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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Elucidating the Impact of Sodium Salt Concentration on the Cathode-Electrolyte Interface of Na-Air Batteries Yafei Zhang, Nagore Ortiz-Vitoriano, Begoña Acebedo, Luke A. O'Dell, Douglas R. Macfarlane, Teofilo Rojo, Maria Forsyth, Patrick C. Howlett, and Cristina Pozo-Gonzalo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02004 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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The Journal of Physical Chemistry

Elucidating the Impact of Sodium Salt Concentration on the Cathode-Electrolyte Interface of Na-air Batteries Yafei Zhanga, Nagore Ortiz-Vitorianob,c , Begoña Acebedob, Luke O’Delld, Douglas R. MacFarlanee, Teófilo Rojob,f , Maria Forsytha, Patrick C. Howletta, Cristina Pozo-Gonzalo *a,

a

ARC Centre of Excellence for Electromaterials Science, Institute for Frontier Materials, Deakin

University, Melbourne, Australia b

c

CIC Energigune, Alava Technology Park, C/Albert Einstein 48, 01510 Miñano, Álava, Spain

IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain

d

Institute for Frontier Materials, Deakin University, Geelong, Australia

e

ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria 3800,

Australia f

Departamento de Química Inorgánica. Universidad del País Vasco UPV/EHU, P.O. Box 664, 48080

Bilbao, Spain *Correspondence: [email protected]

Abstract A promising approach to improve the specific capacity and cyclability in a Na-O 2 cell using a pyrrolidinium-based ionic liquid electrolyte in a half-cell has been explored in this work. Increasing the concentration of sodium salt in an ionic liquid electrolyte produces a significant enhancement in the discharge capacity of up to 10 times, a reduction of the overpotential and an increase in long-term cyclability. Additionally, a distinct discharge morphology is also observed, which is demonstrated to be a result of a different oxygen reduction reaction (ORR) mechanisms. These improvements are likely due to the solvation of Na+ in the electrolyte mixtures containing different Na+ concentrations; the coordination of Na+ by the anion of the ionic liquid dictates the discharge product morphology. At low

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Na+ concentrations, Na+ is strongly coordinated by the anion of the ionic liquid, and this also can have an effect on its mobility; however, at high Na+ concentration this interaction is weakened, and favouring mass transport prior to product deposition. It therefore appears that the concentrated electrolyte strategy is a useful route to enhance the performance of Na-O 2 batteries. Interestingly, when using a pressurized Swagelok-type cell the discharge product presents cubic morphology which is typical of NaO2. This is the first work where this characteristic morphology appears when using an ionic liquid which opens new venues for future research.

1. INTRODUCTION Rechargeable metal–oxygen batteries with high specific high-energy densities (e.g. 3451 Wh kg-1 for Li-air batteries)1-5 have drawn much attention as next generation energy storage technologies, which will be beneficial to support the increasing use of renewable energy and electric vehicles.1, 6 Among the different chemistries of metal-oxygen batteries, Na–O2 batteries have recently attracted much research interest, due to a number of important advantages in comparison with the Li-analogue. 7 For example, in Li-O2 systems, the superoxide radical (O2●-) is unstable and rapidly undergoes a secondary reaction to form lithium peroxide (Li2O2)8. As there is a large kinetic barrier to overcome in the oxidation and reduction of the resultant lithium peroxide, this limits its further cyclability. The superoxide radical complex (Na+ -O2●-) for Na-O2 batteries is more stable than Li+ -O2●- and does not undergo further reactions while still offer high specific energy (e.g. 1108 Wh kg-1)9. The electrochemical formation and dissolution of NaO2 involve only one electron, therefore there is a smaller discharge-charge potential hysteresis, facilitating cyclability in comparison to the electrochemical processes that occur in Li-O2 batteries. 9-11 Moreover, sodium as a resource is more abundant in nature and more cost-effective than lithium. These combined properties support the idea that the Na–O2 system is a promising alternative to its lithium counterpart.


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For Na-O2 batteries, it has been reported that the morphology of the deposit on the air cathode is key in terms of discharge capacity and cyclability. For instance, a series of glymes-ethers of various chain lengths have been studied as potential electrolytes for Na-O2 batteries, leading to different capacities, which were related to the deposit morphology. It was found that large cubic crystal deposits enabled high capacity (ca. 7.5 mAh cm-2), compared to submicrometric crystals (ca. 0.2 mAh cm-2). The different deposit morphology was a result of the nucleation mechanism during discharge, which was controlled by the interaction of the discharge intermediates with the solvent. 12 Therefore, the nature of the electrolyte is found to have a profound effect on the discharge mechanism, and subsequently the discharge product and morphology. In the case of glymes-ethers, the existence of two different discharge mechanisms has been reported. A discharge mechanism dominated by a solution precipitation route in which soluble NaO 2 species precipitate from solution on the electrode, as identified in several literature reports. 11,

13-17

The

alternative is the surface dominated mechanism.12 The discharge mechanism can affect the discharge product crystal structure, morphology and distribution, which are important parameters that influence the voltage profile on cycling, the discharge capacity and the reversibility in metal-air batteries.18 Ionic liquids (ILs) as an alternative electrolyte choice have been explored in rechargeable Na-O 2 batteries due to their superior electrochemical and thermal stability, which can enhance the overall safety of the battery.19-20 Among the various ILs, a pyrrolidinium-based ionic liquid, N-butyl-N-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide [C4mpyr][TFSI] has caught our attention as an alternative electrolyte choice for Na-O2 batteries for the following reasons. Firstly, successful plating and stripping of sodium in [C4mpyr][TFSI] IL have been reported presenting optimum physicochemical properties even in the presence of large sodium salt concentration. 21 Furthermore, a smooth surface, devoid of dendrites, can be obtained in ionic liquid based electrolytes through extensive plating/stripping as recently reported. 22 Secondly, the oxygen reduction reaction 3 ACS Paragon Plus Environment


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(ORR) in the neat [C4mpyr] [TFSI] has been studied, which shows the single-electron reversible reaction O2/O2●-. 23 Recently, we have published several works focusing on understanding the effect of concentration of NaTFSI in [C4mpyr][TFSI] on the ORR electrocatalysis mechanism in the bulk electrolyte.19,24 In summary, it was found that in the absence of NaTFSI, the reduction of oxygen involves the exchange of 1 electron and the stabilization of the electrogenerated superoxide by 4 different [C 4mpyr]+ ions. At low Na+ concentrations (ca. 1.13 mol%), superoxide is stabilized by both Na+ and [C4mpyr]+. However, by increasing the Na+ concentration, the superoxide is preferentially coordinated by the sodium cation and is less available to coordinate with [C4mpyr]+. Additionally, we explored the interaction between Na+ and [TFSI]- upon increasing Na+ which play an important role in Na+- O2●- ion pair formation. In a recent publication by Azaceta et al. 25 the discharge product morphology has been studied using a similar electrolyte mixture, NaTFSI/[C4mpyr][TFSI], with a maximum concentration similar to the one in our studies. In that work, the mechanism of the reduction processes at different NaTFSI concentrations was compared to that recently published in glyme-based systems,

12

either through

solution studies or surface precipitation. However, there was no detailed information presented on the deposition kinetic mechanism on the air cathode, nor was there comprehensive characterization of the deposits. Thus, this article is the continuation of our previous work where we focused in understanding the electrogenerated species in the bulk electrolyte.19, 24 Therefore, building on the knowledge gained during our previous research, we now focus on the discharge performance, mechanism and product deposition using an in-house designed half-cell. The electrochemical results show that increasing the NaTFSI salt concentration in the [C4mpyr][TFSI] IL electrolyte results in an enhancement of the discharge capacity, reduction of the overpotential and increase in the long-term cyclability.


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Significantly, distinctly different discharge morphologies are observed in the electrolyte mixtures with low versus high concentration of Na salt, possibly indicating different ORR mechanisms. Here we explain that these improvements originate from the solvation structures in the bulk electrolyte. The higher Na+ concentration causes the superoxide anion (O2●-) to have a higher probability to interact with Na+ than with IL molecules and reduces the O2●- - [C4mpyr]+ interaction, as previously

mentioned.

Additionally,

weaker

interactions

between

Na+

and

the

bis(trifluoromethylsulfonyl)imide anion, [TFSI]-, is attained due to the monodentate environment at high Na+ concentrations, as already reported in ionic liquid electrolytes with high alkali cation concentrations in the literature.26,27 Finally, we reported, for the first time, the cubic morphology typical of NaO2 when using a pressurized Swagelok-type cell and ionic liquid electrolytes.

2.

EXPERIMENTAL SECTION

2.1 Materials Sodium bis(trifluoromethylsulfonyl)imide (NaTFSI, Solvionic, 99.9%), sodium triflate (NaTfO, SigmaAldrich, 98%) and N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C4mpyr][TFSI] (Solvionic, 99.9%) were stored inside the glovebox, and used as received. The water content in the ionic liquid was less than 50 ppm, as per Karl-Fischer titration measurements (Metrohm KF 831 Karl Fischer Coulometer). The [C4mpyr] [TFSI] IL electrolytes with 1.13 mol% (ca. 0.034 mol kg-1), 4 mol% (ca. 0.12 mol kg-1), 8 mol% (ca. 0.24 mol kg-1) and 16.6 mol% (ca. 0.5 mol kg1

), NaTFSI salt were prepared, and the amount of electrolyte used in each experiment was 200 µL.

The nitrogen and oxygen gas were ultra-high purity grade nitrogen purchased from Supagas Australia. 2.2 Electrochemical Measurements



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An in-house three electrode half-cell was designed for the electrochemical measurements as reported previously 28 and the schematic is depicted in Figure 1.

Figure 1. Schematic of the in-house designed three electrode half-cell used in this work

The carbon paper (Toray Carbon Paper 060 with Microporous Layer) was purchased from Shanghai Hesen Electric Co., Ltd and used as air cathode with the microporous layer in contact with the electrolyte mixture. Before the experiment, the carbon paper was rinsed using ethanol and dried in a vacuum oven for 24 hours at 40 oC. Pt coil wire (XRF) was used as the counter electrode. A Ag/AgTfO reference electrode was manufactured by immersing a Ag wire in a 5 mM AgTfO in [C 4mpyr][TFSI] solution separated from the bulk solution with a porous frit and calibrated vs. sodium (E1/2: +2.65 V). Electrochemical experiments were performed using a Biologic VMP3/Z multi-channel potentiostat. The discharge and charge experiments were performed at a current of ±0.24 mA cm -2 with a potential cut-off between 1.75 to 2.5V vs Na+/Na. The air electrode geometric area is 0.385 cm2. Chronoamperometry (CA) experiments were performed to study the nucleation and growth mechanism of the electrogenerated species on the air cathode. A deposition potential at least 50 mV beyond the onset potential of the discharge process was applied to ensure the reduction process 6 ACS Paragon Plus Environment

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occurred. This approach involves the application of the Sharifker-Hills method 29 that represent 3D instantaneous nucleation and growth (Eq.(1)) and 3D progressive nucleation and growth (Eq.(2)), respectively. i t t ( ) = 1.9542 {1 − exp[−1.2564( )} i t t t t i ( ) = 1.2254 {1 − exp[−2.3367( )} t t i

(Eq. (1))

(Eq. (2))

Where im is the current value of the nucleation peak maximum that occurs at time t m after a potential step in the deposition region. Electrochemical Impedance Spectroscopy (EIS) was measured before and after the discharge process at open circuit potential (OCV) using the same in-house designed three electrode pipette cell. During the test, data was taken over a frequency range of 5 kHz to 50 mHz with eight points per decade using a sinusoidal potential perturbation of 10 mV.

2.3 Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) Discharged carbon paper electrodes were washed thoroughly in the glovebox using dried tetrahydrofuran (THF) prior to SEM imaging. All electrodes were transferred by means of a hermetic transfer chamber to the SEM chamber to avoid exposure to ambient air. SEM measurements were performed using a JSM IT 300 series microscope. The micrographs were obtained in secondary electron mode, with a 5 kV accelerating voltage, 30 nA probe current. EDX were obtained with a 10 kV accelerating voltage. 2.4 Ex situ 23Na and 19F Solid State Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) Spectroscopy A Bruker AVANCE III 500 MHz (11.7 T) wide bore NMR spectrometer equipped with a 2.5 mm magic angle spinning (MAS) probe was used to collect ex situ 23Na NMR spectra. The carbon paper electrodes 7 ACS Paragon Plus Environment


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were washed thoroughly in the glovebox using dried tetrahydrofuran (THF) after discharge and then cut into pieces and packed into 2.5 mm outer diameter MAS rotors inside an argon glove box. 23Na spectra were acquired with a 1.8 μs central transition selective π/2 excitation pulse, a recycle delay of 1 s and up to 200000 scans acquired depending on the signal to noise.

23

Na chemical shifts were

referenced to a 1M NaCl solution at 0 ppm. All spectra were collected at a MAS rate of 20 KHz at room temperature. 2.5 Full Na-O2 cell A pressurized 2-electrode Swagelok-type cell was used for the galvanostatic measurements. The fabricated Na-O2 cells consisted of a sodium metal anode and Toray Carbon Paper 060 with Microporous Layer oxygen electrode. Cells were assembled with sodium metal (99.9%, Sigma-Aldrich) and 200 μL of [C4mpyr] [TFSI] IL electrolyte with 16.6 mol% NaTFSI salt were added dropwise on two microfiber separators (Whatman). A stainless steel mesh (Alfa Aesar) was used as the current collector. The electrochemical cells were assembled inside an argon glovebox (< 0.1 ppm water content, < 0.1 ppm oxygen content). The cells were then purged with pure oxygen and pressurized to ~ 1atm before the electrochemical measurements at a current density of 10 µA cm-2, using a Biologic-sas VSP potentiostat. Morphological analysis was conducted in a Quanta 200 FEG (FEI) scanning electron microscope (SEM).

3. RESULTS AND DISCUSSION 3.1 Discharge - Charge Profiles Figure 2a displays the discharge profiles under flowing oxygen in the neat [C4mpyr][TFSI] and the same IL containing 1.13 mol% of NaTFSI (ca. 0.03 m) using a in-house designed three electrode half-cell 8 ACS Paragon Plus Environment

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(Figure 1). For the neat IL system, there is only one reduction process, occurring at +1.5 V vs. Na +/Na, during discharge, while the discharge profile in the 1.13 mol% NaTFSI/[C4mpyr][TFSI] electrolyte shows two reduction processes. One of these processes occurs at a more positive potential value (+1.9 V vs. Na+/Na) than that in the neat IL due to the presence of Na+. Similar experiments under flowing nitrogen were also performed as control (dashed line) indicating that the ORR reaction was responsible for the observed electrode response when oxygen was present. Figure 2b further illustrates the ORR process in the 1.13 mol% NaTFSI/[C4mpyr][TFSI] electrolyte by reversing the scan after the first reduction process (labelled as A) and during the second reduction process at different stages (labelled as B, C and D). An intriguing observation about the coulombic efficiency is noted. When the cut-off potential is at point A, the coulombic efficiency is 80%. However, when the cut-off potential is at point B, the coulombic efficiency is 28%. Continuing discharge to the potential at point C and D does not increase the coulombic efficiency (27.5% for point C and 26.9% for point D). This could be due to some side reactions at a more negative potentials; however further characterisation is required in order to clarify the reason for the decreased efficiency. Thus, significantly higher coulombic efficiency is attained when the cut-off potential is limited to immediately after the first reduction process (e.g. 1.7 V vs Na+/Na). The distinct discharge profiles shown in Figure 2a demonstrate the considerable influence of the NaTFSI salt on the ORR process in the [C4mpyr] [TFSI] IL electrolyte. This difference can be understood by a general theory for predicting the ORR mechanism and products in nonaqueous electrolytes based on the “hard-soft acid-base” (HSAB) concept.30 According to this theory, all soft acid cation-containing electrolytes, such as [C4mpyr]+, provide a suitable medium for the highly reversible one-electron O2/ O2●- process. When a strong Lewis acid, such as Li+ and Na+ is present, even at a low concentration, it dominates the reaction mechanism, due to a stronger interaction with O2●-. Therefore, we hypothesise that the first reduction process in this study is due to the interaction between Na + and O2●-, while the second reduction process, could be related to the interaction between [C4mpyr]+ and O2●-. This is



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suported by our previous results based on the oxygen electrocatalysis of the bulk electrolyte which is depicted in Figure 2 e.19,31 More in detail, in the neat [C4mpyr][TFSI], the well-known one-electron O2/O2●- redox couple is seen to occur, C/A.32 Upon addition of Na+ ions to this system, a C1/A1 redox couple is observed and at more positive potentials than the redox process C/A. Furthermore, the current density of the redox process C1/A1 increases in intensity with the Na+ concentration.24 Figure 2c compares the effect of the concentration of NaTFSI salt on the first discharge/charge process. Upon increasing the concentration of NaTFSI salt in the ionic liquid, the discharge and charge capacity were both substantially increased. For instance, when the concentration of NaTFSI salt was increased to 16.6 mol% NaTFSI (ca. 0.5 mol kg-1 , close to saturation point), the discharge capacity (ca. 0.13 mAh cm-2) in this electrolyte was more than 10 times that of the lower concentration electrolyte system (ca. 0.011mAh cm-2 for 1.13 mol% NaTFSI). Similar trends have been observed for Li-air batteries using LiTFSI/tetraglyme.33 Additionally, in our study, it is also noted that with increasing salt concentration in the electrolyte, the discharge onset potential becomes more positive, which importantly results in a smaller discharge charge potential hysteresis. This observation contrasts with the less favourable viscosity properties in the electrolyte upon increasing the NaTFSI concentration since the ionic conductivity and viscosity of the electrolyte should delay the Na+ mass transport.21 However, as the discharge onset potential is related to the deposition process during discharge, the positive shift in discharge potential suggests promotion of the discharge process. In addition, there are other factors to consider, such as the superior mobility of Na+ in highly concentrated electrolytes as already reported in the literature. 26 This is due to the TFSI anion coordinating to M+ in a monodentate structure, as opposed to the bidentate structure at low concentrations which will limit the Na + mobility. Therefore it is possible to conclude that the dynamics of Na+ can be decoupled from the physicochemical properties of the high concentration electrolyte, but in the case of the 1.13% electrolyte, the coordination environment and


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low Na ion conductivity and transference may also contribute to greater cell polarisation and the shorter discharge time.

3.0

3.0 a)

2.6

[C4mpyr][TFSI]+1.13 mol% NaTFSI under N2 Neat [C4mpyr][TFSI] under O2

2.4

[C4mpyr][TFSI]+1.13 mol% NaTFSI under O2

2.2 2.0 1.8 1.6

1.2 0.00

2.8

2.6 2.4 2.2 2.0

A

1.8 1.6

B

C

D

1.4

1.4

3.0

b)

2.8

Neat [C4mpyr][TFSI] under N2

Voltage (E vs. Na+/Na)


2.8

0.02

0.04 0.06 Capacity ( mA h cm-2)

0.08

1.2 0.00

0.10

0.04 0.06 Capacity ( mA h cm-2)

0.08

0.10

d)

Discharge Capacity with 16.6 mol% NaTFSI electrolyte

0.05

2.6 2.4 2.2 2.0 1.8 1.13 mol% NaTFSI 4 mol% NaTFSI 8 mol% NaTFSI 16.6 mol% NaTFSI

1.6 1.4 1.2 0.00

0.02

0.06

c)

Capacity ( mA h cm-2)


Charge Capacity with 16.6 mol% NaTFSI electrolyte

0.04 0.03

0.04

0.06

0.08

0.10

0.12

Discharge Capacity with 4 mol% NaTFSI electrolyte

0.02 0.01 0.00

0.02

0.14

Charge Capacity with 4 mol% NaTFSI electrolyte

0

5

10

Capacity ( mA h cm-2)

15 Cycles

20

25

N2-saturated neat IL neat IL 0.7 mol% /IL 1.13 mol%/IL

1.0

e) A Current density / mA cm-2

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


A1

0.5

0.0

-0.5

C1 -1.0

C -2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Potential / V vs Fc0/Fc+

Figure 2. a) 1st discharge profile in the neat [C4mpyr][TFSI] IL electrolyte and 1.13 mol% NaTFSI/ [C4mpyr][TFSI] IL electrolyte mixtures both under N2 and O2; b) discharge-charge profiles in 1.13 mol% NaTFSI/ [C4mpyr][TFSI] IL electrolyte mixtures, reversing the scan after the first reduction process (labelled as A) and during the second reduction process (labelled as B, C, and D) under O2; c) 1st discharge-charge process in [C4mpyr][TFSI] IL –based electrolyte mixtures. Cut off potential: 1.75- 2.5 V; d) Discharge-charge capacity vs. cycles in 4 mol% NaTFSI/[C4mpyr][TFSI] and 16.6 mol% NaTFSI/[C4mpyr][TFSI] IL electrolyte mixtures with the controlled depth of discharge (the discharge time is limited to 5 minutes Cut off potential: 1.752.5 V. Applied current: 0.24 mA cm -2, e) Cyclic voltammograms of a glassy carbon working electrode in nitrogen saturated (dotted line) and oxygen saturated (solid line) [C4mpyr][TFSI] in the absence (―) and in the presence of 0.7 mol% NaTFSI (―) and 1.13 mol% (―).



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Finally, we examine the effect of the concentration of NaTFSI salt on the cycle life of the cell, as shown in Figure 2d. It is observed that for the 4 mol% NaTFSI system, the discharge and charge capacity faded quickly with cycling, whereas a high concentration electrolyte mixture (ca. 16.6 mol% NaTFSI) showed constant discharge and charge capacity over at least 20 cycles, indicating better cyclability (the sudden loss of cycling efficiency in this device at cycle 21 probably reflects the effects of moisture uptake or the accumulation of side reaction products). Overall, the electrochemical results demonstrate that the high concentration of Na+ salt in the [C4mpyr][TFSI] IL electrolyte enhances the performance of the oxygen cathode by increasing the discharge and charge capacity, reducing the over-potential, and improving the cyclability. 3.2 Discharge Morphology - Scanning Electron microscopy To visualize the effect of salt concentration on the ORR process, we examined the morphologies of the carbon paper after the discharge process using ex-situ scanning electron microscopy (SEM). Figure 3 compares the SEM images of the deposits on the carbon paper electrodes after the first discharge cycle in the 4 mol% NaTFSI and 16.6 mol% NaTFSI electrolyte mixtures, low and high concentration. The SEM micrographs show distinct deposition morphologies after discharge in the electrolyte mixtures with different salt concentrations. The diluted electrolyte mixture (ca. 4 mol% NaTFSI in Figure 3) forms more uniform larger crystallites on the electrode surface, whereas for the more concentrated electrolyte mixture (ca. 16.6 mol% NaTFSI in Figure 3b), the surface shows a dense coverage of smaller spherical particles and some large ones. The size of these particles is variable, with a large amount of submicron-scale ( ̴200 nm diameter) particles and some micron-scale particles. This distinct deposition morphology could be the reason for more rapid capacity fade in the electrolyte mixture containing the low NaTFSI concentration (4 mol% NaTFSI) as opposed to the more concentrated electrolyte mixture (16.6 mol% NaTFSI). The compact film across the air cathode in the


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low concentration salt electrolyte mixture may have prevented the O2 from absorbing on the active sites and/or from reaching the gas-electrode-electrolyte three-phase boundaries. Recent studies have proposed two models for the ORR process in non-aqueous organic solvents (e.g. glymes), namely a surface-dominated mechanism and a solution-mediated mechanism.34-37 The surface-dominated mechanism generally induces the formation of film-like deposition products on the electrode surface, which passivates the electrode surface and limits its performance. In contrast, the solution-mediated mechanism leads to larger particle deposits, which enables a higher capacity. Therefore, the specific capacity attained during the discharge processes is closely related to the morphology of the deposits on the air cathode.

Figure 3. SEM image of selected solid deposits on the carbon paper electrode after the first discharge with the cut-off potential of +1.7 V in [C4mpyr][TFSI] ionic liquid electrolyte containing (a) 4 mol% NaTFSI, and (b) 16.6 mol% NaTFSI.



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3.3 Nucleation and growth mechanism To gain insight into the kinetics of the deposition process during discharge and further compare the discharge processes, chronopotentiometry (CP) experiments were performed and analysed using the Sharifker-Hills method of dimensionless current plots. 29 Figure 4 shows the current transient for the oxygen reduction reaction (ORR) held at a constant potential of 1.85 V vs Na+/Na (just after the first reduction process) in the 4 mol% NaTFSI/[C4mpyr][TFSI] and 16.6 mol% NaTFSI/[C4mpyr][TFSI] electrolyte mixtures. Generally speaking, a progressive-type nucleation and growth mechanism would involve solvation, aggregation and desolvation of the species in the electrolyte, prior to precipitation on the surface. 12 Conversely, an instantaneous type of nucleation and growth mechanism on the electrode surface, would lead the accumulation and reaction of electro-generated species on the electrode surface. Interestingly, from Figure 4, the solution effects of Na+ concentration on the deposition mechanism are evident. ORR in the 16.6 mol% NaTFSI/[C4mpyr][TFSI] IL electrolyte mixture is more likely to follow a progressive-type nucleation mechanism, while the ORR in the 4 mol% NaTFSI/[C 4mpyr][TFSI] IL electrolyte mixture is more likely to follow an instantaneous nucleation mechanism. However, there is a discrepancy between theoretical prediction and the experimental data in both cases. It is safe to conclude therefore that the discharge products are attained by different nucleation mechanisms at the different Na contents, however the kinetics cannot be assigned to a specific mechanism. Furthermore, previous studies of the bulk electrolyte by rotating ring-disk electrode (RRDE) have shown the insolubility of the electrogenerated species in the low and high concentrated electrolyte mixtures.24


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1.0 0.8

i/imax

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


0.6 0.4 Instantaneous Progressive 4 mol% NaTFSI 16.6 mol% NaTFSI

0.2 0.0

0

2

4

6

8

10

t/tmax

Figure 4. Experimental curves of current transients of oxygen reduction reaction on a carbon paper electrode discharged at +1.85 V in the 4 mol24% NaTFSI/ [C4mpyr] [TFSI] and 16.6 mol% NaTFSI/[C4mpyr][TFSI] IL electrolyte mixtures are shown in solid lines. Simulated curves of the Sharifker-Hills model of instantaneous and progressive 3D nucleation and growth are present in dash lines.

3.4 Discharge Morphology - Electrochemical Impedance Spectroscopy Electrochemical Impedance Spectroscopy (EIS) was performed on the cathode at OCV before and after the discharge process, in the presence of oxygen gas flowing, with the 4 mol% NaTFSI/[C 4mpyr][TFSI] and 16.6 mol% NaTFSI/[C4mpyr][TFSI] electrolytes to gain more insight into the effect of the deposits on the air cathode. In general, the mid-high frequency region of a Nyquist plot is related to the surface film, while the low frequency region is related to the diffusion of ions from the electrolyte through the electrode and the charge transfer reaction occurring at the interface, as reported by Landa-Medrano et al. 38 As shown in Figure 5, for the pristine air cathode, there is a semicircle observed in the high frequency region related to the capacitive behaviour of the surface of the air cathode. On the other hand, the low frequency region of the Nyquist plot is close to linear with a slope of 45o, which reflects a dominant diffusion behaviour in this region that is likely related to the diffusion of ions from the electrolyte



through the porous carbon paper electrode. A detail of the high frequency and low frequency region is also included in Figure 4. After discharge, the electrode surface was partially covered by the deposits at low and high concentration electrolytes, as evidenced by the SEM observations shown in Figure 3. Generally, the generation of deposits on a porous electrode will lead to an increase in the surface resistance of the electrode and charge transfer resistance between the electrode and electrolyte. A subsequent decrease in the diffusion process of ions through the carbon paper electrode is also anticipated. This is exactly the performance attained for the deposits in the low and high concentration electrolyte. In Figure 5 (upper insert), the Nyquist plots of the electrodes after discharge show a larger semicircle in the high frequency region, which reflects a more resistive surface formed on the electrode in comparison with the bare air cathode. However, when comparing the low frequency region of the Nyquist plots, it is interesting to note differences depending on the electrolyte. For the 16.6 mol% NaTFSI electrolyte mixture, in the low frequency region, a more diffusive behaviour is attained with a slope of 45o, whereas for the 4 mol% NaTFSI electrolyte mixture a more capacitive behaviour is obtained as evidenced by the slope being lower than 45o.

-Im(Z)/Ohm.cm2

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Figure 5. Nyquist plots of Electrochemical Impedance Spectroscopy of pristine carbon paper () and carbon papers after the 1st discharge process in 4 mol% (●) and 16.6 mol% NaTFSI (○) /[C4mpyr] [TFSI] electrolyte mixture. Current density: -0.24 mA cm -2. Cut-off potential: 1.75 V vs Na+/Na.

These findings provide evidence that the porous cathode after discharge in the 4 mol% NaTFSI electrolyte mixture is more occluded in comparison with that in the 16.6 mol% NaTFSI electrolyte mixture, preventing the flow of oxygen to the gas-electrode-electrolyte three-phase boundaries. Thus, this type of deposit can lead to the premature failure of the cell, and thereby is detrimental to battery performance. By comparison, the deposition on the electrode after discharge in the 16.6 mol% NaTFSI electrolyte mixture results in limited passivation and blockage of the electrode surface. Moreover, these findings are in good agreement with previous reports on Li-air cells where the effect of alkali salt in the discharge performance has been studied.33 In that work, the increase in specific capacity was attributed to the morphology of the discharge deposit and efficient use of the electrode surface.

3.5 The nature of discharge products To assess the nature of the discharge products formed during discharge process in 4 mol% NaTFSI and 16.6 mol% NaTFSI electrolyte mixtures, the electrode surfaces after discharge were further examined using energy dispersive X-ray spectroscopy (EDX) and solid-state (SS) MAS NMR. 3.5.1 Energy Dispersive X-ray Spectroscopy As seen in the SEM observation in Figure 3 b, the discharge products formed in the 16.6 mol% NaTFSI/[C4mpyr][TFSI] electrolyte mixture were composed of small and large particles, these two different particles have different elemental compositions as shown by EDX analysis (Figure S1 and Table S1). While the small particles are mainly composed of Na and O, with an approximate ratio of Na:O of 1:2 ±0.2. The larger particles contain Na and O and traces of F, S and N. Although, the Na:O



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ratio in these particles varies across the particle, in every case the Na and O content is lower than that observed in the smaller particles. It is worthy to mention a review by Bender et al 39 that compared the shape of the capacity-voltage plots correlated to the discharge product. NaO2 formation is evidenced as a flat voltage plateau for both the discharge and charge processes as observed in our results, as opposed to other discharge products (e.g. Na2O2 or Na2O2·H2O) characterised by multiple charge plateaus. Furthermore, our previous study that used a pressure cell to monitor the O 2 consumption in the same electrolyte confirmed the generation of NaO2 on the cathode. 31 Therefore, the small particles observed are likely to be NaO2. For the discharge products formed in the 4 mol% NaTFSI/[C4mpyr][TFSI] electrolyte mixture, their elemental compositions are more consistent (Figure S1 and Table S1). Na and O and traces of F, S were also detected in the discharge products. In this case, the ratio of Na: O in the discharged products is 1:3.5 ±0.7, with a significantly larger amount of oxygen. There are several possibilities for the presence of the traces of F, S and N elements: i) a small amount of electrolyte remains on the electrode surface or ii) side reactions due to the interaction of the electrogenerated species (ca. O2●-) and the electrolyte and/or air cathode, resulting in the degradation of the electrolyte and/or air cathode. 3.5.2 Solid-State MAS NMR Spectroscopy Additional information on the nature of the deposition products was revealed from 23Na solid state MAS NMR spectroscopy. In Figure 6, the 23Na MAS NMR spectrum of the discharged air cathode shows a broad peak with a chemical shift distributed from 40 ppm to -60 ppm. Unfortunately, as reported in the literature40, the chemical shifts of the possible reaction products are all in this region (ca. 6.9 ppm for Na2O2, -28 ppm for NaO2, 7 ppm, 7.5 ppm, -4 ppm for Na2CO3, and five unique crystallographic sites ranging from -5 ppm to -30ppm for NaTfO) complicating the identification of the discharge products. 18 ACS Paragon Plus Environment

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The broad 23Na peak observed from the discharged carbon paper (Figure 6) could potentially arise from overlapping signals from these products. A two-dimensional 23Na MQMAS experiment did not provide any improvement in resolution (spectrum not shown). This is probably caused by the following reasons: (a) the broad nature of the

23

Na peak arises from structural disorder; (b) paramagnetic

broadening arises from the electrically-conducting nature of the carbon paper substrate; (c) the discharge products were not produced in a detectable quantity. Similar peaks, in shape and chemical shift, have been observed in

23

Na ss NMR for Na-O2 batteries using NaClO4/DME

41

and

NaTfO/DEGDME40 as electrolytes, which is extremely interesting due to the differences in Na salts and solvents used in each study; but leading somehow to similar NMR spectra. Figure 6 compares the 23Na MAS NMR spectra of carbon paper electrodes after discharge in the 4 mol% NaTFSI and 16.6 mol% NaTFSI electrolyte mixtures. The signal intensity is stronger for the electrode after discharge in the more concentrated solution due to the larger amount of discharge products deposited on the electrode surface. Additionally, it is observed that the signal for the electrode after discharge in 4 mol% NaTFSI electrolyte mixture is broader, suggesting the formation of a more disordered product on the electrode. These observations are in good agreement with the SEM observations shown in Figure 3, larger round particle shaped morphology would lead to hihger crystallinity.34



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Figure 6. Comparison of 23Na Mas NMR spectra of carbon paper electrodes after the 1st discharge process in 16.6 mol% NaTFSI/[C4mpyr][TFSI] IL electrolyte mixture, 4 mol% NaTFSI / [C4mpyr][TFSI] IL electrolyte mixture, and pristine carbon paper

3.6 Na-O2 battery testing Two different Na-O2 battery configurations were prepared in order to study the influence of the carbon on electrochemical performance by placing the microporous side in contact with either the O2 or the electrolyte. We first examine the oxygen reduction reaction kinetics of discharge product formation upon galvanostatic discharge. Figure 7 shows the voltage profiles at a discharge rate of 10 µA cm-2 with a cut-off of 1.4V. The microporous side in contact with O2 delivers the highest capacity (0.19 mAh cm-2) thanks to the porous carbon which facilitates oxygen diffusion into the electrode. A single flat plateau is observed in both cases at 1.94 V corresponding to an overpotential of 270 mV, similar to that obtained by OrtizVitoriano et al. for a glyme-based electrolyte.16 In order to gain insight into the discharge product morphology, SEM analysis was performed (Figure 7b). Micron-scale cubic particles are observed on the surface of the carbon with more particles observable on the electrode discharged to higher capacity (i.e., microporous in contact with O2). This is the first-time cubic particles have been observed in an ionic liquid-based electrolyte, suggesting the formation of NaO2 discharge product. Several studies can be found where this cubic morphology is ascribed to NaO 2 formation. 9,16 Azaceta et al.25, however, observed a conformal coating of the carbon fibers using [C 4mpyr][TFSI] ionic liquid at comparable applied currents (15 µA vs 10 µA on this study) . In addition, galvanostatic discharge in a Swagelok-type cell showed a sloping discharge plateau which differs from the flat plateau observed in this work which is typical of NaO2 formation. However, no SEM data were provided by Azaceta et al. 25

at the cell-level which hinders comparison.


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a)

b)

Figure 7. a) Discharge profile of Na-O2 batteries in 16.6 mol% NaTFSI/[C4mpyr][TFSI] IL electrolyte at 10 µA cm2, b) SEM images of the discharge products

Additional characterization of the discharge products by X-ray diffraction and Raman spectroscopy showed no signal due to the small amount of material present. Further studies to increase the amount of discharge products are in progress to gain further insight into their chemical composition. However, due to the morphological features found in the discharge cells, NaO2 formation is suggested as the most plausible option. Interestingly, only cubic particles were observed using a Swagelok-type cell as opposed to the deposits obtained using an in-house designed three electrode half cell (Figure 3). The influence of key parameters such as discharge rate (200 vs 10 µA cm2), electrolyte volume and oxygen pressure might play an important role in dictating discharge product morphology and performance. For instance, the kinetics when using a Swagelok type cell are slower than in our in-house designed half-cell, which could give rise to a more defined discharge product morphology. These parameters will be carefully examined in further studies.



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3.7 Proposed oxygen reduction reaction (ORR) mechanism in [C4mpyr][TFSI] electrolyte containing different concentrations of NaTFSI salt The results presented here provide strong evidence of the effect of concentration of Na salt on the ORR mechanism as observed by the deposit morphology, specific capacity and cyclability. In summary, by increasing the NaTFSI concentration, superior specific capacity and cyclability are attained. We discuss here a hypothesis of the impact of the species generated in the bulk electrolyte on the discharge morphology and properties. A recent study has investigated the factors responsible for the discharge mechanisms and deposit morphology in a series of glyme-ethers in electrolytes with donor numbers (DN) ranging from 12 to 19 kcal mol-1 in a Na-O2 battery.12 The oxygen reduction mechanism was identified to encompass several key steps: a) reduction of O2 to O2●- on the electrode; b) dissolution of the discharge product in the electrolyte; and c) desolvation and crystallization of the discharge product on the cathode. The differences in the cell performance and deposit morphology were attributed to the interactions between the electrolyte and the discharge products, especially the desolvation energy. Thus, tetraethylene glycol dimethyl ether, TEGDME, has a stronger interaction with the Na+-containing species and also has a higher desolvation barrier to liberate NaO2 onto the electrode surface from the electrolyte. In this case, submicrometric crystallines with low capacities were attained. On the other hand, dimethoxyethane, DME, possesses a weaker solvation ability, and hence could promote the continuous growth of discharge products from the electrolyte. Large macro-cubic crystals enabling high capacities are obtained in this case. Therefore, the solvation structure of the electrolyte and the solvation-desolvation kinetics are major factors in the discharge mechanism. The intermediates’ solubility was studied using RRDE leading to soluble intermediates in TEGDME and low solubility intermediates in DME, and it was concluded that, as opposed to Li-air cells, the solubility of the intermediates is not key to support increased capacities.


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Interestingly, in our work we have obtained similar behaviour by modifying the NaTFSI concentration in the electrolyte and some similarities with the work using glymes electrolytes can be suggested regarding the interactions between the electrolyte and the discharge product. In our previous work, 24 the solvation structure between O2●-and [C4mpyr] + in the presence of different concentrations of Na+ was studied and we envisioned the generation of a complex species with the general structure of [O2•-][C4mpyr+]n [Na+]m.31 It was found that in the electrolyte mixture with a low concentration of Na salt (ca. 1.13 mol%), O2●- was stabilised by both Na+ and [C4mpyr] +. However, by increasing the concentration of Na salt, O2●- prefers to coordinate with Na+ and is less available to coordinate with [C4mpyr]+. It is important to highlight that the coordination of O2●- with Na+ should be stronger than that of O2●- with [C4mpyr]+ due to shorter bond length for the former, favouring ion pair formation. Therefore, the different ORR mechanisms observed in this study are likely due to the differences in the solvation structure with varying concentration of sodium salt in the electrolyte mixtures, and in principle are not related to the solubility of the intermediates. Although the anion, TFSI, has not been included in the general structure mentioned above, it plays a crucial role in the solvation of the Na+ depending on the salt concentration.26-27 The anion coordinates to Na+ in a stronger bidentate structure in low concentration electrolyte mixtures, while the anions coordinate to Na + in a weaker monodentate structure in concentrated mixtures. Therefore, the large particle deposits could be related to a moderate interaction of Na+ with the anion which favor the desolvation process prior the deposition, as opposed to stronger interaction with Na+ at lower concentration which may hinder the desolvation process. This proposed trend in the ORR mechanism at different concentrations of electrolyte mixtures is summarised schematically in Figure 8.



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Figure 8. Simplified schematic illustration of the effect of the concentration of NaTFSI/ [C4mpyr][TFSI] electrolyte on the oxygen reduction reaction (ORR) process in the bulk electrolyte and subsequent deposit.

Additionally, we measured the donor number (DN) of [C4mpyr][TFSI] IL using solution-state 23Na NMR (Supplement Figure S2 and Table S2).36 The measured DN number of [C4mpyr][TFSI] IL is 3, which is lower than the DN of ether-based organic solvents. According to this measurement, this IL electrolyte might be easily saturated with a low concentration of O2●-, and hence the low solubility of the intermediates.

4. CONCLUSIONS In conclusion, we have shown the importance of the tailoring effects of the Na + ion concentration in the electrolyte on the discharge performance and the nature of the discharge products. We have found that increasing the Na salt concentration in the [C4mpyr][TFSI] ionic liquid electrolyte significantly enhances the discharge capacity, reduces the over-potential and increases cyclability of a Na-O2 battery. The reason for this difference in performance is related to the significant difference in


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the discharge morphology of the deposits, which has been characterised by SEM, EIS and EDX. We have also demonstrated the correlation between the species generated in the bulk electrolyte and the type of deposit attained in the air cathode due to the different nucleation and growth mechanism. The difference in the ORR mechanism and nature of discharge products is explained to originate from the solvation structure between O2●-, Na+ and [C4mpyr] +. The higher Na+ concentration causes the superoxide anion (O2●-) to interact with Na+ rather than with [C4mpyr] +. This in turn induces a lower desolvation energy for deposition of NaO2 from the electrolyte and triggers the formation of the discharge products on the electrode surface. Therefore, the concentrated electrolyte strategy is a useful approach to enhance the performance of Na-O2 batteries.

SUPPORTING INFORMATION Additional information related to the composition and morphology of the discharge products at different alkali salt concentrations, as well as donor number for ionic liquid and conventional organic solvents determined by 23Na-NMR.

ACKNOWLEDGEMENTS YZ, CPG, PCH, MF, and DRM gratefully acknowledge the financial support from the Australian Research Council (ARC) through the ARC Centre of Excellence for Electromaterials Science (ACES). MF and DRM also gratefully acknowledge funding through the ARC Laureate program. Deakin University’s Advanced Characterisation Facility is acknowledged for use of the NMR spectrometer. NOV, BA and TR would also like to thank the Basque Government for financial support through ELKARTEK project CICE17. REFERENCES 1. Abraham, K. M.; Jiang, Z., A Polymer Electrolyte ‐ Based Rechargeable Lithium/Oxygen Battery. Journal of The Electrochemical Society 1996, 143, 1-5. 2. Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M., Li-O2 and Li-S Batteries with High Energy Storage. Nat Mater 2012, 11, 19-29. 3. Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G., A Reversible and Higher-Rate LiO2 Battery. Science 2012, 337, 563-566. 4. Ottakam Thotiyl, M. M.; Freunberger, S. A.; Peng, Z.; Chen, Y.; Liu, Z.; Bruce, P. G., A Stable Cathode for the Aprotic Li–O2 battery. Nat Mater 2013, 12, 1050-1056. 25 ACS Paragon Plus Environment


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24. Pozo-Gonzalo, C.; Johnson, L. R.; Jónsson, E.; Holc, C.; Kerr, R.; MacFarlane, D. R.; Bruce, P. G.; Howlett, P. C.; Forsyth, M., Understanding of the Electrogenerated Bulk Electrolyte Species in SodiumContaining Ionic Liquid Electrolytes During the Oxygen Reduction Reaction. The Journal of Physical Chemistry C 2017, 121, 23307-23316. 25. Azaceta, E., et al., Electrochemical Reduction of Oxygen in Aprotic Ionic Liquids Containing Metal Cations: A Case Study on the Na-O-2 System. Chemsuschem 2017, 10, 1616-1623. 26. Forsyth, M.; Yoon, H.; Chen, F.; Zhu, H.; MacFarlane, D. R.; Armand, M.; Howlett, P. C., Novel Na+ Ion Diffusion Mechanism in Mixed Organic–Inorganic Ionic Liquid Electrolyte Leading to High Na+ Transference Number and Stable, High Rate Electrochemical Cycling of Sodium Cells. The Journal of Physical Chemistry C 2016, 120, 4276-4286. 27. Monteiro, M. J.; Bazito, F. F. C.; Siqueira, L. J. A.; Ribeiro, M. C. C.; Torresi, R. M., Transport Coefficients, Raman Spectroscopy, and Computer Simulation of Lithium Salt Solutions in an Ionic Liquid. The Journal of Physical Chemistry B 2008, 112, 2102-2109. 28. Howlett, P. C.; Khoo, T.; Mooketsi, G.; Efthimiadis, J.; MacFarlane, D. R.; Forsyth, M., The Effect of Potential Bias on the Formation of Ionic Liquid Generated Surface Films on Mg Alloys. Electrochimica Acta 2010, 55, 2377-2383. 29. Scharifker, B.; Hills, G., Theoretical and Experimental Studies of Multiple Nucleation. Electrochimica Acta 1983, 28, 879-889. 30. Allen, C. J.; Hwang, J.; Kautz, R.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M., Oxygen Reduction Reactions in Ionic Liquids and the Formulation of a General Orr Mechanism for Li–Air Batteries. The Journal of Physical Chemistry C 2012, 116, 20755-20764. 31. Pozo-Gonzalo, C.; Johnson, L. R.; Jónsson, E.; Holc, C.; Kerr, R.; MacFarlane, D. R.; Bruce, P. G.; Howlett, P. C.; Forsyth, M., Understanding of the Electrogenerated Bulk Electrolyte Species in SodiumContaining Ionic Liquid Electrolytes During the Oxygen Reduction Reaction. The Journal of Physical Chemistry C 2017. 32. Katayama, Y.; Onodera, H.; Yamagata, M.; Miura, T., Electrochemical Reduction of Oxygen in Some Hydrophobic Room-Temperature Molten Salt Systems. J. Electrochem. Soc. 2004, 151, A59-A63. 33. Liu, Y.; Suo, L.; Lin, H.; Yang, W.; Fang, Y.; Liu, X.; Wang, D.; Hu, Y.-S.; Han, W.; Chen, L., Novel Approach for a High-Energy-Density Li-Air Battery: Tri-Dimensional Growth of Li2o2 Crystals Tailored by Electrolyte Li+ Ion Concentrations. Journal of Materials Chemistry A 2014, 2, 9020-9024. 34. Lim, H.-D.; Lee, B.; Bae, Y.; Park, H.; Ko, Y.; Kim, H.; Kim, J.; Kang, K., Reaction Chemistry in Rechargeable Li-O2 Batteries. Chemical Society Reviews 2017, 46, 2873-2888. 35. Aurbach, D.; McCloskey, B. D.; Nazar, L. F.; Bruce, P. G., Advances in Understanding Mechanisms Underpinning Lithium–Air Batteries. Nature Energy 2016, 1, 16128. 36. 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, 1091-1099. 37. 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. 38. Landa-Medrano, I.; Frith, J. T.; Ruiz de Larramendi, I.; Lozano, I.; Ortiz-Vitoriano, N.; GarciaAraez, N.; Rojo, T., Understanding the Charge/Discharge Mechanisms and Passivation Reactions in NaO2 Batteries. Journal of Power Sources 2017, 345, 237-246. 39. Bender, C. L.; Schroder, D.; Pinedo, R.; Adelhelm, P.; Janek, J., One- or Two-Electron Transfer? The Ambiguous Nature of the Discharge Products in Sodium-Oxygen Batteries. Angewandte Chemie 2016, 55, 4640-9. 40. Reeve, Z. E. M.; Franko, C. J.; Harris, K. J.; Yadegari, H.; Sun, X.; Goward, G. R., Detection of Electrochemical Reaction Products from the Sodium–Oxygen Cell with Solid-State 23na Nmr Spectroscopy. J Am Chem Soc 2017, 139, 595-598. 41. Liu, T.; Kim, G.; Casford, M. T. L.; Grey, C. P., Mechanistic Insights into the Challenges of Cycling a Nonaqueous Na–O2 Battery. The Journal of Physical Chemistry Letters 2016, 7, 4841-4846. 27 ACS Paragon Plus Environment


TOC Graphic

1.13 mol% NaTFSI 4 mol% NaTFSI 8 mol% NaTFSI 16.6 mol% NaTFSI

2.8 Voltage (E vs. Na+/Na)

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Capacity ( mA h cm -2)


Elucidating the Impact of Sodium Salt ... - ACS Publications

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