the strongly bound hydroxyl groups, which not only maintain the interlayered spacing for Mg2+ions but also reduce V2O5symmetries.Furthermore, they proved that crystal water is essential for high reversibility of Mg2+ions in their MD simulation work [46].Generally, oxidation state of host cathode materials is changed during insertion of Mg2+ions and resulted local electroneutrality is compensated by neighboring metal ions [47].The local electroneutrality and fast Mg2+ions can be achieved by doping the host cathode material with transition metal ions such as molybdenum, vanadium, or irons, which accommodate the oxidation states and increase the ionic conductivities [47-50].One such effort was made by Kaveevivitchai et al.[51] when they synthesized the Mo2.5+yVO9+δcathodes and tested in Mg(TFSI)2-acetonitrile(AN)based electrolyte against activated carbon cloth.The detailed structure of the Mo2.5+yVO9+δis shown in Fig.8(a).It can be seen that MO6octahedra and (Mo)Mo5O27pentagonal units are corner-shared to form Mo2.5+yVO9+δlayers which are built by MoO7pentagonal bipyramid edge-shared with MoO6octahedra.The electrochemical characterization revealed that Mo2.5+yVO9+δcathode exhibits the insertion and extraction of Mg2+ions when cycled from 2 to 10mAg?1between 3.33 to 1.73V as shown in Fig.8(b).The chargingdischarging profile of Mo2.48VO9.93cathode at a current rate of 4mAg?1revealed that discharge voltage is lowered with increase in current rate attributed to kinetic effects.During charging process,diffused Mg2+ions are extracted and battery is reversible by 114mAhg?1.This is because Mo2.48VO9.93cathode exhibits the large tunnels, which provide the rapid and large number of vacant sites for Mg2+ions ultimately increasing the specifi capacity of MIB.Furthermore, cycle stabilities results show that capacity decays in firs few cycles and then become stable up to 25 cycles as shown in Fig.8(cd) [51].Thus, doping of Mo into Vanadium oxide can may results in more oxidation states thus help to maintain the electroneutrality and fast reaction kinetics.

Fig.7.STEM images of (a) pristine, (b) fully magnesiated, and (c) fully demagnesiated V2O5/RFC composites (scale bars: 50nm); Electrochemical results of V2O5/RFC composites (d) rate performance of the composite, (e) comparison of the specifi capacity of RFC, Mo6S8, and V2O5 for Mg battery cathode,and (f) cyclic stability of RFC/V2O5 composite at 320mA g?1 in 0.2M [Mg2(μ-Cl)2(DME)4][AlCl4]2; and g) Proposed surface mechanism for the formation of highly dispersed vanadium oxide nanoclusters on a porous carbon support and the molecular reversible storage of Mg ions [43].

More recently, FeVO4.0.9H2O-Graphene composite was also employed as cathode material for aqueous rechargeable magnesium ion batteries [52].As shown in Fig.9(a-c), the specifi capacity of graphene-based composites (FeVO4.0.9H2O-Graphene) is better than pristine FeVO4.0.9H2O.The composite electrode exhibits the discharge capacity of 183.8mAhg?1at the current density of 50mAg?1in magnesium sulfate based electrolytes and is stable up to 50 cycles with capacity retention of about 82%.The improvement in electrochemical performance was due to presence of highly conductive graphene which boost up the electron transportations during redox reactions [28].We know that Li3VO4is an effective cathode material for LIBs owing to its high ionic conductivities and have been extensively used [53].It consists of lantern type 3D structure,where VO4and LiO4are corner shared to provide sufficien channels for accommodation of guest ions such as lithium ions [54].Zeng et al.[55] argued that ionic radius of Mg2+ion is 72pm which is close to that of Li+ions (76pm), thus can be used to store magnesium ions.To fulfil this target,they synthesized Li3VO4hollow spheres coated with carbon[56] and used as cathode material for MIBs in Mg(ClO4)2-Acetonitrile based electrolytes.The half-cells cycled between 0-2.5V against Mg foils revealed the reversible capacity of 320mAhg?1at 20mAg?1.Although specifi capacity was significantl high but capacity retentions was found to be very poor.No obvious platforms were observed in charge-discharge profile and specifi capacity of 153mAhg?1was left just after 15 cycles.The low coulombic efficien y and fast decay were due to trapping of Mg2+ions and passivation layer formed on metal magnesium respectively [47].

Fig.8.(a) Structure of Mo2.5+yVO9+δ viewed down the c-axis: MO6 octahedra and MO7 pentagonal bipyramids (M=Mo and V cations predicted theoretically with different oxidation states and occupancies: green, Mo5+/V4+; red, Mo6+/V5+; blue, Mo6+/Mo5+; orange, Mo5+; and purple, Mo6+); (b)Cycling performance of an AC/Mo2.48VO9.93 cell between 3.33 and 1.73V at a rate of C/70 (2mA/g); (c) Electrochemical discharge-charge profil of an AC/Mo2.48VO9.93 cell (Mgx Mo2.48VO9.93, 0≤x≤1) at a rate of C/40 (4mA/g); and (d) Capacity retention data for galvanostatic cycling of AC/Mo2.48VO9.93 cells (MgxMo2.48VO9.93, 0≤x≤1) at different current densities: black, 4mA/g (C/40); red, 10mA/g (C/12) [51].

Apart from V2O5, vanadium dioxide (VO2) has also been considered as potential cathode material for lithium-ion batteries owing to its low-toxicity, high capacity, high working voltage, and high structural fl xibility, diversity, and stability [57].The VO2exists in different forms such as VO2(A),VO2(B),VO2(M1),VO2(M2),and VO2(M3);however their chemical formula is same[58].Among these,the VO2(B)has monoclinic layered structure with space group ofC2/m(12)and resemble to that of V6O13[59].The edge-shared octahedrons form similar layers inc-direction and led to formation of VO2(B) unit cell.The vanadium-oxygen (V-O) tunnel formed due to stacking of corner-shared neighbor cells, facilitates the insertion/extraction of guest ions such as Mg2+, Na+and Li+etc.[60].To insure the accommodation of Mg2+ions inside V-O tunnel, Luo et al.[61] attempted to synthesize the VO2(B) nanomaterials (nano-sheets, nano-rods) and used as electrode material for MIBs in Mg(ClO4)2/AN electrolytes as shown in Fig.10.

Fig.9.The rate performance of FeVO4.0.9H2O (a) and FeVO4.0.9H2O/Graphene (b); the cycle performance of two electrodes in 1.0mol L?1 MgSO4(c) [52].

Fig.10 represents the charge-discharge curves of VO2(B)where nanosheets exhibit a reversible capacity of 356 mAhg?1which decays in firs few cycles.On the other hand, nanorods exhibit a reversible capacity of 391mAhg?1, which is stable compared with that of nanosheets (Fig.10(b)).The chargedischarge curves show clear plateaus, which corresponds to reaction given in thebelow equation:

The anode side of VO2(B)-based Mg-ion battery is dominated by oxidation of magnesium as shown in the below equation:

The cycle stabilities of MIBs are shown in Fig.10(c) and it can be seen that nanorods shows better capacity retentions even after 60 cycles at 12°C.The reversible capacities of 206 and 61mAhg?1were retained for nanorods and nanosheets respectively.To further examine the rate capabilities, the electrodes were tested at 20°C with current rates (100, 50, and 25 mAg?1) as shown in Fig.10(d).The reversible capacities of 341, 370, and 391mAhg?1were achieved for nanorods at the current densities of 100, 50, and 25 mAg?1, respectively.Whereas, nanosheets exhibit the reversible capacities of 333,337, and 356 mAhg?1at the current densities of 100, 50, and 25 mAg?1, respectively.

Besides vanadium oxide based nanosheets and nanorodes,vanadium oxide nanotubes were also employed in rechargeable magnesium ion batteries.It was noticed that the conventional synthesis of vanadium oxide nanotube utilizes amine templates, which are very beneficia for electrochemical properties.However, the specifi capacity of vanadium oxide nanotubes prepared by hydrothermal route, was limited to about 75 mAhg?1in non-aqueous magnesium electrolytes[62,63].To achieve the high reversible capacities, researcher have focused on intercalation of octadecylamine and hexadecylamine inside vanadium oxides, which can alter the valance state of V [64].Inspired from these strategies, Kim et al.[65] investigated the electrochemistry of vanadium oxide nanotubes (V3+/V4+/V5+) for Mg-ion batteries.These nanotubes were fabricated by microware-assisted hydrothermal method,and exhibited the specifi capacity of 218 mAhg?1at the current density of 60 mAg?1with a significan cycle life.The excellent electrochemical performance of vanadium oxide nanotubes was attributed to the generation of vanadium ions and lower charge transfer resistance at the electrode/electrolyte interface.Recently, Xu et al.[66] made successful attempt to synthesize magnesium and water intercalated bi-layered vanadium oxide nanowires (Mg0.3V2O5-1.1H2O)).The nanowire cathode was tested for Mg-ion batteries and experimental results revealed that pre-intercalated magnesium boosts the electrical conductivity, whereas the crystal water enhanced the Mg2+ions transportation through shielding effect.The synthesized nanowire cathode exhibited excellent rate capability (160 mAhg?1at 0.1C) and cycle life for 10,000 cycles.The good electrochemical performance of Mg0.3V2O5-1.1H2O was attributed to synergetic effect of magnesium and crystal water.

Fig.10.Galvanostatic charge/discharge curves of the as-prepared VO2 (B) in the form of (a) nanosheets, and (b) nanorods at 25mA/g; Cycling performance of VO2 (B) nanomaterials at 12°C at (c) 50mA/g, and (d) rate performance of VO2(B) at various current densities at 20°C [61].

4.Closing remarks and future recommendations

This review has successfully debated a post-lithium ion technology i.e.magnesium-ion batteries while mentioning their exciting advantages over lithium-ion batteries.The reasons for delay of magnesium-ion battery in 19th century and firs breakthrough made by Aurbach in 2000, are discussed.This breakthrough has triggered the research on all-phenylcomplex electrolytes along with molybdenum sulfid cathodes.Quick review of various kinds of molybdenum sulfide for magnesium-ion batteries was provided,along with detailed discussion on adaptation of alternative cathodes such as carbon derived and vanadium oxides.The chemical structures of cathode materials, effect of doping, nature of electrolytes and their solvents, cut-off voltages, working potentials and finall electrochemical properties were deeply discussed to obtain high energy and power densities of rechargeable magnesiumion batteries in temperature range of ?30 to 55°C.We also concluded that magnesium-ion battery is one of the best technology that can be cycled without dendrite formation, thus high capacity retentions are expected.Unfortunately, unlike lithium-ion batteries, magnesium-ion batteries still exhibits low working potentials.The recommendations for future research priorities are given below.

1.The graphite fluorid cathode exhibits excellent electrochemical properties during their firs cycle with a stable voltage plateau.However, during second cycle the capacity goes to almost zero with absence of voltage plateau.Chemical researcher should investigate the reason behind this phenomenon, and attempt should be made to retain the electrochemical performance of graphite fluorid during subsequent cycles.

2.The vanadium oxide-graphene composite based Mg-ion battery exhibited the excellent specifi capacities at room,low, and high temperatures.This means that MIBs could be ideal candidate for low and high temperature applications.Further research is needed to explore this fiel on urgent basis.

3.It was found that working potential of vanadium oxide is higher compared to that of molybdenum sulfide in magnesium-ion systems.Thus,higher specifi energies can be obtained by using vanadium oxide and its derivatives doped with transition elements and crystal waters.

4.The research on development of efficien electrolyte solutions for magnesium ion batteries is still far from commercial needs.Future research should focuses on increasing the ionic conductivities of electrolytes by using different organic and inorganic additives.

We hope this review will provide quick and broaden view of magnesium-ion battery and more attempts will be made to conquer the technical setbacks of this technology.

Conflic of interest

There is no conflic of interest.

Acknowledgements

This research was supported by National Natural Science Foundation of China (51601073), Jiangsu Distinguished Professor Project (1064901601), Jiangsu Provincial Six Talent Peaks Project (1062991801), and Jiangsu University of Science and Technology Research Start-Up Fund (1062921905).The authors would like to express our gratitude to the technical staff in the Department of Materials Science and Engineering, Jiangsu University of Science and Technology.