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Revealing the Surface Effect of the Soluble Catalyst on Oxygen Reduction/Evolution in Li-O2 Batteries Zhen-Zhen Shen, Shuang-Yan Lang, Yang Shi, Jianmin Ma, Rui Wen, and Li-Jun Wan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12183 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019
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Journal of the American Chemical Society
Revealing the Surface Effect of the Soluble Catalyst on Oxygen Reduction/Evolution in Li-O2 Batteries Zhen-Zhen Shen1,2‡, Shuang-Yan Lang1,2‡, Yang Shi1,2, Jian-Min Ma3,4, Rui Wen1,2*, Li-Jun Wan1,2 1
Key Laboratory of Molecular Nanostructure and Nanotechnology, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 School of Physics and Electronics, Hunan University, Changsha 410082, China 4 Key Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry of Education, Zhengzhou University, Zhengzhou 450002, China ABSTRACT: Understanding the catalytic mechanism at nanoscale is essential for the advancement of lithium-oxygen (Li-O2) batteries. Using in situ electrochemical atomic force microscopy (EC-AFM), we explored the interfacial evolution during the Li-O2 electrochemical reactions in dimethyl sulfoxide (DMSO)-based electrolyte, further revealing the surface catalytic mechanism of the soluble catalyst 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ). The real-time views showed that during discharge flower-like Li2O2 formed in the electrolyte with DBBQ but small toroid without DBBQ. Upon charge, Li2O2 decomposes at a slow rate from bottom to top in the absence of DBBQ, yet with an outside-in approach in the presence of DBBQ. Larger discharge products and more efficient decomposition pathways in the DBBQ-containing system reveal the catalytic activity of DBBQ straightforwardly. Our work provides a direct insight into the surface effect of soluble catalyst DBBQ on Li-O2 reactions at nanoscale, which is critical for the performance optimization of Li-O2 batteries.
1. INTRODUCTION Rechargeable nonaqueous lithium-oxygen (Li-O2) batteries have attracted worldwide attention because of their ultrahigh theoretical specific energy, 3458 Wh Kg-1. However, the insulation of the final discharge product lithium peroxide (Li2O2) and the slow kinetics of electrode reactions lead to high overpotential, low coulombic efficiency and low rate capability in Li-O2 batteries.1-3 Using suitable catalysts is the key to improve the sluggish oxygen reduction reaction (ORR) and oxygen evolution reduction (OER) in the Li-O2 batteries.46 It is shown that the soluble catalysts, such as viologens,7 phthalocyanines8 and quinones9 et al. play an important role in assisting the occurrence of electrode reactions. Getting insight into the interfacial mechanism of the soluble catalysts in a LiO2 battery is essential for further optimizing the catalytic capability. Sun et al. found that the dissolved iron phthalocyanine (FePc) was easy to coordinate with O2 and Li2O2. Then the Fe−oxygen coordination and the electron dislocation in the big conjugated structure could promote ORR and OER to occur.8 Chen et al. measured standard heterogeneous electron transfer rate constants of several different classes of redox mediators and their rate constants for the oxidation of Li2O2. The results reveal that the oxidation of Li2O2 by the redox mediators is mainly an inner-sphere process. Besides, the steric hindrance as the mediator approaching the surface of Li2O2 plays a key role in influencing the kinetics of Li2O2 oxidation.10 Although the process has been achieved in the study of soluble catalytic
mechanisms in Li-O2 batteries, the catalytic details are not yet fully revealed due to the limitation of the in situ techniques with both high temporal and spatial resolution. In situ electrochemical atomic force microscopy (EC-AFM) with high spatial resolution and low invasion has demonstrated its unique strengths in exploring the detailed interfacial catalytic mechanism at nanoscale and been applied in several studies. Using in situ EC-AFM, Can et al. found that a soluble catalyst tetrathiafulvalene cation (TTF+) could give assistance for removing small Li2O2 particles (10-20 nm in lateral dimension) on the gold electrode by a homogeneous oxidation pathway.11 Recently, Misun et al. reported that the addition of 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO) in the electrolyte could aid in fast decomposition of film Li2O2 (around 3 nm in thickness) and obviously suppress the OER overpotential. 12 However, the mediated reaction pathway and surface dynamics at nanoscale for the soluble catalysts is still far from deep understanding, especially in the case of acting on the individual Li2O2 particle. Here, we focus on exploring the interfacial catalytic mechanism of a typical soluble catalyst, 2,5-di-tert-butyl-1,4benzoquinone (DBBQ),13-14 in a Li-O2 battery with dimethyl sulfoxide (DMSO)-based electrolyte. Using in situ EC-AFM, we observed the nucleation, growth, and decomposition of Li2O2 on a highly oriented pyrolytic graphite (HOPG) surface in a Li-O2 model cell. In the electrolyte without DBBQ, toroid Li2O2 formed at 2.46 V on discharge, and took a bottom-up way to decompose at 3.82 V on charge. In contrast, bigger
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flower-like Li2O2 nucleated at 2.70 V on ORR, and adopted an outside-in approach to oxidize on OER with the present of DBBQ. The monitored results demonstrate that the reaction activity, reversibility and the battery capacity are highly improved in the DBBQ-containing system. Our work provides direct insights into the interfacial catalytic mechanism of DBBQ at nanoscale, giving straightforward guidance for performance enhancement of Li-O2 batteries.
2. EXPERIMENTAL SECTION 2.1. Preparation of the Electrolytes. Dimethyl sulfoxide (DMSO) was purchased from Acros. Lithium bis(trifluoromethane sulfommide) (LiTFSI) and 2,5-di-tertbutyl-1,4-benzoquinone (DBBQ) were purchased from SigmaAldrich. DMSO was further dried over freshly activated molecular sieves (type 3 A) for a week, resulting in a final water content of
