Indian Journal of Chemistry Vo l. 44A, May 2005, pp. 875-890

Advances in magnesium electrochemistry-A challenge for nanomaterials

Doron Aurbach*, Yossi Gofer, Orit Chusid, Elena Lev i, Mikhail D Levi, Yulia Vestfrid, Haim Gizbar & Eli Lancry

Department of Chemi stry, 8 ar-llan University, Ramat-Gan 52900, Israel

Email : aurbach@mail.biu.ac.i l

Received 23 Decel11ber 2004

[n thi s review, very recent studies re lated to magnesium electrochemi stry (in connecti on with R&D of Mg batteri es) have been reported. These include the study of new electro lyte solutions, based on compl exes with the fo rmal sto ichi ometry. Mg(AICI4 _nRn)z in ethers, their unique structures and analysis by electrochemical and spectroscopi c methods. the study of Mg depositi on processes by microelectrodes and mi croscopy, and the study of Mg insertion in to hosts based 0,1 the so-called Chevrel phase structure (MgO_2M06XS, X=S, Se). The ionic Illobility of Mg2+ ions and their ease of diffu sion within these structures are di scussed. It is demonstrated that the use of a nanostructured acti ve Illass may be hi ghl y important for reducing the diffusion length considerably. and hence, for increasing the kinetics of transport of the bi valent cations. We deal herein with some key fac tors that Illay affect the possibility of sillooth and reversibl e Mg insert ion in to inorgani c host materials, and the possible advantages in the use of nanopart icles fo r these systems.

IPC Code: In t. CI.7 8828; C2589/ 10

Introduction One of the most impress ive successes of modern

electrochemistry has been the deve lopment and commerci ali zati on of rechargeable Li-ion batteries in recent years. The field of Li and Li-ion batteri es still attracts thousands of research groups th roughout the world in attempts to propel thi s technology to higher energy densities, rates , stab ility and lower productions costs. The fac t that Li batteries are co mposed of highly reactive materi als, and that thei r stability depends on pass i vation phenomena, pushes the relevant res~arch to the fo refront of materials and surface science.

It should be noted that the current power sources avail able fo r mobile electronic equipment, electric vehicles, load levelling, back-up systems, and storage for sustainable energy sources, etc. , are far behind the needs of modern society . Hence, there is a very strong incenti ve to develop new power sources . The success in R&D of Li batteries focuses attention on R&D of novel battery systems, based on other active metals.

According to the peri odi c table, the next natural candidate as an anode materi al for high energy density batteries after lithium is magnesium metal. The bas ic properties of lithium are a density of 0.53 g/cm3, a capacity of 3.88 Ah/gr, and a potential (S HE) of - 3.05 V. The parameters of magnesium are a density of 1.74 g/('m3

, a capacity of 2.23 Ah/gr, and a

potential (SHE) of -2.36 V. Hence, magnes i um is highly attractive as an anode materi al in rechargeable batteries and also due to its being an active material with low red-ox potenti al, and a very light metal with a hi gh specific capacity . It is abundant and cheap, and many of its compounds can be regarded as being green and safe and can be handl ed under ambient air, in contrast to Li metal , which can only be handl ed in dry rooms, or preferably in glove boxes under inert atmosphere.

Indeed, during the las t two decades there were attempts to study many aspects of magnesium electrochemi stry, and even to develop rechargeable magnesium battery sys tems. Attempts have been made to search and study electrolyte solutions in which magnesium can be dissolved and deposited reversibl/ -s. As an active metal , magnesium is naturally covered by surface films that comprise Mg oxide, M g hydroxide, Mg carbonate, etc6

. Mg reacts readily with protic solvents (to form hydrogen and hydroxides, alkoxides, or carboxylates, etc .. depending on the solvent), and may al so react w ith most of the polar aprotic solvents (e.g. , esters , alky l carbonates, amides, acetonitrile)6.

The products are always insoluble Mg salts. In contrast to the case of Li metal (pass i vated by surface films in most polar aprotic solutions) in which the th in films of Li salts that cover the active metal can

876 INDIAN J CHEM, SEC A. MA Y 2005

conduct Li ions (the SEI model \ Mg salts , even as thin films, cannot conduct Mg ions under an electrical field. Mg dissolution in most polar aprotic solutions occurs on ly via a break and repair mechanism of the surface films 6

, and Mg deposition is completely blocked by surface films comprising M g salts. A well­known exception is that of Grignard salt so lutio ns in ethers (RMgX , R-alky l, aryl and X-halide), in which Mg deposition/di ssol ution may occur reversi bly. However, the e lectrochemical windows of such so lutions are too narrow for any application in battery

t 5.6 sys ems .

Over the years, some groups succeeded in developing e lectrolyte sys te ms of wider electrochemical windows, based on Mg boraness or Mg si licon9 complexes in ethers. For instance, so lutions of :- lg(BPh2Bu2h in THF could provide an e lectrochemical window close to 1.8 V, while Mg electrodes may behave reversibly in them.s There were attempts to study and deve lop Mg inse rti on materi als in a manner similar to that of the intensive and sLlccessful R&D efforts related to positive electrodes for Li batteries H). Reference 8 also reports on a preliminary study of a rechargeable Mg battery system based on an Mg metal anode, and CO, 0 4 host cathode, and an electrolyte solution based o n THF and Mg(BR4h complexes (R-alky l, ary l).

We were the first to develop rechargeable magnesium battery systems based on Mg or Mg alloy

anodes, MgxM06SS (0<x<2) Chevrel phase cathodes and e lectrol yte soluti ons comprising TH F or glyme ethereal so l vents and complexes of the Mg(AICI4_1l Rllh type (R-alkyl , aryl groupS) II .12. It was

also possible to compose solid state, rech argeable magnesium batteries comprising the same e lectrodes and gel electrolytes, based on poly(vinylidene­difluoride) (PVdF) or pol y(ethylene ox ide) (PEO), Mg(A ICl 2R2h co mplexes and tetraglyme CHr (OC2H4)4-0CH, as a plasticizer l

,. The theo retical energy density of these batteries is 135 Wh/kg, which is lower than that of other commonly used rechargeable batteries (lead acid, Ni -Cd, Ni-meta l hydride, and lithium batteries)I-l . However, si nce Mg depos ition/dissolu tion is hi ghly reversible in the above e lectro lyte sys tems (can be used as very thin fo il anodes), the MgxM06SS cathode materia l possesses hi gh electronic conductivity and o nly a very thin layer of e lectrolyte solution/gel is needed. the amou nt/percentage of parasiti c material s needed in the Mg batteries (e.g., anode current co ll ector,

conductive additives) is lower than that in o the r batteries. Hence, it is expected to reach a re lative ly hi gh utility of the energy density \....- -+0% of the theoretical one) in prac tical , well-eng ineered batteries.

The most important property of the above described rechargeable magnesium battery system is the lack of chemical reacti vity between the e lectrodes and the e lectrolyte solutions. In thi s respect, th e Mg batte ry systems presented herci n are unique si nce in all other rechargeable battery systems there are pro nounced reac ti ons between the e lectrodes and the electrolyte so luti on. Thi s is true for the aqueous batteries (L-A, Ni-Cd, Ni-MH )1 4 and critical for Li batteries, whose stabi I ity depends on pass i vation phenomena 15 . Due to the lack of reactivi ty between the e lectrodes and the so lutions, and the absence of side react ions, it was possible to cycle the above Mg batteries thousands of times in coin-type cells. In addition. there are indica­ti ons of excellent hi gh temperature pe rformalice, ve ry low self discharge, and ex tended calendar and cycle life.

The above features may give these Mg batteries some unique advantages compared to other battery systems g iving them practica l impo rtance. During the course of e ffo rts to develop these systems and make them practical. it was di scovered that the electrolyte soluti o ns, Mg deposition/d issol utio n processes, and Mg intercal ation into M06SR Chevrel phase electrodes are a ll very compli cated, and require intensive study, lI si ng state-of-the-art tool s in electrochemi stry. imag ing, spectroscopy and diffraction.

It was again proven that R&D of rechargeab le batteries may prov ide a very good pla tform and impetus for hi ghly interesting, basic surface. electrochemical and materials science. Interca lati on of the bivalent Mg2+ ions into any host mate ri al , including the hosts described here in, suffer from kinetic limitati ons, due to the relatively high barriers to ionic diffusion in the sol id state. The usc of nanoparticles , and hence, red uc ing the diffusion length by orders of mag nitude compared to mic rosizc materials. may accorcli ngly improve the kinetics of Mg ion insertion. The use of nanoparticles fo r ion insertion electrodes is very thoroughl y deal t with in connection with Li- ion batteri es ' 6- ' 8. However, since most of the LixMOy (M -transition metal such as Mn, Co, Ni and the ir combinations) are reac ti ve with the co mmonl y used electrolyte so lutio ns for Li-ion batteri es, the hi gh surface area of th e nanopartic les may mean pronounced parasitic side reactions and

AURBACH el (/1.: ADVANCES IN MAGNESIUM ELECTROCHEMISTRY 877

problems in the interparticles ' electrical contact. In the Mg-based systems described herein , there are no side reactions between the electrodes and the electrolyte solutions. In addition, the Chevrel phases have high electronic conductivity . Thereby, the use of nanoparticles as the acti ve mass of the Mg insertion cathodes may improve their kinetics considerably , wi th no side reactions.

The aim of thi s paper is to rev iew o ur recent di scoveries and new unders tandings, related to nonaqueous mag nesium electrochemi stry , and the possibility of usi ng nanomaterials in Mg batteries . The importance of this li es also in the overview of the multi-process systems that inc lude Mg deposi­tio n/d isso lu tion, and multi equilibrium reactions In soluti ons, and the co mplicated intercalation of bivalent cat ions in inorganic hos t mate ri als.

Materials and Methods Al l chemi cal preparations and e lectrochemical

measurements were carri ed out under pure argon atmosphere in M Braun fnc. g love boxes (less than I ppm of water and oxygen). The typ ical preparation of a complex salt so luti on consis ted o f the c1ropwise addi tion of a carefu ll y measured amou nt of filtered 1M dibuty lmagnesium (Bu2Mg) in hexane (A ldri ch , 97%) to a vigorous ly stirred , chosen amount o f 1M ethyla luminumdichloride (EtAIC I2) solution In heptane (Aldrich , 97%) . fn all the cases, a mildly exothermic reaction look place, y ie lding immediately a powdery white preci pitate. The suspension thus fo rmed was stirred at room temperatu re for an additional 24 hours, afte r which it was vacuum dried . Extra dry ~~trahydrofuran (TH F, Merck, Selectipure, < 10 ppm water, as determined from a Karl-Fi scher ti trator) was added to the dry, off-white solid , usually to a nominal concentrat ion of 0.25 M. T he so lut ions were c lear and colorl ess.

A soluti on o f di ethyl magnesi um (Et2Mg) in hexane was prepared by the addition of a sl ight excess of dry 1,4-d ioxane (Aldrich , 99.9%) to a ethylmagnes ium­chloride (EtMgC I) so lu tion in THF (Aldri ch, 97%), which caused the precipitation of MgCh.dioxane. The soluti on formed was decanted and filtered through a fine g lass frit. An MgCl2'2THF solution in T HF was prepared by reacti n8 an excess of pure magnesium fo il (Merck, 99 .95 %) with a dry solution o f HgCIz in THF.

Triethylaluminum (Et, AI , Aldrich , 97%), diethyl ­aluminumchloride (Et2AICI , Aldrich, 97%). AICI,

(Aldrich, 99.99%), and e thylaluminumdichloride (EtAICh, Aldrich, 97%), were used as received. Acetonitrile (A ldrich 99 .93%+) and propylene carbonate (Tomiyama, hi gh purity) were used as received after several days of drying over molecula r sieves (4;\) . Mg(CI04)2 (Aldrich , anhydrous) was

dried at 170°C under vacuum for fo ur days .

A M06SS Chevrel phase was obtained in two steps 19,20. In the first step , C U2M06Ss was synthes ized by a reaction of the element mixture in evacuated, sea led quartz tubes [for detail s see refs 19 and 20]. In the second step, CU2M06Sg was leached in 6 M HCI IH20 (l: I) in the presence of air for mild ox idation, so that the M06Sg Chevrel phase compound , almost free of C u, was obtained 19.20. M06Seg could be synthesized directly fro m Mo and Se at e levated temperatures in quartz ampoules. The purity of these material s could be well monitored by XRD.

All the e lectrochemical measurements were performed using EG&G, Inc. 273 potentiostats controlled by Corrware Software (Scribner Inc.). or by an Ecochemi e Mode l 20 Autolab System computerized potentiostat-gal vanostat, controlled by Eco Chemie B.V. Software, G PES Version 4.8 (Utrecht, The Netherlands). T he e lectrochem ical cel ls for cyclic voltammetry measurements consisted of mag nesium metal counter and reference electrodes, and either gold o r platinum working electrodes. The scan rate used was 20 m V Is, and the anodic Ii mi t of the electrochemical wi ndow was determi ned at the defl ecti on point, where the I versus E curve showed a rapid increase in the positive currents, usually, at ca.

5.0 mA/cm2 above the backgrou nd currents . Specific so lution conductivities were measured w ith a di gital

conductometer (1 KHz at 25 ± O.5°C).

Magnes ium deposition and dissolution were studied usi ng micro di sc electrodes that were prepared from a Pt wire (0 50 p.m) embedded in the centre of the polypropylene rods . Before each measurement, cutting its epcl renewed the e lectrode. Electrochemical measurements with these e lectrodes were performed in two-electrode cells consisting of a magnesiu m metal counterlreference electrode, and a Pt microdisc working electrode. In control experiments, a third reference Mg electrode was introduced, which yielded identical experimental va lues compared to that of the two-electrode ce ll s. Th is resul t is consistent w ith the expected practical e liminati on of ohmic potential drops in so lutions in contact wi th microsize work ing electrodes . In poten ti al step measurements . each

878 INDIAN J C HEM , SEC A, MA Y 2005

cathod ic step, resu lting in a short-time Mg-depos ition , was fo ll owed by its complete anodi c stripping. Then, the second cathodic step of a hi gher overvoltage was applied . Such steps were repeated until a whole range of overvoltages was covered.

Mo()Ss and M06Seg electrodes were prepared by mixing the ca thode' s active mass (from 3 to 15 mg) with 10% carbon black and 10% PVdF. The mi xtures were applied onto I x I cm" stainless steel foil s. Strips of Mg foil served as counter and reference electrodes in 0.25 M Mg(A ICI2BuEth in THF (DCC/THF). Separate ex periments were done in three component ce ll s fo r Mg inserti on into the Chevrel phases from Mg(C IO-lh di ssolved in acetonitrile (AN) and propylene carbonate (PC). An Hg counter­electrode and Ag/ Ag + reference elect rodes were used in these cases. Li-intercalation into M06Sg was studied in 0.25 M LiCI04 di sso lved in acetonitrile or propylene carbonate (PC). In the latter case, Li strips and wires dipped in the same so lution served as counter and reference electrodes, respecti vely. SSCV (0.1-0.025 m V /sec) and gal vanostatic measurements (0. 1-0.0 I mA/cm") were performed under different temperatures, controlled with an accuracy of ± 0.5°C. We also used submicron ic Chev rel phases, produced by milling under argon. It should be noted that these materials , especially CuM or,S x, may be sensiti ve to mechanical shocks and may undergo unique mechanochemical reacti ons 19 .

NMR spectra were measured with a Bruker DMX-600 spectrometer at 600. 1 (I H), 150.9 ( i3C), 156.3

20. Mg +2 + 2e-~ MgD

1.5

1.0

05

e AI) and 36.ne5M g) MHz. Solutions in THF (0.25-0.5 M) with respect to aluminum were run in 8 mm NM R tubes without a deuterium lock. "c and IH chemical shifts were referenced to the down field THF signal (8 68.17 and 3.68 ppm, respec ti vely)27 . Al chemical sh ifts are reported relative to an external reference: a so lution of AICI} in 0 20 with a drop of concentrated HCI [AI (D20 )63+]. All the experiments were performed at room temperature (25 ± 2°C). XRD measurements were made us ing the advanced 0 8 Diffractometer from Bruker, Inc.

Results and Discussion General aspects related to solutions and M g electrod es

Figure I shows an overview of the processes that take place in the rechargeab le Mg batteries. These include reversi ble Mg deposition/d isso lution (at efficiencies close to 100%) and reversible Mg interca lation into Mg"M06Ss (0<x<2), the crystal structure of wh ich is presented in the in sert. The speci fi c electro lyte solution related to Fig. I was 0.25 M Mg(A ICI 2BuEth in THF. As to be discussed later in the paper, the electrolyte formula refl ec ts only a formal stoich iometry, while the real structure of these solutions is very comp li cated . Fi g. I demonstrates an electrochemi cal window of 2.2 V, into which all the electrochemical activity of the Mg,Moc,Ss electrode fits very well , with suffici ently broad margins. Hence, the operati on of a fu ll Mg- MgxM06Sg battery system does not in volve any side reactions of these electrolyte solutions.

,'; deinterealation

I I I I

1.0.

0.5

~ 00

I l/-';- ~

0.0 E E (.) (.) "-« -05 5

s. :;c E

Mo . --1.0 Mg.

site A -0.5

-1.5 Mg = "'interealati on

-20.

site B

-to. -0.5 0..0. 0.5 1.0 1.5 20 25 3.0

E [Volt) vs. Mg/Mg++ -Fig. I- An overview o f the e lec trochem ical processes in the Mg batte ry system. in the form of cycli c vo lt aml11ograms. [So lid line : vo ita lllillogralll of the e lectrolyte so luti on (Mg(A IC I2BuEtb 0.25 MffHF) with a Pt e lectrode, 20 IllV/sec. Note the revers ible Mg dcpositio n and di ssoluti on. Dashed line: vo italll illog ram of Mg intercalat ion-deinte rcalation into the M06SS C hevre l phase. Same so luti on . S ~I V/sec. The insert shows the c rys tal struc ture o f the MgxMo(,Ssj.

AURBACH et ai.: ADVANCES IN MAGNESIUM ELECTROC H EMISTRY 879

Figure 2 demonstrates the effect of the sy nthesis of the electrolytes on the electrochemical window of the solutions. The complex electrolytes are in fact products of the reaction between the R2Mg Lewis base (L. base) and an AlRnC13_n Lewis acid (L. acid). As seen in Fig. 2, the anodic stabi li ty of these solutions depends mostly on the ratio between the L. base and the L. acid in the complex electro lyte. The higher the amount of the ac id in the reactant mixtures, the hi gher is the anodic stability. We should note that the anodic stabi lity depends also on the R groups where -CH3>-C2Hs>-C4H9. However, the effect of the acid/base ratio on the electrochemi cal window is the

.most pronounced. As discussed later, these differences in the anodic stability of the solutions is due to different structures formed as a functio n of the L. acid/L. base ratios. It should also be noted that as the acid-base ratio is higher, or contains more chloride (e.g., AlCl2R2 versus AlCI3R), the overvoltage for Mg deposition (see in Figs 1 and 2), is higher21. Highly interesting is the impact of the electrolyte composition on the morphology of Mg deposition, as IS demonstrated in Fig. 3.

The Lewis ac id/Lewis base ratio determines the type of Mg crystallites that are deposited. At low and high acid/base ratios (e.g., 0.5, 3), Mg deposition is a submicronic-nanonic phenomenon at a wide di stribution of crystal size, while at moderate A/B ratios (l , 2), Mg is depos ited as micronic-size crystallites. This morphologic effect of the A/B ratios

is discussed later, in connection with the impact of the A/B ratio on the structure of the electrolyte solutions. Another important factor that determines the morphology of Mg deposition is the CUITent density. The higher current density, the small er is the crystallite size. Thus, whil e at moderate CUITent densities (1-2 mA/cm2), Mg deposition is a micrometric phenomenon, at higher current densities (e.g., 4 mA/cm2, Fig. 3), Mg is deposited in nanometric-sizc crystallites.

In any event, Mg deposition on bare metall ic surfaces (passivation , oxide-film free) is not at all dendriti c and is highly reversible. El ectrochemical dissol ution of magnesium deposits on bare metallic surfaces leaves no residual magnesiu m. This behaviour is due to the fact that there are no chemical reactions between bare magnesium and these electrolyte solutions. Hence. Mg deposition (which might be affected by some adsorption processes, see later), takes place (only! ) under passivation-free conditions. Since the properties of Mg deposition processes and the electrochemical properties of the solutions depend so strongly on the composition of the electrolyte, we review below our recent ly concluded understanding of the structure of the THF/Mg(AICI4.nRnh solutions.

On the structure of the electrolyte solutions Despite the understanding that we built up over the

recent years on the solution's electrochemical

Electrolyte Synthesis

~ E "

Ether L bilse L acid Evaporation

R2Mg(sol) + XAIRnCI3-n ---+ ---, ?

2.00

1.00

R :: Butyl, Ethyl, Methyl

./

X:: 1 AJR3THF + MgCI2 I

Cl .§. 0.00 , -'

-1.00

-2.00 +---,.---,---r--- --,---..,.----,---....,

-0.5 0.0 0.5 1.0 1.5 EIV)

2.0 2.5 3.0

Fig. 2-Cyclic voltammograms tha t demonstrate thc el ec trochcmical w indows o f TH F so lut ions con taining the complex e lectro lyte BuzMg+xA IC lzEt at different x va lues (Lewi s acid-Lewis base ratios). 20 mY/sec. T he c lec trolyte concenu·ation was 0.25 M in a ll cases.

880 IND IAN J C HEM. SEC A. MA Y ~005

Lewis base Lewis acid

Cu substrate 2.8 Ccm 2

x = 2, 1 mAlcm2 x =2, 2 mAlcm2

x = 0.5, 1 mAlcm2 x = 1, 1 mAlcm2 x=3, 1 mAlcm2

Fig. 3--SEM images of Mg deposits on Cu substra tes. Deposition charge of 2.8 ClcJ11~ from TI-ll: so luti on~ conrai ning the eompl.;, elec troly te Bu ,Mg+xA ICI ,Et (0.25 M) at different x ratios. as indicated.

behaviour2t, we had littl e information regard ing the chemical identity of the sol uti on species. As already described, the electrolyte soluti ons discussed herein are prepared as acid-base reactions in which the organomagnesium reagent, (the Lewis base), and the organo-halo aluminum (the Lewis acid ) compounds are reacted to form THF solubl e complex salts. The most val uable salt, yielding sol utions with good electrochemical properties, as described above, was prepared by reacting 1 mo le of dibutyl magnesi um with 2 moles of ethyl al uminu m dichloride, to yie ld the 1:2 product named "DeC". The first indicati on of a chemical reaction , rather than just plain mixing, is seen from the very vigorou s reaction that is observed. with the precipitation of a white powder and the di ss ipation of heat upon the addition of the acid to the base (as solut ions in aliphatic hydrocarbons). Moreover, in contras t to the THF solutions of the starting material s, the solutions of the products display ionic conducti vity in a milli siemens/cm range. This not only substantiates the claim for chemical reac tion, but also indicates that the products are ionic species.

When it comes to magnesium, the literature conta ins very limited information on analytical

studies of such compou nds, which is in contrast to a lkal i metal salts with organo- and organo-halo alumin um22.2J or various titaniu m derivatives used extensively as catalysts in the polymer industry. A lthough there are several accounts in the literature of similar compounds, a ll these cases cover the identification of solid materia ls. From our preliminary efforts to identify the soluti on species, we understood that any analysis that we undertake must be carri ed out on the solution phase21

, under conditions similar to those present during the electrochemical studi es. Since the solutions consi st of very delicate complexes, and poss ibly partiCipate in several equi I ibri a, any deviation from "normal" condi tions might change the identity and the concent rations of the solution spec ies. For example, a small a change as the add iti on of 2- methyl THF to the THF solution of Dee in order to enhance the solvent stability, ended up with phase separati on.

The first question that we wanted to reso lve was whether the reaction between the magnesium and the aluminum reagents is stoichiometri c, lead ing to species with definite structures. We soon found th at it would be extremely hard to answer thi s question on a s tatic picture based situatio n, i.e. only for the case of

AU RB ACH el al .: ADV ANCES IN MAGNES IUM ELECTROC HEMISTRY 88 1

the practically interes ting DCC so lu tions in THF. Thus, can'ied out a seri es of experiments in which the soluti on composition was varied in order to obtain a broader vi ew of the sys tem, and possibly some informati on on the reaction schemes. Initially the precursors (Bu2Mg and EtAICh) were reacted at various ratios, between 0: 1 to 1 :5, (Mg:A I), and the ionic conductiv ity of the soluti ons in THF (keepi ng the "salt" concentration constant) was measured. Interestingly, a curve containing two definite maxima and one minimum was obtained (Fig. 4a). The results confirmed our assumption that the precursors react to give products with a definite sto ichiometry, and most probabl y are present in the solution as the so le species in the reaction ratios I: 1, 1:2 and 2: 1. T he two maxima, at 1:2 and 2: 1 reactant ratios, form compounds with a defin ite ionic character, whil e the 1: I ratio, that exhibited the lowest conductivity, probably indi cates the fo rmation of non-ionic compounds. Except for the above general indications, no specific chemical infor mati on could be deri ved from these data.

One of the strongest analytical tools for solu tion che mi stry analys is is NMR. The major constituents of the experimental solution, C, H, Mg and AI , have isotopes that are NMR active. Both 25 Mg and 27 AI , the NMR active isotopes of Mg and AI , contain quadrapolic nucle i. Thus, its NM R features are expected to be broad and weak. In fact, C I, another important constituent of the studied compounds also has an NMR active isotope. However, its spectra are very ill defined and it is hard to extract knowl edge from them.

At first, we ran multinuclear NMR spectra on the same solutions that were prepared for the ioni c conductivity measurements, namely, the series of solutions with vari ous ratios between the reactants24 . The spectra obtai ned indi cated that although informative data can be obtained, the 27 Al spectra were so rich in features th at a simpler, model system had to be used for fine and accurate analys is. The richness of the peaks is caused by the existence of both ethyl and butyl ligands and because the commercial BU2Mg, the precursor used, contained a mixture of It-butyl and sec.-buty l moieties. We fo und that when all the organi c ligands in the precursors are ethyl groups, the products behave in an electro­chemically si mil ar way. Thus, we synthes ized a similar series of salts, using Et2Mg and EtA ICI2 as the precursors, at various reactant proportions. T he ionic

conductivity meas urements exhibited a very simi lar curve, as observed in Fig. 4b, with two maxima at ca . 1:2 and 2: 1 ratios; sharp minimum at the 1: I rati o, and monotonically lowering conductivity at an ever increasing aluminum ratio after the 1:2 ratio. The similarity in the electrochemical characteristics, including the ionic conductivity behavior, confirmed that thi s model system is adequate fo r NMR analys is. Although the complete description of the resul ts is beyond the scope of thi s paper (see refs 2 1 and 24 for more deta ils) , the following po ints should be noted :

From the IJC and the IH spectra it was inferred that no bridging alkyl groups are ever present. Organic ligands bonded to the Al or Mg cores can be d istinguished accord ing to their chemi cal shi ft (the more e lectropositive Mg radical causes greater high field shifts, to the atoms in the a position, observed in the IH and the 13C spectra, then in the AI core). In the

§ 1.8

1.8 en 1.4 .s .~

1 .2

> 1 'fi :::J 0 .8 'C c 0.6 8 u 0 .4 'c

0.2 E 0

0

1

§ 0.75 en .s ~ 0 .5 > 'fi :::J 'C c:

0.25 8 u 'c .2

0

0

(a)

(b)

2 3 4

Acid I Base ratio

/'" I \

../ '-..".

170 150 180 160 140

0.5 1,5

Acid I Base ratio

2

6

2.5 3

Fig. 4-{a) Specific conducti vity o f 0.25 M solu tions of the reacti on products of BU2M g ("base") with EtAIC I} ("acid") in THF, a t vario us reactant ratios. (b) Specific cond uctivity o f 0.25 M solutions of the reacti on products of Et2Mg ("base") with EtA ICI 2 ("acid" ) in T HF, at va rio us reactant rat ios. 27 AI NMR spectra of the reaction products of Et2Mg with EtA ICI2 a t various ra tios are also presented in the insert. T he data of aci d/base ra tios o f I :2, I: 1 and 2: 1 correspond to Eqs 1,2 and 3, respectively. in Scheme 1.

882 INDIAN J CHEM, SEC A. MA Y 2005

1:2 and the I: I complexes no organic ligands are bonded to magnesium atom. From the 27 AI NMR (peak positions between 120 to 180 ppm), it was inferred that alumi num realizes tetra-coordination in all the cases. From the 25Mg NMR (peak positi ons close to 0.0 ppm) it was inferred that magnesium is always hexa-coordinated. Spectra of so lutions in which the reactants were mi xed in proportions other than I: I, 1:2 and 2: I revealed the existence of a mixture of the products. The most i nformati ve data was obtained from the 27 AI spectra. As seen in the inserts of Fig. 4b, the various products, des ignated by the reactant ratios (Mg:AI), are characterized by well­defined peaks that were the basis for the identification of the specific molecules. Based on the 13C spectrum it was possible to identi fy the 2: I ratio product as a tetraethyl aluminum an ion. Its spectrum, seen in Fig. 4b, is of special interest, as the very symmetric structure of this anion leads to an uncharacteri st ically sharp peak, centered at 153 ppm. The spectrum of the 1: I ratio product contains large and broad peak at 178 ppm, that was identified as THF-coordinated triethyl aluminum, as was ascertained by acqu iring the spectrum of a reference material (i.e., tri ethyl alu minum dissolved in THF). The spectrum of the most interesting solution , namely the 1 :2, DCC, exhibits a large and very broad peak , centered at 154 ppm, which was a bit harder to identify with certainty. During the course of the stuci " it was noticed that there is a trend in the 27 AI NMR features of the various model compounds that were measured. In general, in a homologue series of tetracoordinated aluminum compou nds, (as aluminate anions or as THF-coordinated organo-halo aluminum compounds), the hi gher the ratio of the Cl/organic ligands, the lower is the peak position in ppm. Furthermore, the influence of THF on the peak position is similar to that of chlorine. Literature data on similar compou nds show similar trends25.26. Thus, with a peak position at 154 ppm, we could eliminate the possibility of the followi~g possible structures: AICl ~- , EtAIClJ-, EtJAICl , Et4Af, AICI 3THF, Et3AITHF and EtAICI2THF. Furthermore, following the same argument, we hypothesized that the aluminum­containing molecule should be either Et2AICI2-, Et2AICl(THF) or, taking into account the tendency of these complexes to form dimers, Et2ClAI-CI-AICIEt2- . The possibi lity that the peak relates only to the neutral molecule Et2AICI (THF) was not logical since it contradicted the high ionic conductiv ity of the

so lutions. From stoichi ometry considerat ions it also cou ld no! be deduced th at the first compound, namely Et2AICI2 . is the so le product. However, based both on stoichiometry and on the ioni c conduc tivity data, we deduced that DCC solutions contain either the Et2ClA I-CI-AICIEt2- dimer or a mixture of Et2A ICl (THF) and Et2AICl2-, wh ich are both expected to absorb at about the same field.

The identity of the positi ve cations, or other molecules containing the Mg as the core, was deduced from stoichiometry considerations, from ionic conductivity data and from IH and l3C NMR data. For example, the absence of organic ligands bonded to magnesium in the 1:2 and I: I solutions, as wa. mentioned above. led us to determi ne that MgCl+ and MgCl2 are the major magnesium-conta ining species in these solutions, respectively (both coordinated by THF to hexacoord inated complexes). Based on the same considerations, including the detection of organomagnes ium signals in the NMR, we concluded that the 2: 1 solution contains either a mixture of MgCI+ and Et2Mg, MgCI2 and EtMg+. or their dimers. The possibility of the existence of a Grignard compound, e.g., EtMgCI, is excluded, as this compound features a broad peak at 55 ppm in the 25Mg NM R (measured duri ng the course of thi s study). The studied so lutions do not show any 25 Mg NMR peaks at this sh ift.

These results are se lf exp lanatory, and accordingly. a reaction scheme (Scheme I) was formulated.

2RAICI : + R2Mg ~R 2A I CI/+ R2AICI· THF* ( I ) + MgCI+ 5THF

RA ICl2 + R2Mg~RJAI · THf + MgCI2·4THF (2)

RAICI2 + 2R2Mg~ R4M + MgCI+·STHF + Et2MgATHF (3) R= buty l or ethyl radical * 01' EtlCIAI-CI-AICIEt2' dimer

Li st of electrol yte reactions Scheme 1

This reaction scheme is also consistent with the electrochemical wi ndows presented in Fig. 2. The ri ght hand side limit of the electrochemical window, namely, the potential at which the solut ion undergoes irreversible oxidati on, represents the susceptibility of the solution towards ox idation. Thus, it is obv ious that it will be the most oxidizeable spec ie in the sol uti on that will determine the electrochemi cal window. The solution species specified in Eqs I th rough 3 contain organometallic aluminate anions and molecules, with an ever increasing tendency to oxidize. R2AICI2' is less prone to oxid izat ion than the others, as the two

AURBACH el at.: ADVANCES fN MAGNESIUM ELECTROCHEMISTRY 883

electron withdrawing chlorine ligands increase the actual oxidation state of the aluminum core. The product of Eq . 3, R4A(, on the other hand, not only contains four (J bonded organic ligands, but is also negatively charged. This both increases the electron densi ty around the aluminum core, weakening these bonds, and increasing the number of the oxidizeable bonds.

Mg deposition/dissolution processes with microelectrodes: Mechanisms and their possible impact on the morphology of Mg deposits

In developing rechargeable magnesium batteries, knowledge of the electrochemical behavior of the reversible Mg anodes is crucial. It is well known that Mg cannot be deposited from solutions of simple Mg salts such as Mg(CI04h in conventional organic solvents, e.g., acetonitrile, propylene carbonate or dimethylformamide l

-6

, presumably because the electrode surface becomes covered with dense passivating reduction products of solution species, in which the Mg ion conductivity is very low. It appears that this property is typical for all the aprotic organic solutions with si mple Mg salts, which makes the behavior of Mg anodes quite different from that of Li electrodes. However, the facts that the conductivity of the solutions in which Mg electrodes are reversible is only a few millisiemens/cm and that the overvoltage for Mg deposition may not be negligible (see Figs 2 and 4), make the application of a stationary microelectrode technique for the study of the

(a) WE (Pt. ~ so J1IIl)

(d)

a

" :c o of '" u

" :c o c «

(b)

Z'IO

electrochemical behavior of Mg electrodes very advantageous.

The micro disc electrodes that we used (0 50 /lm Pt wire) embedded in the center of a polypropylene cylinder of a diameter of ca. 5 mm are schematically shown in Fig. 5. The theory of diffusion to a micro disc electrode is well established27

. Two major advantages of the microelectrode technique (compared to the classical electroanalytical methods related to macroelectrode systems) should be mentioned in this respect:

(i) While a linear potential sweep voltammetry for a macroelectrode system contall1l11g soluble reversible redox species AI k ·shows a typical peak-shaped response, sweeping the potential of a micro disc electrode with the same rate, results in a steady-state wave with a limiting current of extremely high current density (Fig. 5) . As is well known, the reason is that the specific condition of diffusion of solution species to a micro-size disc electrode is such that the limiting current appears to be inversely proportional to the disc's radius27

.

This means that utilization of the microelectrode technique allows the diffusional- and solution­resistance limitations, which are typical of the electroanalytical responses from macroelectrode systems, to be effectively overcome, especially when nonaqueous solutions of relatively low conductivity are used .

. (c)

" :c o of " u

(e) a .... ~

Z'IC

Fig. 5-- A scheme of the microelectrodes used here (a). Schematic improvement in the ability to measure electrode kinetics (b, c) and impedance (d, e) due to the use of microelectrodes (compared to macroelectrodes). Plots (b) and (d) indicate the response of macro­electrodes and plots (c) and (e) indicate response of the micro-electrodes.

884 INDIAN] CHEM. SEC A, MA Y 2005

(ii ) The next advantage of the microelectrode technique is that it is well adapted to studying electrode systems, having extremely hi gh impedance during polarization to low current densities , and ex hibit a drastic decrease in the impedance when polarized to high current densities (see also in Fig. 5). This was proved, for example, fo r Li electrodes covered with surface films; when the current density is low, the films are perfect and dense, and their ionic conductivity is relatively IOW7

.IS . Application of hi gher current

densities makes the fi lms less regular and dense, resulting generally in Increase In ionic conductivity , thereby enab ling the characterization of the interfacial charge-transfer kinetics (see the second semicircle with the diameter Rei in Fig. 5). This shows that the microelectrode techniques that operate wi th ex tremely hi gh current densities present a viab le tool fo r the elucidation of the nature of the interfacia l processes during metal deposition/d issolution.

The shape of the Tafel plots obtained with the microelectrodes during Mg deposition/ dissolution in a seri es of ethereal solutions of Mg(AICI2BuEt)2 and related moieties, were studied carefu lly by slowly scanning the potential in the cathodic and the anodic direc ti ons (25 mVs-I), or by performing the related small-amplitude potential steps. Figure 6 shows, as an example, the anodic and cathodic branches of the Tafel plots (logarithm of cune!1t density versus overvoltage) for Mg deposition/dissolution fro m MgBu2:AIEtCI2 (I :2) (DCC) (0.25 M) and MgBu2(0.25 M), respectively, obtained by slow potential scanning. It IS seen that in the range of

overpotenti als fro m ± 0.1-0.3 V, the plots in Fig. 6 are approximately li near. However, the typica l slope obtained, 0.25 Vdecade-I, is too hi gh to be refelTed to any particul ar charge-transfer process . From Fig. 6 (the plot for Mg deposition/dissolution for an all ·· organi c solution, which does not contain any ch loride or Al species), one cou ld see that the absolute values of current related to the deposition/dissolution process substantially increase, whereas the range of overvol tage with a li near portion of the Tafel plots drastically decrease. High overvoltage resu lts in the saturation of curren t which, however, cannot be ascribed to a convent ional diffusion limitation since the limi ting diffusion GLI rrent to the microelecrrode under consideration is higher than that observed in the Tafe l plots.

Figure 6 also summarizes Tafel plots (in the fo rm of overvoltage liS. log of current density) obtained by a small poten ti al step techni que (i.e., at a higher resolu tion compared to that for the linear scan voltammetry) for several solution compositions. These included both all -i norgani c and all -organic solutions. Two major trends become obvious from a careful consideration of these plots:

(i) When pass ing from all -inorganic to all -organi c solutions, the absolute va lue of the currents drastically increases (at the same value of the potenti al applied) . This phenomenon is attributed to the changing proporti ons of the electroactive BuMg+ and MgCI+ in the solutions.

(ii ) Only in the li mit of a very small overvo ltage the va lue of the slope of the Tafel plots is compatible with a slow electron-transfer reaction. From the slope of this curve, the transfer coefficients for the cathodi c and the anodic reactions were fo und

3 .2 i---------,.-;;::::================;_]

2.4

> ;;1 .6

0.8

MaCI,:AICl, (1 :1) .atllnlted

o -2.5

-, ·11.11

t

legjt Acm-2

, ,

1.5

-4.2

r-O.lS Vdccadc-1

O.:! 0.8

3.5

Fig. 6--Exampl es of the potellliodynamics o f Li deposition/di sso luti on processes with the mi croelectrodes and a few examp les o f T afel plots. The identil y of Ihe re le vant solu ti ons is indicated in the fi g ure.

AURBACH el ul.: ADVANCES IN MAGNESI UM ELECTROCHEM ISTRY 88S

to be around (Xc-O.S and (X,,- I.S , which is ev idence o f seq uential, two one-electron transfers, with the first electron transfer, being the RDS . An increase in overvoltage results in a corresponding increase in the slope, revealing a li mitat io n due to a step obviously d iffe rent from that of the conventi onal diffusion, as outlined above for the potenti al scanl1ing experiments.

The increase in the Tafel slope up to 0.6-0.7 Vdecade- I in the range of high overvoltage is quite unex pected and has not been noted fo r any o f the electrochemical systems already reported . In trying to co mply with the above features of the Mg depos ition reactio n, a com plex, three-stage e lectrocrystalli zatio n mechanism was adopted, which desc ri bes the growth of the metall ic deposits in terms of processes occurring on the surface, the steps and the kinks o f the substrate metal (the FSK model)28. Scheme 2 (ref. 29) derived fro m these considerati ons, incl udes Mg cation discharge with the fo rmati on of ad-atoms o n the electrode surface, fo ll owed by surface diffus io n of Mg-ad-atoms to locations near the kinks (whi ch are the actual growth sites), and, fin all y, the embedment of the ad-ato ms at the growth sites (s imil ar to c lass ical electrocrystalli zation mechani sm). The last stage can be considered as the fo llow ing chemical reaction with respect to the slow-electron transfer stage. The specific features of Mg-depositio n can be well understood in terms of a multi step, first stage of the

above mechanism, compn slI1g fo ur serial and two paralle l processes (Scheme 2) . An increase in the Tafel slope wi th the increase in the cathodic overvoltage is dominated by interpl ay of the slow, potential-independent rate of the adsorption of the MgCl+ on a surface, with the exponentia lly increasing rate of the firs t inte rfac ia l e lectron transfer to the MgCl\ r. As described e lsewhere, when using ill Silu

FTIR spectroscopy we obtained strong ev idence that Mg depos ition involves the adsorptio n of species with Mg-C l bonds30

. A similar increase in the Tafel s lope with an increase in the anodic overvoltage is explai ned by the change in the nature of the RDS: we suggest an interpl ay which relates to the same potenti al-dependent rate of the firs t-e lec tron transfer (i .e ., second step in the di ssolu tion process) , and the potenti al independent, preced ing chemi cal reaction, i.e., d iffu sion of the Mg-ad-ato ms fro m the growth sites to the metalli c face. T he proposed mechani sm, whi ch explains the peculi ar kinet ic data for Mg depos ition fro m the co mpl ex/ethereal so luti ons, is svmewhat simil ar to th at fo r gold o r Co-AI a lloy depos iti ons fro m cyanide and AICh-BPC-CoC," room temperature molten salt, respectivel/1.32.

An understand ing of the Mg deposition mechani sms refl ected by Scheme 2 explains the effect of the ac id/base ratio of the e lectro lyte in solution o n the morpho logy demo nstrated above (Fig. 3). Since Mg deposition is a multi step process in which adso rption also pl ays a ro le, the nature of the

I. Formation of a single layer:

DiHusion Fast • MgCI+ (near Mg metal f2ce)

MgCI + (near Mg metal face) Adsorption. MgCI+ Slow sf

Slow MgCI + sf + e- ~ MgCI"sf

(a)

(b)

(c)

F~t MgCI"sf + e- (-'- Mgsf + CI - (d)

CI -___ Diffusion to solution bu lk (e)

---MgCI \ f + CI - ~ MgCI 2 sf (f)

II. Layered growth of Mg deposit

Discharge with the formation of Mg-ad-atom on a metal face as in pathways I (9)

surtac: diffusi0'l Mg (near a kink) ast

(h)

Mg (near a kink) ~ Mg (embedded in the growth place) (i) Slow

M echanisms for M g depositi on Scheme 2

886 INDIAN J CHEM, SEC A, MA Y 2005

adsorbed species may attenuate the deposition process due to secondary current di stribution consideration. Because we know that different acid/base ratios mean 'different active species (Scheme 1), it is clear that different species adsorb in each case. We cannot explain how the various adsorption processes lead to the specific morphologies presented in Fig. 3. However, the diversity in morphology can be, at least at the qualitative level, understood as resulting from the different compositions of the electrolyte solutions that we elucidated.

Problems of Mg2+ insertion into inorganic ho~ts and the Chevrel phase phenomenon

rn general, the cathodes for Mg batteries could be 2+

based on Mg ion insertion (intercalation) to the crysta l structure of active materia ls, which are quite similar to those used in Li batteries. However, during efforts to develop rechargeable M g batteries in recent yearsS

.IO, it was realized that the selection of materi als

suitable for the Mg ion insertion presents a great challenge. rn fact, in spite of the expected simil arity between Li and Mg ion intercalation , a lmost a ll inorganic compounds, which prove themselves as suitable cathode materials for Li batte ries, show very poor electrochemical performance regarding Mg ion i nsertion 8. 10.33.40

According to the literature, it is clear that the main problem of Mg ion insertion into the usual hosts is its slow kinetics. A re latively successful Mg intercalation was observed for nano-crystalline materi als4o, thin films41, or nanotubes42 . In such products, the intercalation kinetics should be a priori much higher than in the micro-size products. It is a lso clear that the reason for the slow kinetics is the divalent character of the inserted ions, resulting in strong interactions between the inserted divalent cati ons and the anions and the cations of the host41, or high activation barriers for site changes in the case of inserted ions with high charge densities. As a result, M g insertion was more successful in hydrates3

?, because water or hydroxyl species can shield the strong coulombic interaction between the polyvalent gues t species awl the cations of the host.

A few years ago, we discovered that the Chevrel phase MxM06S8 can insert reversibly two Mg atoms per c luster (i.e., M-Mg, 0<x<2), and hence, can serve as an excellent cathode materi al for secondary Mg b . 11 ·13 Th ' i' nttenes . IS (ISCovery resul ted fro m many unsucc; :sfu l experiments of Mg insertion into well ­knov. :, Li + ion hosts, as well as from the literature

concerning the poss ibility of divalent ion intercalatio n in inorganic materia ls. This analysis revealed that Chevrel phases are unique material s that allow a relatively fast insertion of divalent cations43.45 such as Zn2+ Cd2+ N ·2+ Mn2+ C 2+ d F 2+ , , I , , 0 an e.

In contrast to the common ionic hosts, the crystal structure of Chevrel phases46

-48 is built from the M06Ts clusters, i. e., the assembly of six metal ato ms combined by metal/metal bonds and surrounded by eight chaJcogen ions. In the inte rcalation process, these six Mo atoms can be regarded as a sing le large ion that can accommodate up to 4 e lectro ns (compared to one o r two e lectrons for the usual

transition metal io n) . Upon insertion of o ne Mg 2+ ion per formula unit, the formal charge of any individual Mo ion in the cluster changes by o nly 1/3 e lectron.

Moreover, in the Chevrel crysta l structure. 12 vacant sites per formula unit are available for the inserting ions (see insert in Fig. 1) . The distances between them are very short ( l . i-l .4 A), and thus, only two of the sites can be occupi ed simultaneous ly by divalent ions. Therefore, the crysta l structure of Chevrel phases is ideal for io n mobility because of the large number of the vacant sites, the short di stances between them, and the metall ic c lus te r that ensures the local electl'o-neutrality of the intercalati on compound. Hence, the high acti vity of Chevrel phases in the process of Mg io n insertio n/extraction can be attributed to the unusual crystal structure of these materials 11.1 2. 19,20,

Fortunately , the e lectrochemical window of Mg ion insertion into Chevrel phases, matches the electrochemical windows of the solu tions in which the Mg electrodes behave reversibly (see Fig. I). Figure 7 shows typical CYs of Li+ and Mg2+ ion inserti on into M06SS in the same solvent, aceto nitrile, which enables a direct comparison between the two se ri es o f processes49 . The insert in Fig. 7 shows the CYs of M g2+ ion inserti on into M06Sg in THF/0.25 M

Mg(AICI2BuEt)z solutions at 20°C and 45°C. The relevant reaction schemes are also presented in Fig.7. Mg2+ ion insertion occurs in two steps, both of wh ich involve phase transition 49, A comparison between Mg2+ ion intercalation into M06Sg In the AN/Mg(CI04h and into T HF/Mg(AICI2BuEt)2 sol utions shows di fferences, mostly in the first process (redox potentials around - 1.05 Y versus Ag/Ag+ R.E. , or around 1.3 Y versus Mg R.E. in the AN and THF solutions, respectively) . (See the insert in F ig. 7). The second process (around - 1.3 Y versus

AURBACH ef af.: ADVANCES TN MAGNESIUM ELECTROCHEMISTRY 887

2.5 CP + Mg2+ + 2e -t MgCP

MgCP + Mg2+ + 2e -t M9 2CP 0 -LixMo6SS

~ - MgxMo6SS

1 .0

~ ::::: -0 .5

-2.0

-3.5

CP + Li+ + e -~ LiCP

LiCP + 2Li+ + 2e - > Li3CP

Li 3CP + Li+ + e Li4CP

-1 .25 -1 .00

6-7 mg em-l I 0.251\1 LiCIO~ or 1\lg(CIO~)ll AN, v=50 I-l'" S· l

20 ··_···_-

ID ' ,:;oC' ~ v:::::2SJlYs--J

;. 10 1"""? '00 (' ~ 1L:.l-:: 0 I .' (3 ·10 I

-20 I __ 0.6 0.8 1.0 1.2 IA 1.6 J.8

E/V(vs . Mg)

-0.75 -0.50 ~O . 2f.

E / V (vs. Ag/Ag+)

Fig. 7- Typical steady state CVs of Li and Mg ion insertion into M06SS composite electrodes in AN solutions containing 0. 25 M LiCI04

or Mg(CI04h The experimental conditions were very simil ar. The scan rate was 60 /lV/s; temp. 25°C. The insert shows a typical steady state, slow scan rate CV of Mg insertion into a M06SS electrode at 20°C and 45°C from a THF/O.25 M Mg(A ICI"BuEth so luti on.

Ag/Ag+ in AN solutions or around 1.1 V versus Mg R.E. in THF solutions) seems to depend much less on the solution used. These measurements reflect the fact that the first Mg2+ ion insertion into MO/iSs is much slower than the second one. It should be noted that while the initial magnesiation of these electrodes involves the entire theoretical capacity (i .e., the insertion of two Mg2+ ions per M06SS unit, 122 mAh/gr), during cycling at room temperature part of the Mg2+ are trapped in the electrode, and thus, the first process occurs (reversibly) at about 60-80% of its theoretical capacity. The second process occurs at full capacity (::::60 mAh/gr, one Mg per MO/iSg) during prolonged cycling (>3000 charge-discharge cycles at 100% 00D)49. At elevated temperatures (e.g., >50°C), the first process also occurs at its full capacity, and hence, upon cycling these electrodes at temperatures above 50°C, their theoretical capacity is repeatedly obtained.

As seen in Fig.7, Li+ ion insertion into M06Sg occurs in three stages: in the first, a Li ion is inserted at a similar formal redox potential as the first Mg2+ ion. Then, two Li+ ions are inserted at the same formal redox potential as the second Mg2+ ion. Both processes involve phase transition. The fourth Li ion is inserted at potentials below - 1.35 Y versus Agi Ag + in the Li/Li+ scale) in a process that forms a solid solution (thereby appeari ng in the CYs as broad waves rather than peaks). The comparison between the insertions of the two ions into the Chevrel phase is important as it demonstrates the more sluggish kinetics of the Mg2+ ion insertion compared to the

insertion of the Li ions, especiall y in the first process. In general, the diffusion coefficient for Li ions is higher than that cOITesponding to Mg2+ ions49 . Hence, it is a challenge to utilize the theoretical capacity of this Chevrel phase as a cathode for rechargeable Mg batteries, especially at high rates.

It is highly interesting to see the effect obtained by increasing the polarizability of the anionic framework of the Chevrel phase on the nature of Mg2+ ion intercalation into these compounds. Consequentl y, Mg2+ ion insertion into M06Ses has also been studied. The replacement of sulfur by selenium as the anionic element in the Chevrel phase may reduce the intensity of attractive interactions between the intercalated Mg ions and the anionic framework of the host, thus reducing the diffusion ban'iers.

It should be noted that the M06SS materi al is thermodynamically unstable and can be obtained only indirectly by chemical or electrochemical leaching of the more stable, metal-containing Chevrel phases, e.g., CU2M06S850,5 1. However, the phase diagram of the latter material is well known52, and its high temperature synthesis (lOOO-1200°C) from the elements or the sulfides in evacuated, sealed quartz tubes is relatively simple53 . In contrast, M06Ses can be synthesized directly from the elements at high temperatures54.55.

Next, we started studying Mg ion insertion into M06Ses. Figure 8 shows a comparison between the CVs of MgxM06SS and MgxM06Ses (0 < x < 2) measured with similar electrodes in similar sol uti ons and experimental condi ti ons. The difference between

888 INDIAN 1 CHEM, SEC A, MAY 2005

20 0 -Mg . Mo, Se,

-10 1I = 10pVS·1

-20 L-_~ ____ ~ __ ~_~-'---'

0.8 0.9 1.0 1.1 1.2 1.3 1.4

E IV(vs. Mg)

Fig. 8--Typical steady state, slow scan rate ( 10 /-AV Is C Vs of Mg ion in seltion into composite e lectrodes containing Moc,Ss and Mo6Se8, as indicated. The solution was THF/0.25 M Mg(A ICI2BuEt)2: temp. 25°C.

the behavior of the two material s is spectacular. As '<peeted, and reflected by the CYs in Fig. 8, the kinetics of Mg ions into the selenide is much faster as compared to the sulfide (note the smaller peak­potential separations in Fig. 8). In addition , the capacity of Mg ion insertion into MgxM06Ses in repeated cycling is very close to the theoretical one (i.e., 0 < x ::s 2), even at room temperature. Nevertheless, the M06SS is of course much more prefelTed as a cathode material for rechargeable Mg batteries than the M06Ses due to higher redox potentials and theoretical capacity .

One poss ibility for increasing the performance of the M06SS cathode material is to reduce the size of the particles from micrometric to nanometric size. The fact that this compound does not react with ethers and with Mg-AI-CI-R complexes ensures the lack of complications due to the increase in the surface area of the active mass. The easiest approach for that is milling. However, it shoul d be noted that milling the precursor, CU2M06SS, in any atmosphere, or M06SS in air, leads to irreversible mechanochemical reactions 19 .

We obtained interesting preliminary results with M06SS milled mildly under argon atmosphere (submicronic particle size).

Figure 9 shows the comparison between composite electrodes comprising untreated M06SS particles and electrodes containing submicronic size M06SS in identical experiments (galvanostatic cycling at CIlO in T HF/O.2S Mg(AIChBuEth solutions . It is clearly seen that the behavior of the milled material is much better that the untreated one. Both the practical capacity and the kinetics are improved due to the reduction of the particle size (see the CYs in the insert in Fig. 9). It should be noted that in the present case,

110~-------------~

~"<=;:J' .~":""~ 10

2 1 1

"", 100 a .:. 90 >-

:!:! 80 ~ ~ 70

~ 60 IV .c 50 bl o 40

.,/

-'ti _ t> -" '" v = 100 11,",," 1

2 - 10 ......,..~.,,--~_~_-':-:! o.~ 0.8 1.1 1A 1.7

E/V(vs. Mg) 30~--~---~-----~----~~

o 25 50 Cycle number

75 100

Fig. 9--Capacity versus cycle number curves of Mg, Mo"Ss composite (0<x<2) e lectrodes during galvanostatic cycling (CIl 0) in THFIMg(AICl2BuEt)2 0.25 M for both the milled (sub mi crometric size particles) and regular, micronic size active mass. Typical CVs of these e lec trodes in similar experiments and condition s are presented in the insert. [Curve I : Mi ll ed particles, sub-mi cronic size: Curve 2: Regu lar, micro nic size partic les].

the capacity utility depends mostly on the kinetics of the fi rst stage of the Mg ion deintercalation. Hence, reduction of the particle size, which improves the kinetics, influences very positively the capacity utility as well.

Conclusions The R&D of rechargeable magnesium batteries C<.ill

only be advanced on the basis of thorough electrochemical, surface and structural studies of the relevant systems. To date, there are very limited types of electrolyte solutions known, in which Mg electrodes behave reversibly, and yet possess an anodic stabi li ty that is compatible wi th positi ve­cathodic reactions. A key feature of the electrolyte solutions that are described herein , is their being a product of a reaction between a R2Mg Lewis base and an AIChR Lewis acid, whose different ratios (2: I , 1: 1, 1 :2, etc.) determine the different product and product distributions. The nature of these products determines the anodic stability of the sol ution and changes the morphology of Mg deposition via adsorption processes . Mg deposition , while occurring in these solutions only because Mg surfaces are bare and passivation-free, is complicated and involves several sub-processes in series and in parallel , as demonstrated in Scheme 2. The Mg ion intercalation reactions into Chevrel phases (sulfide and selenide) described herein are also complicated. Comparing Mg ion insertion into M06Sg and M06Seg demonstrates the influence of the anionic framework of the host material. The mobility of the divalent cations is easier

AURBACI-I e: 01.: ADV ANCES IN MAGNESIUM ELECTROCHEMISTRY 889

as the polarizability of the anions is higher, and their charge densi ty is lower. The more important material , namely, M06Sg, shows some kjnetic limitation in the first stage of the process, to form MgM06Sg. These kinetic problems limit the full capacity utilization during cycling at room temperature. The solution to this limitation is the use of nanoparticles. Since there are no side reactions between MgxM06Sg and the relevant ethereal solutions, reduction of the particle size reduces the diffu sion length and, accordingly, increases the diffusion rate of the ions in the host, without the penalty of detrimental side reactions that result from the high surface area of the nanomaterials. The high electrical conductivity of MgxM06Ss ensures a very good interparticle electronic contact, despite the small particle size. Hence, this system is a classic example of the advantageous use of nanomaterials in the field of rechargeable batteries.

Acknowledgement Partial financial support for thi s work was obtained

by the Israel Science Foundation, ISF, and A TU Inc. , Israel.

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