Role of Electrolyte Anions in the Na–O2 Battery: Implications for

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Role of Electrolyte Anions in the Na−O2 Battery: Implications for NaO2 Solvation and the Stability of the Sodium Solid Electrolyte Interphase in Glyme Ethers Lukas Lutz,†,‡,§,∥ Daniel Alves Dalla Corte,†,# Mingxue Tang,⊥,# Elodie Salager,⊥,# Michael Deschamps,⊥,# Alexis Grimaud,*,†,# Lee Johnson,‡ Peter G. Bruce,‡,∥ and Jean-Marie Tarascon†,∥,# †

Chimie du Solide et de l’Energie, FRE 3677, Collège de France, 75231 Paris Cedex 05, France Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, U.K. § Department of Chemistry, Université Pierre et Marie Curie, 4 Place Jussieu, 75005 Paris, France ∥ ALISTORE-European Research Institute, FR CNRS 3104, 80039 Amiens, France ⊥ Université d‘Orléans, CNRS, CEMHTI UPR3079, 1D avenue de la recherche scientifique, 45071 Orléans Cedex 2, France # Réseau sur le Stockage Electrochimique de l‘Energie (RS2E), CNRS FR3459,33 rue Saint Leu, 80039 Amiens Cedex, France ‡

S Supporting Information *

ABSTRACT: Herein we investigate the influence of the sodium salt anion on the performance of Na−O2 batteries. To illustrate the solvent−solute interactions in various solvents, we use 23Na-NMR to probe the environment of Na+ in the presence of different anions (ClO4−, PF6−, OTf−, or TFSi−). Strong solvation of either the Na+ or the anion leads to solvent-separated ions where the anion has no measurable impact on the Na+ chemical shift. Contrarily, in weakly solvating solvents the increasing interaction of the anion (ClO4− < PF6− < OTf− < TFSi−) can indeed stabilize the Na+ due to formation of contact ion pairs. However, by employing these electrolytes in Na−O2 cells, we demonstrate that changing from weakly interacting anions (ClO4−) to TFSi does not result in elevated battery performance. Nevertheless, a strong dependence of the solid electrolyte interphase (SEI) stability on the choice of sodium salt was found. By correlation of the physical properties of the electrolyte with the chemical SEI composition, the crucial role of the anion in the SEI formation process is revealed. The remarkable differences and consequences for long-term stability are further established by cycling Na coin cells, where electrolytes using NaTFSi are absolutely detrimental for metallic sodium, employing NaOTF and NaClO4 leads to short-term stability, and only the combination of 1,2-dimethoxyethane with NaPF6 allows for high efficiency and performance.

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INTRODUCTION

since electrolytes for metal−O2 batteries (M = Li, Na) face extreme conditions.6−8 For instance, the use of highly reducing metallic (lithium or sodium) anodes as well as the reactive oxygen species (superoxide and peroxide) that form at the cathode upon discharge can result in solvent decomposition by nucleophilic attack and proton abstraction, leading to poor cycling.7,9 On the other hand, it has also been shown that the electrolyte plays a critical role in the performance of cathode electrodes in metal− O2 batteries.10−12 In Li−O2 batteries, solvents with high Gutmann donor number (DN) can induce solubility of the discharge product by stabilizing the Li+−O2− intermediate in solution, leading to formation of Li2O2 toroids via a solution

Sodium-air batteries are a promising candidate in the search for more sustainable and efficient electrochemical energy storage technologies.1−3 Besides the geographical distribution, abundance, and low cost of sodium,4 the high theoretical energy density of the Na−O2 battery (1108 W·h/kg for NaO2)1 has attracted significant research interest. Hartmann and others have demonstrated that despite the lower energy density compared to Li2O2 (3485 W·h/kg),1 shifting from the Li−O2 to the Na−O2 system can improve the cell performance by lowering the charge overpotential and achieving better reversibility.1,5 This makes the Na−O2 system an auspicious alternative with regard to practical applications. Unfortunately, both technologies are still confined to fundamental research due to many unanswered questions. One of the key challenges is finding a suitable electrolyte formulation (salt and solvent) © 2017 American Chemical Society

Received: May 11, 2017 Revised: June 26, 2017 Published: June 26, 2017 6066

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Chemistry of Materials precipitation mechanism.10,12 This results in an overall capacity increase compared to low DN solvents, where premature cell death is caused by formation of insulating Li2O2 thin films on the cathode surface.13 In addition, Kwabi et al. reported that increasing the Gutmann acceptor number (AN) of the solvent could also be a promising strategy to increase solubility of Li+− O2− by rather stabilizing the O2− in solution. Recently, Gunasekara et al. demonstrated similar results when using conductive lithium salts with high DN anions, where higher capacities were ascribed to stronger solvation of the Li+ cation due to the high DN of the triflate (OTf−) compared to the low DN hexafluorophosphate (PF6−) salt anion.14 Further, Burke et al. claimed a comparable effect when using the NO3 anion.15 However, others have shown that increased performance when using LiNO3 electrolytes can also result from the unique catalytic behavior of the NO3−/NO2− redox couple.16,17 All together, these studies demonstrate that besides the solvent, the electrolyte salt can influence multiple facets of the battery. In the much younger Na−O2 system, these effects have yet to be investigated. Nevertheless, the anode cannot be neglected here, as the electrolyte composition is expected to have a significant effect on the solid electrolyte interphase (SEI).18−22 Ideally, the SEI should be electronically insulating but ionically conducting to allow effective cycling and protect the anode from continuous degradation.17,18 Nevertheless, the complex phenomenon of SEI formation and in particular the role of the conductive salt anion in this process remains to be understood. Regarding the solvent component of the electrolyte, besides carbonates, glyme ethers have been reported as promising solvents for several battery systems like Na−ion, Na−sulfur, and Na−O2 batteries.1,23,24 In particular, for the Na−O2 system glyme ethers are currently the sole choice of solvent due to their elevated stability toward the superoxide radicals and the metallic sodium anode compared to other solvents.8 Definitively, previous reports about Li−O2 chemistry incite us to investigate the effect of salt anion on the discharge mechanism for Na−O2 cells, bearing in mind based on our own previous experiences with the Na-ion technology that extreme caution must be taken when using metallic sodium anodes in aprotic solvents. In this regard, investigating the effect the conductive salt on the relevant processes occurring at the air electrode while considering the stability of the metallic sodium anode could provide a crucial understanding toward the development of the Na−O2 technology. Hence, in a first step, the complex process of the Na+-cation solvation in various solvents was studied by means of 23Na-NMR and the impact of the salt anion on the Na+ solvation is discussed. By subsequent implementation of different glyme ether electrolytes in Na−O2 batteries, the effect of the conductive salt anion on the electrochemical NaO2 formation process is examined. We further demonstrate that the chemical structure of the salt anion plays a key role in the SEI formation process at the sodium anode and therefore has important implications for the cycling performance of batteries employing metallic sodium anodes.

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(NaClO4, 98%) was bought from Alfa Aesar. Sodium hexafluorophosphate (NaPF6 99.9%) was purchased from Stella Chemifa. The binder free gas diffusion layer (Freudenberg H2315, GDL), here used as electrode, was obtained from Quintech. Prior to use, the GDL was dried under vacuum at 260 °C for 24 h. Solvents were dried over freshly activated molecular sieves (4 Å) for several days. Sodium salts were dried under vacuum at 80 °C for 24 h. The 0.5 M electrolyte solutions were prepared in an argon-filled glovebox (0.1 ppm O2/0.1 ppm H2O). The water content of the electrolyte solutions was analyzed by Karl Fischer titration and was found to be below 10 ppm, except for NaClO4 solutions which showed a water content of approximately 40 ppm. 23 Na NMR Measurements. The 23Na-containing solutions were placed in 4 mm NMR tubes in an argon glovebox, closed tightly with caps and Teflon film. The measurements were performed under a N2 atmosphere to avoid moisture and oxygen contamination. The spectra were acquired on a 4.7 T Avance III HD Bruker spectrometer (23Na frequency of 52.9 MHz) at room temperature. The 23Na spectra were obtained using a simple pulse. Sixty-four transients were accumulated for each sample and a repetition time of 4 s was used for complete relaxation. All spectra were referenced to a 1 mol·L−1 aqueous solution of NaCl at 0 ppm. Na−O2 Cell Assembly and Electrochemical Measurements. Na−O2 cells were built from a modified Swagelok setup.25 The Swagelok cell was connected to a pressure monitor to analyze pressure changes during cycling. The cell was vacuum-dried under elevated temperature and transferred into a glovebox (0.1 ppm H2O, 0.1 ppm O2). The anode was made from metallic sodium (Sigma-Aldrich), cut into a 0.5 cm2 disc. Dried electrolyte solution (0.5 M sodium salt in DME) was imbibed on two Whatman glass fiber filters (QM-A grade) (dried under vacuum at 260 °C, 24 h), about 0.3 mL. A piece of GDL (1.13 cm2 of surface area, 210 μm in thickness, and a weight of 10 mg) was used as cathode and held by an inox mesh current collector containing holes for gas exchange. No further insulating sleeve was used. The assembled cells were transferred from the glovebox to a filling station, and after a first evacuating step, the cells were pressurized with dry, ultrapure O2 to 1.3 bar. To guarantee stable temperature conditions, the cells were mounted inside of a temperature-controlled incubator (25.0 ± 0.1 °C). The electrochemical measurements were performed under temperature-controlled conditions (25.0 °C) after resting for 4 h at the open-circuit voltage and using a Bio-Logic VMP3 potentiostat. Pressure analysis decay analysis and e−/O2 ratios were calculated like described elsewhere.26 X-ray Diffraction (XRD). Samples were washed with dried DME prior to XRD analysis. A Bruker D8 Advance diffractometer with a Cu Kα radiation source (λ1 = 1.5405 Å, λ2 = 1.5443 Å) and a Lynxeye XE detector was used to collect the XRD patterns. The XRD patterns were recorded for 30 min in the 2θ range of 20−65°. A special airtight cell with a beryllium window was used to guarantee no ambient air contamination during XRD measurements. Electrochemical Impedance Spectroscopy (EIS). EIS was performed at open-circuit voltage after 1 h intervals of rest periods with an alternating current (ac) amplitude of 10 mV that was varied from 200 kHz to 10 mHz with 10 points per decade. X-ray Photoelectron Spectroscopy (XPS) Analysis. XPS analysis of the surface of aged sodium samples was performed by a means of a SPECS Sage HR 100 spectrometer with a nonmonochromatic X-ray source (Al Kα line of 1486.6 eV energy and 300 W). The samples were placed perpendicular to the analyzer axis and calibrated using the 3d5/2 line of Ag with a full width at halfmaximum (fwhm) of 1.1 eV. All samples were transferred by means of a gastight transfer chamber to avoid air contact. All samples were further rinsed with dry DME solution to remove excess salt. In the case of sodium aged in TFSi electrolyte, the surface of the sample and not the precipitate was analyzed. The selected resolution for the spectra was 10 eV of pass energy and 0.15 eV/step. All measurements were made in an ultrahigh vacuum (UHV) chamber at a pressure around 5 × 10−8 mbar. An electron flood gun was used to compensate for charging during XPS data acquisition. In the fittings asymmetric and Gaussian−Lorentzian

METHODOLOGY

Chemicals and Materials. 1,2-Dimethoxyethane (DME, 99.9%), acetonitrile (ACN, 99.8%), dimethyl sulfoxide (DMSO, 99.8%), sodium trifluoromethanesulfonate (NaOTf, 98%), and tetrabutylammonium trifluoromethanesulfonate (99%) were purchased from Sigma-Aldrich. Sodium(I) bis(trifluoromethanesulfonyl)imide (NaTFSI, 99.5%) was purchased from Solvionic. Sodium perchlorate 6067

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Chemistry of Materials functions were used (after a Shirley background correction) where the fwhm of all the peaks were constrained while the peak positions and areas were set free. For every anion, XPS was performed on the surface of the metallic sodium. For TFSI, the XPS spectrum was collected as well on the surface of the metallic sodium, while the pilled off part was not analyzed. Coin Cell Assembly and Electrochemical Measurements. 2032-Type coin cells were assembled in an Ar-filled glove and electrochemical tests were perfomed by means of a Bio-Logic VMP3 Potentiostat. A 0.8 cm2 Cu foil was used as a working electrode with a 0.8 cm2 sodium metal counter electrode, a Cellgard separator, and 25 μL of electrolyte. After assembly, galvanostatic plating of sodium was carried out at 0.5 mA/cm2 with a capacity limit of 0.2 mA·h/cm2, followed by stripping until a potential cutoff of 1 V was reached. Na3V2(PO4)3 (NVP) Anodes. The NVP electrodes were prepared as described elsewhere.27 In detail, after drying under vacuum at 80 °C for 24 h, the material was activated and precharged in DME (0.5 M NaPF6) to reach a stable plateau. The NVP was subsequently washed with dried DME and Na−O2 cells using a 0.07 cm2 GDL were assembled with the respective dried electrolyte and discharged at 100 μA·h/cm2. Sodium Aging in O2 Saturated Electrolytes. Sodium metal electrodes were prepared in a glovebox (0.1 ppm H2O, 0.1 ppm O2) and subsequently immersed in O2-saturated DME/DGME/TGME electrolytes containing 0.5 M of dried NaPF6, NaOTf, NaClO4, and NaTFSi, respectively. Images were taken after 48 h of aging.

Scheme 2. Chemical Structures of the Herein Investigated Sodium Salt Anions

reduction of O2 to O2− on the cathode, the Na−O2 complex is solvated away from the electrode and the solid NaO2 discharge product is formed via a solution precipitation mechanism elsewhere in the porous carbon structure. Therefore, it is worth at this point to discuss in detail the physical properties governing the solvation process of metal−anion complexes. When a solute is dissolved in solution, its solvation is determined not only by its own chemical and structural properties but also by the solvent properties. Gutmann introduced in the 1970s the concept of the Gutmann donor (DN) and acceptor numbers (AN) to establish a quantitative measure for the structural changes in solution, induced by the chemical properties of each reacting species.29,30 In this seminal work, the solvation of a solute by a solvent is described as a nucleophilic attack of the solvent molecule on the solute and electron density donation to the area of low electron density in the solute (Scheme 1, blue arrow S → Na+). On the basis of the electron acceptor power of the solvent, it can also simultaneously accept electron density from an electron-rich area of the solute, that is, the anion (Scheme 1 blue arrow A− → S), and thereby separate both ions independently (solventseparated ions). However, it becomes obvious that to achieve good solvation, the strength of the solvent to donate and accept electronic density from the solute has to exceed the electronic interaction between the cation and anion itself which otherwise leads to contact ions pair formation in solution (Scheme 1). Therefore, it is crucial when discussing the solvation power of a solvent to take the DN and AN of the solvent and solute as well as their secondary interactions into account. Furthermore, it is important to state here that the DN and AN are sole measures of the enthalpy contributions of the solvation process and do not include any entropic terms that can be related to configurational and reorganization processes. Lastly, when the solvation strength of a solvent toward M−O2 complexes is described, the entropic term cannot be neglected since it is well-known that modification of the chain length of glyme ethers can modify their ability to chelate small cations.11 To test the above-stated effects, various electrolytes formulations were studied. Three solvents with a wide range of DN and AN numbers were selected (Figure 1): (i) a glymeether solvent, namely, 1,2-dimethoxyethane (DME) having a relatively low AN and DN; (ii) acetonitrile (ACN) as a solvent with high AN and low DN; and (iii) dimethlysulfoxide (DMSO) possessing both a high AN and DN. To visualize the influence of the anion and the solvent on the sodium solvation, 23Na-NMR spectra of the Na+ ions in the different solvent, containing four commonly used anions (ClO4−, PF6−, OTf−, and TFSi−, see Scheme 2) were recorded. This

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RESULTS AND DISCUSSION Several reports have shown that efficient Na−O2 cells are based on a solution-mediated formation of NaO2,2,11,28 where after Scheme 1. Illustration of Electron DN and AN Concept of Gutmann, Where “S” Stands for Solventa

a

Blue arrows illustrate the path for electron donation of the solvent to the solute cation as well as electron donation from anion to the solvent. Grey arrow indicates the ability of the solute anion to donate electrons towards the cation.29

Figure 1. Gutmann donor and acceptor numbers of acetonitrile (ACN), 1,2-dimethoxyethane (DME), and dimethlysulfoxide (DMSO), collected from the literature.11,31,32

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Figure 2. 23Na-NMR visualization of the impact of the ClO4− (blue line), PF6− (green line), OTf− (pink line) and TFSi− (organe line) anion on the Na+ chemical shift in (a) DME, (b) ACN, and (c) DMSO.

Figure 3. Na−O2 cell discharges at current densities of 25 μA/cm2 (a) as well as 100 μA/cm2 (b), using DME with 0.5 M of NaPF6 (green line), NaOTF (pink line), NaCl4 (blue line), NaTFSi (orange line), and a GDL cathode. Consumption of moles of electrons per moles of oxygen, calculated from oxygen pressure measurements during discharge (c). X-ray diffraction patterns of fully discharged electrodes (d), including patterns of sample holder (black line) and a NaO2 reference kindly provided by M. Jansen, MPI-FKF, Stuttgart, Germany (gray line).

technique probes the electron density on the Na+ cation, which is a direct measure of the interaction of the Na+ with the solvent and the anions as previously discussed. Hence, it has been shown that the 23Na-chemical shift of infinitely diluted NaClO4 is linearly dependent on the DN of the solvent.33 The ClO4− anion is a very weakly solvating

counterion so that its interaction with Na+ can be neglected, allowing for the study of the solvent properties. For other anions the interaction with the counterion must also be taken into account. The Na+ solvation sphere (hence the 23Nachemical shift) is a three-body system and therefore it depends on the competition between the cation−solvent interactions, 6069

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the anions appear to have sizable effect on the Na+ chemical shift (Figure 2a, Table S1 of the Supporting Information). This indicates that in the weakly solvating DME, the solute favors contact-ion pair formation where the anion and cation are in close contact with each other and not separately solvated. In other words, the solute is stabilizing itself (Scheme 1, gray arrow) and the different salt anions alter the electronic density on the Na+. However, when using the other low DN solvent ACN, the impact of the anion on the Na+ chemical shift was found to be lower (Figure 2b), and only for OTf−, a slight downfield shift was observed. This result seems rather surprising with respect to the low DN of ACN. On the other hand, ACN has a fairly high AN, which is 2 times larger than that of DME, allowing for a stronger solvation of the anion by electron acceptance of the ACN solvent. This ultimately restricts the impact of the salt anion on the Na+ ion. Interestingly, the 23Na chemical shift in ACN does not follow the same trend as in DME. In ACN, OTf forms a stronger pair with Na+ than TFSi− with Na+. This result emphasizes the high sensitivity of the 23Na chemical shift to the environment of the Na ion and hence the ion pair strength. Hence, it was previously shown for ACN that the presence of nitrogen donor centers can alter the 23Na chemical shift, demonstrating the complex interplay of the interactions at stake.34 Therefore, DN/AN of the solvent or the anion are not sufficient to describe the Na+ environment in the case of ACN. For DMSO, both the cation and the anion are stabilized in solution by means of the high donation and acceptor strength of the solvent, therefore favoring the formation of solvent separated ion pairs in solution. This separation consequently screens the Na+ cation from the anion, leading to an identical chemical Na+ shift for all sodium salts, as observed in Figure 2d. Note that small variations (less than 1 ppm) may also be masked by the width of the 23Na peak in DMSO. To explore the implications of these findings on the Na−O2 system, Na−O2 cells were discharged using DME-based electrolytes containing the above-mentioned sodium salts. DME was selected as it shows the biggest impact of the anion on the 23Na+ shift. In Figure 3a, the discharge performance of such cells at low current densities (25 μA/ cm2) using NaClO4, NaPF6, NaOTf, and NaTFSi as conductive salts is displayed. Interestingly, no correlation between the strength of electron donation from the anion to the cation and the discharge capacity could be made. Despite the fact that using OTf−/TFSi− should increase the electron donation to the Na+ and thereby enhance stabilization of Na+ in solution, the capacities obtained with NaClO4, NaPF6, and NaOTf containing electrolytes were found to be similar (approximately 8−10 mA·h/cm2). However, when NaTFSi is used as the conductive salt, maximum capacities of only approximately 1 mA·h/cm2 could be reached. Very similar findings were obtained when increasing the current density to 100 μA/cm2 (Figure 3b). The rather low effect of the anion chemistry on the Na−O2 cell performance is further supported by rotating ring-disc electrode (RRDE) voltammetry, where no significant increase in NaO2 solubility was found when switching from OTf− to ClO4− for which relatively weak ion pair formation (Figure S1). These findings bolster the contention that stabilization of the Na+−O2− complex resulting from the use of an anion such as OTf−/TFSi does not result in increased capacities for Na−O2 cells. Furthermore, no evident impact of the anion on the formation mechanism of NaO2 was found when analyzing the NaO2 discharge product. Correlating

Figure 4. SEI growth evolution, as deduced by electrochemical impedance spectroscopy (EIS) collected over 48 h, in symmetrical Na−Na coin cells employing electrolytes based on DME and containing constant amounts (0.5 M) of (a) NaPF6, green lines, (b) NaOTf, pink lines, (c) NaClO4, blue lines, and (d) NaTFSi, orange lines. Spectra were recorded every 6 h over a time of 48 h, the colors being dimmed from bright to dark with time increasing. The inserts show pictures of metallic sodium aged for 48 h in the respective oxygen-saturated electrolyte solutions.

the cation−anion interactions, and the anion−solvent interactions. Other effects, such as sterical hindrance or charge delocalization, might also affect the 23Na chemical shift. In Figure 2 we explore the effect of the anion on the system by means of 23Na chemical shift. First, one can see that for DME 6070

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Figure 5. XPS spectra of the surface of metallic sodium anodes aged in DME electrolytes containing 0.5 M (a) NaClO4, (b) NaPF6, (c) NaOTf, and (d) NaTFSi.

Scheme 3. Chemical Reduction of Ether Molecules by Metallic Sodium and Potential Subsequent Reaction Pathways, Leading to the Formation of Organic/Inorganic SEI Compounds

the oxygen pressure change monitored during discharge and the capacity allows for identification of a one- or two-electron reduction of O2.2,25 For every electrolyte considered in this work, a one-electron reduction of oxygen with the consumption of 1 ± 0.1 mol of electrons/mol of oxygen was obtained (Figure 3c). The crystalline nature of the NaO2 discharge product was further confirmed by X-ray diffraction for all cells (Figure 3d). However, in the case of NaTFSi, the anion seems to have a profound impact on the system by drastically limiting the discharge performance. Since the effect could not be ascribed to a different formation mechanism of NaO2 and a modification of the solution process on the cathode side, the anode side (that is, the metallic sodium) and its SEI were investigated in greater detail. For that, Na−O2 cells were assembled using Na3V2(PO4)3 (NVP) as

Figure 6. Electrochemical plating and stripping voltage profiles of symmetric Na−Na coin cells using DME with 0.5 M (a) NaPF6, (b) NaOTf, (c) NaClO4, and (d) NaTFSi as electrolyte. Cells were cycled to a restricted capacity of 0.2 mA·h on discharge and a cutoff voltage of 1 V upon charge at a current density of 0.5 mA/cm2.

anode, as previously proposed by Zhang et al.27 (Figure S3). In this configuration, similar capacities were obtained for NaClO4, and NaTFSi salts, confirming that the limitations observed in Figure 3 are related to the use of metallic sodium at the anode. Nevertheless, smaller capacities were observed with NVP 6071

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Scheme 4. Illustration of the Suggested Sodium SEI Formation Mechanism and Its Composition in Different DME Electrolytes

(Figure 4a). In the case of NaOTf, the SEI takes approximately 48 h to stabilize, reaching a final impedance of ∼2.5 Ω/cm2 (Figure 4b). However, when switching to the NaClO4 electrolyte, the initially small SEI impedance strongly increases over time, with the appearance of several semicircles reaching a constant total impedance value of ∼1000 Ω/cm2 (Figure 4c). We ascribe this increase to the formation of an SEI with relatively low. When NaTFSi is employed as the conductive salt in the system, drastically different impedance behavior was observed (Figure 4d). An initial semicircle with a rather small impedance of ∼15 Ω/cm2 is observed at first, after which the SEI impedance increases rapidly (Figure S5). After 20 h, the spectrum shows several additional semicircles, indicating the formation of multiple new and highly resistive features within the SEI. The impedance continues to grow gradually and does not, even after 48 h, show any sign of stabilization. The differences between SEI formation in these electrolytes can also be visually seen when looking at metallic sodium aged in the aforementioned electrolytes for 48 h (Figure 4 inserts). For electrolytes using NaPF6, NaCl, and NaOTf the samples exhibit a gray or bluish SEI covering the sodium metal. However, in the case of NaTFSi electrolyte, the corrosion reactions are so severe that large parts of the decomposition product peel away from the sodium and form a precipitate, ultimately leading to dissolution of the sodium metal over time. Similar reactions were obtained when sodium was imbedded in other ethereal solvents like diglyme (DGME) and tetraglyme (TGME) (Figure S6). Combining this information with the findings of Figure 3 (orange lines), we postulate that the restricted performances and the early cell death for the NaTFSi based Na−O2 cells arise

anodes, demanding for further studies to fully understand the role of the SEI in such electrodes.27 This control experiment further exemplifies that even though the anion plays a role on the contact-ion pair formation, greater effects must be anticipated on the SEI formation at the metallic sodium anode. Analyzing the stripping and plating reactions of metallic Na anode on copper foil (Cu foil) performed in coin cells (Figure S4), drastic differences were found depending on the anion. When electrolytes containing DME + 0.5 M NaPF6, NaClO4, and NaOTf were used, the anode capacity was exceeding by far the capacity obtained for Na−O2 cells reported in Figure 3a. This indicates that the Na-plating/stripping process is not limiting the Na−O2 cell capacity. In contrast, when NaTFSi is used as the conductive salt, a drastic increase in overpotential for the plating/stripping reaction can be observed. Therefore, we conclude that for NaTFSi the formation of a detrimental SEI is limiting the Na plating and stripping reactions. To gain deeper knowledge about the chemical SEI formation at metallic sodium in DME-based electrolytes and the role of the conductive sodium salt anion in this process, electrochemical impedance spectroscopy (EIS) of symmetric Na−Na cells was performed. Probing the conductive nature of a developing SEI by means of EIS can give valuable insights about the formation mechanism of such an SEI. The remarkable differences in impedance evolution over 48 h found for the herein reported electrolyte formulations, which differ only in the salt anion, demonstrate the strong impact of the anion on the SEI formation process (Figure 4). For NaPF6, the impedance is rather small (∼0.3 Ω/cm2) and relatively constant throughout the measurement, which implies that the metallic sodium is covered by a stable and protective SEI 6072

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may be of importance for the uniformity of the SEI. These conclusions can be made when observing the great instability of the SEI formed when the electrolyte lacks a source of fluoride (NaClO4) or large anions such as the TFSi− anion are used, where we ascribe the absence of a protective SEI due to incorporation of large anion fragments in the SEI. We suggest that this incorporation could increase the porosity of the SEI and would explain the continuous degradation of metallic sodium by the electrolyte over time. At this point it is worth mentioning that when analyzing the pressure and the composition of gases released during the first 24 h of SEI formation, H2 was detected, most likely resulting from the reduction of residual H2O at the metallic anode and the subsequent formation of OH− and H2 as discussed by others (Figure S8).35 Since similar pressure increase was found for all electrolyte combinations, we believe that the different electrolytes possess relatively low H2O contents, as measured by Karl Fischer titration. This observation makes the contribution of H2O as a possible origin for the different SEI stability very unlikely. Further, these findings clearly demonstrate that water can be consumed at the anode even before the first discharge of a Na−O2 cell and caution has to be taken when discussing electrochemical results in light of the role of H2O, especially when such H2O contents are determined prior to embedding the electrolyte solution. To demonstrate the importance of the above-described findings for the electrochemical behavior of anodes in sodium− metal-based batteries, the sodium plating/stripping performances of Na-coin cells was investigated using the herein described electrolytes. From Figure 6, it can clearly be seen that the above-described differences in SEI composition have tremendous implications for the electrochemical cycling behavior. Only for NaPF6 and NaOTf based electrolytes, which have shown similar SEI compositions and reasonable impedances (Figure 4, Scheme 4), does consecutive plating and stripping occur reversibly. Of these, only the NaPF6 electrolytes allowed for long-term efficiencies of >98% as well as a fivefold lower charge hysteresis of approximately 50 mV compared to NaOTf. On the other hand, the unstable SEIs formed for NaClO4 and NaTFSi electrolytes did not allow for effective plating and stripping.

from the heavy decomposition reactions between metallic sodium and DME in the presence of TFSi anions, and does not result from effects of the anion in the NaO2 formation mechanism. Understanding the SEI formation of metallic sodium and its dependence on the salt anion is of great value not only for Na−O2 batteries but also for other sodium-based systems like the Na−sulfur and Na−metal batteries. An ideal SEI has been described to be insoluble and to form a uniform protective layer on the metallic anode with minimum electronic and maximum ionic (Li+ or Na+) conductivity.18 To gain more insight in the chemical composition of the reported SEI, the surface of metallic Na anodes aged in the herein investigated electrolytes was further analyzed by means of X-ray photoelectron spectroscopy (XPS). The XPS spectra in Figure 5 verify the presence of salt anion decomposition products in all analyzed SEIs, indicating the importance of the anion for the decomposition process. In general, two main SEI components were found, inorganic sodium−halides Na−X (X = F, Cl) and organic ether−salts as well as carbonate-like compounds. The Na−X unambiguously results from halide abstraction from the salt anion since this is the only halide source in the system. It is worth mentioning here that no voltage or current was applied during the anode aging and all the described findings result purely from chemical reactions. The organic SEI compounds most likely result from reduction of the DME molecule when oxidizing metallic sodium (step 1 in Scheme 3). The electron generated during the Na0 oxidation attacks an electrophilic center such as carbon atoms (most likely from a methyl group) to form a radical as well as a sodium ether−salt. These highly reactive radical species can subsequently polymerize with other solvent molecules (step 2 in Scheme 3), leading to the formation of ethereal−oligomers, ether sodium−salts, and carbonate species, such as for example alkyl carbonates, in the SEI (O−CO, C− O−C, and C−O−Na in Figure 5, C 1s and O 1s). However, these radicals can also attack the polar salt anions (step 3 in Scheme 3). The high reactivity of these radicals toward fluorinated compounds can also be seen when Mylar is used as an insulating sleeve for Swagelok cells with such electrolyte/ anode combinations (Figure S7). The attack on the fluorinated anion could explain the Na−X signal found by XPS and the presence of anion decomposition products (CF2, CF3, P−F) in the SEI (Na 1s and F(Cl)1S spectra Figure 5, Table 2). Interestingly, the relative intensities of Na0 and Na−halide peaks varied in each electrolyte. For NaClO4 and NaOTf salts, the Na−Cl/Na−F peaks are the dominating feature and the Na0 peak intensity is rather weak, denoting a thick SEI layer. However, in the case of NaPF6, the Na0 is much more pronounced, which we assign to an effective coverage of the Na0 by a thin NaF layer. Furthermore, almost no Na−F signal could be detected, indicating the absence of any protective SEI layer when using TFSi− anions. On the basis of these results and the fact that no significant differences in the organic composition of the SEI were found, we conclude that the formation of a uniform NaF layer is essential to form a protective SEI. In both cases where a large increase in impedance was observed (Figure 4c,d), either the NaF coverage was low (TFSi−) or absent (ClO4−). While the use of ClO4− resulted in a considerable amount of NaCl, we believe that based on the “hard soft acid base” (HSAB) theory, this weaker Na−Cl bond ultimately does not protect the metallic sodium sufficiently from further degradation and leads to a lower SEI stability than NaF. Second, the size of the anion

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CONCLUSION The solution-mediated processes occurring during the discharge of Na−O2 cells are complex. The influence of the conductive salt anion on the solvation of Na+ cannot be easily described by applying empirical values from the literature to this delicate system. Herein, we have demonstrated by means of 23 Na-NMR spectroscopy that the specific Lewis basicity of various salt anions can indeed modify the electronic surrounding (solvation) of Na+ in weakly solvating solvents such as DME. When solvents like DMSO are employed, the anion impacts the Na+ chemical shift less due to the strong solvation of Na+ by DMSO. However, we experimentally verify that increasing the Na+ solvation does not necessarily translate into greater performances for Na−O2 batteries. In fact, the herein reported results imply that the anion has no significant influence in the formation mechanism of NaO2 in sodium air batteries. Nevertheless, we found that the anion actually plays a key role in the SEI formation on the surface of metallic sodium anodes in ethereal solvents. Radical species produced by the chemical reduction of the solvent can lead to the polymer6073

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(3) Larcher, D.; Tarascon, J. M. Towards greener and more sustainable batteries for electrical energy storage. Nat. Chem. 2015, 7 (1), 19−29. (4) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; CarreteroGonzález, J.; Rojo, T. Na-ion batteries, recent advances and present challenges to become low cost energy storage systems. Energy Environ. Sci. 2012, 5 (3), 5884−5901. (5) Peled, E.; Golodnitsky, D.; Mazor, H.; Goor, M.; Avshalomov, S. Parameter analysis of a practical lithium- and sodium-air electric vehicle battery. J. Power Sources 2011, 196 (16), 6835−6840. (6) Chau, V. K. C.; Chen, Z.; Hu, H.; Chan, K.-Y. Exploring Solvent Stability against Nucleophilic Attack by Solvated LiO2− in an Aprotic Li-O2 Battery. J. Electrochem. Soc. 2017, 164 (2), A284−A289. (7) 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. 2015, 5 (1), 1400867. (8) McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Girishkumar, G.; Luntz, A. C. Solvents’ Critical Role in Nonaqueous LithiumOxygen Battery Electrochemistry. J. Phys. Chem. Lett. 2011, 2 (10), 1161−1166. (9) Ottakam Thotiyl, M. M.; Freunberger, S. A.; Peng, Z.; Bruce, P. G. The carbon electrode in nonaqueous Li-O2 cells. J. Am. Chem. Soc. 2013, 135 (1), 494−500. (10) Johnson, L.; Li, C.; Liu, Z.; Chen, Y.; Freunberger, S. A.; Ashok, P. C.; Praveen, B. B.; Dholakia, K.; Tarascon, J. M.; Bruce, P. G. The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li-O2 batteries. Nat. Chem. 2014, 6 (12), 1091− 1099. (11) Lutz, L.; Yin, W.; Grimaud, A.; Alves Dalla Corte, D.; 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−O2Batteries: Key Parameters for SolutionMediated Discharge. J. Phys. Chem. C 2016, 120 (36), 20068−20076. (12) Sharon, D.; Hirsberg, D.; Salama, M.; Afri, M.; Frimer, A. A.; Noked, M.; Kwak, W.; Sun, Y. K.; Aurbach, D. Mechanistic Role of Li(+) Dissociation Level in Aprotic Li-O(2) Battery. ACS Appl. Mater. Interfaces 2016, 8 (8), 5300−5307. (13) Viswanathan, V.; Thygesen, K. S.; Hummelshoj, J. S.; Norskov, J. K.; Girishkumar, G.; McCloskey, B. D.; Luntz, A. C. Electrical conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries. J. Chem. Phys. 2011, 135 (21), 214704. (14) Gunasekara, I.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. Study of the Influence of Lithium Salt Anions on Oxygen Reduction Reactions in Li-Air Batteries. J. Electrochem. Soc. 2015, 162 (6), A1055−A1066. (15) Burke, C. M.; Pande, V.; Khetan, A.; Viswanathan, V.; McCloskey, B. D. Enhancing electrochemical intermediate solvation through electrolyte anion selection to increase nonaqueous Li-O2 battery capacity. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (30), 9293− 9298. (16) Sharon, D.; Hirsberg, D.; Afri, M.; Chesneau, F.; Lavi, R.; Frimer, A. A.; Sun, Y. K.; Aurbach, D. Catalytic Behavior of Lithium Nitrate in Li-O2 Cells. ACS Appl. Mater. Interfaces 2015, 7 (30), 16590−16600. (17) Walker, W.; Giordani, V.; Uddin, J.; Bryantsev, V. S.; Chase, G. V.; Addison, D. A rechargeable Li-O2 battery using a lithium nitrate/ N,N-dimethylacetamide electrolyte. J. Am. Chem. Soc. 2013, 135 (6), 2076−2079. (18) Verma, P.; Maire, P.; Novák, P. A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries. Electrochim. Acta 2010, 55 (22), 6332−6341. (19) Xu, K. Electrolytes and interphases in Li-ion batteries and beyond. Chem. Rev. 2014, 114 (23), 11503−11618. (20) Cheng, X. B.; Zhang, R.; Zhao, C. Z.; Wei, F.; Zhang, J. G.; Zhang, Q. A Review of Solid Electrolyte Interphases on Lithium Metal Anode. Adv. Sci. 2016, 3 (3), 1500213.

ization of the solvent and the decomposition of the salt anions. The presence of organic oligomers, Na−halides, and anion decomposition fragments in the sodium SEI has been verified by means of XPS spectroscopy. Consequently, the structure of the salt anion has profound influence on the nature of the SEI, where NaF seems to be a crucial component for high stability. Our data shows that only the PF6− anion, in combination with DME, delivers a sufficiently stable and conductive SEI, suitable for efficient long-term cycling whereas the TFSi− anion, most probably due to its large size, was found to be absolutely detrimental. These results emphasize that achieving stable cycling of Na− O2 battery cells using ethereal solvents depends heavily on the choice of conductive salt anion and its influence on SEI formation at the metallic sodium anode, rather than increasing the Na+ solvation by using high DN anions. In light of such findings, an obvious continuation of this work calls for means to control the SEI growth on the Na electrodes to improve the Na−O2 chemistry. This is being pursued in our group based on the experience that we have been piling in developing the Na-ion technology. Among the explored routes are coatings and alloying approaches together with attempts to mitigate the SEI growth via the use of less reducing insertion compounds such as NVP.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01953. Research data has been deposited in ORA-data at DOI:10.5287/bodleian:RybBd7MaD. 23 Na chemical shifts for the various salts in DME, ACN, and DMSO; RRDE experiments; sodium metal aging in various ethereal solvents; detailed analysis of XPS data; reactivity of Mylar in battery cells; pressure analysis of H2O consumption by sodium in NaO2cells (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +33 1 44 27 11 89. E-mail: [email protected]. ORCID

Lukas Lutz: 0000-0002-3466-4775 Mingxue Tang: 0000-0002-7282-4100 Elodie Salager: 0000-0002-5443-9698 Alexis Grimaud: 0000-0002-9966-205X Notes

The authors declare the following competing financial interest(s): P.G.B. is indebted to the EPSRC and the RCUK Energy programme including SUPERGEN for financial support.

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ACKNOWLEDGMENTS L. Lutz thanks ALISTORE-ERI for his PhD grant. REFERENCES

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DOI: 10.1021/acs.chemmater.7b01953 Chem. Mater. 2017, 29, 6066−6075