Thermal and Electrochemical Stability of TetraglymeâMagnesium Bis

Article pubs.acs.org/JPCC

Thermal and Electrochemical Stability of Tetraglyme−Magnesium Bis(trifluoromethanesulfonyl)amide Complex: Electric Field Effect of Divalent Cation on Solvate Stability Shoshi Terada,† Toshihiko Mandai,† Soma Suzuki,† Seiji Tsuzuki,‡ Katsuya Watanabe,† Yutaro Kamei,† Kazuhide Ueno,† Kaoru Dokko,*,†,§ and Masayoshi Watanabe† †

Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ‡ National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan § Unit of Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: Phase behavior of binary mixtures of tetraglyme (G4) and Mg[TFSA]2 (TFSA: bis(trifluoromethanesulfonyl)amide) was investigated. In a 1:1 molar ratio, G4 and Mg[TFSA]2 formed a stable complex with a melting point of 137 °C. X-ray crystallography of a single crystal of the complex grown from a G4-Mg[TFSA]2 binary mixture revealed that the G4 molecule wraps around Mg2+ to form a complex [Mg(G4)]2+ cation, and the two [TFSA]− anions also participate in the Mg2+ coordination in the crystal. The thermal stability of [Mg(G4)][TFSA]2 was examined by thermogravimetry, and it was found that the complex is stable up to 250 °C. Above 250 °C, desolvation of the Mg2+ ion takes place and G4 evaporates. On the other hand, the weight loss starts at around 140 °C in solutions containing excess G4 (n > 1 in Mg[TFSA]2:G4 = 1:n) due to the evaporation of free (uncoordinated) G4. The suppression of G4 volatility in the [Mg(G4)][TFSA]2 complex is attributed to strong electrostatic and induction interactions between divalent Mg2+ and G4. In addition, complexation of G4 with Mg2+ is effective in enhancing the oxidative stability of G4. Linear sweep voltammetry revealed that the oxidative decomposition of [Mg(G4)][TFSA]2 occurs at electrode potentials >5 V vs Li/Li+, while the oxidation of uncoordinated G4 occurs at around 4.0 V. This oxidative stability enhancement occurs because the HOMO energy level of G4 is reduced by complexation with Mg2+, which is supported by the ab initio calculations.

1. INTRODUCTION In liquid electrolyte solutions, ion−ion and ion−solvent interactions exert significant effects on salt dissociation, liquid structures (solvation structures of ions and ion-pair formation), and transport properties. Formation of ion pairs is mainly dominated by ion−ion electrostatic interactions. On the other hand, ion-dipole (electrostatic) and ion-induced dipole (induction) interactions jointly drive the solvation of an ion. In electrolyte solutions containing alkali metal salts, alkali metal ions strongly interact with solvent molecules because the charge density of alkali metal ions are high and produce strong electric fields, thereby forming complex (solvate) cations.1−4 Under certain conditions, highly stable solvate cations can be obtained, and the stability of the solvates affects the electrochemical reactions in electrolyte solutions.1,5−12 Glymes (Gl, l in CH3−O−(CH2−CH2−O)l−CH3) with multiple ether-oxygen atoms can form relatively long-lived (stable) solvates with alkali metal ions. Glymes possess strong solvation power (Lewis basicity) against alkali metal ions and can be used as solvents for nonaqueous electrolyte solutions.1,5,8,9,11−13 In such electrolyte solutions, glymes © XXXX American Chemical Society

(Lewis base) interact strongly with alkali metal ions (Lewis acid, M+), resulting in complex formation in certain molar ratios, [M(glyme)x]+.14−19 Henderson et al. studied glyme−Li salt complexes systematically and reported that the coordination structures of [Li(Gl)x]+ change depending on the anion species and chain length of glyme (l).20−25 Some of the glyme− Li salt complexes are low melting and remain in the liquid state at room temperature.20,23,25−28 We studied the physicochemical properties of a series of glyme−Li[TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide) complexes and found that the complexation of Li+ ions with G3 or G4 yields remarkably stable [Li(G3 or G4)]+ cations, owing to the chelate effect.7,8,29−31 This results in the low volatility, nonflammability, and enhanced oxidative stability of [Li(G3 or G4)][TFSA]. The molten [Li(G3 or G4)][TFSA] is now recognized as a representative solvate ionic liquid32 and can be used as a thermally stable electrolyte in lithium batteries.8,9,11,13,33,34 Received: October 6, 2015 Revised: December 29, 2015

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DOI: 10.1021/acs.jpcc.5b09779 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

calorimetry (DSC; DSC6220, Seiko) and TG (TG/DTA 6200, Seiko). For the DSC measurements, the sample electrolytes were hermetically sealed in aluminum pans in a glovebox. The sample pans were first annealed at certain temperatures to avoid hysteresis, followed by cooling to −150 °C at 5 °C min−1, and then heated to specific temperatures at a heating rate of 10 °C min−1. The Mg[TFSA]2/G4 = 1/0.6 mixture was aged at room temperature for 2 days after annealing, since no clear peak was observed in either the cooling or heating traces. The tops of the endothermic peaks were evaluated as the melting point (Tm) and solid−solid transition temperature (Ttr), and the onset of the heat capacity change was evaluated as the glass transition temperature (Tg) in the heating traces. The effect of aging on the thermal behavior of [Mg(G4)][TFSA]2 was investigated by storing the annealed sample at room temperature for 0, 12, 18, 24, and 72 h and then recording the heating trace to 150 °C at a scan rate of 10 °C min−1. The thermal stability of the Mg[TFSA]2/G4 mixtures was evaluated by TG measurements. The samples were heated from room temperature to 500 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere. The thermal decomposition temperature (Td) was defined as the temperature at 5% weight loss. Cyclic voltammetry (CV) and LSV were performed with a typical three-electrode beaker cell using an electrochemical analyzer (PARSTAT MC1000, Princeton Applied Research). A Pt disk (3.3 mm in diameter) and a coiled Pt wire (0.5 mm in diameter) were used as the working and counter electrodes, respectively. A reference electrode was fabricated by soaking Li metal in 1 M Li[TFSA]/G4, confined in a glass tube with a liquid junction. These three electrodes were soaked in 4 mL of electrolyte and hermetically sealed to avoid exposure air. The CV measurements were performed on two different 0.5 M Mg[TFSA]2/G4 electrolytes, with and without molecular sieves 4A, during cycling to clarify the role of water impurities in the electrolytes. All the voltammetric measurements were performed at 30 °C at a scan rate of 1 mV s−1. Single crystals suitable for X-ray crystallography were grown from a 2:1 mixture of G4−Mg[TFSA]2 after storing for several days at ambient temperature. A [Mg(G4)][TFSA]2 single crystal with 0.1 × 0.3 × 0.3 mm3 dimensions was coated with vacuum grease, mounted on a glass pin, and then cooled to −100 °C using a steady flow of nitrogen. All diffraction measurements were performed on Bruker Apex II equipped with a charge-coupled device (CCD) area detector using monochromated Mo Kα radiation (λ = 0.710 73 Å). An empirical absorption correction was applied using a multiscan averaging of symmetry equivalent data using the SADABS program.42 The structure was solved by the direct method SHELXS-97 and was refined full-matrix least-squares anisotropically for non-hydrogen atoms using the SHELXL2013.43,44 All hydrogen atoms were introduced at geometrically ideal positions and refined using an appropriate riding model. The crystallographic data are summarized in Table S1. The crystallographic information file (cif) was deposited in the Cambridge Structure Database (CSD) as CCDC 1420478. The powder X-ray diffraction (PXRD) profile of [Mg(G4)][TFSA]2 was obtained by an XRD system (Ultima IV, Rigaku) with Cu Kα (λ = 1.5418 Å) radiation at ambient temperature. The samples were placed in an airtight chamber with a Be window inside a glovebox and transferred to the PXRD setup to avoid exposure to air.

Likewise, other alkali metal salts such as Na[TFSA] and K[TFSA] form 1:1 complexes with certain glymes, and the crystal structures and fundamental properties of the complexes have been reported recently.18 The oxidative stability and thermal stability of the complexes change depending on the size of the alkali metal ion. As the size of the metal ion decreases, the interaction between the alkali metal ion and glyme is enhanced. These results encouraged us to elucidate the physicochemical properties of glyme−Mg salt complexes. The radius of the hexacoordinated Mg2+ ion is 0.86 Å, comparable to that of Li+ ion (0.90 Å).35 Electrostatic and induction interactions are the major sources of attraction between the metal ion and glyme.36 Therefore, the divalent cation (Mg2+) has stronger electrostatic and induction interactions with glymes owing to the strong electric field around Mg2+. In this study, the phase behavior of G4−Mg[TFSA]2 binary mixtures was explored. It was found that G4 and Mg[TFSA]2 form a stable complex in a 1:1 molar ratio. The Mg2+ coordination structure was analyzed using X-ray crystallography and Raman spectroscopy. The thermal and oxidative stabilities of the [Mg(G4)][TFSA]2 complex were revealed using thermogravimetry (TG) and linear sweep voltammetry (LSV), respectively. In addition, the quantum chemical calculations supported that the strong electric field of the divalent cation plays a major role in the enhancement of glyme−Mg salt solvate stability. The glyme−Mg salt complexes are promising electrolytes for Mg batteries.37−40 Understanding the interactions between solvent and salt will help in developing electrolytes for batteries.

2. EXPERIMENTAL SECTION Materials. Bis(trifluoromethanesulfonyl)amine (H[TFSA]), Na2CO3, and N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([P13][TFSA]) were purchased from Kanto Chemical and used as received. The Mg ribbon was purchased from Nilaco and treated with 0.1 M H2SO4 to remove a surface oxide film prior to use. Tetraglyme (G4) was kindly supplied by Nippon Nyukazai and dried over molecular sieves (4A) for several days. Battery-grade Li[TFSA] was kindly provided by Solvay Japan as a courtesy sample. Mg[TFSA]2 and Na[TFSA] were synthesized by neutralization of H[TFSA] with Mg metal and Na2CO3, respectively, according to previously published procedures.14,41 All the salts were dried under high vacuum at specific elevated temperatures for several days and stored in an Ar-filled glovebox before use. A series of binary mixtures of Mg[TFSA]2/G4 were prepared by mixing appropriate amounts of Mg[TFSA]2 and G4 in an Ar-filled glovebox. To obtain uniform samples, the mixtures were heated to 160 °C and stirred for 1 h. Measurements. The water content of the electrolytes was determined by Karl Fischer titration. The ionic conductivities (σ) of Mg[TFSA]2/G4 were determined by the complex impedance method using an ac impedance analyzer (Biologic, VMP3) in the frequency range of 500 kHz−1 Hz with a sinusoidal alternating voltage amplitude of 10 mV root-meansquare (rms). A cell equipped with two platinized Pt electrodes was utilized for conductivity measurements (CG-511B, TOA Electronics). The cell was placed in a temperature-controlled chamber, and conductivity was measured at 30 °C. The liquid density and viscosity were measured using a viscometer (SVM 3000, Anton Paar). The thermal properties of the binary mixtures of Mg[TFSA]2/G4 were determined using differential scanning B


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The Journal of Physical Chemistry C Raman spectra were recorded using a 532 nm laser Raman spectrometer (RMP-330, JASCO). The instrument was calibrated using a polypropylene standard. The sample temperature was adjusted to 30 ± 0.1 °C (unless otherwise noted) using a Peltier microscope stage (TS62, INSTEC) with a temperature controller (mk1000, INSTEC). A spectral resolution of 4.5 cm−1 and 60 scans with each accumulation time of 30 s were adopted for all Raman measurements. The peak intensities were normalized with respect to the band observed at ca. 740 cm−1 (assigned to the expansion− contraction mode of the entire [TFSA]− anion), based on the salt concentration. For the spectroscopic analysis, the spectra were deconvoluted by Voigt function with suitable Gaussian−Lorentzian ratio. Computational Methods. The Gaussian 09 program45 was used for the quantum chemical calculations. The basis sets implemented in the Gaussian program were used. The geometries of the complexes and glyme monomer were fully optimized at the HF/6-311G** level. The geometries of the complexes in the crystals were used as the initial geometries for the geometry optimizations. The Cartesian coordinates of the optimized geometries are provided in the Supporting Information (Table S2). The intermolecular interaction energies (Eint) were calculated at the MP2/6-311G**//HF/ 6-311G** level by the supermolecule method.46,47 The basis set superposition error (BSSE)48 was corrected for all the interaction energy calculations using the counterpoise method.49 Our previous calculations of the [emim][BF4] ([emim]: 1-ethyl-3-methylimidazolium) and Li[TFSA] complexes50,51 show that the basis set effects on the calculated interaction energies of the complexes are very small if basis sets including polarization functions are used and that the effects of electron correlation beyond MP2 are negligible. Therefore, we calculated the interaction energies of the complexes at the MP2/6-311G** level in this work. The deformation energy (Edef), which is the sum of the increase of energies of glyme and the [TFSA]− anion by deformation of the geometries associated with the complex formation,50 was calculated at the MP2/6-311G** level. The electrostatic and induction energies were calculated using ORIENT version 3.2.52 The electrostatic energy of the complex was calculated as interactions between distributed multipoles of ions. Distributed multipoles,53,54 up to hexadecapole, on all atoms were obtained from the MP2/6-311G** wave functions of an isolated species using the GDMA program.55 The induction energy was calculated as interactions of polarizable sites with the electric field produced by the distributed multipoles of monomers.56 The atomic polarizabilities of carbon (α = 10 au), nitrogen (α = 8 au), oxygen (α = 6 au), fluorine (α = 3 au), and sulfur (α = 20 au) were used for the calculations.57 Distributed multipoles were used only to estimate the electrostatic and induction energies. Vibrational frequencies and Raman activities were calculated by the density functional method using the B3LYP functional58 and 6-311+G** basis set.

Figure 1. Concentration dependencies of ionic conductivity (σ) and viscosity (η) for the binary mixtures of Mg[TFSA]2 and G4 at 30 °C; c is the molar concentration of Mg[TFSA]2, and n is the molar ratio of Mg[TFSA]2:G4 as 1:n.

ionic conductivity of the Mg[TFSA]2/G4 electrolytes increases as the molar concentration increases until a maximum is observed at 0.55 M; however, a further increase in concentration results in a decrease in the conductivity. The viscosity of solution gradually increases as increasing the concentration. The decrease in ionic conductivity at higher than 0.55 M is due to the trade-off between mobility and number of charged species. The highest ionic conductivity of the present system is 1.3 mS cm−1, which is lower than those of Mg[TFSA]2/G1 or G2 electrolytes, possibly due to relatively higher viscosity of G4 than those of G1 and G2.59 The present system also possesses lower conductivity compared to the Li[TFSA]/G4 electrolytes (e.g., 2.1 mS cm−1 at 0.5 M and 3.2 mS cm−1 at 1 M) even at the same salt concentration,15 despite involving divalent ions. This would be caused by the relatively poor dissociativity of Mg[TFSA]2 in the electrolytes. Walden plots, which are well-established as a measure of salt dissociativity in solutions based on the deviation from the ideal line of KCl aqueous solution,60 clearly illustrate much associative nature of Mg[TFSA]2 in G4 than that of Li[TFSA] (Figure S1). Strong interionic interactions arising from strong electric field around Mg2+ ions is a most likely reason (vide inf ra). For the conventional nonaqueous electrolytes containing monovalent salts, the conductivity maxima appear at around 1 M, almost irrespective of the solvent and salt choices. The conductivity maxima are indeed found at around 1 M for the monovalent counterparts, such as Li[TFSA]/G4 and Na[TFSA]/G5 (G5: pentaglyme).14,15 The maximum for the Mg[TFSA]2/G4 electrolytes, however, was observed at a relatively low concentration, ca. 0.5 M. Strong electrostatic interactions due to divalent Mg2+ ions could affect the mobility of charged species. In addition, dissociation of Mg[TFSA]2 in the electrolytes could release divalent Mg2+ cation and two distinct [TFSA]− anions as charge carriers. These complicated features arising from incorporation of divalent Mg2+ ions probably cause characteristic ion transport behavior, consequently leading to the maximum conductivity at a lower concentration. The reversible electrodeposition/dissolution of Mg metal in the glyme- and ionic liquid-based electrolytes are reported elsewhere.37−40,59 A similar result was expected for our system; however, only cathodic current attributable to the deposition of Mg metals is observed at 0.7 V vs Li/Li+, and no anodic

3. RESULTS AND DISCUSSION Ionic Conductivity and Cyclic Voltammetry. Figure 1 shows the ionic conductivity of G4 and Mg[TFSA]2 binary mixtures as a function of the molar concentration of Mg[TFSA]2 in the electrolytes. Because of the precipitation of solid materials in the relatively concentrated electrolytes (>0.65 M) at room temperature, such concentrated mixtures are excluded from the discussion. As shown in Figure 1, the C


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Figure 2. Cyclic voltammetry of 0.5 M Mg[TFSA]2/G4 at 30 °C with Pt as the working and counter electrodes and Li metal soaked in 1 M Li[TFSA]/G4 as the reference electrode at a scan rate of 1 mV s−1. (a) As-prepared electrolyte and (b) electrolyte treated with activated molecular sieves.

response is present during the reverse scan in the as-prepared 0.5 M Mg[TFSA]2/G4 electrolyte (Figure 2a). This irreversible behavior is caused by passivation of the deposited Mg surface due to the presence of water impurities in the electrolytes.61 The water content in the bath electrolyte indeed increases after cycling from 52.8 to 226.5 ppm. Although we prepared the bath electrolyte under a dry atmosphere, Mg[TFSA]2 salt itself is prone to crystallize as a hexahydrate, and complete removal of those hydrated water molecules is extremely difficult to accomplish.62 To address this problem, we used activated molecular sieves as a drying agent during cycling. This treatment works well to achieve reversible deposition/ dissolution of Mg metals, as shown in Figure 2b. An anodic current corresponding to the dissolution of the deposited Mg metals is observed at 1.2 V vs Li/Li+ in the extra-dried electrolyte. The water content of the extra-dried electrolyte after cycling was 98.5 ppm, which was much lower than that of the untreated electrolyte. These results clearly indicate that water impurity has a significant impact on the reversibility of Mg electrodeposition/dissolution, and the reversibility can be improved by controlling the water content in the bath electrolytes. Phase Behavior of G4−Mg[TFSA]2 Binary Mixtures. To understand the complexation behavior of Mg[TFSA]2/G4 binary mixtures, the phase diagram was investigated by DSC (thermograms are shown in Figure S2) and summarized in Figure 3. Comparable to the reported diagrams of various glyme−alkali metal salt binary mixtures,16−18,20−25 this diagram clearly indicates complexation of G4 and Mg[TFSA]2 in a 1:1 molar ratio upon mixing. The Tm observed at around −30 °C for the mixture of a mole fraction x < 0.5 is attributed to the melting of uncoordinated G4 in the mixture. The Tm of the G4−Mg[TFSA]2 complex is clearly observed for the mixtures of x ≥ 0.29 (Figure S2) and shows the highest Tm at x = 0.5. The Tm of [Mg(G4)][TFSA]2 (137 °C) is much higher than those of glyme−M[TFSA] complexes involving monovalent alkali-metal ions ( 1) species, owing to their relatively strong solvation ability, thereby weakening the Mg2+−[TFSA]− interaction and facilitating salt dissociation. Observation of a broad shoulder located at relatively lower frequency side than 890 cm−1 for the 0.5 M Mg[TFSA]2/G4 solution (Figure 7) also supports the presence of various structures other than [Mg(G4)]2+, most possibly [Mg(G4)2]2+.82,86 The representative anionic Raman band is observed for 0.5 M Mg[TFSA]2/G4 solution at 741 cm−1 (Figure 7), and this frequency is lower than those for the crystal (744 cm−1) and molten state (743 cm−1) of [Mg(G4)][TFSA]2 (Figure 6). As mentioned before, SSIP or free [TFSA] − anions are characterized at 742−739 cm−1.18,27,66 Spectral analysis of 0.5 M Mg[TFSA]2/G4 suggests that [TFSA]− anions mainly exist

Table 1. Tds and ΔTds of Pure G4 and [M(G4)][TFSA]m Incorporating Li+, Na+, and Mg2+ Ions as a Coordination Center G4 [Li(G4)][TFSA] [Na(G4)][TFSA] [Mg(G4)][TFSA]2

Tda/°C

ΔTd/°C

134.1 205.3 212.1 265.9

0 71.2 78.0 131.8

a

Td is defined at the temperature where 5% loss of weight was observed in the TG curves. G


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The Journal of Physical Chemistry C Table 2. Interaction Energies (kcal mol−1) of G4 with Mm+ (M = Li, Na, and Mg) +

[Li(G4)] [Na(G4)]+ [Mg(G4)]2+

Einta

Edefb

Eformc

Eesd

Einde

Eotherf

−119.7 −84.7 −312.0

12.0 7.6 25.1

−107.7 −77.1 −286.9

−103.2 −74.4 −216.9

−76.0 −43.4 −290.7

59.4 33.1 195.6

a

BSSE corrected interaction energies calculated for complexes at the MP2/6-311G** level. bThe increase of the energy of glyme by the deformation of geometry associated with the complex formation. See text. cStabilization energy by the formation of complex from isolated ions. Sum of Eint and Edef. dElectrostatic energy. See text. eInduction energy. See text. fEother = Eint − Ees − Eind. Eother is mainly exchange-repulsion energy.

Table 3. Interaction Energies (kcal mol−1) among [M(G4)]m+ Complexes and [TFSA]− Anions (M = Li, Na, and Mg)

dependence of its magnitude by each metal species. In particular, inclusion of Mg2+ ions as the center metal drastically improves the thermal stability by 130 °C from pure G4. A relatively small divalent Mg2+ ion possesses strong electrostatic and induction interactions compared to monovalent Li+ and Na+ ions. The stabilization energies by the formation of [Mg(G4)]2+, [Mg(G4)][TFSA]2, and Mg[TFSA]2 complexes from isolated species were evaluated by quantum chemical calculations. As the desolvation and subsequent evaporation of G4 correspond to the thermal degradation process of [M(G4)][TFSA]m, the stabilization energy should be well-correlated with the thermal stability of such complexes. The stabilization energy by the formation of complexes from isolated components (Eform) can be calculated as follows:36 Eform = E int + Edef

[Li(G4)][TFSA] [Na(G4)][TFSA] [Mg(G4)][TFSA]2

Einta

Eesb

Eindc

Eotherd

−81.8 −79.8 −280.8

−74.1 −78.0 −265.2

−12.6 −14.2 −48.0

5.0 12.4 32.4

a

BSSE corrected interaction energies calculated at the MP2/6-311G** level. bElectrostatic energy. See text. cInduction energy of glyme. See text. dEother = Eint − Ees − Eind. Eother is mainly exchange-repulsion energy.

anion (ca. −140 kcal mol−1) is still substantially larger than the Eint calculated for the [Li(G4)]+ and [Na(G4)]+ complexes. Definitely the strong electrostatic interaction (Ees) between the divalent Mg2+ cation and [TFSA]− anion is responsible for the large Eint. The desolvation energy of G4 from the [M(G4)][TFSA]m complex (ΔEform), which is the difference between the stabilization energies (Eform) by the formation of the [M(G4)][TFSA]m and M[TFSA]m complexes, should correlate with the Td, as the parent salts M[TFSA]m would yield through thermal degradation of [M(G4)][TFSA]m (vide supra). The Eform for the [M(G4)][TFSA]m complexes and parent salts M[TFSA]m, calculated using eq 1, are shown in Table 4. The ΔEform values

(1)

where Eint and Edef are the intermolecular interaction energy and the deformation energy, respectively. Edef is the sum of the increase of the energies of glyme and the anion by deformation of the geometries associated with the complex formation. The Eform calculated for the [Mg(G4)]2+ complex are compared with those calculated for the [Li(G4)]+ and [Na(G4)]+ complexes, as shown in Table 2. The contributions of the electrostatic interaction (Ees), induction interaction (Eind), and other intermolecular interactions (Eother = Eint − Ees − Eind, mainly due to exchange repulsion) are also shown. As expected, the complexes are stabilized significantly by strong attraction between the metal ion and glyme, irrespective of the metal ion species. For the monovalent complexes, the attraction between Na+ and G4 is substantially weaker than that between Li+ and G4, as Ees and Eind are smaller (less negative) owing to the longer distance between the metal ion and oxygen atoms of glyme. An exceptionally large stabilization energy was calculated for the [Mg(G4)]2+ complex. The induction energy is proportional to the square of the electric field. Therefore, the induction energy induced by the electric field of the divalent cation is 4 times as large as the induction energy of monovalent cations, if the intermolecular distance is identical. Indeed, the induction energy in the [Mg(G4)]2+ complex is considerably larger than those in [Li(G4)]+ and [Na(G4)]+ complexes, as shown in Table 2. This remarkably large induction energy is apparently the major source of the exceptionally large Eform for the [Mg(G4)]2+ complex, which is in good agreement with the large ΔTd of [Mg(G4)][TFSA]2. The interaction energies (Eint) among the [Mg(G4)]2+ complex and two [TFSA]− anions are compared with the Eint of the [Li(G4)]+ and [Na(G4)]+ complexes with [TFSA]− anions, as summarized in Table 3. Although the Eint calculated for the [Li(G4)]+ and [Na(G4)]+ complexes are comparable (ca. −80 kcal mol−1) irrespective of the ionic radii, the Eint calculated for the [Mg(G4)]2+ complex (ca. −280 kcal mol−1) is significantly large. The interaction energy for one [TFSA]−

Table 4. Stabilization Energies (kcal mol−1) for [M(G4)][TFSA]m and M[TFSA]m Complexes Eforma M

[M(G4)][TFSA]m

M[TFSA]m

ΔEformb

Li Na Mg

−177.7 −149.1 −543.5

−137.2 −110.5 −490.4

40.5 38.6 53.1

a

Stabilization energy by the formation of complexes from isolated species. Sum of Eint and Edef. bThe difference between the Eform values for the [M(G4)][TFSA]m and M[TFSA]m complexes. The ΔEform corresponds to the energy for removing G4 from the [M(G4)][TFSA]m complex.

calculated for the [Li(G4)][TFSA] and [Na(G4)][TFSA] complexes are close, as well as the experimental ΔTd values (Table 1), although ΔEform and ΔTd display slightly different tendencies. This discrepancy is partly due to the computational conditions used. The ΔEform was evaluated based on the calculations of isolated complexes. Difference of activation energies of a desolvation process in the [Li(G4)][TFSA] and [Na(G4)][TFSA] crystals also contributes to the discrepancy for ΔEform and ΔTd found in [Li(G4)][TFSA] and [Na(G4)][TFSA], as the Eforms of both parent salts and complexes were calculated based on the specific static states while the thermal degradation is dynamic process. On the other hand, the ΔEform of the [Mg(G4)][TFSA]2 complex is obviously larger than H


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mixed with the [M(G4)][TFSA]m complexes to assess the oxidative stability of [M(G4)][TFSA]m in liquid electrolytes. For comparison, a mixture of G4/[P13][TFSA] (without metal cations) was also examined. The LSV curves are shown in Figure 8. In the case of 0.3 M G4/[P13][TFSA], the anodic

those of the [Li(G4)][TFSA] and [Na(G4)][TFSA] complexes, indicating the substantially strong attractive interaction of G4 with Mg[TFSA]2. Altogether, the very strong attraction between G4 and Mg[TFSA]2, mainly arising from induction interactions induced by the divalent Mg2+ cation, is the most likely reason for the exceptionally high thermal stability of [Mg(G4)][TFSA]2. Electrochemical Stability. The complexation of glymes with metal ions also affects the HOMO energy levels of glymes due to polarization induced by the strong electric field around metal ions. Variation in the HOMO energy levels of glymes upon complexation was estimated by ab initio molecular orbital calculations. The HOMO energy level of isolated G4 and those of G4 in the [M(G4)]m+ and [M(G4)][TFSA]m complexes are tabulated in Table 5. The values for isolated G4 (all trans), Table 5. HOMO Energy Levels of G4 in the Equimolar Complexes with Different Coordination Centers, Calculated by HF/6-311** Level ab Initio Molecular Orbital Calculations G4 (all trans) [Li(G4)]+ [Li(G4)][TFSA] [Na(G4)]+ [Na(G4)][TFSA] [Mg(G4)]2+ [Mg(G4)][TFSA]2

HOMO/au

HOMO/eV

ΔHOMO/eVa

−0.421 16 −0.556 25 −0.43357 −0.551 42 −0.437 77 −0.713 30 −0.464 62

−11.46 −15.14 −11.80 −15.00 −11.91 −19.41 −12.64

0 −3.68 −0.34 −3.54 −0.45 −7.95 −1.18

Figure 8. LSV curves of 0.3 M [M(G4)][TFSA]m/[P13][TFSA] at a scan rate of 1 mV s−1 at 30 °C.

current rise is observed at an electrode potential of ca. 4.0 V vs Li/Li+, attributable to the electrochemical oxidation of G4. In stark contrast, the anodic limit (ca. 5.0 V vs Li/Li+) of the studied electrolytes (0.3 M [M(G4)][TFSA]m/[P13][TFSA]) is obviously higher than that of 0.3 M G4/[P13][TFSA], irrespective of the metal species. Complexation of G4 with metal ions lowers the HOMO energy levels of G4 due to strong polarization of the oxygen atoms by the strong electric field of the metal ions, consequently resulting in the enhanced oxidative stability of the complexes. This observation also implies the absence of free G4 molecules, even in such electrolytes mixed with [P13][TFSA]. As shown in Figure 8, the anodic limits of the electrolyte are dependent on the centered metal species, in the following order: [Na(G4)][TFSA] ≤ [Li(G4)][TFSA] < [Mg(G4)][TFSA]2. Although the calculated HOMO energy level of [Li(G4)][TFSA] is higher than that of [Na(G4)][TFSA] (Table 5), the anodic limit of [Li(G4)][TFSA] is slightly higher than that of [Na(G4)][TFSA]. This trivial discrepancy between the calculated and experimental results may originate from the structures used in the calculations. The calculated HOMO energy levels are also substantially influenced by the structures (geometries, conformations) used.18,36 Indeed, the deformed G4 molecules, isolated from the complexes, have different HOMO energy levels from all-trans G4 (Table S4). However, the order of ΔHOMOdeformed based on the levels of each deformed G4 for the studied complexes (Table S4) is still unchanged from that of ΔHOMO shown in Table 5. The conformations of coordinated G4 change dynamically in the liquid due to the low-energy barrier among different conformations.36 Therefore, it is extremely difficult to predict the trivial difference in actual stability of the complexes precisely. In contrast, Mg2+ has a prominent effect on the oxidative stability enhancement of glyme, in accordance with our expectation from the calculations. Exceptionally strong electrostatic and induction interactions of Mg2+ ions with G4 highly polarize the coordinated ether oxygen atoms, making the resulting complex electrochemically stable.

ΔHOMO corresponds to the change in the HOMO energy level upon complexation from that of the isolated all-trans G4. a

[Li(G4)]+, [Li(G4)][TFSA], [Na(G4)]+, and [Na(G4)][TFSA] are taken from ref 18. A large reduction of the HOMO energy levels is expected for the Mg-based complex. As shown in Table 5, the complexation with metal ions (Mm+) lowers the HOMO energy levels of G4, irrespective of the coordination center; however, that magnitude is dependent on the centered metal ions. The ion pair formation with the [TFSA]− anion raises the HOMO energy level of G4 because the negative charge of the anion weakens the electric field around the metal ions; the resulting HOMO energy levels in the ion pairs are, however, still much lower than that of isolated G4. The valence of the center metal ion has a strong impact on the HOMO energy levels of G4. The strong electric fields produced by divalent Mg2+ ions strongly polarize glyme and attracts the lone pairs of ether oxygen atoms, thereby lowering the HOMO energy levels of G4 significantly upon complexation. The divalent [Mg(G4)]2+ complex binds with two [TFSA]− anions to balance the total charge in the system. The change of the HOMO energy level of G4 by binding with two [TFSA]− anions is exceptionally large (+6.77 eV). Although the change of the HOMO energy levels associated with the binding of two [TFSA]− anions is significant, the HOMO energy level of G4 in the [Mg(G4)][TFSA]2 complex is still lower than those in the [Li(G4)][TFSA] and [Na(G4)][TFSA] complexes. To confirm the change of the HOMO energy levels revealed by the computational studies, we applied LSV on [M(G4)][TFSA]m (M = Li, Na, or Mg) complexes. [Na(G4)][TFSA] and [Mg(G4)][TFSA]2 are solid at room temperature; therefore, an ionic liquid, N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([P13][TFSA]), was I


Article


4. CONCLUSIONS A series of binary mixtures of Mg[TFSA]2 and G4 with different salt fractions were prepared, and their electrochemical reactivity, complexation behavior, and solvate structure in the solid and liquid states were investigated. Reversible Mg plating and stripping were achievable in the well-dried bath electrolytes. The phase diagram and TG curves on the mixtures clearly illustrate the formation of a stable equimolar [Mg(G4)][TFSA]2 complex (Tm = 137 °C). The aging time-dependent thermal transition behavior of [Mg(G4)][TFSA]2 indicates the presence of two different solid phases: phase II, formed by aging the sample at room temperature, transforms into phase I at 82 °C. X-ray crystallography revealed the solvate structure of [Mg(G4)][TFSA]2 in the crystalline phase II state, where the G4 molecule wraps around the Mg2+ ion in a manner similar to that observed for [Na(G4)]X, to form a complex [Mg(G4)]2+ cation. Two distinct [TFSA]− anions with a cisoid conformation also participate in the Mg2+ coordination, thereby forming a contact ion pair type solvate with 7-fold coordination of Mg2+. The complex structure of [Mg(G4)]2+ is maintained, even in the molten state, as confirmed by Raman spectroscopy. In contrast, the Raman spectral feature of 0.5 M Mg[TFSA]2/G4 suggests the presence of other species, such as [Mg(G4)y]2+ (y > 1) and SSIPs, in the solution. To demonstrate the effect of the divalent Mg2+ ion on stability enhancement, the thermal and electrochemical stabilities of [Mg(G4)][TFSA]2 were compared with those of monovalent Li- and Na-based counterparts, with the help of quantum chemical calculations. An exceptionally large thermal stability improvement of G4 upon complexation with Mg[TFSA]2, ΔTd > 130 °C, was observed. Analysis of the interaction energies clearly suggested that not only the electrostatic interaction but also the induction interaction between Mg2+ and G4 remarkably contributes to the stabilization of the [Mg(G4)]2+ complex. The stabilization energy by the formation of the [Mg(G4)][TFSA]2 complex from isolated components is larger than those for the [Li(G4)][TFSA] and [Na(G4)][TFSA] complexes, much owing to the very strong attraction between G4 and Mg2+ induced by the strong electric field of the divalent Mg2+ ion. The anodic limit (oxidative stability) of G4 is enhanced upon complexation with metal ions because of the reduced HOMO energy levels of G4 due to strong polarization by the electric field of metal ions.

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G4 solution (Figure S6), and the complete author list for refs 11, 31, and 45 (PDF) Structure of [Mg(G4)][TFSA]2 (CIF)

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax +81-45-339-3942; e-mail [email protected] (K.D.). Present Addresses

T.M.: Department of Applied Physics, Chalmers University of Technology, SE-412 96, Göteborg, Sweden. K.U.: Department of Applied Molecular Science, Graduate School of Medicine, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan. Author Contributions

S.T. and T.M. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported in part by the MEXT program “Elements Strategy Initiative to Form Core Research Center” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan, the JSPS KAKENHI (No. 15H03874 and No. 15K13815) from the Japan Society for the Promotion of Science (JSPS), and the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). The authors thank Prof. Takeshi Abe (Kyoto University) for kind advice on the dehydration procedure of electrolytes.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09779. Crystallographic data of [Mg(G4)][TFSA]2 (Table S1), Cartesian coordinates of the complexes used in the calculations (Table S2), selected torsion angles of the coordinated G4 for [Mg(G4)][TFSA]2 and [Na(G4)][TFSA] (Table S3), HOMO energy levels of deformed G4 isolated from the different complexes (Table S4), Walden plots for Mg[TFSA]2/G4 and Li[TFSA]/G4 solutions (Figure S1), DSC thermograms of Mg[TFSA]2/G4 binary mixtures (Figure S2), DSC traces of [Mg(G4)][TFSA]2 (Figure S3), PXRD profiles of [Mg(G4)][TFSA]2 (Figure S4), Raman spectra of [Mg(G4)][TFSA]2 (Figure S5), deconvolution result of representative anionic Raman spectrum for Mg[TFSA]2/ J


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