Jiazheng Niu ,Meijia Song ,Ying Zhang,Zhonghua Zhang

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials(Ministry of Education),School of Materials Science and Engineering,Shandong University,Jingshi Road 17923,Jinan 250061,P.R.China

Abstract Rechargeable Mg ion batteries(MIBs)have aroused great interests,and using alloy-type anodes and conventional electrolytes offers an effective way to develop high energy density Mg battery systems.However,the dealloying-induced nanoporosity evolution of alloy-type anodes during the charging process has received less attention.Herein,using a magnetron-sputtered Mg3Bi2 film as an example,we investigate its electrochemical dealloying and associated structural evolution in an all-phenyl-complex electrolyte by in-situ and ex-situ characterizations.The microstructures and length scales of nanoporous Bi can be facilely regulated by changing electrochemical parameters,and there exists a good linear correlation between the surface diffusivity of Bi and the applied current density/potential scan rate on a logarithm scale.More importantly,the self-supporting nanoporous Bi electrodes deliver satisfactory Mg storage performance and alloy-type anodes show good compatibility with conventional electrolytes.Furthermore,the charging-induced dealloying in MIBs is a general strategy to fabricate nanoporous less noble metals like Sn,Pb,In,Cu,Zn and Al,which shows advantages over chemical dealloying in aqueous solutions.Our findings highlight the significance of nanoporosity evolution of alloy-type anodes during dealloying,and open opportunities for the fabrication of nanoporous reactive metals.

Keywords: Mg ion batteries;Alloy-type anodes;Dealloying;Nanoporous metals;Surface diffusivity.

1.Introduction

Rechargeable Mg ion batteries(MIBs)have been considered as one of the most promising alternatives to Li ion batteries(LIBs)owing to abundant raw materials,smooth Mg deposition and high volumetric specific capacity [1].Although high voltage/high capacity Mg insertion cathodes and ethereal-based electrolytes in which Mg metal anodes are fully reversible have been developed in the past two decades,the construction of working Mg battery prototypes with high energy density remains a great challenge [2,3].This is largely due to the incompatibility of Mg metal anodes versus conventional electrolytes and high voltage/high capacity cathodes versus advanced ethereal-based solutions [4].Replacing Mg metal with alloy-type anodes offers a feasible strategy to build high energy density MIBs based on high voltage/capacity cathodes and conventional electrolytes [5].In recent years,alloy-type anodes like nanostructured Bi-based materials [6–8],SnSb/graphene composite [9],Mg2Ga5alloy[10],dual phase Bi-Sn electrodes [11–13]have made a very good progress.Furthermore,promising MIBs prototypes have been assembled based on Mg alloy anodes,conventional electrolytes and high-performance cathodes [8,14,15].

Alloying-dealloying occurs during the dischargingcharging process of secondary batteries with alloy-type anodes [5,16,17].The charging(dealloying)process of Mgbased alloy anodes occurring in MIBs could be perfectly integrated with a common corrosion process during which the active element is selectively etched away and the remaining non-active metal reorganizes to form a nanoporous structure[18].So far,dealloying has been growing into the most important method to fabricate nanoporous metals [18],which show great potentials in catalysis [19],sensing [20],actuation [21],full cells [22],supercapacitors [23],rechargeable batteries [11,24],and so forth.Chemical/electrochemical dealloying is the most well-known technique to fabricate nanoporous metals in aqueous solutions [25,26].Novel dealloying strategies have been recently developed,such as liquid metal dealloying [27]and vapor phase dealloying[28].However,these two methods usually require high operating temperatures,and produce relatively coarser porous structure.The general fabrication,structural modulation and self-supporting design of nanoporous metals are still a great challenge,especially at room temperature.

Herein,a charging-induced dealloying strategy was proposed to generally synthesize nanoporous less noble metals(Bi,Sn,Pb,In,Cu,Al,Zn,etc.)in MIBs with sputtered Mg-based alloy anodes and nonaqueous electrolytes.With Bi metal as an example,the dealloying kinetics and nanoporosity evolution were comprehensively investigated usingin-situX-ray diffraction(XRD),electrochemical characterization andex-situscanning electron microscopy(SEM).The microstructure and length scale of nanoporous Bi were further regulated through controlling electrochemical parameters(for example,galvanostatic current density and potential scan rate).More importantly,the self-supporting nanoporous Bi electrodes deliver superior electrochemical performance for Mg storage and show good compatibility with conventional electrolytes,which provides useful information for the design of highperformance alloy-type anodes and the development of high energy density MIBs.

2.Experimental

2.1.Film deposition

Mg-based alloy films were directly deposited onto a Cu foil substrate by magnetron sputtering apparatus(SKY Technology Development Co.,Ltd,China)using high-purity Mg and M(M=Bi,Sn,In,Pb,Cu,Zn,Al,Ge or Si)targets(99.99 wt.%).The sputtering was operated at room temperature to fabricate Mg alloys in an argon atmosphere.The sample holder was rotated at 5 rpm to guarantee the homogeneous deposition on the substrate surface.Subsequently,the Mg-based alloy films were prepared by co-sputtering.Finally,the tailored discs with a diameter of 12 mm were directly used as the precursors for electrochemical dealloying in Mgion half-cells.

2.2.Synthesis of electrolyte solution

The 0.4 M all-phenyl-complex(APC)electrolyte was synthesized by the reaction of AlCl3with PhMgCl(molar ratio of 1:2)according to the following procedure.First,AlCl3(Aldrich,anhydrous,99.999%)was added to the vigorously stirred tetrahydrofuran(THF)solvent(Aldrich,anhydrous,99.9%,inhibitor-free).Then,2 M PhMgCl solution in THF(Aldrich)was added dropwise to the pellucid AlCl3in the THF solution.The whole process was handled under the argon atmosphere.The resulting solution was mildly stirred for additional 24 h at room temperature.The 0.5 M Mg(TFSI)2/diglyme electrolyte was purchased from Suzhou Dodochem Ltd.

2.3.Materials characterization

XRD(Beijing Purkinje General Instrument Co.,Ltd,China)was used to characterize the phase constitution of the sputtered and dealloyed samples.SEM(ZEISS SIGMA300)was used to characterize the morphology of the obtained samples.The energy dispersive spectrometer(EDS)analyzer attached to SEM was used to determine the chemical compositions.Raman spectra were collected using a LabRAM HR Evolution Raman system(HORIBA Scientific,λ=488 nm).The elemental valence states were analyzed using X-ray photoelectron spectroscopy(XPS,ESCALAB 250)with a monochromatic Al KαX-ray source(150 W)at room temperature.Transmission electron microscope(TEM,JEMARM200F)was performed to characterize the microstructure of nanoporous Bi.

2.4.Electrochemical tests

Mg-based alloy films were dealloyed and tested in a two-terminal coin cell configuration versus Mg metal using nonaqueous electrolytes.Cyclic voltammetry(CV)tests were conducted using a CHI660C potentiostat at different scan rates.Galvanostatic discharging-charging tests were performed on a LAND-CT2001A instrument(Wuhan,China)at room temperature.The discharged and charged electrodes at varied states were disassembled from the cells and then washed with THF in the glove box forex-situcharacterizations(XRD,SEM,etc.).For the operando XRD,the Mg3Bi2alloy was uniformly sputtered onto stainless steel mesh for ensuring the electrolyte transport.The operando XRD analysis was conducted by adopting a CR2016 coin cell with one side beryllium(Be)window(12 mm in diameter)for X-ray beam transmission.

3.Results and discussion

Based on binary alloy diagrams and parameter regulations,herein we fabricated a series of flexible Cu foil-supported MgxMy(M=Bi,Sn,Pb,In,etc.)alloy films by a facile magnetron co-sputtering method(Figs.1a and S1).The XRD result(Fig.1b)verifies the successful fabrication of Mg3Bi2alloy(the signals of Cu come from the substrate)and the SEM images(Fig.1c,d)show the formation of columnar grains.The actual composition of the Mg3Bi2film was determined to be 66.1 at.% Mg and 33.9 at.% Bi by EDS,which is consistent with the phase diagram(Figs.S2 and S3).The elemental mapping images indicate the homogeneous distribution of both Mg and Bi in the sputtered film.Similarly,other Mg-based alloy films(e.g.Mg2Sn,Mg2Pb,MgIn,Mg2Ge,Mg2Si,Mg2Cu,Mg7Zn3,Mg/Mg2Al3)were also fabricated by modulating the sputtering time and power,Fig.S4.

Fig.1.(a)Schematic illustration showing the fabrication of MgxMy alloy films by co-sputtering and power modulation.(b)XRD pattern,(c)Plan-view and(d)cross-section SEM images of the sputtered Mg3Bi2 film.The inset photograph in(b)is the tailored disk.(e)Schematic illustration showing the charging-induced dealloying strategy involving electrochemical demagnesiation in MIBs.

Fig.1e schematically illustrates the charging-induced dealloying strategy involving electrochemical demagnesiation in MIBs with the sputtered MgxMyfilms as the cathode and Mg metal as the anode.The general reaction could be described as follows.

Notably,the sputtered MgxMyfilms could be directly employed as electrodes(inset of Fig.1b).During the charging process,Mg atoms in MgxMyare electrochemically extracted by oxidative reaction to form Mg2+ions.Synchronously,the solvated Mg2+ions transfer through the electrolyte,desolvate and deposit onto the Mg metal anode [29,30],coupling with electrons from the cathode.At the cathode side,the released M atoms diffuse and re-organize into a nanoporous structure.

In the following,Mg3Bi2was selected as a model system to investigate the nanoporosity evolution in nonaqueous solutions.Firstly,operando(in-situ)XRD was utilized to monitor the real-time phase evolution during the charging-induced dealloying process in the 0.4 M APC electrolyte(Figs.2a and S5a).Clearly,the diffraction peaks(22.0°,24.0°,25.1°,32.8° and 38.5°)of Mg3Bi2gradually decrease and finally vanish with the ongoing charging(dealloying).Simultaneously,the characteristic peaks(27.2°,37.9° and 39.6°)belonging to the Bi phase(JCPDS no.44–1246)start to appear and continuously strengthen.No metastable or intermediate phase appears and only the Bi phase can be detected at the end of charging.The contour plot in Fig.S5b vividly reveals the phase evolution associated with the electrochemical demagnesiation of Mg3Bi2.Fig.2a,b highlights the intensity changes of Mg3Bi2(011)and Bi(012),indicating the straightforward phase transformation from Mg3Bi2to Bi.Despite a ravined surface,a typical nanoporous structure with small ligaments(18.6±4.0 nm)and nanopores was generated in the grain interior after first demagnesiation(Figs.2c–f and S6).That is,a core-shell structure comprising ravined shell and nanoporous core is formed,which could be attributed to the formation of a dense shell [31],electrode/electrolyte interface film [32,33],or oxidation layer [34].Additionally,the columnar grain structure of the sputtered film is well retained after the first demagnesiation process(Fig.S7).

Fig.2.(a)Operando XRD patterns and(b)corresponding contour plot showing the phase evolution of the sputtered Mg3Bi2 electrode during the first charging process at 10 mA g?1.(c–e)SEM images of the Mg3Bi2 electrode before and after electrochemical demagnesiation and(f)the first charging profile at 100 mA g?1.

To break the ravined shell,multiple galvanostatic chargingdischarging cycles were performed on the sputtered Mg3Bi2film.Notably,the dealloying potential in the first charging is much higher than that in the following cycles due to the formation of nanoporous structure after the first demagnesiation(Fig.S8a–c).Although the columnar grain morphology is retained,the surface structure evolves from the ravined shell to nanoporosity with increasing galvanostatic cycles in the sputtered Mg3Bi2film(Figs.3a–c and S9).Typical bicontinuous ligament-channel structure is formed in both the surface and interior of columnar grains(Fig.3a–f).Furthermore,TEM and scanning TEM(STEM)images reveal the typical nanoporous structure of the electrochemically dealloyed Mg3Bi2film(Figs.3g and S10).The average ligament size of the formed nanoporous Bi is less than 30 nm,but shows minor changes with increasing charging cycles(Fig.S8d).Obviously,galvanostatic cycling treatment could break the surface shell of columnar grains and trigger the formation of nanoporous structure.Volume expansion/shrinkage arising from the alloying-dealloying processes may be favorable to the destruction of surface shell.Fig.3h vividly demonstrates the morphology evolution from core-shell to typical nanoporous structure upon increasing cycles.Noticeably,significant Mg residual(31.7 ?43.6 at.% Mg)was detected by EDS in all the nanoporous Bi samples,regardless of the galvanostatic charging-discharging cycles(Figs.S11 and S12).The magnesiated specific capacities are obviously higher than the demagnesiated capacities during the alloying-dealloying processes(Fig.S8a–c),which suggests that partial Mg cannot be extracted from Mg3Bi2during the charging process.Despite high Mg contents,only the diffraction peaks of Bi can be identified in the electrochemcially demagnesiated samples(the Cu signals come from the substrate),Fig.S13.Owing to infinitesimally small solid solubility of Mg in Bi(Fig.S3)[35],the residual Mg may exist in the form of amorphous Mg-Bi phase in the obtained nanoporous Bi [33,36].In addition,the retained Mg in the electrochemcially demagnesiated samples could be removed by further chemical dealloying in tartaric acid.After such treatment,the residual Mg content is less than 1 at.%(Fig.S14)and the nanoporous structure is well retained(Fig.S15).

Fig.3.SEM images of(a–c)grain surface and(d–f)grain interior for the electrochemically dealloyed Mg3Bi2 film through galvanostatic charging-discharging cycles at 100 mA g?1.(g)HAADF-STEM image of the electrochemically dealloyed Mg3Bi2 film at 100 mA g?1 for 3 cycles.(h)Schematic illustration showing the morphology evolution of nanoporous Bi upon increasing cycles.

To detect the chemical composition and valence state,the nanoporous Bi fabricated by galvanostatic chargingdischarging at 100 mA g?1for 3 cycles was selected for Raman and XPS analyses.Fig.4a shows the Raman spectra of the sputtered Bi,Mg3Bi2and nanoporous Bi.Two sharp Raman peaks centered at 68.4 and 92.1 cm?1of the sputtered Bi film are associated with the characteristic of metallic Bi[37],while the peak located at 76.4 cm?1of the sputtered Mg3Bi2film is ascribed to the Mg3Bi2phase.No Raman peaks of Bi2O3are detected in the sputtered Mg3Bi2film,while they are obviously visible in the nanoporous Bi [38].A broad Raman peak at 57.0 ?117.0 cm?1appears in the nanoporous Bi film.The broad band could be decomposed into three peaks(Fig.4b),which could be attributed to the presence of metallic Bi(68.5 and 90.7 cm?1)and amorphous Mg-Bi phase(78.9 cm?1).In comparison,the Raman signal from the amorphous Mg-Bi phase is absent after removing the residual Mg in the tartaric acid(Fig.S16).Fig.4c illustrates the Bi 4f XPS spectra of the sputtered Bi,Mg3Bi2and nanoporous Bi.Two weak peaks(162.1 and 156.8 eV)in the sputtered Bi film can be well indexed to the 4f5/2and 4f7/2peaks of Bi0,respectively [37].However,the binding energies(160.7 and 155.4 eV)of two peaks in the Mg3Bi2film are lower than those of the Bi0peaks,which signifies the formation of Mg3Bi2,as confirmed by the Mg 2p spectrum(Fig.S17).After demagnetization,the XPS peaks of Bi0shift towards higher binding energies(161.6 and 156.3 eV),and the Mg 2p spectrum confirms the significant Mg residual in the nanoporous Bi(Fig.S17).In addition,surface oxidation occurs for the sputtered Bi and nanoporous Bi,as evidenced by the XPS results.

Fig.4.(a)Raman spectra of the sputtered Bi,Mg3Bi2 and nanoporous Bi obtained by galvanostatic charging-discharging treatments of the sputtered Mg3Bi2 film for 3 cycles at 100 mA g?1.(b)The multi-peak-fitting result for the broad Raman peak of the nanoporous Bi.(c)Bi 4f XPS spectra of the sputtered Bi,Mg3Bi2 and nanoporous Bi.SEM images of(d and e)grain surface and(f and g)grain interior for the electrochemically dealloyed Mg3Bi2 film at different current densities.(h)Average ligament size of the obtained nanoporous Bi and(i)the linear relationship between the logarithm of surface diffusivity of Bi and the logarithm of current density.

To control the surface diffusivity is crucial to regulate the length scale of nanoporous metals.Herein,the current density-dependent nanoporosity evolution in Mg3Bi2was investigated by galvanostatic charging-discharging cycling(Fig.S18a–c).Both the surface and interior of columnar grains exhibit a bicontinuous interpenetrating ligament-channel structure in the electrochemically dealloyed Mg3Bi2film at different current densities(Figs.3a,d and 4d–g).The XRD results verify that the dealloyed Mg3Bi2film is comprised of the single Bi phase(Fig.S18d).Fig.4h displays the characteristic length scale of ligaments/channels in the nanoporous Bi,which shows a dependence on the applied current density.The surface diffusivity(Ds)of the more noble adatoms along the alloy/electrolyte interface during dealloying at a temperature(T)can be calculated by the following equation [39]

whered(t)is the characteristic length scale of nanoporous metals,kis Boltzmann constant,γis the surface energy of metals[40],t is the dealloying time,and a is the lattice parameter.The surface diffusivity of Bi was evaluated and sharply increases with increasing current density(Fig.S19).Furthermore,there exists a good linear relationship between the logarithm of Dsand the logarithm of current density(Fig.4i).For the electrochemical dealloying of the sputtered Mg3Bi2film in the 0.4 M APC electrolyte,the following equation can be determined.

Obviously,we can modulate the surface diffusivity of Bi during electrochemical dealloying and further regulate the ligament size of nanoporous Bi by changing the applied current density.Additionally,significant Mg is retained in the nanoporous Bi,which is independent on the applied current density(Fig.S20)and could be rationalized by electrochemical analysis.Despite different discharge capacities,the charge capacities are almost identical at different current densities(Fig.S18a–c),implying the similar extracted Mg contents during the charging(dealloying)process of Mg3Bi2(Fig.S21).

We also investigated the effect of scan rate on the dealloying-induced morphological evolution of the sputtered Mg3Bi2electrodes(Fig.5).In the CV curves(Fig.S22),the anodic peaks are associated with the demagnesiation(dealloying)of Mg3Bi2and the formation of nanoporous Bi.The morphological feature of nanoporous Bi obtained at 1 mV s?1shows a nanoporous core-ravined shell structure(Fig.5a and d).Unexpectedly,a critical scan rate of 0.1 mV s?1was captured for the breakdown of the ravined shell(Fig.5b and e).A typical nanoporous structure is formed at the lower scan rate of 0.01 mV s?1(Fig.5c and f).Regardless of the scan rate,only the Bi phase can be identified in the dealloyed samples(Fig.5g).However,the EDS results show that the residual Mg content ranges from 30.3 to 41.6 at.% Mg in the obtained nanoporous Bi.The average ligament size of nanoporous Bi is around 30 nm and slightly changes with the scan rate(Fig.5h).Notably,the calculated surface diffusivity of Bi obviously increases with increasing scan rate(Fig.S23).Furthermore,a good linear correlation exists between the logarithm of surface diffusivity and scan rate(Fig.5i).The following equation can be determined for the electrochemical dealloying of Mg3Bi2in the APC electrolyte.

We could regulate the microstructure and length scale of nanoporous Bi through changing the potential scan rate.Fig.5j schematically reveals the transformation process from the core-shell structure to the typical nanoporous architecture and the breakdown of the ravined shell.

Impressively,the present charging-induced dealloying strategy is also applicable to the other sputtered MgxMy(M=Sn,In,Pb,Cu,Zn,Al,Ge and Si)films(Fig.6).Fig.S24 shows the galvanostatic charging-discharging profiles of the MgxMyfilms at 100 mA g?1.As for the Mg2Sn film,the electrochemical characterizations(Figs.S24a and S25)andin-situXRD results(Fig.S26)reveal a simple biphasic transition from Mg2Sn to Sn,different from the previous speculation that the demagnesiation process of Mg2Sn might involve an amorphization transition from MgxSn to Sn [14,32,36,41].A similar scenario occurs for the Mg2Pb and MgIn films,however,the other MgxMy(M=Cu,Zn,Al,Ge and Si)electrodes cannot be cycled due to the inactive nature of the M elements for reversible Mg storage(Fig.S24).After electrochemical dealloying,nanoporous structure is generally generated in Sn,Pb,In,Cu,and even in reactive metals like Zn,Al(Fig.6a–f).In the electrochemically dealloyed Mg2Si and Mg2Ge films,however,the nanoporous structure is not obvious(Fig.S27).The XRD results confirm the formation of single-phase Sn,In and Pb in the electrochemically dealloyed Mg2Sn,MgIn and Mg2Pb,respectively(Fig.6g).In comparison,no characteristic diffraction peaks can be observed in the dealloyed MgxMy(M=Al,Zn,Si and Ge)alloys(Fig.S28),implying the low-crystallinity or amorphous nature of the produced nanostructures.

The average ligament sizes of the obtained nanoporous metals were determined and are presented in Fig.6h and Table S1.Compared to porous metals fabricated by chemical dealloying in the aqueous solution(Figs.S29 and S30),the ligaments are much smaller for nanoporous Bi,Sn,Pb and In obtained by electrochemical dealloying in the APC electrolyte(Fig.6h).Such a pronounced difference is essentially related to the surface diffusion of the more noble element.Based on the ligament sizes,the surface diffusivities of the involved species were calculated and are listed in Table S1.Clearly,for any of the four elements(Bi,Sn,In,Pb),the surface diffusivity for chemical dealloying in the aqueous solution is 4–5 orders of magnitude faster than that for electrochemical dealloying in the APC electrolyte.Additionally,the solid electrolyte interface(SEI)film can alleviate the coarsening rate of ligaments in the porous structure [42].The feature size of nanoporous Bi,Cu,Zn and Al is obviously smaller than that of Sn,In and Pb,which is attributed to their relatively low surface diffusivities.More importantly,the surface diffusivities for electrochemical dealloying could be facilely adjusted by regulating the current density or potential scan rate,which further demonstrates the unique advantages of such an electrochemical demagnetization strategy in nonaqueous electrolytes.

Fig.5.SEM images of(a–c)grain surface and(d–f)grain interior for the electrochemically dealloyed Mg3Bi2 film at different scan rates.(g)XRD profiles of nanoporous Bi obtained at different scan rates.(h)Average ligament size of the obtained nanoporous Bi and(i)the logarithm of surface diffusivity of Bi adatoms versus the logarithm of scan rate.(j)Schematic illustration showing the breaking process for the surface layer with the decrease of scan rate.

Fig.6.(a-f)SEM images of nanoporous metals fabricated by electrochemical dealloying of the sputtered MgxMy(M=Sn,In,Pb,Cu,Zn and Al)films in the 0.4 M APC electrolyte at 100 mA g?1.(g)XRD patterns of the obtained nanoporous Sn,In and Pb.(h)The average ligament size(left coordinate)and lg Ds(right coordinate)for electrochemical dealloying in the APC electrolyte and chemical dealloying in the 0.13 M tartaric acid aqueous solution.

The nanoporous Bi electrodes fabricated by galvanostatic charging-discharging treatments of the sputtered Mg3Bi2film for 3 cycles at 100 mA g?1were selected to further evaluate their Mg storage performance in the 0.4 M APC electrolyte(Fig.7).The operando XRD results(Fig.7a-b)and electrochmeical characterizations(Fig.S31)verify a simple twophase reaction mechanism(2Bi+3Mg2++6e??Mg3Bi2)between Bi and Mg3Bi2.Fig.7c shows the rate performance of nanoporous Bi electrodes at different current densities from 200 to 3000 mA g?1,delivering a discharge capacity of ? 242 mA g?1even at 3000 mA g?1.Compared with advanced alloy anodes in the literature(Fig.S32)[6,7,9–11,43,44],the nanoporous Bi electrodes exhibit excellent rate performance.In addition,the nanoporous Bi electrodes could skip the activation process and deliver a better electrochemical performance than the sputtered Bi film(Fig.S33).Fig.7d shows the cycling stability of nanoporous Bi electrodes at 500 mA g?1.The specific capacity of nanoporous Bi electrodes continuously decays with increasing cycles,which is associated with the alternating strong volume changes [43].Noticeably,the nanoporous Bi electrodes are free of any conductive agents and binders,which partially accounts for the gradual capacity decay.Most importantly,we assembled the cathode-limited(Fig.S34)full cells consisting of the sputtered Mg3Bi2anode,0.5 M Mg(TFSI)2/diglyme electrolyte and Mo6S8cathode.The full cells(Fig.7e–f)can deliver a stable specific capacity of above 50 mA g?1over 15 cycles,which confirms the good compatibility of Mg3Bi2anode with conventional electrolytes.Besides,the Mg2Sn electrodes show good compatibility with conventional electrolytes like Mg(TFSI)2/diglyme(Fig.S35),which suggests that the passivation issue of Mg metal can be well bypassed by using alternative Mg alloy anodes.

Fig.7.(a)Operando XRD patterns and(b)corresponding contour plot of nanoporous Bi electrodes during the alloying-dealloying processes.The dischargingcharging profile is also shown for reference.(c)Rate performance and(d)cycling stability of nanoporous Bi electrodes.The nanoporous Bi electrodes were fabricated by galvanostatic charging-discharging treatments of the sputtered Mg3Bi2 film for 3 cycles at 100 mA g?1.(e)Discharging-charging profiles and(f)cycling stability of Mg3Bi2//Mo6S8 full cell with the 0.5 M Mg(TFSI)2/diglyme electrolyte for different cycles at 20 mA g?1.

4.Conclusions

In summary,we developed a general charging-induced dealloying strategy to fabricate nanoporous metals in a nonaqueous electrolyte.A broad range of nanoporous less noble metals including Bi,Sn,In,Pb,Cu,Zn and Al can be fabricated through electrochemical dealloying of the sputtered Mg-based alloy films in MIBs.Using Mg3Bi2as an example,the nanoporosity evolution during dealloying was revealed,including the formation/breakdown of ravined shell,and the formation of bicontinuous nanoporous structure upon increasing the charging cycle or changing the potential scan rate.In particular,the surface diffusivity of Bi for electrochemical dealloying could be facilely regulated by tuning the current density or potential scan rate.We also unveiled the linear positive correlation of the logarithm of surface diffusivity of Bi with the logarithm of current density or scan rate.Noticeably,the surface diffusivity for electrochemical dealloying in the APC electrolyte is 4–5 orders of magnitude slower than that for chemical dealloying in the aqueous solution.More importantly,the nanoporous Bi electrodes deliver superior electrochemical performance for Mg storage and alloytype anodes show good compatibility with conventional electrolytes.Our results broaden the spectrum of strategies for fabricating nanoporous materials with tunable morphology and size through dealloying,while providing useful knowledge for the development of advanced alloy-type anodes for MIBs.

Declaration of Competing Interest

There are no conflicts to declare.

Acknowledgments

The authors gratefully acknowledge financial support by National Natural Science Foundation of China(51871133),and the support of Taishan Scholar Foundation of Shandong Province,the program of Jinan Science and Technology Bureau(2019GXRC001),and Department of Science and Technology of Shandong Province,China.We thank Dr.Pan Liu(Shanghai Jiaotong University)for TEM characterization.

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2021.04.003.