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Li-Salt Anion Effect on O Solubility in Li-O Battery Electrolytes Jonas Lindberg, Balázs Endr#di, Gustav Åvall, Patrik Johansson, Ann Cornell, and Göran Lindbergh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09218 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017
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The Journal of Physical Chemistry
Li-salt Anion Effect on O2 Solubility in Li-O2 Battery Jonas Lindberg,∗,† Balázs Endrődi,†,‡ Gustav Åvall,¶ Patrik Johansson,¶ Ann Cornell,† and Göran Lindbergh∗,† †Department of Chemical Engineering and Technology, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden ‡Department of Physical Chemistry and Materials Science, University of Szeged, H-6720 Szeged, Hungary ¶Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden E-mail:
[email protected];
[email protected] 1
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Abstract For the promising Li-O2 battery to be commercialized further understanding of its constituents is needed. This study deals with the role of O2 in Li-O2 batteries, both its influence on electrochemical performance and its solubility in lithium-salt containing dimethyl-sulfoxide (DMSO) electrolytes. Experimentally, the electrochemical performance was evaluated using cylindrical ultramicroelectrodes. Two independent techniques, a mass spectrometer and an optical sensor, were used to evaluate the O2 solubility, expressed as Henry’s constant. Furthermore, the ionic conductivity, dynamic viscosity, and density were also measured. Density functional theory calculations were made of the interaction energy between O2 and the different species in the electrolytes. When varying O2 partial pressure, the current was larger at high pressures confirming that the O2 concentration is of key importance when studying the kinetics of this system. Compared with neat DMSO, the O2 solubility increased with addition of LiTFSI and decreased with addition of LiClO4 , indicating that the salt influences the solubility. This solubility trend is best explained in terms of apparent molar volume and interaction energy between O2 and the salt anion. In conclusion, this study shows the importance of O2 concentration, not just its partial pressure, and that the choice of Li-salt can make this concentration increase or decrease.
1
Introduction
The Li-O2 battery, also known as the Li-air battery, has been suggested as a possible technology for the next generation of batteries primarily due to its high theoretical energy density. 1–5 Much of the research has focused on the O2 electrode, with efforts such as trying to find suitable electrode materials, 6,7 understanding the O2 /Li2 O2 reduction and oxidation mechanism, 8–11 characterizing the discharge products and side-products, 12–14 and evaluating galvanostatic cycling performance 15,16 . Less research has focused on the negative electrode and on the electrolyte. The electrolyte in a non-aqueous Li-O2 battery consists of a Li salt dissolved in one or 2
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The Journal of Physical Chemistry
several non-aqueous solvents. During operation, its role is to shuffle Li+ ions between the electrodes. However, this role extends further as the choice of electrolyte has proven to affect the reaction mechanism, whether it stabilizes the LiO2 intermediate or not. 17,18 An often overlooked constituent of the Li-O2 battery is the Li salt. The frequently used Li-ion battery salt lithium hexafluorophosphate (LiPF6 ) is unstable in Li-O2 batteries and decomposes during cycling. 19 Other salts, such as lithium tetrafluoroborate (LiBF4 ) and lithium bis(oxalato)borate (LiBOB) have also shown decomposition. 20 The most studied salts in the literature are lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium perchlorate (LiClO4 ). 19 The LiTFSI salt concentration has been shown to influence the discharge capacity with a maximum seen at 3 M in tetraethylene glycol dimethyl ether (TEGDME) and 1,2-dimethoxyethane (DME). 21,22 Furthermore, highly concentrated electrolytes (LiTFSI:DMSO molar ratio 1:3) have shown increased cycle stability. 23 One property of the electrolyte that has not been extensively studied is its ability to dissolve O2 . It has been mentioned as a key electrolyte property, 19 however, the actual O2 concentration is seldom known. As O2 is a reactant in the reduction reaction its concentration will effect the rate of the reaction. Batteries utilizing propylene carbonate and diglyme have shown larger discharge capacities with increasing O2 pressure. 24–26 Furthermore, as O2 might be depleted from the electrode surface it can be the source of capacity limitation. The relation between the pressure of a gas and its concentration in a solvent, at low concentration and moderate pressure, is described by Henry’s law:
H cp =
c p
(1)
where H cp is Henry’s constant, p partial pressure, and c concentration. For DMSO-based Li-O2 battery electrolytes Henry’s law has been proven valid to at least 1 atm. 27,28 Previously, the O2 concentration has successfully been measured in numerous carbonate-solvent based electrolytes along with some electrolytes based on other solvents, and in electrolytes with fluorinated ether additives. 29,30 However, carbonate based electrolytes decompose and are not 3
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regarded suitable for Li-O2 batteries. 31 For DMSO-based electrolytes, the O2 concentration has been studied using a flow cell coupled to a mass spectrometer 27 and using a rotating ring-disc electrode (RRDE) 32 . However, the results of these studies differ, e.g. a saturation concentration of 1.6 mM was found for a 1 M LiClO4 electrolyte saturated at 1 atm O2 , 27 while the same electrolyte composition showed O2 concentrations of only 0.135 mM when saturated at 0.21 atm O2 , 32 suggesting that there are uncertainties about the solubility of O2 in DMSO-based electrolytes and how it is influenced by the Li salt. The aim of this study was to investigate the O2 concentration and its influence on the performance of Li-O2 batteries. By using cylindrical ultramicroelectrodes, the electrochemical performance was evaluated at different O2 partial pressures. In order to correlate O2 partial pressure to concentration, solubilities of O2 in DMSO-based electrolytes were measured at different Li-salt concentrations of three different Li salts (LiTFSI, lithium trifluoromethanesulfonate (LiTf), and LiClO4 ) using two independent analytical methods. For better understanding, the dynamic viscosity, ionic conductivity, and density of the electrolytes were measured and discussed together with the observed O2 solubility trends. To deepen the discussion, the interaction energies between O2 and the Li-salt anions were calculated using density functional theory (DFT).
2 2.1
Experimental Chemicals
The electrolytes consisted of anhydrous dimethyl sulfoxide (DMSO) (Sigma-Aldrich, Anhydrous), lithium bis(trifluoromethane)sulfonimide (LiTFSI) (Sigma-Aldrich, 99.95 %), lithium perchlorate (LiClO4 ) (Sigma-Aldrich, 99.99 %), lithium trifluoromethanesulfonate (LiTf) (Sigma-Aldrich, 99.995 %), and tetrabutylammonium hexafluorophosphate (TBAPF6 ) (SigmaAldrich, 98 %). The electrolytes were prepared by adding each salt to reach three different concentrations (0.05 M, 0.5 M, and 1 M) in DMSO inside an argon-filled glove box. All 4
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chemicals were used as received.
2.2
Cylindrical Ultramicroelectrodes
An in-house made cylindrical ultramicroelectrode cell, utilizing a single carbon fiber (CF) (diameter 5 µm) or a gold wire (diameter 15 µm) as substrate for the O2 reduction- and evolution reaction, was used to study the influence of O2 concentration on the electrochemical performance (figure 1). The CF was attached, using silver paint, to a copper current collector. In order not to expose the current collector or the silver paint to any electrolyte, a thermoplastic (3M) was hot pressed over the current collector and the paint thus only exposing the CF to the electrolyte. The length of the exposed fiber was 7 mm. A foil of LiFePO4 coated on aluminum (Quallion) was used as counter electrode. It was deliberately dimensioned to have a capacity (1.5 mAh) far excessing the capacity of the CF (capacities in the order of 10 µAh) in order to minimize polarization of the counter electrode as it also served as reference electrode. The counter electrode underwent formation cycling against metallic lithium before usage in order to get a reproducible behavior. At the end of formation, the counter electrode was at 50 % state of charge with a potential of 3.44 V vs. Li+ /Li(m), this potential was used to relate the potential of the CF to Li+ /Li(m). A filter paper (Whatman GF/A) was placed between the CF and the counter electrode in order to prevent short circuiting. The electrodes and filter paper were fixed in a perforated polyethylene film, by hot pressing. The cell was submerged into 3 ml of electrolyte, i.e. there was an excess of electrolyte. Before the measurements the electrolyte was bubbled with 1 atm pure O2 (purity 99.999 %) or technical air (0.21 vol.% O2 and 0.79 vol.% N2 , dried with an adsorption dryer and an APEX gas generator to
