Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Article
Analysis of Degradation Mechanisms in Quinone-Based Electrodes for Aqueous Electrolyte System via In-Situ XRD Measurements Takaaki Tomai, Hiroshi Hyodo, Daiki Komatsu, and Itaru Honma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08124 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018
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.
The Journal of Physical Chemistry C 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 20 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
The Journal of Physical Chemistry
Analysis of Degradation Mechanisms in Quinoneuinone-based Electrodes for Aqueous Electrolyte
in--situ XRD Measurements System via in Takaaki Tomai,* Hiroshi Hyodo,* Daiki Komatsu, Itaru Honma Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan
ABSTRACT: Organic materials are promising electroactive components of energy storage devices such as lithium-ion batteries and electrochemical capacitors. Among them, low-molecular weight organics have attracted attention as higher-energy-density, environmentally friendly, and inexpensive electrode materials, but its poor cycle performance is the main drawback. Using in-situ XRD measurement in aqueous electrolyte system, here we investigated the capacity fading mechanism of an organic electrode based on low-molecular weight quinones. Although the capacity fading of such organic electrodes is generally attributed to their elution into the electrolyte, our structural analysis reveals that the capacity fading is also associated to the expansion of an electrochemically inactive region, which persists in the electrode, but does not take part in the reversible redox reactions. Moreover, the detailed analysis of the XRD patterns suggests that the capacity fading of the electrode is accompanied by the crystal growth of organic component, which occurs through dissolution-reprecipitation processes taking place during charge-discharge cycling. The association between capacity fading and the increased size of these crystalline domains suggests that the elongated electrical/ionic conduction paths in the growing organic crystals (leading to the expansion of the electrochemically inactive region of the electrode) can be a possible capacity fading mechanism in organic electrodes. 1
ACS Paragon Plus Environment
The Journal of Physical Chemistry 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 20
INTRODUCTION Organic compounds have been widely investigated as promising electroactive components in electrochemical energy storage devices,1-3 such as Li-ion,4,5 Na-ion,6-8 and Mg-ion9,10 batteries, electrochemical capacitors11-13, and other devices.14-18 Among various promising candidates, low-molecular weight quinones can deliver a higher energy density than conventional inorganic electroactive components. In addition, they can be synthesized from renewable natural sources, resulting in a low carbon footprint and relatively economic materials. However, the main drawback of electrodes based on these compounds is represented by their poor cycling stability. Many research groups have recently been attempting to address this issue. The capacity fading of electrodes based on low-molecular weight organic active materials has been mostly linked to their elution into the electrolyte. In order to improve the cycle performance, various attempts have been made to reduce their solubility, for instance through polymerization,9,14,19,20 immobilization on carbon surfaces,21-24 and addition of functional groups.25-29 However, treatments such as polymerization, immobilization, and functionalization cause undesirable changes in the redox potential and add to the costs of the materials. Developing the robust electrodes composed of low-molecular weight organic active materials might replace the conventional inorganic electrodes in various types of energy storage devices. In order to reach this target, a detailed understanding of the degradation mechanism is crucial. In several recent studies on the structural and morphological aspects of organic compounds for energy storage applications,29-35 changes in crystal structure and morphology during charge-discharge cycling have emerged as important factors affecting the capacity fading of organic active materials.34,35 These are well-known causes of degradation in the case of inorganic active materials, as was shown using XRD analysis to highlight the structural 2 ACS Paragon Plus Environment
Page 3 of 20 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
The Journal of Physical Chemistry
changes that take place during charge-discharge operation.36-38 However, only a few studies have analyzed the structure of organic-based electrodes using XRD data, and in these studies the XRD analysis was applied only to the first few charge-discharge cycles to check the reversibility of the redox reactions involving organic compounds.32-35 Herein, we conducted in-situ XRD measurements of a quinone-based organic electrode subjected to over 1000 charge-discharge cycles in aqueous electrolyte, in order to elucidate its degradation mechanism from the structural and morphological viewpoints. From the XRD patterns obtained under charged and discharged conditions, the reversible transformation between quinone and hydroquinone at the cycle exhibiting high capacity was confirmed. As the charge-discharge cycling progressed further, the reversible XRD behavior had disappeared, accompanied by the capacity fading. The established mechanism, elution into the electrolyte, could not explain the capacity fading of a quinone-based electrode, but the in-situ XRD analysis revealed that the degradation mainly derives from the expansion of an electrochemically inactive domain of hydroquinones. This inactive portion formed during charge-discharge cycles persists in the electrode, but does not take part in the reversible redox reactions. Moreover, the detailed analysis of the XRD patterns suggests that the capacity fading was accompanied by crystal growth of hydroquinones, induced by dissolution-reprecipitation processes during the charge-discharge cycles. The growth of these organic crystals, resulting in the expansion of the inactive region of the electrode, can be a possible mechanism controlling the capacity fading of low-molecular weight quinone-based electrodes.
EXPERIMENTAL The experimental system consisted of an aqueous electrochemical cell. As a model 3 ACS Paragon Plus Environment
The Journal of Physical Chemistry 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 20
electroactive component, we employed anthraquinone (AQ), which has been shown to exhibit
reversible
redox
behavior
in
processes
involving
protons
and
lithium
ions.12,14-16,18,19,21,28,29,32 AQ was employed as the composite with the nanoporous carbon in the working electrode. The weight ratio of AQ to the nanoporous carbon in the composite was at 7:3. Our previous study39 indicated that the AQ-carbon composite with such high loading ratio of AQ shows inferior capacity retention rate (with a
