Recent advances based on Mg anodes and their interfacial modulation in Mg batteries

Fanfan Liu,Guoqin Cao,Jinjin Ban,Honghong Li,Yan Zhang,Guoshng Shao,Aiguo Zhou,Li zhn Fan,Junhua Hu,??

aSchool of Materials Science and Engineering,Industrial Technology Research Institute of Resource and Materials of Henan Province,Zhengzhou University,Zhengzhou 450001,China

b State Center for International Cooperation on Designer Low-carbon and Environmental Materials(CDLCEM),Zhengzhou University,Zhengzhou 450001,China

c Henan Provincial Key Laboratory for Metal Fuel Battery,Foguang Power Generation Equipment Co.Ltd,50 Holly Street,Zhengzhou 450000,China

d School of Materials Science and Engineering,Nanjing Institute of Technology,Nanjing 211167,China

e Henan Key Laboratory of Materials on Deep-Earth Engineering,School of Materials Science and Engineering,Henan Polytechnic University,Jiaozuo 454000,China

fBeijing Advanced Innovation Center for Materials Genome Engineering,Institute of Advanced Materials and Technology,University of Science and Technology Beijing,Beijing 100083,China

Abstract Magnesium(Mg)batteries(MBs),as post-lithium-ion batteries,have received great attention in recent years due to their advantages of high energy density,low cost,and safety insurance.However,the formation of passivation layers on the surface of Mg metal anode and the poor compatibility between Mg metal and conventional electrolytes during charge-discharge cycles seriously affect the performance of MBs.The great possibility of generating Mg dendrites has also caused controversy among researchers.Moreover,the regulation of Mg deposition and the enhancement of battery cycle stability is largely limited by interfacial stability between Mg metal anode and electrolyte.In this review,recent advances in interfacial science and engineering of MBs are summarized and discussed.Special attention is given to interfacial chemistry including passivation layer formation,incompatibilities,ion transport,and dendrite growth.Strategies for building stable electrode/interfaces,such as anode designing and electrolyte modification,construction of artificial solid electrolyte interphase(SEI)layers,and development of solid-state electrolytes to improve interfacial contacts and inhibit Mg dendrite and passivation layer formation,are reviewed.Innovative approaches,representative examples,and challenges in developing high-performance anodes are described in detail.Based on the review of these strategies,reference is provided for future research to improve the performance of MBs,especially in terms of interface and anode design.

Keywords:Magnesium anode;Dendrite;Passivation layers;Interfacial engineering;Solid electrolyte interphase.

1.Introduction

Climate warming and energy shortages are two intertwined problems in the world today.To address them,the concepts of carbon neutrality and carbon peaking have been proposed one after another by countries around the world.Meanwhile,the development of clean,low-pollution,and sustainableenergy sources,such as wind and solar power,is also imminent.Over the past decade,as renewable energy generation technologies are getting matured,their costs have decreased significantly.However,since renewable energy generation technologies,especially wind and solar,tend to be intermittent,a suitable energy storage system is required to regulate them.Electrochemical energy storage devices are considered one of the most desirable options due to their high energy efficiency,flexibility,and small constraints[1–3].Nevertheless,most existing battery technologies,e.g.,lithium-ion batteries(LIBs),sodium-ion batteries(NIBs),and lead-acid batteries,have some problems in obtaining electrochemical energy storage systems with high safety,all-climate,low cost,and long life[4–7].Multivalent ion batteries with Mg,Zn,and Al as anodes can provide high energy density through multielectron reactions[8–12].Moreover,they offer numerous advantages such as small ion radius,low cost,and relatively low deposition potentials.The relevant parameters of various earth-abundant metal anodes are summarized in Fig.1,which makes them a potential new battery system after LIBs.Certainly,as anodes,they also have serious opportunities and challenges in the corresponding batteries at this stage,more time and effort are still required to meet the gap with LIBs.

Fig.1.A comparison of the general features of earth-abundant metal anodes.

Mg is a divalent metal with a relatively high volumetric capacity and low redox potential[13,14].In the 1990s,the first magnesium battery(MB)consisting of Mg//0.25 mol L–1Mg[B(Bu2Ph2)]2/THF-DMF//Co3O4was reported,and the coulombic efficiency of charging and discharging capacity was close to 99%[15].Subsequently,tremendous progress has been made in the research and development of MB and its key materials(electrolyte and electrode materials)in the past decades.It is worth noting that the successful development of multiple electrolyte systems paves the way for the stability of the Mg metal anode.In particular,the birth of all-phenyl complex(APC)electrolytes boosted the electrochemical window to 3.3 V[16].However,the presence of chloride ions can corrode the electrodes;the use of conventional electrolytes such as Mg(TFSI)2and Mg(CF3SO3)2can also cause the Mg anode passivation and incompatibility of the anode side interface.In addition,although it has generally been believed that dendrite growth does not occur in Mg metal anode,this concept is being debated increasing.The inhomogeneous deposits similar to“Mg dendrites”have been observed in some electrolyte systems or under specific conditions recently.Thus,it may be time to reconsider whether previous studies reflected the presence or growth of dendrites well.Therefore,this leads to two questions:(1)Does the presence of dendrites occur in Mg the metal anode?(2)How to alleviate and attenuate the sluggish ion/electron transport and poor interfacial stability caused by the formation of passivation films or dendrites on the anode side of Mg?To cope with these problems,optimizing conventional electrolytes and designing non-passivated and dendrite-free Mg metal anodes are essential for the development of MBs.

In recent years,several research groups have modified Mg anode[17–19],introduced electrolyte additives[20,21],and designed artificial solid electrolyte interphase(SEI)[22–25]to attenuate the reversible deposition/stripping overpotential ofMg and enhance the stability of anode interface[26–28].However,to the best of our knowledge,most of the current reviews on key materials for MBs focus on the generalization of cathode and anode materials and the development of new electrode materials[28,29–32],while few relevant reviews have been done on the progress of Mg metal anode and their interfaces with electrolyte in the MBs[33],thus prompting us to do a concise review on the interface between Mg metal anode and electrolyte for MBs.

Fig.2.Schematic illustration of Mg batteries(MBs);the failure mechanism of MBs and the practical strategies to suppress passivation layer.Where M in MxOy and MxSy represents transition metal.

In this review,as displayed in Fig.2,the existing problems of Mg anodes are introduced first in detail.Then an in-depth analysis of the formation mechanism of passivation film and dendrites in the Mg anode is provided.In the penultimate part,the current research status of anode-electrolyte interface in MBs from the design of Mg anode structure,interface regulation and development of new electrolyte system and other optimization strategies are discussed.Finally,we conclude with a summary of the current status in the Mg anode side and an outlook for future study.We hope that scientists working on Mg anodes and related materials could find this review helpful in exploring the potential of MBs through various anode modification strategies,promoting the practical application of MBs.

2.Problems on the Mg anode

Compared with Li and Na metal anodes,Mg metal anodes are generally less prone to dendritic deposition,which allows for a safer battery operation.However,due to the low diffusion barrier,a passivation film is produced on the surface of the Mg metal anode in most electrolytes.In particular,due to the relatively high reactivity of Mg with H2O in Mg-air battery,the parasitic reaction(Mg+2H2O→Mg(OH)2+H2)of Mg anode in the aqueous electrolyte in the standby state or under anodic polarization is severe.And as a product of the discharge reaction and parasitic reaction,when the pH of electrolyte is＞10.4,a large amount of Mg(OH)2is formed and stably precipitated on the anode surface,forming a passivation film and resulting in a large potential drop and energy loss.Therein,it causes the poorly reversible deposition of Mg ions on its surface.Moreover,it has been confirmed that at a high current density(10 mA cm-2),dendrites are generated in the Mg metal anode,which can lead to short-circuiting of MB.Therefore,it is necessary to improve the corrosion resistance of Mg electrodes,clarify the mechanism for the formation of passivation layers and dendrites on the Mg anode,and then optimize them in a targeted way.

2.1.Passivation layer

As early as 2003,it was thought that a stable“passivation layer”formed on the fresh Mg metal foils immersed in an electrolyte containing tetrahydrofuran(THF)and Mg(AlEtBuCl2)2,the layer contained physically adsorbed molecules rather than ionic species on the Mg surface.With the deepening of research,the dominant view is that the relatively low reduction potential of Mg metal anode(-2.37 V vs.SHE)means that most common electrolyte components(e.g.,carbonate solvents and a common Mg salt)are readily reduced on the Mg metal anode forming an interfacial passivation layer with the composition of mainly MgO,MgCO3and Mg(OH)2.The passivation layer insulates electrons and ions,thus preventing the reversible stripping/deposition of Mg,resulting in reduced Coulombic efficiency and shortened cycling life.

2.2.What does Mg dendrite look like?

In previous reports,the dendrite free deposition of Mg was confirmed by the study of the atomic diffusion potential on the metal surface[34].Compared with Li(001)and Na(001),the self-diffusion energy of the Mg(0001)surface is lower,which allows Mg to be preferentially deposited along the inplane direction to inhibit dendrite growth.Hence,it was believed that this advantage of Mg metal can enable it to be used directly as a battery anode without posing a threat to battery safety.However,in recent years,several groups have claimed to observe the nonuniform deposits similar to“Mg dendrites”[35,36].Furthermore,this phenomenon has also been confirmed by Kwak et al.simultaneously[19,35,37].Kwak et al.studied the growth of Mg dendrites under electrostatic conditions and by building a phase map,as displayed in Fig.3a.The Mg dendrite growth morphology was influenced by the current density,electrolyte concentration,andadded ligands.The increase in current density amplified the degree of branching,indicating an accelerated electrochemical reaction rate;the increase in concentration induced a transition from a fractal to a dendritic growth state.Notably,the dendritic growth showed an extended single-crystal domain along the(1120)growth direction.The Mg dendrites morphology gradually changes from smaller grains(including aggregated thin hexagonal plates)at lower concentrations(0.25 M MeMgCl/THF)to spherical deposits with tangential hexagonal surface features at higher concentrations(0.5 M).Typical dendrite with a strongly anisotropic directional growth occurred at the highest concentration(1.5 M).Lim et al.demonstrated the dynamic growth of Mg dendrites in APC electrolytes using various dynamicin situobservation techniques,and discussed the growth behavior of Mg dendrites under various electrochemical conditions[19].At a relatively low current density of 2 mA cm-2,a large number of spherical Mg crystal species covered the substrate(Fig.3b).In contrast,lethal needle-like dendrite growth was observed(Fig.3c)at a critical current density of 10 mA cm-2.Short-circuiting due to the direct contact between dendrite and opposite electrode.Dendrite was also confirmed by dynamicin situview techniques combined with EIS analysis.This is in agreement with the results of the Banerjee's group.

Fig.3.(a)Phenomenological map depicting several differentiated growth regimes as a function of reaction variables.Reproduced with permission[35].Copyright 2020,The Royal Society of Chemistry.(b,c)Ex-situ SEM images after the Mg depositions at 2 mA cm-2 and 10 mA cm-2,respectively.Reproduced with permission[19].Copyright 2021,American Chemical Society.

From the above discussion,it can be seen that during the operation of MBs,Mg ions are reversibly deposited and dissolved on the surface of anode material[38].It is easy to form a relatively dense passivation film on the Mg anode surface.It is difficult for Mg ions to pass through the film,which affects the dissolution/deposition of Mg.After many cycles,Mg dendrites are easily formed on the surface of Mg,which deteriorates the battery performance and even causes short circuit in MB.Researchers have employed a variety of approaches to mitigate and eliminate passivation film formation and dendrite growth.The third part of this review will categorize and elaborate the above advances.

3.Interfacial modulation strategies

It can be seen from the above analysis that the problems of Mg metal anode are mainly its poor compatibility with traditional electrolytes,which affects the performance of MBs[27,39,40].Researchers have proposed strategies to address the anode side interface problems,such as(1)structured Mg metal anode,(2)development of novel anode materials,(3)construction of artificial SEI film,(4)introduction of electrolyte additives,and(5)design of solid-state electrolyte for the above problems.

3.1.Mg metal anode structuring strategy

Structural micronization is a way to alleviate passivation film formation and enhance the ion transport performance of Mg anode.Chen and co-workers prepared Mg nano/mesoscale structures by vapor-transport approach,as shown in Fig.4a and b.Compared with bulk Mg(B-Mg),ultrasmall Mg nanoparticles(N-Mg)with an average diameter of～2.5 nm demonstrated better cycling properties than B-Mg.This is attributed to the N-Mg with a larger specific area.The surface passivation film on N-Mg particles is thinner when the same amount of surface passivating species was formed,thus,enhancing the ion diffusion in the anode of MBs.Combined with a graphene-like MoS2cathode,the cell has achieved an operating voltage of 1.8 V and an initial discharge capacity of 170 mAh g-1[41].Hu et al.clarified the effect of grain size on the electrochemical properties of pure Mg anode[42].

In addition to micronized Mg metal anode,Mg metal is composited with various three-dimensional structures to obtain a structured Mg metal anode.The three-dimensional structure can reduce the surface current density and slow down the growth rate of Mg dendrites.It can solve the problem of volume expansion during the deposition of Mg,which can achieve a stable cycle of Mg metal anode.Taking the reports on the structuring of Li/Na anodes as references,the vertically aligned N-and O-doped carbon nanofiber arrays on carbon cloth(VNCA@C)is used as the substrate,and the uniform Mg deposition is achieved through synergistic coupling of current homogenization,geometric confinement and chemisorption effect(Fig.5a)[43].Finally,a reduced nucleation overpotential(647 mV)and an elongated Mg plating/stripping cycle life(110 cycles)have been obtained at 10 mA cm-2(Fig.5b and c).

After studying the growth state of Mg dendrites,Kwak et al.proposed an effective solution to reduce the growth rate of Mg dendrites.Magnesiophilic sites were introduced by coating Au atoms on various substrates[19].Combined with DFT,the results showed that highEads(adsorption energy)of the Au sites make Au highly magnesiophilic.In addition,thedendrite growth would be limited due to the relatively lowEdiff(diffusion barrier)of Mg atoms,as depicted in Fig.5d.Therefore,the introduction of magnesiophilic sites will be a simple and effective strategy to suppress Mg dendrites formation and regulate internal short circuits in future applications of MBs.

Fig.4.(a)Cycling behavior at 20 mA g-1 and(b)galvanostatic discharge-charge voltage profiles of the cells fabricated with B-or G-MoS2 cathode and Bor N-Mg anode.The inset is a TEM image of N-Mg;Reproduced with permission[41].Copyright 2011,Wiley-VCH.

Fig.5.(a)The schematic illustration of Mg electrodepositing on varied substrates;(b)The overpotential of Mg//VNCA@C,Mg//CC,Mg//Cu at 10.0 mA cm-2;(c)Voltage profiles of metal Mg plating/stripping of symmetric cells(Mg@VNCA@C//VNCA@C)at a current density of 10.0 mA cm-2;Reproduced with permission[43].Copyright 2021,Wiley-VCH.(d)Diffusion barrier of Mg ion to an adjacent site on each surface.Reproduced with permission[19].Copyright 2021,American Chemical Society.

The alloying treatment of Mg could reduce the activity of Mg metal anode and improve the corrosion resistance and battery performance.Mg alloying is mainly used in Mg-air batteries to enhance the poor corrosion resistance of Mg anode in the electrolyte.The discharge performance was much improved by adding some special alloying elements.So far,some Mg-based and Mg-Al-based alloys such as Mg-Ca(Ge)[44],Mg-Gd-Zn[45],Mg-Al-Zn[46,47],Mg-Al-Pb-In[48],Mg-Al-Pb-RE[49],and so on[50–52]have been developed.Chen et al.reported that Mg-0.1 wt% Ca binary alloy has the best self-corrosion resistance and discharge performance[44].When the as-extruded Mg-1.5 wt% Ca alloy with fine dynamically recrystallized grains(15±2 μm)was employed as an anode,it displayed a high energy density of 1413 mW hg-1with a satisfactory specific capacity(1296 mAh g-1)at 20 mA cm-2,as depicted in Fig.6a and b[53].By regulating the content of elements,the addition of Ge,In,Bi,or Zn could improve the voltage and relevant discharge performance of Mg-Ca-based alloys and reduce their corrosion rate[54–56].For example,Mg-0.1 wt% Ca-0.1 wt% Ge al-loy showed a low corrosion rate of 0.2 mm y–1in 3.5 wt%NaCl solution[57];Compared with Mg-0.2 wt% Ca-0.4 wt%In,Mg-0.2 wt% Ca-0.2 wt% In anode had a higher anodic efficiency(80.2%)and energy density(2259 Wh kg-1)at 23.75 mA[54].Generally,for Mg-Al-based alloys,Mg-Al-Zn is common(called AZ31,AZ91,according to the content of Al and Zn)[46,58],which has poorer discharge performance than anodes alloyed with rare metal.Although the Mg-Al-Pb alloy containing 2 wt% In with desirable discharge performance is more suitable to serve as the anode for Mg-air battery[48],Pb is toxic and harmful to the environment.According to the report of Xiao et al.thanks to the generation of long-period stacking ordered(LPSO)structure,a T6 treated Mg95.28Gd3.72Zn1.00alloy anode possessed the best discharge capacity and anodic efficiency of 1329 mAh g-1and 59.9%at 40 mA cm-2,respectively,as displayed in Fig.6c,d[45].

Fig.6.(a)Microstructure of the Mg-1.5 wt%Ca anodes;(b)Summarized discharge properties of the as-extruded CP Mg and Mg-1.5 wt%Ca anodes.Reproduced with permission[53].Copyright 2019,Elsevier.(c)SEM image of T6 treated Mg95.28Gd3.72Zn1.00 alloy;(d)Cell voltage and discharge capacity of different Mg95.28Gd3.72Zn1.00 alloys in Mg-air batteries.Reproduced with permission[45].Copyright 2021,Elsevier.

3.2.Novel anode materials

Considering that passivation films or dendrites are easily conformed to grow on the Mg metal anode surface,researchers have developed some new anode materials to replace Mg metal,which can effectively alleviate this phenomenon.In theory,due to the high storage capacities,superior operating voltage,and compatibility with the conventional battery electrolytes,the group XIV elements(Si,Ge,Sn)and Bi,Sb could also be used as the promising insertion type anodes for MBs[59–62].Singh et al.have explored the atomistic mechanisms of Mg ions insertion reactions in the Si,Ge,and Sn anodes for Mg batteries through DFT calculations[62].The results indicated that the insertion of Mg in crystalline X(X=Si,Ge,Sn)anode leads to the breakage of the stronger X-X bond to form an amorphous MgxX phase and weaker Mg-X bonding network with increasing Mg concentration.The theoretical capacities of Mg ion insertion in Si and Sn are 3817 mAh g-1and 903 mAh g-1,respectively[61].The diffusion barriers for an isolated Mg ion in the Sn and Bi are 0.43 and 0.67 eV,and the corresponding average voltages are 0.15 and 0.18 V,respectively[59].Furthermore,the nanostructured Bi anode delivers a specific capacity(350 mAh g-1Bi),good stability with 7.7% of capacity fading,and high Coulombic efficiency(～100%)after 200 cycles(Fig.7)[60].This work lays the foundation for the nanostructure design of the pure metal Mg anode material.

Based on the above researches on metallic anodes,by combining the advantages of each element,multi-element alloying design is another way to prepare anodes for MBs.SnSb alloy can form Mg2Sn and Mg3Sb2when used as the anode of MBs,displaying a high theoretical capacity of 768 mAh g-1[63].For the experiment,thanks to the unique interface formed between the Sn-rich and Sb-rich subdomains,Niu et al.reported that the SnSb alloy was able to deliver an excellent reversible capacity of≈420 mAh g-1with good rate capability and cyclic stability,as displayed in Fig.8.The nanoporous(NP)Bi-Sn alloys(Bi6Sn4and Bi4Sn6)with unique porous structure,dual-phase microstructure,and high density of phase/grain boundaries prepared by chemical dealloying showed excellent Mg storage performance[64].In particular,the Bi6Sn4exhibited a large discharge specific capacity(434 mAh g-1,50 mA g-1)and good cycling stability(280 mAh g-1after 200 cycles)(Fig.9a–c).Similarly,Zhang and his group obtained a porous Bi-Sn(P-Bi3Sn2)composite with controllable composition/size through selective phase corrosion(Fig.9d–f)[65].The P-Bi3Sn2anode manifested a superior discharge capacity of 367 mAh g-1at 1 A g-1and a long cycling performance(200 cycles)with reversible capacity retention of over 93%.Furthermore,through experiments and DFT calculations,Zhang et al.introduced Bi into Sn by magnetron co-sputtering to form a self-supporting,additive-free,nano-sized Sn-Bi film composite[66].This method could lower the defect formation energy of Mg insertion in Sn.On the other hand,it can effectivelypromote the electrochemical reaction between Sn and Mg,and with the increase of Bi content,the electrochemical performance of Sn-Bi has been significantly improved.

Fig.7.(a)Structural transformation of Bi during the discharge/charge process,(b,c)rate performance and cycling stability of Mg/0.1 M Mg(BH4)2-1.5 M LiBH4-diglyme/Bi cell.Reproduced with permission[60].Copyright 2014,American Chemical Society.

Fig.8.(a)Schematic illustration of the electrochemical reaction mechanism of SnSb particles with Mg ions;(b)Specific capacity of SnSb at different current densities as noted and(c)Cycling stability of SnSb(at 500 mA g-1).Reproduced with permission[63].Copyright 2015,Wiley-VCH.

Fig.9.(a)Schematic illustrations of the electrochemical reaction mechanisms of NP-Bi-Sn alloys;(b)Rate performance of NP-Bi and NP-Bi-Sn;(c)Cycling stability of NP-Bi,NP-Bi-Sn at 200 mA g-1 and NP-Sn at 20 mA g-1.Reproduced with permission[64].Copyright 2018,Elsevier.(d)Schematic illustration showing the design strategy of porous dual-phase Bi-Sn alloys;(e)TEM characterization of P-Bi3Sn2;(f)Rate performance of P-Bi-Sn and P-Bi/Sn at various current densities from 0.05 to 1 A g-1.Reproduced with permission[65].Copyright 2019,The Royal Society of Chemistry.

The modified two-dimensional(2D)materials are considered to be a potential anode for MBs.Adsorption of divalent Mg ions on defective graphene and graphene allotropes is predicted by DFT[67].The results suggested that an Mg storage capacity as high as 1042 mAh g-1could be achieved in graphene with 25% divacancy defects.According to the report of Penki et al.[68],the presence of reduced graphene oxide(RGO)layers could effectively alleviate the large volume change and shorten the diffusion length for Mg2+during alloying and dealloying.Thus,Bi nanoparticle-anchored on RGO(Bi/RGO)prepared by thein-situsolvothermal method possessed a high-rate capability with a discharge capacity of 238 mAh g-1at 700 mA g-1.Moreover,it delivered a high discharge capacity of 372 mAh g-1at 39 mA g-1in the 50th cycle,as depicted in Fig.10a–c[68].In addition,there are some graphene-like 2D materials,such as carbonnitrogen materials(C2N,C3N,and g-C3N4)[69],and BSi[70],which can theoretically be used as the anode electrode of Mg batteries.And interestingly,when g-C3N4nanosheets were curled into nanotubes,the storage capacity of Mg-ions could be much improved.Different from the above materi-als,as an Mg-ion insertion-type anode material,the spinel Li4Ti5O12nanoparticles provided a reversible capacity as high as 175 mAh g-1[71].

Fig.10.(a)SEM and(b)TEM images of Bi60;(c)Rate capability of Bi100 and Bi60 sample at different current densities are indicated in mA g-1.Reproduced with permission[68].Copyright 2018,The Royal Society of Chemistry.(d)Optimized structures of Mg2+@AlNNT complexes;(e)Partial density of state(DOS)of Mg@armchair AlNNT complex.Reproduced with permission[77].Copyright 2020,Elsevier.

Recently,researchers have used the first principles theory to predict the electrochemical performance of borophene[72,73]and phosphorene[74,75]as anode materials for MBs.Mortazavi et al.proved that the flat borophene film as electrically conductive and thermally stable anode material had an ultra-high capacity of～2480 mAh g-1for Mg storage,which is not only significantly better than buckled borophene but also than all other 2D materials[73].Han et al.found that the monolayer phosphorene could store Mg atoms via insertion and adsorption with a binding energy of-0.716 eV[74].

Through DFT calculation,it was found that the top of the ring in BN and SiC nanosheets[76]and the N site in AlN and GaN nanosheets[77]are the most favorable sites for Mg ion adsorption,and they have larger adsorption energy.Compared with AlN nanosheets,as displayed in Fig.10d and e,the small curvature of AlN nanotubes was more conducive to the diffusion of Mg on the surface,which could achieve a faster charge/discharge rate[78].N-triphenylenegraphdiyne nanosheets with an ultrahigh capacity(1439 mAh g-1)for Mg-ions storage were predicted by Salavati et al.[79].In addition,Graphyne-like Covalent Triazine Framework(GYCTF)[80],B40fullerene[81],and C24N24cavernous nitride fullerene[82]are also considered to be promising anode materials for MBs.

In general,the Mg storage performance of most of the above-mentioned anode materials is still at the theoretical prediction stage.How to prepare and apply them in MBs needs to be experimentally explored in the future.

3.3.Constructing artificial SEI strategy

Mg anodes suffer from severe passivation and extremely high overpotential in conventional electrolytes.Li et al.designed a durable non-homogeneous SEI with a low surface diffusion potential barrier,which consists of an MgCl2-rich top layer and a silicon-based bottom layer[83].As shown in Fig.11a,this SEI can withstand long anode cycling and permanently protects the Mg anode from passivation in conventional electrolytes.Distinct from the conventional porous accumulation SEI,this hybrid SEI could be agglomerated into nanodomains by monolithic MgCl2attenuation and embedded in Si-O and Si-C reinforced organic matrices without affecting Mg ion diffusion.Benefiting from this anti-passivation strategy,the cell assembled with Mg-Si electrodes achieved a highly reversibility＞600 times(Fig.11b),confirming that the Si-based protective layer allows the quick diffusion of Mg2+.

Lv et al.obtained a modified Mg metal anode with an Snbased artificial layer by ion exchange and alloying reactions[17],which is simple and safe.In the artificial coating layer,the Mg2Sn alloy anode offered a fast ion transport channel,and the insulating MgCl2/SnCl2bestowed the necessary po-tential gradient to prevent Mg deposition on the surface.In Mg(TFSI)2/DME electrolyte,the ion conductivity of the SEI was significantly improved and the overpotential of Mg symmetric cell was reduced.The coated Mg anodes could sustain a stable plating/stripping process over 4000 cycles at 6 mA cm-2,as shown in Fig.11c–e.

Fig.11.(a)Schematic diagrams of SEI evolution and Mg deposition manner based on Mg-Si electrode in the conventional electrolyte;(b)Mg plating/stripping performance of Mg/Mg symmetric cells based on bare Mg electrodes and Mg-Si electrodes at 0.1 mA cm-2 with an areal capacity of 0.05 mAh cm-2.Reproduced with permission[83].Copyright 2021,Elsevier.(c)Schematic illustration of the formation procedure of the modified Mg foils;(d)Cross-sectional image for modified Mg anode plated with 2 mAh cm-2;(e)Voltage profiles in symmetric cells with modified Mg anodes at a current density of 6 mA cm-2.Reproduced with permission[17].Copyright 2021,Oxford University Press.

Furthermore,to address the poor compatibility of the anode-electrolyte interface,Li et al.proposed a widely applicable strategy of forming an artificial SEI(a-SEI)layer on Mg film[84].The a-SEI layer prepared by immersing Mg foil in LiTFSI+AlCl3/TEGDME solution was found to display promising Mg2+conductivity and relatively low and stable overpotentials.More importantly,complete passivation of the Mg surface was avoided,as shown in Fig.12a.The a-SEI layer could also be synthesized by adding appropriate additives to the electrolyte.Through anion-solvent coordination,for the first time,Cui and co-workers adjusted the molecular orbital energy level to an anion Mg salt centered on Al and successfully constructed a stable Mg2+conductive SEI on Mg metal anode[25].Similarly,they reported that a stable Li-species-containing SEI could be generated by the partial decomposition of Li electrolyte during the electrochemical process(Fig.12b)[23],which prevents parasitic reactions between Mg metal anode and electrolyte,and enables the electrolyte to withstand the long-term cycle.This method could build a“super”SEI film on Mg metal anode,which was of great significance to promote the application of Mg metal batteries(Fig.12c and d).Certainly,the artificial Mg2+-conducting interphase on the Mg anode surface could also be realized with a polymeric interphase layer[22],which was prepared by thermal-cyclized polyacrylonitrile(CPAN)and Mg(CF3SO3)2.The polymeric interphase could effectively prevent the electrochemical reduction of electrolyte and water therein,and allow the migration of Mg2+,thus ensuring the use of oxidation-resistant and non-corrosive electrolyte components.Recently,Li et al.co-dissolved the Mg(pftb)2and MgCl2in tetrahydrofuran(THF)to form an electrolyte with[Mg2Cl3?6THF]+[Mg(pftb)3]-as the main electrochemical active material.The lowest unoccupied molecular orbital energy level of the[Mg(pftb)3]-anion allows the formation of a stable SEIin-situon Mg anodes and suppresses the side reactions very effectively in the TFSI-containing electrolytes.The Coulombic efficiency of the assembled Mg//Mo6S8full cell is as high as 99.5% after 800 cycles,as described in Fig.13[85].

Fig.12.(a)Mg K-edge NEXAFS spectra of different Mg foils;Reproduced with permission[84].Copyright 2021,American Chemical Society.(b)The specific mechanism of SEI formation on the Mg anode surface in LBhfp/DME electrolyte;(c)Charge-discharge profiles of the Mo6S8/Mg battery(with LBhfp/DME electrolyte)at different rates;(d)Cycling stability and corresponding Coulombic efficiency of this Mo6S8/Mg battery for 6000 cycles at the rate of 10C(1C=128.8 mA g-1).Reproduced with permission[23].Copyright 2020,Wiley-VCH.

In addition,a pretreatment method for Mg anode to enhance compatibility between the Mg anode and electrolyte was proposed[18].The Mg foil was immersed in a Ti complex solution with Ti(TFSI)2Cl2as the main component.The insulating MgO layer could be effectively removed through a series of chemical reactions(Fig.14a).The pretreated Mg anode had high compatibility with the glyme-based electrolyte and exhibited good electrochemical performance in the Mg battery with Mo6S8as the cathode(Fig.14b).By simply immersing fresh Mg foil in hydrofluoric acid(HF)to form an ion-conducting but electronically insulating MgF2layer(Fig.14c–e)[86],the formation of this layer could alleviate the side reaction between Mg foil and electrolyte and promote the rapid transmission of Mg2+,thereby improving the voltage stability,Coulombic efficiency and cycle performance of a Bi/Mg cell.

3.4.Electrolyte additive introduction strategy

In order to improve the compatibility between Mg anode and electrolyte,various electrolyte systems have been developed,ranging from nucleophilic electrolytes(based on Grignard reagents)[15,87]to non-nucleophilic(APC[87,88],hexamethyldisilazane(HMDS)-based[89,90],magnesium alu-minum chloride complex(MACC)[91–94],borohydride[95–97],boron-cluster[98–101],and glyme-based electrolytes[21,102,103]).However,these electrolytes either have narrow electrochemical windows or contain halogen anions that may corrode metal battery components,creating difficulties in practical device applications.

Fig.13.(a)Long cycling performance of Mg//Mg cells at 0.5,1.0 and 2.0 mA cm-2;(b)Schematic illustrations of SEI protection of the Mg anode in Mg(TFSI)2/DME electrolyte;(c)Structure model of the SEI[85].Copyright 2022,Wiley-VCH.

Fig.14.(a)Scheme for Mg anode pretreatment by employing a Ti complex;(b)Cycling performance of the cell assembled with pretreated Mg anode,DME/DGM-based electrolyte,and Mo6S8 cathode.Inset is monitoring the change of solution as increasing reaction time.Reproduced with permission[18].Copyright 2017,American Chemical Society.(c)Schematic illustration of the formation process of the MgF2 surface coating;(d)Voltage profiles of the symmetric cells cycling of fresh Mg and MgF2 coated Mg at 0.25 mA cm-2;(e)Cycling performance of fresh Mg and MgF2@Mg foil in the range of 0.01–1.0 V.Reproduced with permission[86].Copyright 2019,Elsevier.

Fig.15.Voltage responses of symmetric Mg cells(a)without or(b)with Ge-based protection film under repeated polarization from 1/4 h charge/discharge cycling;Reproduced with permission[24].Copyright 2020,Elsevier.Cycling behavior of(c)Mg-Mg symmetrical at 500 μA cm-2 in LiCl-added electrolyte cell and(d)Mg/S batteries.Reproduced with permission[20].Copyright 2019,Wiley-VCH.

Subsequently,it was found that the addition of a specific additive to electrolytes could inhibit the formation of ionic insulating interfaces.For example,by adding a small concentration of iodine(≤50×10-3M),the overpotential of Mg deposition/peeling in the Mg(TFSI)2-1,2-dimethoxyethane(DME)electrolyte could be reduced(0.55 V)[21].This was because the ion conductor surface layer MgI2is formed on the Mg anode,which was equivalent to the SEI film.The Ge-based protection layer prepared by adding GeCl4into Mg(TFSI)2/DME electrolyte provided a pathway for rapid Mg2+transport and meanwhile prevented the passivation film formation[24].It is worth noting that a self-repair process would occur when the artificial protection film undergoes an unexpected break(Fig.15a and b).Fan et al.found that LiCl is also an excellent additive[20].It could dissolve insoluble MgCl2deposits and activate the solid/electrolyte interface when added to the[Mg·6THF][AlCl4]2electrolytes,thereby reducing the electrochemical overpotential of the Mg electrode(140/140 mV at 500 μA cm-2)(Fig.15c)and exhibiting long cycling performance in Mg/S cell(a capacity of 300 mAh g-1even after 500 cycles at 670 mA g-1)(Fig.15d).Lately,Wang's group found that the multidentate methoxyethyl amine chelate[-(CH2OCH2CH2N)n-]could greatly promote the interfacial charge transfer kinetics,reduce the overpotential of Mg anode,and inhibit the side reactions at the anode interface through solvation sheath reorganization,thus realizing highly reversible Mg anode and high energy density of 412 Wh kg-1[104].

3.5.Solid-state electrolyte

Compared with liquid electrolytes,solid-state electrolytes with excellent safety performance,good mechanical properties,and wide voltage windows are gradually being studied by the public in the continuous development of rechargeable batteries[105].The research on solid-state electrolytes of MBs at this stage is mainly based on polymer-based electrolytes with better flexibility and interfacial stability.Polymer-based electrolytes are obtained by dissolving the polymer host,inorganic salt,and plasticizers/additives in solvents and after evaporation of the solvents,the resulting solutions are cast into thin films.

In terms of polymer-electrolyte systems,the commonly used raw materials are mainly polyethylene oxide(PEO)and magnesium salts,but some Mg salts(Mg(SO3CF3)2,Mg(N(SO2CF3)2)2)were less compatible with Mg metal anode[106].Recently research indicated that introducing the magnesium borohydride[Mg(BH4)2]into electrolyte was as attractive as solid-state Mg ion conductors.For example,Du et al.reported a new nanocomposite polymer electrolyte based on PEO-Mg(BH4)2for reversible Mg plating/stripping with a high coulombic efficiency(98%)and stable cycling.What’s more,the incorporation of MgO nanoparticles into this electrolyte film could improve its electrical conductivity and mechanical stability[107].The solid-state Mg ion conductors with exceptionally high ionic conductivity at low temperature(5×10–8S cm–1at 30 °C and 6×10–5S cm–1at 70 °C)and stable Mg plating/stripping were synthesized by mechanochemical reaction of Mg(BH4)2and ethylenediamine[98].A novel polytetrahydrofuran(PTHF)-borate-based gel polymer electrolyte coupling with glass fiber(PTB@GF-GPE)was prepared by Cui’s group.The obtained electrolyte has highly reversible Mg plating/stripping byin-situcrosslinking reaction of Mg(BH4)2and hydroxyl-terminated PTHF withinthe GF membrane and excellent compatibility with Mg metal anodes,as depicted in Fig.16a.Moreover,the Mo6S8/Mg batteries assembled with this gel polymer electrolyte possessed an operating temperature range from-20 to 60 °C and did not short-circuit even under cutting test conditions[107](Fig.16b,c).In addition,Jayalekshmi et al.investigated the suitability of PEO-PVP hybrid polymer-based solid electrolyte membranes for all-solid-state Mg ion battery applications.Mg(NO3)2salts were mixed with PEO-PVP mixtures using the solution casting method to synthesize flexible self-supporting PEO-PVP-Mg(NO3)2solid polymer electrolyte(SPE)membranes[108].This SPE exhibited high Mg ion conductivity(5.8×10-4S cm-1)and stable voltage window.

Fig.16.(a)Schematic illustration of in situ preparation of PTB@GF-GPE and the cell assembly procedure;(b)Long cycling behavior of symmetrical Mg//PTB@GF-GPE//Mg cell at a current density of 0.025 mA cm-2;(c)The specific capacity at 0.1 C at varied temperatures.Reproduced with permission[107].Copyright 2021,Wiley-VCH.(d)The preparation process of the PPE;(e)Limiting current density test of Mg//PPE//Mg cell;(f)Surface SEM image of Mg metal after Mg2+plating/stripping 400 h with PPE.Reproduced with permission[114].Copyright 2021,Elsevier.

Certainly,in addition to PEO,the reported polymer host could also be poly(ethylene glycol)(PEG)[109,110],poly(vinylidene fluoride)(PVDF)[111]and poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP)[112].Schaefer and her team investigated the performance of cross-linked,single-ion conducting gel polymer electrolytes consisting ofa PEG dimethacrylate(PEGDMA)for MBs[113].The conductivity of Mg ions was improved,but the charge transfer resistance at the electrode-electrolyte interface was large due to the lack of a complex that could effectively stabilize the active Mg cation.Therefore,future studies may require the incorporation of additional species to stabilize the active Mg cation complex at the electrode-electrolyte interface.Fan et al.obtained porous PVDF-HFP-based electrolytes(PPE)by immersing porous PVDF-HFP membranes in MgCl2-AlCl3/TEGDME(tetramethylene glycol dimethyl ether)[114].The test of Mg//Mg symmetric cell with PPE revealed that the Mg2+was more uniformly plated/stripped on the Mg anode side.Even at a high current density of 20 mA cm-2,the cell did not suffer from an internal short circuit,as displayed in Fig.16d–f.The development and research of polymer electrolytes facilitate the reversible plating/stripping of Mg2+,which could effectively mitigate or weaken the formation of a passivation film on Mg metal anode while enhancing the Mg ions conductivity.

Fig.17.Schematic showing the directions for future research of Mg anode and electrolyte.

Based on the above discussion,in the future,for the modification of Mg anode and adjustment of anode side interface,we can refer to the modification methods of monovalent metal anodes(Li,Na),such as(a)modification of electrolyte or separator,(b)development of solid-state electrolyte,(c)construction of artificial SEI,(d)structured anode.This can reduce the generation of surface passivation film,improve thecompatibility with electrolytes,and realize the improvement of battery electrochemical performance.

4.Summary and perspective

As for anode and its interface,the main problems of the Mg anode are the passivation layer,dendrites,and poor compatibility with electrolytes.Further engineering optimization and more work can be done according to the following points in the future,as shown in Fig.17.

Among recent studies,numerous strategies have been made to further improve the electrochemical stripping/deposition of Mg anode,including constructing artificial anode/electrolyte interface layers,developing appropriate compatible electrolyte systems(solid-state electrolyte),designing reasonable anode structures(alloying),and exploring other anode materials(g-C3N4,BN,SiC nanosheets,Li4Ti5O12…).However,it should be considered that the characterization of structure-property relationships,such as the chemistry of SEI and the degradation mechanism of Mg anodes,should be deeply understood by techniques such as XPS,EDS,or synchrotron radiation.Moreover,in-situor dynamic techniques may be of great assistance for monitoring the relationship between Mg deposition/dissolution behaviors and the evolution of SEI.And the effect of SEI composition on electrochemical performance can also be revealed by electrochemical testingvia in situtechniques.Yet,the practicability of Mg anode has only been explored as the tip of an iceberg compared with that of LIBs.

In addition,the further development of MBs would be promoted through the rational design of electrolytes and electrodes.For example,the ionic conductivity of the electrolyte can be further improved by elemental doping/substitution,the addition of plasticizers,and the regulation of Mg salt concentration.Adding wetting agents in electrolyte,and constructing buffer layers or SEI films on the surface of electrode reduces the interfacial impedance and improves electrochemical stability.Reducing the size of active material or anode,and mixing the electrolyte/electrode enhance the stability of electrolyte-electrode interface.The electrochemical performance of MBs is improved by designing ultra-thin,high-voltage resistant solid electrolytes and high-performance anode material structures.In addition,the adsorption energy of Mg ions in the electrode materials is calculated in conjunction with the first principle,and suitable Mg storage cathode materials are developed with high specific energy/electrode potential/electronic conductivity and excellent reversibility of charge/discharge reactions.From the above-mentioned regulation strategies,the overall performance of MBs is eventually improved.

There is no perfect solution to guarantee that Mg metal works under all conditions.Future continuous research on the science and technology of anode interface will create new avenues for its potential commercial application,as well as provide new insights and research methods for the sustainable development of other advanced energy materials.It is anticipated that this review will stimulate more research on Mg anode and its interface,thus contributing to the development and utility of high-energy-density MBs.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Financial support from the National Natural Science Foundation of China(Nos.52171082 and 51001091),the Program for Innovative Research Team(in Science and Technology)in University of Henan Province(No.21IRTSTHN003)and the Development Strategy of New Energy Industry in Henan Province under the Carbon Neutrality Goal(No.2022HENZDA03)are gratefully acknowledged.