Tuning the Morphology of Li2O2 by Noble and 3d metals: A Planar

Article

Tuning the morphology of Li2O2 by noble and 3d metals – a planar model electrode study for Li-O2 battery Yao Yang, Wei Liu, Nian Wu, Xiaochen Wang, Tao Zhang, Linfeng Chen, Rui Zeng, Yingming Wang, Juntao Lu, Lei Fu, Li Xiao, and Lin Zhuang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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ACS Applied Materials & Interfaces

Tuning the morphology of Li2O2 by noble and 3d metals – a planar model electrode study for Li-O2 battery

Yao Yang1,2, Wei Liu1, Nian Wu1, Xiaochen Wang1, Tao Zhang1, Linfeng Chen1, Rui Zeng1, Yingming Wang1, Juntao Lu1, Lei Fu1, Li Xiao1* and 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 In this work, a planar model electrode method has been used to investigate the structure-activity relationship of multiple noble and 3d metal catalysts for the cathode reaction of Li-O2 battery. The result shows that the battery performance (discharge/charge over-potential) strongly depends not only on the type of catalysts, but also on the morphology of the discharge product (Li2O2). Specifically, according to electro-chemical characterization and scanning electron microscopy (SEM) observation, noble metals (Pd, Pt, Ru, Ir and Au) show excellent battery performance (smaller discharge/charge over-potential), with worm-like Li2O2 particles with size less than 200 nm on their surfaces. On the other hand, 3d metals (Fe, Co, Ni, and Mn) offered poor battery performance (larger discharge/charge over-potential), with much larger Li2O2 particles (1 µm to a few microns) on their surfaces after discharging. Further research shows that a “volcano plot” is found by correlating the discharging/charging plateau voltage with the adsorption energy of LiO2 on different metals. The metals with better battery performance and worm-like shaped Li2O2 are closer to the top of the “volcano”, indicating adsorption energy of LiO2 is one of the key characters for the catalyst to reach a good performance for the oxygen electrode of Li-O2 battery, and it has a strong influence on the morphology of the discharge product on the electrode surface.

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Keywords: Li-O2 battery; catalytic activity; Li2O2 morphology; planar electrode; noble and 3d metals; adsorption energy of LiO2; volcano plot Introduction Aprotic Li-O2 batteries has been focused for their high theoretical specific energy density (3500 Wh/kg), which is much higher than that of Li-ion batteries.1-13 Recent attention has been paid to make these Li-O2 batteries rechargeable, which, nonetheless, still have several great challenges to be addressed.14-15 One of the most prominent challenges is the large over-potential on charging at even very low current densities,16-24 which requires a fundamental understanding of the structure-activity relationship between the catalysts and the oxygen reduction/evolution reaction (ORR/OER). Unlike ORR/OER in aqueous media, in which the reaction products are soluble; in aprotic media, the main discharge product of Li-O2 battery is insoluble/non-conductive Li2O2 solids, which may block the gas transportation channels and increase the reaction over-potential. Herein, the morphology and crystalline structure of Li2O2 are expected to play a crucial role in determining the cell capacity, the over-potential and the cyclability.25 Several groups pointed out that the morphology of Li2O2 depends on the discharging current density.26-30 Smaller discharging current density results in Li2O2 nano-crystallites (several hundred nm), while larger discharging current density leads to Li2O2 quasi-amorphous thin films. The latter provides a lower over-potential on charging due to their closer contact with the electrode surface. In contrast, the relationship between the morphology of Li2O2 and the composition of catalysts is still unclear. There are mainly two kinds of research on this topic. First kind of research shows the composition of catalysts has a very strong impact on the morphology of Li2O2. For example, a toroidal morphology of Li2O2 was found on many kinds of carbon materials, including carbon black,27 carbon nanotube (CNT),29 Ketjen Black (KB) carbon,31 and reduced graphene oxide,32 etc. However, by introducing metal or metal oxide catalysts to these carbon materials, the morphology of Li2O2 can be significantly changed.21,33-39 Specifically, after introducing RuO2 to CNT, the morphology of Li2O2 was changed from the irregularly dispersed and crystalline toroid to the well dispersed and poorly crystalline


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nano-sheet.33 Similarly, after introducing PtRu on XC-72 carbon, the morphology of Li2O2 was changed from rod-shape to mud-shape.34 Research for Pd/C catalysts also shows the similar result.35-37 However, the second kind of research shows the composition of catalysts has small impact on the morphology of Li2O2. For instance, by introducing MnRu to carbon,40 the over-potential on charging can be effectively reduced from 4.2 V to 3.7 V, but no obvious difference was found on the morphology of Li2O2 on the surface of the two catalysts. Similarly, same donut-shaped Li2O2 was observed on both Vulcan carbon and Au/C.41 These contradictions in the morphology of Li2O2 on carbon and metal/carbon powder electrodes indicate that the relationship between the morphology of Li2O2, the catalysts, as well as the battery performance has not been fully understood yet. In this work, we attempt to systematically study the intrinsic growth differences of Li2O2 on different catalysts. Instead of using commonly used powder catalysts, we employed magnetron sputtering method to synthesize ten kinds of planar metal electrodes, including Pd, Pt, Ru, Ir, Au, Fe, Co, Ni, and Mn. Graphene films (G) made by chemical vapor deposition (CVD) were used to represent carbon materials and compared with those metal electrodes. The use of planar electrodes brings many benefits: to exclude the morphology influence of the powder catalysts themselves, provide a uniform current density on the electrode, and maintain a similar surface structure of all the catalysts, which is very hard to achieve for the synthesis of powder catalysts. By using a special electro-chemical cell and the planar model electrode method (Figure 1), the relationship between the battery performance and the morphology of Li2O2 on different catalysts was investigated. Catalysts are grouped as “good” and “bad” ones based on their cell performance. The SEM characterization shows that the “good” catalysts favor the worm-like morphology of Li2O2, with size less than 200 nm and high coverage on the electrode surface. On the contrary, the “bad” catalysts prefer much larger sized Li2O2 grains (from 1 micron to a few microns). Deeper understanding of this structure-activity relationship was obtained by DFT calculation, which correlated the plateau of voltage profile with the adsorption energy of LiO2. The result turned out to be a “volcano plot”, and the catalysts with better cell performance and worm-like shaped Li2O2 are closer to the top of the “volcano”, indicating the proper adsorption energy of LiO2 will influence not only the electrochemical activity, but also



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the morphology of the Li2O2 of these electrodes. This result sheds a new insight to design better catalysts for oxygen electrode of Li-O2 batteries. Experimental Section Preparation of planar model electrodes. Planar metal electrodes were prepared by magnetron sputtering (JSD300-II Anhui Jiashuo Vacuum Tech., China) on Ti substrates (0.5mm). After the chamber pressure was reduced to less than 6×10-4 Pa by the sputter-ion pump, the chamber was refilled with Ar (99.99% purity) and the working pressure was fixed at 1Pa. Sputtering process lasted for 20 min at the sputtering current of 0.2 A and the sputtering voltage of 280 V by using metal targets with a purity of 99.99%. As a strong magnetron metal, Fe is hard to sputter so that thin-film Fe (99.9%) was purchased directly. All the sputtered thin-film metals were cut into electrode disks (1×1 cm2). Graphene film was prepared by Chemical Vapor Deposition (CVD) method on Nickel foil substrate. Nickel foils (Alfa aesar, 0.025mm thick, annealed, purity > 99.5 wt.%) were cut into electrode disks (1×1 cm2) and placed in a quartz tube.42 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.). Structural characterization of the oxygen electrode. Field emission scanning electron microscopy (FESEM, ZEISS, EHT=20 kV) was employed to determine the structure and morphology of the catalysts and the electrodes. Confocal Raman microscopy (Reinshaw, inVia, 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 model electrodes, a special electro-chemical cell was built with polytetrafluoroethylene (PTFE) and the gap between two layers was block by O-ring made by fluororubber to prevent possible electrolyte leaking (Figure 1). The planar electrode was arranged in parallel with the lithium electrode so that the electric field could be created as evenly as possible when the batteries worked. This device was assembled inside the glove box


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([H2O] Ru > Ir > Au, which is in consistent with the catalytic activity trends found by H. Gasteiger and Y. Shao-Horn.52 The plateau voltage of these noble metals is obviously higher than the typical plateau voltage (~2.6V) for carbon loaded metal catalyst in most literatures.2-3 We believe that it is because for carbon loaded metal catalysts, Li2O2 can deposit on both active metal surface and less active carbon surface on discharging, so the plateau voltage actually represents the mixed performance of metal and carbon, which is lower than what we observed on pure noble metals. On charging, the potential of the noble metal group (except Au) is around 3.3V, which is



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lower than that of most carbon loaded metal catalysts, but very similar with reported Pd/C electrodes (3.2 V) when the defects of carbon were protected by Al2O335. According to the literature, the formation of byproducts, such as Li2CO3 or LiRCO3 [R=alkyl], will increase the charge voltage. 17 These byproducts are formed by decomposition of electrolyte or reaction between carbon and Li2O2.15 Because these byproducts are much difficult to decompose than Li2O2, thus the charge voltage is usually higher than 3.5 V for metal catalysts supported by carbon. In this work, because the plateau voltage only represents the performance of the metal catalysts, so the discharge over-potential of these noble metals are lower than the carbon loaded metal catalysts. The plateau voltage of Au on charging is 3.68V, which is similar to the observation on pure nonporous Au electrode (3.75 V) by Peter Bruce,16 also indicating the plateau voltage we observed representing the performance of pure metals. In our previous work, we speculated the morphology of Li2O2 is related to the interaction between oxygen containing species and the surface of the catalyst by using Pd and graphene as the model catalysts.49 In these work, based on more systematic morphology data of Li2O2 on varies model catalysts, we can further explore the relationship between the morphology of Li2O2, the battery performance and the intrinsic property of the catalysts. DFT studies of surface interactions and morphology Based on the experimental evidence including SEM and battery performance of ten planar model electrodes, we find that different surface material strongly influences the morphology of Li2O2 on the electrodes and the over-potential on discharging/charging. In order to explore the relationship between the surface electronic structure of these electrodes and their performance at atomic level, density functional theory (DFT) calculations were performed to study the interaction between Li2O2 and the ten electrodes. Considering that the first step of ORR is the oxygen adsorption to form LiO2 on the electrode surface,50-51 it is reasonable to assume that the interaction between oxygen containing species and electrode surface can greatly influence the growth of Li2O2 and further indirectly determine the over-potential on charging. In this work, we chose the adsorption energy of LiO2 on the electrode surface as a descriptor of the surface electronic structure, and the DFT optimized configurations of LiO2 on these electrode surfaces are shown in Figure S2.


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As shown in Figure 5, the discharge/charge plateaus voltage of the ten electrodes exhibit a “volcano shape” as a function of adsorption energy of LiO2. A similar volcano shape was found by H. Gasteiger and Y. Shao-Horn between the Li+-ORR activities and the oxygen adsorption energy of noble metal surfaces.52 In this work, we explored not only the relationship between battery performance and adsorption energy of LiO2 on noble metals, 3d metals and graphene, but also the morphology of Li2O2 on these electrodes. In aprotic media, the first electron reduction most likely proceeds by the formation of superoxide species such as O2¯ and LiO2.53-55 On surface with weak binding with LiO2, such as graphene and Au, LiO2 may disproportionate or undergo a second electron reduction to form Li2O2. Because the interaction between LiO2 and the graphene surface was so weak that LiO2 had enough time to grow as perfect crystals on graphene (Figure 3). For Au, because the interaction between LiO2 and its surface became a bit stronger, some of LiO2 had enough time to grow as big particles on it, but some of LiO2 quickly nucleated on its surface to form smaller particles with worm-like morphology. On surfaces with increasing binding energy with LiO2, such as Pt and Pd (moving right for the left branch of the volcano in Figure 5), LiO2 quickly nucleated on its surface to form smaller particles with worm-like morphology. These worm-like Li2O2 particles are believed to have amorphous structure, which is easy to decompose on charging. Catalysts having this kind of morphology of Li2O2 on its surface will have lower over-potential on charging, which is in agreement with our experimental results. On further increasing the binding energy of LiO2 on surfaces such as Ir and Ru, adsorbed oxygen species may bind a bit strongly to the surface, so the morphology of Li2O2 became different of that was on Pd and Pt: the worm-like particles still exist, but bigger sized grains could be found in between of those worm-like particles. The battery performance was decreased as well. Ru is a little bit special, because its adsorption energy of LiO2 is close to Co and Ni, but its battery performance is much better than these two catalysts. The possible reason may come from its unique hcp crystal structure, while all the other noble metal crystals are fcc crystal structures, which may cause a calculation deviation. When the binding energy of LiO2 on surface became too strong, such as Co, Ni, Fe, the size of the Li2O2 on their surface became large, with randomly shaped particles from 1 micron to a few microns. These large-size Li2O2 are considered to be difficult to decompose, resulting in high over-potential



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on charging, which is consistent with our experimental result. Note that we excluded Mn in our DFT calculation, mainly because the unit cell and crystal facet of Mn is so complicated that it is very hard to find the most stable facet of Mn for calculation, thus the adsorption energy of LiO2 on Mn is not convincing enough to give a reasonable scientific prediction. It is to mention that in our previous work, we proposed two reaction mechanisms and used a figure sketching to explain the connection between the types of electrode (Pd and graphene) and Li2O2 morphology49, we regard this work as a follow-up paper of our previous one thus did not use a similar figure to sketch the proposed mechanisms. Conclusions. In Summary, by using a planar model electrode method, we built up a relationship between the morphology of Li2O2, the battery performance and intrinsic property (adsorption energy of LiO2) of ten electrodes, including noble metals, 3d metals and graphene, by experiments and DFT calculations. Noble metals (Pd, Pt, Ru, Ir) with moderate adsorption energy of LiO2 show the best battery performance and share a worm-like morphology of Li2O2. 3d metals (Fe, Co, Ni, and Mn) with too strong adsorption of LiO2 show low battery performance and share a randomly sharped morphology of Li2O2 with larger size. The surfaces of Au and graphene has weak adsorption of LiO2, and as a result, the morphology of Li2O2 on their surface was different from the above surfaces, with both worm-like and randomly shaped Li2O2 on Au and near perfect crystals on graphene. Their battery performance was in between of the above two groups. In our previous paper49, electrochemical impedance spectroscopy (EIS) was used to prove that those small worm-like amorphous Li2O2 on Pd have much smaller charger transfer resistance than those larger crystalline Li2O2 on graphene, thus making it kinetically much easier to transfer Li+ and O22- when Li2O2 is forming and decomposing in discharge/charge process. Such difference on the morphology of Li2O2 leads to lower over-potential for discharge/charge profile on Pd. This correlation between the morphology of Li2O2 and the battery performance is also applicable on the other 8 kinds of electrodes in this work. The planar model electrode idea combined with the magnetic sputtering method can become a powerful approach to screen the catalysts for Li-O2 batteries, including binary,


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ternary metal alloys or metal oxides as the model electrodes. This work shed a new light on understanding the correlation between the morphology of Li2O2 and battery performance, and provided valuable insights to the development of non-precious and more active catalysts for Li-O2 battery.

Supporting Information Discharge-charge curves of ten planar model electrodes (Figure S1)

ACKNOWLEDGEMENTS

This work was financially supported by the National Natural Science Foundation of China (21573167, 21125312, 21473124), the National Basic Research Program (2012CB932800, 2012CB215500), the Doctoral Fund of Ministry of Education of China (20110141130002) and the Fundamental Research Funds for the Central Universities (2014203020207). We are grateful to Prof. Lei Liao and Dr. Gongwei Wang at Wuhan University for their help in magnetron sputtering. We appreciate the inspiring discussion about DFT calculation with Dr. Bing Huang at Wuhan University. We thank Prof. Xiaodong Zhou at Wuhan University for his generous help in SEM characterization. We also thank Luxi Shen at Cornell University for her help for editing the manuscript.



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Chem. Lett. 2016, 7, 2803−2808.



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Figure1: Schematic of the special electro-chemical cell. (A) Semi-transparent schematic to see the inside of the cell; (B) Outside of the device.


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Figure2: Raman spectra of electrodes at initial, discharge and charge states.



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Figure3: SEM images of pristine electrodes and electrodes after discharge (1 ߤA/cm2). Pristine1: planar Pd electrode made by magnetron sputtering method (represents Pt, Ru, Ir, Au, Co, Ni and Mn) before discharge; Pristine2: graphene electrode before discharge; Pristine3: Ti electrode before discharge. Other images stand for the surface of Graphene film (G), Au, Pd, Pt, Ir, Ru, Fe, Co, Ni and Mn after discharging. All scale bars are 800 nm.


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Figure4: Plateau voltage on discharge/charge of ten planar electrodes. G represents graphene.



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Figure5: Volcano-type relationship between discharge/charge plateau voltage and the adsorption energy of LiO2 on the planar model electrodes.


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Tuning the Morphology of Li2O2 by Noble and 3d metals: A Planar

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