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C: Energy Conversion and Storage; Energy and Charge Transport
Dimethyl Sulfoxide (DMSO)-Based Electrolytes for High Current Potassium-oxygen Batteries Shrihari Sankarasubramanian, and Vijay K. Ramani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03755 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 8, 2018
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The Journal of Physical Chemistry
Introduction Alkali metal-oxygen batteries theoretically offer very high specific energy values (3458 Wh kg-1 for Li-Li2O21, 1108 Wh kg-1 for Na-NaO22, 935 Wh kg-1 for K-KO23) which makes them intriguing candidates for potential next generation batteries. Amongst these systems, the Li-O2 cell, first demonstrated by Abraham and Jiang4, has been the subject of sustained interest over the past decade. However, practical Li-O2 cells have proven to be elusive due to low columbic efficiency and power density
1,5,6
, Li dendrite growth7 and unstable electrolytes8–11. The high
overpotentials have led to an intensive effort to develop catalysts for the Li-O2 system12–17, and water/oxidatively stable11,18,19 electrolytes have been identified. Despite these promising efforts, the fact remains that Li2O2 (Δ = -570.8 KJ mol-1) is the preferred discharge product for Li-O2 cells20 which necessitates the use of a catalyst to reduce the charge overpotential, further worsening an already marginal economic proposition21. On the other hand, the kinetic (but not thermodynamic) stability of NaO2 (Δ =-218.8 KJ mol-1)2 and the thermodynamic stability of KO2 (Δ =-239.4 KJ mol-1)3 offers the possibility of catalyst free, low overpotential cells with these chemistries. Figure 1(a) compares the energy density and specific capacity of these cells. While the K- based cells have lower theoretical energy density and specific capacity compared to Li-O2 cells (but comparable to Li-S cells5), the far greater reversibility (as seen in Figure 1(b)) offsets these losses.
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Figure 1. (a) Comparison of alkali-metal-oxygen chemistries; (b) Effect of oxygen reduction reaction free energy on electrochemical reversibility; (c) Impact of solvent donicity on reaction pathway. The K-O2 cell, first reported by Ren and Wu3, is a promising, catalyst free, low overpotential system. The cell performance22, dendrite mitigation at the metallic anode23 and possible side reactions24 are issues that have been briefly examined. The present study seeks to achieve a more fundamental understanding of the oxygen reduction reaction (ORR) mechanism and kinetics at the cathode of this system. The electrochemistry of the ORR in non-aqueous electrolytes in the presence of K+ ions has only been cursorily examined without a detailed study
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The Journal of Physical Chemistry
of the kinetics3,25,26. Further, while KO2 is thermodynamically stable, K2O2 (Δ =-425.1 KJ mol1
) is also stable and has a more negative Δ , making a study of the kinetics critical for any
potential-window-based control of the discharge product. Figure 1(c) depicts the impact of solvent donicity on the O2 redox reaction. The shift from dimethoxyethane (DME) to DMSO as the electrolyte solvent (employing the same salt KCF3SO3) resulted in the observation of two distinct reduction peaks along with the corresponding oxidation peaks as opposed to broad peaks observed in case of DME. Thus, the high donor number solvent dimethyl sulfoxide (DMSO) is employed in this study to solvate the K+ ions (thus modulating its Lewis acidity), promote its reaction with the soft Lewis base O2- 27–29 produced, and promote the formation of KO2 on the basis of Pearson’s hard-soft acid-base theory28. The detailed study of the ORR kinetics is carried out using the rotating ring-disk electrode (RRDE). The use of a RRDE for mechanistic elucidation and kinetic studies is well established for aqueous systems30–33 and has recently found increasing use in non-aqueous systems29,34–38. The RRDE in combination with a kinetic model of the K-O2 ORR was used herein to examine the mechanism and kinetics of this system. The possibility of the ORR occurring on the electrode surface and the electrolyte bulk is examined in conjunction with any possible side reactions. The polarization characteristics of DME and DMSO based electrolytes were further compared using a symmetric cell and the cathode products were imaged and spectroscopically characterized to verify the mechanistic predictions. Experimental All RRDE measurements reported herein were carried out in 1M Potassium trifluoromethanesulfonate (Sigma-Aldrich, 98%) in dimethylsulfoxide (DMSO) (Sigma-Aldrich, ≥99%). The electrolyte was prepared in a MBraun Ar filled glove box with H2O and O2 levels < 4 ACS Paragon Plus Environment
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0.5ppm. The KCF3SO3 salt was dried in a vacuum oven for 24 hours at 40˚C and the as-received DMSO was opened in the glovebox and allowed to degas in the Ar atmosphere to ensure absence of moisture before use. All the glassware used were cleaned with concentrated HNO3, washed with de-ionized (DI) water, immersed in CrH2O4 for 24 hours and then thoroughly washed again with DI water. The glassware was subsequently dried in a vacuum oven for 24 hours to ensure absence of moisture. The rate of water uptake in DMSO is very low (0.05 wt% water uptake after 1-hour exposure to air at 75% relative humidity (295 K))39 in the absence of stirring at room temperature, with exposure to 98% relative humidity (295 K) air for 5 days resulting in the mole fraction of water in DMSO being 2.17
39,40
. Thus, considering the precautions taken, the water
content is expected to be close to the assay value of
