Catalytic Cycle Employing a TEMPO–Anion Complex to Obtain a

Letter pubs.acs.org/JPCL

Catalytic Cycle Employing a TEMPO−Anion Complex to Obtain a Secondary Mg−O2 Battery Tohru Shiga,* Yoko Hase, Yusuke Yagi, Naoko Takahashi, and Kensuke Takechi Toyota Central Research & Development Laboratories Inc., Nagakute-city, Aichi-ken 480-1192, Japan S Supporting Information *

ABSTRACT: Nonaqueous Mg−O2 batteries are suitable only as primary cells because MgO precipitates formed during discharging are not decomposed electrochemically at ambient temperatures. To address this problem, the present study examined the ability of the 2,2,6,6-tetramethylpiperidine-oxyl (TEMPO)−anion complex to catalyze the decomposition of MgO. It was determined that this complex was capable of chemically decomposing MgO at 60 °C. A catalytic cycle for the realization of a rechargeable Mg− O2 electrode was designed by combining the decomposition of MgO via the TEMPO− anion complex and the TEMPO−redox couple. This work also demonstrates that a nonaqueous Mg−O2 battery incorporating acrylate polymer having TEMPO side units in the cathode shows evidence of being rechargeable.

SECTION: Energy Conversion and Storage; Energy and Charge Transport (TEMPO+X−) and proposed a catalytic cycle using TEMPO+X− for the charging of a Li−O2 battery (Figure 1, MxOy =

B

attery devices with high specific energy values that are also safe to operate are currently in demand for future vehicle applications. The nonaqueous Li−O2 battery is a potential candidate because it offers energy levels of more than 1000 W h/L and thus has received much attention over the past decade.1−4 One of the problems with the Li−O2 battery is that a large amount of lithium peroxide (Li2O2) is precipitated at the cathode during discharging.5−7 Li2O2 is a hazardous, explosive chemical, and it is therefore challenging to safely incorporate this battery into vehicles. The Mg−O2 battery also has the potential to deliver energy levels of 1000 W h/L. In these devices, Mg2+ interacts with oxygen species via a four-electron reaction in organic solvents (2Mg2+ + O2 + 4e− → 2MgO), such that magnesium oxide (MgO) is formed at the O2 electrode during discharging.8 As MgO is both thermodynamically and electrochemically stable, this battery is expected to be much less hazardous. Unfortunately, the MgO formed at the O2 electrode cannot be decomposed by charging at ambient temperatures.9,10 Therefore, although nonaqueous Mg−O2 batteries may operate as primary cells, a charging catalyst that accelerates the decomposition of MgO would be required to allow their use as secondary cells.11 Acrylate polymers having 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO) as a pendent group belong to the category of so-called radical polymers. Because the TEMPO unit undergoes redox reaction with anions (X−) in the vicinity of 3.5 V versus Li+/Li (eq 1),12−14 these polymers have been studied extensively as cathode active materials. TEMPO + X− ↔ TEMPO+X− + e−

Figure 1. Catalytic cycle for the TEMPO−anion complex during decomposition of metal oxide compounds, MxOy. The inset photographs show TEMPO+ClO4− (upper) and TEMPO (lower).

Li2O2). Initially, the TEMPO undergoes complexation with the anion upon charging of the cell, after which the TEMPO− anion complex reacts with Li2O2 and regenerates the TEMPO units. TEMPO units once more form TEMPO+X− via association with anions during charging. This catalytic cycle works by combining the Li2O2 decomposition with the redox couple of the TEMPO unit, which works as a mediator.16,17 In

(1)

Received: March 27, 2014 Accepted: April 23, 2014 Published: April 23, 2014

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In a previous paper, we reported that Li2O2 was immediately decomposed by a TEMPO−anion complex © 2014 American Chemical Society

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the present study, the applicability of this system to MgO rather than Li2O2 was investigated. Assuming that the TEMPO−anion complex is capable of chemically decomposing MgO, the aim was to design a similar catalytic cycle associated with cell charging so as to realize a rechargeable Mg−O2 battery. The ability of the TEMPO−anion complex to catalyze the decomposition of MgO was therefore studied. The reaction of TEMPO+ClO4− with MgO was investigated at 60 °C. In a 9 mL glass tube, 556 μmol of MgO (purity 99.999%, Aldrich) was suspended in a solution of TEMPO+ClO4− (49.6 μmol) and acetonitrile (3 mL). The glass tubes were heated at 60 °C for 96 h. Quantitative analysis of Mg in the iodine solutions was carried out after heating by inductively coupled plasma atomic emission spectroscopy (ICP-AES). It was confirmed that after heating, Mg ions existed in the solution at a concentration on the order of 10 ppm. When the added TEMPO+ClO4− was 49.6 μmol, the amount of dissolved Mg in the solution was 7.98 μmol, corresponding to 1.43% of suspended MgO (556 μmol). As the purity of the MgO was 99.999%, the dissolved Mg could not have originated from impurities in the MgO particles. If all of the dissolved Mg had originated from the decomposition of MgO, the ratio of MgO to TEMPO+ClO4− was 0.16. When the heating was carried out at 25 °C, the concentration of dissolved Mg was 2.2 ppm. The reaction was promoted by higher temperatures. The activation energy calculated by the Arrhenius equation was 76.6 kJ/mol (SI Figure 2 in the Supporting Information). In this work, we fabricated an electrochemical cell using a cathode composed of a radical polymer (PTMA), MgO particles, Ketjen black, and a Teflon binder, so as to investigate the decomposition of MgO by the TEMPO−anion complex. The anode and electrolyte were Li metal and 1 mol/L LiClO4/ 3-methoxy propionitrile (MPN), respectively. The catalyst, TEMPO+ClO4−, was fixed as the radical polymer to avoid its deactivation at the Mg anode. We selected the LiClO 4 electrolyte and Li anode to limit MgO as the source of Mg. Charging of the cell was initiated under a constant current of 0.01 mA. The charging curves of cells with and without PTMA are presented in Figure 2a. The charging capacity for the cell with PTMA was 0.41 mAh, while the cell without PTMA exhibited a capacity of 0.074 mAh. The latter value was obtained due to the effect of electrical double layer polarization (EDLC). In the case of the cell with PTMA, the capacity results from the effects of EDLC and the redox couple formed from the TEMPO in the PTMA. Because the capacity due to the redox couple was calculated to be 0.133 mAh based on the weight of PTMA in the cathode, the balance of 0.203 mAh must have been due to a different storage. It was expected that the TEMPO−anion complex (PTMA+ClO4−) in this cell would exhibit catalytic capability for the decomposition of MgO just as it had for Li2O2. Having considered the possibility of the decomposition of MgO by PTMA+ClO4−, we next mapped the distribution of Mg on the surfaces of the cathodes before and after the charging trials using electron probe microanalysis (EPMA). Figure 2b,c shows scanning electron microscopy (SEM) images and the corresponding Mg distribution maps. The coverage of Mg on the surface of the cathode changed from 15.9 to 12.7% by charging. Thus, it can be clear that the MgO concentration on the cathode is reduced following charging. When the MgO particles in the cathode of the cell incorporating PTMA are decomposed by charging, the amount

Figure 2. (a) Charging curves for electrochemical cells incorporating cathodes with (black) and without (red) PTMA at 25 °C and (b,c) SEM images (left) and Mg distribution maps (right) for cathodes (b) before and (c) after charging.

of Mg2+ in the electrolyte will increase. A qualitative evaluation of Mg2+ was carried out using inductively coupled plasma atomic emission spectrometry (ICP-AES). The results indicated that the Mg2+ concentration in the electrolyte after charging was 3.6 ppm. When the cathode with MgO and PTMA was immersed in the electrolyte for the same period of time but without charging, the resulting concentration of Mg2+ was less than 0.2 ppm. It can therefore be concluded that MgO in the cathode with PTMA decomposed during charging under the influence of the TEMPO−anion complex. When the balance of the 0.203 mAh capacity in Figure 2a is attributed to the decomposition of MgO, a MgO decomposition quantity of 3.78 μmol is calculated. Because the amount of MgO calculated from the ICP-AES analysis (3.6 ppm) is 1.48 μmol, it appears that some Mg2+ ions must have been electrodeposited at the Li anode (SI Figure 4, Supporting Information). When MgO in the cathode is decomposed, the evolution of oxygen will be observed. The gas in the argon-filled bomb (see the inset in Figure 2a) was analyzed by gas chromatography mass spectrometry (GC/MS) following charging and was found to contain 0.036 cm3 of O2 and 0.009 cm3 of H2. The hydrogen was generated by the reaction between the Li anode and MPN. Decomposition of 3.78 μmol of MgO suggested by the ICP-AES data would be expected to produce 0.042 cm3 of O2, which is reasonably close to the value obtained from GC/ MS. On the basis of the experimental data from several analyses, it can therefore be concluded that MgO in the cathode with PTMA was decomposed during charging. 1649

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calculated to be 3.124 V versus Mg2+/Mg. The potential of the cathode should therefore remain close to 3.5 V versus Li+/Li, ideally until MgO at the cathode disappears. Figure 4 shows the

To elucidate the decomposition mechanism, we investigated the interactions between MgO and PTMA+ClO4−. If the TEMPO complex is associated with MgO particles, the former will be adsorbed on the surfaces of the particles. We therefore suspended MgO particles in an acetonitrile solution of PTMA+ClO4− (2 mg/10 mL) at 25 °C for 24 h, following which the filtered MgO particles were analyzed by X-ray photoelectron spectroscopy (XPS). Wide-range XPS spectra of the specimens are provided in the Supporting Information, and XPS spectra due to Mg-2p and O-1s photoelectrons for bare MgO and for MgO suspended in PTMA+ClO4− are shown in Figure 3a and b, respectively. The Mg-2p signal for the bare

Figure 4. Cyclic performance of a nonaqueous Mg−O2 battery incorporating PTMA in the cathode at 60 °C.

Figure 3. XPS spectra of Mg-2p and O-1s electrons in MgO with (pink) and without (black) PTMA+ClO4−. The MgO particles were suspended in acetonitrile solutions (a) with and (b) without PTMA+ClO4− (2 mg/10 mL) over 24 h at 25 °C.

discharge−charge profiles for the Mg−O2 battery incorporating PTMA in the cathode at 60 °C, when the discharge capacity of the first cycle was 737 mAh/g. When the polarity of the current was changed during the charging process, the cell voltage increased rapidly to about 2.0 V, followed by a more gradual rate of increase. The initial cycle charging capacity was 460 mAh/g, and the reversibility against discharging was 62.4%. The charging process was associated with complexation of the PTMA with ClO4− (48 mAh/g) and decomposition of MgO. In the second discharging, the battery showed two steps in the cell voltage. The first stage, near 1.5 V, was due to the reduction of the PTMA+ClO4−, showing the performance of the radical battery, while the second stage demonstrated the discharging capacity of the Mg−O2 battery. In the second cycle, the discharging capacity was 399 mAh/g, and the charging capacity was as high as 175 mAh/g. As shown in Figure 4, the Mg−O2 battery exhibited rechargeable behavior over several discharge/ charge cycles. The capacity fading was caused by incomplete decomposition of MgO and the degradation of PTMA as the catalyst (SI Figure 11, Supporting Information). The cathode and anode potentials and the cell voltage for the Mg−O2 battery during the first cycle are shown in Figure 5.

MgO is at 48.6 eV but is shifted upward by 0.125 eV in the case of the MgO particles suspended in the PTMA+ClO4− solution. The peak assigned to O-1s electrons is also shifted to higher energy by 0.37 eV. These results indicate that the bonding between Mg and O may be weakened by the adsorption of PTMA+ClO4−. Having established that MgO decomposes in the PTMA cathode, we next designed a catalytic cycle, consisting of a charging mechanism for the Mg−O2 electrode based on combining decomposition of MgO under the influence of the TEMPO−anion complex with the TEMPO redox couple. In a Mg−O2 battery with a PTMA cathode, Mg ions should react solely with oxygen during the first discharging. During the initial charging process, the complexion of TEMPO units with anions would first occur at the cathode. The potential of the TEMPO redox is 3.5 V versus Li+/Li (2.83 V versus Mg2+/Mg), while the free energy of formation of MgO is −601.83 kJ/ mol,18 and the decomposition voltage of MgO has been

Figure 5. Potential profiles for the cathode and anode during the first cycle. The black, red, and blue lines correspond to the cell voltage and the potentials of the cathode and anode. The green dotted line represents the Mg2+/Mg potential. 1650

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The cathode potential during discharging was initially 2.9 V versus Li+/Li, indicating four-electron reduction per mole of oxygen. The anode potential was about 1.7 V versus Li+/Li, which is higher than the theoretical value calculated from the potential of Mg2+/Mg (0.67 V versus Li+/Li). The evident drop in voltage was due to the formation of a solid electrolyte interface (SEI) on the Mg anode.19,20 When the polarity of the current was reversed, the cell voltage increased up to the charging capacity of 364 mAh/g, representing 65.7% of the discharge capacity, while the potential of the cathode was constant at 3.5 V. These results indicate oxidation of the polymer (PTMA+ClO4− + e− → PTMA + ClO4−). The capacity due to the redox couple of the TEMPO unit was 48 mAh/g and was determined by the amount of PTMA in the cathode. Therefore, the associated turnover number was 7.6. The anode potential on charging fell below 0.67 V versus Li+/ Li, indicating that the deposition of Mg2+ occurred at the Mg anode and thus confirming rechargeable behavior at the Mg anode. In order to identify the formation and decomposition of MgO in the Mg−O2 battery, we analyzed the cathodes after discharging as well as after discharging and charging. White precipitate particles several micrometers in size were observed in these samples (SI Figure 12, Supporting Information), and it was found that these precipitates were reduced in size upon charging. The precipitates at the cathodes were further examined by the Raman spectroscopic technique and time-offlight secondary ion mass spectrometry (TOF-SIMS). As shown in SI Figure 13 (Supporting Information), the Raman signal assigned to the vibration of MgO2 at 1137 cm−1 was not detected in the white precipitates. The SIMS spectra of the cathodes before and after discharging are shown in SI Figure 14 (Supporting Information). The sample obtained after only discharging exhibited a peak at m/z = 24 in the positive ion spectra and m/z = 16 in the negative ion spectra, both of which are stronger than the corresponding peaks for the sample before discharging. From these experimental data, it was concluded that the white precipitates were primarily MgO and that this material was decomposed during electrical charging. Finally, we set a gas cylinder to fill argon into the cell after the first discharging to detect the evolution of oxygen during the charging. The first discharging and charging capacities were 753 and 423 mAh/g, respectively. As the weight of the cathode materials was 4.1 mg, the charging capacity was 1.734 mAh. If the contribution ratio due to the decomposition of MgO to the total capacity had been 65% (SI Table 1, Supporting Information), 0.47 cm3 of oxygen would have been evolved. The gases detected after the charging were 0.41 cm3 of O2 and 0.52 cm3 of H2. The result suggests that PTMA cycled as a catalyst for the decomposition. To date, TEMPO compounds have been studied as positive active materials. Our work, however, has revealed a new application for TEMPO compounds as catalysts for the decomposition of metal oxides. In addition, we have succeeded in charging a nonaqueous Mg−O2 battery using a TEMPO− anion complex as an O2 electrode catalyst. The basic mechanism for electrochemical charging by use of the complex could be exploited to design multivalent metal−O2 batteries.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedure, CV profiles, cell performance, Raman, XPS, and TOF-SIMS data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

T.S. conceived and carried out the experiments, analyzed the data, and wrote the paper, Y.H prepared catalysts and had an effective discussion for catalytic mechanism, S.Y and N.T made EMPA and XPS analysis in this work, and K.T. discussed data and directed this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Emi Itoh of Toyota CRDL for GS/MS measurements. REFERENCES

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