Insights into the Chemical Nature and Formation Mechanisms of

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Insights into the Chemical Nature and Formation Mechanisms of Discharge Products in Na-O Batteries by Means of Operando X-Ray Diffraction 2

Ricardo Pinedo, Dominik A. Weber, Benjamin J. Bergner, Daniel Schröder, Philipp Adelhelm, and Jürgen Janek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00903 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on April 11, 2016

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The Journal of Physical Chemistry

Insights into the Chemical Nature and Formation Mechanisms of Discharge Products in Na-O2 Batteries by Means of Operando X-ray Diffraction R. Pinedo1, D. A. Weber1, B. Bergner1, D. Schröder1*, P. Adelhelm1,2 and J. Janek1* 1

Physikalisch-Chemisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 17, 35392 Gießen (Germany)

2

Institute for Technical Chemistry and Environmental Chemistry, Center for Energy and Environmental Chemistry (CEEC), Friedrich-Schiller-University Jena, Philosophenweg, 07743 Jena (Germany) * Corresponding authors: [email protected] (D. Schröder); [email protected] (J. Janek)

Abstract The chemical nature of discharge products in non-aqueous Na-O2 batteries attracts attention due to the differing electrochemical behavior of seemingly similar battery cells forming either sodium peroxide (Na2O2) or sodium superoxide (NaO2). The experimental control parameters for the exclusive formation of one or the other phase during discharge are still unknown. In this paper, the important role of gas phase moisture on the chemical nature of these products is demonstrated by means of operando XRD. Operando XRD is rarely reported for alkali-O2 systems, but provides valuable real time information about the solid discharge products, growth/decomposition rates and side reactions in the cell. In addition, the discharge/charge mechanisms of Na2O2·2H2O are monitored in detail evidencing important differences from the cell reactions forming NaO2 and Li2O2, and refuting previous incorrect conclusions. Moreover, the effect of the applied potential window during cycling is discussed, as it provides valuable information for the future development of Na-O2 batteries as electrochemical energy stores.

1. Introduction Safe energy storage devices of lower cost and higher energy densities compared to current lithium ion batteries (LIB) are being extensively researched in order to meet future demands for electrochemical storage systems. Metal-oxygen systems1, with their high theoretical energy densities, have early been regarded as a promising alternative to overcome the limitations of LIBs, and they were considered for potential use in automotive applications2,3. In particular, great effort has been spent to the development of Li-O2 cells, which were described for the first time by Abraham et al. in the mid-1990s 4. Thereafter, numerous publications were dedicated to the understanding and overcoming of main hurdles toward practical use, such as undesired side reactions5, e. g. degradation of electrolyte associated to high overpotentials6-8, poor cyclability9 or high impedances leading to low current densities and low power10. Although some promising advances were achieved, reliable and reproducible solutions are still far away from practical application. Parallel to the study of Li-O2 cells, an alternative based on the substitution of lithium by sodium has emerged, and these Na-O2 cells – in spite of their lower theoretical energy density – can exhibit better reversibility and much lower overpotentials compared to lithium based cells11-12. In addition, the cost and availability of sodium can be considered as further

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advantages13. In contrast to non-aqueous Li-O2 cells, where Li2O2 was unequivocally identified as the final discharge product, at least three different discharge products, sodium superoxide, peroxide dihydrate and dehydrated peroxide (NaO2, Na2O2⋅2H2O,Na2O2), in some cases accompanied by side products such as sodium hydroxide (NaOH) and/or sodium carbonate (Na2CO3), have been reported for Na-O2 cells14. Although the parameters which influence the generation of these different products are still unknown, the thermodynamic driving forces for the formation of NaO2 and/or Na2O2 are reported to be quite close, and thus, subtle parameters such as solvent mixture, the type of conducting salt and the material of the positive electrode, gas purity, applied current densities or potential windows could be decisive for the formation of one or the other compound15. The severe interest in elucidating the key parameters controlling the discharge route either to the formation of super- and/or peroxides, however, is motivated by the difference in energy density (NaO2: 2643 Wh.Kg-1(excluding weight of oxygen) / 1105Wh.Kg-1(including weight of oxygen) and 2431 Wh.L-1 (refer to the discharged state); Na2O2: 2717 Wh.Kg-1 (excluding weight of oxygen) / 1602 Wh.Kg-1 (including weight of oxygen) and 4493 Wh.L-1 (refer to the discharged state)) and the completely different electrochemical performance9. A close correlation between discharge product and shape of the voltage profile is found. And categorizing the different voltage profiles provides a simple method of describing the overall properties of the cell. So far most metal–oxygen batteries show the following behavior when cycled at moderate rates (see reference 9 for details): Type 1B is found for Na-O2 cells with NaO2 as discharge product. Type 2C, 3B, and 3C are found for Li-O2 and Na-O2 cells with either Li2O2, Na2O2, or Na2O2⋅2H2O as a discharge product. In contrast to NaO2 as discharge product, for which low overpotentials and coulombic efficiencies of over 90 % are reported16, the peroxide exhibits several charge plateaus and notably higher overpotentials. Consequently, poor cycle stability is observed in Na2O2 cells, while more than 100 stable (shallow) cycles have been attained with their NaO2 analogs. Hence, only NaO2 based systems might be considered a step forward on the trail away from LiO2 cells, however, with comparably low energy density. In contrast to NaO217, where the oneelectron transfer allows excellent ORR and OER kinetics, the mechanisms associated with the formation of Na2O2 are poorly understood so far, and further efforts are required that combine the identification of the conditions favoring its exclusive formation with accurate analyses of the electrochemical reactions and performance. In the first part of this work, the influence of several variables on the chemical nature of the discharge products is discussed briefly; a more detailed review of all available data in literature can be found in reference 18. The resulting conclusions are the basis for the experimental part of this work, in which both products, peroxide and superoxide, could be independently formed and unequivocally characterized, thus demonstrating the importance of gas moisture for the cell chemistry of Na-O2 cells. Moreover, both formation and reversion processes are monitored by means of operando XRD and gas pressure measurements, revealing not only the value of these techniques, but also allowing a better understanding of the cell reactions, crystal growth rate and electrochemical performance of the systems. Finally, implications on the cycling stability of the different discharge products are discussed. 2. Experimental and methods 2.1. Galvanostatic measurements Cell Assembly: Commercial Freudenberg H2315-C4 (Ø 12 mm) electrodes purchased from Quintech were employed as positive electrodes. The negative electrode consisted of pure sodium metal (donated by BASF SE) with a diameter of 12 mm. Two glass microfiber filters


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(GF/A, Whatman, Ø 12 mm) were used to separate the electrodes. A Na-β-Al2O3 membrane (Ø 12 mm and thickness 0.5 mm, supplied by Ionotec) was located between the separators to avoid short circuits due to dendrite growth. Tetraglyme (anhydrous, 99.5% Sigma Aldrich) was used as solvent and sodium trifluoromethanesulfonate (sodium triflate, NaSO3CF3, 98%, Aldrich) as conducting salt for the electrolyte. Tetraglyme was dried over molecular sieve (3 Å for at least 2 weeks), while sodium triflate was purified as described by McCloskey et al.19. The final water content of the electrolyte was determined with an 831KF Karl Fischer coulometer (Metrohm) to be less than 10 ppm. Every cell contained 90 μL of electrolyte. Preparation and handling of the electrolyte, as well as cell assembly were carried out in an argon-filled-glovebox (GST4, Glovebox Systemtechnik) with water and oxygen contents below 5 ppm. Electrochemical Cell Testing: Cyclability tests were performed using home-made cells reported in previous works15. All cells were galvanostatically cycled in a climate chamber at 25 °C, using a Maccor battery cycling system 4300. The cells were flushed with oxygen (purity 5.0, Praxair) for 10 s at 105 Pa, just before the measurement. The lower and upper potential limits were set to 1.5 V vs. Na+/Na and 2.8-4.5 V vs. Na+/Na respectively. 2.2. Structural and morphological characterization Exsitu measurements: Discharged positive electrodes were analyzed using an Empyrean (PANalytical) X-ray powder diffractometer (Cu-K source, 40 kV, 40 mA). A self-made gas-tight sample holder, sealed with a polyethylene foil inside the glove-box, was used for the measurements. SEM images were taken in a Merlin high-resolution Schottky fieldemission electron microscope (Zeiss SMT) equipped with an X-Max EDS detector (Oxford Instruments). Operando XRD: A home-made cell was developed for the electrochemical operando XRD experiments. A schematic illustration of the cell can be found in the supporting information (Figure S.1). Gas inlet and outlet were integrated in order to perform measurements under flowing O2. For the measurements under dry conditions, O2 was passed through a vessel with excess of P2O5 located just before the gas entrance of the cell. The cell was sealed with a commercial polyimide dome to ensure air-tight conditions. It must be noted that a small amount of water can permeate through the polymide dome20, however, this amount is not only insufficient to influence the chemical nature of the discharge products but also provides further support to the necessity of high water amounts for this feature. In addition, we can assume that the employed oxygen flow rate is higher than that of the water permeation. The cells were assembled according to the aforementioned procedure, but doubling the volume of electrolyte (180 µL), in order to avoid drying-out during the measurements under flowing gas. XRD patterns were recorded in intervals of 0.2 mAh. A schematic illustration of the carried out experiment is presented in Figure 1.



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Figure 1.Schematic illustration of the operando XRD experiments performed with either dried or humid oxygen 2.3. Pressure monitoring measurements Pressure Sensor: All pressure data were recorded using a closed oxygen vessel, a PAA-33X absolute pressure sensor mounted atop, a K104B USB computer adapter, and the software Read30 v2.10 (all from Omega Engineering). The volumes of the different gas reservoirs were determined by parallel use of the pressure sensor and a calibrated syringe (Hamilton). The oxygen consumption and the gas evolution were calculated from the recorded pressure data on the basis of the ideal gas law. 3. The NaO2/Na2O2 controversy Despite the increasing number of research articles onNa-O2 batteries, the parameters which enable the formation of sodium peroxide and/or superoxide remain unknown18. Up to date, unequivocal evidence has only been reported for NaO2 as crystalline discharge product. The formation of Na2O2⋅2H2O under certain conditions has been also claimed but it is based on less reliable crystallographic data. Thus, the reported evidence for the formation of Na2O2 is relatively weak. Some of the potential parameter which could influence the formation of super/peroxides have been already analyzed in previous reports and can be associated to thermodynamic variables, kinetic (cycling) parameters and cell components. In order to summarize and discuss the contradictions reported until now, a short overview is given in the following section.

3.1 Cell components The solvent mixture has been proposed as one of the potential key parameters as the oxygen solubility and diffusion coefficient could reduce the local oxygen partial pressure (pO2) favoring the formation of Na2O2. However, from the data reported by Hartmann et al.21 no significant differences are expected under conventional test conditions (pO2, electrode thickness, conducting salt concentration) and organic solvents (diglyme, tetraglyme, dimethoxyethane), and therefore no noticeable differences in pO2 can be attained22. Thus, the fact that different


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products have been found by different groups when similar or even the same organic solvents have been employed23-25, supports the idea that the key parameter should be attributed to other variables. This conclusion is also valid for conducting electrolyte salts26,27 or different types of positive electrodes15. Regarding the water content in the electrolyte, although the lack of experimental details in some of the published results does not allow an accurate comparison, we assume that water concentrations in the electrolyte are kept in all the cases under reasonable limits (< 20 ppm), and no differences have been observed within these limits28. It is noteworthy that the water activity exerts strong influence on both morphology of the discharge products29 and specific capacity30 in Li-O2 cells. In contrast, in Na-O2 cells the total absence of water leads to quasi-amorphous product films and negligible capacities28.

3.2. (Electro)chemical factors 3.2.1. Current density The effect of current density on the discharge products has only recently been suggested. Yadegari et al. reported that the chemical composition of the discharge product, and consequently the charge overpotential of Na-O2 cells, could be altered via modifying kinetic parameters such as discharge current density31. This conclusion is based on the different charge profiles attained after discharging several batteries to 0.5 mAh with current densities in the range of 0.1 to 1 mA/cm2. Thus, increasing the current density leads to the appearance of a low potential plateau (< 3 V), which the authors associated withNaO2decomposition. This effect was already demonstrated in their previous work32, but the appearance of the low potential plateau at current densities as low as 0.02 mA/cm2 contradicts their own results. In addition, in accordance to previous works, this low potential plateau could be also ascribed to the hydrated peroxide9. Furthermore, no differences have been found by other groups for both NaO2 and Na2O2 products in current density ranges as wide as 0.01-0.5 mA/cm2 33,34, and 200 mA/g-1000 mA/g (no details about the weight of the electrode or total current were provided)24, respectively. Our results also indicate that current density does not influence the chemical nature of the discharge products (see S.I.2), as solely NaO2 was found in the current density range of 0.05 to 0.2 mA/cm2. Indeed, the formation of superoxide, based on a oneelectron transfer, in general appears to be kinetically favored compared to the two-electron transfer of peroxide35. 3.2.2. Parasitic reactions Minimizing parasitic reactions is one of the main challenges in alkali-oxygen batteries. The products of unwanted side reactions have usually been analyzed by XRD and different spectroscopic techniques (IR, Raman, XPS, XANES, NMR, etc.), however, their accurate determination is complicated due to the amorphous nature of some side products, the limited sensitivity to small concentrations of some of these techniques or the beam induced damages of the native compounds. Even though, the presence of side reactions in alkali-O2 batteries becomes evident as the key chemical signatures of ideal rechargeability are not fulfilled so far36: (1) The yield of superoxide or peroxide relative to that expected from the current and ideal cathode reaction should be 1.00, i.e., no other products are formed during discharge either on the cathode or in the electrolyte, e.g., no hydroxides, carbonates, fluorides, carboxylates, etc.;



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(2) During discharge, the electric current only consumes O2, with [n(e-)/n(O2)]dis = 1.00 or 2.00, for superoxide and superoxide, respectively. During charge, the electric current only leads to the evolution of O2, with [n(e-)/n(O2)]ch = 1.00 or 2.00; (3) No parasitic gas evolution (H2, CO2, etc.) occurs during cycling – which is a necessary condition for (2) in case that the parasitic side reaction are directly caused by the electric current; (4) All O2 consumed during discharge (ORR) is released during charge (OER) so that OER/ORR = 1.00. One of the commonly reported parasitic reactions is the formation of water due to electrolyte degradation by the reaction with superoxide species32,37-39. Thus, although undesired reactions could provide the water source for the formation of Na2O2⋅2H2O, we consider this as unreasonable. Obviously, the huge amount of water necessary to achieve a specific capacity of 1 mAh (see calculations in the S.I.) cannot be provided uniquely by parasitic reactions. Our calculations are consistent with a recent work of Grey and coauthors, who show that the proton source for the LiOH formation is the water present in the electrolyte and is not related to the decomposition of the organic solvent40. Moreover, the group of Nazar reported that NaO2 cells cannot be discharged in absence of water, which acts as a proton source28. Evidently, if the proton source could be supplied by the electrolyte, Na-O2 cells with 0 ppm water could be also discharged. In summary, we estimate that the measured water amount in the electrolyte (< 10 ppm) is not sufficient to explain formation of Na2O2⋅2H2O, rather a twofold higher volume of electrolyte should be added to ensure sufficient water to produce the given discharge specific capacity (see S.I). 3.3. Thermodynamic assessments According to the aforementioned considerations, the factor which steers towards superoxide or peroxide might be related to thermodynamic variables, in view of the close equilibrium potentials for the superoxide and peroxide formation (2.33 and 2.27 V respectively). In addition, the fact that different groups have usually found the same discharge product even under different (electro)chemical conditions and cell components, hints that the key factor could be from an external source, such as the supplied oxygen. 3.3.1. Size effect Theoretical studies were executed to clarify which is the most stable phase under conventional operation conditions (pO2 = 1 atm, T = 298 K). Unfortunately, contradictory results have been reported22,41. Ceder et al. confirmed that for bulk material Na2O2 is stable and NaO2 is metastable at standard conditions41. The authors reports that NaO2 has a much lower surface energy than the peroxide, and therefore, it is stabilized when the particle size is in the nanometer regime where nucleation takes place. Moreover, these nuclei may grow homoepitaxially and never transform to Na2O2, remaining metastable, thus explaining the micrometer-sized NaO2 particles reported in literature. The fact that this holds for all discharge potentials does not explain how sodium peroxide can be formed, and further supports the idea of an external influence rather than an intrinsic one. 3.3.2 Oxygen partial pressure It is evident that a high pO2thermodynamically favors formation of NaO2 relative to Na2O2 while a low pO2 facilitates formation of Na2O2. We tested the effect of low pO2 by monitoring


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the pressure of the cell after several cycles. The results evidence that even after several cycles, the one-electron mechanism still operates despite the progressive decay in pO2 caused by the incomplete oxygen recovery (see S.I.9). These results agree with those of Hartmann et al., who also proved that NaO2 was the main discharge product under pO2 = 0.2 atm21. Certain organic solvents (e.g. ionic liquids) could provide lower oxygen concentrations under kinetic load due to their low oxygen solubility. Up to date, these types of solvents have only been reported twice for room temperature Na-O2 systems with contradictory results42,43. In the first case42, authors reported the presence of peroxide, but the ambiguous XRD identification together with the instability against sodium of the employed ionic liquid, limit reliable conclusions. In contrast, the group of Guo demonstrated the presence of NaO2 in ionic liquids, although the performance was significantly lower than that obtained with ether based electrolytes43. Nevertheless, under conventional conditions (usual organic solvents, current densities and positive electrode thicknesses) sufficient O2 for formation of NaO2 is available in the cell44. 3.3.3. Gas moisture Gas moisture has been recently reported to have an effect on both cell performance and chemical nature of the discharge product45. However, the discharge products are often only poorly characterized, while the death of the cells in some cases seems to be caused by dendrite formation4. Considering that air was continuously supplied during the analysis and that previous work reported different products for either static and continuous flow conditions32, we assume that gas moisture might play a relevant role in the control of discharge products. Moreover, the reaction of water with several cell components is thermodynamically favored, and could result in the formation of sodium hydroxide and/or in hydrated forms of sodium peroxide easily, even while transferring samples from cells to analytical devices15. Nevertheless, the moisture provided by the oxygen gas is expected to be absorbed by the organic solvent and therefore, a reaction between NaO2 and water should occur. Interestingly, previous studies have reported the formation of NaO2 even in the presence of large amounts of water in the electrolyte28,33. Thus, continuous sources of water can be only provided by leakages and/or continuous feed of oxygen. The effect of leakages has recently been shown by Ortiz-Vitoriano et al.33. Therefore, the influence of continuous flow of oxygen, together with the role of water in the electrolyte will be examined in detail in the next sections. 4. Results and discussion 4.1. The role of water in gas and liquid phases In order to analyze the effect of gas moisture on the chemical nature of the discharge products, we designed a specific cell, enabling operando XRD analysis. This type of analysis, rarely found in metal-oxygen cells to date47,48, and to the best of our knowledge reported for the first time for Na-O2 systems, provides real time determination of the chemical nature of the crystalline discharge products together with information about their formation/reversion rates. Experiments were conducted with a continuous flow of either humidified or dried oxygen. In both cases oxygen was supplied to the cells by plastic tubings. Such tubings are frequently used in lab research but suffer, in contrast to metal tubings, from gas/H2O leakage. So gas supplied via this tubing will be always humidified, providing a continuous source of water. Thus, we define these measurement conditions as “wet conditions” – irrespective of the water content of the pre-humidified gas (see Figure 2). For real dry conditions, the flowing



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gas was dried with P2O5directly before entering the cell. The obtained results are presented in Figure 2.

Figure 2.Operando X-ray diffraction patterns recorded continuously in intervals of 0.2 mAh for cells 2 discharged with j = 0.2 mA/cm , cut-off of 1 mAh and voltage window of 1.5 V - 3 V under a) dry and b) wet(flowing gas) conditions. The arrows indicate the direction of cycling. The corresponding discharge (blue)/charge (orange) profiles are presented in S.I.2. The reflections associated to the superoxide and peroxide are denoted with asterisk and triangle symbols, respectively. The reflections corresponding to the current collector or external cell components are denoted with open circle symbols.

The diffraction patterns unequivocally prove the key role of wet/dry oxygen atmosphere in controlling the chemical nature of the discharge products, and therefore, of the electrochemical performance. Under (real) dry conditions, NaO2is identified as the sole discharge product (see details in the S.I.). The diffraction pattern under wet conditions reveals the presence of a phase that is usually assigned as Na2O2⋅2H2O. Unfortunately, the insufficient crystallographic data available until date, together with their lack of reliability, do not allow reasonable refinements. Small amounts of side products such as NaOH (38.4°) have been detected and evidence undesired side reactions which also influence the coulombic efficiency. The effect of the gas moisture is consistent with literature since NaO2 has yet never been detected under flowing gas conditions11,31,32,34,45,49, in contrast to stationary conditions (closed cells) for which NaO2 is reported in most of the cases12,15-19,21,28,33,34,43,46,50. Hence, for the first time we were able to form (and prove the formation of) both products in the same cell under identical conditions except the water content of the flowing oxygen gas. This finding highlights the effect of gas moisture and the importance of its control not only for metal-O2batteries but also for practical metal-air battery operation, as indicated elsewhere51. 4.2. Discharge/charge mechanisms for Na2O2⋅2H2O 4.2.1. Discharge process and side reactions during discharge After the key parameter and the product formation conditions under flowing gas have been elucidated, we aim to clarify the reaction mechanisms. For this purpose, pressure sensor measurements are suitable, as they allow the direct determination of the number of electrons transferred to the oxygen molecule (one- or two-electron transfer mechanism)6,12. Unfortunately, pressure sensor measurements are not possible in the XRD cell setup under flowing gas conditions. Therefore, we mimic large amounts of water absorbed from gas moisture by adding a sufficient amount of water to the organic solvent also in order to facilitate peroxide formation. We proved this by means of post-mortem XRD measurements of


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electrodes from closed cells and indeed identified hydrated peroxide (following the usual assignment) and sodium hydroxide as discharge and side products, respectively (see S.I.). The typical discharge profile of a cell with high water amount shows a higher potential plateau by about 200 mV than water-free cells, and it is similar to those reported previously28,33. This higher potential probably corresponds to a mixed potential of the formation of NaOx and side products. The formation of hydrated peroxide (as seen in S.I.8) contradicts these previous studies, where no peroxide products were reported. The reason of this controversy could be related to the superior amount of water (10 vol.-%) employed in the present study, which ensures sufficient water even after reaction with the anode. These results not only provide further evidence of the key role of water, but they also justify monitoring of the gas pressure in the closed cell instead of measuring oxygen consumption in the open cell (Figure 3).

Figure 3. Pressure sensor monitoring of a cell with 10vol.-% water added in the electrolyte and cycled under j = 0.1mA/cm2 and a voltage window of 1.5-4.5 V. a) Evolution of the gas pressure(grey curve) and b) and c) number of electrons exchanged per mol of gas (green), during the discharge (blue) and charge process (orange).

From the pressure sensor analysis, several important conclusions can be drawn. The first effect is observed before the operation of the cell, where the gas pressure is expected to remain constant. At this stage, however, the cell pressure notably increases, due to the chemical reaction between the metallic sodium and water which results in the production of H2 (region I). The increase of the cell gas pressure comes along with a slight increase of the discharge potential, which might be related to the formation of a passivating anode SEI (e. g. formed by NaOH). Once passivation is achieved, the cell pressure starts to decrease linearly during discharge (region II). This decrease is consistent with a two-electron transfer (after about 24 h) and thus, the formation of peroxide species prevails. Moreover, due to the large amount of added water, the influence of side reactions adding to the observed two-electron process cannot be neglected, as the number of electrons consumed per oxygen in the transient phase is higher than 2. The discharge process can be explained by the following two-reaction mechanism (R1 + R2): 2 O2 + 2 Na+ + 2 e- → 2 NaO2

(R.1)

2 NaO2 + 2 H2O →Na2O2⋅2H2O + O2

(R.2)

O2 + 2 Na+ + 2 H2O + 2 e-→Na2O2⋅2H2O

(Overall)

The aforementioned reaction mechanism includes two well identified processes: the oneelectron transfer mechanism for the formation of NaO212,19,28 and the chemical decomposition of this compound in the presence of water33,34, effectively leading to a quasi two-electron process as derived only from pressure monitoring. Clearly, the rate of gas release (region III) is approximately two times higher than that of the discharge in region II, and it surprisingly appears that the decomposition of the formed peroxide proceeds in a one-electron process.



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The oxygen reversion yield (ratio of oxygen evolution during charge to oxygen consumption during discharge) is about 80 % in region III (low overpotential plateau), which is significantly lower than that of NaO2 reported previously (> 90 %)12. It must be noted that even this value (80 %) is probably overestimated for the case of cells with water because the hydrogen production at the beginning alters the real O2 consumption values. Interestingly, the operando XRD analysis presented in Figure 4 shows that the hydrated peroxide is completely decomposed during the first charge plateau, indicating that the all regular discharge product is in fact decomposed during the first charge plateau and that further charge plateaus correspond to the decomposition of side products.

Figure 4. Discharge/charge profile of cells forming a) NaO2and b) Na2O2⋅2H2O during operando XRD together with the integrated intensity of the main XRD reflection of the respective product phase. The points represent measured data while the dashed lines show linear fits to the experimental data. Data related to discharge and charge processes are colored in blue and orange, respectively.

To date, no comprehensive and conclusive data for peroxide formation have been reported in Na-O2 batteries, and we therefore propose a reasonable mechanism in the following. We suggest that the formation of NaOH (R.3), which has also been detected as side product, plays an important in cells with large concentration water. It is important to note that any reaction should also be present during the second and further cycles, as the ratio between the discharge and charge slopes remains constant (see S.I.). 2 NaO2 + 2 H2O → H2O2 + 2 NaOH + O2

(R.3)

Na2O2⋅2H2O 2 NaOH + H2O2

(R.4)

Thus, R.3 may compete with R.2 without changing the number of transferred electrons per mol of oxygen, but by forming NaOH which requires a higher charge potential. In addition, peroxide dihydrate might also be also degraded to hydroxide (R.4) in a disproportionation reaction, as recently reported18, and again no variation of the number of transferred electrons per mol of oxygen is expected during discharge. This appears reasonable as cells with peroxide dihydrate as discharge product always exhibit an extra plateau associated to NaOH, even when this product is not detected in the cathode after discharge. The decomposition of hydrogen peroxide would also result in oxygen evolution (see R.6), and therefore, in a different value of electrons per mol of oxygen. Further studies will be necessary to determine the role of these reactions as several variables such as the amount of water, electrolyte nature, pH value and/or volume could affect the solubility of NaOH and the stability of H2O2 complicating not only their identification but also comparisons between different groups. 4.2.2. Charge process


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Assuming the formation of Na2O2⋅2H2O as a consequence of the disproportionation of electrochemically formed NaO2, it is important to determine whether the peroxide can be electrochemically charged at reasonable potentials or not. The characterization of peroxide discharge products, as reported in the literature, is ambiguous and various charge plateaus have been observed9. In addition, not much information about the charge mechanism has been reported to date, with Kim et al. being the only authors who proposed the following charge mechanism for peroxide dihydrate40: Na2O2⋅2H2O→ 2 NaOH + H2O2

(R.4)

2 NaOH → 2 Na+ +H2O2 + 2e-

(R.5)

2 H2O2→ 2 H2O + O2

(R.6)

which can be represented by the (two-electron delivering) net reaction Na2O2⋅2H2O → 2 Na++ 2e- + 2 H2O + O2 It must be noted that R.4 and R.5 are expected to occur during the first and second charge plateaus respectively. Thus, the gas evolution should continue after NaOH decomposition, while the first reaction should not result in a plateau as no electrons are transferred. Evidently these data, together with the two-electron transfer mechanism, do not match with the pressure sensor data, which indicate a one-electron transfer mechanism and main oxygen evolution during the first stages of charging. Two alternative mechanisms fit better to the experimental observations:(a)The apparent oneelectron transfer reaction during charge, the observed low overpotential and the lack of reliable crystallographic data indicate the presence of a certain amount of superoxide in the discharge product. However, considering that nearly 80 % of the oxygen consumed during the discharge was released during the first charge plateau, large amounts of superoxide would be expected; but no XRD reflections of NaO2 have been detected under wet conditions. Thus, we can safely assume that NaO2 is not involved to a major extent. (b) The second explanation assumes the presence of a parallel reaction at the anode with hydrogen gas evolution during charge. Thus, a possible reaction mechanism could be the following: Na2O2⋅2H2O → 2 Na+ + O2 + 2 H2O +2e-

(R.7)

2 H2O + 2 Na →2 NaOH + H2

(R.8)

It must be noted that R.7 is only reasonable if secondary products are formed during discharge, as the formation and reversion rates significantly differs from each other (Figure 4b). Nevertheless, as mentioned above, the formation of NaOH during discharge can be indirectly evidenced by the 3 V charge plateau, validating therefore the proposed reaction path. We suggest that R.7 and R.8 occur in parallel at the cathode and anode side, and that freshly deposited sodium metal directly reacts with water. In a recent study of Li-O2cells, we found that the lithium metal anode unexpectedly affects reactions at the cathode side52, which supports our current conclusion. Obviously, the reactions at the anode must be considered for cells with high concentration of water and these reactions will be analyzed in detail in further studies. In order to better explain R.8, two factors must be taken into account: i) during the charge process the sodium metal surface is renewed via plating of Na ions on the anode, allowing R.5



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to occur; ii) the produced NaOH is consistent with the 3 V plateau, which is usually observed in cells where peroxide dihydrate is found9. According to the operando XRD analyses and previous work34,40, the decomposition of peroxide dihydrate occurs exclusively during the first charge plateau (2.5 V), while NaOH, continuously formed during discharge (R.3) and also during the first charge plateau (R.4), is exclusively decomposed during the second plateau. In order to validate this reaction, differential electrochemical mass spectrometry (DEMS) measurements are required and will be performed in future studies. This type of measurement will also be effective for determining the reaction mechanism of the second plateau which has been previously ascribed to NaOH decomposition: NaOH → Na+ + e- + ½ H2 + ½ O2

(R.9)32

2 NaOH → 2 Na+ + 2e- + H2O2

(R.10)40

It is worth noting that R.10 is more likely to occur as only negligible gas evolution is observed during this plateau, which leads to a high increase on the number of transferred electrons per mol of oxygen (Region IV). In fact, this high values of [n(e-)/n(O2)]ch appears strange, but it is the natural consequence of its definition – as virtually no gas is being released in region IV. Finally, the last pseudo-plateau observed above 4 V is ascribed to CO2 evolution (Region V) and occurs in both peroxide and superoxide type cells, suggesting the oxidation of the electrolyte and/or graphite electrode at these high voltages – as it also occurs in Li-O2 cells. And again the high values of [n(e-)/n(O2)]ch essentially mean that only a very small amount of gas is released. This fact is in good agreement with the cycling performance at elevated potentials windows which is going to be discussed in the next section. 4.3. Cyclability and potential window 4.3.1. Potential window The influence of the potential window on the cycling behavior is usually omitted in the study of Na-O2 cells. Although Hartmann et al. reported a significant increase of the coulombic efficiency by lowering the upper cut-off voltage from 3.6 to 2.6 V16, most results reported until now were obtained with upper potentials above 3 V, especially for cells with peroxide as main discharge product. In order to investigate the role of the potential window, we cycled conventional cells (static O2 and dried electrolyte) within different potential windows (see Figure 5). We like to note that these conditions have been selected in order to avoid side reactions, which are more likely to occur in wet conditions.


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Figure 5. Shallow cycling of different cells with cutoffs established at 0.5 mAh, j = 0.2 mA/cm2 potential windows of a) 1.5-2.8 V, b) 1.5-3.6 V and c) 1.5-4 V, respectively. d) Coulombic efficiency of the cycled cells.

As expected, the widening of the potential window results in a significant decrease of the efficiency and/or the battery life, indicating the influence of side reactions on the instability of the cell components in the selected potential window. The potential limit ≤ 2.8 V is consistent with the sharp interruption of the linear evolution of O2during the charge process. Therefore extending the upper voltage is expected to favor the degradation of the cell components and the premature failure of the cell, in a similar way as in Li-O2 systems5. In contrast, by keeping the voltage window under reasonable limits (< 3 V), over hundred shallow cycles can be achieved with efficiencies exceeding 90-95 % and combined overpotentials of around 0.5 V. For the cells with peroxide as the main discharge product, adjusting the upper voltage to 3 V also results in a progressive and significant increase of the charge plateau (see Figure 6a).



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Figure 6.a) Cyclability of the hydrated peroxide cell in the potential range 1.5-3 V in Swagelok cell 2 configuration with flowing O2 without drying and a current density of 0.2 mA/cm ; b) Diffraction diagram (post mortem) of the cathode cycled as shown in Figure 6.

However, the large amount of water required for the formation of Na2O2·2H2O promotes also NaOH formation, which cannot be reverted below 3 V. Thus, after few cycles, the cathode is completely covered with accumulated NaOH avoiding further discharges (Figure 6b). These results are further supported by impedance measurements presented in the supporting information (S.I. 12). Widening of the potential window is necessary in cells cycled under wet conditions, since side products are only reverted at higher potentials. However, as mentioned above, this strategy accelerates the degradation of the cell components, explaining the poor cycling stability (limited number of cycles and/or high overpotentials) reported until now for hydrated peroxide species. 5. Conclusions Conflicting results on the chemical composition of the discharge product in Na-air batteries have been reported within the last years. Within the present study we found that this peculiar behavior can be well explained by the different gas supply of the studied cells. For the first time, we were able to deliberately form either NaO2 or Na2O2·2H2O in the same cell by varying how oxygen gas is supplied to the cell, i.e. by using either static or flowing conditions. We found that the key factor determining the discharge product is the amount of available water. When oxygen gas is continuously fed to the cell, water impurities enter the system simply because of leakage through frequently used plastic tubing. This effect can be circumvented by drying the gas with P2O5 directly before entering the cell (or by using sealed metal tubes). Thus, when the gas is properly dried, NaO2 forms as the main discharge product enhancing the performance and cycling stability of the cells. Gas moisture promotes the disproportionation of NaO2 to Na2O2·2H2O limiting its electrochemical behavior notably. Both products have been identified for the first time by means of operando XRD measurements providing significant information not only about the chemical nature of the discharge products but also about the growth and reversion rates of both discharge products. In addition, the discharge/charge reaction mechanisms in cells with peroxide dehydrate have been analyzed in detail by means of pressure monitoring. Finally, the effect of the potential window on the cycle stability of both systems has been shown and discussed, evidencing its critical role in the battery life. Moreover, these results indicate the necessity of developing suitable membranes that completely block out water from the atmosphere to enable long-life Na-air cells with NaO2 as discharge product.


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Supporting Information Supporting information contains further information on the following points: • • • • • • • •

Methods Quantitative comparison of theoretical water amount related to peroxide products and the real water amount calculated from the Karl Fischer measurements. Kinetic Factors: Effect of the current density Operando XRD analyses Discharge product identification and side reactions in Na-O2 cells Effect of water in the electrolyte Effect of the potential window SEM images of the discharge products

Acknowledgements The authors thank M.R. Busche and Dr. Bjoern Luerβen for fruitful discussions and Dr. Pascal Hartmann (BASF SE) for critical reading of the manuscript. The authors acknowledge support by the LaMa (Laboratory of Materials Research) at JLU. This project was supported by the BASF International Network for Batteries and Electrochemistry.

References: [1] Christensen J.; Albertus P.; Sanchez-Carrera R. S.; Lohmann T.; Kozinsky B.; Liedtke R.; Ahmed J.; Kojic A. A Critical Review of Li/Air Batteries. J. Electrochem. Soc. 2012, 159, R1-R30. [2] Girishkumar G.; McCloskey B.; Luntz A. C.; Swanson S.; Wilcke W. Lithium-Air Batteries: Promise and Challenges. J. Phys. Chem. Lett. 2010, 1 2193−2203. [3] Bruce P. G.; Freunberger S. A.; Hardwick L. J.; Tarascon J.-M. Li-O2 and Li-S Batteries with High Energy Density. Nat. Mater. 2012, 11, 19-29. [4] Abraham K. M.; Jiang Z. A Polymer Electrolyte-Based Rechargeable Lithium/Oxygen Battery. J. Electrochem. Soc. 1996, 143(1), 1-5. [5] McCloskey B. D.; Valery A.; Luntz A. C.; Gowda S. R.; Wallraff G. M.; Garcia J. M.; Mori T.; Krupp L. E. Combining Accurate O2 and Li2O2 Assays to Separate Discharge and Charge Stability Limitations in Nonaqueous Li-O2 Batteries. J. Phys. Chem. Lett. 2013, 4, 2989-2993. [6] Bergner B. J.; Schürmann A.; Peppler K.; Garsuch A.; Janek J. TEMPO: A Mobile Catalyst for Rechargeable Li-O2 Batteries. J. Am. Chem. Soc. 2014, 136, 15054-15064. [7] Lim H. D.; Song H.; Kim J.; Gwon H.; Bae Y.; Park K. Y.; Hong J.; Kim H.; Kim T.; Kim Y. H.; Lepró X.; Ovalle-Robles R.; Baughman R. H.; Kang K. Superior Rechargeability and Efficiency of Lithium-Oxygen Batteries: Hierarchichal Air Electrode Architecture Combined with a Soluble Catalyst. Angew. Chem. Int. Ed. 2014, 53, 1-7. [8] Landa-Medrano I.; Olivares-Marín M.; Pinedo R.; Ruiz de Larramendi I.; Rojo T.; Tonti D. Operando UV-Visible Spectroscopy Evidence of the Reactions of Iodide as Redox Mediator in LiO2 Batteries. Electrochem.Comm. 2015, 59, 24-27. [9] Adelhelm P.; Hartmann P.; Bender C. L.; Busche M.; Eufinger C.; Janek J. From Lithium to Sodium: Cell Chemistry of Room Temperature Sodium-Air and Sodium-Sulfur Batteries. Beilstein J. Nanotechnol. 2015, 6, 1016-1055. [10] Luo Z. K.; Liang C. S.; Wang F.; Xu Y. H.; Chen J.; Liu D.; Sun H. Y.; Yang H.; Fan X. P. Optimizing Main Materials for a Lithium-Air Battery of High Cycle Life. Adv. Funct. Mater. 2014, 24, 21012105. [11] Peled E.; Golodnitsky D.; Mazor H.; Goor M.; Avshalomov A. Parameter Analysis of a Practical Lithium- and Sodium-Air Electric Vehicle Battery. J. Power Sources 2011, 196, 6835-6840.



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

[12] 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. Mat. 2013, 12, 228232. [13] Palomares V.; Serras P.; Villaluenga I.; Hueso K. B.; Carretero-González J.; Rojo T. Na-ion Batteries, Recent Advances and Present Challenges to Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5, 5884-5901. [14] Kwak W. J.; Chen Z.; Yoon C. S.; Lee J. K.; Amine K.; Sun Y. K. Nanoconfinement of LowConductivity Products in Rechargeable Sodium-Air Batteries. Nano Energy 2015, 12, 123-130. [15] 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) Batteries. Adv. Energy Mater. 2014, 4(12), 1301863. [16] 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. [17] Hartmann P.; Heinemann M.; Bender C. L.; Graf K.; Baumann R. P.; Adelhelm P.; Heiliger C.; Janek J. Discharge and Charge Reactions Paths in Sodium-Oxygen Batteries: Does NaO2 Form by Direct Electrochemical Growth or by Precipitation From Solution? J. Phys. Chem. C 2015, 119 (40), 22778-22786. [18] Bender C. L.; Schröder D.; Pinedo R.; Adelhelm P.; Janek J. One or Two Electron Electrode Reaction? – On the Ambiguous Nature of the Discharge Products in Sodium-Oxygen Batteries. Angew. Chem. Int. Ed. (in press). [19] McCloskey B. D.; Garcia J. M.; Luntz A. C. Chemical and Electrochemical Differences in Nonaqueous Li-O2 and Na-O2 Batteries. J. Phys. Chem. Lett. 2014, 5, 1230-1235. [20] Chen G. Q.; Scholes C. A.; Qiao G. G.; Kentish S. E. Water Vapor Permeation in Polyimide Membranes. J. Membr. Sci. 2011, 379, 479-487. [21] Hartmann P.; Grubl D.; Sommer H.; Janek J.; Bessler W. G.; Adelhelm P. Pressure Dynamics in Metal-Oxygen (Metal-Air) Batteries: A Case of Study on Sodium Superoxide Cells. J. Phys. Chem. C 2013, 118(3), 1461-1471. [22] Lee B.; Seo D. H.; Lim H. D.; Park I.; Park K. Y.; Kim J.; Kang K. First-Principles Study of the Reaction Mechanism in Sodium-Oxygen Batteries. Chem. Mater. 2014, 26(2), 1048-1055. [23] Sun Q.; Yadegari H.; Banis M. N.; Liu J.; Xiao B.; Wang B.; Lawes S.; Li X.; Li R.; Sun X. SelfStacked-Nitrogen-Doped Carbon Nanotubes as Long-Life Air Electrode for Sodium Air Batteries: Elucidating the Evolution of Discharge Product Morphology. Nano Energy 2015, 12, 698-708. [24] Liu W.; Sun Q.; Yang Y.; Xie J. Y.; Fu Z. W. An Enhanced Electrochemical Performance of a Sodium-Air Battery with Graphene Nanosheets as Air Electrode Catalysts. Chem. Commun. 2013, 49, 1951-1953. [25] Yin W. W.; Shadike Z.; Yang Y.; Ding F.; Sang L.; Li H.; Fu Z. W. A Long-Life Na-Air Battery Based on a Soluble NaI Catalyst. Chem. Commun. 2015, 51, 2324-2327. [26] Yin W. W.; Yue J. L.; Cao M. H.; Liu W.; Ding J. J.; Ding F.; Sang L.; Fu Z. W. Dual Catalytic Behavior of a Soluble Ferrocene as an Electrocatalyst and in the Electrochemistry for Na-Air Batteries. J. Mater. Chem. A 2015, 3, 19027-19032. [27] 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. [28] 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. [29] Aetukuri N. B.; McCloskey B. D.; García J. M.; Krupp L. E.; Viswanathan V.; Luntz A. C. Solvating Additives Drive Solution-Mediated Electrochemistry and Enhance Toroid Growth in NonAqueous Li-O2 Batteries. Nat. Chem. 2015, 7, 50-56. [30] Schwenke K.U.; Metzger M.; Restle T.; Piana M.; Gasteiger H. A. The Influence of Water and Protons on Li2O2 Crystal Growth in Aprotic Li-O2 Cells. J. Electrochem. Soc. 2015, 162(4), A573A584. [31] Yadegari H.; Banis M. N.; Xiao B.; Sun Q.; Li X.; Lushington A.; Wang B.; Li R.; Sham T. K.; Cui X.; Sun X. Three-Dimensional Nanostructured Air Electrode for Sodium-Oxygen Batteries: A Mechanism Study toward the Cyclability of the Cell. Chem. Mater. 2015, 27, 3040-3047. [32] 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.


Page 16 of 18

Page 17 of 18

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


[33] 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(13), 2636-2643. [34] 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. [35] Weiss J. One-Electron Versus two-Electron Transfer Processes in the Mechanism of OxidationReduction Reactions in Solution. Nature 1958, 181, 825-826. [36] Luntz A. C.; McCloskey B. D. Nonaqueous Li-Air Batteries: A Status Report. Chemical Reviews 2014, 114, 11721-11750. [37] Freunberger S. A.; Chen Y.; Drewett N. E.; Hardwick L. J.; Bardé F.; Bruce P. G. The LithiumOxygen Battery with Ether-Based Electrolytes. Angew. Chem. Int. Ed. 2011, 50, 8609-8613. [38] Li F.; Wu S.; Li D.; Zhang T.; He P.; Yamada A.; Zhou H. The Water Catalysis at Oxygen Cathodes of Lithium-Oxygen Cells. Nat. Comm. 2015, 6, 1-7. [39] 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. [40] Liu T.; Leskes M.; Yu W.; Moore A. J.; Zhou L.; Bayley P. M.; Kim G.; Grey C. P. Cycling Li-O2 Batteries Via LiOH Formation and Decomposition. Science 2015, 350, 530-533. [41] Kang S. Y.; Mo Y.; Ong S. P.; Ceder G. Nanoscale Stabilization of Sodium Oxides: Implications for Na-O2 Batteries. NanoLetters 2014, 14(2), 1016-1020. [42] Das S. K.; Xu S.; Archer L. A. Carbon Dioxide Assist for Non-Aqueous Sodium-Oxygen Batteries. Electrochem. Comm. 2013, 27, 59-62. [43] Zhao N.; Guo X. Cell Chemistry of Sodium-Oxygen Batteries with Various Nonaqueous Electrolytes. .J Phys. Chem. C 2015, 119(45), 25319-25326. [44] Read J. Characterization of the Lithium/Oxygen Organic Electrolyte Battery. J. Electrochem. Soc. 2002, 149, A1190-A1195. [45] Sun Q.; Yadegari H.; Banis M. N.; Liu J.; Xiao B.; Li X.; Langford C.; Li R.; Sun X. Toward a Sodium“Air” Battery: Revealing the Critical Role of Humidity. J. Phys. Chem. C 2015, 119, 13433-13441. [46] Bi X.; Ren X.; Huang Z.; Yu M.; Kreidler E.; Wu Y. Investigating Dendrites and Side Reactions in Sodium-Oxygen Batteries for Improved Cycle Lives. Chem. Commun.2015, 51, 7665-7668. [47] Ganapathy S.; Adams B. D.; Stenou G.; Anastasaki M .S.; Goubitz K.; Miao X. F.; Nazar L. F.; Wagemaker M. Nature of Li2O2 Oxidation in a Li–O2 Battery Revealed by Operando X-ray Diffraction. J. Am. Chem. Soc. 2014, 136(46), 16335-16344. [48] Lim H.; Yilmaz E.; Byon H. R. Real-Time XRD Studies of Li-O2 Electrochemical Reaction in Nonaqueous Lithium-Oxygen Battery. J. Phys. Chem. Lett. 2012, 3, 3210-3215. [49] Sun Q.; Yang Y.; Fu Z. W. Electrochemical Properties of Room Temperature Sodium-Air Batteries with Non-Aqueous Electrolyte. Electrochem. Comm. 2012, 16(1), 22-25. [50] Bender C. L.; Bartuli W.; Schwab M. G.; Adelhelm P.; Janek J. Toward Better Sodium–Oxygen Batteries: A Study on the Performance of Engineered Oxygen Electrodes Based on Carbon Nanotubes. Energy Technol. 2015, 3, 242-248. [51] Schröder D.; Krewer U. Model Based Quantification of Air-Composition Impact on Secondary Zinc Air Batteries. Electrochimica Acta 2014, 117, 541-553. [52] Bergner B. J.; Hoffman C.; Schürmann A.; Schröder D.; Peppler K.; Schreiner P. R.; Janek J. Understanding the Fundamentals of Redox Mediators in Li-O2 Batteries: A Case Study on Nitroxides. Phys. Chem. Chem. Phys. 2015, 17, 31769-31779.



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