A Graphene Model Electrode S - ACS Publications - American

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Tuning the morphology and crystal structure of Li2O2 — A graphene model electrode study for Li-O2 battery Yao Yang, Tao Zhang, Xiaochen Wang, Linfeng Chen, Nian Wu, Wei Liu, Hanlin Lu, Li Xiao, Lei Fu, and Lin Zhuang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05660 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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ACS Applied Materials & Interfaces

Tuning the morphology and crystal structure of Li2O2 — A graphene model electrode study for Li-O2 battery Yao Yang1,2, Tao Zhang1, Xiaochen Wang1, Linfeng Chen1, Nian Wu1, Wei Liu1, Hanlin Lu1, Li Xiao1*, Lei Fu1, Lin Zhuang1

1

College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China

2

Present Address: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York, 14853, USA *E-mail: [email protected]

Abstract The performance and the cyclability of the Li-O2 batteries are strongly affected by the morphology and crystal structure of Li2O2 produced during discharge. In order to explore the details of growth and electrochemical decomposition of Li2O2, and its relationship with the cell performance, graphene films were used as model carbon electrodes and compared with electro-deposited Pd nanoparticles (NPs) on graphene. Multiple methods, including transmission/scanning

electron

microscopy

(TEM/SEM),

Raman

spectroscopy,

electrochemical impedance spectroscopy (EIS), and coin cell charge/discharge test were employed for material characterization and reaction monitoring. The results showed that the presence of Pd NPs significantly changed the growth, morphology and crystal structure of Li2O2 and reduced the charge over-potential by 1060 mV. All of these changes are ascribed to the stronger binding energy between LiO2 and the Pd surface, resulting in the generation of -

amorphous Li2O2 with higher ionic conductivity of Li+ and O22 , which in turn improve the cell charging performance.

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KEYWORDS: Li-O2 battery; overpotential; Li2O2 morphology; graphene film; crystalline; amorphous; electrocatalysis; adsorption energy of LiO2

Introduction Li-O2 batteries possess the highest theoretical energy density among all the secondary batteries,1-12 but these devices face several tough challenges for the further development, including high over-potential and poor cyclability.13-18 Recent research has shown that by using proper catalysts, including precious metals,19-26 nonprecious metals and metal oxides,27-36 the over-potential on charging can be significantly reduced. However, the structure-activity relationships of the catalysts are still unclear. There are several contradictory opinions on whether the catalysts can affect oxygen reduction to Li2O2 during discharge. Some researchers believe that the catalysts with different oxygen adsorption energy can reduce the over-potential at the beginning of discharge, and this characteristic may be used to design highly active electrodes.25 Others propose that the catalysts have no effect on the discharge process. They suggest the slight catalytic advantages vanish very fast as the discharging process goes on, since the solid Li2O2 generated during discharge covers the surface of the catalyst, blocks the active sites, and further de-activates the catalysts.1 In contrast, there is no doubt that catalysts are quite useful during evolution of O2 from Li2O2 during charging, but still some uncertainties exist. For example, most of the research indicates that much smaller Li2O2 particles form on highly active electrode surfaces compared to pure carbon surfaces.1 This phenomenon begs a key question: if catalysts can affect the size of Li2O2 produced on discharging, why does the size of Li2O2 particles remain small, even after the surface of the catalysts becomes fully covered by Li2O2? More specifically, the fractional surface coverage of Li2O2 is different on diverse electrodes. For example, active electrode surfaces usually have high Li2O2 coverage, while the pure carbon surfaces have much lower Li2O2 coverage.24 We are interested in studying the origin of decreased discharge over-potentials on certain electrodes. The improved performance may come from the catalytic activity of the catalyst, or from the disparate morphology of the Li2O2 formed upon discharging, or a combination of both. Morphological


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improvement in particular might be due to the small Li2O2 particle size, leading to intimate contact with the electrode that would benefit the electron transport at the Li2O2/electrode interfaces.30 In the present work, a special cell was designed and fabricated to facilitate the use of graphene films as model electrodes to study Li2O2 electrochemistry.36 Using this cell with graphene film electrodes, the growth process of Li2O2 at the initial discharging stage was clearly tracked by SEM and TEM. Ex-situ scanning electron microscopy (SEM) captured the growth process of Li2O2 from small particles to bulk crystallites. For comparison, graphene-loaded Pd NPs (Pd/G) were also investigated. We select Pd and graphene as the electrode material intentionally because these two materials are very representative. Pd was claimed to maintain the lowest over-potential among all the reported catalysts and graphene film represents widely used carbon materials well.21,51 By combining with the planar model electrodes study method, it is hoped to observe the intrinsic growth difference of Li2O2 on these two materials. SEM characterization of these mixed films indicated that only small Li2O2 particles were produced. High-resolution transmission electron microscopy (HRTEM) and selected electron diffraction (SAED) were used for further structure observation of the Li2O2 formed on both graphene and Pd/G electrodes, and two distinct structures comprised of crystalline and amorphous Li2O2 were observed. Cell performance for Pd/G was much better than graphene film electrode. We further used DFT calculations to explain the relevance of morphology and structure of Li2O2 on cell performance, leading to proposed models for growth of Li2O2 on each of the electrode types. In summary, this paper focuses not only on the composition of electrodes but also on the successful application of planar model electrode to the study of structure-activity relationship for the catalysts in Li-O2 battery.

Experimental Section Preparation of graphene electrode Graphene films were prepared by chemical vapor deposition (CVD).38 Nickel foils (Alfa Aesar, 0.025mm thick, annealed, >99.5 wt.%) were cut into electrode disks (1×1cm) and placed in a quartz tube. Then they were heated to 1000 °C in a horizontal tube furnace (Lindberg Blue M, HTF55322C) under Ar (500 s.c.c.m.) and H2 (200 s.c.c.m.), and annealed for 5 min. CH4 (10 s.c.c.m.) was then introduced into the reaction tube. After 5 min of growth, the samples were cooled to room temperature under Ar (500 s.c.c.m.) and H2 (200 s.c.c.m.).



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Three dimensional graphene (3D graphene) was also prepared by CVD on nickel foam, followed by dissolution of the Ni. Nickel foams (Alantum Advanced Technology Materials) were cut into 1×1cm pieces and coated with graphene by the same CVD method as described above.These composites were drop-coated with a polymethyl methacrylate (PMMA) solution, and baked at 180°C for 30 min under air atmosphere. Then the sample was put into HCl (3 M) solution at 80°C for 3 h to dissolve the nickel substrate. Finally, free standing 3D graphene films were obtained by dissolving the PMMA in hot acetone at 55°C. Preparation of Pd/G electrode An electro-deposition method was used to deposit Pd NPs onto graphene substrates. The deposition process was performed in a deaerated 0.1M NaCl solution containing 0.5 mM H2PdCl4. Planar graphene and 3D graphene were used as the working electrodes, and a carbon paper served as the counter electrode and reference electrode. The potentiostat was a CHI660 electrochemical station. All chemicals were of at least analytical grade, and solutions were prepared using deionized water (Millipore, 18 MΩ•cm). Pd deposition proceeded using DC pulses of -0.1 mA/cm2. The pulse width was 2 seconds with a 50% duty cycle. Planar graphene surfaces were deposited with 125 total deposition pulses, whereas the 3D graphene films were deposited with 500 pulses. Structural characterization of the oxygen electrode. Field emission scanning electron microscopy (FESEM, ZEISS, EHT = 20kV) and high-resolution transmission electron microscopy (HRTEM, JEM-2100, U=200kV) were employed to determine the structure and morphology of the catalysts and the electrode. For HRTEM characterization, planar graphene supported by nickel foil cannot be analyzed, so we used 3D graphene without nickel foam. 3D graphene after discharging was sonicated for ten seconds in tetraethylene glycol dimethyl ether (TEGDME) for sample preparation. Confocal Raman microscopy (Reinshaw, inVia+Plus, λ=532 nm) was applied to identify the characteristic peaks of carbons and the formation and disappearance of Li2O2. Electrochemical measurements. For the study on planar electrodes, the experimental device was built from two layers of polytetrafluoroethylene (PTFE) separated by a gap defined by an O-ring made by


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fluororubber to prevent possible electrolyte leakage (Figure S1). The planar electrode was arranged in parallel with the lithium electrode to produce uniform polarization across the electrode surface. This device was then assembled inside the glove box ([H2O]

A Graphene Model Electrode S - ACS Publications - American

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