Electrochemical Film Formation on Magnesium Metal in an Ionic

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Electrochemical Film Formation on Magnesium Metal in an Ionic Liquid that Dissolves Metal Triflate and its Application to an Active Material with Anion Charge Carrier Tohru Shiga, Yuichi Kato, and Masae Inoue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10107 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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ACS Applied Materials & Interfaces

Electrochemical Film Formation on Magnesium Metal in an Ionic Liquid that Dissolves Metal Triflate and its Application to an Active Material with Anion Charge Carrier Tohru Shiga*, Yuichi Kato, and Masae Inoue Toyota Central Research & Development Laboratories Inc. Nagakute-city, Aichi-ken, 480-1192 Japan

ABSTRACT. Irregular metallic growth at the anode during recharging of batteries can seriously influence the safety of batteries. To address this problem, we have attempted to design active anode materials with anion charge carriers and recently observed the formation and dissolution of an electrochemical film by triflate anions (CF3SO3-) at the surface of magnesium in an ionic liquid (IL) electrolyte of Mg(CF3SO3)2, which represents a rare anode material. The effect of heterogeneous cations on film formation was examined in this work. In an IL that dissolves NaCF3SO3, sodium ions with a lower reduction potential than Mg2+/Mg would not be expected to assist film formation. However, to our surprise, we discovered that some sodium ions are involved in film formation. The sodium ions are believed to act as a crosslinking point for the formation of a film network, which resulted in fairly good reversibility for film formation. In a Ce(CF3SO3)3-IL electrolyte, an electrochemically-formed film free of Ce3+ was obtained. The 1 Environment ACS Paragon Plus


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trivalent cerium cations were deactivated and transformed to an oxide on Mg metal. However, the reversibility of film formation in the Ce(CF3SO3)3 system did not meet the expected level. By coupling the film formation and dissolution behavior with a V2O5 cathode, a rechargeable battery was fabricated with dual ion transport species of Na+ or Ce3+ for the cathode and CF3SO3- for the anode. The unique battery with NaCF3SO3 is demonstrated to exhibit good discharge/charge performance with long-term cyclability.

1. Introduction Electrochemical deposition/dissolution of metals such as Li, Mg, and Al produces high specific capacities (Li: 2061 Ah/L; Mg: 3832 Ah/L; Al: 8040 Ah/L), and such metals have been studied over the past several decades as good anode materials with which to design post-conventional Li-ion batteries.1-12 However, one problem with these metal anodes is that uncontrolled metallic crystal growth, which appears upon recharging, causes safety issues. Dendritic lithium metal has been observed in some Li batteries.13-15 In the worst case, short-circuiting from dendrite formation can cause venting of the Li cells, fire, or explosion. In Mg batteries, many spherical crystals with micrometer size are generated,16 which can cause a short-circuit between the anode and cathode. Therefore, there has been a strong need for innovative proposals to achieve homogeneous metal electrodeposition.17-19 The use of anions as an alternative charge carrier is one strategy to avoid metallic crystal growth. In our previous paper,20 a new redox-couple separated from magnesium deposition and dissolution was observed in a Mg deposition test using Mg(CF3SO3)2-Nmethyl-N-propyl piperidinium bis(trifluoromethanesulfonyl)amide (PP13+TFSA-). The redox couple was based on the film formation and dissolution associated with CF3SO3-,

2 Environment ACS Paragon Plus

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Mg2+, and (CF3SO2)2N- (TFSA-), which indicated that the negative active material of CF3SO3- was the main ion transport species. A rechargeable anion battery was fabricated with a radical polymer-cathode involving CF3SO3- as the only charge carrier. The first purpose of this study is to investigate the effect of the cation type in the electrolyte on the formation of an electrochemical film on a Mg plate. According to our previous analysis, the film formed on a Mg plate composed of CF3SO3, Mg with a coordination number of three or four, and TFSA. If the origin of Mg in the film is both the Mg plate and the Mg salt in the electrolyte, then some supporting salts, including polyvalent cations, are expected to strongly affect the chemical and electrochemical properties of the film. Electrochemical film formation at the surface of the Mg plate was investigated in ionic liquid (IL) electrolytes with dissolved metal triflates, Mn+(CF3SO3)n (Mn+: Na+, or Ce3+). Sodium (Na) is an alkali metal with a reduction potential of –2.71 V vs. the standard hydrogen electrode (SHE). Na has a high coordination structure in some compounds;21 therefore, Na+ may be involved in electrochemical film formation on the Mg plate, despite having a lower reduction potential than Mg2+/Mg (–2.37 V vs. SHE). Cerium (Ce) is a rare metal with a reduction potential of Ce3+/Ce (–2.32 V vs. SHE), which is very close to that of Mg2+/Mg (–2.37 V vs. SHE). When the Mg plate is immersed into a solution of Ce supporting salt, some trivalent Ce3+ ions in the electrolyte will deposit at the surface of the Mg plate. Therefore, a complicated film formation process at the Mg plate is possible. Thus, film formation and dissolution will be considerably influenced by the type of cation in the metal triflate of the IL electrolyte. The results of the first part of this study are expected to provide excellent active materials with an anion charge carrier. The second purpose of this study is to fabricate novel rechargeable batteries using dual



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charge carriers of Mn+ for the cathode, and CF3SO3- for the anode. Li-ion capacitors are well-known as typical dual-carrier devices and operate with intercalation of Li+ for the graphite anode and the adsorption of BF4- for the activated carbon cathode. Dual graphite electrode-cells using Li+ for the anode and BF4- for the cathode have also been investigated for realizing high voltage operation.22,23 More recently, a unique battery based on aluminum-deposition/dissolution at the anode and the intercalation of Al(Cl4)into graphite at the cathode was reported by Li et al.24 The dual-carrier batteries in this work, with charge carriers of Na+ or Ce3+ for the cathode and CF3SO3- for the anode, are very dissimilar to cells so far in terms of the charge carriers, i.e., anion carrier for the anode and cation carrier for the cathode. Finally, we demonstrate that these unique dualcarrier batteries show evidence of being rechargeable. 2. Experimental Section Materials. NaCF3SO3 (Kishida Chemicals, battery grade) and Ce(CF3SO3)3 (Aldrich) were used as supporting salts. The latter salt was dried under vacuum at 190 °C for 15 hours before mixing with ionic liquid. The ionic liquid (N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethane-sulfonyl) amide, DEME+TFSA-, Figure S1 in the supporting information) was available from Kanto Chemicals. Mg film (Nilako Corporation, thickness = 0.2 mm) and Pt film (Tanaka Holdings, thickness = 0.1 mm) were cut off and polished by sand paper (#400) in an argon-filled glove box before use in the electrochemical cell. Vanadium oxide powder (V2O5, Aldrich) was used as a cathode active material. Three-electrode test. A three-electrode cell (Figure S2) was fabricated using a V2O5-carbon sheet as cathode, Mg anode, and an Ag+/Ag reference electrode (BAS, RE-7. Electrolyte solution: an acetonitrile solution containing 0.01M AgNO3 and 0.1M (C4H9)4NClO4). The


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electrolytes were prepared by dissolving NaCF3SO3 or Ce(CF3SO3)3 into the ionic liquid electrolyte (DEME+TFSA-). For the Ce(CF3SO3)3 system, 0.5 mL of 1-butyl-3-methyl imidazolium trifluoromethanesulfonate (Kanto Chemicals) was added to 5 mL of DEME+TFSAelectrolyte to assist in the dissolution of Ce(CF3SO3)3. The concentration of Ce(CF3SO3)3 was 0.05 mol/L. The NaCF3SO3 electrolyte had a concentration of 0.2 mol/L. The V2O5-carbon sheet was prepared by dry-mixing V2O5 powder (Aldrich, 57% by weight), carbon black (Tokai Carbon, TB5500, 33%) and Teflon powder as a binder (Daikin, F-104, 10%). The cathode was prepared by compressing 1.2 mg of the V2O5-carbon sheet with Pt mesh grid (Nilaco Corp.) Prior to the test, the cells were stored in a thermostat-controlled chamber (ESPEC Corp., Compact ultralow temperature chamber) at 60 °C for 8 h to prepare the solid electrolyte interface (SEI). The discharging-charging performance of the cells was followed using a Hokuto Denko charge/discharge instrument (HJ1001SM8A) by applying a 0.0065 mA/cm2 (0.01mA per cathode area) discharge current at 60 °C. When the discharge voltage reached 1.1 V, the current was reversed. The cell drove the charge voltage up to 2.35 V. The charging was continued up to a voltage of design for a new rechargeable battery using dual ion transport species. Cyclic voltammetry. Cyclic voltammetry (CV) for our ionic liquid electrolytes was carried out at 60°C to understand the film formation and deposition/dissolution of Na+ or Ce3+. A beaker cell was fabricated with Mg or Pt working electrode, Pt counter electrode, and an Ag+/Ag reference electrode (BAS, RE-7). Before the CV test, the cell was stored in a 60°C chamber for 10 hours. The CV tests were carried out by using a potentiometer (IVIUM Technologies, IVIUMSTST-XR). The sweep rate was 0.2 mV/sec.



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Surface analysis. Raman spectra were acquired with a JASCO laser Raman spectrophotometer (NRS 3300) to study the formation of the film on the Mg anode before and after discharging. The samples were set in an argon-filled quartz cell. The wavelength of the excitation laser was 532 nm and the spot was 30 µm in diameter. Time-of-flight secondary ion mass spectrometry (TOFSIMS) analysis of the Mg anode before and after discharging, and after charging, was also performed using a TOFSIMS-5 spectrometer (ION-TOF GmbH) over an area of 0.5 mm × 0.5 mm. X-ray photoelectron spectroscopic (XPS) measurement was performed in a ULVAC PHI spectrometer (PHI-5500MC) using a monochromatized Mg Kα radiation and a measurement area of 0.8 mm Φ. The TOF-SIMS and XPS spectrometers were directly connected through transfer chambers to an argon dry box in order to avoid moisture exposure of the samples. Measurement of cell performance. An electrochemical coin cell (Figure S3) was fabricated with a carbon cathode incorporating V2O5 powder. The finished carbon cathode (total weight 10 mg, surface area 1.5386 cm2, thickness 0.12 mm) was then sandwiched between two 80-mesh SUS grids (Nilako Corp., SUS304), along with an Mg anode (16 mm in diameter, 0.2 mm thick), the ionic liquid electrolyte, and three 25-µm-thick polyethylene separators (Tonen Chemical Corp.). The electrolyte of 0.3 mL was transferred into the cell. The measurements of the electrochemical cell performance were made at 60 °C. Before the electrochemical test, the cell was stored in a 60 °C chamber for 20 hours. The discharge and charge cycle was repeated between 0.6 and 2.35 V. The discharging-charging performance of the cells was followed using a Hokuto Denko charge/discharge instrument (HJ1001SM8A) by applying a 15.3 µA (0.01 mA/cm2 cathode) discharge current at 60 °C.


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3. Results and Discussion 3.1 Three-electrode test. A discharge-charge test was conducted using a three-electrode cell with a V2O5 cathode, Mg plate, and Ag+/Ag reference electrode to examine the behavior at the Mg plate in detail. The potential of the Ag+/Ag electrode corresponds to +3.168 V vs. Li+/Li. Figures 1a and 1b show the three-electrode test results, where the blue and red lines represent the cathode and anode potentials, respectively, while the cell voltage is denoted by the black line. For the NaCF3SO3-DEME+TFSA- electrolyte (Fig. 1a), the anode potential was maintained at –1.81 V vs. Ag+/Ag for the discharge process and at –2.02 V for the charge process. The two constant values of the anode potential were higher than the reduction potential of Na+/Na (–2.84 V vs. Ag+/Ag). Therefore, it appeared that no sodium deposition/dissolution took place on the Mg plate. The anode potential suggests that film formation takes place at the Mg plate. The Ce(CF3SO3)3DEME+TFSA- system had a different profile for the anode potential (Fig. 1b). Although the profile changed slightly with time near –1.5 V during the discharge process, the anode potential increased immediately after the changing the polarity of the current, contrary to expectation. This strange phenomenon cannot be explained well, but it may be affected by the deactivation of cerium, i.e., precipitation of Ce2O3 at the surface of the Mg plate (see Fig.7b). The anode potential was higher than the reduction potential of Ce3+/Ce (–2.45 V vs. Ag+/Ag), which indicates that no Ce3+ deposition occurred. A V2O5 powder/carbon black composite was used as a working electrode at the cathode side. The insertion of metal ions such as Li+, Na+, and Mg2+ into vanadium oxide has already been investigated.25-27 In addition, the intercalation of Al3+ into V2O5 has also been reported by Smyrl et al.28 It was thus expected that a trivalent working ion, Ce3+,



may intercalate in the same manner into V2O5 in an IL electrolyte environment. The cathode potential in Fig. 1b reflects interaction between Ce3+ and V2O5, i.e., the insertion and extraction of Ce3+ at the V2O5 active material. Cyclic voltammetry (CV) was measured for the Ce(CF3SO3)3-DEME+TFSA- electrolyte using V2O5 as the working electrode at 60 °C. The CV profile is shown in Fig. S4. An increase in the cathodic current, attributed to Ce3+ insertion, was detected below –0.05 V vs. Ag+/Ag. In the anodic scan, one peak located at +0.31 V corresponds to the de-intercalation of Ce3+ from the V2O5 structure. Thus, the intercalation/de-intercalation of Ce3+ in a crystal of V2O5 occurred in the same manner as Al3+. 3

Potential (V vs. Ag) Cell voltage (V)

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Figure 1. Potential profiles and cell voltage in a three-electrode test using (a) NaCF3SO3-DEME+TFSA-, and (b) Ce(CF3SO3)3-DEME+TFSA- at 60 °C.

To examine the discharge behavior in detail, CV measurements were conducted in a cell using Mg or Pt working electrodes (Mg W.E. or Pt W.E.), Pt plate as a counter electrode, and metal triflate electrolytes of Mn+(CF3SO3)n (Mn+: Na+, or Ce3+) and DEME+TFSA-. Figure 2a shows CV profiles between the 1st and 10th cycles for the cell using Mg or Pt W.E. and NaCF3SO3DEME+TFSA-. The reduction current increased gradually for the Pt W.E. sweep up to –2.6 V vs. Ag+/Ag. When the polarity of the sweep was changed, the current decreased gradually and reached zero. The reduction potential of Na+/Na is –2.84 V vs. Ag+/Ag; therefore, no sodium


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deposition occurred. In the case of the Mg W.E., a shoulder in the anodic scan of the 1st cycle was observed near –1.40 V vs. Ag+/Ag (arrow in Fig. 2a), and the corresponding cathodic wave at the 2nd cycle appeared at –1.95 V vs. Ag+/Ag. This redox-coupling reflects film formation by the anion carriers on the Mg W.E. An increase in current after the shoulder at the 1st cycle was observed. Thus, a small amount of magnesium may possibly be dissolved from the working electrode. The CV curves for the cells with the Mg W.E. or Pt W.E. and Ce(CF3SO3)3DEME+TFSA- are shown in Fig. 2b. The reduction potential of Ce3+ is –2.45 V vs. Ag+/Ag; therefore, the deposition of Ce3+ appears in the CV scans. When Pt was used as the working electrode, a large increase in current attributed to Ce deposition was detected below –2.5 V vs. Ag+/Ag. However, the corresponding signal that would reflect the dissolution of Ce was not observed, which indicates that the deposition behavior of Ce3+ was irreversible. In contrast, a small broad peak appeared in the cathodic scan with Mg W.E. at –2.30 V vs. Ag+/Ag, while a sharp signal appeared at –1.75 V in the anodic process. Therefore, it appears that the decrease in the CV scans at –2.30 V in Fig. 2b may not be caused by Ce deposition.

Figure 2. 1st, 2nd, 5th, 8th, and 10th CV curves for (a) NaCF3SO3-DEME+TFSA- and (b) Ce(CF3SO3)3-DEME+TFSA- at 60 °C using Mg (red) or Pt (black) working electrodes. The cycle numbers of the CV curves are labeled. The scan rate was 1 mV/s. The dotted lines represent the reduction potentials of Na+/Na and Ce3+/Ce.



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3.2 Characterization of the film on Mg plate. Raman spectroscopy was employed to elucidate the discharge behavior in the cell with metal triflate-IL electrolytes by analysis of the Mg plates before and after the 1st discharge. Every discharged Mg plate was covered with a blackish film. Figure 3 shows Raman spectra for the discharged Mg plates between 100 cm-1 and 1400 cm-1. Raman spectra for the reagents NaCF3SO3 powder (pink line, c) and DEME+TFSA- (black line, d) are also plotted in Fig. 3. The SO3 rocking, SO3 bending, symmetric C-S stretching, asymmetric SO3 stretching, and symmetric CF3 stretching vibrations of the CF3SO3 anions appear at 372, 591, 787, 1104, and 1247 cm-1, respectively.29,30 The vibrations of the (CF3SO2)2N anion in DEME+TFSA- are characteristic of asymmetric and symmetric S-N-S stretching, located at 681 and 740 cm-1, respectively.31,32 For the discharged Mg electrode with NaCF3SO3-DEME+TFSA- (green line, a), five distinct signals, which were assigned to the vibration of CF3SO3, were observed. In contrast, the Ce(CF3SO3)3-DEME+TFSA- system had a complicated profile (red line, b). The peak at 403 cm-1 is caused by the rocking and stretching modes of SO2 in TFSA. The weak signal at 551 cm-1 reflects SO3 bending. The signal at 723 cm-1 was included with asymmetric and symmetric S-N-S stretching. The broad signal at 1008 cm-1 was due to the asymmetric stretching of CF3 units. Based on the Raman data, the films are formed by CF3SO3 and (CF3SO2)2N anions. Thus, the interface in Ce(CF3SO3)3DEME+TFSA- at the Mg plate is formed mainly by (CF3SO2)2N anions.


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Figure 3. Raman spectra of the films formed on the Mg plate with the (a) NaCF3SO3-DEME+TFSA- and (b) Ce(CF3SO3)3DEME+TFSA- electrolyte systems at 60 °C. The pink and black lines represent Raman spectra for the reagents NaCF3SO3 and DEME+TFSA-, respectively.

Before and after the three-electrode test, the surface of the Mg electrodes was also analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS). All TOFSIMS spectra are shown in Figs. S5–S7. Figure 4a shows TOF-SIMS spectra for the positive ions from the Mg electrodes before and after discharge, and after charge in the NaCF3SO3-DEME+TFSA- system. The mass range was between m/z 10 and m/z 120. The positive ion signal at m/z 23 is assigned to Na+, and was stronger than the corresponding peak for the sample before discharge. However, the signal disappeared after charging, which suggests that Na+ ions assisted film formation on the Mg anode. For the negative ions, characteristic signals were picked up from Figs. S5 and S7. The signal intensities at m/z 149 (CF3SO3), 280 [(CF3SO2)2N], and 281 (C2F6S2O5) before and after discharge, and after charge are summarized in Fig. 4b. The signal at m/z= 281 was assigned to the dimer of CF3SO3. A comparison of the bar graphs for before and after discharge indicated an enhancement in the intensity of the signal at m/z 149 during discharge. A significant increase in intensity at m/z 281 was also detected during discharge. In contrast, the signal at m/z 280 did not change significantly. The TOF-SIMS results suggest that CF3SO3



anions are strongly involved in the formation of the film at the Mg surface. The formation of a CF3SO3- network is also supported by the presence of Mg2+(CF3SO3-)3, as indicated by the signal at m/z 471 (Fig. S5).

Intensity (counts) Intensity (counts)

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Figure 4. TOF-SIMS positive ion spectra in the range of m/z=10 to 120; (a) positive ion spectra for Mg plate in the NaCF3SO3DEME+TFSA- system, and (b) peak intensities for negative ions at m/z=149, 280, and 281 during the discharge/charge process.

Figure 5 shows TOF-SIMS data for Mg electrodes before and after discharge, and after charge in the Ce(CF3SO3)3-DEME+TFSA- system. For cerium positive ions, the signals for CeO+ and its isotope, which are derived from Ce2O3, were observed at m/z 155.9 and 157.9 (Fig. 5a). The intensities of these signals were greater by a degree of magnitude than those of Ce+ and its isotope (m/z 139.9, 141.9). The signals for CeO+ and its isotope decreased during discharge. The dissolution of Ce2O3 into the electrolyte was not observed; therefore, the decrease in the intensity of both signals was attributed to film formation during discharge. Figure 5b shows a comparison of the negative ion signal intensities at m/z 149 (CF3SO3), 280 [(CF3SO2)2N], and 281 (C2F6S2O5), before and after discharge, and after charge. For the two former signals, no significant difference in intensity was observed between the discharge and charge processes. The intensity at m/z 281, attributed to the dimer of CF3SO3, was half of the intensities of the signals at m/z 149 and m/z 280. Moreover, the signal for Mg2+(CF3SO3-)3 at m/z 471 was almost absent, both


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before and after discharge (Fig. S7). The TOF-SIMS results suggested that the composition of the film at the Mg surface was unchanged after discharge. Therefore, the film on the Mg anode is considered to form mainly from (CF3SO2)2N anions. CF3SO3 anions were applied to film formation as a minor charge carrier. 6

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Figure 5. TOF-SIMS positive ion spectra in the range of m/z=10 to 120; (a) Ce positive ion spectra for Mg plate in the Ce(CF3SO3)3DEME+TFSA- system, and (b) peak intensities for negative ions at m/z=149, 280, and 281 during the discharge/charge process.

The films on Mg electrodes in the NaCF3SO3-DEME+TFSA- system were also investigated using X-ray photoelectron spectroscopy (XPS) to obtain information on the coordination of Na+ and Mg2+. Figures 6a and 6b show Mg 2p and Na 1s XPS spectra, respectively. The samples were Mg electrodes before discharge (black), after discharge (green), and after charge (pink). The XPS line position for Mg 2p photoelectrons is located near 50 eV.33 A shoulder in the spectrum for the sample before discharge at 48.4 eV may be assigned to metallic magnesium. The signal at 50.125 eV is due to Mg in the film. XPS analysis showed that the photoelectron signal of Mg 2p at 50.125 eV was shifted by 0.375 eV to the high energy side during discharge, which may reflect a multi-coordinate structure of Mg ions. The metallic luster of magnesium was observed before discharge, which indicated that the solid-electrolyte interphase (SEI) was very thin. In contrast, the sample after discharge was covered with a blackish film, while the Mg anode after recharging



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appeared grayish, which suggests that both surface films had thicknesses beyond the limit of depth detection for the XPS technique. Therefore, the intensities of the Mg 2p signal in the samples diminished compared to the sample before discharge. The cause of the low intensity for the sample after recharge may reflect the packing or arrangement of residue that could not be decomposed. For Na 1s photoelectrons, the XPS signal was observed at 1071 eV (Fig. 6b). The intensity of the Na 1s signal was increased by discharge, which indicates that Na+ ions were assembled at the surface. The XPS line position for Na 1s was shifted by 1.05 eV to the high energy side during discharge, which implies the possibility of a multi-coordinate structure of Na ions. Thus, the formation of the CF3SO3-associated film on Mg was well supported by the Raman, TOF-SIMS, and XPS experimental data. The shift in the XPS line position for the Mg 2p and Na 1s signals reflects highly coordinated Mg and Na in film formation.

Figure 6. XPS (a) Mg 2p and (b) Na 1s spectra for Mg plates in the NaCF3SO3-DEME+TFSA- system.

3.3 Structural models for the film on Mg metal. Brooks et al. reported a polymeric network structure for the LiCF3SO3-acetonitrile (AN) adduct (see Fig. S8).34 According to X-ray crystal structural analysis, the polymeric network was based upon the cleavage of S=O bonding in CF3SO3 and a highly coordinated structure of Li. The opened S-O- single


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bond contacted with a Li ion, where the Li ion is arranged with a four-coordinated structure as a crosslinking point (Fig. S8). The AN molecule acted as an electron donor to elemental Li. Meffre et al. analyzed the crystal structure of the NaCF3SO3-AN adduct.21 Similar to the role of the MCF3SO3-AN adduct (M: Li or Na), we propose a polymeric networking model for CF3SO3 anions (Fig. 7). For the film in the NaCF3SO3DEME+TFSA- system, CF3SO3- anions first migrate toward and adhere to the Mg anode, some of the adhered CF3SO3- anions cleave the bonding of S=O units, and the opened single bonds react with Na+ or Mg2+. The intervention of Na+ was supported by TOFSIMS analysis. These ions then react with other CF3SO3- or TFSA- anions present in the IL. Na+ acts as one of the crosslinking points in the film. The formation of the polymeric network thus proceeds at the interface with the Mg metal electrode. The simple anodic and cathodic reaction was shown in Figure S9. The role of TFSA- anions in the formation of a network is not yet clear. Finally, the polymeric network cannot be decomposed perfectly, as indicated from the Raman and TOF-SIMS analyses. The observed redox coupling behavior is in part irreversible. For the film formed in the Ce(CF3SO3)3DEME+TFSA- system, Ce3+ was not involved in the film formation. A large amount of CeO+ was detected, which suggests that Ce was present at the surface of Mg as cerium oxide (Ce2O3). Therefore, the formation of a polymeric network involving Mg ions, CF3SO3- and TFSA- anions progresses. TOF-SIMS and Raman analyses indicated that many TFSA- anions are included in the film.



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Figure 7. Proposed model for the polymeric networking of CF3SO3 anions formed at the surface of Mg during discharge in the NaCF3SO3DEME+TFSA- system (left), and the Ce(CF3SO3)3-DEME+TFSA- system (right).

3.4 Cell performance. Finally, coin cells composed of a V2O5 cathode, Mg anode, and NaCF3SO3 or Ce(CF3SO3)3-DEME+TFSA- electrolyte were fabricated. Figures 8a and 8b show discharge-charge curves for the batteries with dual carriers at 60 °C. Both of the Mg anodes with each electrolyte system had no metallic luster after discharge but were covered with the blackish film. The battery with the NaCF3SO3-DEME+TFSA- electrolyte exhibited one step in the cell voltage during the 1st discharge process (Fig. 8a). The discharge plateau was observed near 1.3 V, which is in accordance with the potential difference, as shown in Figs. 3a and S5. The 1st discharge capacity was 145.6 mAh/g, which was determined from the amount of V2O5 in the cathode. This indicated that 0.98 mol of Na+ was intercalated into 1 mol of V2O5. When the polarity of the current was reversed, the cell voltage increased to approximately 1.8 V. The total charge capacity was 148.9 mAh/g and the coulombic efficiency was 102.2%. The cycle performance up to the 100th cycle is shown in Fig. 8c. The discharge and charge capacities decreased slightly with the cycle number. The coulombic efficiency was low, at approximately 97%, which reflects a small amount of leakage current attributable to the coin cell structure. The capacity retention was 75% at the 100th cycle. The battery with Ce(CF3SO3)3-DEME+TFSAelectrolyte also showed one step in the cell voltage during the 1st discharge process (Fig. 8b). The discharge plateau was observed near 1.4 V. The 1st discharge capacity was 71.5 mAh/g. When the polarity of the current was reversed, the cell voltage increased to approximately 2.0 V. The


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total charge capacity was 66.2 mAh/g and the coulombic efficiency was 92.5%. The discharge and charge capacities increased with the cycle number, and reached a maximum at the 6th cycle. The maximum discharge capacity was 108.1 mAh/g. This indicated that 0.244 mol of Ce3+ was intercalated into 1 mol of V2O5. The small amount of inserted ions is due to the large ionic radius and slower diffusion of cerium cations. The cycle performance of the Ce(CF3SO3)3DEME+TFSA- system was thus inferior to the NaCF3SO3-DEME+TFSA- system. The capacity retention at the 80th cycle was 10%. The large capacity fade for the Ce(CF3SO3)3-DEME+TFSAsystem is due to irreversible film formation (see Fig. S10), which leads to a loss of Ce3+ as an ion transport species. Figure 8d shows the discharge-rate performance for the dual-carrier batteries. The batteries functioned under a current on the order of 0.01 mA/cm2, which indicates the poor rate performance caused by slow and irreversible film formation and dissolution at the Mg anode, rather than by the low rate of intercalation at the cathode. 3

3 (a) NaCF3SO3

Charge

Cell voltage (V)

2.5

Cell voltage (V)

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2 1.5 Discharge

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0.5

0.5 5th - 1st 0 0

20

40

60

80

100

120

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140

1st → 5th

0 160

180

0

20

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100

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Figure 8. Cell performance; discharge-charge curves for the 1st to 5th cycles at 60 °C for the cells with the (a) NaCF3SO3 and (b) Ce(CF3SO3)3 electrolyte systems. (c) Cycle performance up to the 100th cycle, and (d) discharge rate properties. Cycle number: 1st (black), 2nd (pink), 3rd (orange), 4th (green), and 5th (blue). The open and solid symbols in Fig. 8c represent the discharge and charge capacities, respectively. The numbers in (d) indicate the current density in mA/cm2.

4. Conclusions The deposition and dissolution behavior of metal electrodes has been investigated extensively over the past a few decades. The Mg deposition test at 60 °C in our previous study revealed an unexpected electrochemical reaction with triflate ions in a particular IL electrolyte. This reaction results in the formation of an active material with an anion charge carrier as the ion transport species, and provides a solution to the issue of metallic dendrite growth in batteries. Electrochemical film formation and dissolution at the surface of Mg plate in an IL electrolyte with metal cations (Na+ or Ce3+) was examined to understand the effect of different cations on film formation. Sodium ions were captured in the film as a crosslinking point of the NaCF3SO3-DEME+TFSA- system, which contributed to the reversibility of film formation. In contrast, cerium ions were deactivated as Ce2O3 and were not involved in film formation in the Ce(CF3SO3)3DEME+TFSA- electrolyte. The electrochemical film formation/dissolution behavior was reversible; therefore, these anode materials were combined with a cathode material that can insert and extract Na+ or Ce3+. It was demonstrated for the first time that cells with dual charge carriers can be recharged and have potential toward the realization of an extremely safe battery.


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■ ASSOCIATE CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Test apparatus, Typical CV profiles, TOFSIMS spectra, Photographs of Mg plates before and after discharge, and after charge. ■ AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Tel：+81-561-71-7607 ■ NOTES The authors declare no competing financial interest. ■ Author Contributions T.S.

conceived

and

carried

out

the

experiments, analyzed the data and wrote the paper, Y.K and M.I made Raman and TOFSIMS analysis in this work.

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Chiang, Y-M.; Cui, Y. The Synergetic Effect of Lithium Polysulfide and Lithium Nitrate to Prevent Lithium Dendrite Growth, Nature Comm., 2015, 6, 7436-7443. (20) Shiga, T.; Kato, Y.; Inoue, M.; Takahashi, T.; Hase, Y. Anode Material Associated with Polymeric Networking of Triflate Ions Formed on Mg. J. Phys. Chem. C 2015, 119, 3488-3494. (21) Meffre, A.; Barboiu, M.; Lee, van der. Acta Crystallographica Section E, 2002, E63, m255-m257. (22) Seel, J.A.; Dahn, J.R. Electrochemical Intercalation of PF6 into Graphite, J. Electrochem. Soc. 2000, 147, 892898. (23) Placke, T.; Fromm, O.; Lux, S.F.; Bieker, P.; Rothermel, S.; Meyer, H.-W.; Passerini, S.; Winter, M. J. Electrochem. Soc. 2012, 159, A1755-A1765. (24) Lin, M.-C.; Gong, M.; Lu, B.; Wu, Y.; Wang, D.-Y.; Guan, M.; Angell, M.; Chen, C.; Yang, J.; Hwang, B.-J.; Dai, H. An Ultrafast rechargeable Aluminum-Ion Battery, Nature, 2015, 520, 325-331. (25) Ng, S-H.; Patey, T.J.; Buchel, R.; Krumeich, F.; Wang, J-Z.; Liu, H-K.; Pratsinis, S.E.; Novak, P. Flame Sprayed-Pyrolyzed Vanadium Oxide Nanoparticles for Lithium Battery Cathodes. Phys. Chem. Chem. Phys. 2009, 11, 3748-3755. (26) Yamada, H.; Tagawa, K.; Komatsu, M.; Moriguchi, I.; Kudo, T. High Power Battery Electrodes Using Nanoporous V2O5/Carbon Composites. J. Phys. Chem. C. 2007, 111, 8397-8402. (27) Gershinsky, G.; Yoo, H.D.; Gofer, Y.; Aurbach, D. Electrochemical and Spectroscopic Analysis of Mg2+ Intercalation into Thin Film Electrodes of Layered Oxides: V2O5 and MoO3. Langmuir, 2013, 29, 1096410972. (28) Le, D.B.; Passerini, S.; Coustier, F.; Guo, J.;

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Soderstrom, T.; Owens, B.B.; Smyrl, W.H. Intercalation of Polyvalent Cations into V2O5 Aerogel. Chem. Mater. 1998, 10, 682-684. (29) Gejji, S. P.; Hermansson, K.; Lindgren, J. Ab Initio Vibrational Frequencies of Trfilate Ion, (CF3SO3)-. J. Phys. Chem. 1993, 97, 3712-3715. (30) Rey, I.; Johansson, P.: Lindgren, J.; Lassegues, J. C.; Grondin, J.; Servant, L. Spectroscopic and Theoretical Study of (CF3SO2)2N- (TFSI-) and (CF3SO2)2NH (HTFSI). J. Phys. Chem. A 1998, 102, 3249-3258. (31) Castriota, M.; Caruso, T.; Agostino, R. G.; Cazzanelli, E.; Henderson, W. A.; Passerini, S. Raman Investigation of the Ionic Liquid N-Methyl-N-propylpyrrodinium Bis(trifluoromethanesulfonyl)imide and Its Mixture with LiN(SO2CF3)2. J. Phys. Chem. A 2005, 109, 92-96. (32) Umebayashi, Y.; Mitsugi, T.; Fukuda, S.; Fujimori, T.; Fujii, K.; Kanzaki, R.; Takeuchi, M.; Ishiguro, S-I. Lithium Ion Solvation in Room-Temperature Ionic Liquids Involving Bis(trifluoromethanesulfonyl) Imide Anion Studied by Raman Spectroscopy and DFT Calculations. J. Phys. Chem. B 2007, 111, 13028-13032. (33) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bombe, K. D., Handbook of X-ray Photoelectron Spectroscopy, 1992, Perkin-Elmer Corporation. (34) Brook, N.R.; Henderson, W. A.; Smyrl, W.H Acta Crystallographica Section E, 2002, E58, m176m177.

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Electrochemical Film Formation on Magnesium Metal in an Ionic

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