Anode Material Associated with Polymeric Networking of Triflate Ions

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An Anode Material Associated with Polymeric Networking of Triflate Ions Formed on Mg Tohru Shiga, Yuichi Kato, Masae Inoue, Naoko Takahashi, and Yoko Hase J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5114015 • Publication Date (Web): 21 Jan 2015 Downloaded from http://pubs.acs.org on February 2, 2015

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The Journal of Physical Chemistry

An Anode Material Associated with Polymeric Networking of Triflate Ions Formed on Mg

Tohru Shiga*, Yuichi Kato, Masae Inoue, Naoko Takahashi, and Yoko Hase Toyota Central Research & Development Laboratories Inc. Nagakute-city, Aichi-ken, 480-1192 Japan Supporting Information ABSTRACT: We have examined anode materials with anions as an ion transport species to solve metal deposition in rechargeable batteries intrinsically. Mg deposition-dissolution tests were conducted using some ionic liquid electrolytes at 60 °C, and a new electrochemical reaction was observed in addition to Mg deposition and dissolution in an electrolyte of Mg(CF 3SO3 )2 and N-methyl-N-propylpiperidinium bis(trifluoromethane sulfonyl)amide. The reaction was determined to be based on the formation and release of a polymeric network of triflate ions (CF3SO 3-) on the Mg metal surface, which suggests a novel anode material with anion carriers. An anion battery is also demonstrated using this phenomenon and incorporating an acrylate polymer with 2,2,6,6-tetramethylpiperidine-oxyl side units in the cathode to provide evidence of the rechargeability by the intermediary anion carrier.

Keywords: Mg anode, Anion carrier, Ionic liquid, Radical polymer,

1. Introducti on Magnesium has a higher volumetric capacity of 3832 mAh/mL than lithium (Li:2062 mAh/mL)1,2 and is an abundant earth metal (Clarke number; Mg:1.93, Li:0.006), which makes it the ideal anode for the next-generation secondary cells: however, magnesium deposition and its crystal growth appear upon recharging, which degrades battery performance.3,4 Although the Mg crystal deposited is not dendritic,5,6 it may probably lead to short circuits. Thus, a breakthrough for the smoothing of the electrodeposits is required to keep high safety battery. To address metal crystal growth intrinsically, the use of anions as ion transport species is an alternative approach. For anion batteries, Fitcher et al. reported a rechargeable fluoride ion-battery with a solid fluoride electrolyte 7 that was operated at 150 °C. The same group has also designed chloride-ion batteries using metal oxychlorides such as FeOCl, VOCl2 , and BiOCl as cathode materials, which have demonstrated good cycling performance at 25 °C.8,9 In addition,Sekiguchi and Nakano have proposed pseudocapacitors that used stable persilyl-substituted radicals based on the heavy group 14 elements, (tBu 2 MeSi)3 Si, (tBu: tert-butyl, Me; methyl) as an anode material.10 We have previously proposed a catalytic cycle that employs a mediator to obtain a secondary Mg-O2 battery.11 As

the blocking layer was formed on a Mg anode in aprotic solvents,12-14 an ionic liquid electrolyte was selected for the MgO2 battery to undergo repeatable deposition and dissolution of Mg at the anode. For ionic liquid electrolytes, NuLi et al. have succeeded in the reversible deposition and dissolution of Mg on a Cu plate at 25 °C using 1 mol/ L Mg(CF3 SO3 )2 /ionic liquid (1-butyl-3-methyl-imdazo r iu m tetrafluoroborate, BMImBF4 ).15 However, Aurbach et al. pointed out the lack of an anodic process attributed to mag nesium dissolution in the BMImBF4 media, but instead indicated the irreversible nature of Mg deposition.16 Therefore, in this work, we have conducted a Mg deposition test using various ionic liquids at 60 °C. Here we report a new redo x couple separate from the Mg deposition and dissolution processes, which suggests the potential of Mg as an anode material. The redox couple has been observed in 0.1 mol/ L Mg(CF3 SO3 )2 /N-methyl-N-propylpiperidin iu m bis(trifluo romethanesulfonyl)amide (PP13TFSA) at 60 °C and is associated with the polymeric networking of CF3 SO3 - and Mg 2+. Consequently, we have fabricated a rechargeable anion battery by combining this redox couple as an anode and an acrylate polymer with 2,2,6,6-tetramethylpiperidine-o xy l (TEMPO) units (PTMA +CF3 SO3 - in Figure 1c of the Supporting Information, SI) as a cathode.

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2. Experimetal Section 2.1 Materials. 1-butyl-3-methylimidazo liu m tetrafluoroborate (BMImBF4 ) and N-methyl-N-propylpiperidin iu m bis(trifluoromethanesulfonyl) amide (PP13TFSA) were available from Kanto Chemicals. Radical polymer-anio n complex (PTMA +CF3 SO3 -) was prepared by charging the carbon sheet composed of PTMA (49% by weight), Ketjen black (Mitsubishi Chemicals, ECP-600JP, 36%) and Teflon powder as a binder (Daikin, F-104, 15%). The PTMA was synthesized by the radical polymerization of 2,2,6,6-tet ramethylpiperidine methacrylate monomer with 2,2'-azo b isisobutyronitrile, followed by oxidation with 3-chloropero xybenzoic acid. The molecular weight of the resulting PTMA was 23,800. 2.2 Mg deposition-dissolution test. Electrochemical cells with a Mg plate (Nilako, 16mm in diameter, 0.25mm thickness), a Pt plate (Tanaka Holdings , 14mm in diameter, 0.2 mm thickness), and an electrolyte solution (0.1mo l/ L Mg(CF3 SO3 )2 -Ionic liquid) were fabricated using a coin cell (SI Figure 2a) under argon. One 200-µm-th ick paper sheet (Kiriyama Corp.) was used as a separator. Before the electrochemical test, the cell was stored in a 60℃ chamber for 20 hours. The discharging-charging behavior of the cells was followed using a Hokuto Denko charge/discharge instrumentation (HJ1001SM8A), applying a 10μA (6.5μA/cm2 Pt cathode) discharge current at 60 °C. When the discharge time reached 60 minutes, the current was reversed. The charging was continued up to a voltage of 2.0V with a current of 10μA (6.5μA/cm2 ). The Mg deposition-dissolution test was also carried out by a beaker cell with three electrodes (see SI Figure 2b). The Ag +/Ag electrode (BAS, RE7) was used as a reference electrode. For 0.1mo l/ L Mg(CF3 SO3 )2 -PP13TFSA system, we have tested cells using two separations between Mg and Pt (2mm or 5mm) to estimate discharge capacity. The test was finished at the cell voltage of 0V. 2.3 Cyclic voltammetry (CV). The CV measurement for our ionic liquid electrolytes was carried out at 60℃ by using a beaker cell of Pt working electrode, Pt counter electrode, and a Ag +/Ag reference electrode (BAS, RE-7). The electrolytes were a 0.1mol/L Mg(CF33 O3 )2 -PP13TFSA and a 0.1mol/L Mg(CF33 O3 )2 -BMImBF4 . The scanning rate was 1mV/sec. 2.4 Surface analysis. Raman spectra were acquired with a JASCO laser Raman spectrophotometer (NRS 3300) to study the formation of the solid-electrolyte interface (SEI) on the Mg anode before and after discharging, and after charging. 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 (TOF-SIMS) analysis of the Mg anode and Pt cathode 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 per-

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formed in a ULVAC PHI spectrometer (PHI-5500M C) using a monochromatized Mg Kα radiation and a measurement area of 800 μm 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. 2.5 Measurement of cell performance. Electrochemical coin cells were fabricated with a carbon cathode incorporating PTMA +CF3 SO3 - sandwiched between two SUS mesh grids, along with a Mg anode (18 mm in diameter, 0.2 mm thick) and an electrolyte solution composed of 0.1 mol/ L Mg(CF3 SO3 )2 in an ionic liquid (PP13TFSA). The electrolyte solution was prepared by mixing Mg(CF3 SO3 )2 , (Aldrich) with PP13TFSA, respectively, and 0.3 mL of the solution was transferred into the cell. The measurements of the electrochemical cell performance was made at 60 ℃. The discharge and charge cycle was repeated between 1.10V and 2.35V. Before the electrochemical test, the cell was stored in a 60 ℃ chamber for 20 hours. The cathode containing PTMA +CF3 SO3 - was prepared by charging the carbon sheet composed of PTMA (49% by weight), Ketjen black (Mitsubishi Chemicals, ECP-600JP, 36%) and Teflon powder as a binder (Daikin, F-104, 15%). The finished carbon cathode (total weight 4 mg, surface area 0.49 cm2 , thickness 0.12 mm) was then sandwiched between two 80-mesh SUS grids (Nilako). The charging was performed by a Li anode, 0.5mol/L LiCF3 SO3 –PP13TFSA electrolyte, and three 25µm-thick polyethylene separators (Tonen Chemical Corp.). The charging trials were performed with a Hokuto Denko potentiometer (HJ1001SM8A), applying a current density of 2.04 μA/cm2 at 25 °C. 2.6 Three-electrodes test. Three-electrodes test was carried out at 60℃ by using a beaker cell of Mg anode, a carbon cathode employing PTMA +CF3 SO3 -, an Ag +/Ag reference electrode (BAS, RE-7), and a 0.1mol/L Mg(CF3 O3 )2 PP13TFSA electrolyte. 3. Results and Discussion 3.1 Mg deposition test and CV measurement. The Mg deposition-dissolution test was conducted in ionic liquid electrolytes at 60 °C using a coin cell with Mg and Pt electrodes. Prior to the deposition test, the coin cells were stored in a thermostat-controlled chamber at 60 °C for 20 h to prepare the solid electrolyte interface (SEI). Figure 1 shows discharge-charge voltage-profiles for coin cells with Mg(CF3 SO3 )2 as a supporting salt and PP13TFSA or BMImBF4 as a solvent. When discharge started, the cell voltage for the BMImBF4 system (blue line) immediat ely dropped below 0 V, which suggests that the deposition of Mg to the Pt electrode had begun. When the polarity of the current was changed, the voltage increased and moved to greater than 0 V, which indicated the possibility of Mg dissolution from the Pt electrode. In contrast, the PP13TFSA system had an unconventional curve (red line). When the test started, the voltage remained constant at around +0.65 V and did not diminish below 0 V. After the change of polarity, the voltage increased with time gradually up to 2.2 V, which


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suggested that there was no deposition of Mg on the Pt electrode in the PP13TFSA system.

CV measurement. The CV signal at 1.47V may reflect that SEI formation was in a transition period. 0.3

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Figure 1. Discharge-charge curves for Mg-Pt cells using ionic liquid electrolytes of 0.1 mol/L Mg(CF3 SO3 )2 at 60 °C; BMImBF4 (blue) and PP13TFSA (red). Cyclic voltammetry (CV) was measured for the Mg(CF3 SO3 )2 -PP13TFSA electrolyte system to elucidate the phenomenon observed in Fig. 1. A three-electrode cell was fabricated using two Pt plates as working and counter electrodes, and an Ag +/Ag reference electrode. Thus, the source of Mg was limited to only the electrolyte. Figure 2a shows the CV curves for the BMImBF4 electrolyte at 60℃. At the 1st cycle, the cathodic current attributed to Mg deposition was detected near 0 V vs. Mg 2+/Mg. The corresponding signal that reflects the dissolution of Mg was observed at +1.72 V. The large IR drop for the Mg dissolution in aprotic solvents was reported also by Orikasa et al. They detected the signal due to the Mg dissolution in Mg(TFSA) 2 -trigly me near 1.4V at 100℃.17 For the PP13TFSA electrolyte, the wave current attributed to Mg deposition was detected below 0 V vs. Mg 2+/Mg at the 1st cycle (Figure 2b). The corresponding signal that reflects the dissolution of Mg, as can be seen in the BMImBF4 electrolyte at 1.72 V, was not clear. However, a new signal appeared at around +1.47 V. The reductive wave was observed at +0.71 V at the 2nd cycle. The new redo x couple, separate from Mg deposition and Mg dissolution, was also detected near +1.0V between the 3rd and the 5th cycles. As shown in Figure 2a, this redox couple was not observed in the BMImBF4 electrolyte. The difference between the potentials due to Mg dissolution (+1.72V in Fig.2a) and new redox couple (+1.0V at 3rd cycle in Fig.2b) is approximately equal to the gap of open-circuit voltages in Fig.1 It is inferred that this redox couple is related to the voltage curves detected in the Mg deposition test. The CV signal at 1.47V of the 1st cycle was unidentified. However it may be affected by that SEI on Mg. The Mg deposition test had an annealing time for 20 hours at 60℃, and on the contrary, there was not much time so as to form effective SEI on Mg deposits in the

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Figure 2. CV patterns for the Mg(CF3 SO3 )2 -ionic liquid electrolytes at 60 °C; (a) BMImBF4 and (b) PP13TFSA. Two Pt plates were used as working and counter electrodes with an Ag +/Ag reference electrode. The concentration of Mg(CF3 SO3 )2 was 0.1 mol/L. The scan rate was 1 mV/s. 3.2 Surface analysis. To elucidate the discharge behavior in the Mg(CF3 SO3 )2 -PP13TFSA system, the Mg and Pt electrodes were analyzed before and after discharge, and after charging using Raman spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Figure 3 shows Raman spectra for the Mg electrodes (at points A, B, and C in Fig. 1) between 250 cm-1 and 1500 cm-1 . The Raman spectra for the specimen before discharge provided informatio n on the SEI formed on the Mg metal electrode. The Raman signals at 737 and 751 cm-1 in specimen A have been assigned to CF3 bending and symmetric S-N-S stretching in the TFSA ions, respectively.18,19 The signal at 1249 cm-1 was due to the asymmetric stretching of CF3 units. The peaks at 331 and 1147 cm-1 reflect the rocking and stretching modes of SO2 in TFSA, respectively. The weak signal at 1041 cm-1 was caused by the asymmetric stretching of S-N-S units.20 Based on the Raman data, the SEI was formed mainly by reaction between Mg and TFSA anions.



shown in Figure 5. Especially, the signal at m/Z=471 is due to Mg 2+(CF3 SO3 )3 -. It suggests that Mg ions could possibly have a multi-coordinate structure, i.e. a networking. The intensity of the signal enhanced by discharging, and then decreased upon charging, indicating the formation and release of the network.

Intensity (counts)

In specimen B after discharge, five peaks at 372, 591, 787, 1104, and 1247 cm-1 were detected, which were characteristic of the vibrations in CF3 SO3 - anions:21,22 SO3 rocking, SO3 bending, symmetric C-S stretching, asymmetric SO3 stretching, and symmetric CF3 stretching, respectively. Therefore, the surface of the Mg plate after discharge was covered with a film associated with CF3 SO3 - anions. As a reference, we examined Raman spectra of the Mg(CF 3 SO3 )2 and PP13TFSA reagents, as shown in Figure 3a of the SI. A comparison of the spectra for specimen B and Mg(CF 3 SO3 )2 indicates that the signals for the rocking and bending of SO3 units in specimen B were shifted to lower wavenumber. It appears that the motion of CF3 SO3 units was restricted after the discharge process. Specimen C (after charging) had signals at 751 and 787 cm-1 , which corresponds to the TFSA and CF3 SO3 - anions. The intensities of the five signals due to the CF3 SO3 - unit were weakened or diminished upon charging. The Raman data suggest that the redox couple detected at Mg anode was partially irreversible.

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Figure 4. TOF-SIMS spectra of positive ions from Pt plates . The SIMS data for the specimens A, B, and C in Fig.1 were described in turn from upper to bottom.

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Figure 3. Raman spectra of the Mg electrodes (A) before discharge (black), (B) after discharge (green), and (C) after charging (red) in Fig.1. TOF-SIMS spectra of positive ions for the Pt electrodes at points A, B, and C are shown in Figure 4. The signal intensity at m/Z=24 for Mg in the specimen B was almost equal to that in the specimen A. On the other hand, the signal at m/Z=142 due to PP13+ had high intensity. Therefore, no deposition of Mg occurred at the Pt plate during the discharge process, and PP13+ ions were absorbed at the Pt plate. The negative ions spectra for the Pt electrodes were displayed in Figure 4 of the SI. The intensity of the signal due to CF3 SO3 - diminished by discharging, and then increased upon charging. In the negative ions spectra of the Mg plates (Figure 5), the intensity of the signal at m/Z=149, which corresponded to CF3 SO3 -, increased after the discharge process, but weakened with charging. The TOF-SIMS data corresponded well with the Raman results. The signal at m/Z= 281 was assigned to the C2 F6 S2 O5 dimer of CF3 SO3 , and was stronger than the corresponding peak for the sample before discharge. Some of TOF-SIMS signals of negative ions fro m Mg plates between m/Z=450 and 800 could be assigned as

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x10 4.0 5.0 3.0 4.0 2.0 3.0 2.0 1.0

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with another CF3 SO3 - or TFSA anions of the ionic liqu id . Thus, format ion of the polymeric network proceeds at the surface of Mg metal. The role of TFSA anions in the formation of a network is ambiguous. The BMI mB F 4 system had no redox coupling behavior (Fig.2a). A comparison of TFSA and BF4 anions revealed that the former anion is weakly bound to Mg 2+, and consequently produces a contact ion pair solvate or bridg in g aggregate solvate.24 It appears that these solvates decompose easily during the charge process. Finally , the polymeric network cannot be decomposed perfectly on the basis of Raman and TOF-SIM S results. The red o x coupling behavior which we observed is in part irreversible.

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These experimental data suggest that the surface of the Mg electrode was covered with a CF3 SO3 -associated film after discharge. The surface film then broke up and disappeared partially upon charging; therefore, the formation of this film provides the possibility of a new anode material with CF3 SO3 anions as the ion transport species. Since the TOF-SIMS signal at m/Z= 281 assigned to the C2 F6 S2 O5 dimer of CF3 SO3 is detected after discharging, the surface film may be based upon a polymeric networking of CF3 SO3 -. The formation of the networking of CF3 SO3 - was also supported by the existence of Mg 2+(CF3 SO3 )3 - at m/Z=471. The networking film could not decompose fully, and there is a possibility of the film peeling from Mg anode. We propose a structural model for the CF3 SO3 - network formed on Mg metal. Brooks et al. reported the polymeric network structure of the LiCF3 S O 3 acetonitrile (AN) adduct.23 According to X-ray cryst al structural analysis, the polymeric network was based upon the cleavage of S=O bonding in CF 3 SO3 . Th e opened S-O- single bond contacted with a Li ion, where the Li ion is arranged with a four-coordin ated structu re as a crosslinking point (Figure 6, SI). The acetonit ril e molecule acted as an electron donor to elemental Li. Similar to the role of the LiCF3 SO 3 -A N adduct, we pro pose a polymeric networking model for CF3 SO3 anion s (Figure 6). Firstly, CF3 SO3 - anions migrate toward and adhere to the Mg anode, and then some of the adhered CF3 SO3 - anions cleave the bonding of S=O units. Th e opened single bond reacts with Mg 2+. Mg 2+ then react s

O

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O

Figure 5 TOF-SIMS spectra of negative ions from Mg plates. The SIMS data for the samples A, B, and C in Fig.1 were described in turn from upper to bottom.

O

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Figure 6. Proposed model for the polymeric networking of CF3 SO3 anions formed at the surface of Mg during discharge. 3.3 Cell performance. Having deduced that the poly meric network of CF3 SO3 - anions was formed on the Mg metal surface, we next designed a rechargeable anio n battery with CF3 SO3 - as the ion transport species, which consisted of an anode by the networking of CF 3 SO3 - and a radical polymer cathode (PTMA + CF3 SO3 - ). An understanding of the electrical capacity of the polymeric net work is required to design a rechargeable battery that utilizes this phenomenon as an anode. As the discharg e behavior of 0.1mo l/ L Mg(CF3 SO3 )2 -PP13TFSAsy s t e m was associated with CF3 SO3 anions, the discharge capacities of 0.1mo l/ L Mg(CF3 SO3 )2 -PP13TFSA systems were measured by changing separation between Mg and Pt to understand the effect of the amount of CF 3 SO3 an ion.We defined the discharge capacity up to 0 V as the capacity of the polymeric network. The network con sisted of CF3 SO3 - anions, which were present only in the electrolyte. Therefore, we have measured and estimat ed the discharge capacity of the network using Mg-Pt cells with various amounts of electrolyte. As summarized in


To elucidate the electrochemical reaction with anions at the Mg anode in our anion battery, a three-electrode test was conducted. The cathode and anode potentials, and the cell voltage of the battery during the first discharge and charge cycles are shown in Fig.7b. The cathode potentials of discharge and charge were +0.49 V and +0.52 V vs. NHE, respectively, which indicates the TEMPO redox couple (PTMA +CF3 SO3 - + e- → PTMA + CF3 SO3 -). The anode potential before discharging was ca. -1.8 V vs. NHE, which is higher than the theoretical value calculated from the potential of Mg 2+/Mg (red dotted line in Fig. 8b). The anode potential profile suggests no Mg deposition and dissolution. The anode potential increased up to -1.1 V vs. NHE upon discharging. When the charging started, the potential decreased down and shows a plateau at -1.63 V, which confirmed the rechargeable behavior associated with CF3 SO3 anions at the Mg anode. We have obtained evidence of the rechargeable behavior at Mg anode by the intermediary anion carrier.

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Capacity (mAh/g) Figure 7. (a) Discharge-charge curves for the anion battery at 60 °C. The black, pink, green, blue, and red lines correspond to the first, second, third, fourth, and fifth cycles, respectively. (b) Potential profiles for the cathode (pink) and anode (green) in the three-electrode test. The black line shows the cell voltage. We fabricated an anion battery using by combining polymeric networking of CF3 SO3 anion as anode and radical polymer as cathode. In order to elucidate the interaction by CF3 SO3 anion we analyzed Raman measurements of Mg anode of our anion battery. The results were shown in Figure 8. Raman signals at 368 cm-1 , 594 cm-1 , 779 cm-1 , 1101cm- 1 , and 1247 cm-1 , assigned to vibrations of CF3 SO3 anion were observed in the sample after discharging. On the other hand, these signals disappeared upon charging. The peaks detected at 663 cm-1 and 752 cm-1 after charging corresponded to vibrations of TFSA anion.

650

663

135

752

600

Intensity(a.u.) /

Table I of the SI, the discharge capacity was propo rtional to the amount of CF3 SO3 - anions in the electroly t e. It has been roughly es timated that a quarter of the CF3 SO3 - anions in the electrolyte were involved with the networkin g behavior. Figure 7a shows discharge-charge curves for this anion battery at 60 °C. The battery showed one step in the cell voltage during the 1st discharge process. The discharge plateau was observed near 1.7 V. The 1st discharge capacity of the anion battery was 84.2 mAh/g, which was determined from the amount of PTMA in the cathode. When the polarity of the current was reversed during the charge process, the cell voltage increased to approximately 1.9 V, and a flat voltage was observed at ca. 2.0 V. The total charge capacity was 93.4 mAh/g and the reversibility against discharge was 90.2%. In the 2nd discharge/charge cycle, the electrochemical discharge and charge capacities were 78.4 and 77.4 mAh/g, respectively (pink line), becoming 73.3 and 73.9 mAh/g, respectively (green line), in the 3rd cycle. The capacity fade of the anion battery was caused by the degradation of PTMA as the active material (Figure 9, SI). The capacity of PTMA after the discharge-charge cycle test was about a half of the theoretical value (110mAh/g).

Cell voltage(V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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After charging

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After discharging

779

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Charge

350 300 0

500

1000

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2000

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3500

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Discharge

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Figure 8 Raman spectra of Mg anodes in our anion battery after discharging (green) and charging (red)

1st

0.5 0 0

20

40

60

80

Capacity(mAh/g)

100

120

Mg electrodes were also investigated using X-ray photoelectron spectroscopy (XPS). Figures 9a-9c show XPS spectra for Mg-2p, S-2p, and O-1s photoelectrons, respectively. The samples were Mg electrodes before discharging (black), after discharging (green), and after charging (red).


12000 50.125

(a) 10000 8000

50.375

6000 49.625

4000 2000

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168.125

.

(b)

169.125

Intensity(counts)

8000 6000 4000 169.125

2000 0 162

164

166

168

170

172

174

Binding energy (eV) 12000

531.75

(c)

.

The XPS line position for Mg-2p photoelectron is located near 50 eV.25 A shoulder in the sample before discharging at 48.4 eV may be assigned to metallic magnesium. The signal at 50.125 eV is due to Mg in the SEI. The XPS analysis showed that the photoelectron signal of Mg 2p at 50.125 eV was shifted by 0.25 eV to the high energy side during discharge, which may reflect on a multi-coordinate structure of Mg ions. SI Figures 9a to 9c show photographs of Mg electrode before and after discharge, and after recharge in turn. We could observe metallic luster of magnesium before discharge, indicating that the SEI was very thin. The sample after discharge was blanketed with blackish film. The Mg anode after recharge looked to be grayish. It suggested that the both surface films had thicknesses beyond the limit of detection in depth for XPS technique. Therefore, the intensities of Mg-2p signal in the samples diminished compared to the sample before discharge. The cause of the low intensity in sample after recharge may reflect on the packing or arrangement of the residue which could not be decomposed. For S-2p photoelectron, the XPS signal which harmo nized S-2p 1/2 with S-2p 3/2 photoelectrons was given at 169.125 eV (Fig.9b), and may be assigned to sulfite group (SO3 ). The intensity of S-2p signal was increased by discharging, indicating that the surface was covered with the CF3 SO3 anions. Thus, the formation of the CF3 SO3 associated film on Mg was well supported by the Raman and XPS experimental data. The XPS signal for S-2p photoelectron was shifted by 1.0 eV to the low energy side on charging process. The line position for O-1s photoelectron is observed at 531.0 eV. The XPS signal due to the O-1s photoelectron in our samples was detected near 531.8 eV (Fig.9c). It was also moved to the low energy side by charging. The peak shifts for S-2p and O-1s photoelectrons indicate a possibility of a structural change, i.e. replacement of CF3 SO3 by (CF3 SO2 )2 N. As the result, it seems that the intensities for S2p and O-1s photoelectrons in the sample were almost unchanged during recharging.

Intensity(counts) .

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Page 7 of 8

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531.35 531.85

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Binding energy (eV)

Figure 9. XPS spectra for Mg-2p, S-2p, and O-1s photoelectrons from Mg electrodes. The samples were Mg electrodes in our anion battery before discharging (black), after 1st discharging (green), and after 1st charging (red).

4. Conclusion The deposition and dissolution of Mg has been inves tigated extensively over the past a few decades. The Mg deposition test at 60 °C in this study revealed the un expected electrochemical reaction with triflate ions in a particular ionic liquid electrolyte. The reaction results in the formatio n of a novel active material with anions as ion transport species, which provides a solution to the issue of metal crystal growth. A prototype of an anio n battery that combines this material as an anode wit h PTMA as a cathode demonstrated the potential for a highly safe battery. ■ ASSOCIATE CONTENT

0 46

48

50

Binding energy (eV)

52

54

Supporting Information Experimental procedure, Cell performance, Raman , and TOF-SIMS data. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author E-mail: [email protected]



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■ 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.H prepared radical polymer cathode and had an effective discussion for the anode material, Y.K and M.I made Raman and TOF-SIMS analysis in this work, N.T. carried out XPS study.

■ REFERENCES (1) M ohtadi, R; M atsui, M ; Arthur, T.S; Hwang, S-H. M agnesium Borohydride: From Hydrogen Storage to M agnesium Battery. Angew. Chem. Int. Ed. 2012, 51, 9780-9783. (2) M uldoon, J; Bucur, C.B; Gregory, T. Quest for Nonaqueous M ultivalent Secondary Batteries: M agnesium and Beyond. Chem. Rev. 2014, 114, 11683-11720. (3) Aurbach, D; Lu, Z; Schechter, A; Gofer, Y, Gizbar, H; Turgeman, R; Cohen, Y; M oshkovich, M ; Levi, E. Prototype systems for rechargeable magnesium batteries. Nature, 2000, 47, 724-727. (4) Imamura, D; M iyayama, M ; Hibino, M ; Kudo, T. M g Intercalation Properties into V2O5 gel/Carbon Composites under HighRate Condition. J. Electrochem. Soc. 2003, 150, A753-A758. (5) M atsui, M . Study on electrochemically deposited M g metal. J. Power Sources. 2011, 196, 7048-7055. (6) Doe, R.E; Han, R; Hwang, J, Gmitter, A.J; Shterenberg, I; Yoo, H.D; Pour, N; Aurbach, D. Novel electrolyte solutions comprising fully inorganic salts with high anodic stability for rechargeable magnesium batteries. Chem. Comm. 2014, 50, 243-245. (7) Reddy, M . A.; Fichtner, M . Batteries Based on Fluoride Shuttle. J. Mater. Chem. 2011, 21, 17059-17062.

(8) Zhao, X.; Zhao-Karger, Z.; Wang, D.; Fichtner, M. Metal Oxychloride as Cathode Materials for Chloride Ion Batteries. Angew. Chem. Int., Ed. 2013, 52, 13621-13624. (9) Zhao, X.; Ren, S.; Bruns, M .; Fichtner, M . Chloride Ion Battery: A New M ember in Rechargeable Battery Family. J. Power Sources 2014, 245, 706-711. (10) M aruyama, H.; Nakano, H.; Nakamoto, M .; Sekiguchi, A. High-Power Electrochemical Energy Storage System Employing Stable Radical Pseudocapacitors. Angew. Chem. Int. Ed. 2014, 53, 1324-1328. (11) Shiga, H.; Hase, Y.; Yagi, Y.; Takahashi, N.; Takechi, K. Catalytic Cycle Employing a TEM PO-Anion Complex to Obtain a Secondary M g-O 2 Battery. J. Phys. Chem.Lett. 2014, 5, 1648-1652. (12) Nicholson, M . M . Lithium-M agnesium Electrodes in Propylene Carbonate. J.Electrochem. Soc. 1974, 121, 734-738. (13) Lu, Z.; Schechter, A.; M oshkovich, M .; Aurbach, D. On the Electrochemical Behavior of M agnesium Electrodes in Polar Aprotic Electrolyte Solutions. J. Electroanal. Chem. 1999, 466, 203-217. (14) Cohen, Y. S.; Cohen, Y.; Aurbach, D. M icromorphological Studies of Lithium Electrodes in Alkyl Carbonate Solutions Using in Situ Atomic Force M icroscopy. J. Phys. Chem. B 2000, 104, 12282-12291.

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Sources, 2007, 174, 1234-1240. (17) Orikasa, Y.; M asese, T.; Koyama, Y.; M ori, T.; Hattori, M .; Yamamoto, K.; Okado, T.;Huang, Z-D.; M inato, T.; Tassel, et al. High energy density rechargeable magnesium battry using earthabundant and non-toxic elements. Scientific Reports 2014, 4, 1-6. (18) Rey, I.; Johansson, P.: Lindgren, J.; Lassegues, J. C.; Grondin, J.; Servant, L. Spectroscopic and Theoretical Study of (CF 3SO2)2N(TFSI- ) and (CF 3SO 2)2NH (HTFSI). J. Phys. Chem. A 1998, 102, 3249-3258. (19) Umebayashi, Y.; M itsugi, 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. (20) Burba,C. M .; Rocher, N. M .; Frech, R.; Powell, D. R. CationAnion Interactions in 1-Ethyl-3-M ethylimidazolium Trifluoromethanesulfonate-Based Ionic Liquid Electrolytes. J. Phys. Chem. B 2008, 112, 2991-2995. (21) Gejji, S. P.; Hermansson, K.; Lindgren, J. Ab Initio Vibrational Frequencies of Trfilate Ion, (CF 3SO 3)-. J. Phys. Chem. 1993, 97, 3712-3715. (22) Castriota, M .; Caruso, T.; Agostino, R. G.; Cazzanelli, E.; Henderson, W. A.; Passerini, S. Raman Investigation of the Ionic Liquid N-M ethyl-N-propylpyrrodinium Bis(trifluoromethanesulfonyl)imide and Its M ixture with LiN(SO 2CF3)2. J. Phys. Chem. A 2005, 109, 92-96. (23) Brooks, N. R.; Henderson, W. A.; Smyrl, W. H. Lithium Triffluoromethanesulfonate Acetonitrile Adduct. Acta Crystallographica Section E 2002, E58, m176-m177. (24) Giffin, G. A.; M oretti, A.; Jeong, S.; Passerini, S. Complex M ature of Ionic Coordination in M agnesium Ionic Liquid-Based Electrolytes: Solvates with M obile M g2+ Cations. J. Phys. Chem. C 2014, 118, 9966-9973. (25) M oulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bombe, K. D., Handbook of X-ray Photoelectron Spectroscopy, 1992, PerkinElmer Corporation.

Table of Content graphic

R

eMg

e-

CF3SO3-

X Mg2+

Polymeric networking of CF3SO3 -

(15) NuLi, Y.; Yang, J.; Wu, R. Reversible Deposition and Dissolution of M agnesium from BM IM BF 4 Ionic Liquid. Electrochem . Comm. 2005, 7, 1105-1109. (16) Amir, N.; Vestfrid, Y.; Chusid, O.; Gofer, Y.; Aurbach, D. Progress in Nonaqueous M agnesium Electrochemistry. J. Power


Anode Material Associated with Polymeric Networking of Triflate Ions

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