On the Stability of NaO2 in NaâO2 Batteries - ACS Applied Materials

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 13534−13541

On the Stability of NaO2 in Na−O2 Batteries Chenjuan Liu, Marco Carboni, William R. Brant, Ruijun Pan, Jonas Hedman, Jiefang Zhu, Torbjörn Gustafsson, and Reza Younesi* Ångström Advanced Battery Centre (ÅABC), Department of ChemistryÅngström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden

ACS Appl. Mater. Interfaces 2018.10:13534-13541. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/16/19. For personal use only.

S Supporting Information *

ABSTRACT: Na−O2 batteries are regarded as promising candidates for energy storage. They have higher energy efficiency, rate capability, and chemical reversibility than Li−O2 batteries; in addition, sodium is cheaper and more abundant compared to lithium. However, inconsistent observations and instability of discharge products have inhibited the understanding of the working mechanism of this technology. In this work, we have investigated a number of factors that influence the stability of the discharge products. By means of in operando powder X-ray diffraction study, the influence of oxygen, sodium anode, salt, solvent, and carbon cathode were investigated. The Na metal anode and an ether-based solvent are the main factors that lead to the instability and decomposition of NaO2 in the cell environment. This fundamental insight brings new information on the working mechanism of Na−O2 batteries. KEYWORDS: metal−air battery, in operando X-ray diffraction, sodium superoxide, NaO2, decomposition

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INTRODUCTION Alkali metal−oxygen batteries, such as Li−O2 and Na−O2, have been of great research interest over the past decade as high specific energy alternatives to current state-of-the-art lithiumion batteries. Particularly attractive is the emerging Na−O2 batteries with NaO2 as the discharge product, which have the potential to provide a higher energy efficiency, rate capability, and chemical reversibility compared to Li−O2. Sodium is also cheaper and more abundant than lithium. However, both systems are facing great challenges, such as the instability of all cell components during cycling and the large overpotential originating from the insulating discharge products. To advance these two technologies, a deep mechanistic understanding of the electrochemical processes is crucial. In contrast to nonaqueous Li−O2 batteries, a great deal of debate has centered around the cell chemistry of Na−O2 batteries because of reports of a wide range of discharge products. Thus, a single, main discharge product has not been observed and agreed upon.1 With their similar standard potential of formation (U° NaO2 = 2.27 V vs Na+/Na, U° Na2O2 = 2.33 V vs Na+/Na) but different number of electrons transferred, both NaO2 and Na2O2 as well as Na2O2·2H2O have been identified as discharge products.2−6 Unfortunately, the underlying reasons for these divergent results still remain unclear. The conversion of NaO2 to Na2O2·2H2O was observed experimentally by the influence of moisture, oxygen, or air exposure.7,8 However, other studies show that the as-formed NaO2 can be converted to Na2O2· 2H2O under cycling or resting the cell.5,9,10 Because NaO2 and Na 2O2 are very sensitive to air or moisture, ex situ characterization methods could easily be affected by artefacts © 2018 American Chemical Society

introduced during the cell disassembly and/or measuring procedure. Alternatively, in operando analysis can provide direct insight into the real discharge products and the factors that affect the formation of different products.11 In this work, we investigated the reaction products formed during discharge of Na−O2 cells and their stability over relaxation (pause) time. In operando and ex situ X-ray diffraction (XRD) analysis was performed to quantify the amount of the main reaction products, that is, NaO2, and to understand the possible factors that affect the stability of the discharge products.

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RESULTS AND DISCUSSION In Operando Analysis of the Discharge Product. In operando XRD analysis was carried out to track the formation of discharge products using a newly designed electrochemical cell (details in the experimental section). This analysis provides real-time quantitative determination of the crystalline discharge products. Figure 1a shows the in operando diffraction patterns and the corresponding galvanostatic discharge voltage profile of an Na−O2 cell with the electrolyte of 0.5 M NaOTf in diglyme (G2) at the current density of 0.1 mA cm−2 and the low cutoff potential of 1.8 V. The cell provided a discharge capacity of around 2.2 mA h cm−2. The appearance of the diffraction peaks at 2θ ≈ 32.5° and 46.6°, corresponding to the (200) and (220) reflections of NaO2 (ICSD # 87176), revealed that the Received: January 26, 2018 Accepted: April 4, 2018 Published: April 4, 2018 13534

DOI: 10.1021/acsami.8b01516 ACS Appl. Mater. Interfaces 2018, 10, 13534−13541

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ACS Applied Materials & Interfaces

Figure 1. (a) Waterfall plot of the diffraction patterns collected in operando and the corresponding discharge voltage profile during the first discharge with a 0.5 M NaOTf/G2 electrolyte at a current density of 0.1 mA cm−2 and the low cutoff potential of 1.8 V. The mass evolution of NaO2 as a function of time at in the Na−O2 cells: (b) galvanostatically discharged without relaxation/pause and (c) in the cell with applied pauses during the discharge.

To investigate the stability of NaO2 formed during discharge, a galvanostatic intermittent titration technique (GITT) combined with the in operando XRD method was introduced. Figure 1c shows another Na−O2 cell discharged with the same electrolyte and current density while it was paused for 15 min after every 1 h of discharge during which XRD patterns were obtained. As shown in Figure 1c, the discharge capacity is reduced and the NaO2 formation efficiency is much lower in the cell with pauses during the discharge. The discharge capacity of the continuously discharged cell (Figure 1b) is around 2.2 mA h cm−2, whereas the cell with pauses during the discharge (Figure 1c) delivered a lower capacity of about 1.6 mA h cm−2, which means that about 30% of the discharge capacity is lost. A smaller quantity of NaO2 formed in the GITT experiment compared to a pure galvanostatic run indicates that NaO2 is not stable and undergoes decomposition. Therefore, the detected amount of NaO2 results from a competition between the main electrochemical reaction (i.e., formation of NaO2) and chemical parasitic reactions resulting in the decomposition of NaO2. However, formation of Na2O2·2H2O or similar side products on the cathode is not observed from XRD patterns (Figure S1). Instability of NaO2 during resting of an Na−O2 cell has previously been observed.6,9,10,12 However, the main factors that lead to the decomposition of NaO2 are not yet known. At least five different possible pathways for the spontaneous decomposition of NaO2 could be considered: (1) NaO2 reacts with the electrolyte: salt, solvent, or both; (2) NaO2 itself is unstable at room temperature and a disproportionation reaction takes place (NaO2 + NaO2 → O2 + Na2O2);

electrochemically formed NaO2 is the sole crystalline discharge product. The peaks from metallic aluminum in the pouch and the current collector are also marked in the diffraction patterns. Silicon, added as an internal intensity standard, shows a peak at 2θ ≈ 28.6° and enables us to quantify the amount of NaO2 formed during the discharge reaction. Thus, we are able to discuss the ratio between main and parasitic reactions. To quantify the efficiency of formation of NaO2, the amount of NaO2 was determined from the weight ratio with the constant, a known quantity of Si reference, via a Rietveld refinement using the FullProf program. The details of the Si reference and the weight ratio calculation can be found in the Supporting Information. When the cell is discharged/charged with a constant current, a linear growth/decomposition of NaO2 is expected in the absence of any side reactions. If all of the exchanged electrons are used to generate NaO2 during discharge, the Na+ + e− + O2 ⇋ NaO2 net reaction is assumed and the theoretical amount of NaO2 generated via the reaction would be proportional to the discharge capacity (i.e., 2.05 mg NaO2 per mA h would be expected). In Figure 1b, the theoretical growth of NaO2 is presented as a green dot at each 15 or 30 min interval during discharge. The detected amount of NaO2 (red dots in Figure 1b) is highly efficient up to around 50% of discharge; however, it slows down as the discharge voltage reaches below 2.0 V. The lower efficiency during the end of discharge indicates that (i) side reactions occurred and/ or (ii) produced NaO2 underwent chemical decomposition. Considering the limitation of XRD, we cannot rule out the possibility of the formation of amorphous discharge products during the discharge. 13535


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Figure 2. Time-resolved in operando XRD patterns showing the evolution of the discharge products of Na−O2 cells that rest/paused (a) inside the cell environment with 0.5 M NaOTf/G2 electrolyte and (b) inside the cell environment with 1.5 M NaOTf/G2 electrolyte. (c,d) are the quantity of NaO2 corresponding to (a,b), respectively.

Figure 3. Time-resolved in operando XRD patterns showing the evolution of the discharge products from Na−O2 cells at rest (a) inside the fresh G2 solvent (H2O < 1ppm) and (b) without Na metal inside the cell environment with 1.5M NaOTf/G2. (c,d) are the quantity of NaO2 corresponding to (a,b), respectively.

characterized under different conditions. First, the decomposition of NaO2 in electrolytes with two different salt concentrations was quantitatively demonstrated. Na−O2 cells with 0.5 and 1.5 M NaOTf dissolved in G2 were fully discharged and then rested at room temperature. Without disassembling the cell, the amount of NaO2 was tracked by in operando XRD analysis, as shown in Figure 2. NaO2 was stable for about 1.5 days in the 0.5 M cell and about 2 days in the 1.5 M cell. However, as the pause continued, a linear decomposition of NaO2 was observed in both cells with similar

(3) NaO2 reacts with water from the electrolyte to form NaOH or Na2O2·2H2O; (4) NaO2 reacts with the carbon electrode to produce NaCO3; (5) NaO2 can be dissolved into the electrolyte and the asformed O2− can migrate to and react with some parts of the cell, most likely the Na anode. NaO2 Stability in the Cell Environment. To analyze the factors influencing the decomposition of NaO2 in the cell environment during relaxation, the discharge products were 13536


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Figure 4. Evolution of rinsed and dried NaO2 inside a sealed pouch. Time-resolved XRD patterns of the cathode extracted from a discharged cell with (a) 0.5 M NaOTf/G2 and with (b) 1.5 M NaOTf/G2. (c,d) are the evolution of NaO2 corresponding to (a,b), respectively.

did not produce the X phase. The evolution of the different morphologies of the discharge products, which were stored in a sealed pouch and under G2 solvent (the same electrode used in Figure 3a), respectively, was revealed by scanning electron microscopy (SEM). As shown in Figure S2, the discharge products retained the same cubic morphology when stored in the evacuated pouch for 60 days, whereas cubic particles could no longer be observed in the reference sample kept in the G2 solvent. Thus, when the carbon fiber surface became exposed to the solvent, the discharge products largely dissolved. Elemental analysis shown in Figure S3a further supports this finding. For the sample kept in the G2 solvent, no Na and O could be detected on the carbon fiber. Instead, we observed the formation of an unknown Na−O compound in the pores of the carbon fiber. It has been reported that the as-formed NaO2 can dissolve in a fresh electrolyte.9,13−15 If we assume that all of the missing NaO2 in this cathode had dissolved into the fresh G2 solvent, the concentration of the dissolved NaO2 would be ∼200 mM. This value of dissolved O2− is slightly higher than the value measured by Kim et al. using electron spin resonance spectroscopy.9 As shown in Figure 3a, after a resting period in the fresh solvent that was 50 times longer than that reported by Kim et al., Na2O2·2H2O was finally detected by XRD analysis. In this regard, our result indicates that the liberated O2− from NaO2 can attack the G2 solvent to form Na2O2·2H2O as the side product, which agrees with the observation from Kim et al.9 On the basis of these experiments, it can be concluded that Na2O2·2H2O was more likely formed after the cell disassembly and rinsing the cathode with a fresh solvent. It is worth noting that the disappearance of NaO2 in fresh G2 occurred with a rate of ∼0.171 mg/day (Figure 3c), which is much slower than the decomposition of NaO2 in the whole cell environment. Thus, the factor that promotes the fast decomposition of NaO2 in a cell at rest, therefore, needs to be understood. As reported by Hartmann et al., O2− that is formed at the cathode surface can migrate within the cell. In this case, O2− could migrate to the anode side and react with the Na metal. Therefore, to probe the influence of Na metal, an

decomposition rates (Figure 2c,d). Thus, the concentration of the electrolyte salt does not affect the decomposition rate of NaO2 in the cell environment. Recently, Kim et al. have reported that NaO2 could completely convert to Na2O2·2H2O in an electrochemical cell because of a reaction with the electrolyte solvent, as determined by XRD analysis.9 However, their XRD measurements were carried out after disassembling the cell and rinsing the cathode with a fresh solvent (acetonitrile). The water content in acetonitrile was not specified. However, our in operando XRD data show that without disassembly of the cell, Na2O2·2H2O was not detected after allowing the cell to rest for more than 20 days. As shown in Figures 2a,b, neither NaOH nor Na2O2·2H2O was detected as the side product. Instead, a new unknown phase started to appear after about 4 days resting, and at the same time, NaO2 started to disappear. Unfortunately, because of the insufficient crystallographic data available, identification of this unknown phase was not possible. In this case, we abbreviate this phase as X. As the salt concentration does not affect the decomposition rate of NaO2, it is then worth considering whether the solvent promotes the decomposition. The instability of NaO2 in the G2 solvent was thus investigated. An Na−O2 cell was discharged for 16 h and then disassembled inside a glovebox. The extracted cathode was rinsed by G2 (less than 1 ppm H2O content) to remove the residual salt and solvent. The cathode was then cut roughly in half; one part was dried and sealed in a pouch, and the other half was sealed in a pouch containing 150 μL of G2 (the same as electrolyte volume added to Na−O2 cells). The decomposition of NaO2 in these two samples was followed by XRD analysis, as shown in Figure 3a. The X phase was detected directly after disassembly, cleaning, and placing the cathode in contact with the electrolyte solvent inside a pouch. The decomposition of NaO2 resting in a fresh G2 solvent was observed and it is slower than that in the cell environment. In addition, the X phase reduced in intensity over time. In this regard, once the X phase formed during the washing procedure, the subsequent decomposition of NaO2 in the pure G2 solvent 13537


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under the same conditions as in Figure 4a. The extracted cathode was rinsed by dry G2 to remove the salt and then dried under vacuum. One part of the cathode was transferred directly for SEM study, and the other part was sealed in a pouch under vacuum for future SEM studies. The SEM images revealed that after discharge, the cathode consisted of a cubic structure discharge product, which is reported as NaO2. Figure 5a,b

Na−O2 cell was disassembled directly after a full discharge, and then, the Na metal anode was extracted from the cell and the rest of the cell was sealed in a pouch without adding any new electrolyte or washing procedures. After the Na metal was removed, the NaO2 phase remained in the cell after 25 days of resting (Figure 3b). Only a small amount of NaO2 was observed to decompose at the very beginning of pause with a rate of 0.333 mg/day (Figure 3d, which is about half of the rate that is observed in the full cell environment). As shown in Figure 3b, without rinsing the electrode, the X phase did not seem to appear at the beginning. However, after about 3 days of resting, NaO2 begins to decompose very slowly with a rate of about 0.015 mg/day, and the X phase appears and grows in intensity during the pause procedure. From this observation, it can be concluded that the presence of Na metal promotes the decomposition of NaO2. It is worth noticing that even with the presence of a similar volume of the electrolyte, the decomposition rate of NaO2 in the used electrolyte (without the presence of Na metal) is much slower compared with that in the fresh G2. In addition, as-formed NaO2 maintains a cubic shape inside the cell without Na metal for 80 days (Figure S3 and Figure S4) if the cathode is not rinsed by G2. However, the cubes vanished after resting in the whole cell environment for 80 days, and only some flake-shaped compounds remain on the carbon surface (Figure S4). This suggests that the used electrolyte reaches O2− saturation during the discharge process. However, the decomposition rate of NaO2 in the whole cell environment is still faster than both of these cases (discharged cathode in a fresh solvent and the cell without Na metal). This means that the Na metal in the cell environment directly (Na reacts with O2−) or indirectly (Na reacts with G2 and the product of this reaction could in turn react with O2−) contributed to the migration of O2− to the anode side, which further promotes the dissolution process of NaO2 into the electrolyte. Disproportionation of NaO2. If O2− migration and reaction at the anode surface is the main process for the decomposition of NaO2 in the cell environment, the origin of the slow decomposition of NaO2 in the absence of Na metal must be due to other reasons. Considering the thermodynamic stability of solid NaO2 produced during discharge, one could question the storage of NaO2 at room temperature. To investigate this possibility, two Na−O2 cells were discharged to the same capacity (1.6 mA h cm−2) with the same current density of 0.1 mA cm−2 as used in the previous test. The cathodes were directly extracted from the cells after discharge, rinsed with a “water-free” solvent ( the solvent decomposition > NaO2 disproportionation. Table 1. Summary of the Stability of NaO2 on the Cathode in Different Situations samples

Anode

used electrolyte

fresh electrolyte

decomposition rate (mg/day)

in cell in G2 no Na in pouch

yes no no no

yes no yes no

no yes no no

0.697 0.171 0.015 0.012

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CONCLUSIONS In this study, we demonstrated the main factors that affect the stability of the discharge products within the Na−O2 cell environment. As characterized by in operando XRD, the chemical composition of the discharge product was solely NaO2. The formation of Na2O2·2H2O was not detected in either the complete cell environment or after the removal of Na metal, salt, solvent, or electrolyte, followed by a long pause/ relaxation time. However, an unknown compound was detected after disassembling the cell. The evolution of NaO2 has been quantitatively tracked by XRD analysis in different situations. It is shown that the influence from Na metal is the main factor that promotes the dissolution and migration of O2− and causes the decomposition of NaO2 in the cell environment. Without the influence from Na metal, the electrolyte becomes saturated by O2−. However, when NaO2 is in contact with the saturated solvent for a long time or in contact with the fresh solvent for a short time, a new compound is formed, possibly with the composition intermediate between NaO2 and Na2O2. In addition, this unknown phase can transform to Na2O2·2H2O phase when being in contact with the G2 solvent for a long time or being exposed to a moisture-containing gas. This observation explains why Na2O2·2H2O is detected as the discharge product in some studies. More importantly, decomposition from NaO2 to the unknown intermediate phase was observed under vacuum at room temperature. However, this disproportionation process is about 60 times slower than the dissolution and migration of O2− promoted by Na metal. The findings in this report could help to resolve the conflicting observations of different discharge products in previous studies and also provide guidance in developing strategies to improve the stability of Na−O2 cells. To achieve a better performance in Na−O2 batteries, a strategy to restrain O2− migration to the anode side is necessary.

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METHODS

Materials. The sodium anodes were prepared by pressing sodium cubes in an argon-filled glovebox. After removing the residual surface oxides, the sodium cubes (99.9%, Sigma-Aldrich) were cut into small pieces and pressed on an aluminum foil to a 0.5 mm thickness and 12 or 14 mm diameter discs, which were then used as the anodes. Diglyme (G2, >99.5%, Sigma-Aldrich) was dried over 4 Å molecular sieves (Aldrich) for about 2 weeks before use. Sodium triflate (NaSO3CF3, 98%, Aldrich) was dried at 180 °C for 24 h under vacuum in an Ar-filled glovebox. NaOTf/G2 electrolytes (0.5 and 1.5 M) containing less than 5 ppm H2O were prepared in an Ar-filled glovebox. Determined using an 831KF Karl Fischer coulometer (Metrohm), the water content of the G2 solvent for rinsing the electrodes was about 1 ppm. The cathode was prepared from a 13540


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(10) Sayed, S. Y.; Yao, K. P. C.; Kwabi, D. G.; Batcho, T. P.; Amanchukwu, C. V.; Feng, S.; Thompson, C. V.; Shao-Horn, Y. Revealing instability and irreversibility in nonaqueous sodium-O2 battery chemistry. Chem. Commun. 2016, 52, 9691−9694. (11) Liu, C.; Rehnlund, D.; Brant, W. R.; Zhu, J.; Gustafsson, T.; Younesi, R. Growth of NaO2 in Highly Efficient Na−O2 Batteries Revealed by Synchrotron In Operando X-ray Diffraction. ACS Energy Lett. 2017, 2, 2440−2444. (12) Black, R.; Shyamsunder, A.; Adeli, P.; Kundu, D.; Murphy, G. K.; Nazar, L. F. The Nature and Impact of Side Reactions in Glymebased Sodium-Oxygen Batteries. ChemSusChem 2016, 9, 1795−1803. (13) Schechter, D. L.; Kleinberg, J. Reactions of Some Metal Salts with Alkali Superoxides in Liquid Ammonia. J. Am. Chem. Soc. 1954, 76, 3297−3300. (14) Hartmann, P.; Heinemann, M.; Bender, C. L.; Graf, K.; Baumann, R.-P.; Adelhelm, P.; Heiliger, C.; Janek, J. Discharge and Charge Reaction Paths in Sodium−Oxygen Batteries: Does NaO2 Form by Direct Electrochemical Growth or by Precipitation from Solution? J. Phys. Chem. C 2015, 119, 22778−22786. (15) Lutz, L.; Yin, W.; Grimaud, A.; Corte, D. A. C.; Tang, M.; Johnson, L.; Azaceta, E.; Sarou-Kanian, V.; Naylor, A. J.; Hamad, S.; Anta, J. A.; Salager, E.; Tena-Zaera, R.; Bruce, P. G.; Tarascon, J.-M. High Capacity Na−O2 Batteries: Key Parameters for SolutionMediated Discharge. J. Phys. Chem. C 2016, 120, 20068−20076. (16) Yadegari, H.; Li, Y.; Banis, M. N.; Li, X.; Wang, B.; Sun, Q.; Li, R.; Sham, T.-K.; Cui, X.; Sun, X. On rechargeability and reaction kinetics of sodium−air batteries. Energy Environ. Sci. 2014, 7, 3747− 3757. (17) Xia, C.; Black, R.; Fernandes, R.; Adams, B.; Nazar, L. F. The critical role of phase-transfer catalysis in aprotic sodium oxygen batteries. Nat. Chem. 2015, 7, 496−501. (18) Wang, B.; Zhao, N.; Wang, Y.; Zhang, W.; Lu, W.; Guo, X.; Liu, J. Electrolyte-controlled discharge product distribution of Na-O2 batteries: a combined computational and experimental study. Phys. Chem. Chem. Phys. 2017, 19, 2940−2949. (19) Bender, C. L.; Hartmann, P.; Vračar, M.; Adelhelm, P.; Janek, J. On the Thermodynamics, the Role of the Carbon Cathode, and the Cycle Life of the Sodium Superoxide (NaO2) Battery. Adv. Energy Mater. 2014, 4, 1301863. (20) Hartmann, P.; Bender, C. L.; Vračar, M.; Dürr, A. K.; Garsuch, A.; Janek, J.; Adelhelm, P. A rechargeable room-temperature sodium superoxide (NaO2) battery. Nat. Mater. 2013, 12, 228−232. (21) Hartmann, P.; Bender, C. L.; Sann, J.; Dürr, A. K.; Jansen, M.; Janek, J.; Adelhelm, P. A comprehensive study on the cell chemistry of the sodium superoxide (NaO2) battery. Phys. Chem. Chem. Phys. 2013, 15, 11661−11672. (22) Adams, B. D.; Black, R.; Williams, Z.; Fernandes, R.; Cuisinier, M.; Berg, E. J.; Novak, P.; Murphy, G. K.; Nazar, L. F. Towards a Stable Organic Electrolyte for the Lithium Oxygen Battery. Adv. Energy Mater. 2014, 5, 1400867. (23) Freunberger, S. A.; Chen, Y.; Drewett, N. E.; Hardwick, L. J.; Bardé, F.; Bruce, P. G. The lithium-oxygen battery with ether-based electrolytes. Angew. Chem., Int. Ed. Engl. 2011, 50, 8609−8613. (24) Younesi, R.; Hahlin, M.; Treskow, M.; Scheers, J.; Johansson, P.; Edström, K. Ether Based Electrolyte, LiB(CN)4 Salt and Binder Degradation in the Li−O2 Battery Studied by Hard X-ray Photoelectron Spectroscopy (HAXPES). J. Phys. Chem. C 2012, 116, 18597−18604. (25) Liu, C.; Brant, W. R.; Younesi, R.; Dong, Y.; Edström, K.; Gustafsson, T.; Zhu, J. Towards an Understanding of Li2O2 Evolution in Li-O2 Batteries: An In Operando Synchrotron X-ray Diffraction Study. ChemSusChem 2017, 10, 1592−1599. (26) Younesi, R.; Hahlin, M.; Edström, K. Surface characterization of the carbon cathode and the lithium anode of Li-O2 batteries using LiClO4 or LiBOB salts. ACS Appl. Mater. Interfaces 2013, 5, 1333− 1341. (27) Rodríguez-Carvajal, J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 192, 55−69.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01516. Calculation of theoretical mass of NaO2 and H2O; XRD pattern; SEM images; and schematic drawing of a homemade Na−O2 cell for in operando XRD study (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

William R. Brant: 0000-0002-8658-8938 Jiefang Zhu: 0000-0002-6326-8106 Torbjörn Gustafsson: 0000-0003-2737-4670 Reza Younesi: 0000-0003-2538-8104 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully appreciate the financial supported by the Swedish Research Council, StandUp for Energy, the Swedish Energy Agency, the Ångpanneföreningen’s Foundation for Research and Development, the State Key Laboratory of Fine Chemicals (KF1413), and the China Scholarship Council. Dr. David Rehnlund is gratefully acknowledged for his assistance with SEM measurements.

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REFERENCES

(1) Yadegari, H.; Sun, Q.; Sun, X. Sodium-Oxygen Batteries: A Comparative Review from Chemical and Electrochemical Fundamentals to Future Perspective. Adv. Mater. 2016, 28, 7065−7093. (2) Bender, C. L.; Schröder, D.; Pinedo, R.; Adelhelm, P.; Janek, J. One- or Two-Electron Transfer? The Ambiguous Nature of the Discharge Products in Sodium-Oxygen Batteries. Angew. Chem., Int. Ed. 2016, 55, 4640−4649. (3) Kim, J.; Lim, H.-D.; Gwon, H.; Kang, K. Sodium-oxygen batteries with alkyl-carbonate and ether based electrolytes. Phys. Chem. Chem. Phys. 2013, 15, 3623−3629. (4) Jian, Z.; Chen, Y.; Li, F.; Zhang, T.; Liu, C.; Zhou, H. High capacity Na−O2 batteries with carbon nanotube paper as binder-free air cathode. J. Power Sources 2014, 251, 466−469. (5) Zhao, N.; Li, C.; Guo, X. Long-life Na-O2 batteries with high energy efficiency enabled by electrochemically splitting NaO2 at a low overpotential. Phys. Chem. Chem. Phys. 2014, 16, 15646−15652. (6) Landa-Medrano, I.; Pinedo, R.; Bi, X.; de Larramendi, I. R.; Lezama, L.; Janek, J.; Amine, K.; Lu, J.; Rojo, T. New Insights into the Instability of Discharge Products in Na-O2 Batteries. ACS Appl. Mater. Interfaces 2016, 8, 20120−20127. (7) Pinedo, R.; Weber, D. A.; Bergner, B.; Schröder, D.; Adelhelm, P.; Janek, J. Insights into the Chemical Nature and Formation Mechanisms of Discharge Products in Na−O2 Batteries by Means of Operando X-ray Diffraction. J. Phys. Chem. C 2016, 120, 8472−8481. (8) Ortiz-Vitoriano, N.; Batcho, T. P.; Kwabi, D. G.; Han, B.; Pour, N.; Yao, K. P. C.; Thompson, C. V.; Shao-Horn, Y. Rate-Dependent Nucleation and Growth of NaO2 in Na−O2 Batteries. J. Phys. Chem. Lett. 2015, 6, 2636−2643. (9) Kim, J.; Park, H.; Lee, B.; Seong, W. M.; Lim, H.-D.; Bae, Y.; Kim, H.; Kim, W. K.; Ryu, K. H.; Kang, K. Dissolution and ionization of sodium superoxide in sodium-oxygen batteries. Nat. Commun. 2016, 7, 10670. 13541


On the Stability of NaO2 in NaâO2 Batteries - ACS Applied Materials

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