Subscriber access provided by GRIFFITH UNIVERSITY
Article
Synergistic Integration of Soluble Catalysts with Carbon-Free Electrodes for Li-O2 Batteries Won-Jin Kwak, Sung Hoon Ha, Do Hyung Kim, Kyu Hang Shin, Yang-Kook Sun, and Yun Jung Lee ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02359 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 28
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
ACS Catalysis
Synergistic Integration of Soluble Catalysts with Carbon-Free Electrodes for Li-O2 Batteries Won-Jin Kwak‡, Sung Hoon Ha‡, Do Hyung Kim, Kyu Hang Shin, Yang-Kook Sun* and Yun Jung Lee* Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea
Abstract
The instabilities associated with solid catalysts and carbon electrode materials are one of the challenges that prevent Li-O2 batteries from achieving a truly rechargeable high energy density. Here we seek to achieve reversible Li-O2 battery operations with high energies by tackling these instabilities. Specifically, we demonstrate synergistic integration of dual soluble catalysts (2,5-ditert-butyl-1,4-benzoquinone (DBBQ) for discharging and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) for charging) with antimony tin oxide (ATO) non-carbon electrodes with a porous inverse opal structure. The dual soluble catalysts showed a synergistic combination without any negative interference with one another, leading to higher capacity and rechargeability. Moreover, non-carbon pATO (porous antimony tin oxide) cathodes guaranteed improved stability against catalyst degradation, while KB carbon electrodes severely threatened stability of the soluble catalysts during cycling. We also found that the surface properties of the electrodes influenced the
ACS Paragon Plus Environment
1
ACS Catalysis
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
Page 2 of 28
discharge mechanism even in the presence of a solution phase growth promoter such as DBBQ, which implies that further interface engineering may improve the performance. This study shows the great potential of the integration of soluble catalysts with electrode materials for further improvements in capacity, energy efficiency, and rechargeability for the practical development of Li-O2 batteries.
KEYWORDS: Lithium-oxygen batteries, redox mediator, non-carbon electrode, TEMPO, DBBQ
1. INTRODUCTION As the demand for mid- to large-sized battery systems for electric vehicles and energy storage systems (ESSs) has increased, there have been many studies on next-generation secondary batteries with energy densities beyond that of the current lithium ion batteries. Lithium oxygen (Li-O2) batteries are a representative next-generation high energy battery system that can theoretically deliver an energy density of 11,140 Wh kg -1 based on Li metal and 3,370 Wh kg -1 based on Li2O2 through the electrochemical reaction of lithium and oxygen (2Li + O 2 ↔ Li2O2, Eo = 2.96 V).1-3 The electrochemical reaction of lithium and oxygen occurs at the cathode. As such, air cathodes of Li-O2 batteries need (1) a porous structure to provide sufficient mass transport of the reactants (oxygen and electrolyte), (2) a sufficient surface to accommodate solid discharge products, and (3) a high electronic conductivity to deliver electrons for the electrochemical reaction. Various carbon materials satisfy most of the above requirements, and these have been employed as cathode materials in typical Li-O2 batteries.4-6 However, instabilities have been reported in carbon materials such as corrosion of carbon by reactive reduced oxygen species and reaction of carbon with the discharge products. Such undesirable reactions impede the reversible
ACS Paragon Plus Environment
2
Page 3 of 28
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
ACS Catalysis
Li-O2 operations for truly “rechargeable” systems. 7 Therefore, various non-carbon (carbon free) materials (TiC,8 Au,9 Ag,10 Co3O4,11 ITO,12 ATO,13 TiO214) have been suggested as potentially reversible substitutes for carbon materials. Non-carbon air cathodes are typically equipped with solid catalysts to supplement the catalytic ability for the formation and decomposition of Li2O2.1213
However, the catalytic ability of solid catalysts in Li-O2 cells is not fully operative and does not
completely decompose the Li2O2 during charging since solid catalysts can be inactivated during cycling. The discharge products and by-products in Li-O2 batteries are solids, therefore immovable solid catalysts on the cathode can be covered by these products. Once the catalysts are fully covered, they are isolated and are therefore inactive. Moreover, solid catalysts can catalyze both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and accelerate decomposition of the electrolyte and electrode.15-17 Recently, redox mediators16-22 for OER and discharge product formation mediators23 for ORR have been studied as soluble catalysts. These have been dissolved in electrolyte solvents and have been very effective in decreasing overpotential during charging and increasing the discharge capacity during discharge, respectively. However, there are few studies on systems employing non-carbon electrodes combined with soluble catalysts. Also, the effects and problems of simultaneous utilization of multiple soluble catalysts have rarely been explored. Herein, we demonstrate the synergistic integration of the dual soluble catalysts 2,5-di-tert-butyl1,4-benzoquinone (DBBQ) for discharge and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) for charge with an antimony tin oxide (ATO) non-carbon electrode. DBBQ was reported to promote solution phase formation of Li2O2 during discharge, which significantly increased the capacity. 23 TEMPO has been used as a redox mediator for OER during charge. 17 ATO has been reported to have significant ORR catalytic activity among the non-carbon electrode materials.13, 24 ATO was
ACS Paragon Plus Environment
3
ACS Catalysis
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
Page 4 of 28
engineered to have an inverse opal structure with regular pores (porous ATO, pATO). 25-27 We studied the effects of DBBQ on discharge and TEMPO on charge in a non-carbon pATO electrode when these two soluble catalysts were used individually or simultaneously in Li-O2 batteries. The dual soluble catalysts showed synergistic effects of ORR and OER promotion for enhanced capacity and energy efficiencies without any interference with one another. These results illustrate the importance of addressing both ORR and OER activity simultaneously. The non-carbon pATO showed improved stability against soluble catalyst degradation while KB carbon electrodes severely degraded the soluble catalysts during cycling, emphasizing that the stability of soluble catalysts can be controlled by choosing the proper material for the electrodes.
2. EXPERIMENTAL METHODS 2.1. Synthesis of poly methyl methacrylate templates Deionized water (200 mL) and methyl methacrylate (MMA, 50 mL) were added to a 3-necked round-bottomed flask equipped with a mechanical stirrer (glass shaft with Teflon stirrer blade), a water-cooled reflux condenser, and a thermocouple probe. The reaction was performed under slow nitrogen flow. This mixture was stirred (∼350 rpm) while being heated to 80°C. After the reaction mixture was stabilized for 15 min, 0.1875 g of 2,2′-Azobis(2-methylpropionamidine) dihydrochloride (granular, 97 %, Sigma Aldrich) as initiator was added and the polymerization reaction was started. The colloidal suspension turned milky white after a few minutes and the reaction temperature was then maintained for 1 h. After the polymerization was completed, the round-bottomed flask was stoppered, placed on a cork stand, and permitted to cool for 1 h. The resulting colloidal PMMA spheres were then filtered through glass wool to remove large agglomerates. The PMMA was packed via centrifugation at 13000 rpm for 30 minute (2 times).
ACS Paragon Plus Environment
4
Page 5 of 28
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
ACS Catalysis
Afterward, the supernatant was decanted, and the bulk solid was dried under ambient conditions for at least 1 week. The PMMA colloidal crystals were lightly crushed to form a powder before use.
2.2. Synthesis of porous antimony tin oxide 5 g of pre-synthesized PMMA was added to 4 mL of DI water, and the solution mixture was sonicated for 1 h followed by stirring for 2 h (solution I). 1 g of F-127 (Pluronic® F-127, Sigma Aldrich) was stirred for 3 h in a solution (ethanol 3 ml and DI water 2 ml) containing 0.32 g of SbCl3·xH2O and 3.15 g of SnCl4·xH2O (solution II). The two solutions (solution I and II) were mixed and stirred for 1 h. Then, mixed solution was filtered and dried at room temperature for 2 days. Finally, pATO was prepared by annealing the dried ATO precursors-PMMA mixture at 450°C for 2 h in the furnace under air with a slow heating rate (1.0 °C min-1) to avoid the destruction of the inverse opal porous structure.
2.3. Preparation of Li−O2 batteries To prepare the cathode, a slurry was prepared by mixing 90 wt% synthesized porous antimony tin oxide (pATO) or commercial ATO nanopowder (Alfa Aesar), 10 wt% polyvinylidenefluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). Then, the slurry mixture was coated on a nickel foam (MTI) current collector and it was punched into circular pieces of 1.2 cm in diameter (15~16 mg cm-2 loading). The cathodes were dried at 80 °C under vacuum for 24 h. 1M bis(trifluoromethane)sulfonimide
lithium
salt
(LiTFSI,
99.95%,
Sigma
Aldrich)
in
tetraethyleneglycol dimethylether (TEGDME, >98%, TCI) was used as the electrolyte with or without soluble catalysts (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO, 98%, Sigma Aldrich)
ACS Paragon Plus Environment
5
ACS Catalysis
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
Page 6 of 28
and 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ, 99%, Alfa Aesar). Solvent and salt were thoroughly dried on molecular sieves and vacuum drying, respectively until the final water content
